Lead bromide (PbBr 2 ,99.999%), lead iodide (PbI 2 , 99.999%), lead chloride (PbCl 2 , 98%), manganese (II) bromide (MnBr 2 , 98%), ytterbium (III) nitrate pentahydrate {Yb(NO 3) 3 .5H 2 O, 99.99%}, caesium carbonate (Cs 2 CO 3 , 99.9%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), oleylamine (OLA, 98%), trioctylphosphine (TOP, 90%), methyl acetate (MeOAc, 99.5%), N,N-dimethylformamide (DMF, 99.8%), hydrogen bromide (HBr, 43 wt%, 99.99%) were purchased from Aldrich and were used without further purification. Acetone (99.0%, SDFCL), Hexane (99.99%, Rankem), Toluene (99.5%, Rankem) solvents are used as obtained.

Cs2AgInCl6 double perovskite (DP) is a direct band gap, stable, and environmentally benign material with a three-dimensional perovskite structure. However, the band gap (∼3.3 eV) is too wide for optoelectronic applications in the near-infrared (NIR) region. To achieve optical functionality in the NIR window, we have doped Yb3+ in both microcrystals and colloidal nanocrystals of Cs2AgInCl6 DPs. Our Yb-doped Cs2AgInCl6 DP microcrystals and nanocrystals show NIR emission at ∼994 nm (1.24 eV), whereas the band gap of the hosts is in the UV region. Therefore, the Yb-emission does not suffer from self-absorption. The microcrystals exhibit a single-exponential decay with 2.7 ms long lifetime corresponding to 2F5/2 → 2F7/2 transition of Yb3+ f electrons. The nanocrystals show biexponential decay (3 ms and 749 μs), probably because of the proximity of Yb to the surface of nanocrystals. Interestingly, NIR emission and perovskite structure of samples are stable for >3 months in ambient conditions for future optoelectronic applications.