Photoluminescence of Te‐Doped Double Perovskite Cs2ZrCl6 Nanocrystals Versus Bulk: Insights From Temperature‐Dependent Spectroscopy

Abstract Although bulk crystals of lead‐free vacancy‐ordered double perovskites demonstrated a highly efficient emission, their nanocrystals (NCs) counterparts exhibit inferior optical performance. To understand the reasons behind this phenomenon, Cs 2 ZrCl 6 :Te 4+ double perovskite NCs are synthesized, and their optical properties are compared with their bulk powders. Temperature‐dependent spectroscopy revealed that the NCs sustain a thermal sensitization of the intermediate trap state, which is located between the self‐trapped state of the host (Cs 2 ZrCl 6 ) and the triplet states of the dopant (Te 4+ ). This opens up a pathway for the non‐radiative recombination, and thus decreases the energy transfer efficiency from host to dopant. Importantly, this pathway is suppressed in larger (40 nm) Cs 2 ZrCl 6 :Te 4+ NCs, resulting in their photoluminescence quantum yield of 24%, as compared to 7% for the 22 nm NCs. Furthermore, the emission spectral range of these double perovskite NCs is shown to extended into near‐infrared by incorporating rare‐earth ions as additional dopants. The study has established a crucial relation between the optical properties and the size effect in lead‐free vacancy‐ordered double perovskites and thus lays a foundation for further improvement of their optical performance.