Fermi level positioning in organic semiconductor phase mixed composites: The internal interface charge transfer doping model

Photoemission data, taken on co-sublimed films and on bilayers of the prototypical small molecule semiconductor CuPc and p-type dopants TCNQ or WO3 show similar electronic trends that have to be interpreted by phase separation of the dopant within the matrix material forming a phase mixed composite. High resolution TEM micrographs for CuPc:WO3 co-deposited films clearly prove such phase separation. Therefore the doping models developed for singly dispersed dopant molecules cannot be applied. For the mechanism of the doping induced variations of the host matrix Fermi level in such phase mixed semiconductor:dopant composites we propose the internal interface charge transfer doping model. According to this model the Fermi levels of two mixed phases align at the internal interfaces and the doping limit is defined by the work function difference of matrix and dopant minus the potential drops induced by dipole formation at the internal matrix/dopant interfaces. It is shown that the magnitude of the internal interface dipole potential drops may be estimated from the dipoles measured at matrix/dopant bilayer interfaces and that the maximum dopant induced Fermi level shift may be estimated from the difference of work functions measured on thick films of matrix and of dopant, minus a mean value for the interface dipole.