Charge Generation in Non-Fullerene Donor–Acceptor Blends for

Article pubs.acs.org/JPCC

Charge Generation in Non-Fullerene Donor−Acceptor Blends for Organic Solar Cells Nasim Zarrabi, Dani M. Stoltzfus, Paul L. Burn, and Paul E. Shaw* Centre for Organic Photonics & Electronics, School of Chemistry & Molecular Biosciences and School of Mathematics & Physics, The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: The power conversion efficiencies of solar cells incorporating non-fullerene donor−acceptor blends now approach those of fullerene-based devices in the best performing examples. However, the lack of clear structure−property relationships means that the poor device performance of many novel nonfullerene systems cannot be readily explained. We report a series of non-fullerene acceptors, comprising essentially the same chromophore, that differ in terms of their shape and number of chromophores. To understand the impact of these structural differences on charge generation, we have employed transient absorption spectroscopy to investigate the photophysical properties of the acceptors in blends with the conjugated polymer PTB7. To minimize the impact of morphology, we employed a broad range of acceptor concentrations and compared the results with those of blends containing the fullerene derivative PC70BM. In terms of singlet exciton harvesting, the non-fullerene acceptors exhibited similar performance to PC70BM in blends with PTB7. The rate of singlet exciton quenching as a function of acceptor concentration was consistent with exciton diffusion mediated quenching with a diffusion length of 4−5 nm for the PTB7 singlet exciton. The polaron generation efficiency of the non-fullerene acceptors was comparable to that of PC70BM although the fraction of polarons that subsequently underwent geminate recombination was much greater in the non-fullerene blends. Furthermore, the photophysical properties of the non-fullerene acceptors were not influenced by the shape of the acceptor, the chromophore number, or the donor−acceptor ratio, which indicates that the higher geminate recombination is related to the structure of the acceptor chromophore and the interface formed with the donor. The implication of these results is that despite appropriate energetics and optical absorption, the performance of some non-fullerene donor−acceptor blends will be intrinsically limited by the choice of acceptor chromophore and the nature of the interface it forms with the donor material.

1. INTRODUCTION The continued improvements to organic solar cell performance have been the result of novel material development and the optimization of the bulk heterojunction morphology.1,2 While a broad range of donor materials have been devised, the choice of acceptor material remains dominated by fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM).3−7 The extensive use of fullerenes is due to their historic introduction and the fact that when combined with a broad range of different donors (both polymeric and nonpolymeric) they can yield high-performance bulk heterojunction-based solar cells. This has been attributed to their electron-accepting properties, high charge generation quantum yields, and capacity to form morphologies that facilitate both charge generation and charge extraction.8,9 However, fullerenes do have their limitations, including a low absorption coefficient across most of the solar spectrum and limited tunability of the ionization potential (IP) and electron affinity (EA). In contrast, nonfullerene acceptors can be molecularly engineered to have high © XXXX American Chemical Society

absorption coefficients and tuned IPs and EAs. Hence, donor− non-fullerene-acceptor pairs with complementary absorption and well-matched energy levels in the appropriate morphology have the potential to deliver superior performance to fullerenebased organic solar cells. These factors are the motivation behind the development of an increasing number of nonpolymeric non-fullerene acceptors reported in the literature.10−13 Although some non-fullerene acceptors deliver blends with performance in devices comparable or exceeding that of fullerene-containing counterparts,14−18there are many examples that do not.10−12 Unfavorable morphology in the bulk heterojunction and, in particular, the formation of excessively large non-fullerene domains17,19−21 are often given as reasons for poor performance, but little is known about the photophysical properties of non-fullerene acceptors and how they relate to molecular structure. More generally, it is not clear Received: June 15, 2017 Revised: July 26, 2017

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Figure 1. (a) Molecular structures of the non-fullerene acceptors A1 (note − a proton is attached at the dashed bond for A1), A2, and A3. In the case of A2 and A3, A1 is linked at the position para to the acetylene moiety. (b) Normalized optical absorption of neat films of PTB7 and the acceptors. (c) Energy levels of PTB7 and the acceptor materials where the ground state energy was determined from photoelectron spectroscopy in air (PESA) and the energy of the lowest lying excited state was estimated by adding the value of the optical gap to the ground state energy.

what the “template” is for a high-performance non-fullerene acceptor. The high performance of fullerene acceptors has been linked to their spherical shape, which results in isotropic charge transfer and charge transport. Whether it would be beneficial to replicate this property in non-fullerene acceptors with high dimensionality geometries that facilitate interconnectivity has yet to be determined. Furthermore, fullerene aggregation has been reported to strongly enhance the dissociation of excitons into separated charges.22,23 The reason for this enhancement has not been completely established but has been variously attributed to energetic gradients between mixed interfacial regions and pure domains,24 bandlike energy levels that allow rapid transport of the electron away from the interface,25 and resonant coupling between the polymer singlet exciton and the excited state manifold of fullerene aggregates.26 The latter two hypotheses would imply that fullerenes possess an intrinsic advantage over non-fullerene acceptors; thus, it is essential to understand the photophysical properties of non-fullerene donor−acceptor blends and how these are affected by acceptor structure. To probe the impact of molecular structure on the performance of non-fullerene acceptors in bulk heterojunctions, we investigated the photophysical properties of a series of nonfullerene acceptors, each containing the same chromophore but varying in terms of shape and number of chromophores within the (macro)molecule. Crucially, essentially the same chromophore is shared between the acceptors, so the energetics, and thus the driving force, for charge transfer are similar. To obtain data that was representative of the donor−acceptor interface that was not strongly influenced by blend morphology, we chose to investigate blends of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) containing low acceptor content and compare these with a control blend of the same polymer with PC70BM with broadband transient absorption spectroscopy (TAS). Our results show that each of the non-fullerene acceptors are

capable of quenching PTB7 singlet excitons with similar efficiency to that of PC70BM over more than 2 orders of magnitude of acceptor concentration. In fact, the yield of polarons for the non-fullerene acceptors was slightly higher than that achieved with PC70BM. However, we find that a greater proportion of the polaron population in the nonfullerene blends undergoes geminate recombination, so the yield of separated polarons is consistently lower than that with PC70BM. Unexpectedly, we observe the same trend for all the acceptors, regardless of the number of chromophores, molecular shape, or the presence of aggregates. This suggests that the choice of acceptor chromophore is critical and that some non-fullerene acceptors will have intrinsically high rates of geminate recombination independent of acceptor “structure” that will ultimately limit device performance.

2. EXPERIMENTAL METHODS 2.1. Film Preparation. PTB7 with M̅ w of 105 kDa and a polydispersity of 2.5 was purchased from 1-Material, Inc. PC70BM was purchased from American Dye Source, Inc. An acceptor solution of 4 mg/mL was prepared using chloroform as the solvent. The solution was then serially diluted to obtain the required low-concentration acceptor solutions into which the PTB7 was dissolved. The solutions were stirred at ∼50 °C overnight. Fused silica substrates were sequentially sonicated in acetone for ∼15 min and in 2-propanol for ∼20 min and then blow-dried with nitrogen. The blend solutions were spin-coated in air at 1200 rpm for 60 s onto the clean substrates. The films were then stored under vacuum for ∼2 h to remove any residual solvent before being transferred to a nitrogen-filled glovebox. The samples were loaded into a sealed optical chamber while in the glovebox for the transient absorption measurements. Film thicknesses were determined using a Veeco Dektak 150 surface profilometer. 2.2. Steady-State Spectroscopy. UV−vis absorption was measured with a Varian Cary 5000. Photoluminescence spectra were measured using a Horiba Jobin-Yvon Fluoromax 4 B

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The Journal of Physical Chemistry C fluorimeter. The photoluminescence quantum yield measurements were measured in an integrating sphere under a continuous flow of nitrogen. The excitation source was a HeNe laser attenuated to ∼0.2 mW and modulated with a mechanical chopper at 310 Hz. A longpass dielectric filter was used to block the excitation with the luminescence signal detected with an amplified silicon detector via a lock-in amplifier. The value of the PLQY was calculated using the method described by Greenham et al.,27 with the PL quenching efficiency (PLQE) calculated from PLQE = 1 −

The process of charge generation in a donor:acceptor blend is the product of multiple steps.30 The absorption of the excitation photon can lead (depending on the excitation wavelength) to the formation of excitons in both the donor and/or acceptor materials on a femtosecond time scale that undergo internal conversion in the first picosecond to the lowest lying singlet excited state. Depending on the availability of the donor:acceptor interface the exciton may undergo dissociation via charge transfer before internal conversion is complete. Those excitons that are not generated sufficiently close to the interface may undergo dissociation following exciton diffusion. Hence, the lifetime of the singlet excitons effectively limits the time-scale for dissociation, and thus charge generation, to occur. In order to probe the details of the charge generation process and the impact of varying the shape of the acceptor, we will separately consider the photophysics of the neat materials and blends and then focus on singlet exciton harvesting and polaron formation in the blends. To probe the photophysical properties of the blend, it is essential to also understand the optical properties of the individual components in neat films, such as the spectrum of the singlet exciton absorption and its decay lifetime so that features in the blend can be correctly identified. Whereas in donor−fullerene blends the excited state absorption of the fullerene is much weaker than that of the donor, this is not necessarily the case for non-fullerene acceptors, which can make interpretation of the data difficult due to overlapping signals. The evolution of the photoinduced absorption (PA) spectra for PTB7 and all three non-fullerene acceptors at different time delays after photoexcitation is shown in Figure 2a. For PTB7 at wavelengths between approximately 500−700 nm, we observe a negative signal due to ground-state bleaching (GSB), and in the near-infrared (NIR) we observe a broad positive PA signal extending from ∼900 nm to beyond 1600 nm. Two excited-state species can be identified in the spectra: a strong short-lived signal that peaks ∼1400 nm, which we assign to absorption by the singlet exciton, and a weaker long-lived signal that peaks around 1130 nm that can be assigned to either polaron or triplet exciton absorption of the PTB7, in agreement with existing reports.31,32 For pristine films of the acceptors, we observe the GSB signal between approximately 500−650 nm, which is consistent with the measured absorbance spectrum, and a PA signal extending from 650 nm into the NIR. The latter, which peaks at about 750 nm and decays rapidly, is assigned to absorption by singlet excitons. We also observe a broad weak PA signal in the NIR that decays exponentially with a lifetime of ∼1.2 μs, which is consistent with triplet exciton absorption (see Figure S1). The decay kinetics of the signals assigned to singlet excitons in PTB7 and the non-fullerene acceptors are shown in Figure 2b. None of the materials feature an exponential decay, which is consistent with the presence of intermolecular interactions in the solid state. The decay kinetics of the singlet exciton population in A1, A2, and A3 are similar and decay more slowly than that of PTB7. The photoinduced absorption signal for the non-fullerene acceptors was weaker (see Figure 2a) than that of the PTB7. From the excitation densities, we estimate that the singlet exciton absorption cross sections (calculated using the method described in ref 33) of PTB7 is ∼3.8 × 10−16 cm2 (at 1400 nm) and ∼1.0 × 10−16 cm2 (A1 at 780 nm), ∼1.4 × 10−16 cm2 (A2 at 750 nm), and ∼1.6 × 10−16 cm2 (A3 at 750 nm) for the non-fullerene acceptors. Hence, the singlet exciton absorption of the non-fullerene

PLQYblend PLQY neat

2.3. Time-Resolved Spectroscopy. The femtosecond transient absorption measurements were performed using a Helios system from Ultrafast Systems, Inc. The output from a Spectra-Physics Spitfire amplifier was split to seed a TOPAS optical parametric amplifier and generate the 650 nm pump beam, with the remainder used to generate the white light continuum for the probe. The pump power was attenuated with a neutral density filter and measured with a power meter. For the nanosecond transient absorption measurements, an EOS system from Ultrafast Systems, Inc. was used. The spot size of the pump beam was measured with a Newport LBP beam profiler and used to calculate the laser fluence.

3. RESULTS AND DISCUSSION 3.1. Photophysical Properties of the Non-Fullerene Acceptors. The chemical structures of the non-fullerene acceptors used in this work are shown in Figure 1a. The synthesis and characterization of these compounds is reported separately in ref 28. All three contain the same basic chromophore structure 2-[(7-{4,4-di-n-octyl-6-[phenylethynyl]-4H-silolo[3,2-b:4,5-b’]dithiophen-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile, which is acceptor A1. A2 and A3 both contain four A1 chromophores arranged around a central tetrahedral carbon atom or an adamantyl moiety respectively, neither of which extends the conjugation of the chromophores appreciably. The normalized absorbance spectra of the acceptors are shown in Figure 1b with that of PTB7 and PC70BM. The shape of the thin film absorbance spectra for A1, A2, and A3 are nearly identical, which is consistent with the fact that the conjugation in A2 and A3 does not extend across the central unit and in spite of the A1 chromophores in A2 being linked to the branching unit via an alkoxy moiety. The ionization potentials of the acceptors and PTB7 were measured with photoelectron spectroscopy in air (PESA) with the excited state energies estimated by adding the value of the optical gap energy (see ref 28 for details). The energy levels for each compound are shown in Figure 1c with the values for PC70BM obtained from existing reports in the literature.29 For each acceptor there is sufficient energetic offset between the IPs and EAs to enable both photoinduced electron and hole transfer. Given the shapes of A2 and A3, they will pack differently in the solid state from A1, and the relative proximity and orientation of four chromophores provide a route toward investigating whether such multichromophoric structures lead to enhanced charge dissociation. Furthermore, the shapes of A2 and A3 are expected to result in greater isotropic character, particularly for A3 where the chromophores are held rigidly in a tetrahedral arrangement compared to that of planar A1, akin to the properties of PC60BM and PC70BM. C

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results in blends where the acceptor is more disperse; thus, the TAS data will be representative of the interaction that occurs at the interface between the donor and acceptor molecules without the complication of significant acceptor−acceptor interactions. Furthermore, the TAS signal from donor− acceptor blends with low acceptor content will feature a negligible contribution from the acceptors due to their low absorption relative to the donor, their lower excited state absorption cross section, and high probability of rapid dissociation of the acceptor excitons. The evolution of the PA spectra of the PTB7:acceptor (5 wt %) blends in the NIR are shown in Figure 3a. Two PA species can be identified in these spectra: a rapidly decaying signal that peaks at wavelengths longer than 1400 nm, which is consistent with PTB7 singlet exciton absorption, and a long-lived state with peak absorption centered at ∼1130 nm. The latter is consistent with existing reports of positive polaron absorption in PTB7 and confirms the photogeneration of charge carriers in the PTB7:acceptor blends. Hence, the data for the different blends shows the evolution over time from PTB7 singlet exciton absorption to PTB7 positive polaron absorption. There is a shift in the peak polaron absorption from ∼1115 to ∼1130 nm in the first ∼200 ps for all of the blends. The shape of the polaron absorption at later times (∼1.5 ns) once the singlet exciton population has fully decayed is the same for all the acceptors (see Figure S2), which indicates that the same species is present in all the blends and that there is a negligible contribution from the negative polaron on the acceptor toward the signal in the NIR. The kinetics at 1460, 1129, and 902 nm are shown in Figure 3b for the blends containing 5 wt % of each of the acceptors. The rapid decay of the signal at 1460 nm is consistent with efficient singlet exciton dissociation, indicating that nearly all PTB7 singlet excitons have been dissociated or undergone relaxation within ∼100 ps for all the acceptors. While the polaron signal peaks at ∼1130 nm, at this wavelength there is significant spectral overlap with the PTB7 singlet exciton absorption. However, there is an isosbestic point in the PTB7 excited state absorption at ∼900 nm (see Figure 2a) that allows the polaron signal to be isolated from the singlet exciton absorption.35 The differences between the kinetics at 1129 and 902 nm are evident in the first ∼100 ps where the slow rise of the polaron signal at 902 nm can be clearly seen in Figure 3b. At later times (≳1 ns), when the singlet exciton population has completely decayed, the decay of the polaron signal is the same at 902 and 1129 nm. 3.3. Efficiency of Donor Singlet Exciton Harvesting. To investigate the process of singlet exciton quenching we measured the effect of acceptor concentration on the PTB7 singlet exciton decay dynamics. Figure 4a shows the singlet exciton decay kinetics in a neat PTB7 thin film and in blends with A2 across a wide range of acceptor concentrations. Similar data was obtained for all the acceptors, and A2 was chosen as an exemplar. The decay of the PTB7 singlet exciton population becomes progressively faster as the acceptor concentration increases. The singlet exciton decay dynamics in a neat polymer film can be described by

Figure 2. (a) Photoinduced absorption spectra of thin films of PTB7 (fluence of 1.6 μJ/cm2) and non-fullerene acceptors (fluence of 5.4 μJ/ cm2) at different time delays after excitation with 520 nm pulses. (b) Decay kinetics of the singlet exciton absorption in neat films.

acceptors is comparatively weaker than that of PTB7, particularly in the NIR. 3.2. Photophysical Properties of Blends of PTB7 with the Non-Fullerene Acceptors. To probe the effect of acceptor shape on the photophysics of charge generation, we prepared blend films with low concentrations of the acceptors, ranging from 0.1 to 23 wt %. The primary reason for this is that structurally similar acceptors may form different morphologies when blended with the same polymer,34 making comparison between them difficult. Lowering the acceptor concentration

Ineat = I0 e−kt

(1)

where I0 is the initial amplitude and k is the decay rate. Then, the decay of the exciton population due to the presence of an acceptor will be given by Iblend = I0 e(−(k + ka)t D

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Figure 3. (a) Temporal evolution of the PA spectrum of PTB7:acceptor blends with 5 wt % acceptor content after photoexcitation (650 nm with a fluence of 3.4 μJ/cm2) and (b) associated kinetics at 902, 1129, and 1460 nm.

where ka is the dissociation rate due to electron transfer at the donor−acceptor interface. Taking the ratio of the decay of the blend by that of the neat film therefore allows the decay component that is due to exciton dissociation to be isolated giving Iblend = e−kat Ineat

although the number of molecules per unit volume is the same for A2 and A3 there are four times as many chromophores. The fact that the dissociation rate is essentially the same for all the acceptors implies that there is not necessarily an advantage of having multiple chromophores on a single macromolecular acceptor. The exciton dissociation rate versus number of chromophores is shown in Figure S3a). It is important to note that the calculated acceptor/chromophore concentration is simply the number density and as such represents an upper limit on the number of sites/domains. Aggregation of the acceptors would reduce the actual number of sites/interfaces although on average the number of molecules per cubic centimeter would still be the same. It has been previously reported that PC60BM will cluster in blends at concentrations as low as 1 wt %;35,36 hence, it is unlikely that the acceptors are fully dispersed at all concentrations. Nonetheless, it was unexpected that the trend should be so similar between PC70BM and the non-fullerene acceptors over such a broad range of concentrations given the structural differences. We consistently encountered difficulty preparing high-quality films containing 0.1 wt % of A3 as they appeared to contain grains, so these data have been omitted. The trend for the singlet exciton dissociation rate shown in Figure 5a is approximately linear over 2 orders of magnitude of acceptor concentration. As PTB7 is the dominant absorber, the singlet exciton dissociation rate will be determined by the proximity of the acceptor interface, i.e., the acceptor concentration, and the diffusivity of the PTB7 excitons. Fitting to the data with a diffusion-mediated model enables D, the exciton diffusion coefficient, to be determined,37 with the model given by

(3)

The benefit of this approach is that it can be applied to multiexponential decays where the relaxation of the singlet excitons is the result of a combination of decay processes. The exciton dissociation kinetics for blends of PTB7:A2 across a range of acceptor concentrations are shown in Figure 4b. For very low acceptor content, the kinetics are exponential, which is consistent with a time-independent dissociation rate and a dispersed acceptor. Increasing the acceptor content increases the decay rate of the exciton dissociation kinetics, i.e., ka increases with acceptor content. The decays deviate from exponentials at higher acceptor content, which may be due to the availability of multiple acceptor sites per excitation or the emergence of a more complex morphology. The singlet exciton dissociation rates were obtained by fitting to the decay kinetics with a single exponential (see Figure 4b) with the values plotted as a function of the acceptor concentration (molecules per cm3) in Figure 5a (see Supporting Information for details of the acceptor concentration calculation). The results indicate similar trends for the exciton dissociation rate for blends with comparable acceptor concentration. Crucially, we can conclude that PC70BM does not appear to have a performance advantage compared to the non-fullerene acceptors for the dissociation of thermally relaxed excitons. Furthermore, the results show that the differences in molecular structure between the non-fullerene acceptors do not appear to affect the exciton dissociation rate. It is interesting to note at this stage that

ka = 4πR aDNacceptor

(4)

where Ra is the exciton−acceptor distance at which the exciton will spontaneously be quenched, and Nacceptor is the acceptor E

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Figure 4. (a) Singlet exciton decay in neat PTB7 and PTB7:A2 blends of varying A2 concentration (wt %). Blends were photoexcited at 650 nm with a fluence of 3.4 μJ/cm2, and the signal was probed at 1461 nm. (b) Singlet exciton decay ratio, i.e., quenching kinetics, in PTB7:A2 blends with exponential decay fits as solid lines.

concentration. Fitting to the data for A1 and A3 in Figure 5a, both of which exhibit the greatest linearity and encompass the data for A2 and PC70BM at lower concentrations and assuming Ra = 1 nm, the value of D for PTB7 was determined to be in the range from (1.2 ± 0.1) × 10−3 to (2.1 ± 0.2) × 10−3 cm2/s. For an average singlet exciton lifetime τ of 125 ps for PTB7 (calculated from a weighted two exponential fit to the decay in Figure 2b), the one-dimensional exciton diffusion length (L1D = Dτ ) was calculated to be in the range of ∼4−5 nm. Calculating the dissociation rate from the singlet exciton decay kinetics does not account for instantaneous quenching that may occur on a time scale faster than can be resolved. Such quenching would result in an overall decrease in the amplitude of the signal but cannot be reliably determined from the transient absorption data. To measure the total singlet exciton quenching efficiency of the blends, i.e., instantaneous and diffusion-mediated exciton dissociation, we measured the photoluminescence quantum yields (PLQY) of neat PTB7 and the blends and calculated the photoluminescence (PL) quenching efficiency using PLQE blend =

Figure 5. (a) Singlet exciton quenching rate in PTB7:acceptor blends over a range of acceptor concentrations. The solid line and dashed black lines are fits to the data for PTB7:A1 and PTB7:A3 respectively with eq 4. (b) Steady-state photoluminescence (PL) quenching efficiency of PTB7:acceptor blends with acceptor concentration. (c) Proportion of donor excitons that are quenched within the temporal resolution of the measurements with acceptor concentration.

PL quenching efficiency based on the number of acceptor molecules is highest for PC70BM, followed by that for A2/A3 and that for A1. However, if you take into account the fact that A2 and A3 have four chromophores per acceptor then the quenching efficiency of all three non-fullerene acceptors is essentially the same based on a per chromophore analysis (Figure S3b). To elucidate whether the diffusion-mediated quenching obtained from the time-resolved measurements

PLQY neat − PLQYblend PLQY neat

(5)

with the results shown in Figure 5b. All the acceptors quench the PTB7 PL efficiently at concentrations >1019 cm−3, which corresponds to ∼1 wt % PC70BM. At lower concentrations, the F

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The Journal of Physical Chemistry C could account for the steady-state PL quenching, the PLQY of the films was modeled with PLQY =

kr k r + k nr + ka

(6)

where kr and knr are the radiative and nonradiative decay rates in the absence of the acceptor and ka is as described before. Hence, the percentage of the excitons that are quenched within the instrumental response were estimated from the difference between the measured PLQY and the PLQY calculated with the diffusion-mediated quenching rates (see Figures 5c and S3c). At high acceptor concentrations, where there was efficient quenching of the PL, the diffusion-mediated quenching rate could account for the observed changes in the PLQY. At lower concentrations, the diffusion-mediated quenching was insufficient to account for the observed quenching of the PL, with the difference due to instantaneous quenching via an ultrafast process. However, the differences between the trends for the different acceptors were found to be encompassed by the uncertainties of the measurements; thus, it was not possible to conclude whether the degree of instantaneous quenching varied between the acceptors. The data in Figure 5c suggests that the degree of instantaneous quenching decreases with increasing acceptor concentration, which is counterintuitive. However, we suspect that the data for high acceptor concentrations is limited by the dynamic range of the PLQY measurements, where the differences between two strongly quenched samples are hard to quantify. In contrast, excited state lifetimes can be reliably measured over a wide range of time scales. Hence, measuring the degree of instantaneous quenching will be less reliable at higher acceptor concentrations where the PL of the films is nearly completely quenched. 3.4. Efficiency of Polaron Generation. The singlet exciton quenching data indicated that there was little difference between the exciton quenching rates for the non-fullerene electron acceptors and PC70BM. However, exciton dissociation results in the generation of both free charges and bound charges, i.e., the latter are charge-transfer states (CT-states) that will recombine geminately. Furthermore, the nongeminate recombination of separated charges can result in triplet exciton formation if the energy of the triplet exciton is lower than that of the CT-state.23,30,38 The spectral features observed in the TAS data (see Figure 3a) confirm the presence of polarons with all the acceptors used in this study, and Figure 6 shows the TAS signal measured at 902 nm (where there is no PTB7 singlet exciton absorption) for PTB7:acceptor blends of varying acceptor concentrations. To compare the polaron yields, both bound and separated, for the acceptors in blends with PTB7 the peak polaron signal at 902 nm (corrected for differences in photoexcitation absorption) was plotted versus the concentrations of acceptor (Figure 7a) and chromophore (Figure 7b). At low acceptor concentrations ( 90% Quantum Efficiency. Adv. Mater. 2014, 26, 1923−1928. (25) Gélinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512−516. (26) Savoie, B. M.; Rao, A.; Bakulin, A. A.; Gélinas, S.; Movaghar, B.; Friend, R. H.; Marks, T. J.; Ratner, M. A. Unequal Partnership: Asymmetric Roles of Polymeric Donor and Fullerene Acceptor in Generating Free Charge. J. Am. Chem. Soc. 2014, 136, 2876−2884. (27) Greenham, N. C.; Samuel, I. D. W.; Hayes, G. R.; Phillips, R. T.; Kessener, Y. A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Measurement of Absolute Photoluminescence Quantum Efficiencies in Conjugated Polymers. Chem. Phys. Lett. 1995, 241, 89−96. (28) Stoltzfus, D. M.; Larson, B. W.; Zarrabi, N.; Shaw, P. E.; Clulow, A. J.; Burn, P. L.; Gentle, I. R.; Kopidakis, N. Investigating the Relationship Between Blend Microstructure and Charge Generation in Polymer:Non-Fullerene Bulk Heterojunctions. submitted June 2017. (29) Armin, A.; Kassal, I.; Shaw, P. E.; Hambsch, M.; Stolterfoht, M.; Lyons, D. M.; Li, J.; Shi, Z.; Burn, P. L.; Meredith, P. Spectral Dependence of the Internal Quantum Efficiency of Organic Solar Cells: Effect of Charge Generation Pathways. J. Am. Chem. Soc. 2014, 136, 11465−11472. (30) Brédas, J.-L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: the Challenges. Acc. Chem. Res. 2009, 42, 1691−1699. (31) Basel, T.; Huynh, U.; Zheng, T.; Xu, T.; Yu, L.; Vardeny, Z. V. Optical, Electrical, and Magnetic Studies of Organic Solar Cells Based on Low Bandgap Copolymer with Spin 1/2 Radical Additives. Adv. Funct. Mater. 2015, 25, 1895−1902. (32) Szarko, J. M.; Rolczynski, B. S.; Lou, S. J.; Xu, T.; Strzalka, J.; Marks, T. J.; Yu, L.; Chen, L. X. Photovoltaic Function and Exciton/ Charge Transfer Dynamics in a Highly Efficient Semiconducting Copolymer. Adv. Funct. Mater. 2014, 24, 10−26. (33) Howard, I. A.; Mauer, R.; Meister, M.; Laquai, F. Effect of Morphology on Ultrafast Free Carrier Generation in Polythiophene:Fullerene Organic Solar Cells. J. Am. Chem. Soc. 2010, 132, 14866− 14876. (34) Stoltzfus, D. M.; Clulow, A. J.; Jin, H.; Burn, P. L.; Gentle, I. R. Impact of Dimerization on Phase Separation and Crystallinity in Bulk Heterojunction Films Containing Non-Fullerene Acceptors. Macromolecules 2016, 49, 4404−4415. (35) Zarrabi, N.; Burn, P. L.; Meredith, P.; Shaw, P. E. Acceptor and Excitation Density Dependence of the Ultrafast Polaron Absorption Signal in Donor−Acceptor Organic Solar Cell Blends. J. Phys. Chem. Lett. 2016, 7, 2640−2646. (36) Ferguson, A. J.; Kopidakis, N.; Shaheen, S. E.; Rumbles, G. Dark Carriers, Trapping, and Activation Control of Carrier Recombination in Neat P3HT and P3HT:PCBM Blends. J. Phys. 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DOI: 10.1021/acs.jpcc.7b05862 J. Phys. Chem. C XXXX, XXX, XXX−XXX