Synthesis of thieno[3,4-d]thiazole-based conjugated polymers and HOMO level tuning for high V_OC photovoltaic cell

An electron-accepting building block, thieno[3,4-d]thiazole (TTz) was synthesized through a new synthetic route. When it was alternatively copolymerized with electron-donating fluorene, carbazole, or benzodithiophene derivatives via coupling reaction, all of the three copolymers had the low-lying lowest occupied molecular orbital energy levels around -3.55 eV, while the highest occupied molecular orbital (HOMO) energy level of the copolymer was dependent upon the electron-donating ability of electron-donating unit in the copolymer. When a fluorene derivative, the weakest electron-donating unit, was used as electron-donating building block, the copolymer showed the lowest HOMO energy. The alternating copolymer composed of TTz and fluorene derivative exhibits a high V_OC of 0.91 eV, because a weak electron-donating fluorene lowers the HOMO level of the alternating copolymer.