The next generation of solar cells will be made from common, abundant materials – and devised by Australian researchers.
I was probably in year seven or eight when I realised I didn’t want a routine job – and that I wanted to make a difference through my work. It was during my Masters I realised I can happily stay in front of the computer until midnight if I’m working on some interesting modelling, or going through coding to make it run faster.
My role now is certainly not routine, and I definitely hope my work can make a difference! My title is the professor and head of the New Photovoltaic Materials and Devices Research Group at the School of Photovoltaic and Renewable Energy Engineering at the University of NSW.
The challenge for us now in photovoltaics (PV) is getting more electricity out of our solar cells while using materials that are cost-effective, abundant, and made from non or low-toxicity materials.
About 95% of the PV market now is based on silicon. Many researchers believe silicon will continue its dominance in the PV market in next 10 years. Our “next big thing” is reducing the cost of solar cells even further. It may be realised by stacking thin-film solar cells made by other common raw materials on silicon to extract more electricity out of sunlight.
While silicon is an almost ideal material for photovoltaics, there is a need for similarly environmentally benign thin-film materials that can be used in applications not well suited to silicon solar cells, such as for coatings on buildings or integrated onto the bodywork of solar vehicles. Such cells can also be stacked on the top of standard silicon cells to boost their power output.
For a while now it’s been understood that kesterites are one of the ideal options as the top cell or beyond silicon-based technologies which can directly coat other materials, like glass or steel. Kesterites are a family of materials that are very stable and based on elements that are common and abundant, like copper, zinc, tin, sulphur and selenium, which can be heated to create a crystalline film. All these elements are relatively cheap to extract, and readily available. They work like silicon, but you use much less. For example, the silicon needed in a solar cell is 300 or 400 micrometres thick, but you only need less than one micron of a kesterite, and it still has a quite strong light absorption coefficient.
The problem we’ve been stuck on is trying to improve the efficiency of the kesterite for a solar cell. For a long time, we haven’t been able to improve it higher than 13%.
For the last decade I have wrestled with this problem – and at some stages even considered giving up. Many people have tried different methods to solve the issue, but we still couldn’t get a big jump in efficiency past that 15% to make it economical to produce.
Then a few years ago I realised that we hadn’t really looked at something called the grain boundary, which is how the kesterites are put down in a thin layer of crystalline material. I had the idea that we needed to find a way to look at it in micro-scale to define the problem first, which was a critical first step, but we didn’t have that tool here at the University of NSW. So I talked to the director of the Electron Microscopy Unit at UNSW, and Professor Richard Tilley became excited about my idea, so we worked together to secure funding to buy the machine and install it. Finally, we could see the image of kesterites at nanometre scale, and we discovered where the major problem is.
It’s very satisfying to come up with a new idea that can solve a scientific problem. It gets quite technical to explain, but once we could see this grain boundary, we were able to explore solutions to solve this problem– and that has meant we are able to integrate a new approach with our previous strategies, and push the efficiency to beyond 20 percent.
It’s like being a detective. When a problem arises you want to find the truth. But first you have to see all the different pieces, and put them together to understand the whole picture. We should look at the whole picture as much as possible instead of only seeing part of it. Only in that way can we truly understand the problem.
The “next big thing” is extracting more electricity from the same area of solar cell. We think the answer to that is tandem solar cells – having another layer of high bandgap solar cells on bottom low bandgap cells such as silicon cells.
The sunlight which reaches the Earth consists of high-energy and lower-energy photons. We know that different materials are better at converting these different energies, so if you have one layer which most efficiently converts the high-energy photons, and then the second layer converts the lower-energy photons, you extract more electricity from the same area.
We are working on different top cell options, and the chalcogendie including kesterite is one of the major groups showing great promise.
If we want to reduce carbon emissions and slow the rate of global warming, the future of our energy sources in heavy industries like mining and agriculture is going to be pumped by solar energy, producing electricity and fuel like green hydrogen or ammonia (as a hydrogen carrier). To create this fuel, we can use electricity generated from the sunlight – and a lot of it, so improving the efficiency of solar cells is very important. To achieve this, solar power is our superpower.
Professor Xiaojing Hao is an Australian Research Council Future Fellow, Fellow of AIP, and Head of New PV Materials & Devices Research Group at the School of Photovoltaic and Renewable Energy Engineering, Australian Centre for Advanced Photovoltaics at the University of NSW.