Anomalous photovoltaic effect

The "anomalous" refers to those cases where the photovoltage (i.e., the open-circuit voltage caused by the light) is larger than the band gap of the corresponding semiconductor.

Overall, materials that exhibit the anomalous photovoltaic effect have very low power generation efficiencies, and are never used in practical power-generation systems.

[2] The compound α-In2Se3 can be made to exhibit the bulk photovoltaic effect and outperform traditional solar cells.

Among the first attempts to explain the APE were few that treated the film as a single entity, such as considering the variation of sample thickness along its length[14] or a non-uniform distribution of electron traps.

[15] However, studies that followed generally supported models that explain the effect as resulting from a series of microelements contributing additively to the net photovoltage.

The transfer of carriers at the surface of crystallites is assumed to be hindered by the presence of some unspecified layer with different properties, thus cancellation of consecutive Dember voltages is being prevented.

To explain the polarity of the PV which is independent of the illumination direction one must assume that there exists a large difference in recombination rates at opposite faces of a crystallite, which is a weakness of this model.

A potential barrier is formed due to a combination of the band gap difference and the electric fields produced at the interface.

One should remember that this model can be invoked to explain anomalous PV effect only in those materials that can demonstrate two types of crystal structure.

It was suggested by Starkiewicz [4] that the anomalous PV is developed due to a distribution gradient of positive and negative impurity ions through the microcrystallites, with an orientation such as to give a non-zero total photovoltage.

If it is assumed that the optical absorption depth is much less than the space charge region in the crystallites, then, because of their inclined shape more light is absorbed in one side than in the other.

[2] Theoretical calculations using density functional theory or other methods can predict the extent to which a material will exhibit the bulk photovoltaic effect.

This also explains why large open-circuit voltages tend to be seen only in crystals that (in the dark) have very low conductivity: Any electrons that can freely move through the crystal (i.e., not requiring photons to move) will follow the drift-diffusion equation, which means that these electrons will subtract from the photocurrent and reduce the photovoltaic effect.

[19][21] This is required because if an electron is excited into a mobile, delocalized state, and then it scatters a few times, then its direction is now randomized and it will naturally start following the drift-diffusion equation.

However, in the bulk photovoltaic effect, the desired net electron motion is opposite the direction predicted by the drift-diffusion equation.