Activationless Charge\nTransfer Drives Photocurrent\nGeneration in Organic Photovoltaic Blends Independent of Energetic\nOffset

Organic photovoltaics (OPVs) have recently shown substantial\nprogress\nin enhancing device efficiency, driven in particular by advances in\nthe design of nonfullerene acceptors and the reduction of the energy\noffset driving exciton separation at the donor/acceptor interface.\nHerein, we employ temperature-dependent transient absorption spectroscopy\nto investigate the activation energy for charge generation and recombination\nin a range of bulk heterojunction blends with nonfullerene acceptors.\nRemarkably, we find that in all cases charge generation is almost\nactivationless, in the range of 11–21 meV, independent of energetic\noffset. Geminate recombination is also observed to be almost activationless,\nwith only the kinetics of bimolecular charge recombination being strongly\ntemperature-dependent, with an activation energy >400 meV. Our\nobservation\nof essentially activationless charge generation, independent of energy\noffset, strongly indicates that charge generation in such blends does\nnot follow Marcus theory but can rather be considered an adiabatic\nprocess associated with the motion of thermally unrelaxed carriers.