The #1 Hidden Stress That Kills Solar Modules (And Why ‚Creep‘ Is The Surprising Cure)

Imagine a brand new, high-performance bifacial solar module, fresh off the production line. It passes every quality check with flying colors—flawless, efficient, and ready to generate clean energy for decades. But years later, its performance unexpectedly declines. The culprit? A network of tiny, invisible fractures in the solar cells, known as microcracks.

These cracks didn’t come from a hailstorm or shipping damage. They grew from a hidden, internal battle: a constant push and pull between the materials within the module itself. This battle is called thermomechanical stress, and understanding it is key to unlocking next-generation module reliability. At the heart of this challenge lies a surprising hero—a material property called „polymer creep.“

What is Thermomechanical Stress, and Why Does It Target Bifacial Modules?

Think about a bridge on a hot summer day. The metal expands. On a cold winter night, it contracts. Engineers accounted for this with expansion joints, giving the material room to move without breaking.

A solar module experiences this same cycle every day. As it heats up in the sun and cools down at night, its different components—glass, solar cells, and encapsulant—expand and contract at different rates. This mismatch creates internal forces, or thermomechanical stress.

This problem is especially critical in bifacial modules. With glass on both the front and back, the fragile silicon cells are sandwiched between two rigid, unyielding layers. There’s nowhere for the stress to go. The encapsulant, the polymer layer that surrounds and protects the cells, becomes the sole mediator in this thermal tug-of-war. If it can’t effectively cushion the cells, that stress gets transferred directly to them, leading to the slow, insidious formation of microcracks.

The Encapsulant: Your Module’s Built-In Shock Absorber

The encapsulant is more than just a glue holding the module together. It’s a critical performance layer responsible for:

• Optical Clarity: Letting maximum sunlight reach the cells.
• Electrical Insulation: Preventing current leakage and ensuring safety.
• Mechanical Protection: Cushioning the cells from external impacts and, crucially, internal thermomechanical stress.

For years, EVA (Ethylene Vinyl Acetate) was the industry standard. More recently, POE (Polyolefin Elastomer) has become popular for its superior resistance to moisture and potential-induced degradation (PID). Now, a new generation of encapsulants like EPE (a modified Ethylene Propylene Elastomer) is being engineered specifically to solve the mechanical stress puzzle.

But how do you measure a material’s ability to absorb stress over time? The answer lies in its „creep.“

The Surprising Benefit of ‚Creep‘

In engineering, „creep“ often has a negative connotation. It’s the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stress. Think of a heavy bookshelf causing a wooden shelf to sag slowly over many years—that’s creep.

In a solar module encapsulant, however, a controlled amount of creep is not just good; it’s essential.

An encapsulant with the right creep properties doesn’t stubbornly resist the forces from the expanding and contracting glass. Instead, it slowly yields and deforms, effectively relaxing and dissipating the stress before it can reach the solar cell. It acts like a slow-motion shock absorber. An encapsulant that is too rigid, by contrast, transfers that stress directly to the cells, like a faulty suspension system jarring a car’s passengers.

Engineers can quantify this behavior using a technique called Thermomechanical Analysis (TMA), a highly precise method for measuring how a material’s dimensions change under a specific load as the temperature varies.

POE vs. EPE: A Head-to-Head Comparison

At PVTestLab, we conducted a TMA study to compare the creep behavior of a standard POE encapsulant with a new, advanced EPE formulation designed for high-reliability modules. The materials were subjected to a constant force while the temperature was cycled to simulate the daily life of a module.

The results were revealing.

Thermomechanical Analysis (TMA) results showing the comparative creep (deformation) of EPE and POE encapsulants under simulated stress and temperature cycling.

As the graph shows, the EPE material exhibited significantly more deformation, or creep, than the conventional POE under the same conditions. At peak temperature and stress, the EPE deformed nearly twice as much.

This means the EPE is far more effective at relaxing and absorbing the thermomechanical stress generated by the glass. By yielding to these forces, it shields the solar cells from the damaging push and pull, drastically reducing the long-term risk of microcrack formation.

An electroluminescence (EL) image revealing microcracks (dark lines) in a solar cell, often caused by unresolved thermomechanical stress.

What This Means for Long-Term Module Reliability

Choosing the right encapsulant is no longer just a matter of chemistry—it’s a matter of mechanics. For designers of bifacial and other advanced module types, focusing on the thermomechanical properties of materials is paramount.

1. Reduced Cell Stress is Key: An encapsulant with optimized creep behavior, like the EPE tested here, acts as a superior cushion, directly contributing to lower cell stress and a reduced likelihood of microcracks over the module’s 25- to 30-year lifespan.

2. Lab Data Must Meet Reality: TMA provides critical insight into material behavior, but these properties must be validated in a real-world manufacturing environment. Comprehensive material testing & lamination trials are essential to ensure that the theoretical benefits translate into a stable, reliable production process.

3. Prototyping Bridges the Gap: The final proof is in the finished product. Building and testing complete modules through prototyping & module development allows engineers to confirm that the entire „sandwich“ of materials works in harmony, validating the design before committing to mass production.

The hidden stress inside a solar module is a powerful force, but it can be managed with intelligent material selection and a deep understanding of thermomechanics. By choosing encapsulants that can „go with the flow,“ we can build more resilient, reliable solar modules that stand the test of time.

Frequently Asked Questions (FAQ)

What is an encapsulant and what does it do?

An encapsulant is a polymer-based layer within a solar module that surrounds the solar cells. Its primary jobs are to provide adhesion between the glass and backsheet, offer electrical insulation, and protect the delicate cells from stress, moisture, and impact.

What are POE and EPE?

POE stands for Polyolefin Elastomer, a type of polymer widely used as a solar encapsulant known for its excellent durability and resistance to moisture. EPE is a related but more advanced formulation—an Ethylene Propylene Elastomer—engineered to have enhanced mechanical properties, such as controlled creep, for better stress reduction.

What’s the difference between thermal stress and thermomechanical stress?

Thermal stress is caused purely by temperature changes making a single material expand or contract. Thermomechanical stress arises when two or more different materials, bonded together, try to expand or contract at different rates, which creates mechanical forces between them. This is the primary challenge inside a solar module.

Why are microcracks bad for a solar module?

Microcracks are tiny fractures in the silicon solar cell. While a few might not cause immediate failure, they can grow over time due to thermal cycling. These cracks disrupt the electrical pathways, creating inactive areas on the cell that reduce the module’s overall power output and can eventually lead to hotspots or catastrophic failure.

How do you test for microcracks?

Microcracks are usually invisible to the naked eye. The industry-standard method for detecting them is Electroluminescence (EL) testing. This process involves passing a current through the module in a dark room, causing the solar cells to light up. Cracks and other defects appear as dark or inactive areas, allowing for precise quality control.

Your Path to a More Reliable Module

Understanding the subtle science of polymer creep is the first step toward conquering thermomechanical stress. The next is to apply that knowledge to real-world conditions. For developers exploring new materials or module architectures, the ability to validate these concepts is critical.

Optimizing the interaction between materials, cells, and equipment requires a hands-on approach. Partnering with a facility that specializes in process optimization & training can provide the data and expertise needed to bridge the gap between an innovative concept on paper and a reliable, high-performance product in the field.