High-power femtosecond laser efficiently promotes large-area patterning and low-cost mass production of BC batteries

In recent years, battery technology research and development has been the focus of the development of the photovoltaic industry. In order to pursue higher battery conversion efficiency, BC battery technology, as a technology route that has attracted much attention in the current photovoltaic industry, is considered to be a new technology for crystalline silicon cells in the next 3-5 years. Mainstream products. There has been a series of hot discussions and debates in the market recently, and the BC battery concept continues to heat up.

It is worth noting that although BC battery technology has significant advantages, its technical difficulties and equipment costs are still challenges we are currently facing. Each GW production line of BC batteries requires 2-3 laser processes, with a total equipment value of 6000-7000. Ten thousand yuan. In this context, the importance of laser patterning, the core process of BC batteries, is particularly prominent. With its excellent performance, laser technology will participate in various structural realization technologies of BC batteries. The integration of femtosecond/picosecond laser and BC battery technology will become a new round of development opportunities for photovoltaic cell laser equipment.

What are the advantages of pyramid tip BC batteries?

BC technology was proposed by scientists as early as 1975. However, during the past 48 years, its development has been relatively slow, mainly due to the very high cost of the photolithography process used in solar cell structures, which has limited popularization and application. The so-called BC battery, or Back Contact battery, is a general term for all types of current back contact structure crystalline silicon solar cells. Mainly include IBC, HBC, TBC, ABC, HPBC, etc

The main principle of a BC battery is that there are no grid lines on its surface, and the positive and negative electrodes are prepared in a cross-arranged manner on the back of the battery, thereby reducing the shading loss caused by the electrode grid lines and maximizing the use of sunlight.

Three major advantages of BC battery structure

01 High efficiency, large room for improvement

The PN junction and electrode grid lines on the front side of the battery are transferred to the back side of the battery, thereby reducing the electrode grid line's blocking of 3-5% of the incident light. The front material can better perform light absorption and passivation performance, and improve the overall photoelectric conversion efficiency;

02 Pure appearance, enhanced beauty

There are no grid lines on the front of the battery, and the pure appearance improves the aesthetics and forms product differentiation. It is suitable for distributed photovoltaic scenarios. After further improving the back structure and increasing the bifacial ratio, it is also suitable for mainstream large-scale power stations, and the market is broad.

03 Good versatility, improve efficiency and reduce costs

The BC technology platform is highly versatile and can be combined with various material systems (PERC, TOPCON, HJT, laminated batteries, etc.) to continuously improve efficiency and reduce costs.

With these three major advantages, BC modules have recently occupied the top of the rankings of Taiyang News, the industry's main module efficiency comparison platform, with a maximum efficiency of 24%; domestic BC battery modules will begin mass production in 2022, with a production capacity of 40GW+. It is about to enter a period of rapid growth. With the advancement of mass production by leading manufacturers and the increasing maturity of the upstream and downstream industry chains, more and more powerful TOPCON and HJT manufacturers have included BC technology in R&D and pilot plans, and the industry trend is clear.

Although BC batteries have always occupied the top of the technology pyramid, its process threshold and high cost, which center on large-area complex high-precision patterning on the back, have restricted the popularity of this technology to a certain extent. Laser patterning technology is currently the most economical processing method, which also means that laser processing technology will become the biggest beneficiary on the road to mass production of BC batteries.

BC battery requirements and challenges for laser technology

The main reason why the BC battery process has not been popularized is that its process flow is relatively complex, and the key lies in the local doping on the back of the battery and the metal electrode. With the rapid development of laser technology, people have also seen its potential in industrial processing. Laser has become the main process method for BC battery technology due to its advantages of precision, speed, zero contact and good thermal control effect.

Currently, the mainstream TOPCON poly-Si\HJT a-Si material system requires multiple patterned etching processes on the multi-layer nm film layer on the back when making the BC battery structure, so the processing process requires nm-level etching. Accuracy and thermal diffusion control, um-level graphics control accuracy and second-level single-chip processing time (processing time is mainly inversely proportional to spot size and laser frequency).

The core five requirements of BC battery for laser processing equipment

01 Large spot size

The spot size is large and requires powerful laser pulse energy;

02 Scanning speed is fast

Fast scanning speed requires the use of lasers with high pulse frequency and high total power;

03 Good spot uniformity

To achieve good spot uniformity, it is necessary to improve the quality of laser light and ensure high accuracy of the optical path system;

04 Cost and Upgrade

Low cost and fast upgrade and iteration speed require a high localization rate of lasers

Low thermal impact and damage

High precision, low damage and small thermal impact require ultra-short laser pulses and ultra-high peak power;

Points 1 and 2 above point to high power and high productivity, points 3 and 4 point to high precision and high performance, and point 5 ensures the development and operation efficiency of the entire technology. Starting from first principles, the core of laser process equipment is the laser. Only high-power lasers with independent research and development, technology and cost control can satisfy this requirement.

Ultrafast laser efficiently etches and opens films in BC cells

Ultrafast laser refers to the type of laser with output pulse width below 10-12s, including picosecond, femtosecond, attosecond, etc. Early photovoltaic researchers tried to use nanosecond (10-9s) laser processing, using the heat accumulation of laser energy to melt the material, and the pulse duration was long. However, due to heat accumulation conduction in the material, the edge of the processed material has a large thermal impact, and residues and residues are easily generated. Damage such as chips and micro-cracks cannot meet the needs of BC battery mass production process. Therefore, this process has not been promoted on a large scale. However, according to the author's research, some companies have proposed new solutions and achieved certain results.

At the Shanghai SNEC Photovoltaic Exhibition in May this year, Dongguan Shengxiong Laser Advanced Equipment Co., Ltd. launched a 90W green light picosecond large spot etching equipment. The equipment has a dual-line dual-laser structure with a maximum production capacity of 5000uph. From a technical perspective, due to its extremely narrow pulse width, picosecond laser can rely on its extremely high peak power to vaporize most materials instantly. Different from the thermal processing of nanosecond laser, picosecond laser is a vaporization ablation or modification process. The thermal effect and the generation of molten beads are very small, and the processing edges are neat, breaking the dilemma of nanosecond laser thermal influence and large melting zone.

Recently, thanks to the technological breakthrough of the core high-power femtosecond laser, Shengxiong Laser has once again upgraded and launched a dual-line 4-laser (femtosecond/picosecond) high-capacity model. The structure of the dual-line 4-laser enables It can match the use of different lasers, and the overall production capacity can reach up to 10,000uph. There is no doubt that femtosecond laser, with its GW-level peak power, brings obvious nonlinear absorption effects, which can further compress the material absorption depth based on the wavelength absorption characteristics, achieving nm-level etching accuracy and lower material damage control.

The core of the two products lies in the self-developed ultraviolet/green femtosecond/picosecond laser. Shengxiong Laser introduced a national-level professional team and after years of research and development, this series of lasers has reached industry-leading levels in terms of total power, pulse energy, performance stability, etc. . The average power of its green femtosecond laser reaches 100W, and its green picosecond laser reaches 200W; the average power of its ultraviolet femtosecond laser reaches 60W, and its ultraviolet picosecond laser reaches 100W. The continuous upgrade of the core laser enables it to output a larger spot, achieve higher precision and lower damage processing effects, and help the new generation of BC cells achieve higher efficiency (≥27%) and higher productivity (≥10,000uph)

It can be said that the femtosecond/picosecond laser BC battery etching process has many advantages over traditional etching equipment. It is suitable for large-area film layer removal patterning processes and can effectively promote BC batteries to enter mass production faster. . The author compares and summarizes the actual application effects of the improvement as follows

Figure 1 shows the microscopic picture of the film layer etching sample on the textured and polished surfaces of the green picosecond laser with a large spot (>200um). It can be seen that the overall morphology is regular and complete, and the internal uniformity and consistency are excellent. From the polished surface From the morphology comparison, there is basically no damage to the morphology of the silicon substrate underneath the film layer.

Picosecond laser can meet the patterning requirements of BC batteries to a large extent, but in terms of accuracy and damage control, femtosecond laser has better performance.

From the comparison in the figure, it can be seen that femtosecond laser has obvious advantages in preserving the morphology of the pyramid.

Figure 4 Comparison of composition analysis of etched/non-etched areas of TCO film layer with green light picosecond (a), green light femtosecond (b), and ultraviolet femtosecond (c)

Figure 4. Composition testing shows that both picosecond and femtosecond lasers achieve complete removal of the target film layer.

Figure 5 Comparison shows that there are a large number of residual particles in green light nanosecond etching, and ultraviolet femtosecond has better performance in edge transition zone control than green light femtosecond.

Figure 2-6 shows the etching details through SEM images, which can visually compare the effect difference between femtosecond laser and picosecond laser. Comparing the etching effects of the silicon nitride layer on the textured and polished surfaces, the femtosecond laser (500fs) has smaller melting and morphological changes on the etched surface than the picosecond laser (10ps). Especially when using ultraviolet femtosecond laser, the edge transition zone is almost invisible on the micron scale of the SEM image, so it has the best etching effect.

The reason why femtosecond laser pulses can compress the pulse width to 1/20 of a picosecond while maintaining the same energy is because its peak power is increased by 20 times, reaching the GW level. This high power brings stronger nonlinear absorption, resulting in a lower absorption depth of the material, and more laser energy causes the material to form a plasma and be vaporized. This process makes the heat impact and melting area of etching smaller, resulting in a flatter etching surface and significantly reducing defects such as residue, molten beads and micro-cracks.

Four major advantages of femtosecond laser etching effects

01 Improve etching accuracy

High-performance femtosecond laser can control the etching accuracy to the nm level, which is 1-2 orders of magnitude higher than the picosecond level; the edge width accuracy can be controlled to the um or sub-um level;

02 Reduce material damage

The peak power of femtosecond laser is high and the absorption is strong. Its etching heat-affected zone and melting depth are smaller than that of picosecond laser. Therefore, it has less impact on etching the bottom layer and can significantly reduce material damage or even achieve no damage. ;

03 Avoid damage to underlying materials

Using ultraviolet picosecond/femtosecond laser to etch the transparent film layer can avoid damage to the underlying material;

04 Efficiently complete process requirements

Through the combination of different lasers, the complex pattern production process requirements of various film layers of BC batteries can be efficiently completed.

For battery processes, the use of femtosecond lasers can reduce material damage and simplify the cleaning process, thereby reducing costs and increasing efficiency. The high-performance femtosecond laser can better ensure the efficient mass production of XBC cells while maintaining a processing speed of no less than picoseconds, with an efficiency as high as 26-27%.

Shengxiong Laser integrates complete sets of equipment with independent research and development of lasers as the core. The total value of laser equipment per GW production line can be reduced by 20-30% compared with existing production line equipment.