Fine Tuning of HOMO Energy Levels for Low-Band-Gap Photovoltaic Copolymers Containing Cyclopentadithienopyrrole and Bithiazole Units

Five conjugated LBG polymers P1-P5 consisting of a DTP unit as an electron donor and various bithiazole units as electron acceptors are designed and synthesized for photovoltaic applications. The effects of the electron-deficient bithiazole derivatives on the thermal, optical, electrochemical, and photovoltaic properties are investigated. The molecular structures are confirmed and characterized. The resulting polymers have high thermal stabilities and broad absorption bands with low optical bandgaps. The hole and electron mobilities are calculated. PSC devices are fabricated utilizing the polymers as electron donors and PCBM as an electron acceptor. The PSC device with an active layer of P4:PCBM (1:1 by weight) exhibits the best power-conversion efficiency.