Numerical Analysis of Lamellar Gratings for Light-Trapping in Amorphous Silicon Solar Cells

TL;DR

This study presents a modal method to accurately analyze light absorption in lamellar gratings for amorphous silicon solar cells, revealing how geometry and plasmon effects influence efficiency and suggesting design improvements.

Contribution

Introduces a modal analysis technique for material-specific absorption in light-trapping structures, highlighting the impact of geometry and plasmon interactions on solar cell performance.

Findings

Metallic plasmons cause parasitic absorption in gratings.

Optimizing grating geometry reduces undesired metal absorption.

Maximum silicon absorption occurs with thinner a-Si layers.

Abstract

In this paper, we calculate the material specific absorption accurately using a modal method by determining the integral of the Poynting vector around the boundary of a specific material. Given that the accuracy of our method is only determined by the number of modes included, the material specific absorption can be used as a quality measure for the light-trapping performance. We use this method to investigate metallic gratings and find nearly degenerate plasmons at the interface between metal and amorphous silicon (a-Si). The plasmons cause large undesired absorption in the metal part of a grating as used in a-Si cells. We explore ways to alleviate the parasitic absorption in the metal by appropriate choice of the geometry. Separating the diffraction grating from the back reflector helps, lining silver or aluminum with a dielectric helps as well. Gratings with depth > 60nm are preferred, and periods > 600nm are not useful. Maximum absorption in silicon can occur for less thick a-Si than is standard. We also find that geometric asymmetry can improve the absorption for PV. Lastly, we investigate the modal contributions to absorption, and find that for the most part, the absorption can not be attributed to single modes, but comes from the interplay of a few important modes.