Abstract Lead-free double perovskites are emerging as an excellent candidate material for optoelectronic devices, owing to their unique properties such as widely adjustable band gap and markedly lower toxicity than the lead-halide counterparts. In this work, a series of Mn 2+ -doped Cs 2 NaInCl 6 (Mn 2+ :Cs 2 NaInCl 6 ) DP crystals with tailored Mn 2+ -doping concentrations are synthesized. The effects of the reaction temperatures and Mn/(In + Na) ratios in the raw materials on the Mn 2+ doping concentrations and their photoluminescence (PL) properties are investigated systematically. The resultant DP crystals exhibit well-resolved Mn 2+ 4 T 1 → 6 A 1 d − d emission with an optimal PL quantum yield of 16% and an extremely long excited state lifetime up to ~ 17 ms, which is the longest one of Mn 2+ emission among those of Mn 2+ -doped II–VI semiconductors or perovskite materials ever reported. Furthermore, the temperature-dependent PL and PL decay are studied in the temperatures range from 120 to 480 K. The decay lifetime decreased monotonously with the rise in temperatures, while the PL intensity gradually reaches a maximum and then keeps nearly constant, followed by a sharp decrease. Such abnormal phenomenon is mainly attributed to the combined effect of intrinsic property of octahedrally coordinated Mn 2+ in perovskite lattice and enhanced electron–phonon coupling at elevated temperature. Graphical abstract
Near-infrared (NIR) light-emitting diodes (LEDs) are potential devices that could become the core components in biological imaging, security monitoring, etc., which are currently constrained by suboptimal luminous efficiency due to the absence of excellent luminescent materials. Here, we report a double perovskite Cs2NaInCl6 that is suitable for Cr3+ doping and exhibits broad NIR emission. Through direct absorption and energy transfer (ET) from the self-trapped excitons (STEs) induced in the Cs2NaInCl6 matrix, Cr3+ can be excited to 4T1(P), 4T1(F), and 4T2(F) levels. Then, the broad emission with a full width at half-maximum (FWHM) of ∼150 nm, spanning 800 to 1300 nm, and a high photoluminescence quantum yield (PLQY) of up to 70% can be realized, resulting from the 4T2→4A2 transition of Cr3+. Subsequently, the introduction of Yb3+ endows Cs2NaInCl6:Cr3+,Yb3+ with a higher PLQY of 76% at the optimal doping ratio than Cs2NaInCl6:Cr3+. Ultimately, a miniaturized NIR LED device packaged based on Cs2NaInCl6:15%Cr3+,15%Yb3+ has been fabricated, which demonstrates superior performance for night vision, traceless perspective detection, and biological imaging. This work not only screens out a suitable Cs2NaInCl6 matrix for Cr3+ doping with high PLQY but also promotes its luminescence performance through Yb3+ doping for NIR LED applications.
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