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

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, Cs₂ZrCl₆:Te⁴⁺ 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 (Cs₂ZrCl₆) and the triplet states of the dopant (Te⁴⁺). 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) Cs₂ZrCl₆:Te⁴⁺ 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.