Scientists from Chinese solar manufacturer Trina Solar have proposed a new classification of ultraviolet-induced degradation (UVID) in TOPCon solar cells, distinguishing between reversible (r-UVID) and irreversible (ir-UVID) degradation. The aim is to improve reliability assessments, standardize testing procedures, and better predict long-term performance under real-world operating conditions.
The research team explained that the proposed r-UVID and ir-UVID categories are aligned with the recovery procedures envisaged in the forthcoming International Electrotechnical Commission (IEC) technical specification IEC TS 63624-1, which is being developed to standardize laboratory testing protocols for assessing UV-induced degradation in crystalline silicon solar modules.
“By aligning its proposed definitions with the forthcoming IEC standard, the paper introduces a classification that aims to establish a common language across standardization, reliability testing, warranty assessments, and project financing, enabling more consistent evaluations of long-term performance risks and helping distinguish temporary, recoverable power losses from permanent degradation that affects actual energy yield,” corresponding author Zhiwei Li told pv magazine.
The researchers explained that a defining feature of UVID in TOPCon solar cells is its pronounced metastability. Following UV exposure, cells can experience further performance losses during dark storage, primarily driven by a decline in open-circuit voltage and characterized by double-exponential degradation kinetics. Much of this additional degradation, however, is reversible and can be recovered through light soaking or thermal annealing, which promote hydrogen redistribution and changes in charge trapping at passivation interfaces.
“Field tests have confirmed that this metastable loss does not affect actual outdoor energy yield, as daily sunlight enables dynamic recovery,” the researchers said. “The severity of UVID strongly depends on the thickness and deposition method of the aluminum oxide (AlOₓ) layer, hydrogen content and bonding configuration, and the choice of capping layer, with thicker AlOₓ and silicon oxynitride (SiON) capping providing superior resilience.”
“UVID in TOPCon solar cells shows pronounced manufacturer-to-manufacturer variability, reflecting differences in wafer quality, passivation deposition processes, and emitter doping profiles,” they added.
Unlike conventional UV preconditioning tests, which typically use relatively low UV doses and may fail to capture degradation mechanisms specific to TOPCon solar cells, the proposed approach incorporates substantially higher UV exposure levels followed by a standardized light-soaking recovery procedure. According to the scientists, this additional step allows researchers to distinguish temporary, reversible performance losses from persistent degradation that remains after recovery, providing a more realistic assessment of long-term module reliability and potential impacts on outdoor energy yield.
The researchers said improving front-side passivation will be central to mitigating UVID. One of the most effective strategies involves optimizing the thickness and deposition method of the AlOₓ layer. Excessively thick AlOₓ layers, however, can cause blistering, introduce additional interface defects, and increase tunneling resistance. Alternative capping materials, particularly SiON, also show promise by stabilizing hydrogen, reducing interfacial charge interference, and improving chemical passivation.
They also noted that atomic layer deposition (ALD), particularly plasma-enhanced ALD, generally provides better UV resistance than conventional plasma-enhanced chemical vapor deposition (PECVD), although performance depends on the specific process. In addition, they recommended adjusting the refractive index of silicon nitride (SiNₓ) to enhance UV absorption and protect underlying interfaces, while cautioning that excessive absorption may reduce optical efficiency.
Hydrogen management was identified as another critical mitigation strategy, as hydrogen can both passivate defects and contribute to degradation when released or redistributed under UV exposure. Optimizing deposition temperatures, gas compositions, firing profiles, and annealing conditions can help maintain beneficial hydrogen bonding while limiting the formation of recombination-active defects.
“The combined effects of environmental stresses, particularly heat and ultraviolet radiation, require further investigation and more representative reliability tests,” Li said. “Differences between UV aging light sources, including fluorescent and metal halide lamps, also need standardized equivalence criteria. Moreover, the influence of ultraviolet-B (UVB) radiation on degradation rates relative to ultraviolet-A (UVA) remains insufficiently quantified.”
“Researchers have also observed differing degradation patterns among electrical parameters, particularly between short-circuit current and passivation-related characteristics, but the underlying mechanisms remain unclear,” he added. “Finally, targeted mitigation strategies for ir-UVID are still underdeveloped, highlighting the need for systematic approaches to prevent persistent performance losses and improve long-term module reliability.”
The proposed classification is presented in the paper “Ultraviolet-induced degradation in TOPCon solar cells: Reversible (r-UVID) and irreversible (ir-UVID) degradation, mechanisms and mitigation strategies,” recently published in Solar Energy Materials and Solar Cells.
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