Geometric Light Confinement in a-Si Thin Film Solar Cells on Micro-Structured Substrates

In this work we propose and study a light trapping scheme for thin film silicon solar cells that is based on geometrical light trapping (instead of textures optimized for light scattering), using periodically arranged pyramidal structures with dimensions larger than the effective wavelength of light. We studied the absorption behavior of amorphous silicon (a-Si) layers on such a pyramidal structured substrate using ray tracing calculations. According to the calculations, for pyramids on a square base, the maximum relative light absorption enhancement for normal incident light occurs at pyramid angles between 40° to 45°, which results in an increase of the weighted absorption of 45%. For diffuse light a maximum in light trapping yields an absorption increase of 40% compared to a flat structure. a-Si solar cells in a p-i-n configuration deposited at low temperature (125°C) on a micro-pyramidal structured polycarbonate substrate were used as proof-of-concept of this new light trapping scheme for the current enhancement. The cells deposited on the structured substrates showed an increase in short-circuit current density of 24% compared to the reference solar cell structures on flat glass substrates.