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Abstract
Effective light trapping is critical for polycrystalline silicon (poly-Si) thin-film solar cells to generate sufficiently high photocurrent. Glass substrate texturing is a standard and very effective light-trapping approach for poly-Si solar cells fabricated by plasma enhanced chemical vapour deposition but it cannot be applied to poly-Si cells deposited by electron beam (e-beam) evaporation, which is a preferred deposition process. Attempts to develop effective light trapping for e-beam poly-Si cells, by texturing of either glass substrate or poly-Si film itself has been studied in this thesis. A spray etching method is designed for glass texturing, but it is proved not to be suitable for producing light trapping textures. Light trapping is then implemented by texturing of the rear surface of e-beam poly-Si films deposited on planar glass. Water-based solutions of KOH, NH4F and NH4F/H2O2 are found to be able to texture poly-Si films and, thus, to significantly improve light-trapping. The related texturing processes and resulting textures are characterised by Si etching rates, the surface roughness versus removed Si thickness, texture angle distributions, optical absorption and spectral response enhancement. Both 2.5% KOH and NH4F-only texturing are found to provide roughness over 200 nm RMS, steep peak feature angles up to 20˚ and higher absorption enhancement of 30~40% compared to planar films. With a removed Si thickness of less than 2 µm, NH4F-only texturing provides more advantages than 2.5% KOH etching. In order to apply this etch-back texturing to functional e-beam cells, a new spin-on diffused back-surface-field (BSF) subsequent to the texturing process has been successfully developed and therefore the short-circuit current (Jsc) enhancement with various etch-back texturing can be compared. A 3.6 µm thick e-beam cell with KOH etch-back texturing was demonstrated ~21% Jsc enhancement compared to a reference planar cell with a rear reflector and a 3 µm thick e-beam cell with NH4F-based texturing is achieved even higher Jsc enhancement of 28%. Thus, it is demonstrated that a wet-chemical etch-back texturing is a very promising approach to light-trapping in e-beam cells on planar substrates.