Research Background

HJT solar cells have attracted wide attention because of their high energy conversion efficiency,fewer manufacturing processes,lower preparation temperature and better temperature coefficient. The low temperature preparation characteristics of HJT solar cells limit the selection of paste, resulting in poor electrical conductivity of silver paste and high cost. 

In order to reduce the cost, various solutions have been developed in the industry, such as silver-coated copper pastes, electroplating copper technology, laser transfer  printing technology and non-main grid technology 

Preparation of Solar Cells

Schematic diagram of the HJT solar cells structure

HJT solar cells are fabricated using an n-type silicon wafer as the substrate. The process involves alkali texturing, plasma-enhanced

chemical vapor deposition (PECVD) to prepare amorphous silicon thin films, magnetron sputtering (PVD) to deposit transparent conductive oxide thin films (TCO), and silk screen printing to fabricate metal electrodes in sequence. Metal electrodes are prepared using silver-coated copper paste with a silver content of 50%.

Structural characterization and tensile properties of silver-coated copper grid line

SEM image of the front fine grid lines of the silver-coated copper HJT solar cell

The powder in the silver-coated copper paste is in the form of spherical particles with good dispersibility. The grid lines exhibit a clear structure, and the silver-coated copper particles are evenly distributed in the grid lines, indicating that the paste can form good conductivity during the low-temperature sintering process. This uniform distribution helps to improve the conductivity of the electrodes and their contact performance with the transparent conductive oxide (TCO) thin films.

Topography image of the front grid lines of the silver - coated copper HJT solar cell

The grid line topography image captured by  3D microscope shows the flatness and uniformity of the grid lines. There are differences in the flatness of the grid lines, with uneven highs and lows. This may be due to the mesh knots of the screen, resulting in poor line - type uniformity.

 The average line width of the silver - coated copper grid lines is approximately 43 µm, the average line height is about 14 µm, and the aspect ratio is around 32.5%. This is slightly lower than the performance of the grid lines of conventional low - temperature silver paste HJT solar cells. However, the silver - coated copper grid lines can still achieve good electrode functions and have sufficient mechanical strength.

Tensile force diagram of the front grid lines of the silver-coated copper HJT solar cell 

Tensile test results of the front grid lines of the silver-coated copper HJT solar cell: The tensile force is basically above 2.0 N, reaching a maximum of 7.4 N, and the average tensile force is 3.5 N, indicating that the mechanical properties of the grid lines are qualified.

The tensile properties of the silver-coated copper grid lines are comparable to, and even slightly higher than those of the traditional low-temperature silver paste,It shows that the silver-coated copper grid lines can meet the practical application requirements in terms of mechanical properties. 

Research on Electrical Performance

Distribution diagram of the photoelectric conversion efficiency of the silver-coated copper HJT solar cell

The electrical performance of the silver-coated copper HJT solar cell

The photoelectric conversion efficiency of the silver-coated copper HJT solar cells mainly ranges from 24.3% to 24.7%. The efficiency grade with the highest proportion is 24.5% (accounting for 22.8%), indicating that the silver-coated copper HJT solar cells have good photoelectric conversion performance.

For the photovoltaic module prepared with 72 silver-coated copper HJT solar cells, the encapsulation loss (CTM) is 97.3%, which is comparable to conventional low-temperature silver paste HJT photovoltaic module.

Reliability Testing

Initial Performance Testing

The initial performance parameters of 5 silver-coated copper HJT photovoltaic module samples show that the average front-side output power of the samples is 434.49 watts, the back-side output power is 365.33 watts, and the bifacial power generation efficiency reaches 84.1%.

Damp Heat Test 

The output power attenuation of the samples after the DH test and the EL images.

Low power attenuation：

After 3,000 hours of damp heat testing, the output power attenuation rate of the photovoltaic module is only 2.6%, demonstrating excellent stability.

No internal damage:

No cracks or black spots were found in the EL images, indicating that the internal structure of the module is intact. The silver-coated copper grid lines exhibit excellent oxidation resistance in a damp and hot environment.

Reliability Verification: The silver-coated copper HJT photovoltaic modules perform excellently under damp and hot conditions, verifying their long-term reliability in complex environments. This provides strong support for the practical application of the silver-coated copper paste.

"DH1000 + Load" test

Output power attenuation and EL images after the "DH1000 + Mechanical Load" test.

Low power attenuation:

The front-side output power attenuation is only 0.68%, and the back-side output power has slightly increased, indicating that the module can still maintain high performance under complex environmental conditions.

No internal damage:

 No abnormalities were found in the EL images, further verifying the integrity and damage resistance of the silver-coated copper grid lines under mechanical load.

Reliability Verification:

The silver-coated copper HJT photovoltaic module performs excellently under temperature cycling conditions, verifying its long-term stability and reliability in complex environments.

TC Test

The output power attenuation of the samples after the TC test and the EL images

Low power attenuation:

After 600 temperature cycles, the front-side output power attenuation rate was only 0.72%, and the back-side output power attenuation rate was only 0.03%, demonstrating the excellent durability of the module in an environment with temperature variations.

No internal damage:

No cracks, black spots or other abnormal phenomena were found in the EL images, further verifying the integrity of the internal structure of the module after the temperature cycling test.

Reliability Verification:

The silver-coated copper HJT photovoltaic module performs excellently under temperature cycling conditions, verifying its long-term stability and reliability in complex environments. 

PID Test 

The output power attenuation of the samples after the PID test and the EL images

Low power attenuation: 

After three PID tests totaling 576 hours, the front-side output power attenuation rate was only 1.09%, and the back-side output power attenuation rate was only 0.63%, indicating the excellent stability of the module under potential-induced conditions.

No internal damage:

No cracks, black spots or other abnormal phenomena were found in the EL images, further verifying the integrity of the internal structure of the module after the PID test.

Reliability Verification:

The silver-coated copper HJT photovoltaic modules perform excellently in the Potential Induced Degradation (PID) test, verifying their long-term stability and reliability in high-voltage and humid environments.

Cost Analysis of Silver-Coated Copper HJT Solar Cells

Cost Prediction Analysis of Silver-Coated Copper Paste

The costs of silver-coated copper pastes under different processes were compared, including SMBB + low-temperature silver paste, SMBB + silver-coated copper paste, and 0BB + silver-coated copper paste. Using silver-coated copper paste can reduce the cost of silver paste by approximately 25.8%. Combining it with the 0BB technology can further reduce the cost by 50%.

Through the research on the application of silver-coated copper paste with a silver content of 50% in HJT solar cells, a comprehensive analysis has been carried out on the structure and tensile properties of the silver-coated copper grid lines, the electrical properties of the solar cells, and the reliability of the photovoltaic modules.

HJT solar cells prepared with silver-coated copper paste have excellent photoelectric conversion efficiency, reaching 24.37%, which is comparable to the performance of photovoltaic modules made with conventional low-temperature silver paste.

In addition, the silver-coated copper HJT photovoltaic modules perform excellently in reliability tests such as the Damp Heat (DH) test, Thermal Cycling (TC) test, and Potential Induced Degradation (PID) test. The output power attenuation rates are all lower than 3%, further verifying their stability and reliability in harsh environments.

Millennial Temperature Humidity Test  Chamber

Email：market@millennialsolar.com

Millennial Temperature Humidity Test Chamber adopts an imported temperature controller, which can achieve multi-segment temperature programming. It features high precision and excellent reliability, meeting the testing requirements under different climatic conditions.

✔  temperature range：  20℃~+130℃

✔  Temperature and humidity range：10%RH~98%RH(at+20℃-+85℃）

✔  Meet the test standards：IEC61215、IEC61730、UL1703and so on

The application of silver-coated copper paste in HJT solar cells not only significantly reduces the cost of silver paste but also maintains high-efficiency photoelectric conversion capability and excellent reliability.

Combined with the rigorous testing of the Millennial Temperature Humidity Test Chamber, we believe that the silver-coated copper HJT solar cells will demonstrate strong competitiveness in the future photovoltaic market and provide powerful support for achieving efficient and low-cost clean energy solutions.