Australian team surpasses solar efficiency target

The research team at ANU have achieved a solar cell efficiency of 30.3 per cent in their laboratory.

In an era of rapid climate change three factors have dominated the race to deliver solar electricity: solar cell efficiency; combined manufacturing, deployment and operational costs; and a very short timeframe.

ARENA has set ambitious targets for each of these. Launched in September 2021 and dubbed Solar 30 30 30, ARENA’s goal is to lift solar cell efficiency to 30 per cent and reduce the cost of solar modules in the field to 30 cents a watt. The final 30 represents the target date of 2030, a mere seven years from now.

Could a class of materials first discovered in Russia’s Ural Mountains in the mid 19th century be the key to reaching those targets?

Now, a team of researchers at the ANU led by Dr The Duong, say they have produced in the lab a solar cell with an efficiency of 30.3 per cent, using perovskite.

Perovskites, named after Russian minerologist Lev Perovski, are known to have potentially high solar cell efficiency but are notoriously unstable. Dr Duong’s team has demonstrated a method of stabilising the structure of a perovskite

The ANU team’s work has been financially supported by ARENA through the Australian Centre for Advanced Photovoltaics (ACAP). ARENA in June 2022 announced up to $45 million funding to ACAP to extend operations of their cutting-edge solar photovoltaic (PV) research to 2030. And in January 2023, ARENA announced a total of $41.5 million funding to 13 projects focussed on delivering Ultra Low Cost Solar.

In this interview, Dr The Duong explains his team’s approach and possible future developments.

Q: Can you describe the work you have done and what you have achieved.

We have discovered a simple method to significantly improve the quality of the perovskite material and therefore we can improve both the efficiency and the stability of the perovskite in the cell.

When combining these perovskite solar cells with silicon cells in a 4-terminal tandem configuration, we achieved 30.3 per cent [efficiency]. That means 30.3 per cent of the energy of the sunlight had been converted into electricity.

Our work not only demonstrated perovskite-silicon tandem solar cells with ultra-high efficiency, but we also show that the devices have excellent stability.

Q: Can you explain what “tandem” means in terms of solar cell construction?

We build a tandem arrangement by combining two cells, a perovskite cell with a high band gap sitting on the top and a silicon one with the lower band gap sitting at the bottom. The high band-gap cell is more effective at the short wavelengths [of light] while the low bandgap device is more effective at the long wavelengths.

My work is mainly focused on the perovskite. So, perovskite is usually the top cell on a tandem cell and then absorbs the blue light and converts it to energy. Importantly, the perovskite device is made semi-transparent, which lets the red light pass through to the silicon bottom cell and generates additional energy. In that way, the tandem solar cell can produce significantly more energy than each individual cell.

Tandem is not a new concept. But having perovskite in tandem is low-cost as compared to the gallium arsenide or other high-cost materials. So, hopefully, with this at high density, we can lower the cost of the solar energy.

Q: What are the advantages of using perovskite?

The advantage of perovskite is that it can be made with a very low temperature process.

So basically, for silicon you need very high temperatures, 700 to 1000 degrees C. And the whole process is very energy intensive.

But perovskite [is produced] in a very low temperature process, normally 100 to 150 degrees C.

In addition, you can deposit high quality perovskite materials using many different techniques: you can print perovskite on, or you can just spray it on. So, it is very simple and that’s why you can make perovskite cells in a very short time and at a low cost. So, it is very simple and that’s why you can make perovskite cells in a very short time and at a low cost.

Q: What is the significance of this work?

The theoretical maximum efficiency of a single junction cell is about 29 per cent. And, we are getting very close to that.

So that’s why you need to think about other approaches to improve efficiency.

The very highest efficiency tandem cells have achieved before is more than 30 per cent. But these cells have mostly used gallium arsenide, which is very high cost.

We work on a perovskite tandem layer, so this is kind of the low-cost version of the tandem cell.

But the perovskite layer can be made in different configurations.

You can just monolithically deposit perovskite on top of the lower cell or you can combine the two cells mechanically. And in our work, we have combined the two devices mechanically.

Monolithic tandems need to have current matching. To work properly, the top cell and the bottom cell need to deliver the same current at the same time.

So, let’s say you’ve got the whole spectrum of sunlight that can vary from the morning to the afternoon. In the morning, when you got a lot of blue light, the monolithic top perovskite cell produces a lot of current but the second silicon cell only produces a very small current. And so, the whole device is limited.

So overall, the monolithic tandems can suffer from inefficiencies through the day.

With our perovskite tandem device, each layer can act independently. The top and the bottom cell produce a separate electric current and do not have to match the electric current of the other layer. So, they are not affected in the same way by the changing solar spectrum through the day

So overall, we think that we can expect this tandem to deliver a higher energy yield than the monolithic tandem.

Q: Does that mean that solar cell maintenance would be easier?

The mechanical construction tandem cells have several advantages.

One is that if you’ve just got the silicon module, what you can do is just upgrade into a tandem by placing the perovskite module on top. And if a perovskite layer fails, you can remove it and put another one on.

Also, when you [monolithically] combine the two devices, if one of them stops working, then the whole device will not work.

But when we combine the two devices mechanically, they operate independently. So even if one stops working the other one still works.

Q: Do you think this this will be scalable in terms of building up into big solar panels?

There are three major things that we need to overcome. First is just to demonstrate that this model can be stable to operate in the field for 20 to 25 years or beyond.

At this point, [the technology] is very new. We’ve done a lot of testing in the lab but it’s not long enough.

Secondly, there are still major challenges in terms of scaling up the technology. The lab-scale device is quite small, and in the field you need to be much bigger. So, So, we’re looking at different methods to scale up the perovskite modules.

Thirdly, we need to cut the cost of perovskite modules since some layers in the devices are expensive to make.

There has been great progress being made in terms of commercialising the technology. We are confident that the challenges will be overcome in a couple of years.

Q: Do you think there’s room to increase the efficiency even more?

Yes, of course. In fact, I think close to 34 or 35 per cent can be achieved. It’s feasible.

ARENA has supported solar PV research and development  for more than decade. The agency has provided almost $300 million to around 200 solar research and development projects since 2009 through funding programs inherited from the Australian Solar Institute, or run by ARENA since 2012.

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