Vacuum sublimed $α,ω$-dihexylsexithiophene thin films: Correlating electronic structure and molecular orientation

In order to correlate the molecular orientation of organic thin films with charge injection barriers at organic/metal interfaces, the electronic structure and molecular orientation of vacuum sublimed thin films of $α,ω$-dihexylsexithiophene (DH6T) on the substrates Ag(111), highly oriented pyrolytic graphite (HOPG), and tetratetracontane (TTC) precovered Ag(111) were investigated. Results from metastable atom electron spectroscopy, ultraviolet photoelectron spectroscopy, and x-ray diffraction were used to derive growth models (including molecular orientation and conformation) of DH6T on the different substrates. On Ag(111), DH6T exhibits a transition from lying molecules in the monolayer/bilayer range to almost standing upright molecules in multilayers. This is accompanied by a shift of the molecular energy levels to a lower binding energy by 0.65 eV with respect to the vacuum level. The unit cell of standing DH6T on lying DH6T on Ag(111) is estimated to be similar to the DH6T bulk phase. On HOPG, DH6T grows in the bulk phase with lying orientation, starting already from the monolayer coverage. DH6T on TTC precovered Ag(111) grows in an almost lying orientation and a conformation that allows a strong overlap of the hexyl chains of DH6T with the alkyl chains of TTC. In all cases, the electronic structure and, particulary, the ionization energy of DH6T is dependent on the orientation of DH6T, i.e., lying DH6T has higher ionization energy than standing DH6T.