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

Abstract

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.

Hydrogen induced contact resistance in PERC solar cells

“…It has been suggested that this unstable increase in contact resistance is related to the motion of hydrogen [10]. This is in good agreement with recent simulations on the re-distribution of hydrogen at similar temperatures [11] which predicts that there will be a significant build-up of hydrogen at the metal contacts.…”

“…These processes are often intended to getter or improve passivation of defects [1][2][3][4], or to mitigate degradation effects [5][6][7][8]. While these approaches have proven to be effective in improving the bulk lifetime and longterm stability of devices, an unintended consequence has been a reported increase in device series resistance (RS) [5,9,10].…”

“…Initial observations of this effect on heavily diffused emitters concluded that the increased RS was the result of a thickening of the glass layer surrounding silver crystallites at the Ag-Si interface [9]. However, more recently the authors have reported an unstable component to the RS increase that appears to be related to the motion of charged particles [10]. This unstable component has been shown to be most significant at temperatures between 450 and 500 o C, while a more permanent increase in RS is observed at higher temperatures.…”

“…This unstable component has been shown to be most significant at temperatures between 450 and 500 o C, while a more permanent increase in RS is observed at higher temperatures. Further investigation has revealed that this increase in RS is almost exclusively due to an increase in the contact resistance at the interface between the screen-printed silver fingers and the silicon surface [9,10].…”

High‐voltage p‐type PERC solar cells with anchored plating and hydrogenation

“…However, the hydrogenation appeared to be having another impact on the FF, actually causing it to be worse on the standard cells (72.4), or those with less significant anchor point damage (50% coverage or less). It is well‐known that hydrogen can passivate or neutralize dopants in the bulk or emitter regions, and more recently, it has also been suggested that hydrogen could play a role in increasing front contact resistance on screen print contact cells, which could potentially also occur in the case of plated contacts . It is therefore possible that hydrogenation processes can increase the series resistance of a cell, which could potentially cause the observed FF loss.…”