Pb-free all-inorganic halide double perovskites (DPs) have emerged as an important class of environmentally benign phosphors for optoelectronic applications. The studies on them mainly restricted to the visible spectral region.  

Lanthanide (Ln³⁺) doping is proposed as a valid approach to expand the spectral range of DPs towards the near-infrared (NIR) region. Unfortunately, these Ln³⁺-doped DPs still suffer from low weak NIR emission of Ln³⁺ due to the small absorption cross-section of Ln³⁺. 

In a study published in Angewandte Chemie International Edition, the research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences developed a new class of NIR-emitting DPs based on Ln³⁺-doped Cs₂(Na/Ag)BiCl₆. Benefiting from the Na⁺-induced breakdown of local site symmetry in the Cs₂AgBiCl₆ DPs, efficient NIR emissions of Ln³⁺(e.g. Yb³⁺ and Er³⁺) are realized through Bi³⁺ sensitization. 

The researchers first systematically studied the optical properties of Ln³⁺-doped Cs₂(Na/Ag)BiCl₆ phosphors to determine the optimal Na⁺ content, and then comprehensively surveyed the excited-state dynamics and the energy transfer process in the optimal Ln³⁺-doped Cs₂Ag_0.2Na_0.8BiCl₆ phosphors by means of temperature-dependent steady-state and transient photoluminescent spectroscopy. 

Raman spectroscopy analysis and first-principles density functional theory (DFT) calculations confirmed that Na/Ag alloying caused the change of Bi-Cl bond length, which resulted in the local symmetry breaking of Bi³⁺ in [BiCl₆]^3- octahedron.  

Compared with that of the Na-free Cs₂AgBiCl₆ counterparts, the NIR emission of Yb³⁺ and Er³⁺ can be boosted by 7.3-fold and 362.9-fold in Cs₂Ag_0.2Na_0.8BiCl₆ DPs, with the optimal photoluminescence quantum yields (PLQYs) of 19.0% and 4.3%, respectively. 

Besides, the researchers employed these Ln³⁺-doped NIR-emitting Cs₂Ag_0.2Na_0.8BiCl₆ DPs for 365-nm UV-converted NIR Light Emitting Diodes (LEDs), which indicated their potential applications as high-performance optoelectronic devices.  

These findings provide new insight into the design of efficient NIR luminescent Ln³⁺ doped all-inorganic halide double perovskites materials, which may accelerate the development of new NIR optoelectronic devices.