Created by E.Ö., SUPSI, on 11.03.2026
PV modules installed in Alpine environments are exposed to significantly higher ultraviolet (UV) radiation due to increased solar irradiance and strong UV reflection from snow. At the same time, several new silicon solar cell technologies show increased UV sensitivity [1]. To investigate the impact of these conditions, we performed an indoor UV stress test sequence (Figure 1).
After UV exposure, the modules were stored in the dark and subsequently subjected to light soaking. This step is important because some UV-induced (UVID) performance changes are reversible. Therefore, measuring module performance after light soaking (LS) provides a more realistic estimate of long-term degradation [2]. Under real operating conditions, sunlight typically restores these reversible losses within 1–2 hours during the first period of illumination each day.
Four different module types were included in the UV stress test sequence: three TOPCon (tunnel oxide passivated contact) modules and one BC (back contact) module. One of the TOPCon modules (Module-1) is a standard, conventional design, while the other three are Alpine-relevant modules. These modules incorporate design features intended for Alpine conditions, such as thicker glass, reinforced or specialised frame geometries, and other adaptations that address Alpine stressors.
Figure 1: UV stress test sequence (UV: Ultraviolet, LS: Light Soaking).
Performance Changes After UV Exposure
After the UV60-front test, all four modules showed a measurable reduction in open-circuit voltage (Voc) and fill factor (FF) (Figure 2). These electrical changes were accompanied by a chequered degradation pattern visible in the electroluminescence (EL) images (Figure 3).
After 2 kWh/m² of light soaking, the modules partially recovered their performance loss.
Observed effects after UV60-front:
- 2–4 % reduction in maximum power (Pm)
- Mainly caused by 1–3 % loss in open-circuit voltage (Voc)
- Similar behaviour across all module types
- The standard module showed stronger UV-related instability
The observed electrical losses correlate with the chequered patterns in the EL images.
Extended UV Testing (UV60+UV90-front)
The UV60-front exposure does not represent even one year of outdoor UV exposure at locations such as Burgdorf (533 m), Lugano (283 m), or Birg (2680 m), based on rough estimates. For this reason, the testing campaign was extended to include additional UV90-front exposure.
For the modules that have completed the full sequence UV60-front + UV90-front so far, the following degradation was observed:
- 3.2–8.9 % reduction in maximum power (Pm)
- Voc reduction: 2.2–5.6 %
- Fill factor (FF) reduction: 0.7–3 %
These electrical losses again correspond with the chequered patterns observed in the EL images.
Figure 2: Change in (top-left) maximum power (Pm), (top-right) open-circuit voltage (Voc), (bottom-left) short-circuit current (Isc) and (bottom-right) fill factor (FF) for the modules. (UV: Ultraviolet, LS: Light soaking, TOPCon: Tunnel oxide passivated contact, BC: Back contact)
Figure 3: Electroluminescence images of the modules taken at 100% of short-circuit current (Isc). (UV: Ultraviolet, LS: Light soaking)
References
[1] P. Gebhardt, U. Kräling, E. Fokuhl, I. Hädrich, and D. Philipp, “Reliability of commercial TOPCon PV modules—An extensive comparative study,” Progress in Photovoltaics: Research and Applications, 2024.
[2] P. Gebhardt, E. Fokuhl, S. S. Mujumdar, H. Frey, A. Beinert, A. Stöhr, M. Kaiser, and I. Hädrich, “Stabilization procedures for TOPCon PV modules after UV-induced degradation,” Solar Energy Materials and Solar Cells, 2026.
