Superlattice contact layers for high open circuit voltage a-Si:H solar cells

The maximum theoretical open-circuit voltage of a solar cell is set by its built-in voltage. For amorphous silicon p-i-n cells, the positions of the Fermi levels ill the p and n-contact regions are of the order of 0.4 eV and 0.2 eV from their respective band edges, limiting the built-in voltage to 0.6 eV. The authors propose replacing the p- and n-regions by superlattices in which the Fermi levels in the wide-gap barrier regions are, on an absolute scale, closer to the valence band or conduction band edges of the low-gap material (a-Si:H) than the values indicated above. In order for the p and n-superlattices to yield a larger built-in voltage, the density of states in the doped wideband materials must be much greater at the Fermi levels than that of the undoped low-bandgap quantum wells. To accomplish this, the wide-gap material must be heavily doped and have effective tails much wider that those of the undoped low-bandgap material. For the n-region superlattice the barrier must be in the conduction band; hence, the authors propose a-Si_1-xC_x:H and a-Si:H. For the p-region superlattice the barrier must be in the valence band, and the authors propose a-Si_1-yN_y:H and a-Si:H