Dissecting the nature and dynamics of electronic excitations in a solid-state aggregate of a representative non-fullerene acceptor

Abstract

Understanding electronic excitations and their dynamics in non-fullerene acceptor (NFA) materials is crucial for improving the efficiency of opto-electronic devices. In this study, we use a Frenkel-exciton Hamiltonian, which couples electronic and nuclear degrees of freedom, to investigate the optical properties and exciton dynamics in an extended solid-state aggregate formed by a representative NFA named m-4TICO. Besides the presence of significant H-like interactions, we observed a predominant influence of J-aggregation-like effects on the solid-state absorption spectrum, resulting in a spectral red-shift and a reduced vibronic band compared to the solution phase. We have also identified a significant correlation between energetic disorder—primarily due to low-frequency molecular torsional vibrations in the acceptor–donor–acceptor structure—and the Stokes shift. Through semiclassical non-adiabatic molecular dynamics, we explore how excitonic interaction patterns affect exciton delocalization and diffusion. Based on our results, we discuss design principles derived from the manipulation of energetic disorder and excitonic coupling-sign relationships that substantially modulate exciton transport, thereby offering actionable strategies to enhance the efficiency of opto-electronic devices.