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 x 10(-3) cm(2)/Vs with an on/ off ratio of about 10(5). The maximum power conversion efficiency of polymer solar cell fabricated with the blend of EHex-CPDT-Se2/C-71-PCBM reached 1.86% with all open circuit voltage (V-OC) of 0 55 V, a short circuit current density (J(sc)) of 7.27 mA/cm(2), and a fill factor (FF) of 0 47 under AM 1.5G irradiation (100 mW/cm(2)) (C) 2009 Wiley Periodicals, Inc J Polym Sci Part A: Polym Chem 47 6873-6882, 2009