Activationless Charge Transfer Governs Photocurrent Generation in Organic Photovoltaic Blends

Organic photovoltaics (OPVs) have recently shown substantive progress in device efficiency, driven in particular by advances in non-fullerene acceptor design and suppression of energy offsets between exciton (S₁) and charge-transfer (CT) states. Ultrafast charge transfer (< 200 fs) from polymer donors to fullerene acceptors has often been observed and attributed to the origin of efficient photocurrent generation in polymer:fullerene blends. While efficient charge photogeneration has also been observed in polymer:non-fullerene OPVs, several studies have reported much slower CT rates on the order of tens of ps. As suggested in Marcus theory, higher activation energy barriers could be the origin of slower CT rates. However, it remains unclear whether the suppressed S₁-CT offset and/or the hybridisation/thermal equilibrium between S₁and CT states would lead to higher barriers for charge generation and hinder charge transfer in such small-offset OPV blend systems. Herein, utilizing temperature-dependent transient absorption spectroscopy for a range of polymer:non-fullerene and polymer:fullerene OPV blends, we elucidate that the activation energy for charge generation is below 15 meV, implying that activationless charge generation pathway governs photocurrent generation in archetypal polymer based OPVs including the highly efficient low-offset PM6:Y6 blend. Suppression of the energy offsets in non-fullerene based OPVs does not lead to any increase in activation energy barriers for charge separation. While charge transfer is activationless, bimolecular recombination of charges proceeds with a high activation barrier of several hundreds of meV. Efficient charge photogeneration is universal among the blends studied herein and is not a limiting factor for device performance, while the geminate and/or no-geminate recombination losses appear to be the limiting factors.