Revisiting light trapping in solar cells: new upper bounds and models

We present a universal model of broadband absorption in a slab of semiconductor. The theoretical framework, based on the description of multiple overlapping resonances in the frequency domain, has a very broad domain of validity. We derive simple analytical formulas for reference light-trapping models and for absorption upper bounds. Two light-trapping strategies are compared: multi-resonant absorption achieved with a sub-wavelength periodical pattern, and isotropic scattering obtained with random texturing. We provide an answer to the long-debated question of the best strategy for light-trapping in solar cells, and guidelines for the design of ultrathin solar cells. They apply to both silicon and thin-film solar cells. The new upper bounds on absorption presented in this work could be used to revisit the maximum efficiency of single-junction silicon solar cells.