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2 September 2008 Metal/fullerene electrode structure: physics and device applications
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Fullerene (C60) has been found to form a universal hole injection interface in the Metal/C60 anode structure. This bilayer structure opens up possibilities to select various highly conductive metals as anodes to replace indium tin oxide (ITO). Organic light emitting diodes (OLEDs) utilizing Au/C60 and Mg/C60 bilayer anodes were fabricated. Electroluminescence (EL) efficiencies of devices with Au/C60 anodes surpassed the ITO baseline device by ~ 25%. It was found that the hole injection current for the Au/C60 anode can be tuned via the C60 interlayer thickness in the range of 1 - 5 nm. Single carrier hole-only (HO) devices with different metal and metal/C60 bilayer anodes were studied. With the insertion of a 3 nm thick C60 buffer layer between the anode metal and hole transport layer (HTL), an increase in hole injection current of more than two orders was realized. Based on device modeling we extracted C60-induced dipoles on the metal surfaces. These dipole values agree well with values obtained by Kelvin probe and photoemission measurements. What is more, the dipole values effectively pin the work function of all metals to a common value of ~ 4.6 eV, creating a universal hole injection barrier regardless of the pristine anode metal work function.
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M. G Helander, Z. B. Wang, and Z. H. Lu "Metal/fullerene electrode structure: physics and device applications", Proc. SPIE 7051, Organic Light Emitting Materials and Devices XII, 70510Z (2 September 2008);

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