Defining global testing standards for UV-induced degradation in TOPCon solar cells

As TOPCon silicon solar cells become the dominant technology in global photovoltaic manufacturing, long-term stability has become as critical as peak efficiency. One increasingly important stress factor is ultraviolet (UV) radiation, which can trigger performance losses over time. At UNSW, Professor Bram Hoex and his research group are leading the international effort to understand, quantify and standardise how UV-induced degradation (UVID) should be tested and mitigated in modern silicon solar cells¹˒².

Supported by ACAP-funded infrastructure and a 2025 collaborative research project, the Hoex group has carried out one of the most comprehensive investigations of UVID in TOPCon devices worldwide. Experimental work is led at UNSW, with close industrial collaboration with Canadian Solar. Together, this partnership has developed a practical UV cell-testing protocol that is now informing the emerging IEC standard for UVID testing.

Progressing from observation, to mechanism and then to guidance for industry

In the first study, the team compared exposure to UV-A and UV-B radiation under controlled conditions. Both wavelength ranges produced the same degradation mechanism, but UV-B acted much faster¹. This finding shows that UV-B can be used to accelerate reliability testing without introducing artificial failure modes. This also highlights that areas with high UV-B, such as the Australian outback, are at greater risk for this degradation. The work also demonstrated that the current rear side of TOPCon cells is inherently stable, as the doped polysilicon layer blocks short-wavelength UV from reaching the sensitive tunnel oxide, making them suitable for bifacial applications.

In a second, complementary study, the group developed a comprehensive model describing how UVID develops and recovers². Ultraviolet photons break silicon–hydrogen bonds near the front surface, releasing mobile hydrogen that accumulates at the interface and increases recombination losses. At the same time, UV exposure temporarily traps negative charge in the aluminium oxide layer, masking some of the damage. After illumination stops, performance can continue to drift during dark storage as these charges are released. A brief low-temperature anneal restores electrical performance, even though spectroscopy reveals a permanent rearrangement within the dielectric stack.

As Professor Bram Hoex explains, “By separating what is reversible from what is permanent, we can finally design UV tests that reflect how TOPCon cells really behave in the field — and avoid both under- and over-testing.”

The work has gained strong international visibility. Prof Bram Hoex, on behalf of Dr Muhammed Umair Khan (who was unable to travel), presented this work in a plenary session at EU PVSEC 2025 in Bilbao³, and the findings have been widely reported by PV Magazine and PV Tech.

Reflecting on the impact, Dr Khan notes, “Our results show that UV-induced degradation is not a single effect, but a coupled chemical and electrical process –  once you understand that, you can design cells and tests that are far more resilient.”

Key contributors include Muhammad Umair Khan, Chandany Sen, Alison Ciesla, Aeron Johns, Michael Pollard, Ting Huang, Munan Gao, Ruirui Lv, Yuanjie Yu, Xinyuan Wu, Haoran Wang and Xutao Wang, under the leadership of Professor Hoex.

(l-r) Dr Alison Ciesla, Dr Chandany Sen, Haoran Wang, Dr Muhammad Umair Khan, Prof Bram Hoex, Xutao Wang, Jiexi Fu, Dr Xinyuan Wu. With ACAP support, UNSW's Professor Bram Hoex and his research group are leading the international effort to understand, quantify and standardise how UV-induced degradation (UVID) should be tested and mitigated in modern silicon solar cells. Image: Rob Largent / UNSW

Together, this body of research positions UNSW as a global reference point for UV reliability in TOPCon technology and directly shapes how the next generation of silicon solar cells will be tested and deployed.

References

1. Khan, M. U., Sen, C., Pollard, M., Huang, T., Gao, M., Lv, R., Yu, Y., Wu, X., Wang, H., Wang, X. & Hoex, B. (2025) UV-induced degradation in TOPCon solar cells: hydrogen dynamics and impact of UV wavelength. Solar Energy Materials and Solar Cells, 268, 113895. https://doi.org/10.1016/j.solmat.2025.113895

2. Khan, M. U., Ciesla, A., Johns, A., Sen, C., Huang, T., Song, H., Gao, M., Lv, R., Yu, Y., Wu, X., Wang, H., Wang, X. & Hoex, B. (2025) Charge trapping, hydrogen accumulation and structural rearrangement: a complete model for ultraviolet-induced degradation in TOPCon devices. Solar Energy Materials and Solar Cells, 269, 114149. https://doi.org/10.1016/j.solmat.2025.114149

3. Hoex, B. & Khan, M. U. (2025) UV-induced degradation mechanisms and testing standards for TOPCon solar cells. Plenary presentation, EU PVSEC 2025, Bilbao, Spain.

4. Khan, M. U. & Hoex, B. (2025) Understanding and mitigating UV-induced degradation in TOPCon solar cells. Sinovoltaics Technical Webinar, online.