Photovoltaic cell technology analysis of HJT ( heterojunction solar cell)

    HJT (HeterojunctionwithIntrinsic Thin-film) - intrinsic thin film heterojunction solar cell. It has a symmetrical double-sided battery structure with N-type crystalline silicon in the middle. The front is deposited in order of the original amorphous silicon film and the P-type amorphous silicon film, thus forming a P-N junction. On the back, the original amorphous silicon film and N-type amorphous silicon film are deposited in turn to form a back surface field. In view of the poor electrical conductivity of amorphous silicon, a transparent conductive film (TCO) is deposited on both sides of the battery for electrical conductivity, and finally screen printing technology is used to form a double-sided electrode.

HJT: Advantages

• Advantage 1: The process is short. The HJT battery process mainly includes 4 links: lint, amorphous silicon deposition, TCO deposition, 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 of TCO thin film deposition: RPD (reacting plasma deposition) and PVD (physical and chemical meteorological deposition). Sumitomo Heavy Industries has the patent of RPD, while PVD technology is mature, and there are many manufacturers that provide equipment.

• Advantage 2: High conversion efficiency. It is mainly due to the double passivation 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% has been very clear, that is, the use of doped nanocrystalline silicon, doped microcrystalline silicon, doped microcrystalline silicon oxide, and doped microcrystalline silicon carbide to replace the existing doped on the front and rear surfaces; the conversion efficiency of HJT superimposed IBC and perovskite may be improved more than 30% in the future.

• Advantage 3: No LID and PID, low attenuation. Because the HJT solar cell substrate is usually N-type monocrystalline silicon, while N-type monocrystalline silicon is phosphorus-doped, and there is no boron-oxygen compound and boron-iron composite in P-type crystalline silicon, so the HJT solar cell is immune to the LID effect. The surface of the HJT solar cell is deposited with a TCO film without an insulating layer, so there is no chance for the surface layer to be charged, and PID is avoided structurally. HJT battery attenuates by 1-2% in the first year, and then attenuates by 0.25% per year, which is much lower than the attenuation of PERC solar cells (2% in the first year, and 0.45% attenuation per year thereafter). Therefore, the full life cycle of HJT solar cell is about 1.9%-2 higher than the double-sided PERC solar cell per W power generation. 9%.

• Advantage 4: Low temperature coefficient and high power generation. The power temperature coefficient of HJT solar cell is usually –0.25 to 0.2%/℃, which is lower than -0.45%/°C to -0.35%/°C of conventional and PERC solar cells. The low temperature coefficient of HJT means that in the high-temperature operating environment of the component, the HJT solar cells has relatively high power generation performance, thus achieving power generation gain and reducing the power cost of the system. If the working temperature of the solar cell exceeds the ambient temperature of 10-40℃, and the average ambient temperature of the whole year is 5-10℃ lower than the standard working conditions of the laboratory, the HJT solar cell is about 0.6%-3.9% higher than the double-sided PERC solar cell per W.

• Advantage 5: High double-sided rate. The front and back of HJT are symmetrical, and the TCO film is light-transmitting, which is naturally a double-sided solar cell. The double-sided rate of HJT can reach more than 90% (up to 98%); the double-sided rate of double-sided PERC is only 75%+. According to solarzoom calculations, considering the difference between 10%-20% of back irradiation and the double-sided rate of solar cells, the single-watt power generation of HJT solar cell is about 2%-4% higher than that of double-sided PERC solar cells .

• Advantage 6: Low light effect. The HIT solar cell uses N-type monocrystalline silicon wafers, while the PERC soalr cell uses P-type monocrystalline silicon wafers with irradiation intensity below 600W/m. The N-type power generation performance is about 1%-2% higher than that of P-type; the HJT solar cell is about 0.5-1 higher than the double-sided PERC solar cell per W power generation due to the weak light effect. About .0%.

• In summary, the power generation per W in the full life cycle of double-sided HJT solar cell is significantly higher than that of double-sided PERC solar cells, with a comparative advantage of about 7%.

HJT: Industrialization Progress - Economy

• The current high cost is an important factor limiting the large-scale industrialization of HJT technology.

1. The process route of HJT and PERC is completely different and cannot be extended. It can only be put into new production lines, and HJT is not compatible with mainstream PERC production equipment. Therefore, the input of PECVD and other membrane and vacuum equipment will bring higher conversion costs to enterprises.

2. HJT solar cell cost structure: silicon wafer cost, non-silicon materials (silver slurry, target, gas and chemicals, etc.), equipment depreciation, other manufacturing costs (including labor and power costs), etc.

The high cost of HJT solar cells is mainly reflected in the slurry, target and equipment. 1) Because the conductivity of the low-temperature silver pulp required by HJT is relatively weak and the welding tension is low, the consumption is large. At the same time, the localization rate of low-temperature silver pulp is low, so its price is significantly higher than that of high-temperature silver pulp. 2) HJT needs to deposit an additional transparent conductive layer, and the price of targets such as ITO (PVD route) or IWO (RPD route) is higher. 3) HJT equipment investment is relatively high. At present, the investment in HJT equipment is 400 to 450 million yuan/GW, more than twice that of PERC (about 150 to 200 million yuan/GW).

HJT: cost reduction path - equipment cost reduction

• Before 2019, HJT equipment was mainly provided by Mayerberg, YAC, AMAT, Sumitomo Japan and other foreign brands, and the cost of equipment was about 1 to 2 billion to 2 billion/GW; in 2019, Maiwei, Junshi, Jiejia Weichuang, etc. promoted import substitution, and the equipment cost was reduced to 5-10 billion/GW; 2020 In 20 years, the old European leader Meyerberg withdrew from the competition; in 20 years, the cost of HJT equipment dropped to 500 million/GW.

• At present, domestic equipment manufacturers Maiwei, Jiejia and Junshi have the supply capacity of HJT whole line equipment. With the promotion of the localization of HJT equipment, the current investment cost has been reduced to about 450 million yuan/GW, but compared with PERC's 150-250 million yuan/GW and TOPCon's 2-250 million yuan. Yuan/GW is still much higher. The high investment cost of equipment not only affects the enthusiasm for investment in the early stage, but also means that the depreciation of non-silicon costs in the later stage is higher. In addition, the large-scale production of HJT manufacturers is insufficient at present, resulting in the depreciation cost of HJT being at least 0.03 yuan/W higher than PERC.

• HJT mainly includes four steps: velvet cleaning, amorphous silicon film deposition, TCO film deposition and metallization, corresponding to four equipment: velvet cleaning, PECVD, PVD/RPD, and screen printing/electroplating. The main factors affecting equipment investment include: 1) equipment configuration, such as RPD or PVD for TCO preparation; 2) import or domestic production; 3) improve equipment production efficiency.

HJT: The path of cost reduction - silver paste cost reduction

• Photovoltaic silver paste is divided into two types: high-temperature silver paste and low-temperature silver paste. P-type solar cell and N-type TOPCON use high-temperature silver  paste; while HJT grid electrodes do not allow high-temperature sintering and curing, and must be limited to less than 150℃. Only low-temperature silver paste can be used. In order to ensure low resistance contact and high conductivity transmission, low-temperature silver is required. The concentration of silver paste is high, the thickness of the grid line is high, and the consumption is large.

• High price: The production process of low-temperature silver paste is difficult, and it requires cold chain transportation. The price is usually 10-20% higher than that of conventional silver paste. At present, HJT is still in the early stage of the market. At present, only Japan's KE (90% market share) and one or two domestic enterprises can achieve mass production, and the silver powder used. It is similar to customization, so the processing fee is higher and there is a certain profit bonus.

• Large dosage: low-temperature silver plasma electrical properties and printing performance are poor; and because of its high resistivity and HJT double-sided use, the single consumption of silver  paste is much higher than that of PERC. For example, the amount of silver paste used in HJTM6 is about 200mg, and the amount of silver pulp used by PERCM6 is about 130mg higher, according to the current  usage of HJT silver paste and the price of HJT slurry, the cost of HJT in the metallization link is about 0.12 yuan/W higher than PERC.

HJT: Industrialization Progress - Silicon cost

• The HJT solar cell structure is naturally suitable for silicon wafer thinning. The symmetrical structure of the HJT soalr cell sheet can reduce the mechanical stress in  production, so the fragmentation rate of the silicon wafer is lower; HJT adopts a low temperature process below 200°C, which makes the silicon wafer not easy to warp at low temperature, and the yield is higher; when the silicon wafer becomes thinner, the HJT open circuit voltage rises and the short circuit current decreases. The battery efficiency is basically unchanged.

• At present, the thickness of PERC soalr cells and TOPCON solar cell is 170-180μm, and HJT can reach 150μm. Theoretically, it can reach 100μm, and the thickness reduction space is large. Generally, for every 20μm thinning of silicon wafers, the component cost can be reduced by about 5-6 points/W.