Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption

Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI4[Cu-II((CuInIII)-In-I)(0.5)Cl-8] (1, A = organic cation) may be derived from a Cu-I-In-III double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the Cu-I-In-III double perovskite and Cu-II single perovskite structures. However, whereas the highly insulating Cu-I-In-III and Cu-II perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.

Automated summary

Introducing heterovalent cations can change the optoelectronic properties of halide perovskites, and utilizing the structural flexibility of two-dimensional perovskites allows for placing three distinct stoichiometric cations in octahedral sites. The new layered perovskite exhibits enhanced electronic conductivity due to intervalence charge transfer between mixed-valence Cu centers. This demonstrates the importance of increasing the compositional complexity of halide perovskites for improved properties such as enhanced visible light absorption.