Should heterojunction solar technology be put on hold for now? Can copper plating replace the silver conductive surface of the screen printing in the battery? Why series batteries may be the successor of PERC? Brett Hallam recently sat down with Natalie Filatoff in Sydney to explain the results of a new study by the University of New South Wales (UNSW) aimed at answering these controversial questions.
"In the final analysis, heterojunction batteries need to be indium-free," said Brett Hallam.
Your most recent study is titled "Design Considerations for Multi-Terawatt Scale Manufacturing of Existing and Future Photovoltaic Technologies: Challenges and Opportunities Related to Consumption of Silver, Indium, and Bismuth." It has triggered a lot of discussions in the photovoltaic community-how did you choose this topic for research?
Photovoltaic is very successful-it is now the cheapest form of electricity. This means that we are not only talking about potential terawatt-level electricity, but also about the transformation of the energy sector to transportation, heating, and a range of other energy needs.
Pierre Verlinden wrote a paper on the future challenges of terawatts-we can now see a level of production within 10 years. One of my main concerns for the industry is that many roadmaps are indeed driving the deployment of heterojunction solar cells, because in current industrial implementations, heterojunction designs use twice as much silver as PERC. Then they needed a transparent conductive oxide layer, for which they used very, very scarce indium.
We do need to improve efficiency, but we can't jump all the way to technology that will cause bigger problems.
How do you investigate the reserves of silver, indium and bismuth related to the photovoltaic industry?
I am not sure how this paper will be accepted because it is very simple. It basically means that here is the physical size of the silver contacts, which is what this means for the total silver consumption of the photovoltaic industry as the scale of production expands.
Then, we also calculated what the series resistance on the solar cell finger means if we reduce the amount of silver used. The finger size is always the same, no matter where you put the silver, the distance between your fingers, and all other geometric shapes, if you use X milligrams of silver, you will have the corresponding series resistance-traditional designs cannot solve this problem. However, in terms of sustainability, this means that we cannot use silver fingers as we are currently using.
You also concluded that the small increase in efficiency provided by TOPCon and silicon heterojunction (SHJ) technology does not reduce the use of key materials in a meaningful way, as the industry expands production and installation to a multi-terawatt scale— Yeah?
Ultimately we will need to focus on efficiency, because by improving efficiency, we will improve material consumption across the board. But now it makes no sense to change the technology and production line to use something that doubles the number of key elements.
For example, I think there are 40 to 50 gigawatts of heterojunction manufacturing plans, if all manufacturers use ITO [Indium Tin Oxide], this may occupy up to 60% of the global indium supply.
Within the current limits, there is scope for using such ITO layers of less than a typical 100 nanometers on each side. One option to improve sustainability is to use a double-layer transparent conductive oxide with 20 nanometers of ITO, covered by an indium-free layer such as AZO [Aluminum Zinc Oxide]. But even so, it can only be used for about 100 gigawatts-before we reach the terawatt level, indium will be exhausted. After all, heterojunction batteries will need to be indium-free.
There are proven alternatives, such as copper plating instead of screen printing, which can greatly reduce the use of silver, right?
Yes, but almost every photovoltaic company's roadmap is talking about transitioning to TOPCon or SHJ. If you consider ITRPV (International Technology Roadmap for Photovoltaics), screen printing is getting stronger and stronger-electroplating is rarely talked about. Therefore, electroplating is completely inconsistent with the current forecast. If we want to use screen printing, we must either stick to PERC, or we need to use copper plating entirely. At that time, we can start to discuss whether we will develop an indium-free heterojunction, but only if the choice there does not lead to more material restrictions.
Copper plating is very different from current manufacturing practices, but copper is also much cheaper than silver. Suntech has been very successful in electroplating, as have other companies. SunPower’s Maxeon contacts use copper plating on the back. So it can be done, but compared to what most industries do now, the tools have changed a lot, and the disposal of liquid metal waste will become a challenge related to copper.
The paper concluded that PERC may continue to dominate solar cell technology, but when we consider the sustainability of multi-terawatt scale, its most likely successor is the series cell architecture. How did you get to this point?
PERC is getting stronger and stronger. It is the moving target of everything else. The industry is working to reduce the use of silver in PERC, and due to the current scale of PERC production, this may have a significant impact on the sustainability of the silver used in PV, but we have proven that these reductions must be much larger than anticipated in order to achieve Sustainable terawatt production. Tandem technology provides the next major leap in efficiency. The efficiency of PERC is 24-25%, TOPCon and SHJ only provide 25-26%, but the series connection can reach 30%.
One of the truly exciting opportunities for connecting both ends in series is that the resistance loss is proportional to the ratio of the maximum power point current to the maximum power point voltage. Compared with the series connection, the current is halved and the voltage is approximately three times that of PERC, which means that it is possible to reduce silver consumption by a factor of six.
How would you like the industry to react to your paper? You said that manufacturers have invested heavily in heterojunction production. Do you think they can change at this point?
I certainly hope they will. We know that if we reduce silver consumption, we can manufacture in a terawatt-level sustainable way, and this can be achieved by electroplating. And there is no indium heterojunction solution, but we need to promote it. My current concern is that the company may only be driven by material shortages when it causes prices to rise.
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