Sb ³⁺ /Ag ⁺ Induced Local Structure Engineering of Cs ₂ NaInCl ₆ Enabling Dual‐STE Emissions

Abstract Self‐trapped exciton (STE) emission is an important luminous form in lead‐free double perovskites (DPs). However, the STE emitting mechanism and its tunability remain a big challenge. Herein, Sb ³⁺ /Ag ⁺ induced local structure engineering is developed to construct dual‐STE states in the Cs ₂ NaInCl ₆ DP for the first time. Experimental results and theoretical calculations verify that the broadband orange emission of STE1 is generated by breaking the forbidden transition due to the reduction of electron dimensionality caused by Ag ⁺ alloying. Sb ³⁺ doping, besides the blue emission arising from Sb ^{3+ 3} P ₁ → ¹ S ₀ transition, additionally induces distortion of [AgCl ₆ ] ⁵⁻ octahedra, thereby generating STE2 radiative recombination with broadband far‐red emission. Importantly, an energy transfer channel from Sb ³⁺ dopants to STE2 is identified, and the relative intensities of the two STEs can be tuned by adjusting Ag ⁺ and Sb ³⁺ concentrations. These findings deepen the understanding of the photophysical mechanisms of lead‐free double perovskites doped with ns ² electrons and provide a foundation for more effectively regulating their optoelectronic properties.