Basic Structure of OLEDs

The foregoing section attempted to provide an introduction to the dynamics of singlet excitons, generated either by photoexcitation or by polaron recombination, and the effects of polarons and TEs on the SE dynamics. We now turn to the basic structure and dynamics of OLEDs, which obviously reflect the basic processes described above. 

The cathode is typically a low-to-medium workfunction ( / ) metal such as Ca (0 = 2.87 eV), A1 ( / = 4.3 eV),15 or Mgo.gAgo.i (for Mg, p = 3.66 eV)5 deposited either by thermal or e-beam evaporation. However, in case of A1 or Ca, addition of an appropriate buffer layer between the top organic layer and the metal cathode improves the device performance considerably. This issue is discussed in some detail in Sec. 1.5.8 below. 

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The basic structure of OLEDs consists of multilayers of organic maferials sandwiched befween two electrodes (Figure 9.1). The total thickness of organic materials is usually of fhe order of 100 nm and is comparable to fhe emission wavelengfh. 

Electroluminescence (EL) is the phenomenon by which electrical energy is converted into luminous energy by the recombination of electrons and holes in the emissive material [8], The basic structure of an OLED consists of a thin film of organic material sandwiched between two electrodes, an anode of high-work-function material such as indium tin oxide (ITO) on a glass substrate, and a cathode of a low-work-function metal such as calcium (Ca), magnesium (Mg), or aluminum (Al) or an alloy such as Mg Ag. 

The basic structure of a typical dc-biased bilayer OLED is shown in Figure 1.5. The first layer above the glass substrate is a transparent conducting anode, typically indium tin oxide (ITO). Flexible OLEDs, in which the anode is made of a transparent conducting organic compound, e.g., doped polyaniline (see Fig. 1.2),44 or poly(3,4-ethylene dioxy-2,4-thiophene) (PEDOT)-polystyrene sulfonate (PEDOT-PSS) (see Fig. 1,2)45 deposited on a suitable plastic, e.g., transparency plastic, have also been reported. 

The basic structure of a typical OLED is shown in Fig. 3.1 [35]. It consists of a transparent conducting anode, typically indium tin oxide (ITO) coated on a glass or plastic mechanical support, the organic layers, and a metal cathode. The thickness of OLEDs (excluding the mechanical support) is typically <0.5 j,m. Under forward bias electrons are injected from the low-workfunction cathode into the electron-transport layer (ETL). Similarly, holes are injected from the high-workfunction ITO into the hole-transport layer (HTL). Due to the applied bias, the electrons and holes drift toward each other, and typically recombine in a recombination zone near, or at, the ETL/HTL interface. A fraction of the recombination events forms radiative excited states. The radiative decay of these states provides the electroluminescence (EL) of the device. 

The PD, e.g., a photomultiplier tube or a Si photodiode, can be placed in front of the sensing film ( front detection ) or behind the OLED array ( back detection ). The basic structure of the integrated OLED/sensor him in the back-detection geometry is shown in Fig. 3.2. In this configuration, the PD collects the PL that passes through the gaps between the OLED pixels. The... 

Fig. 3.2. Basic structure of an integrated OLED pixel array/luminescent sensor film module in the back detection geometry...

The organic salt of tetrabutylammonium tetrafluoroborate (BujNBF ) was further dissolved into the basic organic solution at an appropriate concentration. The thickness of the spin-coated organic layer was about 80 nm. Then, an A1 cathode layer (100 nm) was formed on the top of the organic layers via thermal deposition at a rate of 0.7 nm/ s under a base pressure of 2 x 1Q Torr. In this experiment, phosphorescent OLEDs were fabricated and comprared one with BU4NBF4 (0.0050 wt%) annealed electrically at V = +7 V (forward bias) at T = 65°C the other for reference with BU4NBF4 (0.0050 wt%) annealed electrically at V = +20 V (forward bias) at T = 25°C. It should be noted that, except for the emissive layer, the device structure of the reference device was identical to that of the sample device. The structures of the devices and materials used were identical. The devices were prepared in inert Ar gas environments this preparation included electrical and thermal treatments. 

Many variations of the basic OLED device structure have been investigated. The basic OLED structure can also be used for organic photodetectors and photovoltaic cells. 

Based on the three basic OLED structures, the new multilayer structures (MH1-3) are proposed as summarized in Fig. 2.5b. MH-1 and MH-2 structures are composed of double HTLs54-56 and ETLs,57,58 as shown in Figs. 2.5a and 2.5b. The meaning of the double layers is a separation of carrier injection from an electrode and carrier transport. The following are five major requirements that the HTL should have ... 

Fig. 1 Basic set-up of a layered OLED structure. Electrons and holes are injected from the respective electrodes (metal cathode, semiconducting and transparent anode). The charge carriers move from different sides into the recombination/emitter layer, where electrons and holes recombine and excite the doped emitter molecules (asterisks, e.g., or-ganometallic triplet emitters). For more details see Fig. 2. For clarity, light emission is only shown for one direction although the photons are emitted in all directions...

All materials within an OLED structure should meet several basic physical and optical requirements. When traditional vapor deposition techniques are used to deposit thin films, all materials should be thermally stable for sublimation. Also, with the exception of dopants, and possibly injection materials used only in very thin layers, all OLED materials should form amorphous films that are morphologically and thermally stable. Finally, OLED materials present in large quantities—hosts and transport materials—should also have little or no absorption in the light emission region. 

There are two approaches for realizing a white OLED. The first approach consists in using a multilayer structure where the resulting light is composed of three basic RGB components (Figure 15.3). 

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