Floating crystalline Si-foils for photovoltaic applications

A new Float Foil Growth (FFG) technique [1–2] has been demonstrated for growing thin crystalline 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 bath 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 high purity Si-foils. Si-foils with thickness range of 50–200µm were obtained from molten In baths. The Si-foils were mono and quasi-mono-crystalline 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 Si material cost and energy savings beyond 80%, compared with conventional sliced Si wafers, with similar photovoltaic conversion efficiency.