Nondestructive Characterization and Accelerated UV Testing of Browned Field-Aged PV Modules

Encapsulant browning is one of the most common degradation modes found in crystalline silicon field-aged photovoltaic modules. Browning, usually undetected unless severe, reduces the short-circuit current (I_sc) produced by a module. Therefore, field-aged browned modules have been subjected to accelerated UV testing to obtain true end-of-life activation energy for the encapsulant browning mechanism. A novel time- and resource-saving accelerated UV exposure testing method simultaneously allowing multiple module temperatures to be maintained in a single chamber run is presented. Six field-aged crystalline silicon modules of glass/backsheet construction (three each from BP Solar/Solarex MSX 60 and Siemens M55) with similar glass and encapsulant formulation were exposed to a UV dosage of 450 kWh/m². Through passive heating, the average module temperatures for the BP Solar/Solarex modules were 60 °C, 77 °C, and 85 °C and those of Siemens M55 were 75 °C, 80 °C, and 837 °C. To study UV browning, the modules were intermittently characterized through visual imaging, UV fluorescence imaging, electroluminescence imaging, quantum efficiency measurements, module, and cell-level light I-V measurements. An I_sc decrease corroborated the increased extent of browning with increased module temperature. For the BP Solar/Solarex modules, the Low T, Mid T, and the High T modules showed a 1.37%, 2.48%, and 3.26% I_sc drop, respectively. The Siemens modules showed a 4.11%, 5.65%, and 6.95% I_sc drop. The multiple cell-level I_sc data points obtained for each module temperature increased provided statistical significance. Activation energy for encapsulant browning was calculated as 0.44 and 0.72 eV for MSX 60 and M55 modules, respectively.