Physics of potential-induced degradation in bifacial p-PERC solar cells

Abstract

The combination of increasing operational voltages beyond 1000 V in photovoltaic (PV) installations and the emergence of new PV technologies requires a critical assessment of the susceptibility to potential-induced degradation (PID). Since this failure mode can trigger significant and rapid power losses, it is considered among the most critical failure modes with a high financial impact. Insights in the physical mechanism of the performance loss and its driving factors are critical to develop adapted characterization methods and mitigation solutions. PID in p-type solar cells is triggered by sodium (Na) that diffuses into stacking faults of the silicon lattice, causing shunt paths through the pn-junction. In addition, it is hypothesised that for bifacial p-PERC solar cells positive charges, such as Na + , accumulate in/on the negatively charged AlO x rear passivation layer due to the potential difference between the glass and the rear cell surface. This significantly increases surface recombination. However, the degradation behaviour observed in bifacial monocrystalline p-PERC solar cells under PID stress from both sides (bifacial PID stress) does not match with just one of the degradation mechanisms. A comprehensive test matrix was carried out to understand the physical origin of PID in front emitter bifacial p-PERC solar cells in a glass/glass packaging. The results show that bifacial p-PERC solar cells under bifacial PID stress suffer from both shunting of the pn-junction and increased surface recombination at their rear side. Hereby, we prove that the glass/glass packaging in combination with bifacial solar cells can significantly increase the severity of PID.

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

“…Because it has been shown in the literature that PID can be avoided by using a transparent backsheet/frontsheet, no degradation due to PID is expected at this side of the solar cell unless the degradation mechanism under rear-side monofacial PID appears to be the same as under bifacial PID stress. 22 The light I-V curves under front-and rear-side illumination measurements of the first laminate in configuration B during rear-side monofacial PID stress are shown in Figure 6A. Indeed, the results are very clear: Over 130 h of PID stress resulted in a negligible P MAX degradation level of 4% and 2% under front-side illumination measurements for sample one and sample two, respectively.…”

“…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 ).…”

“…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. 22,23 PV module manufacturers, at the final stage of product certification, are mostly interested in PID qualification tests and mainly perform the experiments according to stress method (a) as described in IEC TS 62804-1, 34 that is, testing in damp heat using an environmental chamber, which results in bifacial PID stress with an inhomogeneous electric field distribution over the front and rear covers of the (bifacial) PV module. Alternatively, in the early stage of the cell and/or module material development, quantitative testing is implemented according to stress method (b) as described in the IEC TS 62804-1, that is, contacting the surface with a conductive electrode, hereafter referred to as foil-method.…”

Reduced power degradation in bifacial PERC modules by a rear silicon oxide additive layer

“…10 PID-s is thus represented by a drop in shunt resistance (R sh ) and fill factor (FF). 11 Contrarily, in the case of rear side PID, it is governed by PID of the polarization type (PID-p), whereby Na + ions also drift toward the AlO x /SiN x stack when the rear side of the solar cells is negatively biased. Instead of causing shunting paths, Na + ions and possibly other sources of positive charge reduce the fixed negative charge density of AlO x , which subsequently diminishes the field passivation effect of AlO x.…”

Mitigating Potential-Induced Degradation (PID) Using SiO2 ARC Layer

“…The investigation of bifacial PID in bifacial mono c-Si p-PERC solar cells was developed by Carolus et al [17]. It was evident that there is more considerable reduction in the efficiency of bifacial solar cells that have glass packing renders, which tends to be extremely sensitive to PID testing.…”