Heterojunction (HJT) technology is one of the brightest stars in the solar industry, promising remarkable efficiency and performance. But like any high-performance engine, it has a hidden vulnerability—one that doesn’t show up on a standard datasheet. The weakness lies in the delicate connections holding the cells together, and the trigger can be something as simple as the changing of the seasons.
What if the very material you trust to protect your advanced HJT cells is secretly making them weaker over time?
It’s a counterintuitive idea, but one that’s gaining traction as we push the boundaries of module reliability. The story begins not with the solar cell itself, but with the tiny, brittle solder joints that give it life.
The Achilles‘ Heel of High-Efficiency: Brittle Solders and Temperature Swings
To understand the problem, we need a quick look under the hood of an HJT cell. Unlike traditional PERC cells that can withstand high heat during manufacturing, HJT cells are remarkably sensitive. Their thermal sensitivity requires them to be assembled using low-temperature solders—often alloys like tin-bismuth (SnBi) or tin-bismuth-silver (SnBiAg), which melt below 200°C.
While these solders get the job done, they come with a critical trade-off: they are significantly more brittle and susceptible to fatigue than their high-temperature counterparts.
Now, imagine a solar module in the real world. It bakes in the summer sun and freezes in the winter cold. International standards replicate these conditions through demanding thermal cycling tests, repeatedly taking modules from a frigid -40°C to a sweltering +85°C.
During these temperature swings, every material inside the module expands and contracts, but not at the same rate. This phenomenon, known as Coefficient of Thermal Expansion (CTE) mismatch, puts the copper ribbon, the silicon cell, the glass, and the encapsulant in a constant tug-of-war.
Diagram showing Coefficient of Thermal Expansion (CTE) mismatch in a solar cell layup during thermal cycling.
For a traditional solar module, this is a manageable issue. For an HJT module, however, this CTE mismatch puts immense, repetitive stress directly onto those fragile, low-temperature solder joints. Over hundreds of cycles, microcracks can form and grow, eventually severing electrical connections and killing parts of the module.
The Encapsulant’s Dual Role: Protector and Stress Conductor
This brings us to the encapsulant. Module designers often choose Polyolefin Elastomer (POE) for its excellent properties, including a very low water vapor transmission rate (WVTR) and outstanding resistance to potential-induced degradation (PID). These qualities make it a go-to choice for protecting cells from the elements.
But another property is often overlooked: mechanical modulus, a technical term for stiffness.
While POE is fantastic at blocking moisture, it’s also relatively stiff, especially at low temperatures. When the module gets cold and its materials start to contract, a stiff encapsulant doesn’t absorb the resulting mechanical stress. Instead, it transmits it—efficiently and directly—to the weakest link in the chain: the HJT solder joints.
Graph comparing the mechanical modulus of Silicone vs. POE at different temperatures, showing POE’s increased stiffness at low temperatures.
Over time, the POE acts less like a protective cushion and more like a rigid bar, transferring strain from the thermal tug-of-war right into the solder. This is a crucial consideration during solar module prototyping, as a simple material choice can have decade-long reliability implications.
Expert Insight from Patrick Thoma, PV Process Specialist at PVTestLab:
„We often see materials that perform well on datasheets fail under dynamic, real-world conditions. The stiffness of POE at -40°C is a perfect example. It’s not a ‚bad‘ material; it’s simply not engineered to buffer the specific mechanical stresses HJT solder joints face. The encapsulant must be evaluated as part of a complete system, not just for its individual properties.“
Engineering a „Stress Buffer“ with Flexible Silicone
If a stiff encapsulant is the problem, a more flexible one is the logical solution. Enter silicone.
Silicone has a much lower mechanical modulus, meaning it’s more „rubbery“ and less stiff across the entire operational temperature range. This inherent flexibility allows it to act as a mechanical cushion, or stress buffer.
Instead of transferring strain, the soft silicone absorbs and dissipates it. It decouples the solder joints from the larger forces at play, effectively protecting them from the relentless tug-of-war caused by CTE mismatch.
But how much of a difference does this really make?
At PVTestLab, we put this theory to the test. We constructed identical HJT modules—one with standard POE and one with a flexible silicone encapsulant—and subjected them to 600 rounds of thermal cycling. This type of accelerated lifetime testing is designed to simulate decades of harsh environmental exposure.
Side-by-side Electroluminescence (EL) images of an HJT module with POE vs. one with Silicone after 600 thermal cycles, showing microcracks in the POE module.
The module with POE showed significant degradation. Dark lines and patches revealed widespread microcracks and inactive cell areas, a direct result of solder joint fatigue. In contrast, the module with flexible silicone remained almost pristine. By buffering the thermomechanical stress, the silicone preserved the integrity of the electrical connections, safeguarding the module’s power output and long-term reliability.
It’s All About Context: Making the Right Choice
This doesn’t mean POE is obsolete or that silicone is always the superior choice. The trade-off often involves higher material costs and potentially different processing parameters for silicone.
The decision depends on the module’s design, its intended application, and its long-term reliability goals.
- For a utility-scale project in a moderate climate, the proven PID resistance of POE might be the top priority.
- For a premium residential module sold with a 30-year warranty for use in regions with extreme temperature swings, the added protection of a silicone stress buffer could be a game-changer.
Ultimately, you can’t know for sure without data. Validating these material interactions through controlled lamination trials is the only way to quantify reliability gains and make an informed decision that balances performance, cost, and bankability.
Frequently Asked Questions (FAQ)
1. What are HJT solar cells?
Heterojunction (HJT) cells are a type of high-efficiency solar cell that combines crystalline silicon with thin-film silicon layers. This structure allows them to capture more energy from sunlight but also makes them more sensitive to high temperatures during manufacturing.
2. What is thermomechanical stress?
It’s the internal stress created within a material or assembly when temperature changes cause its components to expand or contract at different rates. In a solar module, this happens daily and seasonally.
3. Why are HJT solder joints so fragile?
Because HJT cells are heat-sensitive, they must be soldered at low temperatures (<200°C). The metal alloys that work at these temperatures are inherently more brittle and less resistant to the repeated stress of expansion and contraction (fatigue) than the solders used in traditional modules.
4. Is POE a bad encapsulant for HJT modules?
Not at all. POE is an excellent material for protecting against moisture and PID. However, its mechanical stiffness, especially when cold, can be a liability for the fragile solder joints in HJT modules. It’s a classic engineering trade-off.
5. How can I know which encapsulant is right for my module design?
The best way is through applied testing. Creating prototypes with different material combinations and putting them through accelerated stress tests like thermal cycling allows you to gather real-world data on which configuration offers the best long-term reliability for your specific application.
The Path Forward
For decades, the solar industry has focused on the encapsulant’s role as a chemical and moisture barrier. But as we enter an era of high-efficiency technologies like HJT, we must also see it as a critical mechanical component.
Choosing an encapsulant is no longer just about PID resistance and WVTR. It’s about understanding how its mechanical properties will interact with the most delicate parts of your module over its entire lifetime. By looking beyond the datasheet and investing in real-world process validation, you can turn a potential point of failure into a source of enduring strength.