With the advantages of high conversion efficiency and low attenuation, heterojunction solar cell has the opportunity to enter the market in the future. Compared to 2019, more than 20 companies announced expansion plans in 2020. Large-scale and breakthroughs in key equipment will become the possibility of breakthrough industrialization.
1. What is a heterojunction solar cell
The English name of heterojunction solar cell is abbreviated as HIT (Heterojunction with Intrinsic Thin-layer), which is also an intrinsic thin film heterojunction cell. HIT was first developed by Japan and registered as a trademark.
The heterojunction solar cell is generally based on N-type silicon wafers, and the typical structure is shown in the figure. On the front side, there are transparent conductive oxide film (referred to as TCO), P-type amorphous silicon film, and intrinsic hydrogen-rich amorphous silicon film.
The HIT cell structure diagram
On the back of the battery, TCO transparent conductive oxide film, N-type amorphous silicon film and intrinsic amorphous silicon film are sequentially displayed. In addition, the feterojunction solar cell is also relatively simple in the manufacturing process, and the thin films in the cell structure mentioned above are formed by deposition.
2. Characteristics of the heterojunction solar cell
Subsequent enterprises entering the field of heterojunction solar cells have adopted different names in order to avoid patent disputes, such as HJT/SHJ/HDT. Although the English name and abbreviation are slightly different, their meanings represent intrinsic thin film heterojunction cells.
Finally, metal electrodes are prepared on both sides of the battery by screen printing or electroplating process, and then the manufacturing of heterojunction solar cell is completed through low-temperature curing process.
The characteristics of HIT determine its market penetration from distributed to power stations. It is not possible to differentiate the pricing of home energy storage, so the price of HIT batteries will gradually decrease during market penetration.
3. Heterojunction solar cell production process
Compared with the traditional lifepo4 battery production process and TOPCon battery process, the process of heterojunction solar cell is relatively short, with only four major links. The following are cleaning and texturing, amorphous silicon deposition, TCO deposition, and screen printing curing.
● Cleaning and texturing
Similar to the conventional P-type or N-type battery manufacturing process, heterojunction solar cells are the first step in cell manufacturing by cleaning and texturing. The main purpose of this step is to remove oil and metal impurities from the surface of the N-type substrate, remove the mechanical damage layer, form a pyramid pile, trap light and reduce surface reflections.
● Preparation of amorphous silicon films
The silicon wafer makes a passivation film and PN junction in a PECVD device. The root of the high efficiency of heterojunction solar cell lies in the excellent passivation effect of intrinsic amorphous silicon films. Due to the large number of suspension bonds on the surface of the crystalline silicon substrate, a small number of carriers excited by light are easily captured by the suspension bonds and recombined after reaching the surface.
Thereby reducing the cell efficiency. In addition, by depositing hydrogen-rich intrinsic amorphous silicon films on the front and back of the silicon wafer, the suspension bond can be effectively hydrogenated and surface defects reduced. This significantly improves the life expectancy, increases the open-circuit voltage, and ultimately improves battery efficiency.
Although the thickness of each layer of film is only 4-10nm, the functions and preparation processes realized by each 1-2nm are very different, so intrinsic and doped amorphous silicon films need to be completed in multiple chambers, and multi-chamber deposition systems need to be introduced in PECVD.
● Deposition of metal oxide conductive layers
After the silicon wafer is deposited with an amorphous silicon film, it enters the SPUTTER (magnetron sputtering) or RPD (ion reaction coating) equipment to deposit the transparent metal oxide conductive film TCO. TCO collects carriers longitudinally and transmits them to the electrodes.
SPUTTER magnetic sputtering principle
Due to the disordered structure of amorphous silicon layer crystals, the migration rate of electrons and holes is low. Moreover, the transverse conductivity is poor, which is not conducive to the collection of photogenerated carriers.
Therefore, a layer of TCO 75-80 nm thick needs to be deposited above the frontal doping layer for longitudinal collection of carriers and transmission to the electrodes, and TCO can be reduced at the same time. TCO film has more than 80% penetration in the visible range and has very low resistance. At present, the most widely used are ITO, SCOT, IWO, AZO.
There are two processes for TCO preparation: SPUTTER (magnetron sputtering) or RPD (ion reaction coating), and most of the SPUTTER (magnetron sputtering) processes are currently chosen due to cost considerations.
● Screen curing
The final step in the production of heterojunction solar cell is screen curing, where metal electrodes are prepared and cured. Considering that HIT is a low-temperature process, the process of screen printing and low-temperature curing is relatively simple.
However, one of the disadvantages of this feature is the high price and large consumption, so some companies in the industry are currently trying to use the copper plating process to make electrodes. Because silver paste is not used in the copper plating process, the cost is relatively low. But even so, the process is not widely used because it is very complex. And there are serious environmental constraints on waste liquid discharge, which limits its promotion.
4. The advantage of heterojunction solar cell
● High conversion efficiency
The high conversion efficiency of heterojunction solar cell comes from the high open-circuit voltage, the open-circuit voltage Voc of heterojunction solar cell can be close to 750mV, while ordinary PERC batteries are generally lower than 700mV.
The high open-circuit voltage of HIT batteries comes from two points:
- Excellent passivation effect of hydrogenated intrinsic amorphous silicon film.
- Photogenerated carriers can penetrate hydrogenated amorphous silicon thin films.
So laser film opening and ohmic contact formation are not required, which can effectively reduce recombination. Due to the introduction of multi-main gate technology and photoregeneration technology, the current R&D efficiency of HIT has generally exceeded 24%.
● Low attenuation
Due to the special structure of heterojunction solar cells, their attenuation degree is significantly lower than that of PERC cells: The TCO on the surface of heterojunction solar cell has conductive properties, the charge does not produce polarization on the surface, and there is no potential induced attenuation PID.
N-type silicon wafers doped with phosphorus, without boron-oxygen complexes and boroferric complexes, photodecay LIDs are small According to relevant data, the first year attenuation and average annual attenuation of heterojunction solar cell are lower than that of PERC battery, and have been verified by power station power generation.
● Low temperature coefficient
The temperature coefficient of a component measures how much output power decreases with increasing temperature. When sunlight hits the surface of the module, the surface of the module continues to heat up, and the open-circuit voltage of the battery continues to drop, resulting in a decrease in output power. HIT has a very low temperature coefficient compared to other types of batteries.
● High double-sided rate
Heterojunction solar cells have a higher bilateral rate. Its front and back three-layer film and TCO are light transmission, and the structure is symmetrical, naturally is a bifacial battery, its bifacial rate can easily achieve more than 90%, or even reach the level of 98%. The double-sided rate of PERC batteries can be 80%-85%.
5. ConclusionAt present, the top 10 lithium battery companies in the world layout HIT because of its benefits, and industrialization is brewing. At present, the world's largest HIT mass production scale is only 600MW, and the new production lines in the past two years are mainly 100-200MW pilot lines.