Thermally activated intermixture in pentacene-perfluoropentacene heterostructures

Using thermal desorption spectroscopy (TDS) the thermal stability of binary pentacene/perfluoropentacene (PEN/PFP) thin films has been investigated for various preparation protocols. Variation of stoichiometry ratio reveals a significantly enhanced thermal stability in comparison to the single compounds only for films with equimolar stoichiometry. The stabilization also depends on the preparation method and was found for co-deposition as well as for multi-stacks and subsequently grown PEN/PFP-stacks but not for stacks grown in the reversed order. By systemically varying the substrate temperature during deposition, we prove that the resulting intermixture is caused by a thermally activated diffusion during film growth and not due to post-deposition diffusion induced upon heating during TDS measurements. The different extents of thermal stabilization are discussed in the context of the film morphology studied by means of atomic force microscopy (AFM). For complementary information, optical absorption spectra of the heterostructures are analyzed, where the arisal of new absorption bands and the extinction of excitonic bands existing in the pure compounds are identified as decisive criteria to judge the efficiency of intermixture.