Hopping model of thermally stimulated photoluminescence in disordered organic materials

An analytical model of thermally stimulated photoluminescence (TSPL) in a r andom hopping system is formulated. The model is based on the assumption th at TSPL originates from radiative recombination of sufficiently long gemina te 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 carrier s is formed after low-temperature hopping relaxation of photogenerated carr iers 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 dat a on molecularly doped polymers if a double-peak energy distribution of loc alized states is invoked for these materials. Comparing theoretical results with existing experimental data also reveals a somewhat slower low-tempera ture energy relaxation of charge carriers in these materials than predicted by the conventional theory of carrier random walk in random hopping system s. (C) 2001 Elsevier Science B.V. All rights reserved.