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  Niederhausen, J.; Amsalem, P.; Frisch, J.; Wilke, A.; Vollmer, A.; Rieger, R.; Müllen, K.; Rabe, J.P.; Koch, N.: Tuning hole-injection barriers at organic/metal interfaces exploiting the orientation of a molecular acceptor interlayer. Physical Review B 84 (2011), p. 165302/1-5

Abstract:
Ultraviolet photoelectron spectroscopy was used to demonstrate organic/metal-contact charge injection barrier tuning by exploiting the orientation-dependent work function φ of a molecular acceptor [hexaazatriphenylenehexanitrile (HATCN)] interlayer on Ag(111). The work function φ of a flat-lying HATCN monolayer on Ag was 4.6 eV (similar to a pristine Ag electrode), whereas a layer of edge-on HATCN on Ag exhibited φ of 5.5 eV (comparable to a pristine Au electrode). The hole-injection barriers (HIBs) between HATCN-modified electrodes and the organic semiconductors tris(8-hydroxyquinoline)aluminum (Alq3) and N,N-bis(1-naphtyhl)- N,N-diphenyl-1,1-biphenyl-4.4-diamine (α-NPD) were reduced by more than 1 eV compared to pristine Ag and Au electrodes. Noteworthy, the HIBs determined with the flat-lying HATCN interlayer were lower than those obtained for pristine Ag substrates (φ of both electrodes is 4.6 eV), and the HIBs with the edge-on HATCN on Ag were lower than those found for pristine Au (φ of both electrodes ca. 5.4 eV). This shows that acceptor interlayers are beneficial for charge injection in electronic devices even when the molecularly modified electrode φ is comparable to that of a pristine metal surface. It is argued that the molecularly modified electrodes are electronically more rigid than their pristine metal counterparts, i.e., the electron spill-out at the organic-terminated surface is less pronounced compared to Ag and Au surfaces.