What is HJT battery
HJT cells, also known as heterojunction cells, are based on N-type monocrystalline silicon, with silicon-based thin film stacks and transparent conductive films with different properties deposited on the front and back surfaces. Standard crystalline silicon solar cells are homojunction cells, that is, the PN junction is formed on the same semiconductor material, while the PN junction of heterojunction cells is composed of different semiconductor materials.
Japan's Sanyo Company invented the HIT battery in 1990 and applied for a registered trademark, so the heterojunction battery is also called HJT (Heterojunction Technology) or SHJ (Silicon Heterojunction).
Advantages of HJT batteries
1. High conversion efficiency
This is mainly due to the dual passivation effect of the N-type silicon substrate and amorphous silicon on substrate surface defects.
At present, the mass production efficiency is generally above 24%; the technical route for above 25% is already very clear, that is, using doped nanocrystalline silicon, doped microcrystalline silicon, doped microcrystalline silicon oxide, and doped microcrystalline silicon carbide on the front and back surfaces to replace the existing doping; in the future, the conversion efficiency of HJT may be increased to more than 30% by superimposing IBC and perovskite.
2. Short process flow
The HJT cell process mainly includes four steps: texturing, amorphous silicon deposition, TCO deposition, and screen printing, which is 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 has a high patent penetration rate, while PVD technology is mature and there are many manufacturers providing equipment.
3. Low temperature process
Since HIT cells use silicon-based thin films to form pn junctions, the highest process temperature is the formation temperature of amorphous silicon thin films (~200°C), thus avoiding the high temperature (about 900°C) of pn junctions formed by traditional heat diffusion type crystalline silicon solar cells. Low temperature processes save energy, and the use of low temperature processes can reduce thermal damage and deformation of silicon wafers, and 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
Since the intrinsic thin film ia-SiH is inserted between crystalline silicon and doped thin film silicon, HIT cells can effectively passivate the defects on the surface of crystalline silicon. Therefore, the open circuit voltage of HIT cells is much higher than that of conventional cells, thus achieving high photoelectric conversion efficiency. Currently, 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°C, but the actual application environment of photovoltaic modules is outdoor, and the performance of cells under high temperatures 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 the HIT cell performance reported earlier was -0.33%/°C. After improvement, the open circuit voltage of the cell was improved, and its temperature coefficient was reduced to -0.25%/°C, which is only about half of the temperature coefficient of the crystalline silicon cell -0.45%/°C, making the HIT cell have better output than conventional cells under light temperature rise. Due to the amorphous silicon thin film in the cell structure, the HIT cell has the advantages of a thin film cell, and its weak light performance is better than that of conventional cells.
6 , No LID and PID, low attenuation
Since the HJT cell substrate is usually N-type monocrystalline silicon, which is phosphorus-doped and does not contain boron-oxygen compound or boron-iron compound in P-type crystalline silicon, HJT cells are immune to the LID effect. The surface of the HJT cell is deposited with a TCO film and has no insulating layer, so there is no chance for the surface layer to be charged, thus avoiding PID from a structural perspective.
7. High double-sided rate
The front and back structures of HJT are symmetrical, and the TCO film is light-transmissive, so it is naturally a bifacial cell. The bifaciality of HJT can reach over 90% (up to 98%); the bifaciality of bifacial PERC is only 75%+.
HJT process
TOPCon main processes and functions
1. Cleaning and velveting
The texturing and cleaning process needs to optimize the light trapping performance of the battery. An effective texturing structure can cause the incident light to be reflected and refracted multiple times on the surface, extending the optical path and increasing the photogenerated carriers. It is necessary to form a clean surface to reduce 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 films and doped amorphous silicon films on the front and back sides of silicon wafers, the silicon wafers can obtain excellent surface passivation capabilities, which is also an important condition for obtaining higher battery efficiency. By utilizing the excellent passivation effect of amorphous silicon, the minority carrier lifetime of silicon wafers can be greatly improved.
②Methods: The cluster RF PECVD equipment currently used in mass production has ultra-high process control accuracy, and the glow starts to stabilize within 0.5s. In order to avoid cross contamination caused by the deposition of doped gases, pa-Si:H, ia-Si:H (p-side), na-Si:H, ia-Si:H (n-side) are deposited by 4 main process reaction chambers respectively. SiH is used as a precursor (and H2 is used to adjust the proportion of SiH4) to deposit ia-Si:H, and doping gases PH3 and B2H are added to deposit the corresponding na-Si:H and pa-Si:H film layers. The thickness of each film deposition is controlled between 5-8nm.
3. TCO deposition
①Purpose: The TCO film layer plays a role in light transmission and conductivity in HJT cells, and must have both optical and electrical properties, that is, it must simultaneously meet the requirements of high transmittance, high mobility and low square resistance, and minimize 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 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, and is a good window layer material.
②Methods:
4. Screen printing
①Purpose: In order to conduct the generated current, it is necessary to make positive and negative electrodes on the surface of the battery cell. The basic requirements for preparing electrodes are: good contact with the 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 the battery.
②Methods: Since HJT batteries 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°C.
