Synthesis of Quinoxaline-Based Donor−Acceptor Narrow-Band-Gap Polymers and Their Cyclized Derivatives for Bulk-Heterojunction Polymer Solar Cell Applications

A series of narrow-band-gap donor−acceptor (D−A) conjugated polymers, with thiophene-substituted quinoxaline monomer 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(5-octylthiophen-2-yl)quinoxaline (TTQx) or its cyclized phenazine derivative monomer 8,11-bis(5-bromothiophen-2-yl)-2,5-dioctyldithieno[2,3-a:3′,2′-c]phenazine (TTPz) as acceptors, were synthesized via Suzuki coupling reaction. It was found that the copolymers based on thiophene-substituted quinoxaline TTQx exhibit obviously red-shifted absorbance compared to previously reported D−A copolymers based on phenyl-substituted quinoxaline. Their analogous copolymers based on the cyclized TTPz acceptor show more pronounced red-shifted absorption spectra with a significantly decreased band gap due to the enlarged planar polycyclic aromatic ring of TTPz. Moreover, compared to the copolymers based on TTQx, the TTPz-based copolymers' mobilities are also significantly increased due to the reduced steric hindrance and improved structural planarity among the copolymers. Bulk-heterojunction polymer solar cells based on the blends of the copolymers with a fullerene derivative as an acceptor exhibit promising performance, and the best device performance with power conversion efficiency up to 4.4% was achieved.