Sharp has established indium gallium zinc oxide (IGZO) as a successful backplane material that enables high aperture ratios in high-pixel-density LCDs in sizes too large for low-temperature polysilicon (LTPS) to be financially viable. IGZO is also an attractive, lower-cost alternative for LTPS in high-pixel-density smart-phone LCDs.
Sharp and Qualcomm are working together on IGZO-driven pixtronix displays, which use in-plane MEMS shutters and field-sequential color.
Sharp is not discussing plans for IGZO-driven OLED displays, although they show an increasing variety of IGZO/OLED prototypes at trade shows. There are good reasons for being cautious. When it became clear that the switching characteristics of amorphous silicon (a-Si) were extremely unstable when a-Si TFTs were used to switch current-driven OLEDs and that LTPS was both expensive and not readily scalable to monitor and TV sizes, the industry set out to find a backplane material for OLEDs that ideally combined the low cost and scalability of a-Si with the stability of LTPS. R&D teams were drawn to the class of materials called transparent metal-oxide semiconductors.
IGZO eventually appeared to be the best suited to the task, but several categories of instabilities raised their ugly heads. One by one, solutions for those instabilities were found, with a couple of exceptions. LG Display made a strategic decision to commit itself to IGZO when most researchers though that at least a couple of years more were needed to make IGZO ready for volume production of OLED-TVs. LG initially paid a price for its leap of faith. Knowledgeable sources believe that LG’s manufacturing yield of IGZO/OLED panels was 10% last year, rising to 50% early this year. LGD has established an internal goal of 70% for its new Gen 8 M2 fab, which is scheduled to begin producing OLED-TV panels in the third quarter of this year. Will the lessons LGD has learned by climbing this painful learning curve ultimately pay off? Time will tell.
Quick summary of the story so far: IGZO is a success for LCDs and is working its painful way forward for OLEDs. There is certainly room for alternatives.
Sharp is working on crystalline IGZO (x-IGZO). The original appeal of a-IGZO was that its carrier mobility was not too much less than the crystalline form, and offered the vision of inexpensive a-Si-like fabrication. But Sharp now feels it understands how to crystallize the amorphous form economically and obtain the greater stability and even greater carrier mobility that crystallinity will impart. That, says Sharp, will provide a material that is more suitable for OLED backplanes, as well as very high-ppi LCDS.
Other possibilities are the wonder materials graphene and carbon nanotubes, but they are still quite a way from being ready for incorporation in commercial panels.
Amorphyx, a development-stage Oregon State University spin-off, is developing the amorphous metal nonlinear resistor (AMNR) for display-switching applications. The AMNR is a two-terminal device that has just three thin films and uses current tunneling for its operational mechanism. Amorphyx claims no sensitivity to light, 40% lower cost than a-Si and better optical performance, and a manufacturing process that leverages a-Si TFT production equipment.
Finally, for the purposes of this column, is CBRITE. CBRITE has a management and technical team that grabs your attention. The Chairman and co-founder is Nobel Prize winner Alan J. Heeger. Former Display Fellow at DuPont Display Gang Yu is CTO and co-founder. Bruce Berkoff, former EVP and CMO at LG.Philips Display is CMO.
CBRITE is using a metal-oxide TFT, but the metal oxide is something other than IGZO. The material and process delivers carrier mobility that is greater than IGZO’s, says Berkoff. The mobility readily goes beyond 30cm²/V·sec, and 80cm²/V·sec has been demonstrated. CBRITE’s switches are OLED-stable, says Berkoff, and I(ON)/I(OFF) ˜ 10¹º @ 10V. A five-mask process reduces cost compared to a-Si. Gang Yu says partners are likely to receive panels for qualification late this year. Berkoff adds the technology will probably appear in shipping products in 2015 or 2016.
Expect lots of discussion about these issues at next month’s SID Display Week. a-IGZO is an important chapter in the development of display backplanes, but it’s certainly not the last one.
Ken Werner is Principal of Nutmeg Consultants, specializing in the display industry, manufacturing, technology, and applications. He consults for attorneys, investment analysts, and companies entering or repositioning themselves in industries related to displays and the products that use them. You can reach him at firstname.lastname@example.org.
Posted by Ken Werner, May 6, 2014 5:22 PM
About Ken WernerKenneth I. Werner is the founder and Principal of Nutmeg Consultants, which specializes in the display industry, display technology, display manufacturing, and display applications. He serves as Marketing Consultant for Tannas Electronic Displays (Orange, California) and Senior Analyst for Insight Media. He is a founding co-editor of and regular contributor to Display Daily, and is a regular contributor to HDTVexpert.com and HDTV Magazine. He was the Editor of Information Display Magazine from 1987 to 2005.