Two-dimensional (2D) Ruddlesden–Popper (RP) organic–inorganic perovskites have emerged as promising candidates for solar cells with technologically relevant stability. Herein, a new RP perovskite, the fifth member (⟨n⟩ = 5) of the (CH3(CH2)2NH3)2(CH3NH3) n−1Pb n I3n+1 family (abbreviated as PA2MA4Pb5I16), was synthesized and systematically investigated in terms of photovoltaic application. The obtained pure PA2MA4Pb5I16 crystal exhibits a direct band gap of E g = 1.85 eV. Systematic analysis on the solid film highlights the key role of the precursor–solvent interaction in the quantum well orientation, phase purity, grain size, surface quality, and optoelectronic properties, which can be well-tuned with addition of dimethyl sulfoxide (DMSO) into the N,N-dimethylformamide (DMF) precursor solution. These findings present opportunities for designing a high-quality RP film with well-controlled quantum well orientation, micrometer-sized grains, and optoelectronic properties. As a result, we achieved power conversion efficiency (PCE) up to 10.41%.

Producing high efficiency solar cells without high-temperature processing or use of additives still remains a challenge with the two-step process. Here, the solution processing of MAPbI 3 from PbI 2 films in N,N-dimethylformamide (DMF) is investigated. In-situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements reveal a sol-gel process involving three PbI 2 -DMF solvate complexes-disordered (P 0 ) and ordered (P 1 , P 2 )-prior to PbI 2 formation. When the appropriate solvated state of PbI 2 is exposed to MAI (methylammonium Iodide), it can lead to rapid and complete room temperature conversion into MAPbI 3 with higher quality films and improved solar cell performance. Complementary in-situ optical reflectance, absorbance, and quartz crystal microbalance with dissipation (QCM-D) measurements show that dry PbI 2 can take up only one third of the MAI taken up by the solvated-crystalline P 2 phase of PbI 2 , requiring additional annealing and yet still underperforming. The perovskite solar cells fabricated from the ordered P 2 precursor show higher power conversion efficiency (PCE) and reproducibility than devices fabricated from other cases. The average PCE of the solar cells is greatly improved from 13.2(±0.53)% (from annealed PbI 2 ) to 15.7(±0.35)% (from P 2 ) reaching up to 16.2%. This work demonstrates the importance of controlling the solvation of PbI 2 as an effective strategy for the growth of high-quality perovskite films and their application in high efficiency and reproducible solar cells.

The two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI3 hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI3, is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI3, requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI2 into the α‐FAPbI3 perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI2 precursor from its different states are compared. α‐FAPbI3 forms spontaneously and efficiently at room temperature from P2 (ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI2) to 17.23(±0.28)% (from solvated PbI2) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA+ cation without the need for Cs+ or mixed halide formulation.

Low-dimensional organic–inorganic hybrid perovskites have triggered many fundamental research studies due to their intrinsic tunable photovoltaic properties, technologically relevant stability, and promising efficiency. However, there is limited information on how ligand size influences inherent structural and electronic properties of perovskites. To gain deeper understanding of ligand-size related structural and film properties, we fabricated a series of (L)2(MA) n‑1Pb n I3n+1 materials by introducing organic spacer ligands of n-CH3CH2NH3 (EA), n-CH3(CH2)2NH3 (PA), and n-CH3(CH2)3NH3 (BA) into the three-dimensional (3D) methylammonium (MA) lead iodide (MAPbI3) system with the same inorganic layer thickness (average ⟨n⟩ = 4). We demonstrate that the increased number of carbon atoms on ligands affects compatibility of ligands with the 3D [PbI6]4– framework, leading to different structural dimensionality and crystal orientation, largely explaining different electronic properties, crystal stability, and the consequent device performance of solar cells. This work provides key missing information on how ligand size influences structural dimensionality and desirable electronic properties for future stable and efficient solar cells.