Perovskite solar cells are the talk of the town, promising record-breaking efficiencies that could redefine solar energy. But for anyone working to bring this technology from the lab to the rooftop, one major hurdle looms large: stability. For years, the industry’s primary boogeyman has been water vapor, and we’ve poured immense effort into developing barriers against it.
But what if the biggest threat isn’t the one we’ve been focused on? What if another, more insidious gas is silently undermining your module’s performance, even when your water barrier seems perfect?
It’s time we talked about oxygen.
The Industry Standard Isn’t the Whole Story
For solar module encapsulation, the Water Vapor Transmission Rate (WVTR) is the gold standard. It’s a critical metric that measures how much water vapor can pass through a barrier material over a given period. For conventional silicon modules, managing WVTR has been a well-understood and largely solved problem.
This focus is understandable: moisture is known to cause corrosion and delamination. But perovskite materials are a different breed. They are uniquely vulnerable not just to water but also to oxygen, especially when exposed to light and heat. This process, known as photo-oxidation, can trigger a rapid, irreversible degradation of the perovskite crystal structure, leading to a catastrophic drop in performance.
The dangerous assumption many developers make is that a material with a low WVTR must also be a good barrier against oxygen. Unfortunately, the physics of gas permeation doesn’t work that way. A material can be excellent at blocking large water molecules while allowing smaller oxygen molecules to sneak through.
Relying solely on WVTR to qualify materials for perovskite modules is like building a fortress with a high wall but leaving the gates wide open.
Both oxygen (O₂) and water (H₂O) can find pathways through the encapsulation stack. While both are harmful, degradation from oxygen ingress is often faster and harder to mitigate once a module is in the field.
Measuring the Invisible: Quantifying Oxygen Ingress
If oxygen is a primary culprit, the next logical question is: how do we measure it?
This brings us to the Oxygen Transmission Rate (OTR)—a metric quantifying how much oxygen gas passes through a barrier. While standard tests exist to measure the OTR of a single film or material sample, that approach falls short when predicting real-world module performance.
Why? Because a solar module isn’t a single film. It’s a complex, laminated stack of glass, encapsulants, and a backsheet. A module’s true barrier performance depends on how these materials interact after the solar module lamination process. Interfaces, edge seals, and the encapsulant itself all create potential pathways for gas ingress.
To get meaningful data, the complete encapsulation system must be tested as a finished unit.
At PVTestLab, we recognized this critical gap and engineered a specialized testing environment to measure the OTR of a full module laminate. Our custom-built test chamber, coupled with a high-precision mass spectrometer, allows us to place a complete, freshly laminated module sample inside and precisely measure the rate at which oxygen molecules permeate the entire stack—from the backsheet to the front glass.
This method moves beyond theoretical material specs to deliver a holistic, real-world OTR value for the entire protective system—providing actionable data on how a chosen material combination will actually perform.
Connecting the Dots: How OTR Predicts Module Lifetime
Measuring OTR is a powerful diagnostic tool, but its true value comes from connecting it to long-term reliability. A high OTR value is worrying, but what does it actually mean for your module’s T80 lifetime—the time it takes to lose 20% of its initial power?
To answer this, we correlate our OTR measurements with data from accelerated lifetime tests (ALT) conducted in highly controlled atmospheres. By testing identical perovskite mini-modules under two different conditions, we can isolate the impact of oxygen:
- Oxygen-Free Environment: The module is tested in a chamber filled with pure nitrogen (N₂).
- Oxygen-Rich Environment: The module is tested in a chamber with synthetic air (a controlled mix of nitrogen and oxygen).
The results are often stark. Modules protected by encapsulants with a high OTR degrade dramatically faster in the presence of oxygen compared to their counterparts in the nitrogen-only environment.
This comparative testing offers undeniable proof of oxygen’s destructive impact and, more importantly, allows us to quantify it. By analyzing these degradation curves, we can help material manufacturers and module developers answer critical questions:
- Does this new POE encapsulant offer a better oxygen barrier than a standard EVA?
- How much does adding an edge sealant like polyisobutylene (PIB) improve the module’s hermeticity?
- Which backsheet material provides the best combined barrier against both H₂O and O₂?
This data-driven approach is essential when prototyping new module designs, as it removes the guesswork from material selection and helps create the kind of truly hermetic package that perovskite cells need to thrive.
Frequently Asked Questions (FAQ)
What exactly is Oxygen Transmission Rate (OTR)?
OTR is a measurement of the amount of oxygen gas that passes through a substance over a given period. It’s typically measured in cubic centimeters of oxygen per square meter per day (cc/m²/day). A lower OTR value indicates a better barrier.
If my encapsulant has a low WVTR, isn’t that good enough?
Not for perovskites. A low WVTR is necessary but not sufficient. The molecular structure of a polymer determines which gases can pass through it. A material can be very effective at blocking larger water molecules but still be permeable to smaller oxygen molecules. For maximum stability, perovskites need a material that is a strong barrier to both.
What is photo-oxidation?
Photo-oxidation is a chemical degradation process that occurs when a material is simultaneously exposed to oxygen and light (photons). In perovskites, this interaction creates highly reactive oxygen species that attack the perovskite crystal lattice, leading to defects that trap charge carriers and reduce the cell’s efficiency and lifespan.
Are certain encapsulants better at blocking oxygen than others?
Yes, absolutely. Different polymers have inherently different barrier properties. For example, materials like polyisobutylene (PIB) are known for their extremely low gas permeability and are often used as edge seals to create a hermetic barrier. Comparing the OTR of different encapsulants like EVA, POE, and TPO is crucial for selecting the right one for a perovskite application.
How can I test the oxygen barrier performance of my own materials?
The most effective way is to test the complete laminated stack. This involves creating prototype modules with your chosen glass, encapsulant, and backsheet combination and then measuring the OTR of the finished laminate in a specialized chamber. This yields data far more representative of real-world performance than testing a single material film in isolation.
Your Next Step Towards Stable Perovskite Modules
The commercial viability of perovskite solar technology hinges on our ability to solve the stability challenge. While the industry has made great strides in managing moisture, it’s clear that oxygen ingress is an equally, if not more, critical factor.
Expanding our focus beyond WVTR to include OTR is no longer optional—it’s essential. By understanding, measuring, and designing for oxygen permeation, we can finally build the robust, long-lasting modules needed to unlock the full potential of this groundbreaking technology.
For anyone developing materials or designing modules for next-generation solar, understanding their true gas barrier properties is the foundational first step toward success.