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Unformatted text preview: Blue organic electrophosphorescence using exothermic hostguest energy transfer R. J. Holmes and S. R. Forrest a) Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 Y.-J. Tung, R. C. Kwong, and J. J. Brown Universal Display Corporation, 375 Phillips Boulevard, Ewing, New Jersey 08618 S. Garon and M. E. Thompson Department of Chemistry, University of Southern California, Los Angeles, California 90089 ~ Received 23 October 2002; accepted 21 February 2003 ! We demonstrate efficient blue electrophosphorescence using exothermic energy transfer from a host consisting of N , N 8-dicarbazolyl-3,5-benzene ~ mCP ! to the phosphorescent iridium complex iridium ~ III ! bis @~ 4,6-difluorophenyl !-pyridinato- N , C 2 8 ]picolinate ~ FIrpic ! . By examining the temperature dependence of the radiative lifetime and the photoluminescence of a film of mCP doped with FIrpic, we confirm the existence of exothermic energy transfer in contrast to the endothermic transfer characteristic of the N , N 8-dicarbazolyl-4-4 8-biphenyl and FIrpic system. In employing exothermic energy transfer between mCP and FIrpic, a maximum external electroluminescent quantum efficiency of (7.5 6 0.8)% and a luminous power efficiency of (8.9 6 0.9)lm/W are obtained, representing a significant increase in performance over previous endothermic blue electrophosphorescent devices. 2003 American Institute of Physics. @ DOI: 10.1063/1.1568146 # The design of an efficient, electrophosphorescent organic light-emitting device ~ OLED ! often begins with the selection of a materials combination that allows for exothermic energy transfer between conductive host and phosphorescent guest molecules. Exothermic energy transfer occurs between a host excited state and a lower-energy guest unoccupied orbital, resulting in an energetically favorable excited state transition between molecules. Unlike the situation for red and green electrophosphorescent OLEDs, 1,2 the challenge to achieving exothermic energy transfer in the blue lies in the lack of suitable high-energy hosts ~ i.e., with a triplet level emitting at wavelengths l< 450 nm). Until now, the only means dem- onstrated to achieve efficient blue electrophosphorescence has been through endothermic energy transfer in which the energy of the host triplet lies below that of the guest triplet state. 3 This allows for the generation of efficient high-energy electrophosphorescence provided that the energy required for the endothermic energy transfer is not significantly greater than the thermal energy. Unfortunately, the endothermic transfer efficiency is strongly affected by defect levels in the host that limit device efficiency, operating temperature, and long-term operational stability....
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This note was uploaded on 03/27/2011 for the course CHEM 2211L taught by Professor T.a. during the Spring '08 term at University of Georgia Athens.
- Spring '08