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Unformatted text preview: High efficiency and low roll-off blue phosphorescent organic light-emitting devices using mixed host architecture Neetu Chopra, 1,2 James S. Swensen, 1 Evgueni Polikarpov, 1 Lelia Cosimbescu, 1 Franky So, 2, a ! and Asanga B. Padmaperuma 1, b ! 1 Pacific Northwest National Laboratory, Energy and Environment Directorate, Richland, Washington 99352, USA 2 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA s Received 4 March 2010; accepted 27 June 2010; published online 20 July 2010 d We report high efficiency and low roll-off for blue electrophosphorescent organic light emitting devices based on a mixed host layer architecture. The devices were fabricated using a mixed layer of di- f 4- s N,N-ditolyl-amino d-phenyl g cyclohexane, a hole transport material, and 2,8-bis s diphenylphosphoryl d dibenzothiophene, an electron transport material, as the host layer doped with the blue phosphor iridium s III d bis fs 4,6-difluorophenyl d-pyridinato-N,C 2 8 g picolinate. Using a mixed layer as the host allowed us to achieve high power efficiency s 59 lm/W at 100 cd / m 2 d , low turn-on voltage s 2.7 V for . 10 cd / m 2 d , and low roll-off in these devices. 2010 American Institute of Physics . f doi: 10.1063/1.3464969 g Organic light emitting devices s OLEDs d are touted as a promising technology for solid state lighting due to their potential for lower manufacturing costs, ease of processing, compatibility with flexible substrates, and reduced power consumption. 1 3 The power efficiency of OLEDs has steadily improved through material and device structure optimization. Red 4 and green 5 phosphorescent OLEDs with high efficien- cies and reasonably long lifetimes have been demonstrated for lighting and display applications. However, achieving a stable and high efficiency blue component still remains a challenge. Most of the work published for blue OLEDs has lower efficiency and lifetimes as compared to their red and green counterparts. 6 Only recently some high efficiency blue phosphorescent OLEDs have been demonstrated. 7 11 Some of the major issues with the optimization of device efficiency in blue phosphorescent OLEDs have been identi- fied as charge balance 8 , 12 in the device and triplet exciton confinement. 7 , 10 , 13 , 14 Charge balance is a very important fac- tor in the operation of OLEDs; not only does it influence the peak efficiency in the device but it also influences efficiency roll-off. 15 In most OLEDs, the hole mobility of the hole transport layer s HTL d is higher than the electron mobility of the electron transport layer s ETL d . 8 Hence, most devices are not charge balanced, leading to lower efficiency and shorter lifetime. Apart from the HTL and ETL, the host also plays a major role in determining the overall charge balance in a device....
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