Exciton Seebeck in Molecular Systems

Various applications relevant to the exciton dynamics, such as the organic solar cell, the large-area organic light-emitting diodes and the thermoelectricity, are operating under temperature gradient. The potential abnormal behavior of the exicton dynamics driven by the temperature difference may affect the efficiency and performance of the corresponding devices. In the above situations, the exciton dynamics under temperature difference is mixed with the charge transportation through exciton dissociation and electron-hole recombination. This problem is not studied yet and the poor understanding of this mechanics hinders the further improvement of the above mentioned applications. On the other hand, the dynamics of electron and spin under temperature gradient have been well understood as Seebeck effect and spin Seebeck effect, respectively. We investigate the exciton dynamics in molecular systems under temperature with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a donor-bridge-acceptor model, we find that the temperature difference causes exciton population redistribution and affects the exciton transfer time though it almost does not have any impact on the light-matter interaction. This phenomenon can be fully accounted by tuning the site energy without changing the transition energies inside each site. In this sense, there exists correspondence between the site energy tuning and the temperature difference. This phenomenon is named as exciton Seebeck effect due to its close similarity to the Seebeck effect and spin Seebeck effect. This finding may offer new strategy to improve the performance of the large-area OLED and thermoelectric materials.