Influence of electrode modification by Ar+ ion beam upon passivation and electrical characteristics in organic light-emi
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Se-Jong Lee Department of Materials Engineering, Kyungsung University, Busan 608-736, Korea
Kie Moon Song Department of Applied Physics, Konkuk University, Chungju 380-701, Korea (Received 18 June 2004; accepted 21 September 2004)
Ion-beam-assisted deposition (IBAD) was used for cathode preparation in organic light-emitting diodes to fabricate dense electrode. Dark spot growth rate was decreased by employing the IBAD process due to a highly packed aluminum structure inhibiting the permeation of H2O and O2. However, undesirable leakage current was generated because energetic particles of Al assisted by Ar+ ion may damage the organic material resulting in reduction of contact resistance. The decrease of contact resistance in the IBAD device may be caused by large contact area, increase of density of states, and Li diffusion to phenyl-substituted poly-p-phenylene vinylene.
I. INTRODUCTION
In recent years organic light-emitting devices (OLED) have attracted increasing attention due to their potential advantages in low power, emissive, flexible, and costcompetitive flat panel displays.1–3 However, limited lifetime, which arises from exposure to oxygen, moisture, and high electric fields, still remains to be solved for the practical applications.4,5 The pronounced morphological change observed in a debased OLED is the delamination of the cathode material, which appears as dark spot.6 Research has revealed that the dark spot had a domelike structure (“bubbles”) filled with gases (mostly oxygen) presumably evolved during electrochemical and photoelectrochemical processes in the presence of water.7 Some recent articles revealed that the bubbles originated from pinholes of metallic electrode in the presence of atmospheric humidity. Gases stimulated by significant heating during the device operation form these bubbles and then cause cathode delamination at the metal/ organic material interface. Therefore, humidity and heat evolution are the primary reasons for bubble formation.8 One of the most important factors limiting the durability of OLED is the degradation of electron injecting contact. In OLED, the metal cathode layer is commonly
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0004 J. Mater. Res., Vol. 20, No. 1, Jan 2005
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deposited by thermal evaporation to minimize the damage of the organic materials. In the case of energetic processes like sputtering, energetic atoms and ions are incident onto the organic surface and transfer their energy to the surrounding organic molecules. The interactions between ions and solids can cause substantial damage to organic material and result in degraded device performance.9 On the other hand, metal cathode layers formed by energetic process are known to have better adhesion to the substrate than those deposited by thermal evaporation. Suzuki et al. reported that the lifetime of their OLEDs prepared by sputtering increased due to the improvement of Al/organic contacts.10 Ho
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