The rapid commercialization of TOPCon (Tunnel Oxide Passivated Contact) and Heterojunction (HJT) solar technologies has transformed the photovoltaic (PV) landscape. While these advanced solar technologies offer higher efficiency than traditional PERC (Passivated Emitter and Rear Cell) modules, they also introduce unique challenges, particularly in terms of quality and reliability. A recent webinar by Sinovoltaics with UNSW highlighted these risks, focusing on the latest research developments and quality control measures for TOPCon and HJT modules.
The panel included insights from leading experts in the field, including Professor Bram Hoex, a researcher from the University of New South Wales (UNSW), and Niclas Weimar, CTO and co-founder of Sinovoltaics, a company specializing in quality control for PV systems. This article summarizes key takeaways from the discussion on quality risks and research in TOPCon and HJT solar modules.
Figure 1. TOPCON cell architecture
Figure 2. SHJ/HJT cell architecture
Increasing Adoption of TOPCon and HJT Technologies
According to the latest ITRPV report, TOPCon and HJT technologies have gained significant traction over recent years. Manufacturers are switching from conventional PERC technology to these advanced alternatives to achieve higher efficiency and improved energy yield. Table 1 gives an insight into the advantages of these new solar cell technology architectures. However, with this transition, concerns about long-term quality and performance have surfaced.
Figure 3. ITRPV 2023 TOPCON & HJT taking greater market share
Niclas Weimar shared observations from recent factory inspections in Asia, highlighting that the pre-lamination rework rates for TOPCon modules have been steadily declining, indicating improved manufacturing processes. For instance, rework rates at Facility A dropped from 15.45% in January 2024 to 8.5% by March, suggesting better raw material handling and equipment calibration. Despite this improvement, Weimar emphasized that many manufacturers still face challenges in ensuring consistent quality across all facilities, necessitating a focus on targeted quality assurance and reliability testing.
Unique Failure Modes in TOPCon and HJT Modules
TOPCon and HJT modules have exhibited new failure modes not previously observed in PERC modules, primarily due to the differences in their structure and material composition. Professor Bram Hoex presented findings from extensive testing on these technologies, showing how these failure modes impact the solar panel degradation rate, particularly under damp heat (DH) conditions, affect both technologies. Their paper can be found here.
For HJT modules, failure modes were identified that were triggered by the combination of bill of materials (BOM) used in the manufacturing process. Professor Hooks noted four new failure modes in HJT modules, including busbar darkening and issues related to contamination. These defects can lead to significant degradation, with some modules experiencing efficiency losses of up to 50% after only 1,000 hours of damp heat testing.
Figure 4. EL Images after 1000 hours of damp heat performance testing
Similarly, TOPCon modules have their own set of vulnerabilities, particularly at the cell edges where moisture can enter and degrade the module. A study presented by Jinko Solar at the EU PVSEC demonstrated that TOPCon modules experienced significant degradation after 3,000 hours of damp testing heat exposure when using ethylene-vinyl acetate (EVA) as the encapsulant. However, the addition of polyethylene (POE) layers improved the stability, reducing the loss to acceptable levels.
Sodium and Metallization Sensitivity
One of the central challenges in TOPCon and HJT module reliability is the sensitivity of these technologies to sodium contamination. Professor Hoex emphasized that sodium is present in almost every component of PV solar panels, including glass and encapsulants. Sodium-related degradation can lead to severe issues, particularly with series resistance and contact corrosion.
Tests showed that TOPCon cells are particularly sensitive to sodium when applied to the front surface, resulting in significant efficiency drops. Meanwhile, HJT cells are susceptible to sodium on both the front and rear sides. The degradation of metallization due to sodium leads to higher series resistance, compromising the electrical performance of the solar module.
The Role of Soldering Flux and Contaminants
Soldering flux used during module manufacturing can be another critical factor in determining module reliability. When systematically testing solar panels, the research team revealed that inappropriate soldering flux could trigger severe degradation in HJT module performance, even in dry conditions. This degradation can occur without the presence of moisture, which was an unexpected finding. For TOPCon modules, however, moisture must be present to activate the chemical reaction between soldering flux residues and the metallization, making them less vulnerable than HJT under dry conditions.
This discovery reinforces the need for manufacturers to carefully select soldering flux and ensure that the module production environment is free of contaminants like sodium chloride, which can exacerbate degradation.
Laser-Assisted Firing for TOPCon Reliability
A breakthrough for improving the stability of TOPCon modules is the adoption of laser-assisted firing (LAF). This technique has led to significant improvements in both efficiency and corrosion resistance. By eliminating aluminum from the contact paste used in TOPCon modules, LAF reduces the sensitivity to sodium, resulting in more stable performance under harsh conditions.
In trials, modules with laser-assisted firing exhibited only a 3.6% efficiency loss after undergoing 1,000 hours of the damp heat test, compared to a 92% loss in modules without this technology. This makes laser-assisted firing a game changer for TOPCon reliability and is being rapidly adopted by manufacturers, particularly in China.
Implications for the Solar Industry
The insights shared in this webinar underscore the importance of adapting manufacturing processes and quality control measures to the unique demands of TOPCon and HJT technologies. While these technologies offer substantial efficiency gains over PERC, they also present new challenges that manufacturers, project developers, and investors must address to ensure long-term reliability.
Key takeaways include:
• Advanced Quality Control: Manufacturers need to implement advanced quality control standards and mechanisms, such as Sinovoltaics' AI-based software Selma, to detect potential defects early in the production process.
• Material Selection: Proper selection of encapsulants, back sheets, and soldering flux is crucial in preventing degradation caused by moisture, sodium, and other contaminants.
• Accelerated Lifetime Testing (ALT): New regimes of photovoltaic module accelerated testing ALT testing, including combinations of damp heat and thermal cycling, are necessary to fully capture the degradation risks in TOPCon and HJT modules.
• Laser-Assisted Firing: The adoption of laser-assisted firing in TOPCon manufacturing is a significant step forward in improving module reliability, reducing corrosion sensitivity, and enhancing performance.
Conclusion
As the solar industry continues to transition towards higher-efficiency technologies like TOPCon and HJT, ensuring the reliability and longevity of these modules is paramount. Research by leading institutions like UNSW and quality control efforts from companies like Sinovoltaics are helping to identify and address the emerging risks associated with these technologies. The results from the lab tests show that advanced testing procedures are crucial for ensuring the long-term reliability of TOPCon and HJT solar modules. However, continued collaboration between industry and academia will be essential to overcoming the challenges and ensuring the success of these advanced PV technologies in the long term.
Reach out to us today to explore advanced PV testing and quality control solutions for TOPCon and HJT solar modules, ensuring long-term reliability and performance.
