Organic solar cells gain ground
R. Colin Johnson, EE Times, August 1, 2007
Source: EETimes Online
http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=201202360
A new composite material for plastic solar cells, formulated at Ohio State University, offers what researchers there claim is the best bet yet for beating the relatively high cost of grid-supplied electricity. Building on the best aspects of previous attempts to construct organic dye-sensitized solar cells, these researchers promise to best today’s inorganic silicon-based solar cells, and beat the cost of traditional electricity generation sources in just a few years.
“The manufacturing cost of our dye-sensitized solar cells is much lower than that of today’s polysilicon solar cells,” said the inventor of the new method, Professor Yiying Wu, at Ohio State University. Today, amorphous polysilicon solar cells typically achieve 10 to 15 percent efficiencies. The organic dye-sensitized solar cells Wu’s group is working on, achieve just under 9 percent, but at a quarter of the manufacturing cost. “However, we believe we can greatly improve our efficiencies in just a few years,” said Wu.
Today, even the most expensive single-crystal silicon-based solar cells can’t beat the cost of electricity from traditional sources, due to the expense of high-temperature vacuum deposition. Amorphous polysilicon solar cells are cheaper than are single-crystal solar cells, but still a more expensive alternative than grid-supplied electricity. Organic dye-sensitized solar cells (DSSCs), on the other hand, can be manufactured using ultra-low-cost, low-temperature roll-to-roll manufacturing methods, and, hence, hold the promise of someday offering a more economical alternative.
Organic dye-sensitized solar cells work by absorbing photons on an inexpensive thin-film, comprising dye molecules attached to a titanium oxide layer on a glass or plastic substrate. When the dye cells absorb a photon, the resultant excitation injects electrons into the titanium, which transports them to the negative electrode. Researchers have so far used two approaches. One maximizes the surface area of the dye-based absorption layer by coating nanoparticles with dye. A second group of researchers has, instead, coated nanotubes or -wires with dye. Now a third approach promises to solve the inefficiencies of both approaches by combining the best of both worlds.
“What we have done,” said Wu, “for the first time is combine those two approaches in a composite material that has the advantages of both nanoscale building blocks.” The advantage of dye-sensitized solar cells using nanoparticles is that they provide the most surface area for photon absorption. Unfortunately, nanoparticles have many grain boundaries, so their electron transport is relatively poor. Nanotubes and nanowires, on the other hand, have excellent electron-transport characteristics. Unfortunately, they have less surface area than do nanoparticles.
The Ohio State researchers’ solution is to mix nanoparticles with nanowires in a composite material that they claim harvests the best aspects of both–large surface areas for maximal absorption, with high electron transport. “First, we manufacture our single-crystalline nanowires, so we have excellent electron-transport properties. Then we mix them with nanoparticles and deposit the composite material over a titanium oxide film on an inexpensive substrate,” said Wu.
For their current prototypes, Wu’s group just allowed the nanoparticles and nanowires to mix together in random orientations in a film about 10 microns thick, resulting in 8.6 percent efficiencies. However, the group is also experimenting with growing the nanowires vertically on the titanium-oxide-coated substrate, then sprinkling the nanoparticles in between, in hopes of upping their efficiency to the 10 to 15 percent levels that would rival polysilicon solar cells.
“We are still learning how to choose the best substrates, the best sensitizer dyes, and to optimize our approach to maximize both the voltage and the current we get from our design,” said Wu. “We also plan to fully characterize our current designs, as well as try to fully understand how our material is functioning so we can maximize electron mobility.” In addition to fine-tuning the methods of combining nanoparticles and -wires, Wu also is experimenting with different architectures (branching nano-trees), different dyes (ruthenium), different titanium oxides (ternary) and alternative materials (zinc stannate), in search of the optimal formulations that will finally make organic dye-sensitized solar cells a commercial reality.
Wu performed the work with his postdoctoral researcher Bing Tan, his doctoral student, Yanguang Li, and an undergraduate student Elizabeth Toman. His research was funded by the American Chemical Society’s Petroleum Research Fund.