Two p-type semiconducting donor-acceptor polymers were designed and synthesized for use in organic solar cells. The polymers combine a benzodithiophene (BDT) donor and a thiazole-fused benzothiadiazole (TzBT) acceptor. Two TzBT acceptor units are compared, one with an alkylthio group (P1) and the other with a more strongly electron-withdrawing alkylsulfonyl group (P2) at the fused thiazole ring. The strongly electron-accepting nature of the TzBT unit lowers the lowest unoccupied molecular orbital (LUMO) energy of P1 and P2 relative to that of the BT analog (PBDT-BT), without altering the energy of the highest occupied molecular orbital (HOMO). Despite the smaller optical band gaps, bulk heterojunction organic solar cells fabricated using these polymers in a PC 71 BM blend showed high open-circuit voltages. The power conversion efficiency (PCE) of the P1-based device reached 6.13%. Though efficiency of the P2-based device was lower, photoelectric conversion extended into the near-IR region up to 950 nm.

2,1,3-Benzothiadiazole (BT) is a widely used electron-accepting unit in organic electronics including organic solar cells (OSCs). As modifications of BT skeleton, two types of azolefused BT units were designed and synthesized; thiazole-fused BT with an electronwithdrawing C=N bond and imidazole-fused BT with an electron-donating nitrogen atom as well as an electron-withdrawing C=N bond. Electrochemical measurements and theoretical calculations suggest that thiazole-fusion enhances the electron-accepting ability, whereas imidazole-fusion endows the BT skeleton with electron-donating ability while maintaining its electron-accepting ability. Moreover, in thiazole-fused BT units, the electronic structure could be further modulated by varying the oxidation state of the sulfur atom in methylthio group at the fused thiazole ring.