PERC & HJT – Reminds me of Telecom technologies evolution. What is really the next big thing in Solar PV ?

With every passing day the mix between module as percentage of total project is changing. Also, technology is changing fast. All solar cell manufacturers that are evaluating capacity expansion in the current investment cycle have to decide if they stick to traditional technology or opt for advanced solutions. I feel that there will be a lot of decentralization & consolidation in this segment in China. India will agument more capacity in terms of manufacturing in the next few years with the new Atmanirbhar drive.

When I read about the evolution of solar technologies, I remember the initial days of GSM and Code Division Multiple Access ( CDMA) in India. When CDMA first came to India, the world was already leading in CDMA technology. Reliance and TATA were the front runners in CDMA but still in India the story is different. GSM took the front seat in India with Airtel, Hutchison and Idea since it was more cost effective.

Similarly, in solar the world is moving towards two main technology in solar cells. Increasing efficiency becoming more and more important. Cell makers are increasingly focusing on increasing efficiencies. Two technologies have evolved technology wise. One is the PERC ( Passive Emmitter Rear Contact) and other is the Heterojunction Technology (HJT). Let us understand in brief both of them and then we can compare better.

PERC ( Passive Emmitter Rear Contact)

PERC, which stands for Passivated Emitter and Rear Cell or Passivated Emitter and Rear Contact, is a new technology aimed to achieve higher energy conversion efficiency by adding a dielectric passivization layer on the rear of the cell.

The structure of a PERC solar cell from front to rear:

• Screen-printed Silver paste front contact;
• Anti-Reflective Coating (ARC);
• Silicon wafers that form the P-N junction;
• Local Aluminum Back Surface Field (Al-BSF);
• Dielectric passivation layer;
• SiNx Capping Layer; and
• Screen-printed Aluminum paste layer

Image Credits: https://www.enfsolar.com

The dielectric passivation layer contributes to the increase of efficiency by:

• Reducing electron recombination – electron recombination blocks the free flow of electrons through the cell, reducing efficiency. The extra passivation layer makes the flow of electrons more steady and consistent thereby producing additional electric current.
• Increasing the solar cell’s ability to capture light – unabsorbed light is reflected by the passivation layer back to the solar cell for a second absorption attempt to produce additional energy making the cells more efficient.
• Reflecting specific wavelengths that normally generate heat out of the solar cells – certain wavelengths are absorbed by the rear layer of the solar cell accumulating heat and reducing efficiency. The additional passivation layer reflects these wavelengths out of the solar cell maintaining the temperature of the solar cell.

Image Credits: https://www.solarworld.com

Heterojunction technology (HJT)

HJT is a fusion of wafer based solar cell technology and thin film PV. Heterojunction technology (HJT) is a not-so-new solar panel production method that has really picked up steam in the last decade. The technology is currently the solar industry’s best option to increase efficiency and power output to their highest levels. HJT combines the best qualities of crystalline silicon with those from amorphous silicon thin-film to produce a high-power hybrid cell that surpasses the performance of the industry’s go-to technology, PERC.

Sanyo, which was acquired by Panasonic, held the key patents for the technology till 2010. The key patents expired in 2010 & attracted the interest of researchers first, then the industry

Non comparable cell efficiency

Heterojunction is the only commercially available c-Si technology that delivers over 24% cell-level production efficiencies today and possibly 25% in the next three to four years. The technology combines the advantages of crystalline silicon (c-Si) solar cells with the good absorption of sunlight and the superior passivation characteristics of amorphous silicon (a-Si) known from thin film technologies. The thin intrinsic a-Si:H layers deposited between the n-type c-Si wafer and the doped layers are vital to achieving maximum performance from the cell structure. That results in reduced interface state density and decreased surface recombination losses.

Image Credit: Updated August 2019. Taying News

Comparison between PERC ( Passivated Emitter and Rear Cell) & HJT (Heterojunction technology )

Heterojunction cell technology combines the advantages of mono crystalline silicon (c-Si) solar cells with the good absorption and the superior passivation characteristics of amorphous silicon (a-Si) known from a-Si thin film technology using readily available materials.

HJT Cell processing has 3 important sections

Wafer conditioning: More than standard texturing applied, cleaning is the key in wet chemical treatment

Deposition: The heart of the HJT processing lies in two deposition steps – core layer deposition of applying intrinsic and doped a-Si layers on both sides of the wafer then applying TCO on top, also on both sides

Metallization: Since the active layers of HJT are thin films of a-Si, the technology requires a special metallization process – different type of pastes and curing method

Total cost of ownership is lower as a result of silver paste con-tent of less than 2.4 g per 60-cell module with screen printing technology or even without silver in the case of Cu plating technology.

Meyer Burger develops the wire bonding technology, known as SmartWire Connection Technology (SWCT) which have given an edge in terms reducing silver paste consumption by 70 to 80%.

HJT is based on low temperature process. Simplifies the process steps – from about 13 for PERC to 8. It has Low degradation & Lower thermal coefficient. But, HJT requires a completely different processing sequence that has little in common with today’s mainstream cell production. So, set up costs are pretty high.

Global Technology Roadmap:

Image Credit: Meyer Burger

As per research published, PERC: n-TOPCon/monoPolyTM likely to be the next evolutionary upgrade technology for existing PERC lines. In HJT n-HJT likely to be the green field technology of choice because of higher CE and lower complexity. 3rd generation solar cell technologies such as perovskite-on-Si in the focus.

Current Scenario and Conclusion

For existing cos, the simplest and most attractive silicon cell technology upgrade is called PERC. It only needs a pair of additional equipment to upgrade a standard line, which can result inefficiency improvements. In terms of efficiency and future evolution, HJT outweighs well above PERC modules. High efficiency potential, CoO, and simplicity are pros, while requiring completely different tools set is on flip side. Similarly, in PERC the things like LeTID ( Light and elevated Temperature Induced Degradation ) and long term evolution road map are flips.

HJT technology is increasingly attracting followers all along the PV value chain – equipment suppliers, cell & module companies and research centers. While initially started with a small number of PV makers the number is growing consistently. But, in terms of cost they are expensive as well. Only volumes and market demand can pressurize the price southwards.

PERC is on everyone's radar, but soon expected to reach its optimum limit. The current average efficiencies are above 22% for PERC and 22.5% for HJT at several PVmakers. PERC is expected to reach the practical efficiency limits in less than 2 years. The life expectancy is almost equal.as per research. As per my understanding larger projects will still follow the PERC route due to economies. Even then they will have to weigh between risk and LCOE based on reliability. The real game play will be in the rooftop segment which will be mostly driven either by LCOE or power density depending on the customer preferences. It is left up-to market to ultimately decide who becomes the winner.

I am not a techie but since I am from the Renewable industry and an green enthusiast, I always wondered what these two technology are. Its good that innovation is on the spree which brings a lot of opportunity along with. So, I have put down my research and understanding from secondary sources with a view that it is easy to understand from the readers. Constructive feedback are always welcome.

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