Star-Shaped Fused-Ring Electron Acceptors with a C 3 h -Symmetric and Electron-Rich Benzotri(cyclopentadithiophene) Core for Efficient Nonfullerene Organic Solar Cells.

Classical fused-ring electron acceptors (FREAs) with linear acceptor-donor-acceptor (A-D-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.