Metal-assisted chemically etched black silicon: morphology and light interaction

Abstract

An important part of the operation of a silicon solar cell is the ability to capture large amounts of incident light, which is subsequently absorbed in the substrate. Therefore, maximisation of light coupling into the semiconductor is desired for highly efficient devices. The existing texturing processes in the PV industry can be further enhanced by fabrication of nanostructures on the surface of the semiconductor. As such, black silicon (b-Si) becomes an excellent antireflective and light-trapping scheme, which has the potential to optically outperform commercial textured surfaces. Over the last decades, several fabrication methods have been proposed, out of which metal-assisted chemical etching (MACE) has become the most promising for industrial integration, due to its inherently low cost, ease of processing, reproducibility and scalability. This thesis presents an investigation into black silicon fabricated by MACE processes, including the effect of etching parameters, as well as morphological and opto-electronic characterisation of the resulting nanostructures. The black silicon surfaces are shown to consistently outperform their micron-scale pyramids counterparts, both in reduced broadband surface reflectance of 1-2% and in scattering of incident photons towards larger polar angles. These properties of black silicon effectively address challenges related to silicon’s large surface reflectance and poor absorption coefficients, particularly for low-energy photons. A trade-off between the optical and electrical performance of the black silicon layers is identified and surface passivation of minority carriers is investigated by means of atomic layer deposition of aluminium oxide. To this end, sufficiently low surface recombination velocities below 30 cm/s are obtained for a range of nanostructure heights, which suggests that black silicon is a suitable and competitive candidate for high-efficiency device integration, especially on back contacted solar cell architectures.

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Identifiers

ORCID for Tudor Emilian Scheul: orcid.org/0000-0003-4517-6096

ORCID for Stuart Boden: orcid.org/0000-0002-4232-1828

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