Activationless Charge Transfer Drives Photocurrent Generation in Organic Photovoltaic Blends Independent of Energetic Offset

Abstract

Organic photovoltaics (OPVs) have recently shown substantial progress in enhancing device efficiency, driven in particular by advances in the design of nonfullerene acceptors and the reduction of the energy offset driving exciton separation at the donor/acceptor interface. Herein, we employ temperature-dependent transient absorption spectroscopy to investigate the activation energy for charge generation and recombination in a range of bulk heterojunction blends with nonfullerene acceptors. Remarkably, we find that in all cases charge generation is almost activationless, in the range of 11-21 meV, independent of energetic offset. Geminate recombination is also observed to be almost activationless, with only the kinetics of bimolecular charge recombination being strongly temperature-dependent, with an activation energy >400 meV. Our observation of essentially activationless charge generation, independent of energy offset, strongly indicates that charge generation in such blends does not follow Marcus theory but can rather be considered an adiabatic process associated with the motion of thermally unrelaxed carriers.