Codependence between Crystalline and Photovoltage Evolutions in

Letter www.acsami.org

Codependence between Crystalline and Photovoltage Evolutions in P3HT:PCBM Solar Cells Probed with in-Operando GIWAXS Daniel Moseguí González,† Christoph J. Schaffer,† Stephan Pröller,‡ Johannes Schlipf,† Lin Song,† Sigrid Bernstorff,§ Eva M. Herzig,‡ and Peter Müller-Buschbaum*,† †

Lehrstuhl für Funktionelle Materialien, Physik-Department, Technische Universität München, James-Franck-Strasse 1, 85748 Garching, Germany ‡ Herzig Group, Munich School of Engineering, Technische Universität München, Lichtenbergstrasse 4, 85748 Garching, Germany § Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14−km 163.5 in AREA Science Park, Basovizza, 34149 Trieste, Italy S Supporting Information *

ABSTRACT: We address the correlation between the crystalline state of photoactive materials in a model organic solar cell based on poly(3-hexylthiophene-2,5diyl):phenyl-C60-butyric acid methyl ester (P3HT:PCBM) and the photovoltage in an in-operando investigation. I−V curves are simultaneously measured together with grazing incidence wide-angle X-ray scattering probing the crystalline state of the device active layer as a function of the operation time. The results show a high degree of correlation between open-circuit voltage VOC and the crystalline state of P3HT.

KEYWORDS: organic photovoltaics, GIWAXS, in-operando, nanomorphology, crystallinity

P

levels. Given the semicrystalline nature of polymers, which can only aggregate partially, HOMO and LUMO levels are described as Gaussian distributions. Figure 1 displays the distributions of the HOMO and LUMO DOS (blue lines). These locally different HOMO and LUMO levels are a consequence of partial aggregation, unequal conjugation lengths, chemical defects, etc. This feature is normally referred to as energetic or Gaussian disorders and it is parametrized by the standard deviation (σn and σp) of the energy distribution.5,8 The occupation of the DOS is of special relevance when the solar device is brought out of equilibrium, i.e., illuminated (blue/gray areas in Figure 1). At standard illumination conditions, the concentration of photogenerated charge carriers lies about 2 orders of magnitude below the amount of available states.5 Thereby, the mean positions of the occupied states are closer to each other than the HOMO and LUMO mean values.5 Moreover, the bimolecular nature of some recombination processes prevents the occupation of upper states despite higher irradiation. One example is the nongeminate recombination, whose recombination rate increases with charge carrier density.11,12 These generally low occupancies restrict the charge carriers basically to the DOS tails. This shifts the (positive/ negative) polaron quasi-Fermi levels toward each other

resently, organic solar cells (OSCs) have reached power conversion efficiencies (PCE) exceeding 10%.1,2 In spite of many promising properties, like internal quantum efficiencies approaching 100% or short-circuit current densities comparable to inorganic devices, OSCs still do not reach PCEs comparable to inorganic technologies.3 Main reasons behind that are displayed by lower fill factors (FF) and smaller open-circuit voltages (VOC). VOC results from a complex interplay of system parameters like recombination rates, density-of-states (DOS) shape, charge carrier and exciton mobility, etc in the OSCs.4−6 In its description, the crystalline state of the active layer is of special relevance.7−9 Vandewal et al. reported links between the charge transfer energy (and therefore VOC) and the crystalline fraction in solution of the photoactive materials.10 Generally, although many of the above-mentioned features have been investigated separately, an unanimous evidence of the codependency of crystallinity with the photovoltage is still missing. Generally, VOC is primarily understood as being determined by the energy of the optical bandgap (energy difference between the donor HOMO and the acceptor LUMO), because the two magnitudes depict a linear relation. However, over time, corrections have been added to improve the energetic description of the VOC. The scaling of VOC with the energy of the optical bandgap suggests that the recombination mainly takes place at the donor/acceptor (D/A) interface. However, the HOMO of the donor and the LUMO of the acceptor are not sharp energy © XXXX American Chemical Society

Received: December 6, 2016 Accepted: January 17, 2017 Published: January 17, 2017 A

DOI: 10.1021/acsami.6b15661 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

thereby yielding scattering signals statistically averaged over large sample areas comparable to common active areas of OSCs. It allows for the determination of lattice distances and grain sizes. In combination with the high X-ray flux available at a synchrotron radiation facility, it allows for in-operando timeresolved experiments and, hence, for real-time tracking of physical processes governing film and/or device evolution.19,20 A more detailed description of the GIWAXS technique and its advantages can be found elsewhere.21 The OSCs are placed inside a custom-made in-operando chamber that provides paths for the X-ray exposure and signal collection on the detector, as well as for the illumination with simulated sun light from a solar simulator positioned below. Simultaneously GIWAXS data displaying the subnanometer crystalline sample structure and current−voltage (I−V) characteristics are collected (see Supporting Information SI.3). The evolution of the solar cell characteristics is tracked under ambient working conditions, i.e., under air atmosphere with a monitored temperature evolution to enable degradation on time scales reasonable for a synchrotron experiment. Given the impact of temperature on the charge carrier statistics, the evolution of the device temperature is monitored in order to allow comparison with OSC indicators, like the FF. The film temperature increases mildly from about 26 to 32 °C during the experiments (see Supporting Information SI.2 for further details) because of illumination with the solar simulator. In a control experiment the in-operando study is repeated with neither illumination, nor with I−V sweeps. Aim of this control experiment is to see which changes are related to the functioning of the OSC during illumination, and to ensure the absence of radiation damage. The X-ray probing is discontinuous and data acquisition times and times with closed X-ray shutter are optimized in test experiments to avoid any potential beam damage (see Supporting Information SI.6 for further details). Measurements are performed at one spot on the sample to ensure that changes in crystallinity are not related to local heterogeneities of the film. For the studied OSCs a strong P3HT (100) Bragg peak is found in the GIWAXS data. PCBM forms crystallites without preferred orientation, which do not undergo changes. Tracking of the P3HT (100) family allows a crystal structure parametrization. The P3HT (100) family is one of the most prominent crystallite families in P3HT thin films, indicating edge-on orientation being dominant in the samples. Crystallites with a face-on orientation are rarer as can be seen from the 20times weaker (020) Bragg peak. Although face-on orientation is assumed to be beneficial for OSC applications,22,23 the use of high-boiling-point solvents typically results in a dominant edgeon orientation.24 It provides a strong indicator for the ordering of P3HT in the P3HT:PCBM blend forming the active layer.21 The [100] lattice vector in a P3HT crystal corresponds to the spacing between neighboring P3HT backbones along the side chains. In the edge-on configuration the P3HT side chains are oriented perpendicular to the surface of the sample substrate (see inset in Figure 2a). More detailed information on the morphology and orientation of P3HT crystals can be found in the Supporting Information and in the literature.21 Figure 2 compares the temporal evolution of the Gaussian parameters extracted from the fitting of the (100) P3HT Bragg peaks of the in-operando study in case of device illumination and in the dark (control experiment). In the initial stages of solar cell operation (first 15 min) with illumination, the edgeon (100) crystalline signal becomes stronger in intensity,

Figure 1. Schematic drawing of a BHJ’s optical bandgap (acceptor HOMO and donor LUMO Gaussian DOS, blue lines) and their corresponding occupied states (colored areas). DOS have certain energy disorders indicated by the deviations σn and σp. The occupied states at both energy bands are determined by equilibrium between photogeneration and recombination. The resulting photovoltage is determined by the splitting between the pseudo-Fermi levels EFn and EFp. The offset energy between the device VOC and the optical bandgap is Δ = Δn + Δp. Dotted lines within the bandgap indicate trap states. Figure adapted with permission from ref 5. Copyright 2010 AIP publishing.

(indicated in Figure 1 by the energy offsets Δn and Δp). As a result, the device VOC appears lower than the voltage associated with the optical bandgap, resulting in an offset between the two magnitudes (compare the optical bandgap with −qVOC in Figure 1). A detailed discussion on the effect of the Gaussian disorder on the VOC can be found elsewhere.5,13 Besides Gaussian disorder, other effects, such as trap-assisted recombination, failure of electrodes, etc. contribute as well to a lowered VOC.14,15 We see how VOC scales with the splitting of the quasi-Fermi levels. The relation between the generation and recombination rates defines the occupancy levels and therefore the extracted device VOC and its offset compared to the optical bandgap. It is important to emphasize that the dynamics of exciton and charge carrier transport and recombination, as well as the DOS shape are closely related to the crystalline arrangement of the materials within the bulk heterojunction (BHJ).7,8,16−18 The degree of crystallinity, as well as crystal sizes and lattice parameters, have a direct impact on different properties such as conjugation length uniformity (and consequentially, DOS shape), charge and exciton transport and thereby recombination rates, etc.6 Thus, although the link between VOC and crystalline state in that way seems very consistent in theory, direct experimental evidence is still missing. In the present work, we close this gap. For the first time, we observe the correlation between OSC crystalline state and VOC in an in-operando investigation of a model OSC based on poly(3-hexylthiophene-2,5-diyl):phenyl-C60-butyric acid methyl ester (P3HT:PCBM). The evolution of the photovoltaic characteristics is simultaneously probed as a function of time along with the crystalline state of P3HT within the active layer during operation. The results show a high degree of correlation and evidence codependence between VOC and the crystalline state of P3HT. We probe the crystalline state of the active layer with grazing incidence wide-angle X-ray scattering (GIWAXS). GIWAXS allows for the characterization of thin films via an X-ray beam impinging on the sample under a shallow incident angle, B


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progressive reduction of crystallite size. Again, in the control experiment the FWHM values remain constant within the experimental errors. The absolute value of the (100) lattice vector stays constant throughout the whole experiment independent of illumination, indicating that the average [100] lattice distance remains unchanged (see Figure 2b). Figure 3a, b show the time evolution of the solar cell characteristics. The power conversion efficiency (PCE) decays as a function of time, mainly governed by the decay in shortcircuit current-density (JSC). The decay of the JSC in OSCs in time has been mainly attributed to physical rearrangement of the BHJ within the active layer and failure of the device electrodes if in contact with oxygen-rich atmospheres.14,25 Both contributions to the decay of JSC are supported by the evolution of the series resistance (RS), which also increases with time (Figure 3b). Moreover, Deschler et al. showed that shorter conjugation lengths and lower chain planarity stemming from photodegradation associated with long exposures to sunlight leads to higher triplet yield, as well as to higher exciton and polaron trapping due to products of the photodegradation, having as well a negative impact on the JSC.26 In the early stages of OSC illumination, the FF increases by about 10−15% of its initial value, indicating a lower recombination compared to the initial stage. This feature is also supported by the increasing shunt resistance (RSH).27 The evolution of VOC roughly depicts a linear decay to 93% of its initial value after 20 000 s. However, a closer look on the temporal evolution of VOC shows a close correlation with the evolution of the GIWAXS signals (see Figure 3c and Figure 3d). Figure 3c displays the evolution of the P3HT (100) Bragg peak intensity along with that of the VOC. In the beginning, both increase with respect to their initial values before turning to a linear decay presenting the same time scales. The observed codependence between VOC and crystal state is further supported by the results displayed in Figure 3d, which displays the evolution of device VOC along with P3HT average crystalline grain size, estimated via the FWHM values obtained from the edge on (100) intensities. Zimmerman et al. presented an investigation seeking for the optimum BHJ morphological configuration. According to them, the most favorable morphology consists of a disordered D/A interface along with a highly crystalline domain bulk. This arrangement hinders the D/A orbital overlap and, therefore, also the recombination (increasing device RSH), whereas the highly crystalline bulk provides efficient exciton and charge carrier transport. This configuration yielded the best VOC values.6 Other studies conducted ex-situ, report links between the degree of crystallinity and the charge carrier mobility.7 In agreement with the work presented here, the codependence between the crystal state and energy disorder has been reported.8,16 In the following we discuss the evolution of the OSC crystal state and its link with VOC, according to our in-operando observations. In a first stage the recombination in the device decreases, partially due to the increasing temperature in the beginning of the experiment caused by the illumination (see Supporting Information SI.2). This is indicated by the evolution of the FF, which presents a similar time profile as the temperature. Because of the lowered recombination, the occupancy of the DOSs increases, separating the quasi-Fermi levels from each other, and thereby contributing to the initial increase in device VOC. At the same time, the increasing temperature enhances chain mobility in the polymer phase,

Figure 2. Time evolution of the edge-on P3HT crystalline family. As a function of time, for the P3HT (100) Bragg peak the (a) peak intensity, (b) peak center position, and (c) peak FWHM are displayed. Y-axes display the changes relative to the initial values. Black data points correspond to the GIWAXS measurements under solar illumination, whereas red data points correspond to a control measurement in the dark. The inset in “a” depicts the position of an edge-on oriented P3HT crystal with respect to the substrate, which is indicated by the gray surface underneath.

indicating an increase in the total degree of crystallization (Figure 2a). At the end of this first stage, the total amount of crystallized P3HT in the OSC reaches a maximum. Beyond this point, the process reverses, indicating a partial loss of the crystalline phase due to illumination. This second stage is defined by the linear decay of the (100) intensity. In contrast, in the control experiment the intensity remains basically constant over time. The evolution of the full-width-halfmaximum (FWHM) values fits the evolution of the (100) intensities (Figure 2c). In the initial stage, the FWHM values of the measurement performed with solar illumination decrease, indicating an increase in the average P3HT crystalline grain size. After reaching a minimum (same time scale) the process reverts and the FWHM values start increasing, indicating the C


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Figure 3. Time evolution of (a) power conversion efficiency (PCE), fill factor (FF), short-circuit-current density (JSC), open-circuit-voltage (VOC), (b) series (RS) and shunt resistances (RSH). Ordinate axes represent the variations normalized to the initial values. (c) Time comparison of the P3HT (100) edge-on intensity vs VOC and (d) estimated crystalline grain size along the crystal direction (100) vs VOC. The red solid line refers to VOC (right axes) and black data points refer to intensity or crystalline grain sizes (left axes).

enabling relaxation toward equilibrium of the system, similarly to the process that occurs during thermal annealing. Thereby, the packing of P3HT chains increases and results in an increased volume fraction of [100]-edge-on P3HT, as depicted by the corresponding increasing intensity (Figure 2a). In addition, the enhanced degree of crystallinity is also displayed by the increased crystallite grain size and the lowered (100) FWHM values, which eventually also indicates a more even and extended distribution of backbone spacing, eventually resulting in narrower HOMO and LUMO distributions as reported.28,29 At some point, the system reaches maximum crystallinity. After the temperature stabilizes the process reverts. The longtime exposure to light and temperature triggers thermal stress and degradation mechanisms like oxidation, photobleaching, etc. that negatively impact on the crystallinity, as systematically proven by Hintz et al.30 The enhanced mobility of P3HT segments due to long-time exposure to increased temperatures and light makes crystalline domains unstable, inducing a “disintegration” of the P3HT crystallites.29 This is displayed by the decrease in (100) peak intensity as well as by the increasing FWHM values, depicting a reducing population of P3HT crystallites as well as a decrease in the average grain size. Motaung et al. reported the same behavior in P3HT:PCBM samples. After an initial increase in the degree of crystallinity, a trend reversion followed.29 In late stages, both Raman and XRD investigations supported the formation of disordered and noncrystalline material, accompanied by a reduction in the conjugation length. This last phenomenon has been also thoroughly observed in UV/vis measurements on P3HT samples. For long annealing times samples showed a blueshift, indicating a decrease in conjugation length for weakly coupled H-aggregates, like P3HT.31,32 According to Raman spectroscopy investigations, the main P3HT degradation pathways feature the modification of chemical structure of chromophores and detachment of light units such as C−S units or P3HT hexyl side chains.33

The fading of crystallites increases the energetic irregularity of the photoactive medium. This induces the appearance and broadening of DOS tail states, reducing the quasi-Fermi levels splitting. After long illumination times the splitting is further reduced by photobleaching, reducing the absorption of the active materials and, thereby, the concentration of photogenerated charge carriers.30 These effects play along with the physical reorganization of the BHJ, which reduces the D/A interface and alongside the likelihood of exciton splitting,25 increasing geminate recombination and contributing to a VOC reduction. We argue that the evolution of the crystalline state results from the competition of two mechanisms. On the one hand, an enhanced crystallinity arising in the early stages due to increased temperatures provided by the sunlight. On the other hand, radiation-induced degradation mechanisms lead to a decrease of crystallinity. The competition of these two effects drives, in turn, the evolution of the device VOC, given the influence of the crystallinity on DOS sharpness, transport properties and recombination dynamics. The effect of the active layer morphology on the aging of the photovoltaic parameters was already reported. However, so far it was difficult to explain especially the mechanisms behind the behavior of the VOC in the initial “burn-in” phase and VOC was typically assumed to be constant. Some comprehensive models are already available for parametrizing the fine structure of the burn-in phase and the aging behavior with reasonable success.34 In this respect, the results presented in this work add a further step in the understanding of the VOC evolution. Overall, innovative contributions to the understanding of the link between crystallinity and VOC are being made, and some groups even reported successful approaches involving ternary active layers that enable tailoring of the main photovoltaic parameters, including VOC, through fine control of aggregation, packing distances and crystallite orientations.35 In conclusion, we present the first direct evidence of the link between the crystalline state of the active layer of an OSC and D


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(5) Garcia-Belmonte, G.; Bisquert, J. Open-Circuit Voltage Limit Caused by Recombination through Tail States in Bulk Heterojunction Polymer-Fullerene Solar Cells. Appl. Phys. Lett. 2010, 96, 113301. (6) Zimmerman, J. D.; Xiao, X.; Renshaw, C. K.; Wang, S.; Diev, V. V.; Thompson, M. E.; Forrest, S. R. Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells. Nano Lett. 2012, 12 (8), 4366−4371. (7) Vanlaeke, P.; Swinnen, A.; Haeldermans, I.; Vanhoyland, G.; Aernouts, T.; Cheyns, D.; Deibel, C.; D’Haen, J.; Heremans, P.; Poortmans, J.; Manca, J. V. P3HT/PCBM Bulk Heterojunction Solar Cells: Relation between Morphology and Electro-Optical Characteristics. Sol. Energy Mater. Sol. Cells 2006, 90 (14), 2150−2158. (8) Spano, F. C. Modeling Disorder in Polymer Aggregates: The Optical Spectroscopy of Regioregular Poly(3-hexylthiophene) Thin Films. J. Chem. Phys. 2005, 122 (23), 234701. (9) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. On the Origin of the Open-Circuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8 (11), 904−909. (10) Vandewal, K.; Oosterbaan, W. D.; Bertho, S.; Vrindts, V.; Gadisa, A.; Lutsen, L.; Vanderzande, D.; Manca, J. V. Varying Polymer Crystallinity in Nanofiber Poly(3-alkylthiophene):PCBM Solar Cells: Influence on Charge-Transfer State Energy and Open-Circuit Voltage. Appl. Phys. Lett. 2009, 95 (12), 123303. (11) Foertig, A.; Wagenpfahl, A.; Gerbich, T.; Cheyns, D.; Dyakonov, V.; Deibel, C. Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells. Adv. Energy Mater. 2012, 2 (12), 1483−1489. (12) Garcia-Belmonte, G.; Boix, P. P.; Bisquert, J.; Sessolo, M.; Bolink, H. J. Simultaneous Determination of Carrier Lifetime and Electron Density-of-States in P3HT:PCBM Organic Solar Cells under Illumination by Impedance Spectroscopy. Sol. Energy Mater. Sol. Cells 2010, 94 (2), 366−375. (13) Shuttle, C. G.; Treat, N. D.; Douglas, J. D.; Fréchet, J. M. J.; Chabinyc, M. L. Deep Energetic Trap States in Organic Photovoltaic Devices. Adv. Energy Mater. 2012, 2 (1), 111−119. (14) Voroshazi, E.; Verreet, B.; Aernouts, T.; Heremans, P. Longterm Operational Lifetime and Degradation Analysis of P3HT:PCBM Photovoltaic Cells. Sol. Energy Mater. Sol. Cells 2011, 95 (5), 1303− 1307. (15) Reese, M. O.; Morfa, A. J.; White, M. S.; Kopidakis, N.; Shaheen, S. E.; Rumbles, G.; Ginley, D. S. Pathways for the Degradation of Organic Photovoltaic P3HT:PCBM based Devices. Sol. Energy Mater. Sol. Cells 2008, 92 (7), 746−752. (16) Spano, F. C. Absorption in Regio-Regular Poly(3-hexyl)thiophene Thin Films: Fermi Resonances, Interband Coupling and Disorder. Chem. Phys. 2006, 325 (1), 22−35. (17) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Molecular and Mophological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices. J. Am. Chem. Soc. 2009, 131 (26), 9281−9286. (18) Credgington, D.; Hamilton, R.; Atienzar, P.; Nelson, J.; Durrant, J. R. Non-Geminate Recombination as the Primary Determinant of Open-Circuit Voltage in Polythiophene:Fullerene Blend Solar Cells: an Analysis of the Influence of Device Processing Conditions. Adv. Funct. Mater. 2011, 21 (14), 2744−2753. (19) Palumbiny, C. M.; Liu, F.; Russell, T. P.; Hexemer, A.; Wang, C.; Müller-Buschbaum, P. The Crystallization of PEDOT:PSS Polymeric Electrodes Probed In Situ during Printing. Adv. Mater. 2015, 27 (22), 3391−3397. (20) Pröller, S.; Liu, F.; Zhu, C.; Wang, C.; Russell, T. P.; Hexemer, A.; Müller-Buschbaum, P.; Herzig, E. M. Following the Morphology Formation In Situ in Printed Active Layers for Organic Solar Cells. Adv. Energy Mater. 2016, 6 (1), 1501580. (21) Müller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26 (46), 7692−7709. (22) Zhao, G.; He, Y.; Li, Y. 6.5% Efficiency of Polymer Solar Cells Based on poly(3-hexylthiophene) and Indene-C60 Bisadduct by Device Optimization. Adv. Mater. 2010, 22, 4355−4358.

its photovoltage using a novel in-operando combination of GIWAXS and I−V tracking. Our observed behavior can be understood in terms of different bulk-heterojunction properties, such as recombination rate, exciton and charge carrier transport, or energetic disorder. The presented technical breakthrough combined with synchrotron radiation enabled time resolution allows for first direct experimental evidence. In future work we can apply this approach to other OSC systems. Thus, in-operando characterizations in controlled sample environments will open the door to gain a better understanding of the influence of particular conditions such as temperature, pressure, or moisture on OSC performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15661. Materials, sample preparation, and experimental details; details on the analysis of the GIWAXS data, evolution of the temperature and the OSC working conditions, details of the in-operando chamber and the measurement setup, initial OSC performance and time evolution of the FF (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daniel Moseguí González: 0000-0002-8397-8834 Sigrid Bernstorff: 0000-0001-6451-5159 Peter Müller-Buschbaum: 0000-0002-9566-6088 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by TUM.solar in the frame of the Bavarian Collaborative Research Project “Solar technologies go Hybrid” (SolTec), by the GreenTech Initiative (Interface Science for Photovoltaics - ISPV) of the EuroTech Universities and by the Nanosystems Initiative Munich (NIM) is acknowledged. L.S. acknowledges the China Scholarship Council (CSC). S.P. and E.M.H. are supported by the Bavarian State Ministry of Education, Science and the Arts via the project “Energy Valley Bavaria”.

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REFERENCES

(1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 48). Prog. Photovoltaics 2016, 24 (7), 905−913. (2) Heliatek Press Release, http://www.heliatek.com/en/press/ press-releases/details/heliatek-consolidates-its-technology-leadershipby-establishing-a-new-world-record-for-organic-solar-technology-witha-cell-effi, accessed October 2016. (3) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3 (5), 297−302. (4) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in BulkHeterojunction Solar Cells − Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18 (6), 789−794. E


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Codependence between Crystalline and Photovoltage Evolutions in

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