Optimization of organic light-emitting devices via electrode modification and new fabrication process

Abstract

Organic light-emitting devices (OLEDs) are emerging from the stage of being a research curiosity to that of becoming important commercial applications, and are gaining acceptance as a promising flat panel display technology in the new millennium. Intensive efforts on the development of new materials and color gamut have been made; however, there is still room for improvements on device efficiency and longevity. This thesis concerns the optimization of OLEDs through novel device architectures and new fabrication process. Particular attention is paid to three areas: 1) the utilization of efficient CsF/Yb cathode; 2) systematic study on the impact of metal overlayer; and 3) the influence of deposited temperature of organic layers on the device performance and stability. Firstly, the application of ultrathin CsF buffer layer and an overlaid rareearth metal Yb as bilayer cathode was studied. Devices with the CsF/Yb cathode exhibited a significantly reduced driving voltage, improved EL efficiency and also operational stability. The performance improvement was creditable to the release of free Cs metal atoms through fluoride dissociation, leading to a sizeable reduction in the cathodic barrier. More importantly, the enhancement effects were independent of the organic materials used, providing strong evidence that the CsF/Yb is a promising candidate for almost all types of OLEDs. The interplays of different metal cathodes and a CsF buffer layer were also studied systematically, in which representative alkaline-earth metals (Mg, Ca, Ba) and lanthanides (Yb, Sm, Ce, Ho) were selected because of their low work functions. In addition to the well-known effects of metal work function, the metal reflectivity and reactivity were also critical to the device efficiency and operational stability. Metals with lower reflectance generally resulted in a lower EL efficiency. On the other hand, upon the insertion of an ultrathin CsF interlayer, device characteristics improved considerably and became almost independent of the metal work function. Another important finding was the dependence of device lifetime on the nature of chemical reaction between the organics and metal cathode. Devices with metal cathodes that reacted detrimentally with Alq3 would incur poor operational stability. In principal, a proper selection of metal overlayers that would not destroy the Alq3 molecular structure is a fundamental requirement for a good device lifetime. Finally, the effects of deposition temperature on device performance were also studied. It was observed that deposition of organic materials at elevated temperature significantly improved the device performance including EL efficiency, storage stability, and also retarded dark-spots growth. In contrary to conventional perception, crystalline NPB films deposited at elevated temperatures showed a better morphological and thermal stability. These consequences combined with a better carrier balancing in the devices were considered to be major factors responsible for the improvement.

Date of Award	15 Feb 2005
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chun Sing LEE (Supervisor)