Energy Level and Molecular Structure Engineering of Conjugated Donor-Acceptor Copolymers for Photovoltaic Applications

A series of novel soluble conjugated copolymers consisting of coplanar donor (bithiophenevinyl)-acceptor (2-pyran-4-ylidenenemalononitrile) (TVM)-based unit coupled to different electron-donating ability moieties were synthesized by Suzuki coupling polymerization. The structures of the copolymers were characterized, and their physical properties were investigated. High molecular weight (M-n up to 43.8 kg/mol) and thermostable copolymers were obtained. The combination of TVM unit building block with gradually increased electron-donating ability moieties results in enhanced pi-pi stacking in solid state and intramolecular charge transfer (ICT) transition hands, which lead to an extension of their absorption spectral range. Cyclic voltammetry measurement displayed that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers can be fine-tuned. The resulting copolymers possessed relatively low HOMO energy levels promising good air stability and high open-circuit voltage (V-oc) for photovoltaic application. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)-phenyl C-61-butyric acid methyl ester (PCBM) as acceptor. It was found that the V-oc reached 0.90 V, and the power conversion efficiencies (PCE) of the devices were measured between 0.04% and 0.99% under simulated AM 1.5 solar irradiation of 100 mW/cm(2). The significant improvement of PCE indicates a novel concept for developing TVM-based donor-acceptor (D-A) Conjugated copolymers with high photovoltaic performance by adjusting electron-donating ability and coplanarity.