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Alternating Current Photovoltaic effect
The alternating current photovoltaic (AC PV) effect is the generation of alternating current (AC) in the nonequilibrium states when the light periodically shines at the unction or interface of material. The first demonstration of the AC PV effect was done by Dr. Haiyang Zou and Prof. Zhong Lin Wang at the Georgia Institute of Technology in 2017. Most photovoltaic effects generate direct current output under equilibrium states and the AC PV is operated at the nonequilibrium conditions.

The first study was based on a p-Si/TiO2 nanofilm. It is found that except for the DC output generated by the conventional PV effect based on a p-n junction, AC current is also produced when a flashing light is illuminated at the interface. The AC PV effect does not follow the Ohm's law, instead, it is based on the capacitive model that the current strongly depends on the frequency of the chopper, but voltage is independent of the frequency. The peak current of AC at high switching frequency can be much higher than that from DC. The magnitude of the output is also associated with the light absorption of materials.

The AC PV effect can not be explained by the conventional PV effects. It is suggested to be a result of the relative shift and realignment between the quasi-Fermi levels of the semiconductors adjacent to the junction/interface under the nonequilibrium conditions. The electron flow in the external circuit back and forth to balance the potential difference between two electrodes. The organic solar cell, which the materials have no initial carrier concentration, does not have the AC PV effect.

The AC PV effect can work as a high-sensitivity broadband photodetector, and as a remote power source providing extra power output in addition to the conventional photovoltaic effect at an appropriate frequency.