Efficient donor-acceptor type polymer semiconductors with well-balanced energy levels and enhanced open circuit voltage properties for use in organic photovoltaics

Three donor-acceptor type copolymers, poly[9,9'-dioctylfluorene-alt-2,5-bis(seleno-2-yl)-2,3-bis(4-hexyloxyphenyl) quinoxaline] (PFSeQ), poly[9-(1'-octylnonylidene)fluorene-alt-2,5-bis(seleno-2-yl)2,3-bis(4-hexyloxyphenyl) quinoxaline] (PAFSeQ), and poly[N-9'-heptadecanyl-2,7-carbazole-alt-2,5-bis(seleno-2-yl)-2,3-bis(4-hexyloxyphenyl) quinoxaline] (PCSeQ), were synthesized through Suzuki coupling reactions for use as organic photovoltaic materials with high V(OC) values. In these polymers, selenophene, which has stronger electron-donating property than thiophene, was adopted as a spacer. The polymers had relatively low optical band gaps (<1.9 eV). We investigated bulk heterojunction type polymer solar cells based on the synthesized polymers used as electron donor materials and [6,6]-phenyl C61 butyric acid methyl ester (PC(61)BM) or [6,6]-phenyl C71 butyric acid methyl ester (PC(71)BM) used as the acceptor. The power conversion efficiency (PCE) of the devices was in the range of 1.1-3.3% under AM 1.5G illumination (100 mW cm(-2)). Among these polymers, PFSeQ exhibited the best device performance with a power conversion efficiency (PCE) of 3.3% upon the introduction of PC(71)BMas the acceptor. This is the highest performance among photovoltaic materials containing selenophene units.