Modeling combined coherent and incoherent scattering structures for light trapping in solar cells

Current structures for solar cells or LEDs often incorporate layers of various optical regimes, with a mixture of coherent, partially coherent or incoherent behavior. We developed a simple and efficient calculation method to study such combined solar cell structures with both wave and ray optics sections. These One-Pass Coherent calculations take wave effects into account where they matter the most, while avoiding a large computational domain to model rough structures. The method simulates a general diffuser by working directly with the reflected wavefronts, instead of using its geometry. We utilize this method to study thin film silicon solar cell structures with a grating on the front and a diffuser at the back. More absorption is obtained with the combined light trapping scheme of appropriate characteristics, compared with grating-only or diffuser-only counterparts. Finally, we report a significant effect of incoherence on the absorption of fairly thin (∼10 μm) cells. We demonstrate that partially incoherent light can be more efficiently absorbed than fully coherent light on average over a broad wavelength range. It turns out that the scarcity of guided modes for fully coherent light can hinder the grating enhancement, leading to a consistently better performance when light coherence is limited or lost.