Enabling Solderability of PVD Al Rear Contacts on High-Efficiency Crystalline Silicon Solar Cells by Wet Chemical Treatment

Results of an electroless displacement process for the deposition of inexpensive solderable metal on physical vapour deposited (PVD) aluminum are presented. The dip - rinse - dry processing sequence for electroless metal deposition on Al is simple and easy to use because it does not harm the SiN antireflection coating and the screen-printed Ag metallization on the cells' front side. Hence, finished cells can entirely be immersed into the electrolyte. The deposited metal layer is highly porous, but forms a massive alloy with common Sn/Pb/Ag solder at low temperature of 245 °C in the presence of no-clean flux. Because the molten alloy wets Al and copper well, we obtained strong solder joints for Sn/Pb/Ag coated copper ribbons on Al. The measured average 90° peel forces of ribbons soft soldered to thermally evaporated Al on shiny etched silicon and on the rear of alkaline textured monocrystalline Si PERC solar cells were up to 3.1 and 2.2 N per mm ribbon width, respectively. These values exceed 1 N/mm that is specified by the standard DIN EN 50461. Climate tests on single-cell glass-glass modules yielded 4.1 and 7.7 % power degradation after 200 thermal cycles from -40 °C to 85 °C and after 1000 h under damp-heat conditions at 85 °C and 85 % relative humidity, respectively. We attribute the too high power degradation after damp-heat test to residuals of the acidic flux which was used for soldering the cells in this first, older version of the developed technology.