Properties of an alternating copolymer and its applications in LEDs and photovoltaic cells

To obtain a dual-functional polymeric semiconductor for efficient red-light emission and high-performance photovoltaic cells, an alternating copolymer containing triphenylamine (TPA; as donors) and cyano-substituted phenylene (as acceptors), TPA-CNPPV, was synthesized by Wittig reaction. The polymer was characterized by FT-IR, 1H-NMR, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Cyclic voltammetry (CV) measurements showed that the polymer presented reversible oxidation and reduction with relative low potentials, suggesting its good electron and hole-injection abilities. The photophysical properties of the light-emitting material were investigated in both solution and spin-coated film. Efficient red photoluminescence (PL; at 641 nm) was observed in the film. TPA-CNPPV possessed good film-forming property and thermal stability, lower band gap, efficient photoluminescence, good hole-injection ability and high electron affinity, which are highly desirable for realizing red-light emission in light-emitting diodes (LEDs) and achieving high-performance photovoltaic cells. This point was supported by the fabrication and characterization of optoelectronic devices. The single-layer light-emitting device of ITO/TPA-CNPPV/Al had relative high external quantum efficiency (0.06%), which was increased to 0.35% with PVK as hole-injection transport layer and Ba as cathode. ITO/PEDOT/TPA-CNPPV/Ba/Al showed dual function in both light-emitting devices and photodiodes. Photovoltaic cells with pristine TPA-CNPPV had energy conversion efficiency of 0.1% at 500 nm. The efficiency was improved to 1.1% under AM1.5 (78.2 mW/cm2) with a TPA-CNPPV/C60 weight ratio of 1:4.