Built OLEDs

Figure 13-4. Encigy level diagnim of a single-layer OLED, where the organic malerial is depicted as a fully depleted semiconductor. The valence band Ey corresponds to the HOMO and the conduction band Ec corresponds to the LUMO. Tile Fermi levels of the two metal electrodes are marked as Et-. Upon contact a built-in potential is established and needs to be compensated for, before the device will begin to operating.

Another way to measure the Vhi is by means of photovoltaic measurements [97, 113. The technique is based on the fact that, at near zero applied bias, the OLED acts as a photovoltaic cell, where photogencraled carriers drift under the influence of Vhi to produce a current in an external cireuit. In a way similar to electroabsorption, an external bias is applied in order to compensate the built-in potential and null the net pholocurrent (Fig. 13-6). However, it has been shown that the measurement produces accurate results only at low temperatures, where diffusive transport of charges that are phoiogcneraled at the interlaces is negligible [97]. 

Recently, Mailiaras et al. [ 1511 have shown that for the analysis of the current-voltage characteristics of single layer OLEDs, it is of fundamental importance to properly account for the built-in potential. The electrical characteristics of MEH-... 

A similar ITO/TPD (see fig. 117) OLED was built, with [Nd(8-Q)3] as luminescent active layer the device has an operative voltage of 30 V, corresponding to a current density of 78 mAcm-2 the threshold voltage is 13 V ( 0.23 mAcm-2) and no visible luminescence is emitted (Khreis et al., 2000). 

The built-in potential plays an important role in determining the OLED performance. The net voltage across the organic layer is therefore the built-... 

The most important process worldwide is the Shell higher-olefin process (SHOP) in which ethylene is oligomerized to higher-molecular-mass, linear, a-ole-fins. The nickel-catalyst, containing a phosphorus/oxygen chelate ligand, is dissolved in the polar solvent 1,4-butanediol, which is not miscible with the a-olefins. Two big plant with a total capacity of 1 Mio. ty 1 are built in Geismar (USA) and Stanlow (UK). 

Even the colours of television screens and computer monitors make use of solids that have been developed by chemists. Gone are the days of power-hungry, bulky cathode-ray tubes. Now we are in the world of liquid crystals, plasma displays, and OLEDs (organic light-emitting diodes). The liquid crystals and OLEDs are formed of molecules built by chemists that respond in special... 

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