structure formation in low-bandgap polymer:fullerene solar cell blends in the course of solvent...

Structure Formation in Low-Bandgap Polymer:Fullerene Solar CellBlends in the Course of Solvent EvaporationBenjamin Schmidt-Hansberg,*,†,# Michael F. G. Klein,‡ Monamie Sanyal,§ Felix Buss,†

Gustavo Q. G. de Medeiros,‡ Carmen Munuera,§ Alexei Vorobiev,∥ Alexander Colsmann,‡

Philip Scharfer,† Uli Lemmer,‡ Esther Barrena,*,⊥ and Wilhelm Schabel†

†Institut fur Thermische Verfahrenstechnik, Thin Film Technology, Karlsruhe Institute of Technology, Kaiserstrasse 12, 76131Karlsruhe, Germany‡Lichttechnisches Institut, Karlsruhe Institute of Technology, Kaiserstrasse 12, D-76131 Karlsruhe, Germany§Max Planck Institut fur Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany∥European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex 9, France⊥Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain

*S Supporting Information

ABSTRACT: The drying process of the bulk heterojunction (BHJ) layerhas a strong impact on the solar cell performance for the well-investigatedmaterial system P3HT:PC61BM. For higher performing low-bandgappolymers and C71 fullerene derivatives, no comprehensive studies of theBHJ structure evolution during film drying are available. In this work weinvestigate the structure formation of the low-bandgap polymer poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]-2,6-diyl-alt-(2,1,3-benzothi-diazole)-4,7-diyl} (PSBTBT) and [6,6]-phenyl C71-butyric acid methyl ester(PC71BM) in the transition from wet to solid by in-situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry simultaneously. Thenucleation and crystallization of PSBTBT differs from the interface-inducedcrystallization of P3HT and occurs partially in the solution. It is shown that PSBTBT:PC71BM blend nanomorphology andoptoelectronic device properties are rather insensitive to the drying process in the investigated temperature range of 40−85 °C.This is beneficial for fast drying at elevated temperatures which enables high throughput fabrication of efficient organicphotovoltaics.

I. INTRODUCTIONThe fast progress in power conversion efficiency (PCE) oforganic solar cells exceeding recently 10% shows the potentialof this technology toward low-cost photovoltaics.1 The intrinsicadvantage of organic semiconductors in contrast to theirinorganic counterparts is their solubility which allows for cost-effective large area solution processing.2−4 Organic solar cellstypically comprise a blend of hole conducting polymer andelectron conducting fullerene as photoactive layer. Bothcomponents are coated in a single step from solution andform an intermixed film upon evaporation of the solvent, in aso-called bulk-heterojunction (BHJ) structure. The optimizedstructure has to provide efficient exciton separation and abalanced charge carrier transport for electrons and holes in thevertical direction which strongly depends on the structuraldetails of the interpenetrating polymer and fullerene network;crystallinity, domain size, orientation, and spatial distribution ofthe phase separated donor and acceptor domains are all factorsaffecting the overall PCE.5−8 For the most investigated materialsystem poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PC61BM), it has been shown that theBHJ structure strongly depends on the drying conditions and

postprocessing treatments9−13 with strong implications on theassociated solar cell properties. Yet only few works haveinvestigated the mechanisms of structure formation duringsolvent evaporation11,14−16 although a fundamental under-standing is of paramount importance for material and processdesign.In this work, we focus on the low-bandgap polymer PSBTBT

(Figure 1e) and PC71BM. For this material system, powerconversion efficiencies of about 5.5% have been reached.17−21 Ithas the specific advantage of color neutral light absorption inthe visible range, allowing the realization of semitransparentsolar cells for building integrated window applications.22,23

Herein, we investigated the structure formation during filmdrying for PSBTBT:PC71BM in situ by grazing incidence X-raydiffraction (GIXD) and laser reflectometry simultaneously.With this experimental approach we are able to correlate thestructural evolution with the film composition. The results arediscussed in conjunction with preceding work on

Received: May 10, 2012Revised: August 4, 2012Published: September 24, 2012

Article

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© 2012 American Chemical Society 7948 dx.doi.org/10.1021/ma300945k | Macromolecules 2012, 45, 7948−7955

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P3HT:PC61BM.11,12,15 The drying process−structure−propertyrelationship for PSBTBT:PC71BM is studied at different dryingtemperatures. It is shown that the structure formation andoptoelectronic properties of PSBTBT are not significantlyaffected by drying process parameters in the investigated rangeof 40−85 °C drying temperature. This is beneficial forincreased drying speeds at increased temperatures in highthroughput processes and simplifies the upscaling of solar cellfabrication.

II. EXPERIMENTAL METHODSIn-Situ Drying GIXD Measurements. The polymer PSBTBT was

provided by Konarka Technologies and used as received. PC71BM(>99%) was purchased from Solenne. The solutions have beenprepared in a 1:2 ratio by weight for PSBTBT:PC71BM at 3 wt % solidfraction in o-DCB and have been stirred at 80 °C for at least 48 h afterpreparation under a nitrogen atmosphere in a glovebox. The doctorblading parameters were 400 μm slit width at 10 mm/s coating speedwith 60 μL solution on 35 mm × 60 mm silicon substrates.Laser reflectometry (λ = 650 nm) drying process measurements

were accomplished at an incident angle of 29° and low laser intensitiesin order to minimize local heating of the wet film.For the analysis of the reflectometry measurements the refraction

index of the blend was derived from variable angle spectroscopicellipsometry measurements (VASE, J.A. Woollam Co., Inc.). Themeasured spectroscopic values Ψ and Δ were analyzed with thesoftware WVASE (J.A. Woollam Co., Inc.). First, PSBTBT andPC71BM were analyzed independently, and two analytical models werederived. In a second step, the blend was characterized by combiningthe optical models of both constituents within the Bruggeman effectivemedium approximation.24 The index of refraction of the blend wasdetermined to n = 1.91 at the probing wavelength λ = 650 nm of thelaser reflectometer. The effective refractive index of the solution was

calculated by averaging the refractive indices of the blend and DCB asa function of drying time.25 The experiments were carried out in adrying channel at 40 °C and 0.15 m/s drying gas velocity.

In-situ GIXD images (λ = 0.9305 Å, critical angle of 0.11°) weretaken in intervals of 18 s at beamline ID10B at ESRF as described inref 11.

Ex-Situ Characterization. For the sample preparation, we used 48mm × 60 mm ITO/glass substrates (12 Ω/□, Visiontek). Thesubstrates were cleaned by sonication in acetone and isopropanol,followed by an oxygen plasma treatment. Subsequently, thePEDOT:PSS dispersion was coated by doctor blading at ambientconditions. 40 μL of PEDOT:PSS dispersion Clevios VP AI 4083(Heraeus), diluted 1:1 by volume with water, was cast with a blade slitwidth of 70 μm and a blade speed of 5 mm/s, resulting in 20−40 nmdry film thickness. This layer was subsequently heated at 120 °C for 20min in a glovebox under a nitrogen atmosphere. PSBTBT andPC71BM solutions were prepared and coated as described before. Thecoating parameters and setup were the same for in-situ GIXDmeasurements and solar cell preparation. After cutting the substratesinto 16 mm × 16 mm pieces they have been dried in a vacuum oven at40 °C overnight. Subsequently, a calcium (50 nm)/aluminum (200nm) cathode was deposited through a shadow mask. The average filmthickness of the active layer was 86 ± 11 nm. Film thicknesses havebeen measured with surface profiling over a scratch with a Dektak 6M.Low polymer solubility demands processing temperatures above roomtemperature in order to prevent large-scale crystallization and gelation.Therefore, PSBTBT:PC71BM solar cells were fabricated at 40−85 °Ccoating and drying temperature and 0.15 m/s gas flow velocity in thepreviously reported experimental setup.26 AFM measurements wereconducted in tapping mode with a Veeco Dimension Icon. Theabsorption was measured in transmission with a spectrophotometer(PerkinElmer, Lambda 1050) using a blank PEDOT:PSS/ITO/glasssubstrate as reference. The absorption spectra were normalized to the

Figure 1. GIXD diffraction patterns of PSBTBT:PC71BM (1:2) films cast from o-dichlorobenzene by doctor blading at (a) PEDOT:PSS/ITO/glass,(b) scheme of the face-on configuration related to the out-of-plane (010) Bragg reflection, and (c) edge-on configuration related to the out-of-plane(h00) Bragg reflection with high hole mobility μh along the polymer backbones and along the π−π-stacking direction as indicated by red arrows.Lines represent polymer chains, and gray planes represent the lamella plane. (d) GIXD diffraction patterns at native silicon substrates. Drying wasaccomplished at 40 °C. (e) Molecular structure of PSBTBT.

Macromolecules Article

dx.doi.org/10.1021/ma300945k | Macromolecules 2012, 45, 7948−79557949

PCBM absorption peak at 375 nm. The samples were taken from thesame 48 × 60 mm substrates that were used for the solar cell devices.

III. RESULTS AND DISCUSSION

In-Situ Drying GIXD Measurements. We have previouslyemployed in-situ GIXD to investigate the crystallizationbehavior of P3HT:PC61BM blends cast by doctor blad-ing.11,12,15As described in preceding work, we simultaneouslyutilized laser reflectometry for monitoring the film thicknessduring film drying. On the basis of the evolution of filmthickness, we can calculate the film composition at each instantof GIXD measurement.25,26

The main diffraction features in such two-dimensional (2D)GIXD measurements of PSBTBT:PC71BM films are labeled inFigure 1a.17 The diffraction patterns reveal the presence ofPSBTBT crystallites in edge-on as well as in face-on orientationas discussed in the following. PSBTBT exhibits a well-pronounced (100) reflection along qz arising from polymerstacking into planar structures known as “lamellae”, which arein the case of P3HT usually oriented perpendicular to thesubstrate, so-called edge-on.18 For edge-on orientation ofPSBTBT, we expect higher hole mobility along the polymerbackbones and along the direction of π−π-stacking, both in thein-plane direction as indicated in Figure 1c.8 The (010)reflection, arising from π−π polymer stacking of the lamellae, isalso observed. Highest intensity of the diffraction ring along thevertical axis reveals the presence of face-on oriented crystallitesas illustrated in Figure 1b. This orientation promotes high holemobilities perpendicular to the substrate along the π−π-stacking direction. Although edge-on and face-on are the twodominant orientations, the large mosaic spread of the (100)and (010) scattered intensity (forming an arc) indicates a broaddistribution of orientations (schemes in Figure 1b,c). Themisorientation of the π−π-stacking is expected to be beneficialfor the required vertical hole transport along the polymerbackbones (Figure 1b) due to larger overlapping of the pzorbitals and better connectivity between adjacent crystallites.8

For direct comparison of the evolution of this blend filmstructure with device performance in solar cells, it would beoptimum to coat the films on the same substrates, whichcommonly are poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS)-coated indium−tin oxide(ITO)-coated glass slides. On the other hand, the diffractionpatterns are more pronounced if native silicon oxide/silicon(SiO2/Si) is used, which is beneficial for the X-ray analysis ofthe structure emerging during the first stages of drying. Wehave examined the structure of PSBTBT:PC71BM blendsformed on SiO2/Si and PEDOT:PSS/ITO/glass. Two-dimen-sional (2D) GIXD patterns obtained on both substrates displayqualitatively similar diffraction features (Figure 1a,d). Thecomparison reveals that choosing silicon substrates is areasonable compromise between well-defined GIXD patternand comparability of the diffraction features on PEDOT:PSS-covered substrates as used for the fabrication of solar cells.Before discussing the structural evolution of the

PSBTBT:PC71BM blends in dichlorobenzene (DCB), we takea look at the film drying process recorded by reflectometryduring the GIXD measurements. Figure 2a depicts thereflectometer raw signal. On the basis of the interferencefringes, we can calculate the evolution of film thickness andsubsequently the solvent mass fraction xDCB, plotted in Figures2b and 2c, respectively. Up to about 10 wt % DCB the filmthickness decreases with a constant evaporation rate.

Subsequently, the drying rate drops rapidly (transition frominterference fringes to slight changes in Vdiode) which iscommonly attributed to a diffusional mass transfer limitation ofthe residual solvent in the film (falling rate period). These lowamounts of solvent act as plasticizer and provide molecularmobility which can lead to slight molecular rearrangements inthis last stage of drying. Such small associated changes ofthickness during the falling rate period cannot be resolvedquantitatively by this experimental setup. A selection of the 2D-GIXD images obtained during film drying is shown in Figure3a. The first image corresponds to the freshly castPSBTBT:PC71BM film from DCB solution at 97 wt % solventand depicts two diffraction rings arising from solvent scattering.The sharp ring at lower q than DCB is a spurious feature alsoobserved for the bare substrate. At 90 s drying time and 95wt % solvent, the appearance of the (100) Bragg reflectionalong the vertical axis reveals the first indication of PSBTBTcrystallinity. The spot-like shape of the (100) reflectionsuggests well-oriented edge-on crystallites nucleated at aninterface: air−film or film−surface. Surface-induced orderinghas been suggested for P3HT12,27,28 and for other polymersbased on the results obtained with different substratetreatments.29−31 Concomitant with the appearance of the(100) peak, a sharp powder diffraction ring emerges with thesame momentum transfer revealing the nucleation of apopulation of PSBTBT crystals with random orientation(better observed in a zoomed area of the 2D pattern shownin Figure 3b). Therefore, we infer that PSBTBT does not onlycrystallize at the substrate interface as P3HT but also partlycrystallizes in the bulk of the solution (Figure 3b−e). Forcomparison, Figures 3b and 3d show a 2D GIXD patterncollected at an earlier stage of blend drying for both PSBTBTand P3HT polymers.In agreement with our real-time GIXD results, Chen et al.

observed by optical absorption that strong π−π interaction ofPSBTBT molecules already occurs in the solution state.According to their simulations, strong intermolecular inter-actions between PSBTBT molecules are expected even insolution.17 The structural evolution with drying time can be

Figure 2. (a) Reflectometer raw data of the PSBTBT:PC71BM in-situGIXD drying experiment indicating the constant and falling rateperiod. Solvent was pure DCB. Calculated evolution of (b) filmthickness and (c) solvent mass fraction based on the reflectometermeasurement.



followed analyzing the distribution of the (100) scatteredintensity as a function of the azimuthal detector angle atdifferent drying times shown in Figure 4a (after backgroundsubtraction for each image). Since in the GIXD geometry weare not exactly satisfying the Bragg condition, the momentumtransfer measured along the detector vertical is not exactlyparallel to the sample z-directionimplying that oriented(100) crystallites with an angle smaller than ∼4° are notprobed.32,33 Still the measured angular plots reflect theevolution of the orientation distribution of PSBTBT duringcrystallization. The evolution of the diffraction ring arising fromthe population of misoriented crystallites manifests itself asbaseline background that becomes more intense with time (seelogarithmic scale in the inset of Figure 4a). The evolution withdrying time of the integrated intensity of both populations ofPSBTBT crystals, with (100)-edge-on orientation and randomorientation, is shown in the Figure 4b. The largestcrystallization rate occurs similarly for both portionsedge-on and misorientedduring the transition from constant tofalling rate period between 180 and 198 s, that is, for solventfractions of 80 down to 13 wt %. Another notable differencewith respect to the crystallization of P3HT:PC61BM blends isthat the orientation distribution of the PSBTBT (100)-orientedportion remains almost constant during crystallization with afwhm of ∼7.5° (see Supporting Information, Figure S1).At a later stage, PC71BM aggregation sets in as depicted in

Figure 3a. The exact determination of the onset of fullereneclustering needs further analysis of the profiles upon

deconvolution from the scattered intensity from the solvent.The important fact is that strong PC71BM diffraction signalsappear only at the very end of the drying process which issimilar to the behavior of P3HT:PC61BM.12

The real-time data shown here reveals the formation of apopulation of misoriented PSBTBT crystallites that evolvesfrom crystallization of PSBTBT in the solution. We have foundsimilar evidence of bulk crystallization on PEDOT:PSS/ITOsubstrates. Although the crystallization kinetics is mainlydominated by the rate of solvent evaporation, there is nodoubt that the surface energy and roughness of the substratemay influence the microstructure of the blend.30,34

Film Structure and Optoelectronic Properties. Theprocess−structure−property relationship of PSBTBT:PC71BMfilms has been investigated for different drying temperatures. Asshown in previous work on P3HT:PC61BM, topography imagesobtained by atomic force microscopy (AFM) reveal micro-meter-length scale features which exhibit a finer substructure inthe phase image (Figure S2). In contrast to P3HT, theroughness of PSBTBT:PC71BM films does not changesignificantly with changing drying temperature.Prior to a thermal treatment of as cast PSBTBT:PC71BM

solar cells, the devices show strong performance deviation asmanifested in a broad distribution of the J−V characteristics.Figure 5a depicts typical J−V curves for a 40 °C processedsample. In comparison to the first characterization run (1. run),the PCE increases with device characterization time. In thesecond characterization run (2. run) one solar cell of the 40 °C

Figure 3. (a) Evolution of GIXD diffraction patterns during the drying of a PSBTBT:PC71BM (1:2) film cast from DCB solution by doctor blade at40 °C on native silicon substrate. While the first image is of the freshly coated film, the second image shows the first indication of crystallinity after 90s drying. Following images show the evolution of film drying and crystallization. Below each image, the drying time and actual solvent fraction isgiven. Enlarged section of the (100) Bragg peak for PSBTBT and P3HT during drying of the polymer:fullerene blend in (b) and (d), respectively.Cartoons in (c) and (e) illustrate the associated mechanisms of nucleation and crystal growth.



processed substrate exhibited 5% PCE while other solar cells atthe same substrate showed poor performance. It is important tonote that in spite of the large variability of the results, all the J−V curves exhibit a similar Jsc. This implies that the nano-morphology of the BHJs is sufficiently “good”, such that chargecarriers can be transported to the electrodes. The strongincrease in Voc obtained in the second characterization run, onthe other hand, suggests that interface effects could beresponsible for initially reduced solar cell efficiency. Thisinterpretation is in agreement with Lu et al., who showed theincrease of PC71BM concentration at the BHJ film−cathodeinterface due to thermal annealing after cathode deposition.35

Figure 5b shows the same devices after thermal treatment at150 °C for 5 min. After the thermal annealing step at 150 °Cfor 5 min, no difference between both characterization runsexists anymore and all J−V curves overlap.Thickness measurements before and after annealing by

surface profiling resulted in 3.4% shrinkage upon thermalannealing (Table S3). The shrinkage occurs mostly due to theevaporation of residual solvent which could promote structuralrearrangement due to its plasticizing effect. The structure of thedried films before and after annealing has been examined byGIXD. The 2D scattering patterns exhibit similar structural

features without apparent signs of enhanced order ororientation distribution changes after annealing the films(Figure 6a,b). The scattering profiles reveal a slight increaseof the PSBTBT scattered intensity after annealing arising fromthe population of disoriented crystallites (Figure 6c). Incontrast, the intensity and width of the PSBTBT (100) peak(i.e., from the population of edge-on crystallites) remainunchanged showing only a small shift of the momentumtransfer, corresponding to a 1.5% increase of the associate layerspacing (from 17.80 to 18.09 Å). This can be appreciated inFigure 6d where a scattering profile along ∼qz is plotted aftersubtraction of an off-axis profile to subtract the intensitybackground and the contribution of the PSBTBT disorderedphase (Figure 6d). Thus, the observed structural changes takingplace in the film during the thermal treatment cannot becorrelated with the improvement and homogenization of theoptoelectronic performance. The thermal treatment probablyled to a sort of equilibration of the film−cathode interfacewhich results in the strong increase in Voc and thehomogeneous device performance. The average PCE stronglyincreased while the maximum value decreased slightly. Thiscould originate from an improved state at the interface butslightly worsened bulk morphology due to excessively increasedPC71BM cluster size upon annealing which is a commonphenomenon for P3HT:PC61BM blends.36

Figure 4. (a) Distribution of (100) orientation as a function of theazimuthal detector angle (background subtracted) at different stages ofdrying. Besides the increase of intensity around the detector angle ∼0°(predominant edge-on orientation), an increase of intensity alsooccurs at all azimuthal angles (observed as an upward shift of thewhole curve) revealing a population of misoriented crystals. (b) Timeevolution of the normalized intensity corresponding to bothpopulations of crystals, with edge-on (100) orientation (whitesquares) and random orientation (black squares).

Figure 5. Current density−voltage (J−V) characteristics of (1:2)PSBTBT:PC71BM solar cells cast by doctor blading from DCB at 40°C drying temperatures under ambient conditions. J−V curves of threesolar cells after the first (dashed lines) and second (solid lines)characterization run for (a) an as-cast device and (b) the same butsubsequently thermally treated device (5 min at 150 °C). Maximumand average efficiencies are given in each graph.



The effect of the substrate temperature during doctor bladinghas also been addressed. The comparison of PCEs of as-castand annealed solar cells is shown in Figure 7a for differentdrying temperatures ranging from 40 to 85 °C. (A summary ofsolar cell characteristics can be found in Figure S3 and TablesS1 and S2 of the Supporting Information.) The strong increaseof reproducibility and average PCE of the solar cells afterannealing occurs for all drying temperatures. Anotherimportant observation is that no correlation between PCEand drying temperature can be observed within the deviation ofdevice performance. This is in agreement with the lightabsorption of the solar cells as shown in Figure 6b, which showssimilar absorption spectra for all film processing temperatures.

While the absorption peak at 700 nm is already present in neatPSBTBT solutions, the peak at 760 nm can be attributed topolymer chain π−π interaction.17 Similar peak positions andshape of absorption spectra indicate a very similar effectiveconjugation length and similar amount of π−π packed polymerchains, respectively. The 2D GIXD patterns obtained forPSBTBT:PC71BM films dried at three different temperatures(25, 40, and 80 °C) corroborate the structural similaritybetween films and evidenced the coexistence of interface-oriented and misoriented crystallites of PSBTBT for all thefilms (Figure S4). The relative insensitivity of the nano-morphology formation to the drying conditions is possibly aresult of the strong interaction of PSBTBT molecules, causingtheir aggregation in solution.17 Hence, the process−structure−property relationship of BHJs of PSBTBT:PC71BM andP3HT:PC61BM are remarkably different, the last being largelyinfluenced by the drying conditions such as temperature anddrying gas velocity.9−12 Since the optoelectronic deviceproperties of PSBTBT do not suffer under increased dryingtemperatures (and short drying times), these materials shouldbe advantageous for fast production speeds.

IV. CONCLUSIONSIn conclusion, in-situ GIXD during film drying revealed thatnucleation and crystal growth of PSBTBT take place at aninterface (substrate/film or air/film) in defined orientation and

Figure 6. 2D scattering pattern before (a) and after (b) the thermaltreatment measured on PSBTBT:PC71BM (1:2) on a native siliconsubstrate. (c) Scattering profiles obtained with an integration apertureof 10° (marked in the inset) before (black) and after annealing (red).(d) Scattering profiles across the PSBTBT (100) peak marked withblack dashed lines in the inset upon subtraction of an off-axis profile(white dashed lines in the inset).

Figure 7. (a) Average power conversion efficiencies (PCEs) for the as-cast solar cells (filled symbols) and after subsequent thermallytreatment (open symbols) of solar cells dried at different temperatures.(b) Normalized absorption spectra of as-cast PSBTBT:PC71BM filmstaken from the same samples. The spectra are normalized to theabsorption peak at 375 nm.



in the bulk in random orientation simultaneously. Dry filmshave two dominant orientations, i.e. edge-on and face-on,coexisting with a large distribution of misoriented domainswhich is possibly beneficial for high hole mobilities in thevertical direction.We identified PSBTBT as an efficient polymer whose

structural properties in the blend are not significantly affectedby means of the different drying temperatures in the range of25−85 °C. Solar cells fabricated by doctor blading underambient conditions exhibited efficiencies up to 4.6−5.0% in theinvestigated temperature range. This is in the range of reportedefficiencies for spin-coated devices under inert conditions whichis promising toward large-scale fabrication under ambientconditions. The insensitivity of PSBTBT:PC71BM filmmorphology and optoelectronic device properties to coatingand drying process parameters allows for an optimization ofcoating quality and fabrication throughput without drawbacksin solar cell performance.

■ ASSOCIATED CONTENT*S Supporting InformationFurther GIXD data, AFM images, J−V curves, and solar cellperformance data. This material is available free of charge viathe Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: (B.S.-H.) [email protected]; (E.B.) [email protected] Address#Cavendish Laboratory, Department of Physics, University ofCambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, U.K.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe present work was supported by the German ResearchFoundation (DFG) within the Priority Program 1355. Financialsupport by the MICINN of the Spanish Government (ProjectMAT2010-20020). H. J. Egelhaaf and M. Morana of KonarkaTechnologies Inc. we thank for their generous supply ofPSBTBT and their support. The authors further thank H.Holscher and the Karlsruhe Nano and Micro Facility (KNMF)for providing access to the AFM, N. Mechau for access to aDektak 6M, and T. Basile for his support.

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