Electrical and electro reflectance studies on ortho-chloranil-doped polyaniline

The addition of ortho-chloranil to the emeraldine base form of polyaniline has been found to increase the dark current and the photocurrent by three orders of magnitude. An electron transfer from polyaniline to ortho-chloranil occurs producing holes in polyaniline. Since ortho-chloranil is an electron acceptor there is a general effect of it acting as an oxidizing agent upon the conductivity properties of polyaniline. The increase in photocurrent upon illumination is due to the transfer of electrons from excited polyaniline molecule to the ortho-chloranil negative ion producing further polyaniline holes and ortho-chloranil ions. The activation energy of polyaniline decreases with increasing doping concentrations. For doping concentrations from 4% (w/w) to 8% (w/w), the activation energy is halved when compared with that of pure polyaniline and it is one fourth of the value of pure polyaniline for 10% doping concentration. The band gap of polyaniline also decreases with increasing doping concentrations. On either side of the band gap, oscillatory structures are observed. On the lower energy level they are ascribed to excitons and on the higher energy level, they indicate structures in band gap system. The dissociation of excitons might take place at the interface at molecular level between polyaniline and ortho-chloranil molecules forming a charge-transfer complex, which explains the increase in conductivity upon the addition of ortho-chloranil. The presence of single transition point in electroreflectance spectra reveals that the mixture of polyaniline and ortho-chloranit behaves as a single entity and not as a simple mixture. The conduction mechanism in this charge-transfer complex is molecular rather than solidstate in nature. (c) 2006 Elsevier B.V. All rights reserved.