Calculating the Universal Energy‐Level Alignment of Organic Molecules on Metal Oxides

Abstract

Recently, Greiner et al. [ Nat. Mater. 2012, 11 , 76 ] published a survey of the level alignment of about 40 metal oxide/organic molecule interfaces. They observed a striking regularity in the electronic level alignment of the highest occupied molecular orbital (HOMO) and the Fermi level that depends solely on the difference between the substrate work function and the ionization energy of the molecule independent of the details of the electronic structure of the oxide. The authors could reproduce their data under the assumption of thermodynamic equilibrium occupation of the HOMO using four adjustable parameters. A model that quantifi es well-established concepts in heterojunction physics and achieves the same result without any adjustable parameters is presented here. This approach explains why the level alignment is rather independent of the experimental details, such as the electronic structure of the oxide, defects in the oxide, and the thickness of oxide and overlayer. are not included in the spectroscopically determined HOMO and LUMO energies as expressed through IE org and EA org , respectively. The unusually large polaronic binding energy is, according to the authors, due to the fact that one is dealing here with interface polarons. We shall return to this question later.Recently, Greiner et al. [ 29 ] expanded the results of Salaneck and co-workers [ 26 ] to encompass small molecules as well. They too observed the upper part of the Zorro curve for a great number of organic/metal oxide molecule interfaces, albeit measured slightly differently. Instead of the work function, they measured the energy of the HOMO orbital relative to E F , (i.e., the hole-emission barrier Φ Bp ), and plotted Δ E H = Φ Bp = E F -E HOMO vs. Φ sub -IE org . This plot is redrawn here as Figure 2 . Substrates and interfaces were prepared in UHV and the substrate work function, the ionization energy of the organic molecules, and Δ E H were measured in situ by photoemission spectroscopy. In this sense, Greiner et al.'s [ 29 ] procedure follows more closely that of Kahn and co-workers, [ 27 ] except for using metal substrates with a well-defi ned oxide layer on top. Nevertheless, the experimental results comply perfectly with those of Salaneck and co-workers and the interpretation is based on the same concept, namely the transition from the Schottky-Mott regime to pinning when the substrate work function equals the ionization energy:The transition at Φ sub -IE org = 0, as suggested by the dashed line, indicates that the relevant ICT + is indeed the HOMO. However, spectroscopically the HOMO is measured about 0.3 eV below E F . Greiner et al. [ 29 ] were able to describe their considerable body of data with an expression that contains four adjustable parameters. In what follows, we present a the electron affi nity and ionization energy of the organic semiconductor, respectively, as they are measured for the isolated constituents of the interface. This, of course, presupposes that the LUMO and HOMO are the transport levels for electrons a...

Regular Energetics at Conjugated Electrolyte/Electrode Modifier for Organic Electronics and their Implications on Design Rules

“…The energy where the Fermi level is subsequently pinned is referred to as EICT+,-depending on if it is positive or negative polarons that are being created and several approaches recently have been developed to calculate the pinning energies of organic semiconductors. [30][31][32][33] Note that the ICT states are essentially polarons interacting with the transferred charge (or image charge) on the other side of the interface. This additional energy, not present for bulk polarons, and variation in intermolecular order typically cause the ICT state distribution to be different than the bulk polaron distribution, [26] see Fig.…”

Energetics at Doped Conjugated Polymer/Electrode Interfaces

“…Furthermore, they do not have the same energy as bulk polarons, mainly due to Coulombic interaction with the opposite charge across the interface and variations in molecular order. Several different approaches have recently been developed to quantitatively treat the effects of screening and molecular order on the E ICT+,-energies in the ICT model [30][31][32][33][34][35] , and the abruptness of the integer charge transfer region also is being intensively researched and may be material dependent, as Neher and coworkers show a region extending beyond 50 nm for two types of polymers [18] whereas Frisch and coworkers found that the integer charge transfer only involved the first monolayer for rr-P3HT. [36] Here, the aim of our study is to explore the formation mechanism of the molecule-doped conjugated polymer/electrode interface by photoemission spectroscopy, and conclusively establish its universal energy level alignment.…”

Energy level alignment of self-assembled linear chains of benzenediamine on Au(111) from first principles

“…[1][2][3][4] As a consequence, electronic energy level alignment at interfaces between organic components (such as individual molecules) and inorganic electrodes, which are critical to charge flow within organic devices, have been the focus of significant recent fundamental work [5][6][7][8][9][10][11][12][13]. When a molecule is in contact with an electrode, its orbital energies are significantly altered relative to the gas-phase by several competing physical contributions.…”