In the relentless race for solar efficiency, we often focus on the solar cells themselves. We celebrate every tenth of a percent gained through new PERC, TOPCon, or heterojunction technologies. But what if a significant power boost was hiding in plain sight, in the very material holding the module together?
Most of us think of encapsulants like EVA or POE as the „glue“ of a solar module—a protective layer that holds everything in place. While true, this view misses a crucial detail: these layers are also optical components. Every photon of light has to pass through them to reach the cell.
This is where a fascinating, if niche, technology enters the picture: Optically Transparent Adhesives (OTAs). While traditional encapsulants are effective, OTAs promise superior performance by allowing more light to pass through. But as with any innovation, this promise comes with a major manufacturing puzzle.
First, A Look at the Glass-Glass Revolution
Before we dive into adhesives, let’s set the stage. Glass-glass modules, which use a sheet of glass on both the front and back, are becoming increasingly popular, especially for bifacial applications. They offer superior durability, fire resistance, and a longer lifespan compared to traditional modules with a polymer backsheet.
Their robust structure makes them an ideal testbed for advanced materials. The stability of two glass panes creates a perfect environment to explore how new encapsulating materials can push performance boundaries.
The quality of a glass-glass module isn’t just in the glass or the cells—it’s in the synergy of all its components. And the component with the most untapped optical potential is the one in the middle.
Understanding Cell-to-Module (CTM) Losses
Every solar module loses a small amount of power during manufacturing. The difference between the sum of the individual cells‘ power and the final module’s power output is known as the Cell-to-Module (CTM) loss (or gain, in some cases). A key contributor to this loss is the optical performance of the encapsulant.
Traditional encapsulants like Ethylene Vinyl Acetate (EVA) and Polyolefin Elastomer (POE) are workhorses of the industry for good reason. They’re cost-effective, durable, and well-understood. However, they aren’t perfectly transparent. They reflect a tiny fraction of light and have a refractive index that doesn’t perfectly match the glass and the solar cell, causing minor optical losses.
Think of it like looking through an old window versus a brand-new, perfectly clean one. You can see through both, but the new one lets in noticeably more light. For a solar module, more light means more power.
The Clear Advantage: How OTAs Enhance Light Transmission
Optically Transparent Adhesives (OTAs) are designed to address these very losses. Originally developed for high-tech displays and touch screens, these materials are engineered for one primary purpose: maximum optical clarity.
When applied to solar modules, their benefits are twofold:
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Higher Light Transmission: OTAs are inherently more transparent than even the best grades of EVA or POE, allowing a higher percentage of photons to reach the solar cell.
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Better Refractive Index Matching: An OTA’s refractive index can be better matched to the glass and cell, minimizing reflection at the material interfaces. Less reflection means more light is absorbed by the cell.
A recent experimental study, Optically Transparent Adhesives (OTAs) in Glass-Glass Solar Modules, validated this potential under industrial conditions. The research found that substituting a traditional encapsulant with a well-chosen OTA could improve the CTM ratio by 0.5% to 1.5%.
That might not sound like much, but for a 100 MW solar farm, a 1% gain translates to an extra 1,000,000 kWh of energy production per year. The promise is undeniable.
The Reality: Why OTAs Are a Process Engineering Puzzle
If OTAs are so great, why aren’t they everywhere? Because their unique properties make them a completely different beast during manufacturing. The very things that make them optically superior also create major challenges in the lamination process.
Our applied research at PVTestLab, which formed the basis of the study, confirmed that realizing the benefits of OTAs is far from straightforward. Here are the key hurdles:
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High Viscosity: Unlike EVA or POE films that melt and flow easily, many OTAs are thicker and more viscous. This makes it incredibly difficult to achieve a uniform, bubble-free layer during lamination without precise control.
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Critical Air Evacuation: The high viscosity of OTAs traps air bubbles tenaciously. A standard lamination vacuum cycle is often insufficient to remove them, leading to voids that can compromise both performance and long-term reliability.
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Sensitive Curing: OTAs have different curing mechanisms (often UV or thermal) that require highly specific temperature profiles and pressure application. Deviate even slightly, and you risk outgassing—where dissolved gases form bubbles within the adhesive—or incomplete curing, which can lead to delamination later.
Successfully integrating OTAs isn’t about simply swapping materials. It requires a fundamental rethinking of the lamination recipe. That’s why hands-on solar module prototyping on a real production line becomes essential. You can’t simulate these dynamic, physical challenges on a computer; you have to solve them on the machine.
The research concluded that while the CTM gains are real, they are achievable only through rigorous process optimization. Any successful adoption of OTAs depends on thorough material validation and the development of a specialized lamination cycle tailored to the specific adhesive.
FAQ: Your Questions on OTAs in Solar
For those just starting to explore this topic, here are a few common questions.
What’s the main difference between an OTA and a POE encapsulant?
The biggest difference lies in their chemical makeup and primary design goal. POE is an elastomer designed for durability, moisture resistance, and adhesion in PV modules. An OTA is an adhesive engineered first and foremost for optical purity and clarity, often with different curing properties and higher viscosity.
Are OTAs more expensive than traditional materials?
Generally, yes. As specialized materials developed for industries like consumer electronics, OTAs typically have a higher cost per square meter than standard EVA or POE. The decision to use them involves a cost-benefit analysis, weighing the higher material cost against the projected lifetime energy gain.
Can any lamination line handle OTAs?
Not without major process adjustments. A standard laminator can physically process the materials, but achieving a void-free, perfectly cured module requires deep process knowledge and the willingness to experiment with vacuum levels, temperature ramps, and pressure profiles. It’s a task for an R&D or pilot-line environment, not a mass-production line.
How do you measure the CTM gain from using an OTA?
It requires a controlled experiment. You would produce two identical sets of modules—one with a standard encapsulant and one with an OTA—using cells from the same batch. Both sets are then tested under standard test conditions (STC) using a high-precision flasher. The difference in the average power output reveals the CTM gain or loss.
The Path from Lab to Fab
Optically Transparent Adhesives represent a real, if challenging, opportunity to squeeze more power out of solar modules. They prove that innovation isn’t limited to the solar cell—it extends to every material in the stack.
However, their story is also a critical reminder: a promising material is only as good as the process used to integrate it. The bridge between a datasheet showing 99% light transmission and a reliable, high-performance solar module is built with hands-on testing, data-driven optimization, and deep process expertise.
For material developers and module manufacturers intrigued by the potential of OTAs, the next step isn’t mass production—it’s controlled experimentation. Understanding how these advanced materials behave under real industrial conditions is the key to unlocking their clear advantage.