A cyclopentadithiophene/thienopyrroledione-based donor–acceptor copolymer for organic solar cells

A new cyclopentadithiophene/thienopyrroledione-based donor–acceptor copolymer (P1) was synthesized using a microwave-assisted Stille coupling procedure and was compared to a known benzodithiophene-based copolymer using the same thienopyrroledione acceptor monomer (PBDTTPD). Cyclopentadithiophene-based copolymers have been known to exhibit lower band gaps than their corresponding benzodithiophene-based counterparts. The polymer showed excellent solubility at room temperature in chlorinated solvents. The absorption onset for P1 is close to 740 nm as compared with ∼685 nm for PBDTTPD, corresponding to an optical band gap of 1.67 eV, which is 0.15 eV lower than PBDTTPD. The photovoltaic characteristics of the polymer were determined under AM1.5 illumination. The P1:PCBM BHJ device showed a high Voc (0.92 V) and Jsc (8.02 mA/cm2) as well as a good PCE (2.43%), while the best device with 2% solvent additive gave a PCE of 3.47%.