Electronic structure of hybrid interfaces for polymer-based electronics

Abstract

The fundamentals of the energy level alignment at anode and cathode electrodes in organic electronics are described. We focus on two different models that treat weakly interacting organic/metal (and organic/organic) interfaces: the induced density of interfacial states model and the so-called integer charge transfer model. The two models are compared and evaluated, mainly using photoelectron spectroscopy data of the energy level alignment of conjugated polymers and molecules at various organic/metal and organic/organic interfaces. We show that two different alignment regimes are generally observed: (i) vacuum level alignment, which corresponds to the lack of vacuum level offsets (Schottky-Mott limit) and hence the lack of charge transfer across the interface, and (ii) Fermi level pinning where the resulting work function of an organic/metal and organic/organic bilayer is independent of the substrate work function and an interface dipole is formed due to charge transfer across the interface. We argue that the experimental results are best described by the integer charge transfer model which predicts the vacuum level alignment when the substrate work function is above the positive charge transfer level and below the negative charge transfer level of the conjugated material. The model further predicts Fermi level pinning to the positive (negative) charge transfer level when the substrate work function is below (above) the positive (negative) charge transfer level. The nature of the integer charge transfer levels depend on the materials system: for conjugated large molecules and polymers, the integer charge transfer states are polarons or bipolarons; for small molecules' highest occupied and lowest unoccupied molecular orbitals and for crystalline systems, the relevant levels are the valence and conduction band edges. Finally, limits and further improvements to the integer charge transfer model are discussed as well as the impact on device design.

Different approaches to adjusting band offsets at intermolecular interfaces

“…For many large molecular adlayers, including a number of organic and metal-organic species, the energy level alignment (of the adsorbate) is dependent upon the interfacial electronic structure and the interfacial dipole layer, as has been readily demonstrated for many large molecules [5][6][7], including the metal phthalocyanines [8]. Charge injection and the dipoles at the electrode interface are surprisingly complex [9] because of interface chemistry. At the interface between the two organic layers, the expected weaker chemical interactions permit better understanding of the molecular band offsets, in spite of usually limited understanding of the in-terface structure.…”

Charge equilibration and potential steps in organic semiconductor multilayers

“…Table 1 summarizes the pinning levels and the work functions of the materials discussed in this paper. Pinning levels are obtained from first-principles calculations [16], or from experiment [7,15]. To establish a pinning level experimentally, one organic material is deposited on a range of metallic substrates with different work functions.…”

“…Pinning levels can be obtained from experiment [7,15], or from first-principles density functional theory calculations [16]. The ionization potential and electron affinitiy I D and A A of a molecule may depend on its local surroundings.…”

“…Consequently, a detailed understanding of interface properties, energy level alignment in particular, is critical for guiding the improvement of device performance. The energy level alignment at all-organic and hybrid organic-inorganic interfaces has been the subject of extensive research efforts in recent years [3][4][5][6][7][8][9][10][11][12].…”

“…It does not involve the formation of chemical bonds at the interface, which determines the band offsets at inorganic heterojunctions [13,14]. To describe the potential offsets observed at organic interfaces, an empirical model has been formulated, based on the transfer of (an integer number of) electrons across an interface [7,15]. Recent theoretical work has shown that the assumptions of such an integer electron transfer, combined with simple electrostatics at the interface, allows one to calculate the potential offset at an interface [16].…”