Understanding the Degradation of Silicon Heterojunction Modules

With silicon heterojunction (SHJ) solar cells being close to their practical efficiency limit, long-term reliability, and stability are the lowest hanging fruit to improve their market adoption. However, the passivation degradation over time under field operating conditions at amorphous silicon (a-Si:H) and crystalline silicon (c-Si) interface remains a concern. Herein, we compare our damp heat accelerated tests and fielded module results to the aging behaviour of the passivation quality of a-Si:H/c-Si. We extract surface recombination velocity (SRV) using the temperature- and injection- dependent lifetime spectroscopy technique and model the expected cell performance over time. Our results show a degradation of the a-Si/c-Si interface through an increase in SRV originating from failing chemical passivation exhibited by an increase in defect density at the interface. The field passivation for the a-Si:H/c-Si stack remains the same through the time of these experiments. Moreover, microstructural analysis shows increase in defect density originates due to changes at the interface and not in the bulk of a-Si:H.