This article is extracted from ACAP 10 Years - Creating a Pipeline of Opportunities
Featuring: Professor Xiaojing Hao
ARC Future Fellow, FTSE, FAIP, School of Photovoltaic and Renewable Energy Engineering, UNSW
Perovskite and kesterite are rising stars in thin film solar cell development, but they’ve each posed significant challenges with stability and efficiency, respectively.
UNSW Professor Xiaojing Hao and her ACAP-supported team have made significant discoveries that are edging these exciting materials closer to commercial-competitive status.
Thin film solar cells can be utilised alone or in tandem with silicon wafer solar cells to achieve higher efficiency. They are potentially cheaper, lighter, flexible and more versatile. These features, together with their short energy pay-back time, means they are crucial to expanding the capacity of solar to meet the world’s energy needs. They have the potential to be ubiquitous and, as Hao says, “We have solar energy everywhere.”
When Xiaojing Hao was 11 years old and in grade 6 at primary school, her uncle gave her a solar-powered calculator from Taiwan. She was fascinated by its ability to draw power from sunlight and work without a battery and wanted to understand the theory behind it.
“I’ve always been very curious,” says Professor Hao, a Fellow of ATSE. “I’m also always trying to find a better way to do things. When I’m driving my daughter to school, I’m often thinking about a faster way to get there. In my research and daily life, I look for patterns to find a way to improve them, just like the patterns in the traffic.”
Professor Hao has devoted 10 highly successful years applying these qualities to the development of low-cost, high efficiency, thin film solar cells and tandem solar cells. Her achievements have garnered multiple prestigious awards including the Australian Academy of Science Pawsey Medal and the Prime Minister’s Prizes for Science; the Malcom MacIntosh Prize for Physical Scientist of the Year.
Increasing the efficiency of earth abundant, non-toxic kesterite solar cells
With ACAP funding, Hao and her team of researchers have achieved five world record efficiencies with high bandgap kesterite (CZTS), a potentially transformative semiconductor material for thin film, flexible solar cells and Si-based tandem cells, and one world record efficiency with low bandgap kesterite (CZTSe). The current highest efficiency for the former CZTS is 11.4% (UNSW) and that of the latter is 14.9% on lab scale cells, moving it ever closer to a stable, commercially acceptable efficiency.
With ACAP funding, Hao and her team of researchers have achieved five world record efficiencies with high bandgap kesterite (CZTS) ... and one world record efficiency with low bandgap kesterite (CZTSe).
The current commercialised high efficiency thin film PV technologies that can be applied to flexible surfaces are either expensive or have toxic components. Kesterite, on the other hand, is a copper zinc tin sulfide mineral (CZTS) whose elements are all earth abundant, and have proven long term stability. And very importantly, it’s non-toxic.
“We all want our children to grow up in a world where the climate is stable and energy is produced without pollution. We are working to accelerate the transition to 100% clean energy,” says Hao.
Kesterite’s theoretical efficiency is over 30% but progress in getting there has stagnated in recent years. Hao and her team have reset the path towards beyond 20% efficiency by changing the focus of their research to control defects and microscale inhomogeneity. The properties of the material can vary significantly from crystal-grain to crystal-grain and the grain boundaries can trap the photo-carriers.
In the past this has been ignored because it is hard to study and measure. But Hao’s team have found it to be a dominating defect through their developed performance diagnosis and optimisation platform via linked advanced characterisations and 3D device simulation.
She says, “We discovered that severe grain boundary recombination is the current limiting factor of these CZTSe solar cells.”
Nonradiative recombination is a process where electrons and holes, that a solar cell needs to separate and send to opposite contacts, recombine, releasing energy in the form of heat, and thereby reducing the energy conversion efficiency of solar cells. Recombination often occurs at grain boundaries, where charged carriers are trapped due to defects and imperfections. Additionally, microscale inhomogeneity within the material further complicates the situation as the defects may vary. To tackle these micro-scale challenges effectively, a fundamental prerequisite is a comprehensive understanding of them.
Hao says, “The results of this work have cleared the mist in the path to >20% efficiency kesterite.”
Improving the stability of perovskite solar cells
Perovskite has shown high efficiencies of 26.1%, but one of the critical issues limiting the development of perovskite solar cells (PSC) is instability. Hao and her team came late to working with perovskites, but they’ve applied the strategies and knowledge they’ve accumulated improving the efficiency of kesterite solar cells.
After just three years of work, they’ve achieved PSC stability rates that are among the best in the world.
Despite being heavily used in world record efficiency perovskite solar cells, the hole transport layer material (HTM) Spiro, usually requires a lithium-containing dopant to improve its electrical properties. Unfortunately, the mobile lithium ions decrease the cell stability. Hao’s team were aware that sulphur and lithium coordination made stable compounds, so they developed a doping method that uses a cheap sulphur-based additive that ‘locks-in’ the lithium in the hole transport layer, improving Spiro’s conductivity and achieving the high efficiencies.
Happily, they then discovered that the addition of sulfur containing material DDT also makes the dopant an oxidation agent with a quick, controllable oxidation process and the potential to speed up production of perovskite cells.
Additionally, the addition of DDT makes the hole transport layer repel water and prevent water ingress. These are all important for cell stability.
Hao and her team have shown that perovskite cells fabricated with the novel additive were able to maintain more than 90% of the initial efficiency after operating at maximum power point under one sun illumination for 1,000 hours.
Further, they maintained more than 93% of the initial efficiency after staying at open circuit condition under one sun illumination for 2,000 hours. This reported stability result is the best in Australia and among the highest in the world. The UNSW team have a provisional patent in place for the DDT additive.
ACAP’s role retaining valuable skills in solar cell development, and in Australia
Professor Hao says she has outstanding ACAP fellows in her team working on kesterite and perovskite solar cell and tandem cells development and stresses the importance of ACAP funding for retaining valuable researchers.
“ACAP funding holds particular importance for Australia as it plays a vital role in retaining local PV skills and talents," says Hao.
"Without adequate funding to secure our early career researchers, we risk losing them to overseas opportunities or even entirely different fields at a rapid pace.”
“ACAP also provided grants that enabled us to delve into new ideas and kickstart fresh research partnerships, especially on an international scale, where a multitude of diverse perspectives and competencies converge to tackle complex research problems."
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