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EVA Encapsulant In Solar Panels: What It Does And How It Degrades

EVA (ethylene-vinyl acetate) is the transparent polymer layer that sandwiches solar cells between the glass and backsheet in virtually every crystalline silicon panel. It provides the optical coupling, moisture barrier, and mechanical cushioning that keep fragile silicon cells producing electricity for 25 years or more. EVA's main weakness is UV-induced browning, a gradual yellowing that reduces light transmission and contributes to the 0.5% per year power degradation typical of crystalline silicon panels.

What EVA does in a solar panel

A solar panel is a laminated sandwich: tempered glass, front EVA, solar cells with metal interconnects, rear EVA, and backsheet (or rear glass). The EVA layers are the glue that holds everything together and the medium through which sunlight passes to reach the cells.

Optical coupling. Fresh EVA has a light transmittance of approximately 90% across the visible spectrum. Its refractive index (about 1.48) falls between glass (1.52) and air (1.0), which reduces internal reflections at the glass-EVA interface. This means more photons reach the cells instead of bouncing back out.

Moisture barrier. Silicon cells and their silver or copper metallization corrode when exposed to water vapor. The EVA encapsulant, combined with the glass front and polymer backsheet rear, creates a sealed environment with low water vapor transmission. However, EVA is not perfectly impermeable. Its water vapor transmission rate (WVTR) is moderate, which is why panels in humid climates can eventually develop moisture-related degradation.

Mechanical cushioning. Silicon cells are brittle and only about 160-180 micrometers thick in modern panels. The EVA absorbs mechanical stress from wind loading, thermal expansion, hail impact, and the vibrations of transport and installation. Without encapsulation, the cells would crack within weeks of outdoor exposure.

Electrical insulation. The EVA layers provide dielectric isolation between the cell circuits and the grounded glass and frame, contributing to the panel's overall electrical safety.

How EVA is manufactured into panels

EVA encapsulant arrives at the panel factory as rolls of clear, flexible film, typically 0.4 to 0.5mm thick. The film contains crosslinking agents, UV stabilizers, and antioxidants mixed into the base ethylene-vinyl acetate copolymer (typically 28-33% vinyl acetate content).

During panel assembly, sheets of EVA are placed above and below the cell string matrix, and the entire stack goes into a vacuum laminator. The laminator heats the assembly to 140-160 degrees Celsius for 10-20 minutes. During this process, the EVA melts and flows around the cells and tabbing ribbons, filling all gaps. Simultaneously, the crosslinking agents trigger a chemical reaction that converts the thermoplastic EVA into a thermoset material, meaning it can never be re-melted. The vacuum removes trapped air bubbles that would otherwise scatter light and create weak points.

The result is a monolithic laminate where the cells are permanently embedded in a clear, rubbery matrix bonded to the glass and backsheet.

How EVA degrades over time

EVA degradation is one of the best-studied phenomena in solar panel reliability. The primary degradation mechanism is UV-induced photodegradation, but heat and moisture also play roles.

UV-induced browning. Ultraviolet light breaks molecular bonds in the EVA polymer, creating chromophores, molecules that absorb visible light and give the encapsulant a yellow or brown tint. This browning reduces the amount of sunlight reaching the cells. NREL studies have measured light transmission losses of 5-10% in heavily browned EVA compared to fresh material. The front EVA layer degrades faster because it is directly exposed to UV, while the rear layer is partially shielded by the cells.

Acetic acid production. As EVA degrades, it releases acetic acid (the same compound in vinegar) as a byproduct. This acetic acid can corrode the metallic components inside the panel, particularly the silver cell metallization, solder joints, and interconnect ribbons. NREL research has documented a correlation between acetic acid concentration inside the laminate and increased series resistance in aged modules.

Delamination. Over time, the bond between the EVA and the glass or backsheet can weaken, particularly at the panel edges where moisture ingress is most likely. Delamination creates visible bubbles or milky areas in the laminate. These areas scatter light, reduce output, and trap moisture that accelerates further degradation.

EVA's role in the 0.5%/year degradation rate

Modern crystalline silicon panels typically degrade at about 0.5% per year after an initial 1-3% first-year drop (light-induced degradation). EVA browning is one of several factors contributing to this ongoing degradation, alongside cell-level degradation (LID, LeTID), solder joint fatigue, and increased series resistance.

In hot, high-UV climates like the desert Southwest, EVA browning can be a more significant contributor to degradation. In cooler, lower-UV climates like the Pacific Northwest, cell-level degradation mechanisms tend to dominate. This is one reason why panels in Arizona may degrade slightly faster than the same panels in Oregon.

POE: the emerging alternative

POE (polyolefin elastomer) encapsulant has gained significant market share as an alternative to EVA, particularly in premium and bifacial panels. POE offers several advantages.

PropertyEVAPOE
Water vapor transmission rateModerateVery low (roughly 1/10 of EVA)
Acetic acid productionYes (degradation byproduct)No
PID resistanceModerateExcellent
UV stabilityGood with stabilizersBetter inherent stability
CostLowerHigher
Volume resistivityLowerHigher (better electrical insulation)

POE's lower moisture permeability makes it especially important for bifacial and HJT panels, where moisture-sensitive cell structures are exposed from both sides. Many manufacturers now use a hybrid approach: POE on the cell-facing side (where moisture resistance matters most) and EVA on the glass-facing side (where cost savings are realized without significant performance compromise).

The ITRPV roadmap projects POE encapsulant share will continue growing, driven by the adoption of n-type cell technologies (TOPCon and HJT) that benefit from POE's superior moisture and PID resistance.

Related terms

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Frequently Asked Questions

What is EVA encapsulant in a solar panel?
EVA (ethylene-vinyl acetate) is a transparent polymer sheet laminated above and below the solar cells inside a panel. It bonds the cells to the front glass and rear backsheet, creating a sealed sandwich that protects the cells from moisture, mechanical stress, and thermal shock. EVA has been the dominant encapsulant material in solar panels since the 1980s and is used in the vast majority of panels manufactured today.
What does EVA encapsulant do?
EVA serves four functions in a solar panel. First, it provides optical coupling between the glass and cells, with a light transmittance around 90% that allows most sunlight to reach the cells. Second, it acts as a moisture barrier. Third, it mechanically cushions the brittle silicon cells against vibration, thermal expansion, and impact. Fourth, it electrically insulates the cells from the glass and backsheet layers.
Why do solar panels turn yellow or brown?
The yellowing or browning of solar panels is primarily caused by UV-induced degradation of the EVA encapsulant. Over years of UV exposure, the EVA undergoes photodegradation, producing chromophores (color-causing molecules) and acetic acid as byproducts. This discoloration reduces the amount of light reaching the cells, directly lowering power output. Browning is most noticeable after 15-25 years and is more severe in hot, high-UV climates.
How much does EVA browning reduce solar panel output?
EVA browning is one contributor to the typical 0.5% per year degradation rate in crystalline silicon panels. Studies by NREL have shown that heavily browned EVA can reduce light transmission by 5-10% compared to fresh encapsulant, though most panels show much less browning under normal conditions. The front EVA layer between the glass and cells degrades faster than the rear layer because it receives direct UV exposure.
What is the difference between EVA and POE encapsulant?
POE (polyolefin elastomer) is an alternative encapsulant with lower moisture permeability and better resistance to potential-induced degradation (PID) compared to EVA. POE does not produce acetic acid during degradation, making it more compatible with moisture-sensitive cell technologies like HJT and certain bifacial designs. POE is more expensive than EVA but is increasingly used in premium and bifacial panels. Some manufacturers use a hybrid approach with POE on the cell side and EVA on the glass side.
How thick is the EVA layer in a solar panel?
Each EVA sheet is typically 0.4 to 0.5mm thick before lamination. A panel has two EVA layers (front and rear), so the total encapsulant thickness is about 0.8 to 1.0mm. During the lamination process at 140-160 degrees Celsius, the EVA melts, flows around the cells and interconnects, and crosslinks into a thermoset material that cannot be re-melted.
Can EVA encapsulant be replaced or repaired?
No. Once laminated, the EVA is permanently bonded to the cells, glass, and backsheet. The crosslinked EVA cannot be re-melted or separated without destroying the panel. If EVA degradation significantly affects performance, the entire panel must be replaced. This is why encapsulant quality is critical, as it is the one component that cannot be serviced or repaired over the panel's lifetime.
How is EVA applied to solar panels during manufacturing?
EVA comes as rolls of clear film that are cut to panel size and placed above and below the cell matrix during layup. The entire stack (glass, front EVA, cells, rear EVA, backsheet) goes into a vacuum laminator that heats to 140-160 degrees Celsius for 10-20 minutes. Heat and vacuum remove air bubbles, melt the EVA around the cells, and trigger crosslinking. The result is a solid, transparent laminate.

Sources

Marko Visic
Physicist and solar energy enthusiast. After installing solar panels on my own house, I built TheGreenWatt to share what I learned. All calculators use NREL PVWatts v8 data and peer-reviewed formulas.