Intermediate Layers for Thin-Film Polycrystalline Silicon Solar Cells on Glass Formed by Diode Laser Crystallization
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Intermediate Layers for Thin-Film Polycrystalline Silicon Solar Cells on Glass Formed by Diode Laser Crystallization Jonathon Dore1, 2, Rhett Evans2, Bonne D. Eggleston1, 2, Sergey Varlamov1, Martin A. Green1 1 University of New South Wales, Kensington, NSW, 2051 Australia. 2 Suntech R&D Australia Pty Ltd, 82 Bay St, Botany, NSW, 2019 Australia. ABSTRACT Intermediate layers between silicon and borosilicate glass are investigated for compatibility with a diode laser crystallization technique for fabrication of thin-film polycrystalline silicon solar cells. SiCx, SiNx and SiOx layers or multilayer stacks of these materials have allowed silicon films of 10μm thickness to be successfully crystallized by diode laser irradiation without dewetting, with each option offering different advantages. SiCx allows the most robust crystallization process, while SiOx is the best barrier to contamination and the most stable layer. SiNx offers the best anti-reflection coating for superstrate configured solar cells. Presently, best device performance is achieved with a SiOx intermediate layer with cells achieving up to ~540 mV open-circuit voltage. INTRODUCTION Thin-film polycrystalline silicon solar cells on glass substrates have long been considered a potential competitor to standard, wafer-based photovoltaics, however, this has yet to be fully realized. Devices using silicon of a few microns thickness formed by solid-phase crystallization or aluminum-induced crystallization have been limited to low open-circuit voltage (VOC)[1, 2], while those using high-temperature, zone-melting processes require high-cost substrates[3]. Laser crystallization has been shown to produce large-grained, low-defect polysilicon seed layers for epitaxial growth methods without necessitating high-temperature substrates, but this has not yet produced devices with high efficiencies[4-6]. Recently, some progress has been made using an electron-beam crystallization method in which around 10 μm of silicon is crystallized in a single step (i.e. without epitaxy), resulting in high VOC[7]. The present work employs a similar technique, although with a diode laser instead of an ebeam. An intermediate layer (IL) between the glass and the silicon has been found to heavily influence the crystallization process and device characteristics. This is not surprising, considering the many functions of this layer, including a wetting layer, a barrier to diffusion of contaminants, a dopant source, a passivation layer and an anti-reflection coating as well as requiring stability at temperatures above the melting point of silicon. EXPERIMENT A boron-doped IL is deposited by RF sputtering onto borosilicate glass (BSG). ~10 μm of a-Si is then deposited by e-beam evaporation. The silicon is melted by scanning an 11 mm long, 808 nm wavelength Limo diode laser beam across the sample. Upon re-solidification, the silicon crystallizes with grains up to several mm long and several hundred μm wide[8]. Silicon carbide (SiCx), silicon nitride (SiNx) and silicon oxide (SiOx) are all candidat
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