exciton
/ EK-sy-ton /
When a flash of light kicks an electron out of its place in a crystal, it leaves behind an empty spot. That spot behaves like a positive charge, and the freed negative electron is still tugged toward it. Like two magnets that drift but stay loosely paired, the electron and the empty spot can orbit one another. That bound, dancing pair is an exciton.
More precisely, an exciton is an electron and the hole it left behind, held together by their electrical attraction and roaming through the material as a single neutral package. It is a little like a hydrogen atom, except the proton is just a missing electron and the whole thing lives inside a solid, where the surrounding atoms weaken the pull and let the partners orbit farther apart. The exciton carries energy but no net charge, so it can shuttle energy across a material without moving any current.
Excitons matter because they govern how light turns into useful energy and how materials glow. They are the first thing formed when light is absorbed in many solar cells and the last thing that emits light in an LED or a glowing screen, so harvesting or splitting them is central to those devices. The honest caveat is that in ordinary silicon the pair is so loosely bound that room-temperature heat tears it apart almost instantly; excitons truly matter in materials where the binding is strong, such as organic dyes, certain two-dimensional sheets, and quantum dots.
In the glowing pixels of an OLED phone screen, an injected electron and hole meet, bind briefly into an exciton, and then collapse together — releasing their stored energy as a single photon of red, green, or blue. Every dot of light on the screen is an exciton dying.
Each pixel of an OLED screen lights up when an exciton forms and then releases its energy as light.
An exciton carries energy but no net charge, so it does not contribute to electric current. To make electricity from absorbed light, a solar cell must pull the electron and hole apart before they recombine and vanish.