Bandgap Engineering of Lead-Free Double Perovskite Cs₂AgInCl₆ Nanocrystals via Cu²⁺-Doping

Lead-free double perovskites (DPs) with excellent moisture, light, and heat stability have been explored as alternatives to toxic lead halide perovskite (APbX₃) (A for monovalent cation and X for Cl, Br, or I). However, the bandgaps of the current DPs are generally larger and either indirect or direct forbidden, which leads to weak visible light absorption and limitation for photovoltaic and other optoelectronic applications. Herein, we demonstrate the first synthesis of Cu²⁺-doped Cs₂AgInCl₆ double perovskite nanocrystals via a facile hot-injection solution approach. The electronic bandgap can be dramatically tuned from ∼3.60 eV (Cs₂AgInCl₆, parent) to ∼2.19 eV (Cu²⁺-doped Cs₂AgInCl₆) by varying the Cu²⁺ doping amount. We conclude that the decrease of bandgap is attributed to the overlap of the Ag-d/In-p/Cl-p orbitals and the Cu-3d orbitals in the valence band. The wide tunability of the optical and electronic properties makes Cu²⁺-Doped Cs₂AgInCl₆ DP NCs promising candidates for future optoelectronic device applications.