Imagine this: a state-of-the-art, glass-glass (G2G) solar module aces all its standard certification tests. It’s durable, powerful, and ready for a 30-year lifespan. Yet, after just a few years in a coastal desert—a region of scorching days, cool nights, and surprising humidity—its power output begins to dip inexplicably. The culprit isn’t a single, dramatic event. It’s a silent, relentless process: thermomechanical fatigue.

While the industry has rightfully celebrated the robustness of G2G modules, their inherent rigidity creates a unique set of challenges that standard tests can overlook. Understanding this hidden stress is key to ensuring long-term module reliability.

The Glass-Glass Paradox: A Story of Strength and Stress

Glass-glass modules, with glass on both the front and back, are renowned for their superior durability, fire resistance, and suitability for bifacial cells. Unlike traditional modules with a flexible polymer backsheet, the G2G structure is incredibly rigid. This is its greatest strength—and its most subtle weakness.

Think of it this way: a module with a flexible polymer backsheet can „breathe“ a little. When materials expand and contract with temperature changes, the backsheet can deform slightly to absorb some of that mechanical stress.

A glass-glass module, however, cannot. It’s a stiff, laminated sandwich. Every component—the glass, the encapsulant, the silicon cells, and the copper interconnects—is locked in place. When these materials expand and contract at different rates (a property known as the Coefficient of Thermal Expansion or CTE), significant internal stress builds up. There’s simply nowhere for it to go. This internalized stress is the seed of thermomechanical fatigue.

What Exactly is Thermomechanical Fatigue?

It’s not a single event, but a slow, cumulative degradation caused by the combined effects of temperature cycles and mechanical stress. In the real world, two primary environmental factors drive this fatigue:

1. Thermal Cycling (TC): The daily swing from the heat of the sun to the coolness of the night. This causes the various materials inside the module to constantly expand and contract.

2. Damp Heat (DH): Sustained high temperature and high humidity. This allows moisture to gradually penetrate the module edges, potentially weakening the bonds between layers.

Crucially, these two stressors rarely happen in isolation. A hot, humid day is often followed by a cool, damp night. This constant interplay of expansion, contraction, and moisture ingress puts a unique and powerful strain on the module’s internal structure, especially the rigid G2G design. Standard tests that evaluate these factors separately can miss the synergistic effect that accelerates degradation.

Simulating Reality: The Combined Damp Heat and Thermal Cycling Protocol

To truly understand how a G2G module will perform over decades, we need to replicate the combined stresses of the real world in a compressed timeframe. Advanced environmental chambers and combined testing protocols are designed for exactly this purpose.

By programming a chamber to cycle through periods of high heat and humidity followed by rapid cooling, we can simulate years of environmental exposure in just weeks. This accelerated aging process doesn’t just test if a module will fail; it reveals how it will fail. It’s a diagnostic tool that helps us pinpoint the weakest link in a new design, making this level of analysis fundamental to developing next-generation products through advanced solar module prototyping.

This rigorous approach allows us to see the early signs of fatigue long before they could lead to a critical failure in the field.

The Telltale Signs: What We Look For

During and after combined stress testing, we use high-resolution imaging and performance measurements to look for specific modes of failure unique to G2G structures.

Cell Microcracking

The silicon solar cells are the heart of the module, but they are incredibly thin and brittle. In a rigid G2G module, the mechanical stress from thermal expansion and contraction is transferred directly to the cells. Over thousands of cycles, this can lead to the formation of tiny, almost invisible cracks. While a few small cracks might not cause immediate power loss, they can grow over time, isolating parts of the cell and creating inactive areas.

Electroluminescence (EL) imaging acts like an X-ray for solar modules, allowing us to see these microcracks clearly.

Solder Bond Fatigue

The solar cells are connected by thin copper ribbons coated in solder. The CTE mismatch between the copper ribbon, the silicon cell, and the solder itself is significant. As the module heats and cools, these materials fight against each other, putting immense shear stress on the solder joints. Eventually, this can cause the bonds to crack and fail, leading to an increase in the module’s series resistance and a direct loss of power.

Loss of Adhesion (Delamination)

The encapsulant (typically EVA or POE) is the glue that holds the module sandwich together. Its ability to maintain a strong bond with the glass is critical for the module’s structural integrity and longevity. When moisture from damp heat testing penetrates the module, it can compromise this adhesive bond. Combined with the mechanical stress from thermal cycling, this can lead to delamination—where the layers of the module begin to separate.

Delamination not only creates a pathway for more moisture and corrosion but also impacts the module’s optical properties, further reducing performance.

Material selection is therefore paramount, as different polymers behave differently under these conditions. Thorough encapsulant material testing is essential to ensure the chosen material can withstand these combined forces over the long term.

From Lab Insights to Real-World Reliability

Understanding these failure modes isn’t just an academic exercise. It’s about building better, more reliable solar modules from the ground up.

„We’re not just breaking modules; we’re deconstructing the failure process. Understanding how and why a G2G module fails under combined stress allows our partners to build resilience directly into their designs from day one.“
– Patrick Thoma, PV Process Specialist

By identifying whether a new design is prone to microcracking, solder fatigue, or delamination, manufacturers can make critical adjustments before committing to mass production. This could involve selecting a more robust encapsulant, adjusting the spacing between cells, or refining the soldering process. Each insight gained through this rigorous testing contributes to more effective lamination process optimization and, ultimately, a more reliable final product.

Frequently Asked Questions (FAQ)

Is standard IEC testing not enough to catch these issues?

Standard IEC tests (like TC200 and DH1000) are excellent for establishing a baseline of safety and reliability. However, they typically test for these stresses in isolation. Combined protocols are designed to reveal synergistic failures that may not appear in sequential testing, providing a deeper understanding of long-term field performance.

Does thermomechanical fatigue affect all glass-glass modules equally?

No. The level of risk is highly dependent on the bill of materials and the manufacturing process. The choice of encapsulant, the design of the cell interconnects, and the quality of the lamination all play a critical role in a module’s ability to withstand internal stresses.

Is this a bigger problem in certain climates?

Absolutely. Regions with high diurnal temperature swings (hot days, cold nights) and high humidity are the most challenging environments. Coastal deserts, tropical regions, and areas with extreme seasonal shifts put the most thermomechanical stress on G2G modules.

How is this different from the stress on a glass-backsheet module?

A flexible polymer backsheet can expand, contract, and warp slightly to dissipate some of the mechanical stress. A rigid G2G module internalizes almost all of that stress, concentrating it on the cells and interconnects. This makes G2G modules more susceptible to these specific fatigue-related failure modes if not designed and manufactured properly.

Building for the Future

The shift towards glass-glass architecture is a positive step for the solar industry, promising longer lifespans and greater energy yield. However, this evolution demands a deeper understanding of the unique physical forces at play within these rigid structures.

By moving beyond standard testing and embracing combined stress protocols, we can proactively identify weaknesses, validate new materials, and engineer the next generation of solar modules to be truly resilient—not just in the lab, but for decades to come in the world’s most demanding environments.