A Novel Alternating Phenylenevinylene Copolymer with Perylene Bisimide Units: Synthesis, Photophysical, Electrochemical, and Photovoltaic Properties

An alternating phenylenevinylene copolymer P bearing perylene bisimide moieties along the backbone was synthesized by Heck coupling. The tert-butyl and hexyloxy side groups enhanced the solubility of the copolymer in common organic solvents. It had glass transition and decomposition temperatures of 72 and 370 degrees C, respectively. The absorption of P was broad, with longer wavelength maximum around 500 nm and optical band gap of 1.66 eV. The solution of P emitted yellow-orange light with photoluminescence (PL) maximum at 555 nm. Moreover, efficient interaction and charge/energy transfer between the phenylenevinylene-donor and the perylene bisimide-acceptor led to PL quenching in the thin film of P. The current-voltage characteristics of ITO/copolymer P/Al indicated that copolymer P behaved as an n type organic semiconductor. The electron current was found to be a space charge limited current (SCLC), providing a direct measure of mobility as a function of temperature and field. The average electron mobility for this copolymer was 0.85 x 10(-2) cm(2)/Vs. The photogeneration mechanism in blends of copolymer P (electron acceptor) with poly(3-phenyl hydrazone thiophene) (PPHT) (electron donor) were investigated. Annealing of the complete device was found to result in an increase in power conversion efficiency from 1.67% to 2.32%. By studying the dependence of photocurrent on effective bias voltage, annealing was found to increase the charge generation efficiency through an increase in the efficiency of separation of exciton following the charge transfer.