A new type of solar cell has overcome the theoretical efficiency barrier of silicon-based cells and allows us to get more energy from the sun's rays.
Almost all commercial solar cells are made of silicon. It can only convert a narrow frequency band of sunlight into electricity. Light that is too far from this range will pass through or be lost as heat, giving silicon cells a theoretical efficiency limit of about 29.4 percent.
Theoretically, this limit could be higher if other materials that generate electricity from light in a different frequency range were placed on top of the silicon layer. Perovskite, titanium, and calcium crystals work well because they better absorb light in the near-infrared spectrum. However, this has proven difficult to do effectively. This is because the unpredictable electrons are absorbed back into the crystal before being converted into electricity.
Now two research groups have found a way to combine perovskite with silicon and thus achieve higher efficiency.
To join silicon and perovskite, Xin-Yu Ching at the Swiss Federal Institute of Technology in Lausanne and colleagues used a two-step process. The silicon cell is first coated with a compact layer of precursor chemicals, and then a second layer of chemicals is added that reacts with the precursors to form perovskites. This process creates fewer defects at the silicon-perovskite interface, Chin said, increasing the number of electrons available for current flow. The team's device has an efficiency of 31.2 percent.
In a separate study, Silvia Mariotti of the Helmholtz-Zentrum Berlin and colleagues injected liquid piperazinium iodide into a perovskite layer, which also reduced unwanted electrons and achieved an efficiency of 32.5%.
"The performance is phenomenal," said Kyle Frona of the University of Cambridge. However, these numbers are currently limited to solar cell sizes much smaller than those needed for commercial use, he said.
In May, solar energy company Oxford PV demonstrated that perovskite silicon tandem cells could be produced commercially, albeit with a slightly lower efficiency of 28 percent.
"If we can do it at scale, which some companies seem to be able to do, that's great," says Frona. "The only caveat is that we want to make sure they are strong enough to last for a long period of time."
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