Your recent study is titled “Design Considerations for Multi-Terawatt Scale Manufacturing of Existing and Future Photovoltaic Technologies: Challenges and Opportunities in Consuming Silver, Indium, and Bismuth”. This sparked a lot of discussion within the PV community – how did you choose this topic for the study?
PV has been so successful – it is now the cheapest form of electricity. This means that we are not just talking about a potential scale of terawatts for electricity, but also for transforming the energy sector to provide transport, heat and a whole range of other energy needs.
Pierre Verlinden had written an article on future challenges at the terawatt scale – a level of production that we could now see in 10 years. One of my main concerns for the industry is that there are many roadmaps that really push heterojunction solar cells to deploy, given that in today’s industrial implementation, heterojunction designs use twice as much money. than PERC. And then they need the transparent conductive oxide layers, for which they use indium, which is very, very rare.
We need to become more efficient, but we cannot move on to technologies that create bigger problems along the way.
How did you study the capacity of silver, indium and bismuth reserves in relation to the photovoltaic industry?
I was not sure how this article would be received as it is very simplistic. It basically says, here are the physical dimensions of, say, silver contacts, and this is what that means for the overall consumption of silver by the PV industry as production increases.
We then also calculated what it would mean for the series resistance on the fingers of a solar cell if we reduced the amount of silver used. The fingers are the same dimensions, all the way down, and it doesn’t matter where you put the money down, what finger spacing and all those other geometries, if you’ve used X milligrams of silver, you’ll have a corresponding series resistance – it doesn’t. There’s no way around this with conventional designs. In terms of durability, however, that means we can’t use silver fingers the way they’re used now.
You also concluded that the small efficiency increases offered by TOPCon and Silicon Heterojunction (SHJ) technologies cannot significantly reduce the use of critical materials as industry adapts production and facilities to a several terawatt scale – right?
Ultimately, we will have to focus on efficiency, because by increasing efficiency we will improve material consumption at all levels. But there’s no point in changing technologies and manufacturing lines now to something that uses double the amount of a critical item.
For example, I think there are 40-50 gigawatts of heterojunction manufacturing planned, which would potentially use up to 60% of the world’s indium supply if all manufacturers used ITO. [indium tin oxide].
Within current limits, it is possible to use less than the typical 100 nanometers of this ITO layer per side. One option to improve durability is to use a transparent double-layer conductive oxide with 20 nanometers of ITO, capped with an indium-free layer like AZO [aluminium zinc oxide]. But even that can only be used for maybe a few 100 gigawatts – indium will run out before we hit the terawatt scale. Ultimately, heterojunction cells will need to be indium-free.
There are proven alternatives, like copper plating rather than screen printing, that could greatly reduce the use of silver, right?
Yes, but virtually every roadmap for PV companies talks about transitioning to TOPCon or SHJ. And if you consider ITRPV [International Technology Roadmap for Photovoltaics], serigraphy is getting stronger and stronger – there is very little talk of plating. The veneer is therefore completely misaligned with current projections. We either have to stick with PERC if we are going to use screen printing, or we have to go with copper plating. And at that point, we can start talking about whether we’re going to develop indium-free heterojunctions, but only if the choices there don’t result in more material constraints.
Copper plating is a massive departure from current manufacturing practices, but copper is also much cheaper than silver. Suntech has done the plating very successfully, as have other companies. SunPower’s Maxeon contacts use a copper plating on the back. So it can be done, but the tooling is such a big change from what most of the industry is doing now, and the problem of handling liquid metallic waste is going to be a challenge compared to copper as well.
The paper concludes that PERC will likely continue to dominate solar cell technology, but that its most likely successor when we consider sustainability at the multi-terawatt scale is the tandem cell architecture. How did you come to this?
PERC is getting stronger and stronger. It’s a moving target for 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 silver used in PV, but we have showed that these reductions must be much greater than expected in order to allow sustainable production at the terawatt scale. Tandem technology offers the next significant leap in efficiency. PERC is 24-25% efficient, TOPCon and SHJ only offer 25-26%, but tandems can reach 30%.
One of the really interesting opportunities for two terminal tandems is that the resistive losses are proportional to the ratio of the maximum power point current to the maximum power point voltage. And with the tandem, the current is halved and the voltage is about three times the PERC, which means that we can potentially reduce money consumption by a factor of six.
How do you think the industry will react to your article? You say that manufacturers have already invested heavily in heterojunction production. Do you think they can change at this point?
I really hope they will. We know that we can sustainably produce at the terawatt scale if we reduce the use of silver, which we can achieve with plating. And there are solutions for heterojunctions without indium, but we have to push that. What concerns me right now is that companies are only dragged into material shortages when they drive up prices.