Molecular structure-property engineering for photovoltaic applications: Fluorene-acceptor alternating conjugated copolymers with varied bridged moieties

A series of novel soluble conjugated copolymers consisting of electron-accepting 2-pyran-4-ylidenemalononitrile (PM) and electron-donating fluorene connected by different electron-donating ability conjugated 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 conjugated bridge between PM and fluorene building block with gradually increased electron-donating ability moieties results in enhanced intramolecular charge transfer (ICT) transition bands, 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 C₆₁-butyric acid methyl ester (PCBM) as acceptor. The power conversion efficiencies (PCE) of the devices were in the range of 0.02-0.52% under simulated AM 1.5 solar irradiation of 100 mW/cm², and the highest V_oc reached 0.82 V. The significant improvement of PCE indicates a novel concept for developing donor-acceptor (D-A) conjugated copolymers with high photovoltaic performance by adjusting electron-donating ability of conjugated bridge.