Even though heterojunction (HJT) cell technology offers high efficiency and power generation compared to its contemporaries, its uptake has been slow owing to high costs. Nonetheless, cost reduction efforts are being made by the industry to ensure industrialization of these products.
One of the prominent proponents and industrial manufacturers of HJT technology, Risen Energy of China has been producing HJT modules since 2019.
Continuous improvements in R&D of this technology led the company to unveil its Hyper-ion module that incorporates ultra-thin silicon wafers, low-silver paste, 0BB cells and low temperature, along with stress-free cell interconnection.
In a whitepaper, the manufacturer tracks its efforts to develop low-silver metallization paste for the Hyper-ion module, which is primarily responsible for cost reduction, while maintaining the high efficiency of this product.
It currently uses 10 mg/W of pure silver for its HJT cell in the industrial application and aims to drop its use by 1 mg/W/quarter in the future.
Considering the cost of wafer currently forms the largest chunk of the cost of an HJT cell with a 55% share, Risen says it has started mass production of ultra-thin wafers with less than 100 µm, and also introduced self-produced wafers. Reuse of ingot trimmings as an experiment is also showing good results.
With about 20% share of its total cost, metallization paste comes next, which is basically silver paste. An expensive material, its price fluctuations have a direct impact on the solar cell production costs. Risen says choosing an alternative is imperative for the industry basing the same on conductivity, higher reserves and low price.
Till a suitable candidate is found, Risen is using low-silver-content paste, which is a paste of low-cost metal encapsulated with silver shell. The company developed the paste in close cooperation with its partner metallization paste suppliers.
The key for wide adoption of this paste is the inner low-cost metal should not come out of the silver shell during the processing and operation. Risen conducted several technical studies to evaluate the reliability of this paste with low-cost metal coated with silver in various processing and operating conditions. These tests found that the thickness of silver shell is uniform and is between 80 nm and 100 nm, which can effectively prevent the exposure of inner low-cost metals during the processing.
The company also studied the effect of light exposure, electrification and heat, simulating operational conditions, which also concluded that none of these testing conditions led to the precipitation of low-cost metal ions, implying the silver shell is intact. The white paper also cited a year-long field test study in Yinchuan, China,; it claims to have found that these modules can fully meet or even exceed the performance of modules with pure silver paste cells.
The Yinchuan test showed, according to Risen, that the power generation of bifacial HJT modules exceeded that of mono–facial PERC modules by approximately 9.6% and was higher than that of bifacial PERC modules by about 6.1%.
"At present, the main challenge that needs to be urgently addressed is cost reduction. As a result of Risen's years of efforts as well as the development and progress in the industry, the cost of HJT modules is expected to be on a par with that of TOPCon or even PERC modules by the end of 2023 or the beginning of 2024," reads the whitepaper.
Higher power generation of HJT modules will significantly reduce the balance of system (BOS) cost and levelized cost of energy (LCOE) of PV systems, thereby leading to higher revenue generation and faster reduction of carbon emissions, it adds.