Imagine this: you’ve just produced a new batch of high-efficiency Heterojunction (HJT) solar modules. The initial flash test results are fantastic, showing impressive power output (Pmax). But a few hours or days later, follow-up tests show a noticeable drop in performance.
It’s a common scenario that can cause concern. But what if that initial drop wasn’t the end of the story, but rather the first step in a unique process that could ultimately lead to the module performing better than its initial test suggested?
With HJT technology, this isn’t just a possibility—it’s a fundamental characteristic. Unlike other cell technologies, HJT modules undergo a distinct two-phase cycle of light-induced degradation (LID) followed by regeneration. Understanding this process—and managing it properly—is the key to unlocking their true, stable performance and ensuring your nameplate ratings are both accurate and competitive.
What is HJT Light-Induced Degradation (and Regeneration)?
Most people in the solar industry are familiar with Light-Induced Degradation (LID), a phenomenon where a solar module’s performance decreases after its first exposure to sunlight. In common technologies like PERC, this is a one-way street of stabilization that manufacturers must account for.
HJT technology, however, behaves differently. Its unique degradation and regeneration cycle unfolds in two phases:
- Initial Degradation: A rapid power drop that occurs within the first few hours of light exposure.
- Performance Regeneration: A subsequent recovery where, under the right conditions, the module not only regains the lost power but can even exceed its original Pmax measurement.
This „power boost“ is a fascinating and commercially critical aspect of HJT. Failing to account for it means you might be undervaluing your own product.
The Two-Phase Power Journey of an HJT Module
To accurately label an HJT module, you must guide it through its entire power journey. This isn’t a process that can be left to chance; it requires precise, controlled conditions to ensure reliable and repeatable results.
Phase 1: The Initial Degradation Dip
As soon as an HJT module is exposed to light, a rapid degradation process begins. This known effect is tied to the amorphous silicon layers fundamental to the HJT cell structure. Our research shows this initial power loss typically occurs within the first 1-3 hours of light soaking.
Phase 2: The Regeneration Climb
After the initial dip, the magic happens. Under sustained light and elevated temperatures, the module begins to regenerate. The key to unlocking this potential lies in creating the right environment. Based on extensive testing, the optimal conditions for regeneration are:
- Light Intensity: Greater than 800 W/m²
- Module Temperature: Between 50°C and 70°C
- Duration: Typically 24 to 72 hours
Under these controlled conditions, the module’s performance will climb, stabilize, and often settle at a power level higher than its initial flash test immediately after lamination.
(Image: A graph showing the characteristic power curve of an HJT solar module during light soaking, illustrating the initial degradation followed by a longer regeneration phase where power exceeds the starting value.)
This graph clearly shows the two-phase journey: a quick dip in performance followed by a steady climb to a stable, higher-power state.
Why Accurate Stabilization is Non-Negotiable
Simply placing modules outside in the sun isn’t enough. Environmental conditions like passing clouds, changing sun angles, and fluctuating temperatures make it impossible to achieve the consistent state required for accurate measurement. Without a controlled stabilization process, you face significant business risks:
- Inaccurate Nameplate Ratings: If you measure power before regeneration is complete, you are undervaluing your module.
- Failing Certifications: Standards like IEC 61215 require modules to be in a stable state before final power measurement. An unstable module will fail this crucial test.
- Customer Disputes: While a customer whose HJT modules perform better than the nameplate rating might seem like a good problem to have, it points to an uncontrolled process and creates uncertainty about data accuracy for future projects.
As PV Process Specialist Patrick Thoma often notes, „An HJT module’s initial flash test is just a starting point. Its true, stable performance is only revealed after a carefully controlled regeneration process.“
A Repeatable Method for Quantifying HJT Performance
At PVTestLab, we’ve developed a systematic methodology to characterize and stabilize HJT modules, giving developers and manufacturers the precise data they need. The process ensures that the final power rating reflects the module’s true, long-term capability.
Step 1: Establish the Baseline (Initial Flash Test)
A new module is first tested with a Class AAA flasher to determine its initial, unstable Pmax. This establishes the „time zero“ starting point before any significant light exposure.
Step 2: Controlled Light Soaking
The module is then placed in our climate-controlled light-soaking chamber, where we maintain the precise conditions—typically 1000 W/m² irradiance and a module temperature of 60°C—needed to drive the regeneration phase. This stage is crucial for any comprehensive material validation, as different encapsulants or backsheets can influence thermal behavior and, consequently, the regeneration rate.
(Image: A photo of a solar module inside PVTestLab’s advanced light soaking chamber, illuminated by powerful lamps to simulate intense sunlight under controlled conditions.)
Step 3: Monitoring and Final Validation
Throughout the light soaking, we monitor the module’s performance until it reaches a stable state—meaning the power output no longer changes over time. This process can take anywhere from 24 to 72 hours. Once stabilized, the module is cooled to standard test conditions (25°C) and undergoes a final Pmax measurement with the Class AAA flasher.
This final number represents the module’s true, stable power output—the value that should inform the nameplate rating and is essential for any serious solar module prototyping.
(Image: A close-up shot of a solar module positioned under the high-intensity lamps of a Class AAA sun simulator for a final, precise power measurement.)
Frequently Asked Questions (FAQ)
How is HJT LID different from PERC LID?
PERC LID (and LeTID) is primarily a degradation process where performance stabilizes at a lower point than the initial measurement. HJT LID is a two-step process of degradation and regeneration, where the final stabilized power can be higher than the initial measurement.
Can I just put my modules out in the sun to stabilize them?
While sunlight will trigger the process, the lack of control over irradiance and temperature makes it impossible to achieve a repeatable, certifiable result. For accurate data, a controlled environment is essential to ensure every module is stabilized to the exact same endpoint.
How long does the stabilization process take?
Under optimized lab conditions (e.g., 1000 W/m² and 60°C), stabilization is typically achieved within 24 to 72 hours. The exact duration can depend on the specific HJT cells and module materials used.
Does every HJT module gain power after stabilization?
Most high-quality HJT modules will demonstrate a net power gain compared to their initial „time zero“ measurement. The exact percentage of gain depends on the cell technology, module design, and quality of the materials used.
From Lab Theory to Production Reality
The unique degradation and regeneration behavior of HJT technology is not a flaw; it’s a characteristic that, when properly managed, highlights its superior performance. By moving beyond simple, initial flash tests and implementing a controlled stabilization process, manufacturers can confidently label their modules with their true, higher power output.
This level of process control is fundamental to successful solar module R&D. It bridges the gap between theoretical potential and bankable, real-world performance, ensuring that the innovation developed in the lab translates directly into reliable and competitive products in the field.