Thermally stimulated luminescence in electrolummescent polymers

Low-temperature. thermally stimulated luminescence (TSL) technique was applied to study distribution of traps in methyl-substituted ladder-type poly(paraphenylene) (MeLPPP), a polymer with greatly reduced disorder. We assume that the localized states within the tail of the, DOS distribution can act as traps in MeLPPP at very low temperatures. The observed characteristic features of TSL in MeLPPP can be explained in terms of the hopping model of carrier thermally-assisted relaxation: Predictions of the model are found to be in good quantitative agree-ment with experimental data and interpretation of the TSL data yields the effective DOS width of 0.055 eV in good agreement with the value of 0.05 eV obtained from the transport measurements. It was found that both fluore-scence and phosphorescence contribute to the TSL spec-trum, whereas only the latter was observed in the long isothermal afterglow following photoexcitation of the sample at helium temperature. This implies,the binding energy of,a short off-chain geminate pair higher than the binding energy of a singlet excitation but lower than that of a triplet exciton.