Photoexcited electrons and holes are bound together to form excitons in molecular assemblies. The separation of these excitons into free charges remains a bottleneck process that can limit the photo-to-electrical conversion efficiency of photovoltaics, light sensors, and photocatalyst. By using time-resolved photoemission spectroscopy, our group found that in some selected molecular systems, charge transfer (CT) excitons residing at donor-acceptor interfaces can undergo charge separation (CS) at a very rapid timescale (~1 - 10 ps) despite the CS is an enthalpy uphill process with an enthalpy increase of ~0.2 - 0.3 eV. The observed CS rate is ~2 - 3 orders of magnitude faster than the expected Arrhenius rate. Recently, we have observed similar CS behaviors in some other systems such as organic/2D-transition metal dichalcogenide (TMD) interfaces, and bulk heterostructures consisting of the high-performance Y6 non-fullerene acceptors as well. The CS mechanism is explained by the interplay between electron delocalization, entropy, and stacking anisotropy. The spontaneous separation of CT excitons can reduce the large energy loss often accompanied with the CS, and can improve the energy conversion efficiency. We propose that in order to trigger this CS channel, the delocalized electron and hole in the CT exciton should be in contact with each other <i>via</i> a point-like junction.

Spontaneous enthalpy uphill separation of excitons induced by electron delocalization, entropy and stacking anisotropy

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