Modelling energy level alignment at organic interfaces and density functional theory

Abstract

A review of our theoretical understanding of the band alignment at organic interfaces is presented with particular emphasis on the metal/organic (MO) case. The unified IDIS (induced density of interface states) and the ICT (integer charge transfer) models are reviewed and shown to describe qualitatively and semiquantitatively the barrier height formation at those interfaces. The IDIS model, governed by the organic CNL (charge neutrality level) and the interface screening includes: (a) charge transfer across the interface; (b) the "pillow" (or Pauli) effect associated with the compression of the metal wavefunction tails; and (c) the molecular dipoles. We argue that the ICT-model can be described as a limiting case of the unified IDIS-model for weak interface screening. For a fully quantitative understanding of the band alignment at organic interfaces, use of DFT (density functional theory) or quantum chemistry methods is highly desirable. In this Perspective review, we concentrate our discussion on DFT and show that conventional LDA or GGA calculations are limited by the "energy gap problem of the organic materials", because the LDA (or GGA) Kohn-Sham energy levels have to be corrected by the self-interaction energy of the corresponding wavefunction, to provide the appropriate molecule transport energy gap. Image potential and polarization effects at MO interfaces tend to cancel these self-interaction corrections; in particular, we show that for organic molecules lying flat on Cu and Ag, these cancellations are so strong that we can rely on conventional DFT to calculate their interface properties. For Au, however, the cancellations are weaker making it necessary to go beyond conventional DFT. We discuss several alternatives beyond conventional LDA or GGA. The most accurate approach is the well-known GW-technique, but its use is limited by its high demanding computer time. In a very simple approach one can combine conventional DFT with a "scissor" operator which incorporates self-interaction corrections and polarization effects in the organic energy levels. Hybrid potentials combined with conventional DFT represent, probably, the best alternative for having a simple and accurate approach for analyzing organic interfaces. The problem then is to find an appropriate one for both the metal and the organic material in a plane-wave formulation; we show, however, how to overcome this difficulty using a local-orbital basis formulation. As examples of these alternatives, we present some DFT-calculations for several organic interfaces, using either the scissor operator or a hybrid potential, which can be interpreted in terms of the unified IDIS-model.

Interplay of local and global interfacial electronic structure of a strongly coupled dipolar organic semiconductor

“…Both short-range and long-range interactions can play an important role, making it at present still difficult to predict or rationally design a desired energy-level alignment at such an interface [2,3]. Further complications arise from an often rich and complex landscape of organic film growth and self-assembly at the metal surface [4], since the interfacial properties arise due to both surface/molecule and molecule/molecule interactions [5,6].…”

Chemical Interaction, Space-Charge Layer, and Molecule Charging Energy for a TiO2/TCNQ Interface

“…20,[24][25][26] This difference is related to the self-interaction energy, and is described by the molecular charging energy term, U mol , with U mol =E g gas -E g LDA =3.6 eV for TCNQ in gas phase. 7 In the case of the organic-oxide interface, additional electron correlation effects reduce the gas phase charging energy, U mol , to U and, consequently, the energy gap of the adsorbed TCNQ molecule. These effects are associated with the image potential induced by the oxide and the other molecules on the electron (LUMO) or the hole (HOMO) of the molecule under consideration.…”

“…The unit cell size defining the molecule-molecule distance has been fixed assuming a good matching between the oxide and the adsorbed TCNQ-structure; we stress that these distances are similar to the ones found in other TCNQ-interfaces. [20][21][22] For a careful discussion of the organic/oxide interface electronic properties, one has to introduce some corrections to the standard DFT-calculations due to limitations of this approach: (a) the Kohn-Sham energy levels yield transport gaps that are usually too small; 7,17,18 (b) although the local-orbital basis set has been optimized in each material to give a reasonable description of the electronic properties of either the oxide or the organic (except for their energy gaps), their initial relative level alignment is not correctly described, a general problem that appears for even well converged LDA-calculations. 23 The energy gap for a TCNQ molecule in the gas phase, E g gas , (measured as the energy difference between its electron affinity and ionization potential) is about 5.3 eV, whereas the energy gap between the Kohn-Sham HOMO and LUMO levels in LDA (or in GGA)…”

“…[1][2][3][4][5][6] A large amount of work has been undertaken in an effort to understand the energy level alignment between different metal/organic and organic/organic interfaces. [7][8][9] In contrast, very few studies have analyzed the energy level alignment at the interface between transition metal oxides and organic semiconductors. In a recent work, Greiner et al 10 analyzed a variety of non-reactive oxide/organic interfaces and concluded that the energy level alignment is determined mainly by one driving force: the electron chemical potential equilibration between the oxide Fermi level and the organic ionization energy.…”

Electronic Structure of the Perylene–Zinc Oxide Interface: Computational Study of Photoinduced Electron Transfer and Impact of Surface Defects

“…In principle, highly accurate quantum chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 interfaces, since the use of Kohn-Sham energy levels, especially from DFT calculations at the GGA level, can lead to errors in this alignment [146][147][148] . However, the very large size of the unit cells for the combined interface (for example, 261 atoms in 1-a) precludes the use of GW 149 or G 0 W 0 methods in the calculations for ZnO, defect-containing ZnO and perylene derivatives.…”