Algaas Top Solar Cell for Mechanical Attachment in a Multi-Junction Tandem Concentrator Solar Cell Stack

Abstract

Free-standing, transparent, tunable bandgap Al/sub x/Ga/sub 1-x/As top solar cells have been fabricated for mechanical attachment in a four terminal tandem stack solar cell. Evaluation of the device results have demonstrated 1.80 eV top solar cells with efficiencies of 18% (100X, AM0) which would yield stack efficiencies of 31% (100X, AM0) with a silicon bottom cell. When fully developed the Al/sub x/Ga/sub 1-x/As/Si mechanically-stacked two-junction solar cell concentrator system can provide efficiencies of 36% (AM0, 100X). AstroPower has fabricated Al/sub x/Ga/sub 1-x/As top solar cells with bandgaps from 1.66 eV to 2.08 eV. Liquid phase epitaxy (LPE) growth techniques have been used and LPE has been found to yield superior Al/sub x/Ga/sub 1-x/As material when compared to MBE or MOCVD. The result of this work is the first demonstration of a free-standing wide bandgap top cell technology which can be readily developed for commercial availability based on the Al-Ga-As mixed crystal system. Free-standing, tunable bandgap top solar cells with over 95% transparency to sub-bandgap photons will be made commercially available. The stack assembly technology will be readily applicable to any mechanically stacked multi-junction (MSMJ) system. Development of a wide bandgap top Solar cell is the only feasible method to obtain stack efficiencies greater than 40% at AM0. System efficiencies of greater than 40% can be realized when the AlGaAs top Solar cell is used in a three solar cell mechanical stack. This is a unique approach to improve the performance of space photovoltaic power systems.