Energy-Level Alignment at Organic–Inorganic Heterojunctions

Abstract

The combination of organic (OSC) and inorganic (ISC) semiconductors in hybrid structures is driven by the potential to combine the best of the two worlds and thereby to achieve optoelectronic performance surpassing that of the single component. Key requirements toward this goal are a thorough understanding of the energy-level alignment at organic–inorganic heterojunctions and a portfolio of tools to control it. This chapter discusses first the factors that determine the electronic structure at pristine ISC–OSC interfaces. Based on that knowledge, two principal approaches are presented to tune the energy-level alignment. The first approach is based on the assembly of a monolayer of molecules with an electric dipole moment on the ISC surface and the second on the introduction of a molecular donor or acceptor interlayer, which undergoes a ground-state charge transfer with the ISC. Thus, a fine-tuning of the energy-level alignment over a huge energy range becomes feasible by proper choice of the interlayer. The presently achievable tuning range of the work function of wide bandgap ISCs (ZnO, GaN) spans from 2.3 eV to 6.5 eV. The chapter also provides examples that demonstrate how the energy-level tuning via a molecular interlayer provides indeed the desired control over the functionality of organic–inorganic heterojunctions.