EL and I-V Correlation for Degradation of PERC vs. Al-BSF Commercial Modules in Accelerated Exposures

Passivated emitter rear contact (PERC) cells offer increased power conversion efficiency but also present several degradation risks compared to the traditional aluminum back surface field (Al-BSF) cell, including instability of the passivation layer and increased light-induced degradation. Newer generations of bifacial PERC cells with localized back contacts (as opposed to full rear side metalization) introduce further vulnerabilities such as mechanical cracking and greater susceptibility to corrosion. In this work, we evaluate these degradation modes through accelerated exposures of full-size modules, using advanced characterization and analysis techniques. Damp-heat and thermal cycling are used to activate distinct degradation modes in Al-BSF, PERC, and bifacial PERC full-size modules. At stepwise intervals during exposure, the modules are measured by $I -V$ curve tracing and electroluminescence (EL) imaging. EL images are standardized to allow for calculation of quantitative image parameters to be compared with standard $I -V$ parameters. Correlation of parameters extracted from $I -V$ and EL reveals statistical relationships between degradation mechanisms and performance, for module and accelerated exposure type. Bifacial PERC modules showed both the greatest power degradation in damp heat, and the most cell cracking in thermal cycling. The greater power degradation through damp heat exposure for modules with bifacial PERC cells is likely the result of both increased susceptibility of this cell type to corrosion, and instability of the rear-side encapsulant, white EVA.