Two-dimensional halide perovskites spacer for stable perovskite solar cells: A review

Perovskite solar cells (PSCs) have reached remarkable efficiencies exceeding 27%, yet achieving long-term operational stability remains a critical challenge. Two-dimensional (2D) halide perovskites have shown promise in addressing this issue due to their enhanced moisture resistance and structural tunability. Most existing works emphasize phase types or device-level performance, but lack a molecular-level classification that links the structure of organic spacer cations to crystallization behavior, interfacial interactions, and device functionality. In this review, we construct a comprehensive, molecular-level framework for the rational design of 2D/3D hybrid perovskite systems. We systematically classify over hundred organic spacer cations across three structural dimensions—spatial geometry (e.g., linear vs. cyclic), functional group chemistry (e.g., hydroxyl, carboxyl, alkynyl), and heteroatom composition (e.g., O, N, S substitutions)—to elucidate how each structural motif governs crystallization behavior, phase formation, defect passivation, and charge transport. This structure-function map is then contextualized within four widely used heterostructure construction methods: bulk incorporation, buried interface modification, surface treatment, and vapor-phase deposition. Each strategy is evaluated in terms of its impact on film morphology, phase alignment, interfacial energetics, and scalability. This review also highlights emerging molecular designs, such as nitrogen-containing spacers lacking classical ammonium groups, which offer new opportunities for inducing layered structures and tuning optoelectronic properties. Finally, we outline future directions involving AI-guided molecular screening, entropy-driven cation engineering, and scalable deposition techniques, aiming to bridge molecular design with practical manufacturing. This framework provides a clearer design guidelines and highlight promising directions for researchers seeking to advance stable and scalable perovskite photovoltaic technologies.