Influence of Difluoro Regio-Substitution Variations of Donor Polymers on the Non-Fullerene Solar Cell Performance

This study presents the synthesis of three donor–π–acceptor-type donor polymers, namely, PcBTT12ffB, PcBTT13ffB, and PcBTT14ffB, for use in polymer solar cells (PSCs). These polymers feature a backbone incorporating a benzo[1,2-b:4,5-b′]dithiophene-based unit as the donor moiety, a thieno[3,2-b]thiophene-based unit as the π-bridge, and three different difluoro-substituted benzene units as the acceptor moieties. They demonstrate relatively wide optical band gaps of approximately 2.1 eV, which exhibit complementary absorption profiles with the Y6 acceptor. PSCs were fabricated and studied using the active layer of a polymer/Y6 blend. Among the three PSCs, the PcBTT14ffB device exhibited the best performance, with a Jsc of 23.34 mA/cm2, a Voc of 0.866 V, an FF of 0.703, and a PCE of 14.15%. Morphology and device physics studies revealed that PcBTT14ffB achieved better crystallinity, mobility, and phase-separation scale compared with the other two substitution analogues in blend films. Additionally, the PcBTT14ffB device showed a relatively small Eloss of 0.544 eV due to the low radiative recombination energy loss. This work not only highlights the synthesis of wide-gap donor polymers using difluorobenzene units but also provides a systematic study of how fluorine regio-substitution variations in conjugated polymers influence the charge transport and PSC performance.