Results tagged “materials science”

(Once again, all references are to MRS paper numbers, abstracts for which can be found at the conference site.)

Today at the MRS Spring Meeting, John Robertson reported (paper A13.1) that his group at Cambridge University has achieved n-channel mobility of 450 cm²/V-sec in microcrystalline silicon TFTs, and 100 cm²/V-sec p-channel mobility. Both those values are very good, and that's a problem. Plenty of models exist to explain why the material's mobility might be bad, and those models break when the mobility is good. More research needed.

(Special thanks to Dr. Robertson for walking me through yesterday morning's session on graphene, too.)

Meanwhile, Yifei Huang and a Princeton University group demonstrated (paper A13.2) a self-aligned process for low temperature polysilicon TFTs. It uses nickel silicide source and drain regions, aligned using the gate structure. At low annealing temperatures, the nickel doesn't react with the gate and can simply be etched away. Results were among the best ever recorded for top gate TFTs.

In the solar cell sessions, Makoto Shimosawa described (paper A14.1) Fuji Electric's FWave flexible solar material. It laminates roll-to-roll amorphous silicon/amorphous SiGe tandem cells (deposited by PECVD on plastic) onto steel foil. Each 2 square meter sheet generates 92 watts at peak output and weighs just 16 kg (including the steel foil). The company is now ramping production to wider rolls, targeting production of 40 MW per year.

The a-Si/a-SiGe tandem cell may be on its way out, though, as Xixiang Xu's group at United Solar Ovonic reported (paper A14.2) better results with small area triple junction a-Si/nanocrystalline-Si/nc-SI cells. Scale-up to large areas and optimization of the nc-Si component cell are the next steps.

The conference's own coverage is definitely worth a look as well.

For those who don't know what the last two posts were talking about, MRS stands for Materials Research Society, the professional society for materials scientists. I'm in San Francisco for the MRS Spring Meeting this week.

Among other things, MRS gives people in the field a handy crib sheet to use when people ask, "so what's materials research?"

Briefly, materials science is the glue that pulls together subjects as diverse as Mayan bronzes, high temperature superconductors, organic and inorganic semiconductors, and ink rheology. All of these areas involve manipulating the structure and processing of materials in order to achieve the desired properties.

References are to MRS paper numbers. All abstracts can be found at the meeting site.

Two papers from Kyoto University, by Hideo Ohkita et. al. (AA5.32) and Jiamo Guo et. al. (AA5.33), presented good fundamental studies of charge generation and transport in fullerene-polymer bulk heterojunction solar cells. Extra points for providing copies of their posters as takeaways.

In paper KK5.14, Hagay Shpaisman, et. al., take a skeptical look at multiexciton solar cells, questioning whether they offer much improvement over the more conventional, and more tunable, tandem cell.

Yong Soo Kang, et. al. (KK5.1) presents a novel electrolyte for dye-sensitized solar cells, in which hydrogen bonds pull oligomers together in situ to form a self-solidified polymer that can still penetrate the cell's titanium dioxide nanostructure. Someone who saw me taking notes added the caveat that one of the components of the electrolyte is still a liquid; the author wasn't around to answer questions.

I'm not sure I quite understand this one, but in paper AA5.86 Janelle Leger and Glenn Bartholomew propose a single-layer polymer-based p-i-n transistor. The semiconducting polymer incorporates ion transport agents and ion-paired monomers, apparently creating an all-in-one electrochemical cell.

Everyone talks about how nice roll-to-roll fabrication would be, but Daniel Tobjork and coworkers (AA5.45) have actually done it. They used reverse gravure coating to put P3HT:PCBM / PEDOT:PSS organic semiconductor structures on plastic. Results were comparable to those achieved with spin-coating; roll speed controlled the coating thickness.

Recommended reading for anyone interested in renewable energy and related issues: the MRS Bulletin's April issue. It's a comprehensive look at everything from controlling fossil fuel emissions, to hydrogen storage and transportation, to energy conservation. There's a lot of material here, but it's well worth the time. Reasonably accessible to nonspecialists and nontechnical people.

The lead editorial (PDF file, free registration required) in this month's issue of the MRS Bulletin observes that few researchers know how much the materials they're investigating actually cost. The author makes an extremely important point. While it's true that cost is negligible for research quantities of most materials, it most certainly is not negligible for industrial quantities. In fact, cost is one of the reasons why silicon dominates both the photovoltaic and integrated circuit markets, and one of the most serious obstacles to any competing technology. Researchers who choose to ignore cost considerations are likely to be rudely surprised when industrial interest in their creations fails to materialize.

I tried three times to write something insightful about percolation theory, and decided I couldn't because I don't know enough about it. Yet. I'll be working on that. Meanwhile, I mention the topic at all because I'm seeing it appear in all sorts of contexts, from pore structures in low-k dielectrics to heterojunctions and carrier conduction mechanisms in advanced solar cells.

Percolation theory, as the name implies, looks at the way liquids infiltrate and work their way through porous media. Much of the early work was done by hydrologists, but the same mathematical formalism turns out to apply to many types of structure formation. In porous low-k dielectrics, for example, percolation affects conversion of the poragen, outgassing of any reaction byproducts, infiltration of moisture or other contaminants into the film, and so forth. Interesting stuff, and it's becoming more relevant as more electronic devices use materials that don't form nice uniform films.