Triplet harvesting

Figure 4.47. Illustration of triplet harvesting. In the absence of a triplet emitter, the triplet excitation energy is converted to heat, losing 75% of the quantum efficiency. Also shown are a variety of organometallic complexes, and their relative spin-orbit coupling values - directly proportional to their use in phosphorescent OLEDs. Reproduced with permission from Yersin, H. Top. Curr. Chem. 2004, 241, 1. Copyright 2004 Springer Science and Business Media.

In conclusion, the process of inter-system crossing in organometallic compounds with transition metal ions is fast and efficient for all compounds shown in Fig. 7. The quantum efficiency for this process is often nearly one (e.g., see [4]). Therefore, an emission from the Si state is not observable. This property is the basis of the triplet harvesting effect discussed above. 

The triplet-harvesting mechanism was confirmed by efficiency analysis of the BIY device spectra. A blue fluorescent (BE) device was constructed similarly to the BIY except that the phosphorescent emitter was omitted. The blue portion of the spectrum (380-512 nm) of the BE device exhibits 4.9% EQE, indicating very efficient fluorescence. Figure 14.52 shows that devices B Y and BE have nearly identical spectral radiance for the blue component of the electroluminescence. Thus, fluorescence remained the same while the addition of the phosphorescent dopant to the BE device resulted in the appearance... 

Kondakova, M. E., Giesen, D. Deaton, J. C. et al. 2008. Highly efficient fluo-rescent/phosphorescent OLED devices using triplet harvesting. SID Inti. Symp. 

YER 11] Yersin H., Rausch A.F., Czerwieniec R. et al., The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs , Coordination Chemistry Review, vol. 255, nos. 21-22, pp. 2622-2652, 2011. 

Explain how triplet harvesting improves the efficiency of OLEDs. 

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