Investigations on the Aluminum/Para-Hexaphenyl Interface in Light Emitting Devices
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ABSTRACT Blue light emitting devices (LED) with para-hexaphenyl (PHP) as the active material and aluminum as cathode exhibit very high quantum efficiencies. To further optimize device performance it is crucial to understand the physical properties of the involved interfaces. We have performed Rutherford-Backscattering experiments on actual devices to show the importance of oxygen in the interface formation at the cathode as this leads to the formation of a layer of AlXOy between PHP and aluminum. In devices, where the organic film is exposed to air before the metal electrode is evaporated, an insulating layer on the metal-side therefore is inherent. It has been shown that the introduction of an intermediate layer between active material and electrodes results in a higher quantum efficiency of the LED, the most common concepts being chargetransport-layers, or insulators on the other hand. Our results underline the need for a better control of the LED processing. Ultraviolet- and X-ray photoelectron spectroscopy in situ growth studies of thin aluminum films on P1P have been made to reveal the change in the electronic structure of the active medium in a LED in the absence of oxygen. Also the direct interaction of oxygen with this organic material is investigated by photoelectron spectroscopy. INTRODUCTION Since it has been demonstrated that it is possible to build highly efficient light emitting devices with conjugated polymers and oligomers sandwiched between two electrodes [1-7], many attempts have been made to increase the device performance by introducing additional layers in the device structure [8-14]. Before being able to tune these layers to an optimum one has to understand the principal processes involved in the interface formation in the ordinary triple systems. The general way to produce a LED with a conjugated organic material as active medium is to evaporate or spin-coat it onto the transparent, high work function electrode (most commonly indium-tin-oxide, ITO), and to evaporate the other, low work function, metal electrode (Ca, Mg, Al, alloys...) on top of it [4]. If ultra-clean conditions are not maintained throughout the production process, or the device is not kept in protective environments afterwards, the simple triple system may be altered into a more complicated one. Interaction of water or oxygen with the organic material may change its electronic structure [15]. Not affected by the environment are the diffusion and the possible doping processes of the metal atoms from the top electrode, which nevertheless may take place [16]. This will result in regions of virtually unknown electronic properties in the LED, most likely confined to the interface of the active material and the low work function electrode, which will strongly modify the characteristics of charge carrier injection. 509 Mat. Res. Soc. Symp. Proc. Vol. 488 0 1998 Materials Research Society
In the present investigation, LEDs with para-hexaphenyl (PHP) as active material, ITO and aluminum as electrodes are considered. As a first step th
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