Unveiling the root causes of metal contact failures in TOPCon solar cells via accelerated damp-heat testing

Xinyuan Wu¹, Chandany Sen¹, Xutao Wang¹, Yuhao Cheng¹, Ruirui Lv², Hao Song², Yuanjie Yu², Baochen Liao³, Sheng Ma⁴, Muhammad Umair Khan¹, Alison Ciesla¹, Bram Hoex^1,*

¹ School of Photovoltaic and Renewable Energy Engineering, UNSW, Australia

² Canadian Solar Inc.

³ School of Microelectronics and School of Integrated Circuits, Nantong University

⁴ Institute of Solar Energy, Shanghai Jiao Tong University

^*E-mail: b.hoex@unsw.edu.au

Abstract

Tunnel oxide passivated contact (TOPCon) solar cells are poised to lead the global photovoltaic market due to rapid gains in power conversion efficiency (PCE). However, concerns remain regarding the reliability of these cells, particularly under hot and humid conditions. Current module-level analyses often provide slow feedback, delaying process optimization. In this study, we investigate the degradation of metal contacts in TOPCon cells under accelerated damp-heat conditions (85°C, 85% relative humidity, DH85) using sodium bicarbonate (NaHCO₃) and sodium chloride (NaCl), two salts commonly involved in solar cell testing. Results show that NaHCO₃ on the front side leads to a ~5% relative PCE drop after 100 hours of exposure, while NaCl causes a drastic ~92% relative reduction, both primarily driven by increased series resistance (R_s) due to electrochemical reactions in the Ag/Al paste. On the rear side, NaHCO₃ results in ~16% relative PCE loss due to increased recombination and contact degradation, whereas NaCl primarily affects open-circuit voltage (V_oc), leading to a ~4% relative PCE loss. This study focuses on rear-side degradation mechanisms, with significant oxidation observed at the Ag-Si interface. The findings highlight the vulnerability of TOPCon solar cells to contact corrosion, underscoring the electrochemical activity of metallisation as a risk for long-term module performance. These insights are vital for enhancing the reliability and durability of TOPCon solar modules through improved process optimization. 

Keywords: Photovoltaics; Tunnel oxide passivated contact (TOPCon); Damp-heat stability; Silicon solar cells; Metallization reliability; Corrosion  

Biography:

Xinyuan Wu is a postdoctoral fellow at School of Photovoltaics and Renewable Energy Engineering, UNSW Sydney, Australia. His research focuses on the industrial solar cell’s reliability and performance enhancement combining nano-scale engineering. He was the process engineer on front surface passivation optimization at Jiangsu Leadmicro Nano-Equipment Technology Ltd. His research work has been published in several international conferences and peer-review journals.