What if the empty space in your solar panel was just as important as the solar cells themselves?

For years, the gap between cells was seen as inactive, a necessary but unproductive part of the module’s real estate. In the world of bifacial solar modules, however, this thinking is being turned on its head.

That small gap is no longer dead space. It’s an active optical component, a hidden engine that can significantly boost a module’s power output—if tuned correctly. This concept, known as Cell-to-Module (CTM) gain, is the key to unlocking the full potential of bifacial technology. The challenge lies in finding the precise spacing that creates a perfect harmony between light capture and active cell area.

The Unsung Hero of Bifacial Modules: The Cell Gap

In a standard monofacial module, any light that doesn’t hit a cell is essentially lost energy. Bifacial modules, designed to capture light from both the front and back sides, change the game entirely.

When sunlight (or reflected light from the ground) enters a bifacial module with a transparent backsheet or a glass-glass design, the photons don’t just have one chance to be converted into electricity. Light that passes through the gaps between the cells can be recycled.

1. Light travels through the transparent front glass and encapsulant, missing the cells.
2. It continues through the module and strikes the white grid pattern printed on the rear glass or the reflective backsheet.
3. This reflective surface acts like a mirror, bouncing the light back up towards the rear side of the solar cells.

This process of internal reflection dramatically increases the amount of light, or irradiance, available to the rear side of the cells. The gap isn’t wasted space; it’s a cleverly designed light-trapping system.

This simple mechanism is a game-changer, but it also introduces a fascinating engineering puzzle.

The Great Trade-Off: Active Area vs. Internal Reflection

If wider gaps create more internal reflection, shouldn’t we make them as wide as possible? Not quite. This is the central trade-off every module designer faces.

• Wider Cell Gaps increase the potential for internal reflection, boosting rear-side energy capture. The trade-off is fitting fewer cells into the module, which reduces the total active (front-side) area.
• Narrower Cell Gaps maximize the number of cells on the module and increase front-side power potential. The downside is choking off the pathway for light, which limits the internal reflection that drives bifacial gain.

Instead of maximizing one factor at the expense of the other, the goal is to find the „sweet spot“—the optimal cell-to-cell spacing that results in the highest overall power output. This is measured by the Cell-to-Module (CTM) gain, a metric that quantifies the net increase in efficiency from the bare cell to the finished module. An optimized gap ensures the gains from internal reflection outweigh any minor losses from a slightly reduced active area.

As the graph illustrates, there is a clear peak where the balance is perfected. Deviating in either direction—too narrow or too wide—results in a lower CTM gain and a less powerful module.

Why One Size Doesn’t Fit All: The Variables That Matter

So, what is the magic number for the perfect cell gap? The answer is: it depends.

The optimal spacing is not a universal constant. It’s unique to the specific components and design of your module. The key variables include:

• Rear-Side Material Reflectivity: A highly reflective glass-frit pattern or backsheet will bounce more light back to the cells, potentially allowing for a slightly narrower gap to achieve the same effect.
• Cell Technology: Different cell types (e.g., PERC, TOPCon, HJT) have varying levels of bifaciality—the efficiency of their rear side compared to their front side. A cell with higher bifaciality will benefit more from the increased rear-side irradiance.
• Encapsulant Properties: The optical properties of the EVA or POE encapsulant also influence how light travels and reflects within the module.

Because of these interconnected factors, the sweet spot can’t be calculated on a spreadsheet. It must be found through building, testing, and measuring. This is where hands-on module prototyping becomes indispensable. By conducting structured lamination trials with different materials and configurations, you can gather the real-world data needed for an informed design decision.

From Theory to Reality: How to Find Your Module’s Sweet Spot

Finding the optimal cell spacing for your module design is a systematic process, moving from a theoretical model to a physically validated product.

The most effective approach involves these steps:

1. Establish a Baseline: Start with your current module design or a standard industry benchmark for cell spacing.
2. Create Prototypes: Manufacture a series of identical mini-modules or full-size prototypes where the only variable is the cell-to-cell gap. For example, you might create modules with 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm spacing.
3. Conduct Performance Testing: Using calibrated equipment like a AAA flasher, measure the power output and calculate the CTM gain for each prototype under identical, controlled conditions.
4. Analyze and Optimize: Plot the CTM gain against the cell spacing (just like the graph above) to identify peak performance. This data-driven result is your module’s sweet spot.

Of course, once you’ve optimized for performance, it’s equally important to validate module durability to ensure that your new design can withstand decades of real-world conditions without degradation or failure.

This iterative process of building, measuring, and refining is the cornerstone of modern solar module development. It transforms a good design into a great one.

Frequently Asked Questions (FAQ)

What exactly is CTM (Cell-to-Module) gain?

CTM gain is the net power gain or loss that occurs when individual solar cells are assembled into a finished module. A positive CTM gain means the module’s design—including optical effects like internal reflection—enhances the power output beyond the simple sum of the individual cells‘ power.

Does cell spacing matter for traditional monofacial modules?

For monofacial modules, the primary goal is to pack cells as tightly as possible to maximize the active area, as there is no mechanism for capturing light from the rear. The „active gap“ concept is unique to bifacial technology.

Is a bigger gap always better for bifacial gain?

No. While a wider gap increases internal reflection, it simultaneously reduces the number of cells in the module. After a certain point, the power lost from having fewer cells becomes greater than the power gained from extra reflection, causing the overall CTM gain to drop.

How much can CTM gain improve by optimizing cell gaps?

The exact improvement depends on the module’s materials and baseline design, but optimizing cell spacing can often boost CTM gain by several percentage points. In an industry where every fraction of a watt counts, this is a significant and highly cost-effective enhancement.

Your Next Step in Module Optimization

The space between your solar cells is no longer an afterthought—it’s a powerful design lever for maximizing energy yield. By understanding the trade-off between active area and internal reflection, you can begin to see your module not just as an assembly of parts, but as a finely tuned optical system.

Realizing this potential moves beyond theory and into the realm of physical testing. The path to a higher-performing, more competitive product lies in rigorous prototyping, precise measurement, and data-driven optimization. By treating the cell gap with the attention it deserves, you can unlock hidden power and set a new standard for performance.