Determining Exciton Diffusion Length in Organic Semiconductors: Unifying Macro- and Microscopic Perspectives

Abstract

Long exciton diffusion length (L_D) is key to maximizing excitation harvesting in organic solar cells, but contradicting values are reported for non-fullerene acceptors (NFA). To understand the factors enabling large L_D, experimental observation of exciton decay by transient absorption spectroscopy (TAS) is combined with microscopic Kinetic Monte Carlo (KMC) simulations on 4 ITIC derivatives. Exciton decays are fitted considering singlet exciton-singlet exciton annihilation (SSA) and the intrinsic exciton's lifetime τ, resulting in L_D from 20 to 70 nm. The critical importance of an independent estimate of τ is discussed and its measurements from pristine NFA films is found to be more relevant than from NFA molecules embedded in an inert polystyrene matrix. From experimental parameters, the microscopic Förster Resonant Energy Transfer hopping rate and the annihilation rate in a cubic lattice are determined, considering a Gaussian energetic disorder. KMC simulation of those rates are able to reproduce the experimental transients and L_D, provided a lattice constant a close to the molecular π-π stacking distance is used. It is found that this tight packing and a low disorder are critical to reach large L_D, and empirically relate linearly such that 40 meV more disorder can be compensated by 1 Angstrom tighter packing (shorter a).