Efficient Inverted Top-Emitting Organic Light Emitting Diodes with Transparent and Surface-Modified Multilayer Anodes

Abstract

Highly transparent and efficient red phosphorescent inverted top-emitting organic light emitting diodes were investigated by using a surface-modified tungsten oxide ͑WO 3 ͒/silver/WO 3 ͑WAW͒ anode. A thin buckminsterfullerene ͑C60͒ dipole layer was introduced for the surface treatment of the WAW anode, and the thickness of the surface-modified WO 3 was controlled to optimize the WAW anode for hole injection. The optimum thickness of the surface-modified WO 3 was 5 nm, and the C60 dipole layer further improved the hole injection from the WAW anode to the hole transport layer.In recent years, organic electronic devices have been actively studied because of the merits of a large printing area, 1 a low cost fabrication process, 2 and flexibility. 3 In particular, there have been many researches about organic light emitting diodes ͑OLEDs͒ 4,5 for applications as a backlight for liquid crystal displays and flexible displays. Since the discovery of high efficiency OLEDs during the 1980s and 1990s, 4,6 many studies have been conducted to improve the electroluminescent properties of the OLEDs.In the past decade, inverted top-emitting organic light emitting diodes ͑ITOLEDs͒ 7-10 with reflective bottom cathodes have been developed. Common top-emitting OLEDs are compatible with p-type transistors, while the ITOLEDs can be combined with n-type thin film transistors. 7-11 In general, ITOLEDs require a high quality transparent anode, and commonly gold ͑Au͒ semitransparent anodes have been used because of their adequate hole injection. 7,11-13 However, the thickness of the Au film is limited in practical usage because of low transmittance. Several types of hole injection mechanisms have been proposed. Chen et al. 14 observed that the ITOLED with a surface-modified silver ͑Ag͒ anode treated with a UV/ozone exhibited better device performance than a bottom-emitting device. Silver oxide ͑Ag 2 O͒ was generated during the UV/ozone treatment, and it played a role of the interfacial layer. Thin tungsten oxide ͑WO 3 ͒ ͑ϳ5 nm͒ 11 was also used as the interfacial layer to reduce the potential barrier between the organic layer. Meyer et al. 12 showed that a thin ͑5 nm͒ WO 3 layer enhanced the device performance through an adequate band alignment between the organic layers and the anode. Recently, Leo et al. 9,15 also demonstrated the top-emitting OLEDs with a reasonable efficiency using Ag top electrodes.In this work, the surface morphology of the WO 3 /Ag/WO 3 ͑WAW͒ multianode was modified by reducing the thickness of WO 3 in contact with the organic layer from 40 to 5 nm. This thickness reduction affected the band alignment between the organic layer and the WAW anode. It was demonstrated that holes were directly injected from the Ag layer to the organic layer through the thin WO 3 layer ͑5 nm͒ in the WAW multianode.Multielectrodes 10,16-18 are another alternative for the electrodes in transparent OLEDs. Indium zinc oxide ͑IZO͒/Ag/IZO, indium tin oxide ͑ITO͒/Ag/ITO, and WAW have been used as highly transparent multielectrodes with low she...

Linear and nonlinear optical properties of Ag/Au bilayer thin films

“…For the second term in Eq. 3 (5) where the summation is composed of M interband terms and each contribution term is denoted by the subscript j, x is the electron energy, κ is a constant related to effective electron mass, E g is the minimum transition energy from a valence band to an ideal parabolic conduction band, γ ee is the inverse scattering time, F(x,T e (t)) is the electron occupation number, E f is the electron distribution function, E fd is the transition energy from the band to the Fermi level, k B is the Boltzman constant, ℏ is the plank constant, and γ a and γ b are constant coefficients [24].…”

“…These values were obtained by fitting the measured steady-state permittivity at room temperature [25], T l (t) = T e (t) = 300 K, with Eqs. (3)- (5). Figure 4 shows the comparison between simulated Au permittivity and that obtained by spectroscopic ellipsometry.…”

“…This negative permittivity also enables artificial metamaterial structures which allow remarkable control over the dispersion and sign of the refractive index of a material and consequently over the flow of electromagnetic energy throughout its structure [2]. In addition, highly transparent metallic structures [3] have been reported and are attractive as optical filters with high out-of-band rejection [4] and thin film metal electrodes in organic lightemitting diodes [5]. Noble metals have also attracted great attention because their extremely large and ultrafast non-linear optical (NLO) response [6,7] can be exploited to achieve alloptical control of metallic nanostructures [8][9][10][11].…”