Abstract

A series of nonfullerene acceptors (IOTC, IETC, IOPC and IEPC) have been designed and synthesized by using asymmetric indenothiophene as an electron-donating core. Different side chains (linear 4-n-octylphenyl and branched 4-(2-ethylhexyl)phenyl), and electron-withdrawing units (2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile and 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile) were used to tune the optoelectronic properties of the resulting indenothiophene-based nonfullerene acceptors. With a benchmark p-type copolymer of PBDB-T, the best-performance binary devices based on IOTC and IETC (both with the ending group of 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile) exhibited power conversion efficiencies (PCEs) of 7.94% and 7.40%, respectively. Whereas, the devices based on IOPC and IEPC (both with the ending group of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile) showed relatively low PCEs of 4.86% and 5.26%, respectively. The improved photovoltaic properties of IOTC- and IETC-based binary devices are mainly attributed to their higher hole- and electron-mobilities compared to those of IOPC- and IEPC-based counterparts. In order to improve the light-harvesting ability of the binary PBDB-T:IOTC in the short wavelength region, PC71BM was introduced as the second acceptor to form a ternary blend of PBDB-T:IOTC:PC71BM. After optimization, the ternary blend of PBDB-T:IOTC:PC71BM with a weight ratio of 1:1:0.5 achieved a best PCE of 9.85% with a short-circuit current density (Jsc) of 16.35 mA/cm2, and a fill factor (FF) of 64.90%, much higher than the corresponding binary blend based on either PBDB-T:IOTC or PBDB-T:PC71BM. We further demonstrated that the addition of PC71BM not only improved the light absorption but also enhanced the carrier mobilities of the ternary blend thereby increasing the final PCE.

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