| As organic semiconductor technology matures, enhancement requires understanding/engineering of the cathode/organic interface. In this work, using X-ray photoelectron spectroscopy (XPS) and common materials for organic light emitting diodes (OLEDs), the expected interfacial structure in conventionally fabricated devices has been described and some simple predictive methods developed.; The buried electrode/active layer interface was examined by analysing: (1) both sides of the interface in conventionally fabricated devices under high vacuum with the unique peel-off technique, and (2) monolayers of one material grown atop another. Connections were drawn between the interfacial structures in devices, those observed during traditional surface science investigations, and the device behaviour. A critical insight is that no one metal or metal/interlayer combination may be used as a universal cathode. Rather, certain criteria for interfacial structure and stability must be confirmed to ensure adequate performance. This can be determined through simple material property information, such as lattice constants, or with inorganic analogues for organic molecules.; For combinations of metals and 8-tris(hydroxyquinoline aluminum) (Alq 3), interfacial reactions can be predicted by assuming Al2O 3 as an inorganic analogue. Using this analogue, molecular fragmentation may be described as a simple metal-exchange oxidation-reduction reaction.; As cathode complexity increases, such simple descriptions lose validity. This work shows that all three components (organic/LiF/metal) are required to adequately describe the interfacial structure of bi-layer cathodes. The major conclusions regarding the role of LiF are: (1) that 5-10A LiF changes the cathode oxidation behaviour, predicted by the lattice mismatch of the interlayer with the metal. Oxidation is suppressed for Al, which is well matched to LiF; for Mg, which has poor matching, preferential formation of carbonates occurs. Device behaviour is related to the metal oxidation, such that Al/LiF cathodes are superior to Mg/LiF ones. (2) that near the interface, LiF forms charge transfer complexes with electron transporting molecules. (3) that the cathode should be considered a metal-insulator-metal capacitor with the organic layer acting as the bottom electrode. The usable thickness of LiF is dependent on the conductivity of the layer.; These insights indicate some of the conditions necessary for adequate device performance and longevity, useful for future device optimization. |
