Browsing by Author "Yokoyama, Meiso"

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Effects of hole injection layer thickness on the luminescent properties of white organic light-emitting diodes
(2006-12-30)
Li, Jian-Feng
;
Chen, Shih-Fang
;
Su, Shui-Hsiang
;
Hwang, Kao-Shing
;
Yokoyama, Meiso
This work investigates how the thickness of the hole injection layer (HIL) influences the luminescent characteristics of white organic light-emitting diodes (WOLED). Experimental results indicate that inserting a thin HIL (< 200 angstrom) into a WOLED without an HIL reduces the brightness and clearly changes the chromaticity because the surface of the 4,4',4"-tris{N,-(3-methylphenyl)-N-phenylamino}-triphenylamine) (m-MTDATA) film is extremely rough. In contrast, a dense film structure and the fine surface morphology of m-MTDATA of moderate thickness (350-650 angstrom) provides a uniform conducting path on which holes cross the indium tin oxide (ITO)/HIL interface, improving luminescent performance, associated with the relatively stable purity of the color of the emission, with Commission Internationale 1'Eclairage (CIE) coordinates of (x = 0.40, y = 0.40). However, inserting a thick HIL (> 650 angstrom) reduces the luminescent performance and causes red-shift, because the holes and electrons in the effective emissive confinement region become less optimally balanced. Moreover, optimizing the device structure enables a bright WOLED with CIE coordinates of (x = 0.34, y = 0.33) to reach a luminance of 7685 cd/m(2) at a current density of 100 mA/cm(2), with a maximum luminous efficiency of 1.72 Im/W at 5.5 V. (c) 2006 Elsevier B.V. All rights reserved.
Enhancement on the efficiency and contrast of phosphorescent white organic light-emitting diodes
(2007-01-01)
Li, Jian-Feng
;
Su, Shui-Hsiang
;
Wu, Chung-Ming
;
Hwang, Kao-Shing
;
Yokoyama, Meiso
High efficiency and high contrast phosphorescent white organic light-emitting diodes (WOLEDs) using a zinc selenide (ZnSe) buffer layer and a copper phthalocyanine (Cupc)/ titanium oxide phthalocyanine (TiOPc) anti-reflection layer have been fabricated and investigated. The contrast ratio of the device with a ZnSe and a CuPc/TiOPc layer is about double that of the conventional device. Furthermore, the power efficiency of the proposed device is higher than the conventional device.
Full-wavelength white organic light emitting diodes with blue fluorescence and phosphorescent iridium complexes
(2006)
Li, Jian-Feng
;
Chen, Shih-Fang
;
Su, Shui-Hsiang
;
Hwang, Kao-Shing
;
Yokoyama, Meiso
A highly efficient white organic light emitting diode (WOLED) was fabricated and possible emission mechanisms of a WOLED configured with blue fluorescence and phosphorescent iridium complexes are proposed. The white light emitting layer is comprised of a blue fluorescent emitting layer anthracene and a green and red phosphorescent emitting layer TPBI:11% Ir(ppy)(3): 0.5% Ir(piq)(2)(acac). The device exhibits white emission with a Commission Internationale de l'Eclairage coordinate of (x = 0.33, y = 0.33) and the luminance and luminous efficiency was 6389 cd/m(2) and 6.4 cd/A, respectively, at 100 mA/cm(2). Evidence indicates that the good energy overlap between the absorption spectrum of Ir(ppy)(3) and Ir(piq)(2)(acac) and the emission spectrum of TPBI results in an effective energy transfer from TPBI to Ir(ppy)(3) and Ir(piq)(2)(acac). The optimal thickness of the green and red emitting layer promotes the injection and emitting of carriers in that layer. (c) 2006 The Electrochemical Society.