# UVID in TOPCon Solar Cells: Non-Contact IV Tes··· | Ooitech > A study on UV induced degradation (UVID) in TOPCon solar cells using non-contact IV testing, ToF-SIMS and depth-profiling XPS to reveal hydrogen and oxygen dual mechanisms behind 6.72% efficiency loss. ![UVID in TOPCon Solar Cells: Non-Contact IV Testing Tracks Passivation Degradation and 6.72% Efficiency Loss](https://cdn.ooitech.com/static/upload/image/20260804/17aeb9818f648b0a8226d34423b2a293.webp) - ** 2026-08-04 - ** 0 Views - ** [Blog](/Blog.html) ### UVID in TOPCon Solar Cells: Non-Contact IV Testing Tracks Passivation Degradation and 6.72% Efficiency Loss ##### Introduction TOPCon solar cells now dominate the PV market thanks to strong surface passivation and carrier-selective contacts. But outdoor operation exposes a real weak spot: UV induced degradation, or UVID. After UV60 irradiation, efficiency drops by as much as 6.72%. Most earlier work blamed UVID on high-energy photons breaking Si–H bonds and releasing free hydrogen. That is only part of the story. The solar UV spectrum spans a wide 3.1–4.43 eV energy range, so its interaction with the front-surface layers goes well beyond a single Si–H rupture pathway. And the front Al₂O₃ / SiNₓ stack is far weaker against UV than the rear side. A non-contact IV tester helps a lot here. It is non-destructive, uses zero consumables, runs at millisecond speed, works with BC and busbar-free designs, and pulls IV, EQE and PL parameters in a single shot. This study uses ToF-SIMS and depth-profiling XPS to track element distribution and chemical bond changes in the Al₂O₃ / SiNₓ layers before and after UV60. The finding: N–H bond breaking is a second hydrogen source alongside Si–H. Al–O bonds in amorphous Al₂O₃ get broken by UV and generate a large number of oxygen vacancies. The hydrogen and oxygen mechanisms stack up and worsen surface recombination, giving a clear direction for improving passivation reliability. ##### Experimental Method (a) TOPCon solar cell structure schematic (b) symmetric front-structure sample (c) symmetric rear-structure sample Symmetric front-structure samples (SiNₓ/Al₂O₃/n-Si/Al₂O₃/SiNₓ) and symmetric rear-structure samples (with SiOₓ/n⁺-poly-Si layer) were prepared. UV aging was carried out at module level. The light source was mainly UV-A with about 11% UV-B, under 60°C and 30% humidity. Spectral irradiance of the UV light source ##### Front vs Rear Structure: UV Resistance Compared Changes in (a) τeff at an injection carrier concentration of 1×10¹⁵ cm⁻³, (b) iVoc, and (c) single-side J₀ for symmetric front- and rear-structure samples during UV exposure The symmetric rear structure barely degraded after 60 kWh·m⁻² of UV exposure. Its 120 nm thick poly-Si layer absorbs almost all the UV light and provides effective shielding. The front structure told a different story. τeff dropped from 2895.6 μs to 1552.8 μs. iVoc fell from 735.5 mV to 715.2 mV. Single-side J₀ shot up to 18.4 fA·cm⁻², roughly three times the initial value. The Al₂O₃ / SiNₓ passivation degraded heavily. ##### Chemical Composition Evolution Intensity depth profiles of (a) hydrogen and (b) oxygen in the polished symmetric front-structure sample before and after UV60, measured by ToF-SIMS ToF-SIMS shows hydrogen concentration rising clearly after UV exposure, especially inside the SiNₓ layer. N 1s XPS depth profiling shows that at the SiNₓ / Al₂O₃ interface, the N–H bond fraction fell from 9.64% to 5.97%. So N–H bond breaking is the second hydrogen source beyond Si–H. The freed hydrogen diffuses toward the SiNₓ surface region and recombines with silicon there, which is why the N–H fraction actually rises near that surface. Oxygen tells an equally important story. Oxygen in the Al₂O₃ layer dropped slightly, while oxygen at the SiNₓ/Al₂O₃ and Al₂O₃/Si interfaces increased. This points to Al–O bonds breaking and free oxygen migrating outward. Al–O bond energy in an ideal crystal is about 5.3 eV. But in amorphous Al₂O₃, under-coordinated aluminum ions and oxygen vacancies trap electrons and turn negatively charged, dropping the activation energy for adjacent Al–O bond rupture to about 2.4 eV. That means 400 nm UV light (3.1 eV) is enough to break them. N 1s depth-profiling XPS spectra at different interfaces of the Al₂O₃/SiNₓ layer before and after UV60, with fitted curves for N–Si (397.5 eV) and N–H (399.5 eV) bonds O 1s XPS shows lattice oxygen (OI) converting into oxygen vacancies (OII). In the Al 2p spectra, the Al₂O₃ fraction fell from 69.99% to 60.36% while Al–OH rose from 30.01% to 39.64%. In the Si 2p spectra, Si²⁺ increased while higher-valence silicon dropped. The electrons released when oxygen vacancies form reduce silicon from higher oxidation states. Taken together, these changes in oxygen, aluminum and silicon bonding show the Al₂O₃ film quality has already suffered. ##### Electrical Performance Degradation EQE and reflectance curves of the TOPCon solar cell before and after UV60 Change in front contact resistivity of the TOPCon solar cell during UV60 exposure Reflectance stayed basically unchanged after UV60. EQE showed a moderate drop in the short-wavelength region below 500 nm, matching stronger front-surface recombination. Front contact resistivity climbed nearly linearly with UV dose, reaching 4.1 mΩ·cm², about 8.6 times higher. The cause: free hydrogen and oxygen generated in the passivation layer diffuse to the electrode interface and take part in redox reactions. UV also converts water molecules on the silver surface into hydroxyl radicals, and together they corrode the metal electrode. Electrical performance degradation rates during UV60 exposure: (a) Voc (b) Jsc (c) FF (d) Eff Voc dropped 3.46% relative, driven by front-surface passivation degradation and rising J₀. FF fell 2.82%, driven by higher front contact resistivity. Jsc only dropped 0.64%, since the short-wavelength EQE loss was limited. The final photoelectric conversion efficiency loss came to 6.72%. ##### Conclusion The front SiNₓ/Al₂O₃ structure of TOPCon cells is far weaker against UV than the rear structure. Under UV, Si–H and N–H bonds break in sequence and release free hydrogen. Al–O bonds in amorphous Al₂O₃ break under UV, releasing free oxygen and creating a large number of oxygen vacancies. Al₂O₃ converts toward Al–OH and silicon valence shifts. These two degradation mechanisms, hydrogen and oxygen, stack up and worsen surface recombination. Add the sharp rise in front contact resistivity, and the result is 6.72% efficiency loss. The work offers a useful reference for understanding TOPCon UVID and for improving the long-term reliability of passivation layers. ##### Ooitech's View UVID keeps proving that the front stack is where reliability really gets tested, so how you build and control the SiNₓ/Al₂O₃ deposition and encapsulation on a module line matters just as much as the cell recipe. On our turnkey TOPCon and PERC module lines we see the same lesson play out on the layup, lamination and EL side, small process choices decide whether panels hold up outdoors for years. 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