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

Abstract

Thermally stimulated luminescence (TSL) was studied in molecularly doped polymers (MDPs) consisting of substituted triphenylamine charge-transporting molecules and the polymer binder polystyrene at different doping concentrations. TSL measurements on some neat photoconducting polysilylenes and carbazole 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 approach was based on analysis of the fractional TSL data, from which we extracted a parameter characterizing the energetic disorder in MDPs. This parameter agreed well with the width of the DOS as determined from charge transport studies. In the present work, we studied the effect of dopant concentration 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 the mean activation energies as measured by fractional TSL, and a large decrease in the frequency factor. These effects are discussed in terms of decreased hopping frequency, and we estimated the wave function decay parameter.