Progress in making better or cheaper solar cells has become so rapid that developments are announced on a weekly basis. Nevertheless, it is not often that three teams independently report advances on the same day, as happened on Monday.
The overwhelming majority of solar panels in use are made from silicon. Producing the high-grade silicon necessary for this purpose is no longer at the prohibitive cost it was just 10 years ago, but it still places a limit on how cheap these sorts of cells can become, creating a push for alternatives.
Existing thin-film cells, however, have their own problems, using toxic heavy metals or rare elements that would be difficult to acquire in bulk. Consequently, a team at the European Union's Institute of Photonic Sciences are excited by the potential of the AgBiS2 nanocrystal cells they have made to capture the Sun's light.
Very thin silver-bismuth-sulfur cells provide an alternative option for the future of solar cells. ICFO
Silver, bismuth, and sulfur don't sound like a cheap combination, but compared to elements like tellurium used in some existing cells, they are dirt cheap and also non-toxic. "A very interesting feature of AgBiS2 solar cells is that they can be made in air at low temperatures using low-cost solution processing techniques without the need for the sophisticated and expensive equipment required to fabricate many other solar cells,” the Institute's Dr Nicky Miller said in a statement.
Miller is the author of the paper in Nature Photonics announcing the production of AgBiS2 cells with 6.3 percent efficiency. This is far below what more developed solar technologies are managing, but impressive as a first step, with team members announcing a target of 12 percent for the near future.
The major advantage of thin films like the ones Miller is working on is that using less material potentially means lower costs. However, others have spotted additional benefits. A team at the Gwangju Institute of Science and Technology, South Korea, are among those hoping to use the flexibility of some ultra-thin films to create wearable solar cells that could power electronic devices in remote locations.
"Our photovoltaic is about 1 micrometer thick," Professor Jongho Lee, co-author of the second study on solar cells, said in a statement. This is hundreds of times thinner than silicon cells and 2 to 4 times thinner than cells that are usually called thin. The team are so anxious to avoid unnecessary thickness that they cold weld their gallium arsenide cell onto a substrate rather than using an adhesive.
The result is a cell so flexible, it can wrap around a cylinder with a radius of 1.4 millimeters (0.06 inches). "The thinner cells are less fragile under bending, but perform similarly or even slightly better," said Lee.
In Applied Physics, Lee and co-authors announce that their flexible cells achieve up to 15.2 percent efficiency; still well short of the best roofing panels, but ideal for a backpack or tent.
The third announcement, in Nature Chemistry, concerns much earlier stage research. Dr Curtis Berlinguette of the University of British Columbia has uncovered the causes of inefficiency at semiconductor interfaces, showing that it is not just the distance electrons need to travel that matters. Instead, electron transfer is influenced by molecules that form a bridge between donors and acceptors.
"If electrons go in the wrong direction, we lose much of the Sun's energy as heat before it can be converted into electricity or fuel,” said Berlinguette in a statement. "Now we can design molecules to act as a gate and keep electrons moving forward in one direction and not reverse their direction."
The findings may one day improve the efficiency of solar cells built using a wide range of materials, edging ever closer to a world where sunlight becomes the primary source of electricity.
The construction of the molecule attached to the semiconductor determines how easily electrons are transmitted. UBC Chemistry
3