Control of back surface reflectance from aluminum alloyed contacts on silicon solar cells

A process for forming highly reflective aluminum back contacts with low contact resistance to silicon solar cells is described. By controlling the process conditions, it is possible to vary the silicon/aluminum interface from a specular to a diffuse reflector while maintaining a high interface reflectance. The specular interface is found to be a uniform silicon/aluminum alloy layer a few angstroms thick that has epitaxially regrown on the silicon. The diffuse interface consists of randomly distributed (111) pyramids produced by crystallographic out-diffusion of the bulk silicon. The light trapping ability of the diffuse contact is found to be close to the theoretical limit. Both types of contacts are found to have specific contact resistivities of 10^-5 Ω2-cm². The process for forming the contacts involves illuminating the devices with tungsten halogen lamps. The process is rapid (under 100 s) and low temperature (peak temperature <580°C), making it favorable for commercial solar cell fabrication