You’ve heard the buzz around Heterojunction (HJT) solar cells. Their efficiency numbers are impressive, promising more power from every panel. But as manufacturers and researchers are discovering, unlocking that potential isn’t just about the cell itself—it’s about how you connect them.
The hidden weakness of these high-performance cells is their sensitivity to heat. This single characteristic changes the entire game of module assembly, turning a standard process like soldering into a major liability. This introduces a critical choice that impacts everything from quality and reliability to production speed: how to apply the adhesive that holds it all together?
Why Traditional Soldering Falls Short for HJT Cells
For decades, soldering has been the go-to method for connecting solar cells. It’s robust, well-understood, and cost-effective. The problem? It requires temperatures well over 200°C.
HJT cells, with their delicate amorphous silicon passivation layers and transparent conductive oxide (TCO) coatings, simply can’t take the heat. Exposing them to temperatures above 180°C risks irreversible damage, compromising the very efficiency gains you’re trying to achieve. It’s like trying to weld a plastic toy—the tool is too powerful for the material.
This reality forces a shift to a gentler, more advanced approach: Electrically Conductive Adhesives (ECAs).
Enter Electrically Conductive Adhesives (ECAs): The Low-Temperature Solution
Think of an ECA as a high-tech glue infused with silver particles. It creates a strong mechanical bond and a reliable electrical connection, all while curing at temperatures safely below 160°C. This makes it the ideal solution for temperature-sensitive HJT cells.
But choosing to use an ECA is only half the battle. The real challenge lies in the application method. Two dominant techniques have emerged, each with its own set of trade-offs:
- ECA Dispensing: A precision jetting process that applies tiny, controlled dots of adhesive.
- ECA Screen Printing: A high-speed process that uses a stencil and squeegee to apply adhesive lines.
The choice between them isn’t trivial. It fundamentally shapes the uniformity of your electrical contacts, your material consumption, and your overall production throughput.
The Core Debate: How You Apply the ECA Matters More Than You Think
At PVTestLab, we’ve run countless trials comparing these two methods under real industrial conditions. The data reveals a clear story of precision versus speed, which means the right choice depends entirely on your project’s goals—whether you’re in the R&D phase or scaling up for mass production.
Method 1: ECA Dispensing – Precision at a Price
ECA dispensing uses a nozzle to „jet“ or dispense small, precise amounts of adhesive onto the cell’s contact points. It’s a highly controlled, digital process.
Our validation data shows that dispensing offers exceptional control over the connection points, or „joints.“ It consistently achieves joint heights of 70-100 µm with very high reproducibility. This precision is critical during the development phase, ensuring each connection is uniform, minimizing mechanical stress on the delicate cell and providing a stable electrical contact.
But this precision comes at a cost: speed. Cycle times for dispensing are inherently slower, as the machine must address each contact point individually. This can create a bottleneck in a high-volume production line.
Best For: Prototyping, material validation, and small-batch production where quality and consistency are top priorities.
Method 2: Screen Printing – The Path to Scale
Screen printing involves pressing the ECA paste through a mesh screen or stencil onto the cell. It’s the same basic principle used to print t-shirts, but with microscopic precision. This method applies adhesive to all contact points simultaneously.
This approach enables significantly higher throughput, a critical factor for any industrial-scale manufacturing plan. That speed, though, introduces variability. Our tests show that screen printing can lead to greater variation in adhesive deposition, with joint heights ranging from 90-150 µm. While perfectly acceptable for many applications, this wider range requires tighter process control to avoid issues like paste smearing or inconsistent contact heights, which can introduce stress into the cell laminate.
Careful optimization is also needed to manage material usage and prevent waste—a lesser concern with the additive nature of dispensing.
Best For: High-volume manufacturing where throughput and cost-per-unit are the primary drivers.
A Side-by-Side Comparison: What the Data Reveals
Let’s break down the key differences observed in our R&D production line.
Joint Uniformity and Contact Quality
The goal is a perfect connection. Dispensing’s narrow 70-100 µm height range creates a more uniform, lower-stress bond. Screen printing’s wider 90-150 µm range means some joints may be thicker than others, a potential mechanical reliability concern if not managed during lamination. Ultimately, the lower, consistent height from dispensing often leads to a more reliable electrical contact.
Throughput and Production Speed
Here, screen printing has a clear advantage. By addressing all points at once, it can outpace dispensing significantly, making it the only viable option for mass production lines aiming for thousands of modules per day. If you are focused on process optimization trials, understanding this trade-off is fundamental.
Flexibility for Prototyping and R&D
For developers creating new designs, dispensing is the undisputed champion. It requires no physical stencils and allows for on-the-fly adjustments to adhesive patterns and volumes. This is invaluable when prototyping new solar module concepts or testing different materials, allowing for rapid iteration without the cost and lead time of ordering new screens.
Making the Right Choice for Your HJT Project
So, which method is better? The answer is neither. The right question is: which method is right for your current stage?
If you are in the R&D or pilot phase: Start with dispensing. It gives you the control and precision needed to validate your materials and module design without process variables getting in the way.
If you are scaling to mass production: Your target is screen printing. The key is to use the data from your dispensing trials to define the precise parameters needed to make screen printing reliable, repeatable, and cost-effective.
The journey from a lab concept to a factory floor often involves starting with dispensing to perfect the recipe, then transitioning to screen printing to scale it.
Frequently Asked Questions (FAQ)
What exactly is an Electrically Conductive Adhesive (ECA)?
An ECA is a composite material, typically a polymer resin filled with conductive particles like silver flakes. It acts as both an adhesive to bond components and a conductor to allow electricity to flow between them. It’s cured at low temperatures, making it safe for sensitive electronics and solar cells.
Why can’t I just use a lower-temperature solder?
Even so-called low-temperature solders often require heat levels that can degrade the sensitive layers of an HJT cell. The TCO layer, in particular, is susceptible to damage from the flux and heat associated with soldering, which can reduce the cell’s overall efficiency and long-term reliability. ECAs eliminate this risk entirely.
What is the TCO layer you mentioned?
TCO stands for Transparent Conductive Oxide. It’s an ultra-thin layer on HJT cells that is essential for collecting the electrical current generated by the cell while still allowing sunlight to pass through to the active layers. It’s highly effective but also very delicate.
How does inconsistent joint height affect the final module?
Inconsistent joint heights can create internal mechanical stress when the module is laminated under heat and pressure. A taller-than-average joint can become a pressure point, potentially causing micro-cracks in the ultra-thin HJT cell. This stress can impact long-term reliability and performance.
Your Next Step: From Theory to Validation
Understanding the difference between ECA dispensing and screen printing is the first step. The next step is seeing how they perform with your specific materials and cell architecture. While theoretical data is helpful, true validation only comes from testing, measuring, and optimizing these processes in a controlled, industrial environment.
The only way to truly de-risk your HJT module production is to base your decisions on applied data. Whether you are validating a new adhesive or perfecting your interconnection strategy, testing under real-world conditions is the key to success.
Ready to see how your materials perform? Explore PVTestLab’s full-scale R&D production line and discover how applied research can accelerate your path from concept to production.