Imagine holding a brand-new, high-performance solar module. It looks flawless, gleaming in the light, ready to generate clean energy for decades. But hidden from the naked eye, a microscopic battle is about to begin. Tiny, invisible fractures, known as microcracks, lurk in every solar cell. Under the relentless stress of daily and seasonal temperature swings, will they remain dormant, or will they spread like a spiderweb, silently draining the module’s power?
This is the critical question facing today’s two leading high-efficiency solar technologies: TOPCon and HJT. Both are pushing the boundaries of what’s possible, but their fundamental structures mean they respond to stress in dramatically different ways.
This analysis, based on high-resolution Electroluminescence (EL) imaging before and after rigorous testing, illuminates this hidden battle. We’ll explore how their unique designs dictate their resilience and what that means for the long-term reliability of your solar assets.
The Invisible Threat: Why Microcracks Matter
So, what are microcracks? They are tiny fractures in a solar cell’s silicon wafer that can form at any stage—from cell manufacturing to module assembly, transportation, and installation.
While a small, stable microcrack might have a negligible impact, the real danger is propagation. When a crack grows and spreads, it can:
- Sever Electrical Connections: The crack can cut through the delicate metal grid on the cell’s surface, creating „dead“ or inactive areas that no longer generate power.
- Increase Electrical Resistance: This leads to energy loss and can create localized „hotspots,“ which accelerate the degradation of the module’s materials.
- Cause Catastrophic Failure: In severe cases, a propagating crack can lead to a complete loss of power from a significant portion of the module.
As manufacturers push for thinner wafers and larger cells to reduce costs and boost efficiency, the risk of microcracks and their propagation becomes an even more critical factor in module design and long-term performance.
Understanding the Architects: TOPCon vs. HJT Cell Structures
To understand why these cells behave so differently under stress, we have to look at their construction. Each technology’s manufacturing process creates a fundamentally different internal architecture.
The TOPCon Approach: High Temperatures and Rigid Layers
TOPCon (Tunnel Oxide Passivated Contact) cells achieve their high efficiency through a process involving extremely high temperatures, often exceeding 800°C. This heat creates a rigid, crystalline structure, including a critical ultra-thin tunnel oxide layer and a polysilicon layer.
Think of it like firing a piece of pottery in a kiln. The process makes it incredibly strong and efficient, but also inherently brittle.
The HJT Approach: Low Temperatures and Flexible Layers
HJT (Heterojunction Technology) takes a completely different path. Its manufacturing process uses much lower temperatures—typically below 250°C—to sandwich a standard crystalline silicon wafer between ultra-thin layers of flexible, non-crystalline (amorphous) silicon.
This structure is more like laminated safety glass. It has a strong, rigid core, but the flexible outer layers are designed to absorb impact and stress, preventing cracks from spreading.
The Real-World Gauntlet: Simulating Stress with Thermal Cycling
How do we see this difference in practice? We can’t wait 25 years. Instead, we use a standardized reliability test called thermal cycling. A module is placed in a climate chamber and subjected to hundreds of cycles between extreme temperatures, typically from -40°C to +85°C. This process simulates and accelerates decades of thermomechanical stress caused by the natural expansion and contraction of the module’s different materials.
To see the results, we use Electroluminescence (EL) imaging. EL is like an X-ray for solar modules. By passing a current through the module, we cause the active areas of the cells to illuminate, instantly revealing defects like microcracks as dark, inactive lines. This type of rigorous testing is a cornerstone of modern solar module prototyping and development, ensuring new designs are truly field-ready.
The Verdict from the Lab: A Comparative Analysis
By comparing high-resolution EL images taken before and after thermal cycling, the distinct behaviors of TOPCon and HJT cells become strikingly apparent.
Initial State (Before Stress)
Before testing, EL images show that both TOPCon and HJT modules typically have some minor, pre-existing microcracks—a normal result of the cell manufacturing and module assembly process. At this stage, their impact on performance is minimal.
The Impact of Thermal Cycling
After undergoing thermal cycling, the differences are stark:
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TOPCon Cells: The EL images frequently reveal that the initial microcracks have propagated significantly. The rigid, brittle nature of the cell structure offers little resistance to the stress. Small, isolated cracks grow into long, branching fractures that sever electrical pathways and create large, dark, inactive zones on the cell.
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HJT Cells: In contrast, the EL images of HJT cells show remarkable stability. The initial microcracks have either not grown at all or have propagated only minimally before being halted. The flexible amorphous silicon layers act as a buffer, absorbing the thermomechanical stress and effectively stopping the cracks in their tracks.
The Consequence for Power Output
This visual evidence translates directly to performance and reliability.
The propagated cracks in the TOPCon module lead to a measurable drop in power output as significant portions of cells become electrically isolated. This degradation will likely continue as the module faces more stress in the field.
The HJT module, by containing the cracks, maintains its power output far more effectively. This inherent mechanical resilience points to a lower degradation rate and more reliable energy production over the module’s lifetime. Since the interaction between the cell and the encapsulant is also crucial, lamination trials for new materials are essential to further mitigate these stress effects.
As our PV Process Specialist, Patrick Thoma, often notes, „The initial EL image tells you the history of the cell’s handling; the final EL image tells you the future of its performance.“
What Does This Mean for Module Developers and Investors?
These laboratory findings have profound real-world implications.
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For Module Developers: The choice of cell technology directly impacts design freedom and long-term durability. HJT’s superior mechanical flexibility may allow for more innovative module designs—such as using thinner glass or different encapsulants—without sacrificing reliability.
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For Asset Owners and Investors: A module’s ability to resist degradation is paramount. HJT’s demonstrated resistance to microcrack propagation suggests a more stable power output over time, leading to more predictable energy yields, a lower Levelized Cost of Energy (LCOE), and a more secure return on investment.
Understanding these nuances is precisely why a dedicated PV test lab for applied solar research is invaluable, as it allows developers to validate these critical performance characteristics before committing to mass production.
Frequently Asked Questions (FAQ)
What exactly is a microcrack?
A microcrack is a tiny, often microscopic, fracture in a solar cell’s silicon wafer.
Are all microcracks bad?
Not necessarily. The concern is not the presence of a microcrack but its potential to propagate. A small, stable crack may have no noticeable effect, while a growing one can lead to significant power loss.
Does this mean TOPCon modules are unreliable?
No. TOPCon is an excellent, high-efficiency technology. This analysis, however, highlights a specific mechanical vulnerability. This risk must be carefully managed through optimized module design, high-quality material selection (especially encapsulants), and precise process control during manufacturing to minimize stress on the cells.
Why can’t I see microcracks with my eyes?
Most microcracks are too small to be seen without specialized equipment. Electroluminescence (EL) testing is the industry standard for revealing them, as it shows the electrically active and inactive parts of a cell.
What other tests are important for module reliability?
Besides thermal cycling, other key tests include Damp Heat (testing for moisture resistance), PID (Potential Induced Degradation), and Mechanical Load tests (simulating wind and snow loads).
The Path from Research to Real-World Reliability
While both TOPCon and HJT technologies are driving the solar industry forward, they offer different levels of mechanical resilience. Their fundamental architectures dictate how they handle the unavoidable stresses of the real world. This analysis shows that HJT’s low-temperature manufacturing process and flexible amorphous silicon layers give it a clear advantage in resisting microcrack propagation.
Understanding the interplay between cell technology, module materials, and process parameters is key to building the next generation of durable, high-performance solar products. Validating these characteristics in a controlled, real-world production environment is the critical step that turns a concept into a reliable, market-ready module.