We investigated the electrical properties and charge transport mechanisms of a rubidium-carbonate ͑Rb 2 CO 3 ͒-doped 4,7-diphenyl-1,10-phenanthroline ͑Bphen͒ electron transporting layer ͑ETL͒. The electron-only devices and photoemission spectroscopy analysis revealed that the formation of doping-induced gap states dominantly contributes to the improvement of carrier transport characteristics of the doped system. High-efficiency green phosphorescent p-doping/intrinsic/n-doping ͑p-i-n͒ organic light emitting diodes were demonstrated using the Rb 2 CO 3 -doped Bphen ETL and rhenium oxide ͑ReO 3 ͒-doped N, ,4Ј-diamine hole transporting layer, exhibiting an external quantum efficiency of 19.2%, power efficiency of 76 lm/W, and low operation voltage of 3.6 V at 1000 cd/m 2 . © 2008 The Electrochemical Society. ͓DOI: 10.1149/1.3007239͔ All rights reserved.Manuscript submitted August 26, 2008; revised manuscript received October 6, 2008. Published October 27, 2008 Organic light emitting diodes ͑OLEDs͒ have been recognized and partially realized as next-generation flat-panel displays and solid-state lighting.1,2 Highly efficient OLEDs with external quantum efficiency ͑EQE͒ over 20% have been demonstrated through the development of materials and device structures.2-4 However, a relatively high driving voltage causes loss of power efficiency. To overcome the limitations, a doping concept has been applied to the conventional OLED structure and created promising p-doping/intrinsic/ n-doping ͑p-i-n͒ OLEDs exhibiting low operation voltage, high efficiency, and long lifetime. 5-8The doping technology of charge-transporting layers is a key issue in fabricating high-performance p-i-n OLEDs with low power consumption. Various p-dopants for a hole transporting layer ͑HTL͒ have been developed, which include an organic-based dopant of F 4 -TCNQ, 5-8 metal halides such as FeCl 3 and SbCl 5 , 9,10 and metal oxides like WO 3 and MoO 3 .11,12 Recently, our group has also developed another metal-oxide p-doping system based on rhenium oxide ͑ReO 3 ͒, showing an efficient p-doping property coming from the formation of charge-transfer complex within the HTL. 13 Furthermore, the developed p-doping system facilitates easy codeposition with organic molecules by a conventional thermal evaporator due to low-temperature deposition ͑ϳ350°C͒ of ReO 3 . In contrast, there are fewer material systems reported to date for n-doping of an electron transporting layer ͑ETL͒. Alkali metals such as Li and Cs 5-7 or alkali metal carbonate like Cs 2 CO 3 14,15 have been applied as n-dopants. However, limited work [14][15][16] has been carried out for the application of a metal carbonate-based n-doping system, requiring further research and development on the efficient n-doping system.In this work, we report on the electrical properties and possible charge-transport mechanisms of a newly developed rubidium carbonate ͑Rb 2 CO 3 ͒-doped ETL system by means of a single carrier device test and photoemission spectroscopy analysis. Highefficiency p-i-n OLEDs are also ...

We report high efficiency phosphorescent green p-i-n top-emitting organic light-emitting diodes consisting of metal anodes (Ag and Al), a rhenium oxide p-dopant, a rubidium carbonate n-dopant, and a semitransparent Ag cathode. Significantly high peak current efficiencies of 88 and 73 cd/A are demonstrated for the devices using Ag and Al anodes, respectively, through the optimization of organic layer thickness. The electroluminescence intensities of the Ag-based device with viewing angles show a nearly Lambertian distribution, whereas those of the Al-based device exhibit a relatively strong angular dependence, which is mainly attributed to the change in the resonance wavelength of the microcavity of the devices. Efficiencies, emission spectra, and angular dependence of the emission of the devices are further successfully analyzed using an optical model.