Synthesis of random copolymers based on 3-hexylthiophene and quinoxaline derivative: Influence between the intramolecular charge transfer (ICT) effect and π-conjugation length for their photovoltaic properties

A series of low band gap, donor-acceptor polymers composed of regioregular 3-hexylthiophene segments and quinoxaline derivative units were synthesized by Stille coupling polymerization. The polymers had relatively low optical band gaps ranging from 1.61 to 1.83 eV. A bulk-heterojunction structure of glass/indium-tin oxide (ITO)/PEDOT:PSS/polymer-PCBM (1:3)/BaF2/Ba/Al was fabricated to examine the photovoltaic properties. 1-(3-Methoxycarbonyl)propyl-1-phenyl-[6,6]-C-71 (PC71BM) was used as the acceptor material owing to its increased absorption property in the visible region compared to 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PC61BM). Among these polymers, P1 showed the best device performance with a PCE of 0.88%. These results provided an effective strategy for the design and synthesis of low band gap conjugated polymers with a broad range of absorption.