You’ve made the investment. Your facility is ready to work with the latest high-efficiency TOPCon or HJT cells, promising unprecedented power output. The datasheets look fantastic, and the sum of the individual cells’ power is impressive. But when the final module comes off the line and goes through the flasher, the number is… lower.

It’s not by a huge amount—maybe 1%, 2%, or even 3%—but it’s a gap. This difference is the frustrating reality of Cell-to-Module (CTM) power loss, and for advanced cell architectures, it’s a challenge old assumptions can’t solve.

That lost wattage isn’t just a number; it’s lost revenue, a lower efficiency rating, and a competitive disadvantage. The good news? It’s not inevitable. By understanding exactly where these losses occur, you can reclaim that power.

The CTM Challenge: Why High-Efficiency Cells Change the Game

Cell-to-Module loss is the gap between the theoretical maximum power of all the cells in a module and the actual power the finished module produces. Every module has some CTM loss, but TOPCon (Tunnel Oxide Passivated Contact) and HJT (Heterojunction) cells introduce new variables.

Their incredible efficiency comes from highly engineered, sensitive structures. The very passivation layers and anti-reflective coatings that make them so powerful are also more susceptible to degradation during the rough-and-tumble module assembly process—especially lamination.

Think of it like building a high-performance race car. You can’t just drop a Formula 1 engine into a standard street car frame and expect record-breaking speed. You need a chassis, suspension, and aerodynamics all designed to work in perfect harmony with the engine. Similarly, TOPCon and HJT cells require a module „recipe“—encapsulants, interconnections, and lamination parameters—perfectly tuned to their unique characteristics.

The Two Thieves of Power: Optical and Resistive Losses

CTM losses in TOPCon and HJT modules primarily come from two culprits: optical losses (photons that don’t make it in) and resistive losses (electrons that struggle to get out).

1. Optical Losses: The Path of Light

Before a photon can generate an electron in a solar cell, it has to navigate a gauntlet of materials: glass, encapsulant, and the cell’s own coatings. Any photon that gets reflected or absorbed along the way is a lost opportunity.

With TOPCon and HJT cells, this challenge is magnified by their advanced anti-reflective coatings. A standard EVA (Ethylene Vinyl Acetate) encapsulant that worked perfectly for PERC cells might have a refractive index that creates an optical mismatch with these new coatings, causing more light to reflect away.

The choice of polymer encapsulant also has long-term implications. Some materials are more prone to UV degradation, which can cause yellowing or clouding over time, slowly choking off the light that reaches the cells and reducing power output over the module’s lifetime. Creating a seamless optical path ensures every possible photon gets to work.

2. Resistive Losses: The Electron Traffic Jam

Once an electron is generated, it needs a clear, low-resistance path to exit the module. Any bottleneck creates a „traffic jam,“ generating heat and wasting energy. This is resistive loss.

TOPCon and HJT cells often feature ultra-fine busbars to maximize the active cell area exposed to sunlight. This is great for photon collection but makes interconnection a delicate process. Traditional soldering ribbons and temperatures can be too aggressive, causing thermal stress and micro-cracks.

This creates a critical manufacturing conflict:

• The Encapsulant Needs Heat: Polymers like EVA and POE (Polyolefin Elastomer) require specific temperatures and times to cure properly, ensuring module durability.
• The Cells Hate Heat: The highly sensitive passivation layers on TOPCon and HJT cells can be damaged by excessive heat. This damage, known as thermal degradation, permanently reduces the cell’s efficiency.

This dilemma necessitates low-temperature interconnection technologies, such as specialized solders or electrically conductive adhesives, that can form a reliable bond without „cooking“ the cell. Finding the right combination of interconnection material and lamination cycle is crucial to minimizing resistive losses. A deep dive into your process through Material Testing & Lamination Trials is the only way to validate these new material pairings.

Making the Invisible Visible: How to Quantify CTM Losses

You can’t fix a problem you can’t see. To truly understand and minimize CTM losses, you need to move beyond simple flasher tests and adopt a more diagnostic approach. This means isolating variables and pinpointing the exact points of failure using advanced analytical tools.

Electroluminescence (EL) Testing: The Solar Cell X-Ray

An EL test is one of the most powerful tools in module diagnostics. By running a current through the module in a dark room, the cells illuminate, revealing their condition. A perfect module glows with uniform brightness.

An EL image taken after lamination can reveal a wealth of information about CTM losses:

• Micro-cracks: Tiny fractures in the cells caused by mechanical or thermal stress during stringing and lamination.
• Inactive Areas: Dark patches indicating parts of the cell that are no longer generating power.
• Interconnection Failures: Dark lines or spots along the ribbons showing poor soldering or adhesive contact.

By comparing EL images from before and after lamination, engineers can see exactly how the manufacturing process is impacting the cells, turning guesswork into a data-driven improvement cycle.

IV Curve Tracing: The Module’s Performance Fingerprint

If EL testing is the X-ray, then an IV (Current-Voltage) curve tracer is the detailed medical report. It measures the module’s electrical characteristics under precise conditions, providing critical data points like open-circuit voltage (Voc), short-circuit current (Isc), and Fill Factor (FF).

A drop in the Fill Factor, for example, is a classic sign of increased series resistance—a direct measurement of resistive losses. By analyzing the IV curve, we can quantify the electrical impact of different interconnection materials or lamination profiles.

When combined, EL and IV analysis create a complete picture. EL shows you where the physical damage is, and the IV curve tells you how much that damage is costing you in electrical performance. This systematic approach is the key to effective Prototyping & Module Development, allowing you to test, measure, and refine your design.

The Payoff: A Validated 1-2% Power Gain

By systematically testing different combinations of encapsulants (e.g., specialized POE vs. standard EVA) and interconnection technologies against a specific TOPCon or HJT cell, you can build a „process recipe“ that minimizes CTM losses.

The result? Through a structured R&D process, manufacturers can reclaim 1-2% of lost module power. This isn’t a theoretical gain; it’s a measurable, validated improvement that comes from matching the right materials and processes to the unique needs of high-efficiency cells. This level of fine-tuning is what separates market leaders from the rest of the pack and is a key goal of our Process Optimization & Training programs.

Frequently Asked Questions (FAQ)

What is the main difference between TOPCon and HJT cells?

Both are high-efficiency technologies that build upon the traditional PERC architecture. TOPCon adds an ultra-thin tunnel oxide layer and a layer of polysilicon to reduce recombination losses. HJT uses layers of amorphous silicon to passivate the crystalline silicon wafer, achieving excellent surface passivation and high voltages. Both are known for their high efficiency and temperature sensitivity.

What is a „good“ or „bad“ CTM loss percentage?

For traditional PERC modules, CTM losses are often in the range of 2-4%. With high-efficiency TOPCon and HJT modules, the goal is to get this number as low as possible, ideally below 1.5%. Anything above 2.5% suggests there is significant room for process and material optimization.

Why can’t I just use the same EVA I’ve always used?

You might be able to, but you could be leaving power on the table. The anti-reflective coatings on TOPCon/HJT cells may not be optically matched to standard EVAs. More importantly, the acetic acid that EVA can release during lamination can, in some cases, corrode the sensitive TCO (Transparent Conductive Oxide) layers on HJT cells, leading to long-term degradation. POE is often preferred for its lower water vapor transmission rate and lack of acid byproduct, but it requires different lamination parameters. Testing is the only way to be sure.

How long does it take to test a new material combination?

In a dedicated R&D environment like PVTestLab, a set of lamination trials for a new encapsulant or ribbon can often be completed in just one or two days. This timeframe covers building prototype modules, conducting EL and IV tests, and analyzing the data to provide clear recommendations for process optimization.

Your Next Step: From Guesswork to Guarantee

The era of one-size-fits-all module manufacturing is over. Unlocking the full potential of TOPCon and HJT technology requires a scientific approach to module design and assembly. Every component and every process parameter must be deliberately chosen and validated.

Stop letting preventable CTM losses erode your product’s performance and your bottom line. The first step is to understand the unique interactions between your chosen cells, encapsulants, and interconnection materials.

Ready to see what a fully optimized process looks like? Explore how an applied research environment at PVTestLab can help you quantify your CTM losses and build a roadmap to higher module power.