Optoionics – Controlling ions with light

Optoionics has recently emerged at the intersection of optoelectronics and solid state ionics, triggered by fundamental work on light-induced ionic conductivity enhancement in methylammonium lead iodide (MAPI). This perspective traces the evolution of optoionics from early 20th century studies on photoionics to contemporary research, elucidating the semantic nuances and historical development of light–ion interactions. We follow the first observations such as copper photoionization and subsequent conceptual extensions such as molecular photoionics and photo-ionic cells, leading on to the current definition and understanding of optoionics. We then proceed to apply this understanding on light–ion interactions in carbon nitrides, distinguishing between intrinsic and extrinsic optoionic effects depending on whether one or more distinct phases are involved. This nuanced understanding is essential for the design of optoionic devices that exploit light–ion interactions to couple light harvesting and electrochemical energy storage. Finally, we provide an outlook on emerging optoionic devices at the intersection of energy conversion and storage and discuss smart circuit elements that integrate optoionic principles for advanced technological applications. • The use of the terms “optoionics” and “photoionics” in the literature over the last decades is critically assessed. • A coherent understanding of prototypical optoionic effects is introduced based on recent literature. • Key features of optoionic mechanisms are evaluated across different materials classes and length scales. • The community's efforts at advancing optoionics are streamlined by defining critical terms and concepts. • An outlook on optoionic applications is provided, ranging from energy conversion and storage to smart circuit elements.