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 solarcell efficiency. The existence of interface states is required in the absence of a Ge p-n junction in order 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. Our 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. If the short-circuit current from the heterojunction is of 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 to 1.5 percent over a wide range of GaAs/Ge doping concentrations. Very few interface states tend to yield a diodelike dark I-V relation, and this behavior in the reverse-biased region will degrade the fill factor while soft breakdown in the reverse-biased region will improve the fill factor. The efficiency is controlled by the forward-biased part of the dark I-V curve. However, if the short-circuit current of the heterojunction is greater than that of the p-n junction cell, the more the interface charge, the better the combined solar cell efficiency. Our results differ from the assertion that a Ge p-n junction is responsible for the open-circuit voltage increase.


Electrical and Computer Engineering


This work was supported by the Air Force Systems Command and the Air Force Office of Scientific Research.

Keywords and Phrases

Photovoltaic Cells; Semiconducting Aluminum Compounds--Applications; Semiconducting Gallium Arsenide--Applications; Semiconducting Germanium--Applications; Semiconductor Devices--Heterojunctions, Dark Current Voltage Characteristics; Heteroface Heterojunction; Heterojunction Solar Cell; Interface States; Short Circuit Current, Solar Cells

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Article - Journal

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© 1989 Institute of Electrical and Electronics Engineers (IEEE), All rights reserved.