Recombination phosphorescence devices

Some of these carriers may recombine within the emissive layer yielding excited electron-hole pairs, termed excitons. These excitons may be produced in either the singlet or triplet states and may radiatively decay to the ground state by phosphorescence (PL) or fluorescence (FL) pathways (Fig. 1-2). An important figure of merit for electroluminescent materials is the number of photons emitted per electron injected and this is termed the internal quantum efficiency. It is clear, therefore, that the statistical maximum internal efficiency for an EL device is 25% as only one quarter of the excitons are produced in the singlet state. In practice, this maximum value is diminished further because not all of the light generated is visi-... 

TCTA, TPBI, and SBFK were used as model cohost materials to demonstrate that mixed EML architecture offers a significant advantage in device performance compared to the conventional neat-host devices and can be used as an alternative approach to the design of triplet OLEDs. The most important feature of this architecture is that the simple variation of the cohosts ratio "tunes up" the location of recombination in the mixed EML, allowing simplification of required properties of the materials used in phosphorescent OLEDs. 

Phosphorescent EL materials are shown in Fig. 14-19 [116]. Singlet and triplet excitons are formed in the ratio of 1 3 by the recombination of electrons and holes. For this reason, fluorescent materials can give a maximum luminescence yield of only 25% internal efficiency. Since the yield of transmission of the luminescence through the layer is around 20%, the total external efficiency is around 5%. Phosphorescent compounds can utilize the triplet excited state to emit luminescence, so that the use of both fluorescent and phosphorescent compounds could give theoretically 100% internal quantum efficiency. Actually a device with 15% external luminescence efficiency has been reported. One of the topics in EL-device research is how to realize luminescence of 100% internal quantum efficiency. The efficiency of fluorescence is often near 100%, but that of phosphorescence is usually not very high. Eu or Tb ions give efficient phosphorescence, and so some complexes of Eu or Tb have been studied for EL devices, but there has been no report of efficient luminescence. 

---