Enhanced Photovoltaic Performance of Ternary Polymer Solar Cells

Sep 12, 2017 - Enhanced Photovoltaic Performance of Ternary Polymer Solar Cells by Incorporation of a Narrow-Bandgap Nonfullerene Acceptor. Wenkai Zho...
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Enhanced Photovoltaic Performance of Ternary Polymer Solar Cells by Incorporation of a Narrow-Bandgap Nonfullerene Acceptor Wenkai Zhong,† Jing Cui,‡ Baobing Fan,† Lei Ying,*,† Yu Wang,† Xue Wang,† Guichuan Zhang,† Xiao-Fang Jiang,† Fei Huang,*,† and Yong Cao† †

Institute of Polymer Optoelectronic Materials and Devices and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China S Supporting Information *

ABSTRACT: We developed a novel nonfullerene electron acceptor, IffBR, that consists of electron-rich indaceno[1,2b:5,6-b′]dithiophene as the central unit and an electrondeficient 5,6-difluorobenzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group. IffBR exhibits peak UV−vis absorbance at 658 nm, which is complementary with the absorption profiles of the wide-bandgap conjugated polymers poly[4,8-bis(4,5-dihexylthiophen-2-yl)benzo[1,2b:4,5-b′]-dithiophene-alt-2-(2-butyloctyl)-5,6-difluoro-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole] (PBTA-BO) and the fullerene acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The ternary device constructed with PBTABO/PC71BM/IffBR as the light-absorption layer exhibited significantly better photovoltaic performance than those obtained from devices based on a bulk-heterojunction layer comprised of binary components. This improvement was attributed to the broadened absorbance, formation of cascade charge-transfer pathways, reduced nongeminate recombination, enhanced charge extraction, and more favorable morphologies of the bulk-heterojunction films. The optimized ternary device exhibited a power conversion efficiency of 9.06%, which is significantly higher than those of binary devices based on either PBTA-BO/IffBR (6.24%) or PBTA-BO/PC71BM (4.73%). These results indicate that IffBR is an outstanding electron acceptor, suitable for the fabrication of nonfullerene or multicomponent-blend polymer solar cells. containing benzo[c][1,2,5]thiadiazole (BT) flanked rhodanine as the peripheral unit have been constructed that exhibit outstanding long-term operation and thermal stability.17,27−31 It has been established that the incorporation of fluorine atoms into the conjugated molecular framework can result in enhanced intramolecular and intermolecular interactions because of its strong electronegativity.32−36 Specifically, the fluorination of the 5- and 6-positions of BT unit, to produce 5,6-difluorobenzo[c][1,2,5]thiadiazole (ffBT), has been conducted to improve its electronic and absorption properties, aggregation behavior, and charge-carrier transportation.37,38 Hence, the photovoltaic properties of polymers made of ffBT are superior to those of their nonfluorinated counterparts. In this study, we developed an ffBT-based narrow-bandgap A-D-A-type nonfullerene acceptor (IffBR), consisting of the indaceno[1,2-b:5,6-b′]dithiophene (IDT) as the central D unit and ffBT flanked with rhodanine as the A group. IffBR exhibits a UV−vis absorption profile that is complementary to that of the wide-bandgap polymer poly[4,8-bis(4,5-dihexylthiophen-2-

1. INTRODUCTION Polymer solar cells (PSCs) continue to attract increasing academic and industrial interest because of their potential in the fabrication of flexible and lightweight products using low-cost roll-to-roll technology.1−3 Typically, the light-harvesting layers of PSCs are composed of an electron-donating polymer and an electron-accepting fullerene derivative. Therefore, their light absorption is limited by the absorption profiles of their components. This limitation can be overcome by incorporating an additional component that exhibits complementary absorbance and suitable energy levels into the light-harvesting layer.4−12 This component can extend the absorption profile and achieve cascade charge transfer in the bulk-heterojunction (BHJ) layer.13,14 Recently emerged high-performance nonfullerene acceptors (NFAs) have been considered as excellent candidates as such components, and a range of highly efficient ternary PSCs, have been developed based on this strategy.15−19 Typically, high-performance NFAs have acceptor−donor− acceptor (A-D-A)-type structural characteristics.20−26 Thus, their optoelectronic properties can be precisely tailored by structural modification of either the central electron-donating or the flanked electron-accepting species. On the basis of this molecular design strategy, a variety of high-performance NFAs © 2017 American Chemical Society

Received: May 31, 2017 Revised: September 9, 2017 Published: September 12, 2017 8177

DOI: 10.1021/acs.chemmater.7b02228 Chem. Mater. 2017, 29, 8177−8186

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Chemistry of Materials Scheme 1. Synthesis of the Nonfullerene Acceptor IffBR

Figure 1. PES curves of the (a) IDT-ffBT and (b) ffBT-R model fragments; (c) optimized geometry and (d) unoccupied (top) and occupied (bottom) NTOs with the largest eigenvalues of S0−S1 transition for IffBR based on TD-DFT calculations.

The synthesis and characterization of IffBR are detailed in the Supporting Information. The decomposition temperature with 5% weight loss (Td) of IffBR was determined by thermogravimetric analysis to be 325 °C (Figure S1). No discernible thermal transition was observed in the differential scanning calorimetric measurement up to 300 °C (Figure S2). 2.2. Theoretical Calculations. Density functional theory (DFT) calculations were carried out using Gaussian 09 software at the ωB97X-D/6-31G* level to explore the molecular geometry of the ground state of IffBR. To simplify the calculations, the alkyl chains of IffBR were truncated as methyl groups. Initially, a potential energy surface (PES) scan was performed to evaluate the conformational freedom of each molecular fragment (Figure 1a, b). The PES plot of benzothiadiazole−rhodanine (BT-R, here “R” represents the rhodanine unit) segment shows a minimum energy at the dihedral angle of 0°, while the ffBT-R segment shows a minimum energy at the dihedral angle of 180° (see Figure S3). This can be understood as the minimum energy of BT-R being correlated to the C−H···N interaction, while for ffBT-R, the rhodanine ring is more likely to flip over relative to ffBT due to

yl)benzo[1,2-b:4,5-b′]-dithiophene-alt-2-(2-butyloctyl)-5,6-difluoro-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole] (PBTA-BO). Moreover, it can facilitate cascade charge transfer when integrated with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Relative to binary devices, the resulting ternary PSCs exhibit significantly greater power conversion efficiencies (PCEs) of >9%, which is attributable to their broadened absorption, favorable charge transfer, suppressed charge recombination, and optimized morphology.

2. RESULTS AND DISCUSSION 2.1. Synthesis of IffBR. Scheme 1 shows the synthetic route for the target electron-acceptor IffBR. The critical intermediate 7-bromo-5,6-difluorobenzo[c][1,2,5]thiadiazole4-carbaldehyde (6) was synthesized via a multistep reaction using 2-bromo-3,4-difluoro-6-nitroaniline (1) as the starting material. The Stille coupling reaction of compound 6 and 2,7bis(trimethylstannane)-4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene gave the dialdehyde intermediate (7) in a good yield of 62%, which was treated with 3-ethylrhodanine to yield IffBR via Knoevenagel condensation. 8178

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Figure 2. (a) Chemical structures of PBTA-BO and PC71BM; (b) normalized UV−vis absorption spectra of PBTA-BO, IffBR, and PC71BM; (c) UV−vis absorption spectra of PBTA-BO/PC71BM/IffBR blend films; (d) IP and EA alignment of three components.

increased with the increasing IffBR weight ratio (Figure 2c), indicating that the incorporation of IffBR into PBTA-BO/ PC71BM resulted in broader absorption. We note that the UV− vis absorption profile of IffBR was blue-shifted in both ternary and binary blends, which can be attributed to the suppressed aggregation of IffBR. Additionally, the absorption profiles of PBTA-BO/PC71BM/IffBR (1:1:0.6 and 1:0:1.6, wt/wt/wt) blend films processed with or without diphenyl ether (DPE) (0.5 vol %) are nearly identical (see Figure S5), which indicates that the addition of DPE has negligible effects on the absorption profiles of the blend films. The ionization potential (IP) and electron affinity (EA) of IffBR are estimated to be −5.71 and −3.77 eV, respectively, based on cyclic voltammetric (CV) measurements (Figure S6b). Regarding the compound IDT-2BR, the UV−vis absorption profiles of IffBR were blueshifted and both IP and EA were slightly reduced (Figure S6). Nevertheless, IP and EA of IffBR lie between those of PBTABO and PC71BM (Figure 2d). Thus, the addition of IffBR into the PBTA-BO/PC71BM blends may facilitate a charge-transfer pathway from PBTA-BO to IffBR and from IffBR to PC71BM, resulting in greater charge-transfer efficiency in the ternary blend films. 2.4. Photovoltaic Performance. The photovoltaic performances of the ternary blend PSCs were evaluated using devices with a conventional architecture: ITO/poly(3,4-ethylenedioxythiophene)/polystyrenesulfonate (PEDOT/PSS)/ photoactive layer/PFN-Br/Al (Figure S7a). The alcoholsoluble poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br, Figure S7b) was used as the cathode interfacial layer to facilitate electron extraction.40 The photoactive layer was composed of PBTA-BO, PC71BM, and IffBR with various weight ratios and was fabricated using spin-casting from chlorobenzene (CB) solution with a film thickness of ∼85

the simultaneous noncovalent bonding interactions of C−H···F and N···S. Consider that the difference between BT-R and ffBT-R is the two fluorine atoms in the ffBT unit, this observation clearly demonstrates that these fluorine atoms have noncovalent bonding interactions with the adjacent hydrogen atoms. The different conformer distributions of ffBT-R and BTR end-capping groups for IffBR and its nonfluorinated analogous IDT-2BR28 (see chemical structure in Figure S4) may influence the self-organization of the molecules in the solid state. In this respect, we choose the most probable conformation, with the lowest energy, of each model fragment for the subsequent calculations. The molecular geometry of IffBR was determined to exhibit a coplanar backbone configuration, as observed from the front and side views (Figure 1c). In this configuration, natural transition orbitals (NTOs) were performed on the basis of time-dependent (TD) DFT functional at the ωB97X-D/6-31G* level. Both the unoccupied and occupied NTOs with the largest eigenvalues of S0−S1 transition are delocalized across the entire backbone (Figure 1d). Under the same calculation conditions, it is also noted that the IDT-2BR presents coplanar conjugated backbone with NTOs well-distributed within the backbone (Figure S4). 2.3. Optical and Electrochemical Properties. The chemical structures of the wide-bandgap electron-donating PBTA-BO and the electron-accepting PC71BM copolymers are illustrated in Figure 2a. The absorption profile of IffBR shows the maximum absorption at 658 nm, with the onset at ∼740 nm, which is complementary with those of PBTA-BO39 and PC71BM (Figure 2b). The optical bandgap of IffBR is estimated to be 1.71 eV, based on the UV−vis absorption of the thin film. The UV−vis spectra of the ternary blend films are a combination of the absorption profiles of the three individual components. The absorption peaked at ∼650 nm and gradually 8179

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Figure 3. (a) J−V and (b) EQE curves and photovoltaic parameters, (c) VOC and FF, and (d) JSC and PCE of devices based on PBTA-BO/PC71BM/ IffBR.

Table 1. Photovoltaic Parameters of Ternary Blend Polymer Solar Cells Based on PBTA-BO/PC71BM/IffBR; Statistic Parameters were Obtained from 10 Individual Devices weight ratio (wt/wt/wt) 1:1.6:0 1:1:0.6 1:1:0.6a 1:0.8:0.8 1:0.6:1 1:0:1.6 a

VOC (V)

JSC (mA cm−2)

± ± ± ± ± ±

9.31 ± 0.09 14.03 ± 0.13 14.52 ± 0.14 13.78 ± 0.19 13.64 ± 0.14 11.77 ± 0.36

0.903 0.932 0.908 0.947 0.950 1.013

0.005 0.006 0.005 0.004 0.004 0.000

FF (%) 55.65 64.69 67.47 58.83 55.05 52.29

± ± ± ± ± ±

0.53 1.09 0.65 0.94 0.77 1.39

PCE (%)

PCEbest (%)

± ± ± ± ± ±

4.73 8.56 9.06 7.95 7.22 6.34

4.68 8.45 8.90 7.67 7.13 6.24

0.03 0.08 0.12 0.21 0.10 0.09

The active layer was solution-processed with 0.5 vol % DPE.

increased from 0.903 to 1.013 V with increased IffBR content (from 0 to 1.6) in the PBTA-BO/PC71BM/IffBR blend film. The EAs of PC71BM and IffBR were −3.92 and −3.77 eV, respectively, while the EAs of PC71BM/IffBR blends with weight ratios of 1:0.6, 1:1, and 0.6:1 were measured to be −3.88, − 3.85, and −3.82 eV, respectively (see the CV curves in Figure S8). The gradually enhanced EAs may result in continuously increased VOC values, which was consistent to the “alloy model”.41,42 In addition, both the JSC and fill factor (FF) were initially enhanced with increasing IffBR content and then gradually decreased (Figure 3c, d), which resulted in a plateau of the PCE of 8.45 ± 0.08% (VOC = 0.932 ± 0.006 V, JSC = 14.03 ± 0.13 mA cm−2, FF = 64.69 ± 1.09%) at a PBTABO/PC71BM/IffBR weight ratio of 1:1:0.6. To further inspect the potential PCEs of the solar cells, the reference devices based on binary blends (1:1.6, wt/wt) were fabricated with 0.5 vol % of DPE as the solvent additive (see the photovoltaic

nm. The current density−voltage (J−V) characteristics were recorded under AM 1.5G illumination (100 mW cm−2). Relevant J−V curves and external quantum efficiency (EQE) spectra are shown in Figure 3. Initial screening of binary devices based on PBTA-BO/PC71BM and PBTA-BO/IffBR provided an optimized weight ratio of 1:1.6 for both of the binary devices (Table S1 and Figure S9), which exhibited moderate PCEs of 4.68% ± 0.03% and 6.24% ± 0.09%, respectively. The higher PCE of the PBTA-BO/IffBR device was attributed to its higher open-circuit voltage (VOC) and short current density (JSC), resulting from the higher EA and broader absorption profile of IffBR, respectively. Further optimization of the device was carried out based on an overall donor (PBTA-BO)-to-acceptor (PC71BM and IffBR combined) weight ratio of 1:1.6, where the weight ratio of the two acceptors was changed from 1.6:0 to 0:1.6 (Table 1). The observed VOC values of the ternary solar cells gradually 8180

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Figure 4. PL spectra of pristine PBTA-BO and IffBR films and PBTA-BO/PC71BM/IffBR blend films with various weight ratios, excited at (a) 570 and (b) 650 nm.

parameters in Table S3 and representative J−V curves in Figure S12). The resulting binary devices based on PBTA-BO/ PC71BM (1:1.6, wt/wt) and PBTA-BO/IffBR (1:1.6, wt/wt) showed enhanced PCEs of 6.49 ± 0.12% (VOC = 0.858 ± 0.009 V, JSC = 10.81 ± 0.12 mA cm−2, FF = 69.95 ± 0.93%) and 6.28 ± 0.16% (VOC = 0.982 ± 0.008 V, JSC = 12.01 ± 0.12 mA cm−2, FF = 53.24 ± 1.19%), respectively. The combination of the increased JSC and FF led to an average PCE of 8.90 ± 0.12 (VOC = 0.908 ± 0.005 V, JSC = 14.52 ± 0.14 mA cm−2, FF = 67.47% ± 0.65%), with the highest PCE value of 9.06% for the ternary blend devices processed with DPE. In addition, to inspect the effects of fluorination on device performances, we fabricated devices based on IDT-2BR for comparison. The binary device based on PBTA-BO/IDT-2BR (1:1.6, wt/wt) showed a moderate PCE of 5.42 ± 0.28%, and the ternary devices processed with DPE (0.5 vol %) showed an obviously enhanced PCE of 7.96 ± 0.10% (see Table S4 and Figure S13). These findings demonstrated that devices based on IDT-2BR have lower photovoltaic performances than those obtained from devices based on IffBR as a result of lower JSC and FF. The obviously enhanced performances of the ternary blend film of PBTA-BO/PC71BM/IffBR regarding the binary blend PBTA-BO/PC71BM primarily stem from the simultaneous enhancement of all photovoltaic parameters including VOC and JSC. The gradually enhanced VOC after the incorporation of IffBR can be attributed to the formation of alloy in the blend films, as the PC71BM/IffBR blends showed gradually enhanced EAs upon the increased weight ratio of IffBR. The enhanced JSC can be attributed to the improved light-harvesting properties of the ternary blend films because the absorption profiles can be broadened upon the incorporation of IffBR, as the IffBR has the complementary absorption profile with PBTA-BO/PC71BM. Additionally, the appropriate IP and EA alignment would also facilitate the charge transfer, resulting in more efficient charge separation in the ternary blend system. The accuracy of the observed current densities was verified using the integrated values of the EQE spectra; the differences between the two sets of values were 70% were observed between 440 and 600 nm, which agrees with the broader absorption profiles and higher absorptivities of the ternary BHJ films compared with those of the binary blend films.

It has been established that the PC71BM has a large tendency to form great aggregation upon thermal annealing that is detrimental to the device performances,43,44 and the addition of NFAs can effectively prevent the aggregation and maintain morphological stability.14 However, in our current case, we note that devices based on PBTA-BO/PC71BM (1:1.6, wt/wt) demonstrated good thermal stability, for which the PCE slightly decreased from 4.67 ± 0.03% to 4.18 ± 0.29% upon thermal annealing the film at 100 °C for 300 min (Figure S14 and Table S6), and the surface optical micrographs of the blend films remain nearly unchanged after thermal treatment (Figure S16). The more stable film morphology than the previously reported PBTA-BO/PC61BM blend film14 might be correlated to the lower thermal annealing temperature. Nevertheless, the fabricated ternary blend devices based on PBTA-BO/ PC71BM/IffBR (1:1:0.6, wt/wt/wt) with DPE also exhibited excellent stability, for which the PCE slightly decreased from 8.90 ± 0.12% to 8.18 ± 0.17% upon thermal annealing the film at 100 °C for 300 min (Figures S14 and S15 and Table S5). These observations clearly demonstrated the remarkable thermal stability of these ternary blend films. 2.5. Charge Transfer, Dissociation, Recombination, and Extraction. Charge transfer was investigated by measuring the photoluminescence (PL) spectra of the blend films (Figure 4). Excitation wavelengths of 570 and 650 nm were used, corresponding to the maximum absorption of PBTA-BO and IffBR films, respectively. When these films were excited at 570 nm, the PL of PBTA-BO peaked at 700 nm and was significantly quenched (∼90%, relative to its initial intensity) when PBTA-BO was blended with PC71BM and IffBR. After excitation at 650 nm, the PL of IffBR peaked at ∼720 nm and was also significantly quenched (∼80%, relative to its initial intensity) in the PBTA-BO/IffBR blend film. These observations indicated the electron transfer was efficient in the blend films (Figure 4). Furthermore, the PL emissions from both PBTA-BO and IffBR were almost completely quenched in the blend films that incorporated PC71BM. To examine the charge transfer from IffBR to PC71BM, we fabricated a device that contained IffBR/PC71BM (1:1, w/w) as the photoactive layer. The resulting device exhibited poor photovoltaic performance, with a low PCE of 0.12% (VOC = 0.558 V, JSC = 0.67 mA cm−2, FF = 31.89%) (Figure S17). Nevertheless, this device confirmed the occurrence of charge transfer in an IffBR/ 8181

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Figure 5. (a) Jph−Veff curves, (b) JSC under different light intensities, and (c) TPC characteristics of devices fabricated using PBTA-BO/PC71BM/ IffBR ternary blend films with ratios of 1:0:1.6, 1:1:0.6, 1:1:0.6 (+DPE, 0.5 vol %), and 1:1.6:0.

Figure 6. (a−d) AFM topography (5 × 5 μm) and (e−h) TEM images of blend films consisting of PBTA-BO/PC71BM/IffBR with weight ratios of 1:1.6:0, 1:1:0.6, 1:1:0.6 (+DPE, 0.5 vol %), and 1:0:1.6.

PC71BM blend film that is favorable for the achievement of improved short-circuit current density.45 To characterize the exciton generation and dissociation processes of these devices, we plotted the photocurrent density (Jph) against the effective voltage (Veff) (Figure 5a). Jph values were calculated according to the following equation: Jph = JL − JD, where JL represents the current density under illumination and JD represents the current density in dark conditions. Veff values were calculated according to the following equation: Veff = V0 − Va, where V0 is the voltage when JL = JD, and Va is the applied bias.46 It was assumed that the photogenerated excitons were almost dissociated into free charge carriers at high Veff values (in this case, ∼3.0 V) and that the saturation current density (Jsat) was only limited by the total amount of absorbed photons. The maximum exciton generation rate (Gmax) can be calculated using Jsat/qL, where q is elementary charge and L is the thickness of the active layer. The Gmax value for the ternary device processed with DPE is 1.11 × 1028 m−3 s−1, which is significantly higher than that for binary devices based on PBTABO/PC71BM (0.77 × 1028 m−3 s−1), suggesting that exciton generation was greater in the device with incorporation of IffBR. The overall efficiency of exciton dissociation and charge collection can be evaluated by using P(E, T), which can be calculated by normalizing Jph with Jsat. Under short-circuit conditions, the ternary device based on PBTA-BO/PC71BM/ IffBR with a weight ratio of 1:1:0.6 was found to have a P(E, T) value of 93.8%, which was increased to 95.3% when the BHJ film was processed with DPE. These P(E, T) values are

significantly higher than those of the binary devices tested to be 88.9% for PBTA-BO/PC71BM and 81.4% for PBTA-BO/IffBR, indicating improved charge dissociation and collection in the ternary devices. Detailed parameters are summarized in Table S4. Thus, the ternary devices have the advantages of the enhanced Jsat of PBTA-BO/IffBR device and the improved P(E, T) value of the PBTA-BO/PC71BM device, which resulted in favorable exciton generation and dissociation processes and therefore led to the obviously improved JSC values of the ternary devices. Charge-carrier recombination in the ternary blend PSCs was examined by measuring the dependence of JSC on light intensity (10−100 mW cm−2), which can be described by JSC ∝ PinS (Figure 5b), where Pin is the intensity of the incident light and S is the slope. The value of S is close to unity when bimolecular recombination of charge carriers occurs inside a device.47,48 The S value of the ternary device processed with DPE was 0.980, which was higher than those of the binary devices based on PBTA-BO/PC71BM (0.962) and PBTA-BO/IffBR (0.946). This indicated that bimolecular combination was suppressed in the ternary devices, relative to the binary devices, which was consistent with the higher JSC and FF values of the ternary devices. We surmise that the suppressed bimolecular recombination of the ternary blends might be attributable to the more favorable film morphology that can lead to improved charge transport and charge-carrier extraction in the ternary blend films, which will be discussed in the following section. Transient photocurrent (TPC) measurements were used to investigate the charge-extraction efficiency of the devices 8182

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Figure 7. (a−f) 2D-GIWAXS patterns and (g) line-cut profiles of the in-plane (dash line) and out-of-plane (solid line) direction of neat PBTA-BO, IffBR, and PBTA-BO/PC71BM/IffBR blend films with various weight ratios and processing conditions.

phase separation in the ternary blend film processed without DPE (Figure 6f), which is not present in the film processed with DPE as a solvent additive (Figure 6g). The smoother and more uniform morphologies of the ternary blend films are more favorable for the charge separation than those of the binary films, which agree with the observed higher JSC values of the ternary devices. Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to investigate the molecular stacking in the BHJ films. The two-dimensional (2D) patterns and corresponding line-cut profiles are demonstrated in Figure 7, and the peak fitting curves of the line-cut profiles in the out-of-plane direction are plotted in Figure S19. As shown in Figure 7a, the neat PBTABO film exhibited strong (010) diffraction peak at 1.74 Å−1 (with d-space of 3.61 Å) in the out-of-plane (OOP) direction, and corresponding (100) peak located at 0.30 Å−1 (d = 20.93 Å) in the in-plane (IP) direction, which indicates the formation of the predominant “face-on” orientation of PBTA-BO backbone. The neat IffBR film showed two sharp peaks at 0.38 and 0.40 Å−1 in the OOP direction and a broad (010) diffraction ring fitted at 1.54 Å−1 (d = 4.08 Å) (Figure 7b). The observed sharp peaks at 0.38 and 0.40 Å−1 in the OOP direction of IffBR may be attributable to the specific conformation of rhodanine end-group due to the selforganization. In contrast, no sharp peaks emerged in the GIWAXS pattern of IDT-2BR (Figure S18). These findings clearly indicated the effects of fluorination of BT moiety on the film morphology of these compounds. For the binary blend films of PBTA-BO/PC71BM (1:1.6, wt/wt, Figure 7c) and PBTA-BO/IffBR (1:1.6, wt/wt, Figure 7d), one can clearly observe the (010) diffraction signals fitted at 1.71 Å−1 (d = 3.67

(Figure 5c). The charge extraction time (τ) can be obtained by nonlinear fitting of the photocurrent as a function of time. The binary blend devices based on PBTA-BO/PC71BM and PBTABO/IffBR exhibited τ values of 0.39 and 0.30 μs, respectively. The ternary devices based on PBTA-BO/PC71BM/IffBR processed with and without DPE exhibited shorter extraction times of 0.22 and 0.21 μs, respectively. This indicates that the ternary devices exhibited significantly higher charge-extraction efficiency than the binary devices, which can lead to suppressed bimolecular recombination and thus higher JSC values of the ternary devices. 2.6. Film Morphology. It has been well-established that film morphology has a significant influence on photovoltaic performance of PSCs.49 In this study, we examined the morphologies of the blend films using atomic force microscopy (AFM, Figure 6a−d) and transmission electron microscopy (TEM, Figure 6e−h). For the PBTA-BO/PC71BM blend film, the AFM image shows a relatively coarse surface with a large root-mean-square (RMS) value of 8.11 nm, indicating the presence of large aggregations across the entire film (Figure 6a). The corresponding TEM image shows high phase separation and severe aggregation (Figure 6e), which may be attributed to the poor miscibility of PBTA-BO with PC71BM. In contrast, the PBTA-BO/IffBR blend film exhibited significantly smoother film morphology (RMS = 1.23 nm) and a fibrous nanostructure (Figure 6d). Moreover, uniform film morphology can be observed in the TEM image (Figure 6h). The ternary blend films of PBTA-BO/PC71BM/IffBR with a weight ratio of 1:1:0.6 processed with and without DPE exhibited very smooth and uniform surface morphologies (RMS ≈ 0.5 nm, Figure 6b, c). The corresponding TEM images show large-scale 8183

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Å) in the OOP direction and (100) peaks at ∼0.33 Å−1 (d = 19.03 Å) in the IP direction, indicating slightly larger d values of molecular π−π stacking but lower d-space of lamellar stacking in these films than those of PBTA-BO film. For ternary blend films based on PBTA-BO/PC71BM/IffBR (1:1:0.6, wt/wt/wt) processed with or without DPE, the (010) peaks in the OOP direction presented a slightly higher q value of 1.75 Å−1 (d = 3.59 Å), which indicates that π−π stacking distances of the ternary blend films are smaller than those of the binary blend films and PBTA-BO. It is worth noting that such reduced π−π stacking distance can effectively improve charge carrier transport and thus lead to suppressed bimolecular recombination in the ternary blend film. Further investigation of the GIWAXS data was carried out by azimuthal profile analysis of the (010) diffraction (q = 1.75 ± 0.15 Å−1) (see the pole figure in Figure S20). The stronger intensities at ∼90° than those at 0°/180° indicated that the pristine PBTA-BO exhibited almost exclusively “face on” orientation of polymer chains, while the IffBR showed random orientation. It is also worth noting that the ternary BHJ film processed with DPE exhibited slightly sharper distributions than that without DPE, indicating more ordered π−π stacking of the former. The combination of GIWAXS patterns and TEM images reveals that the utilization of DPE as the solvent additive can form more uniform film with more ordered π−π stacking of polymer chains, both of which are favorable for charge separation and transport. These observations are consistent with the higher JSC and FF values of the ternary device processed with DPE.

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lei Ying: 0000-0003-1137-2355 Fei Huang: 0000-0001-9665-6642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (no. 2014CB643501), the Natural Science Foundation of China (nos. 51673069, 21634004, 91633301, and 21520102006), and Foundation of Guangzhou Science and Technology Project (201707020019). L.Y. thanks the Pearl River S&T Nova Program of Guangzhou (Grant no. 201710010021).

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REFERENCES

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3. CONCLUSION In summary, we developed a novel nonfullerene electron acceptor, IffBR, that was incorporated into PBTA-BO/PC71BM BHJ film to fabricate ternary PSCs. The ternary devices exhibited slightly higher VOC values than devices fabricated with fullerene-based binary acceptors because of the higher EA of IffBR relative to PC71BM, and they had significantly improved JSC values as a result of the extended absorbance band and the occurrence of cascade charge transfer. The optimized ternary device, based on a photoactive layer processed with DPE as a solvent additive, exhibited a PCE > 9%. The ternary devices exhibited more efficient charge dissociation, reduced charge carrier recombination, and more efficient charge extraction than the binary devices. Overall, these results indicate that the incorporation of IffBR into PSCs is suitable for the fabrication of highly efficient ternary polymer solar cells.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02228. Experimental section including synthesis details; TGA, DSC, PES, CV, and J−V curves; chemical structures of IffBR, IDT-2BR, and PFN-Br; UV-vis absorption spectra; device architecture of solar cell; photovoltaic parameters of solar cells; optical micrographs; GIWAXS data of pristine IDT-2BR and IffBR films; 1H, 13C, and 19F NMR of IffBR; and MALDI-TOF mass spectrometry of IffBR (PDF) 8184

DOI: 10.1021/acs.chemmater.7b02228 Chem. Mater. 2017, 29, 8177−8186

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