Subtle structural modification of thiophene-fused benzotriazole unit to simultaneously improve the JSC and VOC of OSCs

Abstract

In this work, a new electron-accepting unit has been designed through introducing two F atoms simultaneously at α,β-positions of the thiophene fused benzotriazole unit (ffBTAZT) to build a new D–A type polymer donor (PffBTAZT-fBDT) with -4,8-bis(5-alkyl-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene unit (fBDT) as an electron-donating unit. Different from the mono α-fluorinated thiophene-fused benzotriazole (fBTAZT) unit, another F atom introduced at the β-position of the fused thiophene ring of the fBTAZT unit can build an “F⋯HC” non-covalent interaction with its adjacent thiophene ring as a “π-bridge” to partially lock the conformation of the conjugated backbone and strengthen the rigidity of the molecular skeleton, which is favorable for the light harvesting efficiency and the charge mobility to improve the short-circuit current density (J_SC). At the same time, the introduction of a strongly electronegative F atom can further reduce the HOMO energy level of the polymer donor, which is also favorable for the enhancement of the open-circuit voltage (V_OC). Therefore, the photovoltaic performances of the PffBTAZT-fBDT based OSCs with Y6 acceptor have been comprehensively improved compared with PfBTAZT-fBDT based OSCs, in which the corresponding J_SC, V_OC, fill factor (FF) and photoelectric conversion efficiency (PCE) have been increased from 23.68 to 25.43 mA cm⁻², 0.633 to 0.817 V, 58.46 to 69.94% and 8.77 to 14.53%, respectively.