Etch-Back Silicon Texturing for Light-Trapping in Electron Beam Evaporated Thin-Film Polycrystalline Silicon Solar Cells

摘要

Effective light trapping is critical for polycrystalline silicon 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 evaporation, which is a preferred deposition process. In this study light-trapping is 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. The RMS roughness increases with the removed thickness and can be as large as 276 nm. Also, the texture angle distribution can reach its maximum at about 20° and has a long tail of larger angles. The absorption at 800 nm can increase up to 75% compared to 30-40% in planar films. The short-circuit current of 26.6 mA/cm~2 was demonstrated for a cell made of 3.6 nm thick poly-Si film textured by the KOH solution, which is -21% enhancement compared to a reference planar cell with a rear reflector. A larger roughness and steeper texture angles produced by NH4F-based etching solutions compared to KOH-based textures indicate that even higher currents are achievable for e-beam poly-Si thin-film cells on planar glass.