We investigated the optical properties of a dielectric-metal-dielectric multilayer for the transparent top cathode in top-emitting organic light emitting diodes (TOLEDs). The optical transmittance of the metal layer was enhanced by depositing a dielectric material which had a high refraction index n below and above the metal (Ag) layer. Due to multiple reflections and interferences, the Ag layer sandwiched between dielectric materials with a high value of n can show improved transmittance. Because the WO 3 had a high value of n (>2.0), a thin WO 3 layer could fulfill the optimum zero-reflection condition with an Ag metal layer. Thus, a WO 3 /Ag/WO 3 multilayer should have high transmittance with a low sheet resistance. The optimum thicknesses of both Ag and WO 3 to obtain the best transmittance value were determined by theoretical calculation, and they agreed well with the experimental results. The best results were obtained for the thermally evaporated WO 3 (300 Å)/Ag (120 Å)/WO 3 (300 Å) structure, a high transmittance of ∼93.5% and a low sheet resistance about ∼7.22 ohm/sq were obtained. When the top Al cathode was replaced with the WO 3 /Ag/WO 3 multilayer, the maximum luminance value (J = 220 mA/cm 2 ) increased from 8400 to 11700 cd/m 2 , and the power efficiency increased about 26%. To improve the electron injection efficiency at the cathode region, a 20-Å thick Al layer was introduced as an electron injection interlayer between the organic materials and the WO 3 /Ag/WO 3 cathode. Using the Al interlayer decreased the operation voltage at J = 10 mA/cm 2 by 6.9 V. Thus, a WO 3 /Ag/WO 3 with an Al interlayer could promote the transparency of the top cathode and lower the electron injection barrier, enhancing the electroluminescent properties of TOLED.
We report the enhancement of the electron injection by inserting a 1-nm-thick magnesium oxide (MgO) buffer layer between Al cathode and tris (8-hydroxyquinoline) aluminum in an inverted top-emitting organic light-emitting diode (OLED). The turn-on voltage of OLEDs decreased from 10 to 6 V and the luminance increased about 61% as the MgO interfacial layer was employed. The MgO interfacial layer played a role in reducing the energy barrier of electron injection, leading to the reduction of the turn-on voltage and the enhancement of luminance.
We report the enhancement of hole injection using AgOx layer between Ag anode and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in top-emitting organic light-emitting diode (OLED). The turn-on voltage of OLEDs decreased from 17 to 7V as Ag changed to AgOx by the surface treatment using O2 plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased about 0.4eV by the O2 plasma treatment. This led to the decrease of the energy barrier for hole injection, reducing the turn-on voltage of OLEDs.
The effect of magnesium oxide (MgO) buffer layer between cathode and emitting materials on performance of inverted top-emitting organic light-emitting diodes (ITOLEDs) was investigated. The operation voltage at the current density of 100mA∕cm2 decreased from 14.9to9.7V for ITOLEDs with 1nm thick MgO buffer layers. The maximum luminance value increased about 78% in ITOLEDs using MgO buffer layer, which is 1000cd∕m2 at the current density of 191mA∕cm2. Synchrotron radiation photoelectron spectroscopy results revealed that the atomic concentration of Al–O bond increased after deposition of MgO on Al, indicating the oxidation of Al surface. Secondary electron emission spectra showed that the work function increased about 0.8eV by inserting the insulating MgO buffer layer. Therefore, the enhancement of device performance results from the decrease of the energy barrier for electron injection based on the tunneling model.
We report the correlation between charge injection and charge balance using lithium fluoride/aluminum
(LiF∕Al)
cathode and iridium oxide
(IrnormalOx)
-coated indium tin oxide (ITO) anode in organic light emitting diodes (OLEDs). The turn-on voltage decreased from
6.7to1.0V
and luminous efficiency at
80mA∕cm2
increased from
1.24to4.60cd∕A
due to the presence of LiF and the high work function of
IrnormalOx
. When
IrnormalOx
was used in the anode region, the luminous efficiency increment rate decreased from 40% in Al cathode OLED to 7% in
LiF∕Al
cathode OLED. The value of the carrier density suggested that the majority carrier changed from electrons in ITO-
LiF∕Al
OLEDs to holes in ITO/
IrnormalOx-LiF∕Al
OLEDs, reducing the rate of increase. Therefore, it has been considered that charge injection should be increased and balanced in order to maximize the luminous efficiency.
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