Process of making thin film materials for high efficiency solar cells on low-cost silicon substrates. The process comprises forming a low-cost silicon substrate, forming a graded transition region on the substrate and epitaxially growing a thin gallium arsenide film on the graded transition region. The process further includes doping the thin gallium arsenide film and forming a junction therein. The graded transition region preferably is a zone refined mixture of silicon and germanium characterized by a higher percentage of germanium at the surface of the region than adjacent the substrate. The process also includes the formation of homojunctions in thin gallium arsenide films.Solar cells made from the materials manufactured according to the process are characterized by a high conversion efficiency, improved stability and relatively low unit cost.