The Bifacial Balancing Act: Could Fewer Solar Cells Mean More Power?

It sounds like a paradox: to get more energy from a solar module, you might actually need to use fewer solar cells. For developers of next-generation bifacial PV modules, this counterintuitive idea is at the heart of a critical design challenge—the trade-off between front-side power density and rear-side bifacial gain.

Get this balance wrong, and you leave significant energy production and revenue on the table. Get it right, and you unlock a lower levelized cost of energy (LCOE) and a powerful competitive advantage. But how do you find that perfect balance? The answer isn’t in a simulation; it’s in building, testing, and measuring under real-world conditions.

The Two Competing Forces in Bifacial Design

At first glance, designing a solar module seems straightforward: pack as many high-efficiency cells as possible into a given area to maximize the nameplate power. For traditional monofacial (one-sided) modules, this is largely true.

But bifacial modules play by a different set of rules. They generate energy from both the front (direct sunlight) and the back (reflected light from the ground, or albedo). This introduces a new variable that changes the entire equation: the spacing between the cells.

This creates two competing goals:

1. Maximize Power Density: A smaller gap between cells (e.g., 0.5 mm) means you can fit more active cell area on the module. This boosts the standard front-side power rating—the number on the datasheet.

2. Maximize Bifacial Gain: A wider gap between cells (e.g., 2-3 mm) allows more reflected light to pass between them and reach the transparent back of the module, where it can be captured by the rear side of the cells. This increases the extra energy generated from the back, known as bifacial gain.

These two goals are fundamentally at odds. A tight cell-to-cell spacing chokes off light to the rear side, crippling bifacial gain. A wide spacing maximizes bifacial gain but sacrifices precious front-side nameplate power. This is the bifacial balancing act.

From Theory to Tangible Data: A Prototyping Study

Moving beyond theory requires empirical data. At PVTestLab, we put this exact trade-off to the test on our full-scale R&D production line. We manufactured a series of identical 60-cell M10 PERC modules, with one crucial difference: the cell-to-cell spacing was varied at 0.5 mm, 1.0 mm, 2.0 mm, and 3.0 mm.

By holding all other materials and process parameters constant, we could isolate the precise impact of the cell gap. Here’s what the data revealed.

The Impact on Front-Side Power

As expected, increasing the cell gap reduced the module’s front-side power (Pmax).

Cell-to-Cell Spacing: 0.5 mm (Reference)
Pmax (Front): 450.4 Wp
Relative Pmax Change: 0%

Cell-to-Cell Spacing: 1.0 mm
Pmax (Front): 448.9 Wp
Relative Pmax Change: -0.33%

Cell-to-Cell Spacing: 2.0 mm
Pmax (Front): 445.4 Wp
Relative Pmax Change: -1.11%

Cell-to-Cell Spacing: 3.0 mm
Pmax (Front): 442.1 Wp
Relative Pmax Change: -1.84%

While a loss of nearly 2% in nameplate power might seem alarming, it’s only half the story. The real question is whether the gains on the back side can compensate for this loss.

The Boost in Bifaciality

This is where things get interesting. As we widened the cell gap, the module’s ability to capture light from the rear side—its bifaciality factor—increased significantly.

Wider spacing allowed more light to reach the back of the cells, resulting in a higher bifacial gain. This gain is the extra percentage of power you get from the rear side under real-world conditions.

The results paint a clear picture of the trade-off.

Finding Your Optimal Economic Balance

So, is a 2 mm gap better than a 1 mm gap? The data shows there isn’t a single universal answer. The optimal cell spacing is not just an engineering decision; it’s an economic one that depends on several factors:

• Target Albedo: A module installed over highly reflective ground (like sand or snow) will benefit much more from a wider cell gap than one installed over dark soil or grass.

• Module Materials: The choice of a transparent backsheet or dual-glass construction is fundamental. The properties of these materials are critical, and our facility allows for structured experiments on encapsulants and backsheets to see how they perform in a complete module assembly.

• Market Strategy: Are your customers valuing datasheet power (Pmax) or the actual energy yield (kWh) over the project’s lifetime? The answer shapes your design philosophy.

This is where prototyping becomes invaluable. By creating physical modules, you can gather the precise data needed to validate new solar module concepts before committing to mass production. With the right data, analysis, and process expertise, you can confidently define the cell spacing that delivers the best return on investment for your target application.

The bifacial balancing act is a complex puzzle, but it’s one that can be solved with the right methodology. By systematically testing and quantifying the trade-offs, module developers can engineer products that are not only powerful on paper but also superior performers in the field.

Frequently Asked Questions (FAQ)

1. What is a „bifaciality factor?“
   The bifaciality factor is a percentage that represents how efficient the rear side of a solar cell is compared to its front side. For example, a bifaciality factor of 80% means the rear side can generate 80% of the power the front side does under identical illumination. Our study showed that wider cell gaps can improve this factor at the module level.

2. What is albedo and why does it matter for cell spacing?
   Albedo is a measure of how much light is reflected off a surface. A white surface like snow has a high albedo, reflecting most light, while dark soil has a low albedo. For bifacial modules, a high-albedo installation site means more light is available for the rear side to capture. In these scenarios, a wider cell spacing designed to maximize bifacial gain becomes much more valuable.

3. Is there one „best“ cell spacing for all bifacial modules?
   No, and this is the key takeaway. The optimal spacing depends on the module design, the materials used, and, most importantly, the intended installation environment. The best spacing for a solar farm in the Arabian desert will likely be different from one in a field in Germany.

4. How does the backsheet material (e.g., transparent backsheet vs. dual-glass) affect this trade-off?
   The rear material is critical. A highly transparent and durable material allows more light to enter the module and reflect internally, maximizing the light available to the rear of the cells. The optical properties of the backsheet or rear glass interact directly with the cell spacing, making co-optimization essential.

5. Why can’t I just use a simulation for this?
   Simulations are a great starting point, but they rely on assumptions about material properties and light behavior. A physical prototype provides concrete data from a real, industrially produced module. It accounts for subtle manufacturing variations and material interactions that simulations can miss, dramatically reducing the risk of scaling up a sub-optimal design.