Incorporating a dipole interlayer has been one of the most crucial interfacial engineering strategies in organic and perovskite solar cells. An interfacial dipole brings steep shifts in electronic band structure across interfaces and thus effectively tunes charge carrier transport. However, the origin of the interfacial dipole and its effects on device performance are not entirely clear; they are even controversial in some cases. We devote this Perspective to identifying the electric dipole of various interlayers and correlating the interfacial dipole with device performance on the basis of classical semiconductor device theory. It is important to further consider the chemical nature of interlayers beyond the simplified model of an interfacial dipole to develop a full understanding of interfacial structure, energy bands, and device operation mechanism. Researchers are encouraged to integrate <i>in situ</i> and <i>in operando</i> characterizations with numerical simulations in future studies.