Crystalline silicon silicon wafer solar cells occupy about 97% of the global market share, and reducing silver consumption has become critical to meet future production and cost targets. Copper paste has become an ideal alternative to silver due to its low cost, abundant reserves, and similar resistivity, but it suffers from oxidation susceptibility, copper diffusion to silicon, and reduced minority carrier lifetime. In this study, we investigate how optimizing copper paste properties can reduce degradation of copper metallization cells and improve the reliability of PERC solar cells. By applying screen-printed copper paste to PERC solar cells, we demonstrate that copper paste can achieve competitive efficiency while maintaining long-term reliability for PERC solar cells.

Schematic diagram of selective emitter PERC cell

Selective emitter PERC solar cells were prepared on monocrystalline p-type silicon wafers of M6 size (166 mm × 166 mm). The front-side grid lines were screen-printed using copper paste, and the back-side was aluminum (Al) contact with localized contact openings. This screen printing approach enables cost-effective copper metallization for PERC solar cells.

Copper metallization was performed in two steps. First, aluminum is printed on the backside of the silicon wafer, dried and sintered at ~751°C, followed by front-side grid printing with copper paste and sintering at ~630°C. A reference cell with front-side silver printing was prepared using the same silicon wafers and a co-sintering sintering temperature of 751°C for comparison with copper paste PERC solar cells.

Scanning electron microscope image of a cross-section of a copper finger strip

The copper paste was optimized for a peak sintering temperature of ~630°C and a resistivity of ~2-3 × 10-5 Ω-cm. The cross-section of the copper paste finger strip was analyzed by SEM (scanning electron microscopy) and EDS (energy dispersive X-ray spectroscopy). It was found that the width of the copper paste finger strip was ~100 μm, the height was 28.7 μm, and the aspect ratio was 0.27.

EDS analysis of different versions of copper pastes

The initial version of copper paste has a thick oxide layer between copper and silicon wafer. This thick oxide layer increases the series resistance of the PERC solar cell and hinders carrier transmission, leading to lower efficiency. Newer versions of copper paste have a thinner oxide layer that facilitates carrier transport while still acting as a barrier against copper diffusion into silicon, ensuring the PERC solar cell maintains good reliability after 1500 hours of damp heat testing.

Distribution of IV parameters for a group of 100 PERC cells at different peak sintering temperatures

100 PERC solar cells with copper paste copper metallization were divided into three groups, each corresponding to a different peak sintering temperature: 590°C, 610°C, and 630°C.

l Voc: There is a slight tendency for Voc to increase as the sintering temperature increases, suggesting higher sintering temperature may help increase the open-circuit voltage of PERC solar cells.

l Jsc: Jsc does not change significantly at different sintering temperatures, indicating sintering temperature has a small effect on short-circuit current density of copper paste cells.

l Fill Factor (FF): FF is highest at a sintering temperature of 630°C, indicating that optimized sintering temperature can significantly increase the fill factor of PERC solar cells, thus improving overall efficiency.

Series Resistance Distribution of Copper Metallized M6 Size Cells

Efficiency at different sintering temperatures for copper paste PERC solar cells:

· 590°C: Efficiency is lower than 19.25%.

· 610°C: Efficiency improved but still lower than at 630°C sintering temperature.

· 630°C: The highest efficiency of 19.25% for copper paste PERC solar cells.

PL images before and after accelerated testing (85°C/85% humidity hot and humid environment)

Series resistance (Rs) color coding shows series resistance values across the PERC solar cell. Darker colors indicate higher series resistance. Series resistance is an important parameter affecting the fill factor and overall efficiency of copper paste PERC solar cells. Higher series resistance increases voltage drop, reducing power output.

Color coding quickly identifies areas with higher series resistance in copper paste cells. These areas may be due to uneven copper paste finger strips, uneven oxide layers, or other copper metallization defects.

Damp heat testing was conducted for 1500 hours at 85°C and 85% humidity. PL images were observed every 500 hours to monitor copper paste PERC solar cell reliability:

l Initial state: Before the damp heat test, PL images showed brighter overall glow, indicating good initial performance of copper paste cells.

l Performance degradation: As damp heat test time increases, PL images show gradually decreasing luminous intensity, especially in cracked areas of PERC solar cells.

l Improvement effect: After improving copper paste properties, PL images show significant reduction in performance degradation, confirming optimized copper paste improves reliability of PERC solar cells.

Image observed by DLIT after damp heat test

The PERC solar cell underwent 500 hours of damp heat testing at 85°C/85% humidity, followed by DLIT testing at reverse bias . DLIT image bright spots indicate bypass diode locations — defective areas causing current to bypass its normal path, degrading PERC solar cell performance. DLIT images show visible bypass diodes after damp heat testing, indicating degradation at the Cu/Si interface of copper paste cells.

SEM and EDS analysis of copper contacts after ultrasonic welding

Cross-sectional SEM images show the bonding of copper paste contacts to solder. EDS analysis provides chemical composition information of the soldered area, where distribution of copper (Cu), tin (Sn) and indium (In) can be observed. Uniform elemental distribution in SEM/EDS results indicates good mixing during soldering of copper paste contacts, with no obvious copper diffusion or elemental segregation.

A: The optimal peak sintering temperature for copper paste on PERC solar cells is 630°C sintering temperature, which achieves the highest efficiency of 19.25% and the best fill factor. At 630°C sintering temperature, the copper paste forms optimal contact with the silicon wafer while maintaining a protective oxide layer against copper diffusion.

A: Copper paste performance on PERC cells demonstrates that copper paste is a viable cost-effective alternative to silver. Screen-printed copper paste achieves 19.25% efficiency on PERC solar cells, with copper metallization offering similar resistivity at significantly lower material cost.

A: Damp heat reliability testing 1500 hours was conducted at 85°C/85% humidity conditions. PL image and DLIT analysis confirmed that optimized copper paste PERC solar cells maintain good performance after 1500-hour damp heat testing, demonstrating excellent reliability for screen-printed copper metallization.

A: The oxide layer between copper paste and the silicon wafer acts as a barrier against copper diffusion into silicon. Newer versions of copper paste maintain a thin but sufficient oxide layer that prevents copper diffusion into silicon while facilitating carrier transport, ensuring PERC solar cell reliability.

A: Screen-printed copper metallization on selective emitter PERC solar cells achieves cell efficiency 19.25% at the optimal 630°C sintering temperature. This selective emitter PERC architecture combined with optimized copper paste demonstrates strong potential for cost-effective PV production.

The optimized copper paste achieves a PERC solar cell efficiency of 19.25% at a sintering temperature of 630°C, and the PERC solar cell maintains good performance after 1500 hours of damp heat testing at 85°C/85% humidity. These results show that copper paste has great potential for copper metallization to reduce PV production costs while meeting high efficiency and reliability requirements for PERC solar cells. The SEM/EDS and DLIT analysis confirms that the oxide layer effectively prevents copper diffusion, making screen-printed copper paste a proven solution for next-generation PERC solar cells.

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