16 July 2007--Scientists in Korea and California have invented a new way of boosting the efficiency of cheap plastic solar cells, making them more competitive with traditional silicon solar cells. The key is to make the solar cells in pairs.
The tandem cells, as they are called, consist of two layers of different types of light-absorbing, electron-emitting plastics, and the combination converts a broader spectrum of light into energy than either could do alone.
The layers are linked by a metal that conducts the electrons from one cell to the other. In the past, metal links were problematic because they were only semitransparent--absorbing some of the photons passing from the front cell to the back cell, thereby reducing overall energy output and efficiency. But now a team of researchers from the University of California, Santa Barbara, and the Gwangju Institute of Science and Technology, in Korea, has found a way to build polymer tandem cells by laying down the metal links using electrically conducting polymer on top of transparent titanium oxide. The scientists, including Nobel Prize-winning chemist Alan J. Heeger, reported their discovery in the 13 July issue of Science .
Polymer-based, or organic, solar cells are different from the conventional solar cells you see at solar farms in the desert. Those cells are generally 180- to 240-micrometer-thick wafers of crystalline silicon that perform the light-to-energy conversion with about 15 percent efficiency. During the past 25 years, the trend has been to go thinner. Thin-film cells are made by depositing various photovoltaic materials such as amorphous silicon, inorganic compounds, polymers, or organic dyes onto durable, 100-nanometer-thick supporting substrates like plastic or glass. Their superthin profile means it takes less photovoltaic material to make a solar cell. Most thin-film cells are less efficient than those made from silicon wafers, but there are exceptions, such as cells based on gallium arsenide, which produce energy with 30 percent or more efficiency.
Solar-cell efficiency is the product of a couple of factors. One is the energy conversion efficiency of the circuit as a whole--which is basically how much of the light absorbed by the cell is converted into electrical power. That depends mostly on the intensity of light shining on the cell, as well as the amount of resistance in the electrical system connecting the cell. The second factor is the quantum efficiency of the material, or what percent of photons reaching the surface of the cell actually produce electrons. Certain materials have a greater quantum efficiency when exposed to some colors, or wavelengths of light, than others. To get the most out of the sun's rays, a solar cell needs to be sensitive to most of the wavelengths in sunlight. And while some materials are better at that than others, the higher-efficiency materials tend to be more expensive. Gallium arsenide prices have been reported to be as high as US $40 per square centimeter.
That's where tandem cells come in. By linking together two inexpensive cells, each preferring different wavelengths of light, the effect is similar to having one material that is highly sensitive to a broad range of wavelengths. The titanium oxide layer that links the cells allows electrons from one material to flow into the other and from there out of the cell. Importantly, the titanium oxide layer serves as a foundation during the construction of the tandem cell, providing the stability to build the back cell.
Traditionally, the interfacial metal link is produced using a microchip manufacturing process, thermal vapor deposition. In a vacuum chamber, a heated material evaporates and the evaporated material condenses on the surface of the solar cell. Unfortunately, the vapor-deposited layer was only semitransparent, absorbing photons and reducing overall energy output.
According to Kwanghee Lee, one of the leaders of the research team and coauthor of the Science paper, the breakthrough was being able to create the entire tandem cell, including the metal link, from solution. The solution-produced metal layer was less opaque than the one made using thermal deposition. What's more, solution processing, more akin to printing newspapers than making microchips, should lead to cheaper solar cells, Lee says.
Solar cells made using the new process yield a power-conversion efficiency greater than 6 percent. That may not seem like much, but it breaks polymer solar cells out of the doldrums of 5 percent efficiency, in which they've been stuck.
Lee says he expects the process will eventually lead to solar cells having three or more layers, and that three cells in tandem could yield an efficiency of nearly 10 percent. Lee's collaborator Heeger is the cofounder of Konarka Technologies , in Lowell, Mass., a well-funded start-up developing plastic solar cells.