Impact of Junction Breakdown in Multi-Crystalline Silicon Solar Cells on Hot Spot Formation and Module Performance

Localized diode breakdown in mc-Si solar cells is suspected to be potentially critical to module encapsulation when occurring during the operation of a shaded solar cell in reverse. By modelling the operating point of two model cells under varying shading conditions, we show that every cell, breaking down or not, can suffer from significant power dissipation in a standard industrial module. We discuss, that early breakdown can even be beneficial concerning worst case total power dissipation in a shaded cell and module output power. Experimentally, we show that type I, II and III breakdown sites which were identified by DLIT and EL on solar cells from umg and virgin-grade feedstock have not led to critical hot spot temperatures. However, a newly observed edge effect which was activated at approximately 160°C after a significant shading time severely damaged the investigated module. Partial shading of solar cells revealed that the dark part of a cell, even when exhibiting the dominant breakdown mechanism during full shading, can be dominated by the illuminated part, which is supported by modelling a partially shaded cell with a parallel connection of a fully shaded and a fully illuminated cell.