Light-trapping in solar cells by directionally selective filters

The radiative recombination limit described by Shockley and Queisser (SQ) provides the maximum efficiency for the conversion of sunlight into electricity by a solar cell. This maximum efficiency depends only on the radiation balance between the sun, the solar cell, and ambient. This chapter summarizes the theory of energy and solid angle restriction and the theoretical limits of light-trapping and cell efficiency. The calculated efficiency limits of c-Si solar cells with directionally and spectrally selective filters are presented. The gain in annual yield is estimated taking into account the loss in diffuse irradiation for same types of solar cells. The chapter considers the filter systems to be a 1D layer stack (Rugate filter). The chapter reports an experimental realization of a spectrally and directionally selective Bragg-like filter covering the front glass of hydrogenated amorphous silicon solar cells and of a similar filter covering the front of a germanium solar cell.