Hydrogenated amorphous silicon schottky solar cells: The effects of contamination by dopants in the undoped layer

Abstract

The metal-semiconductor solar cell offers a simple structure to determine the photovoltaic behaviour of novel semiconductors, such as thin films of hydrogenated amorpuous silicon (a-Si:H). It is known that solar cells made from this semiconductor must contain a wide built-in field region to collect the photogenerated carriers before they recombine; in contrast with crystalline silicon cells, the diffusion of non-equilibrium carriers is not significant in a-Si:H cells. Thus, whatever the solar cell structure, a-Si:H cells must at present contain an undoped layer to provide a wide space charge region, although heavily doped layers may be required to make low resistance contacts. In this paper we describe the importance of ensuring that no residual dopant enters the undoped layer during its deposition, when this follows the deposition of an n + layer. Slight doping of this i layer will reduce the resistivity of the layer, so reducing the field region width. In the Schottky cells there will be band-bending regions at the front (barrier metal) interface and at the i-n + interface. An increase in the doping of the i layer will ensure that the band bending is mainly at the front interface, but a pure i layer will have a Fermi level midgap and there will then be substantial band bending at the i-n + junction. These effects will change the photocurrent spectral response. Another consequence of slight dopant contamination from the n + layer is that the cell becomes susceptible to photo-induced conductivity changes, which can affect the long-term performance of illuminated cells. In this paper we describe the conductivity, gap state density at the Fermi level, sensitivity to photo-induced conductivity changes and photovoltaic behaviour of Schottky cells with i/n + structures having various degrees of contamination.