Supporting Information for

Transcription

1 Supporting Information for Molecular Rectification in Conjugated Block Copolymer Photovoltaics Christopher Grieco 1, Melissa P. Aplan 2, Adam Rimshaw 1, Youngmin Lee 2, Thinh P. Le 2, Wenlin Zhang 2, Qing Wang 3, Scott T. Milner 2, Enrique D. Gomez 2,4*, and John B. Asbury 1* 1. Department of Chemistry, The Pennsylvania State University 2. Department of Chemical Engineering, The Pennsylvania State University 3. Department of Materials Science and Engineering, The Pennsylvania State University 4. Materials Research Institute, The Pennsylvania State University Table of Contents: Device J-V Characterization Results page S2 Selection of the mid-ir Region for Measuring Polaron Absorption Kinetics page S3 Method for Combining Ultrafast and Nanosecond mid-ir Kinetics page S5 Fitting Routine and Results for Polaron Absorption Kinetics page S9 Structures of P3HT-b-PFTBT and PFT6BT page S11 Steady-State Absorption and Emission Spectra of Films page S12 Pump Energy Density Dependence of Ultrafast Polaron Absorption Kinetics page S14 References page S15 S1

2 Table S1. Device J-V Characterization Results Sample JSC (ma/cm 2 ) VOC (V) FF PCE (%) P3HT-b-PFTBT, 165 o C 4.9 ± ± ± ± 0.07 P3HT-b-PFTBT, 100 o C 2.9 ± ± ± ± 0.02 P3HT/PFTBT, 100 o C 1.9 ± ± ± ± 0.03 P3HT/PFTBT, 165 o C 1.3 ± ± ± ± 0.03 Device results are the average of at least 5 devices. Blend films (P3HT/PFTBT) were 1:1 wt/wt. The temperatures listed are the thermal annealing temperatures. S2

3 Selection of the mid-ir Region for Measuring Polaron Absorption Kinetics To demonstrate the importance of using the mid-ir spectral region for measuring polaron dynamics in the P3HT-b-PFTBT polymer system, we used an as-cast P3HT/PFTBT blend film as a representative sample. (a) (b) PFTBT P3HT/PFTBT P3HT/PFTBT P3HT Instrument Response PFTBT Instrument Response P3HT (c) mid-ir + near-ir controls P3HT/PFTBT (mid-ir) P3HT/PFTBT (near-ir) 50% P3HT + 50% PFTBT (near-ir controls) Figure S1: Nanosecond transient absorption kinetics for a P3HT/PFTBT blend film and the homopolymer controls in the (a) near-ir and (b) mid-ir spectral regions using 532 nm excitation (~30 μj/cm 2 incidence). The kinetics are all normalized to the number of photons absorbed for each sample. (c) An overlay of the mid-ir and near-ir kinetic features, showing that the near-ir kinetics of P3HT/PFTBT can be described as a linear combination of the PFTBT and P3HT homopolymer features in the near-ir, and the mid-ir polaron kinetics of the P3HT/PFTBT blend. S3

4 Transient species having unimolecular decay in homopolymer PFTBT (Figure S1a) have absorptions in the near-ir that overlap significantly with polaron absorption. This is often attributed to triplet states that form in high yields in amorphous polymer films (Ref. S1). On the other hand, the mid-ir region (Figure S1b) appears to feature only polaron absorption on the basis of a clear power law decay form for both the homopolymers and the blend films. The homopolymers also show weak signals, which is consistent with reports on the photogeneration of polarons in neat polymer films (Ref. S2.). The large difference in magnitude of the mid-ir transient absorption feature between the blend and homopolymer films signifies higher polaron yield for the blend. We assign this additional signal to polarons that have formed as a result of charge separation at an electron donor-acceptor interface (i.e. P3HT/PFTBT interface). Most importantly, these data suggests (at least on the nanosecond timescale) that the mid-ir region exclusively features polaron absorption for the P3HT/PFTBT polymer system as compared to the near-ir region. Consequently, we chose to measure the transient absorption in the mid-ir range in order to most clearly monitor the polaron dynamics for the films described in the main text. S4

5 Method for Combining Ultrafast and Nanosecond mid-ir Kinetics It must be noted that the spectral ranges for the measured kinetics in the mid-ir were not identical between the ultrafast and nanosecond measurements. This was done because of two experimental constraints: (1) The ultrafast mid-ir probe, which was generated using an OPA, has only ~200 cm -1 FWHM before dispersion in a monochromator; and (2) for the nanosecond spectroscopy, spectral resolution was sacrificed for higher light levels needed to measure sub- mo.d. signals. In order to achieve the highest time resolution possible for connecting the nanosecond to the ultrafast kinetics, we focused the probe onto a 100 MHz MCT detector with a 0.1 mm diameter element, without using a monochromator. Because of the small element size, it is difficult to focus enough of the incoherent probe light to achieve reasonable light levels to make the transient absorption measurement for the expected low signals (<<1 mo.d.). Eliminating the use of a monochromator by using an optical longpass filter allowed us to focus more light on the detector. The resulting spectral range monitored in the ultrafast timescale was ~ ev, while that measured in the nanosecond timescale was ~ ev. Because there was a large discrepancy in spectral range used to measure the polaron absorption kinetics, we considered the possibility of the mixing of absorption kinetics of different transient species. We assigned the major absorbing species in our spectral ranges to delocalized polarons, as reported by others (Ref. S3). We expected only a minimal contribution from overlapping absorption of localized polarons because the localized polaron peak for P3HT is centered around 0.5 ev (Ref. S3). Furthermore, regioregular P3HT is known to present minimal absorption of localized polarons (Ref. S3) because of its nature to pistack in the solid state as it crystallizes. We show (Figure S2b) the suppression of the peak around 0.5 ev upon annealing of a P3HT/PFTBT blend film, which is correlated with an increase in P3HT (and possibly also PFTBT) crystallinity as it anneals (as evident in the steady-state UV-Vis spectra shown in Figure S2a). As such, we assigned the nanosecond mid-ir kinetics to arise primarily from delocalized polarons and with little spectral contamination from localized polarons. We also assign the ultrafast transient absorption to delocalized polarons because the wavelength probed (4000 nm; 0.31 ev) is far-removed from the region in S5

6 which localized polarons absorb. Thus we can connect the two regions despite a difference in the wavelength probed. Figure S2: (a) Normalized UV-Vis spectra for P3HT/PFTBT films with (100 o C) and without ( as-cast ) thermal annealing. Transient absorption spectra in the mid-ir region at (b) 0 ns and (c) 1 μs time delays measured using 532 nm excitation (~50 μj/cm 2 ). The as-cast film shows a peak maximum around 0.5 ev at early time-delay, which is characteristic of localized polaron absorption. The annealed film shows a polaron absorption spectrum that resembles that of delocalized polarons. Because the wavelength range was not identical for the ultrafast and nanosecond measurements, the magnitude of the transient absorption of the raw data should not accurately describe the true polaron absorption intensity for the kinetics at ~0.31 ev. This is because the nanosecond kinetics are spectrally integrated over a large wavelength range (~ ev) in which the polaron absorption is not constant. Applying a scaling factor should correct for this discrepancy in magnitude provided that the shape of the polaron absorption spectrum does not change between each sample. We make this assumption on the fact that the mid-ir spectrum is nearly identical between the P3HT-b-PFTBT 165 o C and P3HT/PFTBT 100 o C films (Figure 2 in the main text). We corrected our nanosecond kinetics using the following steps: (1) Normalize the data to the number of photons absorbed for each sample; (2) truncate the data starting at 20 S6

7 ns, which is the earliest resolvable time-delay (immediately following the decay of the instrument response function); and (3) scale every sample s nanosecond kinetic traces by the same constant value until the kinetics lined-up with the ultrafast kinetics and were well described by a smoothly varying function (i.e. power law functions). To exemplify the scaling procedure, we show the transient absorption data using a variety of scaling factors for the entire data set in Figure S3. Figure S3: Transient absorption kinetics for all 6 samples reported in the main text. Legend entries are the multiplicative scaling factor used on the nano-to-microsecond kinetic data to connect them with the ultrafast data. A scaling factor of 0.2 was selected for the presentation of the data and fits. S7

8 Power law fits using the 0.2 scaling factor are displayed as solid gray lines. Again, we emphasize that the scaling factor selected was the same for all samples. Selection of the scaling factor was qualitative, and so it is important to assess the error associated with the fits due to the scaling factor as a major source of error. We show this in a later section below. S8

9 Fitting Routine and Results for Polaron Absorption Kinetics The film data presented in Figure 3 in the main text were fit with empirical power laws as shown in equation 1 (adapted from Ref. S4): A(t) = n(1 + at) α (eqn. 1) where n, a, and α are constants specific to each power law term, t is time, and A is the measured change in absorbance. In order to understand the fit results, it is useful to consider how each parameter changes the shape of the power law decay function. In Figure S4, we show how the power law function changes with variations in n, a, and α. Figure S4: Visualization of the power law decay function with varying parameter values. (a) Variations in n change the initial amplitude. (b) Variations in a change the onset of the decay. (c) Variations in the exponent, α, change the steepness of the decay (or slope on the log-log scale). The default parameter values used were n = 1, a = 10 11, and α = 0.3. It is important to highlight the effect that the parameter α has on the decays. Unlike in exponential decays, the time constant, a, in the power law function does not change overall steepness of the decays on the log-log scale. The a values primarily affect the onset of the decay, as seen in Figure S4(b). The exponent, α, affects the steepness of the decays on the log-log scale as seen in Figure S4(c). Closer-to-ideal behavior (α = 1) exhibits the steepest decay form, while trap-limited bimolecular recombination exhibits shallower decay profiles (e.g. α = 0.3). S9

10 One power law was used to describe the majority absorbing species in all film data. The results of the fits are shown in the main text (Figure 4). We next considered how the scaling factor used to connect the nanosecond to ultrafast decays (detailed above) is a major source of error for the fits. Previously we showed why we selected a scaling factor of 0.2 (qualitative) for presentation of the data. Here, we investigate the impact of selecting variations of the scaling factor on the resulting fit parameters. Each set of data was refit for scaling factors in the range After confirming that the resulting parameter values moderately and systematically changed between all of the samples, we calculated the average and errors (as the standard deviations) of the parameters over this parameter range. The results are shown below: Table S2. Power law fit parameters. Sample n a α P3HT-b-PFTBT, 165 o C 3.3 ± 0.3 (1.1 ± 0.5) x ± 0.01 P3HT-b-PFTBT, 100 o C 3.6 ± 0.3 (1.0 ± 0.5) x ± 0.02 P3HT/PFTBT, 100 o C 4.4 ± 0.2 (0.9 ± 0.3) x ± 0.02 P3HT/PFTBT, 165 o C 5.0 ± 0.2 (1.0 ± 0.3) x ± 0.02 P3HT, 165 o C 3.8 ± 0.4 (0.8 ± 0.3) x ± 0.02 PFTBT, 165 o C 3.3 ± 0.1 (1.1 ± 0.1) x ± 0.02 S10

11 Chemical Structures of P3HT-b-PFT6BT and PFT6BT used for solutions studies S11

12 Steady-State Absorption and Emission Spectra of Films Figure S5: Normalized steady-state absorption (solid lines) and emission spectra (dashed lines) for all the film samples presented in the main text. The excitation wavelength for the emission spectra was 532 nm. Emission spectra were obtained by measuring the time-resolved emission spectra and integrating them over the entire kinetic decays. The emission spectra were not corrected for spectrometer efficiency. It is important to rule out the possibility of turning on CT state interactions in the block-copolymer samples in the solid state as a result of polymer planarization, which changes their energy levels. It is possible for these changes in energy levels to affect the intramolecular coupling strength. From the emission spectra, the spectral shape of the P3HT/PFTBT (blend) 165 o C film (blue dashed) is well-described as a linear combination of homopolymer P3HT 165 o C and PFTBT 165 o C emission spectra. The block-copolymer films at both annealing temperatures (red and magenta, dashed) have a significantly different spectral shape (more rounded and broadened) that may be due to the presence of amorphous polymer emission and/or differences in emission peak position due to variations in polymer molecular weight. As such, the presence of CT state emission cannot be concluded from the data shown. In fact, the P3HT/PFTBT (blend) 100 o C (green, dashed), which lacks an intramolecular interface, exhibits emission peak broadening relative to P3HT/PFTBT 165 o C, which is consistent with the presence of amorphous domain emission. Altogether, S12

13 these observations support the conclusion of weak intermolecular coupling and lack of an intramolecular CT transition in the block-copolymers in the solid state. S13

14 Pump Energy Density Dependence of Ultrafast Polaron Absorption Kinetics To ensure that nonlinear effects on polaron generation and/or recombination were suppressed in our kinetic traces, we performed a pump laser fluence dependence study on one of the film samples (P3HT-b-PFTBT, 100 o C). The results are shown below in Figure S6. Figure S6: Effect of pump energy density on polaron absorption kinetics for P3HT/PFTBT 100 o C film excited at 532 nm. Kinetics are shown in the left panel, while the amplitude dependence on energy density is shown in the right panel. Pump energies tested were 194 μj/cm 2 (red), 30 μj/cm 2 (magenta), 20 μj/cm 2 (green), and 7 μj/cm 2 (blue). The amplitudes of the kinetic traces were integrated from -0.3 ps to 0.3 ps. The red dashed line shows a linear fit to the lowest energy data points. S14

15 References: S1. Ohkita, H.; Cook, S.; Astuti, Y.; et al. Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy. JACS , S2. Miranda, P.B.; Moses, D.; Heeger, A.J. Ultrafast photogeneration of charged polarons on conjugated polymer chains in dilute solution. Phys. Rev. B , S3. Jiag, X.; Osterbacka, R.; Korovyanko, O.; et al. Spectroscopic Studies of Photoexcitations in Regioregular and Regiorandom Polythiophene Films. Adv. Funct. Mat , S4. Ohkita, H. and Ito, S. Transient absorption spectroscopy of polymer-based thin-film solar cells. Polymer , S15