Control in energy levels of conjugated polymers for photovoltaic application

A series of copolymers based on thieno [3,4-b]thiophene and thiophene unit have been synthesized. By controlling the ratio of thieno [3,4-b]thiophene to alkyl thiophene in the copolymer composition, the electrooptic properties of the copolymers can be fine tuned. It was shown that the energy gap of copolymers narrowed when the content of thieno [3,4-b]thiophene increased, brought by the decrease in lowest unoccupied molecular orbital and increase in highest occupied molecular orbital energy levels. When these copolymers were blended with [6,6]-phenyl-C-61-butyric acid methyl ester to form solar cell's active layer, an optimized copolymer composition was found that gave the best photovoltaic performance.