Device Architectures

In addition to improving structural order, reliable improvements of device performance have been demonstrated through optimization of device interfaces and architectures. The architectures are perhaps more easily addressed, and are also related to improving the structural order. Several possible constructions of TFT are known from the long history of silicon-based devices. These were the first architectures adopted for OTFTs also. Top and bottom contact, indicating the location of the source and drain electrodes with regard to the semiconductor, are the most widely used. 

The performance of bottom-contact devices, fairly easy to fabricate with traditional lithographic equipment, can also be improved by control of the interface where the metal source and drain layers make contact with the semiconductor. 

The consistent use of top-contact architecture with development of a variety of surface modifications to alter the interface where the semiconductor is deposited, detailed below, have contributed to reports of steadily increasing performance in pentacene and other organic materials. 

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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 ... 

By changing the device architecture e.g. by building multi- instead of single layer structures the physical and chemical processes in the LED can be greatly altered. For that reason the fundamental properties of the LED, such as threshold voltage, efficiency, emission color, brightness, and lifetime can be optimized in multilayer structures [43J. 

Poly(l,4-phenylene vinylcne) and its Derivatives 2 The Basic Polymer LED Device Architecture 4 Substituted Poly(phcnylene vinylcne)s 6 Poly(anthrylenevinylcne)s 10 Step-Growth Routes to PPV Derivatives 10 PPV Copolymers 11... 

Despite the frequent use of arc-discharge and laser ablation techniques, both of these two methods suffer from some drawbacks. The first is that both methods involve evaporating the carbon source, which makes it difficult to scale up production to the industrial level using these approaches. Second, vaporization methods grow CNTs in highly tangled forms, mixed with unwanted forms of carbon and/or metal species. The CNTs thus produced are difficult to purify, manipulate, and assemble for building nanotube-device architectures in practical applications. 

Tour JM (2003) Molecular electronics commercial insights, chemistry, devices, architecture and programming. World Scientific, Singapore... 

Section 1.2 gives a brief review of conjugated polymers in semiconducting and metallic phases. Section 1.3 discusses device architectures and their corresponding processes. Section 1.4 introduces some novel devices and their functions in thin-film polymer devices. Section 1.5 is devoted to technical merits of SMOLEDs and PLEDs used as emitter elements in flat-panel displays. 

Vapor-Deposited Organic Light-Emitting Device Architectures.529... 

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. 

VAPOR-DEPOSITED ORGANIC LIGHT-EMITTING DEVICE ARCHITECTURES... 

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). 

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. 

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. 

If a p-i-n (i.e., LEC) device architecture is used, the choice of the ionic dopant and surfactant additives to the light-emitting organic ink are critical to the print uniformity and resulting device performance and stability. The ionic dopants have limited solubility and can prematurely fall out of solution during the printing process. Most of the previous works in... 

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