Electron-transporting layer conductivity

A typical vaponr deposited EL device consists of a glass snbstrate coated with a conducting transparent indinm tin oxide electrode, on top of which is a 100-500 A hole transport layer (HTL), followed by a thin (= 100 A) light emitting layer (EML), then a 100-500 A electron transport layer (ETL) and finally a cathode of an alloy such as Mg Ag. This is illustrated in Fignre 3.32. 

As you can see, the fabrication is rather simple. Yet, the light output rivals many of the LED s and is certainly more than the output of phosphor screens in certain devices. A conductive layer is required for electron injection into the electron transport layer (ETL). The metal electrode (ME) is composed of Al, Ag or Ag-Mg, and is usualfy light reflective. The hole transport layer (HTL) Injects holes from the transparent electrode (TE) into the HIL. The point where the excitons, i.e.- pairs of holes electrons, recombine is the b ht emitting (LE) region. This can vary according to the type of compounds used to form the two organic layers. These devices can be classified as ... 

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. 

In particular the transition to multi-layer devices should bring additional possibilities for improvement in these figures, especially regarding the applied voltage and the long-term stability. Moreover, the first promising experiments have been conducted using soluble poly(2,5-dialkyl-l,4-phenylene-l,3,4-oxadiazole)s 12 as electron transport layer [33]. 

To ensure an efficient separation of the electrons and holes, transporting layers are often applied between the active layer and the different electrodes. As a holetransporting layer (HTL), the highly conductive thiophene-based polymer poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT PSS) [12] has been the most utilized and as electron transporting layer (ELT), ZnO is often used [13]. 

A ZnO nanoparticle layer is widely used as an electron transport layer. The ZnO nanoparticle has excellent electron transport behavior because it has favorable electron mobility and the conduction band level is nicely matched with the LUMO level (lowest unoccupied molecular orbital) of the PCBM. It... 

Figures 12-12 and 12-13 document that trap-free SCL-conduction can, in fact, also be observed in the case of electron transport. Data in Figure 12-12 were obtained for a single layer of polystyrene with a CF -substituted vinylquateiphenyl chain copolymer, sandwiched between an ITO anode and a calcium cathode and given that oxidation and reduction potentials of the material majority curriers can only be electrons. Data analysis in terms of Eq. (12.5) yields an electron mobility of 8xl0 ycm2 V 1 s . The rather low value is due to the dilution of the charge carrying moiety. The obvious reason why in this case no trap-limited SCL conduction is observed is that the ClVquatciphenyl. substituent is not susceptible to chemical oxidation.

If the film is nonconductive, the ion must diffuse to the electrode surface before it can be oxidized or reduced, or electrons must diffuse (hop) through the film by self-exchange, as in regular ionomer-modified electrodes.9 Cyclic voltammograms have the characteristic shape for diffusion control, and peak currents are proportional to the square root of the scan speed, as seen for species in solution. This is illustrated in Fig. 21 (A) for [Fe(CN)6]3 /4 in polypyrrole with a pyridinium substituent at the 1-position.243 This N-substituted polypyrrole does not become conductive until potentials significantly above the formal potential of the [Fe(CN)6]3"/4 couple. In contrast, a similar polymer with a pyridinium substituent at the 3-position is conductive at this potential. The polymer can therefore mediate electron transport to and from the immobilized ions, and their voltammetry becomes characteristic of thin-layer electrochemistry [Fig. 21(B)], with sharp symmetrical peaks that increase linearly with increasing scan speed. 

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