Influence of Intermixed Donor and Acceptor Domains on the

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Influence of Intermixed Donor and Acceptor Domains on the Ultrafast Charge Generation in Bulk Heterjunction Materials Chengmei Zhong, Jonathan A. Bartelt, Michael D. McGehee, Derong Cao, Fei Huang, Yong Cao, and Alan J. Heeger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09572 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 14, 2015

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The Journal of Physical Chemistry

Influence of Intermixed Donor and Acceptor Domains on the Ultrafast Charge Generation in Bulk Heterjunction Materials

Chengmei Zhong† , Jonathan A. Bartelt ǁ, Michael D. McGehee ǁ , Derong Cao§, Fei Huang†, Yong Cao†, Alan J. Heeger*‡ † State Key Laboratory of Luminescent Materials and Devices, South China University of

Technology, Guangzhou 510641, China. ‡ Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106–

5090 , USA §. School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou 510641, China.

ǁ Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.

ABSTRACT Understanding how intermixed domains and pure domains in bulk heterojunction (BHJ) thin films affect charge generation and recombination in polymer solar cells (PSCs) is important to further improve PSC device performance. We control the proportion of intermixed domains and pure domains in an amorphous polymer and fullerene BHJ thin films by varying the fullerene concentration. By studying these thin films with ultrafast spectroscopy we found that the coherent ultrafast charge generation process is non-existent in this BHJ system due to the lack of pure polymer domains. We also found that the BHJ thin film with 100% intermixed domains 1

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begins charge recombination immediately after the initial charge generation process, while the BHJ thin film with pure fullerene domains showed an additional slow excitonic charge generation process before a slower charge recombination takes place. These results clearly demonstrate that pure domains are important for optimal BHJ solar cell performance.

1. INTRODUCTION After twenty years of development, solution-processed polymer solar cell (PSC) technology 1 continues to attract more attention because of the potential application in large-area and flexible photovoltaic modules via low-cost roll-to-roll processing. 2 The active layer of a PSC is a bulk heterojunction (BHJ) thin film, which is a blend of a conjugated polymer acting as electron donor and a fullerene derivative as electron acceptor. The morphology of the BHJ layer is known to critically influence the PSC device performance. It was long believed that the typical morphology of BHJ thin films comprises phase separated domains of the pure donor polymer phase and the pure acceptor. However more thorough studies in recent years 3 revealed that the BHJ morphology is more complicated. For most polymer-fullerene systems, fullerene derivatives are miscible in disordered polymer phases 4, 5 but are not miscible in the ordered pure polymer domains. 6 As a result, there are two types of distinct phases in a typical BHJ thin film: (1) pure domains with a low level of intermixing such as polymer aggregates or crystallites and/or fullerene clusters and (2) intermixed domains, which are disordered and composed of amorphous polymer and fullerene intimately mixed at the molecular level. More recent work has attempted to establish connections between these two types of phases in BHJ morphology and the charge generation 7,8 charge transport 9 and charge recombination 10 properties in BHJ PSCs. Thus a new interesting question has emerged: What roles do the intermixed domains and pure domains 2


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play respectively in these critical electronic processes? State of the art experimental 11,5 as well as simulation results 12 show that the intermixed domains can facilitate charge separation more efficiently compared to pure domains, albeit at the expense of more rapid charge recombination. However, the vast majority of high efficiency BHJ materials have pure domains in addition to intermixed domains, so understanding how both types of domains affect device performance is of paramount importance. In a series of papers 13, 14, 15 , our group demonstrated ultrafast charge transfer (time scale less than 100 fs) in polymer-fullerene BHJ blends. The interpretation of this work has been criticized as resulting from local charge transfer in the intermixed domains rather than indicative of ultrafast charge transfer between pure domains. In this paper we address this issue directly by studying the ultrafast charge generation dynamics in BHJs with selected morphologies and a select number of phases using femtosecond transient absorption spectroscopy (TAS). It is now widely accepted that an ultrafast charge generation process, which occurs within 100 fs after light excitation without the need for exciton diffusion, is present in many high performance BHJ materials 13,16,17,18 The mechanism behind this ultrafast process, however, is still not fully understood. Recent results have shown that this process is strongly related to the quantum coherence properties of the materials, 19,20 and new breakthroughs in PSC performance could potentially be made if researchers could understand and exploit this charge generation mechanism. Initial TAS studies on multiple high efficiency BHJ systems by L. Kaake et al 13, 14 suggested that the ultrafast charge generation process is insensitive to morphology, but follow-up studies have shown that the contribution of ultrafast charge generation process to total charge generation (ultrafast generation and exciton diffusion generation combined) can be affected by morphology. 12, 21 It is therefore our intention to clarify the relation between BHJ morphology 3



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and the ultrafast charge generation process, and whether this process is indeed a coherent process different from classical Marcus electron transfer theory. We note that several recent TAS studies 22, 23

have examined the charge generation process in BHJ solar cells; however, how the

morphology and presence of pure and/or intermixed domains affects the ultrafast charge generation process is still not well understood. Charge generation in this ultrafast time regime and the role of morphology and intermixed domains is the focus of this paper. We chose to study the BHJ system comprised of regiorandom poly(3-hexylthiophene2,5-diyl) (rra-P3HT) as the donor polymer and [6,6]phenyl- C61-butyric acid methyl ester (PCBM) as the acceptor. (The molecular structures are shown in Figure S1 of the Supporting Information.) Regiorandom P3HT is an amorphous isomer of the more commonly used regioregular P3HT (rr-P3HT). The rationale behind the choice of rra-P3HT in this study is that it readily forms intermixed domains with PCBM because of the high miscibility 4 between the two materials. By using an amorphous polymer, our BHJ samples are entirely intermixed and do not have pure polymer crystallites or aggregates. Changing the blending ratio between rra-P3HT and PCBM, alters the composition of the intermixed phase, and if enough PCBM is added to the blend, a pure PCBM phase is formed. Therefore, by studying rra-P3HT:PCBM thin films with different blend ratios using TAS as the measurement technique, the correlation (or anticorrelation) between ultrafast charge generation and intermixed domains can be clearly revealed. In addition, we measured the influence of intermixed domains on the charge recombination rates in the BHJ films.

2. EXPERIMENTAL METHODS GIXD sample preperation and measurement. Thin-films of rra-P3HT (Sigma Aldrich) and PCBM (Nano-C) of varying composition were spin-cast onto PEDOT:PSS-coated silicon 4


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substrates with chlorobenzene solutions. GIXD measurements were performed at SLAC beamline 11-3 using photon energy of 12.7 keV, a MAR345 image plate area detector, a heliumfilled sample chamber, and an incident X-ray beam angle of ∼ 0.12°. TAS sample preparation and measurement. rra-P3HT:PCBM thin films were spin-cast onto glass substrates with chlorobenzene solutions. The TAS measurement system is described in our previous publications, 13,14 however the pump beam is altered to go through an optical parametric amplifier (Spectra Physics OPA 800) instead of a frequency doubling crystal to produce 488 nm femtosecond pulses (pulse width 150 fs). The pump intensity can be varied in the range of 1−90 µJ/cm2. Thin film samples were kept in a cryostat chamber (Janis ST-100) with a vacuum level of 4×10-6 mbar at room temperature. Steady-state UV-Vis Absorption measurements were performed on a commercial system (Olis Cary 14).

3. CONTROLLING BHJ MORPHOLOGY WITH PCBM CONCENTRATION We first characterize the evolution of intermixed domain in BHJ morphology while changing the PCBM concentration in the films with grazing incidence X-ray diffraction (GIXD) 24

measurements. Figure 1a shows the X-ray diffraction intensity as a function of the scattering

vector for the different blended BHJ films. rra-P3HT has diffraction peaks at q ≈ 0.3 and q ≈ 1.4 Å-1 and PCBM has three diffraction peaks at q ≈ 0.7, q ≈ 1.4, and q ≈ 2 Å-1. By determining at which blend ratio the PCBM diffraction peaks begin to appear in the GIXD data, we can estimate when the PCBM begins to phase separate and form pure PCBM clusters. Figure 1b shows the diffraction intensity at q = 0.7 Å-1 as a function of PCBM concentration. The data are normalized to the background diffraction intensity so that a value of 1 means there is not a 5



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diffraction peak at q = 0.7 Å-1. In Figure 1b, there are two areas with different slopes. From 0 to 50 wt% PCBM, the slope is lower and from 50 to 100 wt% the slope is higher. From this data, we estimate that pure PCBM clusters large enough to diffract X-rays are formed once the concentration of PCBM in the blends increases beyond ≈50 wt%. In other words, below 50 wt% PCBM, the rra-P3HT:PCBM BHJ films are likely 100% intermixed domains with the composition of the mixed phase being the overall rra-P3HT:PCBM blend ratio of the film. When the PCBM concentration increases beyond 50 wt%, the overall morphology is a combination of intermixed domains and pure PCBM clusters. Our findings agree with previous studies, which also showed that rra-P3HT and PCBM are highly miscible to levels of mixing ≥40 wt% PCBM 25, 26

Based on these GIXD results, we chose two representative BHJ films for the TAS study: rra-P3HT:PCBM=3:1 (25 wt% PCBM,) and rra-P3HT:PCBM=1:4 (80 wt% PCBM,). The morphology of the former sample consists of a single amorphous intermixed phase with an intermediate PCBM concentration (≈25 wt% PCBM), while the morphology of the latter sample consists of an amorphous intermixed phase with a higher PCBM concentration (≥40 wt% PCBM) and small interdispersed pure PCBM clusters. A pristine rra-P3HT thin film was also studied to provide the exact spectral assignment of charge (polaron) absorption. (Various thin films with different PCBM concentrations are shown in Figure S3 of supporting information)

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(a)

(b)

Figure 1. (a) GIXD intensity vs q for rra-P3HT:PCBM BHJs with differing PCBM concentration. The GIXD intensity was calculated by taking cake segments (10 ° < χ < 25 °) through GIXD images; (b) Normalized GIXD intensity at q = 0.7 Å-1 as a function of PCBM concentration.

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4. TAS RESULTS AND DISCUSSIONS Figure 2a shows the normalized TAS spectra of pristine rra-P3HT thin film at time delay of 1 ps under pump excitation at 488 nm (Close to absorption peak of rra-P3HT). The negative photo-induced bleaching features at wavelengths below 720nm in the pristine rra-P3HT spectra are a combination of ground state bleaching (GSB, 400~600nm, as determined by UV-vis absorption) and stimulated emission (SE, 600~720nm) 27. The positive photo-induced absorption (PIA) features beyond 720nm in the spectra correspond to the absorption of rra-P3HT excitons, as the dynamics are highly correlated to the SE dynamics. (See Figure S4 in the supporting information) As seen in the spectra of the two BHJ samples in Figure 2b, when PCBM is introduced into the thin film, a new PIA peak centered around 800nm emerges, which corresponds to the charge absorption of rra-P3HT (The peak intensity increases relative to the GSB peak with PCBM concentration as seen in Figure S2 in the Supporting Information), in agreement with previous TAS studies 16, 17 on rra-P3HT. Based on the pristine rra-P3HT spectrum in Figure 2a, we choose to integrate the wavelength region from 700nm to 740nm to obtain the charge population dynamics. In this wavelength region the TAS signal contribution from exciton absorption and stimulated emission are minimized because they are canceled by each other in the integration process.

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Wavelength (nm) Figure 2. (a) Comparison of normalized TAS spectrum at time delay of 1ps of pristine rra-P3HT thin film with its steady state UV-Vis absorption spectrum; (b) TAS spectra of rraP3HT:PCBM=3:1 and rra-P3HT:PCBM=1:4 at time delay of 1 ps.

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The normalized charge population dynamics of the two rra-P3HT:PCBM BHJ films during the first 2 ps after excitation are shown in Figure 3 and compared with similar data obtained from a regio-regular-P3HT:PCBM (rr-P3HT:PCBM) thin film. The charge generation processes in both rra-P3HT:PCBM samples are similar to our previous TAS studies 13, 14, 15 on high efficiency BHJ systems. Note, however that the rise time is ~500 fs which is clearly different from typical coherent ultrafast process observed in rr-P3HT:PCBM where the rise time is resolution limited (< 160 fs). We deconvoluted the experimental TAS dynamics data in Figure 3 with the instrument response function to acquire a more accurate time for the ultrafast process according to literature 28, and the results are shown in Table 1. From Table 1 we see that both rra-P3HT:PCBM samples take 450 fs to finish the "ultrafast" charge generation process, which is significantly slower than the charge generation process in rr-P3HT:PCBM (< 160 fs). As a reference we also included the GSB dynamics in Figure 3 to show that the slow initial rise time is not due to dispersion effects in the thin film, because the rise time of the GSB dynamics is within the limit of time resolution in our femtosecond laser setup (160 fs). To address this unusually slow "ultrafast process" in rra-P3HT:PCBM BHJ, we examined the properties of the donor material. The amorphous nature of rra-P3HT and its high miscibility with PCBM results in lack of pure donor domains in rra-P3HT:PCBM BHJ films. Therefore the spatial delocalization of the excited states is likely small due to the disordered energetic landscape in the intermixed domains. Since coherent ultrafast charge generation processes require a certain degree of spatial delocalization of the excited states, 13, 14 we believe that this ultrafast process is disrupted by the lack of order in the intermixed domains in rra-P3HT:PCBM BHJs. Therefore the ~500 fs initial charge generation process in Figure 3 is likely not the coherent ultrafast charge generation process observed in our previous studies. Guo et al 16 10


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examined the dynamics of rra-P3HT photoexcitations with TAS and found that the primary photoexcitations rapidly relaxed and formed singlet excitons within ~200 fs. Banerji et al 29,30 performed a similar study on the more ordered rr-P3HT and determined that the primary photoexcitation began to self-localize within ~200 fs, but a relaxed singlet exciton was not formed until ~1 ps after excitation. Thus, excited state localization occurs much more rapidly in disordered rra-P3HT polymer thin films when compared to the more order rr-P3HT thin films. As a result, the charge generation process in our rra-P3HT:PCBM BHJs likely occurs after the primary photoexcitation has relaxed and formed a singlet exciton. After formation, tthe exciton is able to quickly dissociate without the need for long range exciton diffusion because the rraP3HT is intimately mixed with a large number of PCBM molecules in the amorphous, intermixed phase. Gélinas et al 18 proposed that the large spatial delocalization of charges in pure PCBM domains play a major role in the coherent ultrafast charge generation process. This seems not to be the case, however, in the rra-P3HT:PCBM BHJ system, as the coherent ultrafast charge generation process is absent in both rra-P3HT:PCBM thin films regardless of the existence of pure PCBM domains. Table 1. Deconvoluted rise time of the ultrafast processes from Figure 3 according to Ref 28 Dynamics

Experimental Rise Time (fs)

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Charge Dynamics (~750nm) rra-P3HT:PCBM=3:1 rra-P3HT:PCBM=1:4 rr-P3HT:PCBM=1:1

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Bleaching Dynamics (~480nm) rra-P3HT:PCBM=3:1 rra-P3HT:PCBM=1:4 rr-P3HT:PCBM=1:1

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Time (ps) Figure 3. Comparison of normalized charge population dynamics and GSB dynamics of rraP3HT:PCBM=3:1, rra-P3HT:PCBM=1:4 and rr-P3HT:PCBM=1:1 from -1 to +2 ps at pump intensity of 10 µJ/cm2 .The instrument response function of the TAS system is also displayed to show the time resolution of our setup.

Though the existence of pure PCBM domains in the rra-P3HT:PCBM=1:4 sample does not facilitate efficient coherent ultrafast charge generation as analyzed above, it does significantly reduce recombination in the BHJ film compared to the rra-P3HT:PCBM=3:1 sample. This can clearly be seen in the pump intensity dependent dynamics shown in Figure 4. In Figure 4a the charge population decay of the rra-P3HT:PCBM=3:1 sample starts immediately 12


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after the initial charge generation process. This decay process is fitted well with a single exponential decay function with a decay time of 256 ps (For fitting function see Eq S1 in supporting information) independent of pump intensity, which could be geminate recombination or very fast bimolecular recombination due to the limited spatial separation of electron-hole pairs in the intermixed domains. On the other hand, the charge population in the rra-P3HT:PCBM=1:4 sample does not decay immediately after the initial charge generation process, but rather first goes through another slow charge generation process which peaks around ~10-100 ps dependent on pump intensity. The population decay process afterwards displayed a strong pump intensity dependence, which is a clear sign of bimolecular recombination. This is confirmed by the good fit obtained with the bimolecular recombination model 31 (For fitting function see Eq S2 in supporting information) with the Langevin recombination rate of 8×10-9 cm3 s-1 ,which roughly translates to a decay time of 100 ns if the initial excitation density is 1016cm-3 (short circuit condition in actual PSC devices). Since excitons can form in the pure PCBM domains in the rraP3HT:PCBM=1:4 sample, they can avoid the fate of fast recombination in the intermixed domains as is in the case of the rra-P3HT:PCBM=3:1 sample. The excitons created inside the PCBM domain must diffuse to the PCBM/ intermixed domain interface to dissociate, thus explaining the extra slow charge generation process in the rra-P3HT:PCBM=1:4 sample which is absent in the rra-P3HT:PCBM=3:1 sample. Our previous studies showed that geminate recombination is negligible in high efficiency BHJ systems, as the donor materials in these systems all possess different degrees of crystallinity to be able to form pure donor phases in the BHJ morphology 21, 32. These previous results and the observations gleaned from this study verify that pure domains are essential to limiting recombination loss in BHJ PSC devices. It should be noted that by increasing PCBM concentration, the effective excitation density in the 13



rra-P3HT domains in the BHJ films is lowered because of competing PCBM absorption, which could also help reduce charge recombination though at the expense of losing photocurrent (weaker absorption coefficient for PCBM compared to rra-P3HT).

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Time(ps) Figure 4. (a) Charge population dynamics of (a) rra-P3HT:PCBM=3:1 and (b) rraP3HT:PCBM=1:4 from -2 ps to 1.5 ns at pump intensity of 10, 30 and 90 µJ/cm2 respectively. The dots in both plots represent data and the solid lines represent fits to the data (single exponential decay fitting in Figure 4a and bimolecular recombination decay fitting in Figure 4b).

5. CONCLUSIONS In conclusion we have studied the effect of intermixed domains and pure domains on the charge generation and charge recombination processes in polymer-fullerene BHJ solar cells. By changing the PCBM concentration we were able to produce two rra-P3HT: PCBM BHJ films with distinct morphologies: rra-P3HT: PCBM =3:1 which predominantly consists of intermixed domains, and rra-P3HT: PCBM=1:4 which is a mixture of intermixed domains and pure PCBM domains. Through TAS measurements, we found that the coherent ultrafast charge generation 15



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process is non-existent in both samples due to the lack of pure polymer domains. Instead an "ultrafast process" is observed in the first ~500 fs after light excitation which is most likely limited by exciton generation in the intermixed domains. The two samples also exhibited completely different charge population dynamics: recombination in rra-P3HT: PCBM =3:1 begins immediately after the initial charge generation process, while in rra-P3HT: PCBM =1:4 there is another slow excitonic charge generation process after excitation and then bimolecular recombination takes place. These results clearly demonstrate that pure domains are important for optimal BHJ solar cell performance. Pure domains are not only critical to the coherent ultrafast charge generation process, but also significantly slow the recombination rate. Moreover, recent new findings 33suggest that pure domains can also help raise the charge carrier mobility, which is needed for high fill factor in PSC devices. Together with our new findings it is clear that in order to fully exploit the coherent ultrafast charge generation process in PSCs, the crystallinity of the donor material must not be overlooked.

ACKNOWLEDGEMENTS Support for this work was obtained from the Center for Advanced Organic Photovoltaics with funding from Department of the Navy of USA, Office of Naval Research Award No. N0001414-1-0580. (Dr. J. Paul Armistead, Program Officer), the Ministry of Science and Technology of China (No. 2014CB643501) and Guangdong Natural Science Foundation of China (Grant No. S2012030006232).

AUTHOR INFORMATION Complete Affiliations of Authors 16


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Dr. Chengmei Zhong 1)

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China.

2)

Center for Polymers and Organic Solids, University of California, Santa Barbara, CA 93106–5090 , USA

3) School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China. Prof. Derong Cao 1) State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China. 2) School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China. Corresponding Author *Email: [email protected].

Tel: +1 (805) 893-2001

ASSOCIATED CONTENT Supporting Information: Chemical structure of rra-P3HT, rr-P3HT and PCBM; Normalized TAS spectra of various rra-P3HT:PCBM thin films with different blend ratios; Normalized charge population dynamics of rra-P3HT:PCBM thin films with different blend ratios; Normalized TAS dynamics at different wavelength for rra-P3HT:PCBM=3:1 thin film; Fitting Functions for Figure 4 in the manuscript.

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Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater. 2011, 1, 82-89.

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Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney, M.; Stingelin, N.; Durrant, J. R. Fullerene crystallisation as a key driver of charge separation in polymer/fullerene bulk heterojunction solar cells. Chem. Sci. 2012, 3, 485-492.

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Burke, T. M.; McGehee, M. D. How High Local Charge Carrier Mobility and an Energy Cascade in a Three-Phase Bulk Heterojunction Enable >90% Quantum Efficiency. Adv. Mater. 2014, 26, 1923-1928.

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TOC GRAPHIC

2.0

Photocarrier Population

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Ideal Morphology 100% Intermixed Domains Intermixed Domains + Pure Acceptor Domains 1.5

1.0

0.5

0.0 6

-2

0

2

4

2

10

Time (ps)

23


4 6

2

100

4 6

1000

2


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Figure 1 167x233mm (300 x 300 DPI)


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Figure 2 170x189mm (300 x 300 DPI)



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Figure 3 98x75mm (300 x 300 DPI)


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Figure 4 134x210mm (300 x 300 DPI)


Influence of Intermixed Donor and Acceptor Domains on the

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