HIT is an abbreviation of Heterojunction with Intrinsic Thin-layer, meaning intrinsic film heterojunction. HIT was first successfully developed by Japan Sanyo Corporation in 1990. Now it has been registered as a registered trademark by Sanyo Corporation of Japan, so it is also called HJT or SHJ. (Silicon Heterojunction solar cell).
1, HIT battery structure and principle
At the time of HIT Solar's initial development, the battery conversion efficiency could reach 14.5% (4mm2 battery). Later, under the continuous improvement of Sanyo, the conversion efficiency of Sanyo HIT battery reached 25.6% in 2015. Sanyo’s HIT patent protection ended in 2015. The elimination of technical barriers is a great opportunity for China to vigorously develop and promote HIT technology.
On the front surface of the battery, due to the energy band bending, 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. At 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 selective deposition on both sides of the battery Layers, so that photo-generated carriers can only be enriched in the absorbent material and then flow out from one surface of the battery, so as to achieve the separation of the two.
2, HIT battery process
One of the advantages of HIT batteries 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 amorphous silicon thin film, which requires extremely high process cleanliness. Reliability and repeatability are a major challenge in the mass production process. Currently, PECVD is usually used to prepare the core process.
HIT battery preparation process is simple, and the process temperature is low, can avoid high-temperature process damage to the silicon, and effectively reduce emissions, but the process is difficult, and the production line is incompatible with traditional batteries, equipment investment is relatively large.
3, HIT battery advantages and characteristics
HIT batteries have the advantages of high power generation and low cost. The specific features are as follows:
(1) Low temperature process
HIT batteries incorporate thin-film solar cells at low temperatures (<250℃)制造的优点, 从而避免采用传统的高温(>900 °C) diffusion process to obtain pn junction. This technology not only saves energy, but also makes a_Si: H-based thin film doping in low temperature environment, the forbidden band width and thickness can be controlled more precisely, and the device characteristics can be easily optimized in process; During the deposition process, the single-piece silicon wafer has small bending deformation, so its thickness can be used as the minimum required by the background light-absorbing material (about 80 μm); at the same time, the low temperature process eliminates the deterioration of the performance of the silicon substrate in high-temperature processing, thereby allowing Use 'low quality' crystalline silicon or even polysilicon as the substrate.
High power generation in high temperature environments. At noon of the day, the power generation of HIT batteries is 8-10% higher than that of general 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 double-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 90% or more, up to 96%, and the advantages of backside power generation are obvious.
(3) High efficiency
The HIT cell's unique heterojunction structure with intrinsic thin layers, completes the surface passivation of monocrystalline silicon at the same time as the pn junction forms a junction, greatly reducing the surface, interface leakage current, and improving the cell efficiency. Currently HIT cells The laboratory efficiency has reached 23%, and the battery efficiency of commercially available 200W modules reaches 19.5%.
(4) High stability
The illumination stability of the HIT battery is good. 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, compared with the temperature coefficient of -0.5%/°C of single crystal silicon cell, the temperature coefficient of HIT battery can reach -0.25%/°C, 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. However, HIT cells have no natural attenuation, and their efficiency has increased to some extent even under light. Shanghai Microsystems found that after HIT photo-induced attenuation experiments, the HIT was irradiated with light. The battery conversion efficiency increased by 2.7%, and there was no attenuation after continuous lighting. The photocompatibility of HIT cells was also confirmed by the joint publication of CIC, Swiss EPFL and CSEM on APL in Japan.
(6) Symmetrical structure suitable for thinning
The perfectly symmetrical structure and low temperature process of HIT cells make them very suitable for thinning. A large number of experiments conducted by Shanghai Microsystems have found that the wafer thickness is in the range of 100-180μm and the average efficiency is almost the same. 100μm-thick silicon wafers have been realized. More than 23% conversion efficiency, currently batch preparation of 90μm wafers. Battery singulation can not only reduce the cost of the wafer, its application can also be more diversified.
(6) Low cost
The thin thickness of the HIT battery can save silicon materials; the low temperature process can reduce the energy consumption, and allows the use of inexpensive substrates; the high efficiency makes it possible to reduce the battery area under the same output power conditions, thereby effectively reducing the cost of the battery.
4, HIT battery industrialization status
The data shows that, in terms of mass production, it is certainly Japan's Sanyo that is second to none, with an existing capacity of 1GW and mass production efficiency of 23%. In addition, Keneka, Sunpreme, Solarcity, and Fujian have more mature HIT technologies. Stone, Jinneng, Xinao and other companies.
At present, the difficulties in mass production of HIT products mainly include the following aspects:
(1) High-quality silicon wafers: Compared to conventional N-type products, HIT batteries have higher requirements for silicon wafer quality and need to be carefully selected by silicon wafer suppliers.
(2) Control of the surface cleanliness of the silicon 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 silicon wafer and the consumption of related chemicals and water.
(3) Q-time control of each process: Prior to the completion of the amorphous silicon coating, the time and environmental requirements for the exposure of the silicon wafer to the silicon wafer are severe. 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 put into production, maintaining production continuity is a major challenge.
(5) Continuous Printing Stability of High-viscosity Slurry: In the preparation of HIT batteries, the viscosity of the paste caused by the large viscosity of the ink is too large and it requires several times the attention of conventional production lines.
(6) The stability of the welding strip tension: The window with stable tension force is narrow, and the double-glass double-side power generation module structure 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 first four items of BOM cost for HIT batteries are silicon wafers, conductive silver paste, target materials, and texturing additives. In response to these high-cost components, Special reductions can be made, including the reduction of raw material consumption, the localization of key equipment, the localization of key raw materials, and the introduction of new technologies.
5. Prospects for HIT Battery Market
Reducing costs and increasing efficiency have always been the eternal theme of the photovoltaic industry. With the constant technological advancement and policy promotion in the industry, the public's attention has gradually shifted to the cost of electricity. As a result, high-efficiency batteries have attracted attention. Following the PERC battery became a hot spot in the industry, HIT The initial breakthrough in battery technology, cost-effectiveness advantages began to show, the future will be the era of P-type PERC battery and N-type HIT battery for the photovoltaic industry.
