Roles of 2D halide perovskites in 2D / 3D heterojunctions for high‐performance perovskite solar cells

Perovskite solar cells (PSCs) have achieved power conversion efficiencies comparable to crystalline silicon, yet their commercial deployment is hindered by inherent susceptibility to environmental stressors. To overcome this, the integration of two‐dimensional (2D) layered perovskites with three‐dimensional (3D) absorbers to form 2D/3D heterojunctions has emerged as a definitive strategy for enhancing device longevity. However, current research has predominantly focused on defect passivation, often leaving the broader structural and electronic interactions between 2D and 3D lattices less explored. This review moves beyond the traditional perspective of 2D layers as passive encapsulants and establishes a design‐oriented framework for their strategic integration into 2D/3D heterojunctions. We classify 2D structures from macroscopic inorganic slab thickness to microscopic interlayer connectivity defined by spacer cation chemistry. Centering on the 2D/3D heterojunction architecture, we elucidate the multifaceted roles of 2D perovskites ranging from thermodynamic stabilization and defect passivation to active electronic functions such as built‐in potential modulation and structural templating. Furthermore, we discuss practical strategies for forming 2D/3D heterojunctions, categorizing them into surface post‐treatments for top‐junction engineering and emerging techniques for buried‐interface control to minimize non‐radiative recombination. This review confirms that the 2D layer functions as an active semiconductor component that fundamentally dictates the device physics of high‐performance PSCs.