Dimensionality and Chirality in Halide Double Perovskites: A Coupled Design Framework for Polarization Control

Halide double perovskites have emerged as chemically versatile alternatives to lead halide perovskites, yet their functional behavior cannot be understood through simple structural analogy. In these materials, orbital decoupling at the band edges reconstructs electronic dispersion and defect energetics, establishing a distinct regime of exciton localization and symmetry sensitivity. Dimensional reduction amplifies dielectric confinement, while chirality introduces controlled inversion asymmetry. When combined with spin–orbit interaction, these structural parameters enable polarization‐sensitive optical responses under ambient conditions. This Review integrates electronic structure, dimensional architecture, and chiral symmetry into a unified design framework. We show that performance arises from a coupled parameter space rather than isolated optimizations: confinement enhances chiroptical effects but increases disorder sensitivity; heavy‐element substitution strengthens spin‐dependent transitions but modifies defect energetics; and bio‐adjacent deployment requires structural containment rather than compositional substitution alone. By defining realistic performance envelopes and clarifying the distinction between polarization control and long‐lived spin coherence, this work positions double perovskites as solution‐processable, polarization‐aware photonic materials. Within this framework, their functional capabilities are best understood in the context of quantum‐enabled photonics, where spin‐selective optical responses can be achieved without requiring extended coherence times. The resulting design map provides guidance for compositional selection, symmetry engineering, and scalable device integration.