Ultrasonically tinned PVD AL rear contacts on high-efficiency crystalline silicon solar cells for module integration
We investigated ultrasonic tinning of physical vapor deposited (PVD) Al rear contacts on p-type passivated emitter and rear cells and n-type back junction cells, respectively. By means of electroluminescence (EL) imaging after ultrasonic tinning we identified small local damage of the cells' rear side passivation only at the positions where the soldering irons were initially placed on the Al. Though measured interconnector peel force was < 1 N/mm and hence lower then required by the standard DIN EN 50461, we obtained power degradation < 5 % after 400 thermal cycles, 2000 h in damp heat test and 20 cycles in the humidity freeze test, respectively, which is double the load demanded by the IEC 61215 standard. EL images revealed no local degradation induced by the climate tests. The microstructure of the tinned PVD Al is investigated by secondary electron microscopy and energy dispersive X-ray spectroscopy.