Float Foil Growth of Si-Foils for Solar Cell Applications

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 <111> 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.