Optical Interference as a Tool to Describe the Spectral Changes Created by a Tunable and Controllable Excitons Confineme
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Optical interference as a tool to describe the spectral changes created by a tunable and controllable excitons confinement Christophe Féry, Salvatore Ciná, Henri Doyeux , Bernard Geffroy1, Christine Denis1, Pascal Maisse1 Thomson R&D France 1, Avenue Belle Fontaine – CS 17616 35576 Cesson-Sévigné – France CEA/DRT/LITEN/DSEN/GENEC/L2C CEA/Saclay, 91191 GIF SUR YVETTE CEDEX, France ABSTRACT In this paper we address the optical process occurring in a standard Alq3 based OLED device using a classical electromagnetic framework. A comprehensive comparison with experimental results is made with an excellent agreement. The insertion of a hole blocking layer (HBL) is used to confine excitons formation and relaxation within a well defined region allowing us to separate optical effects from other mechanisms. By tuning the cavity thickness, we can demonstrate destructive interference into the wavelength region corresponding to Alq3 peak emission. Electroluminescence measurements in an integrating sphere further supports the idea that the interferences scheme used here describes well the optical behaviour of our samples.
INTRODUCTION Optical phenomenon in OLEDs have been intensively investigated driven both by the fundamental interest for the physics underlying light emission in thin films devices and by the need for industry to reach very high efficiency OLED. Two main theoretical approachs (classical and quantum mechanics) have been proposed considering the relaxing exciton as an oscillating dipole. Both treatments are efficient to explain different issues and several studies are available that provide a complete description of the OLEDs emission [1, 2]. The optical interference framework used in this paper was proposed by Drexhage [4] and can be considered as a simplification of the model by Chance et al. [5] when non-radiative energy transfer to the metallic electrode vanishes (i.e. the distance between the dipole and the reflecting electrode above 70 to 100 nm). Crawford [6] suggested a formalism which provides a convenient way to compute interferences in a thin films multilayer. However, from previous studies, it is not obvious to find out how accurate this model is for OLEDs. By comparing a set of Alq3 based devices with or without a Hole Blocking Layer (HBL) we will clearly evidence interferential mechanisms. It will be demonstrated that emission pattern change with device structure can be solely ascribed to them. Then, it will be possible to separate the part of radiative process from others (i.e. excitons formation or non-radiative relaxation…) into the overall device performances.
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Figure 1. Luminous efficiencies in the forward direction for devices with and without HBL EXPERIMENT We made two kind of Alq3 based OLEDs having the following structure : . (1) ITO/NPB(60nm)/Alq3(z=60-160nm)/LiF-Al (100nm) (2) ITO/NPB(60nm)/Alq3(10nm)/BCP(10nm)/Alq3 (z’=40nm-140nm)/LiF-Al (100nm) Where z
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