The Million-Dollar Mistake in PV Testing: Why the First 100 Hours Are Non-Negotiable

Imagine this: your team has just developed a groundbreaking new solar module. The initial power measurement (Pmax) is outstanding, exceeding all expectations. You send it for critical reliability testing—like a Damp Heat or Thermal Cycling test—only to get a report back showing a significant, unexpected power loss. Was it the new encapsulant? A flaw in the backsheet?

Before you start questioning your materials, consider a more likely culprit: an invisible yet predictable phenomenon within the solar cells themselves. This initial power drop, known as Light-Induced Degradation (LID), is a natural characteristic of modern cell technologies. Measuring your module’s performance before this process has stabilized is one of the most common—and costly—mistakes in PV module development.

What’s Really Happening? A Quick Intro to LID and LeTID

When a brand-new solar module is first exposed to sunlight, a series of complex physical changes begins within the silicon. This initial „break-in“ period causes a drop in power output.

• Light-Induced Degradation (LID): A rapid power loss that occurs within the first few hours or days of light exposure. It’s particularly prevalent in p-type cells like PERC (Passivated Emitter and Rear Cell), which dominate the market today.
• Light and elevated Temperature Induced Degradation (LeTID): A similar phenomenon affecting both mono- and multi-crystalline PERC cells, LeTID is driven by both light and heat. It typically causes degradation over a longer period, though some power recovery can occur later on.

Crucially, these effects are not necessarily signs of a faulty module. They are a predictable phase of stabilization. The real problem arises when this initial, expected drop is mistaken for long-term, unpredictable degradation from a stress test.

The Million-Dollar Mistake: Measuring Power Too Soon

Testing a module’s power output is all about establishing a reliable baseline. If that baseline is a moving target, your entire R&D process can be thrown off course.

Think of it like measuring a person’s resting heart rate while they’re still jogging. The data you collect is technically accurate for that moment, but it’s completely useless for judging their actual cardiovascular health.

Our internal research at PVTestLab confirms just how significant this initial drop can be. When testing modern PERC modules, we consistently observe a power loss of up to 2.5% within the first 100-200 kWh/m² of light exposure. After this initial drop, the power output settles into a stable, predictable state.

Measuring Pmax before this stabilization is complete means you are comparing apples to oranges. Any subsequent power loss seen during a certification test (like IEC 61215) will be a mix of two different factors:

1. The initial LID/LeTID stabilization.
2. The actual degradation caused by the stress test.

This overlap makes it impossible to isolate the test’s true impact, so you’re left unable to determine if your new material or design truly passed or failed.

The Anatomy of a False Negative

Let’s walk through a common scenario:

1. Initial Measurement: You build a prototype with a new material and its Pmax measures a perfect 400 W right off the lamination line.
2. Certification Test: You send it for a 1,000-hour Damp Heat test. During the test, the module is exposed to heat and ambient light.
3. The Hidden Effect: Unseen, the module undergoes its initial LID stabilization, causing a 2% power drop. The Damp Heat test itself causes another 1% drop.
4. The Result: The final measurement shows a 3% power loss. You might wrongly conclude your new material failed the Damp Heat test, when in reality, two-thirds of the degradation was just the module „settling in.“

This can lead to abandoning promising materials, wasting R&D budgets, and delaying your time-to-market—all because the initial measurement wasn’t performed correctly.

The Solution: A Controlled ‚Sunbath‘ Before the Real Test

The only way to get trustworthy data is to separate the stabilization phase from the testing phase. This requires a controlled pre-conditioning process often called light soaking.

The protocol is simple in concept but requires absolute precision in execution: before any baseline Pmax measurement is taken, the module is exposed to a specific, uniform dose of light under controlled temperature conditions. This forces the LID and LeTID effects to occur and stabilize before the official testing begins.

Once the module’s output is stable, we take our „time-zero“ Pmax measurement. This value becomes the true, reliable baseline. Now, any degradation measured after a Damp Heat, Thermal Cycling, or UV test can be confidently attributed to the stressor itself. It’s the true starting point for any serious prototyping and module development.

Beyond the Basics: Why a Professional Protocol Matters

While the idea of light soaking sounds straightforward, achieving a scientifically valid and repeatable baseline requires more than just leaving a module out in the sun. An uncontrolled environment introduces too many variables—fluctuating light intensity, changing temperatures, and inconsistent exposure—making the results unreliable for certification.

A professional stabilization protocol demands:

• Controlled Irradiance: A consistent and uniform light source (e.g., 1000 W/m²) across the entire module surface.
• Temperature Management: Keeping the module at a constant, defined temperature to prevent other variables from influencing the outcome.
• Precise Energy Dosage: Accurately measuring the cumulative energy the module receives (measured in kWh/m²) to ensure it has fully stabilized according to industry standards.

This level of control is essential for accurate material testing and lamination trials, where you need to isolate the performance of a new encapsulant or backsheet from any underlying cell behavior. By using an industrial-scale, climate-controlled testing environment, we ensure that the data you receive reflects the true performance of your components, not the randomness of the weather.

Frequently Asked Questions (FAQ) about LID/LeTID Stabilization

What exactly causes LID and LeTID?

In the simplest terms, LID in p-type cells is often caused by the interaction of boron (a dopant in the silicon) and oxygen impurities, which creates light-activated defects that reduce cell efficiency. LeTID is thought to be related to hydrogen interactions within the cell structure, especially at elevated temperatures.

Does this affect all solar cell types?

LID and LeTID are most prominent in p-type technologies like PERC and TOPCon. N-type cell architectures are generally more resistant to these effects, but as new cell variations emerge, validation is always a crucial step.

How long does stabilization take?

It’s not about time, but about the total energy dose received by the module. Stabilization protocols typically require exposing the module to a cumulative energy dose of 50 to 200 kWh/m². The exact amount depends on the specific cell technology and the test standard being followed.

Can’t I just put the module outside in the sun?

For a quick, informal check, perhaps. But for certification or R&D, absolutely not. The sun’s intensity changes constantly due to clouds, time of day, and season. The module’s temperature will also fluctuate. This lack of control makes it impossible to create a repeatable, scientific baseline that would be accepted by any certification body.

Is LID/LeTID permanent?

The initial LID drop is considered a permanent stabilization. Once the module has settled, its power output remains stable under normal conditions. Some LeTID effects have been shown to be partially reversible under certain conditions, but the goal of pre-conditioning is to move the module beyond this volatile phase and into its long-term stable state.

Your Next Step: From Uncertainty to Accurate Data

In the competitive world of solar technology, data is everything. Don’t let a simple, avoidable measurement error derail your innovation. By understanding and properly addressing LID and LeTID stabilization, you ensure that your test results are accurate, your R&D decisions are sound, and your products are truly reliable.

Stabilize first, test second. This foundational principle is the key to moving your solar module concepts from the lab to the market with confidence. If you’re working on a new module design or evaluating new materials, establishing a stable baseline isn’t just good practice—it’s essential for success.