A Comparative Study on Hole Transfer Inversely Correlated with

Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 4110−4116

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A Comparative Study on Hole Transfer Inversely Correlated with Driving Force in Two Non-Fullerene Organic Solar Cells Jianqiu Wang,†,‡ Jianqiu Xu,§ Nannan Yao,∥ Dongyang Zhang,† Zhong Zheng,‡ Shenkun Xie,†,‡ Xuning Zhang,† Fengling Zhang,*,∥ Huiqiong Zhou,*,‡ Chunfeng Zhang,*,§ and Yuan Zhang*,†

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†

School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, No. 37 Xueyuan Road, Beijing 100191, P. R. China ‡ Key Laboratory of Nanosystem and Hierachical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China § School of Physics, Nanjing University, Nanjing, Jiangsu 210093, P. R. China ∥ Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping 58183, Sweden S Supporting Information *

ABSTRACT: We report a faster rate of hole transfer under a smaller ΔHOMO in a comparative study of two group organic solar cells (OSCs) consisting of IT-4F as an acceptor and PBDBT and PBDBT-SF as donors. In the OSCs based on PBDBTSF:IT-4F, a higher short-circuit current (JSC) was observed with a ΔHOMO of 0.31 eV compared to a lower JSC in PBDBT:IT-4F OSCs with a larger ΔHOMO (0.45 eV). Intensive investigation indicates that the rate of transfer of a hole from IT-4F to PBDBT-SF or PBDBT is inversely proportional to the ΔHOMO between IT-4F and donors. The larger JSC in the PBDBT-SF:IT-4F device is attributed to a synergy of faster hole transfer, slower recombination, and rapid charge extraction enabled by desired morphology and balanced charge carrier mobilities with PBDBT-SF, suggesting that under a sufficiently high ΔHOMO, comprehensive considerations of the transport, film morphology, and energy levels are needed when designing new materials for high-performance OSCs.

T

PCE.5,6,16−23 Generally, the rate of transfer of a hole from a hole donor (Dh) to a hole acceptor (Ah) is believed to be affected by the HOMO−HOMO offset (ΔHOMO).24−27 However, the physics of hole transfer in NF-OSCs is still not very clear, although several BHJ systems with a fast hole transfer or a larger JSC under a small ΔHOMO were reported recently.23 So far, the efficiencies of the hole transfer have been commonly demonstrated to be proportional to the ΔHOMOs in the literature, that is, the larger ΔHOMO in a BHJ, the larger photocurrent in the OSC. The question is whether the ΔHOMO is the sole parameter to determine the hole transfer in NFOSCs. If not, what else needs to be taken into account? To answer these questions, a comparative study of two kinds of OSCs based on IT-4F as a Dh and PBDBT28 and PBDBT-SF29 as Ahs was conducted. In this contribution, contrary to the results in the literature, we will report an inverse correlation between JSC and ΔHOMO observed in the comparative study. A larger JSC of 20.3 mA/ cm2 achieved in fluorinated PBDBT-SF devices via a more efficient hole transfer under a smaller ΔHOMO of 0.31 eV compared to a JSC of 19.6 mA/cm2 with PBDBT (ΔHOMO =

he performance of fullerene-based organic solar cells (OSCs) is mainly limited by a small photocurrent due to the weak absorption of fullerene derivatives and voltage losses for dissociating excitons at the interfaces between an electron donor (D e ) and an electron acceptor (A e ) in bulk heterojunctions (BHJs).1−4 A significantly advanced power conversion efficiency (PCE) over 15% was enabled by introducing non-fullerene (NF) Aes with extending absorption of BHJs to the near-infrared region for enhancing the photocurrent and aligning the energetic levels of the two photoactive materials in the BHJs to reduce the photovoltage losses.5−7 In fullerene-based OSCs, studies on enhancing the PCEs have been mostly focused on energetic offsets of LUMOs (ΔLUMO) for dealing with the trade-off between JSC and VOC because the photocurrents dominantly originated from exciton dissociation via electron transfer from Des to fullerenes, while the contribution from fullerenes via hole transfer was almost neglected.8−14 However, unlike the fullerene-based OSCs, efficient hole transfer actually plays a more important role than electron transfer for charge generation in NF-OSCs because representative NF acceptors are highly absorptive in the near-infrared region where the photon flux of the solar spectrum is high.15,16 Therefore, understanding the process of hole transfer to enhancing photocurrent in NF-OSCs is crucial for further increasing the © 2019 American Chemical Society

Received: May 14, 2019 Accepted: July 1, 2019 Published: July 1, 2019 4110

DOI: 10.1021/acs.jpclett.9b01383 J. Phys. Chem. Lett. 2019, 10, 4110−4116

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Figure 1. (a) Chemical structures of three materials used. (b) Energy diagram of PBDBT, PBDBT-SF, and IT-4F where the HOMO and LUMO levels are extracted from cyclic voltammetry measurements. The work functions of two interfacial layers used in the solar cells are also shown. (c) Current density vs voltage (J−V) of the NF-OSCs with two BHJs under AM 1.5 G solar irradiation. The inset shows the device architecture of the OSCs used in this study. (d) External quantum efficiency (EQE) of the OSCs. The inset shows the results of ultraviolet−visible−near-infrared absorption spectroscopy of the two Ahs and IT-4F Dh.

energetics in PBDBT:IT-4F [ΔHOMO = 0.45 eV, and Eg(eff) = 1.07 eV]. The OSCs were fabricated in the conventional ITO/ PEDOT:PSS/active layer/PFN-Br/Al structure (Figure 1c, inset); the current density−voltage (J−V) characteristics of two solar cells under the optimal conditions are provided in Figure 1c with extracted photovoltaic parameters listed in Table 1. The device based on PBDBT-SF:IT-4F achieved a higher PCE of 13.2% with a JSC of 20.3 mA/cm2, a VOC of 0.88 V, and an FF of 0.73, compared to a PCE of 10.1% with a JSC of 19.6 mA/cm2, a VOC of 0.72 V, and an FF of 0.71 in the PBDBT:IT-4F device. It is reasonable that the smaller energetic offset in PBDBT-SF:IT-4F reduced the energy loss and thus achieved a higher Voc. However, it is surprised that a slightly higher JSC was achieved in the device with PBDBT-SF, despite the fact that PBDBT-SF possesses a deeper-lying HOMOD level (5.38 eV) and a smaller ΔHOMO of 0.31 eV with IT-4F than the PBDBT:IT-4F blend (HOMOD = 5.24 eV, and ΔHOMO = 0.45 eV), which means that there is no trade-off between VOC and JSC in PBDBT-SF:IT-4F OSCs. The external quantum efficiencies (EQEs) of two OSCs clearly present the significant contribution from IT-4F at longer wavelength regions (Figure 1d), which confirm the efficient hole transfer from IT-4F to the two Ahs. It is noteworthy that the main EQE differences in the two devices lie in the IT-4F Dh absorption region where the photocurrent of the PBDBT-SF-based cell exceeds that of the one with PBDBT Ah. This indicates that efficient hole transfers from the IT-4F Dh to the two Ahs play critical roles in the ultimate JSC. Moreover, the JSC of the two cells based on the integration of EQEs agrees well with the values extracted from J−V curves (deviation of 90%) indicates efficient charge dissociation in the two BHJs. It is interesting that the comparable PL quenching efficiencies are achieved in the two BHJs despite the reduced ΔLUMO and ΔHOMO in the PBDBTSF:IT-4F blend. To identify the process of hole transfer from IT-4F to the two Ahs, TA spectroscopy was performed at different time delays. The differential transmittance (ΔT/T) versus energy probed at different time delays is plotted in Figure S3 for neat polymer films (pump = 500 nm) and IT-4F (pump = 710 nm). As one can see, we observe two main TA signals in neat Ah and IT-4F Dh films (582 and 640 nm in PBDBT, 584 and 636 nm in PBDBT-SF, and 660 and 770 nm in IT-4F). These peaks arise from the main optical transition from the photobleaching (PB) of ground states that progressively decreases with delay time.31,32 The negative signals of ΔT/T are due to the photoinduced absorption involving the absorption of photogenerated charge carriers or excitons by excited states.31 For neat PBDBT-SF, PBDBT, and IT-4F, the spectra present an excitonic feature, typically found for organic semiconductors with low dielectric constants.31,33 To understand the charge transfer relevant to the solar cell operation, we measured the TA of blend films with the results shown in panels a and b of Figure 2. Here we mainly focused on hole transfer processes

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μe/μh

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DOI: 10.1021/acs.jpclett.9b01383 J. Phys. Chem. Lett. 2019, 10, 4110−4116

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The Journal of Physical Chemistry Letters that the utilized pumping energy can excite only IT-4F, the rising TA signals in the two-Ah absorption region should arise from the hole transfer from IT-4F.34 Moreover, we found that the bleaching of the IT-4F absorption peak in the BHJ films decays faster, compared to that of neat IT-4F, which is further evidence of the occurrence of hole transfer in these two blends. Direct assessments of the rate of hole transfer are enabled by comparing the decay dynamics of the rising PB peaks in the two-Ah absorption region and the lower-energy PB signals related to IT-4F. It should be mentioned that the signal of photoinduced absorption related to polarons may contaminate the signal of ground-state bleaching. To decouple these species, we performed the global analysis to extract the temporal and spectral features of different excited species. The deconvoluted spectra and kinetic curves are included in Figure S5. The decay rate in Figure 2c can be mutually affected by the exciton lifetime and hole transfer rate. As the same IT-4F Dh was used, the differences in decay dynamics should be impacted by the efficiency of the latter process. As for the rising TA signals shown in Figure 2d, given that only the IT-4F is excited, the rate of the TA increase should manifest the rate of hole transfer from IT-4F to Ahs, as well. From the results shown in panels c and d of Figure 2, the BHJ with PBDBT-SF presents slightly faster decays when probed at the PB of IT-4F associated with expedited increases when probed at the PBs of the two Ahs. These observations imply a more efficient hole transfer in PBDBT-SF:IT-4F, which seems to contrast the reduced ΔHOMO with fluorinated PBDBT-SF (see Table 1). In a previous study, we found that introduction of fluorine into both the polymer donor and the small molecule acceptor leads to an increase in the dielectric constant in the BHJ,33 which may reduce the exciton binding energy at charge transfer states. In addition to this factor, the promoted interfacial hole transfer may be ascribed to the local intermolecular dipole at the D/A interface or local carrier mobility with fluorination.29,35 In addition, the possibility that the enhanced hole transfer can also be promoted by the higher hole mobility in PBDBT-SF:IT-4F will be addressed. The TA measurements were also carried out on the basis of higher-energy photoexcitation at 500 nm (see the results in Figure S5). Disentangling the exact origins of the positive TA signals appears to be difficult due to the coexistence of excitons in the Ahs and Dh. By comparing the dynamics probed at 760 nm, we observe a consistent trend that the BHJ with PBDBT-SF exhibits a decay (see Figure S5) that is faster than that of nonfluorinated PBDBT. To correlate the ultrafast charge transfer to the transport properties, we determined the charge carrier mobility in these BHJ films by single-carrier device measurements. The devices were fabricated on the basis of the identical blend films with charge-selective electrodes (see device sketches in the insets of Figure S6). By fitting the dark current with the Mott−Gurney law,36,37 we extracted the mobility of holes (μh) and electrons (μe) in the SCLC region with the results provided in Table 1. The determined mobilities of the two carriers both fall in the range of 10−4 cm2 V−1 s−1, showing relatively good balance. It is worth noting that the μh and μe in PBDBT-SF containing BHJ exceed those with PBDBT. The higher μh is consistent with the trend of hole transfer efficiencies revealed by TA. The change in μe in the two BHJ films could be related to the interpercolated nanomorphology in the BHJs where the electron-transporting pathway in the IT-4F domain may be affected.38 As we will discuss, the increase in both μh and μe

can be correlated to the enhanced phase purity of PBDBTF:IT-4F assessed by differential scanning calorimetry (DSC). This morphologcal feature is consistent with the atomic force microscopy phase images (Figure S7) from which the PBDBTSF blend film tends to have a larger phase separation, compared to that of PBDBT:IT-4F. Apart from charge dissociation, the operation of OSCs and current extraction are subject to the competition between Fdependent charge carrier extraction and recombination,33,39,40 which fundamentally differs from the operation in highly crystalline inorganic or hybrid solar cells, e.g., perovskites.41,42 In this context, it will be important to understand the impact of nongeminate losses with the two Ahs to further understand the differentiated device parameters. Here TPV and TPC measurements were employed to shed more light on the charge extraction and bimolecular recombination. Figure 3a

Figure 3. (a) TPV under open-circuit conditions and (b) TPC under short-circuit conditions measured on different solar cells under excitation at 488 nm. (c) Charge extraction time (τext) vs internal potential with different donors. (d) Carrier lifetime vs light intensity determined by impedance spectroscopy.

shows photovoltage decay kinetics of solar cells excited at 488 nm under open-circuit conditions. In this situation, the net current flow is zero and the kinetics are mainly dictated by charge recombination with the decay time manifested by the recombination lifetime (τrec).43 The devices with PBDBT-SF exhibit a lifetime (τrec = 51.1 μs) that is longer than that of the PBDBT device (τrec = 44.66 μs). On the basis of irradiationdependent JSC measurements (see Figure S8), a power law dependence is observed with the power approaching unity. It indicates that bimolecular recombination is indeed the dominant process.44 On this basis, the longer τrec in PBDBTSF:IT-4F can be ascribed to the suppressed bimolecular recombination. Figure 3b displays photocurrent decay kinetics under short-circuit conditions probed by TPC. It is obvious that faster decay in the PBDBT-SF cell is associated with a faster carrier sweepout time compared to that with PBDBT. To further investigate the charge extraction at various biases, we analyzed the TPC at different biases (see the results in Figure S9). On the basis of the fittings with a monoexponential decay model, the time for charge extraction (τext) was determined and is shown in Figure 3c. In both devices, the τext becomes shorter with an increased internal electrical field, due to the Fassisted electron−hole separation with suppressed recombina4113

DOI: 10.1021/acs.jpclett.9b01383 J. Phys. Chem. Lett. 2019, 10, 4110−4116

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The Journal of Physical Chemistry Letters tion.39 Remarkably, the τext of the PBDBT-SF cell (77.54 ns) is much shorter than that of the PBDBT device (115.11 ns), indicating more efficient charge extraction. The TPV and TPC results suggest that PBDBT-SF:IT-4F BHJ shows reduced recombination and more efficient charge extraction. It is noteworthy that the bimolecular recombination strength is actually affected by the carrier density in the photoactive layer. In our TPV measurements with monochromatic laser excitation, the density of photocarriers could be much lower than that under 1 sun irradiation, which to some degree smears out the difference in recombination.5,45 For this reason, we also estimated the carrier lifetime (τcarr) of solar cells by light intensity (Plight)-dependent impedance spectroscopy (IS) (see Figure S10). On the basis of the chemical capacitance (Cμ) and recombination resistance (Rrec) extracted from IS in conjunction with equivalent circuit modeling,39,46 the τcarr was determined according to the equation τcarr = CμRrec.46 Figure 3d shows the τcarr of the two solar cells as a function of Plight. Indeed, larger differences in τcarr are observed in the two OSCs, which is consistent with the τrec determined by TPV analysis (Figure 3a). The determined density of photocarriers (Nd) in the cell with PBDBT-SF is visibly higher than that in the PBDBT device (see Figure S10c and the detailed calculation method in the Supporting Information). This trend well agrees with the Jsc in respective solar cells. Grazing incidence wide-angle X-ray scattering (GIWAXS) was performed to investigate the order structure in the BHJ films (see the two-dimensional GIWAXS patterns in Figure S11). As comparatively shown by the line-cut profile curves in Figure 4a, both PBDBT and PBDBT-SF BHJs adapt to a face-

Finally, we assessed the phase miscibility between IT-4F and Ahs by DSC. This analysis methodology has been successfully applied to explain the FF in an array of OSCs.49 On the basis of the model proposed by Ye et al. (see the detailed formula in the Supporting Information),48 we determined parameter χ describing the degree of miscibility between the Ahs and Dh domains by DSC (see the details in the Supporting Information). The values of χ were averaged on the basis of multiple sets of samples under the respective conditions to ensure their accuracy (see the results in Figure S12 and Table S1). As shown in Figure 4b, χ was found to be 4.04 in PBDBT:IT-4F and increases to 4.61 for PBDBT-SF:IT-4F blends. The increased χ points to the likelihood that the PBDBT-SF:IT-4F has the higher phase purity. This morphological feature can reconcile the enhanced charge transfer and reduced recombination losses in the PBDBT-SF cell due to the reduced miscibility between the Ah and Dh.48,49 At the molecular level, the increase in phase purity may be attributed to the enhanced intermolecular dipole interactions in the presence of fluorine heteroatoms introduced into both the donor and the acceptor.50 The combined results highlight the fact that PBDBT-SF with fluorination can achieve a favorable film morphology with a higher phase purity. With regard to hole transfer, increasing both the hole transport and the charge extraction efficiency is critical, especially when the HOMO energy offset between D and A is relatively small. To summarize, we observe larger values of JSC, VOC, and FF in a NF-OSC based on PBDBT-SF:IT-4F with a smaller ΔHOMO of 0.31 eV, compared to those of OSCs based on PBDBT:IT-4F with a larger ΔHOMO of 0.45 eV. TA measurements evidence that faster hole transfer occurs in PBDBT-SF:IT-4F, and this tendency is consistent with the larger JSC. The TPV and TPC results reveal the reduced bimolecular recombination loss and the longer carrier lifetime in PBDBT-SF:IT-4F devices, which is also confirmed by light intensity-dependent IS. As a result, suppressed recombination and faster charge sweepout are found in the PBDBT-SF:IT-4F devices, facilitated by the higher carrier mobility and phase purity enabled by fluorination. The eventual increases in JSC and PCE are also relevant to the competition of change extraction with bimolecular recombination in the OSCs. This work suggests that with a sufficiently high ΔHOMO, the rate of hole transfer at the Dh/Ah interfaces may not be subject to the energetic offset but other attributes. Thus, to further increase the PCE of NF-OSCs, molecules and/or polymers with appropriate energy levels and good transport properties are needed.

Figure 4. (a) Line-cut profile curves of two-dimensional GIWAXS measured on thin films of different blends. (b) Differential scanning calorimetry characteristics of various blend films used to assess to the miscibility of donor and acceptor domains.

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on molecular orientation with respect to the substrate, manifested by the observed strong π−π stacking peak in the out-of-plane direction. The π−π stacking peaks are located at 1.73 and 1.76 Å−1 in the films containing PBDBT and PBDBTSF, respectively, corresponding to a π−π stacking d spacings of 3.63 and 3.57 Å, respectively. In general, the reduced π−π stacking distance promotes vertical charge hopping,47 which is consistent with the higher hole mobility in the PBDBT-SF blends. In addition, we determined the crystal correlation length (CCL) according to the Scherrer equation [CCL = 2πk/fwhm (fwhm is the full width at half-maximum of the diffraction peak)].33,47 The increased CCL of 2.57 nm in the PBDBT-SF:IT-4F film (compared to a CCL of 2.09 nm in PBDBT:IT-4F) points to an overall increased range of ordering in π−π stacking. These results confirm the enhanced molecular ordering in the BHJ film with PBDBT-SF.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01383.

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Experimental section, supporting figures, and supporting notes (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail: 4114

[email protected]. [email protected]. [email protected]. [email protected]. DOI: 10.1021/acs.jpclett.9b01383 J. Phys. Chem. Lett. 2019, 10, 4110−4116

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(12) Gao, K.; Li, L.; Lai, T.; Xiao, L.; Huang, Y.; Huang, F.; Peng, J.; Cao, Y.; Liu, F.; Russell, T. P.; Janssen, R. A. J.; Peng, X. Deep absorbing porphyrin small molecule for high-performance organic solar cells with very low energy losses. J. Am. Chem. Soc. 2015, 137 (23), 7282−7285. (13) Sulas, D. B.; Yao, K.; Intemann, J. J.; Williams, S. T.; Li, C.-Z.; Chueh, C.-C.; Richards, J. J.; Xi, Y.; Pozzo, L. D.; Schlenker, C. W.; Jen, A. K.-Y.; Ginger, D. S. Open-Circuit Voltage Losses in SeleniumSubstituted Organic Photovoltaic Devices from Increased Density of Charge-Transfer States. Chem. Mater. 2015, 27 (19), 6583−6591. (14) Vandewal, K.; Ma, Z. F.; Bergqvist, J.; Tang, Z.; Wang, E. G.; Henriksson, P.; Tvingstedt, K.; Andersson, M. R.; Zhang, F. L.; Inganas, O. Quantification of Quantum Efficiency and Energy Losses in Low Bandgap Polymer:Fullerene Solar Cells with High OpenCircuit Voltage. Adv. Funct. Mater. 2012, 22 (16), 3480−3490. (15) Xie, Y.; Yang, F.; Li, Y.; Uddin, M. A.; Bi, P.; Fan, B.; Cai, Y.; Hao, X.; Woo, H. Y.; Li, W.; Liu, F.; Sun, Y. Morphology Control Enables Efficient Ternary Organic Solar Cells. Adv. Mater. 2018, 30 (38), 1803045. (16) Zhou, Z.; Xu, S.; Song, J.; Jin, Y.; Yue, Q.; Qian, Y.; Liu, F.; Zhang, F.; Zhu, X. High-efficiency small-molecule ternary solar cells with a hierarchical morphology enabled by synergizing fullerene and non-fullerene acceptors. Nat. Energy 2018, 3 (11), 952−959. (17) Zhang, J.; Zhang, X.; Li, G.; Xiao, H.; Li, W.; Xie, S.; Li, C.; Bo, Z. A nonfullerene acceptor for wide band gap polymer based organic solar cells. Chem. Commun. 2016, 52 (3), 469−472. (18) Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K. Y.; Marder, S. R.; Zhan, X. Non-fullerene acceptors for organic solar cells. Nat. Rev. Mater. 2018, 3, 18003. (19) Zhang, H.; Yao, H.; Hou, J.; Zhu, J.; Zhang, J.; Li, W.; Yu, R.; Gao, B.; Zhang, S.; Hou, J. Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene Small-Molecule Acceptors. Adv. Mater. 2018, 30 (28), 1800613. (20) Liu, J.; Chen, S. S.; Qian, D. P.; Gautam, B.; Yang, G. F.; Zhao, J. B.; Bergqvist, J.; Zhang, F. L.; Ma, W.; Ade, H.; Inganas, O.; Gundogdu, K.; Gao, F.; Yan, H. Fast charge separation in a nonfullerene organic solar cell with a small driving force. Nat. Energy 2016, 1, 16089. (21) Li, Y. X.; Qian, D. P.; Zhong, L.; Lin, J. D.; Jiang, Z. Q.; Zhang, Z. G.; Zhang, Z. J.; Li, Y. F.; Liao, L. S.; Zhang, F. L. A fused-ring based electron acceptor for efficient non-fullerene polymer solar cells with small HOMO offset. Nano Energy 2016, 27, 430−438. (22) Qian, D. P.; Zheng, Z. L.; Yao, H. F.; Tress, W.; Hopper, T. R.; Chen, S. L.; Li, S. S.; Liu, J.; Chen, S. S.; Zhang, J. B.; Liu, X. K.; Gao, B. W.; Ouyang, L. Q.; Jin, Y. Z.; Pozina, G.; Buyanova, I. A.; Chen, W. M.; Inganas, O.; Coropceanu, V.; Bredas, J. L.; Yan, H.; Hou, J. H.; Zhang, F. L.; Bakulin, A. A.; Gao, F. Design rules for minimizing voltage losses in high-efficiency organic solar cells. Nat. Mater. 2018, 17 (8), 703−709. (23) Chen, S.; Wang, Y.; Zhang, L.; Zhao, J.; Chen, Y.; Zhu, D.; Yao, H.; Zhang, G.; Ma, W.; Friend, R. H.; Chow, P. C. Y.; Gao, F.; Yan, H. Efficient Nonfullerene Organic Solar Cells with Small Driving Forces for Both Hole and Electron Transfer. Adv. Mater. 2018, 30 (45), 1804215. (24) Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The role of buffer layers in polymer solar cells. Energy Environ. Sci. 2011, 4 (2), 285−310. (25) Sommer, M.; Lindner, S. M.; Thelakkat, M. MicrophaseSeparated Donor−Acceptor Diblock Copolymers: Influence of HOMO Energy Levels and Morphology on Polymer Solar Cells. Adv. Funct. Mater. 2007, 17 (9), 1493−1500. (26) Li, Y.; Qian, D.; Zhong, L.; Lin, J.-D.; Jiang, Z.-Q.; Zhang, Z.G.; Zhang, Z.; Li, Y.; Liao, L.-S.; Zhang, F. A fused-ring based electron acceptor for efficient non-fullerene polymer solar cells with small HOMO offset. Nano Energy 2016, 27, 430−438. (27) Gong, X.; Tong, M.; Brunetti, F. G.; Seo, J.; Sun, Y.; Moses, D.; Wudl, F.; Heeger, A. J. Bulk Heterojunction Solar Cells with Large Open-Circuit Voltage: Electron Transfer with Small Donor-Acceptor Energy Offset. Adv. Mater. 2011, 23 (20), 2272−2277.

Huiqiong Zhou: 0000-0003-2124-6563 Chunfeng Zhang: 0000-0001-9030-5606 Yuan Zhang: 0000-0003-0670-2428 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21875012 and 21674006). H.Z. is thankful for the financial support from the National Natural Science Foundation of China (NSFC) (21773045), the National Key Research and Development Program of China (2017YFA0206600), and the Chinese Academy of Science (100 Top Young Scientists Program). Y.Z. thanks the “111” Project. N.Y. and F.Z. acknowledge funding from the Knut and Alice Wallenberg Foundation under Contract 2016.0059, the Swedish Government Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU 200900971), and the China Scholarship Council (CSC) (201708370115). The authors thank Prof. Defeng Zhou and Hui Zhang at Changchun University of Technology for the DSC measurements.

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REFERENCES

(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions. Science 1995, 270 (5243), 1789−1790. (2) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design rules for donors in bulkheterojunction solar cellsTowards 10% energy-conversion efficiency. Adv. Mater. 2006, 18 (6), 789−794. (3) Zhang, F. L.; Inganas, O.; Zhou, Y. H.; Vandewal, K. Development of polymer-fullerene solar cells. Natl. Sci. Rev. 2016, 3 (2), 222−239. (4) Inganäs, O. Organic Photovoltaics over Three Decades. Adv. Mater. 2018, 30 (35), 1800388. (5) Cui, Y.; Yao, H.; Hong, L.; Zhang, T.; Xu, Y.; Xian, K.; Gao, B.; Qin, J.; Zhang, J.; Wei, Z.; Hou, J. Achieving Over 15% Efficiency in Organic Photovoltaic Cells via Copolymer Design. Adv. Mater. 2019, 31 (14), 1808356. (6) Fan, B.; Zhang, D.; Li, M.; Zhong, W.; Zeng, Z.; Ying, L.; Huang, F.; Cao, Y. Achieving over 16% efficiency for single-junction organic solar cells. Sci. China: Chem. 2019, 62, 746. (7) Yuan, J.; Zhang, Y.; Zhou, L.; Zhang, G.; Yip, H.-L.; Lau, T.-K.; Lu, X.; Zhu, C.; Peng, H.; Johnson, P. A.; Leclerc, M.; Cao, Y.; Ulanski, J.; Li, Y.; Zou, Y. Single-Junction Organic Solar Cell with over 15% Efficiency Using Fused-Ring Acceptor with ElectronDeficient Core. Joule 2019, 3, 1140. (8) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. On the origin of the open-circuit voltage of polymer-fullerene solar cells. Nat. Mater. 2009, 8 (11), 904−909. (9) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. Relating the open-circuit voltage to interface molecular properties of donor: acceptor bulk heterojunction solar cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81 (12), 125204. (10) Albrecht, S.; Vandewal, K.; Tumbleston, J. R.; Fischer, F. S.; Douglas, J. D.; Fréchet, J. M.; Ludwigs, S.; Ade, H.; Salleo, A.; Neher, D. On the efficiency of charge transfer state splitting in polymer: fullerene solar cells. Adv. Mater. 2014, 26 (16), 2533−2539. (11) Burke, T. M.; Sweetnam, S.; Vandewal, K.; McGehee, M. D. Beyond Langevin Recombination: How Equilibrium Between Free Carriers and Charge Transfer States Determines the Open-Circuit Voltage of Organic Solar Cells. Adv. Energy Mater. 2015, 5 (11), 1500123. 4115

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The Journal of Physical Chemistry Letters

via Conjugated Rotatable End Groups. Adv. Energy Mater. 2018, 8 (31), 1802131. (46) Zhang, Y.; Dang, X.-D.; Kim, C.; Nguyen, T.-Q. Effect of Charge Recombination on the Fill Factor of Small Molecule Bulk Heterojunction Solar Cells. Adv. Energy Mater. 2011, 1 (4), 610−617. (47) Zhang, X.; Zuo, X.; Xie, S.; Yuan, J.; Zhou, H.; Zhang, Y. Understanding charge transport and recombination losses in high performance polymer solar cells with non-fullerene acceptors. J. Mater. Chem. A 2017, 5 (33), 17230−17239. (48) Ye, L.; Hu, H.; Ghasemi, M.; Wang, T.; Collins, B. A.; Kim, J.H.; Jiang, K.; Carpenter, J. H.; Li, H.; Li, Z.; McAfee, T.; Zhao, J.; Chen, X.; Lai, J. L. Y.; Ma, T.; Bredas, J.-L.; Yan, H.; Ade, H. Quantitative relations between interaction parameter, miscibility and function in organic solar cells. Nat. Mater. 2018, 17 (3), 253−260. (49) Lyons, B. P.; Clarke, N.; Groves, C. The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics. Energy Environ. Sci. 2012, 5 (6), 7657−7663. (50) Mukherjee, S.; Proctor, C. M.; Tumbleston, J. R.; Bazan, G. C.; Nguyen, T.-Q.; Ade, H. Importance of Domain Purity and Molecular Packing in Efficient Solution-Processed Small-Molecule Solar Cells. Adv. Mater. 2015, 27 (6), 1105−1111.

(28) Qian, D.; Ye, L.; Zhang, M.; Liang, Y.; Li, L.; Huang, Y.; Guo, X.; Zhang, S.; Tan, Z. a.; Hou, J. Design, Application, and Morphology Study of a New Photovoltaic Polymer with Strong Aggregation in Solution State. Macromolecules 2012, 45 (24), 9611− 9617. (29) 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 (21), 7148−7151. (30) Azzouzi, M.; Kirchartz, T.; Nelson, J. Factors Controlling Open-Circuit Voltage Losses in Organic Solar Cells. Trends in Chem. 2019, 1 (1), 49−62. (31) Xue, L.; Yang, Y.; Xu, J.; Zhang, C.; Bin, H.; Zhang, Z.-G.; Qiu, B.; Li, X.; Sun, C.; Gao, L.; Yao, J.; Chen, X.; Yang, Y.; Xiao, M.; Li, Y. Side Chain Engineering on Medium Bandgap Copolymers to Suppress Triplet Formation for High-Efficiency Polymer Solar Cells. Adv. Mater. 2017, 29 (40), 1703344. (32) Hou, T.; Liu, F.; Wang, Z.; Zhang, Y.; Wei, Q.; Li, B.; Zhang, S.; Xing, G. Trap-Filling-Induced Charge Carrier Dynamics in Organic Solar Cells. Adv. Opt. Mater. 2018, 6 (8), 1800027. (33) Zhang, X.; Zhang, D.; Zhou, Q.; Wang, R.; Zhou, J.; Wang, J.; Zhou, H.; Zhang, Y. Fluorination with an enlarged dielectric constant prompts charge separation and reduces bimolecular recombination in non-fullerene organic solar cells with a high fill factor and efficiency > 13%. Nano Energy 2019, 56, 494−501. (34) Shivanna, R.; Shoaee, S.; Dimitrov, S.; Kandappa, S. K.; Rajaram, S.; Durrant, J. R.; Narayan, K. S. Charge generation and transport in efficient organic bulk heterojunction solar cells with a perylene acceptor. Energy Environ. Sci. 2014, 7 (1), 435−441. (35) Gautam, B.; Klump, E.; Yi, X.; Constantinou, I.; Shewmon, N.; Salehi, A.; Lo, C. K.; Zheng, Z.; Brédas, J.-L.; Gundogdu, K.; Reynolds, J. R.; So, F. Increased Exciton Delocalization of Polymer upon Blending with Fullerene. Adv. Mater. 2018, 30 (30), 1801392. (36) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19 (12), 1551−1566. (37) Pasveer, W. F.; Cottaar, J.; Tanase, C.; Coehoorn, R.; Bobbert, P. A.; Blom, P. W. M.; de Leeuw, D. M.; Michels, M. A. J. Unified Description of Charge-Carrier Mobilities in Disordered Semiconducting Polymers. Phys. Rev. Lett. 2005, 94 (20), 206601. (38) Kong, J.; Hwang, I.-W.; Lee, K. Top-Down Approach for Nanophase Reconstruction in Bulk Heterojunction Solar Cells. Adv. Mater. 2014, 26 (36), 6275−6283. (39) Wang, J.; Xie, S.; Zhang, D.; Wang, R.; Zheng, Z.; Zhou, H.; Zhang, Y. Ultra-narrow bandgap non-fullerene organic solar cells with low voltage losses and a large photocurrent. J. Mater. Chem. A 2018, 6 (41), 19934−19940. (40) Zheng, Z.; Hu, Q.; Zhang, S.; Zhang, D.; Wang, J.; Xie, S.; Wang, R.; Qin, Y.; Li, W.; Hong, L.; Liang, N.; Liu, F.; Zhang, Y.; Wei, Z.; Tang, Z.; Russell, T. P.; Hou, J.; Zhou, H. A Highly Efficient Non-Fullerene Organic Solar Cell with a Fill Factor over 0.80 Enabled by a Fine-Tuned Hole-Transporting Layer. Adv. Mater. 2018, 30 (34), 1801801. (41) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506. (42) Marchioro, A.; Teuscher, J.; Friedrich, D.; Kunst, M.; van de Krol, R.; Moehl, T.; Grätzel, M.; Moser, J.-E. Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells. Nat. Photonics 2014, 8, 250. (43) Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de Mello, J.; Durrant, J. R. Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell. Appl. Phys. Lett. 2008, 92 (9), 093311. (44) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in polymerfullerene bulk heterojunction solar cells. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82 (24), 245207. (45) Yu, R.; Yao, H.; Hong, L.; Xu, Y.; Gao, B.; Zhu, J.; Zu, Y.; Hou, J. Enhancing the Photovoltaic Performance of Nonfullerene Acceptors 4116

DOI: 10.1021/acs.jpclett.9b01383 J. Phys. Chem. Lett. 2019, 10, 4110−4116