Electron injecting materials

In order to achieve better hole or electron injection for enhancement of device performance, introduction of appropriate hole-transport and electron-injection materials into the interfaces of anode/PFs and PFs/cathode, respectively, is another useful approach in addition to the chemical and physical modifications of PF chain structure. Besides, choosing a cathode material with a work function close to LUMO levels of PFs has been proven to be a practical method for performance improvement. 

Introduction of a material into an OLED can affect the performance of the device in regions that are remote from the new material. For example, a new electron-injection material (EIM) can make electron injection from the cathode more facile, thus causing the electrons to drive further into the device. This can cause the recombination zone to move to a more (or less) favorable location. Alternatively, a highly efficient dopant might also be a charge trap or transporter, which can lead to an expansion or contraction of the... 

Xue, S.,Yao, L., Shen, E, Gu, C., Wu, H., Ma,Y, 2012. Highly efficient and fully solution-processed white electroluminescence based on fluorescent small molecules and a polar conjugated polymer as the electron-injection material. Adv. Funct. Mater. 22,1092-1097. 

Wakimoto, T., et al. 1997. Organic EL cells using alkaline metal compounds as electron injection materials. IEEE Trans Electron Dev 44 1245. 

Fukada, T, Kanbara, T, Yamamoto, T., Ishikawa, K., Takezoe, H., and Fukada, A., Polyquinoxaline as an excellent electron injecting material for electroluminescent device, Appl. Phys. Utt., 68, 2346-2348 (1996). 

Due to the high commercial potential of CP-LEDs, the search for better fabrication methods as well as better materials has also continued aggressively. For example, Fukuda et al. [788] recently studied poly(quinoxaline-5,8-diyl) (P(Qx), structure shown in Fig. 16-27. as a superior electron injection material. A device configuration such as ITO/P(PV)/P(Qx)/Metal was shown to be more efficient than alternative configurations without P(PV) and P(Qx). Fig. 16-27b compares these several configurations schematically, whilst Fig. 16-27c shows representative luminescence characteristics of the first configuration. 

A more effective carrier confinement is offered by a double heterostructure in which a thin layer of a low-gap material is sandwiched between larger-gap layers. The physical junction between two materials of different gaps is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in figure C2.l6.l0. The electrons, injected under forward bias across the p-n junction into the lower-bandgap material, encounter a potential barrier AE at the p-p junction which inliibits their motion away from the junction. The holes see a potential barrier of... 

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... 

There are many organic compounds with useful electronic and/or optical properties and with sufficiently high volatility to be evaporable at a temperature well below that at which decomposition occurs. Since thermal evaporation lends itself to facile multilayering, organic compounds may be selected for use in one or more function electron injection, electron transport, hole injection, hole transport, andI or emission. A complete list of materials that have been used in OLEDs is too vast to be included here. Rather, we list those that have been most extensively studied. 

PPV and its alkoxy derivatives are /j-type conductors and, as a consequence, hole injection is more facile than electron injection in these materials. Efficient injection of both types of charge is a prerequisite for efficient LED operation. One approach to lowering the barrier for electron injection is the use of a low work function metal such as calcium. Encapsulation is necessary in this instance, however, as calcium is degraded by oxygen and moisture. An alternative approach is to match the LUMO of the polymer to the work function of the cathode. The use of copolymers may serve to redress this issue. 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

Introducing heteroaromatic moieties (mainly with N and, to a lesser extent, with O and S) in the backbone of the polymer or as a pendant group, can substantially modify the LUMO level of the materials, improving their electron-transport properties and facilitating electron injection in PLEDs, but the efficiencies still lag behind the other systems. 

Q. Pei and Y. Yang, 1,3,4-Oxadiazole-containing polymers as electron-injection and blue electroluminescent materials in polymer light-emitting diodes, Chem. Mater., 7 1568-1575, 1995. 

Due to the relatively high mobility of holes compared with the mobility of electrons in organic materials, holes are often the major charge carriers in OLED devices. To better balance holes and electrons, one approach is to use low WF metals, such as Ca or Ba, protected by a stable metal, such as Al or Ag, overcoated to increase the electron injection efficiency. The problem with such an approach is that the long-term stability of the device is poor due to its tendency to create detrimental quenching sites at areas near the EML-cathode interface. Another approach is to lower the electron injection barrier by introducing a cathode interfacial material (CIM) layer between the cathode material and the organic layer. The optimized thickness of the CIM layer is usually about 0.3-1.0 nm. The function of the CIM is to lower... 

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