Halide perovskites have become one of the most influential semiconductor materials platformssince 2012, combining outstanding optoelectronic performance with an unusually versatile structural and chemical design space. I will focus on how the field moved rapidly from the margins to the mainstream, and on the key early milestones that defined its trajectory. Three- and two-dimensional (3D and 2D) halide perovskites are an exceptional class of organic-inorganic semiconductors, distinguished by their remarkable carrier lifetimes and structural adaptability. Over the past15 years, these materials have achieved record efficiencies in solar cells, light-emitting devices, and radiation detection, driving rapid advancements in optoelectronic technologies. A critical next step is to deepen our understanding of how organic spacers influence their structure, properties, and performance. This presentation will explore the origins of the field, examine the current state of structure-property relationships, and provide guidelines for the selection and integration of organic spacers into crystalline materials and optoelectronic devices. Recent insights are shedding light on which organic spacer cations can effectively stabilize different perovskite structures.
