Depth--Analysis of the current status of the industrialization of HIT batteries
1, HIT battery structure and principle
HIT is the abbreviation of Heterojunction with Intrinsic Thin-layer, meaning intrinsic film heterojunction. Because HIT has been registered as a trademark by Sanyo Corporation, it is also called HJT or Silicon Heterojunction Solar Cell (SHJ). This type of solar cell was first successfully developed by Japan's Sanyo Corporation in 1990, when the conversion efficiency could reach 14.5% (4mm2 battery). Later, under the continuous improvement of Sanyo Corporation, the conversion efficiency of Sanyo HIT battery reached 25.6% in 2015. . Sanyo’s HIT patent protection in 2015 ended and the elimination of technical barriers was a great opportunity for China to vigorously develop and promote HIT technology.
The figure below shows the basic structure of a HIT solar cell, which is characterized by a pi-type a-Si:H film (5-l0 nm in film thickness) on the light irradiation side and an in-type a-Si:H film on the back side (5-thickness film thickness). L0nm) clamps the crystalline silicon wafer and forms transparent electrodes and collectors on both sides of the top layer, forming a HIT solar cell with a symmetrical structure.
On the front surface of the battery, since the energy band bends, the movement of electrons to the front surface is blocked, and the holes can tunnel through the highly doped p+-type amorphous silicon to form the hole transport layer because the intrinsic layer is very thin. Similarly, on the back surface, the band is bent to block the movement of holes to the back surface, and electrons can tunnel through the highly doped n+ type amorphous silicon to form the electron transport layer. By depositing the selective transport layer on both sides of the battery, the photocarriers can only be enriched in the absorbing material and then flow out from one surface of the battery, thereby achieving the separation of the two.
2, HIT battery process
One of the advantages of the HIT battery is that the process steps are relatively simple and can be divided into four steps: wool cleaning, amorphous silicon thin film deposition, TCO preparation, and electrode preparation.
The preparation of the core process is the deposition of an amorphous silicon film, which requires a very high degree of cleanliness of the process, and reliability and repeatability are a major challenge in the mass production process. Currently, the PECVD method is generally used for preparation.
The preparation process of the HIT battery is simple, and the process temperature is low, which can avoid the damage of the silicon chip caused by the high temperature process and effectively reduce the emission. However, the process is difficult, and the production line is incompatible with the traditional battery, and the investment in equipment assets is relatively large.
3, HIT battery advantages and characteristics
HIT batteries have the advantages of high power generation and low cost of electricity. The specific features are as follows:
(1) Low temperature process
HIT cells combine the advantages of low temperature (<250°C) fabrication of thin film solar cells, thereby avoiding the use of conventional high temperature (>900°C) diffusion processes to obtain p-n junctions. This technology not only saves energy, but also makes the a-Si:H-based thin film doping, band gap and thickness can be controlled more precisely in a low-temperature environment, and it is easy to optimize the device characteristics in the process; in the low-temperature deposition process, the single-piece silicon wafer bends and deforms. Small, so that its thickness can be used as the minimum required for the background light absorbing material (about 80μm); at the same time the low temperature process eliminates the performance degradation of the silicon substrate in high temperature processing, allowing the use of "low quality" crystalline silicon or even polysilicon For the substrate.
High power generation in high temperature environments. At noon of the day, the power generation capacity of HIT batteries is 8-10% higher than that of ordinary crystalline silicon solar cells, and the power generation of dual-glass HIT modules is more than 20% higher, with higher user added. value.
(2) double-sided battery
HIT is a very good two-sided battery. There is basically no color difference between the front and back sides, and the double-sided rate (referring to the ratio of the backside efficiency of the battery to the frontal efficiency) can reach over 90%, up to 96%, and the advantages of backside power generation are obvious.
(3) High efficiency
The unique heterojunction structure with an intrinsic thin layer of the HIT cell completes the surface passivation of the monocrystalline silicon at the same time when the p-n junction is formed, which greatly reduces the surface and interface leakage current and improves the cell efficiency. At present, the laboratory efficiency of HIT batteries has reached 23%, and the battery efficiency of commercially available 200W modules has reached 19.5%.
(4) High stability
The illumination stability of the HIT battery is good, and theoretical studies have shown that the amorphous Si film in the non-silicon/crystalline silicon heterojunction does not have the Staebler-Wronski effect, so that the conversion efficiency of the amorphous silicon solar cell does not appear to be due to light. Degradation phenomenon; HIT battery temperature stability is good, and the temperature coefficient of HIT battery can reach -0.25%/°C compared with the temperature coefficient of -0.5%/°C of single crystal silicon battery, making the battery even under the condition of increasing light temperature. Have a good output.
(5) No photo-induced attenuation
One of the most important problems plaguing crystalline silicon solar cells is photo-induced attenuation, while the HIT cells have no natural attenuation and even increase their efficiency to a certain extent under the light. Shanghai Microsystems found that after the HIT photo-induced decay experiment, the HIT was irradiated with light. The battery conversion efficiency increased by 2.7%, and there was no attenuation after continuous lighting. The joint publication of Japan CIC, Swiss EPFL, and CSEM on APL also confirmed the photo-enhancement of HIT batteries.
(6) Symmetrical structure suitable for thinning
The perfect symmetrical structure and low temperature process of HIT cells make them very suitable for thinning. After extensive experiments by Shanghai Microsystems, the thickness of silicon wafers is in the range of 100-180μm and the average efficiency is almost unchanged. 100μm-thick silicon wafers have been realized. More than 23% conversion efficiency is currently in batch preparation of 90μm silicon wafers. Battery thinning can not only reduce the cost of silicon wafers, but its application can also be more diversified.
(6) Low cost
The thin thickness of the HIT battery can save the silicon material; the low temperature process can reduce the energy consumption and allow the use of inexpensive substrates; the high efficiency makes it possible to reduce the area of the battery under the same output power, thereby effectively reducing the cost of the battery.
4, HIT battery industrialization status
According to data from OFweek Industrial Research Institute, the largest production volume, of course, is Japan’s Sanyo, with an existing capacity of 1GW and mass production efficiency of 23%. In addition, there are more mature HIT technology, including Keneka, Sunpreme, Solarcity, Fujian Junshi, Jinneng, Xinao and Hanergy.
Chart: Industrialization of HIT solar cells at home and abroad (Unit: %, MW)
Depth | Status Analysis of HIT Battery Industrialization
Source: OFweek Industry Research Center
At present, the difficulties in mass production of HIT products mainly include the following aspects:
(1) High-quality silicon: Compared with conventional N-type products, HIT batteries have higher requirements on the quality of silicon chips and need to be carefully selected by silicon suppliers.
(2) Control of the surface cleanliness of the wafer after texturing: The HIT battery requires a very high cleanliness of the surface of the silicon wafer, and it is necessary to balance the cleanliness of the wafer and the consumption of related chemicals and water.
(3) Q-time control of each process: Before the completion of the amorphous silicon coating, the time and environmental requirements for the exposure of the silicon wafer to the silicon wafer in the HIT cell are severe, and it is necessary to pay attention to the control of each process Q-time.
(4) Influence of production continuity on TCO coating equipment: TCO coating must ensure continuous feeding, otherwise, the yield and equipment condition will be affected. Especially when the production line is just put into production, maintaining production continuity is a big challenge.
(5) Continuous printing stability of high-viscosity pulp: In the preparation of HIT batteries, the viscosity of the paste caused by the large viscosity of the ink is large and it requires several times the attention of the conventional production line.
(6) The stability of the welding strip tension: The window of the stable tension force is narrow, and the assembly structure of the double-glass double-sided power generation further increases the difficulty of battery series connection.
In addition, one of the important factors affecting the industrialization of HIT is the cost issue. According to Dr. Yang Liyou, the top four BOM costs for HIT batteries are silicon wafers, conductive silver pastes, target materials, and velvet additives. For these high-cost parts, special reductions can be made, including reducing the consumption of raw materials, the localization of key equipment, the localization of key raw materials, and the introduction of new technologies.
5, HIT battery market outlook
Reducing costs and increasing efficiency have always been the eternal theme of the photovoltaic industry. With the continuous technological advancement and policy promotion of the industry, the public's attention has gradually shifted to the cost of electricity. Therefore, high-efficiency batteries have attracted much attention. After PERC battery became a hot spot in the industry, HIT battery technology broke through at the beginning, and the cost-effectiveness advantage began to show. In the future, it will be the era of P-PERC battery and N-type HIT battery for the photovoltaic industry.
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