THE ROLE OF FRET IN NON-FULLERENE ORGANIC SOLAR CELLS

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A: Spectroscopy, Photochemistry, and Excited States

THE ROLE OF FRET IN NON-FULLERENE ORGANIC SOLAR CELLS: IMPLICATIONS FOR MOLECULAR DESIGN Bhoj Gautam, Robert Younts, Joshua H. Carpenter, Harald Ade, and Kenan Gundogdu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12807 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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The Journal of Physical Chemistry

The Role of FRET In Non-Fullerene Organic Solar Cells: Implications For Molecular Design

Bhoj R. Gautam†, §, Robert Younts†, Joshua Carpenter†, Harald Ade†, Kenan Gundogdu†*

†

Department of Physics and Organic and Carbon Electronics Laboratory, North Carolina State University, Raleigh, NC 27695, USA §

Department of Chemistry and Physics, Fayetteville State University, Fayetteville, NC 28301, USA *

E-mail: [email protected]

Abstract Non-fullerene acceptors (NFAs) have been demonstrated to be promising candidates for highly efficient organic photovoltaic (OPV) devices. The tunability of absorption characteristics of NFAs can be used to make OPVs with complementary donor acceptor absorption to cover broad range of solar spectrum. However, both charge transfer from donor to acceptor moieties, and energy (energy) transfer from high band gap to low band gap materials is possible in such structures. Here we show that when charge transfer and exciton transfer processes are both present, the co-existence of excitons in both domains can cause a loss mechanism. Charge separation of excitons in a low band gap material is hindered due to exciton population in the larger band gap acceptor domains. Our results further show that excitons in low band gap material should have a relatively long lifetime compared to the transfer time of excitons from higher band gap material in order to contribute to the charge separation. These observations provide significant guidance for design and development of new materials in OPV applications.

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Introduction Organic photovoltaic cells (OPV) continue to be promising and rapidly improving, lead-free energy harvesting technology. As the usable solar spectrum spans a large range from the visible to the infra-red (IR), an obvious direction for improved light harvesting in OPVs is to utilize donor and acceptor materials with complementary absorption. Polymer/nonfullerene bulk heterojunctions (BHJ) with small molecule acceptor (SMA) are emerging for this direction and have significant advantages compared to polymer: fullerene BHJs.1-7 These structures, in general, exhibit high open circuit voltage (Voc)4, 8-9, partly due to tuning and optimization of the lowest occupied molecular orbital (LUMO) of SMA for a specific donor,4, 10

and higher energetic order compared to the polymer:fullerene BHJs.11-12 Moreover, most

non fullerene SMAs are strong absorbers in the visible5, allowing both components of the BHJ to complementarily harvest solar radiation.2, 4, 6, 8-9, 13-14 Therefore they have potential to deliver significantly higher short circuit current (Jsc) compared to polymer/fullerene blends of similar active layer thickness.15-16 However, so far reported Jsc values do not exhibit a substantial improvement11 even with the ternary polymer non fullerene structures.17 Instead a comparison of various polymer based OPV structures show that increasing the spectral coverage does not lead to a substantial increase in the Jsc.15, 17-21 This suggests that not all the excitations are efficiently contributing to charge generation in polymer/SMA systems. We show that in at least some of these new systems, a loss-mechanism is present due to inefficient charge separation of excitons when Förster energy transfer mechanism exists. Our results provide directions to overcome this loss mechanism via molecular design.


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Figure 1: a) Chemical structure of polymers and SMA. (b) and (c) are potential pathways for exciton evolution in polymer:SMA blends without (b) and with (c) the possibility of energy transfer. In Figure a, process1: singlet exciton generation upon photoexcitation, process 2: charge transfer (CT) exciton formation at the interface, process 3: charge carrier formation, process 4: recombination to ground state. In Figure c, process1: singlet exciton generation upon photoexcitation, process 2: energy transfer to low band gap material, process 3: charge transfer from the low band gap material, process 4: recombination to ground state. “?” in Figure c refers to charge transfer possibility from low band gap material to high band gap material.



The initial steps in OPV operation involve the creation of neutral excitons through photo absorption, diffusion of excitons to donor acceptor interface and separation into free charges.22-23 Subsequent triplet formation and or other forms of recombination can also impact performance.24-27 In the novel polymer:NF SMA BHJ structures with complementary absorption, in addition to the charge generation, exciton transfer mechanisms such as Forster resonant energy transfer28-30 (FRET) between the donor and acceptor can occur (Figure 1) and act as a loss mechanism. FRET is a well-known process that takes place when the emission spectrum of one material overlaps with the absorption of the other.31 The impact of FRET on polymer OPVs have been a focus of research. For instance Nelson and coworkers reported that highly luminescence polymers can have poor photovoltaic performance on fullerene based solar cells as energy transfer from polymer to fullerene can potentially act a a loss pathway preventing charge photogeneration.32 There are several reports showing FRET as a beneficial process for improving light harvesting in solar cells including polymer/quantum dot33 and fullerene based ternary solar cells.34-36 In case FRET exists in a heterojunction, an exciton created in the higher band gap material has three potential paths for evolution (i) charge separation by donating a charge to the low band gap material at the interface, (ii) radiative or non-radiative recombination within its own domain, and (iii) energy transfer to the low band gap material via FRET. The first mechanism where photogenerated excitons are dissociated by charge transfer leads to an immediate charge generation (process 1, 2 and 3 in Figure 1b). If charge collection is efficient, separated charges contribute to the photovoltaic efficiency directly. The second mechanism, recombination, leads to loss (process 4 in Figure 1). In the third mechanism, an exciton in the larger bandgap material vanishes and a new FRET transported exciton forms in lower bandgap material (process 1 and 2 in Figure 1c). FRET excitons in the low bandgap material have to charge-separate at the interface by donating a charge to the high bandgap material in order to contribute to the photovoltaic process. The impact on device performance 4 ACS Paragon Plus Environment

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of these FRET events are not well understood and might very well lead to a novel loss mechanism. In this work, we investigated the charge generation, exciton recombination and transfer kinetics in two polymer/SMA blends. We paired two polymers with different exciton lifetimes, but with absorption spectra matched to the emission spectrum of a wide-bandgap NF-SMA. We first studied the emission characteristics of the blends of these polymers with the NF SMA, and showed the existence of FRET when the wide bandgap NF-SMA is excited. Then by using time resolved photoluminescence we estimated the efficiency of the FRET process and population of the excitons transferred to the polymer domains via FRET process. Finally by using time resolved absorption experiments, we traced the evolution of the excitons and polarons. We found that when FRET process is present; the population of excitons in both domains can cause a previously unknown loss mechanism for charge photogeneration. Our results indicate that low bandgap materials with longer exciton lifetimes can minimize the recombination losses, a target for optimized materials design. Experimental Section Blend solutions of donor polymer and small molecule acceptor were prepared (D/A ratio 1:1.5, polymer concentration 9 mg/ml) in 1,2,4-trimethylbenzene (TMB) with 2.5% of 1,8-octanedithiol (ODT) and heated at 100 °C for an hour. Before spin-coating, both the solution and glass substrate are preheated on a hotplate at approximately 90 °C. Blend films with resulting thickness of ~120 nm were prepared and encapsulated using UV curable glue before measurement. Neat films were prepared using same procedure. The room temperature steady state absorption spectra were measured with Cary win 50 UV-spectrophotometer from Varian. Photoluminescence (PL) spectra were recorded using FS920 Edinburgh luminescence spectrometer equipped with visible and NIR single photon counting detectors. 2.55 eV and 1.97 eV excitation were achieved using a xenon lamp (Xe



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900, xenon arc lamp that emits continuous radiation from 230 nm to 2600 nm) for selective acceptor and donor excitation. The photoluminescence life time was measured using the time correlated single photon counting (TCSPC) technique. Life spec II from Edinburgh instruments equipped with 4MHz variable excitation pulsed laser source was used for this measurement. Excitation energy was set at 2.55 eV for selective acceptor excitation and 1.80 eV for selective donor excitation. Femtosecond transient absorption spectroscopy (TAS) data were collected using 800 nm Ti:Saphhire laser (1 KHz repetition rate) in combination with an optical parametric amplifier (OPA) and ultrafast Helios spectrometer system. The output of the Ti:sapphire laser was splitted into two paths. The first path is used as a source for OPA to generate the pump. The second path is used to generate whitelight continuum probe pulses. 1.80 eV and 2.55 eV pump pulses were used for the predominant donor and SMA excitation, respectively. Pump excitation intensity was kept low (~3 µJ/cm2) to avoid possible exciton-exciton annihilation and other nonlinear effects. The probe range from 0.8 eV to 1.6 eV was used to monitor exciton and charge dynamics. The instrument response of the setup was ~ 100 fs. Resonant Soft X-ray Scattering data is a Lorentz-corrected 1D circular average scattering intensity vs. q profile (assuming globally an approximately isotropic 3D sample morphology at relevant length scales) calculated by azimuthally integrating a 2D scattering pattern about the beam center and multiplying by q2. Data processing and analysis were performed using a custom modified version of the NIKA software package in Igor Pro.37 The 2D scattering pattern was measured on an in-vacuum CCD camera (Princeton Instrument PIMTE) at beamline 11.0.1.2. at the Advanced Light Source.

38

The energy of the beam was

283.8 eV, near the maximum of the scattering contrast function of the two materials but below the Carbon 1s absorption edge. Approximate domain size quoted is the long period calculated by converting the q position of the maximum of the profile to the corresponding real space 6 ACS Paragon Plus Environment

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value. Samples are thin films, prepared as they would be in devices, cast onto a water soluble layer and subsequently floated off and mounted on 1.5 mm x 1.5 mm, 100 nm thick Si3N4 membrane within a 5 mm x 5 mm, 200 µm thick Si frame (Norcada Inc.).

Results and Discussion The two polymer donors investigated (PffBT3T-E,A; named as G17 hereafter, and PffBT4T-2OD; named as Y5 hereafter) have a complementary absorption spectrum to the high band gap SMA, SF-PDI2 (named as JS1 hereafter) (Figure 2). The emission of the SMA overlaps with the absorption band of these polymers, making these systems ideal for investigating the impact of FRET (Figure 2a and 2b). Furthermore, the complementary absorption allows the comparative study of the charge generation process in different materials by utilizing selective excitation of components in the blends. It is noted that the polymer Y5 showed high crystallinity and strong lamellae stacking39 and is suitable for fullerene based solar cells40 whereas SMA prefers polymer with lower crystallinity and very poor performance is reported when blended with JS1.39 G17 polymer, on the other hand, is less crystalline and maintains a better balance between crystallinity and small domain size. The suitable nanomorphology with small domain size results efficient exciton splitting and charge generation dynamics.20



Figure 2. Absorption and photoluminescence (PL) spectra. (a,b) Absorption spectra of Neat G17 and neat Y5 films, and absorption and emission spectra of neat JS1 films. (c,d) Photoluminescence spectra of G17:JS1 blend (c), and Y5:JS1 blend (d) for polymer SMA and polymer excitation. Polymer SMA weight ratio is 1:1.5 in both blends.

Although both polymers exhibit similar emission and absorption spectra in their neat form, the PL emission of their blends (1:1.5 weight ratio) with the same SMA differs significantly. Figures 2c and 2d show the PL emission in the blends when excited at 1.97 eV and 2.55 eV excitation. At 1.97 eV excitation energy, only the polymer domains are excited. At 2.55 eV the, polymer and SMA domains are co-excited, but the SMA more preferentially (Supporting Information 1). In G17-based blend the PL emission intensity from the polymer domains is at similar intensities regardless of the excitation energy. In contrast in Y5 blend polymer PL intensity exhibits significant difference in two different excitation energies. When SMA-rich domains are preferentially excited (2.55 eV excitation), the emission from 8 ACS Paragon Plus Environment

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the polymer domain is an order of magnitude larger compared to when only the polymer domain is excited. This observation suggests the existence of FRET from SMA domains to the polymer domains and it also suggests in the Y5-based blend at high excitation energy the charge separation of excitons is suppressed. In order to quantitatively analyze the change in the PL quenching at different excitation energies (Supporting Information 2), we first calculated the difference in the PL intensity from the selected excitation energies. Then we compared it with the number of excitons that are in the polymer domains. These excitons can either be directly excited in the polymer domains or be the results of a FRET transfer from the SMA. The population of excitons that are directly excited in the polymer domains is relatively straightforward to calculate as it only depends on the absorption strength of the polymer at a particular excitation energy and photon flux in the excitation beam. The sample-averaged FRET efficiency and hence the population of FRET excitons, can be estimated by using the change in the PL lifetime of the SMA excitons between the neat and the blended forms. The exciton lifetime in the neat SMA is longer compared to that of the blend, because charge transfer and FRET adds additional loss mechanism for the SMA exciton population in the blend. The charge transfer mostly takes place within the first few 100 ps,13,14, 41 Therefore the decay dynamics that takes place in longer time periods are mostly due to FRET and radiative recombination. This difference in the time scales of charge generation, FRET and recombination kinetics provides means to approximately estimate the FRET efficiency in the sample using time resolved PL experiments. FRET efficiency “ ” is given by34-35

= (1 −

) (1)



Here is the lifetime of the SMA exciton without the presence of polymer, and , is the lifetime of the SMA exciton in the blend when FRET is present. Figure 3 shows the time resolved PL of the SMA in neat and blended thin films at 2.55 eV. The characteristic lifetimes (Table 1) are obtained by fitting the transient PL data using exponential decay functions. In the neat thin film, SMA excitons have a very long lifetime (~8 ns). In the blends it is substantially shorter and exhibit multiexponential decay characteristics for both samples. The fast decay components i.e 70 ps for G17 and 130 ps for Y5 are primarily associated with charge transfer and charge generation. These components correspond to 42% of excitons in G17:JS1 whereas it is only 14% in Y5:JS1. Therefore we assume that for the remaining 58% of excitons in G17:JS1 and 86% in Y5:JS1, the primary decay mechanisms are FRET and radiative recombination. In order to calculate the FRET efficiency, we calculated the effective PL lifetime by using the longer decay components (Supporting Information 3) in both blends. The corresponding FRET efficiencies using these lifetimes are 92% and 62% in G17 and Y5 blends. This finding suggests 92 % of the excitons that did not charge transfer in G17 blend undergo FRET transfer to the polymer domains from SMA. This is ~53% of the excitons created in the SMA domains. Similarly in in Y5 JS1 blend 62%, of excitons that did not initially charge transfer undergo FRET transfer. This is approximately 54% of the excitons created in the SMA domains. Therefore at 2.55 eV excitation, in addition to the excitons created in the polymer domains, about half of the SMA excitons transferred to the polymer (Supporting Information 3) in both blends.


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Table 1: PL lifetimes of donor, neat acceptor and blended films, obtained by fitting the decays in Figure 3 to multiexponential functions. The percentile values are the amplitudes of each expontial function. PL Lifetimes of Samples

τ1 (% α1)

τ2 (% α2)

G17 (Eem=1.67 eV)

130 ps (6%)

1.07 ns (94%)

Y5 (Eem=1.67 eV) JS1 (Eem=1.91 eV) G17:JS1 (Eem=1.91 eV) Y5:JS1 (Eem=1.91 eV)

100 ps (66%) 620 ps (4%) 70 ps (42%), 130 ps (14%)

310 ps (34%) 8.17 ns (96%) 450 ps (41%) 1.7 ns (17%) 1.06 ns (39%) 4.55 ns (47%)

τ3 (% α3)

Figure 3. PL dynamics of neat and blended films (a) SMA emission dynamics of neat JS1, G17:JS1 and Y5:JS1 films monitored at 1.91 eV at 2.55 eV excitation. (b) Emission dynamics of neat G17 and Y5 films monitored at 1.67 eV at 1.80 eV excitation. The instrument response (IRF) is in purple color in (a) and (b).

In Y5 blend the polymer PL intensity with polymer SMA excitation is about an order of magnitude larger compared to only polymer domain excitation (Figure 2b). Therefore, the increase in the polymer PL intensity cannot be explained by the increase in the number of excitons transfered. Therefore, the substantial increase in the polymer PL emission in Y5:JS1 blend, when both chromophore species are excited, is primarily due to the increase in the radiative recombination in the polymer. In order to quantitatively analyze the increase in the radiative recombination, we modeled the PL emission intensity from the polymer domains. In this model (Equation 2), the total PL emission from the polymer domains is the sum of the contributions from the 11 ACS Paragon Plus Environment


emission of the excitons that are created initially in polymer domains and excitons transferred into the polymer domains via FRET.

() = (_) + (_). (2) In Equation 2 each term on the right hand side is proportional to the number of excitons and a recombination factor “r“ that relates the PL intensity of the polymer to the number of excitons created in the polymer (Supporting Information 2). This analysis reveal that the radiative recombination of excitons increased by 21.5% in G17 and 323% in Y5 blends with both domain excitation compared to only polymer domainexcitation (Supporting Information 4). Basically, the increase in the PL emission in Y5 is not due to the number of excitons FRET transfered but due to the decrease in charge transfer and the resulting increase in radiative recombination of excitons. This increase in polymer exciton recombination probability in blended samples with both domain excitation is very surprising, because the exciton dynamics in the neat polymer thin films are excitation energy independent (Figure S5). In addition, charge separation efficiency should increase at high excitation energies in the blends.42-43 Therefore, for polymer excitons, radiative recombination resulting in PL should not increase with both domain excitation. However, in these blends, the charge transfer and separation of excitons is hindered at high-energy excitation. It is also striking that the increase in exciton recombination probability in the Y5:JS1 blend (323%) is much larger compared to G17:JS1 blend (21.5%). While both polymers exhibit very similar optical properties, these results suggest G17 blend has better charge separation characteristics. For Y5, the recombination losses increase when the SMA is excited. In order to understand the mechanism behind this increased recombination losses and its relation to the material properties, we studied the morphology using resonant soft X-ray scattering (R-SoXs) and the exciton dynamics using time resolved absorption experiments and compared the material characteristics in the two blends. R-SoXs measurements indicate that 12 ACS Paragon Plus Environment

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the two blends have morphological differences. The typical domain size is 39.6 nm for Y5 blend (Figure S6) and 14.3 nm for G17 blend.22This domain size difference correlates well with the high initial charge generation efficiency in the G17 blend. As excitons need to diffuse to interfaces, large domains can hinder charge generation. As discussed above a significant portion of the SMA excitons (46%) undergoes charge separation in G17 blend right after excitation. The small domain also correlates well with the higher FRET efficiency in the G17 blend. When the domain size is small, FRET acceptor and donors are close to each other, increasing the FRET efficiency. Therefore it is consistent with our interpretation of the PL decay dynamics. In the Y5 blends since the domain sizes are larger initial charge generation, i.e hole transfer from SMA to polymer, and electron transfer from polymer to SMA is relatively inefficient. However this difference in the domain sizes does not explain the 323% increased luminescence efficiency of the polymers domains in Y5 blend, when there is FRET. Time resolved absorption experiments provide further insight into the differences exciton recombination, and charge generation kinetics in both blends. In the time resolved absorption experiments, we measured the evolution of the polaron population after exciting the sample using an ultrashort laser pulse. Similar to the PL experiments we used low energy energy pump pulses to measure the charge generation when only polymer domains are excited. Then we repeated the measurements with high energy pulses which creates excitons in both SMA and polymer domains. The resulting dynamics are monitored by measuring the differential transmission of broadband probe pulses in the near IR region of the spectra at different time delays after the excitation. Transient absorption spectra of both blends at different delays with 2.55 eV are shown in Figure 4a and 4b. The negative bands centered around 0.9 eV and 1.2 eV are due to the excited state absorption of polymer singlet excitons and polarons in the charge separated state, respectively.13, 20 Figure 4c shows the evolution of the polaron population in the Y5 blend at two different excitation energies. The two transients 13 ACS Paragon Plus Environment


are closely matched suggesting similar charge generation dynamics, regardless of the initial excitation energy. On the other hand for the G17 blend there is a stark contrast at two different excitation energies (Figure 4d). When only the polymer is excited there is a fast charge generation. However, when the SMA is excited, in addition to the initial fast charge generation, there exists an additional slow growth in the charge population, which extends over 100 ps. This delayed charge generation most likely results from excitons that are initially created in the SMA and transferred into the polymer via FRET or other exciton transport processes and then separated into charges from the polymer domain by donating an electron to the SMA. The time resolved experiments indicate that although FRET is taking place in both samples, with similar density of FRET excitons as indicated by the PL results in the earlier discussion, the delayed charge generation does not efficiently occur in the Y5 blends. We suppose that this arises from time scale differences intrinsic exciton lifetime in the two polymers and the rate at which FRET takes place in both blends. In the G17 blend, FRET is relatively fast, which is evident from the SMA exciton lifetime in the blend in Table 1. The smaller domain sizes in the G17 blend significantly enhance the FRET process. In contrast, the intrinsic exciton lifetime in the G17 polymer is significantly longer compared to Y5. The characteristic PL lifetimes are 130 ps (6%) and 1.07 ns (94%) for (G17) and 100 ps (66%) and 310 ps (34%) for (Y5) (Table 1). Hence in G17 blends FRET excitons form much faster in the polymer and they survive to relatively longer times. During this enhanced lifetime, they can charge separate after they transfer to the polymer domains. In contrast, in the Y5 blend FRET is slow and after the excitation transfer, the excitons have very short lifetime in the polymer. Here we studied the charge generation dynamics in two different polymers blended with the same high band gap SMA. The Y5 polymer forms larger domains with the SMA. The intrinsic exciton lifetime is relatively short in Y5 polymers. The PL emission yields suggests 14 ACS Paragon Plus Environment

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that, when the SMA is excited a significant portion of the excitons FRET transfer from SMA to Y5 domains. Since the exciton lifetime is short, they recombine in the Y5 domains increasing the PL emission. On the other hand G17 polymers form smaller domains with SMA. The intrinsic exciton lifetime is relatively longer in the G17 polymer. Although FRET is very efficient in G17 blends, a significant portion of the SMA excitons charge separate immediately after the initial excitation prior to FRET. After initial charge separation, the remaining excitons mostly FRET transfer to the G17 domains. Since these FRET excitons have relatively long lifetime in G17, they also contribute to charge generation as indicated by the time resolved absorption experiments. Our observations imply that a direct excitation in the low band gap polymer has a different charge separation yield than one created by the FRET transfer. It is plausible that the local dielectric environment at the interface is altered by interfacial charge population formed due to initial charge separation and/or the exciton population and hence further impacting charge separation of FRET transported excitons. In such cases, a FRET exciton has to find an available interfacial state to charge separate or wait until the local dielectric response relaxes to a state favoring charge generation, both of which require a long exciton lifetime42, 44 and smaller domain sizes.20 Hence the G17:JS1 blend outperforms Y5:JS1 in charge generation. Further studies are needed to fully understand the origin of the observed loss mechanism.



Figure 4. Transient absorption spectra and dynamics. Transient absorption spectra of (a) Y5:JS1 and (b) G17:JS1 blends at different delays at excitation energy 2.55 eV. Comparison of dynamics of polymer polaron for Y5:JS1 (c) and G17JS1 (d) blends pumped at 2.55 eV and 1.80 eV.

Conclusion In summary, efficient light harvesting and charge generation are prerequisites for high efficiency OPVs. Polymer/non-fullerene BHJs provide a significant opportunity to increase light harvesting via large coverage of the solar spectrum through engineering of the optical properties of the materials. However, so far systems with large spectral coverage do not exhibit significant advancement in terms of improving the current density. Our results show that in cases when both acceptor and donor domains are excited this transfer mechanism 16 ACS Paragon Plus Environment

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causes reduction in charge generation. Furthermore, longer exciton lifetime in the low band gap material and smaller domain sizes can improve the charge generation efficiency significantly.

Hence

these

results

provide

significant

guidance

for

improving

polymer/nonfullerene device efficiencies. Acknowledgements Optical spectroscopy work was supported by Office of Naval Research (ONR) grant N000141310526 P00002 (B.R.G, R.Y and K.G). R-SoXS data were acquired at beamline 11.0.1.2. at the ALS in Berkeley National Lab, which is supported by the U.S. Department of Energy (DE-AC02-05CH11231). R-SoXS data acquisition and analysis by J.H.C and and H.A. were supported by the ONR grant N000141512322. Prof. He Yan is acknowledged for supplying donor polymers and non- fullerene acceptor.

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