Photoelectric cells.

To dye for.

A new type of cell may bring "solar" energy indoors.

THE phrase "indoor solar power" sounds like an oxymoron. But there is growing interest in the idea of using photoelectric cells to run gadgets as well as power grids-and doing so even when those gadgets are inside buildings. Much of the light these cells used would, of necessity, come from incandescent bulbs, fluorescent tubes and light-emitting diodes (LEDs) rather than through the window from the sun. But if the right sorts of cells were available this could be cheaper than constantly replacing the batteries that currently power electronic gizmos.

On April 8th G24 Innovations, a firm based in Wales, announced that it may have come up with just such a cell. The latest version of its special, dye-based photoelectric devices has set a new record for the conversion of light from bulbs into electricity: an efficiency of 26%, compared with the 15% which previous ones can manage. That lifts dye-based cells to the point where they might be widely deployable for indoor power.

Dye-based cells are similar to the silicon-based variety found on rooftops around the world in that both rely on a semiconductor to assist the conversion of luminous energy into the electrical sort. The difference is that in the case of silicon cells, this conversion happens directly. That means the frequency of light absorbed is constrained by the physical properties of silicon itself.

In the case of dye-based cells, which were invented at the Federal Polytechnic School of Lausanne, in Switzerland, in the 1990s, the light is first captured by molecules of a photosensitive dye. Tinker with the composition of this dye and you change the frequency of light that can be captured. This makes dye-based cells more flexible than silicon ones.

The dye molecules themselves are bound to tiny particles of titanium dioxide, a less-famous (but cheaper) semiconductor than silicon, and the whole assembly is immersed in an electrolyte and sandwiched between two electrodes. When a photon of light is absorbed by a dye molecule, an electron is knocked into the titanium dioxide. From there it travels to one of the electrodes and a current is thus generated.

The flexibility brought by the dye-based approach makes this sort of cell particularly useful indoors. Silicon-based solar cells have been optimised for sunlight. But artificial light, whether of the incandescent, fluorescent or LED variety, contains a different mixture of frequencies from that put out by the sun. By changing the composition of the dye, G24's engineers ensure that the maximum sensitivity of the cell coincides with whatever frequency mix is appropriate for the artificial light concerned.

Besides being tweakable to match the spectrum of a light-bulb, dye-based cells also work well in dim or diffuse light of the sort often found indoors. Silicon-based systems do not. And dye-based cells, having no rigid parts, can bend, and are mechanically robust compared with the silicon sort. That gives them a further advantage over silicon cells, especially for powering consumer gadgets.

Steven Burt, G24's chief financial officer, talks of light-bulb-powered TV remote controls, smoke detectors and computer keyboards. (A non-removable internal battery would store charge for use when the lights are off.) The company already offers prototypes of bags and jackets with photosensitive panels woven in, designed to charge digital cameras and mobile phones, and a hotel in Las Vegas is using G24's products to run its electric window-blinds. Mr Burt also sees a market for powering the networks of sensors needed to monitor things like temperature and humidity in modern "smart" buildings.

Eventually, says Mr Burt, the ability of dye-based cells to produce useful quantities of power even in dim and diffuse light could see them used outside, perhaps on rooftops in cloudier parts of the world-a market at present dominated by traditional, silicon-based cells, even though they are not well-suited to the purpose. But for now, G24's factory in Cardiff, not a city known for its sunshine, remains powered by a wind turbine.