Ambipolar Organic Semiconductor Material Claims High Charge Mobility28 May 2009
The benzodifuran derivative
announced in 2007 (left) and the
structure of new ambipolar CZBDF
The voltage and external quantum efficiency
characteristics (graph on the left) of homojunction OLED
devices with the intermediate layer doped with a dye, and
the emission of the device (picture on the lower right)
A Japanese research group claimed that it developed a new ambipolar organic semiconductor material with the "highest level" charge mobility as an amorphous material.
The material, "CZBDF," was developed based on a derivative announced by the research group, which is led by Tokyo University and the Japan Science and Technology Agency (JST), in 2007. The derivative has a mother nucleus of "benzodifuran," an annulated π-conjugated compound containing oxygen atoms. The benzodifuran derivative is an amorphous thin film p-type semiconductor material with a high hole mobility.
This time, the research group replaced the "amine" portion in the benzodifuran derivative with "carbazole" and realized the CZBDF ambipolar material with high charge mobility. Also, the group produced a homojunction OLED device with the use of CZBDF and succeeded in achieving EL emission by using both fluorescence and phosphorescence, as well as EL emission of three primary colors of blue, green and red.
The CZBDF amorphous thin film has a hole charge mobility of 3.7 x 10-3cm2/Vs and an electron charge mobility of 4.4 x 10-3cm2/Vs. Both has a high mobility and exhibited well-balanced values, the group said. These values were measured by using the time-of-flight (TOF) method (at a field intensity of 2.5 x 105V/cm).
Furthermore, the research group produced a homojunction OLED device by vacuum vapor deposition with the use of the newly developed ambipolar material CZBDF. Specifically, indium tin oxide (ITO) on a glass substrate is used as the positive electrode, and a 150-200nm-thick organic thin film and an aluminum (Al) metal (negative electrode) are sequentially formed on the positive electrode by vacuum vapor deposition.
Using CZBDF as a single host material, the organic thin film was subjected to p-type doping in the range of 30nm from the positive electrode by co-deposition with vanadium pentoxide (V2O5), which is an inorganic oxidant. Meanwhile, an area in the range of 20nm from the negative electrode is subjected to n-type doping by co-deposition with a reductant (metal cesium). This facilitated the charge injection and transport from the electrodes to CZBDF, the research group said.
To generate three primary colors, an intermediate layer (thickness: 50-100nm), which was doped with no oxidant or reductant, was doped with a blue or green fluorescent dye, or a red phosphorescent dye. The green fluorescent dye showed a high external quantum efficiency of 4.2% at a luminance of 60,000cd/m2.
According to the research group, the following characteristics of CZBDF were believed to have contributed to the emission of three primary colors by the newly developed OLED device and its high luminous efficiency. (1) It is an ambipolar material with high balance and mobility. (2) It is a wide gap semiconductor material that has a sufficiently high energy gap (approximately 3eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). (3) It can effectively capture electric charges into the emitting dyes.
Thus far, OLED devices with a heterojunction structure composed of organic thin film layers made of five or six different types of materials have been pervasive. The research group achieved the emission of three primary colors and a high luminous efficiency by using an OLED device with a simple homojunction structure.
It expects that this achievement leads to the development of low-cost, highly-efficient OLED displays and lighting equipment. Also, the group intends to apply the new material to organic thin-film solar cells with a multilayer structure like OLEDs.
The achievement was published in the online version of German scientific magazine "Advanced Materials" May 25, 2009.
--------------------------------------------------------------------------------------SMD unveils production ready OLED-Tvs and AMOLEDs at SID 20091 June 2009
Samsung Mobile Display (SMD), exhibit an extensive range of OLED and mobile LCD displays at the SID-2009 Display Week 2009.Production-ready AM OLED-TV
SMD is exhibiting 14.1-inch and 31-inch diagonal OLED TV panels. The 31-inch is the world's first OLED display that features full HD resolution (1920 x 1080), a contrast ratio of 1,000,000:1, a color gamut of over 100% NTSC and a ultra-slim design of only 8.9mm – providing outstanding brightness and exceptional image quality. The OLED TV panels can be mass produced through the use of Fine Metal Mask (FMM) technology.Active Matrix AMOLED for the Future
SMD is showing the world’s thinnest “flapping” OLED panel, one that can flutter in a breeze. The super-thin panel is only 0.05mm thin, about one tenth the thickness of OLED panels with a normal glass substrate. It features a high contrast ratio, is polarizer-free and has a pixel resolution of 480 x 272.
SMD also is showing 4.82-inch and 12.1-inch transparent, foldable and ID card displays as well.AM OLED for Mobile
OLEDs have become an important consideration in mobile design as set makers require smarter displays to accommodate multi-functions.
SMD will exhibit a full line up of mobile displays from a 3.2-inch “real” WVGA to a 7-inch WSVGA. The 3.2-inch WVGA on exhibit
is the world’s first OLED-Display with 310ppi. (pixels per inch).
Check out also the new Samsung Mobile Display Website
--------------------------------------------------------------------------------------Cheaper Big-Screen OLEDs5 June 2009
Longer life: DuPont has developed
New organic display materials can be printed with ink-jets.
longer-lasting OLED materials that can be printed
using cheap, simple techniques to make displays
like this one. Credit: DuPont
High-end displays made from organic materials are lightweight, energy efficient, and crisp--but it has proved difficult to manufacture them cheaply and durably.
Now the chemical giant DuPont
is reporting the development of long-lasting organic-display materials that can be printed cheaply over large areas, much like ink. DuPont says that these materials can be used to make cheaper high-end displays with existing equipment, and the company says that it is in talks with display manufacturers to bring them to market.
Each pixel in an organic light-emitting diode (OLED) is made up of materials that emit red, green, and blue light in response to electrical stimulation via a thin-film transistor backplane.
OLED displays on the market today are made by depositing organic materials in a vapor through a mask. This setup ensures that differently colored subpixels are properly aligned, but the process is expensive, because some material inevitably gets lost, and difficult to do over large areas. For this reason, OLEDs have so far found their way into only a few products, including a Sony television and some Samsung cell phones.
An alternative approach is solution deposition, which involves printing liquid organic materials onto a surface. Several companies and university research groups have been trying to develop such printable OLED materials, but it's difficult to make light-emitting materials that last long enough to bring them to market: the display quality tends to degrade too quickly.
"If one could get high performance from solution-deposition methods, it would be very attractive: it would solve the scaling issues" associated with making these displays, says Nick Colaneri
, director of the Flexible Display Center at Arizona State University, in Tempe. "Now DuPont claims to have solved that problem."
This week at the Society for Information Display (SID) Symposium
, in San Antonio, DuPont is presenting OLED materials that can be printed in solution and that make longer-lasting displays. DuPont is disclosing not the composition of the materials or how they are printed. However, the lifetimes of the materials, which the company has disclosed, "are indeed impressive," says Samson Jenekhe
, a professor of chemical engineering at the University of Washington, Seattle. For example, the lifetime of the green material involved is more than a million hours, which DuPont says is a record. The efficiency and color purity of the materials, says Jenekhe, are comparable to those of the state-of-the-art organic displays on the market.Vladimir Bulović
, an associate professor of electrical engineering at MIT and a cofounder of QD Vision
, a startup company that makes lighting and displays using quantum dots, says, "Since they aim to produce displays, the key will be to understand the deposition and pixelation method they intend to use." DuPont says that the materials are laid down using a high-speed nozzle printer developed with Dainippon Screen
, a Kyoto electronics company.
Colaneri adds that, to his knowledge, no solution-printed OLED displays are currently on the market. But other companies are also trying to tackle the problem. Indeed, Sumitomo executives reported at the SID event that they have been shipping solution-printable polymers for displays. Sumitomo also recently acquired U.K. company Cambridge Display Technologies
, which makes polymer-based displays. And Universal Display Corporation
of Ewing, NJ, is also reporting long-lifetime green display materials at the conference.
William Feehery, global business director of DuPont OLED Displays
says that DuPont is currently in discussions with several display companies interested in commercializing its new OLED materials. "They already have the manufacturing infrastructure to make these on glass," he says. The company also plans to look into making flexible displays using the technology.
--------------------------------------------------------------------------------------Corning shows Silicon-On-Glass (SiOG) tech to enable cheaper, larger OLEDs5 June 2009
Corning SiOG prototype
Corning is showing their latest Silicon-On-Glass (SiOG) technology. SiOG is used to transfer a thin-film of silicon into a display substrate. SiOG is scalable, and currently Corning can make it on a Gen2 substrate, Gen4 by the end of the year.
The SiOG process would permit the fabrication of stable OLED pixel switches with higher yield and much greater performance than LTPS, and on larger substrates. Corning claims that costs will be lower, because this enables manufacturers to integrate the circuitry on the display substrate easily.
Some panel makers are already trying this out, and hopefully we'll see prototype OLED displays based on SiOG by the end of 2009.
--------------------------------------------------------------------------------------Scientists eliminate precise doping limits from the OLED manufacturing process5 June 2009
Scientists from the RIKEN Advanced Science Institute in Wako have developed a way to eliminate precise doping limits from the OLED manufacturing process. By using a metal dopant containing molecular groups that block the self-quenching interactions, the scientists have, for the first time, fabricated high-efficiency OLEDs with a wide range of doping concentrations.
Hou and colleagues modified a phosphorescent iridium metal complex with a class of molecules known as amidinates. These molecules bind to iridium through a nitrogen atom that localizes electrons near the center of the metal complex. Bulky carbon groups on the edges of the complex are inert and prevent the materials from attaching and self-quenching their phosphorescence.