Key Takeaways & Executive Findings
- •• • Dark storage of un-screen-printed TOPCon cells for 60 days reduces minority carrier lifetime by 13% (from 4040 µs to 3522 µs), with i-Voc and i-FF dropping by 0.17% and 0.30%, respectively; this metastable decay necessitates controlled storage or pre-treatment before module assembly to avoid irreversible efficiency losses. • • A 1-second UV pre-treatment (200 W/m², 365 nm) on 4677 industrial cells yields an average PCE gain of 0.07%, Voc increase of 0.89 mV, and FF improvement of 0.15%, demonstrating a scalable, low-cost method to recover dark-storage losses and enhance cell performance. • • Cumulative UV irradiation up to 10^6 s (56 kW·h/m²) initially improves cell parameters but subsequently degrades passivation, leading to net PCE reduction; this threshold defines a critical process window where UV exposure transitions from beneficial to detrimental. • • The observed anomalous efficiency enhancement is linked to hydrogen metastability: UV exposure likely redistributes hydrogen species (H⁰/H⁻) and temporarily passivates defects, but prolonged exposure generates interface states that outweigh the benefits, as evidenced by the reversal in i-Voc and i-FF trends.
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Abstract
Tunnel oxide passivated contact (TOPCon) solar cells, despite achieving commercial efficiencies up to 25.6% and a theoretical limit of 28.7%, exhibit ultraviolet-induced degradation (UVID) that threatens long-term reliability. This study investigates the metastable behavior of industrial-scale TOPCon cells under dark storage and cumulative ultraviolet (UVA-365 nm) irradiation at 200 W/m². Dark storage over 60 days revealed a decay in minority carrier lifetime (τ) by 13%, with implicit open-circuit voltage (i-Voc) and implicit fill factor (i-FF) decreasing by 0.17% and 0.30%, respectively. Critically, short-term UV exposure (approximately 1 s) produced an anomalous efficiency enhancement: average power conversion efficiency (PCE) increased by 0.07%, open-circuit voltage (Voc) by 0.89 mV, and fill factor (FF) by 0.15% across a batch of 4677 cells. Prolonged UV irradiation, however, reversed these gains, causing passivation degradation and net PCE loss. This dual behavior—initial improvement followed by deterioration—is attributed to the interplay between hydrogen-mediated defect metastability and UV-induced interface damage. The findings establish a previously unreported UV-induced recovery mechanism and provide a quantitative basis for optimizing UV pre-treatment in TOPCon manufacturing, potentially enabling efficiency gains without additional capital expenditure.
1. Introduction
TOPCon solar cells have rapidly gained industrial traction due to their high conversion efficiency, excellent passivation, and compatibility with existing production lines. However, their susceptibility to ultraviolet-induced degradation (UVID) poses a significant reliability challenge, with commercial modules showing power losses exceeding 5% after UV doses of 60 kW·h/m². While extensive research has focused on long-term UV degradation, the impact of short-term UV exposure remains largely unexplored, leaving a critical gap in understanding the metastable behavior that could be harnessed for performance gains.
This study addresses that gap by systematically investigating the effects of dark storage and cumulative UV irradiation (0.5 s to 10^6 s) on semi-finished TOPCon cells. We uncover a previously unreported anomalous phenomenon: short UV exposure enhances efficiency, while prolonged exposure degrades it. Through a combination of lifetime measurements, implicit Voc analysis, and a large-scale batch validation (4677 cells), we quantify the optimal UV pre-treatment window and elucidate the role of hydrogen metastability. These findings provide a foundation for integrating a low-cost UV pre-treatment step into TOPCon manufacturing, potentially boosting output without altering existing processes.
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LI Yuepeng, YUAN Zhengguo, HUANG Xinyu, XIE Taihong, XIE Yi, YU Jian (2026). Cumulative Anomalous Behavior of TOPCon Solar Cells Under Ultraviolet Irradiation. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9679
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Frequently Asked Questions
What is the exact mechanism behind the initial efficiency enhancement under short-term UV exposure?
The enhancement is attributed to UV-induced redistribution of hydrogen species within the silicon and passivation layers. Specifically, UV photons likely break weak Si-H bonds, allowing hydrogen to passivate dangling bonds at the Si/SiOx interface, thereby increasing minority carrier lifetime and reducing recombination. This is supported by the observed 0.89 mV increase in Voc and 0.15% FF gain after 1 s of UV exposure, which correlates with improved passivation quality.
How does the degradation rate under prolonged UV compare to industry-standard UVID thresholds?
After cumulative UV irradiation of 10^6 s (56 kW·h/m²), the cells exhibit a net PCE loss, with i-Voc and i-FF declining beyond their initial values. This aligns with literature reporting >5% power loss at 60 kW·h/m² for TOPCon modules. The degradation is driven by UV-induced interface defect generation, which outweighs the initial hydrogen passivation benefit. The critical dose for transition from enhancement to degradation is approximately 10^4–10^5 s under our conditions.
Can the UV pre-treatment be integrated into high-volume production without compromising throughput?
Yes. The optimal exposure is approximately 1 s at 200 W/m², which is compatible with inline processing speeds. Batch validation on 4677 cells demonstrated a consistent 0.07% PCE gain, indicating scalability. The UV equipment (45 W, 365 nm) is commercially available and can be inserted after sintering and before metallization, adding negligible cost per cell.
What are the long-term stability implications of UV pre-treated cells under field conditions?
While short-term UV pre-treatment improves initial efficiency, subsequent field UV exposure may accelerate degradation if the total dose exceeds the critical threshold. Accelerated aging tests (UV 2000 h) show that pre-treated cells degrade at a similar rate to untreated ones after the initial gain, suggesting that the benefit is temporary. Encapsulation with UV-blocking glass or filters could mitigate this, but further studies are needed to optimize the trade-off.
How does the metastable decay during dark storage affect the reliability of TOPCon modules?
Dark storage for 60 days reduces lifetime by 13% and i-Voc by 0.17%, which translates to a potential PCE loss of 0.1–0.2% in final modules. This decay is reversible via UV pre-treatment, but if modules are stored for extended periods before installation, the initial performance may be lower than rated. Manufacturers should either minimize storage time or implement a UV recovery step before shipping.
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