Imagine a perfect row of solar panels on a sunny day. Suddenly, a single leaf falls, covering just one cell. While it seems harmless, for a high-efficiency Heterojunction (HJT) module, this tiny shadow can trigger a thermal chain reaction that could lead to catastrophic failure. This isn’t a theoretical problem. It’s a real-world risk that standard tests can overlook, a risk rooted in a phenomenon known as reverse bias.
Understanding how a module behaves under these conditions can be the difference between a product that performs for 25 years and one that fails prematurely.
What Happens When a Solar Cell is Shaded? The Basics of Reverse Bias
In a functioning solar module, all the cells in a string work together, pushing current forward. When one cell is shaded, however, it stops producing power. Worse, the current from all the other healthy cells in the string tries to force its way through this shaded, „offline“ cell.
This forces the shaded cell into a state called reverse bias. Instead of generating power, it consumes it, acting like a resistor in a circuit. That electrical energy has to be dissipated, and it converts directly into heat—the birth of a hot spot.
The Unique Challenge with HJT Cells
While hot spots can occur in any solar technology, HJT cells present a unique challenge.
Research into their reverse bias behavior reveals a critical difference: unlike traditional PERC cells that often have localized weak points (shunts) where heat concentrates, HJT cells tend to exhibit a more uniform reverse bias breakdown.
In practice, this means that instead of heat concentrating on a tiny, focused point, the entire cell area begins to heat up evenly. While this uniformity might sound beneficial, it hides a significant danger. The cell can reach incredibly high temperatures—often over 200°C—across its whole surface well before the module’s safety systems can react, posing a severe risk of permanent damage to the encapsulant, backsheet, and overall integrity of the module.
Visualizing the Danger: Thermal Imaging in Action
A thermal image from a controlled reverse bias test illustrates this phenomenon perfectly. The image below reveals the uniform heating pattern characteristic of an HJT cell under stress.
As you can see, there isn’t one small, dangerously hot point. The entire cell is glowing, revealing a widespread thermal load that can silently degrade the surrounding materials. This level of thermal analysis is a critical part of ensuring module quality and reliability, especially when evaluating new materials or designs.
The Role of the Bypass Diode: Your Module’s Safety Valve
Every modern solar module is equipped with bypass diodes, which act as a safety valve. When a cell or group of cells is shaded, the bypass diode provides an alternative path for the current to flow around the underperforming section, preventing the massive heat buildup that leads to hot spots.
However, the diode only activates when the reverse bias voltage reaches a certain threshold. But testing reveals a critical weakness: the bypass diode’s activation voltage must be perfectly matched to the cell’s behavior. If the threshold is too high, an HJT cell can reach damaging temperatures across its entire surface before the diode even kicks in.
„Think of the bypass diode as a firefighter,“ explains Patrick Thoma, PV Process Specialist at PVTestLab. „Its job is to respond before the fire gets out of control. Our testing ensures the alarm (the activation voltage) is set correctly for the specific thermal behavior of HJT cells, preventing a small shading issue from becoming a catastrophic failure.“
A Closer Look at the Hot-Spot Endurance Test Setup
To guarantee a module’s resilience, engineers can’t simply wait for a cloudy day. In a controlled lab environment, they perform a hot-spot endurance test that simulates a worst-case shading scenario.
Engineers use a precision power supply to intentionally force a reverse current through a targeted cell, mimicking the effect of shading in the field. At the same time, high-resolution thermal cameras and sensors precisely monitor the cell’s temperature distribution.
This setup allows engineers to replicate and analyze field conditions with engineering precision, removing variables like passing clouds or changing sunlight. It’s an essential process for anyone involved in prototyping and developing new solar modules, as it validates the design’s resilience against real-world stresses.
What the Test Results Tell Us: From Data to Durability
This detailed testing provides crucial, actionable insights that go far beyond a simple pass/fail grade.
- Validation of Diode Integration: The test confirms that the chosen bypass diodes are not just present but are correctly integrated to protect the specific HJT cells being used.
- Material Endurance: It shows whether the encapsulant and backsheet can withstand the peak temperatures reached just before diode activation, preventing long-term degradation like browning or delamination.
- Process Consistency: Data shows that even small variations in cell manufacturing can affect reverse bias behavior. The test is therefore an essential quality check and highlights the need for batch-specific validation.
Ultimately, this test validates that a module can withstand worst-case partial shading over long periods without degrading—a crucial step for ensuring its safety, reliability, and lifetime energy yield.
Frequently Asked Questions (FAQ)
Why is this a bigger issue for HJT than for PERC or other cell types?
It comes down to their heating pattern. The localized hot spots in PERC cells, while intensely hot, often trigger the bypass diode’s voltage threshold more quickly. With HJT’s uniform heating, the entire cell’s temperature can rise to dangerously high levels before that same voltage threshold is met, putting the surrounding materials at greater risk for a longer period.
Can’t I just rely on the cell manufacturer’s datasheet?
A datasheet provides a crucial baseline, but it doesn’t capture the full picture. It cannot account for the interactions within a complete module system—including the specific encapsulant, glass, backsheet, and bypass diode. A full-module hot-spot test is designed to validate precisely how these components perform together under stress.
What is considered a „catastrophic failure“?
This ranges from performance degradation to serious safety hazards. It can include irreversible encapsulant browning (which reduces light transmission), delamination (where the layers of the module separate), backsheet burn-through, and in the most extreme cases, fire.
How does this test relate to IEC 61215 certification?
The IEC 61215 standard for module certification includes a hot-spot endurance test. However, the diagnostic approach described here goes deeper. Instead of just checking a box for pass/fail, it involves a detailed characterization of the reverse bias behavior to understand the why behind the results. This is essential for optimizing the design of high-performance modules like HJT, not just certifying them.
Your Path to Building More Resilient HJT Modules
The superior efficiency of HJT technology introduces unique engineering challenges. Acknowledging and designing for its distinct thermal behavior under reverse bias isn’t optional—it’s fundamental to creating a durable, reliable, and safe product.
Don’t leave a module’s long-term reliability to chance. By moving beyond datasheets and standard certifications to embrace this kind of applied testing, manufacturers can validate that their designs are truly built to withstand the rigors of the real world.
For a deeper look into the tools and techniques used for this kind of validation, explore our resources on solar module process optimization and quality control.