Thermally activated electron-hole-lattice cooperative effect on the excited state in an organic nonfullerene acceptor molecule.

Thermal effects have been experimentally observed to promote spontaneous charge separation in nonfullerene acceptor (NFA) based organic solar cells, yet a comprehensive theoretical description accounting for the coupled electron-hole-lattice response to temperature is still lacking. Herein, we develop a theoretical framework based on an extended Su-Schrieffer-Heeger tight-binding model that simultaneously incorporates lattice thermal perturbations and electron-hole thermal excitations. Calculations reveal that, owing to the intrinsic push-pull electronic structure of a NFA molecule, electrons and holes exhibit distinctly different thermal excitation behaviors. Specifically, when the molecular electronic push-pull potential is strengthened (characterized by the electron-push and electron-pull abilities of the molecular central- and terminal-groups), the energy barrier for electron thermal excitation decreases, whereas that for the hole increases, thus enabling selective modulation of their thermal excitation probabilities. Furthermore, we uncover the essence of the thermally activated electron-hole-lattice cooperative effect, which synergistically combines lattice thermal perturbations and asymmetric electron-hole thermal excitations under the instantaneous lattice configuration. This cooperative effect significantly enhances the charge transfer character of an excited state, thereby weakening the binding energy and promoting spontaneous charge separation. Notably, this effect can be effectively amplified by strengthening the electronic push-pull potential. These findings provide a microscopic understanding of how thermal effects promote spontaneous charge separation in NFA molecules, and underscore the crucial role of molecular push-pull electronic structure in modulating the thermal response of excited states.