Al/Ni : V/Ag metal stacks as rear‐side metallization for crystalline silicon solar cells

Rear sides of crystalline silicon solar cells are usually covered with aluminum on which it is difficult to solder. To ease soldering, we present a durability study for a Ni : V/Ag stack on evaporated Al as rear‐side metallization. We adapt this cost‐effective metallization stack from the microelectronic industry and investigate it as metallization for silicon solar cells. Here, a long‐term stability of the metallization and of the solder joint must be guaranteed for 25 years and is therefore evaluated in detail by thermal aging experiments. During this experiment, the mechanical stability of the solder joints is measured. The chemical stability and the intermetallic compound (IMC) growth within the solder joints are examined by secondary electron microscopy, backscattered electron imaging, and energy dispersive X‐ray analysis. Experiments with either a Sn–Ag‐coated copper tab or pure Sn–Ag solder show two different sorts of IMCs at the Ni : V/Solder interface. With the copper tab, a Cu–Ni–Sn compound, presumably (Cu_{1 ‐ x}Ni_x)₆Sn₅, grows at the Ni/solder interface, whereas in case of a pure Sn–Ag solder, a Ni–Sn compound grows, which is likely to be Ni₃Sn₄. Analysis of the reaction kinetics leads to activation energies of 77 and 42 kJ/mol, respectively, for a diffusion‐controlled IMC growth. By using temperature histograms of PV modules in the field, the necessary minimum Ni : V layer thickness is estimated: without a copper tab up to 1.6 µm Ni and with a copper tab less than 0.2 µm may be consumed by IMC formation during 25 years of lifetime. Copyright © 2012 John Wiley & Sons, Ltd.