Abstract
We report an ultrahigh vacuum scanning tunneling microscopy study of thermally driven interface rearrangement in binary films of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and zinc phthalocyanine (ZnPc), a model electron acceptor–electron donor system for organic solar cells. Neat PCBM films have been previously shown to undergo a transition from a disordered (glassy) phase to a crystalline hexagonal close-packed (hcp) arrangement above a critical packing density of 0.9 molecules/nm2. We now show how local PCBM density has a critical impact on binary film structure evolution. Bilayer films of PCBM and ZnPc undergo a spontaneous vertical phase separation to PCBM/ZnPc/Au(111) stacking at lower (<0.9 molecules/nm2) PCBM densities. This vertical phase separation is shown to be electrostatically preferred, with the PCBM/ZnPc/Au(111) dimer stacking lower in energy by 0.16 eV/pair relative to ZnPc/PCBM/Au(111) stacking. At higher local PCBM densities, sufficient to nucleate hcp PCBM domains, ZnPc molecules do not displace PCBM to the second layer. PCBM density variations in binary films thus give rise to heterogeneous interface structures.
Published Version
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