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.

The benefits of excess PbI 2 on perovskite crystal nucleation and growth are countered by the photoinstability of interfacial PbI 2 in perovskite solar cells (PSCs). Here we report a simple chemical polishing strategy to rip PbI 2 crystals off the perovskite surface to decouple these two opposing effects. The chemical polishing results in a favorable perovskite surface exhibiting enhanced luminescence, prolonged carrier lifetimes, suppressed ion migration, and better energy level alignment. These desired benefits translate into increased photovoltages and fill factors, leading to high-performance mesostructured formamidinium lead iodide-based PSCs with a champion efficiency of 24.50%. As the interfacial ion migration paths and photodegradation triggers, dominated by PbI 2 crystals, were eliminated, the hysteresis of the PSCs was suppressed and the device stability under illumination or humidity stress was significantly improved. Moreover, this new surface polishing strategy can be universally applicable to other typical perovskite compositions.

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.