From Cell to Module: Taming the Lamination Process to Unlock HJT’s Full Potential
You’ve selected Heterojunction (HJT) cells for your new solar module design, and for good reason. Their lab-tested efficiency is outstanding, promising a significant leap in power output. The prototypes are designed, the materials sourced, and you’re ready to build. But when the first modules roll off the line, their power output is disappointingly lower than the sum of the cells inside.
Where did that power go?
This frustrating gap between a cell’s potential and a module’s reality is a common challenge known as Cell-to-Module (CTM) power loss. For HJT technology, this loss is often a hidden thief, and its favorite hiding place is the lamination process.
What is CTM Loss and Why Are HJT Cells So Sensitive?
Simply put, CTM loss is the performance drop that occurs when individual solar cells are assembled into a complete, laminated module. While every module type experiences some CTM loss, the percentages vary widely. A standard PERC module might lose around 0.5-1% of its power during this transition, but HJT modules can see losses of 1-3% or even more, according to industry analysis from TaiyangNews.
Why the big difference? It comes down to HJT’s unique and delicate architecture.
Unlike conventional cells, HJT cells are built like a precise sandwich. A crystalline silicon wafer is coated on both sides with ultra-thin layers of amorphous silicon and a transparent conductive oxide (TCO). These layers are the secret to HJT’s incredible efficiency, but they are also its Achilles‘ heel.
Research from leading institutions like the Fraunhofer Institute for Solar Energy Systems (ISE) confirms that these sensitive layers are easily damaged by the heat and pressure of the lamination cycle. This vulnerability makes the journey from a high-performance cell to a high-performance module a far more delicate operation.
The Lamination Process: A High-Stakes Balancing Act
Lamination uses heat and pressure to bond a module’s layers—glass, encapsulant, cells, and backsheet—into a single, durable unit built to withstand decades in the field. For most solar technologies, it’s a robust, straightforward process.
For HJT, it’s a tightrope walk.
The standard lamination profiles developed for PERC and other technologies often involve high temperatures to ensure the encapsulant material (like EVA) cures properly. But apply that same heat to an HJT cell, and you risk damaging its critical passivation and TCO layers. This thermal stress can lead to:
- Increased Recombination: The „leaks“ where electrons are lost before they can contribute to the electrical current, directly reducing the module’s efficiency.
- Reduced Voc and Fill Factor (FF): In simple terms, this means the module produces less voltage and operates less efficiently, kneecapping its total power output.
- Microcracks and Defects: The physical stress can create tiny fractures in the cells, which act as dead zones for power generation.
This is why a „one-size-fits-all“ approach to lamination fails HJT technology. Pushing these cells through a standard process is like trying to bake a delicate soufflé in a pizza oven—the results are predictably disastrous.
A detailed EL (electroluminescence) image showing microcracks and defects in a solar module after lamination.
The Path to Optimization: Finding the Perfect Lamination Recipe
The key to preserving HJT’s impressive efficiency is to treat lamination not as a brute-force step, but as a precise, scientific process. That means developing a customized lamination profile that provides just enough heat and pressure to ensure robust encapsulation without degrading the cell.
Success depends on a careful, data-driven approach:
- Material-Specific Testing: Not all encapsulants behave the same. New materials like Polyolefin Elastomers (POE) often require different temperature and time cycles than traditional EVA. Understanding how they interact with HJT cells under real industrial conditions begins with structured experiments on encapsulants.
- Iterative Process Trials: The ideal profile emerges from systematic experimentation. This involves adjusting process parameters for lamination, such as peak temperature, dwell time, and pressure, and then meticulously measuring the impact of each change on the module’s performance.
- Prototyping Under Real Conditions: Lab simulations can only tell you so much. True optimization happens on full-scale industrial equipment that mimics the reality of a production line. This real-world prototyping is essential when developing new solar module concepts, as it ensures the design is both efficient and manufacturable.
Finding this perfect „recipe“ turns the lamination process from a source of power loss into a step that reliably protects and preserves the cell’s inherent power.
A photo of an engineer carefully placing an HJT module into a full-scale industrial laminator.
Measuring What Matters: How to Quantify and Minimize CTM Loss
You can’t fix what you can’t measure. As NREL research emphasizes, optimizing HJT lamination requires precisely quantifying CTM loss at each stage through robust pre- and post-lamination characterization.
By measuring the electrical performance of cells before they enter the laminator and comparing it to the final module’s output, you can pinpoint exactly how much power is being lost and why.
Key diagnostic tools include:
- IV Curve Tracing (Flasher Test): This measures the module’s voltage and current characteristics to determine its maximum power point (Pmax). Comparing pre- and post-lamination IV curves immediately reveals any drop in performance.
- Electroluminescence (EL) Imaging: This technique acts like an X-ray for solar modules, revealing hidden defects like microcracks, inactive cell areas, or soldering issues that are invisible to the naked eye.
Comparing the IV curve of a module from a standard, high-temperature process against one from an optimized, lower-temperature profile reveals an undeniable difference. The optimized module retains more of its power, validating the process changes and providing a clear path to higher yields.
A graph comparing the power output (IV curve) of an HJT cell before and after a standard vs. an optimized lamination cycle.
By combining iterative process trials with precise measurement, manufacturers can systematically close the gap between cell efficiency and module power, finally unlocking the full promise of HJT technology.
Frequently Asked Questions (FAQ)
What exactly is a „lamination profile“?
A lamination profile is a set of instructions for the lamination machine, detailing the precise temperature, pressure, and time for each stage of the process (e.g., heating, pressing, and cooling). A well-designed profile ensures the encapsulant melts and cures perfectly without damaging the cells.
Can I use the same encapsulant for HJT as for PERC modules?
While you can use similar materials like POE or EVA, their behavior with sensitive HJT cells requires re-validation. The curing properties of the encapsulant must be perfectly matched with a lower-temperature lamination cycle to avoid cell degradation, making material-specific testing essential.
What are the first signs of lamination-induced damage?
The most immediate signs are a lower-than-expected power output (Pmax) and a drop in the module’s open-circuit voltage (Voc) and fill factor (FF) during the final flash test. EL imaging can then provide a visual diagnosis, often revealing new microcracks or darkened areas that were not present before lamination.
How long does it take to find an optimal lamination profile?
This depends on the materials and module design, but it typically involves a structured series of tests over several days. In a dedicated R&D environment, an optimized profile for a new material or cell type can often be developed and validated within a one- to three-day engagement, thanks to the ability to run rapid, back-to-back trials.
The First Step to Higher Yields
Heterojunction technology represents the future of solar efficiency, but harnessing its full potential requires moving beyond outdated processes. Lamination is no longer just an assembly step; it is a critical control point that can either preserve or destroy a cell’s performance.
Understanding the unique sensitivities of HJT cells is the first step. The next is to apply a scientific, data-driven approach to process optimization, transforming your production line into a system that consistently delivers modules as powerful as the cells within them.