OLED construction

Although PPPs and its derivatives reveal extraordinarily high thermal and oxidative stabilities, corresponding single-layer OLEDs exhibit only low electroluminescence efficiencies. Higher efficiencies have been achieved by preparing polymer blends or by virtue of two-layer OLED-constructions. External efficiencies up to 3% were determined for an ITO/PVK/poly(2-decyloxy-l,4-phenylene)/Ca — OLED [86,87]. 

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. 

CDT, based in Cambridge, U.K., was founded after initial work done at the Cambridge University. Researchers in Cambridge discovered that poly(/>-phenylenevinylene) (PPV, Figure 11.16) and its derivatives can be used as emitters to construct OLEDs, which were the first examples of PLEDs [4], The invention has brought flat-panel displays to the verge of commercialization. 

Figure 4.33. (a) Sketch of the basic construction of an OLED. (b) Schematic diagram showing the energy-level alignment for the OLED ITO/CuPc/NPB/Alqs/Mg. Adapted from Lee et al, 1999. 

The configuration and construction of monolayer and multilayer OLEDs have undergone substantial changes and modifications since these first reports of organic electroluminescence from low-molar-mass materials. Several types of OLEDs using small organic and organometallic molecules are described schematically below. 

There are several possibilities for constructing bilayer OLEDs with a more balanced charge injection. These include an electron-transport layer (ETL) and a combined hole-transport (HTL) and emission layer. Conversely a hole-transport layer and a combined electron-transport and emission layer is also effective. 

The thulium (III) ion exhibits spectrally narrow light emission at about 480 nm. Li and coworkers were the first to use the Tm + ion in OLEDs [65]. They prepared a Tm complex Tm(acac)3(phen) and constructed double-layer cells with structure ITO/PVK/Tm complex/Al. The electroluminescence spectrum of the OLED with drive voltage 10 V and the photoluminescence spectrum with excitation wavelength at 350 nm are shown in Figure 11.29. The emitting intensity of 6.0cdm was achieved when a 16 V forward bias voltage was applied. 

Based on the above requirements, it is easy to recognize that it is very difficult to satisfy a single HTL with all of these assignments. In particular, requirement 1 is incompatible with requirement 4, resulting in a construction of double HTLs. In a similar manner, using a double ETL is necessary to enhance OLED performance. 

In the scope of this chapter, organometallic triplet emitters are of particular interest due to their promising use in electro-luminescent devices such as OLEDs (organic/organometallic light emitting diodes). (See for example [11-16].) In Sect. 2, the construction and working principle of an OLED is... 

Another interesting finding based on the thermodynamic data is the low A fH0A (-28.9 kcal/mol or -121.0 kj/mol) of the Ca/CsF system. The combined used of Ca and CsF should conceptually construct a novel cathode configuration in OLEDs. Our device studies in Section 5.5 have clarified this presumption. The device performances are also dependent of other factors, e. g. they may be deteriorated by exciton quenching that arise from the gap-state formation over a rather thick interface region due to the highly exothermic reaction between the Ca and CsF. 

A recent OLED design involved a three-color emission to produce a white-light emitting OLED, as shown in 7,2.35., given on the next page. Note that the construction is similar to that we have already presented. The only difference is that three different colored emitting strata are involved ... 

Microcavity OLEDs fabricated on distributed Bragg reflectors (quarter wave stacks) [94]. Such OLEDs are fabricated on dielectric layers with significant dielectric contrast, so they narrow the emission spectrum by constructive interference. The narrow emission spectra also result in more efficient and more stable devices than regular OLEDs. The emission spectrum can be tailored to the specific sensor requirements (i.e., the absorption peak of the sensing element). 

Using both low-molecular evaporated films and polymer films, multilayer OLEDs of high efficiency have been constructed. The structures of some molecules often used for evaporated films are shown in Fig. 11.4, and the monomers for polymer films in Fig. 11.5. The purpose of the fabrication of multilayer OLEDs is the independent optimisation of the individual processes which were described in Sect. 11.1.2, with the goal of achieving high-efficiency OLEDs injection and transport of the electrons and holes, balance between the currents of electrons and holes, recombination, use of triplet states, and reduction of reflections in the transmission of the luminescence to the outside of the OLED. In the following, we will treat a few typical examples of OLEDs prepared with low-molecular evaporated films. We emphasize, however, that also multilayer OLEDs made with polymers can yield comparable results and information. 

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