Isomerism: Minor Changes in the Bromine Substituent Positioning Lead to Notable Differences in Photovoltaic Performance

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Sep 2021Isomerism: Minor Changes in the Bromine Substituent Positioning Lead to Notable Differences in Photovoltaic Performance Huan Wang†, Liang Han†, Jiadong Zhou†, Tao Liu†, Daize Mo, Hui Chen, Haijian Lai, Nan Zheng, Zengqi Xie, Wenhua Zheng and Feng He Huan Wang† Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 Faculty of Health Sciences, University of Macau, Macao 999078 , Liang Han† Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 , Jiadong Zhou† State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Tao Liu† Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 , Daize Mo Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 , Hui Chen Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 , Haijian Lai Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 , Nan Zheng State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Zengqi Xie State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640 , Wenhua Zheng Faculty of Health Sciences, University of Macau, Macao 999078 and Feng He *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen 518055 Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 https://doi.org/10.31635/ccschem.020.202000540 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail An isomerism strategy was employed to develop single, end‐group bromine-substituted non‐fullerene two isomeric acceptors, 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"3′′:4′,5′] thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(4-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)dimalononitrile (BTIC-2Br-β) and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2,"3′′:4′,5′] thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5-bromo-3-oxo-2,3-dihydro-1H-inden-1-ylidene)dimalononitrile (BTIC-2Br-γ), for organic solar cells, aimed to examine the improvement of power conversion efficiency (PCE). The effects of isomerism on their optical, electronic, charge dynamics, morphological, and photovoltaic properties were systematically investigated. When blended with a donor poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-TF), BTIC-2Br-γ-based devices exhibited an outstanding PCE of 16.52%, they exhibited an outstanding PCE of 16.52%, which was the highest recorded value among brominated acceptors, compared with 8.11% obtained for BTIC-2Br-β-based devices. Crystallographic analysis of BTIC-2Br-γ single-crystal demonstrated that the entire molecular backbone presented a plane structure between the core and end groups. Moreover, multiple intermolecular interactions such as Br⋯π and CN⋯H existing in the solid-state allowed BTIC-2Br-γ to form a three-dimensional (3D) network-packing structure, providing more electron transport channels. Our morphology investigations revealed that the BTIC-2Br-γ-blend films displayed tailored crystallite with distinct fibrillary nanostructures, and the low miscibility of PBDB-TF and BTIC-2Br-γ obtained by the contact angle could assist the formation of the fibrillary interpenetrating networks to achieve effective charge transport pathways. Compared with BTIC-2Br-β, BTIC-2Br-γ possessed a higher extinction coefficient, more balanced charge transport, and weaker bimolecular recombination. This work shows that subtle changes in bromine position can significantly improve the photovoltaic efficiencies of organic solar cells (OSCs); therefore, it provides a new guideline for the rational design of efficient fused-ring electron acceptors (FREAs). Download figure Download PowerPoint Introduction Solution-processed bulk heterojunction (BHJ) polymer solar cells (PSCs) have been investigated extensively because of their significant potential for the production of light-weight, low-cost, semitransparent, and flexible solar panels in commercial applications.1–5 In recent years, the emergence of the fused-ring electron acceptor (FREA) has extended the power conversion efficiencies (PCEs) up to 18%,6,7 far surpassing the 12% attained by fullerene acceptor-based solar cells.8,9 FREAs dominate the field of PSCs due to their advantages, including strong absorption in the visible and near-infrared (NIR) regions, as well as easy adjustment for energy levels and morphology.10,11 The rapid development of high-performance FREA is credited to the tuning of optimal optoelectronic properties by side-chain engineering, core modulations, and functional group modification.12–14 For instance, halogenation has an electron-withdrawing effect, able to enhance the intramolecular charge-transfer (ICT) effects15,16 and reducing the bandgaps of efficient FREAs.17,18 Until now, a large number of top-performing FREA such as IT-4F,16 IEICO-4F,19 Y6,20 BTP-4Cl (BTIC-4Cl)21,22 and BTIC-2Br-m (TPT10),23,24 contain fluorine, chlorine, or bromine atoms as end groups of their molecular structures. Bromine atoms have been used successfully in the development of the FREA and have boosted the PCEs up to >16%.23,24 Still, the top-performing brominated acceptor BTIC-2Br-m is a mixture of three isomers; hence, interpretation of the results is challenging when considering and discussing intermolecular interactions induced by bromine atoms. Isomerization strategy plays an effective role in adjusting compound characteristics such as the absorption spectra, molecular packing, and charge transport of semiconductors, resulting in different device performance. Wang et al.25 designed two isomeric fused nine-ring-core electron acceptors, FNIC1, with a thiophene-fused benzo[1,2-b:4,5-b′]dithieno[3,2-b]thiophene core and FNIC2, with a thieno[3,2-b]thiophene-fused benzo-[1,2-b:4,5-b′]dithiophene core. FNIC2 showed a 42 nm redshift absorption spectrum, compared with FNIC1, leading to a significantly enhanced short-circuit current density (Jsc) and a higher PCE. Yang et al.26 reported the non-fullerene acceptor solar cell, m-ITIC, based on ITIC isomer, formed by replacing p-hexylphenyl with m-hexylphenyl, which exhibited higher electron mobility and improved π–π stacking. Wu et al.27 developed two isomeric acceptors (NBDTP-Fout and NBDTP-Fin) with different oxygen-substituted positions in the benzodi(thienopyran) (BDTP) core, which had different surface tension, and thus, influenced the miscibility and morphology when blended with donor materials. Our previous work investigated the influence of chlorine atom position on the properties of the FREA when introduced to the 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) end group. The single-crystal X-ray structures revealed that the two fabricated isomers possessed different molecular planarity and aggregations, which led to differences in optical, charge transport, and photovoltaic properties.28 From the design perspective, the control of the isomeric composition of the active materials, particularly the precise positioning of the halogen atoms, could be an effective pathway to construct a more promising and efficient material systems in organic solar cells (OSCs). In this work, we developed two non‐fullerene isomeric acceptors (BTIC-2Br-β and BTIC-2Br-γ) with bromine in different positions of the end groups. Single-crystal X-ray crystallographic analysis of BTIC-2Br-γ revealed that the whole molecular backbone presented a plane structure with relatively small 1.45° and 2.92° torsion angles between the core and end groups. Multiple intermolecular interactions such as Br⋯π and CN⋯H existing in the solid-state allowed BTIC-2Br-γ to form a three-dimensional (3D) network-packing structure, which could provide more electron transport channels. Its isomer, BTIC-2Br-β, showed low crystallinity, and it was difficult to capture the single-crystal diffraction signals successfully, despite significant efforts made to grow single crystals of this isomer. Favorably, using the polymer PBDB-TF as a donor, BTIC-2Br-γ-based solar cells exhibited an outstanding PCE of 16.52%, which is the highest recorded value among brominated acceptors; meanwhile, the BTIC-2Br-β-based device showed only a relatively low PCE of 8.11%. The only reason for this difference in PCE values is the different positions of the bromine atom substituents. Morphology investigations, including grazing incidence wide-angle X-ray scattering (GIWAXS) and transmission electron microscopy (TEM), revealed that BTIC-2Br-γ-blend films possessed tailored crystallite with distinct fibrillary nanostructures. The isomerism caused by the position of the bromine substituent significantly influenced the donor–acceptor interfacial tension and resulted in variations in blending miscibility. Low miscibility of PBDB-TF and BTIC-2Br-γ could, in turn, assisted the formation of the fibrillary interpenetrating networks, thus, achieving effective charge transport pathways. In contrast, the high interaction parameter of PBDB-TF with BTIC-2Br-β led to remarkable miscibility, which disfavored transport pathways and resulted in severe recombination. From the investigated dynamics behavior of the carrier, it was found that BTIC-2Br-γ-based devices exhibited higher quenching efficiency, more efficient exciton dissociation, and lower bimolecular recombination than BTIC-2Br-β-based devices. This finding has contributed to a comprehensive understanding of the molecular structure and device performance, delivering a reliable message concerning the importance of isomerism in high-efficiency organic solar conversion. Experimental Section Details of synthesis and characterization of the target materials, the fabrication and characterization of solar cells device, as well as charge-carrier mobility measurement, absorption spectra measurement, cyclic voltammetry (CV) measurements, atomic force microscopy (AFM), TEM, transient photovoltage (TPV) and transient photocurrent (TPC) measurements, photoluminescence (PL) measurement, GIWAXS, contact angle measurements, highly sensitive external quantum efficiency (EQE) and external electroluminescence quantum efficiency (EQEEL) measurements, are presented in the Supporting Information. Results and Discussion Materials synthesis and packing information The synthetic routes and the molecular structures of the target acceptors BTIC-2Br-γ and BTIC-2Br-β are shown in Figure 1a and Supporting Information Scheme S1. The intermediate and the targeted product were tested by the proton nuclear magnetic resonance (1HNMR, as shown in Supporting Information Figure S10–S13). Both of these acceptors exhibited good solubility in common solvents, including chlorobenzene, chloroform, dichloromethane, and tetrahydrofuran. The polymer PBDB-TF with a number-average molecular weight (Mn) of 48.0 kDa and a polydispersity index (PDI) of 2.41 was selected as the donor in the devices' preparation. Figure 1 | The synthetic routes for brominated acceptors and single-crystal structure of BTIC-2Br-γ. (a) The synthetic routes and chemical structures of BTIC-2Br-γ and BTIC-2Br-β. (b) The single-molecule structure of BTIC-2Br-γ with thermal ellipsoids. (c) The intermolecular interactions of BTIC-2Br-γ in one elliptical frame from the side view. (d) The molecular arrangement of BTIC-2Br-γ in one elliptical frame from the top view. (e) The interactions of BTIC-2Br-γ between molecules in adjacent layers. (f) The three-dimensional (3D) network packing of BTIC-2Br-γ was observed along the a-crystallographic axis. (g) 3D network packing of BTIC-2Br-γ observed from different directions. The alkyl chains of BTIC-2Br-γ were omitted to observe the packing details clearly. Download figure Download PowerPoint A single crystal of BTIC-2Br-γ was obtained and submitted to X-ray crystallography in an effort to understand its molecular packing and intermolecular interactions. Figure 1b shows the monomolecular configuration of BTIC-2Br-γ. The entire molecular backbone exhibited an essentially planar structure with relatively small torsion angles (1.45° and 2.92°), which might be attributable to the formation of conformational lock provided by S⋯O interactions (2.68 Å). It was observed from the molecular arrangement in the crystal that the BTIC-2Br-γ molecules were stacked layer-by-layer. In a single layer, molecular chains were formed by CN⋯H hydrogen bonds between end groups. Besides, each molecule was connected with six other molecules in the adjacent layers through π–π interactions across a 3.24 Å separation and Br–π interactions across 3.54 Å (shown in Figures 1c and 1e). With these intermolecular interactions, the BTIC-2Br-γ molecules eventually formed a 3D network structure in an elliptical frame. Each elliptical frame could be regarded as a building block in a network skeleton whose length was 16.98 Å and width, 12.25 Å, and basically composed of four molecules in three adjacent layers, with Br⋯π interaction, and H-aggregation between interlayer molecules (shown in Figures 1d and 1f). The 3D network packing of BTIC-2Br-γ encouraged the formation of more charge transport channels in multiple directions (shown in Figure 1g).29,30 Due to the weak crystallinity of BTIC-2Br-β, irregular aggregates were formed instead of crystals, though many attempts were made to generate crystals from this isomer. Optical and electrochemical properties The UV–vis–NIR absorption spectra of the two isomers are shown in Supporting Information Figure S1. BTIC-2Br-β and BTIC-2Br-γ in a 10−5 mol L−1 solution in chloroform exhibited similar absorption peaks between 700 and 800 nm, with maximum absorption peaks at 739 and 732 nm, respectively ( Supporting Information Figure S1 and Table S1). As shown in Figure 1a, the absorption peaks of BTIC-2Br-β and BTIC-2Br-γ in the thin film were both located at 815 nm. Compared with the related solution spectra, a bathochromic 83 nm shift in the BTIC-2Br-γ film was observed, which was larger than the 75 nm observed with BTIC-2Br-β, suggesting stronger π–π interactions and more ordered packing of the FREAs in the film states of the former isomer. In films, ITIC-2Br-β exhibited an extinction coefficient of 4.73 × 104 L mol−1 cm−1, whereas BTIC-2Br-γ displayed the higher extinction coefficient of 6.44 × 104 L mol−1 cm−1. These results indicate that the absorption properties of the FREA of both isomeric forms were indeed influenced by the position of the bromine atom. Moreover, Supporting Information Figure S1b revealed that the blended films based on PBDB-TF:BTIC-2Br-γ exhibited stronger absorption than those based on PBDB-TF:BTIC-2Br-β in the 750–850 nm wavelength range, and the stronger absorption could enhance the light-harvesting efficiency improving Jsc in the corresponding solar cells.31,32 The electrochemical properties of the donor and acceptors were determined by CV measurements, as shown in Figure 2b and Supporting Information Figure S2, revealing the calculated highest occupied molecular orbital (HOMO) energy levels of BTIC-2Br-β and BTIC-2Br-γ to be −5.60 and −5.57 eV, respectively, and the lowest unoccupied molecular orbital (LUMO) energy levels of BTIC-2Br-β and BTIC-2Br-γ to be −4.01 and −4.03 eV, respectively. The LUMO and HOMO energies of PBDB-TF were −5.45 and −3.65 eV, respectively. As expected, the LUMO offsets between PBDB-TF and the two brominated acceptors were large enough for efficient exciton dissociation. Additionally, the relatively small HOMO offsets (0.12 and −0.15 eV) might have led to a reduced energy loss that supported a charge separation.33,34 Figure 2 | The absorption spectra, energy levels, and photovoltaic properties of the bromine-substituted isomeric devices. (a) Calculated absorption spectra of BTIC-2Br-β and BTIC-2Br-γ in films. (b) Energy level alignments of PBDB-TF, BTIC-2Br-β, and BTIC-2Br-γ. (c) J–V characteristics of BTIC-2Br-β and BTIC-2Br-γ-based devices. (d) EQE spectra of BTIC-2Br-β and BTIC-2Br-γ-based devices. Download figure Download PowerPoint Photovoltaic properties To explore an in-depth understanding of the influence of different substituent positions of the bromine atoms, we fabricated the solar cells with the conventional structure, indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/ poly{[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]} (PBDB-TF):acceptors/poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-5,5′-bis(2,2′-thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N′-di(2-ethylhexyl)imide] (PNDIT-F3N)/Ag. The average PCE of each isomeric solar cell was measured using 15 devices/cell-type. The optimization conditions of the solar cells were shown in Supporting Information Table S2–S4. The photovoltaic parameters of the OSCs and the corresponding J–V curves for the superior devices are summarized in Table 1 and shown in Figure 2c. Upon successive thermal annealing (TA) treatments and solvent vapor annealing (SVA), the BTIC-2Br-β-based device showed a PCE of 8.11%, while the best performance with a PCE of 16.52% was achieved by BTIC-2Br-γ-based devices. Notably, a 50% enhancement in PCE was realized simply by changing the substitution position of the bromine atom of the end group. Due to the lower LUMO energy levels of BTIC-2Br-γ, the corresponding BTIC-2Br-γ-based device exhibited an open-circuit voltage (Voc) of 0.88 V, which was slightly lower than that of BTIC-2Br-β (0.91 V). Nevertheless, a high Jsc of 25.98 mA cm−2 and a fill factor (FF; expressed as the ratio of maximum obtainable power of the solar cell) of 72.23% from BTIC-2Br-γ-based devices resulted in a higher PCE of 16.52%. This outstanding performance benefited from higher light-harvesting efficiency and optimized morphology of the polymer blend films. Figure 2d shows the EQE curves for PBDB-TF:acceptor-based devices, demonstrating that the BTIC-2Br-γ-based device exhibited a higher EQE response (reaching 80% at 600 nm and surpassing 70% from 500 to 900 nm). The integrated Jsc of BTIC-2Br-γ was 24.65 mA cm−2, with a mismatch error of 5%. Comparatively, the BTIC-2Br-β-based devices exhibited a low photovoltaic response and a low EQE of 16.16 mA cm−2. This remarkable difference shows that the spatial arrangement of bromine atoms on the end groups was one of the critical factors required to improve the photovoltaic performance. Table 1 | Detailed Photovoltaic Parameters of the Optimized Acceptor-Based Devices Acceptors Voc (V) Jsc (mA·cm−2) Jcal (mA·cm−2) FF (%) PCEmax(PCE) (%)a BTIC-2Br-β 0.91 (0.91 ± 0.01) 16.94 (16.56 ± 0.47) 16.16 52.70 (52.15 ± 0.51) 8.11 (7.98 ± 0.31) BTIC-2Br-γ 0.88 (0.88 ± 0.01) 25.98 (25.78 ± 0.31) 24.65 72.23 (71.17 ± 1.29) 16.52 (16.32 ± 0.47) aThe average efficiency value was calculated from 15 devices each. Morphology characterization To analyze the origin of the effects on the photovoltaic performance caused by the bromine substitution position, we carried out a series of morphological characterizations, including GIWAXS, AFM, and TEM. The crystalline packing information of the two blend films was measured using GIWAXS. Supporting Information Figures S3 and S4 exhibited the patterns and the corresponding intensity values of both polymer blend films along the in-plane (IP) and out-of-plane (OOP) directions. Both of the acceptor-based blend films exhibited strong π–π stacking diffraction peaks in the OOP direction, indicating that the two acceptors mainly adopted a face-on orientation in the blend films. This could benefit the charge transfer in the vertical direction and improve the photovoltaic performance in OSCs. In the blend film, the same lamellar packing distance (100) of BTIC-2Br-β and BTIC-2Br-γ was observed in the molecular packing, but BTIC-2Br-γ was found to have a larger crystallite size, according to a narrower full width at half maximum (FWHM) of BTIC-2Br-γ (0.076 Å−1), than that of BTIC-2Br-β (0.080 Å−1). This phenomenon indicated that the appropriate positioning of the bromine atom could facilitate the formation of the crystallinity, which might have resulted, in part, from the 3D network packing of BTIC-2Br-γ. The larger crystallite size likely facilitated the charge transport in the devices and further enhanced the high Jsc and FF.35 On the contrary, the neat films of BTIC-2Br-β and BTIC-2Br-γ exhibited the π–π distances of 3.67 and 3.63 Å calculated from the peak q = 1.71 and 1.73 Å−1 in the OOP direction, respectively. Meanwhile, the PBDB-TF:BTIC-2Br-β blend device, the GIWAXS pattern showed a π–π distance of 3.70 Å from q = 1.70 Å−1, which was larger than that of PBDB-TF:BTIC-2Br-γ blend with 3.65 Å from q = 1.72 Å−1, respectively. Thus, it is possible that the closer π–π distance might have enhanced the crystallinity and intermolecular interactions of the latter blend device. AFM experiments were performed to investigate the two different blend films under the optimized conditions. The height images are shown in Supporting Information Figure S5. The two blend films both showed a smooth and uniform surface morphologies, and BTIC-2Br-β and BTIC-2Br-γ had small root mean square (RMS) roughness values of 0.94 and 1.01 nm, respectively. As shown in Figures 3a and 3b, we performed TEM experiments to compare the featureless morphology of BTIC-2Br-β- and BTIC-2Br-γ-based blend films. We observed that compared with the featureless morphology of BTIC-2Br-β, the TEM image of BTIC-2Br-γ -based blend films exhibited distinct fibrillary nanostructure and interpenetrating networks, consistent with the larger crystallite size and stronger intermolecular interactions observed in the GIWAXS, which undoubtedly provided more channels to the nanostructure to enhance its charge transport, and thus, contributed to the Jsc and FF of the solar cells.36,37 Figure 3 | Bright-field TEM images of the blend thin films. (a) (b) Download figure Download PowerPoint As in of the outstanding photovoltaic performance of BTIC-2Br-γ-based devices from two the crystallite size tailored to form the fibrillary nanostructure and the optimized of donor and acceptor in the vertical The appropriate miscibility of the to have a significant role in achieving an network between and the contact angle were in an effort to the of the bromine substitution position on the of the surface of the molecular and the miscibility between the and The surface tension values and the corresponding interaction parameters of the were obtained from the measured contact angles in and The in Figures and Table showed that the surface tension of BTIC-2Br-β, was larger than that of BTIC-2Br-γ, When blended with donor PBDB-TF with a surface tension of a larger surface tension between PBDB-TF and BTIC-2Br-γ was observed, suggesting the lower miscibility of PBDB-TF:BTIC-2Br-γ blend compared with the PBDB-TF:BTIC-2Br-β have shown that the interaction parameters of each of can be through the As shown in Table showed that the interaction parameter between PBDB-TF and BTIC-2Br-β was suggesting high miscibility between the two consistent with separation from morphology The between PBDB-TF and BTIC-2Br-γ was calculated to be indicating low miscibility, which strong separation and higher This was in with that of the morphology characterization and facilitated charge transport and Figure | contact angles of donor and contact angles of on film (a) PBDB-TF, (b) BTIC-2Br-β, and (c) BTIC-2Br-γ. contact angles of on on films (d) PBDB-TF, (e) BTIC-2Br-β, and (f) BTIC-2Br-γ. Download figure Download PowerPoint Table 2 | Parameters for PBDB-TF, BTIC-2Br-β, and BTIC-2Br-γ PBDB-TF BTIC-2Br-β BTIC-2Br-γ aThe interaction parameters between the PBDB-TF and BTIC-2Br-β or BTIC-2Br-γ were calculated the = ( A 2 dynamics properties It is well that the charge dynamics a significant role in the of photovoltaic performance. To further investigate the reason the distinct performance of two acceptors, we performed quenching to investigate the charge-transfer behavior and exciton in two blend films PBDB-TF:BTIC-2Br-γ and The of the BTIC-2Br-β and BTIC-2Br-γ acceptors was at 800 nm, according to their maximum As shown in Figure the peaks of both acceptors were observed in the of nm. For the acceptor-based blend films, the spectra were by for BTIC-2Br-β and for BTIC-2Br-γ. The differences in quenching efficiency indicated that the BTIC-2Br-γ-based blend film exhibited a more efficient electron transfer than that of the BTIC-2Br-β-based blend film, an that was consistent with higher Jsc and FF of the BTIC-2Br-γ-based devices. Additionally, the charge = values of the acceptor-based devices were by the current the effective as shown in Supporting Information Figure revealing that a higher charge was obtained from the BTIC-2Br-γ-based device than that of the BTIC-2Br-β-based device This more efficient exciton at the of PBDB-TF and BTIC-2Br-γ, the FF and efficiency, consistent with the quenching The charge recombination behavior of the devices was investigated by the between the of Jsc and the intensity as Jsc Figure shows that the bimolecular recombination in PBDB-TF:BTIC-2Br-β blend is more than in PBDB-TF:BTIC-2Br-γ with its relatively We performed the and to the of the two blend films. Figure shows the of the two brominated acceptor-based devices. The charge-carrier was for the PBDB-TF:BTIC-2Br-γ which was slightly than that of PBDB-TF:BTIC-2Br-β The could enhance the charge

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