Time OLEDs

Pete Rickard Inc. CobleskiU, N.Y. Ole Time Woodsman Kampers lotion 50... 

The characteristics of ITO described in the previous paragraph make it a useful material for use as the anode in an OLED. At the same time, they arc the cause of many difficulties which have been observed in the reproducibility and stability of OLED devices. We shall return to this topic in more detail later, but suffice it... 

The fifth of the color methods places the three emitting structures in a stack one on top of the other, rather than side by side ]20l ]. Clearly there is a requirement here that the two electrodes in the middle of the structure must be transparent. The advantages are that the display can be made much brighter with up to three times the luminance from each pixel, and the requirements for high resolution patterning are relaxed by a factor of three. The disadvantages are that three times as many layers must be coated (without defects) over the area of the display and electrical driving circuitry must make contact with four sets of elec- trades. It will be extremely difficult to incorporate a stacked OLED into a active matrix array. 

Arnold, R. J. Reilly, J. P. Fingerprint matching olE. coli. strains with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of whole cells using a modified correlation approach. Rapid Comm. Mass Spectrom. 1998,12, 630-636. 

As an extension of the fluorescent sensitizer concept, Forrest et al. have applied this approach to phosphorescent OLEDs, in which the sensitizer is a phosphorescent molecule such as Ir(ppy)3 [342]. In their system, CBP was used as the host, the green phosphor Ir(ppy)3 as the sensitizer, and the red fluorescent dye DCM2 as the acceptor. Due to the triplet and the singlet state energy transfer processes, the efficiency of such devices is three times higher than that of fluorescent sensitizer-only doped device. The energy transfer processes are shown in Figure 3.21. 

As noted above, the whole field of OLEDs began with the green SMOLED emitter Alq3. At this time the leading candidate emitters that have emerged from an enormous number of experiments fall, once again, into two camps — fluorescent and phosphorescent. 

If there is one clear need in the field of OLED materials it continues to be in the area of blue emitters. A blue emissive material with good color coordinates CIE (0.10, <0.10) coupled with long device lifetime (>10,000 h) and high electrical efficiency (>5 cd/A) is the holy grail of materials chemists in this field. A major effort to find such materials continues in many laboratories including our own and the current sets of available materials may be supplanted at any time. However, the current best candidate blue emitters in the SMOLED area compromise many desirable properties — the most troublesome being long lifetime. 

Over the last 50 years, remarkable improvements in the performance of vapor-deposited OLEDs have been made. Operating voltages have been decreased from a few kilovolts to a few volts, at the same time efficiencies are now approaching 100 lm/W. These improvements in device performance have made commercial displays based on vapor-deposited OLEDs viable. This technology is now poised to compete with liquid crystal displays (LCDs) in an expanding flat panel display marketplace. 

The aim of this chapter is to give the reader a broad overview of the field of vapor-deposited small-molecule OLEDs. It is beyond the scope of this chapter to cover every aspect of these devices, however key references are given throughout the text for those readers who are interested in delving more deeply into this topic. Section 7.2 describes the key elements of a typical OLED. Alternative device architectures are also briefly described. Section 7.3 describes the typical fabrication methods and materials used in the construction of vapor-deposited OLEDs. Section 7.4 describes the physics of an OLED in addition to the improvement of the performance over time made through advances in device architectures and materials. Section 7.5 discusses OLED displays and Section 7.6 looks at the future exciting possibilities for the field of vapor-deposited organic devices. 

The films must be stable for long periods. Some materials, particularly those with a low glass transition temperature (Tg) may crystallize over time [22,23], Crystallization may be accelerated when the temperature of the thin film is raised during device operation [24-26]. Therefore, a high Ts is often desirable for the long-term durability of the OLED, e.g., rg>85°C. 

The organic deposition sources are made of a variety of materials including ceramics (e.g., boron nitride, aluminum oxide, and quartz) or metallic boats (e.g., tantalum or molybdenum). Deposition is carried out in high vacuum at a base pressure of around 10-7 torr. The vacuum conditions under which OLEDs are fabricated are extremely important [41] and evaporation rates, monitored using quartz oscillators, are typically in the range 0.01 0.5 nm/s in research and development tools. In manufacturing, higher rates or multiple sources may be used to reduce tact times. 

Displays based on OLEDs may be addressed either passively or actively [124], and the drive requirements are quite different in each case. In passive-matrix addressing, the display is addressed one line at a time, so if a display has 480 lines then a pixel can only be emitting for... 

---