Polycrystalline silicon solar cells on low cost foreign substrates

Abstract

The use of polycrystalline silicon layers on low-cost substrates is a promising approach for the fabrication of low-cost solar cells. Using low-carbon steel and graphite as substrates, solar cell structures have been deposited by the thermal decomposition of silane and appropriate dopants. Steel was selected as a substrate on the sole basis of its low cost. However, steel and silicon are not compatible in their properties, and an interlayer of a diffusion barrier, such as borosilicate, must be used to minimize the diffusion of iron from the substrate into the deposit. The deposited silicon on borosilicate/steel substrates is polycrystalline with a grain size of 1–5 μm, depending on deposition conditions. P- n junction solar cells were found to have low open-circuit voltages and poor current-voltage characteristics, and Schottky-barrier solar cells were found to show negligible photovoltages. Graphite is more compatible with silicon in properties than steel, and silicon deposited on graphite substrates shows considerably better microstructures. A number of solar cells, 2·5×2·5 cm in area, have been fabricated from n +-silicon/ p-silicon/ p +-silicon/graphite structures. The best cell to date had a V oc of 0·35 V and an AMO efficiency of 1·5% (no antireflection coating). This type of solar cell is very promising because of the simplicity in fabrication.