Improvement of the PID problem of bifacial PERC solar modules based on rear passivation coating optimization

Bifacial silicon PERC solar module can increase the electric energy production in photovoltaic power station through utilizing the ambient reflected light. However, the rear finger grid electrode permits the Na+ to penetrate into the rear passivation coating, which results in a much more severe PID problem. Therefore, in this report three methods of adjusting the rear passivation coating were attempted to mitigate the degradation after PID stress. Through increasing the refractive index of the third SiNx:H layer in rear passivation coating, a much lower degradation ratio of rear maximum power has been achieved, which can be explained by the smaller positive charge accumulated in the Al2O3 layer. Besides, a thermally grown SiO2 layer was introduced under Al2O3. However, the underneath SiO2 cannot reduce the influence of accumulated positive charge on the field-effect passivation of Al2O3. Therefore, SiOx was deposited onto the Al2O3 layer by PECVD. In line with the result of refractive index increase of the third SiNx:H layer, the degradation ratio of rear maximum power declines remarkably as well. The spread of Na+ in the silicon oxide layer may result in a low value of positive interface charge density between SiOx and Al2O3, which suppresses the deterioration of rear surface passivation and rear maximum power. Besides, the lower efficiency of solar cell with PECVD-deposited SiOx, which may be attributed to the influence of SiOx on Al2O3, should be considered as part of the reason for the improvement of PID problem as well.