Heterojunction (HJT) solar modules are celebrated for their remarkable efficiency and superior performance, especially in low-light conditions. They represent a significant leap forward in photovoltaic technology, but this impressive performance comes with a unique set of vulnerabilities that often fly under the radar of conventional testing protocols.

If you develop or evaluate HJT modules, you’re likely familiar with standard degradation culprits like PID (Potential-Induced Degradation) and LID (Light-Induced Degradation). But what if the real threats to long-term performance are hiding in plain sight, within the very materials that make HJT so powerful? Relying solely on standard tests for this technology is like using a road map for a country that hasn’t been fully explored—you’re bound to miss the most critical turns.

Beyond the Basics: The Unique Anatomy of an HJT Cell

To understand why HJT modules require a different approach to reliability, we first need to examine their unique structure. Unlike traditional PERC cells, an HJT cell consists of a crystalline silicon wafer „sandwiched“ between ultra-thin layers of amorphous silicon. This elegant design is the secret to its high efficiency.

However, two key components in this structure are particularly sensitive:

1. The Passivation Layers: These amorphous silicon layers are crucial for minimizing energy losses at the surface of the crystalline silicon. Think of them as a perfect seal that keeps electrons moving where they should.

2. The Transparent Conductive Oxide (TCO) Layer: This microscopic layer sits atop the passivation layer. It has two critical jobs: it must be transparent enough to let sunlight pass through to the silicon, and conductive enough to efficiently collect the generated electricity. It’s the module’s window and its wiring, all in one.

This sophisticated architecture, while brilliant for performance, also introduces new pathways for degradation fundamentally different from those found in conventional modules.

The Real Culprits: HJT-Specific Degradation You Can’t Ignore

While HJT modules show excellent resistance to traditional PID, they are susceptible to other environmental stressors that can cause significant power loss over time. Relying on standard certification tests alone creates a false sense of security. Here are the hidden threats that demand closer attention.

1. The Fragile TCO Layer: When the Window Cracks

The TCO layer is a marvel of material science, but it can be surprisingly vulnerable to humidity and heat. When moisture penetrates a module, a destructive chemical reaction can begin.

Our research at PVTestLab, backed by extensive material analysis, shows that certain encapsulants—particularly EVA (Ethylene Vinyl Acetate)—release acetic acid as they age. This acid aggressively corrodes the TCO layer. The result is an increase in the layer’s electrical resistance, which cripples the module’s ability to extract power. This degradation manifests as a drop in the Fill Factor (FF), one of the key indicators of a module’s health.

This slow-burn problem often evades a standard 1,000-hour Damp Heat (DH) test, requiring a more nuanced approach to see how different encapsulants, such as the more stable Polyolefins (POE), interact with the TCO layer under prolonged stress.

2. Passivation Under Attack: The UV Threat (UVID)

The amorphous silicon passivation layers are essential for HJT’s high open-circuit voltage (Voc). However, these layers can be damaged by prolonged exposure to ultraviolet (UV) radiation, especially when combined with high temperatures—a condition known as UV-Induced Degradation (UVID).

UV light can break the chemical bonds within the passivation layer, creating defects that allow energy to escape. This compromises the module’s voltage and overall efficiency. This type of degradation is particularly concerning for modules deployed in high-insolation regions like the Middle East or Australia, where intense UV and heat are the norm. A simple light-soaking test for LID won’t reveal this vulnerability; it requires specialized test cycles that combine UV exposure with thermal stress to simulate real-world conditions.

3. Material Mismatch: The Adhesion and Delamination Challenge

Building a durable solar module is like assembling a multi-layer cake—every layer must stick perfectly to the next. In HJT modules, ensuring strong adhesion between the TCO-coated cell, the encapsulant, and the glass is a major engineering challenge.

Moisture ingress is the primary enemy here. If water vapor finds its way into the module laminate, it can weaken the adhesive bonds, leading to delamination. This not only creates an entry point for more moisture and corrosion but can also cause optical distortions that reduce the amount of light reaching the cell. The choice of encapsulant and its compatibility with the TCO surface are paramount. Testing a single material in isolation isn’t enough; you must validate the entire material stack through Material Testing & Lamination Trials to ensure long-term stability.

Uncovering Hidden Flaws with Smarter Testing

So, how can you de-risk your HJT module design and ensure it will withstand 25+ years in the field? The answer lies in moving beyond basic certification tests and embracing advanced, combined-stress protocols that mimic real-world environments.

At PVTestLab, our German process engineers have developed specialized test sequences to pinpoint these HJT-specific weaknesses before they become costly field failures.

• Cyclic Damp Heat (cDH): Instead of a static test, we cycle temperature and humidity to create a „breathing“ effect in the module. This accelerates moisture ingress and more accurately simulates day/night environmental changes, making it far more effective at revealing TCO corrosion and delamination risks.

• UV Preconditioning Followed by Climate Chamber Tests: To assess UVID and material stability, we expose modules to a significant dose of UV radiation before putting them through thermal cycling and humidity freeze tests. This one-two punch reveals weaknesses in the passivation layer and encapsulant that isolated tests would miss.

• Full-Scale Prototyping: The ultimate validation comes from building and testing a complete module. This allows you to assess the complex interplay between all components—cells, encapsulants, backsheets, and glass. Prototyping & Module Development reveals exactly how your material stack performs under industrial lamination conditions and subsequent stress testing.

By analyzing data from these advanced tests—specifically changes in Fill Factor, Voc, and series resistance—we build a comprehensive picture of a module’s long-term reliability. This provides actionable feedback for material selection and design optimization.

FAQ: Your HJT Degradation Questions Answered

What is a TCO layer and why is it so important in HJT cells?
The Transparent Conductive Oxide (TCO) layer is a thin, clear film on the surface of an HJT cell. It must be transparent enough to let light in and conductive enough to get electricity out. If it degrades, the module’s efficiency drops significantly because it can no longer extract power effectively, even if the cell itself is still generating it.

Is POE always better than EVA for HJT modules?
Not necessarily. POE (Polyolefin Elastomer) is generally more stable and doesn’t produce corrosive acetic acid like many EVA formulations, making it a popular choice for protecting the sensitive TCO layer in HJT modules. However, the best option depends on the specific TCO, cell coating, and lamination process. Comparative testing is the only way to be certain.

Can’t standard IEC certification tests catch these issues?
Standard IEC tests like DH 1000h or TC 200 provide an essential baseline for safety and quality, but they weren’t designed to detect the specific, long-term degradation modes unique to HJT technology. They often lack the duration or combined stresses needed to trigger failures like TCO corrosion or UVID, which can take years to show up in the field.

What’s the difference between UVID and standard LID?
LID (Light-Induced Degradation) typically occurs in the first few hours or days of light exposure and then stabilizes. UVID (UV-Induced Degradation) is a longer-term effect caused by high-energy photons in UV light breaking down the passivation layers over months and years, leading to a steady decline in voltage.

How can I test my materials before building a full module?
You can start with coupon lamination. This involves creating small, sample-sized laminates with different combinations of glass, encapsulant, and cell pieces. These coupons can then be put through accelerated stress tests to screen for issues like discoloration, delamination, and TCO corrosion before you invest in full-scale prototyping.

The Path to Reliable HJT Performance

HJT technology holds immense promise for the future of solar energy, but realizing its full potential requires a deeper understanding of its unique failure modes. By looking beyond standard tests and focusing on the intricate interactions between materials, you can design and manufacture modules that not only deliver day-one efficiency but also stand the test of time.

The first step is recognizing the risks. The next is implementing a robust validation strategy that combines advanced testing with expert Process Optimization & Training to ensure your innovation is built to last.