What is HJT cell? This article will give you a comprehensive understanding!
What is HJT cell
HJT cell, also known as heterojunction cell, is based on N-type single crystal silicon, and deposits silicon-based thin film stacks and transparent conductive films with different characteristics on the front and back surfaces. Standard crystalline silicon solar cells are homojunction cell, that is, PN junctions are formed on the same semiconductor material, while the PN junctions of heterojunction cell are composed of different semiconductor materials.
Japan's Sanyo Company invented the HITcell in 1990 and applied for a registered trademark, so heterojunction cell are also called HJT (Heterojunction Technology) or SHJ (Silicon Heterojunction).
The HJT cell structure is shown in the figure below. First, a very thin intrinsic amorphous silicon film (i-a-Si:H) and a p-type amorphous silicon film (p-a-Si:H) are deposited on the front of the N-type single crystal silicon wafer (c-Si). Then, a very thin intrinsic amorphous silicon film (i-a-Si:H) and an n-type amorphous silicon film (n-a-Si:H) are deposited on the back of the silicon wafer to form a back surface field; then a transparent oxide conductive film (Transparent Conductive Oxide, TCO) is deposited on both sides of the cell by PVD. TCO can not only reduce the series resistance when collecting current, but also play a role in anti-reflection; finally, a metal electrode is made on the TCO.
The PN junction of the heterojunction uses different semiconductor materials to form a double heterojunction: a thin layer of narrow bandgap material is inserted between the wide bandgap p-type and N-type semiconductor materials.
Hjt Technology inserts intrinsic amorphous silicon as a buffer layer between PN junctions, which has a good passivation effect on the crystalline silicon surface, and solves the problem of high carrier recombination loss in the contact area between the conventionalcell doping layer and the substrate, achieving a higher minority carrier lifetime and open circuit voltage.
Development stages of HJT cell
The development history of HJT cell can be divided into four stages: initial development stage, industrialization stage, commercial initial stage, and commercial mass production explosion stage.
Advantages of HJT cell
1.High conversion efficiency
Mainly due to the dual passivation effect of N-type silicon substrate and amorphous silicon on substrate surface defects.
At present, the mass production efficiency is generally above 24%; the technical route of more than 25% is very clear, that is, doped nanocrystalline silicon, doped microcrystalline silicon, doped microcrystalline silicon oxide, and doped microcrystalline silicon carbide are used on the front and back surfaces to replace the existing doping; the future superposition of IBC and perovskite conversion efficiency of HJT may be increased to more than 30%.
2.Short process flow
The HJTcell process mainly includes 4 links: texturing, amorphous silicon deposition, TCO deposition, and screen printing; far less than PERC (10) and TOPCON (12-13). Among them, amorphous silicon deposition mainly uses the PECVD method.
There are currently two methods for TCO thin film deposition: RPD (reactive plasma deposition) and PVD (physical chemical vapor deposition). RPD patents have a high penetration rate, while PVD technology is mature and there are many manufacturers providing equipment.
3.Low temperature process
HIT cell use silicon-based thin films to form p-n junctions, so the highest process temperature is the formation temperature of amorphous silicon thin films (~200℃), thereby avoiding the high temperature (about 900℃) of traditional thermal diffusion type crystalline silicon solar cells to form p-n junctions. Low temperature processes save energy, and the use of low temperature processes can reduce thermal damage and deformation of silicon wafers. Thin silicon wafers can be used as substrates, which is conducive to reducing material costs. The high-efficiency HIT cells recently obtained by Sanyo (now Panasonic) are all obtained on silicon wafers with a thickness of less than 100um.
4.High open circuit voltage
HIT cells insert intrinsic thin film i-a-SiH between crystalline silicon and doped thin film silicon, which can effectively passivate defects on the surface of crystalline silicon. Therefore, the open circuit voltage of HIT cells is much higher than that of conventional cells, thereby achieving high photoelectric conversion efficiency. At present, the V of HIT cells has reached 750mV.
5.Low temperature coefficient
The performance data of solar cells are usually measured under standard conditions of 25℃. However, the actual application environment of Photovoltaic Modules is outdoor, and the performance of cells under high temperature is particularly important. Due to the amorphous silicon thin film/crystalline silicon heterojunction in the HIT cell structure, its temperature characteristics are more excellent. The temperature coefficient of HIT cell performance reported in the early stage is -0.33%/℃. After improvement, the open circuit voltage of the cell is improved, and its temperature coefficient is reduced to -0.25%/℃, which is only about half of the temperature coefficient of crystalline silicon cells -0.45%/℃, making HIT cells have better output than conventional cells under light temperature rise. Due to the amorphous silicon thin film in the cell structure, HIT cells have the advantages of thin film cells, and their weak light performance is better than conventional cells.
6.No LID and PID, low attenuation
Since the HJT cell substrate is usually N-type monocrystalline silicon, and N-type monocrystalline silicon is phosphorus-doped, there is no boron-oxygen compound, boron-iron compound, etc. in P-type crystalline silicon, so HJT cells are immune to the LID effect. The surface of the HJT cell is deposited with a TCO film and there is no insulating layer, so there is no chance of the surface layer being charged, which avoids PID from the structure.
7.High bifaciality
The HJT has a symmetrical structure on the front and back, and the TCO film is light-transmitting, so it is a natural bifacial cell. The bifaciality of HJT can reach more than 90% (up to 98%); the bifaciality of bifacial PERC is only 75%+.
8.Low carbon footprint
According to statistical data, heterojunction cells can reduce the use of raw material silicon, reduce energy consumption, and effectively reduce carbon emissions. The carbon footprint in April 2024 has been reduced to 397eq/W, which is 200eq/W less than that of PERC components. In the future, a series of cost-cutting and efficiency-improving measures will be taken to reduce the carbon footprint to below 300eq/W.
HJT cell process
Compared with the production process of monocrystalline PERC and TOPCon cells, the process flow of HJT cell technology is greatly shortened, and the main process has only four steps: cleaning and texturing, amorphous silicon deposition, TCO deposition, and screen printing.
HJT main process and function
1.Cleaning and texturing
The texturing and cleaning process needs to optimize the light trapping performance of the cell. The effective velvet structure can make the incident light reflect and refract multiple times on the surface, extend the optical path, and increase the photogenerated carriers; it is necessary to form a clean surface to reduce the defects and impurities introduced by the unclean silicon wafer surface, thereby reducing the recombination loss of carriers at the junction interface.
2.Amorphous silicon deposition
①Purpose: By depositing intrinsic amorphous silicon film and doped amorphous silicon film on the front and back of silicon wafers, the silicon wafer can obtain excellent surface passivation ability, which is also an important condition for obtaining highercell efficiency. The excellent passivation effect of amorphous silicon can greatly increase the minority carrier lifetime of silicon wafers.
②Method: The cluster RF radio frequency PECVD equipment currently used in mass production has ultra-high process control accuracy and can achieve stability within 0.5s after glow starting. In order to avoid cross contamination caused by the deposition of doped gases, four main process reaction chambers are used to deposit p-a-Si:H, i-a-Si:H (p-side), n-a-Si:H, i-a-Si:H (n-side), SiH is used as a precursor (and H2 is used to adjust the SiH4 ratio) to deposit i-a-Si:H, and doping gases PH3 and B2H are added to deposit the corresponding n-a-Si:H and p-a-Si:H film layers. The thickness of each film deposition is controlled between 5-8nm.
3.TCO deposition
① Purpose: The TCO film layer plays a role of light transmission and conductivity in HJT cells. It must have optical and electrical properties, that is, it must meet the requirements of high transmittance, high mobility and low square resistance at the same time, and minimize the damage to the amorphous silicon film layer during the coating process. To obtain low resistivity, it can be achieved by increasing the carrier concentration and improving the carrier mobility.
Since amorphous silicon has poor conductivity, adding a layer of TCO film between the electrode and the amorphous silicon layer during the production process of HJT cells can effectively increase the collection of carriers. The transparent conductive oxide film has the dual functions of optical transparency and conductivity, plays a key role in the collection of effective carriers, can reduce light reflection, and has a good light trapping effect. It is a good window layer material.
②Method:
4.Screen printing
①Purpose: In order to export the generated current, it is necessary to make positive and negative electrodes on the surface of the cell. The basic requirements for preparing electrodes are: good contact with ITO film, good conductivity, high current collection efficiency, etc. At present, the most commonly used method for preparing electrodes in the industry is screen printing, which uses screen printing to print silver paste on the front and back of thecell.
②Method: Since HJT cell are not resistant to high temperatures, the silver paste used in this project is different from conventional products. Low-temperature silver paste printing and low-temperature curing are used in the process. The specific process flow includes back electrode printing, drying, positive electrode printing, drying and low-temperature curing. The curing temperature is generally controlled at around 200℃.
HJT cell enterprise layout
In 2023, with the iteration of photovoltaiccell technology, HJTcell technology has also exploded. According to PV Headlines , 39 companies invested in HJT projects this year, with a total production capacity of 265GW and an investment of nearly 200 billion yuan.
After 2024, with the mass production of products of various heterojunction companies, the heterojunction industry has entered a new stage. In March 2024, heterojunction companies such as Huasheng New Energy, Risen Energy, Quanwei Technology, Guosheng Technology, Mingyang Photovoltaic, and Liansheng Photovoltaic established the 740W+ Club. These companies are the main layout companies of heterojunctioncell technology.
Future development trend of HJT cell
According to data from the China Photovoltaic Industry Association, the market share of HJT will be 3% in 2023, which will increase to 11.7% in 2024, and the market share is expected to exceed 30% by 2030.
According to statistics, from 2024 to date, China Green Development, Datang, Guotou, China Power Construction, Huaneng, Huadian, Energy Construction, State Power Investment, China Railway Construction, etc. have issued 20.789GW of heterojunction module bidding projects. It can be seen that heterojunction technology has been generally recognized by central state-owned enterprises.
For HJT technology, another major advantage is that it forms a stackedcell with perovskite cell. At present, the current highest efficiency of heterojunction/perovskite stacked cell is 30.09%, and the theoretical limit efficiency can exceed 40%. Therefore, heterojunctioncell technology has broad development prospects.