Synthesis and characterization of low‐bandgap cyclopentadithiophene‐biselenophene copolymer and its use in field‐effect transistor and polymer solar cells

Novel series of conjugated copolymers, incorporating cyclopentadithiophene (CPDT) and the biselenophene (R‐CPDT‐Se2), were synthesized by Pd‐catalyzed Stille coupling polymerization. The optical, electrochemical, field‐effect carrier mobilities, and photovoltaic properties of the R‐CPDT‐Se2 were investigated and compared with cyclopentadithiophene (CPDT) and the bithiophene (EHex‐CPDT‐T2). The highest hole mobility of thin film transistor devices fabricated with new p‐type polymer semiconductors, Oct‐CPDT‐Se2, was 1.3 × 10−3 cm2/Vs with an on/off ratio of about 105. The maximum power conversion efficiency of polymer solar cell fabricated with the blend of EHex‐CPDT‐Se2/C71‐PCBM reached 1.86% with an open circuit voltage (VOC) of 0.55 V, a short circuit current density (Jsc) of 7.27 mA/cm2, and a fill factor (FF) of 0.47 under AM 1.5G irradiation (100 mW/cm2).