Selenium-Containing Medium Bandgap Copolymer for Bulk Heterojunction Polymer Solar Cells with High Efficiency of 9.8%

In this work, a new D–A copolymer based on m-alkoxyphenyl substituted benzodithiophene (BDT-m-OP) as donor unit and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) as acceptor unit was designed and synthesized, in which selenophene unit as π-conjugated spacer was incorporated into the polymer backbone to broaden the absorption spectrum, enhance the charge transport properties, and even improve the photovoltaic properties. Compared with PBPD-Th with thiophene as π-conjugated spacer, PBPD-Se exhibits an evidently extended absorption spectrum and an enhanced hole mobility with a slightly raised HOMO energy level. The PBPD-Se:PC71BM-based PSCs exhibits a significantly improved PCE of 9.8% with an enhanced Jsc of 14.9 mA cm–2 and a slightly lower Voc of 0.90 V in comparison with a PCE of 8.4% with a Voc of 0.95 V and a Jsc of 12.4 mA cm–2 for PBPD-Th:PC71BM-based devices. These results indicate that the rational selection of π-conjugated spacer in the D–A copolymer backbone is very essential to achieve high efficiency PSCs.