Light-trapping in Thin Film Silicon Solar Cells with a Combination of Periodic and Randomly Textured Back-reflectors

ABSTRACTLight trapping is essential to harvest long wavelength red and near-infrared photons in thin film silicon solar cells. Traditionally light trapping has been achieved with a randomly roughened Ag/ZnO back reflector, which scatters incoming light uniformly through all angles, and enhances currents and cell efficiencies over a flat back reflector. A new approach using periodically textured photonic-plasmonic arrays has been recently shown to be very promising for harvesting long wavelength photons, through diffraction of light and plasmonic light concentration. Here we investigate the combination of these two approaches of random scattering and plasmonic effects to increase cell performance even further. An array of periodic conical back reflectors was fabricated by nanoimprint lithography and coated with Ag. These back reflectors were systematically annealed to generate different amounts of random texture, at smaller spatial scales, superimposed on a larger scale periodic texture. nc-Si solar cells were grown on flat, periodic photonic-plasmonic substrates, and randomly roughened photonic-plasmonic substrates. There were large improvements (>20%) in the current and light absorption of the photonic-plasmonic substrates relative to flat. The additional random features introduced on the photonic-plasmonic substrates did not improve the current and light absorption further, over a large range of randomization features.