The promise of bifacial solar modules is compelling: generate more energy from the same footprint by capturing sunlight from both sides. This innovation has pushed new materials to the forefront, particularly the transparent backsheet—a critical component that allows light to reach the rear side of the solar cells.
Yet this innovation comes with a hidden risk. A transparent backsheet that looks perfect in the lab can become a major liability in the field, turning yellow, brittle, and cracked after just a few years under the sun. This degradation doesn’t just look bad; it systematically strangles the module’s energy output, undermining the very bifacial gain the module was designed to deliver.
The question for every material supplier and module developer is: How can you be certain your chosen backsheet will remain clear and stable for 25 years? As it turns out, standard testing protocols may not be giving you the full picture.
The Bifacial Promise and a Transparent Problem
A bifacial module is an elegant piece of engineering. Instead of an opaque backsheet, it uses a transparent material to allow reflected and diffuse light (albedo) to enter from the rear, boosting total energy production by anywhere from 5% to 30%.
This design places enormous responsibility on the transparent backsheet. It must provide decades of durable electrical insulation and protection from the elements, all while remaining optically clear.
Unfortunately, many polymers are vulnerable to the combined assault of ultraviolet (UV) radiation, heat, and humidity. Over time, UV rays break down the polymer chains in the backsheet material. This process, known as photodegradation, can lead to:
- Yellowing: A discoloration that directly blocks light from reaching the cells.
- Hazing: A cloudiness that scatters light, reducing its intensity.
- Cracking and Brittleness: Physical failure that compromises the module’s integrity and allows moisture to seep in, leading to catastrophic failure.
A yellowed backsheet isn’t just a cosmetic issue—it’s a direct tax on your module’s performance, year after year.
Are Standard IEC Tests Enough for New Materials?
The cornerstone for photovoltaic module qualification is the IEC 61215:2021 standard. It’s a robust set of tests designed to ensure modules are safe, reliable, and perform as expected. Within this standard, two key tests address environmental stress:
- UV Preconditioning (MQT 10): The module is exposed to a total UV dose of 15 kWh/m², followed by thermal cycling and humidity freeze tests.
- Damp Heat (MQT 13): The module is placed in a climate chamber for 1000 hours at a constant 85°C and 85% relative humidity.
These are excellent, necessary tests. However, they evaluate these stresses in isolation. That approach works well for traditional materials with decades of field data, but it can miss crucial failure modes in newer polymers used for transparent backsheets.
Think of it like this: leaving a raincoat out in the sun for a month and then spraying it with a hose on a separate occasion doesn’t tell you how it will perform in a real, prolonged rainstorm. In the real world, UV radiation, heat, and moisture attack a module simultaneously. This combined stress can create a synergistic effect, where one stressor weakens the material and makes it more vulnerable to the others.
A Better Approach: The Combined UV and Damp Heat (UV-DH) Protocol
To more accurately predict long-term performance, we need a test that better simulates the harsh realities of the field. At PVTestLab, we use an extended and combined protocol: accelerated UV exposure followed immediately by Damp Heat (UV-DH) testing, all within the same climate chamber.
Here’s why this approach is so effective:
- Initiating Damage: The initial, intense UV exposure phase mimics years of sunlight, breaking down the polymer bonds at the surface of the backsheet material. This is the first step in the degradation process.
- Accelerating Failure: In the 1000-hour Damp Heat cycle that immediately follows, high temperature and humidity penetrate the UV-weakened material. This combination rapidly accelerates aging mechanisms, revealing potential for yellowing, delamination, or cracking that isolated tests might take much longer to uncover.
This combined stress methodology provides a far more realistic assessment of how a material will behave over its 25-year lifetime. It’s a crucial part of a comprehensive accelerated life testing program designed to de-risk material selection before committing to expensive, full-scale production.
From Visuals to Data: How We Quantify Long-Term Stability
A „pass“ or „fail“ is not enough. To make informed R&D decisions, you need objective, quantitative data. After a sample completes the UV-DH protocol, we analyze its degradation in several ways.
The process begins with a simple visual inspection. Obvious cracking, delamination, or severe yellowing are immediate red flags.
But to truly understand the impact on performance, we must measure what the eye can’t see. Using a spectrophotometer, we measure the material’s transmittance—the amount of light that successfully passes through it. We also calculate the Yellowness Index (YI), a standardized ASTM metric that quantifies discoloration.
This data allows us to plot the degradation over time and directly compare the stability of different materials under identical stress conditions.
The results are often striking. A material that looked fine after a standard UV test might show a 10% loss in transmittance after a combined UV-DH cycle. For a module designed for 20% bifacial gain, that 10% rear-side loss could translate into a 2% absolute loss in total energy yield—a significant factor that compounds every year for 25 years.
Why This Matters for Your R&D
For material manufacturers and module developers, this level of testing isn’t just about compliance; it’s about confidence.
- For Material Manufacturers: You can validate the long-term stability of your products, providing your customers with reliable data that builds trust and sets your materials apart.
- For Module Developers: You can de-risk new bifacial module concepts by screening various encapsulant and backsheet materials early in the design phase. This prevents you from investing in a material that is destined to fail, saving millions in potential warranty claims and protecting your brand’s reputation.
Ultimately, understanding how a material will perform under real-world combined stresses is fundamental to engineering a solar module that delivers on its 25-year promise.
Frequently Asked Questions (FAQ)
What is a transparent backsheet?
A transparent backsheet is the rearmost layer of a bifacial solar module. Unlike traditional opaque backsheets, it is made from a clear polymer (like PET, PVDF, or co-polymers) that allows sunlight to pass through to the rear side of the solar cells, increasing total energy generation.
What causes backsheet yellowing?
Yellowing is primarily caused by photodegradation. Ultraviolet (UV) radiation from the sun contains enough energy to break the chemical bonds in the polymer structure of the backsheet. Over time, this chemical change manifests as a yellow or brown discoloration that blocks light.
How long does a UV-DH test take?
A typical cycle involves an initial UV exposure phase followed by a 1000-hour Damp Heat test. The entire sequence, including measurements, can take several weeks, providing data that simulates years of outdoor exposure.
Can you test other materials besides backsheets?
Absolutely. The same principles of combined stress testing apply to other module components, especially encapsulants (like EVA and POE), which are also prone to yellowing and degradation. We test a wide range of encapsulant and backsheet materials to validate their long-term performance.
Is this test a replacement for full IEC certification?
No. The UV-DH test is a critical research and development tool used for material validation and de-risking before you submit a final module design for full IEC 61215 certification. It helps ensure the materials you choose have a high probability of passing certification and, more importantly, performing reliably in the field.
Your Next Step in Material Validation
Selecting the right materials is the foundation of a high-performance, long-lasting solar module. While standard tests provide a baseline, truly understanding a material’s resilience requires looking deeper and testing for the combined stresses it will face every day for a quarter of a century.
By moving beyond isolated tests and embracing a more realistic UV-DH protocol, you can make data-driven decisions that reduce risk, accelerate innovation, and build a more reliable product.
If you’re developing a new module or validating a new material, discussing your specific challenges with expert process engineers can provide clarity and help define a robust testing strategy.