Side-chain effects on phenothiazine-based donor-acceptor copolymer properties in organic photovoltaic devices

A series of new phenothiazine-based donor-acceptor copolymers, P1 and P2, were synthesized via a Suzuki coupling reaction. The weight-averaged molecular weights (M _w) of P1 and P2 were found to be 16,700 and 16,100, with polydispersity indices of 1.74 and 1.39, respectively. The UV-visible absorption spectra of the polymer thin films contained three strong absorption bands in the ranges 318-320 nm, 430-436 nm, and 527-568 nm. The absorption peaks at 320 and 430 nm originated mainly from the phenothiazine-based monomer units, and the longer wavelength absorption band at 527-568 nm was attributed to the increased effective conjugation length of the polymer backbones. Solution-processed field-effect transistors fabricated with these polymers exhibited p-type organic thin film transistor characteristics. The field-effect mobilities of P1 and P2 were measured to be 1.0 × 10 ^-4 and 7.5 × 10 ^-5 cm ² V ^-1 s ^-1, respectively, with on/off ratios in the order of 10 ⁴ for all polymers. A photovoltaic device in which a P2/PC ₇₁BM (1/3) blend film was used as the active layer exhibited an open-circuit voltage (V _OC) of 0.70 V, a short-circuit current (J _SC) of 6.79 mA cm ⁽², a fill factor of 0.39, and a power conversion efficiency of 1.86% under AM 1.5 G (100 mW cm ^-2) illumination.