The optimal scattering structures for light-trapping in ultra-thin photovoltaic devices

Abstract

The introduction of light scattering structures for efficient absorption of incident illumination is essential in ultra-thin solar cells given their reduced optical path length. The growing interest in these devices demands attaining high efficiencies through the identification of optimal designs to maximise the absorption of incident photons. A pathway towards such high efficiencies is the implementation of transparent scattering structures to minimise parasitic losses. We study the performance of these structures by focusing on dielectric/high-band-gap semiconductor scattering layers in an ultra-thin (80 nm) GaAs solar cell. Comparisons with absorptive scattering layers are enabled by presenting data for an equivalent device with metal/dielectric structures. Following a previously reported light management optimisation method which is guided by the dispersion of the avail- able waveguide modes, we find an improved performance for the transparent scattering layers. Our study also compares the light absorption enhancement offered by transparent photonic crystal diffractive arrays to that of transparent quasi-random geometries which target the diffracted power to the most favourable waveguide modes in the device. We find the former designs to have a superior performance in our device of interest, and the latter to suffer from greater reflection losses. Finally, our results also demonstrate the effectiveness of the optimisation method used and its applicability to multiple device architectures for the design of high-efficiency photovoltaics.