In situ interfacial analysis of evaporated potassium on the electroluminescent fluorene-thiophene copolymer

X-ray photoelectron spectroscopy (XPS) was used to investigate the interface formation during in situ thermal evaporation of potassium on thin films of an electroluminescent conjugated polymer: poly[2,7-(9,9-dihexylfluorene)-co-alt-2,5-(decylthiophene)] (PFT). The chemical state and composition of the metal/polymer interfaces were studied as a function of the potassium coverage. Potassium was found to interact extensively with the bulk-adsorbed oxygen to form a layer of metal oxide at the K/PFT interface. Migration of the bulk-adsorbed oxygen to the surface occurred in response to the deposition of potassium onto the polymer. The changes in the S 2p lineshape suggested the formation of a thiolate species at low potassium coverages and also a sulphide species at higher metal coverages. The presence of change transfer from the deposited potassium atoms to the carbon atoms of PFT suggested that the polymer had undergone n-type doping. The deposited potassium atoms also diffused into the subsurface region of the polymer film. Thus, the interface of K/PFT contained mainly the oxidized metal, metallic potassium and metal-polymer charge transfer complexes.