Charge generation dynamics in efficient non-fullerene organic solar cells

Abstract

Organic solar cells (OSCs) based on non-fullerene acceptors can achieve high charge generation yields despite near-zero donor-acceptor energy offsets to drive electron-hole separation. In this talk I will present experimental data to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer excitons (CTEs) that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron-hole encounters at later times, CTEs and emissive excitons are regenerated, thus setting up an equilibrium between excitons, CTEs and free charges. This endothermic charge separation process enables these systems to operate close to quasi-thermodynamic equilibrium conditions with no requirement for energy offsets to drive charge separation and achieve greatly suppressed non-radiative recombination.