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Identifying Charge-Transfer States in Polymer:Fullerene Heterojunctions by Their Emission Polarization Anisotropy Andreas P. Arndt, Marina Gerhard, Martin Koch, Uli Lemmer, and Ian A. Howard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12853 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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The Journal of Physical Chemistry

Identifying Charge-Transfer States in Polymer:Fullerene Heterojunctions by Their Emission Polarization Anisotropy

Andreas P. Arndt,a Marina Gerhard,b Martin Koch,b Uli Lemmer,a,c and Ian A. Howard*,c a

Light Technology Institute, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, D-76131

Karlsruhe, Germany b

Faculty of Physics and Materials Science Center, Philipps-Universität Marburg, Renthof 5, D-35032

Marburg, Germany c

Institute of Microstructure Technology, Karlsruhe Institute of Technology (KIT), Hermann-von-

Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany;

* Corresponding author: [email protected] (e-mail); +49 0721 608 28398 (phone); +49 721 60842590 (fax)

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ABSTRACT We investigate the time-resolved photoluminescence (PL) polarization anisotropy of the organic photovoltaic model systems P3HT:PC61BM and PTB7:PC71BM, and their corresponding neat polymer films. Both blends show a strong emission from interfacial charge-transfer states (CTS); the CTS emission can be uniquely identified by its negative polarization anisotropy, which is indicative of a significant rotation of the transition dipole moment upon charge-transfer at the donor-acceptor interface.

We also observe a spectral region showing negative anisotropy in the PL of the neat PTB7 film; this spectral region is at slightly higher energy than the blend CTS emission. This negative anisotropy is not observed in the emission spectrum of the polymer in solution. This suggests that some emissive states in the pristine PTB7 film have transition dipole moments rotated relative to the absorbing states; we interpret these to be states with a significant interchain charge-transfer character.

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INTRODUCTION The relationship between photophysical processes and mesoscale morphologies is matter of significant interest in the field of organic photovoltaics (OPV). Numerous experimental and theoretical studies have been recently conducted to address the influence of various aspects as morphology,1-3 exciton delocalization and ultrafast charge separation,4-10 interfacial dipole moments,11-13 and the role of excess energy14-17 on the charge separation process. All this work refines the simplified picture of photoexcitations that diffuse within the bulk heterojunction (BHJ) to the donor-acceptor interface, and subsequently form charge-transfer states (CTS) that either split into separated charges or geminately recombine. The CTS refers to a state that is formed by charge-transfer from the electron donor to the electron acceptor at a heterojunction within the BHJ film.18 Polarization anisotropy studies have proven to be a powerful tool for identifying exciton diffusion and relaxation pathways since a reorientation of the transition dipole moment goes along with the migration of excitations. Transient absorption spectroscopy (TAS) tracking the polarization anisotropy has been performed on pristine P3HT,19,20 and on P3HT:PC61BM films,21,22 yielding valuable insights into excitonic relaxation in these systems. Recently, polarization anisotropy measurements of the emission from P3HT and PTB7 in solution revealed coherent as well as incoherent exciton diffusion dynamics.23 Polarization sensitive photoluminescence (PL) measurements have been also conducted on pristine and blend films but those experiments could not resolve any CTS emission.24,25 However, an unique polarization anisotropy characteristic of the CTS emission is expected since a substantial change of the transition dipole moment accompanies the charge-transfer. In this work, we study the polarization anisotropy of the time-resolved emission of singlet excitons and CTS in P3HT:PC61BM and PTB7:PC71BM solar cells, and additionally in the corresponding neat P3HT 3



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and PTB7 polymer films. We find that the CTS are easy to identify with this technique; uniquely, they have a negative anisotropy. The negative anisotropy is due to the rotation of the emission transition dipole moment of the CTS with respect to the dipole moment of the absorbing chromophores (with the transition dipole moments responsible for absorption lying along the polymer backbone). This unique signal allows not only an easy and robust identification of CTS population but also further insights into the relaxation processes of the emissive CTS manifold. We study these two model systems in particular because they differ from each other in two aspects that we expect could effect the change in the emission transition dipole moment upon charge-transfer. Firstly, P3HT tends to crystallize in the condensed phase whereas PTB7 remains comparably amorphous in the optimal blend morphology.26-29 Secondly, in contrast to the homopolymer P3HT the copolymer PTB7 is composed of alternating BDT (benzodithiophene) and TT (thienothiophene) moieties which show different electron affinities.30,31 Chen and coworkers state that this leads to a significant exciton polarity on the polymer backbone, and argue on basis of their TAS data that this polarity facilitates the formation of a potentially intramolecular ‘pseudo’ CTS or even an intermolecular CTS between PTB7 polymer chains.31-33 For both blend systems we find CTS emission, which we can identify by its unique negative anisotropy. In addition, our work supports the idea of an intermolecular CTS within the pure PTB7 phase as we also observe a negative anisotropy within a certain spectral region of the emission from the pristine PTB7 films.

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EXPERIMENTAL METHODS Sample fabrication All organic films were spin coated from solution onto structured indium tin oxide coated glass, and integrated into a device stack with front and back contacts in order to obtain photovoltaic cells. Regioregular poly(3-hexylthiophene) (rr-P3HT, purchased from Rieke Metals) and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM, purchased from Solenne BV) were dissolved in dichlorobenzene (anhydrous, purchased from Sigma-Aldrich), spin coated on top of a 20 nm thick layer of poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), and finally dried under solvent atmosphere. In a last step we thermally evaporated 0.7 nm lithium fluoride and 200 nm aluminum onto the stack. According to our previous work, we chose a blend ratio of 1:2 with a clear excess of the fullerene derivative. This donor to acceptor ratio leads to a poor power conversion efficiency (PCE) of only 0.85 % (see Figure S1a for the current-voltage characteristic in the Supporting Information) but reveals a much more pronounced signature of the CTS emission compared to the optimized blend ratio with regard to the PCE.34 In case of thieno-[3,4-b]thiophene-alt-benzodithiophene (PTB7, purchased from 1-Material Inc.) and its blend with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM, purchased from Solenne BV), we fabricated samples in an inverted architecture: The active film was spin coated from a chlorobenzene solution on top of a solution processed ZnO film made from zinc acetate dehydrate (ZAD), and finally covered by thermally evaporated molybdenium oxide and an aluminum electrode. The blend was additionally laced with 4 vol% diiodooctane (DIO) in order to decrease the domain size of the fullerene agglomerates, and therefore to enhance the solar cell performance.28 Our solar cells, consisting of a

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1:1.5 blend ratio, show a PCE of 7.5 % (the current-voltage characteristic is given in Figure S1b in the Supporting Information). Time-resolved photoluminescence We recorded the time-resolved photoluminescence spectra with a near-infrared (NIR) sensitive streak camera (Hamamatsu, C5680) which was synchronized to the output pulses of a titanium:sapphire laser with a repetition rate of 80 MHz (Spectra Physics, Tsunami). The temporal resolution is between 12 ps and 24 ps depending on the time window chosen for detection. In order to avoid sample degradation we recorded all spectra under dynamic vacuum with a pressure less than 10-4 mbar and kept the excitation fluence between 0.016 µJ/cm² and 2.7 µJ/cm². The excitation wavelengths were chosen for preferential excitation of the polymer. In case of P3HT and P3HT:PC61BM, we excited with 460 nm and 435 nm, respectively, making use of the second harmonic generation by a lithium triborate (LBO) crystal. We excited both PTB7 and PTB7:PC71BM at 705 nm. A helium flow cryostat enabled us to control the sample temperature between 10 and 290 K. The time-resolved emission polarization anisotropy is calculated according to:35

() =

ǁ () − () ǁ () + 2 ()

The fluorescence polarized parallel (ǁ ) and perpendicular ( ) to the linearly polarized excitation beam were detected with a polarizer between sample and spectrograph. The perpendicular polarized emission is corrected by a wavelength dependent correction function that takes into account the different transmission efficiencies of our instrument (primarily stemming from the grating within the spectrograph) for parallel and perpendicular polarized light. We determined the correction function from 6


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the recording of the parallel and perpendicular polarized spectrum of the non-polarized emission of a halogen lamp. For verification of the non-polarized and isotropic nature of the radiation, the lamp itself was also rotated, and the correction function recalculated. The correction function was invariant under lamp rotation, confirming that the lamps emission was unpolarized and that our correction function was appropriately obtained.

RESULTS AND DISCUSSION Emission polarization anisotropy of pristine polymer films At 10 K the fluorescence spectra of the pristine P3HT film show distinct vibronic bands with an intensity maximum for the 0-1 transition at 1.71 eV (see Figure 1a). The spectrum smooths out at room temperature due to the increased vibrational induced broadening. The overall behavior of the emission spectra follows the description of weakly coupled H-aggregates.36,37 We observe a time-dependent bathochromic shift of the emission peaks by 50 meV, and a fluorescence lifetime of 470 ps at room temperature. Also, the time-resolved anisotropy of P3HT in Figure 1b shows pronounced temperature dependence. The initial anisotropy changes only slightly with temperature (from 0.1 at high to 0.15 at low temperature, see also Figure 5 for initial anisotropy values at different emission energies), which has been explained in previous work by excitonic self-trapping in H-aggregates within a sub-picosecond scale after excitation.19 But the anisotropy decay is clearly influenced by thermal energy: At 290 K, the emission is almost completely depolarized after 700 ps with time constants = 24 and

= 317 from a biexponential fit to the anisotropy decay (see Figure 1b). However, the 7



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depolarization process is slowed at 10 K leading to decay times of = 35 and = 1738 , so, that at low temperature even for the slower emission a significant polarization anisotropy remains. The full results of the biexponential fits are given in Table 1. In contrast to Paquin et al., who found almost no dynamic decay of the anisotropy at low temperature, we observe a minor component of dynamic anisotropy decay.19 This could perhaps be related to our slightly higher excitation energy of 2.70 eV, which excites more preferentially the amorphous instead of the aggregated phase of P3HT, or due to variations in material quality or processing.

Figure 1. P3HT at 10 and 290 K. a) Time-integrated (0 – 710 ps) spectra polarized parallel (ǁ) and perpendicular () to the excitation polarization, and b) time-resolved emission polarization anisotropy. (Excitation energy of 0.05 µJ/cm² per pulse at 460 nm) 8


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Since torsional relaxation, vibrational relaxation, and coherent diffusion proceed within our instrument response time, the dynamic polarization loss observed in our experiment is purely due to energy migration in the inhomogeneously broadened density of states.22,23,38 Thus, the observed rate of the delayed emission anisotropy decay can be taken as a measure for the rate of movement between energy sites that involve a change of the transition dipole moment.

A1 τ1 A2 τ2

10 K 0.040 ± 0.004 (35 ± 7) ps 0.097 ± 0.002 (1738 ± 137) ps

290 K 0.045 ± 0.005 (24 ± 6) ps 0.065 ± 0.003 (317 ± 17) ps

Table 1. Parameters of the biexponential decay function () = ∙ exp (−/ ) + ∙ exp (−/ ) fitted to the time-resolved anisotropies shown in Figure 1b.

The time-resolved anisotropy of the low band gap co-polymer PTB7 differs in two qualitative respects from the observations on the homopolymer P3HT. Firstly, we observe a weak but distinct negative anisotropy near the red-edge of the PTB7 emission; we will discuss this in further detail below. Secondly, the kinetics of the anisotropy decay are entirely different. We find that the anisotropy left after the instrument response time for the PTB7, which is much lower than for P3HT, stays constant over the remaining emission lifetime. Figure 2b shows that the anisotropy for the high energy emission decays to a value of 0.05 within the instrument response time but then does not decay further, regardless of temperature. These observations indicate that in PTB7 most of the anisotropy of the exciton population is lost with fast initial motion, but that some anisotropy is held in the position where the exciton spends the majority of its life. This finding is consistent with transient absorption anisotropy 9



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observations of PTB7 in solution; they reveal that most of the anisotropy decays within 20 ps, but that once the anisotropy reaches about 0.1 its decay ceases (this is also confirmed by our solution anisotropy measurements, see Figure S2 in the Supporting Information).23 That the initial anisotropy of P3HT is substantially higher than for PTB7 can be explained by the partly crystalline and lamellar structure of the P3HT film morphology which ensures a longer conservation of the excitation polarization even when the excitons are mobile, and therefore slows down the initial anisotropy decay in P3HT. PTB7 on the other hand does not crystallize and rather forms amorphous morphologies within a solid film. That the anisotropy decay stops at a certain point in PTB7 (both in solution and film) indicates that, for at least a subset of the total population, after a given period of initial motion the exciton stalls, or the motion is confined to regions wherein the transition dipole moment does not change. The fluence dependence of the exciton lifetime (see Figure S3 in the Supporting Information) indicates, however, that not all PTB7 excitons are immobile on the timescale of constant anisotropy. The decreasing exciton lifetime with higher excitation fluence reveals that some excitons are mobile, or at least close enough to annihilate, while the anisotropy is constant. These mobile excitons could have already completely lost their initial anisotropy, and therefore their motion no longer affects the anisotropy on this timescale. The precise origin of the retained positive anisotropy in the PTB7 film remains an open question; that the kinetics of the positive anisotropy are similar across the entire emission spectrum suggests that this observation is not due to restricted mobility in a low energy subset of the population, but a process relating to excitons in PTB7 in general.

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Figure 2. PTB7 at 10 and 290 K. a) Time-integrated (0 – 1950 ps) spectra polarized parallel (ǁ) and perpendicular () to the excitation polarization (the spectral range of the CTS is marked in orange). b) Time-resolved polarization anisotropy of the charge-transfer state (CTS) emission (open symbols, 1.21 - 1.40 eV at 10 K, and 1.28 – 1.42 eV at 290 K), and of the singlet exciton (S1) emission (filled symbols, 1.40 - 1.70 eV at 10 K, and 1.42 – 1.70 eV at 290 K). (Excitation energy of 0.016 µJ/cm² per pulse at 705 nm)

Returning to the most pronounced way in which the anisotropy of the PTB7 film differs from the one of the P3HT film, we note that the anisotropy of PTB7 is distinctively negative (-0.02) between 1.21 and 1.40 eV at 10 K, and between 1.28 and 1.42 eV at 290 K (see Figure 2). The angle

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transition dipole moment of the absorption and emission can be calculated by the Perrin equation, "#$%& '( 35 ).

= ( !

Thus, an anisotropy of -0.02 corresponds to a 57° rotation between the absorbing

and emitting dipole moments. This large rotation of the emission dipole from the absorbing dipole indicates that some charge has to be relocate within or between molecules prior to emission. Chen and co-workers argue that such local charge-transfer character on the PTB7 polymer is much more likely than for P3HT as the local exciton polarity estimated by the difference in the excited-state and groundstate dipole is about a factor ten higher in PTB7.39 But in contrast to their postulated intramolecular CTS for PTB7,32 our observed PL signature of negative anisotropy is rather due to an intermolecular CTS within the PTB7 phase since we do not observe any negative anisotropy in diluted PTB7 solution; in solution the anisotropy in this region is, as for all other regions, strictly positive (see Figure S2 in the Supporting Information). Interestingly, the observed angle of rotation between absorption and emission dipole is similar to what we see in the next section for the CTS emission at the polymer:fullerene heterojunction.

Emission polarization anisotropy of polymer:fullerene blends Having characterized the emission anisotropy of the pristine polymer films, we now turn to examine how anisotropy measurements can be used to identify interfacial CTS emission in polymer:fullerene blends. Whereas CTS emission can be identified from its dependency on the blend composition, temperature and excitation conditions in both blends as previously reported by our earlier work,34,40 the emission anisotropy measurements give a clear and immediate signal establishing the spectral region of the CTS

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emission. In addition, further information about the formation of the CTS can be gained by time resolving the emission anisotropy. The time-integrated blend PL spectra parallel and perpendicular to the excitation polarization for the two polymer blends under investigation are shown in Figure 3a and 4a.

Figure 3. P3HT:PC61BM at 10 K and 290 K. a) Time-integrated (0 – 700 ps) spectra polarized parallel (ǁ) and perpendicular () to the excitation polarization, and b) time-resolved polarization anisotropy of the charge-transfer state (CTS) emission between 1.18 and 1.33 eV, and the higher energetic polymer singlet (S1) excitons between 1.46 and 1.91 eV. (Excitation energy of 1.6 µJ/cm² per pulse at 435 nm)

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In the regions of polymer singlet exciton emission the anisotropy is, as expected, positive. On the other hand, around 1.35 eV for P3HT:PC61BM, and 1.30 eV for PTB7:PC71BM a broad emission feature appears wherein the luminescence polarized perpendicularly to the excitation polarization is more intense than the parallel emission; as a consequence the anisotropy in this region is negative. Obviously, these two distinct regions of anisotropy indicate that at least two different fluorophores are responsible for the emission, with one fluorophore significantly rotated with respect to the dipole moment responsible for the absorption. The spectral regions that show negative anisotropy match the spectral regions identified as CTS emission in our previous work on the basis of blend-composition- and temperature-dependent measurements of the emission lifetime.34,40 In Figure 3b and 4b the time-dependent decay of the anisotropy is presented for the region of the singlet exciton and CTS emission. The singlet exciton lifetimes are strongly decreased within the blends, their decay rates increase by about one order of magnitude compared to their lifetimes within the neat polymer films, meaning that an accurate measurement of the singlet exciton anisotropy in the blends is challenging at late emission times due to the very small singlet exciton populations remaining. For the P3HT blend we can measure accurate singlet exciton anisotropies until 150 to 200 ps, depending on temperature, but for the PTB7 blend the time-resolved exciton anisotropy becomes noisy directly after the instrument response, and is consequently not presented. Yet the time-integrated spectra clearly show the positive anisotropy in the higher energy region of the pure singlet exciton emission in Figure 4a. In the case of the P3HT blend, shown in Figure 3b, the positive anisotropy of the singlet exciton decays on a hundred-picosecond timescale. Interestingly, the initial anisotropy of the P3HT singlet excitons in the blend is substantially higher than in the pristine polymer film by a factor of two higher (see Figure 1b). Initial anisotropy values in the blend are 0.3, which is close to the initial anisotropy of 0.4 expected for 14


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excitation of randomly distributed transition dipole moments. This indicates that the emissive excitons preserve most of the excitation polarization. The larger initial anisotropy of the P3HT singlet excitons in the blend suggests that the quenching of the singlet excitons that would rapidly lose their polarization anisotropy occurs extremely rapidly due to charge-transfer across the donor-acceptor interface in the blend. This leaves only the slower-moving singlet excitons to emit in the blend whose anisotropy does not decay that quickly; i.e. those excitons that are either in the central region of a crystalline phase or the ones which are relatively immobile. So the higher anisotropy in the blend can be explained by extremely rapid charge-transfer selectively removing the emission of mobile singlet excitons. The further decay of the singlet exciton anisotropy on the hundred-picosecond timescale indicates that the P3HT singlet excitons in the blend that are not immediately quenched are still mobile, similar to how they are in the pristine film. This further motion of the singlet excitons could lead to further charge separation, but the significantly higher value of the singlet exciton anisotropy is in agreement with the established notion that the majority of the exciton quenching happens on a timescale significantly faster than the 12 ps instrument response time of our system. The kinetics of the CTS anisotropy in the P3HT:fullerene blend shown in the lower part of Figure 3b are different from those of the singlet excitons as the anisotropy starts at a negative value, and remains approximately constant over the lifetime of the CTS. The initial anisotropy decay in the CTS region within the first 200 ps could stem either from the motion of singlet excitons within the low energy tail whose emission energetically overlaps with the CTS emission, or from the changing ratio between singlets (with positive anisotropy) and CTS (with negative anisotropy) in the first 200 ps. Irrespectively, the instantaneous negative anisotropy in the CTS region again suggests that a large number of CTS are created within the instrument response of our setup. The kinetics of the anisotropy of the CTS does not

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depend on temperature, indicating that there is not a temperature-activated motion of the CTS in P3HT:PCBM, consistent with our earlier analysis of the temperature-dependent emission kinetics.34

Figure 4. PTB7:PC71BM at 10 K and 290 K. a) Time-integrated (0 – 1950 ps) spectra polarized parallel (ǁ) and perpendicular () to the excitation polarization, and b) time-resolved polarization anisotropy of the charge-transfer state (CTS) emission between 1.10 and 1.31 eV. (Excitation energy of 2.7 µJ/cm² per pulse at 705 nm) According to the Perrin equation a negative anisotropy of -0.05 refers to a tilt angle of 60°. Although this value is lower than the 90° flip of the transition dipole moment upon charge-transfer at an organic heterojunction suggested by transient absorption spectroscopy,21,22,41 it still strongly indicates the 16


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interchain character of the emission. Our result shows a good agreement to the theoretically calculated value of 70° for the P3HT:PC61BM interface by Grancini et al.21 That the rotation of the emission dipole moment is less than the perhaps intuitive 90° flip could be explained by the fact that there is likely a broad distribution of angles between the electron and hole across the polymer:fullerene heterojunctions in a BHJ device, which hence leads to an expectation value for the rotation angle between CTS emission and polymer absorption of less than 90° when averaged over the entire interface configurations. In addition to this, intensity borrowing of the CTS transition dipole moment from the polymer excited-state can also play a relevant role in reducing the observed rotation.25,42-44 The intermixed character of the CTS wave functions contain fractions of the pure donor and acceptor excited state wave functions, this can lead to transition dipole moments of the CTS which contain a significant contribution of the alongbackbone transition dipole moment of the excited donor state. For a few other material systems the intensity borrowing of the CTS from the excited donor state was found to be even that strong that a dominant alignment of the CTS transition dipole moment along the direction of the excited polymer transition dipole was observed.25,45 These results emphasize that the specific tilt angle of the transition dipole moment upon charge transfer depends highly on the electronic properties of the involved donor and acceptor moieties. Importantly, the change in anisotropy is large enough to unambiguously distinguish the CTS from the singlet exciton emission, with the negative anisotropies we observe providing particularly clear contrast. Figure 5 summarizes the key finding of this paper: the polarization anisotropy as a function of emission energy can unambiguously distinguish the spectral region of CTS emission from the spectral region of singlet exciton emission. We present the initial anisotropy calculated from the emission within the instrument response of our setup, which furthermore shows that these CTS are created immediately 17



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upon excitation, and are not selectively created by excitons that slowly diffuse to an interface. Looking across the anisotropy as a function of emission energy, the regions wherein the emission is immediately and drastically rotated from the absorption transition dipole are clearly apparent. Also the emission signature of the PTB7 pristine film shows a region of emission from an interchain species, but this spectral region is distinct from the PTB7:fullerene CTS (these emissions are also significantly different in terms of kinetics, see Figure S4 in the Supporting Information). We note that the interchain emission feature observed in the neat PTB7 is not seen in the PTB7:PC71BM blend; this could support the idea that the CTS within the PTB7 phase could facilitate the further interfacial charge-transfer between the polymer and fullerene.

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Figure 5. Initial emission polarization anisotropies for a) P3HT and P3HT:PC61BM, and for b) PTB7 and PTB7:PC71BM at 10 and 290 K.

CONCLUSION We have shown for both P3HT:PC61BM and PTB7:PC71BM that an interfacial CTS emission becomes immediately apparent in the time-resolved emission polarization anisotropy. From the instantaneously present negative anisotropy in the spectral region of the CTS emission, and especially from the much higher initial anisotropy of the P3HT singlet exciton emission in the blend than in the pristine film, we conclude that these CTS are formed on a sub-picosecond timescale. Our data indicates a typical change

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of 60° between the absorption and emission dipole moment upon charge-transfer at the polymer:fullerene interface on average. Moreover, the polarization anisotropy of the CTS emission remains constant over its picosecond lifetime suggesting that those emissive CTS are effectively immobile and localized at the donor-acceptor interface. Additionally, we find an emission feature characteristic of an interchain CTS in the neat PTB7 film. This feature offers strong corroboration of the CTS in neat PTB7 films previously suggested by transient absorption spectroscopy.32 In summary, emission polarization anisotropy studies, especially those that are time-resolved, provide a simple method of unambiguously identifying emission from CTS in polymer:fullerene blends. The robust identification of CTS emission is important to the organic photovoltaic community, as correctly identified CTS emission can provide direct insight into the excited-state dynamics at the internal heterojunctions. This information, difficult to obtain by other methods, should be useful in testing and developing detailed models that strive to rigorously describe the physics of organic semiconductor heterojunctions. Especially, the evaluation of the CTS transition dipole moment orientation with respect to the initial polymer excitation could, in conjunction with complementary investigation techniques, lead to a further understanding of the relevant requirements for free charge generation in organic solar cells.

Supporting Information Current density-voltage characteristics of the investigated solar cells; emission polarization anisotropy of a diluted PTB7 solution; excitation fluence dependent PL transients of PTB7 film; PL transients of the CTS signatures in PTB7 and PTB7:PC71BM.

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The Supporting Information is available free of charge on the ACS Publications website.

ACKNOWLEDGEMENTS The authors acknowledge the German Research Foundation (DFG) for funding within the SFB 1176 and SFB 1083. IAH thanks the BW-Stiftung for financial support.

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