This paper investigates the cell-to-module (CTM) ratio of back-contact (BC) solar cells during module encapsulation, an innovative and increasingly important research focus in the photovoltaic industry. By comparing the CTM loss factors of bifacial and back-contact modules, the study reveals the differences in compatibility between crystalline silicon solar cells and advanced encapsulation materials and interconnect technologies.

BC Technology Overview

Front and back views of a typical BC solar cell and bifacial solar cell

BC cells are able to capture incident light more efficiently and achieve higher conversion efficiency and better aesthetics due to the absence of electrodes on the front side, but their manufacturing process is complex and the optical gain depends on the encapsulation material.

Bifacial cells are able to utilize reflected light from the backside to generate electricity, although there is the problem of front electrode shading, and the manufacturing process is mature and the production cost is low.

Gap interconnection structures for BC solar cells and bifacial solar cells 

BC cells have a simplified interconnection process due to all electrodes being located on the back side, and are able to achieve high density packaging through overlap soldering technology, reducing inactive areas and increasing the power density of the module.

Bifacial cells have a complex interconnection process due to the need for alternating connections between the front and back sides, and the gap design is limited, but module performance can still be improved through zero or small gap interconnection techniques. 

Packaging Loss Analysis

Factors affecting CTM:

In descending order of influence: solder tape resistance, busbar resistance, cell current matching, junction box resistance, solder tape shading, and contact resistance.

Comparison of influencing factors:

Overlap soldered interconnection structures for BC cells and double-sided cells

BC Cells: All electrodes are located on the back side and interconnections are made only on the back side, with solder tape connecting the positive and negative electrodes of neighboring cells in sequence. This design simplifies the interconnection process and reduces the number of solder joints.

Dual Sided Cells: The electrodes are located on the front and back sides, and the interconnections need to alternate between the front and back sides. This design increases the number of solder joints and process complexity.

Overlapping soldered interconnections for typical BC cells

Simplified Interconnect: With all electrodes located on the backside, the interconnect process is simplified, reducing the number of solder joints and process complexity.

High Density Packaging: Through overlap welding technology, zero or negative gap interconnections can be realized, reducing inactive areas and increasing the power density of the module.

Reduced Optical Loss: With no frontal electrodes, BC cells are able to capture incident light more efficiently, reducing optical loss.

Cross-sectional view of the inactive region of the module

Effect of inactive regions:

Inactive regions can result in a portion of the solar radiation not being captured by the cell, thus reducing the CTM ratio of the module.

By optimizing the module design, the inactive regions can be reduced and the power density of the module can be increased.

 Comparison of CTM Ratio Influencing Factors for Bifacial and BC Solar Modules

Difference between optical gain and optical loss:

Bifacial cell module: Optical gain is mainly from the reflection of frontal welding belt and frontal electrode, and optical loss is mainly from the shading of frontal welding belt and electrode.

BC cell module: Since there is no front electrode and welding tape, BC cell module has no optical gain from the reflection of front metal and no shading loss from front electrode and welding tape. Its optical gain is mainly dependent on the choice of encapsulation material.

Similarity of Geometric and Electrical Losses:

The geometric and electrical loss mechanisms are similar between the two assemblies, but the BC cell assembly may have an advantage in geometric and electrical losses due to the back-side electrode design. 

Influence of encapsulation material:

Double-sided cell module: optical gain and loss are mainly related to the front metal structure, and the encapsulation material plays a relatively minor role.

BC cell modules: Optical gain and loss are mainly dependent on the choice of encapsulation material. By optimizing the glass, adhesive film and backsheet, the CTM ratio of BC cell modules can be significantly improved.

Experiments and Analysis

Glass Matching Verification

Glass transmittance, cell quantum efficiency, and glass variation with different transmittance-enhancing films

Trend of glass transmittance curve with transmittance enhancement film thickness: When the thickness of the transmittance enhancement film on the glass surface is changed, the peak of the transmittance curve shifts to longer wavelengths. A suitable thickness of the transmittance enhancement film can reduce light reflection, increase light absorption and utilization, and thus improve the CTM ratio.

Quantum efficiency curves of different types of crystalline silicon solar cells: BC solar cells show a peak response after a wavelength of about 450nm, and reach a maximum response at 500 - 650nm. This indicates that BC solar cells have higher light absorption and conversion efficiency in this wavelength range.

Comparison of transmittance curves of different transmittance-enhancing glass: double-layer high-transmittance coated glass has the highest transmittance in the full wavelength range, which can effectively increase the transmission of light and enhance the power output of the module; double-layer colorless coated glass has different transmittance in different wavelength ranges, with a transmittance lower than that of the single-layer coated glass below 560nm, and higher than that of the single-layer coated glass above 560nm.

IBC solar modules with single-layer coated glass and double-layer colorless coated glass

Modules using single-layer coated glass appear blue because the single-layer coating, at a given thickness, enhances the reflection of certain wavelengths of light, increasing the blue light component of the reflected light and resulting in a blue appearance of the module. Modules with double-layer, colorless coated glass achieve an all-black appearance, which enhances aesthetics.

CIE Lab Color Space and Color Difference Visualization

The CIE Lab Color Space, illustrates how the change in glass color can be represented by a and b values and how the color difference (ΔE) is calculated. When ΔE < 1.5, it is difficult for the human eye to detect the color difference. In the study, the graph was used to analyze solar module color to help understand the relationship between glass coating thickness and color, as well as the effect of color on module appearance and CTM ratio.

Curves of a and b values and the effect of coating thickness on color display

The trends of a and b values under different coating thicknesses are shown to illustrate the effect of coating thickness on glass color.

Taking single-layer coating as an example, when approaching 110nm, a tends to +1 and b tends to 2, and the glass is dark blue; when approaching 90nm, a and b both tend to +1, and the glass is orange-yellow.

For double-layer coating, the change of film thickness will change the reflection spectrum of the glass, and the transition between red and blue tones, and the thickness of the bottom layer and the surface layer is negatively correlated with the values of a and b, respectively. By adjusting the film thickness, the chromaticity can be controlled, and the difference in color can be reduced, so as to achieve the visual effect of colorless glass. 

Film Matching Verification

Transmittance curves of different encapsulated films

Transmittance difference: In the wavelength range of 290-380nm, the transmittance of POE encapsulation film is slightly higher than that of EPE; in the range of 380-1100nm, the transmittance of EPE encapsulation film is higher than that of POE by about 0.6%. Overall, EPE film has the advantage of transmittance in the 380 - 1100nm spectral range, which is commonly used in solar cells.

Difference in module performance: For IBC solar cells with 23.7% and 24.0% efficiencies, the module power and CTM ratio of the EPE encapsulant film are higher than those of the POE encapsulant film. 

Backsheet Matching Verification

Reflectance curves for different backsheets

Difference in backsheet reflectivity: Early black backsheets had almost no reflectivity on the inner surface, while high-reflectivity black backsheets, with the addition of high-reflectivity black pigments, have greatly increased reflectivity in the 780 - 1100/1400nm near-infrared light range.

Impact on module performance: With the white backsheet, the reflectivity reaches 80% in the 400 - 1100nm range, and the module power and CTM ratio have certain values; with the high reflectivity black backsheet, the module power and CTM ratio are further improved.

Process Defect Verification

welded interconnection

The quality of the weld has a significant impact on the performance and CTM ratio of BC solar modules. Welding defects can lead to increased power loss, resulting in a decrease in module power, CTM ratio and key electrical performance parameters. During the production process, the welding process must be strictly controlled to improve the welding quality and minimize defects such as empty welds, broken fingers, and slippage, in order to ensure the efficient and stable operation of BC solar modules, and to improve their overall performance and CTM ratio.

In this paper, through a systematic study of the cell-to-module (CTM) ratio in the encapsulation process of back-contact (BC) solar modules, key factors affecting the CTM ratio are revealed and optimized encapsulation strategies are proposed. The results show that the choice of encapsulation materials and interconnection technologies has a significant impact on the performance of BC solar modules. Despite the advantages of BC cells in eliminating the shading loss of frontal electrodes, the limitation of their optical gain needs to be compensated by optimizing the encapsulation materials. The enhancement of CTM ratios by different types of PV glass, encapsulation film and backsheets is verified, while the importance of soldering quality and cell matching on module performance is emphasized. 

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