An analysis of GaAlAs-GaAs solar cells - A study of the limiting mechanisms of heterojunction systems

Design and manufacturing techniques are presented for the production of optimized heterojunction GaAlAs-GaAs solar cells. The design goal is to obtain multispectral solar cells with enhanced efficiency and reliability at lower cost than now possible. Window layers with Al doping levels of 40 percent are shown to have efficiencies equivalent to layers with Al doping levels of 80-90 percent. Liquid phase epitaxial techniques are described for producing the cells. Supercooling methods are used to minimize the defect densities. Laboratory tests have yielded 0.04 sq cm cells with 20.2 percent efficiency under an illumination of 0.4 W/sq cm. In extending the study to bispectral cells, a cascade configuration is found promising for lowering production costs. Ternary and quaternary materials are determined to be the most suitable for obtaining the cascade cell, i.e., GaAlAsSb and GaInAsP substrates with GaAlAsSb window layers. A metal grid in the layer connection is demonstrated to give efficiencies of up to 25.4 percent, which are augmented to 27.3 percent with an AR coating. Finally, optimization of the metal contacts further enhances the cell efficiencies to 31 percent in AM0.