Fluorescent OLED device

In the following sections, we will review the status of fluorescent OLED devices of red, green, blue, and white colors. 

The phenomenon of organic EL was first demonstrated using a small-molecule fluorescent emitter in a vapor-deposited OLED device. The Kodak group first used metal oxinoid materials such as the octahedral complex aluminum tris-8-hydroxyquinoline (Alq3) (discussed above as an ETM) as the fluorescent green emitter in their pioneering work on OLED architectures [167],... 

The merit of these chromene dopants is their relatively long emission wavelength peaks compared to DCM or DCJTB materials due to the more conjugated chromene moiety, and this contributes to the more saturated red emission. In fact the EL spectra of OLED devices of ITO/TPD/Alq3 chromene-dopants/Alq3/Mg Ag exhibited satisfactory red emission color, especially for Chromene-1 and Chromene-2 dopants. However, these chromene-based red emitters showed lower fluorescent quantum yield (18%, 15%, and 54% for Chro-... 

One of the most common uses for peri-substituted pentacene is as a red emitter in organic light-emitting diodes (OLEDs). Diphenylpentacene, for example, has a fluorescence quantum yield of 30% as a 0.55% dopant in Alq3, yielding OLED devices with efficiencies near the theoretical maximum [34]. Variation of the aryl substituents improves solubility and processing and can increase fluorescence quantum yield (for example, pentacene 25 has a composite fluorescence quantum yield of 32%) [35]. There is one report of the use of diaryl pentacenes in FET devices, but the performance was generally poor (hole mobility for vapor-deposited 23 was of the order of 10-8 cm2 V-1 s-1) [30]. 

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 1982.26-29 He used the 500-nm-thick PVCz thin films doped with fluorescent molecules as an emissive center, equipped with the efficient hole-injection electrode (SbCls/PVCz) 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 mA/cm2. 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. 

The first example of using the [2+2] cycloaddition for crosslinking of organic semiconductor layers was reported in 1997 by Remmers et al. [10]. It was a derivative of poly-p-phenylene (PP) deposited by the Langmiur-Blodgett (LB) technique (Fig. 9.5(a)). Polarized absorption and fluorescence of the films was reported, but no OLED devices were fabricated. 

As discussed in Section 7.1, NPB Alq devices may not be representative of many different OLED device structures. It is therefore natural to ask whether the role deep traps play in operational degradation is unique for the NPB Alq devices. To answer this question, we examined a diverse set of fluorescent and phosphorescent OLED devices spanning more than two orders of magnitude difference in stability. The results are summarized in Table 7.1 and the following is noteworthy. Despite the structural and functional dissimilarities in the experimental set, the operation-induced accumulation of fixed positive charge densities and correlation with luminance loss were invariably observed. Although we observed some nonlinearity in luminance-charge... 

This section covers the background information for OLED devices that applies to both fluorescent and phosphorescent devices. In addition, materials and architectures specific to fluorescent devices are discussed. Phosphorescent structures are described in Section 14.4. 

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