A team of scientists has invented a new type of solar cell that converts both the sun's heat and light into electricity, potentially giving a boost to the efficiency of solar energy harvesting devices. The cell combines a photovoltaic process that turns light into electricity with another that converts heat; combined, they beat the current record for solar energy efficiency, as well as the theoretical efficiency limit of a cell of this design.

The most popular type of solar generator in use today is the photovoltaic cell. Photovoltaic cells operate by taking in solar photons of certain energies and using them to excite electrons into racing to conductors, creating current. While photovoltaic cells have been an important step towards harnessing the sun's energy, they are fairly inefficient and leave a lot of room for improvement.

The average cell can harvest only about 20 percent of the solar energy that lands on it, and the fanciest of photovoltaic cells can't capture much more than 40 percent. This isn't the fault of the sun, as there is a phenomenal amount of energy beating down on the earth all the time. That leaves a lot of photons ripe for plucking.

The problem with photovoltaic cells is how picky they are about which photons they will work with. Photovoltaic cells use light that's typically limited to the visible spectrum, some of which is reflected, while any photons that are not energetic enough are lost as heat. A group of researchers realized that they might be able to harvest the wasted heat photons and put them to work in the cell, if they could find a suitable way to rework the design.

They noticed that there was another mechanism, called a thermionic energy converter (TEC), that can be used to turn heat energy into electricity. The converters work by sending electrons from a hot cathode to a cooler anode (these were developed under the aegis of NASA and the Soviet space program).

TECs aren't used very frequently, though. They require the cathode to be quite hot, a state that isn't easily achieved naturally. But the researchers realized that this type of converter would thrive sitting in direct sunlight. Once modified, it could be used in tandem with the photovoltaic process, putting all that normally wasted heat energy to use.

The new device the researchers came up with combines photovoltaics and TECs into a process called photon-enhanced thermionic emission (PETE). The PETE process uses an anode and cathode like a TEC, but the cathode is a semiconductor rather than a metal plate. In the PETE cell, light energy gets photons only halfway to their destination by exciting them to the surface of the cathode. Once there, they build even more energy by absorbing incoming heat.

The heat distributes evenly along the cathode and, as the electrons become more and more excited, they are propelled off the cathode and travel through a vacuum toward the anode, creating a usable electric current. By requiring the system's electrons to use both kinds of energy to reach their final destination, researchers found that much less energy was wasted.

If the sun's energy was concentrated 100 times, researchers found the PETE process had an efficiency of 32 percent, well above the standard photovoltaic average of about 20 percent. Concentrating the sunlight to 3,000 times its normal intensity, the cells had an efficiency of 47 percent, well above the Shockley-Quiesser limit for an ideal single-junction solar cell, which is 33.7 percent.

Still, there are some downsides to the device. For example, its highest efficiencies were reached at temperatures of 800 to 1000 degrees Celsius. You can get there with sunlight concentration, but probably only in the hottest, driest parts of the world—otherwise, you need to use an external engine to maintain enough heat in the cathode. Likewise, directing the equivalent of 3,000 suns at the cell would require a pretty sizable, specially shaped mirror to maintain a high enough temperature.

PETE sets no absolute records—multiple junction cells still win out, efficiency-wise—but the authors speculate that even less-optimized versions could outstrip many of the photovoltaic or thermal systems currently in use. Likewise, they note that, if more modern materials were used, such as plasmonic devices and nanostructures, the efficiency could be pushed even higher, potentially putting the cells on the level of their beefier multiple junction counterparts.

Nature Materials, 2010. DOI: 10.1038/NMAT2814 (About DOIs).