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

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 x 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 x 10(-3) cm(2) 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.