Star-Shaped Fused-Ring Electron Acceptors with a C3h-Symmetric

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Organic Electronic Devices

Star-Shaped Fused-Ring Electron Acceptors with C3hSymmetric and Electron-rich Benzotri(cyclopentadithiophene) Core for Efficient Nonfullerene Organic Solar Cells Xiaofu Wu, Weijie Wang, Hao Hang, Hua Li, Yonghong Chen, Qian Xu, Hui Tong, and Lixiang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08017 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

Star-Shaped Fused-Ring Electron Acceptors with C3hSymmetric and Electron-rich Benzotri(cyclopentadithiophene) Core for Efficient Nonfullerene Organic Solar Cells Xiaofu Wu, † Weijie Wang, †, ‡ Hao Hang, †, § Hua Li, †, ‡ Yonghong Chen, †, § Qian Xu, # Hui Tong*, †, ‡ and Lixiang Wang*, †, ‡ † State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. ‡ University of Science and Technology of China, Hefei, 230026, PR China. §University of Chinese Academy of Sciences, Beijing 100039, P. R. China. # School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022, P. R. China. KEYWORDS: Star-shaped fused-ring electron acceptors; Nonfullerene organic solar cells; Benzotri(cyclopentadithiophene); C3h-symmetry; Donor–acceptor

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ABSTRACT. Classical fused-ring electron acceptors (FREAs) with linear acceptor-donor-acceptor (AD-A) architecture continuously break records of power conversion efficiency (PCE) in nonfullerene organic solar cells. In contrast, the development of star-shaped FREAs still lags behind. Herein, a new C3h-symmetric and electron-rich core, benzotri(cyclopentadithiophene) (BTCDT) in which the central benzo[1,2-b:3,4-b’:5,6-b”]trithiophene fused with three outer thiophenes via three cyclopentadienyl rings, are synthesized and used for the construction of star-shaped FREAs (BTCDT-IC and BTCDT-ICF). Owing to the strong electron-donating ability of BTCDT unit, both acceptors exhibit the effective intramolecular charge transfer, leading to the strong absorption in the region of 500-800 nm with narrow bandgaps below 1.70 eV as well as suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Comparing with non-fluorinated BTCDT-IC, fluorinated BTCDT-ICF red-shifts the absorption peak to 688 nm and reduces the bandgap to 1.62 eV, which induces a broader EQE response ranging from 300 to 800 nm and a higher maximum EQE of 70% when blending with wide bandgap polymer donor J61. The J61: BTCDT-ICF blend film exhibits more appreciate phase morphology than the J61: BTCDT-IC blend film, which is responsible for the enhanced EQE value, increased short circuit current (JSC) and fill factor (FF) in organic solar cell device. As a result, J61: BTCDT-ICF-based device yields a best PCE of 8.11% with a high JSC of 16.93 mA cm−2 and a high FF of 65.6%, demonstrating that BTCDT-based star-shaped FREAs hold great potential for nonfullerene organic solar cells.


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INTRODUCTION Bulk heterojunction organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) have gained great progresses in recent years,1-3 and the power conversion efficiencies (PCEs) have already exceeded 15%.4-7 Among various NFAs, the fused-ring electron acceptors (FREAs) have been proven to be the most efficient for achieving high PCEs. The typical FREAs, such as the benchmark molecules IEIC and ITIC pioneered by Zhan and co-workers,8, 9 feature an acceptordonor-acceptor (A-D-A) architecture, which consists of a bulky electron-donating fused aromatic core and two electron-deficient terminal groups. Such an A-D-A framework can induce strong intramolecular charge transfer (ICT), resulting in extended absorption range and adjustable energy level alignment that could better meet the requirement of variable donor materials as compared to fullerene-based acceptors.10, 11 To pursue more efficient FREAs, significant efforts have been devoted to the modifications of the electron-donating cores12-18 and the electron-withdrawing end groups.19-25 With respect to the modulation of the fused-ring cores, enlarging the planar πconjugated skeleton with the aromatic or heteroaromatic fused rings is an efficient strategy to facilitate π-electron delocalization and molecular packing.26-32 Especially, thieno-fusion as a part of the planar skeleton is of crucial importance in red-shifting absorption, reducing the ionization energy and increasing crystallinity.12, 27-29 Guided by these design principles, a range of fascinating linear FREAs with novel fused-ring cores with π-extended and thiophene-rich structures have been developed and displayed extended absorption, reduced bandgap and increased mobility, consequently affording the remarkable improvement of photovoltaic performance. Despite most efforts being focused on the linear FREAs, star-shaped electron acceptors also attract particular interests currently for their extended π-conjugation in two or three dimensions through a central core unit, which leads to stronger light absorption and higher isotropy in charge transportation than linear acceptors.33-36 When building a star-shaped electron acceptor, a major concern is the trade-off between planar structure and twisted structure, which directly correlates with balancing electron mobility and exciton dissociation.3,

37-41

With this consideration, a

straightforward strategy to achieve star-shaped electron acceptors with enhanced rigidity and coplanarity is the transformation of nonfused star-shaped acceptors into fused ones through direct fusion of the periphery units into the core unit. Based on this strategy, star-shaped fused-ring PDI acceptors have been constructed and demonstrated increased molecular rigidity and planarity,


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extended π-conjugation and lowered miscibility between the polymer donor and the acceptor, thus leading to the improved device performance as comparing with their nonfused counterparts.42-48 However, these acceptors commonly have wide bandgaps and weak absorption at long wavelength due to reduced ICT between the core and PDI moieties, which may be detrimental for solar energy harvesting. Besides of star-shaped fused-ring PDI acceptors, the star-shaped FREAs with D-A architecture have also been designed through the incorporation of electron-withdrawing groups into star-shaped and planar polycyclic aromatic cores, following the A-D-A motifs of linear FREAs. A common choice of star-shaped polycyclic aromatic core is C3h-symmetric and rigid truxene,49, 50 which can be facilely attached three electron-withdrawing units on their periphery to modulate electron affinity, ICT interaction and intermolecular packing. These truxene-based acceptors are endowed with strong ICT absorption with high absorption coefficients, enhanced πelectron delocalization and effective charge transport, yielding the best PCE exceeding 10%. 50 However, truxene is still a weak electron-donor unit with the limited π-conjugation length, which is unfavorable for the construction of acceptors with narrow bandgaps. Up to now, few examples have been reported on more extended polycyclic aromatic cores, especially those fused with thiophene rings, to target highly efficient star-shaped FREAs probably owing to tedious and challenging syntheses. With these issues in mind, we are motivated to search for alternative star-shaped polycyclic aromatic cores with highly planar, electron-rich and symmetric structure for the design of new families of D-A type star-shaped FREAs. Recently, we applied C3h-symmetric coplanar trindeno[1, 2-b: 4, 5-b': 7,8-b'']trithiophene (TITT) as a core to construct star-shaped FREAs.51 Owing to thiophene-rich fused-ring structure and enhanced ICT effect, TITT-based acceptors show redshifted absorption and reduced optical bandgaps (below 1.9 eV) as comparing with the truxenebased acceptors (around 2.0 eV).47, 50, 51 Given the limitation of the central benzene ring in TITT architecture on effective conjugation length and electron-donating ability, in this article, we replace the central benzene ring of TITT unit with benzo[1,2-b:3,4-b’:5,6-b”]trithiophene (C3hBTT) unit to develop a novel star-shaped polycyclic aromatic benzotri(cyclopentadithiopene) (BTCDT) unit (Chart 1). C3h-BTT is a highly sought after building block for constructing highmobility p-type organic semiconductors and D-A type star-shaped molecules and conjugated polymers in OSCs, since it possesses a highly planar, C3h-symmetric and terthieno-fused structure.52-55 Through covalently locking C3h-BTT unit and three adjacent thiophene rings with


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three cyclopentadienyl rings in a plane, we believe that the decacyclic BTCDT will be a better skeleton than TITT based on the following reasons: (1) Electron-rich structure and long effective conjugation length of BTCDT allow for efficient π-electron delocalization and strong ICT effect to red-shift the absorption and reduce the optical bandgaps, which would harvest photons efficiently to increase short circuit current (JSC). (2) The large planar π-conjugated framework of BTCDT favors intermolecular packing to enhance charge transport property. (3) C3h-symmetric scaffold of BTCDT facilitates the modification of electron-deficient terminal groups to tune ICT interaction and molecular frontier orbital energy levels. (4) Multiple bulky side chains out of plane will guarantee good solubility as well as inhibition of over aggregation for the target acceptors. Taking BTCDT core to couple with electron-deficient 3-(dicyanomethylidene) indan-1-one (IC) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (ICF), two star-shaped FREAs, BTCDT-IC and BTCDT-ICF, are synthesized. As expected, both BTCDT-based acceptors with planar π-conjugated backbones exhibit intense absorption in the region of 500-800 nm with narrow bandgaps below 1.70 eV. Especially, BTCDT-ICF presents further red-shifted absorption, reduced optical bandgap and improved intermolecular packing relative to BTCDT-IC, owing to the strong electron push-pull effect of ICF terminal group and the fluorine-induced noncovalent interactions.56, 57 Benefiting from the strong photoresponse in a broad spectral range, suitable morphology and enhanced structure order in the blend of BTCDT-ICF and polymer donor J61,58 BTCDT-ICF-based photovoltaic device achieves a high JSC of up to 16.93 mA cm-2 and a FF of 65.6%, resulting in a higher PCE of 8.11% than BTCDT-IC-based device. RESULTS AND DISCUSSION The synthetic routes to BTCDT-IC and BTCDT-ICF are depicted in Scheme 1. Initially, compound 3 was prepared by Stille coupling reaction of 2,5,8-trimethylstannyl benzo[1,2-b:3,4b':5,6-b”]trithiophene (1) and methyl 2-bromothiophene carboxylate (2) in a high yield of 80%. A triple nucleophilic addition reaction of the ester groups in compound 3 with freshly prepared (4hexylphenyl)magnesium bromide afforded the intermediate 4, which was used directly in next step without further purification. Through intramolecular triple Friedel-Crafts-type annulation of 4 in acetic acid, the central core BTCDT could be obtained in a moderate yield of 30%. The trialdehyde compound BTCDT-CHO was prepared by the Vilsmeier-Haack reaction in a yield of 84%. Finally, BTCDT-CHO further reacted with IC and ICF via Knoevenagel condensation to produce the


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desirable products BTCDT-IC and BTCDT-ICF in the yields of 55% and 39%, respectively. All the compounds are characterized by 1H NMR,

13

C NMR and mass spectra (see supporting

information). BTCDT-IC and BTCDT-ICF have good solubility in common solvents, such as chloroform, chlorobenzene and o-dichlorobenzene. They also have excellent thermal stability with decomposition temperatures (Td, 5% weight loss) above 300 °C under nitrogen atmosphere as measured by thermogravimetric analysis (TGA) (Figure S1). The ultraviolet-visible (UV-vis) absorption spectra of BTCDT-IC and BTCDT-ICF in chlorobenzene solution and thin films were measured (Figure 1a, 1b and Table 1). In solution, BTCDT-IC showed an absorption peak at 634 nm with a molar extinction coefficient of 2.8×105 M-1 cm-1; while BTCDT-ICF showed red-shifted absorption peaked at 643 nm with a higher molar extinction coefficients of 3.0×105 M-1 cm-1 due to more effective ICT. Comparing with linear acceptor analogues,12 the higher molar extinction coefficient are possibly due to their extended dimensionality. The absorption peaks of BTCDT-IC and BTCDT-ICF films red-shift to 666 nm and 688 nm, respectively, which can form a complementary absorption with respect to wide bandgap polymer donor J61. The absorption extinction coefficients of BTCDT-IC and BTCDTICF films were 1.59 ×105 cm−1 and 1.68 ×105 cm−1, respectively, which were comparable or even higher than many highly efficient linear FREAs.5, 21, 59 Calculated from the absorption onset, the optical bandgaps also changed from 1.69 eV for BTCDT-IC to 1.62 eV for BTCDT-ICF, which were markedly lower than reported TITT-based acceptors (around 1.9 eV). Relative to BTCDTIC, more red-shifted absorption band of BTCDT-ICF with higher extinction coefficient will benefit light harvesting, and consequently the higher short-circuit current could be expected for its OPV device. The electrochemical properties of BTCDT-IC and BTCDT-ICF were investigated by cyclic voltammetry (CV) measurement (Figure 1c and S2). Calculated from the onset of oxidation and reduction potentials relative to the reference energy level of ferrocene/ferrocenium (Fc/Fc+), the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were located at -5.75 eV and -3.87 eV for BTCDT-IC neat film, and simultaneously downshifted to -5.83 eV and -3.98 eV for BTCDT-ICF neat film owing to the electronwithdrawing effect of fluorine. As comparing with TITT-based acceptors, the raised energy levels of BTCDT-based acceptors also implied stronger electron-donating ability of BTCDT core. The HOMO and LUMO offsets between the acceptors and the donor J61 (HOMO: -5.32 eV, LUMO: -3.08 eV) were suitable for exciton dissociation in OSCs.58


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The optimized molecular geometries and the frontier molecular orbitals of BTCDT-IC and BTCDT-ICF were calculated based on the density functional theory (DFT) calculation at CAMB3LYP/6-31G(d) level (Figure 1d).13, 60 The CAM-B3LYP long-range corrected hybrid functional can describe more accurately the localization and delocalization of the frontier energy levels in planar π-conjugated structures.61, 62 All the hexylphenyl groups were replaced by phenyl groups in order to simplify the calculation. Both BTCDT-IC and BTCDT-ICF exhibited highly planar and C3h-symmetric π-conjugated backbones with six hexylphenyl side chains flanked the main plane. Their C3h-symmetric geometry resulted in the doubly degenerate HOMOs and LUMOs. The HOMO levels distributed on the BTCDT core and extended to dicyano-vinyl moieties of terminal groups, and LUMO levels were equally delocalized over the whole conjugated backbones.51, 54 The calculated HOMO/LUMO energy levels of BTCDT-ICF (-6.88/-2.41 eV) were lower than those of BTCDT-IC (-6.72/-2.22 eV), which was consistent with the trend of CV measurement. The doubly degenerate LUMO energy levels of these acceptors would also allow for the generation of more nearly degenerate lowest charge transfer (CT) state. Since the excitons dissociation rate would be the sum of all dissociation rates of final CT states, these CT states would facilitate the more rapid and efficient charge separation at the donor-acceptor interface.63, 64 Bulk heterojunction organic solar cells (OSCs) derived from BTCDT-IC and BTCDT-ICF blending with J61 were fabricated with a conventional device structure of ITO/PEDOT: PSS/J61: acceptor/Ca/Al. Device processing conditions including donor/acceptor (D/A) weight ratio, solvent additive and thermal annealing were optimized (see supporting information). The active layers of the devices were spin-coated from the chlorobenzene solution with a D/A weight ratio of 1:1 and the total concentration of 20 mg mL-1. Through using 1 % (vol) 1-chloronaphthalene as solvent additive and annealing at 160ºC for 10 min and 200 ºC for 10 min, respectively, the optimal photovoltaic performances of the devices based on J61: BTCDT-IC and J61: BTCDT-ICF could be achieved. The current density-voltage (J–V) curves of two devices with the best performance were shown in Figure 2a, and the corresponding photovoltaic parameters were listed in Table 2. Under the optimized conditions, the device based on J61: BTCDT-IC yielded a PCE of 5.30% with an open-circuit voltage (VOC) of 0.92 V, a JSC of 9.75 mA cm-2 and a fill factor (FF) of 59.0%. In contrast, J61: BTCDT-ICF-based device exhibited a better PCE of 8.11% with a much higher JSC of 16.93 mA cm-2 and a FF of 65.6% in despite of a lower VOC of 0.73 V due to downshifted LUMO level of BTCDT-ICF. In comparison with J61: BTCDT-IC-based device, the higher JSC of J61:


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BTCDT-ICF-based device was attributed to lower bandgap of BTCDT-ICF and better coverage of absorption spectra of J61: BTCDT-ICF blend film. The external quantum efficiency (EQE) spectra of two devices confirmed the difference of JSC between two devices, as shown in Figure 2b. According to the absorption spectra, the EQE spectra at short-wavelength and long wavelength originated from J61 donor and BTCDT-based acceptors, respectively. Comparing with J61: BTCDT-IC-based device with the EQE spectral range from 300 to 750 nm, J61: BTCDT-ICFbased device displayed a broader EQE response extended to 800 nm due to the lower bandgap of BTCDT-ICF. Particularly, the EQE values of J61: BTCDT-ICF-based device were around 70% in the 500-700 nm region, whereas the EQE values of J61: BTCDT-IC-based device were only below 50% in the entire spectral region. The broader and stronger EQE response of J61: BTCDT-ICFbased device implied the more efficient charge generation, which led to the boost of JSC and the improved device efficiency. Steady-state photoluminescence (PL) quenching studies on J61: BTCDT-IC and J61: BTCDTICF blends were performed to investigate exciton generation and charge transfer behaviors. As shown in Figure 3, the PL emission peaks of the neat J61, BTCDT-IC and BTCDT-ICF films were at around 634, 705 and 734 nm, respectively, when selectively exciting at 550, 590 and 620 nm, respectively. In blend films, both BTCDT-IC and BTCDT-ICF could efficiently quench the PL emission of J61 by 97 % and 99 %, respectively, indicating effective electron transfer from J61 to both acceptors (Figure 3a). When monitoring the PL emission of the acceptors quenched by J61, the PL quenching efficiencies for BTCDT-IC and BTCDT-ICF were 38% and 87%, respectively (Figure 3b and 3c). The much higher quenching efficiency for BTCDT-ICF suggested the better hole transfer feature from acceptor to polymer donor, possibly owing to larger HOMO energy difference between BTCDT-ICF and J61 and more suitable phase morphology of donor-acceptor blend, which partially explained the higher EQE and greatly enhanced photocurrent in J61: BTCDT-ICF-based device. To further evaluate the charge dissociation and charge collection efficiency in devices based on BTCDT-IC and BTCDT-ICF, the relationship between the photocurrent density (Jph) and the efficient applied voltage (Veff) were measured (Figure 4a). At high voltage (>2 V), all the photogenerated excitons were completely dissociated into the free charge carriers and collected by electrodes. The saturation photocurrent density (Jsat) was only correlated to incident photons, and


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therefore the value of Jph/Jsat was adopted to estimate the charge dissociation and charge collection probability (P(E, T)).65 In our case, the Jph of two optimal devices reach saturation (Jsat) at relatively high voltages close to 3 V. Under the short-circuit conditions, the Jph/Jsat ratios of BTCDT-IC and BTCDT-ICF based devices were calculated to be 92% and 97%, respectively. The higher Jph/Jsat ratio of BTCDT-ICF based devices suggested more efficient excition dissociation and charge collection process, leading to the higher EQE and JSC. Charge recombination and charge mobility in devices with BTCDT-IC and BTCDT-ICF were explored. By measuring the JSC with different incident light intensities (Plight), which followed the formula of JSC ∝Plightα,66 the α values were determined to be 0.93 and 0.95 for J61: BTCDT-IC based device and J61: BTCDT-ICF based device, respectively (Figure 4b), suggesting weak bimolecular recombination. The bulky charge mobility properties of J61: BTCDT-IC and J61: BTCDT-ICF blend films were measured by the space charge limited current (SCLC) method (Figure S3 and S4). The hole mobility (µh) and electron mobility (µe) were calculated to be 4.08×10-4 and 1.06×10-5 cm2 V-1 s-1 for J61: BTCDT-IC blend film and 5.11×10-4 cm2 V-1 s-1 and 1.89×10-5 cm2 V-1 s-1 for J61: BTCDT-ICF blend film, respectively. The higher charge carrier mobilities of J61: BTCDT-ICF blend film favored better charge extraction, which were contributed to the increased JSC and FF in the device. To gain the deep insight of the difference between BTCDT-IC and BTCDT-ICF-based devices, morphological characteristics of the two blend films were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). In the AFM images, J61: BTCDT-ICF blend film showed a smooth surface with the root-mean-square (RMS) roughness value of 2.39 nm, which was smaller than J61: BTCDT-IC blend film (4.27 nm) (Figure 5a and b). The smoother J61: BTCDT-ICF blend film favored the better contact between active layer top and electrode. The obvious distinction of J61: BTCDT-IC and J61: BTCDT-ICF blend films could also be observed in the TEM images, which matched well with the corresponding AFM images (Figure 5c and d). J61: BTCDT-IC blend film displayed the overlarge phase separation as evident by obvious dark regions. As known, in active layer of BHJ OSCs, the bicontinuous interpenetrated morphology and the ideal phase-separated domain size of 10−20 nm were desirable for efficient charge separation and transport.67, 68 Thus, the large phase separation scale of BTCDT-IC blend film was detrimental to exciton dissociation and charge transport, which might account for the lower PL


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quenching efficiency. In sharp contrast, J61: BTCDT-ICF blend film showed more finely dispersed phase separation with interpenetrating structures, which contributed to charge separation and transport, in turn leading to the improved photovoltaic performance. The X-ray diffraction (XRD) patterns were measured to investigate the molecular packing behaviors of the neat films and the optimized blend films (Figure S5). The neat J61 film took a preferential face-on orientation with a (010) diffraction peak at 1.75 Å−1 in out-of-plane (OOP) direction and a (100) diffraction peak at 0.23 Å−1 in the in-plane (IP) direction, corresponding to the π–π stacking region and the lamellar stacking, respectively. Both BTCDT-IC and BTCDT-ICF neat films showed the similar (100) diffraction at 0.61 Å−1 in OOP direction, while the π–π stacking diffraction located at 1.75 Å−1 for BTCDT-IC and 1.81 Å−1 for BTCDT-ICF, respectively, indicating that BTCDT-ICF had shorter π–π stacking distance (3.47 Å) than BTCDT-IC (3.59 Å). The optimized J61: BTCDT-ICF blend film presented a stronger π–π stacking diffraction peak than J61: BTCDT-IC blend film, as seen from stronger (010) diffraction intensity in OOP direction and (100) diffraction intensity in IP direction, and similar phenomena had been observed in many linear FREAs with fluorinated IC terminal groups.13, 15, 19 These results demonstrated that the better structure order and more suitable phase separation of the J61: BTCDT-ICF blend film were beneficial for exciton dissociation and charge transport, agreeing well with the improved charge-carrier mobilities and the promoted JSC, FF and PCE relative to J61: BTCDT-IC blend film. CONCLUSIONS In summary, a new C3h-symmetric and coplanar fused BTCDT unit has been successfully designed and synthesized, which is employed in preparation of star-shaped FREAs. The BTCDTbased accepters possess an extended π-conjugation and enhanced ICT effect, leading to intense absorption in the region of 500-800 nm with the bandgaps below 1.70 eV. Especially, BTCDTICF exhibits a maximum absorption at 688 nm with a narrow bandgap of 1.62 eV. In combination with wide bandgap polymer donor J61, the optimized J61: BTCDT-ICF-based device acquires an EQE response range from 300 nm to 800 nm with the maximum value of 70%, which is stronger and broader than J61: BTCDT-IC-based device. The appropriate phase separation morphology and enhanced crystalline characteristics of J61: BTCDT-ICF blend film contribute greatly to efficient exciton dissociation and charge transport as comparing with J61: BTCDT-IC blend film. Consequently, J61: BTCDT-ICF-based device shows a best PCE of 8.11% with a high JSC of 16.93


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mA cm−2, a high FF of 65.6% and a moderate VOC of 0.73 V, which is markedly contrast to BTCDT-IC-based device (a PCE of 5.30% with a low JSC of 9.75 mA cm-2, a FF of 59.0% and a VOC of 0.92 V). These results reveal that BTCDT unit is a greatly promising electron-donating building block for the construction of high performance star-shaped FREAs. EXPERIMENTAL SECTION Measurement and Characterization. 1H and 13C NMR spectra were recorded on a Bruker AV400 spectrometer with CDCl3 as solvents. IR spectra were obtained on a FT-IR Bruker Vertex 70 spectrometer at a nominal resolution of 2 cm-1. HR-MALDI-TOF MS was measured using a Bruker Daltonics flex Analysis. Thermal gravimetric analysis (TGA) was performed under an N2 flow on a Perkin-Elmer-TGA 7 system. The temperature of degradation (Td) corresponded to 5% weight loss. UV–visible absorption spectra was measured using a Perkin-Elmer Lambda 35 UVvis spectrometer. Out-of-plane XRD of thin-film was recorded on a Bruker D8 Discover thin-film diffractometer with Cu K α radiation (λ = 1.54056 Å) operated at 40 kV and 30 mA. In-plane XRD data were obtained by using a Rigaku SmartLab X-ray diffractometer with an X-ray generation power of 40 kV tube voltage and a 30 mA tube current. The measurement was obtained in a scanning interval of 2θ between 2° and 30°. Cyclic voltammetry (CV) was performed with a solution of 0.1 M Bu4NClO4 in acetonitrile on a CHI660a electrochemical analyzer system using a glassy carbon working electrode, a platinum gauze counter electrode and a Ag/AgCl reference electrode. The onset potential of Fc/Fc+ was measured to be 0.36 V, and the HOMO and LUMO levels from the onset oxidation (Eox equations: EHOMO = - e(Eox

onset

onset

) and reduction (Ered

onset

) potentials were calculated by

- EFc/Fc++4.8) eV and ELUMO = -e(Ered

onset

-EFc/Fc++4.8) eV,

respectively. TEM was recorded on a JEOL JEM-1011 transmission electron microscope operated at an acceleration voltage of 100 kV. AFM in tapping model was performed with an SPI 3800N Probe Station (Seiko Instruments Inc., Japan).


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Materials. All chemicals and reagents were used as received from commercial sources without further purification. Solvents for chemical synthesis were purified according to standard procedures. 2,5,8-tris(trimethylstannyl)benzo[1,2-b:3,4-b':5,6-b”]trithiophene was synthesized as previously described.52 Synthesis of Compound 3. To a 250 mL two-neck round-bottom flask was introduced 2,5,8tris(trimethylstannyl)benzo[1,2-b:3,4-b':5,6-b'']trithiophene (2.8 g, 3.8 mmol), methyl 2bromothiophene-3-carboxylate (3.8 g, 17.1 mmol), Pd(PPh3)4 (0.22 g, 0.19 mmol) and degassed toluene (150 mL). Under argon atmosphere, the mixture was heated to refluxed for 72 h. The reaction solution was poured into water and extracted with chloroform. The combined organic layer was dried over anhydrous Na2SO4 and filtered. After removing the solvent under reduced pressure, the residue was purified by column chromatography on silica gel (petroleum ether/ethyl acetate=5/1) to give the yellow solid 1 (2.0 g, 80%). 1H NMR (400 MHz, CDCl3) : δ 7.96 (s, 3H), 7.55 (d, J = 5.4 Hz, 3H)，7.31 (d，J = 5.4 Hz, 3H), 3.88 (s, 9H). 13C NMR (125 MHz, CDCl3): δ 163.47, 142.71, 133.74, 133.46, 130.81, 130.68, 128.35, 124.91, 123.97, 51.95. HR-MS(MALDITOF): calcd, 665.9428; found, 665.9413 (M+). Synthesis of Compound 4. A Grignard reagent was prepared by the following procedure. To a suspension of magnesium turning (2.1g, 86.4 mmol) and 1, 2-dibromoethane (0.1 mL) in dry THF (50 mL), was slowly added 1-bromo-4-hexaylbenzene (10.4 g, 43.2 mmol) dropwise, and the mixture was stirred for 2 h. To a solution of compound 3 (2.4 g, 3.6 mmol) in dry THF (100 mL) under argon was added the prepared Grignard reagent at room temperature. The resulting mixture was heated to reflux overnight. The reaction solution was poured into 0.5 M HCl solution, and extracted with chloroform. The combined organic layer was washed with NaCl saturated aqueous


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solution, and dried with anhydrous Na2SO4. After removal of the solvent under the reduced pressure, a brown solid was obtained and directly used for the next step without further purification. Synthesis of BTCDT. The crude compound 4 was dissolved in boiling acetic acid (260 mL) and then refluxing for 12 h. The reaction solution was poured into water, and extracted with chloroform. The combined organic layer was washed with NaCl saturated aqueous solution and then dried with anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel (petroleum ether/dichloromethane = 10/1) to give the yellow solid BTCDT (0.6 g, 30%). 1HNMR (400MHz, CDCl3): δ 7.45 (d, J = 8.1 Hz, 12H), 7.13 (d, J = 4.9Hz, 3H), 7.08 (d, J = 8.1Hz, 12H), 7.01 (d, J = 4.9 Hz, 3H), 2.58 (t, J = 7.5Hz, 12H), 1.58 (m, 12H), 1.27 (m, 36H), 0.84 (m, 18 H). 13C NMR (125 MHz, CDCl3): δ 163.16, 149.97, 141.68, 139.04, 135.57, 134.67, 131.95, 129.88, 129.38, 128.02, 126.45, 122.83, 63.10, 35.52, 31.69, 31.12, 29.06, 22.58, 14.06. HR-MS(MALDI-TOF): calcd, 1488.6775; found, 1488.6722 (M+). Synthesis of BTCDT-CHO. A Vilsmeire reagent was prepared by the following procedure. The POCl3 (0.38 g, 2.4 mmol) was added dropwisely to anhydrous DMF (4 mL) at 0 °C, and then stirred at room temperature for 30 min. To a solution of BTCDT (0.60 g, 0.4 mmol) in dry 1,2dichloroethane was added the prepared Vilsmeire reagent in one portion under the protection of argon. The resulting mixture was heated to 85 °C overnight. The reaction solution was poured into ice-water, neutralized with Na2CO3 aqueous solution, and then extracted with chloroform. The combined organic layer was washed with NaCl saturated aqueous solution and then dried with anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel (petroleum ether/dichloromethane = 1/1.5) to afford the yellow solid BTCDT-3CHO (0.54g, 84%). 1H NMR (400 MHz, CDCl3): δ 9.75 (s，3H)，7.62 (s，3)，7.40


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(d, J = 7.9 Hz, 12H), 7.11 (d, J = 7.9 Hz, 12H), 2.58 (t, J=7.7 Hz, 12H)，1.58 (m, 12H), 1.24 (m, 36H), 0.81 (t, J=6.3 Hz, 18 H). 13C NMR (125 MHz, CDCl3): δ 181.44, 162.30, 152.92, 144.11, 143.51, 141.60, 137.35, 134.42, 132.81, 130.10, 128.61, 128.16, 127.42, 62.41, 34.46, 30.64, 30.06, 27.99, 21.56, 13.02. HR-MS(MALDI-TOF): calcd, 1572.6623; found, 1572.6679 (M+). Synthesis of BTCDT-IC. Under the argon atmosphere, BTCDT-CHO (0.40 g, 0.25 mmol) and 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (0.73 g, 3.8 mmol) was dissolved in dry chloroform, and then pyridine (2 mL) was added to the mixture. The resulting solution was heated to 65 °C for 12 h. The reaction mixture was diluted with chloroform, washed with water and NaCl saturated aqueous solution and then dried with anhydrous Na2SO4. After removal of the solvent, the residue was purified by column chromatography on silica gel (petroleum ether/ chloroform = 1/3) to give the dark red solid (0.29 g, 55%). 1H NMR (400 MHz, CDCl3) : δ 8.82 (s, 3H)，8.69 (d, J = 6.9 Hz, 3H)，7.92 (d, J = 5.8 Hz, 3H)，7.76 (m, 3H)，7.59 (s, 3H)，7.42 (d, J=8.1 Hz, 12H)，7.15 (d, J = 8.1 Hz, 12H), 2.59 (t, J=8.2 Hz, 12H), 1.61 (m, 12H)，1.26 (m, 36H)，0.81 (t, J = 6.9 Hz, 18 H). 13C NMR (125 MHz, CDCl3): δ 188.67, 164.20, 160.26, 156.18, 153.51, 142.88, 140.26, 139.97, 139.04, 138.22, 138.15, 137.81, 136.81, 135.14, 134.40, 133.24, 129.52, 129.15, 128.61, 125.32, 123.66, 121.42, 114.76, 68.73, 63.44, 35.52, 31.66, 31.07, 29.11, 22.58, 14.06. HR-MS(MALDI-TOF): calcd, 2100.7746; found, 2100.7772 (M+). Synthesis of BTCDT-ICF. Under the argon atmosphere, BTCDT-CHO (0.3 g, 0.2 mmol) and 5,6-difluoro-2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (0.55 g, 2.4mmol) was dissolved in dry chloroform, and then pyridine (1.5 mL) was added to the mixture. The resulting solution was heated to 65 °C for 12 h. The reaction mixture was diluted with chloroform, washed with water and NaCl saturated aqueous solution and then dried with anhydrous Na2SO4. After removal


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of the solvent, the residue was purified by column chromatography on silica gel (petroleum ether/ chloroform = 1/3) to give the dark red solid (0.16 g, 39%). 1H NMR (400 MHz, CDCl3): δ 8.79 (s, 3H), 8.53 (m, 3H), 7.66 (t, J = 7.5 Hz, 3H), 7.59 (s, 3H), 7.39 (d, J = 8.2 Hz, 12H), 7.14 (d, J = 8.2 Hz, 12H)，2.58 (t, J = 8.2 Hz,12H), 1.59 (m, 12H), 1.24 (m, 36H), 0.79 (t, J = 6.9 Hz, 18 H). 13C NMR (125 MHz, CDCl3): δ 186.29, 164.45, 158.11, 156.64, 155.58, 154.41, 153.45, 143.01, 140.14, 139.01, 138.64, 138.34, 138.23, 136.58, 134.43, 133.03, 129.51, 129.12, 128.67, 120.55, 115.16, 114.98, 114.27, 112.64, 112.50, 69.32, 63.45, 35.51, 31.65, 31.07, 29.09, 22.58, 14.06. HR-MS(MALDI-TOF): calcd, 2208.7181; found, 2208.7131 (M+). Fabrication and Measurement of OSCs Devices. All devices were fabricated with the conventional structure of ITO/PEDOT:PSS/J61: acceptor/Ca/Al. The ITO-coated glass substrates were cleaned in an ultrasonic bath using isopropyl alcohol, deionized water, acetone, and isopropyl alcohol consecutively, followed by dried at 130 °C for 30 min in the oven. After 25 min UV-ozone treatment, a thin layer of PEDOT: PSS was spin-coated on ITO at 5000 rpm for 40 s and then dried at 120 oC for 30 min in the oven. After that, all PEDOT: PSS coated glass substrates were transferred to the nitrogen-filled glovebox. The J61: BTCDT-IC or J61: BTCDT-ICF active layer was spin-coated from the solution (J61: acceptor=1:1, 1% CN, 20 mg/mL in total) in chlorobenzene at 1600 rpm for 2 min, which resulted in the active layer thickness of 100 nm. Then the active layer was thermally annealed at 160 °C (J61: BTCDT-IC) or 200 °C (J61: BTCDT-ICF) for 10 minutes. Finally, a layer of Ca (20 nm) and Al (100 nm) electrode was sequentially deposited by thermal evaporation at the pressure of about 2×10−4 Pa in the vacuum chamber with an active area of 8.0 mm2. For the optimal devices, an aperture with an area of 2.0 mm2 was also used to measure the performance of the OSCs. The


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current density (J-V) curves of J61: BTCDT-IC and J61: BTCDT-ICF-based devices were measured with a computer-controlled Keithley 2400 source meter under 100 mW cm–2 AM 1.5G simulated solar light illumination provided by a XES-40S2-CE Class Solar Simulator (Japan, SAN-EI Electric Co., Ltd.). The external quantum efficiency (EQE) spectra were measured with a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech Co., Ltd.), which was calibrated by monocrystalline silicon solar cell before use. ASSOCIATED CONTENT Supporting Information. TGA and DSC curves, SCLC data, XRD data; device fabrication, optimization and characterization. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (H.T.) * E-mail: [email protected] (L.W.) Notes The authors declare no conflict of interest. ACKNOWLEDGMENT This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12010200), the 973 Project (2015CB655001), the Science Fund for Creative Research Groups (Grant No. 20921061), and the National Natural Science Foundation of China (Grant Nos. 21574131, 91333205, 21674111, and 21322403). REFERENCES (1) Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-Generation Organic Photovoltaics Based on NonFullerene Acceptors. Nat. Photon. 2018, 12, 131-142.


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Conjugated

Polymer

Nanoparticles

Based

on

C3h

-Symmetric

Benzotrithiophene for Polymer Solar Cells. Macromol. Rapid. Commun. 2017, 38. (56) Huang, H.; Yang, L.; Facchetti, A.; Marks, T. J. Organic and Polymeric Semiconductors Enhanced by Noncovalent Conformational Locks. Chem. Rev. 2017, 117, 10291-10318. (57) Zhao, F.; Dai, S.; Wu, Y.; Zhang, Q.; Wang, J.; Jiang, L.; Ling, Q.; Wei, Z.; Ma, W.; You, W.; Wang, C.; Zhan, X. Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency. Adv. Mater. 2017, 29. (58) Bin, H.; Zhang, Z. G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657-4664. (59)Liu, Y.; Li, M.; Zhou, X.; Jia, Q.-Q.; Feng, S.; Jiang, P.; Xu, X.; Ma, W.; Li, H.-B.; Bo, Z. Nonfullerene Acceptors with Enhanced Solubility and Ordered Packing for High-Efficiency Polymer Solar Cells. ACS Energy Lett. 2018, 3, 1832-1839. (60) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange− Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51-57. (61) Brédas, J.-L. Organic Electronics: Does a Plot of the HOMO–LUMO Wave Functions Provide Useful Information? Chem. Mater. 2017, 29, 477-478. (62) Körzdörfer, T.; Brédas, J.-L. Organic Electronic Materials: Recent Advances in the DFT Description of the Ground and Excited States Using Tuned Range-Separated Hybrid Functionals. Acc. Chem. Res. 2014, 47, 3284−3291. (63) Liu, T.; Troisi, A. What Makes Fullerene Acceptors Special as Electron Acceptors in Organic Solar Cells and How to Replace Them. Adv. Mater. 2013, 25, 1038-1041.


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(64) Ma, H.; Troisi, A. Modulating the Exciton Dissociation Rate by up to More than Two Orders of Magnitude by Controlling the Alignment of LUMO + 1 in Organic Photovoltaics. J. Phys. Chem. C 2014, 118, 27272−27280. (65) Mihailetchi, V. D.; Koster, L. J. A.; Blom, P. W. M.; Melzer, C.; Boer, B. d.; Duren, J. K. J. v.; Janssen, R. A. J. Compositional Dependence of the Performance of Poly(p-phenylene vinylene):Methanofullerene Bulk-Heterojunction Solar Cells. Adv. Funct. Mater. 2005, 15, 795-801. (66) Koster, L. J. A. Origin of the Light Intensity Dependence of The Short-Circuit Current of Polymer/Fullerene Solar Cells. Appl. Phys. Lett. 2005, 87, 203502. (67) Cowan, S. R.; Banerji, N.; Leong, W. L.; Heeger, A. J. Charge Formation, Recombination, and Sweep-Out Dynamics in Organic Solar Cells. Adv.Funct. Mater. 2012, 22, 1116-1128. (68) Treat, N. D.; Varotto, A.; Takacs, C. J.; Batara, N.; Al-Hashimi, M.; Heeney, M. J.; Heeger, A. J.; Wudl, F.; Hawker, C. J.; Chabinyc, M. L. Polymer-Fullerene Miscibility: a Metric for Screening New Materials for High-Performance Organic Solar Cells. J. Am. Chem. Soc. 2012, 134, 15869-15879.


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Chart 1. (a) Molecular structures of TITT and BTCDT. (b) Molecular structures of BTCDTbased acceptors and J61 donor.


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Scheme 1. Synthetic routes of BTCDT, BTCDT-IC and BTCDT-ICF.


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Figure 1. (a) UV−vis absorption spectra of BTCDT-IC and BTCDT-ICF in chlorobenzene solution; (b) UV−vis absorption spectra of BTCDT-IC, BTCDT-ICF and J61 thin films; (c) Energy level diagram of J61, BTCDT-IC and BTCDT-ICF; (d) Optimized molecular geometries and frontier molecular orbitals obtained by DFT calculations for BTCDT-IC (top) and BTCDT-ICF (bottom).


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Figure 2. (a) J-V characteristics and (b) EQE spectra of the optimized OSCs based on J61/BTCDTIC and J61/BTCDT-ICF blends under illumination of an AM 1.5G at 100 mW cm-2.


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Figure 3. (a) PL spectra of neat J61 film, J61: BTCDT-IC blend film and J61: BTCDT-ICF blend film; (b) PL spectra of neat BTCDT-IC film and J61: BTCDT-IC blend film; (c) PL spectra of neat BTCDT-ICF film and J61: BTCDT-ICF blend film.


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Figure 4. The curves of photocurrent versus effective voltage (a) and JSC versus light intensity (b) in optimized devices.


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Figure 5. AFM height images and TEM images of J61: BTCDT-IC blend films (a, c) and J61: BTCDT-ICF blend films (b, d). The scale bar is 200 nm.


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Table 1. Absorption and electrochemical data of BTCDT-IC and BTCDT-ICF. UV (λmax [nm])

Eg opt b （eV）

HOMO c (eV)

LUMO c (eV)

Egec d (eV)

Solution

Film

εmax a (M−1 cm−1)

BTCDT-IC

634

666

2.8×105

1.69

-5.75

-3.87

1.88

BTCDT-ICF

644

688

3.0×105

1.62

-5.83

-3.98

1.85

Compound

a

Molar extinction coefficient at λmax in solution. b Optical bandgaps calculated from the absorption edge of thin film. c HOMO/LUMO energy level estimated from the onset of oxidation potential and reduction potential by CV measurement, respectively. d Electrochemical bandgaps calculated from HOMO/LUMO energy levels.


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Table 2. Photovoltaic parameters of the optimized BTCDT-IC and BTCDT-ICF-based OSCs devices. active layer

Voc [V]

Jsc [mA cm-2]

FF

PCE [%]

J61:BTCDT-IC a

0.92 (0.92 ±0.01)

9.75 (9.34 ±0.24)

59.0 (60.2 ±1.0)

5.30 (5.16 ±0.09)

J61:BTCDT-ICF b

0.73 (0.73 ±0.01)

16.93 (16.75 ±0.12)

65.6 (65.8 ±0.5)

8.11 (8.01 ±0.09)

a

Using 1% CN and thermal annealing at 160 oC for 10 min. annealing at 200 oC for 10 min.

b

Using 1% CN and thermal


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Table of Contents.


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Star-Shaped Fused-Ring Electron Acceptors with a C3h-Symmetric

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