High-power Femtosecond Laser Efficiently Boosts BC Cell Large-area Patterning For Low-cost Mass Production

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 the current photovoltaic industry, the technology route of great concern, is considered to be the next 3-5 years of crystalline silicon batteries, the new mainstream products. Recently, a series of hot discussions and debates in the market, BC battery concept continues to heat up.

It is worth noting that despite the significant advantages of BC battery technology, its technical difficulties and equipment costs are still our current challenges. BC batteries require 2-3 laser processes per GW production line, and the total value of equipment is 60-70 million RMB. In this context, the importance of the core process of BC battery - laser graphic processing is particularly prominent. By virtue of its excellent performance, laser technology will be involved in various structure realization technologies of BC battery. The integration of femtosecond / picosecond laser and BC cell technology will become a new round of photovoltaic cell laser equipment development opportunities.
What are the advantages of BC cell at the tip of the pyramid?
BC technology as early as 1975, scientists have put forward this concept, and in the 48 years, the development has been relatively slow, mainly due to the solar cell structure of the photolithography process used in the cost is very high, resulting in the popularization of the application is limited. The so-called BC battery, that is, Back Contact (back contact) battery, is the current general term for all types of back contact structure of crystalline silicon solar cells. Mainly includes IBC, HBC, TBC, ABC, HPBC and so on.
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The principle of BC cell is that there is no grid line on its surface, and the positive and negative electrodes are prepared on the back of the cell in a cross-arrangement manner, thus reducing the shading loss caused by the electrode grid line and maximizing the use of sunlight.
Three advantages of BC battery structure
01 High efficiency, large room for improvement
The PN junction and electrode grids on the front side of the cell are transferred to the back side of the cell, which reduces 3-5% of the incident light blockage by the electrode grids, and the front side material can better perform the light absorption and passivation performance to improve the overall photoelectric conversion efficiency;
02 Pure appearance, enhance the aesthetics
No grid lines on the front side of the battery, pure appearance, enhance the aesthetics, product differentiation, applicable to distributed PV scenarios; further improve the structure of the back side to enhance the bifacial rate is also applicable to mainstream large-scale power stations, the market is broad.
03 Good versatility, improve efficiency and reduce costs
BC technology platform has good versatility and can be combined with various material systems (PERC, TOPCON, HJT, stacked cells, etc.) to continuously improve efficiency and reduce costs.
With these three advantages, BC components recently occupied the industry's main component efficiency comparison platform - Taiyang News rankings of the top, with a maximum efficiency of 24%; domestic BC battery module mass production from 2022, there are 40GW + production capacity, is about to enter a period of rapid growth. With the leading manufacturers to promote mass production, the industry chain upstream and downstream maturity is increasing, there are more and more TOPCON and HJT strength manufacturers will be included in the R & D and pilot program BC technology, the industry wind direction is clear.
Although BC batteries have been occupying the tip of the technology pyramid, but its back large complex high-precision graphic as the core of the process threshold and high cost to a certain extent restricts the popularity of this technology. The laser graphic processing process is the most economical processing method nowadays, which also means that the laser processing technology will become the biggest beneficiary on the way to realize the mass production process of BC batteries.
Requirements and Challenges of Laser Technology for BC Batteries
The main reason why the BC battery process has not been popularized is that the process is more complex, the key lies in the back of the battery local doping and metal electrodes. With the rapid development of laser technology also let people see its potential in industrial processing. With the advantages of precision, speed, zero-contact and good thermal control effect, laser has become the main process means of BC battery process.
At present, the mainstream TOPCON poly-Si ＼ HJT a-Si material system requires multiple graphical etching treatments of the backside multilayer nm film layer in the fabrication of the BC cell structure, and thus requires a processing process with nm-level etching accuracy and thermal diffusion control, um-level graphical control accuracy, and second-level Single chip processing time (processing time is mainly inversely proportional to the spot size, laser frequency).
BC battery on the core of the five major requirements of laser processing equipment
01 Large spot size
Large spot size requires powerful laser pulse energy;
02 Fast scanning speed
Fast scanning speed, the need to use a high pulse frequency, the total power of the laser;
03 Good spot uniformity
Good spot uniformity, need to improve the quality of the laser light, and to ensure that the optical path system is highly accurate;
04 Low thermal impact and damage
High precision, low damage and low thermal impact require ultra-short laser pulses and ultra-high peak power;
05 Cost and Upgrade
Low cost and upgrade iteration speed, requiring high localization rate of lasers.
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 whole technology. From the first principle, the core of the laser process equipment is the laser, only independent research and development, technology and cost-controllable high-power laser can meet.
Ultra-fast laser in the BC battery efficient etching open film
Ultra-fast laser refers to the type of laser whose output pulse width is below 10-12s, including picosecond, femtosecond, and attosecond. Early photovoltaic personnel tried to use nanosecond (10-9s) laser processing, the use of laser energy heat buildup to melt the material, the pulse duration is long, but due to the generation of heat buildup conduction in the material, the edge of the processed material has a large thermal impact, and is prone to residue, debris, microcracks and other damages, which is not able to meet the needs of the mass production process of the BC battery. Therefore this process has not been massively promoted. However, according to the author's research, there are companies that have proposed new solutions and achieved certain results, and it is believed that in the near future, these programs can be universally applied.