Fullerene-Free Polymer Solar Cells with Highly Reduced Bimolecular

Aug 1, 2014 - Fullerene-Free Polymer Solar Cells with Highly Reduced Bimolecular Recombination and Field-Independent Charge Carrier Generation. Steffe...
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Fullerene-Free Polymer Solar Cells with Highly Reduced Bimolecular Recombination and Field-Independent Charge Carrier Generation Steffen Roland,† Marcel Schubert,† Brian A. Collins,‡,§ Jona Kurpiers,† Zhihua Chen,∥ Antonio Facchetti,∥ Harald Ade,‡ and Dieter Neher*,† †

Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States § National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899, United States ∥ Polyera Corporation, Skokie, Illinois 60077, United States ‡

S Supporting Information *

ABSTRACT: Photogeneration, recombination, and transport of free charge carriers in allpolymer bulk heterojunction solar cells incorporating poly(3-hexylthiophene) (P3HT) as donor and poly([N,N′-bis(2-octyldodecyl)-naphthelene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)) as acceptor polymer have been investigated by the use of time delayed collection field (TDCF) and time-of-flight (TOF) measurements. Depending on the preparation procedure used to dry the active layers, these solar cells comprise high fill factors (FFs) of up to 67%. A strongly reduced bimolecular recombination (BMR), as well as a field-independent free charge carrier generation are observed, features that are common to high performance fullerene-based solar cells. Resonant soft X-ray measurements (R-SoXS) and photoluminescence quenching experiments (PQE) reveal that the BMR is related to domain purity. Our results elucidate the similarities of this polymeric acceptor with the superior recombination properties of fullerene acceptors. SECTION: Energy Conversion and Storage; Energy and Charge Transport romising features like flexibility, solution processability, and light weight have drawn attention to polymer-based organic solar cells (OSCs) over the past years. Through physical understanding and chemical design, power conversion efficiencies (PCEs) exceeding 10% in polymer tandem solar cells have been achieved.1 The most common and highest efficiency systems consist of conjugated polymers as electrondonating and fullerene derivates as electron-accepting materials. The superior performance of fullerene-based solar cells has been attributed to low recombination losses of free charge carriers, a field-independent charge carrier generation, and high bulk mobilities, resulting in internal quantum efficiencies and fill factors (FFs) approaching 100% and 80%, respectively.2−4 However, if the fullerene is replaced by a polymeric acceptor, the blend devices often suffer from strong field-dependence and low free charge carrier yields. Consequently, the majority of allpolymer solar cells display low FFs and photocurrents.5−8 Nevertheless, there is an ongoing search for alternative, polymer-based, electron acceptors due to their higher electronic and optical variability, and their PCE already exceeds 6%.9 Due to the large number of available data on the optical, charge transport, and structural properties,10−21 the low band gap naphthalene diimide-based copolymer poly([N,N′-bis(2octyldodecyl)-naphthelene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,5′-(2,2′-bithiophene)) (P(NDI2OD-T2)),22 can be viewed as a new model system for polymeric electron acceptors

P

© XXXX American Chemical Society

(for chemical structure and absorption spectra, see Figure 1a and Figure S1 in the Supporting Information). Initially, this polymer gained attention due to its high electron field effect mobility of 0.85 cm2 V−1 s−1.20 Later, it was found that also bulk mobilities, measured perpendicular to the electrodes by timeof-flight (TOF) and single carrier diodes, are on the order of ≈5 × 10−2 cm2 V−1 s−1 and belong to the highest n-type mobilities measured so far for organic semiconductors.10,19 The excellent charge transport capability has been attributed to the strong aggregation and structure formation in films of P(NDI2OD-T2), which shows a semicrystalline morphology with exceptional long-range order as well as fast transport along the π-stacking direction.10,16,17,19,20,23,24 First attempts to incorporate P(NDI2OD-T2) as acceptor in organic solar cells with poly (3-hexylthiophene) (P3HT) yielded solar cells with low photocurrents and with a PCE of only 0.2%.14 Later, the performance was increased to 1.4% by applying the solvent chloronaphthalene (CN) in the preparation of the active layers.18 CN was found to change the orientation of the P(NDI2OD-T2) crystals, which introduces a face-to-face orientation of donor and acceptor crystallites beneficial for the dissociation of geminate electron−hole pairs.25 A Received: July 18, 2014 Accepted: August 1, 2014

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of new acceptor polymers, as they signify the great potential and variability of polymeric acceptors in organic solar cells. In this study we compare an optimized P3HT:P(NDI2ODT2) blend that has been dried at 200 °C immediately after spin coating to a room temperature (RT)-dried P3HT:P(NDI2ODT2) blend that exhibits a poor FF of only 50% (Figure 1b)). Active layers were prepared from a 1:1 (by volume) xylene:CN solvent mixture, and a detailed description of the solar cell preparation can be found in our former publication.18 The rapid drying recipe consists of 5 s spin coating and the subsequent transfer of the substrate onto a hot plate at 200 °C, where the solvent-soaked film dries within about 10 s and is kept at that temperature for 1 min.18 The slow drying recipe also includes a 5 s spin coating step, but the subsequent drying is performed under vacuum at RT, which increases the drying time to about 8 to 10 min. The RT-dried cells are kept under vacuum for 16 h. The thickness of the active blend is on average 310 nm for both preparation schemes. The samples comprise the same amount of material after spin coating, but the RTdried film is quite rough. The different height variations on the RT- and 200 °C-dried films (see Figure 5) are a typical feature in polymer blends that show a strong phase separation.30,31 Figure 1b displays the current density−voltage characteristics of the two cells, the 200 °C- and the RT-dried solar cell devices, under 1 sun simulated AM 1.5 G illumination. The power conversion efficiency differs by almost a factor of 2 due to the variation in the FF and short-circuit current, with PCE equal to 1.18% and 0.6% for the 200 °C- and the RT-dried solar cell, respectively. To understand the performance of the differently dried solar cell systems, charge carrier dynamics are probed using the time delayed collection field (TDCF) technique.28,32 TDCF is an electro-optical pump probe experiment where optically excited, free charge carriers are probed after a variable delay time with a rectangular (collection) voltage pulse. From the photocurrent transients, the amount of total charge (Qtot) is obtained by integrating over the entire current transient. During excitation and before applying the collection voltage pulse (Vcoll), different prebiases (Vpre) were applied to tune the internal electric field under which free charge carriers are generated. The shortest delay time that can be applied is 10 ns. This is long enough to ensure that generation of free charge carriers is completed.14,25,33 Furthermore, excitation intensities were adjusted to a value where no bimolecular recombination is observed during the shortest delay (see also Figure S2 in the Supporting Information). Excitation was at a wavelength of 532 nm, corresponding to the maximum absorption of the blends (see Figure S1 in the Supporting Information). Different measurement protocols were applied to extract the relevant recombination and charge transport parameters. Additionally, TOF measurements were performed to determine the charge carrier mobilities of electrons and holes, where the variation of the electric contacts and the field direction provides selective choice over the charge carrier type. At first, the influence of the electric field on charge carrier generation will be discussed. To measure the field dependence of the free charge carrier generation, charges are generated at different prebiases Vpre by a laser pulse and extracted by a high collection bias after a short delay of −5 V and 10 ns, respectively. This ensures that transients are free of recombination effects. The high collection bias guarantees the extraction of all free charges in the device. We then measure the total charges generated by the laser for different prebiases (−2.5 to 0.6 V). The measurements were performed for three

Figure 1. (a) (left) Chemical structure of P3HT; (right) chemical structure of P(NDI2OD-T2). (b) J−V characteristics of 200 °C(magenta) and RT-dried (cyan) P3HT:P(NDI2OD-T2) solar cells measured under AM 1.5 G illumination at 100 mW cm−2.

remarkable feature of P3HT:P(NDI2OD-T2) solar cells is the high FF, reaching 65%. However, the moderate short circuit current densities and the maximum external quantum efficiency of about 20% in the optimized cells demonstrate that this system still suffers from high internal quantum efficiency losses.18,25 That these low quantum efficiencies can be overcome was recently demonstrated by optimizing the donor component of P(NDI2OD-T2)-based solar cells and achieving a current density of 10 mA cm−2 and a PCE of 4.2%.9 Furthermore, Mori et al. presented a maximum external quantum efficiency of 50% for a quinoxaline-based donor polymer with P(NDI2OD-T2) as acceptor, resulting in a PCE above 4%.26 Another all-polymer solar cell system reaching a PCE of over 4% has recently been published by Zhou et al. which mainly benefits from high open circuit voltages of 1 V.27 In this contribution we show strongly suppressed bimolecular recombination in P3HT:P(NDI2OD-T2) with a bimolecular recombination (BMR) coefficient as small as 5 × 10−18 m3/s. This is one of the lowest values reported for all-polymer devices so far. P3HT-based cells, in which the fullerene derivative [6,6]-phenyl C71 butyric acid methyl ester (PCBM) acts as the electron acceptor, show a similarly reduced BMR.28,29 The influence of the processing conditions on generation and recombination were studied and related to electron and hole mobilities. We find that purer phases lead to slightly lower mobilities but also suppress BMR. Our results reveal strong similarities with the P3HT:PCBM model system with regard to the kinetics of extraction and recombination. These similarities encourage further efforts in the development 2816

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different light intensities (0.1 μJ cm−2, 0.2 μJ cm−2, 0.6 μJ cm−2), which are all within the linear regime (see Figure S2 in the Supporting Information). The results are shown in Figure 2 together with the JV-characteristics, measured under conditions corresponding to 0.5, 1, and 3 suns.

Figure 3. Electron (closed symbols) and hole (open symbols) bulk mobilities in the 200 °C-dried (magenta) and RT-dried (cyan) blends measured with TOF (stars) or extracted from TDCF transients (squares). Error bars are calculated assuming a relatively large uncertainty of 20% for the film thickness and the transit times determined from the TOF transients, which are shown in the Supporting Information (Figure S4), respectively. Figure 2. (left axis) J−V characteristics (solid lines) at 0.5, 1, and 3 suns. (right axis) Total collected charge (Qtot) as a function of the prebias at 0.1 μJ cm−2, 0.2 μJ cm−2 and 0.6 μJ cm−2 pulse fluence, corresponding to 0.5, 1, and 3 suns, respectively, for the RT-dried (cyan dots) and the 200 °C-dried (magenta dots) solar cells. One sun data corresponds to the characteristics shown in Figure 1. The uncertainty of the measured values of the total charge is estimated to be 10% and is mainly introduced by fluctuations in the laser intensities.

measurements. The photocurrent transients and the corresponding fits are presented in the Supporting Information, exemplary for the 200 °C-dried solar cells (Figure S3). We note that this method can in general not assign which carrier type is the faster one. Furthermore, the uncertainty of the slower charge carrier type is relatively large due to the strong dispersion of the transients. However, by comparison of the TOF and the TDCF mobility data, we are able to allocate the higher and the lower mobility values extracted from the TDCF photocurrents to the electrons and holes, respectively, and to exclude major mobility differences due to the thickness variation between TOF and TDCF samples. Figure 3 displays the mobilities obtained from both techniques. For the 200 °C-dried device (Figure 3, left, symbols in magenta), TDCF transients have been evaluated for several electric fields, and the resulting mobilities are comparable to the TOF mobility values. Furthermore, TOF mobilities for the RT-dried solar cell system are displayed in Figure 3 (right) and again are confirmed by TDCF transient simulations for high and low electric fields. In both blend systems, a high electron mobility of μe ≥ 3 × 10−3 cm2 V−1 s−1 is found, which is close to the bulk mobility determined for pure P(NDI2OD-T2) films.10,19,23 Neither the blending with P3HT nor the different preparation methods significantly distorts the electron transport in P(NDI2OD-T2). Besides the excellent electron transport, high hole mobilities are obtained, which reach approximately 1 × 10−3 cm2 V−1 s−1 and 2 × 10−3 cm2 V−1 s−1 in the 200 °C- and RT-dried blend, respectively. Similar hole mobility values have been reported for annealed P3HT:PCBM blends and were related to a well ordered P3HT phase.28,40,41 To summarize, we find high mobilities for both charge carrier types and for both drying conditions with slightly higher mobilities in the RT-dried sample. These findings rule out the idea that the difference in FF arises from a significantly altered charge transport, which is even emphasized by measuring higher mobilities in the solar cell with the lower FF. A characteristic in several of the high-performance, fullerenebased solar cells is a suppressed recombination between free electrons and holes.28,42,43 Bimolecular recombination coefficients as low as 10−19 m3 s−1 to 10−18 m3 s−1 have been determined for blends of P3HT and PCBM.28,43−45 Interestingly, this coefficient is by a factor of 100 to 1000 lower than the Langevin recombination coefficient, calculated from the

Free charge carrier generation is found to be always more efficient for the 200 °C-dried cell, rationalizing the higher JSC measured under steady-state. Furthermore, we observe that the total amount of charges extracted with the TDCF setup under pulsed excitation scales in the same way as the steady-state current density at high reverse biases, measured with a sun simulator, demonstrating that TDCF measurements are able to capture the physics of the charge generation process correctly. Figure 2 further reveals that Qtot for either cell does not depend on the applied prebias but stays constant over the entire range of internal fields investigated. The field-independent charge carrier generation is consistent with the high FF observed in the 200 °C-dried solar cell systems. However, the fact that the RTdried solar cell also shows field-independent charge carrier generation reveals that the difference in FF does not stem from a change of the generation mechanism. This is in strong contrast to many other polymer−polymer blends, where low FFs have been attributed to a strongly field-dependent free charge carrier generation, e.g., for systems with MDMO-PPV as donor or F8TBT as acceptor polymer.34−36 Low charge carrier mobilities can have a severe influence on the fill factor, as this leads to the build-up of space charges and increased bimolecular recombination.3,37−39 To determine the electron and hole mobilities, we performed TOF measurements on 5 μm thick blend layers. In contrast to TDCF, TOF offers the advantage to selectively determine electron and hole mobilities. A description of the sample preparation and device parameters as well as corresponding TOF transients for both charge carrier types is presented in the Supporting Information (Figure S4). The mobility values, extracted for different applied fields, are shown in Figure 3. The shape of the photocurrent transients obtained from TDCF measurements has been analyzed with a drift diffusion model to extract the mobility of the faster and slower carriers for the actual solar cell devices, which require significantly thinner films than the TOF 2817

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of free and uncorrelated charge carriers. We were able to iteratively fit all the measured values of Qcoll for the different devices and prebiases with a recombination order of 2 according to Kniepert et al. and Albrecht et al.28,32 Therefore, we measure the bimolecular recombination in the devices, and we can extract the bimolecular recombination coefficient κBMR from the fitting model. Figure 4 (left) shows the BMR fit to the measured charge as an example for a prebias of 0.6 V. The corresponding bimolecular recombination coefficients for the RT- and the 200 °C-dried solar cells are displayed in Figure 4 (right). The κBMR shows no field dependence for both temperature treatments, and the averaged values are (5.4 ± 1.6) × 10−18 m3 s−1 for the 200 °C-dried solar cell and (6.1 ± 1.7) × 10−17 m3 s−1 for the RT-dried one. The BMR-coefficient measured for the optimized solar cell is notably low, comparable to the coefficient reported for optimized P3HT:PCBM blends that also show high FFs over 60%.28 This low value of κBMR for the 200 °C dried solar cell and the concomitant slow free carrier recombination is fully consistent with the observed high FF for this system. Interestingly, the RT drying procedure increases the BMR by one order of magnitude, although it has little effect on the electron and hole mobility. In both cases, κBMR is strongly reduced compared to the theoretical Langevin-type recombination coefficient, the latter being given by κL = e (μe+μh)/ε. Using the measured bulk mobilities (Figure 3), κL is calculated to be ca. 3 × 10−15 m3 s−1 and 6 × 10−15 m3 s−1 for the 200 °C-dried and the RT-dried solar cells, respectively. Strongly suppressed recombination has been frequently reported in fullerene-based blends and represented by the reduction factor ζ, being defined as the ratio of the measured BMR-coefficient to the Langevin coefficient. For the 200 °C-dried sample, we find a ζ of ca. 0.002, which is among the smallest values ever reported in the literature for organic donor−acceptor blends. Apparently, highly suppressed nongeminate recombination is not limited to fullerene-containing blends but is a more general phenomenon. For the RT-dried cell, ζ is calculated to be 0.015, ca. 1 order of magnitude higher than for the high temperature dried sample (see Figure S7). It has been proposed that ζ is related to morphological parameters such as domain size and purity.44,49 In fact, we observe distinct differences in morphology as a result of the different drying conditions. Figure 5 compares the topography

charge carrier mobilities and which has been shown to determine the recombination rate of free charges in pristine organic semiconductors. Despite the importance this effect has on the recombination processes in bulk heterojunction solar cells, the origin of the discrepancy between measured and theoretical recombination coefficients in donor/acceptor blends is still unclear and under debate.46,47 In order to investigate the recombination kinetics in our blends, nongeminate recombination coefficients κ are determined by measuring the amount of charge left in the device after a given delay time (Qcoll). This analysis also takes into account the charge leaving the device during prebias (Qpre) (before the extracting pulse inserts). κ is obtained by an iterative calculation of Qcoll as a function of the delay time tdel,32,48 which is varied from 20 ns to 10 μs. Photocurrents were measured for a fixed collection voltage (Vcoll = −5 V) and for prebiases ranging from 0.1 to 0.6 V. The increase in Qpre with delay time for two different prebiases is displayed in Figure S6. The stronger increase in Qpre with delay time for lower prebiases reflects the enhanced extraction of charge carriers due to the increased internal electric field. The charges leaving the device after the delay time (Qcoll), due to the applied collection voltage, sum up with the precharge (Qpre) to the total charge (Qtot). A decrease in total charge with delay time, as it is apparent in Figure 4 (left), is indicative for the recombination

Figure 4. (left) Delay dependent total, collection, and pre-charge for a pre bias of 0.6 V. The recombination coefficient has been extracted from the BMR fit (dashed line). (right) BMR coefficients for the vacuum-dried and the 200 °C dried solar cells for different pre biases. Error bars were obtained according to procedure described in the Supporting Information (Figure S5).

Figure 5. AFM topography of 200 °C-dried (left) and RT-dried (middle) P3HT:P(NDI2OD-T2) blend films. Both images show an area of 10 × 10 μm. (right) R-SoXS scattering profiles for the RT- (cyan) and the 200 °C- (magenta) dried films. The pictures sketch a cross-section of the films indicating the domain size and purity of the RT- and 200 °C-dried films, respectively. 2818

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of both films measured by atomic force microscopy (AFM). The RT dried film comprises micrometer large, irregularly shaped domains and a very large root-mean-square surface roughness of about 35 nm. In contrast, the 200 °C-dried film is much smoother with a roughness of only about 4 nm and also exhibits a much finer domain structure. Although these images do not provide chemical contrast, it is apparent that the different drying procedure introduced a large-scale phase separation between both polymers. We also note that we do not observe the typical surface texture of pure P(NDI2OD-T2) in any of the investigated films,13,15 which has been previously connected to a vertical phase separation in blends of P3HT and P(NDI2OD-T2).13,50 This means that phase separations occur predominantly in lateral direction in films prepared from xylene:CN, as we have recently demonstrated for the 200 °C dried film.25 To analyze the microstructure of both blends, resonant soft X-ray scattering (R-SoXS) and photoluminescence quenching experiments were conducted. R-SoXS scattering profiles were acquired at energies of 270 and 285.3 eV, where scattering primarily originates from roughness/mass−thickness and chemical composition, respectively. In a recent study, we demonstrated that the optimized, 200 °C-dried blends, are composed of pure P(NDI2OD-T2) domains, which are embedded in a rather impure matrix of P3HT.25 The R-SoXS profiles in Figure 5 can be interpreted in a similar fashion. We previously showed that the domains have a characteristic size of about 70 nm, separated by a distance of about 120 nm.25 On the other hand, the dramatic rise in scattering intensity below q ≈ 0.01 nm−1 indicate that the RT-dried films exhibit much larger domains in the order of micrometers (see Figure 5 right), consistent with the observed topographic features (Figure 5 middle). The coalescence of the nonresonant and resonant scattering profiles in this region indicates that much of the scattering comes from this topological roughness and that the roughness and composition variations are correlated. The scattering feature at q ≈ 0.2 nm−1 exists in both profiles and arises from the presence of polymer crystals with its intensity suppressed in the RT-dried sample. Besides the information on the domain size, the total scattering intensity (TSI) can be estimated by integration of the scattering profiles over the q-range with the results in Table 1

and P(NDI2OD-T2)-rich phases.51 These findings are also supported by photoluminescence quenching efficiency (PQE) measurements, which probe the yield of excitons that are quenched at a discrete heterojunction. The complementary absorbance of both polymers allows us to determine the PQE for the donor and for the acceptor separately. Experimental details and an estimation of the error made by evaluating the photoluminescence spectra can be found in the Supporting Information. Table 1 lists the PQE of P3HT and P(NDI2ODT2) in both blends. We find that the PQE values are similar for the 200 °C- and RT-dried blends, showing that about 95% of the excitons generated on P3HT are quenched at a heterojunction. The lower PQE of the acceptor of roughly 55% is a result of the exciton trapping on aggregated or crystalline phases within the P(NDI2OD-T2) domains.25 Thus, despite the large difference in the domain size, the similar PQE values again highlight the lower purity of the RT-dried blend, fully consistent with the reduced TSI obtained from the RSoXS experiment. By combining the information on the nanomorphology with that of the charge transport and recombination properties, we now aim to explain the different recombination kinetics in this all-polymer solar cell system. Given that we do not observe a significant difference in the bulk mobility, we conclude that the increased bimolecular recombination coefficient of the RTdried blend is a result of the lower domain purity. More generally, our results imply that the phase separation between donor and acceptor components is the origin of the reduced recombination. This is of particular importance for fullerenecontaining photovoltaic blends, as has been pointed out before,52,53 due to the ability of fullerenes to molecularly intermix and even intercalate with common donor polymers.51,54−56 We conclude that the bimolecular recombination can be strongly suppressed if an expedient nanomorphology is reached, which implies for this all-polymer system that fine intermixing of the acceptor polymers with the donor component can be prevented, and the domain purity is high. To conclude, the charge carrier dynamics of the polymer− polymer solar cell system consisting of P3HT and P(NDI2ODT2) was investigated with regard to the high FF of this model all-polymer solar cell system. Extraordinary reduced BMR coefficients are presented for an all-polymer solar cell system. Furthermore, high electron and hole mobilities comparable to values for highly efficient polymer−fullerene based solar cells could be measured, highlighting that efficient charge carrier transport can be realized in fullerene-free solar cells. Independent of the drying procedure, the free charge carrier generation is field independent but, in general, too low to enable high currents and efficiencies. By analyzing the domain structure and purity, we were also able to disclose the connection between domain purity and recombination kinetics, where we observe a decreased recombination rate with higher domain purity. These findings are consistent with Monte Carlo simulations by Lyons et al. connecting OSC performance with domain purity.49 Nevertheless, for the RT-dried solar cell, the recombination is still reduced compared to Langevinrecombination, depicting the remarkable charge carrier transport properties of this particular all-polymer solar cell system. Finally, we present a comprehensive picture of the charge transport and recombination properties of this particular fullerene-free organic solar cell system. Although many data exist for the pure acceptor P(NDI2OD-T2), this is one of few studies that address the transport properties of the acceptor

Table 1. Photoluminescence Quenching Efficiencies for the Donor and the Acceptor Polymer in the Blenda PQE

RT-dried

200 °C-dried

P3HT P(NDI2OD-T2) TSI

(95 ± 1) % (55 ± 5) % 0.32(3)

(96 ± 1) % (55 ± 5) % 1.00

The quenching data for the 200 °C-dried films are also part of our former study.25 Bottom row are the TSI values from the resonant scattering profiles (285.3 eV) with error bars derived from the uncertainty in the power law extensions to high-Q. a

normalized to that of the 200 °C-dried sample. At the resonant energy of 285.3 eV, the TSI is sensitive to domain contrast and with that, domain purity.51 Scattering from roughness is also reduced at this energy relative to nonresonant energies like 270 eV. At resonance, the TSI of the 200 °C-dried blend is about 3 times larger compared to that of the RT-dried sample. This indicates much purer domains, as the TSI is proportional to the square of the composition difference between the P3HT-rich 2819

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polymer in mixture with the donor polymer P3HT. These findings are in good agreement with the work by Fabiano et al., who reported improved bulk mobilities in P3HT:P(NDI2ODT2) films cast from xylene and chloronaphthalene, which they attributed to better intermixed donor/acceptor domains in comparison to films from other solvents.50 We were able to investigate this favorable morphology in more detail and furthermore relate domain purity to the bimolecular recombination losses. On the basis of our results, the high FFs of the optimized blend, which are the most interesting feature of the P(NDI2OD-T2)-based solar cells, are in good agreement with a severely decreased recombination of free charge carriers, a field independent generation of charge carriers, and high electron and hole bulk mobilities. These properties have so far been demonstrated only for fullerene-derivatives. This highlights the great potential of special polymeric acceptors, such as P(NDI2OD-T2), for replacing fullerenes in high-performance organic solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

Details on materials and device preparation. Details on TDCF, TOF, JV, R-SoXS, and PL-Quenching measurement techniques. Absorption spectra of the used polymers. Total extracted charge as a function of the pulse fluence. TDCF transient simulations. TOF transients. Estimated error for the BMR coefficient. Extracted precharge as a function of the delay time for two different pre biases. Reduction factor for the differently dried samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Antonio Facchetti and Zhihua Chen are employees of Polyera Corporation, which produces P(NDI2OD-T2). The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Federal Ministry of Education and Research (BMBF) within the project PVcomB (FKZ 03| S2151D) and by the German Science Foundation (DFG) within the priority program SPP 1355. X-ray characterization by Brian A. Collins while at NCSU and Harald Ade was supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under Contract DE-FG02-98ER45737. R-SoXS data was acquired at beamline 11.0.1.2 at the ALS,57 a National user facility supported by the U.S. Department of Energy (DEAC02-05CH1123).

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REFERENCES

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