Mysterious ‘quantum dots’ to revolutionise solar energy
UQ researchers are harnessing the quantum realm to create solar cells that may appear in the unlikeliest of places.
Professor Lianzhou Wang
Professor Lianzhou Wang
Though you may never have heard of them, quantum dots are vital components of technology that you probably use every day. They pop up everywhere, from high-tech laboratories, to LED lighting, to inside your new TV (yes, the Q in QLED stands for ‘quantum’).
Professor Lianzhou Wang and his lab at UQ’s Australian Institute for Bioengineering and Nanotechnology (AIBN) and School of Chemical Engineering are harnessing these properties to make solar cells that capture a wider range of light for energy, are more stable in their energy production, and can be applied to curved surfaces. They could even be used to generate energy from artificial light, or low light in cloudy conditions.
This could revolutionise how we currently use solar technology – meaning that existing, everyday objects like windows, car roofs and even your smartphone could become handy, portable solar cells in future.
And best of all – their method is easy to scale up, meaning solar cells could be produced in future for a fraction of the cost they are now.
But what is a quantum dot, and how are they going to change the way we make solar energy?
The world of modern electronics has been transformed by a class of materials called ‘semiconductors’. Used in electrical circuits and transistors, semiconductors allow some electricity to pass through them, or conduct electricity under certain circumstances (unlike metal, which always conducts electricity, or glass, which is an insulator).
So fundamental are semiconductors to the world of technology today that Silicon Valley – America’s central tech hub – takes its name from the superstar semiconductor, silicon.
“A quantum dot is a semiconductor material that, because of its small size, has some very unique features,” says Dr Yang Bai, who helped develop the solar cell technology in Professor Wang’s laboratory.
These quantum dots are tiny artificial crystals that are so inconceivably small that they fall within the quantum range. At such small sizes, the quantum world almost seems magical – rules that apply to most matter in the universe don’t apply here. The normal properties of matter change, as well as how it interacts with the world.
At the most basic level, a quantum dot’s purpose is to absorb light. This causes the dot to change the light’s properties and emit it back out, making the dots glow (fluoresce) when hit by light.
In a solar cell, light is absorbed and turned chemically into electricity. One of the most useful properties of quantum dots in solar technology is they allow scientists to flexibly change the kind of light their solar materials will absorb.
“By changing the size of the quantum dots we make, we can finely tune the light absorption ranges,” Dr Bai says.
The sun produces light of all kinds of wavelengths, from the light of very high wavelengths that we can’t see (like UV light), to the mid-length waves of visible light, and below, to infrared light. Traditional materials used in solar cells can only capture light in one set range, because of their composition.
“The traditional materials have an intrinsic absorption property that cannot be easily tuned,” Dr Bai says.
By making quantum dots from different materials and of different sizes, researchers can capture a range of light types, making them more efficient.
“For example, a blue quantum dot solution has dots of a smaller size. With this we can only absorb light made of waves from 400–500 nanometres in length,” Dr Bai explains.
“If we slightly increase the particle size to use a green solution, this will absorb light up to 600 nanometres.
“We use a dark solution of larger dots in our solar cells so we can absorb lights of up to 800 nanometres.”
Making quantum dots of the right size and material to harness low light means could open up solar power possibilities in countries where the inclement weather normally causes solar cells to fail.
Quantum dots could also be engineered to harness power from artificial light sources – even from indoor lighting. Eventually, this could mean that a cell using quantum dots could charge your phone while it is sitting on an office desk.
It is the way quantum dots absorb and emit fluorescent light that make them useful in the world of lighting and display technology.
“On the market, you now see QLED TVs where the flat screen display is made of quantum dots,” Dr Bai says.
In a normal LED TV, a blue LED is wrapped in yellow phosphor to create a white light. This white light is then shone through red, green and blue filters to create the colours we see. But converting light to white then filtering it back to other colours is not an efficient process.
In a quantum dot LED, a plain blue LED illuminates quantum dots, some of which are tuned to give off red or green light. The glowing quantum dots augment the light shining through them, wasting less light through extra filtering and creating a more colourful picture.
Because quantum dots can be spread in thin, translucent sheets, they can also be used to make a more efficient kind of solar cell, known as a ‘tandem’ solar cell.
“Stack-by-stack, you can place one layer of quantum dots upon another layer of different quantum dots to make tandem solar cells,” Dr Bai says. “With a couple of layers stacked together, we can extend the light absorption range.”
Dr Bai says this translucence also means quantum dots can be easily coated onto windows.
“This means we can make a semi-transparent window that can also generate electricity at the same time,” he says.
Looking to solar generation on a large scale outside the lab, the researchers see more benefits to using quantum dots.
“The quantum dots can be synthesised in a simple solution process, which is very low cost. It is also an easy way to scale up the size of the solar technology,” Dr Bai says.
“When synthesising traditional solar cell materials, if we want to scale up the production of the material, we have to change the recipe. For quantum dots, however, we can maintain the optimal recipe by using continuous flow production processes to make as much as we need.
The crystallisation process is also very quick – only five or six seconds. Our method of making quantum dot film is also very compatible with industry-level manufacturing.”
Dr Bai says unlike traditional methods, quantum dots can be easily used on curved surfaces – massively widening their possibility for application.
“Most solar panels are made of silicon. The silicon wafer is relatively thick and brittle, so if you bend it, it can easily damage. But our small particle method, quantum dots, have relatively good mechanical strength. We also use an organic ligand substance on the surface, which makes it a bit sticky and improves the flexibility.”
The development of next generation solar power technology moved a step closer recently when Professor Wang’s lab set a new world record for the conversion of solar energy to electricity using quantum dots.
Professor Wang says the development represents a significant step towards making the technology commercially viable and supporting global renewable energy targets.
“The near 25 per cent improvement in efficiency we have achieved over the previous world record is important,” Professor Wang says.
“It is effectively the difference between quantum dot solar cell technology being an exciting ‘prospect’ and being commercially viable.
Eventually, it could play a major part in meeting the United Nations’ goal to increase the share of renewable energy in the global energy mix.”
Professor Wang and the team have recently been awarded funds from UniQuest, UQ’s main commercialisation company. They are using this award to further improve the efficiency and stability of their technology, as well as develop the first large-scale panels which will be needed for future industry applications.
The team is pushing our understanding of the future of energy: from their ‘artificial leaf’ project creating hydrogen to replace fossil fuels, to their prototype screen-printed, flexible batteries being rolled out for real-world trials this year.
Professor Lianzhou Wang is an Australian Research Council Laureate Fellow based in the School of Chemical Engineering at UQ’s Faculty of Engineering, Architecture, and Information Technology (EAIT) and Australian Institute for Bioengineering and Nanotechnology (AIBN).
The National Renewable Energy Laboratory (NREL) in the US recognised UQ’s world record for quantum dot solar cell efficiency, after verifying independent testing. Professor Wang’s team achieved 16.6 per cent efficiency – the previous world record in quantum dot solar cell category was 13.4 per cent.