Dimensional Phase Reconstruction in 2D/3D Perovskite Solar Cells: Stability Implications Under Thermal, Light, and Bias Stresses

Two‐dimensional/three‐dimensional (2D/3D) perovskite heterostructures have emerged as a promising strategy to enhance the stability of halide perovskite solar cells (PSCs), entailing an efficient energy landscape and defect‐passivation capabilities of the 2D perovskite layer. However, mounting experimental evidence shows that the 2D/3D phase interface is not a static structural entity, but rather a dynamic system susceptible to operational stresses. In this review, we systematically examine how thermal exposure, sustained illumination, and applied bias drive counter‐directional interdiffusion of spacer cations and A‐site cations across the 2D/3D interface, inducing a gradual increase in the dimensionality ( n ‐value) of the 2D perovskite layer. This stress‐induced 2D phase reconstruction erodes the photoactive 3D perovskite domains, disrupts the interfacial funneling energy landscape, and compromises the surface moisture barrier, collectively accelerating device degradation. We further survey emerging mitigation strategies, including polymer interlayers, steric and reactivity engineering of spacer cations, and n ‐value‐tailored Dion–Jacobson frameworks. Finally, we outline critical unresolved challenges and future research directions toward achieving stable 2D/3D perovskite photovoltaics.