Role of Copper in Enhancing Visible Light Absorption in Cs<inf>2</inf>Ag(Bi, In, Sb)Cl<inf>6</inf>Halide Double-Perovskite Materials

Halide perovskite materials with ABX3 show excellent photovoltaic properties, but their stability remains a serious concern. On the other hand, the double perovskite with two formula unit A2BB′X6 shows excellent stability, but their visible light absorption properties are inferior, which restricts them from being employed in solar energy conversion devices. The incorporation of dopant ions in the crystal structure is found to be an effective strategy to introduce the visible light absorption to these materials. In this work, copper is systematically substituted in the place of Ag+ (B site) in three different halide double perovskites, Cs2Ag(Bi/In/Sb)Cl6, and we explored their structural, optical, and electrochemical properties in detail. The role of copper in tuning the properties is found to be different in each of these materials. Only a small fraction of the dopant could be introduced in Cs2AgInCl6 (CAIC) and Cs2AgSbCl6 (CASC), and no copper could be incorporated in Cs2AgBiCl6 (CABC). The presence of Cu+ in the CAIC and CASC crystals (i) distorted the octahedra formed by [InCl6]3- and [SbCl6]3- and (ii) introduced states above the valence band affecting the optical properties. The Cu 3d orbitals are introduced very close to the valence band of CAIC, leading to a change in the absorption onset from 400 to 580 nm. However, for CASC the copper orbitals are pushed toward the conduction band, resulting in near IR absorption with an onset at ∼1300 nm. The color of CAIC changed from white to yellow, and CASC changed from yellow to black upon substitution of copper. Combining structural, optical, and electrochemical characterization experiments, we explained the role of copper in improving the visible light absorption in halide double perovskites.