Transparent organic light-emitting devices with LiF/Mg : Ag cathode

School of Electrical and Electronic Engineering, Nanyang Technological University, Tumasik, 00, Singapore
Optics Express (Impact Factor: 3.49). 03/2005; 13(3):937-41. DOI: 10.1364/OPEX.13.000937
Source: PubMed


Transparent organic light-emitting devices (TOLED) based on a stacked transparent cathode of a LiF/Mg:Ag were investigated. The device has a structure of indium-tin-oxide (ITO)/ N, N'-diphenyl-N, N'-(3- methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) (90 nm)/ tris-(8- hydroxyquinoline) aluminum (Alq3) (80 nm)/LiF (0.5 nm)/Mg:Ag (20 nm)/Alq3 (50 nm), where the transparent capping layer of 50 nm Alq3 acts as refractive index matching layer to optimize optical output. The turn-on voltage of the device is as low as 2.8 V. The device also shows high optical transparency and low reflectivity in the visible region, approximately 40% of light can emit from the top cathode side and 60% of the light from bottom ITO glass. At injection current density of 20 mA/cm2, the current efficiency, for bottom emission from ITO anode side and top emission from metal cathode side, is 3.4 cd/A and 2.2 cd/A, respectively. The lower turnon voltage and higher efficiency of device are due to enhancing electron injection by using LiF/Mg:Ag cathode.

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    • "According to the several reports, the key characteristics of TOLEDs or top-emitting OLED (TEOLED) such as external quantum efficiency (EQE) and spectral distribution can be effectively modulated and improved by introducing dielectric layers or capping layers (CLs) on the outermost surface of the cathode [2] [3] [4] [5]. As an effort to enhance the total emission of TOLEDs, majority of researches have been focused on improving the transmittance of top contact by using CLs [6] [7] [8] [9]. However, reports on the augmenting the emission selectively to one face in white TOLEDs with CLs are scarce. "
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    ABSTRACT: The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminium with a thin layer of MgF2 inserted at the indium-tin-oxide (ITO) and organic interface or the organic and Mg:Ag cathode interface are investigated. A 1.0 nm MgF2 thin layer can enhance electron injection when it is inserted only between organic electron transporting layer and Mg:Ag alloy cathode, but can block hole injection when it is inserted only between the ITO anode and organic hole transporting layer. By inserting MgF2 at both sides on the ITO anode and under the Mg:Ag cathode, the current efficiency of the device is improved by 74%, and power efficiency is also improved by 18% at a current injection of 20 mA cm−2, compared to the standard device without MgF2 buffer layer. This is due to the increased electron and decreased hole injection, which results in more balanced electron and hole injection, and more efficient exciton formation. The increased electron injection can be well understood by the tunnelling effect model.
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