Fine-Tuning of Molecular Energy Level of Alternating Copolymers On the basis of [1,2,5]Thiadiazolo[3,4-g]quinoxaline Derivatives for Polymer Photovoltaics

A series of low-bandgap alternating copolymers consisting of electron-accepting [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) derivatives and electron-donating fluorene or carbazole were synthesized via the Suzuki coupling reaction. For the purpose to fine-tune the molecular energy level of alternating copolymers and thus to improve charge transfer between polymers and PCBM, two different TQ derivatives substituted with strongly electron-donating butoxy group or weakly electron-donating thienyl group were synthesized and used as a building block of alternating copolymers. Copolymers with butoxy-substituted TQ have proper lowest unoccupied molecular orbital (LUMO) energy levels for effective charge dissociation between polymer and PCBM, whereas the LUMO levels of copolymers with thienyl-substituted TQ are too close to that of PCBM to be effective for charge dissociation. The power conversion efficiency was achieved up to 2.17%, which is the highest value among the TQ-based polymer solar cells, when the blend of copolymer with butoxy-substituted TQ and [6,6]-phenyl-C71-butyric acid methyl ester was used as an active layer material in bulk heterojunction solar cells.