Light trapping is crucial to increase efficiency in thin-film solar cells and to reduce the cost of advanced photovoltaic devices. It is especially important to enhance light absorption in the spectral region close to the electronic band gap of the semiconductor, where material absorption is low - and to approach the ultimate limit to absorption, which is usually taken to be the Lambertian limit. Light trapping at the wavelength-scale can be performed with ordered photonic lattices (photonic crystals, diffraction gratings), or with disordered structures, or with a combination of both. Here we report on a theoretical study of thin-film silicon solar cells with randomly rough surfaces described by a Gaussian disorder, which is characterized by the root mean square (RMS) deviation of the height and the lateral correlation length. We show that this model describes very well the scattering properties of actual rough substrates in terms of angular distribution function and haze, and we demonstrate that optimization of the disorder parameters by means of rigorous coupled-wave analysis, and with the short-circuit current density of the solar cell as a figure of merit, allows to reach the Lambertian Limit.
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