Supporting Information

Transcription

1 Supporting Information On the Field Dependence of Free Charge Carrier Generation and Recombination in Blends of PCPDTBT/PCBM: Influence of Solvent Additives Steve Albrecht, Wolfram Schindler, Jona Kurpiers, Juliane Kniepert, James C. Blakesley, Ines Dumsch, Sybille Allard, Konstantinos Fostiropoulos, Ullrich Scherf, and Dieter Neher * Universität Potsdam, Institute of Physics and Astronomy, Soft Matter Physics, D- Potsdam, Germany Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz, D- Berlin, Germany Bergische Universität Wuppertal, Macromolecular Chemistry and Institute for Polymer Technology, Gauss-Strasse, D- Wuppertal, Germany The supporting information contains the following data: - details on sample fabrication and measurement techniques - Fig. S: (a) electron energy loss spectra and (b) thin film absorption - Fig. S: total extracted charges and EQEs from TDCF measurements with different pulse fluences - Fig. S: comparison of ratios between the two blends: current under AM.G and total extracted charge from TDCF for different bias - Fig. S: total-, pre- and collected charges for blends with and without DIO measured with different delay times at.;. and. V pre bias together with the corresponding bimolecular recombination fits - Fig. S: drift diffusion fits to TDCF transients with different delay times for blends (a) with and (b) without DIO. * corresponding author, neher@uni-potsdam.de

2 Sample fabrication: The solar cell-, TDCF- as well as photo-celiv devices were fabricated on structured ITO coated glass slides (Optrex) pre-cleaned in acetone, detergent, DI-water, isopropanol and dried with a nitrogen gun. The pre-cleaned ITO substrate was plasma-cleaned and a - nm layer of PEDOT:PSS (Clevios AI ) was spin cast ontop. The sample was subsequently transferred into a nitrogen filled glove-box followed by annealing at C for min. The active layer was spin cast from solutions containing : (by weight) blend ratios of PCPDTBT (M w = g/mol, PDI=., prepared in a Stille type polycondensation following a procedure described in literature) and PC BM (%, Solenne). Chlorobenzene was used as the solvent. Samples were prepared with and without vol% diiodooctane (DIO) as processing agent. Finally nm Ca and nm Al were thermally evaporated with a base pressure below - mbar trough shadow masks to define the active area to be. mm². Such small area was used to realize a small RC-constant of the device. Due to the high boiling point of DIO, all devices processed with DIO have been dried in vacuum at room-temperature for at least h prior to evaporation of Ca and Al, since residual DIO functions as a hole trap. Samples for Photo-CELIV or TDCF have been encapsulated with two component epoxy resin and a glass lid prior to air exposure. For the TEM measurements, blends processed with and without DIO where prepared on ITO/PEDOT:PSS identical to the solar cell samples but with smaller thicknesses of to nm. These films where then floated off in de-ionized water and picked up by the TEM-grid. Plasmon maps based on EFTEM images: Phase separation was imaged using spectroscopic contrast in the TEM as recently demonstrated for blends of PHT and PCBM. Thin films were analyzed in a Zeiss Libra TEM equipped with an energy filter. By electron energy loss spectroscopy plasmon energies of. ev and. ev were measured for pristine PCPDTBT and PC BM, respectively. A series of energy filtered images of the blend films was recorded in the region from ev to ev using a. ev window and an energy

3 increment of. ev. Plasmon spectra were extracted pixel-by-pixel from the spatially corrected image stack and adjusted for energetic drift and non-isochromaticity. Peak centers were determined by an automatic Gaussian fitting routine and laterally mapped. TDCF measurements: The measurement-scheme was described in detail elsewhere. The pulsed excitation (, ns pulse width, Hz repetition rate, ns jitter) was done with a diode-pumped, Q-switched Nd:YAG laser (NT,EKSPLA). The current through the device was measured via a Ω resistor in series with the sample and was recorded with a Yokogawa DL oscilloscope. For decreasing the time delay between the laser pulse and the start of the collection voltage ramp to ns, the following measures were taken. First, Agilent A pulse generator with a very fast slew rate of. ns was used to apply the preand collection bias to the sample. Second, to reduce the effect of the laser jitter on the measurement, the pulse generator was triggered via a fast photodiode (EOT ET TTL). Also, to compensate the internal latency of the pulse generator, the laser pulses were delayed with a m long multimode fiber (LEONI) with respect to the first trigger diode. The pulses broadened to. ns after the fiber. A second fast photodiode (EOT ET TTL) was placed after the fiber to trigger the oscilloscope. The pulse fluence was measured with a Ophir Vega power meter equipped with a photodiode sensor. Data analysis: The bimolecular recombination coefficient was iteratively calculated from the TDCF-data with equation S. (S) Here, and are the integrals of the photocurrent during delay and collection, respectively, d the device thickness and A the active area. Note that (S) also considers the density of dark charge carriers due to injected charges at forward bias. An analysis of the data without considering will slightly overestimate. The dark charge density has been measured with dark CELIV experiments for each individual pre-bias (see below).

4 Figure S shows, and with the corresponding bmr-fit according to equation (S) for different pre bias settings. Photo-CELIV measurements: Measurements employing the current extraction under linearly increasing voltages (CELIV) technique were realized with the same laser and excitation wavelength as used for TDFC. The linear increasing voltage ramp was applied with an Agilent A wave form generator and a fast custom-built amplifier. The resulting current transients were measured with a fast current amplifier (Femto DHPCA-) and a digital oscilloscope (Yokogawa DL). To vary the field with being the time with maximum photocurrent, the voltage slope was increased by only varying the pulse length. Note that although decreased with increasing, the mobility calculated according to μ ². (S) decreased with higher. The dark density needed for correct calculation of the bmr-coefficient from TDCF transients has been determined via dark-celiv. Devices were held at a pre-bias to to realize steady state conditions and then the voltage ramp was applied. The dark density was determined by subtracting the capacitive current from the CELIV transient. Values for the dark charges were about - cm -. Increasing the dark charge in Eq. S artificially to values much larger than those measured by dark-celiv led to very poor fits of the collected charge versus delay time. We, therefore, rule out that the increase of the bmr-coefficient when the bias approaches as described in the main part of the paper is caused by an exceptional large dark charge. Solar cell characteristics: The solar cell characteristics were measured with an Oriel class A simulator calibrated to mw/cm², the samples were temperature controlled to C during measurement. The calibration of the sun simulator was done with a KG filtered silicon

5 reference cell calibrated at Fraunhofer ISE. All shown data are corrected for spectral mismatch with a mismatch factor of. for PCPDTBT:PC BM processed with and without additive. Figure S Intensity [a.u.]. PC BM. PCPDTBT Blend. ev. ev. ev. Energy Loss [ev] Absorption [a.u.] w/o with DIO Wavelength [nm] Fig. S (a) Electron energy loss spectra from single component films and blends. Shown is the energy region of the plasmon absorption. The PCPDTBT plasmon center peak appears to be at lower energies than that for PC BM. Therefore, the dark areas (loss at higher energy) in Figure refer to the PC BM domains. (b) Thin film absorption spectra on glass substrates of PCPDTBT: PC BM blends processed with (filled symbols) and without DIO (open symbols) for nm thickness. TDCF excitation was at nm, where both blends have almost identical absorption coefficients.

6 Figure S.. EQE.. n tot [cm - ] w/o with V pre [V].... Pulse Fluence [µj/cm²] Fig. S Total charges extracted from TDCF transients measured with a delay of ns at different pulse fluences for blends with (filled symbols) and without DIO (open symbols) at different pre bias of.;. and. V. The corresponding EQEs are plotted in the upper panel.

7 Figure S. Ratio (with / without)..... Q tot steady state current Voltage [V] Fig. S Comparison of the ratio between blends with and without additive in steady state photocurrent (blue solid line) and total extracted charges (black stars) for each bias. The arrows indicate that at conditions of and. V the difference in charge generation is not solely causing the difference in solar cell performance. From the ratio in charge generation, the loss caused by increased non-geminate recombination and less generation of free charges for blends without DIO can be quantified individually. Assuming only difference in charge generation, the photocurrent for additive processed blends can be reduced by the ratio of charge generated for each bias. This would end up in a hypothetical device with J sc =. ma/cm², FF=.%, V oc =. V and PCE=.% for blends without DIO. The measured PCE is.% (.%) for blends without (with DIO). Thus, approximately.% of efficiency is lost due to less efficient generation, whereas ca..% loss can be addressed to the higher imbalance between non-geminate recombination and extraction in blends processed without the additive.

8 Figure S Q [ - C] w/o;.v Q [ - C] w/o;.v Q [ - C] w/o;.v Q pre Q tot Q coll Q [ - C] with;.v with;.v Q [ - C] Dealy Time [ns] Q [ - C] with;.v Q pre Q tot Q coll Fig. S Total-, pre- and collected charges extracted from transients for blends without DIO (upper row) and with DIO (lower row) measured with different delay times and pre bias of. V (left column);.v (middle column) and. V (right column). The pulse fluence was adjusted to. µj/cm². This fluence is almost in the linear regime but the transients have a better signal to noise ratio for higher accuracy fit-results.

9 Figure S Current [ma] Vpre. V Fluence. µj/cm² Vbi. V Qinitial * m - µe(e=).* - cm²/vs µh(e=).* - cm²/vs µe(t=).* - cm²/vs µh(t=).* - cm²/vs t relax ns PF factor -.* - (cm/v) / BMR coeff.* - m³/s Time [ns] Current [ma] Vpre.V Fluence. µj/cm² Vbi. V Qinitial.* m - µe(e=).* - cm²/vs µh(e=).* - cm²/vs µe(t=).* - cm²/vs µh(t=).* - cm²/vs t relax ns PF factor -.* - (cm/v) / BMR coeff.* - m /s Time [ns] Fig. S Drift diffusion fits to TDCF Transients at. µj/cm² and. V pre bias for different delay times between - ns for (a) with DIO and (b) without DIO. The fit parameters initially, homogeneous created charges (Qinitial) and bimolecular recombination coefficient (BMR coeff) have been adopted from measurements. The zero field mobilities µ(e=) and the Poole Frenkel factor (PF factor) have been adopted from mobility measurements and are indicated by the crosses for the corresponding field at the pre bias and the collection voltage in Figure. A slight mobility relaxation was needed to fit the initial slope of the transients. The mobility exponentially decays from the start values µe,h (t=) in ns to the field determined mobility. The built-in field (Vbi) is assumed to be flat at a bias mv higher than. Supplementary References () Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M. et al. Panchromatic conjugated polymers containing alternating donor/acceptor units for photovoltaic applications. Macromolecules,, -. () Cho, S.; Lee, J. K.; Moon, J. S.; Yuen, J. et al. Bulk heterojunction bipolar field-effect transistors processed with alkane dithiol. Org. Electron.,, -. () Herzing, A. A.; Richter, L. J.; Anderson, I. M. D Nanoscale Characterization of Thin-Film Organic Photovoltaic Device Structures via Spectroscopic Contrast in the TEM. The Journal of Physical Chemistry C,, -. () Kniepert, J.; Schubert, M.; Blakesley, J. C.; Neher, D. Photogeneration and Recombination in PHT/PCBM Solar Cells Probed by Time-Delayed Collection Field Experiments. The Journal of Physical Chemistry Letters,, -. () Bange, S.; Schubert, M.; Neher, D. Charge mobility determination by current extraction under linear increasing voltages: Case of nonequilibrium charges and field-dependent mobilities. Physical Review B,,. () Shrotriya, V.; Li, G.; Yao, Y.; Moriarty, T. et al. Accurate Measurement and Characterization of Organic Solar Cells. Adv. Funct. Mater.,, -.