Upper limits to absorption enhancement in thick solar cells using diffraction gratings

The application of diffraction gratings to solar cells is a promising approach to superseding the light trapping limits of conventional Lambertian structures. In this paper a mathematical formalism is derived for calculating the absorption that can be expected in a solar cell equipped with a diffraction grating, which can be applied to any lattice geometry and grating profile. Furthermore, the formalism is used to calculate the upper limit of total absorption that can theoretically be achieved using a diffraction grating. The derived formalism and limits are valid when the solar cell thickness is greater than the coherence length of the illuminating solar spectrum. Comparison is made to the upper limit achievable using an angularly selective Rugate filter, which is also calculated. Both limits are found to be considerably higher than the Lambertian limit within the range of sunlight concentration factors practically employed in photovoltaic systems (1-1000x). The upper limit of absorption using the diffraction grating is shown to be equal to the thermodynamic limit for all absorbances and concentration factors. The limit for the Rugate filter is generally lower, but tends to the thermodynamic limit for lower cell absorbances. Copyright (C) 2011 John Wiley & Sons, Ltd.