Built-in potential and band alignment of matter

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 electrochemistry, electrical engineering, and materials science because it determines the band alignment at the interfaces. Despite its importance, its exact sign and magnitude have generally been recognized as an ill-defined quantity for more than half a century. Here, we provide a universal definition of the built-in potential. Furthermore, 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 the interface plays a role only in the absence of equilibrium. Our quantitative 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 open circuit voltages and electrode potentials in electrochemical cells.