We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from Ͼ20 to Ͻ10 V, the external electroluminescence quantum efficiency ͑backside emission only͒ increases by a factor of 4 ͑from 0.28% to 1%͒, a much higher drive current can be applied to achieve a much higher brightness ͑maximum brightness ϳ10,000 cd/m 2 at 1000 mA/cm 2 ), and the forward-to-reverse bias rectification ratio increases by orders of magnitude ͑from 10 2 to 10 6 -10 7 ). The lifetime of the device is also enhanced by two orders of magnitude.Because of its transparency, high conductivity, and efficiency as a hole injector into organic materials, indium tin oxide ͑ITO͒ has been widely used as the anode contact for organic light emitting devices ͑OLEDs͒. These devices usually consist of a sandwich structure with the organic thin film deposited onto the ITO-coated glass substrate and covered by patterned top metal cathode contacts. 1,2 Since the organic thin film is in direct contact with the ITO, the surface properties of the ITO are expected to directly affect the characteristics of the device. Abnormal device behaviors such as shorting, unstable I -V characteristics, and damage on the surface of the top cathode contact after continuous operation of the device have been observed in OLEDs built on bare cleaned ITO surfaces. 3-5 Furthermore, as-grown ITO contacts have been found to be less efficient for hole injection than low work function metal cathodes for electron injection, resulting in hole-limited devices. 6,7 The mitigation of these problems has so far involved changing the properties of the organic materials or introducing an intermediate stabilization layer with proper carrier injection/transport characteristics between the ITO and the active luminescent layers. 7,8 The alternative of modifying the ITO itself, however, has not been extensively investigated. In this paper, we report that an oxygen plasma treatment is an effective way to modify the surface of ITO. We perform a comprehensive investigation of the correlation between ITO surface properties such as morphology and chemical composition, and the device characteristics. We demonstrate that ITO surface modification leads to good performance even in single layer polymer based OLEDs, where the ITO surface properties are more critical than in multilayer devices.The ITO coated glass substrates used in this study were purchased from Donnelly Applied Films Co. The 1.1-mmthick polished soda lime float glass was coated with a 200 Å SiO 2 barrier layer and a 1400 Å ITO film. ITO was sputtered from an In 2 O 3 -SnO 2 ͑90 wt %-10 wt %͒ oxide target in an Ar/O 2 ambient at an elevated temperature using a planar dc magnetron sputtering system. The ITO was annealed in situ during the deposition and no postdeposition annealing was performed. The sheet resistance and transmittance of the ...

The emission of light and external coupling after the appropriate excitons have been formed in the organic light-emitting devices ͑OLEDs͒ has been investigated. The internally emitted light can be classified into three modes: externally emitted, substrate waveguided, and indium-tin-oxide ͑ITO͒/ organic waveguided. A combined classical and quantum mechanical microcavity model is used to calculate the distribution of light emission into these three modes in an OLED on planar substrates. The ITO/organic modes maybe suppressed due to the thinness of the ITO/organic layers. Consequently, as much as over 50% of the internally generated light is emitted externally in some structures, much greater than the ϳ20% figure given by classical ray optics. This model is used to examine how this distribution varies with exciton to cathode distance, the thickness of the ITO layer, and the index of refraction of the substrate. It can also be applied to OLEDs on shaped substrates where an increase in the total external emission up to a factor of 2.3 has been demonstrated. The numerical results agree well with experimentally measured far-field intensity profiles, edge emissions, and increase in external emission due to shaped substrates. Finally, based on these results, we discuss different approaches to device optimization, depending on the fluorescence efficiency of the emitter and whether a shaped substrate is used.

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