Two-dimensional perovskites: Impacts of species, components, and properties of organic spacers on solar cells

Two-dimensional (2D) perovskites have been attracting extensive attention due to their intrinsic stability compared with their three-dimensional (3D) counterparts. These materials are widely tailorable in composition, structure, and bandgap, and provide an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In the field of photovoltaic, the fabricated 2D perovskite solar cells (PSCs) have achieved high stability as well as sustainable breakthrough in power conversion efficiency (PCE). However, the PCE of 2D PSCs still lags far behind their 3D counterpart, which is attributed to the special physicochemical properties of organic ligands. This review focuses the 2D halide perovskites from a structural perspective, namely the Ruddlesden-Popper (RP) phases, Dion-Jacobson (DJ) phases, alternating cation in the interlayer space (ACI) phases and mixed organic ligands phases, which stems from the diversity and versatility of spacers. Then the impacts of the species, chemical compositions, and physical characteristics of spacers on 2D perovskites, especially on the structure, carrier behavior, and the specific properties of solar cells, were discussed. Finally, several strategies on the rational selection of novel spacers are elucidated, and an outlook toward high-performance of 2D PSCs is presented.