The formation of the interface between a thin film of a ladder-type poly(para-phenylene) m-LPPP and aluminum was investigated with x-ray and ultraviolet photoemission spectroscopy. The physical properties of this interface are of actual interest as m-LPPP is successfully applied as active material in organic light emitting devices (LED). Almost no changes in the core-level and valence electronic structure of the polymer upon increasing coverage with aluminum (in situ) are found. This gives indication for a weak interaction of Al with m-LPPP, ruling out the formation of chemical bonds between the two materials. The rapid occurrence of metallic aluminum at rather low coverage in the presented experiment is an important finding for the understanding of charge injection and the interfacial electronic structure in organic LEDs.
We present the fabrication and characterization of organic light emitting devices (OLEDs) using thin insulating layers for improved electron injection. The OLEDs are constructed with an ITO anode and an aluminum cathode. For the active layer we use either ladder-type Polyparaphenylene (m-LPPP) or Parahexaphenyl (PHP). A thin film of an insulating material is applied between the active layer and the cathode, in order to achieve a better tunnel injection due to a higher electric field at the interface. We compared different insulating materials with various thickness. The best results are obtained by using a LiF-layer with a thickness between 10 Å and 15 Å. Thereby the onset voltage decreases and the current density in the device increases significantly.
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