Unveiling universal trends for the energy level alignment in organic/oxide interfaces

Abstract

In this perspective we present a comprehensive analysis of the energy level alignment at the interface between an organic monolayer (organic = perylenetetracarboxylic dianhydride, PTCDA, zinc tetraphenylporphyrin, Zn-TPP, and tetracyanoquinodimethane, TCNQ) and a prototypical oxide surface, TiO 2 (110), looking for universal behaviours. PTCDA shows a physisorbed interaction with TiO 2 and a small interface dipole potential with its highest occupied molecular orbital (HOMO) energy level located in the oxide energy gap and the lowest occupied molecular orbital (LUMO) energy level located above the oxide conduction band minimum, E C . We analyse how the interface barrier depends on an external bias potential between the organic layer and the oxide surface, Δ, and find for this interface that the screening parameter S = d|(E C -HOMO)|dΔ is close to 1. In the second case, the Zn-TPP monolayer shows a moderate chemisorbed interaction with some charge transfer from the molecule to the oxide and a significant interface dipole potential, in such a way that S decreases to around 0.8. In the TCNQ/TiO 2 (110) case, the TCNQ molecules present a strong chemical interaction with the oxide; the LUMO energy level is located in the oxide energy gap in such a way that one electron is transferred from the oxide to the organic molecule; we also find that in this case S ≈ 0.5. All these cases can be integrated within a universal behaviour when (E C -HOMO) is calculated as a function of Δ; that function presents a zig-zag curve with a central part having an S-slope, and two plateaus associated with either the LUMO or the HOMO energy levels crossing the oxide Fermi level. In these plateaus, a Coulomb blockade regime arises and a space charge layer develops in the oxide surface.

Using Electronic Sum-Frequency Generation to Analyze the Interfacial Structure of Singlet Fission-Capable Perylenediimide Thin Films

“…For example, band bending or the presence of deleterious surface states at the interface between the organic sensitizer and silicon could lead to changes in the energetic landscape that prevent efficient exciton transfer. ,,, Likewise, the morphology of the organic SF sensitizer itself at a junction could be a bottleneck, as triplet energy transfer necessitates wave function overlap between the sensitizer and silicon. Because molecular wave functions are anisotropic, transport properties and energetics at the interface are likely sensitive to molecular orientation. ,,− Moreover, when deposited as a thin film, organic dyes can experience strong energetic coupling between individual molecules, causing the electronic states of the film to greatly differ as a function of how these molecules arrange themselves. ,, Thus, any changes in the morphology of the molecular system at a silicon junction could induce a shift in the organic solid’s triplet energy relative to the film’s bulk. If such a shift places the triplet state lower in energy than silicon’s bandgap, this will create an energetic barrier for triplet transfer. ,,,− …”

On surface chemical reactions of free-base and titanyl porphyrins with r-TiO2(110): a unified picture

“…Consistently, zinc-porphyrins deposited at room temperature on r-TiO 2 (110) are observed by scanning tunnelling microscopy (STM) to adsorb atop the protruding O br rows, 18,31,32 where weak chemisorption (charge transfer) was inferred from the measured interface dipole. 33 In contrast, Ni-TPP molecules, synthesized by sublimation of Ni on previously deposited 2H-TPP, have been reported to lie atop the Ti 5f rows. 34 Remarkably, these adsorption sites on TiO 2 are matching predictions for MTPPs deposited on the (1 Â 1) O-Fe(001) surface, where Zn-TPP and Co-TPP display a preferential adsorption site atop an oxygen atom, while Ni-TPP rather adsorbs atop a substrate Fe atom.…”

Morphology dependent interaction between Co(ii)-tetraphenylporphyrin and the MgO(100) surface

“…30 Such a variety of adsorption scenarios does not allow any a priori prediction about the resulting physical properties of the adsorbed molecules, which explains the great interest in those systems both at experimental and theoretical level. 6,31 It is thus important to understand the extent to which an adsorbed porphyrin is affected by the contact with the surface in terms of geometry and electronic structure.…”