Bandgap Modulation of Cs₂AgInX₆ (X = Cl and Br) Double Perovskite Nano- and Microcrystals via Cu²⁺ Doping

Recently, the double perovskite Cs₂AgInCl₆, which has high stability and low toxicity, has been proposed as a potential alternative to Pb-based perovskites. However, the calculated parity-allowed transition bandgap of Cs₂AgInCl₆ is 4.25 eV; this wide bandgap makes it difficult to use as an efficient solar absorber. In this study, we explored the effect of Cu doping on the optical properties of Cs₂AgInCl₆ double perovskite nano- and microcrystals (MCs), particularly in its changes of absorption profile from the ultraviolet (UV) to near-infrared (NIR) region. Undoped Cs₂AgInCl₆ showed the expected wide bandgap absorbance, but the Cu-doped sample showed a new sharp absorption peak at 419 nm and broad absorption bands near 930 nm, indicating bandgap reduction. Electron paramagnetic resonance (EPR) spectroscopy demonstrated that this bandgap reduction effect was due to the Cu doping in the double perovskite and confirmed that the Cu²⁺ paramagnetic centers were located on the surface of the nanocrystals (NCs) and at the center of the perovskite octahedrons (g_∥ > g_⊥ > g_e). Finally, we synthesized Cu-doped Cs₂AgInCl₆ MCs and observed results similar to those of the NCs, showing that the application range could be expanded to multidimensions.