Soluble narrow‐band‐gap copolymers containing novel cyclopentadithiophene units for organic photovoltaic cell applications

Five novel conjugated copolymers (P1–P5) containing coplanar cyclopentadithiophene (CPDT) units (incorporated with arylcyanovinyl and keto groups in different molar ratios) were synthesized and developed for the applications of polymer solar cells (PSCs). Polymers P1–P5 covered broad absorption ranges from UV to near infrared (400–900 nm) with narrow optical band gaps of 1.38–1.70 eV, which are compatible with the maximum solar photon reflux. Partially reversible p‐ and n‐doping processes of P1–P5 in electrochemical experiments were observed, and the proper molecular design for highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of P1–P5 induced the highest photovoltaic open‐circuit voltage in the PSC devices, compared with those previously reported CPDT‐based narrow‐band‐gap polymers. Powder X‐ray diffraction (XRD) analyses suggested that these copolymers formed self‐assembled π‐π stacking and pseudobilayered structures. Under 100 mW/cm2 of AM 1.5 white‐light illumination, bulk heterojunction PSC devices containing an active layer of electron donor polymers P1–P5 mixed with electron acceptor [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) in the weight ratio of 1:4 were investigated. The PSC device containing P1 gave the best preliminary result with an open‐circuit voltage of 0.84 V, a short‐circuit current of 2.36 mA/cm2, and a fill factor of 0.38, offering an overall power conversion efficiency (PCE) of 0.77% as well as a maximal quantum efficiency of 23% from the external quantum efficiency (EQE) measurements.