You’re holding the future in your hands: a high-efficiency Heterojunction (HJT) solar cell. It promises incredible performance, converting more sunlight into electricity than almost any other technology on the market. But this power comes with a critical vulnerability. Like a world-class athlete with a delicate constitution, its advanced layers are extremely sensitive to heat.
The standard soldering process, which uses temperatures over 200°C to connect cells, would instantly degrade these sensitive layers and destroy the very efficiency you’re trying to capture.
For years, the industry has relied on a workaround: low-temperature Bismuth-Tin (BiSn) solders. They solve the heat problem, melting around 140°C. But in solving one issue, we inadvertently created another, more insidious one—a hidden flaw that can lead to micro-cracks and power loss over the module’s lifetime. This is the brittleness problem.
The Temperature Tightrope of Advanced Solar Cells
To understand the challenge, we need to look inside an HJT cell. It’s a sophisticated sandwich of crystalline silicon layered with thin, amorphous silicon films. These delicate layers, along with the transparent conductive oxide (TCO) coating, are responsible for the cell’s stellar performance—and are also the source of its heat sensitivity.
Exposing them to the high temperatures of traditional tin-lead-silver solders is simply not an option. This led the industry to adopt BiSn alloys as the go-to solution for connecting HJT, TOPCon, and other temperature-sensitive cell architectures.
On the surface, it’s a perfect match. The low melting point protects the cell, and the connection is made. But what happens years later, when the module is on a rooftop, enduring blistering summer days and frigid winter nights?
The Brittleness Problem: Bismuth-Tin’s Hidden Cost
The biggest drawback of Bismuth-Tin alloys isn’t chemical; it’s mechanical. They are inherently brittle.
Imagine connecting a series of delicate computer chips with tiny glass rods instead of flexible copper wires. The initial connection might be fine, but any stress—from vibration, expansion, or contraction—would risk a fracture. This is precisely the scenario unfolding inside solar modules connected with BiSn solder.
As a module heats up in the sun and cools down at night, its various materials expand and contract at different rates. This constant thermal cycling puts immense stress on the solder joints connecting the cells. While a ductile, flexible solder can absorb this movement, a brittle BiSn solder cannot. Over time, this unresolved stress leads to the formation of micro-cracks.
These cracks are a triple threat:
- They increase series resistance, leading to a gradual loss of power output.
- They can propagate into the cell itself, causing physical damage.
- They represent a critical failure point, compromising the long-term reliability and bankability of the entire module.
This poses a major concern for developers, who need to guarantee module performance for 25 years or more. How can you build a reliable, long-lasting product on a foundation that’s prone to cracking?
A More Flexible Future? Putting Indium-Based Alloys to the Test
This challenge prompted our engineers at PVTestLab to investigate alternatives. Could another low-temperature alloy provide the same thermal safety as BiSn but without the crippling brittleness? Our research focused on a promising candidate: Indium-based solder alloys.
Indium alloys offer two potential game-changing properties:
- An even lower melting point (around 118-120°C), providing a larger safety margin during soldering.
- Superior ductility, meaning they are far more flexible and resistant to cracking under stress.
To validate this, we designed a feasibility study comparing a standard BiSn alloy with an Indium-based alloy (In52Sn48) under real-world industrial conditions. Using our full-scale R&D production line and a multi-busbar stringer, we tested both materials on HJT cells, analyzing everything from the quality of the bond to the impact on the cell.
Here’s what we discovered.
Finding 1: Superior Wetting at Lower Temperatures
„Wetting“ is a term for how well a molten solder flows and bonds to a surface. Poor wetting results in a weak, unreliable connection.
Our cross-section analysis revealed that the Indium alloy achieved excellent, uniform wetting at a process temperature of just 150°C. It created a smooth, void-free connection to the cell’s TCO layer. The BiSn alloy, by contrast, required temperatures closer to 180°C to achieve a comparable bond, pushing the cell much closer to its thermal danger zone.
[Image 1: A side-by-side microscopic cross-section comparison showing the smooth, uniform solder layer of an Indium alloy connection versus a less uniform BiSn connection on a TCO layer.]
This wider processing window with Indium is a significant advantage, reducing the risk of thermal damage during mass production.
Finding 2: Strong and Consistent Adhesion
A good bond needs to be strong. In pull-off tests, where we measured the force required to break the solder joint, the Indium alloy showed consistently high adhesion forces. The BiSn connections, while sometimes strong, displayed far greater variability, suggesting a less stable and predictable process.
Finding 3: Happier Cells, Zero Thermal Stress
The ultimate test is the health of the cell itself. Using high-resolution Electroluminescence (EL) imaging, we inspected the cells post-soldering.
The cells soldered with the Indium alloy at its optimal low temperature were flawless. They showed no signs of thermal stress, no dark spots indicating damage, and absolutely no micro-cracks. This visual evidence confirms that the gentler process keeps the cell in perfect condition.
[Image 2: An electroluminescence (EL) image of a solar cell soldered with Indium alloy, showing a perfectly uniform and crack-free surface, highlighting the absence of thermal stress.]
„Indium alloys offer a compelling pathway to enhance the long-term reliability of HJT and other heterojunction modules. Their inherent ductility directly addresses the brittleness that has been a persistent concern with BiSn solders.“
— Patrick Thoma, PV Process Specialist at PVTestLab
What This Means for Next-Generation Module Design
Our findings suggest that Indium-based alloys are not just an alternative to BiSn; they are a significant upgrade. By solving the brittleness problem, they offer a path to manufacturing solar modules that are both highly efficient and mechanically robust.
The key takeaways for module developers and material manufacturers are clear:
- Enhanced Long-Term Reliability: Indium’s ductility can drastically reduce the risk of micro-crack formation, leading to better power generation over the module’s lifetime. This is a critical factor confirmed by comprehensive lamination trials.
- Improved CTM Ratios: A superior, low-resistance electrical contact combined with zero cell damage can reduce Cell-to-Module (CTM) power loss, meaning more of the cell’s impressive efficiency makes it into the final product.
- Safer Manufacturing: The lower process temperature provides a wider, safer window for production, potentially increasing yield and reducing waste.
While cost and supply chain considerations for Indium are important factors, this feasibility study proves that a technically superior solution to the brittleness problem exists.
Frequently Asked Questions (FAQ)
What exactly is a Heterojunction (HJT) solar cell?
An HJT cell combines two different solar technologies: traditional crystalline silicon and amorphous thin-film silicon. This „heterojunction“ allows the cell to capture more energy from the light spectrum, resulting in exceptionally high efficiency.
Why is solder joint brittleness a major concern?
A solar module on a roof expands and contracts daily due to temperature changes. A brittle solder joint can’t absorb this movement and can develop micro-cracks over time. These cracks increase electrical resistance, reduce power output, and can even physically damage the cell, shortening the module’s effective lifespan.
What is „wetting“ in soldering and why does it matter?
Wetting describes the ability of molten solder to flow and create a complete, uniform bond with a surface (in this case, the solar cell). Good wetting is essential for a strong mechanical bond and an efficient electrical connection. Poor wetting leads to weak joints and higher electrical resistance.
Are Indium-based solders a drop-in replacement for BiSn?
Not exactly. While they are highly compatible, using Indium alloys requires process optimization. Soldering parameters like temperature, speed, and flux application must be fine-tuned to leverage their full benefits. This is where applied testing in a controlled environment is crucial.
From Lab Insight to Production Reality
The journey to unlock the full potential of advanced solar cells is paved with material innovations. The choice of solder, once a simple consideration, has become a critical factor in determining the long-term performance and reliability of next-generation modules.
Our research shows that moving beyond Bismuth-Tin to more ductile materials like Indium alloys presents a powerful opportunity to build more resilient, efficient, and durable solar products. The next step is bridging the gap from promising results to mass production.
If you are developing new materials or module designs and facing similar challenges, applied testing is the fastest way to validate your concepts. To learn how real-world process validation can accelerate your path to market, discuss your material testing needs with our engineers.