Extremely thin silicon solar cells are of interest for space applications. Performance models show that a surface-passivated, light-trapping silicon solar cell can have optimum thicknesses as thin as 5 pm, and maintain high efficiency down to thicknesses of under 2 ~zm. Such cells have specific power and extremely good radiation damage tolerance, as well as high efficiency. New methods of fabricating and handling such thin light-trapping cells using epitaxial growth on a removable substrate are discussed. The substrate provides mechanical support to reduce breakage. When the substrate is removed, a transparent glass superstrate provides the mechanical support. By using a process technology in which a sacrificial lattice-matched CaF2 is used, it is possible to re-use the single-crystal substrate.
Background
A very high efficiency, high radiation tolerance and light weight are desirable goals for silicon solar cells. All of these characteristics can be enhanced by use of very thin, light-trapping cells, with thickness which may be as low as several microns. In this paper, methods of fabricating and handling such thin cells without undue breakage are discussed.
Conventional silicon solar cells have thicknesses of approximately 250 #m. In the interest of increasing the power to weight ratio, or specific power, advanced silicon cells for use in space have been made thinner, with thicknesses of approximately 60 #m. Surface passivation and light-trapping techniques allow the possibility of considerably thinner cells to be formed. Such cells would potentially have extremely high power to weight ratio and good radiation damage tolerance [ 1 ], as well as being highly efficient.
Recent advances in single-crystal silicon solar cells have greatly decreased the losses due to recombination at surfaces and contacts [2,3]. Bulk recombination could be potentially lowered by making the devices