Australian researchers have developed a new microwave technology that could boost the production of solar cells by improving manufacturing processes and making them easier to recycle.
During the manufacture of solar panels, silicon undergoes several high-temperature processes known as annealing. At this time, the cells are cooked in the oven. But in a fresh paper Applied Physics Lettersa team led by senior lecturer Binesh P Veettil show that heating with a microwave oven is almost as efficient, but at the same time saves considerable time and energy and offers other advantages.
The microwave radiation selectively heats the silicon, leading to almost immediate effects and huge energy savings, the researchers said.
“Microwave annealing of semiconductor devices has not been extensively studied and is rarely used in industry, but it has the potential to significantly reduce the time and cost associated with high-volume processing of semiconductors, such as various heating and annealing processes for the manufacture of photovoltaic modules,” the researchers wrote.
The article describes microwave annealing of silicon solar cells, effective passivation of light-induced defects and reduction of light-induced degradation. The researchers found that silicon solar cells heat up quickly in a microwave field and that efficient passivation of boron-oxygen defects can be achieved by microwave treatment in less than two seconds. They found that microwave annealing produces similar results to rapid thermal annealing.
The use of microwave radiation leaves the glass, plastic and aluminum laminated panel largely intact. This asset has led to an unexpected recycling advantage for which the group has a patent pending.
Microwave treatment softens the plastic coating (ethylene vinyl acetate) that protects the silicone sheet from moisture and dirt, which enables its mechanical peeling. The plate can therefore be easily removed and reused without strong chemicals.
“Until now, it made economic sense to just put the panels in the landfill,” Veettil said. “In the rare cases when they are recycled, the panels are crushed, heated to around 1400C and washed with chemicals to remove the plastic – a very energy intensive process. But now, as solar panels, which began to be installed in huge numbers about 20-30 years ago, are reaching the end of their useful life and being retired, governments are demanding their recycling.
Other advantages offered by microwave annealing include the ability to focus microwave radiation. The heating it causes can be selective and highly tuned. The researchers note, for example, that more directional annealing is highly advantageous in newer panels that use heterojunction technology, where crystalline and amorphous silicon are interleaved.
“Precise focusing also means that the annealing can be directed to specific parts of the solar panel, making it ideal for annealing solar panels with more complex internal structures that are made for special purposes,” the team adds.
Unlike an oven, where all kinds of chemical substances come off the walls, microwave annealing takes place in a clean environment. “So there’s less pollution,” Veettil explained. “And the whole process can be carried out at room temperature.”
Macquarie has several other solar and sustainable energy projects underway. One of the authors of the annealing paper, Shujuan Huang, leads a group studying microwave annealing in perovskite solar cells. In this case, microwave radiation produced more efficient solar cells compared to traditional annealing methods, but the reason is not clear. Scientists are currently looking to solve this mystery.
Veettili’s research in collaboration with the University of New South Wales School of Photovoltaics in Sydney was initiated with funding from the Australian Center for Advanced Photovoltaics and support from the Australian Government through the Australian Renewable Energy Agency.
