Electron-transport layer

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

Figure 11-18. Calculated current (solid line) and iccuinbiiuuion current (dashed line) density as a function of voltage bias for a single-layer structure, a two-layer structure with a hole-blocking layer and a two-layer structure in which the hole-blocking layer also serves as an electron transport layer.

Another approach to molecular assembly involves siloxane chemistry [61]. In this method, the electrically or optically active oligomers are terminated with tii-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular monolayer on the electrode surface, in order to modify its injection properties. 

Using a stable dopant as the emissive dye has been shown to greatly enhance the lifetime of small molecule LEDs. Rubrene doped into the Alq, electron transport layer ] 184] or into the TPD hole transport layer 1185] can extend the lifetime by an order of magnitude. Similarly, dimclhylquinacridone in Alq has a beneficial effect ]45 ]. The likely mechanism responsible for this phenomenon is that the dopant acts as a trap for the excilon and/or the charge. Thus, molecules of the host maLrix are in their excited (cationic, anionic or cxcitonic) states for a smaller fraction of the time, and therefore have lower probability to undergo chemistry. 

Figure 13-1. Encigy level diagrams under forward bias, (a) Single-layer device Iransports both holes and clccu ons and emits (b) iwo-layer device with hole and electron transport layers, one or both of which may emit (c) three-layer device with emitting dye doped (here) into a thin region of the electron transport layer.

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

N. Donze, P. Pechy, M. Gratzel, M. Schaer, and L. Zuppiroli, Quinolinate zinccomplexes as electron transporting layers in organic light-emitting diodes, Chem. Phys. Lett., 315 405-410 (1999). 

A higher efficiency, yet simpler structure PPLED device fabricated with the same dopant and host materials was almost simultaneously reported by Yang and Tsutsui [35]. The highest EQE of their device ITO/PVK 5%Ir(ppy)3 /OXD-7/Mg Ag (where ITO is indium tin oxide) (using OXD-7 (7) as an electron-transporting layer (ETL), Chart 4.3) reached the value of 7.5%, which was the first reported PLED with external efficiency above 5%, an upper limit of the fluorescent PLEDs. The power efficiency was 5.8 lm/W at the luminance of 106 cd/m2. 

In OLEDs, the positive and the negative charges are injected from the anode and the cathode, respectively. These charges then move through hole and electron transport layers, and meet in... 

Electron-transfer sensitization, 19 109 Electron transport, between photosystem inhibitors, 13 288 Electron-transport layer (ETL)... 

Figure 3.26. Structure of an OLED. S = substrate (glass), ANO = anode (e.g., ITO — indium tin oxide), HIL = hole injection layer (e.g., Cu phthalocyanine), HTL = hole transport layer, EML = emission layer, ETL = electron transport layer, EIL = electron injection layer (e.g., LiF), KAT = cathode (e.g., Ag Mg, Al). The light that is generated by the recombination of holes and electrons is coupled out via the transparent anode.

Figure 3.27. Energy level scheme of the device in Figure 3.26, consisting of the electrode work functions and the molecular HOMOs and LUMOs. The relative energy level of HOMOs and LUMOs can he determined hy cyclic voltammetry and optical spectroscopy. Note the hole blocking character of the electron-transport layer. This feature is important since holes that proceed via the HOMO levels have much higher mobilities than electrons proceeding via the LUMO levels.

The electron mobility of oxadiazoles have been measured in a polymer matrix, values of 10 7 up to 10 3 cm2/Vs have been obtained [262, 263], These values are exceeded by starburst phenylquinoxalines (30) that approach 10-4 cm2/Vs at 106 V/cm [264]. Other material classes that are very interesting candidates for electron-transport layers comprise naphtalene-, 60, and perylenetetracarboxylic diimides, 59 [265], as well as bathophenanthroline [266] with reported electron mobilities of 10 3 and 4.2 x 10 4cm2/Vs, respectively. 

Fournet P, Coleman JN, Lahr B, Drury A, Blau WJ, O Brien DF, Horhold HH (2001). Enhanced brightness in organic light-emitting diodes using a carbon nanotube composite as an electron-transport layer. J. Appl. Phys. 90 969-975. 

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