Stability of Cs2NaBiBr6 and Cs2NaBiCl6.

Bismuth-based halide perovskites are nontoxic alternatives to widely studied lead-based perovskites for optoelectronic applications. Here, we synthesized Cs2NaBiCl6 thin films and attempted to synthesize Cs2NaBiBr6 using physical vapor deposition. While Cs2NaBiCl6 forms a stable cubic structure with a 3.4 eV band gap and could be synthesized successfully, Cs2NaBiBr6 does not form and is unstable with respect to dissociation into Cs3-xNaxBi2Br9 and Cs3-xNaxBiBr6. Furthermore, the close X-ray diffraction patterns of Cs3-xNaxBi2Br9 and Cs2NaBiBr6 raise doubts about the previous reports of the latter's formation based on X-ray diffraction alone.

High photoluminescence quantum yield near-infrared emission from a lead-free ytterbium-doped double perovskite.

When excited by photons with energies greater than 2.2 eV, the bandgap energy, Yb-doped Cs2AgBiBr6 thin films synthesized via physical vapor deposition emit strong near-infrared luminescence centered at ∼1.24 eV via the Yb3+ 2F5/2 → 2F7/2 electronic transition. Robust, reproducible, and stable photoluminescence quantum yields (PLQY) as high as 82.5% are achieved with Cs2AgBiBr6 films doped with 8% Yb. This high PLQY indicates facile and efficient energy transfer from the perovskite host, Cs2AgBiBr6, to Yb, making Cs2AgBiBr6 the most promising lead-free down-conversion material.