Electrophosphorescence devices

Red electrophosphorescent devices employing exciton blocking have been demonstrated.222 These devices contained the luminescent dye Pt(OEP)], doped into a 4,4 -/V, N dicarbazolebi-... 

W Zhu, Y Mo, M Yuan, W Yang, and Y Cao, Highly efficient electrophosphorescent devices based on conjugated polymers doped with iridium complexes, Appl. Phys. Lett., 80 2045-2047, 2002. 

X Gong, JC Ostrowski, MR Robinson, D Moses, GC Bazan, and AJ Heeger, High-efficiency polymer-based electrophosphorescent devices, Adv. Mater., 14 581-585, 2002. 

Electron-transporting materials for organic electroluminescent and electrophosphorescent devices G. Hughes and M.R. Bryce J. Mater. Chem., 15 94-107... 

Y. Su, H. Huang, C. Li, C. Chien, Y. Tao, P. Chou, S. Datta, and R. Liu, Highly efficient red electrophosphorescent devices based on iridium isoquinoline complexes remarkable external quantum efficiency over a wide range of current, Adv. Mater., 15 884-888 (2003). 

There is no reason why the same principle cannot be applied for light-emitting polymers as host materials to pave a way to high-efficiency solution-processible LEDs. In fact, polymer-based electrophosphorescent LEDs (PPLEDs) based on polymer fluorescent hosts and lanthanide organic complexes have been reported only a year after the phosphorescent OLED was reported [8]. In spite of a relatively limited research activity in PPLEDs, as compared with phosphorescent OLEDs, it is hoped that 100% internal quantum efficiency can also be achieved for polymer LEDs. In this chapter, we will give a brief description of the photophysics beyond the operation of electrophosphorescent devices, followed by the examples of the materials, devices, and processes, experimentally studied in the field till the beginning of 2005. 

X Yang, D Neher, D Hertel, and TK Daubler, Highly efficient single-layer polymer electrophosphorescent devices, Adv. Mater., 16 161-166, 2004. 

DF O Brien, MA Baldo, ME Thompson, and SR Forrest, Improved energy transfer in electrophosphorescent devices, Appl. Phys. Lett., 74 442 144, 1999. 

FC Chen, SC Chang, G He, S Pyo, Y Yang, M Kurotaki, and J Kido, Energy transfer and triplet exciton confinement in polymeric electrophosphorescence devices, J. Polym. Sci. B Polym. Phys., 41 2681-2690, 2003. 

The ability to transfer the ligand TE energy to an efficient emissive lanthanide atom state removes the 25% internal quantum efficiency barrier on such OLEDs, enabling very efficient electrophosphorescent devices-see Sec. 1.5.6 below. 

Yang. X. H. Neher. D. (2004). Polymer electrophosphorescence devices with high power conversion efficiencies. Applied Physics letter, vol. 84, no. 14,2476-8. 

In the OLEDs there is a high possibility that exciplex formation occurs at the ETL/EML or HTL/EML interfaces because HTL and electron transport layer (ETL) usually have an electron-donating and an electron-accepting nature, respectively. There have been some researches on the application of exciplexes for the tuning of emission colors (Li et al 2006, Liang and Choy 2006) and white emitting OLEDs (Tong et al 2007). Extensive studies on exited bi-molecular complexes and their application in electrophosphorescent devices have been done by Kalinowski et al (2007) and Cocchi et al (2006). 

Organic Light-Emitting Materials and Electrophosphorescent Devices... 

Fig. 11.1 The electro-phosphorescent device structure used by Ikai et al. [17], Also shown are the chemical structures of CBP, BCP, TCTA, CF-X and CF-Y. The power efficiency of the organic electrophosphorescent devices is shown in the lower graph.

In 1998, Baldo et al. [15] showed that the efficiency of organic LEDs can be improved by using phosphorescent dyes. In their device, the phosphorescent oactaethyl-porphine platinum (PtEOP) dye was doped into an appropriate small molecule host at a low concentration. This electrophosphorescent device emitted red light with an external quantum efficiency of 4%. 

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