excitons

Excitons are bound pairs of electrons and the gaps they leave behind in a material's structure when they are excited by energy, such as light. When a material absorbs energy, an electron is promoted to a higher energy state, leaving behind a positive "hole." The electron and hole are attracted to each other through electrostatic forces, forming an exciton. This quasiparticle behaves like a single unit, traveling through the material until it recombines, releasing energy, often as light. Excitons play a crucial role in the optical and electronic properties of materials like semiconductors and solar cells.