Evidence of intermolecular species formation with electrical aging in

Evidence of intermolecular species formation with electrical aging in

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Evidence of intermolecular species formation with electrical aging in anthracene-based blue organic light-emitting devices Qi Wang, Yichun Luo, and Hany Aziz a ! Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada s Received 6 November 2009; accepted 11 March 2010; published online 27 April 2010 d Electrical aging mechanism in blue emitting organic light-emitting devices s OLEDs d based on 9,10-bis s 2-naphthyl d -2- t -butyl anthracene s TBADN d fuorescent emitter is investigated using a number oF techniques, including delayed electroluminescence measurements. The studies reveal that electrical aging is associated with an increasing concentration oF an intermolecular species with a weak characteristic luminescence at around 535 nm. This species is capable oF charge trapping, and thus plays a role as an electron-hole recombination center with prolonged electrical driving. Weak green luminescence From this species leads to an increased green/blue emission ratio, and causes the color purity loss in aged devices. The results also suggest that this species is also eF±cient in dissipating excitation energy nonradiatively, hence is capable oF quenching TBADN singlet excitons, contributing to the observed eF±ciency loss with electrical aging. © 2010 American Institute of Physics . f doi: 10.1063/1.3386519 g I. INTRODUCTION Organic light-emitting devices s OLEDs d have attracted signi±cant attention because oF their unique advantages For fat panel display applications. 1 , 2 However, the relatively lim- ited electroluminescence s EL d stability oF blue emitting OLEDs continues to limit the commercialization oF Full color OLED displays. 3 , 4 In blue OLEDs, EL eF±ciency decreases rapidly during operation, usually limiting device liFetime to only a Few hundred hours. In most cases, the decrease in brightness is also accompanied by a loss in blue color purity. 5 Several classes oF blue emitter materials have been developed and studied For OLED applications. 6 , 7 Among these, anthracene has been one oF the earliest and most widely studied, owing to a number oF attractive Features, 8 but its high susceptibility to crystallization is a major limita- tion. To circumvent this limitation, anthracene derivatives with bulky substituent side groups are oFten used. 6 Among these derivatives, 9, 10-bis s 2-naphthyl d -2- t -butylanthracene s TBADN ds ReF. 7 d emerges as one oF most promising deriva- tives, displaying good blue color purity, high quantum yield, and a relatively higher EL stability versus other materials. However, the EL liFetime oF a TBADN OLED is still gener- ally much lower in comparison to red and green OLEDs. A number oF studies have addressed the EL stability oF anthracene derivatives in general and oF TBADN in particu- lar. In their studies, Jarikov and Kondakov 9 ruled out the role oF unipolar current on TBADN EL degradation, which is diFFerent From the case oF green OLEDs based on Alq 3 s ReF.
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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.

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Evidence of intermolecular species formation with electrical aging in

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