AbstractsEngineering

Crystalline silicon thin-film solar cells on solid-phase crystallised seed layer

by Wei Li




Institution: University of New South Wales
Department: Photovoltaics & Renewable Energy Engineering
Year: 2013
Keywords: Solar cells; Polycrystalline silicon thin film; Epitaxy; Solid-phase crystallisation; Seed layer
Record ID: 1034862
Full text PDF: http://handle.unsw.edu.au/1959.4/53082


Abstract

Performance of polycrystalline silicon thin film solar cells is limited by high defect density solid-phase crystallised material. A common approach to obtaining polycrystalline silicon films with fewer defects is epitaxy on a high crystal quality seed layer. Such a seed layer can be produced by aluminium-induced crystallisation but it results in films with heavy metal contamination. This work investigates an alternative seed layer prepared by solid-phase crystallisation of heavily phosphorous doped (���1��1020 cm-3) ultra-thin (��� 200 nm) silicon films which then can serve as the cell emitter. The effects of the a-Si precursor thickness and phosphorous concentration on the solid-phase crystallisation kinetics, structural and electronic qualities of the crystallised seed layer are analysed. By increasing the film thickness from 50 nm to 200 nm or increasing the phosphorous concentration up to 3��1020 cm-3, the time needed for complete crystallisation decreases and the structural and electronic quality of the seed layer improves. Polycrystalline silicon thin film solar cells on glass are successfully fabricated by solid-phase epitaxy and vapour-phase epitaxy on the solid-phase crystallised P-doped polycrystalline silicon seed layer. Solid-phase epitaxial layers achieve much better structural quality, longer minority diffusion length and thus better solar cell performance than the vapour-phase epitaxial layers. Therefore, solid-phase epitaxy on the randomly oriented solid-phase crystallised polycrystalline silicon seed layer is more suitable than the vapour-phase epitaxy. Seed layer thickness and phosphorous concentration can influence its crystal quality and thus affect the epitaxial solar cell performance. Transmission electron microscopy analysis proves that the intragrain defects (stacking faults, twins, and dislocations) originate from the seed layer thus limiting the solar cell performance. In order to eliminate the intragrain defects in the seed layer, two different approaches are attempted: rapid thermal and diode laser annealing. Increasing peak temperature of rapid thermal annealing can effectively lower the intragrain defect density and thus improve the structural and electronic qualities of the seed layer. Consequently, both solid-phase and vapour-phase epitaxial polycrystalline solar cells on the rapid thermal processed seed layer show better performance with higher cell efficiency, open-circuit voltage, short-circuit current and spectral response than the cells on the seed layer without rapid thermal annealing. Therefore, epitaxially grown solar cell performance is directly determined by the quality of the seed layer. Line-focus diode laser annealing allows achieving a higher Si film temperature without overheating the glass substrate. Due to efficient defect elimination by laser annealing, a higher dopant activation efficiency, carrier mobility and lower sheet resistance are achieved. However, after epitaxial growth on the diode laser annealed seed layer, the interface between seed layer and…