Efficient Emission of Lanthanide Ion in Double Perovskite Nanocrystals Enabled by Synergistic Effect of Energy Level Modulation and Crystal‐Field Engineering

Abstract Trivalent lanthanide (Ln 3+ ) ions with ladderlike 4f energy levels, as dopants, can endow luminescent matrix with more diverse optoelectronic properties, and a wider application. However, their luminescent intensity is usually limited by the parity forbidden transition rule and the energy level mismatch between the host and dopant. Herein, Ln 3+ ions doped Cs 2 Na 1‐x K x Y 0.97 Sb 0.03 Cl 6 nanocrystals (NCs) are prepared, and the photoluminescence quantum yields of all samples with visible and near‐infrared emissions have surprisingly reached over 50%, therein, the value of Tb 3+ doped Cs 2 Na 0.6 K 0.4 Y 0.97 Sb 0.03 Cl 6 NCs is dramatically up to 83.3%, which is a new breakthrough in the field of lead‐free perovskite NCs. From the first principles calculations, crystal structure, and spectroscopic analysis, this benefits from the synergistic effect of deformation energy levels provided by Sb 3+ ions doping and crystal field environment distortion induced K + ions doping. That is, the localized distortion of the crystal field induced by K + ions doping causes energy level splitting of Sb 3+ ions, resulting in more effective energy transfer efficiency from the host to Ln 3+ ions, simultaneously introducing opposite odd parity, and breaking the forbidden transition rule, promoting luminescence intensity of Ln 3+ ions. This work will open a route to develop efficient Ln 3+ ions doped luminescent materials.