If solar cells were Olympic athletes, this is the point where there'd be demands for drug testing. Records are falling so rapidly that, in sport, we'd know competitors had to be doping. In fact, the scientists building the world's clean energy future are doing just that, but with exotic metals rather than illegal drugs.
Professor Kylie Catchpole of the Australian National University and her PhD student The Duong are the latest world beaters. They've produced a 26.4 percent efficient solar cell by placing a high-band gap semi-transparent perovskite onto an established silicon cell. The high-band gap means the perovskite captures the blue and violet light from sunlight, while letting through most of the reds and yellows to the silicon cell. Last year, a similar combination achieved 25.2 percent.
“Until now efficiencies of this kind have only been achieved using high cost materials normally used on satellites,” Duong said in a statement. “We are now a step closer to a low cost alternative.”
In Advanced Energy Materials, Catchpole and Duong report they achieved the mark despite using silicon cells that fall about 10 percent short of world's best.
Perovskite cells are based on calcium-titanium-oxygen crystals, but these can be doped with metals, and the combination of elements used shapes their behavior, including which wavelengths of light they convert. Catchpole told IFLScience the key to her latest work was adding rubidium. Although not widely used, rubidium is a relatively common metal in the Earth's crust – more so than copper – and the quantities required are small enough that Catchpole told IFLScience it “shouldn't be the limiting factor for cost.”
When solar cells were first invented, most were quite inefficient, as well as very costly. The exception were gallium arsenide cells, which turned more than 20 percent of sunlight into electricity, but were even more hideously expensive than their silicon equivalents. Over two decades, silicon cells have more than doubled in efficiency in the lab, laying the foundation for the solar revolution years later. Alternative cells also improved, but not enough to shake silicon's hold on 90 percent of the commercial solar market. Then, for a long time, efficiency records dried up.
Recently, however, perovskite cells have been breaking barriers at previously unimaginable rates. Perovskite cells have done in four years what silicon did in 20. Meanwhile combining the two, as Catchpole did, has created a whole new set of records to break, which has been happening every few months.
Catchpole does not expect this run to end anytime soon, telling IFLScience: “We can see a near-term path to 30 percent, and a longer term path to 35. After that we would need to do something different.” Although she said the cells' longevity still needs work, she anticipates commercial perovskite cells soon.
It doesn't look like much, but it could save the world