Novel narrow‐band‐gap conjugated copolymers containing phenothiazine‐arylcyanovinyl units for organic photovoltaic cell applications

A series of novel narrow‐band‐gap copolymers ( P1 ‐ P12 ) composed of alkyl‐substituted fluorene (FO) units and six analogous mono‐ and bis(2‐aryl‐2‐cyanovinyl)‐10‐hexylphenothiazine monomers ( M1 ‐ M6 ) were synthesized by a palladium‐catalyzed Suzuki coupling reaction with two different feed in ratios of FO to M1 ‐ M6 (molar ratio = 3:1 and 1:1). The absorption spectra of polymers P1 ‐ P12 exhibited broad peaks located in the UV and visible regions from 400 to 800 nm with optical band gaps at 1.55–2.10 eV, which fit near the wavelength of the maximum solar photon reflux. Electrochemical experiments displayed that the reversible p‐ and n‐doping processes of copolymers were partially reversible, and the proper HOMO/LUMO levels enabled a high photovoltaic open‐circuit voltage. As blended with 6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) as an electron acceptor in bulk heterojunction photovoltaic devices, narrow‐band‐gap polymers P1 ‐ P12 as electron donors showed significant photovoltaic performance which varied with the intramolecular donor‐acceptor interaction and their mixing ratios to PCBM. Under 100 mW/cm² of AM 1.5 white‐light illumination, the device of copolymer P12 produced the highest preliminary result having an open‐circuit voltage of 0.64 V, a short‐circuit current of 2.70 mA/cm², a fill factor of 0.29, and an energy conversion efficiency of 0.51%. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4285–4304, 2008