Ultrafast Long-Range Charge Separation in Nonfullerene Organic

Nov 17, 2017 - Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom. ‡ Center for Organic Photoni...
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Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells Yasunari Tamai,†,∥ Yeli Fan,‡,§ Vincent O. Kim,† Kostiantyn Ziabrev,‡,⊥ Akshay Rao,† Stephen Barlow,‡ Seth R. Marder,‡ Richard H. Friend,*,† and S. Matthew Menke*,† †

Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom Center for Organic Photonics and Electronics and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States

‡

S Supporting Information *

ABSTRACT: Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron−hole pairs as they separate after ionization at the donor−acceptor interface. We show that charges separate rapidly (13%.11−18 In contrast to the rapid improvement of the device efficiency, relatively little is known about the fundamental mechanisms for charge separation and recombination in NFA-based OSCs. On photoexcitation, singlet excitons are photogenerated in either the donor or acceptor material and quickly diffuse to the DA heterojunction. As a result of the offset in molecular orbital energy levels at the DA interface, singlet excitons dissociate, pushing holes to the donor polymer and electrons to the acceptor. If the electron and hole were to move no further than the DA interface, they would form Coulombically bound

rganic solar cells (OSCs) depend on a donor− acceptor (DA) heterojunction to ionize photogenerated excitons to electrons and holes. Efficient solar cell operation requires that the electron and hole need to move beyond their Coulomb capture separation, generally considered to be >5 nm, very quickly to avoid geminate recombination. This requires specific energy landscapes near the DA heterojunction. For the now well-studied polymer donor fullerene acceptor systems, achieving a fast separation time scale requires the presence of pure fullerene regions, typically of size up to 10 nm.1−3 This morphological feature is specific to the fullerenes, and it is now interesting to understand whether this rapid (50%) recombination of tightly bound CT states on the sub-ns time scale.24 Similarly, all-polymer blends of PFB and F8BT show ∼75% geminate recombination to lower-lying F8BT triplet states.21 A hallmark of high-efficiency OSCs based on fullerene acceptors is that the electron and hole are separated into free carriers very efficiently on the sub-ps time scale.1,2,25−28 Recent spectroscopic studies have revealed that ultrafast long-range charge separation occurs through access to band-like electron states in fullerene aggregates and is the primary characteristic for achieving efficient free carrier generation.1−3 In these studies, the degree of aggregation and size of fullerene aggregates were closely correlated with the rate of free carrier 12474

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ascribed to hole polarons on the donor polymer.24,39,40,46 We note that the PDI dimer has an anion absorption band at 700 nm with a moderate absorption cross-section (Figure S14)39,47,48 and that it was not clearly observed due to overlapping with the large GSB band. The hole polaron band remains almost unchanged until 1 ns. For comparison, we also measured the PTB7-Th:PC71BM blend as shown in Figure 1e. In this case, singlet absorption was negligible at 0.5 ps after the laser excitation due to faster charge transfer, suggesting that the PTB7-Th:PC71BM blend forms a more fine scale morphology. As in the PDI blend, the polaron band also remains unchanged until 1 ns. An additional PIA band was observed at 1300 nm at later times only for the PC71BM blend, which we attribute to absorption from triplet excitons on the polymer formed through bimolecular recombination as discussed in the Supporting Information. Ultrafast Long-Range Charge Separation. Figure 2a,b shows the normalized TA spectra for PTB7-Th:bay-di-PDI (red) and PTB7-Th:PC71BM (black) blends at 1 and 10 ps. A

description underscores the importance of redissociation of CT states into free charge carriers.37 In this “reduced Langevin” description, measured bimolecular recombination rates are typically 2−4 orders of magnitude lower than the diffusionlimited Langevin recombination rate, allowing for high fill factors (FFs) to be obtained even with thicker (>200 nm) active layers.6 Turning to NFA-based OSCs, the FF tends to be lower than for fullerene-based OSCs,9,10 indicating that nongeminate recombination loss is more serious in NFAbased devices. As will be shown later, we find that spin plays an important role for nongeminate recombination. It is important to understand whether models developed for fullerene acceptor systems (that depend on the presence of pure-fullerene regions in the OSC structure)1 are also applicable to these efficient NFA systems. Here, we study the charge separation and recombination dynamics in blends of a benchmark donor polymer PTB7-Th4,5 (also known as PBDTTT-EFT or PCE-10, Figure 1a) and a PDI dimer, baydi-PDI (Figure 1b) by transient absorption (TA) spectroscopy. We find clear signals of sub-ps, long-range charge separation and negligible geminate recombination, signaling that deviations for ideal behavior only occur on the longest of time scales.

RESULTS AND DISCUSSION A PDI dimer was chosen as a model NFA material since PDIs in general have been widely studied as promising replacement for fullerenes,9,10,24,38−41 and the twisted bay-to-bay-linked PDI dimer (bay-di-PDI), in particular, offers a combination of a reasonably straightforward synthesis and moderate PCEs.42−45 Simple PDI derivatives, including the PDI dimers employed in this study, have absorption bands complementary with those of low-bandgap polymers, making it possible to excite each component selectively and exhibit a lowest unoccupied molecular orbital (LUMO) energy similar to PC71BM (∼3.8 eV for both bay-di-PDI and PC71BM),28,42 allowing to hold the excess energy constant for a straightforward comparison of charge dynamics. PTB7-Th shows a high PCE when paired with fullerene acceptors, up to 10.95%,5 and up to 5.90% with the particular PDI dimer used here.44 Figure 1c shows the steady-state absorption spectrum of a PTB7-Th:bay-di-PDI blend with the blend ratio of 1:1 by weight. The absorption spectra of a PTB7-Th:PC71BM blend (1:1.5) and a pristine PTB7-Th film are also shown in Figure 1c. TA Spectra after Polymer Selective Excitation. In order to study charge generation and recombination dynamics, we performed broadband TA measurements. Figure 1d shows the sub-ps TA spectra for the PTB7-Th:bay-di-PDI blend. The excitation wavelength was set at 650 nm to selectively excite the polymer (steady-state absorption spectra of pristine films are shown in the Supporting Information). The large positive band in the visible region is ascribed to ground-state bleaching (GSB). A broad photoinduced absorption (PIA) tail at around 1400 nm is attributed to polymer singlet excitons by comparing with the TA spectra of a pristine polymer film (Figure S4). Singlet excitons rapidly decay with a lifetime of 1.3 ps (Figure S5), which is much faster than the intrinsic singlet exciton lifetime of ∼220 ps in the pristine PTB7-Th film, suggesting that almost all excitons are converted into charges within a few ps. This is consistent with the strong photoluminescence quenching measured for the blend film (Figure S3). We note that we observe no evidence of quenching to PDI excimers under selective donor or acceptor excitation. PIA centered at 1150 nm becomes dominant within 10 ps, which can be

Figure 2. Time evolution of EA signal. (a,b) Normalized TA spectra of the PTB7-Th:bay-di-PDI (red) and PTB7-Th:PC71BM (black) blends averaged over the time window shown in each panel. Broken lines represent the TA spectra of the pristine PTB7-Th as a reference. (c) Time evolutions of the EA amplitude per unit charge obtained by dividing the EA signal (ΔT/TEA) by the charge signal (ΔT/TC). The right axis shows total energy stored in the electric field per electron−hole (e−h) pair. The time-resolved energy per e−h pair is obtained assuming that the half of the field is in the 1 donor phase as 2 ε0εr ∫ |E|2 dV and taking a value of 3 for εr. The spatially integrated square of the electric field was converted from the diode-based EA amplitude (see the Supporting Information). 12475

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cm−2 (see Figure S11a,b which shows the excitation intensity dependence of GSB and polaron decays in the PTB7-Th:baydi-PDI blend). This suggests that only a small amount of electron−hole pairs decay through geminate recombination channels and a large majority remain free charge carriers. By comparing the intensity of the GSB at 10 ps and 1.8 ns, we estimate the efficiency of charge carrier separation to be ∼90%, comparable to that in fullerene-based state-of-the-art blends.25−28 Note that the polaron signal increases slightly, even after singlet excitons are completely dissociated within 10 ps. We consider that this is due to a spectral shift of the polaron band, most probably due to downhill relaxation in the polaron density of states (DOS)25,26 as shown in Figure S13. As shown in Figure 3a, we found that there is no apparent difference

negative signal is already observed at 1 ps in both the PDI and PC71BM blends, suggesting ultrafast charge separation. Taking a closer look at the TA spectra in the visible region, as shown in Figures S6 and S7, we find that both the peak position and onset of the main GSB band blue-shift, even after singlet excitons, have been fully quenched. In other words, there is at least one more species in this region with dynamics different to both singlet excitons and charges. In line with past results, we find that the third component in this region is due to electroabsorption (EA) of PTB7-Th.1,3 When an exciton dissociates to form a separated electron−hole pair, a dipolelike, local electric field is generated in the surroundings, which results in a Stark shift of the absorption spectrum, adding a firstderivative like component to the overall TA spectrum. Since the EA amplitude depends on the length of the induced dipole, we can directly quantify the separation of charges from the DA interface by modeling the dynamics of the EA component.1,3 In order to obtain the EA amplitude, we performed a global analysis based on a genetic algorithm (GA), which enables us to decompose a 3D surface of the TA data into its principal spectra and kinetic traces.1,32,36 As shown in Figure S9, TA spectra in the visible region can be decomposed into contributions from three species, which we attribute to the GSB associated with singlets, the GSB associated with charges, and the EA. Figure 2c shows the EA amplitude obtained after deconvolution. The kinetic trace of the EA can be normalized by that of the charges to obtain the time evolution of the EA amplitude per unit charge since the EA amplitude depends not only on the electron−hole separation distance but also on charge population. On the right axis, we also show the total energy stored in the electric field per electron−hole pair. The 1 energy stored in the field was calculated as 2 ε0εr ∫ |E|2 dV and calibrated against quasi steady-state EA measurements on diode structures (see the Supporting Information).1 For the PC71BM blend, the EA amplitude is already large at 200 fs (the earliest time resolution for this experiment) and reaches its maximum by ∼400−500 fs. The stored energy reaches 300 meV, well above the thermal energy at room temperature (∼25 meV) and on the order of the CT state binding energy.49−51 This fast time scale means that the electron−hole pair undergoes rapid spatial separation, despite the opposing Coulomb attraction. This picture cannot be rationalized within the Onsager framework,49 but is consistent with ballistic charge separation through delocalized wave function before thermalization.1,2,25 Importantly, the EA amplitude for the PDI blend is as large as for the PC71BM blend and is reached on the same time scale, meaning that ultrafast spatial separation occurs in the PDI blend as well. Since the ballistic charge separation in previous reports was only observed in blend systems that contain relatively large fullerene aggregates, application of this model has been limited to fullerene-rich blends.1,2 However, this study gives clear experimental evidence that ballistic charge separation can also occur in NFA-based blends, and hence this model that charges separate coherently through delocalized states can be more widely applied for OSCs. Note that the further growth beyond 1 ps observed in the PDI blends is most likely due to subsequent, incoherent hopping of charges in the relatively larger donor and/or acceptor phases.1 Efficient Free Charge Carrier Generation. The GSB signal decays only slightly, and the decay dynamics are independent of the excitation intensity to at least ≤1.6 μJ

Figure 3. Polaron dynamics on the sub-ns time scale. (a) Normalized time evolution of the polaron band in the PTB7Th:bay-di-PDI (red) and PTB7-Th:PC71BM (black) blends. (b) Excitation wavelength dependence of polaron dynamics of the PTB7-Th:bay-di-PDI blend. TA spectra after the 500 nm excitation are shown in the Supporting Information.

between PC71BM and PDI blends, suggesting that there is no intrinsic disadvantage for the NFAs in terms of free charge carrier generation. We now move our attention to charge generation upon direct excitation of the NFA. Figure 3b shows the time evolution of the polaron band in the PTB7-Th:bay-di-PDI blend after the photoexcitation at 650 nm (red line, polymerselective excitation) and 500 nm (green line, PDI-selective excitation). After singlet excitons disappear (∼10 ps), the polaron dynamics are independent of the excitation wavelength, suggesting that both electron transfer from the polymer and hole transfer from the acceptor result in efficient charge separation. This is consistent with previously reported external quantum efficiency (EQE) spectra for PTB7-Th:bay-di-PDI blends where the EQE spectra closely resemble the absorption spectra.42−44 There is a range of views in the literature on the factors that control charge generation. Delocalization of the mobile charges during exciton dissociation is considered to be important.52 Since the photocurrent yield in fullerene blends scales with the presence and size of fullerene aggregates,1 rapid (coherent) motion of electrons from the heterojunction has been considered to be more important.30 In this study, we find similar coherent charge separation in a nonfullerene system, for photoexcitation of either donor or acceptor. It is interesting to explore the separate roles of electrons and holes in the charge separation process. Generally speaking, we can see that the choice of acceptor significantly affects the charge dissociation 12476

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PC71BM blend, this carrier lifetime is sufficiently long to extract the charge carriers before bimolecular recombination. Indeed, PTB7-Th:PC71BM blend cells have shown excellent FF of >70%.4,5 In contrast, PTB7-Th:PDI dimer-based devices have shown poorer FF of W is necessary so that a charge carrier injected ballistically into the band, though it slows down as it rises up the Coulomb well, is still in a bandlike state as it reaches the top (when the Coulomb well is less than kBT). The bandwidth B within simple Hückel models is 2nt, where n is the coordination number and t is the transfer integral. For fullerene acceptors, it is relatively easy to achieve this condition because the coordination number is high (12 for simple C60 but a little lower for PCBM). However, for the planar donor and acceptor molecules or polymers used here, π stacking gives n = 2, and it was far from clear that this would allow sufficient π bandwidth to allow the ballistic charge separation that we observe here. On longer time scales, these NFA-based devices still suffer from strong bimolecular charge decay, and it is this that limits PCE to 5.9% in contrast to the values of >10% with fullerene acceptors. We note that there are more opportunities to tune materials and device properties to improve carrier mobility so that extraction can out-pace recombination.61,62 What is critically important is that the early time charge separation is fast and efficient. This work represents a direct measurement of the time scale and energy stored for electron−hole separation in a nonfullerene blend. We find no evidence that the reduced coordination of this PDI as compared to PC71BM fundamentally hinders ultrafast, long-range separation. The design of future NFAs should focus on chemical modifications that maintain high transfer integrals and coordination numbers on the interfacial length scales while also increasing the bulk mobility of the blend.

Figure 6. Proposed model for charge separation and recombination. (a) Ballistic charge separation thorough delocalized states leads to efficient free charge generation for both the PDI and PC71BM blends. (b) Formation of bound CT states leading to geminate recombination is negligible. (c) Bimolecular charge recombination then leads to the formation of nongeminate, singlet and triplet CT states. (d) Some of the CT states undergo redissociation into free carriers. (e,f) The remainder decay to either the ground state or to lower-lying triplet states on the donor polymer. CT states in the PDI blend are more likely to decay compared to the PC71BM blend. (g) Once triplets are formed, they are then rapidly quenched by holes (triplet−polaron annihilation, TPA) or other triplets (triplet−triplet annihilation, TTA), meaning that their formation is a terminal loss process. The triplet lifetime in the PDI and PC71BM blends should be almost the same since TPA and TTA occurs within polymer domains.

lower-lying triplet states for triplet CT states (arrow f). In the PDI blend, both these channels compete with charge recycling (arrow d). Indeed, the Langevin reduction factor is estimated to be only ∼0.1 for the bay-di-PDI blend as discussed in the Supporting Information. In the fullerene blend, CT states can redissociate more easily, leading to suppressed nongeminate recombination. The difference in the reduction factor most probably arises from differences in nanomorphology near the interface. Formation of fullerene aggregates would prevent charges from getting close, and form more delocalized CT states, allowing CT states to be easily recycled back to free carriers.33,52 To rationalize the pronounced triplet formation in the fullerene blend, it is important to note that decay from singlet CT states is more suppressed than from triplet CT states, leading to decay in triplets being the major channel that competes with the redissociation of CT states formed in the PC71BM blend. In other words, there is still room for suppressing nongeminate recombination even in state-of-theart OSCs. Further studies into the role of spin and optimal nanomorphologies are needed to shut off recombination to triplets. Note that observation of negligible triplet absorption in the PDI blend does not mean negligible recombination to triplets in that blendfast triplet decay after formation would lead to a negligible accumulation of triplets (e.g., via tripletcharge annihilation). We further note that correlating the observed photophysical changes with the interfacial nanomorphology is difficult with conventional spectroscopic techniques which are sensitive to either the surface or bulk of the film rather than sensitive to the donor−acceptor interface. Recently, Jakowetz et al. have shown that direct investigation is possible using an all-optical, three-

MATERIALS AND METHODS Film Preparation. PTB7-Th and PC71BM were purchased from 1Material Inc. and Solenne BV, respectively. The PDI dimers were synthesized as reported elsewhere.42,45 Films were spin-coated onto quartz substrates from chlorobenzene solution with additives of 1,8diiodooctane (DIO) (1 vol %) and 1-chloronaphthalene (CN) (2 vol %) for the bay-di-PDI blend and an additive of DIO (3 vol %) for the PC71BM blend. The weight ratio of donor and acceptor was 1:1 for the PTB7-Th:bay-di-PDI blend and 1:1.5 for the PTB7-Th:PC71BM blend. These blend ratios and additives were optimized elsewhere.4,42−44 Transient Absorption Spectroscopy. Sub-pico to microsecond TA measurements were performed by an optical pump−probe technique. The broadband probe beam was generated in a homebuilt noncollinear optical parametric amplifier and delayed using a mechanical delay stage (Newport). Excitation pulses were generated from two different sources. For short-time measurements ( 1 V Open Circuit Voltages. Energy Environ. Sci. 2016, 9, 3783−3793. (18) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables Over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (19) Hodgkiss, J. M.; Campbell, A. R.; Marsh, R. A.; Rao, A.; AlbertSeifried, S.; Friend, R. H. Subnanosecond Geminate Charge Recombination in Polymer-Polymer Photovoltaic Devices. Phys. Rev. Lett. 2010, 104, 177701. (20) McNeill, C. R.; Westenhoff, S.; Groves, C.; Friend, R. H.; Greenham, N. C. Influence of Nanoscale Phase Separation on the Charge Generation Dynamics and Photovoltaic Performance of Conjugated Polymer Blends: Balancing Charge Generation and Separation. J. Phys. Chem. C 2007, 111, 19153−19160. (21) Westenhoff, S.; Howard, I. A.; Hodgkiss, J. M.; Kirov, K. R.; Bronstein, H. A.; Williams, C. K.; Greenham, N. C.; Friend, R. H. Charge Recombination in Organic Photovoltaic Devices with High Open-Circuit Voltages. J. Am. Chem. Soc. 2008, 130, 13653−13658. (22) Moore, J. R.; Albert-Seifried, S.; Rao, A.; Massip, S.; Watts, B.; Morgan, D. J.; Friend, R. H.; McNeill, C. R.; Sirringhaus, H. Polymer Blend Solar Cells Based on a High-Mobility NaphthalenediimideBased Polymer Acceptor: Device Physics, Photophysics and Morphology. Adv. Energy Mater. 2011, 1, 230−240. (23) Schubert, M.; Collins, B. A.; Mangold, H.; Howard, I. A.; Schindler, W.; Vandewal, K.; Roland, S.; Behrends, J.; Kraffert, F.; Steyrleuthner, R.; Chen, Z.; Fostiropoulos, K.; Bittl, R.; Salleo, A.; Facchetti, A.; Laquai, F.; Ade, H. W.; Neher, D. Correlated Donor/ Acceptor Crystal Orientation Controls Photocurrent Generation in allPolymer Solar Cells. Adv. Funct. Mater. 2014, 24, 4068−4081. (24) Gehrig, D. W.; Roland, S.; Howard, I. A.; Kamm, V.; Mangold, H.; Neher, D.; Laquai, F. Efficiency-Limiting Processes in LowBandgap Polymer:Perylene Diimide Photovoltaic Blends. J. Phys. Chem. C 2014, 118, 20077−20085. (25) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-Hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154−6164. (26) Etzold, F.; Howard, I. A.; Mauer, R.; Meister, M.; Kim, T.; Lee, K.; Baek, N. S.; Laquai, F. Ultrafast Exciton Dissociation Followed by Nongeminate Charge Recombination in PCDTBT: PCBM Photovoltaic Blends. J. Am. Chem. Soc. 2011, 133, 9469−9479. (27) Tamai, Y.; Tsuda, K.; Ohkita, H.; Benten, H.; Ito, S. ChargeCarrier Generation in Organic Solar Cells using Crystalline Donor Polymers. Phys. Chem. Chem. Phys. 2014, 16, 20338−20346. (28) Kawashima, K.; Tamai, Y.; Ohkita, H.; Osaka, I.; Takimiya, K. High-Efficiency Polymer Solar Cells with Small Photon Energy Loss. Nat. Commun. 2015, 6, 10085. (29) Monahan, N. R.; Williams, K. W.; Kumar, B.; Nuckolls, C.; Zhu, X. Y. Direct Observation of Entropy-Driven Electron-Hole Pair Separation at an Organic Semiconductor Interface. Phys. Rev. Lett. 2015, 114, 247003. (30) Savoie, B. M.; Rao, A.; Bakulin, A. A.; Gelinas, 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. (31) Brédas, J. L.; Sargent, E. H.; Scholes, G. D. Photovoltaic Concepts Inspired by Coherence Effects in Photosynthetic Systems. Nat. Mater. 2016, 16, 35−44. (32) Chow, P. C. Y.; Gelinas, S.; Rao, A.; Friend, R. H. Quantitative Bimolecular Recombination in Organic Photovoltaics through Triplet Exciton Formation. J. Am. Chem. Soc. 2014, 136, 3424−3429.

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DOI: 10.1021/acsnano.7b06575 ACS Nano 2017, 11, 12473−12481

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DOI: 10.1021/acsnano.7b06575 ACS Nano 2017, 11, 12473−12481