Rigorous definition of built-in potentials at interfaces*

The built-in potential is the interfacial potential difference due to electric dipole at the interface of two dissimilar materials. It is of central importance to the understanding of many phenomena in interface/surface science because it determines the band alignment between the materials. Despite more than 50 years of investigation, however, its exact sign and magnitude have generally been generally recognized as an ill-defined quantity. In this work, we provide a universal definition of the built-in potential. We find that the built-in potential is explicitly determined by the bulk (i.e., innate) properties of the constituent materials when the system is in electronic equilibrium, while it is subject to the properties of the interface only in the absence of equilibrium. Our theory enables a unified description of a variety of important properties in all types of interfaces, ranging from work functions and Schottky barriers in electronic devices to electrode potentials and redox potentials in electrochemical cells.