Barriers OLEDs

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

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... 

It is known that most metals possess lower gas permeability than plastics by 6-8 orders of magnitude. An unbreakable and lightweight thin stainless steel foil substrate has been used for flexible OLEDs [75]. Therefore, a several micrometers thick metal layer can serve as a highly effective barrier to minimize the permeation of oxygen and moisture. Hence, the... 

The criteria for good electron transport materials are that they should transport electrons, block holes, and have a small barrier to electron injection from the metal cathode. The most commonly used ETL in vacuum-deposited OLEDs is tris-(8-hydroxyquinoline) aluminum (Alq3), as shown in Figure 7.7. Alq3 for example, has a LUMO energy level of 3 eV [65] and an electron mobility of 5x 10 5 cm2/(V s) [66]. 

In demanding applications, for example barriers for OLED displays, control of polymer smoothness and cleanliness by the routes mentioned above may not be sufficient. In such circumstances it is necessary to lay down a further coating under clean-room conditions which acts both as a planarizing coating and as a hard coat to prevent scratching on subsequent processing. This has been discussed elsewhere [6, 12, 13]. 

If we compare the data in Table 8.5 with the barrier requirements set in polymer electronics (Fig. 8.11), it is evident they cannot be met with metallized films, not even with ultra-high-barrier films, multi-layer structures from metal evaporation, and polymeric layers. For transparent barriers, as required for OLEDs, displays, organic solar cells, etc., evaporated oxide layers are even further from meeting the values required. 

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