Instability of Increased Contact Resistance in Silicon Solar Cells Following Post-Firing Thermal Processes

Recently, there have been reports of increased series resistance as a consequence of thermal processes applied after the co-firing of screen-printed silicon solar cells. A previous observation of this effect on very heavily diffused emitters concluded that the increased series resistance is the result of a thickening of the glass layer surrounding silver crystallites at the Ag-Si interface. Here, large increases in the front silver contact resistance after particular thermal anneals are reported that have been used to mitigate carrier-induced degradation (CID) in multi-crystalline solar cells that cannot be fully explained by a thickening of the glass layer. Remarkably, under certain conditions the contact resistance immediately after annealing is found to be unstable - decreasing when a forward current is applied to the solar cell, and gradually increasing again once the forward current is removed. It is speculated that the movement of charged particles, most likely hydrogen, could be the cause of this instability.