Layer Cross-Fading at Organic/Organic Interfaces in OVPD-Processed Red Phosphorescent Organic Light Emitting Diodes as a
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1154-B05-07
Layer Cross-Fading at Organic/Organic Interfaces in OVPD-Processed Red Phosphorescent Organic Light Emitting Diodes as a New Concept to Increase Current and Luminous Efficacy Florian Lindla1*, Manuel Boesing1, Christoph Zimmermann1, Frank Jessen1, Philipp van Gemmern2, Dietrich Bertram2, Dietmar Keiper3, Nico Meyer3, Michael Heuken1,3, Holger Kalisch1, Rolf H. Jansen1 1
Chair of Electromagnetic Theory, RWTH Aachen University, Kackertstraße 15-17, 52072 Aachen, Germany 2 Philips Technologie GmbH, Philipsstraße 8, 52016 Aachen, Germany 3 AIXTRON AG, Kaiserstraße 100, 52134 Herzogenrath, Germany * Corresponding author: [email protected] ABSTRACT The current and luminous efficacy of a red phosphorescent organic light emitting diode (OLED) with sharp interfaces between each of the organic layers can be increased from 18.8 cd/A and 14.1 lm/W (at 1,000 cd/m2) to 36.5 cd/A (+94%, 18% EQE) and 33.7 lm/W (+139%) by the introduction of a layer cross-fading zone at the hole transport layer (HTL) to emission layer (EL) interface. Layer cross-fading describes a procedure of linearly decreasing the fraction in growth rate of an organic layer during deposition over a certain thickness while simultaneously increasing the fraction in growth rate of the following layer. For OLED processing and layer cross-fading organic vapor phase deposition (OVPD) is used. The typical observation of a roll-off in current efficacy of phosphorescent OLED to higher luminance can be reduced significantly. An interpenetrating network of a prevailing hole and a prevailing electron conducting material is created in the cross-fading zone. This broadens the recombination zone and furthermore lowers the driving voltage. The concept of layer cross-fading to increase the efficacies is suggested to be useful in multi-colored OLED stacks as well. INTRODUCTION OLED have the potential to play a dominant role in solid state lighting. One of the most important tasks to achieve higher efficiencies in OLED is to investigate and understand the injection at the electrodes and the electronic interaction at the interfaces between the different organic layers and materials. An optimized structure, in which the recombination zone is not only located at the interfaces, but significantly broadened has to be found to increase OLED efficacy [1]. This can be done using a mixture of electron and hole conducting host materials or an unipolar conducting host material in combination with an emitter, which acts as an hole or electron conductor [2]. As well as an increased lifetime, the typical roll-off of phosphorescent OLED due to triplet-triplet annihilation processes is reduced by a broadened recombination zone [3]. In this work, the effect of layer cross-fading on current and luminous efficacy, roll-off and lifetime is studied and compared with mixed host interlayers of constant ratio. IV measurements of mixtures of the different matrix materials are performed to gather an insight in the charge carrier mobility change at different mixture ratios.
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