Dresden 2003 – scientific programme
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HL: Halbleiterphysik
HL 25: Organische Halbleiter II
HL 25.10: Talk
Tuesday, March 25, 2003, 17:30–17:45, BEY/81
Liquid Metals as Electrodes in Polymer Light Emitting Diodes — •Gunther Andersson1,2, Hans Gommans2, Arnoud Denier van der Gon2, and Hidde Brongersma2 — 1Institut für Physikalische und Theoretische Chemie Universität Leipzig, Linnestr.2, 04157 Leipzig — 2Department of Applied Physics, Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands
We demonstrate that liquid metals can be used as cathodes in light emitting diodes (pLEDs). The main difference between the use of liquid cathodes and evaporated cathodes is the sharpness of the metal-polymer interface, which is shown by ion scattering techniques. Liquid metal cathodes result in significant sharper metal-organic interfaces than vapor deposited cathodes, due to the high surface energy of the metals.
The influence of interface sharpness on device performance was studied by comparing current-voltage-light characteristics of devices with PPV as electroluminescent polymer and different cathodes. Comparison of devices showed that light emission for the liquid Ga cathode is two orders of magnitude larger than for the evaporated Al cathode, and that the external light efficiency is increased by an order of magnitude, although the work function of Ga and Al is nearly the same.
Low work function liquid metal cathodes were studied by using liquid Ca and Ba amalgams. The improved performance of liquid amalgam pLEDs is attributed to the different structure of the metal - polymer interface. Devices with a liquid Ca amalgam cathode showed an increase of the current and brightness compared to evaporated Ca cathodes, which is ascribed to reduced diffusion of Ca into the emissive PPV layer.