What Causes Solar Cell Efficiency Drop After Panel Assembly?

The efficiency of a solar cell is one of the most critical factors in determining how much electricity a solar module can generate. In laboratory conditions, solar cells often achieve high efficiency values. However, once these cells are assembled into a complete solar module, a noticeable drop in efficiency is commonly observed. This phenomenon is well known in the photovoltaic (PV) industry and is referred to as cell-to-module (CTM) loss.

This article explains, in simple and clear language, why solar cell efficiency drops after assembly, the technical reasons behind it, and how solar panel manufacturers work to reduce these losses.

Understanding Cell Efficiency vs Module Efficiency

A solar cell is the smallest functional unit that converts sunlight into electricity. Cell efficiency is measured under ideal laboratory conditions, where light exposure, temperature, and electrical connections are carefully controlled.

A solar module, on the other hand, is made by connecting multiple solar cells together and encapsulating them using glass, EVA sheets, backsheets, frames, and junction boxes. Module efficiency represents the real-world performance of the entire panel, not just the individual cells.

During this transition from cell to module, several mechanical, optical, electrical, and thermal factors contribute to efficiency loss.

1. Optical Losses Due to Glass and Encapsulation Materials

One of the primary reasons for efficiency drop after assembly is optical loss.

When sunlight reaches a bare solar cell in the lab, nearly all usable light enters the cell surface. After assembly, light must pass through:

• Front glass
• Anti-reflective coatings
• EVA (ethylene vinyl acetate) encapsulant

Each layer absorbs or reflects a small portion of sunlight. Even high-quality solar glass reflects around 2–3% of incoming light. Additional reflection and absorption by encapsulation materials further reduce the amount of light reaching the cell.

As a result, less sunlight is available for electricity generation, leading to lower module efficiency compared to cell efficiency.

2. Electrical Resistance and Interconnection Losses

In a module, solar cells are connected using metal ribbons, busbars, and solder joints. These connections introduce electrical resistance, which does not exist when a single cell is tested independently.

Key electrical losses include:

• Resistance in interconnecting ribbons
• Solder joint resistance
• Current mismatch between connected cells

When current flows through these connections, a small amount of energy is lost as heat. This resistive loss reduces the overall power output of the module.

3. Cell Mismatch and Sorting Limitations

In mass production, no two solar cells are perfectly identical. Cells may vary slightly in:

• Efficiency
• Open-circuit voltage (Voc)
• Short-circuit current (Isc)

When cells with different electrical characteristics are connected in series, the weakest cell limits the performance of the entire string. This is known as mismatch loss.

Even with advanced cell sorting techniques, some mismatch is unavoidable, contributing to efficiency reduction after assembly.

4. Mechanical Stress During Module Manufacturing

Solar cells are thin and fragile. During the module assembly process, cells are exposed to:

• Soldering heat
• Lamination pressure
• Thermal expansion and contraction

These mechanical stresses can create:

• Microcracks in the silicon wafer
• Broken fingers or busbars
• Hidden structural damage

Microcracks reduce the effective active area of the cell, causing power loss that may not be immediately visible but affects long-term performance.

5. Temperature Effects in Real Operating Conditions

Cell efficiency measurements are typically taken at 25°C under Standard Test Conditions (STC). However, solar modules operate at much higher temperatures in real environments.

After assembly:

• Heat dissipation becomes less efficient
• Encapsulated cells retain more heat
• Operating temperatures rise

Higher temperatures reduce voltage output, which directly lowers efficiency. This temperature-related loss becomes more pronounced at the module level compared to individual cell testing.

6. Shading and Frame-Related Losses

Once cells are assembled into modules, additional components can create partial shading:

• Module frames
• Junction boxes
• Mounting structures

Even small shaded areas can significantly impact performance, especially in series-connected cells. These shading-related losses do not exist during individual cell testing.

7. Light Reflection from Intercell Gaps

In a solar module, small gaps are maintained between cells to accommodate thermal expansion and mechanical tolerance. These gaps reflect sunlight that could otherwise be used for power generation.

Although this loss seems minor, across an entire module surface it contributes to a measurable reduction in efficiency.

8. Encapsulation Aging and Material Degradation

Over time, encapsulation materials may undergo:

• Yellowing of EVA
• Moisture ingress
• Delamination

While these effects are more related to long-term performance, even initial material properties can slightly reduce light transmission compared to direct cell exposure.

9. Measurement and Rating Differences

Cell efficiency is often measured using small-area, highly controlled equipment. Module efficiency is measured over a larger area and includes all inactive regions such as:

• Cell spacing
• Borders
• Frame edges

This difference in measurement area alone results in lower efficiency values at the module level, even if the cells themselves perform well.

10. Cell-to-Module (CTM) Loss Explained

All the factors mentioned above combine into what the industry calls Cell-to-Module (CTM) loss, which typically ranges from 1.5% to 3%, depending on:

• Cell technology (Mono PERC, TOPCon, HJT)
• Module design
• Manufacturing quality

Advanced technologies like half-cut cells, multi-busbar designs, and improved encapsulation materials are helping reduce CTM losses in modern solar modules.

How Manufacturers Reduce Efficiency Loss After Assembly

To minimize efficiency drop, leading manufacturers adopt:

• High-transmittance solar glass
• Low-resistance interconnection ribbons
• Precise cell binning and sorting
• Stress-free lamination processes
• Advanced cell layouts like shingled or half-cut designs

These innovations help ensure that the final solar module delivers performance as close as possible to the original cell efficiency.

Conclusion

The drop in solar cell efficiency after module assembly is a natural and unavoidable part of PV manufacturing. Optical losses, electrical resistance, mechanical stress, temperature effects, and material limitations all contribute to this reduction.

However, with continuous advancements in cell technology and module design, the gap between cell efficiency and module efficiency is steadily decreasing. Understanding these losses helps manufacturers, installers, and customers make informed decisions and appreciate the engineering behind high-performance solar panels.