Introducing Relatively Isolated In/Out‐Gap Bands in Cs 2 XCl 6 (X = Sn, Hf, Zr, Ti) via B‐Site Substitution: A Route to Brighter Luminescence and Tunable Emission Wavelengths

Both theoretical and experimental approaches are utilized to investigate the fluorescence mechanism of B‐site substituted Cs 2 XCl 6 (A 2 BX 6 ‐type perovskite, X = Sn, Hf, Zr, Ti), aiming to enhance the luminescence and tune the emission wavelengths. Using Te‐monosubstituted Cs 2 SnCl 6 as a model system, thecomputational discovery that the introduction of out‐gap and in‐gap bands by Te can significantly enhance its transition dipole moment is reported. This is further experimentally confirmed, showing a single emission peak resulting from the radiative transition between the out‐gap and in‐gap bands. The broadening of the emission peak is attributed to self‐trapped exciton (STE), while additional absorption/excitation peaks arise from composition segregation. Additionally, the high‐throughput first‐principles calculations indicate that substituting B‐sites of Cs 2 XCl 6 with Se, Te, Po, As, Sb, and Bi may also significantly enhance their light emission with the introduced bands. Thus, fine‐tuning the emission wavelengths by controlling the position of out‐gap and in‐gap bands through cation selection is proposed. Furthermore, tunable white‐light emission is achieved with excellent color stability by adjusting the Te and Bi composition in co‐substituted Cs 2 SnCl 6 material. The findings highlight the potential of utilizing out‐gap and in‐gap bands to tune luminescence in this perovskite family for advanced device applications, including white‐light‐emitting diodes (WLEDs).