Left to right: Dr Heping Shen, Dr Daniel Jacobs and Professor Kylie Catchpole. Image credit: Lannon Harley, ANU.
Study co-author Dr Heping Shen from the Australian National University School of Engineering says the current solar cell market is dominated by silicon-based technology, which is nearing its efficiency limit. Tandem solar cell technology is a more efficient new alternative.
“In order to continue the transition to a renewable energy based economy, we need to keep reducing the cost of solar energy, and the best way to do that is to increase the efficiency of solar cells,” Dr Shen said.
“If we can have a cheap source of energy that is also clean – who wouldn’t want to use it?”
ANU engineers, in collaboration with researchers from the California Institute of Technology, have developed a way to combine silicon with another material (known as perovskite), to more efficiently convert sunlight into electricity.
The key is the way the materials are joined together to form what’s known as a ‘tandem solar cell’ – essentially one solar cell on top of another. The ANU researchers say theirs is one of the simplest ever developed.
“We have constructed a tandem structure that is unconventional. When engineers combine two cells they usually need to have an interlayer to allow electrical charge to be transferred easily between the two cells, so they can work together,” Dr Shen said.
According to co-author Dr Daniel Jacobs, this is a bit like making a club sandwich with extra bread in the middle – it plays a structural role, but the sandwich would taste better without it.
“We’ve found a new way to simply stack the two cells together so they’ll work efficiently with each other – we don’t need the interlayer, or extra bread, anymore,” Dr Jacobs said.
The tandem solar cell technology minimises energy waste and simplifies the structure, hopefully making it cheaper and easier to produce.
“With tandems it’s crucial to demonstrate a fabrication process that is as simple as possible, otherwise the additional complexity is not worthwhile from a cost perspective”, Dr Jacobs said.
“Our structure involves one less fabrication step, and has benefits for performance too.”
Dr Jacobs says while it can be difficult to combine two materials in a tandem solar cell arrangement, once you get it right the efficiency goes up very quickly, well beyond what is possible with silicon by itself.
“We’ve already reached 24 per cent improvement in efficiency with this new structure, and there’s plenty of room left to grow that figure.”
This study was funded by an Australian Renewable Energy Agency (ARENA) grant, as part of a project in collaboration with UNSW and Monash University.
Featured image above: World record holder Xiaojing Hao with CZTS thin-film cells atop the Tyree Energy Technologies Building at UNSW’s Kensington campus.
Xiaojing Hao couldn’t sleep. Two weeks earlier, the UNSW engineer had sent a thin black tile, barely the size of a fingernail, to the US for testing, and she was waiting anxiously for the results. Her PhD students were equally on edge.
It was midnight when Hao checked her email one more time. It was official: her team had broken a solar cell world efficiency record. “I was full of joy at the achievement,” Hao recalls. “I shared the good news with my team immediately – we made it!”
Hao’s thin black tile had become the newest champion in the solar cell race: one of seven world records UNSW photovoltaics researchers broke in 2016. Efficiency records are not just notches in the scientists’ belts. The more sunlight solar cells can convert, the less manufacturing, transport, installation and wiring is needed to deliver each watt – moving solar energy closer and closer to knocking coal off its perch as the cheapest form of energy.
UNSW photovoltaics researchers, led by Martin Green – often dubbed the ‘father of photovoltaics’– have held world records for efficiencies in solar cells in 30 of the past 33 years. And with its strong track record in research commercialisation, UNSW’s prototype technology is setting the trends for the commercial solar market.
Meanwhile, their focus is on developing the next generation of solar cells – pushing forward to a zero-emission future.
Making a commercially viable product
Hao moved to Sydney in 2004 from China, where the solar industry is booming. A materials engineer by training, Hao was intrigued by the frontline photovoltaic research on thin-film solar cells at UNSW.
These cells have benefits over the more traditional silicon cells. The manufacturing process doesn’t require high temperature steps. They can also be much thinner than bulky wafer silicon, and so could engender new solar applications: imagine solar-powered electric cars,building-integrated solar cells or photovoltaic glazing on windows.
So far, the thin-film uptake in the markets has been sluggish: commercial thin-film cells make up only around 8% of the solar market. The problem is that the commercial products available, cadmium telluride and copper indium gallium selenide (CIGS), are made of toxic or rare materials: cadmium is highly toxic and tellurium is about as abundant as gold.
So Hao decided to go back a step. “We’re trying to make the whole world ‘green’, right?” she says. “So, we should choose materials that are non-toxic and cheap, and that would ensure their deployment in the future – without constraint on raw materials.”
Finding a material worth investigating
Her quest for a greener world began in 2011, after she returned from maternity leave. Hao and her PhD supervisor, Martin Green, knew what they were looking for: a mix of elements that would absorb and conduct energy from sunlight, and are commonly found in nature.
“We worked our way through the periodic table for materials that met those criteria – CZTS was the one that popped out at you as worthy of investigation,” Green explains.
In 2012 CZTS – copper, zinc, tin and sulphide – was recorded for the first time in the solar cell efficiency tables, an internationally curated list of solar cell performance. Inclusion in the tables means a new cell has been independently tested for efficiency by a recognised test centre, and indicates the new cell has features that will be interesting for the photovoltaic community.
Hao began making her own version of the CZTS cell, looking for defects, ironing out the kinks and pushing efficiencies, bit by bit.
At the basic level, all solar cells absorb photons from sunlight and funnel them into an electric current. Hao discovered that tiny holes in her CZTS cells, formed as the components were baked during production, acted like a roadblock for that charge. By adding a microscopic grid layer through the cells, her team stopped these holes from forming, and raised their efficiency to 7.6% in a 1cm2 cell.
That was Hao’s first world record. By changing the buffer that helps the CZTS cell collect charge, the team could further tweak the current flow and voltage output. This buffer netted Hao another world record in September 2016 – a 9.5% efficiency for a 0.24cm2 cell, beating a 9.1% record previously held by Toyota.
“We’re completely leading CZTS solar cell technology at the moment,” Hao says with a smile.
According to Hao, these records have already sparked interest from Chinese, US partners China Guodian Corp – one of the five largest power producers in China – and Baosteel, the giant state-owned iron and steel company based in Shanghai.
Hao is also in talks with thin-film manufacturers MiaSolé of the US, Sweden’sMidsummer and Solar Frontier in Japan. The companies are commercial producers of CIGS cells and their production lines use similar methods; Hao says they could easily adapt them
for CZTS production.
Hao believes efficiencies of above 15% will start moving CZTS to the commercial market. She is already well on her way, aiming to bring her CZTS cells to 13% efficiency by 2018.
Taking on the solar cell market
After four decades in photovoltaics research at UNSW, Martin Green has a healthy scepticism when it comes to marrying new breakthrough technologies with commercial markets. “The solar industry is just so huge that you need enormous resources to introduce a new product to the market – and there’s a huge risk associated with that,” he says.
With a firm grip on 90% of the commercial solar cell market, “the situation with silicon is a bit like that of the internal combustion engine,” Green explains. “That engine is not the best fossil fuel engine, but the huge industry supporting it means it has been very difficult to displace.”
But CZTS does not need to compete with silicon – the two can complement each other. Silicon absorbs red light better than blue, while CZTS absorbs blue wavelengths better. A CZTS layer on top of a silicon cell can catch the wavelengths silicon does not use efficiently. Green says the big silicon manufacturers could trial the new CZTS technology by selling these ‘stacked cells’ as a premium product line.
“Companies that are well established would be interested in exploring that space – it just seems like a natural evolutionary path for photovoltaic technology,” he says.
Collaborating with the competition
Just a few labs down the corridor of the Tyree Energy Technologies Building at UNSW’s Kensington campus, Anita Ho-Baillie is working with Green to put another ‘stackable’ thin-film solar cell through its paces.
In 2009, a material called perovskite arrived on the thin-film solar cell stage with an efficiency of 3.8%. Perovskites have since shot up in efficiency ratings faster than any other solar cell technology.
After Ho-Baillie’s team found a new way to apply perovskite to a surface in an even layer, their solar cells broke three more world records in 2016. Her next step is to make perovskites more durable to match the current lifetime of silicon solar cells – an essential prerequisite for large-scale commercial deployment.
As the leader of the perovskites project in UNSW-based Australian Centre for Advanced Photovoltaics (ACAP), Ho-Baillie stands at the nexus of Australia’s greatest cluster of scientists pushing thin-film technologies forward.
This alliance consists of six research organisations around Australia: the national research agency, CSIRO; Melbourne’s Monash University and the University of Melbourne; the University of Queensland in Brisbane; the Australian National University in Canberra; and UNSW in Sydney.
ACAP director Martin Green says, “We’ve been able to draw on the expertise of all these groups and come at problems from different angles, so it’s really put us in a good spot internationally”.
Ho-Baillie admits balancing collaboration with competition is tricky in a field where everyone is trying to claim the top spot. “It’s hard, but we find working together really helps,” she says.
Much like CZTS and other thin films, perovskite cells are flexible, making them a perfect candidate for energy-harvesting glazes on building materials, cars or windows. But Ho-Baillie has even greater ambitions: with their low weight-to-power ratio, perovskites would be perfect for supplying precious energy to spacecraft, where every kilo counts.
“Perovskites came from nowhere,” she says. “Now I think they will lead us to something that we never even thought would work.”
Improving the cost of solar energy by 150 fold
Thin films are making their mark, but Green is also working to squeeze more energy from sunlight using silicon, smashing two more world records in 2016. Using specialised mirrors and prisms, Mark Keevers from Green’s team pushed silicon cells to collect concentrated sunlight with 40.6% efficiency, and unconcentrated sunlight at 34.5%.
Although these prototypes are perfect for soaking up photons on solar tower ‘concentrators’ with heavy-duty efficiency, their manufacturing costs are too high to make them viable in the consumer market.
But on the rooftop, silicon is still king. And it’s thanks to plunging costs made possible by a UNSW-led boom in silicon solar cell production in China, which now provides more than half the world’s solar cells.
In 1995, Green and his long-term collaborator Stuart Wenham – along with (then) PhD student Shi Zhengrong – started solar cell company Pacific Solar in Australia.
After six years racking up a wealth of management and manufacturing know-how, Zhengrong returned to his native China and founded the silicon solar manufacturing company Suntech Power in 2001, using technology developed at UNSW to dramatically reduce costs.
By 2005, Zhengrong became the world’s first ‘solar billionaire’, and a wave of Chinese companies hit the market, following Suntech’s recipe. The global solar industry was growing at an average 41% year-on-year. And within a decade, China’s market share of the global photovoltaic industry had grown from near zero to over 55%. Suntech itself delivered more than 13 million solar panels to 80 countries.
Where photovoltaic solar cells used to deliver one watt for US$76.67 in 1977,that’s down to just US49¢ today. That’s a 150-fold improvement in the 40 years Green has been in the field.
“Shi was the right person at the right place and the right time to move in both Chinese and Western cultures,” Green says.
“It’s interesting to ponder what would have happened if UNSW hadn’t kick-started the Chinese industry.”
Breaking through the next barrier of photovoltaic research
With plunging module prices, rising efficiencies and more durable cells, why is the world still relying on coal for the lion’s share of its electricity needs?
Perhaps it’s not the solar technology that we’re waiting for. A fundamental challenge remains: how to store the energy we can now capture from sunlight for later use.
“I think photovoltaics has already reached the tipping point – the efficiency and cost is already able to compete with fossil fuels,” says Wenham. “I think the next breakthrough needs to be in energy storage, to bring down that cost enough to make photovoltaics usable everywhere at any time.”
This doesn’t mean UNSW photovoltaics scientists are calling it a day. Instead, they continue to push silicon to its limits, while new technologies, such as Hao’s record-breaking CZTS tile, are racing to catch up to silicon’s powerhouse.
“Solar technology will continue to be higher-efficiency, lower-cost – and will keep getting better,” says Wenham. “The more we develop photovoltaic technology, the easier the transition will become.”
“We’ve reached a new era where coal is no longer the cheapest way of making electricity – it’s solar,” says Green. “And the exciting thing about that is – I regard solar as still in a very primitive stage of development, so there is plenty more cost reduction to come.”
– Viviane Richter
Photography: Quentin Jones
For more stories at the forefront of engineering research, check out Ingenuity magazine.
“They’re reliable, energy-diverse and environmentally friendly, and these advantages are driving microgrid research and development.”
Because urban microgrids can connect or disconnect from the main grid as required, they can also provide backup when the main grid goes down, Ghosh says.
For example, when Japan’s 2011 tsunami knocked out Sendai City’s power grid for weeks, the microgrid at its local university didn’t blink, using fuel cells, solar panels and natural gas turbines to power its way through the entire disaster.
But any grid can be knocked out when demand exceeds supply.
Cooperative resource sharing
“The main problem with microgrids is that you have limited resources,” Ghosh says.
“You might not have sufficient backup to cope with peak energy loads, which means there’s the possibility that your grid will go down.”
The answer is to create microgrid clusters, Ghosh says.
His research indicates that connecting independently managed microgrids enables mutual support during peak demand periods.
“Say you know you’re able to supply your microgrid with four generators, but for some reason—maintenance or failure—you lose one generator, you might have a shortfall of twenty or thirty kilowatts, and that’s enough for your microgrid to collapse,” he says.
“That’s when you need to ask your neighbour for help.”
If your microgrid is connected with a neighbour’s microgrid, you could fill your shortfall with their excess supply, but managing this sharing can become complicated, especially where grids are connected using a simple switch.
Ghosh’s simulations employed the more sophisticated option of connecting with a back-to-back converter.
“With a back-to-back converter, I have control over how much power I can take from my neighbour, and how much power I can send…it allows me to give you ‘X’ amount of power, but to keep the rest for myself,” he says.
Ghosh says reducing power demand during peak times is also essential.