Introduction of Innovative Dopant Concentration Profiles to Broaden the Recombination Zone of Phosphorescent OVPD-Proces
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1154-B11-05
Introduction of Innovative Dopant Concentration Profiles to Broaden the Recombination Zone of Phosphorescent OVPD-Processed Organic Light Emitting Diodes M. Bösing1, C. Zimmermann1, F. Lindla1, F. Jessen1, P. van Gemmern2, D. Bertram2, N. Meyer3, D. Keiper3, M. Heuken1,3, H. Kalisch1, R. H. Jansen1 1 Chair of Electromagnetic Theory, RWTH Aachen University, Kackertstr. 15-17, 52072 Aachen, Germany 2 Philips Technologie GmbH, Philipsstr. 8, 52068 Aachen, Germany 3 AIXTRON AG, Kaiserstr. 100, 52134 Herzogenrath, Germany
ABSTRACT OLED with non-constant dopant concentration profiles have been processed by means of organic vapour phase deposition (OVPD) and were compared with regard to their luminous current efficiencies. Especially when driven at ultra-high luminance (>10,000 cd/A), OLED with a dopant concentration profile starting with a rather high dopant concentration on the anode side of the emissive layer showed improved luminous current efficiencies compared to their conventional counterparts. To further investigate this effect, the width and location of the recombination zone have been simulated for all investigated concentration profiles by numerical solution of the semiconductor device equations using experimentally determined doping-dependent charge carrier mobilities. The obtained theoretical results are discussed with regard to the accomplished experiments.
INTRODUCTION The introduction of phosphorescent emitters has established a basis for the development of OLED with impressive luminous efficiencies [1][2]. However, to exploit the full potential of phosphorescent light emission, it is crucial to develop device structures which lead to a relatively broad exciton recombination zone, in order to avoid triplet-triplet-annihilation (TTA) processes [3][4][5]. In many OLED, matrix and emitter contribute differently to the mobility of holes and electrons within the emissive layer, so the concentration of the dopant can be used to influence the position and shape of the recombination zone within the emissive layer. In contrast to vacuum thermal evaporation (VTE), OVPD intrinsically offers the opportunity to vary the concentration of the dopant during the deposition of the emissive layer, so that complex concentration profiles can be realized. In this work, we use non-constant dopant concentration profiles, to locate and broaden the recombination zone of green phosphorescent OVPD-processed OLED in order to increase their luminous efficiency.
EXPERIMENT To test the concept of non-constant dopant concentration profiles, two different OLED structures were employed: A rather simple three-layer structure (structure 1) consisting of a 40 nm emissive
layer sandwiched between NPB (N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'diamine) and Alq3 (tris-(8-hydroxyquinoline) aluminum) as hole and electron transport layer respectively, as well as a five-layer structure (structure 2) with a 10 nm emissive layer and additional exciton blocking layers (Fig. 1). Six OLED based on structure 1 and four OLE
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