Molecular LED: Design Concept of Molecular Materials for High-Performance OLED

Abstract

The recent rapid development of organic light-emitting diodes (OLEDs) has re-sulted in the commercialization of simple dot-matrix OLED displays.! The great success of OLED devices has also introduced abrand new organic semiconductor. Now, we are in the process of designing the physical and chemical makeup of this new type of organic semiconductor. In this chapter, first we will briefly describe the OLED development from the 1960s to the 1980s. Second, we will define the new organic semiconductor especially utilized for OLED devices and c1arify what the organic semiconductor is and how it differs from the conventional inorganic semi-conductor. Then, we will review the recent OLED progress by taking the following aspects into consideration: the design of multilayer cell structures, the design of each molecular constituents, hole transport, electron transport, and emitter and dopant molecules. Fina11y, we will discuss the future prospects of OLEDs. 2.2 OLED Development from the 19608 to the 1980s The recent progress in OLEDs can be dated back to the early research of the 1950s. Thus, it has taken 40 years for this technology to become commercialized. Bemanose et al. firstreported on electroluminescence (EL) by utilizing molecularly J. Shinar (ed.), Organic Light-Emitting Devices © Springer Science+Business Media New York 2004 44 C. Adachi and T. Tsutsui dispersed polymer films. 2 -4 Although they insisted on the similar excitation and emission mechanisms that had been established in inorganic EL in those days, it was understood by Hercules et al. that the emission was induced from the secondary ultraviolet light by a glow discharge between two electrodes. s During these early experiments, a very interesting chance is the aluminum quinolinol (Alq), which is one of the most promising compounds in the recent OLED devices, used in this experiment. In the 1960s, research moved on to the carrier-injection type of electrolumi-nescence, namely OLED, by using a highly purified condensed aromatic single crystal, especially an anthracene. Pope et al., 6, 7 and W. Helfrich et al.,8,9 in particu-lar, experimented on carrier recombination and the emission mechanism, and their physical interpretation is still very useful today. Whereas a highly purified zone-refined anthracene single crystal essentially shows a conductivity of 10-20 Stcm, double injection of holes and electrons, however, were achieved efficiently based on space-charge-limited current (SCLC) with the equipment of charge-carrier-injection electrodes, and resulted in a successive carrier recombination, the creation of single and triplet excitons, and the radiative decay of them. Thus, the basic EL process has been established since the 1960s. From the 1970s to the 1980s in addition to the studies on the EL mech-anism 10-20 the focus of research shifted from single crystals to organic thin films. Based on the successful research on anthracene single crystals, various aromatic compounds were examined by using vacuum vapor deposition. 21 -24 The morphology of polycrystalline films, however, was insufficient for stable current injection and transport. Moreover, although another thin-film fabrication method-the Langumir-Blodgett method-was examined, similar unstable be-havior prevented further consideration. 2S Thus, in the thin-film devices, two major target areas for efficient EL were pointed out: improving the carrier-injection electrodes, in particular, electron injection, and forming pinhole-free thin films. This basic research was extremely important and provided a foundation for the continuation of EL development. Judging from the present OLED status, the most important research was Partridge's report on the EL utilized poly(vinyl carbazole) (PVCz) thin films in 1982.2 6 -29 He used the 500-nm-thick PVCz thin films doped with fluores-cent molecules as an emissive center, equipped with the efficient hole-injection electrode (SbClsIPVCz) and the electron-injection electrode (cesium) as a low-workfunction metal. Although no quantitative measurement of luminance was described, surprisingly the injection current density reached 1 mAlcm 2 • Nowadays, we can fabricate very similar OLED devices with superior EL performance. Thus, Partridge's device contributed to establishing the prototypes of present OLED devices. Next, in the 1980s, the organic multilayer structures, which are another key technology of present high-performance OLEDs, appeared. In 1986, Hayashi et al. observed a remarkable reduction of the driving voltage by inserting a polythiophene-electropolymerized thin film between an indium-tin-oxide (lTO) anode and a perylene-deposited film. 3o The insertion of the poly thiophene thin