Self-doping and partial oxidation of metal-on-organic interfaces for organic semiconductor devices studied by chemical analysis techniques

The performance of organic electronic devices, such as organic light emitting diodes, transistors, or organic solar cells, depends critically on the chemical composition of the metal/organic and organic/metal interfaces which inject or extract charges into or from the device. By combining a number of techniques, such as x-ray photoemission spectroscopy (XPS) sputter depth profiling, XPS itself, secondary ion mass spectrometry, and laser desorption/ionization time-of-flight mass spectrometry, we investigate the reasons for differences in charge injection from metallic bottom and top contacts into either preferentially hole or preferentially electron transporting materials. We find that the deposition of metal onto organic semiconductors creates an organic-inorganic mixed interlayer in between the organic bulk material and the metal. In the case of electron injection, this interlayer acts as highly doped injection layer, while for hole injection, no significant improvement is visible. In addition to the self-doping, some cathode materials form partially oxidized metal-on-organic interfaces caused by oxygen in the residual gas. Depending on the evaporation conditions, the oxygen content varies. The effect of the oxygen incorporation, the origin, and the binding behavior in between the metal-on-organic interlayer is investigated and discussed. In contrast, organic materials evaporated on top of metals create an abrupt interface, where no self-doping effect is observed.