Last week, I talked about the nozzle printing process developed by DuPont and Dainippon Screen as being a promising way of applying OLED materials to larger substrates than is possible with current processes, and to do so less expensively. DuPont says its cost models show they will be able to make small OLED displays at a cost that is more than 10% less than LCDs. Kodak and its partners are taking a very different approach to the same end.
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But even if DuPont/Dainippon and Kodak are both successful in making cheaper OLED frontplanes, that doesn’t address the problem of low-temperature polysilicon (LTPS) backplanes. When used in pixel switching circuits for active-matrix OLED (AMOLED) displays, LTPS TFTs have the advantage of being quite stable over time. Unfortunately, current laser annealing processes produce LTPS TFTs with characteristics that vary across the substrate. The variability becomes greater as the processing area becomes larger, which limits substrate size. That contributes to high cost, and limits the maximum size of the panels that can be made. If that’s not bad enough, the process for making these TFTs has a relatively low manufacturing yield. That drives the cost up even more.
There is a silicon TFT process that, unlike LTPS, has high yields and high consistency over large substrates. That’s the amorphous-silicon (a-Si) process that’s used for most medium- and large-size LCDs. While a-Si TFTs are very stable when used with voltage-driven LCD pixels, they exhibit unstable switching thresholds with current-driven OLED pixels.
A variety of laboratories have tried to devise pixel-switching circuits consisting of 2 to 6 or more a-Si TFTs to stabilize the threshold voltage, with varying degrees of success. Although a-Si AMOLEDs have been shown as technology demonstrators, none have made it to market - at least, not yet.
Yesterday at the SID show in Los Angeles I spoke with Dr. IB Kang, Head of LG Display’s LCD R&D Lab. Kang said LGD has developed an a-Si pixel circuit that has a 20,000-hour lifetime in smaller displays. That’s a significant advance over previous efforts - significant enough so that LGD is commercializing it’s a-Si AMOLED panel. Expect to see a 3-inch version in a small portable TV before the end this year, Kang said.