Investigation of Degradation Mode Spreading Interconnectors by Pressure-Cooker Testing of Photovoltaic Cells

We investigated the mechanism of the degradation mode spreading from interconnectors, a phenomenon that is frequently observed for crystalline silicon photovoltaic modules after damp heating or pressure-cooker testing (PCT). Modules tabbed by soldering or conductive film (CF) with glass- backsheet or glass-glass structure were subjected by the PCT at 110 °C and 85% relative humidity for 1000 h, then evaluated by means of current-voltage measurements, electroluminescence imaging, cross-sectional scanning electron microscopy, and energy-dispersive x-ray analysis, whereas tabbing of interconnectors with solder showed the degradation spreading from interconnectors, tabbing with a CF or the use of glass-glass structures suppressed the degradation. In the degraded area, cross-sectional observations revealed the degradation of the interfacial glass layer between the electrode and the silicon, the precipitates on inside walls of pores in the electrode, and the presence of deposits composed of tin and oxygen on the surface of the silver finger electrode. On the basis of these results, in this article we discuss the degradation mechanism in terms of preferential corrosion of the interfacial glass layer under the silver finger electrode covered by deposits.