OLED Development Follows the Familiar Pattern
In May 2005, Samsung stunned the display world when it first demonstrated a 40-in. prototype OLED TV. This breathtaking prototype used a white emitting layer with color filters and utilized amorphous-silicon (a-Si) TFTs in the active backplane – at a time when many others were working low-temperature polysilicon (LTPS). In an article in Information Display in February 2006, Kyuha Chung, Senior VP for OLED Development at Samsung, pointed out that a-Si TFTs posed some challenges that needed to be addressed before OLED TV could become a commercial success. These included relatively low electron mobility, wide variations in performance over temperature, and long-term degradation effects that lead to lower net current densities. And yet, a-si TFTs are the workhorse of the LCD-TV community: ease of fabrication on very large substrates, high uniformity across adjacent cells, and relatively inexpensive to produce. The problem lies in the fact that OLED cells, unlike liquid-crystal cells, require significant amounts of electrical current in order to generate bright light. OLEDs are analogous to earlier electroluminescent (EL) displays because they generate their own light, rather than modulating external light. In early 2006, much of the attention toward OLEDs understandably was focused on materials research: small molecule vs. large molecule; white emitters vs. RGB vs. blue with secondary phosphor; methods of material deposition and sealing; and organic contamination, emission efficiencies, and lifetimes. Relatively little attention was focused on backplane switches, although in that same February 2006 issue of ID, Amal Ghosh and Steven Van Slyke reported in their article that LTPS and a-Si backplanes were both being evaluated for AMOLED displays. They predicted that "LTPS will likely remain the technology of choice for small displays, where the high mobility enables integrated drivers on the display substrate, reducing module size. Whether a-Si is acceptable as an AMOLED backplane remains to be determined." They were right in also noting that significant efforts were under way to fully explore multiple options.
Today, backplane technologies, including the switches, are clearly the focus of numerous development efforts and part of the required solution set before OLED TV can become a wide-range commercial success. Our Guest Editor this month, Julie Brown of Universal Display Corp., has done a stellar job in bringing us three interesting articles detailing the latest advances in backplane technology from LG Display, Samsung SDI, and Sony. Both Sony and Samsung report that poly-Si is no longer a long-term focus, and neither is traditional a-Si. Sony reports on a new process it has developed to produce "microcrystalline-silicon TFTs," which has a mobility improvement up to 10 times that of a-Si while retaining a-Si's good uniformity characteristics, according to the article. Samsung describes its development of amorphous indium-gallium-zinc-oxide (InGaZnO) TFTs, building on earlier work by LG and Cannon. With InGaZnO, Samsung achieves similar improvements of 10x or more over a-Si, and the process is compatible with existing vacuum deposition and masking steps. LG describes its work to fabricate a-Si TFTs on a stainless-steel flexible backplane. LG uses a-Si because it requires lower process temperatures than high-temperature poly-Si, but the article freely admits that because of the low mobility, very high luminous efficiency is required in its OLED materials. I have little doubt one of their next steps is going to focus on alternate TFTs on their flexible backplanes.
I'm focusing on this component of the stories because I think it represents a major theme in the natural evolution of new display technologies. Whether it is OLED, FED, plasma, LCOS, or any other similar technology, the roadmap usually starts with the supposition that an existing supporting platform of manufacturing processes and underlying technologies can be leveraged to attain commercial success as rapidly as possible. It's an appealing story and, more often that not, is true at least in part. Liquid-crystal–on–silicon (LCOS) developers touted the fact that they could utilize trailing-edge semiconductor fabrication lines and processes to achieve a very short time to market. In fact, they could, except for an almost endless array of seemingly minor details that resulted in long delays and very expensive process modifications.
As development progresses, the depth and scope of the resulting efforts grows way beyond the original fundamental technology involved. Numerous side projects crop up to solve secondary process and technology problems that emerge along the way. In the end, the total scope of the effort looks much greater than what was originally considered. I don't know if Samsung, LG, or Sony specifically anticipated the need for a new TFT technology when they began commercializing OLED TVs, but I'm sure a lot more money and time has been spent on oxide TFTs than ever would have been without OLED technology. I can also imagine that out in the world somewhere, there could be a totally unrelated application of InGaZnO that would otherwise never be possible without this effort.
In the end, that's the good news. The cycle of technological development produces ever greater opportunities that come with ever greater challenges and surprises at the same time. What appears as a setback to one endeavor can end up creating the crucial enabler for a totally different endeavor. The universe is funny that way.
We welcome back Matt Brennesholtz from Insight Media with the second part of his historical perspective on the evolution of projection technology which takes the story from 1992 to the present, offering a number of insights into the various innovations that rose up so rapidly in the last 20 years to build the rich landscape of products we see today. Part I can be found online in the May 2008 issue of ID at www.informationdisplay.org.