Precise Intergranular Voids Control of MgF₂ via Solidifying Micelle‐Carried Precursor for Tunable Refractive Index

Porosification of fluoride can be effectively utilized in various applications, based on its abundance of exposed active sites. However, previous approaches have drawbacks in terms of application in fields that require meticulous control over porosity. Herein, an innovative strategy is suggested for achieving precise porosity tuning using a micelle‐carried MgF₂ precursor intermediate. As the size of the MgF₂ precursor clusters increases through micellization, the sizes of the resulting solidified MgF₂ grains and the intergranular voids between the grains increase. Controlled MgF₂ with intergranular voids can achieve very low‐refractive indices (≈1.04) with extremely fine refractive index control intervals (0.01–0.04). By employing extreme controllability of refractive index, graded refractive index antireflective coating is applied to the quartz window, and an average transmittance of ≈97.96% (250–1100 nm) is obtained. Finally, the coating is applied to a perovskite half‐cell enabling a ≈22.46% increase in the light‐absorption efficiency.