A low-bandgap polymer bearing the N-octyl-2,7-dithia-5-azacyclopenta[alpha]pentalene-4,6-dione electron-withdrawing unit

Abstract

A low-bandgap PBDTDTPD polymer is synthesized. It is a donor-acceptor-type polymer that consists of dialkylthiophenyl benzodithiophene (BDT) donor unit and N-octyl-2,7-dithia-5-aza-cyclopenta[a]pentalene-4,6-dione (DTPD) acceptor unit. The thienothiophene-fused strong-electron-withdrawing DTPD effectively reduces the bandgap of the resulting polymer and keeps its highest occupied molecular orbital low. Moreover, PBDTDTPD is thermally stable and dissolves well in typical organic solvents. PBDTDTPD:PC71BM(1:1) blend film showed a power conversion efficiency of 3.18% and a largely elevated open-circuit voltage of 0.75 V compared to that of well-known P3HT:PC61BM blend film. In this work, we prove that the DTPD moiety can be utilized to develop a future low-bandgap material that can absorb a broad wavelength of light and have high thermal stability, which are important characteristics for next-generation high-performance organic solar cells.