Chemical Origin of the Stability Difference between Copper(I)‐ and Silver(I)‐Based Halide Double Perovskites

Abstract

AbstractRecently, CuI‐ and AgI‐based halide double perovskites have been proposed as promising candidates for overcoming the toxicity and instability issues inherent within the emerging Pb‐based halide perovskite absorbers. However, up to date, only AgI‐based halide double perovskites have been experimentally synthesized; there are no reports on successful synthesis of CuI‐based analogues. Here we show that, owing to the much higher energy level for the Cu 3d10 orbitals than for the Ag 4d10 orbitals, CuI atoms energetically favor 4‐fold coordination, forming [CuX4] tetrahedra (X=halogen), but not 6‐fold coordination as required for [CuX6] octahedra. In contrast, AgI atoms can have both 6‐ and 4‐fold coordinations. Our density functional theory calculations reveal that the synthesis of CuI halide double perovskites may instead lead to non‐perovskites containing [CuX4] tetrahedra, as confirmed by our material synthesis efforts.