Energy level alignment at the porphyrin/cobaltocene interface: From transfer doping to cobalt intercalation

N-type doping of the organic semiconductor zinc tetraphenylporphyrin (ZnTPP) by overlayers of the reducing molecule decamethylcobaltocene (CoCp2*) is demonstrated using photoelectron spectroscopy. A transfer doping model involving integer charge transfer between molecules reproduces quantitatively all measured level shifts as a function of CoCp2* coverage using the ionisation potential of CoCp2* and the electron affinity of ZnTPP as sole input parameters. The model yields the experimentally observed limitation of doping to the first monolayer of cobaltocene while further layers remain neutral without the need to resort to special bonding arrangements for the first monolayer. Temperature-dependent studies reveal that doping is still present at room temperature, despite the high vapour pressure of CoCp2*. Higher annealing temperatures initiate CoCp2* molecular dissociation and diffusion of Co atoms into the ZnTPP film. Hence, the nature of doping changes from surface molecular transfer doping to bulk metallic doping as a function of temperature. (C) 2013 Elsevier B.V. All rights reserved.