Currently, most two‐dimensional (2D) metal halide perovskites are of the Ruddlesden–Popper type and contain the thermally unstable methylammonium (MA) molecules, which leads to inferior photovoltaic performance and mild stability. Here we report a new type of MA‐free formamidinium (FA) based low‐dimensional perovskites, featuring a general formula of (PDA)(FA)n−1PbnI3n+1 with propane‐1,3‐diammonium (PDA) as the organic spacer cation. The perovskite films with well‐oriented crystal grains are attained under the assistance of the FACl additive, where the role of Cl is investigated through the grazing‐incidence X‐ray diffraction technique. The photovoltaic device based on the optimized (PDA)(FA)3Pb4I13 film demonstrates a remarkable power conversion efficiency of 13.8 %, the highest record for the FA‐based 2D perovskite solar cells. In addition, compared to (PDA)(MA)3Pb4I13, the MA‐containing analogue and a renowned stable 2D perovskite, both the (PDA)(FA)3Pb4I13 films and their derived devices exhibit exceedingly higher thermal stability.

Two-dimensional (2D) perovskite materials have exhibited great possibilities toward the fabrication of highly efficient and stable solar cell devices. The large degree of structural versatility due to the viable choices of organic interlayer spacers promises new and valuable 2D perovskite species. Herein, phenyltrimethylammonium (PTA+) is successfully employed as the organic interlayer spacer to prepare the 2D Ruddlesden–Popper perovskite films that exhibit exceptional optoelectronic properties. By adding Cl– ions during film growth, the (PTA)2(MA)3Pb4I13 (MA = methylammonium) perovskite films are effectively prepared with a tunable crystal orientation and film morphology. The optimized devices fabricated with the assistance of Cl– ions deliver the power conversion efficiency up to 11.53%, which is ascribed to the simultaneous reductions of charge transfer resistance and defect-induced charge recombination. Moreover, the PTA-based 2D perovskite solar cells demonstrate remarkable environmental and thermal stabilities.

The heterogeneous stacking of a thin two-dimensional (2D) perovskite layer over the three-dimensional (3D) perovskite film creates a sophisticated architecture for perovskite solar cells (PSCs). It combines the remarkable thermal and environmental stabilities of 2D perovskites with the superior optoelectronic properties of 3D materials which resolves the chronic stability issue with no compromise on efficiency. Herein, we propose the vapor-assisted growth strategy to fabricate high-quality 2D/3D heterostructured perovskite films by introducing long-chain organoamine gases in which the 2D layers have a uniform and tunable thickness. The 3D to 2D transformation of the widely adopted MAPbI3 (MA = methylammonium) film is initiated by the butylamine vapor and monitored through the in situ grazing-incidence X-ray diffraction technique. A variety of 2D species are observed and rationalized by the different collapsing and reconstruction models of the Pb–I octahedra. The PSC devices based on the optimized 2D/3D heterostructures show significant improvements in photovoltaic performances, owing to better energy level alignments, longer carrier lifetimes, and less defects as compared to their 3D analogues. In addition, both the butylamine vapor-treated perovskite films and the derived PSC devices demonstrate exceptional long-term stabilities.

Metal halide perovskites have revolutionized the development of highly efficient, solution-processable solar cells. Further advancements rely on improving perovskite film qualities through a better understanding of the underlying growth mechanism. Here, a systematic in situ grazing-incidence X-ray diffraction investigation is performed, facilitated by other techniques, on the sequential deposition of formamidinium lead iodide (FAPbI 3 )-based perovskite films. The active chemical reaction, composition distribution, phase transition, and crystal grain orientation are all visualized following the entire perovskite formation process. Furthermore, the influences of additive ions on the crystallization speed, grain orientation, and morphology of FAPbI 3 -based films, along with their photovoltaic performances, are fully evaluated and optimized, which leads to highly reproducible and efficient perovskite solar cells. The findings provide key insights into the perovskite growth mechanism and suggest the fabrication of high-quality perovskite films for widespread optoelectronic applications.

Metal halide perovskite solar cells (PSCs) have advanced to the forefront of solution-processed photovoltaic techniques and made stunning progress in power conversion efficiency (PCE). Further improvements in device performances rely on perfecting the structure and morphology of perovskite films. However, undesirable defects such as pinholes and grain boundaries are often created in film preparations due to lack of knowledge of the precise reaction mechanism. Here, in situ grazing-incidence X-ray diffraction (GI-XRD) investigations are performed, facilitated by other techniques, on the formation of the widely adopted MAPbI (MA = methylammonium) perovskite films from their intermediate adduct (IA) phases. The influences of solvent vapor atmospheres on MAPbI films are also systematically investigated, where the dynamic conversion processes between different phases are visualized in real time. Further in situ GI-XRD and infrared spectroscopy measurements reveal that the IA phases contain both N,N-dimethylformamide and dimethyl sulfoxide (DMSO) as coordinating molecules. By tuning the DMSO concentration in perovskite precursors, the ideal perovskite film is formed and the best PCE is achieved for the planar MAPbI -based PSCs. These findings highlight the role of IA phases and the effect of solvent atmospheres on the quality of perovskite films, providing direct insights into their growth mechanism.