This is an extract from ACAP 10 Years - Creating a Pipeline of Opportunities
Professor Daniel Macdonald co-leads the silicon solar research at ANU as well as the University’s engagement with ACAP. He says ACAP’s sustained funding support has enabled critical planning, investment, coordination and collaboration amongst silicon PV researchers.
“The real advantage of ACAP comes from the fact that it enables collaboration and provides continuity. During ACAP1.0 we built up capacity in terms of skills, people and equipment,” says the world-leading expert in silicon materials and solar cells.
“Now we’ve got that critical mass, we’re ready to go and start making the most of ACAP2.0 from day one.”
The silicon solar cell research team at ANU consists of 25 researchers, including academics, post-doctoral fellows, PhD students and technical staff. Support through ACAP1.0 has enabled ANU to build and maintain critical mass in several key areas of silicon research. These include silicon materials, device modelling and characterisation, new materials and architectures for high efficiency solar cells, and transfer of technology to industry.
One of Macdonald’s major contributions to solar PV development has been demonstrating that many metal impurities are much more detrimental in p-type silicon (used in the mainstream PERC cell technology) than n-type silicon, because of their charge state. This is one of the reasons why n-type cells (TOPCon and heterojunction) are now emerging as the dominant technology in industry.
His team at ANU showed that trace quantities of iron impurities limit the electronic quality of Ga-doped p-type Cz wafers, which are used in industrial PERC cells. Fortunately, these iron impurities move to the surface of the wafer during high temperature cell process steps, where they are trapped by heavily doped layers, rendering them inactive.
“I’m proud to think that through basic scientific discovery, we were able to have an impact on the direction of the industry,” says Dan Macdonald.
This beneficial ‘gettering’ effect is also present in TOPCon cells fabricated with n-type Cz wafers, in which iron has a reduced, but still important, impact. The presence of this iron means that a special pre-gettering step is required for the fabrication of heterojunction cells, which do not have their own high temperature processing steps.
Dr Anyao Liu from the ANU silicon group won the 2022 Ulrich Gösele Young Scientist Award for her pioneering work on impurity gettering. They are also working to better understand the formation and impact of so-called ‘ring’ defects in Cz wafers, which are often observed during cell processing, and can be highly detrimental to cell performance.
These defects are caused by oxide precipitates, but the conditions required to trigger their formation remain poorly understood. The silicon team at ANU also has a strong presence in device modelling and characterisation. They use 3-dimensional simulations to identify the primary sources of power losses in solar cells. Combined with advanced luminescence-based characterisation methods, some of which were developed jointly between the ANU and UNSW teams, they can accurately trouble-shoot cell fabrication processes.
In the first round of ACAP Infrastructure funding, ANU installed an advanced optical characterisation cluster, expanding the ability to study the many structural, morphological, and optical properties of solar cells to understand the causes of losses in efficiency. ANU’s silicon group also explore new materials and architectures to further improve the efficiency of silicon solar cells.
With the second round of funding ANU was able to purchase a cluster of next generation processing tools that enable rapid development and testing of new materials and designs for solar cells. Researchers can create thin films of materials with precise control over their thickness and composition. The facilities are available for use by other ACAP nodes.
“Our ultimate goal here is to discover the ideal passivating contact structure, which simultaneously allows outstanding surface passivation, electrical contact, and optical transparency – a combination which is very challenging to achieve in practice,” says Macdonald.
“That’s a long term quest, a five-to-eight year mission. And under ACAP, we can go ahead and get started. Whereas, if we were funding it through smaller projects, it stops and starts, staff move on, and it would be a lot more disjointed.”
Most of this work is focused on doped polysilicon films formed at high temperatures, and doped metal-oxides such as titania and copper oxides formed at low temperatures. Macdonald says they apply these new materials to small-area prototype solar cells made in the laboratories at ANU.
The ANU silicon team also works closely with industry to bring new solar cell technology to mass production.
ANU have a longstanding partnership with Jinko Solar and contributed to the commercialisation of n-type TOPCon cells and modules, leading to multiple world record efficiencies for fullsize cells using this technology, including 24.8% in 2020, 25.4% in 2021, and 26.4% in 2022.
Macdonald sums up, “ACAP’s systemic funding to several different nodes has helped us work together.
"ACAP’s research plan means all the important parts are being tackled and brought together through collaborations between the partners in a productive way.”
“It really helped to strengthen our collaboration with UNSW,” says MacDonald. “We are now more visible on the global stage, and we’re more attractive to industry partners to work with.”
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