Polarization-type Potential-induced Degradation of Bifacial p-PERC Solar Cells: Impact of Dark Storage

Currently, polarization-type potential induced degradation (PID-p) is still a challenge for p-type PERC bifacial solar cells due to the polarization effect which causes an accumulation of charges in the passivation layer on the rear side of the cells. These accumulated charges decrease short circuit current (I<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</inf>), open circuit voltage (Voc), and power (P<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf>) in certain p-PERC modules. In this study, we investigated the behavior and progression of PID-p, with intermittent dark storage intervals, by applying high voltage on the rear side of the bifacial silicon solar cell for a long period. Tests were conducted under -1000V bias voltage, 35 °C for 10h. A total of 10 tests (100h) were conducted with intermittent 2-4 days of dark storage intervals, and PL images were taken before/after each test to observe degradation. We closely monitored Isc and observed a slight increase in Isc during the 1<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> stress for 10s, then a total of 5% degradation happened for 30h. Then, the cell showed a recovery to 99% of the initial value. After that, the I<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</inf> started to decrease again. Notably, more degradation was observed after the dark storage, with a highest of 16.5% degradation which was recoverable with bias voltage. Collaborations at Arizona State University (ASU) and University of Central Florida also conducted PID experiments on custom minimodules fabricated using identical solar cells but different front and rear encapsulants under indoor (-1500V on the rear, 45 °C, 168h) and outdoor field conditions (-1500V for a couple of months), respectively, followed by dark storage afterwards. However, only the ASU minimodules exhibited degradation during stress and further degradation after dark storage. The key finding of our study is that while PID-p generally causes degradation in bifacial cells, additional degradation can also occur during dark storage, and it should be considered when characterizing commercial bifacial p-PERC modules.