ON THE desk of Chris Case, chief technology officer of Oxford Photovoltaics, there sits a small but heavy vial filled with a canary-yellow liquid. “That’s enough for a kilowatt,” he says. The material in the vial is called methylammonium lead iodide, and enthusiasts such as Dr Case believe it, and materials like it—known collectively as perovskites—could lead to a dramatic increase in the world’s use of power from the sun.

Oxford Photovoltaics is one of many firms, both small and large, that see promise in perovskites. These are compounds that share a crystal structure and are named, collectively, after the mineral that was the first substance found to have this structure. Often, they are semiconductors. This means that, like the most famous semiconductor of all, silicon, they can be used in solar cells.

The first perovskite solar cells were made in 2009. They converted 3.8% of the light falling on them into electricity. Now, the best hoover up around 20%. This rate of conversion is similar to the performance of commercial silicon cells, and researchers are confident they can push it to 25% in the next few years.

Moreover, unlike silicon, perovskites are cheap to turn into cells. To make a silicon cell, you have to slice a 200-micron-thick wafer from a solid block of the element. A perovskite cell can be made by mixing some chemical solutions and pouring the result onto a suitable backing, or by vaporising precursor molecules and letting them condense onto such a backing. If these processes can be commercialised, silicon solar cells will have a serious rival.

Light work

Solar cells, perovskite ones included, all function in broadly the same way. When light hits a crystal of the material they are made from, it frees electrons (which are negatively charged) and leaves behind what are, in effect, positively charged holes in the crystal lattice. This formation of electron-hole pairs is characteristic of semiconductors exposed to light. Neighbouring materials then capture the positive and negative charges and transport them to electrode layers on the cell’s outer faces, where they generate a current.

This general theme, though, is capable of variation. Last year, for example, Michael Grätzel of the Swiss Federal Institute of Technology, in Lausanne, devised a cell in which the perovskites were infused into the electron-capturing material, rather than being a separate layer. That design, he thinks, will make cells cheaper to manufacture, and more stable. Dyesol, an Australian firm with which Dr Grätzel is collaborating, agrees. It is building a factory in Turkey, planned to open in 2017, to manufacture solar cells that are based on the Grätzel infusion principle.

But the variations on the photovoltaic theme that most excite researchers at the moment are tandem cells, which have layers of both perovskite and silicon in them. These will permit more of the spectrum to be converted into electricity. Perovskites can be made to many different formulae, which means they can be tuned to absorb different parts of the spectrum. The top layer of a tandem cell is a perovskite that has been tweaked to absorb strongly at the blue end of the spectrum. Beneath it is a layer of silicon, which mops up the red.

The first such tandem was unveiled in March by researchers from Stanford University and the Massachusetts Institute of Technology. It had an efficiency of 13.7%. This week, Oxford Photovoltaics showed off one that has an efficiency of 20%. It hopes to see its first commercial tandems roll off production lines in 2017. This marriage of convenience between the old and new ways of doing photovoltaics may not, however, last long. Henry Snaith, Oxford Photovoltaics’ founder, looks forward to all-perovskite tandems that have cells of different composition, each tuned to harvest a particular part of the solar spectrum.

The main obstacle to the march of perovskites is water: they decompose in it. Perovskite solar panels must thus be totally watertight. But technology exists to make effective seals on solar cells. The standard tests for cells, including those for watertightness, are set by a body called the International Electrotechnical Commission. One of these tests requires that cells sustain their performance for more than 1,000 hours at 85°C and 85% humidity. Others put cells through drastic temperature swings, artificial hailstorms and so on. Dr Snaith says that Oxford Photovoltaics’ cells have passed the 1,000 hour test and are well on the way to making 2,000 hours.

Another way around the problem of a potentially limited lifetime is to find applications where it does not matter. In these, perovskites should do well. Some firms, for example, hope to enter the mobile-device market—reasoning that such devices are usually replaced by their owners every few years and so do not require a long-life cell. Saule Technologies, in Poland, and VTT, in Finland, are experimenting with flexible perovskite cells intended to charge mobiles. Olga Malinkiewicz, Saule’s founder, says that her company has made prototype flexible cells which are 3% efficient, and she thinks its engineers can get to 10% in the next two years. When Saule’s cells are commercialised, she plans to make them using inkjet printers that spray perovskite precursors onto adhesive backings. This will permit the cells to be stuck onto any device that needs power.

Oxford Photovoltaics and Dyesol, though, are looking at larger-scale, longer-lasting products. They hope to glaze buildings with panes covered in semi-transparent perovskite cells, allowing such edifices to declare, if not independence then at least autonomy from the power grid. The ultimate dream, though, would be to feed that grid itself with power from perovskite cells. At the moment, solar energy contributes a little over 1% to the world’s electricity production. Silicon cells are teetering on the brink of being properly competitive with fossil fuels, but a new way of doing things might push costs down dramatically. Perovskites could be that innovation.