A unified mechanism for ultraviolet-induced degradation in TOPCon solar cells

Bram Hoex¹, Muhammad Umair Khan¹, Alison Ciesla, Aeron Johns¹, Chandany Sen¹, Xinyuan Wu¹, Haoran Wang¹, Xutao Wang¹,

Ting Huang², Hao Song², Munan Gao², Ruirui Lv², Yuanjie Yu²

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

² CSI Solar Co. Ltd.

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

Abstract

Tunnel oxide passivated contact (TOPCon) solar cells, among the most efficient silicon architectures, face a critical challenge: degradation under ultraviolet (UV) exposure. Here we uncover the physical origin and dynamic evolution of UV-induced degradation (UVID) in TOPCon lifetime structures containing thin (4 nm) and thick (7 nm) AlOₓ layers. By cycling the samples through UV exposure, dark storage, and dark annealing, we disentangle how chemical and field-effect passivation respond to light stress. High-energy UV photons break Si–H bonds in the SiNₓ capping layer, releasing mobile hydrogen that accumulates at the Si/AlOₓ interface and generates recombination-active defects. This loss of chemical passivation, seen as a rise in interface defect density, is partly masked by a transient enhancement of field-effect passivation as trapped charges increase the negative fixed charge in AlOₓ. During subsequent dark storage, these charges de-trap, revealing the underlying chemical damage. Dark annealing drives the hydrogen back into the silicon bulk, restoring the interface passivation, but FTIR spectra shows that the dielectric stack undergoes a permanent structural rearrangement. Devices with thicker AlOₓ exhibit markedly greater UV resilience, acting as a stronger barrier to hydrogen transport. Our findings establish a complete mechanistic picture of UVID in TOPCon structures and provide clear design rules to engineer UV-stable passivation stacks for long-lived, high-efficiency silicon photovoltaics.

Keywords: TOPCon; UV induced degradation

Biography:

Bram Hoex is a Professor and Deputy Head at UNSW Sydney's School of Photovoltaic and Renewable Energy Engineering. His research focuses on solar cell efficiency improvement, reliability and performance, financial and performance yield modelling, and the application of artificial intelligence for material discovery. He published over 300 papers and has received multiple international awards, including the 2008 SolarWorld Junior Einstein and 2016 IEEE PVSC Young Professional. In 2018, Renewable Energy World featured him on their global "Solar 40 under 40" list.