ALD coatings for next generation solar cells
The 2022 Millennium Technology Prize has been awarded today, 25 October, to Scientia Professor Martin Green of UNSW Sydney, Australia, for his innovation that has transformed solar energy production.
Most commercial solar cells are silicon-based and use PERC (Passiveed Emitter and Rear Cell) technology, originally launched by Martin Green in 1983, a recent Millennium Award winner. However, more and more efficient, cheaper and more durable solar cells are being developed all over the world. Even in the case of silicon-based cells, there is a move to new technologies, including the tunnel oxide passivated contacts (TOPCon) concept, where multiple layers of silicon and oxides are added to the cell.
Transparent and flexible solar cells
In addition to silicon, other solar cell technologies are being investigated. The most promising new technology is based on the use of halide perovskites as a light-absorbing material. The general chemical formula for halide perovskites is ABX3, where A is an alkali metal or amine, B is tin or lead, and X is a halide. The most commonly studied compound is methylammonium lead iodide CH3NH3PbI3. Perovskite solar cells are on the verge of commercialization, and some manufacturers believe they will be mainstream in a few decades.
“Since these new types of solar cells can be transparent, they can be installed on windows, for example. They are also flexible, which increases their use,” says the university lecturer Marianna Kemellwho leads a research project funded by the Academy of Finland.
Although halide perovskite solar cells have achieved high efficiency, cell stability issues and the lack of industrial-scale production techniques have been bottlenecks preventing their widespread adoption.
Breakthrough with metal iodides
While completing his master’s degree in chemistry, Ph.D George Popov bravely chose halide perovskites and their atomic layer deposition (ALD) as the subject of his thesis. There were doubters, because there was little information based on previous research.
“We identified the appropriate chemicals and were able to design a reaction that allowed us to create a metal iodide coating for the first time through deposition. We were able to show that this can actually be done using atomic layer deposition. The first successful experiment was performed with lead iodide, which was then processed into CCH3NH3PbI3 perovskite through an additional reaction,” says Popov. “The research article was published in a refereed journal Chemistry of Materials scientific journal. Later we also developed ALD processes for cesium iodide and CsPbI3 perovskite.”
Coatings produced by atomic layer coating are used in approximately 30% of silicon-based solar panels. ALD group led by a professor Mikko Ritala The University of Helsinki has achieved promising results regarding the applicability of the technology to perovskite solar cells. The advantage of coatings made with atomic layer coating is that they form a uniform and comprehensive layer even on rough surfaces.
“If at some point we start manufacturing tandem solar cells that combine a silicon cell and a perovskite cell, we will know how to make that perovskite. We are developing the recipes and chemistry used to grow perovskite,” says Popov.
While basic research, recipe development and experiments are currently carried out on small areas, the technology is suitable for large-scale production.
“Current solar cell factories in China and elsewhere are able to adapt their equipment to produce ALD-coated solar cells,” says Popov.
The future of solar cells
More than 80% of solar cells are manufactured in China, where industrial-scale ALD devices are also manufactured. Wei-Min Li, PhD, an alumnus of the Department of Chemistry at the University of Helsinki, works as the CTO of Leadmicro, a leading Chinese ALD equipment manufacturer. This connection gives the department a solid understanding of where the field is going. ALD equipment used to manufacture silicon-based solar panels can also be expanded to manufacture next-generation solar cell materials.
“We are developing future technical solutions that will gradually replace and supplement current production. In the future, production will require fewer resources and, thanks to more efficient cells, also more surface area. When solar cells can be installed not only on flat surfaces but also on uneven surfaces, we no longer need to build solar parks on fields, because the fields are needed for other purposes,” says Popov.
However, Popov points out that we cannot afford to wait for new technical solutions, as the use of renewable energy sources must be increased now. When the current energy sources are replaced as much as possible with solar or wind power, the pressure increases and the entire field advances.
“The best part about silicon-based cells is that they last about 20-30 years and will continue to work after that, although possibly less efficiently. Since PERC solar cells are the current technology and available, you should get as many as possible. They pay for themselves back, university lecturer Kemell says.
The “Atomic Layer Deposition as a key enabler of scalable and stable perovskite solar cells” project funded by the Academy of Finland will continue until 2024. In addition to Marianna Kemelli and Georgi Popov, doctoral researchers participate in the project. Alexander Weiss and master’s student Mariia Terletskaia.