Local Corrosion of Silicon as Root Cause for Potential‐Induced Degradation at the Rear Side of Bifacial PERC Solar Cells

Abstract

Industrial bifacial PERC solar cells are exposed to a high‐voltage stress at the rear side. The tested cells show a potential‐induced degradation (PID) in Voc and Isc. From a front side current–voltage measurement, a power loss of about 12%rel is observed, however, without a significant change in the fill factor. The degradation is traced back to micron‐sized hole‐shaped damages of the rear surface, which correlate with localized regions of increased recombination. A focused ion beam (FIB) cross‐section through a hole‐shaped damage is performed for subsequent transmission electron microscopy (TEM) and energy‐dispersive X‐ray spectroscopy (EDXS). EDXS shows an accumulation of Na, K, and Ca impurities in the cavity. Thus, it is assumed that corrosion of the Si surface occurs beneath the AlOx/SiNy passivation layer stack and is accompanied by the formation of a SiO2 layer.

Why and how to adapt PID testing for bifacial PV modules?

“…For one of the PID-c mechanisms observed in bifacial solar cells, it is assumed that beneath the AlO X /SiN Y passivation layer stack a corrosion process of the Si surface occurs, resulting in the formation of a SiO 2 layer. 23 It has been shown that this degradation mode only affects I SC and V OC similarly to the behavior under PID-p. However, PID-c shows an irreversible behavior whereas PID-s and PID-p are shown to be reversible with thermal treatment and/or the application of reverse bias between the active cell circuit and the grounded module frame.…”

“…19 Until now, research has identified three different PID modes for bifacial c-Si solar cells, namely, PID of the shunting type (PID-s), PID of the polarization type (PID-p), and PID of the corrosive type (PID-c). [20][21][22][23] PID-s has been shown to be caused by sodium (Na) diffusing into silicon stacking faults through the pn-junction and thus shunting the cell. 20,[24][25][26] This degradation mode affects primarily the fill factor (FF), next the open-circuit voltage (V OC ), and lastly the short-circuit current (I SC ).…”

“…However, PID-c shows an irreversible behavior whereas PID-s and PID-p are shown to be reversible with thermal treatment and/or the application of reverse bias between the active cell circuit and the grounded module frame. 21,23,32 In addition, degradation due to PID-p has been shown to be recoverable by light, which might slow down or even mitigate the degradation process in the field. 21,23,33 Thoroughly understanding the physics and the underlying electrical/thermal/optical "signatures" of such PID mechanisms (both theoretically and experimentally) is of key importance for defining vsufficient and suitable reliability testing, as well as best practices for characterization and optimal material selection for PV module components and cell technology.…”

“…21,23,32 In addition, degradation due to PID-p has been shown to be recoverable by light, which might slow down or even mitigate the degradation process in the field. 21,23,33 Thoroughly understanding the physics and the underlying electrical/thermal/optical "signatures" of such PID mechanisms (both theoretically and experimentally) is of key importance for defining vsufficient and suitable reliability testing, as well as best practices for characterization and optimal material selection for PV module components and cell technology. Particularly, bifacial PV modules have been proven susceptible to the aforementioned PID mechanisms, which might occur in isolation or in combination on both the front and the rear side of the cell/module.…”

Bifacial p-Type PERC Solar Cell with Efficiency over 22% Using Laser Doped Selective Emitter

“…In recent years, more photovoltaic companies and researchers [1,2] began research and production of silicon bifacial solar cells and passivated emitter and rear cells (PERC) based on p-type crystalline Si wafers are mainstream high-efficiency cell technology although most of the advanced cell technology has their own dual-generation properties. The performance of industrial-type screen-printed 6-inch PERC solar cells has been significantly increased over the past few years.…”