Optimizing the geometry of plasmonic reflection grating back contacts for improved light trapping in prototype amorphous silicon thin-film solar cells

Abstract

In this study, we experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. The plasmonic reflection grating back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. By varying the geometrical parameters of these back contacts, design strategies for optimized light trapping are identified. First, a general correlation between a reduction of the period of the plasmonic reflection grating back contact and an increase of the absorptance as well as external quantum efficiency is found for various unit cells of the nanostructures i.e. square unit cell, hexagonal unit cell and face-centered unit cell. Second, the width of the nanostructures is varied. With increasing width, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, an optimized design for improved light trapping in the studied thin-film solar cells is a combination of a period of 600 nm and a structure width of 350 nm. Solar cells fabricated on plasmonic reflection grating back contacts with this optimized configuration yield enhanced power conversion efficiencies as compared to reference solar cells processed on state-ofthe- art randomly textured substrates. In detail, the power conversion efficiency is enhanced by around 0.2 % from 9.1 % to 9.3 %. This increase is largely due to the enhancement of the short-circuit current density of around 7 % from 14.7 mA/cm² to 15.6 mA/cm².