Synthesis and photovoltaic properties of low band gap copolymers containing (bithiophenevinyl)-(2-pyran-4-ylidenemalononitrile) (TVM) moieties

Three novel conjugated copolymers containing a coplanar bithiophenevinyl-(2-pyran-4-ylidenemalononitrile) unit linked with different electron-donating moieties by a double bond were synthesized via Heck polycondensation reactions and characterized using nuclear magnetic resonance, gel permeation chromatography and elemental analysis. Thermogravimetric analysis of the copolymers revealed that they have good thermal stability, with a decomposition temperature >330?°C. By eliminating torsional interactions between donor and acceptor units, combination by double bonds resulted in an enhancement of the conjugated length, extension of the absorption spectral range and a low band gap of the polymers. Cyclic voltammetry measurements suggest that the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels of the copolymers can be fine-tuned by introducing donor units with different electron-donating ability. Bulk-heterojunction photovoltaic cells were fabricated with the copolymers as the donors and (6,6)-phenyl C71-butyric acid methyl ester as the acceptor. The cells based on the three copolymers exhibited power-conversion efficiencies of 0.36, 0.41 and 0.47% under one sun of air mass 1.5 solar simulator illumination (100?mV?cm-2).