"The basic problem is that when you put solar cells in sunlight, the efficiency starts to decrease by as much as 15 percent to 20 percent over a period of several days," said Rana Biswas, a physicist at Ames Laboratory and the MRC. "Obviously, that's not good."
Solar cells made from hydrogenated amorphous silicon, a noncrystalline form of silicon, absorb light far more effectively than traditional crystalline silicon solar cells. "Instead of a thick, 20-micron crystalline silicon film, you can just deal with a very thin, half-micron amorphous silicon film," said Biswas. "These cells are more cost-effective as they involve much less material and processing time - driving forces for industry. However, although amorphous silicon absorbs light very efficiently, it suffers from this degradation effect - that's the bad news."
Biswas and his co-workers have been studying the troublesome degradation effect, also known as the Staebler-Wronski effect, for the past few years. The effort includes investigations into the atomic origins of the S-W effect and the subsequent exploration of possible new solar cell materials through computer molecular dynamics simulations.
Biswas explained that exposure to light can cause changes in hydrogenated amorphous silicon, resulting in defects known as metastable dangling bonds - bonds that can go away only when heated to a high temperature. Dangling bonds are missing a neighbor to which they can bond. To remedy the situation, they will "capture" electrons, reducing the electricity that light can produce and decreasing solar cell efficiencies. "The question," Biswas said, "is how does light create the dangling bonds?"
The answer has long been a mystery, but now Biswas and his co-workers, Bicai Pan and Yiying Ye, are helping resolve many puzzling aspects of the problem with their three-step atomistic rebonding model. The model is based on rearrangements of silicon and hydrogen atoms in the hydrogenated amorphous silicon material. In the first step, sunlight creates excited electrons and holes (vacant electron energy states) in the material. When the electrons recombine, they pair up with holes on the weak silicon bonds. The recombination energy causes the weak silicon bonds to break, creating silicon dangling bond-floating bond pairs. During the second step, the floating bonds break away from the dangling bonds and move freely throughout the material. This occurs when the extra floating bond from one silicon atom moves to a neighboring silicon atom. The third step reveals that the short-lived floating bonds disappear. Some recombine with the silicon dangling bonds, which results in no material defects. Others "hop" away from the dangling bonds and are annihilated when hydrogen atoms in the network move into the floating bond sites.
Biswas' three-step rebonding model shows that defect creation in hydrogenated amorphous silicon solar cells is initially driven by the breaking of weak silicon bonds followed by the rebonding of both silicon and hydrogen sites in the material. The research represents a significant achievement in understanding the atomic origins of the light-induced degradation effect in hydrogenated amorphous silicon and so provides a vantage point for eliminating this effect in the development of new solar cell materials - a task on which Biswas is now focusing his efforts.
To improve the efficiency and reliability of solar cells, Biswas and his co-workers are investigating mixed-phase solar cell materials - a mixture of clusters of nanocrystalline silicon embedded in an amorphous matrix. "One of the most promising developments has been the success of hydrogen-diluted materials grown at the edge of crystallinity - the phase boundary between microcrystalline and amorphous film growth," said Biswas. "These materials and solar cells made from them have a much greater stability to light-induced degradation than traditional amorphous material."
By developing molecular dynamics computer simulations, Biswas hopes to learn more about this mixed-phase material at the atomic level and discover what aspect of the mixture is responsible for the improved material properties. His research efforts may even extend to manipulating the nanoscale structure of the material, allowing the design and creation of improved materials.
The research is funded by DOE's Energy Efficiency and Renewable Energy Office through the National Renewable Energy Laboratory and is administered by the Institute for Physical Research and Technology, a network of research and technology-transfer centers at ISU. The American Chemical Society provided start-up funds. Ames Laboratory is operated for the DOE by ISU. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.