Temperature OLEDs

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

There have been numerous studies of the electrical and emission properties of conjugated polymer-, small molecule-, and molecularly doped polymer-based OLEDs. The current-voltage and radiance-voltage characteristics have been nica sured as a function of thickness of the organic layer, temperature, different metal electrodes, etc. in an attempt to understand the device physics. A major factor in hibiting progress is the purity of the organic impurities that are incorporated dur-... 

Another way to measure the Vhi is by means of photovoltaic measurements [97, 113. The technique is based on the fact that, at near zero applied bias, the OLED acts as a photovoltaic cell, where photogencraled carriers drift under the influence of Vhi to produce a current in an external cireuit. In a way similar to electroabsorption, an external bias is applied in order to compensate the built-in potential and null the net pholocurrent (Fig. 13-6). However, it has been shown that the measurement produces accurate results only at low temperatures, where diffusive transport of charges that are phoiogcneraled at the interlaces is negligible [97]. 

The discovery of the use of A1Q3 as an electron-transport-emitting layer is undoubtedly the most significant achievement in the research that led to the development of stable OLEDs.180,181 It is very stable and can be sublimed without decomposition at 350 °C,188 and its thin-film PL quantum efficiency at room temperature is about 32%, independent of film thickness between 10 nm and 1,350 nm.189... 

FIGURE 6.21 J-V (a), L-V (b), and E-V (c) characteristics measured for OLEDs made with a commercial ITO-coated 1.1-mm rigid glass substrate and a low-temperature ITO-coated reinforced ultrathin glass sheet. 

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. 

SW Yin, Z Shuai, and Y Wang, A quantitative structure-property relationship study of the glass transition temperature of OLED materials, J. Chem. Inf. Comput. Sci., 43 970-977, 2003. 

VI Adamovich, MS Weaver, RC Kwong, and JJ Brown, High temperature operation and stability of phosphorescent OLEDs, Curr. Appl. Phys., 5 15-18, 2005. 

Today, many red and green OLEDs have lifetimes exceeding 10,000 h (see one example in Table 10.3). Instead of testing the OLED at room temperature, an accelerated experiment may be useful. The operational lifetime of PLED was studied at several temperatures (25-85°C) by Parker et al. [110]. They found the changes in luminance were significantly accelerated at higher temperatures. From the lifetime data at several temperatures, the OLED lifetime at room temperature can be readily estimated. 

Kodak is commercialising its low molecular weight OLEDs for use in both passive and active matrix display architectures. It has also licensed its technology to Pioneer Corp who have commercialised passive matrix displays for car radios and cellular phone displays. TDK has displays for cellular phones, personal digital assistants and car instrumentation clusters. Perhaps the most significant collaboration to date has been with Sanyo. Sanyo s capabilities in low-temperature polycrystalline silicon have been married with Kodak s low MW materials to produce a full colour, 5 inch active matrix display, commercialisation of which was expected in 2001. 

Ziegler catalysis involves rapid polymerization of ethylene and a-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide (1,2). Catalysts of this type operate at low pressures (up to 30 atm), but often at 8-10 atm, and, in special cases, even under reduced pressure, and at temperatures up to 120°C, but often as low as 20-50°C. Approximately 2,200,000 tons of polyethylene and 2,900,000 tons of polypropylene are produced per year with the aid of such catalysts. The polyeth-... 

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