Optimization of generalized dielectric nanostructures for enhanced light trapping in thin-film photovoltaics via boosting the local density of optical states.

Recent work has shown that using a high-index cladding atop a lower-index photovoltaic absorber enables absorption of light beyond the ergodic (4n2) limit. In this paper, we propose a generalized optimization method for deriving optimal geometries that allow for such enhancement. Specifically, we adapted the direct-binary-search algorithm to optimize a complex 2-D multi-layer structure with the explicit goal of increasing photocurrent. We show that such an optimization results in enhancing the local density of optical states in an ultra-thin absorber, which forms a slot-waveguide geometry in the presence of a higher-index overcladding. Numerical simulations confirmed optical absorption approaching 100% and absorption-enhancement beyond the ergodic (4n2) limit for specific spectral bands of interest. Our method provides a direct, intuitive and computationally scalable approach for designing light-trapping nanostructures.