Spacer cations: molecular switches for 2D and 2D/3D perovskite photovoltaics

Over the past decade, perovskite solar cells (PSCs) have advanced rapidly, with power conversion efficiencies (PCE) exceeding 27%. Nevertheless, conventional three-dimensional (3D) perovskites still face serious stability challenges, posing a key obstacle to commercialization. Introducing bulky organic spacer cations into 3D perovskites to construct two-dimensional (2D) perovskites can markedly enhance stability. As molecular switches in 2D and 2D/3D PSCs, spacer cations enable precise control over the crystal structure, optoelectronic properties, and stability. This review systematically outlines the classes of spacer cations and the principles guiding their design, provides an in-depth analysis of the mechanisms by which they regulate perovskite properties, summarizes advances in 2D and 2D/3D PSCs enabled by spacer-cation engineering, and highlights core challenges and future directions, thereby offering theoretical support for the development of efficient and stable perovskite photovoltaic devices.