Limits of light-trapping efficiency of prototypical lamellar 1-d metal gratings for amorphous silicon PV cells

TL;DR

This paper calculates the theoretical maximum light absorption in amorphous silicon PV cells using lamellar metal gratings, showing how geometry and polarization affect efficiency limits.

Contribution

It introduces an analytical method to determine ideal light-trapping limits for lamellar gratings in PV cells, accounting for polarization and geometric asymmetry.

Findings

Absorption limits reach 79% for E-polarization with asymmetry.

Absorption limits reach 90% for H-polarization with sine-like gratings.

Planar reflectors have a maximum absorption of 62%.

Abstract

One-dimensional lamellar gratings allow a particularly efficient way for solving Maxwell's equations by expanding the electromagnetic field in the basis of exact eigenmodes of the Helmholtz equation. Then, the solution can be expressed analytically as a superposition of these eigenmodes and the accuracy depends only on the number of modes $N$ included. On this basis, we compute ideal limits of light-trapping performance for prototypical lamellar metal surface relief gratings in amorphous silicon (a-Si) PV cells assuming that light absorption in the metal and front surface reflection can be suppressed. We show that geometric asymmetry can increase absorption. For large enough $N$, convergence of absorption spectra for E polarisation is reached. For H polarisation it is reached for wavelengths $\lambda<$680-700 nm, while the integrated AM1.5-weighted absorption varies by less than 1\% at large $N$. For an a-Si layer with height 200 nm and normal incidence, we obtain upper limits of the total absorption of 79\% for E-polarisation with asymmetric- and 90\% for H-polarisation with sine-like lamellar grating reflectors, whereas a planar reflector yields a limit of 62\%, where 100\% stands for complete absorption for $350<\lambda<$770 nm.