Imagine launching a revolutionary new bifacial solar module. Its heterojunction (HJT) cells are pushing efficiency records, and the sleek glass-glass design promises decades of performance. But a few years into deployment, field reports trickle in: power output is dropping faster than expected. The culprit isn’t the cells themselves, but something seemingly simple—the clear, adhesive layer holding it all together.
This scenario highlights a quiet fear in the solar industry, one that points to an invisible threat: moisture. The core of the problem is how well the chosen encapsulant can protect highly sensitive HJT cells from water vapor. Winning this battle comes down to a single, critical metric: the Water Vapor Transmission Rate (WVTR).
WHAT IS WATER VAPOR TRANSMISSION RATE (WVTR) AND WHY DOES IT MATTER?
Think of WVTR as the breathability of a material. It measures how much water vapor can pass through a given area over a specific period. A high-performance raincoat, for example, needs to block water from coming in while still allowing moisture to escape from the inside. For a solar module encapsulant, however, the goal is the lowest possible WVTR, creating an almost impenetrable barrier against environmental moisture.
While all solar cells are susceptible to moisture, HJT cells are uniquely vulnerable. Their architecture relies on ultra-thin layers of Transparent Conductive Oxide (TCO) to efficiently extract electricity. But these TCO layers have an Achilles‘ heel: they are highly susceptible to corrosion when exposed to water.
THE ACHILLES‘ HEEL OF HJT: TCO CORROSION
When water vapor sneaks past the encapsulant and reaches the TCO layer, it triggers a chemical reaction. This corrosion degrades the TCO’s conductive properties, which increases the module’s series resistance and directly reduces its power output. Over time, this damage can lead to more severe problems like delamination, where the layers of the module begin to separate.
This isn’t just a cosmetic issue; it’s a direct assault on the module’s performance, reliability, and ultimately, its bankability. The promise of a 30-year lifespan can be cut short by an encapsulant that fails to hold the line against humidity.
CHOOSING YOUR SHIELD: A GUIDE TO SOLAR ENCAPSULANTS
The encapsulant is the first and most important line of defense against moisture ingress. The three most common materials used—EVA, POE, and EPE—offer vastly different levels of protection.
1. EVA (ETHYLENE VINYL ACETATE)
For decades, EVA has been the industry workhorse. It’s cost-effective and offers excellent adhesion. However, it has a relatively high WVTR, making it a poor choice for moisture-sensitive HJT cells. Worse, during the lamination process, EVA releases acetic acid as a byproduct, which can actively accelerate TCO corrosion.
2. POE (POLYOLEFIN ELASTOMER)
POE is the modern gold standard for protecting sensitive cells. It boasts an extremely low WVTR, creating a superior barrier against moisture. Unlike EVA, POE is chemically stable and does not produce acidic byproducts, ensuring the TCO layers remain pristine. While typically more expensive, its protective properties are considered essential for ensuring the long-term reliability of HJT modules.
3. EPE (EVA-POE-EVA)
EPE is a co-extruded „sandwich“ film that attempts to offer a compromise. A central layer of low-WVTR POE is flanked by two outer layers of EVA. The idea is to get the barrier properties of POE with the processing ease of EVA. However, the outer EVA layers still pose a risk of releasing acetic acid, potentially compromising the very cells it’s meant to protect.
As PV Process Specialist Patrick Thoma explains, „For HJT, choosing an encapsulant isn’t just a material spec—it’s the most critical decision you’ll make for long-term reliability. We often see that a low-WVTR POE is non-negotiable for preventing premature degradation.“
THE BIFACIAL CHALLENGE: DOUBLE THE GLASS, DOUBLE THE RISK?
Bifacial modules, with their glass-on-glass construction, present a unique challenge. Unlike a traditional module with an impermeable backsheet, a glass-glass design has two potential entry points for moisture: the front and the back.
This dual exposure intensifies the need for a best-in-class encapsulant. The entire system’s longevity now hinges on the encapsulant’s ability to form a perfect, void-free seal. This is where advanced solar module prototyping becomes critical to validate that the chosen material and design can withstand decades of environmental stress from both sides.
BEYOND THE DATASHEET: WHY LAMINATION MATTERS
A material’s properties on a datasheet are one thing; its performance after being laminated into a module is another. The lamination process—a carefully controlled sequence of heat, pressure, and vacuum—is where the encapsulant is cured to form its final protective structure.
Even the best POE film can fail if the lamination parameters are wrong. An incorrect temperature profile or insufficient vacuum can lead to improper curing, trapping bubbles or creating pathways for moisture to invade. This is why rigorous encapsulant material testing under real-world production conditions is so important. Selecting the right material isn’t enough; you must also perfect the process. Perfecting this recipe through structured lamination process optimization is the key to unlocking an encapsulant’s full protective potential.
FREQUENTLY ASKED QUESTIONS (FAQ)
WHAT EXACTLY IS WVTR?
Water Vapor Transmission Rate is a measurement of how easily water vapor can pass through a material. It’s typically measured in grams per square meter per day (g/m²/day). For solar modules, a lower number is always better.
IS POE ALWAYS BETTER THAN EVA?
For moisture-sensitive cells like HJT and TOPCon, POE’s superior moisture barrier makes it the clear winner for long-term reliability. For less sensitive PERC cells in monofacial designs with a backsheet, EVA can still be a cost-effective and reliable choice. Ultimately, the right choice depends on the specific cell technology and module design.
CAN YOU SEE TCO CORROSION WITH THE NAKED EYE?
Not in its early stages. The damage begins at a microscopic level, increasing series resistance and reducing power long before it becomes visible. By the time you see physical effects like yellowing or delamination, significant and irreversible performance degradation has already occurred.
HOW CAN I TEST WHICH ENCAPSULANT IS RIGHT FOR MY MODULE DESIGN?
The most reliable method is to build and test functional mini-modules or full-sized prototypes. This allows you to subject the complete module system—cells, encapsulant, glass, and interconnects—to accelerated aging tests like damp heat testing and see how all the components perform together under stress.
YOUR NEXT STEP IN MODULE RELIABILITY
Choosing the right encapsulant is more than a line item on a bill of materials—it’s a strategic decision that directly impacts the longevity, performance, and bankability of your solar modules. For HJT and other advanced cell technologies, the science is clear: a low-WVTR encapsulant like POE is a fundamental requirement, not a luxury.
Understanding these material interactions is the first step. The next is to validate them under real-world conditions, ensuring your design is truly built to last from the inside out.