You’ve selected advanced bifacial cells, but the final module datasheet doesn’t reflect their full potential. This gap between cell efficiency and module power—the Cell-to-Module (CTM) ratio—is where promising designs either thrive or fail.
While academic papers detail theoretical gains, the real challenge lies in translating those concepts into measurable, repeatable performance on a production line. The key isn’t just knowing what to optimize, but how to validate those changes under real industrial conditions.
This guide moves beyond simulation to explore the critical design levers that influence CTM gains and losses, helping you convert theoretical advantages into bankable module power. With bifacial modules now accounting for over 50% of new utility-scale installations, mastering the CTM ratio has become the defining factor for competitive advantage.
What is the Cell-to-Module (CTM) Ratio?
The CTM ratio is a critical measure of your module’s design and manufacturing efficiency. It’s calculated by dividing the final module power (Pmax) by the sum of the power of all individual cells used.
CTM Ratio = Pmaxmodule / (Σ Pmaxcells)
Historically, this ratio was always below 100%, held back by inevitable losses like interconnection resistance, optical shading from ribbons, and inactive edge areas. With advanced bifacial designs, however, CTM ratios are now exceeding 100%.
This „free energy“ comes from recapturing light that would otherwise be lost, transforming module assembly from a process of loss mitigation into one of tangible power gain. Achieving a CTM ratio of 98.4% or higher isn’t an accident. It’s the direct result of a systematic approach that connects design choices with process validation—a methodology we’ve perfected at PVTestLab.
From Theory to Production: Correlating Design with Performance
Simulations provide a valuable starting point, but they can’t replicate the nuanced interplay of materials and process parameters. At PVTestLab, we bridge this gap by using a full-scale, climate-controlled R&D production line to generate empirical data. Our approach is built on a clear cycle:
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Isolate the Variable: We focus on a single design parameter, such as ribbon width or cell spacing.
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Prototype & Test: We build module prototypes under real industrial conditions, precisely controlling all other process parameters.
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Measure & Correlate: Using AAA Class flashers and high-resolution EL inspection, we measure the exact impact of the design change on module power and reliability.
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Define Process Windows: We deliver not just a result, but an optimized process window that ensures your CTM gains are stable and repeatable in mass production.
This applied methodology is how we turn design theory into measurable CTM performance.
Ribbon & Busbar Design—Minimizing Shading, Maximizing Gain
The interconnection strategy is a classic CTM battleground. While traditional flat ribbons create significant optical shading, modern multi-busbar (MBB) and zero-busbar (ZBB) architectures change the equation entirely. The deciding factors are the geometry and material of the cell interconnection. Using round wires instead of flat ribbons, for example, reduces direct shading on the cell surface—a seemingly small change with a cascading effect on CTM.
In our lamination trials comparing modules with conventional 5BB flat ribbons against those using 12BB round wires, we consistently measure a power gain of 1.5-2.5% for the MBB design. The round wires not only reduce the shaded area but also scatter incident light back onto the cell surface for a secondary optical gain.
However, the transition to round wires requires precise lamination process control. The encapsulant flow and curing cycle must be optimized to prevent wire movement and ensure perfect encapsulation without voids. Our process engineers fine-tune lamination recipes to achieve high yield and long-term reliability for these delicate structures.
CTM Optimization Insight: Adopting MBB with round wires offers a dual CTM benefit: it reduces resistive losses and converts potential optical losses into real power gains through light scattering.
Cell Spacing & Optical Reflection—The Surprising Power of the Gap
In monofacial modules, the space between cells is dead space. In a bifacial module, it’s an opportunity. The key is to transform the backsheet from a passive component into an active optical element.
The critical factors here are the distance between cells and the reflectivity of the backsheet or encapsulant. By intentionally spacing the cells, you allow light to pass through the gap, reflect off the high-albedo backsheet, and enter the cell from the rear.
To quantify this, we manufacture modules with cell gaps varying from 0.5 mm to 3.0 mm, using backsheets with different reflectivity coefficients. Our data shows a clear inflection point: for a typical PERC or TOPCon cell, increasing the gap from 0.5 mm to 2.0 mm can boost module power by up to 1%. Beyond that, the gains diminish as the loss of active area begins to outweigh the optical benefits.
This strategy demands exceptional stringing and layup accuracy to maintain module geometry. Our automated stringers and layup stations ensure that a 2.0 mm gap is maintained with sub-millimeter precision, guaranteeing the designed optical gain is realized consistently across every module.
„We often see clients focus heavily on cell efficiency, but forget that a well-designed gap and a highly reflective backsheet can provide over 1% of ‚free‘ module power. It’s one of the most cost-effective CTM gains available.“ — Patrick Thoma, PV Process Specialist
Encapsulants & Backsheets—Material Choices for Maximum Light Capture
Your choice of encapsulant and backsheet directly impacts how much light reaches the cell. This is a critical decision in bifacial design, where materials must manage light on both sides of the module. Key factors include the optical transmission of the front encapsulant and the reflectivity and UV stability of the rear materials.
Highly transparent, UV-stable POE (Polyolefin Elastomer), for instance, is often preferred over traditional EVA for bifacial modules due to its lower water vapor transmission rate and resistance to PID.
Our material and lamination trials involve creating test laminates with various material combinations and measuring light transmission before and after damp-heat and UV stress tests. These findings show that certain POE formulations can maintain up to 2% higher light transmission over the module’s lifetime compared to some EVAs, directly protecting your CTM gains from long-term degradation.
Because POE has a different rheology and curing profile than EVA, it requires a carefully managed lamination cycle to ensure proper cross-linking without causing cell stress. At PVTestLab, we develop custom lamination recipes tailored to your specific material stack, validating the process for both peak initial performance and 25-year reliability.
CTM Optimization Insight: CTM is not just about initial power. Selecting the right encapsulant/backsheet combination, validated through accelerated aging tests, ensures your day-one gains don’t fade over the project’s lifetime.
Frequently Asked Questions
- How do your real-world test results differ from software simulations like SmartCalc.CTM?
Simulations are an excellent tool for modeling theoretical outcomes and are part of a good design process. However, they operate on ideal assumptions. Our applied testing on a full-scale production line uncovers the real-world variables that simulations miss: subtle material interactions, the impact of lamination pressure on cell spacing, and the actual curing behavior of encapsulants. We provide the empirical data that validates—or corrects—the simulation, de-risking your transition to mass production.
- Is optimizing for CTM gains expensive?
Not necessarily. Some of the most effective strategies, like optimizing cell spacing or selecting a backsheet with 1-2% higher reflectivity, add minimal cost while delivering measurable power gains. The key is identifying the changes with the highest ROI. Renting our R&D line for a few days to test multiple variables is far more cost-effective than committing to a flawed design for millions of modules.
- What about CTM gains for advanced cell architectures like TOPCon or HJT?
These architectures are excellent candidates for CTM optimization. Because they have higher bifaciality factors, they benefit even more from techniques that enhance rear-side light capture. However, they can also be more sensitive to process-induced stress during lamination. Our Process Optimization & Training services are specifically designed to help manufacturers develop gentle, high-yield processes for these next-generation cells.
- How do you ensure confidentiality for our proprietary designs?
Confidentiality is fundamental to our operation. All projects are conducted under strict Non-Disclosure Agreements (NDAs). Our facility can be booked exclusively for your team, ensuring your materials, designs, and process data remain completely private. We operate as a secure extension of your own R&D department.
Your Roadmap to Validated CTM Performance
Achieving a CTM ratio over 100% is no longer a theoretical novelty; it’s a commercial necessity. It requires moving beyond datasheets and simulations to embrace a culture of empirical validation.
By systematically testing and correlating each design choice with its real-world impact on module power, you can unlock significant performance gains and build a more competitive, profitable product.
The path from concept to a fully optimized module begins with data. Validate your next design innovation and fine-tune your production process with our team of German process engineers.
Contact PVTestLab today to discuss your module development project.