"The king of conversion efficiency" IBC battery in the end where powerful?
What is the core of photovoltaic technology competition? The answer is to increase conversion efficiency!
With regard to photovoltaic technology competition, various companies are chasing after you and exhausting all efforts. What kind of technology is the deciding advanced technology in the future? Some people say PERC battery technology, others say IBC battery technology, and others say MWT component technology... But no matter what kind of technology, first of all, conversion efficiency is the foundation of winning the future.
In the past few years, both single crystal and polycrystalline cells have maintained an annual efficiency increase of about 0.3% to 0.4%. At present, China's photovoltaic equipment industry has entered an era of total quality and efficiency. The improvement of conversion efficiency has been extremely difficult, and a significant technological breakthrough is required for every 0.5-percent increase.
Among various leading technologies, IBC batteries are one of the things that have to be mentioned. In this technical study, Trina Solar has achieved the most advanced results.
On April 26, 2016, the State Key Laboratory of Photovoltaic Science and Technology of Trina Solar announced that it has been independently tested by a third party authority, JET, and created a large area of ​​156×156 mm2 of N-type monocrystalline silicon with a photoelectric conversion efficiency of 23.5%. IBC battery world record. The company has broken the world record of IBC batteries 15 times.
Where is IBC's battery technology? In particular, we carefully reviewed the structural principles, process technologies, and development of IBC batteries.
The principle and characteristics of IBC battery
IBC battery (full back-contacted crystalline silicon photovoltaic cell) is a technology that moves positive and negative two-pole metal contacts to the back of the cell, so that the front of the cell facing the sun is completely black, and the front of most photovoltaic cells cannot be seen at all. metal wire. This not only brings more effective power generation area to the user, but also helps to improve the power generation efficiency and the appearance is also more beautiful.
The biggest feature of the IBC battery is that the PN junction and the metal contact are on the back of the battery. There is no metal electrode shielding on the front side, so it has a higher short-circuit current Jsc, while the back can tolerate a wider metal gate to reduce the series resistance Rs. Increasing the fill factor FF; adding the open-circuit voltage gain from the front surface field (FSF) of the cell and good passivation results in a high conversion efficiency for this frontal, unobstructed cell.
IBC Battery Process Technology
The process flow of IBC batteries is much more complex than traditional solar cells. The key issue for the IBC battery process is how to make P and N regions on the backside of the cell arranged in interdigitated spaces and to form metallized contacts and gate lines on top of each other.
Mask method
There are many kinds of IBC battery processes. The common method of localized doping includes a mask method. The required pattern can be formed on the mask by photolithography. This method is expensive and not suitable for large-scale production. However, by screen printing an etching paste or a blocking paste to etch or block portions of the mask that do not require etching to form a desired pattern, this method is less costly and requires a separate two-step diffusion process to separately form P-type area and N-type area.
In addition, the doped impurity source (boron or phosphorus source) can also be directly added into the mask. The doped mask layer can generally be formed by chemical vapor deposition. In this way, after only a high temperature is required to diffuse the impurity source into the interior of the silicon wafer, thereby saving one step high temperature process.
Moreover, a layer of boron-containing interdigitated diffusion masking layer can also be printed on the backside of the battery. Boron on the masking layer is diffused and then enters the N-type substrate to form a P+ region, and the region where the mask layer is not printed is diffused by phosphorus. Form N+ zone.
However, the limitations of the screen printing method itself, such as alignment accuracy issues, printing repeatability issues, etc., impose certain requirements on the design of the battery structure, and under a certain parameter conditions, a small PN spacing and metal contact area It can bring about the improvement of the battery efficiency. Therefore, the screen printing method needs to find a balance between the repeatability of the process and the battery efficiency.
In addition, laser is also a way to solve the limitations of screen printing. Whether it is an indirect etching mask or direct etching, the laser method can obtain a finer cell unit structure than screen printing, a smaller metal contact opening and a more flexible design.
Ion implantation has also been transferred from the semiconductor industry to the photovoltaic industry. The biggest advantage of ion implantation is that it can precisely control the doping concentration so as to avoid the presence of diffusion dead layers in the furnace tube diffusion. Selective ion implantation doping can be done through the mask. After the ion implantation, a one-step high temperature annealing process is required to activate and push the impurities into the silicon wafer, and at the same time repair the surface lattice damage of the silicon wafer caused by the high energy ion implantation. Therefore, the key to the mass production of ion implantation technology is equipment and operating costs.
2. Surface passivation technology
For crystalline silicon solar cells, the optical properties and recombination of the front surface are crucial. For IBC high-efficiency cells, better optical loss analysis and optical defection design are especially important. Electrically, compared to conventional batteries, the performance of IBC batteries is more affected by the front surface because most of the photogenerated carriers are generated at the incident surface, and these carriers need to flow from the front surface to the back of the battery until the contact electrodes. Therefore, better surface passivation is needed to reduce the recombination of carriers.
In order to reduce the recombination of carriers, the surface of the battery needs to be passivated, and the surface passivation can reduce the surface state density, and there are usually chemical passivation and field passivation methods. Chemical passivation is more often used for hydrogen passivation, such as the H bond in SiNx films, which enters the silicon under the effect of heat, neutralizes surface dangling bonds, and blunts defects.
Among them, field passivation is the use of a fixed positive or negative charge in the film to shield minority carriers. For example, a positively charged SiNx film will attract negatively charged electrons to the interface. In N-type silicon, minority carriers The flow is a hole, and the positive charge in the film has a repulsive effect on the hole, thus preventing the hole from recombining on the surface.
Therefore, a positively charged thin film such as SiNx is more suitable for the passivation of the N-type silicon front surface of the IBC cell. For the back surface of the battery, since both P and N diffuse simultaneously, the ideal passivation film can passivate both P and N diffusion interfaces. Silica is an ideal choice. If the ratio of the back Emitter/P+ silicon is large, a negatively charged film such as AlOx is also a good choice.
3. Metal grid lines
The grid lines of the IBC battery are all on the back side, and there is no need to consider the light shielding, so the grid line can be designed more flexibly and the series resistance can be reduced. However, since the front surface of the IBC battery is not shielded by the metal gate line, the current density is large, and the external series resistance loss at the back contact and the gate line is also large. The recombination of the metal contact areas is usually large, so the smaller the proportion of the contact area in a certain range, the less the recombination, resulting in a higher Voc. Therefore, the metallization of IBC batteries generally involves the step of opening the contact holes/lines.
In addition, the contact hole regions of N and P need to be aligned with their respective diffusion regions, otherwise battery leakage failure may occur. Similar to the method for forming the alternating diffusion regions, the passivation film in the contact region may be removed by screen printing etching slurry, wet etching or laser, etc. to form a contact region.
Moreover, vapor deposition and electroplating have also been applied to the metallization of highly efficient batteries. For example, ANU’s 24.4% IBC battery uses metallization to form metal contacts. The SunPower company uses electroplated Cu to form the electrodes. Since the metal paste generally contains precious metal silver, not only the cost is high, but silver's natural resources are far less abundant than other metals. Although it is not yet a bottleneck for the development of the solar cell industry, the search for a more inexpensive metal with better performance is also Solar cell research hotspot.
Past, Present and Future of IBC Battery Technology
How did IBC battery technology develop so far?
IBC batteries were first proposed by Lammert and Schwartz in 1975 and were originally used in high-concentration systems. After nearly 40 years of development, the conversion efficiency of IBC batteries under a solar standard test has reached 25%, far exceeding that of all other monocrystalline silicon solar cells.
The earliest mass production of IBC batteries is the United States SunPower Corporation, which is the leader in industrialization. In 2014, the United States SunPower Corporation held an IBC battery with an annual capacity of 1.2GW, including a third-generation high-performance IBC battery production line with an annual production capacity of 100MW. The average efficiency of the battery produced by this line has reached 23.62%.
In addition, Japanese R&D personnel combine IBC with heterojunction (HJ) technology to break the efficiency of crystalline silicon cells beyond 25% in 2014. Among them, Sharp and Panasonic of Japan combined IBC and HJ technology, and the efficiency of the developed polysilicon multi-junction cells reached 25.1% and 25.6% respectively.
Seeing that IBC battery technology has begun to occupy the photovoltaic market, more and more photovoltaic companies are investing in the R&D of IBC battery technology, such as Tianhe, Jingao, and Hairun. In 2013, Hairun Photovoltaic developed IBC battery efficiency of 19.6%.
In 2011, Trina Solar also joined the research and development of the technology, and established partnerships with the Singapore Solar Energy Institute and the Australian National University to research and develop low-cost, industrialized IBC battery technologies and processes. In 2012, Trina Solar undertook the National 863 Program "Key Technology Research and Demonstration Production Line for Industrialization of Low-cost Crystalline Silicon Cells with Efficiency of Over 20%", and launched systematic research and development of IBC battery technology.
After unremitting efforts by researchers, in 2014, the efficiency of the small-area IBC battery developed by the Australian National University (ANU) and Changzhou Trina Solar Energy Co., Ltd. reached 24.4%, setting a world record for battery efficiency at the time of the IBC structure.
In addition, Changzhou Trina Solar Photovoltaic Science and Technology State Key Laboratory also independently developed a 6-inch large-area IBC battery with an efficiency of 22.9%, which is the highest conversion efficiency of a 6-inch IBC battery. After that, Trina Solar was able to build a pilot production line relying on the national 863 project, adopting the newly developed technology and breaking the world record of IBC batteries 15 times.
In addition, in June 2016, the University of New South Wales, Australia (UNSW) once again broke the energy efficiency record of photovoltaic cells using Trina Solar's IBC High-Efficiency Battery and raised the solar energy conversion efficiency to an astonishing 34.5%, shocking the industry.
However, although IBC batteries have high conversion efficiency, they also have superior actual power generation capabilities compared with conventional batteries. However, the manufacturing process is complicated and the cost of the used N-type high quality single crystal silicon wafer is relatively high, which results in a high technical threshold and a high manufacturing cost.
At present, the IBC battery cost is about 2 times that of ordinary batteries, which restricts the large-scale application of IBC batteries. With the entry of China's first-line photovoltaic manufacturers, as well as the development of new processes and new materials, IBC batteries will continue to move forward in the direction of improving battery conversion efficiency and reducing battery manufacturing costs. The commercialization and promotion of IBC solar cells has broad prospects. (author/ Wing Chi)
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