Oligomers Comprising 2-Phenyl-2H-benzotriazole Building Blocks for

Article pubs.acs.org/JPCC

Oligomers Comprising 2‑Phenyl‑2H‑benzotriazole Building Blocks for Solution-Processable Organic Photovoltaic Devices Michael F. G. Klein,*,† Felix Pasker,‡ Henning Wettach,‡ Immanuel Gadaczek,§ Thomas Bredow,§ Peter Zilkens,§ Peter Vöhringer,§ Uli Lemmer,† Sigurd Höger,‡ and Alexander Colsmann*,† †

Light Technology Institute, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76131 Karlsruhe, Germany Kekulé-Institute für Organische Chemie und Biochemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany § Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Wegelerstrasse 12, 53115 Bonn, Germany ‡

S Supporting Information *

ABSTRACT: In the past, benzotriazole acceptor units have been incorporated successfully into donor−acceptor polymers for highly efficient photovoltaic devices. Here, we explore 2phenyl-2H-benzotriazole oligomers as model systems for donor−acceptor polymers. We designed and synthesized acceptor−donor−acceptor oligomers incorporating 2-phenyl-2Hbenzotriazole acceptor and naphthodithiophene donor units. Supported by quantum chemical calculations we investigated the optoelectronic properties, such as electronic levels and intramolecular charge transfer, by means of optical and fluorescence spectroscopy. We built bulk-heterojunction solar cells comprising these oligomers and [6,6]-phenyl C61-butyric acid methyl ester. In doing so, the film-forming conditions were optimized as the absorber layer morphology is strongly affected by the deposition conditions, in particular drying time and drying temperature.

1. INTRODUCTION Due to the manifold possibilities of synthesizing new materials, organic semiconductors are a promising material class to tailor materials with properties meeting special requirements for photovoltaic applications. Today, mainly two approaches for fabrication of organic solar cells (OSCs) are followed: deposition of conjugated polymers utilizing low-cost solution processes and vacuum sublimation of low-molecular-weight organic molecules. Both material classes allow for building OSCs with certified power conversion efficiencies (PCE) exceeding 10% in single and multijunction devices.1,2 One of the most important challenges for further improvement of the PCE is the enhancement of the spectral coverage of sunlight in order to harvest more photons. Extension of the absorption to the near-infrared requires organic semiconductors with low band gaps. One approach for synthesis of low-band-gap polymers utilizes the concept of alternating electron donor and acceptor units along the polymer chain which leads to a smaller energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymer and hence to a reduction of the band gap as compared to the respective homopolymers.3,4 For example, in the copolymer poly[2,6-(4,4bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b[1]]-dithiophene)alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), benzothiadiazole acts as an acceptor unit that alternates with electron-rich dithiophene donor units.5 In a similar approach, acceptor units were attached to both ends of donor molecules, leading to low bandgap oligomers.6,7 Replacing the sulfur atom in the © 2012 American Chemical Society

benzothiadiazole acceptor unit leads to new degrees of freedom for the design of acceptor units due to the possibility of fine tuning the electronic properties or adding solubilizing side chains.8 Polymers based on fluorinated 2-alkyl-2H-benzotriazoles and benzodithiophene units have shown very high efficiencies in OSCs.9 Alkylated benzotriazoles exhibit little electron-accepting behavior.10 Hence, we examined the electron-poor 2-aryl-2H-benzotriazole polymers11 in donor− acceptor−donor oligomers12 to investigate the tunability of the electronic properties by variation of the aryl substituents.11−13 We focus our investigations on a well-defined oligomer utilizing naphthodithiophene as a central donor unit flanked by two 2phenyl-2H-benzotriazole acceptor units. Naphthodithiophene is a moderately electron rich building block for high-performance donor polymers.14−16 Contrary to polymers, oligomers possess a defined and defect free structure. This allows a detailed analysis of the spectroscopic properties that, in combination with sophisticated quantum chemical calculations, will allow us to understand the photophysical behavior of this acceptor unit in detail. We further investigated the optoelectronic material properties and the applicability of the oligomer as light absorbing material in organic solar cells. Received: April 24, 2012 Revised: June 29, 2012 Published: July 6, 2012 16358

dx.doi.org/10.1021/jp3039384 | J. Phys. Chem. C 2012, 116, 16358−16364

The Journal of Physical Chemistry C

Article

Scheme 1a

(a) 2-Ethylhexylbromide, K2CO3, KI, DMF, 65 °C, 2 d, 84%; (b) n-BuLi, TMEDA, I2, THF, −78 °C to higher than room temperature, 84%; (c) HBr/HOAc (45% w/v), Br2, 43%; (d) bispinacolatodiboron, PdCl2(dppf), KOAc, DMF, 90 °C, 18 h, 86%; (e) PEPPSI-iPr, KOH, THF, 60 °C, 24 h, 64%. a

package ORCA.20 For treatment of the excited states we applied time-dependent perturbation theory (TD-DFT). The COSMO model21 was used to simulate the solution environment of the molecules in THF and CH2Cl2. 2.4. Device Fabrication and Characterization. Solar cells were fabricated according to the device architecture depicted in Figure 1. Structured indium tin oxide (ITO)

2. EXPERIMENTAL SECTION 2.1. Synthesis. All details on the synthesis can be found in the Supporting Information. 2.2. Optical Spectroscopy and Electrochemistry. UV/ vis absorption spectra were obtained from a Shimadzu UV2100. Time-resolved fluorescence measurements were carried out with the time-correlated single-photon counting technique using either the second harmonic of a mode-locked Ti:sapphire laser whose repetition rate was reduced to 2 MHz by means of an acousto-optic pulse picker or the 250 kHz signal pulses of a 400 nm pumped femtosecond optical parametric amplifier as an excitation source. Instrument response was determined to 90 ps (full width at half-maximum). Cyclic voltammetry was performed with a computer-controlled VA-Standard 663 and a μAutoLab type III potentiostat of Metrohm AG, Suisse, in dried and oxygen-free dichloromethane using 0.1 M tetrabutylammonium hexafluorophosphate (electrochemical grade, Fluka) as supporting electrolyte, a platinum disk Φ = 5 mm) as working electrode, a glassy carbon counter electrode, and an Ag/AgCl reference electrode. Redox potentials were referenced against ferrocene/ferrocenium (Fc/Fc+, −4.8 eV vs vacuum level17). 2.3. Quantum Chemical Calculations. As a starting point the geometries of 2,5-bis(4-yl-2-phenyl-2H-benzotriazole)-8,9bis(2′-ethylhexyloxy)naptho[2,1-b;3,4-b′]dithiophene (7) and its two building blocks 5,6-dimethoxynaphtho[2,1-b:3,4-b′]dithiophene (8) and 2-phenyl-2H-benzotriazole (4) (Scheme 1 and Figure 2) were optimized with the B3LYP functional18 using a TZVP basis set19 with the quantum-chemical program

Figure 1. (a) Device architecture of the photovoltaic devices. (b) Proposed energy levels.

covered glass slides were sufficiently cleaned with a cleaning agent, rinsed with water, acetone, and isopropyl alcohol, and then dried in a nitrogen stream in a class 1000 clean room. After exposing the samples to an oxygen plasma for 120 s, samples were transferred to a glovebox and kept under nitrogen atmosphere for the remaining process. After spin casting a 60 nm thick layer from filtered poly(3,4ethylenedioxythiophene):polystyrenesulfonate aqueous dispersion (PEDOT:PSS) (Clevios P VP AI 4083, Heraeus), residual 16359



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water was removed by drying the substrates in a vacuum oven. 7 and [6,6]-phenyl C61-butyric acid methyl ester (PCBM, Solenne BV, 99%) were separately dissolved in 1,2dichlorbenzene (DCB, Sigma-Aldrich, anhydrous, 99%) with a concentration of 30 mg/mL. Solutions were stored at 80 °C for 1 h, mixed in a 1:1 ratio, and stirred for further 10 h. The hot blend solution was spin cast on hot substrates (T = 80 °C), resulting in an average dry film thickness of 94 nm. A calcium (30 nm)/aluminum (235 nm) electrode was evaporated through a shadow mask in a Lesker Spectros system at 7 × 10−6 mbar. The active area was A = 10.5 mm2 as defined by the cross-section of both electrodes. Current density−voltage characteristics (JV) of the solar cells were recorded with a source measure unit (Keithley 238) under nitrogen atmosphere. A spectrally monitored Oriel 300 W solar simulator was used to simulate sunlight according to the ASTM-G173-03e1 standard for hemispherical solar irradiance.22 For external quantum efficiency (EQE) measurements outside the glovebox, the solar cells were encapsulated. The EQE setup features a 450 W xenon light source, a 300 mm monochromator (LOT-Oriel GmbH & Co KG), a customdesigned current amplifier (DLPCA-S Femto Messtechnik GmbH), and a digital lock-in (eLockin 203 Anfatec). A modified photoreceiver (OE-200-S Femto Messtechnik GmbH) with a Si/InGaAs sandwich diode was used to monitor the stability of the monochromatic light beam. Initial calibration was done with a reference silicon diode (NIST traceable calibration, Thorlabs). Following standard test conditions we measured the EQE while applying white light bias illumination. The deviation of the device short-circuit current density Jsc as derived from the EQE measurement and the JV curve was below 5%, which is within the measurement uncertainty of the reference diode’s calibration.

photophysics associated with the electronic excitations of the hybrid as well as of the donor and acceptor species alone. A solution of 7 in tetrahydrofuran (THF) exhibits absorption maxima at 435 and 457 nm (see Supporting Information, Figure S1) and appears orange. The optical gap (Egopt) in THF solution is 2.54 eV, as determined from the absorption onset, which is in very good accordance to the calculated S1 energy. The thin film absorption of 7 is red shifted when compared to solution and shows two well-separated maxima at 471 and 505 nm and an Egopt of 2.29 eV. The fluorescence excitation and fluorescence dispersion spectra of the acceptor unit 4 in THF solution are displayed in Figure 2A. Its first excited singlet state, S1, absorbs around 310 nm, and resonances due to higher lying states are clearly discernible from multiple absorptions appearing below 280 nm. The fluorescence of 4 peaks at 365 nm if it is excited at the wavelength of maximum absorbance (310 nm). The very large

3. RESULTS AND DISCUSSION 3.1. Oligomer synthesis. 5,6-Di(ethylhexyloxy)naphtho[2,1-b:3,4-b′]dithiophene (2) is available from 5,6-di(hydroxy)naphtho[2,1-b:3,4-b′]dithiophene (1) by Williamson-ether synthesis with 2-ethylhexylbromide (Scheme 1).23,24 Iodination of the phenyl-bridged dithiophene donor unit 3 can be achieved by successive treatment with n-BuLi and iodine. Synthesis of 2phenyl-2H-benzotriazole (4) was performed as described recently.11 Treating a solution of 4 in HBr/acetic acid (45% w/v) with neat bromine results in formation of 4-bromo-2phenyl-2H-benzotriazole (5) in 43% yield. The corresponding boronic ester (6) was synthesized under Miyaura-coupling conditions from 5 and bis(pinacolato)diborane in 86% yield. 7 was obtained by Suzuki cross coupling of 3 with 2 equiv of 6 using the PEPPSI-iPr Pd catalyst. Purification of 7 was achieved by column chromatography and subsequent recycling gel permeation chromatography (recGPC), yielding the pure product as an orange solid. All experimental details can be found in the Supporting Information. 3.2. Spectroscopic Properties. Optical spectroscopy is a valuable tool to gain experimental insight into the electronic structure of organic molecules. Compound 7 was therefore characterized by stationary UV/vis absorption and emission spectroscopy. Furthermore, for optoelectronic applications, the lifetimes of the optically prepared excited states are important quantities because they ultimately affect the efficiencies of charge transfer. Therefore, picosecond time-resolved fluorescence spectroscopy was carried out to investigate the

Figure 2. Fluorescence excitation spectra () and dispersion spectra (···) of 4 (A), 8 (B), and 7 (C) in THF solution. Calculated absorption energies (TD-B3LYP/TZVP/THF) and intensities are shown as vertical lines. 16360



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Stokes shift of nearly 4900 cm−1 originates most likely from an intramolecular nuclear relaxation such as rotation of the phenyl group relative to the triazole moiety.25 The fluorescence lifetime was determined by time-correlated single-photon counting (TCSPC) to 2.55 ns in THF solution. No evidence for dynamic Stokes shifting was obtained in the time-resolved fluorescence data. This gives credence to our assignment of the large stationary Stokes shift to an intramolecular relaxation process, which occurs much faster than our time resolution of 90 ps (FWHM of the instrument response function). Corresponding spectra of the 5,6-dimethoxynaphtho[2,1b:3,4-b′]dithiophene (8) in THF are shown in Figure 2B. At a first glance, the excitation profile consists of two general regions around 255 and 320 nm, respectively.26 The long-wavelength region itself consists of a sharp red-shifted feature peaking at 350 nm and having a bandwidth of only 10 nm and a broader blue-shifted absorption at 320 nm that exhibits a pronounced high-frequency Franck−Condon progression. The TCSPC data obtained at excitation wavelengths of 310 and 350 nm reveal a single component with fluorescence lifetimes of 0.86 and 0.84 ns, respectively. Dynamic spectral Stokes shifting was observed for neither excitation wavelength. Excitation and emission spectra of 7 in THF are shown in Figure 2C. As for its constituent 8 the excitation spectrum can be formally decomposed into two regions. The low-frequency part extends from about 360 nm all the way into the visible region at roughly 500 nm and peaks at a wavelength of 440 nm. The peculiar spectral profile of this near-UV/Vis band in conjunction with its enormous spectral width (∼4700 cm−1) strongly suggests contributions from multiple electronic transitions rather than from the first excited singlet state only. Additionally, higher lying electronic transitions are the origin of the UV band centered at 320 nm. The dispersed fluorescence exhibits a maximum intensity at 495 nm, thus resulting in a peak-to-peak Stokes shift of almost 4000 cm−1. Again, conformational relaxation along the internal donor (NDT)− acceptor (BTA) torsion or/and alternatively, along the phenyl− triazol torsional degree of freedom of the acceptor might be held responsible for this large red shift of the emission. Altogether, the stationary spectra of 7 bear no resemblance to the spectra of its constituent compounds. As judged purely by the spectral positions, neither of the two absorptive features can be attributed to a localized excitation on any of the two moieties, i.e., to the phenyl-bridged dithiophene or the benzotriazole. This indicates that the two electronic manifolds are heavily mixed in the linked hybrid. When 7 is excited at 400 nm, the emission decays clearly in a monoexponential fashion with a fluorescence lifetime of 2.35 ns. This is a factor of 3 longer than the lifetime of the isolated donor, which is advantageous for a photovoltaic application of compound 7. The strong coupling between the donor and the acceptor moieties in 7 shifts the absorption spectrum significantly into the visible part of the electromagnetic spectrum. 3.3. Quantum Chemical Calculations. In order to get better insight into the electronic structure of 7 in the ground and excited states we performed theoretical calculations based on density-functional theory (DFT). The calculated excitation energies and their intensities are depicted as vertical lines in Figure 2. The agreement between calculated and measured S 1 excitation energies of 4 and the experimental spectra in THF is good (experiment 4.01 eV versus theory 4.15 eV; Figure 2A).

For 8 the difference between theory and experiment is larger (calculated 3.93 eV versus experimental 3.56 eV) but still within the typical error range of TD-DFT (Figure 2B).27 The calculated excitation energy of 7 for the S1 state is 2.54 eV in the gas phase and 2.44 eV in THF (Figure 2C). Both values are close to the experimental result of Egopt = 2.54 eV in THF solution (see Supporting Information, Figure S1). The transition dipole moment indicates that the first excitation is the most intensive in the spectrum. For this excitation one electron is excited from the HOMO to the LUMO of 7 as illustrated in Figure 3. The S2 and S3 states are nearly degenerated in the gas phase (2.74 and 2.78 eV).

Figure 3. HOMO (a) and LUMO (b) of 7 calculated with B3LYP/ TZVP.

Comparing the calculated gas-phase spectrum of 7 (Supporting Information, Table S3) with the experimental thin film spectrum it can be easily seen that this splitting is also present. Inclusion of solvent effects (CH2Cl2 and THF) shifts the gas-phase S3 state (from 2.78 to 2.48 eV in THF) more than the gas-phase S2 state (from 2.74 to 2.59 eV in THF, see Supporting Information, Table S3) in a way that the former S3 state is lower than the former S2 state and nearly degenerates with the S1 (2.44 eV in THF) state. The S2 and S3 states have similar oscillator strengths and are both predicted to become visible in the spectrum, which is indeed the case, especially in the spectrum of the thin film (Supporting Information, Figure S1). These states correspond to the excitation from the HOMO to the LUMO+1 (S2) and HOMO−1 to LUMO (S3). In the thin film the splitting between the S1 and the S2/S3 (0.18 eV) states is much more pronounced than in solution (calculated 0.11 eV for the S1/S2 and S3 splitting in THF). Comparing the orbital shapes in Figure 3 it can be seen that the HOMO of 7 is mostly located in the central electron-rich dithiophene unit, while the LUMO is located on the electrondeficient benzotriazole periphery and has the same topography as the LUMO of 4 (Supporting Information, Figure S2). The excitation therefore corresponds to an intramolecular charge transfer from the inner to the outer units. This is supported by the difference density of the S1 state (Figure 4). Electrons are transferred from the blue region (mostly located at the electron-rich inner part of the molecule) to the red regions, which are located at the benzotriazole units. The reduction of the excitation energy S1 in 7 as compared to the isolated building blocks accounts for a red-shifted absorption of the oligomer. 3.4. Electrochemical Characteristics. Besides the optical gap, the HOMO and LUMO levels are essential for designing a proper device architecture. Cyclic voltammetry measurements of 7 in dichloromethane show a reversible oxidation (E1/2ox1 = 16361



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of 2.52 eV is in excellent agreement with the optical gap of Eg = 2.51 eV in CH2Cl2 (see Supporting Information, Figure S1). 3.5. Solar Cell Characteristics. The HOMO and LUMO levels of the oligomer as derived from the electrochemical material characterization indicate that a blend of 7 and the fullerene derivative PCBM enables an efficient exciton dissociation at the interface. Consequently, we built solar cells comprising 7:PCBM bulk-heterojunction layers between ITO/PEDOT:PSS anodes and calcium/aluminum cathodes as depicted in Figure 1a. Figure 1b shows the proposed energy levels of the respective materials. Apart from the proper alignment of electronic levels, solar cell performance strongly depends on the active layer morphology.28−31 Hence, we investigated three different active layer treatments which are commonly used for enhancing the performance of solution-processed OSCs: Deposition from hot solution, active layer drying under solvent atmosphere (solvent annealing), and thermal post-treatment of the entire device.32−34 Atomic force microscope (AFM) images of the active layers in Figure 6a show that 7:PCBM layers that were spin cast at elevated temperatures (T = 80 °C) exhibit a smooth surface with an average roughness Ra = 0.5 nm. If we increase the drying time by casting the solution at room temperature and subsequently anneal the active layer on a hot plate at 150 °C for 7 min, we find cracks at the surface. Consequently, the average roughness increases to Ra = 2.4 nm (Figure 6b). If the drying time is further increased by solvent annealing, AFM measurements reveal crystal-like aggregates while the average roughness increases to Ra = 6.3 nm (Figure 6c). Usually such aggregates are formed when bulk-heterojunctions demix and pure material domains grow. They protrude from the surface, and the respective solar cells are nonfunctional. Comparison of the three deposition methods indicate that “freezing” the layer morphology in a nonequilibrium state by accelerating the 7:PCBM drying process leads to a much more homogeneous active layer and a better distribution of the respective materials. According to the poor morphology in thermally annealed or solvent annealed active layers, solar cells comprising 7:PCBM blends do not show any photovoltaic effect. However, OSCs with a fast-dried active layer spin cast from hot solution (T = 80 °C) show a photovoltaic response as it becomes evident in the EQE measurements depicted in Figure 7. At 343 nm both polymers and fullerenes contribute to the photocurrent, while the maxima at 463 and 493 nm can be attributed to absorption of 7. This clearly evidences that the synthetic coupling of the high-band-gap donor units 2 and

Figure 4. Difference density ρ(S1)−ρ(S0) of 7 calculated with B3LYP/ TZVP.

0.52 V, EHOMO = −5.32 eV) and an irreversible oxidation signal (Eonsox2 = 0.99 V, maximum 1.13 V) vs Fc/Fc+ (Figure 5).

Figure 5. Reduction cycles (solid line, left, ν = 100 mV/s) and oxidation cycles (dotted line, right, ν = 50 mV/s) of 7 in 0.1 M (Bu)4NPF6/CH2Cl2 solution.

Furthermore, an irreversible reduction signal with an onset at Eonsred1 = −2.0 V (giving ELUMO = −2.80 eV) and a second irreversible reduction signal at Eonsred2 = −2.42 V can be observed. These energy levels are in good agreement with the calculated HOMO (−5.10 eV) and LUMO (−2.27 eV) energies. The electrochemical gap (EgCV = E1/2ox1 − Eonsred1)

Figure 6. AFM images (7.5 μm × 7.5 μm) of 7:PCBM layers that are treated differently during or after deposition, hence affecting the active layer drying time: (a) solution is spin cast at 80 °C, (b) solution is spin cast from cold solution at room temperature and the device is postbaked afterward, (c) solvent annealing. Ra is the average roughness of the film, while Rt is the peak to valley roughness. 16362



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Information, Figure S4). In a double-logarithmic representation of the photocurrents vs I, (Supporting Information, Figure S5), the sublinear behavior (≈ I0.75) of the intensity-dependent photocurrent becomes obvious. Such intensity dependence corresponds to higher order loss mechanisms which are commonly attributed to bimolecular or surface recombination.35−38 With respect to the morphological considerations above and the necessity to freeze the layer morphology in a nonequilibrium state, we attribute these strong bimolecular recombination losses within 7:PCBM blends to a demixing of both phases. On the other hand, this finding clearly points out the potential of this material class. Future modifications should aim at the improvement of the layer morphology, e.g., by modifying the solubility side groups, in order to extract a reasonably high number of charges that are generated within the device. Figure 7. EQE of a typical ITO/PEDOT:PSS/7:PCBM/Ca/Al device comprising an active layer that was cast from hot solution, measured while applying white light bias illumination.

4. CONCLUSIONS 2-Phenyl-2H-benzotriazole acceptor units and the corresponding oligomeric and polymeric materials are valuable pieces for the chemical toolbox for a target-oriented design of new materials in the future. Chemically, benzotriazole monomers are easily accessible and will allow for further functionalization. The corresponding acceptor−donor−acceptor oligomer comprising a phenyl-bridged dithiophene donor unit was synthesized and investigated in detail to understand the basic photophysical properties of the 2-phenyl-2H-benzotriazole building block and its electronic coupling within a donor− acceptor copolymer. In addition, oligomer 7 was incorporated as a donor material in bulk-heterojunction solar cells. The active layer smoothness and correlated solar cell performance shaped best upon deposition at elevated temperature and short drying time. Dissociation of excitons and extraction of charges, however, will need further improvement in future research. Quantum chemical calculations based on the B3LYP functional using a TZVP basis set were very well in accordance with the results from spectroscopic investigations, making the simulative approach a valuable tool for future investigations of related oligomers.

acceptor units 4 results in a photoactive oligomer with a lower band gap and a reasonable optoelectronic performance. Finally, we investigated the electrical key performance parameters of the 7:PCBM solar cells comprising an active layer that had been deposited at elevated temperatures by measuring the JV curve (Figure 8).

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of all new compounds; stationary absorption spectra of 7 in solution and thin film; computational data of 4, 7, and 8 based on the TD-DFT calculations and the respective HOMO and LUMO shapes of 4 and 8. Illumination intensity-dependent JV characteristics and double-logarithmic representation of photocurrent vs illumination intensity. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. JV characteristic of a ITO/PEDOT:PSS/7:PCBM/Ca/Al solar cell in the dark and under illumination with 100 mW/cm2 according to ASTM G173-03 A.M.1.5 G.

The best OSC exhibited a power conversion efficiency η = 0.6% and an open-circuit voltage Voc = 0.72 V. In good agreement with the restricted spectral range of the material, the solar cell exhibits a moderate short-circuit current density Jsc = 2.8 mA/cm2. However, under a reverse bias of V = −1.2 V the solar cell current density doubles. The moderate Jsc in conjunction with a poor fill factor FF = 30% reflects the recombination of photogenerated charge carriers within the device. These field-dependent charge carrier losses further become evident in the strong slope of the solar cell current density under illumination and reverse bias. In order to investigate the charge carrier losses in more detail, JV characteristics for various illumination intensities between I = 10 and 100 mW/cm2 were recorded (Supporting

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected](M.F.G.K.); alexander. [email protected] (A.C.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was funded by the German Research Foundation (DFG) through the collaborative research centres 16363



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(27) Dreuw, A.; Head-Gordon, M. Chem. Rev. 2005, 105, 4009− 4037. (28) Sanyal, M.; Schmidt-Hansberg, B.; Klein, M. F. G.; Munuera, C.; Vorobiev, A.; Colsmann, A.; Scharfer, P.; Lemmer, U.; Schabel, W.; Dosch, H.; Barrena, E. Macromolecules 2011, 44, 3795−3800. (29) Sanyal, M.; Schmidt-Hansberg, B.; Klein, M. F. G.; Colsmann, A.; Munuera, C.; Vorobiev, A.; Lemmer, U.; Schabel, W.; Dosch, H.; Barrena. Adv. Energy Mater. 2011, 1, 363−367. (30) Schmidt-Hansberg, B.; Sanyal, M.; Klein, M. F. G.; Pfaff, M.; Schnabel, N.; Jaiser, S.; Vorobiev, A.; Müller, E.; Colsmann, A.; Scharfer, P.; Gerthsen, D.; Lemmer, D.; Barrena, E.; Schabel, W. ACS Nano 2011, 5, 8579−8590. (31) Klein, M. F. G.; Pfaff, M.; Müller, E.; Czolk, J.; Reinhard, M.; Valouch, S.; Lemmer, U.; Colsmann, A.; Gerthsen, D. J. Polym. Sci., Polym. Phys. 2012, 50, 198−206. (32) Ballantyne, A. M.; Chen, L.; Nelson, J.; Bradley, D. D. C.; Astuti, Y.; Maurano, A.; Shuttle, C. G.; Durrant, J. R.; Heeney, M.; Duffy, W.; McCulloch, I. Adv. Mater. 2007, 19, 4544−4547. (33) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. Adv. Funct. Mater. 2007, 17, 1636−1644. (34) Müller, C.; Ferenczi, T. A. M.; Campoy-Quiles, M.; Frost, J. M.; Bradley, D. D. C.; Smith, P.; Stingelin-Stutzmann, N.; Nelson, J. Adv. Mater. 2008, 20, 3510−3515. (35) Mandoc, M. M.; Veurman, W.; Koster, L. J. A.; de Boer, B.; Blom, P. W. M. Adv. Funct. Mater. 2007, 17, 2167−2173. (36) Hodgkiss, J. M.; Albert-Seifried, S.; Rao, A.; Barker, A. J.; Campbell, A. R.; Marsh, R. A.; Friend, R. H. Adv. Funct. Mater. 2012, 22, 1567−1577. (37) Riedel, I.; Parisi, J.; Dyakonov, V.; Lutsen, L.; Vanderzande, D.; Hummelen, J. C. Adv. Funct. Mater. 2004, 14, 38−44. (38) Street, R. A.; Schoendorf, M.; Roy, A.; Lee, J. H. Phys. Rev. B 2010, 81, 205307.

SFB 813 and within the priority program SPP 1355 “Elementary processes of organic photovoltaics”. The authors thank the DFG Heisenberg Group Nanoscale Science, contract no. DFG 442/3-1, of the Light Technology Institute (LTI, Karlsruhe Institute of Technology) for the support in the AFM measurements. We also want to mention the support of Markus Schröder (LTI) and Matthias Isen (LTI), particularly for assistance when measuring the EQE.

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