[Focus] Laser micro-nano processing technology helps the industrialization of a new generation of high-efficiency solar thin film cells

The new generation of solar thin film cells has huge development prospects due to its many advantages. In terms of processing methods, because laser has the advantages of non-contact processing, regional selectivity, high processing precision, strong adjustability, improved material utilization, and effective control of heat-affected zones, it is now indispensable in the preparation process of thin-film batteries. In short, the application scenarios continue to expand. This article mainly introduces new high-efficiency solar thin-film cells that have received much attention and research in recent years. Compared with traditional crystalline silicon solar cells, new thin-film cells have greater room for efficiency improvement and lower production costs. At the same time, this article also discusses laser The mechanism of action, process effects, and development trends in the processing of thin film batteries are described

Progress in high-efficiency solar thin film battery technology

Solar cell technology has developed for nearly two centuries since its birth. As an efficient, environmentally friendly, and renewable energy source, solar cells are becoming increasingly important in the future development trend of the energy field. Judging from the development history of solar cells, the industry usually divides solar cells into the following three categories: 1) The first generation is crystalline silicon cells (polycrystalline silicon and monocrystalline silicon cells). At present, the technical maturity and commercialization level of this type of cells rank first among all solar cells. Crystalline silicon cells themselves have also gone through many iterations, from the earliest aluminum backfield cells to the later PERC cells, which quickly occupied the market and became The well-deserved mainstream of the market is now close to the theoretical efficiency ceiling. Starting from 2021, compared with traditional P-type monocrystalline cells and P-type polycrystalline cells, N-type cells have high conversion efficiency, high bifacial rate, low temperature coefficient, no light attenuation, good low-light effect, and load carrying capacity. Advantages include longer flow life. Judging from the current technological development, P-type PERC batteries are approaching the efficiency ceiling, and the speed of cost reduction has also slowed down. N-type batteries have a higher efficiency ceiling, and battery technology and efficiency improvements have been significantly accelerated. There is greater room for improvement in conversion efficiency in the future. P-type batteries have begun to iterate to N-type batteries; 2022 can be said to be the first year of the development of N-type batteries (with TOPCon and HJT as the market leaders). The current expansion plan for TOPCon batteries is about 800GW, which is much larger than HJT. The rapid development of TOPCon batteries is mainly due to the application of laser technology in its process chain. The current record peak conversion efficiency of crystalline silicon cells is 26.8%, and its theoretical efficiency limit can reach 29.4%. How to further break through this theoretical limit? The future development route is to design a stacked route for crystalline silicon cells and perovskite cells. Such stacked cells are expected to exceed the theoretical conversion efficiency of 45%.

2) The second generation of solar thin film cells is based on the ability to save raw materials and achieve better economic effects (including cadmium telluride, copper indium gallium selenide, gallium arsenide, etc.). The theoretical efficiency value of this type of battery can be have reached a higher level, are lightweight, and have more flexible application fields. However, their active layers contain some rare elements and heavy metal elements. They also suffer from low localization of equipment, limited material reserves, low large-area efficiency, and low light transmittance of the film layer. Restricted by factors such as differences, it is difficult to achieve large-scale mass production and will cause certain pollution to the environment. The current market share is far smaller than that of crystalline silicon cells.

3) The third generation of new solar cells includes perovskite solar cells (PSC), dye-sensitized solar cells (DSSC), organic solar cells (OSC), quantum dot solar cells, etc. This type of battery has the advantages of abundant raw material reserves, low cost, simple process and flexible preparation. It has great potential for industrialization development and is gradually moving towards mass production. It is currently the most concerned third-generation photovoltaic solar cell type.

Perovskite solar cells are solar cells that use perovskite-type organic metal halide semiconductors as light-absorbing materials. Perovskite solar cells have significant advantages over crystalline silicon and other thin-film cells. First, it is more efficient. Perovskite is a synthetic material with high defect tolerance and low thermal recombination efficiency loss. Therefore, it has a high carrier lifetime and high theoretical efficiency. The band gap can be adjusted according to different formulas, which can improve spectral utilization and achieve higher photovoltaic efficiency. Conversion efficiency.

Secondly, the cost is lower, the preparation process is simple, and the process is short. It only takes 45 minutes from the entry of glass, target materials, and chemical raw materials to component molding, and all preparation processes can be completed in just one factory. In comparison, traditional crystalline silicon components are divided into four production links: silicon material, silicon wafers, cells, and modules. They require four specialized factories for production. Even if all links are seamlessly connected, it will take more than 3 days. to complete production. At the same time, perovskite cells have high material impurity tolerance, raw material purity requirements are much lower than crystalline silicon cells, low material costs, and no need for high-temperature production.

Third, more application scenarios. Perovskite materials have good light absorption properties, and the film layer can be made very thin. Its material properties provide a wealth of development space for the appearance of the battery, such as color adjustment, light transmission, flexibility and bendability, etc., while also ensuring a certain power generation efficiency. At the same time, the stronger low-light power generation capacity of perovskite cells can also show advantages in common scenarios such as indoors, morning and evening. At present, perovskite cells are finding broad development space in application scenarios such as photovoltaic vehicle integration, photovoltaic building integration, flexible wearables, outdoor power stations, and indoor photovoltaics.

Since 2009, the development of perovskite batteries has gone through multiple rounds of technological innovation and iterations. 2009-2012 is the technology gestation period: dozens of papers in the field of perovskites are published around the world every year. Perovskites gradually evolve from dye-sensitized batteries to all-solid-state battery structures, with efficiency at a low level of about 10%. 2013-2019 is the technology growth period: by continuously improving the perovskite film preparation process, light-absorbing layer components and adjusting the device structure, the conversion efficiency has been improved from 10% to 25%.

2020-2024 is the technology transition period: as researchers deepen their understanding of perovskite and the industrialization process arrives, it continues to make breakthroughs in conversion efficiency and stability; and the gradual maturity of laboratory technology lays the foundation for industrialization. With a solid foundation, we will gradually begin to carry out demonstration projects, large-scale product production and verification. In addition to benefiting from breakthroughs in domestic perovskite scientific research, the rapid development of perovskite batteries also benefits from domestic dual-carbon related policies that support the track. For example, the "Guiding Opinions on Promoting the Development of the Energy Electronics Industry" issued by the Ministry of Industry and Information Technology and six other departments further clarified the need for large-scale mass production capabilities while focusing on the development of perovskite technology. The specific development goals of solar thin film cells are: to achieve major breakthroughs in key low-carbon core technologies in key industries and fields by 2025: to further research and break through a number of carbon-neutral cutting-edge and disruptive technology developments in 2030, including efficient and stable calcium titanium mining technology.

It is predicted that the technology will mature after 2025: the production line will be complete, product production will form standard specifications, product stability, lifespan, quality will pass, production capacity will increase, the market will be fully rolled out, and the technology will mature. There are three main existing perovskite technology routes, namely, rigid single-junction cells are the majority, and the future will mainly take the power station and BIPV route; laminated cells, mainly crystalline silicon players enter the game, and are combined with crystalline silicon cells to achieve more High conversion efficiency, difficult process; flexible batteries, taking the route of consumer electronics, indoor photovoltaics, and flexible wearables, account for a minority.

Laser application in new generation thin film batteries

In the process of preparing perovskite thin film batteries, it mainly involves four laser processing processes, which are three selective film layer etchings, namely the scribing process (called P1, P2 and P3), and one full film layer removal. , that is, laser edge cleaning process (called P4). The entire battery is divided into n sub-cells in series structure, aiming to reduce the impact of performance differences in different areas on the components caused by the inhomogeneity of the perovskite material, while also increasing the output voltage of the component and reducing the output current of the component. , Reduce the series resistance between sub-batteries and the heat loss of external circuit resistance.

Laser P1 scribing: Laser etches the bottom TCO film layer to form independent TCO substrates;

Laser P2 scribing: Laser etches other film layers above TCO to provide transmission channels for the positive and negative electrodes of two adjacent sub-cells;

Laser P3 scribing: After depositing the back electrode, laser etches other film layers above the TCO to separate the sub-cells;

Laser P4 edge cleaning process: Laser removes the deposited film on the edge of the battery to prevent leakage and ensure battery packaging reliability.

The necessity of laser scribing for perovskite cells:

Mechanical scribing is one of the traditional battery interconnection methods. Although it has high efficiency, it is widely used in the P2P3 process of copper indium gallium selenide batteries. However, there are also obvious limitations, including the fact that mechanical scoring is contact type, the mechanical needle tip will continue to be worn, has a short lifespan, and needs to be replaced regularly, severe edge chipping, insufficient depth control accuracy, and poor line consistency. , large dead zone and other disadvantages. Therefore, this scribing method is completely unsuitable for perovskite thin film batteries with a total film thickness of less than 1 μm.

In comparison, laser marking has the advantages of non-contact, easy maintenance, small edge heat impact, easy control of marking depth, good line consistency, and smaller dead zones, making it more accurate, cleaner, and The scribing process is completed more efficiently, and the removal of the film layer can be precisely controlled, and the bottom and edges of the groove are clean and smooth.

Regarding the choice of pulse length, nanosecond lasers have obvious thermal effects during processing, and have shortcomings such as rough edges, surface debris, and slow processing speeds. However, ultrashort pulses represented by picosecond lasers can exhibit high peak power and small thermal effects. , the advantages of smooth marking edges and high precision. During the laser etching process, the picosecond laser can focus on an ultra-fine space area and quickly vaporize the material, avoiding the material's linear absorption of the laser, energy transfer, conversion, and the existence and thermal diffusion of heat, making it almost seamless. Thermal influence, realizing laser "cold" processing.

As far as the selection of laser wavelength is concerned, different materials have different absorption rates for lasers of different wavelengths. The higher the absorption rate, the smaller the thermal impact produced.

As for the selection of the scratching direction, there are generally two ways: direct scratching on the film surface and scratching through the glass surface. The former refers to focusing the laser beam on the top of the film layer to be removed, and the material absorbs the laser energy and vaporizes to form a score line. This method has a large thermal impact and is easy to form a "crater", requiring precise control of the process. Scratching through glass focuses the laser beam on the interface between the glass and the removed film. The interface absorbs energy and vaporizes the film. The volume rapidly expands to form an "explosion shock wave", thereby forming the engraved line. This process has a smaller heat-affected area and is less likely to form a "crater", but requires stronger laser energy and higher process window requirements.

Laser scribing should compress the width of the dead zone as much as possible on the premise of ensuring that the thin-film module can divide the entire battery into n sub-batteries connected in series. When the laser acts on the film layer, it may produce obvious heat-affected zones and craters. , bottom debris burrs, film delamination and other undesirable process effects.

Therefore, in addition to the above-mentioned laser pulse width (ns/ps/fs); laser wavelength (355/532/1064nm); processing method (film surface processing/glass surface), the dead zone control and a series of factors that affect the processing effect In addition to processing), it also covers the corresponding laser parameters including power, beam quality, focus spot size and overlap rate selection, etc., as well as subsequent process effects such as scribing width, heat effect, crater, and consistent line scribing of P123 sex, parallelism, etc. In addition, factors that need to be considered include beam shaping and dust handling during processing.

Thin film battery laser precision micro-nano processing solution

In recent years, JPT has jointly innovated with industrial partners and successfully developed a series of laser intelligent equipment and precision micro-nano processing solutions for high-precision scribing of thin-film batteries according to different production capacity needs and product types. These include: laboratory and small test line equipment, 100 MW pilot mass production line equipment, etc.

First of all, the thin film photovoltaic laser scribing machine (four-in-one equipment) is the first choice of laser equipment for experiments and small test lines in the perovskite industry. Through the verification of small-format thin-film battery products in the early stages of product development, high-quality processing effects such as low craters and no thermal impact can be achieved. This thin film photovoltaic laser scribing machine can verify the feasibility of transplanting the technology for preparing thin film components to large-scale mass production lines. This equipment is compatible with P1~P3 laser scribing and P4 edge cleaning for film and glass surfaces; it adopts a scientific research-grade marble high-speed motion platform to ensure system stability, processing accuracy and efficiency; and can be customized according to the spectral absorption characteristics of the customer's film material. Choose laser light sources of different wavelengths.

Secondly, the 100MW/GW level thin film photovoltaic laser micromachining scribing machine is used in the mid-stage of perovskite product development and in the pilot line before large-scale mass production. It is suitable for large-sized rigid calcium oxide with high production capacity requirements and large processing range. For the P1, P2 and P3 scribing operations of titanium battery components, infrared, green light and ultraviolet picosecond light sources can be selected. The optical system can mechanically divide multiple beams of light for simultaneous processing, which greatly shortens the scribing CT and comes with multiple auxiliary functions. , effectively ensuring the quality of marking. The pilot line with a production capacity of 100MW requires an independent automated laser marking equipment for P1/P2/P3/P4. JPT's 100MW laser marking equipment can provide 8-16 laser beam splitting devices for a single device at the same time depending on the production capacity. processing.

Other core advantages of this equipment include: the product remains stationary during laser processing to ensure the flatness of the processing area; the focus and spacing of each optical path are automatically adjusted with an accuracy of ±1μm; the processing speed is 0.8-3m/s, 3g acceleration; the optical design of mechanical spectroscopy can Ensure the power consistency of each optical path, the difference is far less than 3%, which can greatly improve the processing efficiency while ensuring the consistency and stability of the marking effect; with auxiliary functions such as height tracking, trajectory tracking, power detection, and dead zone monitoring, It ensures the process effect of large-format scribing and minimizes the dead zone width. The self-developed software has a friendly UI interface, complete functions and convenient operation. It can be iteratively upgraded according to customer needs; there is also room for self-development for customers with wider P2 line width requirements. Optical shaping technology can achieve rapid scribing of large line widths and compress CT.

It is worth mentioning that in 2023, JPT successfully won its first hundred-megawatt order and cooperated with a leading perovskite photovoltaic cell company to build a complete set of laser marking for its 100MW perovskite photovoltaic cell mass production line. equipment, which marks a new milestone in JPT perovskite photovoltaic cell laser scribing technology.

Author of this article: Li Yini | Project Manager of Shenzhen JPT Optoelectronics Co., Ltd.