User:Hadarl/My Draft Article

The phenomenon of a photovoltage larger than the band gap of the corresponding semiconductor is known as the anomalous photovoltaic effect (APE). This effect was discovered by Starkiewicz et al. in 1946 on PbS films and was later observed on other semiconducting polycrystalline films including CdTe, Si , Ge , ZnTe and InP , as well as on amorphous silicon films. It has not been reported for nanocrystals systems. Observed photovoltages were found to reach hundreds, and in some cases even thousands of volts. The films in which this effect was observed were generally thin semiconducting films that were deposited by [vacuum evaporation] onto a heated insulating substrate, held at an angle with respect to the direction of the incident vapor. However, the photovoltage was found to be very sensitive to the conditions and procedure at which the samples were prepared. This made it difficult to get reproducible results which is probably the reason why no satisfactory model for it has been accepted thus far. Several models were, however, suggested to account for the extraordinary phenomenon and they are briefly outlined below.

The oblique deposition can lead to several structure asymmetries in the films. 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 or a non-uniform distribution of electron traps. 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 more popular models used to explain the photovoltage are reviewed below.

• The Dember effect. When photogenerated electron and holes have different mobilities a potential difference can be created between the illuminated and non-illuminated faces of a semiconductor slab. Generally this potential is created through the depth of the slab, whether it is a bulk semiconductor or a polycrystalline film. The difference between these cases is that in the latter a photovoltage can be created in each one of the microcrystallites. As was mentioned above, in the oblique deposition process inclined crystallites are formed in which one face can absorb light more than the other. This may cause a photovoltage to be generated along the film, as well as through its depth. 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.

• The structure transition model. This model suggests that when a material crystallizes both in cubic and hexagonal structures, an asymmetric barrier can be formed by a residual dipole layer at the interface between the two structures. 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.

• The p-n junction model. It was suggested by Starkiewicz 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. This is equivalent to an array of p-n junctions. However, the mechanism by which such p-n junctions may be formed was not explained.

• The surface photovoltage model. The interface between crystallites may contain traps for charge carriers. This may lead to a surface charge and an opposite space charge region in the crystallites, in case that the crystallites are small enough. Under illumination of the inclined crystallites electron-hole pairs are generated and cause a compensation of the charge in the surface and within the crystallites. 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. Thus a difference in the reduction of the charge is created between the two sides. This way a photovoltage parallel to the surface is developed in each crystallite.