Electrodeposition of Aluminum onto Silicon in Molten Chloride Salt

Abstract

One-step electrodeposition of aluminum metal onto silicon from molten salt is described herein as an alternative to conventional multi-step forming electrical conductors on silicon, like the screen-printing of silver paste, firing to sinter silver metal and form electrical contact to the silicon. Applications include metal current-collectors in solar cells and electronics. Treatment of a silicon surface with 1% hydrofluoric acid before plating is effective to remove silicon oxides that inhibit electrical contact and adhesion of aluminum metal to silicon surfaces. Plating is done with a silicon working electrode, aluminum counter electrode, and an aluminum/aluminum-ion reference electrode. The plating bath is a eutectic (melting point = 91.4°C) molten salt consisting of NaCl-KCl-AlCl3 with molar fractions of 26.25%-15%-58.75%, respectively. Aluminum is potentiostatically plated onto silicon in a range of 0.156 to 0.336 volts vs NHE, resulting current densities between 4-12 mA/cm2 with smooth and uniform plate thickness, electrical efficiencies of up to 94% and aluminum metal resistivities as low as 13.67×10-6 Ω·m. The aluminum plate has good adhesion to silicon, as evidenced by no delamination by ASTM D3359 tape pull test. Adhesion of aluminum on silicon does not show any noticeable change whether the silicon was smooth (real/projected area < 1.1) or sanded with a 600 grit.