Microwaves are advancing the production and recycling of solar cells
(Nanowerk News) Microwave technology invented at Macquarie University will improve the manufacture of solar cells and make them easier to recycle.
During the manufacture of solar panels, silicon goes through several high temperature processes known as annealing. At this time the cell is cooked in the oven.
But in a paper published in Applied Physics Letters (“Microwave annealed silicon solar cells”), a team led by senior lecturer Dr Binesh Puthen Veettil from the School of Engineering has shown that heating using microwave radiation is almost as efficient. Plus, it saves a lot of time and energy and has other advantages.
Because microwave radiation selectively heats silicon, it causes an almost instantaneous effect with enormous energy savings. This is partly because the rest of the laminated panels of glass, plastic, and aluminum have been largely unaffected. And properties that have generated windfall recycling benefits for which the group’s patents are still pending.
Under microwave treatment, the plastic coating (ethylene vinyl acetate) that protects the silicone plate from moisture and contamination softens until it can be mechanically peeled off. That means plates can be easily delaminated and components reused without the use of harsh chemicals.
“Until now it made economic sense to dispose of the panels in landfill,” says Dr Veettil. “In the rare case of recycling, you crush the panels, heat them to about 1400°C and wash them with chemicals to remove the plastic — a process that is very energy intensive. But now, as the solar panels that began to be installed in large quantities about 20-30 years ago are reaching the end of their useful life and being decommissioned, the government is demanding that they be recycled.”
Microwave annealing has several other advantages. The ability to focus microwave radiation means that the heating it generates can be selective and highly tuned. Some newer panels, for example, use what is known as heterojunction technology, in which crystalline and amorphous silicon is interleaved. In these cells, more rapid and directional annealing is advantageous.
Precise focusing also means that the anneal can be directed to a specific part of the solar panel, making it ideal for annealed solar panels with more complex internal structures made for special purposes.
And, unlike an oven where all kinds of chemicals are released from the walls, microwave annealing is done in a clean environment. “So there’s less contamination,” says Dr Veettil. “And the whole process can all be done at room temperature.”
There are several other projects involving solar cells and sustainable energy ongoing at Macquarie. One of the co-authors of the annealing paper, Associate Professor Shujuan Huang, led the group observing microwave annealing in perovskite solar cells. Perovskite is a group of crystalline minerals with semiconducting properties that may in the future be used for solar cells because they are flexible, lightweight, and inexpensive to manufacture.
In this case, microwave radiation produces a more efficient solar cell than conventional annealing methods, but the reasons for this are unclear. Work is currently underway in part to answer that question.