Unveiling the origin of metal contact failures in TOPCon solar cells through accelerated damp-heat testing

Abstract

Tunnel oxide passivated contact (TOPCon) solar cells are expected to dominate the global photovoltaic market in the coming decade thanks to rapid advancements in power conversion efficiency (PCE). However, there are concerns about the reliability of TOPCon modules, particularly in hot and humid conditions. The current module-level fundamental analysis strategies for TOPCon solar cells provide too slow feedback for rapid process development. This study explores the degradation of metal contacts in TOPCon solar cells under accelerated testing conditions of 85 °C and 85 % relative humidity (DH85). The degradation was induced by two commonly used sodium-related salts, sodium bicarbonate (NaHCO₃) and sodium chloride (NaCl), in the testing of the solar cells. When applied to the front side, NaHCO₃ caused a ∼5%_rel PCE reduction after 100-h DH85 exposure, while NaCl leads to a more significant ∼92%_rel PCE reduction. The primary cause of degradation is a considerable increase in series resistance (R_s), likely due to electrochemical reactions within the Ag/Al paste. When the salts are applied to the rear of the TOPCon solar cell, the degradation becomes more complex. NaHCO₃ increases recombination and results in a deterioration of the contact, resulting in a ∼16%_rel PCE reduction after 100-h DH85 testing. Conversely, NaCl primarily causes a decline in open-circuit voltage (V_oc) and a ∼4%_rel PCE loss. This manuscript primarily investigates degradation mechanisms related on the rear side, with a focus on significant oxidation occurring at the interface between Ag and Si. These findings highlight the susceptibility of TOPCon solar cells to contact corrosion, emphasizing the electrochemical reactivity of metallisation as a potential risk for long-term TOPCon module operation. This study provides crucial insights into TOPCon cell degradation mechanisms, which are essential for optimising performance and enhancing the long-term reliability of TOPCon modules.