In organic semiconductors, optical excitation does not necessarily produce free carriers. Very often, electron and hole are bound together to form an exciton. Releasing free carriers from the exciton is essential for the functioning of photovoltaics and optoelectronic devices, but it is a bottleneck process because of the high exciton binding energy. Inefficient exciton dissociation can limit the efficiency of organic photovoltaics. Here, nanoscale features that can allow the free carrier generation to occur spontaneously despite being an energy uphill process are determined. Specifically, by comparing the dissociation dynamics of the charge transfer (CT) exciton at two donor–acceptor interfaces, it is found that the relative orientation of the electron and hole wavefunction within a CT exciton plays an important role in determining whether the CT exciton will decompose into the higher energy free electron–hole pair or relax to the lower energy tightly‐bound CT exciton. The concept of the entropic driving force is combined with the structural anisotropy of typical organic crystals to devise a framework that can describe how the orientation of the delocalized electronic wavefunction can be manipulated to favor the energy‐uphill spontaneous dissociation of CT excitons over the energy‐downhill CT exciton cooling.

Halide perovskites intrinsically contain a large amount of point defects. The interaction of these defects with photocarriers, photons, and lattice distortion remains a complex and unresolved issue. We found that for halide perovskite films with excess halide vacancies, the Fermi level can be shifted by as much as 0.7 eV upon light illumination. These defects can trap photocarriers for hours after the light illumination is turned off. The enormous light-induced Fermi level shift and the prolonged electron trapping are explained by the capturing of photocarriers by halide vacancies at the surface of the perovskite film. The formation of this defect−photocarrier complex can result in lattice deformation and an energy shift in the defect state. The whole process is akin to polaron formation at a defect site. Our data also suggest that these trapped carriers increase the electrical polarizability of the lattice, presumably by enhancing the defect migration rate.

At organic donor−acceptor (D−A) interfaces, electron and hole are bound together to form charge transfer (CT) excitons. The electron and hole wave functions in these CT excitons can spatially delocalize. The electron delocalization opens up possibilities of extracting free charges from bound excitons by manipulating the potential energy landscape on the nanoscale. Using a prototype trilayer structure that has a cascade band structure, we show that the yield of charge separation can be doubled as compared to the bilayer counterpart when the thickness of the intermediate layer is around 3 nm. This thickness coincides with the electron delocalization size of CT excitons typically found in these organic films. Tight-binding calculation for the CT states in the trilayer structure further demonstrates that electron delocalization, together with the energy level cascade, can effectively flatten the energetic pathway for charge separation. Hence, it is possible to add nanometer-thick layers between the donor and the acceptor to significantly enhance the charge separation yield.

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