Float Foil Growth of Si-Foils for Solar Cell Applications
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1245-A11-03
Float Foil Growth of Si-Foils for Solar Cell Applications Uri Cohen1 and Michael Roitberg1 1 Ribbon Technology LLC, 4147 Dake Avenue, Palo Alto, CA 94306, U.S.A. ABSTRACT A new Float Foil Growth (FFG) technique has been demonstrated for growing thin Si-foils from molten metal solvent, such as molten indium (In) or tin (Sn), at temperatures below 1,000°C. Si-source is first dissolved to saturation (or close to saturation) in a molten metallic bath (or solvent) at a temperature T2 (T2 ≤ 1,000°C), and the molten bath is then cooled to T1, where T2 » T1. Due to lower solubility of Si at T1 than at T2, Si separates (or is driven) out of solution and, due to its much lower density than that of the molten metallic bath, it floats to the top of the melt to form a floating thin Si-foil. The thickness of the Si-foil is determined primarily by T2, the dissolution temperature (i.e., Si solubility at T2), and the depth of the molten bath. This paper reports preliminary results demonstrating the utility of the FFG technique for growing Si-foils. Si-foils with thickness range of 50-200µm were obtained from molten In baths. The Si-foils were multicrystalline with crystalline (or grain) size of several millimeters, having a strong preferred orientation. The Si-foils were very pure; with In (solvent) content as low as 14ppb. Other metallic impurities were below 0.1ppm, oxygen content was as low as 1.8ppm, and carbon content was below the detection level (50ppb). It is expected that large FFG thin Si-foils, when produced on large scale, will offer significant Si material cost and energy savings (> 80%), compared with conventional sliced Si wafers, with similar photovoltaic conversion efficiency. INTRODUCTION Silicon solar cell panels are usually fabricated from polished round wafers, sliced from single-crystalline or polycrystalline Si ingots. The sawing, polishing, and etching of the wafers results in a costly loss, or "kerf", of Si material. Also, the process of growing single-crystalline or polycrystalline ingots is energy-intensive and costly. In addition, the size of the round wafers is limited, having a typical size of less than 200mm or 300mm in diameter. Therefore, many wafers are required to assemble one panel. To minimize efficiency losses due to unused area between round wafers on a panel, the round ingots are first machined into elongated semi-square rods prior to slicing them into semi-square wafers. This machining results in a further Si material loss. Growing large (single-crystalline, or large-grain polycrystalline) thin Si-foils, at lower temperatures (≤ 1,000°C) than conventional Si growth from its melt (.1,414°C), will offer significant (> 80%) Si material cost reduction and energy savings (compared with conventional Si wafers), along with high solar cell conversion efficiency. This paper reports preliminary results obtained in a feasibility study of a new Floating Foil Growth (FFG) technique [1-2] for growing thin Si-foils at temperatures below 1,000°C. Si-source is first dissolved to saturation (or cl
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