Research progress on 2D and 3D mixed-phase perovskite thin films: Fabrication, materials, optimization and characterization

Three-dimensional (3D) perovskite solar cell (PSC) has emerged as promising candidate for next-generation photovoltaic beyond silicon due to its remarkable power conversion efficiencies (PCE). However, its poor stability remains a major obstacle. Dimensional reduction of 3D perovskite into two-dimensional (2D) structure has proven effective in enhancing stability, leading to growing interest in 2D and 3D mixed-phase PSC that potentially combine durability with high efficiency. Despite these advantages, the incorporation of 2D phases still faces a series of challenges, such as the introduction methods of the 2D phase, the selection of suitable 2D cations, the control of the crystallization behavior in 2D and 3D mixed-phase films, and the development of advanced characterization techniques for identifying and analyzing the 2D phase. Unlike heterojunction structures where 2D perovskite layers are deposited as capping layers atop 3D films, 2D and 3D mixed-phase perovskites incorporate 2D components directly within the 3D matrix, forming localized heterojunctions. In this review, we summarize the fabrication methods for 2D and 3D mixed-phase films, including modified one-step and two-step solution approaches as well as the more recent inverse temperature crystallization (ITC) method. We further discuss the development of novel 2D materials across various perovskite hosts (CH3NH3⁺ (MA+), CH(NH2)2⁺ (FA⁺), Cs⁺, multi-cations, and Sn-based systems), although cross-comparisons of identical 2D materials within different host lattices are still lacking. Key optimization strategies, such as additive engineering to improve carrier transport, crystallization control to promote oriented 2D growth, and exploiting the unique nanoplate structures of 2D phases, are highlighted. Finally, we review conventional and advanced characterization techniques that enable structural and stability analysis of 2D and 3D mixed-phase perovskites. This comprehensive overview provides insights into synthesis strategies, targeted material design, performance optimization, and characterization methodologies, thereby guiding the development of high-performance and commercially viable PSCs.