Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

M Peters,K Forberich,B Bläsi,N Sahraei,A.G Aberle,C Battaglia

doi:10.1364/oe.20.029488

Abstract

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector.

Highlights

Light trapping is of key importance for the further development of thin-film silicon photovoltaics (PV)
Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature
Based on the structures defined in the preceding section, we show a theoretical analysis of their light trapping properties and how they change if the geometry of the structure is varied

Summary

Introduction

Light trapping is of key importance for the further development of thin-film silicon photovoltaics (PV). A widely used and efficient approach to realize light trapping in thin-film silicon solar cells is to apply scattering textures at the interface between the transparent front electrode and the silicon layers These textures are typically stochastically modulated transparent conductive oxide (TCO) surfaces onto which the photoactive silicon layers are deposited. The effectiveness of scattering textures has been shown by various authors [2,3] and the current world record for thin-film silicon solar cells has been achieved with a sophisticated hierarchical random texture [4] Another promising approach is light trapping with diffractive, periodic structures like 1D or 2D gratings or photonic crystals. This concept was first proposed by Sheng et al [5] for binary linear gratings for thin-film silicon solar cells. Considerable absorption enhancements could be achieved; the performance of gratings has not yet surpassed that of scattering textures

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Results

Conclusion