Polymer LED devices

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

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... 

For typical polymer LED device parameters, currenl is space charge limited if the energy barrier to injection is less than about 0.3-0.4 eV and contact limited if it is laiger than that. Injection currents have a component due to thermionic emission and a component due to tunneling. Both thermionic emission and tunneling... 

In a typical polymer LED device, the anode is the electrode with a high work-function used for hole injection. Usually, the polymer thin film is spun on top of the anode. On the other hand, the cathode of the device is a metal electrode with a low workfunction used for electron injection. It is usually produced by the thermal evaporation of the metal on top of the polymer film. For the hole-only devices that will be discussed here, both electrodes consist of the same metal. Therefore, there is no logical cathode and anode. However, for the sake of consistency, we still... 

FIGURE 5.27 Schematic structure of a single-layer polymer LED device formed with polyfp-phenylene vinylene). (After Burroughes, J. H., Bradley, D. D. C, Brown, A R., Marks, R. N, Mackay, K., Friend, R. H., Bum, R L., and Holmes, A. B. 1990. Nature, 347, 539.)... 

Fig. 29.2 Schematic structure of a single-layer polymer LED device formed with poly(p-phenylenevinylene) (PPV).

In addition to depth-profiling the active polymeric components of elertronic devices, IBA has proved to be a useful tool to study the diffusion of indium from the adjacent indium tin oxide (ETO) electrode into the polymer layers. Although the diffusion of In into active polymer layers does not predude the use of this comhination, the instability of the interface that was established by these experiments is certainly a cause for some concern when considering the operational lifetime of polymer LED devices. 

Polymers. The Tt-conjugated polymers used in semiconducting appHcations are usually insulating, with semiconducting or metallic properties induced by doping (see Flectrically conductive polymers). Most of the polymers of this type can be prepared by standard methods. The increasing use of polymers in devices in the last decade has led to a great deal of study to improve the processabiUty of thin films of commonly used polymers. 

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.

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. 

Polymer LEDs are similar to thin film organic molecular LEDs first reported in 1987 17). Organic molecular LEDs utilize thin films of small organic molecules rather than polymer films as the light-emitting layer. The films of small organic molecules are undoped and have electronic properties comparable to the polymer films used in polymer LEDs. In general, the device physics of polymer LEDs is... 

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. 

Recent work with multi-layer polymer LEDs has achieved impressive results and highlights the importance of multi-layer structures [46]. Single-layer, two-layer and three-layer devices were fabricated using a soluble PPV-based polymer as the luminescent layer. The external quantum efficiencies of the single-layer, two-layer, and three-layer devices were 0.08%, 0.55%, and 1%, respectively, with luminous efficiencies of about 0.5 hn/W, 3 lm/W, and 6 lm/W. These results clearly demonstrate improvement in the recombination current because of the increase in quantum efficiency. The corresponding increase in luminous efficiency demonstrates that the improvement in recombination efficiency was achieved without a significant increase in the operating bias. 

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. 

State-of-the-art polymer LEDs now have operating lifetimes and luminous efficiencies suitable for a wide variety of commercial applications. Furthermore, it is clear that the fundamental limits of polymer LED performance have not yet been reached. With improvements in material synthesis, fabrication techniques, and device design, significant increases in LED performance are to be expected. These improvements should lead to the extensive use of polymer LEDs in future display applications. 

Although the exact mechanism of the fluorenone formation is not known, it is believed that the monoalkylated fluorene moieties, present as impurities in poly(dialkylfluorenes), are the sites most sensitive to oxidation. The deprotonation of rather acidic C(9)—H protons by residue on Ni(0) catalyst, routinely used in polymerization or by metal (e.g., calcium) cathode in LED devices form a very reactive anion, which can easily react with oxygen to form peroxides (Scheme 2.26) [293], The latter are unstable species and can decompose to give the fluorenone moiety. It should also be noted that the interaction of low work-function metals with films of conjugated polymers in PLED is a more complex phenomenon and the mechanisms of the quenching of PF luminescence by a calcium cathode was studied by Stoessel et al. [300],... 

Other fluorene-oxadiazole copolymers, such as fully conjugated 268a,b [360] or 269a-c, with conjugation interrupted by o-links [361], have been synthesized. For both series of polymers, emission was in the blue region at very similar wavelengths, but no LED device was reported. 

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