"Buried" metal/dielectric/semiconductor reflectors for light trapping in epitaxial thin-film solar cells

Effective light trapping can be realized in thin film solar cells by including a reflector between the substrate and device structure. Light trapping can significantly improve the performance of almost any solar cell, although the practical attainment of high levels of light trapping has been elusive in at least several important types of solar cells. In many cases, the full potential of light trapping has not been realized due to difficulties in combining a thin solar cell structure with an effective backside reflector. We report a new method for incorporating so-called "buried mirrors" between the supporting substrate and epitaxial device layers that is widely applicable to thin-film crystalline silicon and Ill-V solar cells. We form the mirror as a multilayer coating of amorphous or polycrystalline dielectrics, metals, and/or semiconductors deposited as a mask on the substrate. The thin-film solar cell structure (emitter and base layers) is formed over the mask by epitaxial lateral overgrowth. This approach provides very high backside reflectivities over a wide wavelength bandwidth and over a wide range of angles of incidence. The technique is compatible with different mirror structures and materials, and is superior to the more common epitaxial AlAs/GaAs quarter-wavelength stack Bragg reflectors used for backside mirrors in GaAs-based solar cells, LEDs, and vertical cavity lasers.