Polaron effects on thermally stimulated photoluminescence in disordered organic systems

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Abstract
Thermally stimulated photoluminescence (TSPL) in disordered organic materials subjected to the polaron formation was studied both theoretically and experimentally. Since TSPL measurements were performed after some dwell time on samples, which have been photoexcited at helium temperature, the initial energy distribution of polarons was formed in the course of low-temperature energy relaxation. In order to calculate this distribution, an analytic model of low-temperature energy relaxation of polarons in a disordered hopping system was formulated. This model proves that energy relaxation of polarons is much slower than that of charge carriers in a similarly disordered but rigid hopping system. At moderate values of the polaron binding energy the suppressed rate of low-temperature relaxation leads to a shift of TSPL curves to lower temperatures as compared to what could be expected in the absence of the polaronic effects. This shift is both obtained from analytic calculations and observed experimentally in some polysilylene derivatives.