Kinetic Monte Carlo Simulation of Exciton Dynamics in Organic Non-Fullerene Electron Acceptors

The Long exciton diffusion length is revealed to be a desirable feature of nonfullerene electron acceptors (NFA) utilized in organic solar cells by exciton-exciton annihilation measurements. In order to reveal its underlying reason, we employed Kinetic Monte Carlo (KMC) method to simulate the time evolution of the exciton density upon photo-excitation in pristine NFA materials such as ITIC, IT4F and ITM. The photogenerated excitons were assumed to hop in a cubic lattice with energetic disorder described by Gaussian disorder model, and the hopping rates were given by the Forster resonant transfer mechanism. Through reproduction of the experimental transient absorption (TA) kinetics, some of the key parameters such as the exciton hopping prefactor and the energetic disorder were determined, with which the exciton diffusion lengths were calculated using the KMC method, and the results turned out be good approximations for the experimental values. It is found that the energetic disorder parameters in all the simulated materials are ubiquitously 40 meV or lower, being consistent with the first principle results; and the hopping prefactors are in the order of 10¹² nm⁶/s, all of which contribute to the high diffusion lengths. This work also establishes a theoretical framework for predicting exciton diffusion lengths and estimating the exciton transport parameters which are difficult to determine via experimental measurements.