Study on Encapsulation Losses of Back Contact (BC) Solar Cell Modules: From Material Selection to Process Optimization

1. Overview of BC Technology

BC Solar Cells feature no electrodes on the front surface, enabling more efficient capture of incident light, higher conversion efficiency and superior aesthetic performance. Nevertheless, they involve complex manufacturing processes, and their optical gain is highly dependent on encapsulation materials.

(Figure: Front and Rear Views of Typical BC Solar Cell and Dual-Sided Solar Cell)

Dual-sided solar cells suffer from front electrode shading loss, yet they can generate power by utilizing reflected light from the rear side. Additionally, they boast mature manufacturing processes and low production costs.

(Figure: Gap Interconnection Structure of BC Solar Cell and Dual-Sided Solar Cell)

With all electrodes arranged on the rear side, BC cells simplify the interconnection process. Overlap soldering technology enables high-density packaging, reduces inactive areas, and improves the power density of modules.

For dual-sided cells, interconnection requires alternating connection between the front and rear surfaces, leading to a complicated interconnection process and restricted gap design. Even so, module performance can be enhanced via zero-gap or small-gap interconnection technologies.

2. Analysis of Encapsulation Losses

2.1 Key Factors Affecting CTM Ratio

Ranked by impact degree in descending order: ribbon resistance, busbar resistance, cell current matching, junction box resistance, ribbon shading, and contact resistance.

2.2 Comparative Analysis of Influencing Factors

• BC Solar Cells: All electrodes are placed on the rear side, and interconnection is implemented solely on the rear with ribbons connecting the positive and negative electrodes of adjacent cells in sequence. This design streamlines the interconnection workflow and reduces the number of welding joints.
• Dual-Sided Solar Cells: Electrodes are distributed on both front and rear sides, requiring alternating front-rear interconnection. This structure increases welding joint quantity and process complexity.

(Figure: Overlap Soldering Interconnection Structure of BC Cell and Dual-Sided Cell)

(Figure: Overlap Soldering Interconnection of Typical BC Cell)

Advantages of BC Cell Interconnection Design

• Simplified Interconnection: Rear-only electrode layout simplifies procedures, cuts down welding joints and reduces process complexity.
• High-Density Packaging: Overlap soldering realizes zero-gap or negative-gap interconnection, minimizing inactive areas and boosting module power density.
• Reduced Optical Losses: The absence of front electrodes optimizes incident light capture and lowers optical losses.

(Figure: Cross-Section of Module Inactive Area)

Impact of Inactive Areas

Inactive regions block part of solar radiation from being absorbed by cells, thereby lowering the module CTM ratio. Optimized module design can shrink inactive areas and improve power density.

(Figure: Comparison of CTM Ratio Influencing Factors Between Dual-Sided and BC Solar Cell Modules)

Differences in Optical Gain and Optical Loss

• Dual-Sided Modules: Optical gain mainly derives from reflection by front ribbons and electrodes, while optical loss is primarily caused by shading from front metal structures.
• BC Modules: Without front electrodes and ribbons, BC modules have no optical gain from front metal reflection nor shading loss from front metal components. Their optical gain mainly relies on the selection of encapsulation materials.

Similarities in Geometric and Electrical Losses

The formation mechanisms of geometric loss and electrical loss are similar for both module types. However, BC modules possess inherent advantages in geometric and electrical loss control due to their rear electrode design.

Influence of Encapsulation Materials

• Dual-Sided Modules: Optical gain and loss are predominantly associated with front metal structures, with encapsulation materials playing a relatively minor role.
• BC Modules: Optical performance is highly dependent on encapsulation material selection. Optimizing Photovoltaic glass, encapsulation films and backsheets can significantly elevate the CTM ratio of BC modules.

3. Experiments and Result Analysis

3.1 Glass Matching Verification

Adjusting the thickness of the anti-reflection (AR) coating on glass shifts the peak transmittance toward longer wavelengths. An optimal AR coating thickness reduces light reflection, enhances light absorption and utilization, and further improves the CTM ratio.

(Figure: Glass Transmittance, Cell Quantum Efficiency and Performance Variation of Glass with Different Anti-Reflection Coatings)

BC solar cells show a spectral response peak at around 450 nm and reach the maximum response within 500–650 nm, indicating high light absorption and conversion efficiency in this wavelength band.

Double-layer high-transmittance coated glass achieves the highest transmittance across the full spectrum, effectively boosting light transmission and module power output. Double-layer colorless coated glass exhibits wavelength-dependent transmittance: lower than single-layer coated glass below 560 nm and higher above 560 nm.

(Figure: IBC Solar Cell Modules with Single-Layer Coated Glass and Double-Layer Colorless Coated Glass)

Modules adopting single-layer coated glass appear blue, as the specific coating thickness enhances reflection of blue light wavelengths. In contrast, modules with double-layer colorless coated glass achieve an all-black appearance with upgraded aesthetics.

(Figure: CIE Lab Color Space and Color Difference Visualization)

The CIE Lab color space characterizes glass color variation via a and b values and defines the calculation method of color difference (ΔE). A color difference ΔE ＜ 1.5 is imperceptible to the human eye. In this research, it is adopted to analyze module color performance, clarify the correlation between glass coating thickness and color, and explore the influence of color on module appearance and CTM ratio.

(Figure: Variation Curves of a & b Values and Coating Thickness Effect on Color Presentation)

The graph illustrates the changing trend of a and b values under different coating thicknesses and reveals the coating thickness impact on glass color:

• For single-layer coating: At a thickness near 110 nm, a approaches +1 and b approaches 2, presenting a deep blue color; at around 90 nm, both a and b tend to +1, showing an orange-yellow hue.
• For double-layer coating: Coating thickness adjustment alters the glass reflection spectrum, enabling color transition between red and blue tones. The thickness of the bottom and surface coatings is negatively correlated with a and b values respectively. Tuning coating thickness can regulate chromaticity, minimize color deviation and realize a colorless visual effect.

3.2 Encapsulation Film Matching Verification

(Figure: Transmittance Curves of Different Encapsulation Films)

In the 290–380 nm band, POE film shows slightly higher transmittance than EPE film; in the 380–1100 nm band, EPE film outperforms POE film by approximately 0.6%. Overall, EPE film has prominent transmittance advantages in the 380–1100 nm spectral range commonly used for solar cells.

For IBC solar cells with efficiencies of 23.7% and 24.0%, modules encapsulated with EPE film deliver higher output power and CTM ratio than those with POE film.

3.3 Backsheet Matching Verification

(Figure: Reflectance Curves of Different Backsheets)

Traditional black backsheets show nearly no inner surface reflectance, while high-reflectivity black backsheets adopt high-reflection black pigments to substantially improve near-infrared reflectance in the 780–1100/1400 nm range.

White backsheets maintain 80% reflectance in the 400–1100 nm range with decent module power and CTM ratio; high-reflectivity black backsheets further improve both indicators.

3.4 Process Defect Verification

(Figure: Welding Interconnection Structure)

Welding quality exerts a decisive impact on the performance and CTM ratio of BC solar cell modules. Welding defects such as cold solder, finger breakage and cell slippage will aggravate power loss and degrade module power, CTM ratio and key electrical parameters. During production, strict control of welding processes is required to improve welding quality, eliminate common defects, ensure efficient and stable operation of BC modules, and optimize their overall performance and CTM ratio.

4. Conclusion

This paper conducts a systematic study on the CTM ratio of BC solar cell modules during encapsulation, identifies the core influencing factors of CTM ratio, and proposes targeted encapsulation optimization strategies.

The research results verify that the selection of encapsulation materials and interconnection technologies significantly determines the performance of BC modules. Although BC cells eliminate front electrode shading loss, their inherent limitations in optical gain need to be compensated by optimizing encapsulation materials. Experiments confirm that customized photovoltaic glass, encapsulation films and backsheets can effectively improve the CTM ratio. Meanwhile, welding quality control and cell current matching are emphasized as critical guarantees for superior module performance.