Solar cells that work in the dark.

Solar cells that work in the dark

Immerse a sliver of semiconducting material in an electrically conducting chemical soup and the result is a photoelectrochemical cell capable of converting sunlight directly into electrical or chemical energy. Recently, a group of researchers in Israel constructed a single device that combines photoelectrochemical conversion and electrochemical storage. The final product is a solar cell that includes the equivalent of a built-in storage battery. The storage system allows the cell to draw on energy stored during daylight hours so that it continues to generate electricity even at night.

In the cell, developed by Stuart Licht and his colleagues at the Weizmann Institute of Science in Rehovot, the light-absorbing electrode is a single crystal of the semiconductor cadmium selenide telluride. This light-sensitive electrode and its companion counter-electrode are immersed in an aqueous polysulfide solution. A permeable membrane separates the photoelectrochemical cell from the storage part of the device, which consists of a tin-sulfide electrode dipped in an alkaline sulfide solution.

Light shining on the cadmium selenide telluride electrode starts a chain of events that forces electrons to flow in a wire joining the photoelectrochemical cell's two electrodes. The photoelectrochemical half of the device produces more than a volt of electrical potential at a respectable solar conversion efficiency of 11.8 percent. At the same time, part of the generated current is used to convert tin ions into tin metal in the storage half of the device. In darkness or below a certain level of light, the storage unit delivers power by converting tin back into tin ions. The net result is that the cell continues to work at an overall efficiency of 11.3 percent regardless of the light level.

"It's a wonderful system in its simplicity,' says Licht, who is presently at MIT. "There's no electronic switching. There's no computer control. It's just a chemical system that stores energy and spontaneously releases it when it's needed.' Licht provides a detailed description of the chemistry involved in this and similar photoelectrochemical cells in the Nov. 12 NATURE. "This is the first chemical description of how these cells work,' he says.

Photoelectrochemical cells are potentially more versatile than solid-state photovoltaic devices because they can generate fuels and other useful chemical products in addition to producing electricity. "These are still in the future,' says Licht, "but they're being worked on.'