Unveiling the degradation mechanisms in silicon heterojunction solar cells under accelerated damp-heat testing

Abstract

Silicon heterojunction (SHJ) solar cells have become one of the mainstream solar cells in the current photovoltaic market due to their high efficiency. Still, concerns about their long-term reliability are seen as a potential issue restricting further marketisation. In particular, the sensitivity of silicon heterojunction solar cells to high temperatures and moisture is a concern. Sodium (Na) in combination with humidity is widely considered one of the causes of degradation in silicon heterojunction solar cells. Yet, a comprehensive understanding of the mechanisms behind Na-induced decay remains lacking. This study will investigate humidity-induced degradation of industrial SHJ solar cells at elevated temperatures using various sodium-containing salts [sodium bicarbonate (NaHCO₃), sodium chloride (NaCl), and sodium nitrate (NaNO₃)] to improve our fundamental understanding of Na-induced degradation. We will show that SHJ solar cells exposed to NaHCO₃ and NaCl show a significant reduction in efficiency, while solar cells exposed to NaNO₃ show minimal degradation. Further analysis indicates that NaHCO₃ may interact with the transparent conductive oxide (TCO) layer, leading to a reduction in surface passivation and a deterioration of the metal-TCO interface. NaCl primarily affects the Ag contact, resulting in a reduction of the adhesion of the screen-printed contact. Moreover, the TCO composition, particularly the oxygen content, influences its chemical tolerance. These results show that Na-related degradation is more complicated than initially thought. The chemistry is strongly influenced by the negative ions, as well as the composition of TCO and metal paste. These factors are determined by the bill of materials and the contaminants introduced during cell/module fabrication and operation of the SHJ module. The findings of this paper may lead to the development of new accelerated testing protocols for SHJ technology to ascertain long-term reliability in the field.