Stabilized Blue Emission from Polyfluorene-Based Light-Emitting Diodes: Elimination of Fluorenone Defects

Authors


  • This work was partially supported by the Mitsubishi Chemical Center for Advanced Materials at UCSB and the Air Force Office of Scientific Research through the MURI Center (“Polymeric Smart Skins”), Charles Lee, Program Officer. Synthesis of the CF3OXD and related tetrahedrally coordinated molecules was supported by a grant to G. Bazan from DuPont Displays. The XPS results were obtained by using the MRL (Material Research Laboratory at UCSB) Central Facilities supported by the National Science Foundation under DMR 96-32716. We thank Prof. W. R. Salaneck of Linköping University (Sweden) for important comments. X. Gong is grateful to Dr. Gang Yu of Dupont Display for valuable discussions. Dr. Tom Mates (UCSB-MRL) and Dr. Jiang Wang (Dupont Displays) are gratefully acknowledged for assistance in various aspects of the experimental work and for valuable discussion.

Abstract

Polyfluorene (PF)-based light-emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low-energy emission band (at ∼ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low-energy emission band originates from fluorenone defects which are introduced by photo-oxidization, thermal oxidation, or during device fabrication. X-ray photo-emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron-transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.

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