Plasma nanotexturing of silicon surfaces for photovoltaics applications: influence of initial surface finish on the evolution of topographical and optical properties.

Guillaume Fischer,Thomas A Germer,Martin Foldyna,Erik V Johnson,Etienne Drahi

doi:10.1364/oe.25.0a1057

Abstract

Using a plasma to generate a surface texture with feature sizes on the order of tens to hundreds of nanometers ("nanotexturing") is a promising technique being considered to improve efficiency in thin, high-efficiency crystalline silicon solar cells. This study investigates the evolution of the optical properties of silicon samples with various initial surface finishes (from mirror polish to various states of micron-scale roughness) during a plasma nanotexturing process. It is shown that during said process, the appearance and growth of nanocone-like structures are essentially independent of the initial surface finish, as quantified by the auto-correlation function of the surface morphology. During the first stage of the process (2 min to 15 min etching), the reflectance and light-trapping abilities of the nanotextured surfaces are strongly influenced by the initial surface roughness; however, the differences tend to diminish as the nanostructures become larger. For the longest etching times (15 min or more), the effective reflectance is less than 5% and a strong anisotropic scattering behavior is also observed for all samples, leading to very elevated levels of light-trapping.

Highlights

Despite significant improvements in the efficiency of competing technologies, crystalline silicon (c-Si) solar cells are leading the market in the photovoltaic industry, benefiting from high reliability, improving production module efficiencies, and continuing cost reduction
Plasma-assisted c-Si nanotexturing is typically obtained with an SF6/O2 discharge in a reactive ion etching (RIE) reactor [5]: more precisely, fluorine radicals are responsible for the etching of Si at the surface, while the addition of oxygen to the plasma enables the formation of in-situ micro-masking species of type SiOxFy responsible for the spontaneous formation of nanostructures
Evolution of the surface morphology Scanning electron microscopy (SEM) images, obtained with a Hitachi S-4800, of the initial surface morphologies are shown in Fig. 1

Summary

Introduction

Despite significant improvements in the efficiency of competing technologies, crystalline silicon (c-Si) solar cells are leading the market in the photovoltaic industry, benefiting from high reliability, improving production module efficiencies, and continuing cost reduction. Low cost c-Si solar cells, plasma assisted texturing appears to have significant advantages, notably the ability to process large area, thin wafers (thickness below 100 μm) on a single side, with reduced silicon removal. Plasma-assisted c-Si nanotexturing is typically obtained with an SF6/O2 discharge in a reactive ion etching (RIE) reactor [5]: more precisely, fluorine radicals are responsible for the etching of Si at the surface, while the addition of oxygen to the plasma enables the formation of in-situ micro-masking species of type SiOxFy responsible for the spontaneous formation of nanostructures. Results show that improvements of light management in silicon solar cells from front surface nanotexturing depend upon the initial surface roughness

Studied substrate finishes and nanotexturing process

Results and discussion

Evolution of the optical properties

Conclusion