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.
We report on the resistive switching effect in metal/organic semiconductor/metal structures and present a general explanation of the switching mechanism in these devices. The J-V characteristics of metal/tris(8- hydroxyquinolinato)aluminum (Alq3)/metal devices will be discussed and it will be further shown that these sustain only a limited number of switching cycles. Besides Alq3, we also investigate other organic semiconductor materials and obtain a bistable behavior, which is independent of the organic material but dependent on the current injection conditions of the interface between the organic material and the metal top electrode. Further, we investigate the material independent switching effect using impedance spectroscopy and can disclose a transition from capacitive to resistive behavior at switching voltages. We propose that the switching in metal/organic semiconductor/metal structures is caused by the growth and rupture of resistive filaments in the organic semiconductor.
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