Solid state conductivity and photoconductivity studies of an ion-exchange polymer/dye system

摘要

Abstract: Various methods have been used to investigate photocurrent generation and charge transport in a system composed of zinc meso-tetraphenylporphyrin (ZnTPP) incorporated in an ion- exchange polymer which is a blend of polyvinylpyridine-costyrene and a random ternary copolymer with cationic sites. Thin films of this mixture were cast onto SnO$-2$/ optically transparent electrodes and the conductivity cells were then completed by the evaporation of gold contacts on top of the films. Measurements of the dark current as a function of applied voltage, performed on polymer films with or without ZnTPP, show the existence of traps at a density of approximately 4 $MUL 10$+14$/ cm$+$MIN@3$/ which is left unchanged by the presence of the dye molecules. This also has little effect on the charge mobility which was found to vary from 3 to 4 $MUL 10$+$MIN@9$/ cm$+2$/V$+$MIN@1$/s$+$MIN@1$/. The temperature dependence of the dark conductivity, however, shows that the activation energy for the conduction process changes from a $Delta@E of approximately 4 eV for the polymer alone, to a $Delta@E of approximately 2 eV for the dye/polymer system, in the same temperature range. Photoconductivity measurements performed in the visible on the dye/polymer system clearly indicate that light absorption by ZnTPP is responsible for the observed photocurrents. The response times for the photoinduced currents, during a light- on/light-off sequence, support a trap limited conduction process. The isothermal time dependence of photocurrent decay indicates the existence of a narrow trap distribution with an E$-t$/ of approximately 0.8 eV, which is assumed to lie below the first excited singlet state of ZnTPP. A model based essentially on the energetic relation between the system's individual components, which invokes the distributions of donor and acceptor states associated to the polymer blend's pendant groups, is proposed to explain these observations. !