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Unformatted text preview: Concentration quenching of electroluminescence in neat Ir ppy 3 organic light-emitting diodes Y. Q. Zhang, G. Y. Zhong, and X. A. Cao a ! Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, West Virginia 26506, USA s Received 1 June 2010; accepted 21 September 2010; published online 22 October 2010 d We studied concentration quenching of electroluminescence s EL d in organic light-emitting diodes with a neat fac-tris s 2-phenylpyridinato-N, C2 8 d iridium s III d f Ir s ppy d 3 g emitting layer of different thicknesses sandwiched between electron and hole blocking layers. The intensity of the green emission decreased rapidly with increasing Ir s ppy d 3 thickness and was reversely correlated with the tail band emission. The overall light output power reached the minimum at 4 nm, and attained a saturated value for Ir s ppy d 3 thicker than 6 nm. These results are interpreted as evidence that concentration quenching in Ir s ppy d 3 originates from both short and long-range energy transfer between excited and ground states of molecules. The EL quenching magnitude was found to be independent of the injection current, indicating that biexcitonic annihilation plays a minor role. 2010 American Institute of Physics . f doi: 10.1063/1.3504599 g I. INTRODUCTION The quantum efficiency s QE d of organic light-emitting diodes s OLEDs d has been dramatically improved by the use of heavy metal phosphorescent emitting materials. 1 17 Heavy atom-induced spin orbit coupling enables efficient intersys- tem crossing from the singlet to triplet states, so both singlet and triplet excitons contribute to light emission. 1 Particularly, cyclometalated Ir s III d complexes are promising candidates for room-temperature phosphorescence due to highly effi- cient emission from mixed metal-to-ligand charge transfer s 3 MLCT d triplet state. 2 , 3 OLEDs with 6.2 wt % green- emitting fac-tris s 2-phenylpyridinato-N, C2 8 d iridium s III d f Ir s ppy d 3 g doped in wide energy gap host 4,4 8 ,4 9 ,-tris s N-carbazolyl d triphenylamine s TCTA d have achieved 19.2% external QE s EQE d , 8 which, considering the out-coupling losses, is close to the theoretical device effi- ciency limit. However, the performance of OLEDs doped with phosphorescent dyes including Ir s ppy d 3 severely suffers from quenching as the dopant concentration is increased. 2 7 The EQE of OLEDs with a neat Ir s ppy d 3 emitting layer s EML d is reduced to , 1% due to strong concentration quenching. 2 , 3 To date, little effort has been directed toward the under- standing of the nature of concentration quenching in Ir s ppy d 3 . It is generally believed that, due to the relatively slow phosphorescent process, excitons generated on Ir s ppy d 3 can be easily quenched through triplet-triplet annihilation s TTA d ....
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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