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 electro-optic 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-C61-butyric acid methyl ester to form solar cell’s active layer, an optimized copolymer composition was found that gave the best photovoltaic performance.