A narrow-bandgap donor polymer for highly efficient as-cast non-fullerene polymer solar cells with a high open circuit voltage

Abstract A new narrow-bandgap copolymer PBFTT , based on 4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDT-2F) as donor unit and octyl-3-fluorothieno[3,4- b ]thiophene-2-carboxylate (TT) as acceptor unit, was synthesized as donor material for non-fullerene (NF) polymer solar cells (PSCs) application. Compared to the analogue polymer PTB7-Th, PBFTT with two fluorine atoms in the conjugated side chains of BDT possesses a deeper HOMO energy level of −5.47 eV and a slightly enhanced hole mobility of 3.13 × 10 −3  cm 2  V -1  s -1 versus ( vs .) a HOMO energy level of −5.30 eV and a hole mobility of 1.11 × 10 −3  cm 2  V -1  s -1 for PTB7-Th. Hence, the corresponding PSCs based on PBFTT :ITIC exhibited an optimal PCE of 9.1% with a V oc of 0.94 V, a J sc of 16.0 mA cm −2 , and a FF of 60.5% compared to a PCE of 6.8% with a V oc of 0.81 V, a J sc of 14.2 mA cm −2 and a FF of 59.1% for PTB7-Th:ITIC-based PSCs. Notably, this PCE is among the highest values for the NF-PSCs based on narrow-bandgap polymer without extra treatments. These results indicate that PBFTT is a promising narrow-bandgap polymer donor material for NF-PSCs application.