Charge OLEDS

The processes of charge injection, transport, and recombination dictate the recombination efficiency h(/), which is the fraction of injected electrons that recombine to give an exciton. The recombination efficiency, which is a function of the device current, plays a primaty role in determining the amount of emitted light, therefore determining the OLED figurcs-of-meril. For example, the quantum efficiency /y(/) (fraction of injected electrons that results in the emission of a photon from the device) is, to a first approximation, given by ... 

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]. 

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... 

Closely related to the anode modifications described above, the use of a HIL material to improve charge injection into the OLED device has spawned a number of materials, which have been shown to provide benefits, particularly in terms of lower operating voltages and extended lifetimes of devices. 

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... 

Schiff bases with intramolecular charge transfer complexes such as 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile zinc (II) (BDPMB-Zn, 187) emit red fluorescence with fluorescent quantum yields up to 67%. OLEDs with a structure of ITO/TPD/ TPD BDPMB-Zn/Alq3 BDPMB-Zn/Alq3/Mg-Ag showed very bright saturated red emission with CIE (0.67, 0.32) with a luminance of 2260 cd/m2 at 20 V and a current efficiency of 0.46 cd/A (at 20 mA/cm2). In addition, the EL spectra do not change with the doping concentration in the range of 0.5—3% [229]. 

The holes are the majority charge carriers in OLEDs and the hole current is much larger than the electron current in OLEDs, that is,. /h > Je. Thus, the total current Jtot =, /h I /e can be simplified as Jtot, /h. Therefore, Equation 6.2 can be written as... 

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