You’ve invested in Heterojunction (HJT) solar cells, drawn in by their chart-topping efficiency ratings and the vision of a solar module that outperforms everything on the market. But after lamination, you run a flash test and your heart sinks. The power output is lower than expected. The cells, once pristine and powerful, have lost their edge.
What happened?
The answer often lies in a hidden conflict inside your laminator—a battle between temperature, time, and the chemistry that holds your module together. HJT cells are the thoroughbreds of the solar world: powerful, but also sensitive. Subjecting them to the intense heat of a standard lamination cycle is like asking a sprinter to run a marathon in a sauna, compromising their performance before they ever see the sun.
This is the central challenge of HJT module manufacturing, and solving it requires a new approach to a critical component: the encapsulant.
The Promise and Peril of HJT Technology
To understand the problem, we have to look at what makes HJT cells so special. Unlike traditional PERC cells, Heterojunction cells use a unique sandwich-like structure of crystalline silicon layered with thin films of amorphous silicon. This design is exceptionally effective at reducing electron loss—the very key to their high conversion efficiencies.
However, this sophisticated structure has an Achilles‘ heel: the Transparent Conductive Oxide (TCO) layer. This ultra-thin layer is essential for conducting electricity out of the cell, but it’s extremely sensitive to heat.
When exposed to temperatures above 150°C, the TCO layer can degrade. This harms the cell’s passivation quality, creating pathways for energy to escape and ultimately reducing the module’s overall power and long-term reliability.
A Classic Lamination Cycle Won’t Cut It
In a typical solar module factory, lamination is a high-heat, high-pressure process. Standard encapsulants, particularly the Polyolefin Elastomers (POE) favored for durability and moisture resistance, require temperatures around 165°C to achieve proper chemical cross-linking. As it cures, the encapsulant transforms from a soft film into a durable, protective cushion that bonds the module layers together for decades.
Here’s the standoff:
- HJT Cells Demand: Stay below 150°C to protect the sensitive TCO layer.
- Standard POE Encapsulants Require: Go up to ~165°C to cure properly.
Trying to use standard materials and processes for HJT technology forces a devastating compromise: either you cook the cells to cure the encapsulant, or you lower the heat to save the cells and end up with an under-cured, unreliable module.
Low-Temperature Cure POE: A Solution That Demands Verification
Fortunately, material scientists have developed a solution: low-temperature cure POE encapsulants. These advanced materials are formulated with special catalysts that allow them to achieve full cross-linking at temperatures safely below the 150°C danger zone.
Problem solved, right? Not so fast.
Simply swapping your standard POE for a low-temp version and hoping for the best is a recipe for failure. Every new material behaves differently, and „low-temperature“ is not a universal setting. Without rigorous validation, you risk catastrophic failures in the field. An under-cured encapsulant is a ticking time bomb that can lead to:
- Delamination: The layers of the module begin to separate over time.
- Moisture Ingress: Water vapor can penetrate the module, causing corrosion and short circuits.
- Degradation: The module becomes highly susceptible to failures like potential-induced degradation (PID), severely shortening its productive lifespan.
The Qualification Blueprint: Two Numbers You Must Get Right
Qualifying a new low-temperature POE isn’t about guesswork; it’s about data. The process hinges on measuring and verifying two key performance indicators (KPIs) to ensure your modules are built to last.
KPI #1: Degree of Cure (DoC)
The Degree of Cure (DoC), sometimes called gel content, is a direct measure of how successfully the encapsulant has cross-linked during lamination.
- Why it Matters: An insufficient DoC means the encapsulant hasn’t formed a stable, protective structure. It remains soft, gummy, and unable to provide the mechanical strength or moisture barrier needed for a 25-year lifespan.
- The Target: For long-term module reliability, the industry benchmark is a Degree of Cure greater than 80%.
- How it’s Measured: This is determined using precise laboratory equipment like a Differential Scanning Calorimetry (DSC) machine or a Rheometer. These instruments analyze small samples of the cured encapsulant to measure the extent of the chemical reaction.
KPI #2: Peel Strength
While DoC measures the encapsulant’s internal chemical state, Peel Strength measures its external performance: how well it adheres to the other layers. This KPI quantifies the adhesion between the encapsulant, the glass, and the backsheet.
- Why it Matters: Strong adhesion is what literally holds the module together. Poor peel strength is a direct indicator of future delamination. It means the layers can be pulled apart with minimal force, leaving the fragile cells exposed to mechanical stress and the elements.
- The Target: A robust bond is indicated by a peel strength greater than 40 N/cm.
- How it’s Measured: This is measured with a physical test defined by standards like ASTM D903. A strip of the laminated material is placed in a machine that pulls the layers apart at a constant speed, precisely measuring the force required for separation.
Finding the Sweet Spot: The Process Window Matrix
So, how do you achieve both a high DoC and strong peel strength without overheating your HJT cells? The key is to find the optimal process window—the perfect recipe of temperature, time, and pressure for your specific combination of materials and equipment.
This is achieved through systematic lamination trials. Rather than a single test, a matrix of experiments is designed. For example, you might test lamination cycles at 140°C, 145°C, and 150°C, each with holding times of 10, 12, and 15 minutes.
Each resulting sample is then meticulously tested for DoC and peel strength. By analyzing the data from this matrix, you can pinpoint the exact parameters that ensure a robust, reliable bond. This data-driven approach removes uncertainty and forms the foundation for successful solar module prototyping and, eventually, mass production.
Frequently Asked Questions (FAQ) about HJT Lamination
What exactly does a solar encapsulant do?
Think of it as the „glue“ and „cushion“ of a solar module. It’s a thin polymer film that, when heated, melts and flows around the solar cells. Once cured, it bonds the glass, cells, and backsheet into a single, durable unit, protecting the cells from moisture, oxygen, and mechanical shock for decades.
Why is POE often preferred over EVA for HJT modules?
While EVA is a common encapsulant, POE offers superior resistance to moisture and is not prone to producing acetic acid during curing, which can cause corrosion. For high-efficiency cells like HJT and TOPCon, POE’s enhanced durability and low water vapor transmission rate make it a better choice for preventing degradation mechanisms like PID.
What are the real-world consequences of a low Degree of Cure?
In the short term, nothing might seem wrong. But after a few years in the field, a module with a low DoC can begin to delaminate, especially around the edges. Bubbles may appear and moisture will find its way in, leading to rapid power loss and complete module failure long before its warranted life is over.
Do I need a special laminator for low-temperature processes?
Not necessarily. Most modern industrial laminators can be programmed for lower temperature cycles. The critical factor isn’t the laminator itself, but having the process expertise and testing capability to define and validate the correct recipe for that specific machine and material set.
From Theory to a Production-Ready Process
The incredible efficiency of HJT solar cells presents a massive opportunity. But unlocking that potential means moving beyond traditional manufacturing mindsets and adopting a scientific, evidence-based approach to lamination.
Successfully qualifying a low-temperature cure POE encapsulant is the bridge between a promising cell technology and a reliable, high-performance solar module. By focusing on the critical data—Degree of Cure and Peel Strength—you can build with confidence, ensuring your innovative designs deliver on their promise for decades to come.