OLEDs device architecture

Figure 7.5 shows a schematic example of the electroluminescent process in a typical two-layer OLED device architecture. When a voltage is applied to the device, five key processes must take place for light emission to occur from the device. 

The presented results show that the properties and performance of the various organic OLEDs are quite different from each other. This is due to the different types of the active organic layer, the electrodes, the device architecture, and the preparation conditions ... 

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 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 following sections will first describe the major components of a typical bottom-emitting OLED the transparent anode, the organic layers, and the metal cathode. Alternative device architectures are also briefly described. 

Other device architectures include inverted OLEDs. Here the cathode is in intimate contact with the substrate. The organic layers are then deposited onto the cathode in reverse order, i.e., starting with the electron transport material and ending with the HIL. The device is completed with an anode contact. In this case, as above, one of the electrodes is transparent, and light exits from the device through that contact. For example, Bulovic et al. [38], fabricated a device in which Mg/Ag was the bottom contact and ITO the top electrode. The advantage of this type of architecture is that it allows for easier integration with n-type TFTs (see Section 7.5 for a discussion of active-matrix drive OLED displays). 

Light emission Light is observed from photons that exit the OLED structure. Typically many photons are lost due to processes such as total internal reflection and selfabsorption of the internal layers [71]. In typical bottom-emitting device architectures, only 20-30% of the photons created exit the device through the front of the substrate. 

Another potential application for LEDs is in illumination. The requirements for devices that serve as illumination sources are somewhat different than the monochromatic OLEDs described above. OLEDs targeted for RGB displays have to give electroluminescent spectra with a relatively narrow line shape centered on the peak wavelength. On the other hand, an illumination source is meant to approximate the blackbody solar spectrum and needs to have a broad line shape with roughly equal intensity across the entire visible spectrum. Therefore, in order to attain complete coverage across the visible spectrum, an OLED used for illumination purposes typically employs multiple emitters are that are either co-deposited into a single emissive layer or distributed into different layers or regions of the device. A number of the different device architectures have been reported to achieve efficient white EL and are discussed below. 

OLED Materials and Device Architectures for Full-Color Displays and Solid-State Lighting... 

The luminous efficiency of OLEDs can be improved Imther by use of phosphorescent emissive materials. Incorporated into OLED devices starting in the 1990s, phosphorescent dopants have a potential to achieve 100% internal quantum efficiency. We will describe materials and architecture developments of phosphorescent OLED devices in Section 14.4. Finally, we discuss the future outlook of the OLED technology in Section 14.5. 

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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