New developments in organic electroluminescent devices

Abstract

The operation mechanism, the technology and the new developments of the components of organic light-emitting diodes (OLEDs) were reviewed and investigated. Four main areas have been studied in detail. They are: (i) the deposition of the transparent conductive anode, ITO, (ii) the development and fabrication of next-generation OLEDs: flexible OLEDs, (iii) the exploration of new transparent conductive anodes for OLEDs, and (iv) the technique to enhance the display contrast of OLEDs. These four aspects are very important in the display performance of OLEDs. (i) The Deposition of the Transparent Conductive Anode, ITO The radio-frequency sputtering deposition technique of indium tin oxide was studied and investigated. Different heat treatments were performed during the deposition of ITO on the glass substrate. The heat treatment includes: high temperature deposition, vacuum post annealing and air post annealing. Also, the effects of different oxygen flow rates, film thicknesses, levels of sputtering power and substrate materials on the performance of ITO films were investigated and optimized. (ii) The Development and Fabrication of Next-Generation OLEDs, Flexible OLEDs A radio-frequency sputtering deposition method was applied to prepare indium tin oxide (ITO) on a plastic substrate, polyethylene terephthalate (PET). The correlation of deposition conditions and ITO film properties was systematically investigated and characterized. The optimal ITO film has a transmittance of over 90% in the visible range (400-700 nm) and a resistivity of 5.0x10-4 ohm-cm. Sequentially α-napthylphenylbiphenyl diamine, tris-(8-hydroxyquinoline) aluminium, and magnesium-silver were thermally deposited on the ITO-coated PET substrate to fabricate flexible organic light-emitting diodes (FOLEDs). The fabricated devices have a maximum current efficiency of ~4.1 cd/A and a luminance of nearly 4100 cd/m2 at 100 mA/cm2. These values showed that the FOLEDs have comparable performance characteristics to the conventional OLEDs made on ITO-coated glasses with the same device configuration. (iii) The Exploration of a New Transparent Conductive Anode for OLEDs Highly transparent conductive, aluminum-doped zinc oxide (ZnO:Al) films were deposited on glass substrates by midfrequency magnetron sputtering of metallic aluminum-doped zinc target. Due to the high ion flux and ion energy, and the effective activation of plasma species, good film properties can be obtained at a deposition rate of approximately 10 nm/s and substrate temperature lower than 200 °C. The prepared films have vastly improved film properties. ZnO:Al films with surface work functions between 3.7 and 4.4 eV were obtained by varying the sputtering conditions. OLEDs were fabricated on those ZnO:Al films. A current efficiency of higher than 3.7 cd/A was achieved. For comparison, 3.9 cd/A was achieved by the reference OLEDs fabricated on commercial indium–tin–oxide substrates.(iv) The Technique to Enhance the Display Contrast of OLEDs A black conductive electrode with a resistivity of 6.75×10-4 ohm-cm was fabricated by doping silicon monoxide into aluminum by simple thermal evaporation. The relative optical reflectance of such electrode layers within the visible spectral range was between 0.12 and 0.05. The high contrast OLEDs were fabricated by sequential deposition of α- napthylphenylbiphenyl diamine, tris-(8-hydroxyquinoline) aluminum and a black conductive layer of silicon-doped aluminum on indium-tin-oxidecoated glass substrates. The black layer reduced the reflection of ambient light entering the device and resulted in a significant increase in the OLED display contrast ratio. The electroluminescence properties of the device incorporating the black layer were investigated. In summary, investigation and study of new types of device fabrication technology have been performed in the present study aiming to achieve new developments of OLEDs.

Date of Award	4 Oct 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)