Two-dimensional complex metal halides: influence of restricted dimensionality on functional properties

The field of perovskite solar cell research has recently experienced a shift from structurally 3 dimensional materials, primarily halide perovskites, to materials with a 2D structural dimensionality, such as Ruddlesden-Popper (RP) phases and Dion-Jacobson (DJ) phases. This review introduces the term complex metal halides (CMHs) as a composition-based alternative to the commonly used perovskite term, avoiding discrepancies in their classification. Furthermore, a classification table is presented that separates CMHs based on their form factor and structural dimensionality. These compounds, which are two important sub-categories of CMHs, have received significant attention in recent years due to their enhanced stability as solar absorbers. Research has shown that integration of films consisting of structurally 2D compounds with 3D halide perovskite films can boost the photovoltaic performance and at the same time improve its device stability. We aim to provide an overview of the recent progress in RP and DJ phases, and their potential as materials in the field of light-absorbing and light-emitting applications. We use the term "complex metal halides" to refer to compounds such as halide perovskites. Our focus includes exploring various dimensionality types, with a specific emphasis on structurally 2D materials, including RP and DJ phases.