Interface contribution to GaAs/Ge heterojunction solar cell efficiency

A solar cell formed by growing a p-on-n AlGaAs/GaAs heteroface homojunction on a thin Ge substrate is studied by investigating the contribution of the GaAs/Ge heterostructure to the solar-cell efficiency. The existence of interface states is required in the absence of a Ge p-n junction to produce the photovoltaic effect with an open-circuit voltage enhancement as experimentally observed. Dark current-voltage characteristics of the GaAs/Ge heterojunction are calculated when the carrier transport is by thermionic emission and tunneling mechanisms. The evaluations correctly explain the observed changes of efficiency, the decrease of fill factor, the increase of open-circuit voltage, and the insignificant change of short-circuit current as compared to a GaAs/GaAs solar cell. It the short-circuit current from the heterojunction is on the order of 25 mA/cm², which is less than that of the p-n junction cell, the reduction of the solar cell efficiency is about 0.5-1.5% over a wide range of GaAs/Ge doping concentrations. Very few interface states tend to yield a diode-like dark I-V curve