Phase transformations during the Ag–In plating and bonding of vertical diode elements of multijunction solar cells

The conditions of the bonding of silicon multijunction solar cells with vertical $p$–$n$ junctions using Ag–In solder are studied. The compositions of electrodeposited indium films on silicon wafers silver plated by screen printing and silver and indium films fabricated by layer-by-layer electrochemical deposition onto the surface of silicon vertical diode cells silver plated in vacuum are studied. Studying the electrochemical-deposition conditions, structure, and surface morphology of the grown layers showed that guaranteed bonding is provided by 8-min heat treatment at 400$^\circ$C under the pressure of a stack of metallized silicon wafers; however, the ratio of the indium and silver layer thicknesses should not exceed 1: 3. As this condition is satisfied, the solder after wafer bonding has the InAg$_3$ structure (or InAg$_3$ with an Ag phase admixture), due to which the junction melting point exceeds 700$^\circ$C, which guarantees the functioning of such solar cells under concentrated illumination.