High Open Circuit Voltage of MQW Amorphous Silicon Photovoltaic Structures

The maximum theoretical open circuit voltage of a solar cell is set by its builtin voltage. For amorphous silicon p-i-n cells, the position of the Fermi levels in the p-and ncontact regions are on the order of 0.4 eV and 0.2eV from their respective band edges, limiting the built-in voltage to Eg 0.6eV. We propose replacing the pand 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 nsuperlattices to yield a larger build-in voltage, the density of states in the doped wide band materials must be much greater at the Fermi levels than that of the undoped low band gap quantum wells. To accomplish this, the wide gap material must be heavily doped and have effective tails much wider than that of the undoped low band gap material. For the n-region superlattice the barrier must be in the conduction band, hence we propose a -Si1–xCx: H and a -Si:H: For the p-region superlattice the barrier must be in the valence band, and we propose a -Si1–yNy: H and a -Si:H.