Device structure

The materials described in the preceding section may be combined in an OLED device in a variety of different geometries and compositions. The simplest of these is a single organic layer sandwiched between two electrodes. In contrast to the convention used in surface science, it is customary to list the layers in the order of deposition. Thus, anode/organic/cathode (for example, ITO/PPV/Al) implies that die anode (ITO) is deposited first on the (presumably transparent) substrate. 

By using multiple organic layers one may immediately overcome the restrictions imposed on a single material. This realization was the breakthrough in the field 

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an exciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Thus, for example, an electron trap in the ETL will capture and hold an electron until a hole is injected nearby from the HTL. In tliis case, the dopant is the recombination 

13 The Chemistry, Physics and Engineering of Organic Light-Emitting Diodes 

Clearly additional layers may be used to accomplish other benefits, tailoring the energy profiles and mobilities across the entire organic stack. Splitting the transport layer(s) into two separate layers permits the optimization of injection into the layer nearest the electrode (sometimes called the injection layer), and transport in the farther layer [101]. Layers of insulator (charge confinement layers) have also been used in an attempt to control the motion of the charges and ensure recombination in the desired region [102]. 

---

In real device structures like heterojunction bipolar transistors, certain features in the PR spectrum can be correlated with actual device performance. Thus PR has been employed as an effective contacdess screening technique to eliminate structures that have imwanted properties. 

Figure 1-3. In Ihis improved bilaycr device structure lor a polymer LED an extra ECHB layer has been inserted between the PPV and the cathode metal. The EC11B material enhances the How of electrons but resists oxidation. Electrons and holes then accumulate near the PPV/EC1113 layer interface. Charge recombination and photon generation occurs in the PPV layer and away from the cathode.

In the many reports on photoelectron spectroscopy, studies on the interface formation between PPVs and metals, focus mainly on the two most commonly used top electrode metals in polymer light emitting device structures, namely aluminum [55-62] and calcium [62-67]. Other metals studied include chromium [55, 68], gold [69], nickel [69], sodium [70, 71], and rubidium [72], For the cases of nickel, gold, and chromium deposited on top of the polymer surfaces, interactions with the polymers are reported [55, 68]. In the case of the interface between PPV on top of metallic chromium, however, no interaction with the polymer was detected [55]. The results concerning the interaction between chromium and PPV indicates two different effects, namely the polymer-on-metal versus the metal-on-polymer interface formation. Next, the PPV interface formation with aluminum and calcium will be discussed in more detail. 

The current-voltage and luminance-voltage characteristics of a state of the art polymer LED [3] are shown in Figure 11-2. The luminance of this device is roughly 650 cd/m2 at 4 V and the luminous efficiency can reach 2 lm/W. This luminance is more than adequate for display purposes. For comparison, the luminance of the white display on a color cathode ray tube is about 500 cd/m2l5J. The luminous efficiency, 2 lm/W, is comparable to other emissive electronic display technologies [5], The device structure of this state of the art LED is similar to the first device although a modified polymer and different metallic contacts are used to improve the efficiency and stability of the diode. Reference [2] provides a review of the history of the development of polymer LEDs. 

Figure 11-1. Chemical structure of poly(pura-phenylene vinylene) (PPV) and schematic polymer LED device structure.

The utility and importance of multi-layer device structures was demonstrated in the first report of oiganic molecular LEDs [7]. Since then, their use has been widespread in both organic molecular and polymer LEDs [45, 46], The details of the operating principles of many multi-layer structures continue to be investigated [47—49], The relative importance of charge carrier blocking versus improved carrier transport of the additional, non-luminescent layers is often unclear. The dramatic improvements in diode performance and, in many cases, device lifetime make a detailed understanding of multi-layer device physics essential. 

The paper is oiganized to describe, first, the materials that have been used in OLEDs, then the device structures that have been evaluated. After a description of the methods used to characterize and evaluate materials and devices, we summarize the current stale of understanding of the physics of device operation, followed by a discussion of the mechanisms which lead to degradation and failure. Finally, we present the issues that must be addressed to develop a viable flat-panel display technology using OLEDs. Space and schedule prevent a comprehensive review of the vast literature in this rapidly moving field. We have tried to present... 

Figure 15-29. Chemical structures of the conjugated polymers used in the device and the device structure of the laminated solar cell. For the top half of the device, A1 or Ca was evaporated on glass substrates, and the acceptor material MEH-CN-PPV (and a small amount of POPT, usually 5%) was spin coaled. The half with the POPT (and a small amount of MEH-CN-PPV, usually 5%) was spin coaled on 1TO substrates and heated to 200"C under vacuum belore the device was laminated together by applying a light pressure.

In this section the electronic structure of metal/polymcr/metal devices is considered. This is the essential starting point to describe the operating characteristics of LEDs. The first section describes internal photoemission measurements of metal/ polymer Schottky energy barriers in device structures. The second section presents measurements of built-in potentials which occur in device structures employing metals with different Schottky energy barriers. The Schottky energy barriers and the diode built-in potential largely determine the electrical characteristics of polymer LEDs. 

Maurice H. Francombe, Non-Crystalline Films for Device Structures, Volume 29, 2002. 

RESEARCH UNIVERSITIES ARE IN THE MIDST OF MAJOR CHANGE. Historically, the research universities have been supported by the Government with two theories in mind (1) national security is important, and science and technology are critical to a strong defense and (2) human health is important. The interest in human health persists, an interest in national security persists, but the adversary has given up. The Soviet Union no longer exists. The question now is, What is the rationale for the support of universities—support in the post-Cold War era The Department of Defense, which has nurtured an important set of activities, has a role in electronics and devices, structural materials, and high-performance or advanced-performance materials. 

Figure 11. The effect of reflow nearby the glass transition temperature for a PMMA-DR1 waveguide (a) device structure, (b) SEM picture of waveguide before reflow, (c) idem after reflow.

Ashwell GJ, Berry M (2005) Hybrid SAM/LB device structures manipulation of the molecular orientation for nanoscale electronic applications. J Mater Chem 15 108-110... 

Fig. 12. The time dependence of the threshold voltage shift for the same device structure as in Fig. 11 after removal of the bias voltage (Jackson et al., 1989a). 

Researchers have many obstacles to overcome in the quest to make macroelectronics the next big thing. The keys to achieving the desired levels of functionality for a wide range of large-area electronic functions are advances in materials and processes and device structures that can get cost down to pennies (rather than dollars) per square centimeter.Tools and process methods... 

Figure 10.13. (a) SEM image of ZnO nanorods coated with octylamine. Scale bar, 200 nm. (b) Uniform nanorod film fabricated by spin coating of ZnO nanorods. Scale bar, 500 nm. The nanorods assemble into domains with nematic ordering, (c) Saturated transfer characteristics for a thin-film transistor fabricated by spin coating of ZnO nanorods with different ligands octylamine (solid line), butylamine (dashed line). Vi = 60V. (d) Output characteristics of a spin-coated device made from octylamine-stabilized ZnO nanorods.The device structure is shown in the inset in (c). Reproduced from Ref. 83, Copyright 2006, with permission from the American Chemical Society. 

---