Heat Induced Degradation and Chemical Analysis of Silicon Heterojunction Solar Cells

Silicon heterojunction cell performance degradation was investigated by accelerated lifetime testing (ALT) by applying 130°C heat or 1-sun light stress. Cell VOC was found to lose approximately 2 to 10% of initial value after 10 days under heat stress. Cells exposed to light stress, however, showed no degradation in VOC. Our results indicate that cell FF loss after heat stress is the primary culprit towards cell efficiency loss. Cells lose 25% to 60% of their initial FF value under heat stress after a 10 day period has elapsed. Surprisingly, ToF-SIMS depth profiling revealed hydrogen effusion and/or movement only within the p-type amorphous silicon layer. This result points to band alignment at the intrinsic/p-type junction, or p-type/ITO interface, as primarily responsible for increased FF loss and subsequent loss of cell efficiency. Our results also indicate that tuning intrinsic amorphous silicon cell layers to increase hydrogen content reduces the propensity of cells to lose VOC and FF.