PERC (Passivated Emitter Rear Cell, emitter and backpassing) as a high-efficiency battery technology, since the beginning of the introduction of mass production, it has been rapidly introduced by photovoltaic cell companies, replacing the conventional back-electric field (BSF) technology has rapidly expanded and is still used today. According to the data in the "China Photovoltaic Industry Development Roadmap (2021 Edition)" released by the China Photovoltaic Industry Association (CPIA), the PERC battery market will account for 91.2% in 2021, and its market position will rank first.
In view of this, the next period of time will introduce PERC technology and products from the three aspects of the development of PERC batteries, the improvement of PERC components and product optimization and selection.
As the first in a series, this article will focus on the reasons why PERC batteries can be mass-produced for ten years and will still dominate the market in the next three years.
1. The ten-year history of the rise of PERC batteries
Generally speaking, the industry will be in 1989 Dr. Zhao Jianhua for the first time officially announced the PERC battery as the starting point of the technology, but until more than 20 years later in 2013, through many manufacturers and institutions long-term technical reserves and research, PERC technology has a basis for mass production and commercialization, and in 2015 officially launched large-scale mass production, after experiencing several process of mass production acceleration, since 2019 for the first time beyond BSF technology to become the most mainstream photovoltaic cell technology. Figure 1 shows several important time nodes since the birth of the PERC battery.
Figure 1 The main development history of PERC batteries
2. Structure and process of PERC battery
PERC battery is upgraded from the early BSF battery, and has been widely used in the industry, its battery structure as shown in Figure 2, by adding a back passivation layer on the basis of the BSF structure, reducing the composite of a small number of carriers, improve the conversion efficiency of the battery.
Figure 2 PERC cell structure diagram
The process flow of PERC batteries is relatively simple, and the equipment is mature, in the past two years, some efficiency improvement processes have been equipped as standard, such as laser SE, alkali polishing, light injection/electrical injection, etc., and its overall process flow is shown in Figure 3.
PERC technology to the back of the passivation layer deposition and laser slotting-based, follow-up on this basis on this process improvement optimization when the addition of positive SE laser and light injection / electrical injection annealing and other processes, the future with the continuous progress of photovoltaic technology, in order to improve the mass production efficiency of PERC batteries, will also be further improved and optimized battery process.
Figure 3 PERC Battery Process Flow Diagram
3. The efficiency improvement route of PERC batteries
PERC battery technology has a strong inclusiveness, can be compatible with all kinds of battery technology and even silicon end technology, in the PERC technology was invented and applied, its mass production efficiency has also been increased from about 20% at the beginning to about 23.5% now, the maximum efficiency of a single year increased by 0.6%, since 2013 PERC battery mass production efficiency trend see Figure 4. It is expected that in the next three years, the mass production efficiency of PERC can reach about 24%.
Figure 4 PERC battery mass production efficiency
Looking back at the decade of PERC development, the path of its efficiency improvement mainly starts from two aspects: increasing the absorption of light and reducing the composite of a small number of carriers, which is mainly reflected in the following four aspects:
(1) Optimization of metal grid line: Since the emergence of photovoltaic cells, reducing the shade of light by the grid line and improving the efficiency of collecting current has always been one of the main directions for improving efficiency, and the main path in this regard is to increase the number of main grid wire roots, adopt multi-main grid (MBB) scheme, adjust the width ratio of the grid line and secondary printing technology, etc.;
(2) The promotion of double-sided technology: PERC technology has a natural double-sided advantage, changing the back-side electric field printing to a local aluminum grid line, saving the cost of slurry, and at the same time effectively using the diffuse reflected light in the environment, thereby improving the actual power generation capacity of the battery;
(3) The use of selective emitter (SE): by re-doping the part where the metal grid line on the front of the battery is in contact with the silicon wafer, the electrodes are lightly doped, further reducing the composite of the diffusion layer, improving the short-wave response, and reducing the contact resistance between the metal electrode and the silicon on the front surface, thereby improving the conversion efficiency;
(4) Solution of battery attenuation problem: The light decay problem caused by boron-oxygen composite of PERC batteries is solved by using gallium-doped silicon wafers, and the attenuation problem can be further improved by light injection/electrical injection after sintering.
In addition, the improvement of battery efficiency can also be achieved through the optimization of the passivation layer on the back, the trap technology of the multilayer anti-reflection film, and the high-quality silicon wafer with higher and less child life.
In terms of theoretical research, BYUNGSul Min et al. of ISFH analyzed the optimization path of new emitter structure, boron-doped aluminum backfield, reducing the width of the front electrode gate, improving the quality of the silicon wafer, multi-main gate and other technologies, and studied the effect of different optimization methods on the efficiency of PERC batteries, and concluded that the mass production efficiency of PERC batteries can be increased to more than 24%, as shown in Figure 5.
Figure 5 PERC Battery Efficiency Improvement Path
来源：B Min, M Müller , H Wagner, A Roadmap Toward 24% Efficient PERC Solar Cells in Industrial Mass Production, IEEE Journal of Photovoltaics , 2017 , PP (99) :1-10
4. The NEXT three years perC battery will remain the protagonist
PERC technology based on its strong cost-effective advantages, as well as capacity layout, at least the next three years is still the mainstream technology in the industry, manufacturers will also actively improve efficiency to reduce the original extension of its life cycle, according to CPIA's forecast, by 2025, PERC battery market share is still more than 50%, the specific data as shown in Figure 6.
Figure 6 Market share trends of various battery technologies from 2021 to 2030
The update iteration of technology is the driving force for the healthy and stable development of the photovoltaic industry, and it can also be seen from Figure 6 that the share of n-type batteries is also increasing year by year due to their higher efficiency potential, especially TOPCon cells and heterojunction cells. However, these two types of technologies need to build new or transform the production line, compared to the PERC battery production line 150 million to 200 million yuan / GW investment cost, TOPCon's new line investment cost is about 30% higher, even if the upgrade of the existing PERC production line, its comprehensive investment cost is also about 20% higher than the PERC production line, the unit investment cost of heterogeneous junction production line is even higher, more than 2 times that of PERC.
Based on high investment costs, its expansion rate is much slower than the previous PERC replacement of BSF batteries, with the market share of n-type technology in 2021 being only 3%. It is expected that by 2025, the market share of TOPCon batteries and heterojunction batteries will only reach about 40%. Of course, in addition to the cost problem, n-type technology also needs to be accumulated step by step and verified many times in order to take over the industry-led baton in the future and provide more value to the industry and customers. Until then, PERC will continue to be the protagonist of the photovoltaic market.