Root cause defect identification in multicrystalline silicon for improved photovoltaic module reliability

Abstract

To meet climate targets by 2030, manufacturing capacity for photovoltaic (PV) modules must be scaled at 22-25% annual growth rate while maintaining high performance and low selling price. The most suitable material substrate to enable this scale-up is cast multicrystalline silicon (mc-Si) due to its low operating cost and capital requirements compared to other technologies. However, a new form of light-induced degradation was discovered when transitioning mc-Si to the latest high efficiency device architecture. Light- and elevated temperature-induced degradation (LeTID) causes performance to decrease by about 10% (relative) under field-relevant conditions within only four months. In this work, the root cause of LeTID is investigated in three parts: (1) Candidate hypotheses are developed for LeTID; (2) Targeted experiments are carried out toward developing a defect-based description of LeTID; and (3) The basis for a predictive model of LeTID is proposed. Techniques including minority carrier lifetime spectroscopy, synchrotron-based X-ray fluorescence, intentional contamination, and process simulation are employed to probe the defect causing LeTID. The results indicate that LeTID is caused by at least two reactants-hydrogen and one or more reactants that can be modified by high-temperature processing-and that the defect at the point of maximum degradation has recombination characteristics similar to a deep-level donor in silicon. By providing the basis for a predictive model, this work enables both identification of the root cause of LeTID and de-risking of novel solar cell architectures based on mc-Si, allowing assessment of the impact of LeTID on the future of the PV industry. This work also enables development of mitigating strategies for LeTID.