Stacking OLEDs

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. 

Fig. 8. Schematic diagram of (a) horizontally and (b) vertically stacked OLED.

In fact, there are still some uncertainties about the applicability of Schottky-Mott model to 0/0 heterojunctions. For example, it is not clear whether the energy-level alignment is controlled only by the constituents of the heterojunction and is independent of the substrates and the formation sequence of the junction. In this chapter, we also discuss the electronic structures of some representative 0/0 heterojunctions and investigate the substrate effects on the energy-level alignment. In Section 6.7, we address the implications of the aforementioned for the design of ambipolar OFETs and stacked OLEDs. 

To date, a variety of connecting units have been introduced for stacked OLEDs, which include Cs BCP/ITO [65], Cs BCP, LirBCP/VzOs [66,67], LirAlqg, Li l,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBIj/FeClgrNPB [68], MgrAlqg/WOg, V2O5, 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane... 

Recently, a tandem (or stacked) OLED structure was developed in order to improve luminous efficiency and to increase lifetime. This was accomplished by vertically stacking several individual electroluminescent (EL) units, each of which comprise a HTL EML ETL, and driving the entire device with a single power source. The schematic structure of the tandem device is shown in Figure 14.19. 

In a stacked OLED having N EL units N > 1), the luminous efficiency (cd/A) can be about N times as high as that of a conventional OLED containing only one EL unit (the drive voltage will also be about N times as high as that of the conventional OLED). Therefore, the stacked OLED needs only about 1/N... 

Bimstock, J., He, G., Murano, S. et al. 2008. White stacked OLED with 351m/W and 100,000 hours lifetime at lOOOcd/m for display and lighting application. SID Inti. Symp. Dig. Tech. Papers 39 822. 

The fluorescence color converter technique [32] can, in principle, overcome much of this power loss by replacing the white light emitter with a blue-emitting organic stack, and the absorbing filters with green and red fluorescent dyes. Thus when a green pixel is desired, the OLED underneath is turned on and the blui... 

OLED devices are fabricated on a glass, plastic, metal, or ceramic substrate as a multilayer-stacked structure represented in Figure 3.1. 

Figure 6.14 illustrates an OLED microcavity structure that comprises a stack of organic layers for providing EL, an upper electrode, and a bottom bilayer electrode of metal transparent conductive layer. The thickness of the transparent conductive layer (e.g., ITO) in the OLED structures can be varied across the substrate surface so as to achieve color tuning. One typical structure of the devices is glass/Ag/ITO (with a graded film... 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

In the last century, many microstructural characterization techniques have been developed, such as electron microscopy, atomic tunneling microscopy, photoelectron spectroscopy, Raman spectroscopy, etc. The structure of the OLED-based displays is such that many pixels are arranged orderly in the x-y plane. The size and number of pixels determine the resolution and size of the display. Along the z-axis, several layers are stacked on each other. These layers... 

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