Hopping model of thermally stimulated photoluminescence in disordered organic materials

An analytical model of thermally stimulated photoluminescence (TSPL) in a random hopping system is formulated. The model is based on the assumption that TSPL originates from radiative recombination of sufficiently long geminate pairs of charge carriers created during photoexcitation of the sample at a low (helium) temperature. Since TSPL measurements are normally performed after some dwell time the initial energy distribution of localized carriers is formed after low-temperature hopping relaxation of photogenerated carriers and, therefore, first thermally assisted jumps of relaxed carriers are considered as rate-limiting steps in the present model. Predictions of the model are found to be in good quantitative agreement with experimental data on molecularly doped polymers if a double-peak energy distribution of localized states is invoked for these materials. Comparing theoretical results with existing experimental data also reveals a somewhat slower low-temperature energy relaxation of charge carriers in these materials than predicted by the conventional theory of carrier random walk in random hopping systems. (C) 2001 Elsevier Science B.V. All rights reserved.