When a photon interacts with a material, an interaction occurs that causes its atoms to change their quantum state (a description of the physical properties of nature at the atomic level). The resulting state is called, aptly, photoexcitation. These photoexcitations are conventionally assumed to kill one another when they come near each other, radically limiting their density and mobility. This in turn limits how efficient tools that rely on photoexcitation such as solar cells and light-emitting devices can be.