The Pennsylvania State University. The Graduate School. Eberly College of Science DEVELOPMENT OF VISIBLE, NEAR-INFRARED, AND MID-INFRARED

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1 The Pennsylvania State University The Graduate School Eberly College of Science DEVELOPMENT OF VISIBLE, NEAR-INFRARED, AND MID-INFRARED TRANSIENT ABSORPTION SPECTROSCOPY ON A FEMTOSECOND TO MILLISECOND TIMESCALE AND ITS APPLICABILITY TO ORGANIC ELECTRONIC MATERIALS A Dissertation in Chemistry by Adam Rimshaw 2016 Adam Rimshaw Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2016

2 ii The dissertation of Adam Rimshaw was reviewed and approved* by the following: John B. Asbury Associate Professor of Chemistry Dissertation Advisor Chair of Committee Enrique Gomez Associate Professor Mark Maroncelli Professor of Chemistry Benjamin Lear Assistant Professor of Chemistry Kenneth Feldman Graduate Program Chair *Signatures are on file in the Graduate School

3 iii ABSTRACT Nanosecond transient absorption instruments capable of 10-5 sensitivity were developed for visible, near-infrared, and mid-infrared spectral regions. These instruments were developed around new advances in analogue to digital (ADC) conversion called flexible resolution. Unlike the traditional ADC architecture, which interleaves multiple 8-bit ADCs to achieve higher resolution, the digitizers used within this manuscript allows multiple high-resolution ADCs to be applied to the input channels in different time-interleaved and parallel combinations to boost either the sampling rate or the resolution. The result is instrumentation capable of changing resolution (8-14 bit), which helps to increase the signal-to-noise ratio by more than an order of magnitude. For each instrument, the spectral and temporal range is explored with an emphasis on the sensitivity and collection time. The detection limit of both nanosecond instruments was calculated to be 10-5 O.D. Example materials were studied on both nanosecond instruments to show their capability in real-world applications with systems such as P3HT:PCBM, CN-MEH-PPV and TIPSpentacene. The mid-ir instrument provided some of most sensitivity spectra ever recorded in the mid-ir, such as alkyne stretches measuring < 100 μo.d. in TIPS-pentacene films on a nanosecond timescale. The ability to record such data within a reasonable time frame makes possible for significantly lower energy densities to be used in studying organic electronic materials. Our ultrafast laser system was expanded to perform visible pump visible pump spectroscopy and our preexisting visible pump mid-ir pump was redesigned with new technologies such as remote connectivity. All of the instrument s programming was redone using MATLAB streamlining our data collection and analysis. Important advances were made in terms of data acquisition time and statistical filtering using the new software as well. The

4 iv redesigned system allowed us to start exploring perylene diimide solutions for unique vibrational modes due to excimer formation. Lastly, these advances have opened the door to developing a new kind of ultrafast technique termed pump-push-probe, which allows for photophysics to be studied on functioning organic electronic devices.

5 v TABLE OF CONTENTS List of Figures... viii List of Tables... xiv Acknowledgements... xv Chapter 1 Fundamentals of Organic Photovoltaic Materials Nature of the Absorption Process Photophysics of Organic Photovoltaics Excitons Charge Transfer Charge Separation Polarons Charge Recombination Charge Extraction How an Organic Photovoltaic Device Operates Thesis Overview References Chapter 2 General Principals of Absorption Spectroscopy Steady-State Absorption Spectroscopy Nanosecond Transient Absorption Spectroscopy Optical Scheme Lasers and Light Sources Detectors Electronics Excitation Geometry Calculation of the Transient Absorption Signal Relationship between the Time Resolution and Signal-to-Noise Ratio Ultrafast Transient Absorption Spectroscopy Pump-Probe Technique Ultrafast Pulse Generation and Amplification Wavelength Tunability of Ultrafast Pulses Array Detectors References Chapter 3 Developing a High Performance Nanosecond Visible/Near-Infrared Transient Absorption and Photoluminescence Spectrometer Challenges with OPV materials Instrument Design Light Sources Optics... 48

6 vi Detectors Data Collection Hardware Data Collection Software Instrument Performance OPV Sample Preparation Spectral Range Temporal Range Sensitivity The Importance of Sequential Subtraction for Signal-to-Noise Enhancement Common Transient Absorption Artifacts for Organic Photovoltaic Materials Photoluminescence Artifacts Thermal Artifacts Conclusion References Chapter 4 Developing a High Performance Nanosecond Mid-Infrared Transient Absorption Spectrometer Challenges with Mid-Infrared Transient Absorption Light Sources and Collection Efficiency Detector Elements Previous State-Of-The-Art Dispersive Systems Step-Scan FTIR Instrument Design Optics Detectors Data Collection Software Instrument Performance Sample Preparation Spectral and Temporal Range Signal Detection Limit Examples of Performance in OPV Systems Vibrational Modes in OPV Materials Electronic Mid-IR Polaron Absorption in P3HT:PBCM Films Conclusion References Chapter 5 Development of Ultrafast Visible and Mid-Infrared Transient Absorption System Designing the Visible Pump Mid-IR Probe Ultrafast Transient Absorption Instrument Optical Layout Hardware Software Signal Measurement and Statistics Visible Pump Visible/NIR Probe Ultrafast Transient Absorption Optical Layout

7 5.2.2 Hardware Software Perylene Diimides Previous work Current Work: Excimers in Solution Conclusion References Chapter 6 Advancements and Future Directions Time-Resolved Mid-IR Spectroscopy of Block Copolymers Pump-push-probe (Electrooptical) Transient Absorption Spectroscopy Performing Electrooptical Measurements Advances in Data Collection through Sequential Subtraction Conclusions References vii

8 viii LIST OF FIGURES Figure 1-1. Jablonski diagram depicting the photophysical processes common to OPV materials and organic materials in general. The electronic states are label S 0 (singlet ground state), S 1 (singlet excited state), and T 1 (triplet excited state). Each electronic state has superimposed vibrational states (gray lines) and rotational states (not shown) that give the electronic states their bandwidth. The absorption process is depicted as a green line, fluorescence (red) and phosphorescence (maroon). Thermal relaxation (internal conversion) is depicted as a teal line and intersystem crossing between singlet and triplet states is shown in blue. In theory, there are many fluorescence and phosphorescence transitions; however, only one of the transitions is shown for clarity Figure 1-2. Energy offset of a donor and acceptor. The CT state is formed after exciton migration to the D/A interface where the electron (e - ) is transferred to the acceptor and the hole (h + ) remains on the donor. E binding is used as an approximation of the charge-transfer exciton binding energy Figure 1-3. Diagram depicting Marcus theory. The state S 0 refers to the donor ground state, S 1 is the donor excited state and CT is the D + /A - state, Q is the reaction coordinate, ΔG is the free energy, and λ is the reorganization energy. The intersection of the parabolas is the point where the geometry of the CT and S 1 states are the same Figure 1-4. Geminate recombination in OPVs is described through Onsager theory. The figure depicts the recombination and dissociation processes on an electron/hole pair based on the Coulombic capture radius r c. The thermalization length is labeled L, and k B is the Boltzmann constant, T is temperature, e - is an electron, and hν is the energy of a photon. The y-axis is potential energy Figure 1-5. Basic device architecture of an OPV device showing the top electrode (aluminum), an electron accepting/hole blocking layer (LiF), and the active layer, which is a heterogeneous mixture of the donor and acceptor molecules. The layer PEDOT:PSS is a hole accepting/electron blocking layer that also helps match the work function of the organic to the metal to increase charge injection into the bottom electrode ITO. The entire device is constructed on a glass substrate Figure 2-1. Basics of nanosecond transient absorption spectroscopy illustrating a pulsed laser source optically exciting a sample and a continuous wave probe (lamp) providing the detected signal. The monochromators are denoted λ, and PD = photodiode. The signal is recorded using a digital oscilloscope Figure 2-2. Nanosecond transient absorption beam geometries depicting quasi-parallel (left) and perpendicular (right). Time resolution is identical for both geometries Figure 2-3. Overview of the ultrafast pulse generation, amplification and wavelength conversion processes. Ultrafast pulses are generated via a mode-locked laser (seed)

9 ix and sent into an ultrafast amplifier that increases the energy of the pulse by ~10 6. After amplification, the pulse can be used to generate a white light continuum probe (used in visible pump-probe spectroscopy), and/or to pump an OPA or multiple OPAs. Depending on the OPAs signal and idler frequencies (adjustable by the user), an OPA can produce a very large range of wavelengths utilizing sum frequency generation (SFG), second harmonic generation (SHG), or difference frequency generation (DFG) Figure 2-4. Basics of ultrafast pump-probe spectroscopy. A pump is modulated by a chopper to separate blocked and unblocked pump shots. The probe is delayed on a mechanical delay stage relative to the pump, which creates the time delay Δt that serves as the x-axis for data plots. The monochromator separates different probe wavelengths and the light intensity of a particular wavelength is recorded using a photodiode Figure 2-5. Simplified schematic of a regenerative mode-locking ultrafast laser cavity. HR is a high reflective mirror, AOM = acoustic optical modulator, OC = output coupler, and PD is a photodiode. The photodiode signal provides feedback for the AOM. The bottom figure depicts the amplitude and frequency of longitudinal modes in a mode-locked laser Figure 2-6. Principle of pulse stretching in an ultrafast laser amplifier. The pulse is chirped using two gratings that allow for the separation of the blue and red portions of the pulse. The pulse makes two passes on the grating which serves to diffract and then recollimated the beam; however, the recollimated beam is significant longer in duration Figure 3-1. Diagram for the nanosecond transient absorption spectrometer. Lenses are labeled L and filters F. Photodiodes are labeled PD and PA is the preamplifier Figure 3-2. Data acquisition schematic for the transient absorbance instrument. The digital oscilloscope triggers on each laser pulse signal from the reference photodiode. A synchronized mechanical chopper TTL signal, as well as the detector signal are collected for each laser shot. Each shot is identified and tagged as on or off based on the chopper signal. The small changes in the lamp transmission (ΔT) are collected using AC coupling and with low gain to increase voltage resolution. The lamp transmission (T) is collected using DC coupling. Photoluminescence may be collected by blocking the lamp beam, and subtracted from the overall change in transmission signal if desired. The overall transient signal is obtained by dividing ΔT by T. All signals are collected on a shot-by-shot basis using rapid triggering mode Figure 3-3. (a) Steady-state absorption and emission spectra for a P3HT:PCBM blend film. (b) TA spectra for the same film; the pump energy was 40 µj/cm Figure 3-4. Transient absorbance decay of polarons in the P3HT:PCBM film at 1020 nm (1.22 ev) using 60 µj/cm nm laser excitation. Data was collected over a timescale ranging from nanoseconds to 1 millisecond. Note the semi-logarithmic

10 x scale. The inset shows the decay along with the instrument response on a linear scale Figure 3-5. Power-dependent study of polaron absorption decay at 1020 nm (1.22 ev) in the P3HT:PCBM film using 532 nm laser excitation. Signals are well-resolved even at low excitation energy densities, allowing for facile kinetic modeling for samples exposed to light levels that are close to device operating conditions Figure 3-6. RF noise associated with the Q-switch of the pump laser Figure 3-7. Comparison of a transient absorbance signal measured at 1.22 ev (1020 nm) for the P3HT:PCBM film excited using a 7 µj/cm2 laser pulse at 532 nm, detected using either repeat triggering or rapid triggering modes. Each trace is the result of the average of 3 scans using 200 laser shots each (~30 seconds in real time) Figure 3-8. Transient absorbance decays of polaron absorption in P3HT:PCBM in the presence of phololuminescence (PL) artifacts at (a) 850 nm (1.46 ev) and (b) 950 nm (1.3 ev). The excitation wavelength was 532 nm. The artifact is suppressed by measuring and subtracting the PL component from the overall observed signal. Panel (b) emphasizes the need to routinely subtract PL because it is not always directly observed and can attenuate the kinetic amplitudes associated with the true transient absorbance signal Figure 3-9. Transient absorbance decay observed for a neat P3HT film at 1100 nm (1.13 ev) in the presence of a thermal artifact. The excitation wavelength was 532 nm. The thermal artifact is suppressed by using lower laser energy densities Figure 4-1. Instrument diagram for a dispersive mid-ir nanosecond transient absorption spectrometer. M = mirror, SH = shutter, EM = ellipsoidal mirror, FC = flow cell, CH = chopper, S = source, and PA = preamplifier Figure 4-2. Step scan FTIR spectrum of Re[(CO) 3 bpycl] collected at 30 ns with a collection time of 100 min Figure 4-3. Optical layout for the nanosecond mid-ir transient absorption spectrometer instrument. The cw probe source is housed in a custom built water cooled aluminum block in ensure temperature stability. The removable mirror allows for either detector to be used. For kinetics, the 100 MHz MCT is used, whereas for spectral data, the 16 MHz detector is used. All lenses are CaF Figure 4-4. Data collection scheme for the mid-ir instrument. The Picoscope is triggered by the reference photodiode, followed by the sorting of the waveforms into ON (pump laser unblocked) and OFF (pump laser blocked) shots based on the chopper signal. The program sequentially subtracts the ON and OFF shots and then divides by the DC signal Figure 4-5. (a) Spectrum of P3HT:PCBM film pumped at 532 nm (< 50 μj/cm 2 ) showing the broad mid-ir polaron absorption. (b) Power law decay of the mid-ir polaron

11 xi absorption from (a). The kinetics were obtained using bandpass filters. The inset shows the kinetic trace on a linear scale to better illustrate the instrument response Figure 4-6. Polaron kinetics for P3HT:PCBM at various energy densities. The instrument response is shown in black. The inset shows the average amplitude for each energy density calculated from the first 30 ns of each trace. The red line shows the fit to the amplitudes to illustrate the linearity of the energy density Figure 4-7. Normalized transient absorption kinetics for P3HT:PCBM illustrating the effect of energy density on the polaron decays. The energy densities were 8.5 (blue), 3.4 (teal), 2.1 (green), 0.9 (pink), and 0.4 μj/cm 2 (red). Inset shows the nonnormalized data Figure 4-8. Nanosecond vibrational spectra for the alkyne stretch of a TIPS-pentacene film. The film was pumped at 642 nm using a home-built dye laser at an energy density of 100 uj/cm 2. The positive and negative features arise due to thermochromism of the alkyne peak. The bottom spectrum is the steady-state FTIR spectrum for reference Figure 4-9. Transient absorption of TIPS-pentacene triple at 525 nm after the sample was excited at 642 nm. The decay shows the triplet-triplet annihilation, which is the source of the thermochromism mentioned in Figure Figure Nanosecond spectra for the OPV film CN-MEH-PPV:PCBM pumped at 532 nm (<100 μj/cm 2 ). The vibrational bleach at ~1740 cm -1 corresponds to the C=O stretch of PCBM, while the bleach at 1690 cm -1 corresponds to the C=C stretching of CN-MEH-PPV Figure Spectra for P3HT:PCBM film for the visible, NIR, and mid-ir regions. The visible and NIR regions were collected using the instrument in Chapter 3. The visible region shows the electronic ground state bleaches while the NIR region is a convolution of localized and delocalized polarons. The mid-ir region is primarily comprised of localized polarons and the gray area depicts the integrated region that was used for the kinetics in Figure Figure Power law fits for the kinetics that were obtained via integration of the wavelength range specified in Figure The alpha values of the fits are shown to illustrate the influence energy density has on the shape of bimolecular decays. The increasing alpha values are indicative of decreasing trap-limited bimolecular recombination Figure 5-1. Ultrafast optical layout for the visible pump mid-ir probe (bottom) and the visible pump visible probe systems. L denotes lenses Figure 5-2. Ultrafast beam overlap between the pump and probe pulses. The angle between the pulses is α Figure 5-3. Data collection scheme for the visible pump mid-ir probe system showing the details of the modified National Instruments Driver structure. MATLAB controls

12 the get/set requests for the PCI card with communicates with the IR FIFO memory. Different Ports are designated from particular functions by MATLAB Figure 5-4. Data collection scheme for the visible pump visible probe ultrafast instrument. The reference signal is a 5V TTL pulse from the laser amplifier, the AC coupled signals are from the transimpedance amplified balanced photodiode. Note the amplitude difference in the Probe Signal between ON and OFF laser shots. After the signals are sorted by ON and OFF shots from the chopper, the waveforms are integrated and then averaged to form ΔT and CF (correction factor). Both are used to calculate ΔA, which is plotted as the output. The x-axis is produced by the position of the delay stage Figure 5-5. Previous work by the Asbury group using the SAVS technique with PCBM and a PDI acceptor. (a) Cartoon depicting SAVS technique. (b) SAVS used to measure the barrier of charge separation in between PDI:P3HT and PCBM:P3HT. PCBM exhibits activationless separation due to 3D delocalization of the polaron Figure 5-6. Steady-state UV-vis spectra for PDI-um at difference concentrations. The absorptions arise from the perylene C=C stretching modes (0-0, 0-1, 0-2, 0-3) coupled to the π-π* electronic transition. The photograph (top) shows the concentration series, note that the sample 1 mg/ml is missing due to the solution dilution giving inadequate signal Figure 5-7. Steady-state FTIR measurements for the PDI-um concentration series. The 1690 cm -1 mode is the C=O asymmetric stretch, 1650 cm -1 is the C=O symmetric stretch, and the two C=C core stretching modes are at ~1580 and 1590 cm Figure 5-8. Steady-state florescence measurements for the PDI-um concentration series. The broad, structureless fluorescence indicative of excimers is present once the concentration increases passes 1 mg/ml Figure 5-9. Ultrafast visible pump mid-ir probe spectra for 1 mg/ml PDI-um sample. The ground state bleaches (GSB) of the carbonyl asymmetric (1690 cm -1 ) and symmetric (1660 cm -1 ) modes are present along with their excited state absorptions (ESA). The GSB and ESA of the perylene core C=C modes are also present. At 1 mg/ml, there is no evidence of new vibrations suggesting excimers do not form at this concentration Figure Ultrafast visible pump mid-ir probe spectra for 10 mg/ml PDI-um sample. The ground state bleaches (GSB) of the carbonyl asymmetric (1690 cm -1 ) and symmetric (1660 cm -1 ) modes are present along with their excited state absorptions (ESA). The GSB and ESA of the perylene core C=C modes are also present. Unlike the 1 mg/ml sample, a new feature begins to form in the region of the C=C modes. These new broad ESA is assigned to the excimer based on the concentration dependence (see text). The rate of formation of excimer in the 10 mg/ml was measured to be ~879 ps Figure Ultrafast visible pump mid-ir probe spectra for 15 mg/ml PDI-um sample. The excimer vibrations are visible as they were in the 10 mg/ml sample, xii

13 however they have appeared faster, suggesting a concentration dependence. The rate of formation of excimer in the 10 mg/ml was measured to be ~206 ps Figure Ultrafast visible pump mid-ir probe spectra for 20 mg/ml PDI-um sample with a rate of formation of ~176 ps Figure Species Associated Spectra for the 20 mg/ml sample and kinetic fit to the disappearance of the C=C ESA at 1504 cm -1. The fits were done using a 2x2 k- matrix with target analysis. The model is described in the text. The black trace in the SAS spectrum corresponds to the monomer unit, the red corresponds to the intermediate state between monomer and excimer and the blue corresponds to the excimer state. The model assumes the experimental spectrum is a linear combination of the SAS spectra Figure 6-1. (a) Chemical structure for the block copolymer and homopolymer. (b) Transient absorption spectra of P3HT-b-PFTBT films, homopolymer blends, and their neat homopolymer controls for the visible, near-ir, and mid-ir regions. The visible and NIR regions were collected using the instrument described in Chapter 3 and the mid-ir region were collected using the instrument described in Chapter 4. Each spectra were time averaged between 25 and 50 ns (upper panel) and 1 2 μs (lower panel). The excitation wavelength was 532 nm and each trace has been normalized to the number of absorbed photons. The NIR region consists of overlapping spectral features (polarons and pseudo-charge transfer states) while the mid-ir absorption is from polarons without the present of other species, providing a unique spectral region to probe the evolution of polarons. [Reprinted with permission from Grieco, C. et al. J. Phys. Chem. C, 2016, 120(13), Copyright American Chemical Society] Figure 6-2. Mid-IR kinetics for the block copolymer, blends, and neat homopolymers. The kinetics have been normalized by the number of absorbed photons from the pump at 532 nm. The ultrafast portion of the kinetics were collected using our ultrafast instrument (Chapter 5) and connected to the mid-ir kinetics collected with the nanosecond mid-ir instrument discussed in this Chapter. The kinetics are nearly identical between the block copolymer and blend materials, suggesting that the covalent linkage as little effect on charge recombination. [Reprinted with permission from Grieco, C. et al. J. Phys. Chem. C, 2016, 120(13), Copyright American Chemical Society] Figure 6-3. Mid-IR kinetics for the block copolymers (a,b), homopolymers (c,d) and the blends (e,f). The fits were produced using single power laws. The deviations from the fits seen in e and f arise from the increased phase-separation found in the blends. [Reprinted with permission from Grieco, C. et al. J. Phys. Chem. C, 2016, 120(13), Copyright American Chemical Society] Figure 6-4. Basic photophysics of the pump-push-probe technique. An optical pump creates excited states that form CT states and the push pulse excites the CT state giving the necessary energy to undergo charge separation. The charges can then be optically or electronically (or both) detected xiii

14 Figure 6-5. Optical layout for (a) electronic detection in pump-push and (b) optical detection in pump-push-probe spectroscopy. PD refers to photodiode Figure 6-6. Pump-push electrical signal from a P3HT:PCBM device measured using the sequential subtraction method discussed in Chapter xiv LIST OF TABLES Table 3-1. Photodetectors for nanosecond transient absorption spectrometer Table 5-1. Excimer Formation Rate. The rates of excimer formation were calculated using target analysis for 1, 10, 15, and 20 mg/ml concentrations of PDI-um in chloroform Table 6-1. Device J-V Characterization Results

15 xv ACKNOWLEDGEMENTS Completing this doctoral degree would not have been possible without the help of many people in and outside of the Asbury Lab. I would like to first thank Prof. Asbury for his guidance and direction over the past five years in helping me become a better researcher. He provided me with the financial support to pursue my research interests and was always available to help me. If he had not trusted me to build so many new instruments in the lab, I am quite sure I would not be where I am today. I would also like to thank my committee members Profs. Marconcelli, Gomez, Mallouk, and Lear. I am especially thankful for the guidance Prof. Gomez has provided over the years in x-ray and TEM of organic materials. I am very thankful for being able to work with Chris, Rob, Eric, Grayson, Kayla, and Alec in the Asbury lab. If it was not for the ridiculously long nights and weekends, I am very sure most of this dissertation would not have been possible. The members of the Asbury lab made those nights bearable, and I am grateful for their friendship. I am especially grateful to Chris, who is easily one of the hardest working people I have ever met; it was a pleasure being able to work alongside him. I would also like to thank Anna, who has been a great friend since my first year in graduate school. Lastly, I d like to thank Ryan Pensack, the man who originally trained me on the ultrafast instrument all those years ago. He has always been available to discuss anything and everything over the years and without his help, I would not know half of what I know today. To you all, I thank you.

16 1 Chapter 1 Fundamentals of Organic Photovoltaic Materials The potential applications of organic semiconductors were first illustrated by Tang et al. in the 1970s with the demonstration of the first organic light emitting diode (OLED). 1 Organic semiconductors offer several unique properties such as flexibility, thinness, and simple fabrication. For example, OLEDs are commercially employed in a number of flexible and ultrathin displays. 2 Another important application of organic semiconductors involves solar energy, where organic photovoltaic (OPV) materials may promise cheap, flexible solar cells. In contrast to OLEDs, OPVs make use of organic semiconductors to absorb light and convert it to electrical energy. The properties of OPVs have been under active research for the last 20 years, and currently top performing OPV devices are beyond 8% efficient. 3 From a spectroscopic standpoint, OPV materials have complicated photophysics that require unique and challenging experiments to unravel their inner workings and lend crucial fundamental insight into what factors limit their performance. The rest of this Chapter is dedicated to providing the reader with a fundamental understanding of the photophysical processes in OPV materials. 1.1 Nature of the Absorption Process When atoms bond to form molecules, their individual atomic orbitals combine to form molecular orbitals, where the distribution of electrons dictate the type of bond (single, double, triple) formed between atoms. 4 When discussing spectroscopy, we usually refer to states rather than orbitals, and more specifically, what are termed electronic states. Electronic states are concerned with the properties of all of the electrons in all of the molecular orbitals. In other

17 2 words, the wave function for an electronic state is a combination of the wave functions of each of the electrons in each of the molecular orbitals of a molecule. 5 When an electron moves from one orbital to another, the electronic state of the molecule changes and this is termed an electronic transition. 4 The absorption process occurs when the energy of a photon (E = hc/λ) matches the energy difference between two different electronic states of a molecule. In the case of a molecule undergoing an initial absorption process, the electronic transition occurs between the ground state and an excited state of the molecule. The ground state is defined as the lowest energy state (most stable) of a molecule. The process of absorption is depicted in Figure 1-1, where the excitation of a molecule with photon wavelength λ 1 causes the molecule to undergo an electronic transition from the ground state S 0 to the first excited electronic state S 1. The timescale for the absorption process is very fast, typically (femtosecond) or less. 5, 6 The S denotes a singlet state, meaning that the electron spins are opposed, whereas T is used to denote a triplet state, meaning the electron spins are parallel. Each electronic state has superimposed vibrational states on it, which correspond to different vibrational configurations of the molecule. These vibrational states give rise to the bandwidth of electronic states, meaning that there are more than one λ 1 that would satisfy the condition for the electronic transition between S 0 and S 1. Once the molecule is in an excited state, there are a number of different processes that can occur depending on the molecular structure and the molecular environment. After an electron has been promoted to a higher electronic state, it is rarely initially in the lowest vibrational state of that electronic state. Usually the excitation puts the molecule into a higher vibrational state of the excited electronic state as depicted in Figure 1-1. The molecule can either radiatively lose its energy (fluorescence) by emitting a photon or it can vibrationally relax (nonradiative). Vibrational relaxation, also called internal conversion, is the transfer of excess vibrational energy from the molecule to its surroundings (solvent). 5 This process is highly efficient, making the

18 3 radiative path of emitting a photon from an excited vibrational state highly improbably, except in gases at low pressure. Therefore, for all systems we will consider, fluorescence can be assumed to occur from the lowest vibrational state on an electronic state (called Kasha s rule in spectroscopy). Once the molecule has vibrationally relaxed to the lowest vibrational level of the excited state it can either undergo fluorescence or it can continue to nonradiatively decay (internal conversion) to the ground state. Which of these processes is dominant depends on the environment of the molecule, but the efficiency of internal conversion from one electronic state to another is typically very low for aromatic molecules such as those studied in this dissertation. 5 There are additional pathways for the energy in S 1, such as intersystem crossing, where the energy is transferred from a singlet state to a triplet state (e.g. S 1 T 1 ), followed by emission from the triplet state to the ground state (phosphorescence). These processes are not important for the systems we will later be discussing and the reader should see references 5, 6 for a more detailed account. The instrumental techniques involved in the study of absorption and fluorescence can be found in Chapter Photophysics of Organic Photovoltaics In the preceding discussion of absorption, we assumed isolated molecules; however, in solid-state systems like OPVs, that is rarely the case. Usually, molecules are packed aggregated depending on the molecular structure and how the molecules were deposited on the substrate. Therefore, when we speak about a molecule absorbing light, what we really mean is that an ensemble of molecules undergoes the absorption process. Many, but not all, spectral properties of small aggregates of molecules and crystals are directly traceable to properties of the individual molecules. 4 However, the energy of interaction between molecules, although usually weak, imposes a communal response upon the molecular behavior of the aggregate. The

19 4 collective response of the molecules to light forms a quasi-particle called an exciton, first induced by Frenkel in the 1930s. 7, 8 As molecules begin to pack, their electronic energy levels couple together, and the individual molecular orbits begin to form bands of orbitals. These bands are termed valence and conduction bands in inorganic semiconductors and their organic semiconductor analogues are termed highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) respectively. 9 The HOMO and LUMO of organic semiconductors refer to energy bands that correspond to different hybridization states of the p- bonds which will result in different energy levels of an organic semiconductor. When an electron is excited from the HOMO to the LUMO of an organic semiconductor, the molecule itself is excited into a higher energy state, as opposed to the actual excitation of a free electron from the valence band to the conduction band in inorganic semiconductors Excitons The formation of excitons in OPVs happens when the energy of a photon matches the energy difference between the HOMO and LUMO levels (Figure 1-2). Because of the low dielectric constant of organic semiconductors and their localized electron and hole wavefunctions, the exciton (electron hole pair) is Coulombically bound. These excitons are referred to as Frenkel excitons in OPVs. 4 The energy to dissociate the electron-hole pair is termed the exciton binding energy and is usually ev for organic semiconductors. In contrast, inorganic semiconductors typically have binding energies of a few mev, where dissociation of the electron and hole often happens from excess thermal energy from absorption. 10 In order to dissociate the exciton in OPVs, it is necessary to create an energetically favorable environment to overcome the Coulombic attraction between the electron and hole.

20 5 The most efficient method to date for overcoming the binding energy is two create a heterojunction between two materials. 11 To achieve exciton dissociation, two materials with proper band alignment are placed adjacent to each other. The proper band alignment is shown in Figure 1-2, where the difference between the HOMO of material A and the LUMO of material B has to be lower than the energy between the bound electron-hole pair. Material A is termed the donor material, since it gives an electron to material B, which is called the acceptor material. Therefore, when an exciton is generated and it diffuses to the interface between the donor and acceptor, 12 if the energy difference between LUMO B and HOMO A is lower than the energy of the exciton, the transfer of an electron from the exciton to LUMO B is an energetically favorable process. 10 Typical exciton diffusion lengths are ~ 10 nm of organic semiconductors, and places strict limitations on the nanomorphology This process is called charge transfer and occurs on a much faster timescale ( s) than the competing process of luminescence, which involves the radiative recombination of excitons (~1 ns). Once transferred, the electron and hole are not yet free of their Coulombic attraction, instead they are loosely bound and referred to as a charge transfer or CT state Charge Transfer The process of charge transfer is typically addressed with nonadiabatic electron transfer theory, or more commonly referred to as Marcus theory in honor of Rudolph Marcus who pioneered much of the theory in the 1950s. 12 The basics of Marcus theory are depicted in Figure 1-3, where the reactant and product potential energy surfaces are drawn as two intersecting harmonic oscillators with the horizontal axis as the reaction coordinate the motion of all nuclei in the system. Electron transfer must occur at the intersection of the potential energy surfaces in order to conserve energy and satisfy the Franck-Condon principle, which states that electron

21 transfer occurs much faster than the nuclei can respond, thereby the immediate geometry (nuclear 6 configuration) of the excited state is unchanged relative to the ground state. 4 In Figure 1-3, S 0 refers to the donor ground state, S 1 is the donor excited state and CT is the D + /A - state, Q is the reaction coordinate, ΔG is the free energy, and λ is the reorganization energy. Because the intersection point represents the energy level and nuclear configuration that the reactant state must achieve (through vibrational motion) in order for isoenergetic electron transfer to occur, the process of electron transfer is an activated process with barrier ΔG and the relationship: (1) The reorganization energy is the energy required to bring the reactant and its surrounding medium to the equilibrium geometry of the product state. It is usually split into an inner and outer component, where the inner reorganization energy is the vibrational contribution reflecting the change in geometry upon electron transfer. 12, 13 The outer component is the solvent contribution, meaning changes in polarization of the surroundings to stabilize the product state. The rate of electron transfer (k ET ) is described using Fermi s Golden rule as ( ) (2) where V refers to the electronic coupling between reactant and product and depends on the wavefunction overlap of the donor and acceptor. Equation 2 is called the nonadiabatic limit, meaning electronic coupling is weak and splitting of the two potential energy surfaces is small relative to kt. 12 If that is not the case, then the potential energy surfaces split into a lower and upper one, this process is termed adiabatic, and only occurs if strong electronic coupling is present not the case with OPV materials. The exponential term is the Franck-Condon factor and predicts that as ΔG 0 increases, so does the rate of electron transfer. When ΔG 0 = λ, there is no barrier to transfer, however if ΔG 0 continues to increase pass the ΔG 0 = λ intersection point, then the transfer will occur in what is termed the

22 7 inverted region. In the inverted region, an increasing value of ΔG 0 actually decreases the electron transfer rate. Once charge transfer as occurred between the D/A in OPVs, the electron/hole still need to be dissociated from the CT state Charge Separation The mechanism for CT state dissociation is the focus of ongoing research by numerous groups. Important questions exist as to how the bound electron/hole pairs are able to overcome, in some cases with near unity quantum yields, their significant Coulombic attractions (~ ev) One possible explanation involves the role of interfacial or applied electric fields assisting in the dissociation process. Application of an applied electric field to OPV devices showed that photoluminescence quenching of the CT state was field dependent, but only under high bias (10V), corresponding to field strengths of 10 8 V/m. Such high bias are well beyond the normal operation range of OPVs (~1V), whereas within normal operating voltages, changes in quenching and charge extraction were less than 10%. These experiments indicated that macroscopic fields are probably not significant contributors to CT state dissociation. 12, 19, 20 However, questions were raised about whether electric fields might be involved in facilitating CT state dissociation in cases where such fields are predicted to be on the order of 10 7 V/m, such values are estimated for interfacial fields. Another point of contention involves the role of excess thermal energy. Some groups contend that excess thermal energy is insignificant on the basis of no observed temperature dependence for CT state emission or internal quantum efficiencies. 24, 25 Others, believe that excess thermal energy from the initial transfer of the "hot" electron into the LUMO of the acceptor plays an important role in CT state dissociation by providing the additional energy needed to overcome the Coulombic barrier These studies have shown clear correlations for

23 8 the free energy difference between donor/acceptor and the percentage of charge separation, which also led to better device performance. Most notably is the recent work by Jailaubekov et al. where "hot" CT states were directly observed using time-resolved two-photon photoelectron spectroscopy. 29 These conflicting results may indicate that such a process is system dependent, although since all materials studied formed reasonable solar cells, there may be additional underlying factors influencing performance involving molecular structure. Regardless of the exact charge separation mechanism, once the charges are no longer Coulombically bound, they are referred to as polarons Polarons After charge separation, electrons are transported within the acceptor regions and holes are transported in donor regions. When a charged particle moves through a lattice, it is altering the local state of the lattice, and therefore local electronic state. 30 Polarons can be described as a charged particle (electron or hole in the case of OPVs) and the local lattice distortion (polarization) that the charged particle creates. Polarons are quasi-particles and their formation is a consequence of dynamic electron-phonon interactions some researchers describe them as electrons (or holes) surrounded by a cloud of phonons. 30 Free carriers not do truly exist in OPV materials because of the low dielectric constant of organics; therefore, when we speak of carriers or charge carriers in OPVs we mean polarons. The transport mechanism for polarons moving through a material is a thermally activated hopping mechanism where the polarons must thermally hop (transfer) between adjacent donor or acceptor molecules. Polaron formation typically happens on a picosecond timescale while their recombination and transport is usually on the nano-microsecond timescale. 4

24 Charge Recombination There are two predominate recombination mechanisms in OPV materials. The first is termed geminate recombination, which is a unimolecular process and refers to the recombination of the electron and hole pair shortly after their creation. 12 The theory of geminate recombination was put forth by Onsager while studying recombination in electrolytic solutions in An overview of Onsager theory is depicted in Figure 1-4, where Onsager proposed that the electronhole pair is formed with a hot (excess thermal energy) electron that cools (thermalizes) for some distance termed the thermalization length (L). The variable r c refers to the Coulombic capture radius and is defined as (3) where ϵ r is the dielectric constant of the material, ϵ 0 is the permittivity of free space, k B is the Boltzmann constant and T is temperature and e is charge. If the thermalization length L is greater than r c, charges can dissociate, otherwise they will recombine. The escape probability of the electron depends on the applied electric field, distance between the electron and hole and on the temperature of the material. Electric fields lower the Coulombic potential and increase the escape probability. 12 From an OPV perspective, the interfacial electrical field that exists between the donor and acceptor materials due to the band alignment provides the necessary energetic landscape to make the escape probability high enough for OPV devices to generate charges. 10 Geminate recombination can also occur once an electron/hole has separated, but they are confided by the physical sizes of their domains such that each electron can only recombine with its original hole. The process of geminate recombination occurs on the order of pico-nanoseconds and is studied using ultrafast transient spectroscopy (Chapter 5). Once polarons are formed they can recombination through the mechanism of bimolecular (Langevin) recombination. 4 Therefore, bimolecular recombination deals with the recombination

25 10 of fully dissociated charge carriers that did not previously belong to the same CT state. Bimolecular recombination differs from geminate recombination in that the charges must diffuse to within their Coulombic capture radius of each other before recombination. Except for the case of very high charge densities, this diffusion process significantly retards the overall kinetics, making bimolecular recombination significantly slower (nano-millisecond) than geminate recombination. 32 Besides their timescale differences, geminate and bimolecular recombination can be separated based on their kinetics, and their excitation density dependence. Geminate recombination exhibits monoexponential decay and no dependence on excitation density with the exception of very high excitation densities, where the CT states begin to interact with each other. Whereas bimolecular recombination involves two fully dissociated charge carriers and follows second-order kinetics (power law behavior), while being strongly dependent on the excitation density. For this reason, any excitation energy density above ~100 μj/cm 2 should be avoided (see Section 4.5.2) Charge Extraction After the charge carriers transport to the active layer/electrode interface, they are extracted from the active layer to the electrodes. To achieve high efficiency in charge extraction, the potential barrier at the active layer/electrode interfaces has to be minimized. Thus, the work function of the anode is ideally expected to match the donor HOMO, while the work function of the cathode is expected to match the acceptor LUMO. When these occur, the contacts are called ohmic contacts and V oc correlates positively with the difference between the acceptor LUMO and donor HOMO. 10

26 How an Organic Photovoltaic Device Operates Using the previously discussed photophysical understanding of OPV materials, we can construct a model case of how an OPV device functions. The device consists of a transparent electrode, commonly indium tin oxide (ITO), followed by an interlayer such as PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) to help match the work function of ITO and facilitate charge injection from the organic to the metal electrode (Figure 1-5). Next is the active layer material, which is comprised of the donor and acceptor material, in this case it is the prototypical OPV material poly(3-hexylthiophene) (P3HT) as the donor and phenyl-c 61 -butyric acid methyl ester (PCBM) as the acceptor material. An interlayer (e.g. LiF) is put between the active layer and the top electrode to decrease the barrier to charge injection. The top electrode is commonly aluminum, but other metals can be used depending on their work function. Light is absorbed by the donor material produces Frenkel excitons, which can undergo radiative or nonradiative decay with lifetimes of 100 ps 1 ns, or migrate to a D/A interface. The latter step is a diffusion-controlled process that requires the exciton be within ~10 nm of the interface for most organic materials, thereby, placing strict limitations on the nanomorphology. 15 Excitons that migrate to the D/A interface can form charge-transfer (CT) states, where depending on interface energetics, they may dissociate into charge-separated (CS) states, or undergo geminate recombination. The separated electrons and holes can be viewed as polarons hopping from site to site until they are extracted rather than free electrons and holes with band transport characteristics. 13 The dominate loss mechanism after dissociation is bimolecular recombination. The extraction process is highly dependent upon the morphology, where bicontinuous pathways for acceptor and donor material (ideal morphology of a bulk-heterojunction) are considered best.

27 Thesis Overview In the present Chapter, an introduction to the fundamental photophysics of OPV materials was given which will provide the necessary basis for understanding the processes discussed in subsequent Chapters. An outline and brief summary of the remaining Chapters is presented below. Chapter 2 develops the experimental techniques needed to measure the photophysical processes discussed in Chapter 1. Specifically, Chapter 2 deals with steady-state, nanosecond transient, and ultrafast transient absorption spectroscopies. The information discussed with be useful for understanding the later Chapters that deal with the design and development of instruments and techniques for measuring OPV photophysics. The focus of the Chapter is on hardware optical design for transient spectrometers. The design of a high performance nanosecond visible pump visible/nir probe transient absorption spectrometer is the focus of Chapter 3. The instrument was developed to study OPV materials in particular by utilizing flexible resolution USB digitizers to significantly the enhance signal-to-noise ratio and allow for 10-5 sensitivity. The instrument was also designed around the necessity of having to utilize a large range of pump ( nm) and probe ( nm) wavelengths, which requires multiple detectors and a home built dye laser to accomplish. The instrument s performance (spectral range, time resolution, sensitivity) were measured using the common OPV blend P3HT:PCBM. Chapter 4 discusses the development of a nanosecond visible pump mid-ir probe transient absorption spectrometer. The instrument is unique since there are no commercially available instruments capable of performing nanosecond-millisecond transient absorption spectroscopy with 10-5 sensitivity. The instrument allows for some of the first ever nanosecond time-resolved spectra of OPV materials that can be connected to NIR and visible spectral of the

28 13 same films using the instrument in Chapter 3. In Section 4.4 and 4.5, relevant OPV systems such as CN-MEH-PPV, P3HT:PCBM, TIPS-pentacene, and block copolymer materials were selected to showcase the instrument s capabilities. Chapter 5 details advancements made to our ultrafast system, including the development of visible pump visible probe to compliment the visible pump mid-ir probe techniques previously done in the Asbury lab. The Chapter focuses on changes made to the ultrafast system that have improved the signal-to-noise. Focus is also given to hardware/software advancements that have been made, which allows for much easier incorporation of future ultrafast techniques. The final Chapter discusses recent work with electro-optical (pump/push/probe) ultrafast spectroscopy on functioning OPV materials. This work highlights the future directions of the lab as well as OPV research in general. Particular focus is given to the measurement technique and instrumentation as well as what new kinds of information can be accessed using electro-optical techniques. 1.4 References 1. Tang, C. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51 (12), LG Corporation. OLED TVs. (accessed April 20, 2016). 3. Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv. Mater. 2010, 22, E-135-E Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999; pp Hercules, D. M. Fluorescence and Phosphorescence Analysis; John Wiley and Sons:

29 14 New York, 1966; p Harris, D. Fundamentals of Spectrophotometry. In Quantitative Chemical Analysis, 8th ed.; W.H. Freeman and Company: New York, 2010; pp Frenkel, J. The Quantum Theory of the Absorption of Light. Nature 1929, 124, Frenkel, J. On the transformation of light into heat in solids. PHYSICAL REVIEW 1931, 37 (1), A., H. W. Solid State Theory; McGraw-Hill: New York, Choy, W. C. H., Ed. Organic Solar Cells; Springer: London, Tang, C. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51 (12), Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, Brédas, J.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc. Chem. Res. 2009, 42, Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer, U.; Feldmann, J.; Scherf, U.; Harth, E.; Gugel, A.; Müllen, K. Phys. Rev. B 1999, 59, Hallermann, M.; Kriegel, L.; Coma, E. D.; Berger, J. M.; Hauff, E. V.; Feldmann. Adv. Func. Mat. 2009, 19, Hou, J.; Chen, H. Y.; Zhang, S.; Chen, R. I.; Yang, Y.; Wu, Y.; Li, G. J. Am. Chem. Soc. 2009, 131, Park, S.H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, M.; Moon, J.S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A.J.. Nat. Photonics 2009, 3, Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li,

30 15 G. Nat. Photonics 2009, 3, Veldman, D.; Meskers, S. C. J.; R.A.J., J. Adv. Funct. Mat. 2009, 19, Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.; Silva, C. Phys. Rev. Lett. 2004, , Arkihipov, V. I.; Heremans, P.; Bassler, H. Appl. Phys. Lett. 2003, 4605, Aarnio, H.; Sehati, P.; Braun, S.; Nyman, M.; de Jong, M. P.; Fahlman, M.; Österbacka, R. Adv. Energy. Mat. 2011, 1, Avilov, I.; Geskin, V.; Cornil, J. Adv. Funct. Mat. 2009, 19, Lee, J.; Vandewal, K.; Yost, S. R.; Bahlke, M. E.; Goris, L.; Baldo, M. A.; Manca, J. V.; Van Voorhis, T. J. Am. Chem. Soc. 2010, 132, Grzegorczyk, W. J.; Savenije, T. J.; Dykstra, T. E.; Piris, J.; Schins, J. M.; Siebbeles, L. D. A. J. Phys. Chem. C. 2010, 114, Shoaee, S.; Clarke, T. M.; Huang, C.; Barlow, S.; Marder, S. R.; Heeney, M.; McCullock, I.; Durrant, J. R. J. Am. Chem. Soc. 2010, 132, Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.; Silva, C. Phys. Rev. Lett. 2004, 92, Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.; McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, Jailaubekov, A. E.; Willard, A. P.; Tritsch, J. R.; Chan, W. L.; Sai, N.; Gearba, R.; Kaake, L. G.; Williams, K. J.; Leung, K.; Rossky, P. J.; Zhu, X.-Y. Nat. Mat. 2013, 12, Bogolubov, N. N.; Bogolubov, J. N. N. Aspects of Polaron Theory; World Scientific

31 16 Publishing: Singapore, Onsager, L. Phys. Rev., 54, Clarke, T. M.; Jamieson, F. C.; Durrant, J. R. Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/Fullerene Blend Films. J. Phys. Chem. C 2009, 113,

32 Figure 1-1. Jablonski diagram depicting the photophysical processes common to OPV materials and organic materials in general. The electronic states are label S 0 (singlet ground state), S 1 (singlet excited state), and T 1 (triplet excited state). Each electronic state has superimposed vibrational states (gray lines) and rotational states (not shown) that give the electronic states their bandwidth. The absorption process is depicted as a green line, fluorescence (red) and phosphorescence (maroon). Thermal relaxation (internal conversion) is depicted as a teal line and intersystem crossing between singlet and triplet states is shown in blue. In theory, there are many fluorescence and phosphorescence transitions; however, only one of the transitions is shown for clarity. 17

33 Figure 1-2. Energy offset of a donor and acceptor. The CT state is formed after exciton migration to the D/A interface where the electron (e - ) is transferred to the acceptor and the hole (h + ) remains on the donor. E binding is used as an approximation of the charge-transfer exciton binding energy. 18

34 Figure 1-3. Diagram depicting Marcus theory. The state S 0 refers to the donor ground state, S 1 is the donor excited state and CT is the D + /A - state, Q is the reaction coordinate, ΔG is the free energy, and λ is the reorganization energy. The intersection of the parabolas is the point where the geometry of the CT and S 1 states are the same. 19

35 Figure 1-4. Geminate recombination in OPVs is described through Onsager theory. The figure depicts the recombination and dissociation processes on an electron/hole pair based on the Coulombic capture radius r c. The thermalization length is labeled L, and k B is the Boltzmann constant, T is temperature, e - is an electron, and hν is the energy of a photon. The y-axis is potential energy. 20

36 Figure 1-5. Basic device architecture of an OPV device showing the top electrode (aluminum), an electron accepting/hole blocking layer (LiF), and the active layer, which is a heterogeneous mixture of the donor and acceptor molecules. The layer PEDOT:PSS is a hole accepting/electron blocking layer that also helps match the work function of the organic to the metal to increase charge injection into the bottom electrode ITO. The entire device is constructed on a glass substrate. 21

37 22 Chapter 2 General Principals of Absorption Spectroscopy Studying OPV materials involves needing to analyze steady-state and dynamic data from several different kinds of spectroscopic techniques. For example, understanding charge transfer, charge separation, geminate recombination, and bimolecular recombination requires information from visible, near-infrared and mid-infrared wavelengths. Furthermore, the timescales range from femtoseconds to milliseconds for these different spectral ranges, putting considerable demands on the experimentalist and the instruments needed. Entire books could be written on each of these subjects. The purpose of this Chapter is to provide the reader with a working practical knowledge of the techniques associated with studying OPV photophysics. 2.1 Steady-State Absorption Spectroscopy Steady-state absorption, sometimes referred to as simply UV-vis, is the measurement of the absorbance spectrum of a molecule. It is called steady-state because the experiment does not allow for processes (absorption, emission, thermal relaxation, etc) to be temporally resolved. In its simplest form, the technique requires a lamp, monochromator, sample, and a detector. The instrument needs to be able to measure the light intensity before and after the sample to calculate the transmittance T = I out /I in or the absorbance A = log(i in /I out ). 1 The lamp (e.g. tungsten) produces a continuum of light (usually nm) and the monochromator selects a wavelength of that continuum to illuminate the sample. The detector records the intensity of the light as a function of wavelength. Because absorption is a relative measurement, before the sample is measured a blank must be measured, which in the simplest case is the lamp spectrum. The process must be repeated for each wavelength, which is achieved by moving the diffraction grating within the

38 23 monochromator to select the excitation wavelength. Steady-state spectroscopy is used for determining where to optically pump an OPV sample for transient studies, determining solid-state packing changes (crystallization), and determining if a sample has degraded. 2.2 Nanosecond Transient Absorption Spectroscopy Nanosecond transient absorption spectroscopy (ns-ta), also known as flash-photolysis, is a powerful tool in modern photochemistry and photophysics. Unlike steady-state absorption techniques, ns-ta gives researchers access to the timescales for charge transport and recombination found in most solid-state semiconductor materials. 2 The information obtained through the TA signal gives crucial insight into the lifetime, decay rates, and concentration of charge carriers in a material all of which can influence a material s performance in an electronic device. 3 The fundamental idea in ns-ta is to use a short excitation pulse to disturb the system under study and to follow the course of the photoreaction by monitoring absorption properties of the system. 4 As an example, consider a solution of dye molecules. Under normal conditions the dye molecules are in equilibrium with the solvent molecules, which means that electronic subsystem is in its lowest energetic state (ground state). When a molecule absorbs a photon, an electron is promoted to a higher molecular orbital and the overall energy of the molecule has increased. The excited molecule can relax back to the ground state via a few intermediate states, such as singlet and triplet excited state (Chapter 1), or it can participate in a reaction (photoreaction) such as charge transfer. In any case, the excited state and the following transient states have their own absorption spectra and can be monitored by measuring the absorption change at some specific wavelengths. Although the photoreactions will take place under both continuous and pulsed excitation, under continuous excitation the longest-lived transient state

39 dominates hiding any faster intermediate products. Time-resolved nanosecond experiments allow for the monitoring of fast reactions so the whole reaction scheme can be recovered Optical Scheme A general optical scheme of an instrument for a ns-ta is presented in Figure 2-1. The instrument consists of an excitation source (nanosecond laser), a continuous wave (CW) probe source, a monochromator or filter for selecting a band of light from the probe source, and a monochromator with a detector. The monochromator or filter prior to the sample helps protect the sample from degradation, which can occur if the entire output of the CW source is incident on a sample. The detector must have a nanosecond response time, which is common for silicon photodiodes used for visible light detection and indium gallium arsenic (InGaAs) photodiodes for NIR detection. The signal is collected using a digitizer typically in the form of an oscilloscope. The rest of this Chapter is devoted to explaining each of these components in detail and how the transient signal is captured Lasers and Light Sources A nanosecond laser is the basic requirement for an excitation source. A frequency doubled Nd 3+ :YAG laser (532 nm) is one of the most common types of nanosecond lasers used for ns-ta measurements. Other lasers such as diode-pumped solid-state (DPSS) lasers are becoming more common as their prices decrease. The advantage of YAG lasers is their high power output (>200 μj/pulse), easily allowing them to pump dye lasers or optical parametric oscillators (OPOs), thereby, achieving tunability of the pump wavelength for transient absorption measurements. The downside is their low repetition rate (10-30 Hz), which considerably

40 25 increases experiment time to achieve adequate signal-to-noise ratio for the transient absorption signal. DPPS lasers feature much higher repetition rates (>50 khz), 5 however, their lower power is not sufficient to operate most OPOs or dye lasers, making the user limited to the fundamental or harmonics of the DPPS laser. In theory faster lasers (picosecond or femtosecond) can be used, however, the detector and associated electronics will then be the limiting factor in the achievable time resolution. The most common probe light source is an incoherent source such as a lamp with the spectral output for the wavelength region the transient absorption experiment requires. Typical examples include tungsten-halogen ( nm), metal halide ( nm), and xenon lamps ( nm). Xenon lamps are typically pulsed to achieve higher power output by applying a high voltage for a short period of time to drastically increase the temperature (~10,000K) compared to their usual 6,000K operating temperature. 4 The downside is that pulsed xenon lamp sources have lifetimes of less than 1,000 hours. Recently, laser driven plasma sources have become popular where a CW laser initiates discharge from a xenon plasma generating a broadband output ( nm, Energetiq, Inc). 6 For mid-ir emission a Nernst Glower cylindrical rod made from materials such as zirconia, yttria, and thoria is used because it exhibits a near black body emission when current is passed through the rod. 7 Another popular rod material is silicon carbide (Globar), which exhibits a very similar spectrum as a Nernst Glower, but usually has a more stable output (more uniform operating temperature) Detectors The most common detector for ns-ta are silicon photodiodes, which are readily available with <10 ns rise times, good spectral range ( nm) and usually can be

41 26 purchased for less than $ For NIR detection, InGaAs is the most common detector due to its good temporal response (typically <25 ns) and wide spectral range ( nm). Germanium detectors are sometimes used, but usually have long (>300 ns) rise times and a more limited spectral range than InGaAs. Options are very limited for mid-ir detectors, the only detectors that have adequate rise times for nanosecond spectroscopy are small element mercury cadmium telluride (MCT) detectors, which usually have element sizes of <1 mm diameter and need to be cooled with liquid nitrogen. 9 Photomultiplier tubes (PMTs) are also quite common, and feature excellent sensitivity but are considerably more expensive than photodiodes. Recently, avalanche photodiodes have become very popular, which achieves much better sensitivity that traditional photodiodes, but without the costs associated with PMTs. Arrays of single elements of Si or InGaAs form what are called charge coupled devices (CCDs) for detection of visible and NIR light respectively. In CCD terms, each detector element is called a pixel, and the size of arrays can vary from a few dozen to thousands of elements. The issue with array detectors is their time response is usually mirco-milliseconds, making most unusable for nanosecond spectroscopy. Specialized CCDs are available that allow the user to individually read out the temporal response of each element after the CCD is triggered with a gated pulse from the laser. These fast CCDs are typically very expensive (>$50,000) and therefore have not seen much use in nanosecond transient absorption instruments Electronics Digitalization of the analog signal from the detector requires that the digitizer have a bandwidth comparable to that of the detector and a sampling rate of at least 5 times that of the digitizer bandwidth to avoid aliasing artifacts. Therefore, a 5 ns Si detector would have a bandwidth of 70 MHz and require a digitizer with about 100 MHz bandwidth and sampling rate

42 27 of at least 500 Ms/s. 10 Typically, oscilloscopes have sampling rates many times higher than the bandwidth to avoid any aliasing issues that might occur, for example a commercial Lecroy 500 MHz oscilloscope has a sampling rate of 2.5 Gs/s. 11 Digital oscilloscopes are commonly used to record and save the transient signal from the detector. Another method involves PCI digitizer cards, which offer very fast data transfer rates (Gb/s); however, they are usually very expensive compared to oscilloscopes with the same specifications. Regardless of the digitization hardware employed, the digital waveform is saved to a computer and manipulated by the user usually using Labview, MATLAB, or software written in a C language Excitation Geometry There are two primary geometries for performing ns-ta measurements. The first is termed quasi-parallel due to the pump and probe being nearly parallel to each other (Figure 2-2 left). The second, and more common method, is the perpendicular geometry (Figure 2-2 right), where the pump and probe beams are nearly 90º to each other. The advantage of the perpendicular geometry is that it helps to eliminate stray light from the pump beam that may otherwise hit the detector. In terms of time resolution, both sample geometries are equivalent Calculation of the Transient Absorption Signal The aim of any transient absorption measurement is to record the temporal change in the sample absorbance, A(t), induced by the excitation pulse. Absorption cannot be measured directly, therefore a monitoring light is needed, and the light intensity, I(t), is the parameter which is going to be recorded during the experiments. The relation between the light intensities before and after the sample is

43 28 (1) where I in and I are the monitoring light intensity before and after the sample, respectively. There is an initial absorbance of the sample that changes in time as the result of the photoexcitation and relaxation. Therefore, it is convenient to present the absorbance as a sum of two parts A(t,λ) = A 0 (λ) + ΔA (t,λ), where A 0 (λ) is the sample absorbance before the excitation and ΔA(,t,λ) is the absorbance change due to some photoreaction. The value ΔA(t,λ) is called the differential absorbance. Then (2) (3) (4) Since the photo detector signal is proportional to the light intensity, I(λ) is the signal from the detector. Thus, in order to calculate the differential absorbance (as a function of time), one needs to measure the signal before the excitation, I 0, then measure the time dependence I(t) in response to the excitation flash and calculate the difference ΔA = I o I(t). Afterwards, the differential absorbance can be calculated as ( ) (5) By repeating the measurements at different wavelengths one can collect a two dimensional data array ΔA(t,λ), which can be used to create time-resolved differential absorption spectra.

44 Relationship between the Time Resolution and Signal-to-Noise Ratio In transient absorption spectroscopy, the signal-to-noise ratio (S/N) of a signal is proportional to the intensity of the probe source (W/nm). The relationship arises due to the connection between the time resolution, light level (intensity) and S/N. The relationship between S/N and frequency of the probe source can be seen using the noise equivalent power (NEP) of a detector to calculate the minimum amount of light needed to generate a signal equal to the noise level. 4 For example, using a silicon photodiode (Thorlabs Det10A) with a NEP power of W/Hz 1/2, the minimum light level would be 1 nw if we wanted to measure with the time resolution of 1 ns. Assuming a minimum S/N of 25, we can calculate approximately how much light we need per nm (I) to achieve that S/N using (6) where h is Planck s constant, ν is the frequency of light used, δ 2 is the accuracy which is related to S/N, θ f and θ m are the filter and monochromator efficiencies respectively, T is the sample transmittance and τ is the time resolution required. Assume a 700 nm probe, S/N of 25, and filter/monochromator efficiencies of 0.5 (50%), and all other optics about 0.5 (50%) efficient. The sample as a transmittance of 0.3 (30%) and the time resolution required is 1 ns. Therefore, the minimum light intensity (I) would have to be 12 μw at 700 nm. From this relationship, we see that the higher the probe light, the higher the S/N of the signal. The above estimation is a useful starting place when deciding what kind of optical elements, detectors, and light sources to consider when designing a transient absorption spectrometer.

45 Ultrafast Transient Absorption Spectroscopy Femtosecond (ultrafast) transient absorption spectroscopy has become the main technique for studying fast photophysical processes since Zewail s Nobel Prize winning research on chemical bonds in the 1980s using femtosecond pulses. Femtosecond pulses have adequate resolution to analyze fundamental photophysical processes such as charge transfer and charge separation. This Section focuses on establishing the necessary basic understanding about ultrafast instrumentation for later Chapters of this manuscript. The entire process of generation, amplification, and wavelength conversion is summarized in Figure 2-3. Each of the following Sections takes a more in-depth look at the various processes and instruments displayed in Figure Pump-Probe Technique In ns-ta, the time resolution is electronically resolved, meaning that the response of the detector (ns) is fast enough to temporally resolve the transient waveform associated with the experiment. The fastest photodiodes are ~10-10 s, and clearly they cannot be used in to resolve a femtosecond (10-15 s) transient process in the same manner as ns-ta. 4 Ultrafast TA overcomes the time resolution barrier with detectors by removing the time resolution of the experiment from the detector s time resolution. This is achieved by using two pulses of laser light (both femtosecond) and mechanically delaying one pulse relative to the other pulse. The basic instrument schematic of a pump-probe system is depicted in Figure 2-4. Femtosecond pulses are generated from a mode-locked laser and then split via a beam splitter into two paths. The wavelengths of the pump and probe can be the same or different, usually, they are different wavelengths. How different wavelengths are generated is discussed in Section

46 31 One beam path is a fixed distance (Figure 2-4, pump path), while the other path (probe) has variable length due to a delay stage. The delay stage, which has mirrors on it, is moved by a computer to user defined positions. Because the delay stage is changing the length of the probe beam path, the time it takes for the probe beam to travel to the sample will be different than the time for the fixed pump beam. Both beams are focused onto a sample, and the probe beam intensity is detected with a monochromator and detector (e.g. photodiode). The photodetector signal is proportional to the total number of photons in the probe pulse the detected signal is an integral value. The signal acquisition from the detector is depicted in the bottom right of Figure 2-4. When the pump and probe arrive at the same time at the sample (t=0), the photodetector measures the probe intensity, then the stage is moved to a new time position (new time point), and the process is repeated. Because transient absorption techniques measure the difference in a signal, the signal must be measure with and without the pump present. The mechanical chopper in Figure 2-4 modulates the pump beam (block/unblock) allowing for the probe signal to be capture with and without the pump. This process is done for each time point and the signal plotted in pump-probe data is (7) where ΔT is the change in transmission as a function of delay time (t) between the pump and probe beams, and T pumped and T not pumped refer to the sample being not blocked, and blocked by the chopper respectively. The signal is normalized to T not pumped to correct for any sample variations.

47 Ultrafast Pulse Generation and Amplification There are many different components involved in the creation of ultrafast pulses. This Section is meant to illustrate the most commonly used components and techniques and serves as a general overview rather than a comprehensive guide. The most common ultrafast laser is the mode-locked Ti:sapphire laser. The gain medium Ti:sapphire is an ideal choice due to its broad luminescence band in the range nm, high Ti 3+ doping levels (higher amplification possible), and high damage threshold. 12 Figure 2-5 is a simplified schematic of a regenerative mode-locking laser cavity, where a photodiode and acoustic optical modulator (AOM) are used to generate an RF signal to drive the AOM. Initially, the laser is operating in CW mode with oscillations from several longitudinal modes. These are partially phase-locked, and mode beating generates a modulation of the laser output at a frequency of c/2l. This mode beating is detected by a photodiode and then amplified. Since this signal is twice the required AOM modulation frequency (ω ml ), it is divided by two, and then the phase is adjusted such that the modulator is always at maximum transmission when the pulse is present. Finally, the signal is amplified again and fed to the AOM. Figure 2-5 also shows a graphical representation of the mode-locked pulses. Depending on cavity design, ultrafast lasers can output pulses from a few femtoseconds to a few hundred femtoseconds, with the most common commercially available lasers being ~ 100 fs. These 100 fs pulses are a few nj/pulse and usually have MHz repetition rates and exhibit a central wavelength of about 800 nm. 12 As is, these specifications make the laser rather limited in terms of spectroscopy, for example, it would be impossible to use them to pump most dye molecules, and the energy/pulse is rather low. To be of use to a spectroscopist, these pulses must be amplified.

48 33 Ultrafast amplifiers take the output of an ultrafast laser (termed the seed) and amplify the energy /pulse by This high level of amplification allows the pulses to drive a number of nonlinear processes such as second harmonic generation (SHG), sum frequency generation (SFG) and difference frequency generation (DFG). Modern ultrafast amplifiers work on the principle of chirped pulse amplification (CPA). In CPA, the seed pulse is stretched in time (chirped) by a diffraction grating. The grating separates the red and blue portions of the pulse, which increases the time duration of the pulse and therefore the peak power necessary to protect optical components in the amplifier. The fundamental relationship between the width of a laser pulse and its bandwidth is that a very short pulse exhibits a broad bandwidth, and vice versa. For a Gaussian pulse, this relation is given as 4 (8) where dν is the bandwidth and dt is the laser pulse width. For example, for a 100 fs duration pulse at λ = 800 nm, the corresponding bandwidth is more than 9 nm. Therefore, a device that can delay certain frequencies (or wavelengths) relative to others can stretch a short pulse so that it lasts a longer time. Likewise, such a device should also be able to compress a long pulse into a shorter one by reversing the procedure. The phenomenon of delaying or advancing some wavelengths relative to others is called Group Velocity Dispersion (GVD) or, less formally, chirp. The process of pulse stretching is illustrated in Figure 2-6. A light pulse incident on a diffraction grating experiences dispersion; that is, its component wavelengths are spatially separated, and so too are its frequency components. The dispersed spectrum can be directed through a combination of optics (usually the same diffraction grating can be used) to send the different frequencies in slightly different directions. Longer (or redder) wavelengths can be made to travel over a longer path than the shorter (or bluer) wavelengths components of the beam, or vice versa. The result is to lengthen the duration of the pulse, which reduces its peak power (it is

49 34 the same energy under the curve, only spread out more now). The grating and the routing mirrors can be chosen so that, in the stretcher, the bluer frequency components of the spectrum travel further than the redder components, causing the redder frequency components to exit the stretcher first. Once the pulse has been stretched it is amplified in a Ti:sapphire crystal in what is called a regenerative amplifier. Regenerative (regen) amplifiers are designed to recirculate and amplify low-energy laser pulses from the seed laser. Thus, instead of allowing the energy in the amplifier crystal to amplify random spontaneous emission, the seed pulses (having an energy that exceeds the spontaneous emission energy) are selectively amplified. A regen first selects and then optically confines an individual pulse from the train of mode-locked seed pulses that have already been lengthened in duration in the stretcher. The selection and confinement process uses the polarization of the pulse and a Pockels Cell to trap a pulse in the regen, meaning that only if the pulse is the correct polarization can if enter or leave the regen the Pockels cell controls the polarization. Immediately before the selected pulse passes through the Ti:sapphire crystal, the crystal is excited to a condition of population inversion by a high energy pulse from a pump laser, usually a Nd:YLF Q-switched laser. Most of the energy from the pump pulses is retained in the Ti:sapphire crystal and converted to energy in the amplified pulses. The selected seed pulse is passed through the crystal 10 or more times until the pulse energy level is high enough to saturate the population inversion in the amplifier rod. Having thus saturated the gain, that is, absorbed all the available energy, the pulse is ejected into the compressor. An input pulse of only a few nj of energy may be amplified to several mj by a single Ti:sapphire crystal through regenerative amplification. The regenerative amplifier can produce an amplification greater than 10 6 at the output of the compressor. 4 In the compressor, the spatially spread beam is flipped so that the redder components have to take the long path, thereby allowing the bluer frequencies to catch up. This recompresses the pulse, which now has considerably higher peak power than the original

50 35 seed laser pulse. These recompressed pulses have GW peak powers and can easily drive the nonlinear processes require to achieve tunability of their wavelength Wavelength Tunability of Ultrafast Pulses White Light Continuum The easiest way to achieve tunability of an ultrafast pulse is to use what is referred to as a white light continuum. Femtosecond pulses have very high (GW) peak powers, which are easily capable of driving numerous nonlinear processes. White light generation is one such process, where a femtosecond pulse is focused a material, usually sapphire, and a spectrum (continuum) is produced from nm. The amount of input light can be quite small (<1 μj/pulse), meaning only a small fraction of the energy from the laser amplified needs to be utilized. The continuum generated is typically used as the probe beam for visible pump-probe spectroscopy. The downside of the technique is that the continuum does not contain a lot of energy per wavelength (nj/nm), therefore, it cannot be used as a pump for most samples Optical Parametric Amplification An optical parametric amplifier (OPA) is the primary instrument used to tune the wavelength of ultrafast pulses. OPA s utilize nonlinear wave mixing to generate new, amplified photons. In terms of photon-photon interaction, wave mixing can be written 13 (4) where p 1, p 2 and p 3 are photons of frequency ω 1, ω 2 d ω 3. The sum of two photons can c e e ew f e e c ω 3. The process is also reversible, meaning if a photon of

51 36 ω 3 interacts with a nonlinear crystal, it can dissociation and produce two photons of f e e c ω 1 and ω 2. The process can be extended to a two photon reaction: (5) Two photons at ω 1 are generated and the reaction can be considered as the reaction of stimulated emission of the second photon at ω 1. For stimulated reactions, the second photon has the same frequency and phase as the initial photon at ω 1, thus the reaction results in amplification of the light at ω 1. In such cases, the light at ω 1 is called the signal field, and the light at ω 2 is called the idler field and the light at ω 3 is called the pump. Usually the shorter wavelength is called signal and the longer idler either wavelength can be amplified. 4, 12 This process allows for an immense number of output wavelengths from a single 800 nm pump wavelength the output of the laser amplifier. A summary of the available output wavelengths of an OPA pumped at 800 nm is given in Figure Array Detectors It was mentioned in the beginning of Chapter 2 that the detector can be a simple photodiode. Because the delay stage needs to be moved to each new time position (delay between pump and probe) an ultrafast experiment can take hours to perform, especially if the change in absorbance of the sample is small. If a single channel detector is used (e.g. photodiode), the process will take significantly longer because the entire process of collecting the time axis needs to be done at each new wavelength picked by the monochromator. To overcome this, most research groups use array detectors that can capture entire regions of a spectrum, or perhaps the entire spectrum depending on the wavelength range required. For detection of visible light (visible probe experiment), silicon CCDs are used, usually TEM or liquid nitrogen cooled to

52 37 reduce noise. Because the time resolution of an ultrafast experiment is independent of the detector, many of the concerns discussed in are not an issue here. The one requirement is that the readout time of the CCD array is fast enough so that the CCD can capture each laser pulse. Because ultrafast lasers typically have 1-5 khz repetition rates, the CCD and electronics must be able to capture and reset faster than the repetition rate. For mid-ir detection, liquid nitrogen cooled MCT arrays are used; however, unlike the large number of pixels CCD array can have, MCT arrays are less than 128x128 pixels. The limitation arises from the increased readout time and temporal response of each element, too many elements and the array could not be read prior to the next khz laser shot hitting the detector. 2.5 References 1. Harris, D. Fundamentals of Spectrophotometry. In Quantitative Chemical Analysis, 8th ed.; W.H. Freeman and Company: New York, 2010; pp Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999; pp Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, Tkachenko, N. V. Optical Spectroscopy: Methods and Instrumentation, 1st ed.; Elsevier: Amsterdam, Spectra-Physics. DPPS Lasers. (accessed May 10th, 2016). 6. Energetiq. EQ-99X LDLS. (accessed May 10th, 2016).

53 38 7. Rabek, J. F. Optical Radiation Sources. In Experimental Methods in Photochemistry and Photophysics; John Wiley and Sons: Belfast, 1982; pp Thorlabs. High-Speed Photodetectors. (accessed May 8th, 2016). 9. Kolmar Technologies. HgCdTe Photodiode Sensors. (accessed May 8th, 2016). 10. National Instruments. Acquiring an Analog Signal: Bandwidth, Nyquist Sampling Theorem, and Aliasing. (accessed April 16th, 2016). 11. Teledyne Lecroy. Wavesurfer 3000 Oscilloscopes. (accessed May 4th, 2016). 12. Abramczyk, H. Introduction to Laser Spectroscopy; Elsevier: Amsterdam, Zernike, R. M. J. E. Applied Nonlinear Optics; Dover: Mineola, A., H. W. Solid State Theory; McGraw-Hill: New York, 1979.

54 Figure 2-1. Basics of nanosecond transient absorption spectroscopy illustrating a pulsed laser source optically exciting a sample and a continuous wave probe (lamp) providing the detected signal. The monochromators are denoted λ, and PD = photodiode. The signal is recorded using a digital oscilloscope. 39

55 Figure 2-2. Nanosecond transient absorption beam geometries depicting quasi-parallel (left) and perpendicular (right). Time resolution is identical for both geometries. 40

56 Figure 2-3. Overview of the ultrafast pulse generation, amplification and wavelength conversion processes. Ultrafast pulses are generated via a mode-locked laser (seed) and sent into an ultrafast amplifier that increases the energy of the pulse by ~10 6. After amplification, the pulse can be used to generate a white light continuum probe (used in visible pump-probe spectroscopy), and/or to pump an OPA or multiple OPAs. Depending on the OPAs signal and idler frequencies (adjustable by the user), an OPA can produce a very large range of wavelengths utilizing sum frequency generation (SFG), second harmonic generation (SHG), or difference frequency generation (DFG). 41

57 Figure 2-4. Basics of ultrafast pump-probe spectroscopy. A pump is modulated by a chopper to separate blocked and unblocked pump shots. The probe is delayed on a mechanical delay stage relative to the pump, which creates the time delay Δt that serves as the x-axis for data plots. The monochromator separates different probe wavelengths and the light intensity of a particular wavelength is recorded using a photodiode. 42

58 Figure 2-5. Simplified schematic of a regenerative mode-locking ultrafast laser cavity. HR is a high reflective mirror, AOM = acoustic optical modulator, OC = output coupler, and PD is a photodiode. The photodiode signal provides feedback for the AOM. The bottom figure depicts the amplitude and frequency of longitudinal modes in a mode-locked laser. 43

59 Figure 2-6. Principle of pulse stretching in an ultrafast laser amplifier. The pulse is chirped using two gratings that allow for the separation of the blue and red portions of the pulse. The pulse makes two passes on the grating which serves to diffract and then recollimated the beam; however, the recollimated beam is significant longer in duration. 44

60 45 Chapter 3 Developing a High Performance Nanosecond Visible/Near-Infrared Transient Absorption and Photoluminescence Spectrometer The technique of nanosecond transient absorption spectroscopy has existed since the 1970s, and now plays a large role in fundamental and applied studies of emerging electronic materials. 1-3 As discussed in Chapter 1 and 2, transient absorption spectroscopy is capable of measuring the dynamics of excited state populations including excitons and charge carriers through ground state bleach recovery and other nanosecond processes. The enormous growth of the electronic materials field has greatly increased the demand for instrumentation capable of routinely measuring these photophysical processes. Although several commercial nanosecond transient absorption instruments exist, most are not suitable for thin film studies due to design limitations such as limited detector range or sample cell types/sizes. 4-6 The limitations of such instruments may not be obvious to researchers new to the field. As such, we developed a specialized transient absorption instrument to meet specific challenges facing researchers in the field of electronic materials. This Chapter is based on the publication: Rimshaw, A.; Grieco, C.; Asbury, J.B. Using Fast Digitizer Acquisition and Flexible Resolution to Enhance Noise Cancellation for High Performance Nanosecond Transient Absorbance Spectroscopy Rev. Sci. Instr. 2015, 86, Challenges with OPV materials Thin films of optoelectronic materials can present unique challenges due to their atmospheric sensitivity (e.g. light and air), overlapping transient spectral features, and small

61 46 signal levels. Although numerous homebuilt systems meeting the above criteria exists, ours is the only to our knowledge to feature flexible digital resolution and a subsequent subtraction noise elimination technique that has improved S/N by over an order of magnitude (see Section 3.4). These advances are important because they allow data to be collected faster due to better S/N, and allow for detailed exploration of artifacts that adversely affect transient absorption signals. Artifacts often result from photoluminescence or heating of the sample. In particular, many thin-films can be very emissive and can render particular parts of the spectrum unusable for transient absorption. This is especially problematic for low-bandgap polymers, which can have very strong emission in the NIR inconveniently overlapping or obscuring the transient absorption of polarons. Thin-films are also very susceptible to thermal artifacts. Neat materials, which are often used as control experiments, are especially susceptible to thermal artifacts due to few to no electron/energy transfer events. Thermal artifacts have been observed by us and others at laser pump energies below ~30 µj/pulse, and may arise from poor thermal conductivity of the film and/or the deposition substrate, which is typically glass. 7, 8 The need to use excitation densities in the range of 10 µj/cm 2 or lower pushes the sensitivity limits of conventional transient absorption instrumentation commonly used in the field. For example, changes in absorption in the 100 µo.d. range (corresponding to 200 parts per million change in transmission) are needed for routine measurements of transient absorption signals. A consequence of pushing the sensitivity limits of the transient absorption instrumentation is that it can be challenging to accurately measure thermal and photoluminescence artifacts with sufficient fidelity to permit accurate isolation of the transient absorption spectral features. Unlike photoluminescence measurements, for which the zero background permits high sensitivity by use of high quantum yield detectors, thermal and photoluminescence artifacts cannot be eliminated by using a more sensitive detector in transient

62 47 absorption experiments. High intensity light sources based on lasers and laser-driven plasmas are approaches that have been used to enhance the sensitivity of transient absorption instrumentations. While effective, these approaches create significant financial barriers to construction of instrumentation capable of high-sensitivity transient absorption measurements and may increase film degradation due to higher probe light levels. 3.2 Instrument Design The entire instrument schematic is shown in Figure 3-1, the following Sections explain the various components while referring to Figure Light Sources The pump source is a frequency doubled Nd 3+ :YAG laser (Surelite II, Continuum) operating at 30 Hz with a FWHM of 8 ns, or a homebuilt dye laser pumped by either the Nd 3+ :YAG laser or a nitrogen laser (Optical Building Blocks, 10 Hz). The instrument may be coupled to nearly any nanosecond laser provided that the repetition rate is less than 1 MHz limited by the trigger rearm time of the digitizer. The continuous wave probe is a 150W halogen element (Spectral Products, ASBN-150W-L) with an element size of 5.8 mm x 3.0 mm, providing broadband visible and NIR light.

63 Optics All lenses are uncoated BK7 glass (1 inch diameter). Broadband dielectric coated lenses may be used if more light is needed in particular spectral regions for an experiment. Lenses L1 L5 (Figure 3-1) are plano-convex with 100 mm focal lengths. The lenses used for focusing on the detectors (L7-9) are aspheric condenser lenses with 16 mm focal lengths. The monochromator (Spectral Products, DK240) features three gratings allowing for full coverage from ultraviolet (UV) to near-ir (NIR). For NIR spectra we use a 600 grooves/mm (1200 nm blaze) grating that covers from nm. For the visible spectra we use a 1200 grooves/mm (750 nm blaze) grating for nm. For UV and visible spectra we use a 1200 grooves/mm (500 nm blaze) that covers a nm range. A series of longpass filters are used in a motorized filter wheel (Figure 3-1, F2) after the monochromator to block higher orders of diffraction from the grating. Filters in F1 are interference filters used for selecting portions of the lamp spectrum to avoid heating or degrading the sample. The largest effective bandwidths supported are 10 or 20 nm for the 1200 or 600 groves/mm gratings, respectively. However, subnanometer effective bandwidths are achievable, which are used for laser characterization Detectors Our instrument features three photodiodes (Table 3-1) which are chosen via the insertion of a silver mirror. The DET-36A and 10N detectors are fed into a voltage preamplifier (FEMTO, HVA-200M-40-B) that is 200 MHz with variable (20 or 40 db) gain. The output of the preamplifier is sent to a digital oscilloscope for signal digitalization and subsequent computer processing. Detector PDA10D is a transimpedance amplified photodiode and does not require any further amplification by the FEMTO preamplifier. Being able to cover a spectral range of 350-

64 nm allows the instrument to capture any visible or NIR transient signal with nanosecond time resolution. Switching between detectors is easily done via a kinematic cube mirror Data Collection Hardware Data collection is achieved using a PC oscilloscope (Picoscope, 5244A) with three analog inputs. The oscilloscope is triggered off the excitation laser pulse signal from the reference photodiode, while two channels (detector and chopper signals) are collected simultaneously for each trigger event. The oscilloscope has the ability to perform rapid trigger acquisition (~1 µs trigger rearm time) allowing every laser shot to be collected. A commonly overlooked aspect of using digital oscilloscopes for data collection is that every scope has significant dead time between trigger events when running in a traditional repeat (aka normal) trigger mode. Unless the scope manufacturer explicitly states the ability to collect subsequent triggers, which has only become commercially available in the past ten years, the scope does not actually detect every trigger event (Section 3.4). The Picoscope oscilloscope also features a new innovation in analogue to digital (ADC) conversion called flexible resolution. Unlike the traditional ADC architecture, which interleaves multiple 8-bit ADCs to achieve higher resolution, the Picoscope allows multiple high-resolution ADCs to be applied to the input channels in different time-interleaved and parallel combinations to boost either the sampling rate or the resolution. 9 In our case, we exploit the ADC parallel sampling configuration, in which multiple ADCs are sampled in phase and their output combined to effectively change the resolution for each channel. This allows the user to select the resolution 8-12 bit (with two channels enabled) at the cost of lower sampling rates, although still satisfying the Nyquist condition to avoid aliasing.

65 Data Collection Software We use MATLAB s instrument control toolbox to interface with the monochromator, filter wheels and Picoscope for complete control of the instrument using a graphical user interface (GUI). The input parameters are wavelength range, number of laser shots/scan, automated photoluminescence (PL) subtraction, and number of scans. The wavelength range and number of shots may be linear or nonlinear. If nonlinear, the user selects individual wavelengths of interest and defines at those wavelengths, how many laser shots to collect. If linear, the number of laser shots is the same for all wavelengths and the wavelengths are evenly spaced user-defined between the extrema. The instrument s data acquisition flow is given in Figure 3-2. The Picoscope is triggered off a photodiode laser pulse for a user-defined number of times. Two channels are stored in the buffer memory (detector signal and chopper signal) for the specified number of laser shots. At wavelengths where PL is present we utilize a homebuilt automated shutter to block the probe beam (Figure 3-2, shutter closed), allowing the only PL data to be collected. Then the probe is unblocked (Figure 3-2, shutter open) and we collect a signal that contains both PL and TA contributions. The subtraction of the pure PL component from the mixed PL/TA signal allows us to recover the TA kinetics. The chopper signal enables the program to sort the on/off shots to calculate ΔT. A typical data collection scheme involves averaging all the on shots together and off shots together followed by subtraction of the averaged on and off shots to calculate ΔT. We term this bulk subtraction (Section 3.4). Such a method is necessary if the digitizer cannot rearm fast enough for the next trigger event. Instead of the above approach, we subtract each on shot with its corresponding off shot, then take the average of those and subtract to calculate ΔT, which we call subsequent subtraction. This may seem trivial, however, subsequently subtracting the on s and

66 51 off s in time greatly reduces the electromagnetic interference or other non-white noise, which plagues many nanosecond kinetics commonly seen in the literature (see Section 3.4). After calculating ΔT for one wavelength, the program switches the Picoscope to DC coupling and measures the DC offset (due to lamp transmission) at that same wavelength. The resultant ΔT/T is plotted along with the steady-state spectrum in real time. This method of acquiring ΔT and T close in real time is advantageous because it reduces errors in the transient signal due to long-term instabilities or drifts of the laser or lamp beam intensities. The program repeats the process until the wavelength range is done and the requested number of scans is completed. The program selects the appropriate longpass filters to use depending on scan parameters, such as the current monochromator position. 3.3 Instrument Performance OPV Sample Preparation The well characterized organic photovoltaic system regioregular poly(3-hexylthiophene) (P3HT) blended with phenyl-c61-butyric acid methyl ester (PCBM) was chosen to demonstrate the performance of our instrument. A chlorobenzene solution containing 25 mg of P3HT (Sigma- Aldrich) and 25 mg of PCBM (Sigma-Aldrich) was stirred and heated (70 ºC) for 2 hours. The solution was spin-casted while hot onto a 2.5 mm thick circular sapphire substrate (1 inch diameter) at 1000 rpm for two minutes. The film was annealed for 10 minutes at 100 ºC on a hot plate after spin coating then transferred to a cryostat (Janis Research, ST-100) and held under vacuum (~10-5 ) during all measurements. The film thickness was estimated to be around 80 nm based on its optical density.

67 Spectral Range Broad spectral ranges are necessary for any transient absorption spectrometer when studying thin-films due to the broad, often overlapping features they exhibit. Figure 3-3a shows the steady-state absorption and PL spectra for the P3HT-PCBM blend. The absorption was collected using a commercial UV-vis spectrometer (Beckman, DU520), while the PL measurement was made using the transient absorption instrument, which may double as a laserexcited fluorometer if needed. The broad ground state absorption is typical of annealed P3HT:PCBM showing its characteristic π π* electronic transition and two features at 2.25 ev (550 nm) and 2.07 ev (610 nm). These are assigned to P3HT interchain excitation resulting from lamellar structure formed via π-stacking. 10 The transient absorption (TA) spectrum in Figure 3-3b was collected over three regions using a variety of photodiodes (Section 3.2.4) and a pump energy density of 50 µj/cm 2. To minimize collection times, we use nonlinear wavelength steps and a nonlinear number of laser shots per wavelength. For example, in regions where spectral features are known to arise, smaller wavelength steps are used. In a similar fashion, we increase the number of laser shots in regions where signals are low to improve the S/N. Such parameters are read into the program via an excel spreadsheet the user creates. The program also utilized PL subtraction for ev ( nm). The region about 2.33 ev (532 nm) was not measured due to the strong scatter hitting the detector from the pump laser. The TA spectra show two typical features, the ground state bleach region (2-3 ev) along with a photo-induced absorption feature (1-2 ev). The rising feature at 0.75 ev (1653 nm) corresponds to additional polaron optical transitions that are located in the mid-infrared region (see Chapter 4). 11

68 Temporal Range The ability to collect kinetics into the millisecond timescale is crucial for fitting longlived species like polarons and modeling the effects of trapped states. Our instrument is capable of collecting data for hundreds of milliseconds, although usually a millisecond or two is sufficient. Figure 3-4 shows the NIR polaron TA at 1.22 ev (1020 nm) collected on a millisecond timescale at a fluence of 40 µj/cm 2. The total number of scans was 120, with 200 laser shots/scan, for a total runtime of 30 minutes. This particular wavelength was chosen for the lack of PL observed, thereby not requiring PL subtraction. For our setup, the upper limit to the temporal range (~33ms) was limited by the repetition rate of the laser source (30Hz). The time resolution was limited by the laser pulse duration (~10ns) Sensitivity Perhaps the most important aspect is the sensitivity of the instrument. As we discuss below in the thermal artifact Section, low fluence is essential for performing quantitative analysis of thin-films. Even 40 µj/cm 2 can be very high in particular situations. These low pump energies give rise to correspondingly low (~100 µv) detector signals. Such cases are not ideal for traditional 8-bit real-time digitization. The ability to change digitization resolution and collect data with rapid (burst) triggering has allowed such signals to be measured more quickly and more reliably (Section 3.4). We showed this experimentally using a power dependence study from 40 to 0.6 µj/cm 2 to illustrate the importance of low pump energy when performing quantitative analysis. As seen in Figure 3-5, even the lowest signals observed using low pump energies are well-resolvable by the instrument in run-times as short as 30 minutes. Being able to routinely detect ~10-5 signals allows us to probe materials at much lower energies than previously possible.

69 54 This is especially important on the nanosecond timescale where bimolecular recombination is the primary decay mechanism because the energy density is proportional to the number of polarons. Therefore device irrelevant energy densities will produce irrelevant polaron populations and recombination rates. 3.4 The Importance of Sequential Subtraction for Signal-to-Noise Enhancement Achieving low sensitivity requires high resolution digitization along with a minimal electronic noise level. The primary contributor for electronic noise is the pump laser, which for ns transient absorption measurements is usually a Q-switch YAG laser. Even properly shielded systems exhibit enough noise to make low (µv) measurements very difficult. The electronic noise has a fast dampened sinusoidal shape, which is essentially constant, while the amplitude of its individual peaks have a random component (Figure 3-6). Furthermore, we have observed a long term jitter of the waveform. The random component can be improved through averaging; however, care must be taken to avoid long acquisition times due to the sensitivity of most organic materials to laser light. The systematic shape and jitter, are more problematic, however modulation of the pump can greatly help in reducing it (see Figure 3-7). The caveat is that the subtraction between the detector signal when light hits the sample (on) and when the laser light does not (off), should be performed as close in time to the previous laser shot as possible, ideally, on a shot-by-shot basis. The jitter shift between two subsequent shots is essentially zero, and the since the overall waveform shape of the noise is essential constant, subtraction close in time nearly removes the systematic electronic noise completely.

70 55 In order to implement the above subtraction scheme it is necessary capture and sort every trigger event (Figure 3-2). Digital oscilloscopes are typically used for the collection of transient signals common in many spectroscopic instruments. Although they are well suited for such applications, it is important to understand that only a few models, and only available in the past ten years, have the capability to collect subsequent triggers fast enough for the repetition rate of most ns lasers (~ Hz). We compare what we have termed bulk and subsequent subtraction methods (see Data Collection Software and Schemes) to illustrate the difference in signal quality between the two methods. The test between the two methods was done using 200 laser shots, 3 scans total, collected at 1.22 ev (1020 nm). For the bulk collection, an oscilloscope (Lecroy Wavesurfer 432, 350 MHz) was used in the normal trigger mode. By normal we mean the oscilloscope is triggered on the reference PD, followed by digitization of the signal, and subsequent display of the waveform before the oscilloscope is rearmed for the next trigger event. The subsequent subtraction method was performed using the same PC oscilloscope (see methods) used in all other experiments with our instrument. The results of the comparison can be seen in Figure 3-7. The Q-switch noise is a dominating contributor, nearly obscuring the transient signal for the bulk subtraction technique. Even though we are still subtracting on s and off s, not doing so subsequently clearly limits instrument performance. The Q-switch noise is nearly gone when utilizing subsequent subtraction, allowing us to analyze a 500 µod absorbance signal in only 3 scans (~30 s) with an S/N improvement of over an order of magnitude.

71 Common Transient Absorption Artifacts for Organic Photovoltaic Materials The instrument characteristics highlighted above are necessities for overcoming artifacts commonly encountered while measuring low signals typically observed for thin films of optoelectronic materials. Unfortunately, these artifacts can lead to erroneous physical interpretations following their kinetic modeling. Here, we show how our instrument s high sensitivity and wide spectral range are important for identifying and overcoming artifacts arising from physical phenomena such as photoluminescence and heat Photoluminescence Artifacts As mentioned previously, for collecting TA of P3HT:PCBM we utilized what was called automated PL subtraction to collect the data. The subtraction works by using a shutter to block the probe beam from the sample while letting the laser hit the sample, thereby capturing the isolated PL kinetics and allowing for its subtraction (see software collection). Figure 3-8 illustrates the consequences of not systematically using PL subtraction. At 1.46 ev (850 nm) the PL artifact signal is of comparable size to the TA and can easily be distinguished. However, it is not obvious at 1.31 ev (950 nm) (Figure 3-8b) deep in the tail of the photoluminescence spectrum (see Figure 3-3). In this case, the true TA signal is attenuated by ~20% although no PL feature is evident. Just like thermal artifacts (discussed below), these invisible PL artifacts can lead to errors in kinetic modeling if overlooked. Similar artifacts created by laser scatter can also be eliminated using this subtraction method, since like PL, these signals contribute additively to the overall observed transient signal. Thus our instrument is also capable of measuring transient absorbance in regions close to laser wavelengths.

72 Thermal Artifacts It is common when comparing the performance of various thin films of organic photovoltaic materials to perform control experiments with the neat materials. Often a difference in polaron yields made evident by signal amplitude changes or elucidating decay pathways are of interest to the researcher. Neat films pose subtle but dangerous wavelength-independent thermal artifacts that are system dependent. They primary arise from the low thermal conductivity of most polymer/small molecular OPVs that causes heat retention in the film thereby modulating the absorption via the refractive index. 7, 12 They can be exaggerated in neat materials due to the lack of energy/charge transfer pathways, making heat dissipation the dominant mechanism for energy relaxation. Another source of thermal artifacts is the poor thermal conductivity of some substrates used for film deposition. However, by using sapphire or calcium fluoride, this is generally not a concern, although it has been observed on glass substrates. 12 Nevertheless, if a film is sufficiently thick, the thermal conductivity of the sample itself may be limiting rapid dissipation of heat, and so the best way to eliminate the contribution of thermal artifacts to observed transient signals is by using lower laser energy densities. Significant changes to transient signals can occur due to these thermal artifacts. A neat P3HT film was chosen to illustrate the manifestation of thermal artifacts, which can often be invisible in blend film samples. Figure 3-9 shows a neat P3HT film on a sapphire substrate pumped at 170, 70 and 2 µj/cm 2. The fast rise is instrument-limited TA from photoexcited states probed at 1.13 ev (1100 nm) 12, while the negative feature is the thermal artifact. Even at 70 µj/cm 2 there is a contribution from the thermal artifact. This artifact is present in the blend films but to a lesser extent due to the presence of charge-transfer pathways. 12 In addition, the higher TA signal of polarons in blend films obscures the direct observation of a thermal artifact.

73 58 However, it is still reasonable to assume thermal artifacts are present and attenuating the TA signal amplitude to some extent the blended films. 3.6 Conclusion We built a transient absorbance instrument designed to specifically meet challenges facing researchers in the field of optoelectronics. Through a combination of increased spectral range and higher sensitivity made possible by a rapid data acquisition scheme coupled with flexible resolution, small transient signals could be measured easily. Furthermore, these advantages allow for the exploration and elimination of artifacts capable of leading to erroneous quantitative analysis. 3.7 References 1. Porter, G.; Topp, M. R. Nature 1968, 5173, Dalby, F. W. J. Phys. Chem. 1964, 8, Dogliott, L.; Hayon, E. J. Phys. Chem. 1967, 8, Edinburgh Instruments. Laser Flash Photoylsis Spectrometer. (accessed March 13th, 2014). 5. Leukos. Time-Resolved Spectroscopy. (accessed Januray 12th, 2015). 6. Hammamatsu. Flash Photolysis System.

74 59 (accessed May 1st, 2016). 7. Rao, A.; Wilson, M. W. B.; Albert-Seifried, S.; Di Pietro, R.; Friend, R. H. Phys. Rev. B. 2011, 84, Manceau, M.; Rivaton, A.; Gardette, J.-L.; Guillerez, S.; Lemaitre, N. Sol. Energy. Mater. Sol. Cells. 2011, 95, Lecklider, T. The Many Paths to High Resolution. (accessed December 1st, 2015). 10. Ohkita, H.; Shinzaburo, I. Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy. In Organic Solar Cells; Choy, W. C. H., Ed.; Springer: London, 2013; p Drori, T.; Holt, J.; Vardeny, Z. V. Optical studies of the charge transfer complex in polythiophene/fullerene blends for organic photovoltaic applications. Phys. Rev. B. 2010, 82, Albert-Seifried, S.; Friend, R. H. Appl. Phys. Lett. 2011, 98, A., H. W. Solid State Theory; McGraw-Hill: New York, Harris, D. Fundamentals of Spectrophotometry. In Quantitative Chemical Analysis, 8th ed.; W.H. Freeman and Company: New York, 2010; pp Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers, 2nd ed.; Oxford University Press: New York, 1999; pp Clarke, T. M.; Durrant, J. R. Charge Photogeneration in Organic Solar Cells. Chem. Rev. 2010, 110, Tkachenko, N. V. Optical Spectroscopy: Methods and Instrumentation, 1st ed.; Elsevier:

75 60 Amsterdam, Spectra-Physics. DPPS Lasers. (accessed May 10th, 2016). 19. Energetiq. EQ-99X LDLS. (accessed May 10th, 2016). 20. Rabek, J. F. Optical Radiation Sources. In Experimental Methods in Photochemistry and Photophysics; John Wiley and Sons: Belfast, 1982; pp Thorlabs. High-Speed Photodetectors. (accessed May 8th, 2016). 22. Kolmar Technologies. HgCdTe Photodiode Sensors. (accessed May 8th, 2016). 23. National Instruments. Acquiring an Analog Signal: Bandwidth, Nyquist Sampling Theorem, and Aliasing. (accessed April 16th, 2016). 24. Teledyne Lecroy. Wavesurfer 3000 Oscilloscopes. (accessed May 4th, 2016). 25. Abramczyk, H. Introduction to Laser Spectroscopy; Elsevier: Amsterdam, Zernike, R. M. J. E. Applied Nonlinear Optics; Dover: Mineola, 2006.

76 Figure 3-1. Diagram for the nanosecond transient absorption spectrometer. Lenses are labeled L and filters F. Photodiodes are labeled PD and PA is the preamplifier. 61

77 62 Figure 3-2. Data acquisition schematic for the transient absorbance instrument. The digital oscilloscope triggers on each laser pulse signal from the reference photodiode. A synchronized mechanical chopper TTL signal, as well as the detector signal are collected for each laser shot. Each shot is identified and tagged as on or off based on the chopper signal. The small changes in the lamp transmission (ΔT) are collected using AC coupling and with low gain to increase voltage resolution. The lamp transmission (T) is collected using DC coupling. Photoluminescence may be collected by blocking the lamp beam, and subtracted from the overall change in transmission signal if desired. The overall transient signal is obtained by dividing ΔT by T. All signals are collected on a shot-by-shot basis using rapid triggering mode.

78 Figure 3-3. (a) Steady-state absorption and emission spectra for a P3HT:PCBM blend film. (b) TA spectra for the same film; the pump energy was 40 µj/cm 2. 63

79 Figure 3-4. Transient absorbance decay of polarons in the P3HT:PCBM film at 1020 nm (1.22 ev) using 60 µj/cm nm laser excitation. Data was collected over a timescale ranging from nanoseconds to 1 millisecond. Note the semi-logarithmic scale. The inset shows the decay along with the instrument response on a linear scale. 64

80 65 Figure 3-5. Power-dependent study of polaron absorption decay at 1020 nm (1.22 ev) in the P3HT:PCBM film using 532 nm laser excitation. Signals are well-resolved even at low excitation energy densities, allowing for facile kinetic modeling for samples exposed to light levels that are close to device operating conditions.

81 Figure 3-6. RF noise associated with the Q-switch of the pump laser. 66

82 Figure 3-7. Comparison of a transient absorbance signal measured at 1.22 ev (1020 nm) for the P3HT:PCBM film excited using a 7 µj/cm2 laser pulse at 532 nm, detected using either repeat triggering or rapid triggering modes. Each trace is the result of the average of 3 scans using 200 laser shots each (~30 seconds in real time). 67

83 Figure 3-8. Transient absorbance decays of polaron absorption in P3HT:PCBM in the presence of phololuminescence (PL) artifacts at (a) 850 nm (1.46 ev) and (b) 950 nm (1.3 ev). The excitation wavelength was 532 nm. The artifact is suppressed by measuring and subtracting the PL component from the overall observed signal. Panel (b) emphasizes the need to routinely subtract PL because it is not always directly observed and can attenuate the kinetic amplitudes associated with the true transient absorbance signal. 68

84 Figure 3-9. Transient absorbance decay observed for a neat P3HT film at 1100 nm (1.13 ev) in the presence of a thermal artifact. The excitation wavelength was 532 nm. The thermal artifact is suppressed by using lower laser energy densities. 69

85 70 Table 3-1. Photodetectors for nanosecond transient absorption spectrometer. Model (Thorlabs) Material Range (nm) Rise Time (ns) DET-36A Si DET-10N InGaAs PDA10D InGaAs

86 71 Chapter 4 Developing a High Performance Nanosecond Mid-Infrared Transient Absorption Spectrometer 4.1 Challenges with Mid-Infrared Transient Absorption The general principles of visible pump mid-infrared (mid-ir) probe transient absorption (mid-ir-ta) are identical to those of visible pump visible/nir probe transient absorption (ns-vis-ta). However, there are numerous practical aspects that make mid-ir-ta significantly harder, evident by the fact that there are no commercially available nanosecond transient absorption spectrometers. These practical issues will be discussed below along with the previous state-of-the-art in mid-ir-ta spectrometers. This Chapter is based on the paper Rimshaw, A.; Grieco, C.; Asbury, B. J. High Sensitivity Nanosecond Mid-Infrared Transient Absorption Spectrometer Enabling Low Excitation Density Measurements of Electronic Materials. Appl. Spec. June 2016 (Online first before print, ) Light Sources and Collection Efficiency There are many options for visible and NIR probe light sources. In Section 2.2.2, we listed halogen-tungsten, pulsed xenon, and laser-driven plasma sources. These options give experimentalists plenty of options in terms of optical power and spectral range. In the mid-ir region, options are considerably less, for example, Nernest glowers (zirconia, yttria, and thoria) and Globars (silicon carbide) are the best performing materials in terms of spectral power and range. However, these cannot be pulsed; furthermore, they ideally should be liquid cooled to

87 72 ensure stable operating temperature and therefore stable light output. Unlike some visible sources, mw/nm optical power is not possible with mid-ir sources and from the discussion in 3.4 about light level and sensitivity, it becomes obvious that designing an instrument capable of comparable performance to ns-vis-ta instruments (Chapter 3) will be difficult. Designing a mid-ir-ta instrument capable of delivering 10-5 sensitivity requires careful consideration of the optical layout to collect as much light as possible from the mid-ir probe source. Conventional wisdom for light collection is to use low F-number optics, which enables the collection of as much light as possible for a point source. F-number (F/#) refers to the focal length of an optic divided by the diameter of the optic assuming the incident angle of the light is <15, a valid assumption in all of our cases. For example, in Fourier Transform Interferometer (FTIR) instruments a large (1-2 inches length) Globar is housed inside of a large ellipsoidal reflector. Low F/# optics are great for collecting light (gathering efficiency varies as 1/(F/#) 2 )), but the collection of light is only important if the light can be refocused to a spot size comparable to the detector element. In the case of Globars and ellipsoidal reflectors, because Globars are very bad point source approximations, the actual refocusing of the Globar image has points of the source that are not exactly at the focus of the ellipse will be magnified and defocused at the image. Because of this effect, ellipsoids are most useful when coupled with a small source and a system that requires a lot of light without concern for particularly good imaging. We adopted a rather counter-intuitive approach for solving the light collection issue. Because we knew the detector would need to have a very small element size (< 1 mm diameter) for nanosecond time resolution with an MCT detector, we used high F/# lenses, which collect less light but nearly all the light collected can be refocused onto a small detector. The difference between low F/# lenses and high F/# lenses is their quality of collimation, in other words the percentage of marginal rays rays collected near the edge of an optic change depending on the F/# due to spherical and chromatic aberrations. High F/# optics gather less light, but the light

88 73 gathered is largely in paraxial rays (collected near the center of the lens), and is usually well collimated and therefore able to be refocused. Low F/# optics have high collection efficiency, but a large percentage of the light is combined in marginal rays, and is therefore not very collimated. The design of our probe light is discussed Section Detector Elements Unlike visible and NIR detectors, mid-ir detectors with nanosecond time resolution are quite rare. The only mid-ir detectors that can achieve nanosecond time resolve are MCT elements; however, they must be quite small < 1 mm diameter. Most MCTs are microsecond detectors featuring large >5 mm diameter elements. As a comparison, 5 mm Si detectors have ~10 ns rise times, therefore, ns-vis-ta systems do not suffer from needing tiny elements to achieve nanosecond time resolution. Our instrument features two detectors (Section 4.2.3). It has a small (200 μm diameter, 3.5 ns rise time) element MCT for performing kinetic measurements and a large element (1.2 mm diameter, 23 ns rise time) element MCT with a monochromator for collecting mid-ir spectra. The larger diameter element of the 16 MHz detector is better matched to the limited spot size that can be achieved when focusing long-wavelength incoherent mid-ir radiation permitting the 16 MHz detector to be used with a monochromator. 4.2 Previous State-Of-The-Art Dispersive Systems The work of Hamaguchi et al. in mid-ir nanosecond time-resolved spectroscopy throughout the 1990s represented the previous state-of-the-art in the field. 1-3 Their approach

89 74 involved the use of a MoSi 2 Globar for more optical power in the 5-10 μm spectral range, and featured a nanosecond MCT detector with 50 ns time-resolution. An overview of their instrument is given in Figure 4-1. Their detection system consisted of a 7 MHz MCT detector, two preamplifiers and boxcar averaging. The signal from the MCT was amplified (80 db gain, 20 MHz) then amplified again (55 db gain, 100 MHz) prior to being integrated by a boxcar averager. The boxcar averager was set with a gate width (~ns) and then stepped along the transient waveform. Because of this detector scheme, the signal was an integrated time value, furthermore, in order to construct the x-axis (time), the boxcar would need to step move the gate until the entire waveform was captured. This technique is very slow, and would not be practical for OPV materials where waveforms are often nano-millisecond in time. In theory, it is possible to change the gate width once the fast transient features have disappeared, but that may remove transient information from the wavelength at later times. In terms of sensitivity, the instrument could resolve changes in absorption, however the length of the acquisition period (hours) for even short timescale (nano-mircoseconds) would not make the technique practical for OPVs Step-Scan FTIR In step-scan FTIR, the moving mirror of the interferometer does not move in a continuous fashion. Instead, it is stepped to a fixed position and held there until data collection is complete, and then stepped to another position. Each mirror position corresponds to a frequency or range of frequencies depending on resolution. The time resolution is determined by the detector and the time base of the digitizer (usually ns). The advantage of steady-state FTIR over steady-state dispersive IR is that the entire spectrum is collected very quickly (~1 second), which is termed multiplex or Fellgett s advantage. 4 However, that advantage is gone in

90 75 step-scan FTIR, since the mirror must be stopped at different positions (frequencies). Furthermore, the throughput (Jacquinot) advantage is also not present, because although more radiation is able to illuminate the sample (no slits), not all of that light can be focused onto a nanosecond MCT detector because of its small element size. It is important to note that FTIR detectors are usually microsecond detectors (large element size), since they are designed for mostly steady-state measurements. The addition of a nanosecond MCT significantly decreases the throughput advantage. In terms of OPV measurements, the S/N and time required to collect spectra makes it very unlikely that step-scan FTIR could be useful for OPV materials. For example, Figure 4-2 shows the IR dye Re[(CO) 3 bpycl] spectrum at 30 ns, and it required 100 minutes to collect. 5 This example is a highly idealistic case a very strong CO vibration with a pathlength of 250 μm. Typical OPV materials are <100 nm and will not have three CO stretches to measure. 4.3 Instrument Design We focused on the dispersive technique of mid-ir time-resolved spectroscopy because if the S/N per scan can be improved, then a boxcar averager would not be needed, and the time required to collect a spectrum could be significantly reduced. Utilizing new advances in digitizer technology and recognizing the noise-limiting source to our S/N (laser Q-switch) allowed us to improve S/N by over an order of magnitude and remove the boxcar averager. The instrument is described in detail in the following Sections. The instrument scheme is depicted in Figure 4-3 and is referred to below.

91 Optics The water cooled housing for the IR element contains two uncoated one inch diameter plano-convex CaF 2 lenses (100 mm focal length), which serve as the condenser and focusing lenses respectively. Typically, condenser lenses are low F/# optics which enables more light to be collected, however it s important to consider that the performance bottleneck in our instrument is small element size of the nanosecond detector. Using low F/# optics will allow more light to be collected from the source, but that light cannot be collimated or focused as well, therefore using higher F/# optics is advantageous because it enables nearly all the light collected to be focused on the detector. The collection and focusing lenses after the sample are also uncoated plano-convex CaF 2 lenses (100 mm focal length), which were chosen to match the F/# of the monochromator (Spectral Products, DK240). The final two lenses are an uncoated plano-convex CaF 2 (100 mm focal length) and a uncoated biconvex CaF 2 (25.4 mm focal length). All mirrors are protected gold. Longpass filters are used after the monochromator to block higher orders of diffraction from the grating. These filters are housed in a homebuilt motorized filter wheel, which the software program automatically controls based on the scan parameters. Bandpass filters are used before the 100 MHz detector to select regions of interest in the mid-ir for kinetic analysis Detectors Our instrument features two detectors depending on the needs of the experiment. The 100 MHz (Kolmar Technologies) detector features a 3.5 ns rise time and is used primarily for measuring fast bimolecular charge recombination in photovoltaic films. Such studies primarily

92 77 are concerned with kinetics and not spectral features, therefore, wavelength resolution is not important and we are justified in using bandpass filters to select wavelength regions of interest. If spectral features are important, the 16 MHz (Kolmar Technologies) detector (23 ns rise time) is used in conjunction with the monochromator. The small element of the 100 MHz detector (200 μm) prevents it from being used after the monochromator unless the sample has large differential absorption, which is usually not the case for OPVs. Sending light to either detector is done via a kinematic cube mirror (Figure 4-3) Data Collection Hardware Data collection is achieved using a PC oscilloscope (PicoScope, 5443A) with three analogue inputs similar to the data acquisition used in Chapter 3. 6 The Picoscope is triggered by a reference photodiode that is illuminated by the 532 nm pump beam, while two channels (detector and chopper signals) are collected simultaneously for each trigger event. A MATLAB graphical user interface (GUI) was constructed to interface with all aspects of the instrument. The Picoscope has the ability to perform rapid trigger acquisition (1 us trigger rearm time) allowing every shot to be collected. This hardware scheme is identical to that employed with the nanosecond visible system (Chapter 3). In our experiments we specify the number of shots per scan and the total number of scans. For example, 100 laser shots per scan, total of 20 scans means the Matlab GUI creates and segments the Picoscope buffer memory for 100 lasers and repeats the function call to the Picoscope 20 times. The total number of shots the user can collect depends on the sampling rate; however a practical limit is set by the repetition rate of the pump laser. The Picoscope also features a new innovation in analogue to digital (ADC) conversion called flexible resolution. Unlike the traditional ADC architecture, which interleaves multiple 8- bit ADCs to achieve higher resolution, the Picoscope allows multiple high-resolution ADCs to be applied to the input channels in different time-interleaved and parallel combinations to boost either the sampling rate or the resolution. 7 In our case, we exploit the ADC parallel sampling configuration, in which multiples ADCs are sampled in phase and their output combined to

93 78 effectively change the resolution for each channel. This allows the user to select the resolution 8-15 bit (with two channels enabled) at the cost of lower sampling rates. For the 16 MHz detector, using 15-bit resolution, the Picoscope has a sampling rate of 125 MS/s, well above the Nyquist condition. If using the 100 MHz detector, a resolution of 12- bit cannot be exceed due to the sampling rate. The ability for the user to select the resolution given the bandwidth of the detector and size of the signal allows for significant time savings Data Collection Software We use MATLAB s instrument control toolbox to interface with the monochromator, Picoscope, and motorized filter wheel. The Picoscope collects two channels in the buffer memory (detector signal and chopper signal) for the specified number of shots. The ability to hold large buffer sizes of 512 million samples (time points) per waveform allows for nanosecond to second timescales depending on the desired time resolution. For most of our experiments, the digitizer is set for 12-bit digitization, which provides four nanoseconds per point time resolution. If better time resolution is needed, the user can switch to 8-bit digitization for 2 ns/time point. The chopper signal enables the program to sort the on/off shots to calculate ΔT. The scheme for how the instrument collects data can be seen in Figure 4-4. Most TA instruments average all the on shots together and off shots together followed by subtraction of the averaged on and off shots to calculate ΔT. Indeed, this is the only way to do so unless collecting subsequent triggers. We subtract each on shot with its corresponding off shot, then take the average of those and subtract to calculate ΔT. This may seem trivial, however, subsequently subtracting the on s and off s in time greatly reduces the RF noise which plagues may nanosecond literature kinetics. After calculating ΔT for one wavelength, the program switches the Picoscope to DC coupling and measures the DC offset at that same wavelength. The resultant ΔT/T is plotted

94 79 along with the steady-state spectrum. The program repeats the process until the wavelength range is done and the requested number of scans is completed. The program selects the appropriate longpass filters to use depending on the scan parameters. 4.4 Instrument Performance Sample Preparation We used the prototypical organic photovoltaic system: regioregular poly(3- hexylthiophene) (P3HT) blended with phenyl-c61-butyric acid methyl ester (PCBM) spin coated onto a calcium fluoride substrate (80 nm film thickness). The film was annealed at 100 C for ~10 minutes prior to measurements. The CN-MEH-PPV film was made by dissolving CN-MEH-PPB (20 mg) with PCBM (20 mg) in 1mL of chlorobenzene and spin-coated at1000 rpm onto a CaF 2 substrate Spectral and Temporal Range The spectral range of the instrument is technically 500 cm -1 (20 μm) to cm -1 (1 μm) based on the spectral output of the IR source. However, the practical limits are set by the lens materials, grating efficiency, detector sensitivity and the strength of the optical transition being probed. In its current configuration, the spectral limit is about cm -1, set by the CaF 2 lenses and the grating efficiency. This range can be changed with different gratings or by changing the lens material to ZnSe, which is transmissive below 1000 cm -1. To illustrate the instrument s spectral range, we examined the polaron absorption in the P3HT:PCBM sample described above. The spectra were collected using the 16 MHz detector and

95 monochromator (Figure 4-7a). It should be noted that we used a low (~10 µj/cm 2 ) energy density, which is essential when studying OPV materials due to their ability to undergo nonlinear processes (e.g. exciton-exciton annihilation). Previously, only time integrated PIA spectra using a 11, 12 lock-in amplifier were measured for such films. Although time-resolved nanosecond spectra have been reported in the near-ir, 11 the advantage of examining charge carrier kinetics in the mid-ir is that the spectra do not display as many overlapping features compared to transitions in the near-ir range. For example, near-ir spectra of P3HT:PCBM contain localized P3HT polarons, delocalized P3HT polarons, P3HT singlets and triplets along with PCBM anion absorption. 13 Using the mid-ir allows for the simplification of transient spectra since the mid-ir region consists mostly of polarons. Therefore, the mid-ir range can significantly improve the clarity of spectral assignments in studies of optoelectronic materials. Figure 4-5b shows the time-dependence of the mid-ir polaron absorptions measured in the P3HT:PCBM sample using the 100 MHz MCT detector. The kinetics exhibit a clear power law dependence that is expected for polarons undergoing bimolecular charge recombination. 14 The five decades of time allows for accurate modeling of long lived processes such as chargetrapping and polaron recombination (Section 4.5.2) Signal Detection Limit We measured the instrument s signal of detection limit (SDL) with the same P3HT:PCBM film as above. The film was pumped at various energy densities below 3 µj/cm 2 (Figure 4-6), where the signal amplitude is in the linear response region (Figure 4-7). Using Figure 4-7, we calculated the standard deviation of the lowest energy density sample (Figure 4-6,

96 81 blue) to obtain the SDL of the instrument. Knowing the SDL allows experiments to be planned to balance the S/N needed versus sample exposure time. We calculated the SDL using: (1) where s is the standard deviation and y blank is the signal of the blank (Figure 4-6, gray line). 15 The instrument s SDL was calculated to be 10-5 O.D. Unlike ultrafast vibrational studies, for which vibrations often have amplitude changes on the order of 1 mo.d., on the nanosecond timescale most vibrational peaks have already decayed significantly, making their amplitudes <<1 mo.d. This highlights the need to be able to measure μo.d. changes in absorption if we want to be able to measure vibrational modes in OPV materials. 4.5 Examples of Performance in OPV Systems Vibrational Modes in OPV Materials To test the instrument s capability to measure vibrational features, we chose two systems of current interest to researchers. The first is 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPSpentacene) thin films. Recently, TIPS-pentacene is of interest due to its ability to undergo singlet fission. 16 The purpose of this example is to show that vibrational dynamics can be used to probe electronic deactivation processes such as exciton-exciton annihilation since they provide additional in-depth information about these mechanisms. Singlet fission is a type of multiexciton generation mechanism resulting in a population of triplet excitons whose primary deactivation mechanism in the solid state is known to be triplet-triplet annihilation (TTA). Typically, this information is obtained through laser fluence dependence measurements of T-T excited state absorptions. 17 With the mid-ir instrument, we can show direct evidence for triplet-triplet

97 82 annihilation through time-dependent thermochromism of the alkyne stretch mode of TIPSpentacene (Figure 4-8). The sample heats up during early time (few ns) when TTA occurs, giving rise to the blue-shift bleach and the appearance of a positive feature both signatures of thermochromism. Note the bottom of Figure 4-8 contains the steady-state FTIR spectrum for the TIPS-pentacene film for reference. The spectral changes persist until around 200 ns, at which most of the triplet population has decayed. The alkyne features decay slowly on a hundreds of nanoseconds timescale, which is the time scale for the ground state to completely recover. Most importantly, is the ability of the instrument to clearly resolve transient vibrational features as low as ~10 µo.d., which are the smallest vibrational features reported on a nanosecond time scale to our knowledge. Figure 4-9 contains the decay of the triplet state via triple-triple annihilation, where the film was pumped at 642 nm and probed at 525 nm to directly measure the triplet decay. The second vibrational system we used to demonstrate the sensitivity of the instrument was the photovoltaic blend poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-(1-cyanovinylene-1,4- phenylene)] (CN-MEH-PPV):PCBM. The bleach of the carbonyl mode on the electron acceptor PCBM is a convenient vibration probe that has been associated with charge transfer. 18 The thickness of polymer blend film (~ 100 nm) and excitation energy density of the 532 nm pump pulses (~100 µj/cm 2 ) were chosen to be relevant to prior ultrafast vibrational spectroscopy studies of this polymer blend system. 18 When the donor (CN-MEH-PPV) is excited, charge transfer occurs between the donor and acceptor that depopulates (bleaches) the carbonyl mode at 1740 cm -1 (Figure 4-10). Additionally, the C=C stretch is visible around 1690 cm -1. We note that the relatively low energy density of <100 µj/cm 2 ensured that thermal effects would not contribute to the observed vibrational bleaching. This example serves to show that the instrument is capable of resolving ~10 µo.d. vibrations features without averaging portions of the time axis. For example, Tanaka et al. reported nanosecond vibrational spectra by averaging 400 ns portions of their time 19, 20 axis to achieve good S/N.

98 Electronic Mid-IR Polaron Absorption in P3HT:PBCM Films Previously (Section 3.3) we looked at the NIR transient absorption of P3HT:PCBM where we saw localized and delocalized polarons absorbing. Polarons also have mid-ir absorptions, which in some cases are easier to use than NIR due to the elimination of photoluminescence artifacts. We measured the time-resolved mid-ir absorption of P3HT-PCBM and connected it to the NIR spectrum to record an entire spectrum from visible to mid-ir. As previously mentioned, the NIR portion of the spectrum for P3HT:PCBM, and for most organic electronic materials, is congested. Usually the absorptions of singlets, triplets, localized and delocalized polarons and anions are present. 13 Although some species can be determined based on their time dependence, it is not uncommon for several components to be overlapping on the nano-microsecond timescale. Utilizing the mid-ir region can provide similar information with less spectral congestion because it is spectrally removed from fluorescence and phosphorescence. Figure 4-11 shows the time-resolved vis-nir-mid-ir spectrum for P3HT:PCBM. The visible region ( nm) shows the ground state bleaches with the region around 532 nm removed due to overlap with the pump wavelength. The near-ir feature is a combination of overlapping features arising from delocalized P3HT polarons (~700 nm) and localized polarons (~1000 nm). Note that the PCBM anion is generally not present on this time scale for regioregular P3HT, although it is usually present for regiorandom P3HT. 13 In the case of P3HT:PCBM, the broad mid-ir photoinduced absorption (PIA) is associated with a low energy absorption from intrachain P3HT polarons. To verify that the mid- IR features arise due to polarons, we measured the kinetics for the P3HT:PCBM film at several power energies (Figure 4-14). The mid-ir kinetics were fit using a single power law f(t) = A/(1+t/τ) α, where A is amplitude, t is time, τ is lifetime and α is the exponent describing the

99 84 power. 13 The data were fit using a convolution of this model with the instrument response function. At higher energies, exciton-exciton annihilation becomes important and is primarily reasonable for the decreased lifetimes. Nonunity alpha values are typical for electronic materials due to trap-limited bimolecular recombination. However, the alpha values can be drastically influenced by the pump energy. For example, in Figure 4-12, the alpha values approach unity at higher energy densities due to less trap-limited recombination. The density of polarons exceeds the density of traps, thereby making the bimolecular recombination more ideal (i.e. α 1). Such conditions are no longer device relevant and Figure 4-12 highlights the need for low energy densities when studying OPV materials. 4.5 Conclusion We report for the first time a mid-ir dispersive instrument capable of measuring nanosecond time-resolved vibrational spectra on thin (<100 nm) thick electronic materials with low (<10 µj/cm 2 ) energy densities with nanosecond time resolution. We highlighted the spectral range, sensitivity and time resolution using the prototypical organic photovoltaic system P3HT:PCBM showing time scales of nanoseconds to milliseconds and with the ability to resolve sub 100 μo.d. signals. We followed the decay of the transient peak appearing at ~0.5 ev with nanosecond time resolution, assigning it to polaron absorption. The utility of measuring vibrational features on a nanosecond time scale was demonstrated using the singlet fission material TIPS-pentacene. We observed the alkyne bleach evolve as a function of triplet-triplet annihilation. This instrument allows researchers to extend spectroscopic studies of thin film materials to less congested regions of the electromagnetic spectrum and to resolve transient vibrational signals on the order of 10-5 O.D. with nanosecond time resolution.

100 References 1. Yuzawa, T.; Kato, C.; George, M. W.; Hamaguchi, H. Nanosecond Time-Resolved Infrared Spectroscopy with a Dispersive Scanning Spectrometer. Appl. Spec. 1994, 48 (6), Yuzawa, T.; Takahashi, H.; Hamaguchi, H. Submicrosecond time-resolved infrared study of the structure of the photoinduced transient species of salicylideneaniline in acetonitrile. Chem. Phys. Lett. 1993, 202 (3), Iwata, K.; Hamaguchi, H. Construction of a Versatile Microsecond Time-resolved Infrared Spectrometer. Appl. Spec. 1990, 44, Robinson, J. W.; Frame, E. M. S.; Frame, G. M. IR Spectroscopy. In Undergraduate Instrumental Analysis, 6th ed.; Marcel Dekker: New York, 2005; p Palmer, R. A.; Smith, G. D.; Chen, P. Breaking the nanosecond barrier in FTIR timeresolved spectroscopy. Vib. Spec. 1999, 19, Rimshaw, A.; Grieco, C.; Asbury, J. B. Note: Using fast digitizer acquisition and flexible resolution to enhance noisecancellation for high performance nanosecond transient absorbance spectroscopy. Rev. Sci. Instr. 2015, 86, Lecklider, T. Resolving Finer Detail, Evaluation Engineering, Special Report: Oscilloscopes Shuttle, C. G.; O'Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.. D. J.

101 86 R. Phys. Rev. B. 2008, 78, Shoaee, S.; Eng, M. P.; Espildora, J. L.; Delgado, B.; Campo, M.; Vanderzande, D.; Durrant, J. R. Energy Environ. Sci. 2010, 3, Rao, A.; Chow, P. C. Y.; Gelinas, S.; Schlenker, C. W.; Li, C. Z.; Yip, H. L.; Ginger, D. S.; Durrant, J. R. Nat. Lett. 2013, 500, Drori, T.; Holt, J.; Vardeny, Z. V. Optical studies of the charge transfer complex in polythiophene/fullerene blends for organic photovoltaic applications. Phys. Rev. B. 2010, 82, Jiang, X.; Osterbacka, R.; Korovyanko, O.; An, C. P.; Horovitz, B.; Janssen, R. A. J.; Vardeny, Z. V. Spectroscopic Studies of Photoexcitations in Regioregular and Regiorandom Polythiophene Films. Adv. Func. Mat. 2002, 12 (9), Ohkita, H.; Shinzaburo, I. Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy. In Organic Solar Cells; Choy, W. C. H., Ed.; Springer: London, 2013; p Clarke, T. M.; Jamieson, F. C.; Durrant, J. R. Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/Fullerene Blend Films. J. Phys. Chem. C 2009, 113, Harris, D. Quality Assurance and Calibration Methods. In Quantitative Chemical Analysis, 8th ed.; W.H. Freeman and Company: New York, 2010; pp Ramanan, C.; Smeigh, A. L.; Anthony, J. E.; Marks, T. J.; Wasielewski, M. R. Competition Between Singlet Fission and Charge Separation in Solution-Processed Blend Films of 6,13-Bis(triisopropylsilylethynyl)-pentacene with Sterically- Encumbered Perylene-3,4:9,10-bis(dicarboximide)s. J. Am. Chem. 2011, 134, 386-

102 Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, Pensack, R. D.; Asbury, J. B. Beyond the Adiabatic Limit: Charge Photogeneration in Organic Photovoltaic Materials. J. Phys. Chem. Lett. 2010, 1, Yabumoto, S.; Sato, S.; Hamaguchi, H. Vibrational and electronic infrared absorption spectra of benzophenone in the lowest excited triplet state. Chem. Phys. Lett. 2005, 416, Tanaka, S.; Kato, C.; Horie, K.; Hamaguchi, H. Time-resolved infrared spectra and structures of the excited singlet and triplet states of fluorenone. Chem. Phys. Lett. 2003, 381,

103 Figure 4-1. Instrument diagram for a dispersive mid-ir nanosecond transient absorption spectrometer. M = mirror, SH = shutter, EM = ellipsoidal mirror, FC = flow cell, CH = chopper, S = source, and PA = preamplifier. 1 88

104 Figure 4-2. Step scan FTIR spectrum of Re[(CO) 3 bpycl] collected at 30 ns with a collection time of 100 min. 5 89

105 Figure 4-3. Optical layout for the nanosecond mid-ir transient absorption spectrometer instrument. The cw probe source is housed in a custom built water cooled aluminum block in ensure temperature stability. The removable mirror allows for either detector to be used. For kinetics, the 100 MHz MCT is used, whereas for spectral data, the 16 MHz detector is used. All lenses are CaF 2. 90

106 Figure 4-4. Data collection scheme for the mid-ir instrument. The Picoscope is triggered by the reference photodiode, followed by the sorting of the waveforms into ON (pump laser unblocked) and OFF (pump laser blocked) shots based on the chopper signal. The program sequentially subtracts the ON and OFF shots and then divides by the DC signal. 91

107 Figure 4-5. (a) Spectrum of P3HT:PCBM film pumped at 532 nm (< 50 μj/cm 2 ) showing the broad mid-ir polaron absorption. (b) Power law decay of the mid-ir polaron absorption from (a). The kinetics were obtained using bandpass filters. The inset shows the kinetic trace on a linear scale to better illustrate the instrument response. 92

108 Figure 4-6. Polaron kinetics for P3HT:PCBM at various energy densities. The instrument response is shown in black. The inset shows the average amplitude for each energy density calculated from the first 30 ns of each trace. The red line shows the fit to the amplitudes to illustrate the linearity of the energy density. 93

109 Figure 4-7. Normalized transient absorption kinetics for P3HT:PCBM illustrating the effect of energy density on the polaron decays. The energy densities were 8.5 (blue), 3.4 (teal), 2.1 (green), 0.9 (pink), and 0.4 μj/cm 2 (red). Inset shows the non-normalized data. 94

110 Figure 4-8. Nanosecond vibrational spectra for the alkyne stretch of a TIPS-pentacene film. The film was pumped at 642 nm using a home-built dye laser at an energy density of 100 uj/cm 2. The positive and negative features arise due to thermochromism of the alkyne peak. The bottom spectrum is the steady-state FTIR spectrum for reference. 95

111 Figure 4-9. Transient absorption of TIPS-pentacene triple at 525 nm after the sample was excited at 642 nm. The decay shows the triplet-triplet annihilation, which is the source of the thermochromism mentioned in Figure

112 Figure Nanosecond spectra for the OPV film CN-MEH-PPV:PCBM pumped at 532 nm (<100 μj/cm 2 ). The vibrational bleach at ~1740 cm -1 corresponds to the C=O stretch of PCBM, while the bleach at 1690 cm -1 corresponds to the C=C stretching of CN-MEH-PPV. 97

113 Figure Spectra for P3HT:PCBM film for the visible, NIR, and mid-ir regions. The visible and NIR regions were collected using the instrument in Chapter 3. The visible region shows the electronic ground state bleaches while the NIR region is a convolution of localized and delocalized polarons. The mid-ir region is primarily comprised of localized polarons and the gray area depicts the integrated region that was used for the kinetics in Figure

114 Figure Power law fits for the kinetics that were obtained via integration of the wavelength range specified in Figure The alpha values of the fits are shown to illustrate the influence energy density has on the shape of bimolecular decays. The increasing alpha values are indicative of decreasing trap-limited bimolecular recombination. 99

115 100 Chapter 5 Development of Ultrafast Visible and Mid-Infrared Transient Absorption System The Asbury lab s ultrafast system was originally constructed in 2005 to perform 2D IR spectroscopy and visible pump mid-ir probe spectroscopy. The success of the instrument led to a number of publications dealing with mid-ir ultrafast spectroscopy in OPV materials. 1-5 The instrument s capabilities needed to be expanded to perform visible pump visible probe spectroscopy, which would enable more insight into the electronic processes occurring in OPV materials. The redesign also focused on improving the visible pump mid-ir probe in terms of easier usability, faster data acquisition, and more sophisticated statistical filtering of data. All of this was done with the end goal of creating a more modular system allowing for easy expandability of techniques, such as the lab s current efforts in electrooptical ultrafast spectroscopy (Chapter 6). 5.1 Designing the Visible Pump Mid-IR Probe Ultrafast Transient Absorption Instrument Optical Layout Pump Optical Layout The pump path is depicted in Figure 5-1 (green). The output of the ultrafast amplifier pumps an OPA where tunable wavelengths can be generated using sum frequency generation or second harmonic generation (Section 2.3.3). For most OPV materials, the output is nm,

116 101 since that is usually where these materials absorb the strongest. The pump beam is delayed relative to the probe path using a computer-controlled mechanical delay stage. The pump beam uses a 150 mm glass lens (L3) on a movable stage to control to spot size at the sample. A spot size of 350 μm is typical for pump probe experiments. The lens position was chosen to minimize the angle between the pump and probe beams (the probe is normal to the sample surface). In ultrafast spectroscopy, the overlap angle between two beams influences the time resolution of the experiment. For example, in Figure 5-2 the pump and probe beams overlap with an angle α. The pulse duration τ determines the spatial width of the pulse in detection of its propagation d = cτ. At some point in time the pulse will be overlapped (Figure 5-2, green area) where the pump and probe have the same timing. For part of the sample of thickness D, the probe will reach the sample after the pump and for the other part it will reach the sample after the pump. So, the front of the probe must be limited to keep its delay relative to the pump pulse with accuracy equal to the pulse width: The beams should be kept as collinear as possible because bigger spot sizes or angles will (1) decrease the time resolution. The pump beam is modulated at 500 Hz using a mechanical chopper (Newport, model 3502) that enables measurement of blocked and unblocked pump shots for calculating ΔT (Section 2.3.1). A portion of the pump beam is constantly monitored using a photodiode for analyzing the pump beam stability for statistics filtering (Section 5.1.4) Probe Optical Layout The optical layout of the system was designed to allow for visible pump probe and electrooptical techniques to coexist with the previous mid-ir probe techniques. The mid-ir probe

117 102 beam path is shown in Figure 5-1 (red), where the mid-ir pulses are generated via different frequency generation in an OPA (Section 2.3.3) using a AgGaS crystal. The mid-ir beam path length is very long, allowing for a time delay of 8 ns between pump and probe via the delay stage. The duration of the mid-ir pulse is measured using an homebuilt mid-ir intensity autocorrelator. The autocorrelator works by splitting the mid-ir beam into two paths and mechanically delay one path relative to the other. The two beams are then recombined within a nonlinear crystal for second harmonic generation (SHG). The efficiency of the second harmonic generation resulting from the interaction of the two beams is proportional to the degree of pulse overlap within the crystal. Monitoring the intensity of this second harmonic generation as a function of relative delay between the recombining (overlapping) pulses produces a correlation function directly related to pulse width. Usually, the beam is positively chirped and germanium disks are inserted into the IR beam path to compensate for the positive chirp. Germanium has a negative GVD for mid-ir light. The mid-ir pulses are routinely measured and can be kept at ~100 fs. Because the mid-ir beam path is very long (>30 feet), collimation becomes an issue, which is why there are lenses (L15 and L16, Figure 5-1) after the OPA to compensate for imperfections in the beam shape. The beam collimation is determined with the irises R1 and R2, which are in front of pyroelectric detectors. These detectors measure the IR signal as a function of iris size. If the beam is well collimated, then the signal intensity should drop by the same relative amount on each detector when R1 and R2 are the same size. The lenses after the OPA are on mechanical stages for fine adjustment. Once the beam is collimated (~7 mm diameter), it travels to the sample only interacting with gold mirrors in ensure minimal chirping. The IR beam is split using a ZnSe beamsplitter into a signal and reference path (Figure 5-1). After the beams are split, both beams interact with the sample, however, only the signal beam is overlapped with the pump beam. The signal beam is focused to a ~200 μm spot size with a 150 mm CaF 2 lens. The reference beam is also focused, but its size is not measured since it does not interact with the pump beam.

118 103 The reference beam transmits through the sample about 15 mm away from the IR signal beam. Both signal and reference beams are collected using lenses after the sample and then focused into a monochromator (Horiba, Triax 320) and detected using a MCT array. Each beam is detected on the MCT array detector separately (two rows of elements on the detector). This process significantly improves S/N because it allows for correction factors to be generated Hardware IR Digitizer The IR digitizer was designed by Infrared Development Systems. It is an 80 channel analogue to digital converter specially designed to work with MCT array detectors. The first 64 channels are reserved for digitizing the elements in the array detector. The array detector consists of two rows of 32 elements each individually read out by the IR Each element is digitized (16-bit) and stored in First In First Out (FIFO) memory. The stored value for each element is an integrated value because the IR is also a gated integrator allowing for the user to pick a gate width for signal integration of each element. The additional 16 channels (65-80), which are BNC inputs, are likewise digitized at 16-bits. BNC inputs 1-8 (channels 65-72) are also integrated signals like the MCT elements (channels 1-64). BNC inputs 9-18 (channels 73-80) are not time integrated, rather the signal for each of these inputs is sampled once 40 μs after the IR is triggered. The additional BNC channels can be used with other photodetectors that an experiment may require. All of the digitized values are held in the FIFO memory for a user defined number of samples (trigger events). For example, in our pump probe measurements we usually collect 1,000 laser shots (trigger events) at each time point (delay stage position). The

119 104 values from the FIFO memory are transferred to a PCI card (National Instruments PCI-6533) located on a computer. The data transfer is a rather complicated process. A scheme of the process is depicted in Figure 5-3. The PCI-6533 card has 4 ports, each is an 8-bit wide bus (labeled lines 0-7). Each port has a different function that MATLAB interacts with via a driver. MATLAB utilizes a National Instruments driver to make calls to the PCI card, which controls the data flow from the IR Although MATLAB supports National Instrument Products via the Data Acquisition Toolbox, the toolbox does not allow setting clock polarity levels, which are crucial to interacting with the IR Therefore, the National Instruments C++ driver needs to be loaded into MATLAB and code must be written to execute various features rather than using a MATLAB toolbox. MATLAB loads the National Instruments library for the PCI-6533 and then creates digital channels corresponding to the 4 ports on the card: Port 0: Timing port Port 1: Used for setting timing data Port 2: Detector data Port 3: Detector data Each port has 8 lines (bits), Port 0 contains the data for time delay and gate width, while Port 1 tells the IR whether to save Port 0 to its volatile memory or not. For example, if the user wants a 200 ns delay, MATLAB writes (200 in binary) to Port 0, and 1 to Port 1/Line1 which tells the IR to save Port 0 as the delay time between data collection and the trigger pulse. Port 1/Line 0 is used in a similar fashion for setting the integration time and Port 1/Line 3 can be toggle low to high (0-1) to reset the values of delay and integration in the IR memory. Port 0 and 1 are output ports (send data to IR 64-16). Data is read from the IR s FIFO memory through Ports 2:3. Each Port is only 8-bit, however the data from the IR is 16-bit. In order to reconstruct the data value, Port 2 and 3

120 105 must be concatenated to form the unsigned 16-bit integer corresponding to the intensity value of an element. Note that this process needs to be done for each value in memory, in other words, the process loops 64 times to extract the data from each element for the array detector. That would be for one laser shot (trigger event). The number of execution times must be multiplied by the number of trigger events to calculate the total number of calls to FIFO memory. This execution is done on a low level in the C++ driver. The concatenated value is then saved by MATLAB in an array. The same process is done for each of the BNC input channels (65-80), in the end an array of 80xN is obtained, where N is the number of laser shots collected. The FIFO memory does not save data in a sequential fashion, in other words, the second channel read out does not correspond to the second detector element of the array. The readout sequential is 1, 17, 33, 49, 65, 2, 18 therefore, MATLAB needs to decimate and rearrange the array so the 80xN array reads in a logical 1-80 order in the row dimension. After these steps are performed, the code then applies statistics to the array to filter out noisy laser shots (Section 5.1.4) Array Detector and Monochromator The liquid nitrogen cooled 64 element MCT array detector has two rows of 32 elements each with a spectral range of 4-16 μm. Each element has a microsecond time response, however because the time resolution of ultrafast spectroscopy is independent of the detector (Section 2.3), the time response only needs to be fast enough to ensure each laser shot can be captured (1 khz). In our experiments, the top array of 32 elements detects the reference beam, while the bottom row of elements detects the signal beam. The monochromator is a Horbia Triax 320 with two gratings to span the entire mid-ir range. The 150 grooves/mm grating is used for 3-9 μm range and has an effective bandwidth of 2.8 nm/pixel. The 75 grooves/mm grating is used for the 4-10 μm range and has an effective

121 106 bandwidth of 7.8 nm/pixel. The 75 grooves/mm grating is rarely used due to the poorer resolution. In the experiments discussed in Section 5.3, the 150 grooves/mm grating was used. The array detector and monochromator provide about ~ 70 cm -1 to be captured without moving the grating. Capturing more than ~70 cm -1 spectral range requires the grating be moved. Extreme care must be taken when collecting data require multiple spectral regions because artifacts can easily arise due to differences in the normalization of the pixels to light. We normalized the array by scanning the monochromator in steps of 1 cm -1 across the entire OPA mid-ir output (~200 cm -1 ) and generate correction factors for each pixel that is then used to normalize the response of the arrays Pyroelectric and Photodiode Detectors Several detectors are used throughout the instrument to monitor different beams for stability. The pyroelectric detectors are used for alignment and measurement of the collimation of the mid-ir light (Figure 5-1). Pyroelectric detectors have poor sensitivity, but a flat spectral response across a very wide wavelength range (nm-μm) making them ideal for IR applications. Each pyroelectric detector is connected to a channel of the IR-6416 that integrates and digitizes the signal. Users see the signal in real-time allowing for the adjustment of the IR lenses until the beam has been collimated from the OPA. The pyroelectric detectors are not used in the actual pump-probe measurement. The Si-PD reference detector (Figure 5-1) is used to detect the pump beam and its signal is integrated by the IR and captured along with the MCT array signal. The software uses the pump reference signal to record the stability of the pump beam and reject data that was collected if the pump beam intensity was outside of a user defined deviation (Section 5.1.4). This ensures that the power density of the experiment does not vary, nor that bad laser shots are

122 107 averaged with good laser shots. The pump intensity should not deviate by more than 5% if properly optimized Delay Stage The mechanical delay stage (Newport, IMS600CCHA) is a 600 mm long stage with a linear encoder. The pump beam makes four passes on it, for a total beam travel length of eight 2400 mm. The stage s position is controlled by a motion controller (Newport, XPS) that is capable of controlling up to 8 different stages. The stage features backlash compensation allows the stage to make 0.5 μm steps (1.6 fs) reproducibly. The XPS controller is connected via TCP/IP with the computer, where MATLAB uses the XPS driver to get and set functions on the XPS to move the stage to user define time points (motor positions) Software Originally, the ultrafast instrument was set up using Labview code, however, in 2012 the process of converting all that code to MATLAB started. The process was motivated by recent advancements made by Mathworks to improve MATLAB s hardware communication abilities. Since we use MATLAB for nearly all of our data analysis, it made sense to try and streamline the data collection/analysis processes by incorporating MATLAB directly into our instruments. The ultrafast instrument uses MATLAB along with the Instrument Control and Statistics Toolboxes to create a user friendly experience. The graphic user interface (GUI) was made using MATLAB s GUI tools, which are essentially Java Swing embedded into MATLAB. Nearly every aspect of the instrument is controlled through MATLAB, and the following Sections will discuss the most important of those interactions.

123 Signal Measurement and Statistics Calculation of the transient absorption signal (ΔT/T) was given in Section 2.3.1, equation 7. Although correct, obtaining those values is not as straight forward as taking the raw signal from the detector and doing the arithmetic. In pump probe spectroscopy, the noise floor is set by the shot-to-shot noise from the lasers. Each pulse (shot) of the laser should be the same energy, however, that is never the case for a real system because there are a number of factors (i.e. room temperature, humidity, thermal expansion of optical elements) that will contribute to the shot-toshot fluctuations of a laser. Noisy laser shots will significantly decrease the sensitivity of the experiment and it will also require more averaging to achieve a desired S/N. The signal collection works by taking the trigger is from the laser amplifier (1 khz, 5V TTL) which starts the data collection on the IR The signals digitized by the IR are the probe signal (SIG), probe reference (REF), pump PD (PUMP), and CHOPPER. Each signal has its own 32xN array, where N is the number of laser shots that were collected. The software calculates the mean of each signal and filters the array based on the user defined relative standard deviation (1-10% usually). The process occurs in a serial fashion because if a noisy shot is removed in one signal, it must be correspondingly removed in all the other signals. Furthermore, because of the modulation of the pump (on/off), if an on/off shot is removed, its corresponding on/off must be removed to ensure an even number of array elements. This process is termed pair-wise rejection. The complete process is: 1) Filter noisy PUMP laser shots from all signals and perform pair-wise rejection. 2) Filter noisy SIG laser shots from all signals and perform pair-wise rejection 3) Filter noisy REF laser shots from all signals and perform pair-wise rejection The process leaves the user with four arrays (the three listed above plus the chopper signal) that have been filtered based on the relative standard deviation criteria given by the user. The filtering

124 109 process is done using logical indexing masks, which are constructed based on the standard deviation criteria. Once filtered, the SIG and REF values are normalized based on the responsively of each detector element. MCT array elements do not have nearly identical responsivities like Si CCDs elements, and differences in the response of each element must be taken into account or spectral features will exhibit oddities small spikes, weird dips, etc. The final step is to apply the chopper array (32xN logical) to the other signals to sort the on and off shots. Once sorted, a correction factor (CF) is generated based on the SIG OFF and REF OFF values: (2) The CF takes into account non-ideal features in the SIG and REF beams and it is not a single value, but an array of size 32xN (CF for each element of each laser shot). The process is repeated for the SIG ON and REF ON arrays: (3) Then ΔA is calculated by ( ( )) (4) This value is what appears on the y-axis of the ultrafast plots in this dissertation. The above process occurs for each time point (delay stage position) that the user selects in the MATLAB GUI. The user can create a spreadsheet with the desired time points and number of laser shots to collect at each time point that is then loaded into MATLAB. 5.2 Visible Pump Visible/NIR Probe Ultrafast Transient Absorption The ultrafast system was expanded to allow for visible pump (via an OPA) and visible probe transient absorption measurements. Being able to probe OPV materials in the visible and

125 110 NIR regions can provide evidence for spectral assignments in the mid-ir region. It also greatly expands the number of systems we can study to include materials that do not exhibit strong vibrational modes or species that do not exhibit mid-ir electronic transients (e.g. triplets) Optical Layout Pump Optical Layout The pump beam takes the same path as the pump of the mid-ir experiments until after the chopper (Figure 5-1). A kinematic mirror is placed after the chopper that intercepts the beam beam and sends it to the visible setup. The pump beam is focus using a 200 mm glass lens (L10) through the hole of the 90 hole off-axis parabolic mirror. This geometry ensures maximum overlap between the pump and probe beams, which allows for large beam spots while still maintaining temporal overlap between the beams (Figure 5-2). The pump beam is focused to a spot size of ~400 μm. The pump beam is blocked after the sample and monitored for stability using the same photodiode as the mid-ir step up labeled in Figure Probe Optical Layout A small amount of the laser amplifier output (μj/pulse) is sent to a sapphire c-axis cut plate for white light generation with spectral range of ~ nm with energies of nj/nm (Section 2.3.3). Prior to being focused into the crystal for white light generation, the probe beam is attenuated using a polarizer (Figure 5-1, P) so the energy can be carefully adjusted to avoid multi-filamentation of the beam. The lens (L8) used for focusing is in the sapphire crystal is a 150 mm BK7 glass lens that is attached to a moveable stage. The precise focusing of the beam is

126 111 achieved using the stage s micrometer. The diverging white light is collected using a 50 mm lens (L9) with a filter attached. The glass filter is used to block 800 nm light from the beam since the white light generation is only ~30% efficient. The probe light is split via a 50:50 ultrafast beam splitter into a signal path and reference path analogous to the mid-ir signal and reference paths. A 90 off-axis parabolic silver mirror with a center hole drilled out (Thorlabs, MPD269H-P01) is used to focus the probe signal beam onto the sample. The probe reference beam is focused using a 225 mm glass lens (L11). Each beam is collected after the sample using lens and sent to a transimpedance amplified photodiode (Thorlabs, PDB210A). Dielectric filters are used after the sample to select a narrow band of light for detection Hardware The system uses three detectors to monitor the light level of the signal, reference, and pump (Figure 5-1). The pump laser is detected using the same photodiode as the mid-ir probe setup, the probe signal and reference beams are detected using a transimpedance amplified balanced Si photodiode (Thorlabs, PDB210A) with a spectral range of nm. The detector was modified to be AC coupled, so stray light from the room would not produce a DC offset for the measurement. The signal and reference signal are BNC outputs from the PD that are digitized by a 4-channel oscilloscope (Picoscope, 3404A 70MHz). The pump signal and chopper signal is also digitized by the Picoscope. The Picoscope is trigger off the Pockels Cell of the laser amplifier (1 khz).

127 Software The software is very similar to the MATLAB code designed for the nanosecond instrument that also uses Picoscopes (Chapters 3-4). A MATLAB GUI controls the communication to the Picoscope using a series of functions compatible with the Picoscope s driver. The MATLAB GUI also controls the delay stage via the XPS controller. The data collection scheme is outlined in Figure 5-4. MATLAB sets the Picoscope to collect m number of laser shots using its segmented memory mode (saves waveforms to Picoscope s internal buffer) before transferring to the PC. That enables the Picoscope to capture waveforms on every laser shot (1 khz). The Picoscope is triggered off the TTL pulse from the laser amplifier to begin data collection. The chopper TTL pulse is also captured that tells MATLAB when the pump beam was blocked of unblocked. There are two probe signals labeled signal and reference, the signal beam is overlapped with the pump beam at the sample and contains the transient absorption signal, the reference beam does not overlap with the pump (Figure 5-1). The probe signal will always give a transient response on the detector because it is pulsed, however the amplitude of the response will change depending on whether the pump is block/unblocked (Figure 5-4, note the amplitude difference in probe signal). The probe reference amplitude does not change since it does not interact with the pump. These two channels are sorted into ONs/OFFs pulses (meaning pump unblocked/blocked) and then the waveforms are integrated. Remember that the time resolution in an ultrafast experiment is not determined by the detector. Once integrated, the signal ON and reference ON are divided to provide ΔT and then averaged. The correction factor (Figure 5-4, CF) is obtained by dividing the integrated signal OFF and reference OFF signals and then averaged. Finally, the change in absorption is calculated as: (5)

128 113 The above is for one delay stage position (time point). The entire process needs to be repeated for the number of time positions the users wants (e.g fs to 5000 fs in steps of 50 fs). The end result is a transient absorption plot as shown in Figure Perylene Diimides Previous work Perylene Diimides (PDIs) have been around for nearly a hundred years, and have been used extensively as pigments for automobile paints due to their robustness, brightness, and low cost. 6-8 They are particularly interesting as electron acceptors due to their high absorption coefficients for visible light (~ ), and high electron mobility (10-2 cm 2 /Vs) Their use in solar cells as acceptor molecules has been rather limited, where they typically perform much worse than the acceptor PCBM. Recent visible pump visible probe studies by Friend et al. suggests two factors contributing to poor device performance: (1) very fast bimolecular recombination when a PDI molecular is finely dispersed in the donor material, and (2) PDI excitons relaxing into intermolecular states (excimers) between PDI molecules rather than undergoing charge transfer. 6, 11 Therefore, suppression of the excimer state, while not hindering charge transport, must be realized if PDIs are to become competitive with fullerene-based acceptors. A thorough understanding of how charge separation dynamics and charge transport are related to different PDI modifications is needed provide a clearer understanding of why PDIs have failed in comparison to fullerene acceptors. Previous work by Pensack in the Asbury group began addressing PDIs and charge separation by hypothesizing the 2D network of PDIs causes a larger reorganization needed to charge-separation compared to the 3D structure of PCBM. 4 This delocalization argument was

129 114 pursued by Pensack et al.by performing time-resolved vibrational spectroscopy of the carbonyl bleach in blends of P3HT:PDI and P3HT:PCBM. 4 Ultrafast vibrational spectroscopy provided a unique opportunity to directly measure charge dynamics at interfaces because vibrational modes are sensitive to their local molecular environments and charge distributions, 12 known as vibrational solvatochromism. An experimental technique based on this effect, termed solvatochromism assisted vibrational spectroscopy (SAVS) was pioneered in the Asbury lab to identify molecules at electron donor-acceptor interfaces and directly measure the energetic barriers to charge transfer (CT) state dissociation to form charge separated (CS) states through the ultrafast charge transfer dynamics of the molecules using traditional ultrafast transient absorption techniques. 4 Figure 5-5a demonstrates the spatial correlation of frequency and proximity to a donor-acceptor interface observed for the C=O stretch mode of the methyl ester group of the functionalized fullerene, PCBM. Because CT states initially form at donoracceptor interfaces, electrons occupy acceptor molecules having unique vibrational frequencies. As electrons dissociate from CT states to form CS states, they occupy molecules having lower vibrational frequencies. This process gives rise to transient vibrational features exhibiting time-dependent frequency shifts that can be monitored to directly measure the time scales for CT state dissociation. Figure 5-5b shows SAVS being applied to study two different acceptor molecules blended with the donor material P3HT used in organic solar cells. Figure 5-5b (top plots) shows the time-resolved frequency shift of the carbonyl vibration at 300 K. These experiments were performed at a series of temperatures shown in the bottom plots, where the frequency shift is shown as a function of time at different temperatures. The temperature dependence can be understood in terms of electronic wavefunction delocalization. Electrons transferred to PCBM molecules are capable of delocalizing over the entire conjugated framework of the fullerene, resulting in activationless

130 115 charge separation. In contrast, the conjugated framework of the perylene diimide (PDI) molecule is smaller, planar, and anisotropic requiring that electron density be delocalized over many more PDI molecules to obtain similar delocalization as can be achieved with fullerenes. Consequently, higher molecular order is required to support the same degree of charge delocalization Current Work: Excimers in Solution The new ultrafast system described in this Chapter was used to expand on the group s previous PDI work. To simplify the process of using vibrational modes to understand charge separation dynamics we needed to first understand the vibrational modes of neat PDIs and electronically coupled (excimer) PDI systems. The PDI of choice was synthesized by reacting perylenetetracarboxylic acid bisanyhdride (PTCDA) with 3-aminopentane to form N,N -Bis(3- pentyl)perylene-3,4,9,10-bis(dicarboximide) (PDI-um). The yield was appromatixely 90%, and the product was purified using zone sublimation in the Jackson group (PSU, Elec. Eng.). We chose PDI-um (Figure 5-6) due to its reasonable solubility in common organic solvents (i.e. chloroform, acetone, benzene), which was essential due to the need to perform concentration studies for excimer formation. We hypothesized that if two PDI cores were electronically coupled, their core vibrational modes (C=C) could be perturbed and form new vibrational features. This could easily be examined through time-resolved vibrational spectroscopy of excimers. Excimers are dimers that are stable only in electronically excited states. 13 Dye molecules, like PDIs, usually aggregate together as their concentration in solution increases. Exciting a solution of PDIs will produce excited PDI molecules that will then find PDI molecules in the ground state to electronically couple to thereby forming excimers. Excimer formation is a diffusion controlled process,

131 meaning that the timescale for excimer formation is dependent on the concentration of PDIs in solution, the higher the concentration the faster the formation rate of excimers Steady State FTIR, UV-vis, and Florescence Measurements For all the PDI-um concentrations, a liquid cell with CaF 2 windows and a 100 μm spacer was used (Harrick, DLC-M25). The steady-state UV-vis spectra of a series of PDIs is given in Figure 5-6. The concentration set consisted of 1, 5, 10, 15, and 20 mg/ml of PDI-um in chloroform. As the concentration increases the absorption profile did not change until 20 mg/ml where the spectra began red-shifting. This is caused by aggregation of dye molecules in the ground state. Therefore, we limited our ultrafast time-resolved excimer study to 20 mg/ml to ensure we did not have ground state aggregation. Further support for the lack of ground state aggregation can be seen in Figure 5-7, which contains the FTIR data for the PDI solutions. The FTIR peaks correspond to C=O asymmetric stretching (~1693 cm -1 ), C= symmetric stretching (~1653 cm -1 ), and C=C core stretching (~1596 and 1578 cm -1 ). 14 Over the concentration range 1-20 mg/ml, no shifts in the ground state vibrations occur, indicating that ground state aggregation is not occurring in the solutions. Steady-state fluorescence measurements were performed on the PDI concentration set (Figure 5-8). Excimer florescence is characterized by a broad, structureless fluorescence that is red-shifted relevant to the monomer emission. 15 Figure 5-8 shows that as the concentration of PDI-um increases, the emission begins to broaden and shift between 10 and 15 mg/ml. These results along with the UV-vis data and FTIR data helped determine the useable concentrations for the ultrafast experiments.

132 117 Ultrafast Visible Pump Mid-IR Probe Spectroscopy Ultrafast measurements were performed on the new system described within this Chapter. The spectral range for each PDI concentration set was cm -1. Because the array is only capable of capturing a ~ 70 cm -1 bandwidth without moving the grating, we needed to move the grating multiple times to collect each dataset. As discussed in Section 5.2.2, this process must be done very carefully to ensure artifacts are not created by poorly normalized pixels. We normalized each pixel using the procedure described in Section Data was connected using a Matlab script to produce the final spectral range. The energy density for all samples was 100 uj/cm 2 and the samples were pumped at 532 nm. Figure 5-9 shows the time-resolved vibrational spectra for the 1 mg/ml sample. The spectra contian ground state bleaches (GSB) for the C=O modes at ~1660 and 1690 cm -1 as well as their excited state absorptions (ESA). Likewise, GSB and ESA are evident for the C=C core stretching modes. The shifts in the C=O frequencies are caused by the convolution of the GSB and ESA modes changing in time and not from a solvatochromic shift. The FTIR spectrum in Figure 5-7 confirms the expected GSB modes with no new features appearing. This is expected since at a concentration of 1 mg/ml excimer formation should not exist based on the steady-state fluorescent measurements (Figure 5-8). As the concentration increases to 10 mg/ml new vibrational features begin to appear (Figure 5-10). New absorptions appear at ~1525 and 1575 cm -1 neither of which are present in the 10 mg/ml FTIR spectrum Figure 5-7. At 15 mg/ml and 20 mg/ml both features are more pronounced and they appear within the first 100 ps (Figures 5-11 and 5-12). The ESA at 1575 cm - 1 overlaps the GSB of the C=C modes, however, the ESA at 1525 cm -1 is conveniently between the ESA of the C=C modes and allows for clearer examination. We believe the features appearing at 1575 cm -1 and 1525 cm -1 are actually the same ESA, which is a very broad ESA encompassing

133 118 a >100 cm -1 spectral range. The C=O modes shift in much the same manner as with the 1 mg/ml sample, however, due to the spectral congestion it is difficult to determine if there is any influence of the excimer formation on these modes. The excited state absorption most clearly pronounced at 1525 cm -1 seems to be a unique mode corresponding to the excimer. It true, the formation rate of this ESA should be concentration dependent with higher concentrations giving a faster rate of formation. We proposed the following kinetic model for the PDI-um excimer formation: (6) PDI* is the excited state and PDI is for the ground state. This model was chosen because it is the simplest mode that can fit the ultrafast vibrational spectral using target analysis. Each sample was fit using a 2x2 k-matrix target analysis approach with the software Glotaran. The Species Associated Spectra (SAS) shown in Figure 5-13 (top) for the 20 mg/ml sample. Three distinct spectral features were extracted using the target analysis, which assumed that the spectrum was comprised of three unique components linearly combined based on our modeling criteria (kmatrix). The three compounds correspond to the monomer (black), and intermediate state where the excimer is forming (red) and the full excimer (blue). The model s fit to the kinetics at 1504 cm -1, which is the decay of the monomer s C=C ESA mode is shown in Figure 5-13 (bottom). The same model was used to fit the 10 and 15 mg/ml samples to calculate the rate of formation of the excimer by fitting the kinetic component at 1525 cm -1 (Table 5-1). The increasing rate of formation with concentration along with the spectral information suggests that the ESA feature at 1525 cm -1 arises from the excimer formation perturbing the C=C core stretches of the PDI molecule. The unique identification of vibrational modes associated with the excimer may make it possible to extend such studies to solid state samples, where different packing structures could

134 119 likewise have unique C=C vibrational modes which could be used to qualitatively assign different packing motifs. The systematic variation of PDI molecular structure provides a useful testing ground for exploring delocalization effects within a homologous molecular architecture. The facile variation of the PDI structure allows for tuning important molecular properties, such as molecular packing and morphology. The hope is that because PDI packing strongly affects the charge separation and charge transport performance of PDI OPVs, vibrational spectroscopy could provide crucial insights into developing design rules for PDI-based electronic devices. 5.4 Conclusion The ultrafast instrument s visible pump mid-ir probe layout was redesigned to allow for the development of visible pump visible probe spectroscopy. In the process, all of the software was rewritten using MATLAB, enabling us to utilize newer hardware and software technologies. A custom driver was developed to handle the data transfer between the IR and MATLAB. The instrument is now capable of probing the visible and mid-ir regions for solid and liquid samples on the femto-nanosecond timescale with a sensitivity of An example of the instrument s performance was shown through the preliminary work with PDI-um and excimer formation in solutions of chloroform. These studies showed a unique excited state absorption around 1525 cm -1 corresponding to the formation of the excimer with a ~100 ps rate.

135 References 1. Pensack, R. D.; Asbury, J. B. Barrierless Free Carrier Formation in an Organic Photovoltaic Material Measured with Ultrafast Vibratiional Spectroscopy. J. Am. Chem. Soc. 2009, 131, Pensack, R. D.; Asbury, J. B. Beyond the Adiabatic Limit: Charge Photogeneration in Organic Photovoltaic Materials. J. Phys. Chem. Lett. 2010, 1, Barbour, L. W.; Pensack, R. D.; Hegadorn, M.; Arzhantsev, S.; Asbury, J. B. Watching Electrons Move in Real Time: Ultrafast Infrared Spectroscopy of a Polymer Blend Photovoltaic Material. J. Am. Chem. Soc. 2007, 129, Pensack, R. D.; Guo, C.; Vakhshouri, K.; Gomez, E. D.; Asbury, J. B. Influence of Acceptor Structure on Barriers to Charge Separation in Organic Photovoltaic Materials. J. Phys. Chem. C. 2012, 116, Jeong, K. S.; Pensack, R. D.; Asbury, J. B. Vibrational Spectroscopy of Electronic Processes in Emerging Photovoltaic Materials. Acc. Chem. Res. 2013, 46, Li, C.; Wonneberger, H. Adv. Mater. 2012, 24, Y., A.; Li, C.; Müllen, K. J. Mater. Chem. 2010, 20, Tatemichi, S.; Ichikawa, M.; Koyama, T.; Taniguchi, Y. Appl. Phys. Lett. 2006, 89, Kamm, V.; Battagliarin, G.; Howard, I. A.; Pisula, W.; Mavrinskiy, A.; Li, C.; Müllen, K.; Laquai, F. Adv. Energy Mater. 2011, 1, Wen, Y.; Liu, Y. Adv. Mater. 2010, 22, 1331.

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139 Figure 5-1. Ultrafast optical layout for the visible pump mid-ir probe (bottom) and the visible pump visible probe systems. L denotes lenses. 124

140 Figure 5-2. Ultrafast beam overlap between the pump and probe pulses. The angle between the pulses is α. 125

141 Figure 5-3. Data collection scheme for the visible pump mid-ir probe system showing the details of the modified National Instruments Driver structure. MATLAB controls the get/set requests for the PCI card with communicates with the IR FIFO memory. Different Ports are designated from particular functions by MATLAB. 126

142 Figure 5-4. Data collection scheme for the visible pump visible probe ultrafast instrument. The reference signal is a 5V TTL pulse from the laser amplifier, the AC coupled signals are from the transimpedance amplified balanced photodiode. Note the amplitude difference in the Probe Signal between ON and OFF laser shots. After the signals are sorted by ON and OFF shots from the chopper, the waveforms are integrated and then averaged to form ΔT and CF (correction factor). Both are used to calculate ΔA, which is plotted as the output. The x-axis is produced by the position of the delay stage. 127

143 Figure 5-5. Previous work by the Asbury group using the SAVS technique with PCBM and a PDI acceptor. (a) Cartoon depicting SAVS technique. (b) SAVS used to measure the barrier of charge separation in between PDI:P3HT and PCBM:P3HT. PCBM exhibits activationless separation due to 3D delocalization of the polaron. 128

144 Figure 5-6. Steady-state UV-vis spectra for PDI-um at difference concentrations. The absorptions arise from the perylene C=C stretching modes (0-0, 0-1, 0-2, 0-3) coupled to the π-π* electronic transition. The photograph (top) shows the concentration series, note that the sample 1 mg/ml is missing due to the solution dilution giving inadequate signal. 129

145 Figure 5-7. Steady-state FTIR measurements for the PDI-um concentration series. The 1690 cm -1 mode is the C=O asymmetric stretch, 1650 cm -1 is the C=O symmetric stretch, and the two C=C core stretching modes are at ~1580 and 1590 cm

146 Figure 5-8. Steady-state florescence measurements for the PDI-um concentration series. The broad, structureless fluorescence indicative of excimers is present once the concentration increases passes 1 mg/ml. 131

147 Figure 5-9. Ultrafast visible pump mid-ir probe spectra for 1 mg/ml PDI-um sample. The ground state bleaches (GSB) of the carbonyl asymmetric (1690 cm -1 ) and symmetric (1660 cm -1 ) modes are present along with their excited state absorptions (ESA). The GSB and ESA of the perylene core C=C modes are also present. At 1 mg/ml, there is no evidence of new vibrations suggesting excimers do not form at this concentration. 132

148 Figure Ultrafast visible pump mid-ir probe spectra for 10 mg/ml PDI-um sample. The ground state bleaches (GSB) of the carbonyl asymmetric (1690 cm -1 ) and symmetric (1660 cm -1 ) modes are present along with their excited state absorptions (ESA). The GSB and ESA of the perylene core C=C modes are also present. Unlike the 1 mg/ml sample, a new feature begins to form in the region of the C=C modes. These new broad ESA is assigned to the excimer based on the concentration dependence (see text). The rate of formation of excimer in the 10 mg/ml was measured to be ~879 ps. 133

149 Figure Ultrafast visible pump mid-ir probe spectra for 15 mg/ml PDI-um sample. The excimer vibrations are visible as they were in the 10 mg/ml sample, however they have appeared faster, suggesting a concentration dependence. The rate of formation of excimer in the 10 mg/ml was measured to be ~206 ps. 134

150 Figure Ultrafast visible pump mid-ir probe spectra for 20 mg/ml PDI-um sample with a rate of formation of ~176 ps. 135

151 Figure Species Associated Spectra for the 20 mg/ml sample and kinetic fit to the disappearance of the C=C ESA at 1504 cm -1. The fits were done using a 2x2 k-matrix with target analysis. The model is described in the text. The black trace in the SAS spectrum corresponds to the monomer unit, the red corresponds to the intermediate state between monomer and excimer and the blue corresponds to the excimer state. The model assumes the experimental spectrum is a linear combination of the SAS spectra. 136