Influence of Cavity Q-factor on Near-Infrared Emitting Microcavity Organic Light Emitting Diodes
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Influence of cavity Q-factor on near-infrared emitting Microcavity Organic Light Emitting Diodes C. H. Cheung,1 A. B. Djurišić,1 C. Y. Kwong,2 H. L. Tam,3 K. W. Cheah,3 Z. T. Liu,1 W. K. Chan,4 and P. C. Chui2 1
Dept. of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Dept. of Electrical & Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong 3 Dept. of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong 4 Dept. of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, Hong Kong 2
ABSTRACT
In this work, we fabricated near-infrared emitting MOLEDs with two organic layers. The hole transporting layer was N, N’ – di(naphthalene-1-yl) - N,N’- diphenylbenzidine (NPB), while tris(8-hydroxyquinoline) aluminum (Alq) was the emissive and electron transporting layer. The bilayer structure was sandwiched between two silver mirrors. In order to investigate the influence of cavity Q-factor to the emission spectra, devices with different thickness of bottom mirror (anode) were fabricated. The influence of the choice of the bottom mirror (anode) was also investigated, and the devices with copper anode were also fabricated. The devices were characterized by angular dependent electroluminescence, photoluminescence, and transmittance measurements. Possible origins of the observed phenomena are discussed.
INTRODUCTION
The discovery of luminescence in organic materials has stimulated the intense research in the field of organic light emitting diodes (OLEDs) [1]. Organic materials usually exhibit broad emission spectra, which would result in the inferior color purity of the devices. For example, photoluminescence spectrum of Alq, a commonly used electron transporting and emissive material for OLEDs, has full width half maximum of ~ 85 nm. Narrowing of the emission spectrum can be achieved by using a microcavity structure [2-4]. The microcavity can modify the spontaneous emission of materials by redistributing the photon density of states such that only the wavelengths corresponding to the cavity mode can be emitted out of the cavity. Multiple peak emissions can also be achieved with microcavities [3,5]. Different color emissions were reported from Alq based microcavity organic light emitting diodes (MOLEDs) [3]. Due to the negligible emission from Alq at near-infrared spectral range (>700 nm), little work have been reported on near-infrared emission from Alq based MOLEDs. We recently reported near-infrared emitting MOLED, which has an emission peak centered at ~750 nm [6]. N-N’- di(naphthalene-1-yl)-N,N-diphenylbenzidine (NPB) and tris(8-hydroxy-quinoline) aluminum (Alq) were used as hole transporting material and emissive/electron transporting material respectively. The materials were sandwiched between two Ag mirrors. The infrared
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emitting property was demonstrated, but the influence of cavity Q-factor on the emission spectra had not been investigated. In this work, we report the fabrication and characterization of Alq based MOLEDs which s
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