The origin of thermally stimulated luminescence in neat and molecularly doped charge transport polymer systems

Thermally stimulated luminescence (TSL) was studied in molecularly doped po lymers (MDPs) consisting of substituted triphenylamine charge-transporting molecules and the polymer binder polystyrene at different doping concentrat ions. TSL measurements on some neat photoconducting polysilylenes and carba zole containing polymers were also carried out and are shown for comparison . The observed trends in the TSL can be reasonably well understood in terms of the Gaussian disorder model, as the thermal release of charge carriers from the tail states of the density-of-states (DOS) distribution. Our appro ach was based on analysis of the fractional TSL data, from which we extract ed a parameter characterizing the energetic disorder in MDPs. This paramete r agreed well with the width of the DOS as determined from charge transport studies. In the present work, we studied the effect of dopant concentratio n on the TSL properties of MDPs. The most remarkable feature in the TSL of dilute MDPs is a considerable weakening of the temperature dependence of th e mean activation energies as measured by fractional TSL, and a large decre ase in the frequency factor. These effects are discussed in terms of decrea sed hopping frequency, and we estimated the wave function decay parameter. (C) 1999 Elsevier Science B.V. All rights reserved.