halide perovskites have emerged as outstanding semiconducting materials thanks to their superior stability and structural diversity. However, the ever-growing field of optoelectronic device research using 2D perovskites requires systematic understanding of the effects of the spacer on the structure, properties, and device performance. So far, many studies are based on trial-and-error tests of random spacers with limited ability to predict the resulting structure of these synthetic experiments, hindering the discovery of novel 2D materials to be incorporated into high-performance devices. In this review, we provide guidelines on successfully choosing spacers and incorporating them into crystalline materials and optoelectronic devices. We first provide a summary of various synthetic methods to act as a tutorial for groups interested in pursuing synthesis of novel 2D perovskites. Second, we provide our insights on what kind of spacer cations can stabilize 2D perovskites followed by an extensive review of the spacer cations, which have been shown to stabilize 2D perovskites with an emphasis on the effects of the spacer on the structure and optical properties. Next, we provide a similar explanation for the methods used to fabricate films and their desired properties. Like the synthesis section, we will then focus on various spacers that have been used in devices and how they influence the film structure and device performance. With a comprehensive understanding of these effects, a rational selection of novel spacers can be made, accelerating this already exciting field.

Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP2+) as the spacing cation and a mixture of methylammonium (MA+) and formamidinium (FA+) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA0.75FA0.25)3Pb4I13 perovskite has a narrower bandgap, less distorted inorganic framework, and larger PbIPb angles than the single‐cation (3AMP)(MA)3Pb4I13. Furthermore, the (3AMP)(MA0.75FA0.25)3Pb4I13 films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA0.75FA0.25)3Pb4I13‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)3Pb4I13 cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.

Low-dimensional halide perovskites have recently attracted intense interest as alternatives to the three-dimensional (3D) perovskites because of their greater tunability and higher environmental stability. Herein, we present the new homologous 2D series (NHC HNH)(CHNH) PbI ( m = 4-9; n = 1-4), where m represents the carbon-chain number and n equals layer-thickness number. Multilayer ( n > 1) 2D perovskites incorporating diammonium cations were successfully synthesized by the solid-state grinding method for m = 4 and 6 and by the solution method for m = 7-9. Structural characterization by single-crystal X-ray diffraction for the m = 8 and m = 9 series ( n = 1-4) reveals that these compounds adopt the Cc space group for even n members and Pc for odd n members. The optical bandgaps are 2.15 eV for two-layer ( n = 2), 2.01 eV for three-layer ( n = 3), and 1.90 eV for four-layer ( n = 4). The materials exhibit excellent solution processability, and casting thin-films of the n = 3 members was successfully accomplished. The films show a clear tendency for the higher- m members to have preferred orientation on the glass substrate, with m = 8 exhibiting almost perfect vertical layer orientation and m = 9 displaying both vertical and parallel layer orientation, as confirmed by grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements. The vertical layer orientation for the (NHCHNH)(CHNH)PbI member results in the best thermal, light, and air stability within this series, thus showing excellent potential for solar cell applications.

Two-dimensional (2D) hybrid halide perovskites are promising in optoelectronic applications, particularly solar cells and light emitting devices (LEDs), and for their increased stability compared to 3D perovskites. Here, we report a new series of structures using propylammonium (PA +) which results in a series of Ruddlesden-Popper (RP) structures with the formula (PA) 2 (MA) n-1 Pb n I 3n+1 (n = 3, 4) and a new homologous series of "step-like" (SL) structures where the PbI 6 octahedra connect in a corner-and face-sharing motif with the general formula (PA) 2m+4 (MA) m-2 Pb 2m+1 I 7m+4 (m = 2, 3, 4). The RP structures show a blue-shift in bandgap for decreasing n (1.90 eV for n = 4 and 2.03 eV for n = 3) while the SL structures have an even greater blue-shift (2.53 eV for m = 4, 2.74 eV for m = 3, and 2.93 eV for m = 2). DFT calculations show that, while the RP structures are electronically 2D quantum wells, the SL structures are electronically 1D quantum wires with chains of corner-sharing octahedra "insulated" by blocks of face-sharing octahedra. Dark measurements for RP crystals show high resistivity perpendicular to the layers (10 11 Ω•cm) but a lower resistivity parallel to them (10 7 Ω•cm). The SL crystals have varying resistivity in all three directions, confirming both RP and SL crystals' utility as anisotropic electronic materials. The RP structures show strong photoresponse, whereas the SL materials exhibit resistivity trends that are dominated by ionic

This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/smll.adma202002812.

The halide perovskite Ruddlesden−Popper (RP) phases are a homologous layered subclass of solution-processable semiconductors that have aroused great attention, especially for developing long-term solar photovoltaics. They are defined as (A′) 2 (A) n−1 Pb n X 3n+1 (A′ = spacer cation, A = cage cation, and X = halide anion). The orientation control of low-temperature selfassembled thin films is a fundamental issue associated with the ability to control the charge carrier transport perpendicular to the substrate. Here we report new chemical derivatives designed from a molecular perspective using a novel spacer cation 3-phenyl-2-propenammonium (PPA) with conjugated backbone as a lowtemperature strategy to assemble more efficient solar cells. First, we solved and refined the crystal structures of single crystals with the general formula (PPA) 2 (FA 0.5 MA 0.5 ) n−1 Pb n I 3n+1 (n = 2 and 3, space group C2) using X-ray diffraction and then used the mixed halide (PPA) 2 (Cs 0.05 (FA 0.88 MA 0.12 ) 0.95 ) n−1 Pb n (I 0.88 Br 0.12 ) 3n+1 analogues to achieve more efficient devices. While forming the RP phases, multiple hydrogen bonds between PPA and inorganic octahedra reinforce the layered structure. For films we observe that as the targeted layer thickness index increases from n = 2 to n = 4, a less horizontal preferred orientation of the inorganic layers is progressively realized along with an increased presence of high-n or 3D phases, with an improved flow of free charge carriers and vertical to substrate conductivity. Accordingly, we achieve an efficiency of 14.76% for planar p−i−n solar cells using PPA-RP perovskites, which retain 93.8 ± 0.25% efficiency with encapsulation after 600 h at 85 °C and 85% humidity (ISOS-D-3).

The advent of the two-dimensional (2D) family of halide perovskites and their demonstration in 2D/three-dimensional (3D) hierarchical film structures broke new ground toward high device performance and good stability. The 2D Dion−Jacobson (DJ) phase halide perovskites are especially attractive in solar cells because of their superior charge transport properties. Here, we report on 2D DJ phase perovskites using a 3-(aminomethyl)piperidinium (3AMP) organic spacer for the fabrication of mixed Pb/ Sn-based perovskites, exhibiting a narrow bandgap of 1.27 eV and a long carrier lifetime of 657.7 ns. Consequently, solar cells employing mixed 2D DJ 3AMP-based and 3D MA 0.5 FA 0.5 Pb 0.5 Sn 0.5 I 3 (MA = methylammonium, FA = formamidinium) perovskite composites as light absorbers achieve enhanced efficiency and stability, giving a power conversion efficiency of 20.09% with a high open-circuit voltage of 0.88 V, a fill factor of 79.74%, and a short-circuit current density of 28.63 mA cm −2 . The results provide an effective strategy to improve the performance of single-junction narrow-bandgap solar cells and, potentially, to give a highly efficient alternative to bottom solar cells in tandem devices.

Two-dimensional (2D) perovskites are a class of halide perovskites offering a pathway for realizing efficient and durable optoelectronic devices. However, the broad chemical phase space and lack of understanding of film formation have led to quasi-2D perovskite films with polydispersity in perovskite layer thicknesses, which have hindered devices performance and stability. Here, we demonstrate a scalable approach involving dissolution of single-phase crystalline powders with homogeneous perovskite layer thickness in desired solvents, to fabricate 2D perovskite thin-films with high phase purity. In-situ characterizations reveal the presence of sub-micron-sized seeds in solution that preserve the memory of the dissolved single-crystals and dictate the nucleation and growth of grains with identical thickness of the perovskite layers in thin-films. Photovoltaic devices fabricated with such films, yields an efficiency of 17.1% and 1.20V open-circuit voltage, while preserving 97.5% of their peakperformance after 800 hours under illumination without any external thermal management.