Electronic structure of hybrid interfaces for polymer-based electronics

Abstract

The fundamentals of the energy level alignment at anode and cathode electrodes in organicelectronics are described. We focus on two different models that treat weakly interactingorganic/metal (and organic/organic) interfaces: the induced density of interfacial statesmodel and the so-called integer charge transfer model. The two models are comparedand evaluated, mainly using photoelectron spectroscopy data of the energy levelalignment of conjugated polymers and molecules at various organic/metal andorganic/organic interfaces. We show that two different alignment regimes are generallyobserved: (i) vacuum level alignment, which corresponds to the lack of vacuumlevel offsets (Schottky–Mott limit) and hence the lack of charge transfer acrossthe interface, and (ii) Fermi level pinning where the resulting work function ofan organic/metal and organic/organic bilayer is independent of the substratework function and an interface dipole is formed due to charge transfer acrossthe interface. We argue that the experimental results are best described by theinteger charge transfer model which predicts the vacuum level alignment when thesubstrate work function is above the positive charge transfer level and belowthe negative charge transfer level of the conjugated material. The model furtherpredicts Fermi level pinning to the positive (negative) charge transfer level when thesubstrate work function is below (above) the positive (negative) charge transferlevel. The nature of the integer charge transfer levels depend on the materialssystem: for conjugated large molecules and polymers, the integer charge transferstates are polarons or bipolarons; for small molecules’ highest occupied and lowestunoccupied molecular orbitals and for crystalline systems, the relevant levels are thevalence and conduction band edges. Finally, limits and further improvements tothe integer charge transfer model are discussed as well as the impact on devicedesign.