Multilayer 3D/2D Perovskite Heterojunctions: Structural Engineering, Interfacial Modulation, and Photovoltaic Applications

Perovskite solar cells combine excellent optoelectronic properties with solution processability, but their long-term performance remains limited by interfacial defects, ion migration and structural instability. Multilayer 3D/2D perovskite heterojunctions offer a promising route to address these limitations by integrating efficient light absorption and charge transport of three-dimensional perovskites with defect passivation effect and high stability of two-dimensional perovskites. However, poorly controlled formation of low-dimensional phases usually introduce mixed dimensionalities, unfavorable orientations, excessive organic spacers and energetic barriers, thereby compromising charge extraction and long-term stability. Here, we review the evolution of multilayer 3D/2D heterojunctions in terms of interface location, phase composition and crystallographic orientation, and compare solution, thermal evaporation, imprint-transfer and vapor-assisted fabrication strategies. We further discuss their roles in defect passivation, energy-level regulation, charge transport, phase-structure control and phase stabilization, and highlight their emerging applications in large-area modules, tandem and flexible devices. Finally, we outline key opportunities in deterministic phase control, interface-specific asymmetric design, operando characterization, and scalable fabrication to advance multilayer 3D/2D heterojunctions from empirical interface modification toward rationally engineered and manufacturable perovskite photovoltaics.