Temperature Effects on the Dynamics of Photoexcitations in Conjugated Polymers

Within a tight-binding electron phonon interaction model modified to include temperature effects, the dynamics of photoexcitations in conjugated polymers are investigated using a nonadiabatic evolution method. For the photoexcited state from the highest occupied energy level (HOMO) to the lowest unoccupied energy level (LUMO), it is found that with increasing temperature, the stability of the polaron-excitons becomes weak. Furthermore, by showing time evolution of the localized energy levels at different temperatures, a blue-shift in the photoluminescence spectrum is observed. Yields of the polaron-excitons are calculated over the temperature range 5-300 K. It is found that the yield of polaron-excitons decreases with increasing temperature, and involves two different physical processes at lower and higher temperatures. For the photoexcited state from HOMO-1 to LUMO+1, we focus on the charged polaron pair lifetimes and the yield of polaron-excitons. The results show that the temperature effects shorten the lifetime of the charged polaron pair and cause the charged polaron pairs to easily merge into polaron-excitons, while the stability of the polaron-excitons formed from merging of the charge polaron pairs is influenced by temperature. As a result, the yield of the polaron-excitons first increases and then decreases in the processes.