engleski

Correlation of the nanostructure and absorption coefficient of the silicon thin films for solar cells

Series of thin silicon films were prepared by decomposition of silane gas, diluted with hydrogen, in radiofrequency glow discharge. Films with nano-crystallites of different sizes were achieved by varying silane to hydrogen ratio. The nanostructure of silicon thin films was studied by Raman spectroscopy (RS) and high resolution transmission electron microscopy (HRTEM) while spectral distribution of absorption coefficients was calculated from transmittance and reflectance data. Raman spectrum of the microcrystalline Si shows one intensive sharp band at 521 cm-1. For crystallites smaller than 30 nm this band (TO mode) is broadened and its position is shifted to lower frequencies. The shift is dependent on the average crystallite sizes. The size of nano crystallites in investigated samples was estimated from the shift of TO mode in Raman spectra and was compared with HRTEM results. The excellent agreement of the results achieved by these two completely different methods was found. Since the shift of TO mode can be also caused by the strain in the thin film, the match of RS and HRTEM results indicates a relaxed structure. The optical gap depends on the size of nano-crystal due to the quantum effects, so the structural results were correlated to spectral distribution of absorption coefficients. The variation between obtained and theoretically expected values for absorption coefficients will be discussed in the framework of expected possibilities of tailoring the optical properties of nano-crystalline thin films for multi-layer solar cells structures.

solar cells; nanostructure; absorption coefficient

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