Two-dimensional (2D) hybrid organic−inorganic halide perovskites are a preeminent class of low-cost semiconductors whose inherent structural tunability and attractive photophysical properties have led to the successful fabrication of solar cells with high power conversion efficiencies. Despite the observed superior stability of 2D lead iodide perovskites over their 3D parent structures, an understanding of their thermochemical profile is missing. Herein, the calorimetric studies reveal that the Ruddlesden−Popper (RP) series, incorporating the monovalentmonoammonium spacer cations of pentylammonium (PA) and hexylammonium (HA): (PA) 2 (MA) n-1 Pb n I 3n+1 (n = 2−6) and (HA) 2 (MA) n-1 Pb n I 3n+1 (n = 2−4) have a negative enthalpy of formation, relative to their binary iodides. In contrast, the enthalpy of formation for the Dion−Jacobson (DJ) series, incorporating the divalent and cyclic diammonium cations of 3-and 4-(aminomethyl)piperidinium (3AMP and 4AMP respectively): (3AMP)(MA) n-1 Pb n I 3n+1 (n = 2−5) and (4AMP)(MA) n-1 Pb n I 3n+1 (n = 2−4) have a positive enthalpy of formation. In addition, for the (PA) 2 (MA) n−1 Pb n I 3n+1 family of materials, we report the phasepure synthesis and single crystal structure of the next member of the series (PA) 2 (MA) 5 Pb 6 I 19 (n = 6), and its optical properties, marking this the second n = 6, bulk member published to date. Particularly, (PA) 2 (MA) 5 Pb 6 I 19 (n = 6) has negative enthalpy of formation as well. Additionally, the analysis of the structural parameters and optical properties between the examined RP and DJ series offers guiding principles for the targeted design and synthesis of 2D perovskites for efficient solar cell fabrication. Although the distortions of the Pb−I−Pb equatorial angles are larger in the DJ series, the significantly smaller I•••I interlayer distances lead to overall smaller band gap values, in comparison with the RP series. Our film stability studies on the RP and DJ perovskites series reveal consistent observations with the thermochemical findings, pointing out to the lower extrinsic stability of the DJ materials in ambient air.

Mechanical stability of hybrid organic–inorganic perovskites (HOIPs) is essential to achieve long-term durable HOIP-based devices. While HOIPs in two-dimensional (2D) form offer numerous options in the structure and composition to tune their mechanical properties, little is known about the structure–mechanical-property relationship in this family of materials. Here, we investigated a series of 2D lead halide HOIPs by nanoindentation to explore the impact of critical factors controlling the properties of both the organic and inorganic layers on the materials’ out-of-plane mechanical performance. We find that the lead–halide bond in the inorganic framework can significantly influence the mechanical properties of 2D Ruddlesden–Popper (RP) HOIPs with n = 1. Like 3D HOIPs, stronger lead–halide bond strength leads to a higher Young’s modulus in these 2D HOIPs, i.e., E ⊥ Cl ≳ E ⊥ Br > E ⊥ I. In contrast, the hardness of 2D RP HOIPs follows a trend of H Br 2D > H Cl 2D > H I 2D, which is different from that found in 3D HOIPs, probably due to the combined effects from the Pb–X bond strength and inorganic framework structural change (e.g., symmetry and distortion). We further show that the interface between the organic layers in 2D HOIPs can be an effective route to engineer the materials’ mechanical properties. Replacing the weak CH3–CH3 van der Waals forces by covalent bonds or phenyl–phenyl interactions in the interface can lead to a much stiffer and harder 2D HOIPs. Finally, we discover that the mechanical performance of 2D HOIPs with linear aliphatic diammonium spacer molecules is affected by the two basic structural parameters, i.e., the thicknesses of the organic and inorganic layers, in a similar way compared to that of 2D RP HOIPs with linear aliphatic monoammonium spacer molecules. A thinner organic layer and a thicker inorganic layer can result in 2D HOIPs with larger elastic modulus and hardness values. Our results offer intriguing insights into the structure–property relationship of 2D HOIPs from a mechanical perspective, providing guidelines and inspirations to achieve material design with required mechanical properties for applications.

Transformative and reconstructive reactions impart significant structural changes at particle boundaries of hybrid perovskites, which influence environmental stability and optoelectronic properties of these materials. Here, we investigate the moisture-induced transformative reactions in formamidinium­(FA)-based perovskites FAPbX3 (X = I, Br) and show that the ambient stability of these materials can be adjusted from a few hours to several months. For FAPbI3, roles of water vapor, particle size, and light illumination on the kinetic pathways of the cubic (α) transformation to the hexagonal (δ) phase are analyzed by X-ray diffraction, optical microscopy, photoluminescence, and solid-state (ss) NMR spectroscopy techniques. The grain and subgrain boundaries exhibit different α- → δ-FAPbI3 phase transformation kinetics. Our study suggests that the dynamic transformation involves the local water-induced dissolution of the cubic phase occurring at the crystal surfaces followed by precipitation of the hexagonal phase. Insights into structures and dynamics of a kinetically trapped α-|δ-FAPbI3 are obtained by 1H, 2H, and 207Pb ssNMR spectroscopy.

hybrid lead iodide perovskites have gained prominence due to their remarkable structural tunability, optoelectronic features, and moisture stability, which have rendered them as attractive alternatives to 3D MAPbI 3 for optoelectronic devices. 2D multilayer lead bromide perovskites remain an unfathomed phase space with the lack of systematic studies to establish the structure, photophysical properties and stability behavior of this family of 2D halide perovskites. Herein, we present new m e m b e r s o f b i l a y e r l e a d b r o m i d e p e r o v s k i t e s (C m H 2m+1 NH 3 ) 2 (CH 3 NH 3 )Pb 2 Br 7 (m = 6−8) that belong to the Ruddlesden−Popper structure type, incorporating long chain alkylmonoammonium cations (C m H 2m+1 NH 3 ) of hexylammonium (m = 6), heptylammonium (m = 7), and octylammonium (m = 8). A universal solution synthetic methodology for bulk multilayer lead bromide perovskites is presented with all structures solved and refined using single crystal X-ray diffraction. The studied bilayer lead bromide perovskites demonstrate a decrease in the lattice rigidity and lattice match of the inorganic perovskite layer−organic layer, as the alkyl-monoammonium chain length increases. In comparison to their iodide analogues, the bilayer lead bromide compounds exhibit elongation of their stacking axis despite the smaller dimensions of the [PbBr 6 ] 4− lattice, while their internal lattice strain was calculated to be reduced, inferring a greater lattice match between the inorganic [PbBr 6 ] 4− perovskite layer and organic layer. The (C m H 2m+1 NH 3 ) 2 (CH 3 NH 3 )Pb 2 Br 7 (m = 4, 6−8) compounds exhibit narrow-band emission near 2.5 eV. Time-resolved photoluminescence (PL) displays longer carrier lifetimes on the nanosecond time scale comparing to their iodide analogues, where electronic structure calculations indicate that the increase of the alkyl chain length and, thus, lattice softness enhances nonradiative recombinations. A complete set of air, light, and heat stability tests on unencapsulated thin films of (C m H 2m+1 NH 3 ) 2 (CH 3 NH 3 )Pb 2 Br 7 (m = 4, 6−8) and MAPbBr 3 show they are stable in ambient air for at least 5 months, exhibiting greater extrinsic stability than the 2D lead iodide congeners. Extraordinarily, 3D MAPbBr 3 films prove to be more stable than films of 2D lead bromide perovskites, in contrast to MAPbI 3 which is less stable than the 2D lead iodide perovskites.

Two-dimensional (2D) semiconductors are attractive candidates for a variety of optoelectronic applications owing to the unique electronic properties that arise from quantum confinement along a single dimension. Incorporating nonradiative mechanisms that enable directed migration of bound charge carriers, such as Förster resonance energy transfer (FRET), could boost device efficiencies provided that FRET rates outpace undesired relaxation pathways. However, predictive models for FRET between distinct 2D states are lacking, particularly with respect to the distance d between a donor and acceptor. We approach FRET in systems with binary mixtures of donor and acceptor 2D perovskite quantum wells (PQWs), and we synthetically tune distances between donor and acceptor by varying alkylammonium spacer cation lengths. FRET rates are monitored using transient absorption spectroscopy and ultrafast photoluminescence, revealing rapid picosecond lifetimes that scale with spacer cation length. We theoretically model these binary mixtures of PQWs, describing the emitters as classical oscillating dipoles. We find agreement with our empirical lifetimes and then determine the effects of lateral extent and layer thickness, establishing fundamental principles for FRET in 2D materials.