Thin cells for solar energy

A new type of very thin solar cell made from inexpensive materials has been invented by researchers at the Hahn Meitner Institute in Berlin, Germany, in collaboration with a colleague now at Portland State University, Oregon, USA. The new device will be much cheaper to make because it uses less expensive semiconductor materials than conventional solar cells. The researchers publish details of their invention in the Institute of Physics journal Semiconductor Science & Technology on 14 April 2003.

German physicists Rolf Koenenkamp, now at Portland State University, and former colleagues Katja Ernst and Abdelhak Belaidi have used a layer of titanium dioxide that is full of tiny pores to make a much more efficient device for harvesting the sun’s energy.

They formed an extremely thin layer of the light-sensitive cadmium telluride (CdTe) which forms the material for the solar cell on top of the porous titanium dioxide, which was itself supported on a sheet of glass. They connected electrical contacts to the back of this sandwich. When sunlight hits the cadmium telluride layer, the energy is converted into an electrical current which is tapped off via the electrical contacts. Any stray light bounces around inside the tiny pores in the titanium dioxide layer and is scattered back into the cadmium telluride layer, making the device more efficient. This boosts efficiency by a factor of fifty over a similar cell based on a non-porous support material.

The prototype solar cell produces a voltage of 0.67 V and a current of 8.9 milliamps per square centimetre when illuminated with just 100 milliwatts per square centimetre of light typical for a sunny day. The researchers also found that if they alloyed the CdTe with mercury they could boost the current to 15 milliamps. “Solar cells typically produce between 0.5 and 1 V voltage,” explains Koenenkamp, “In applications, several cells are connected in series to provide higher voltage as needed.”

There are a few wrinkles yet to be smoothed out in the prototype solar cell, but its properties offer the promise of renewable energy at much lower cost than current solid state devices. While the present device still uses hazardous or expensive materials, such as gold, cadmium and mercury in small amounts, the simple design principle could be used for other cell types, such as silicon and compound solar cells, says Koenenkamp.