Systems and articles including perovskites and related methods

Abstract

Systems and articles including perovskites and related methods arc generally described.

Description

[0001]SYSTEMS AND ARTICLES INCLUDING PEROVSKITES AND RELATED METHODS RELATED APPLICATIONS This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No.63 / 800,805, filed May 6, 2025, and entitled “Systems and Articles Including Perovskites and Related Methods,” and to U.S. Provisional Patent Application No. 63 / 662,739, filed June 21, 2024, and entitled “Systems and Articles Including Perovskites and Related Methods,” each of which is incorporated herein by reference in its entirety for all purposes. GOVERNMENT SPONSORSHIP This invention was made with government support under DE-EE0009512 awarded by the U.S. Department of Energy. The government has certain rights in the invention. TECHNICAL FIELD Systems and articles including perovskites and related methods are generally described. SUMMARY Systems and articles including perovskites and related methods are generally described. The subject matter of the present disclosure involves, in some cases, interrelated products, alternative solutions to a particular problem, and / or a plurality of different uses of one or more systems and / or articles. Some aspects are related to articles. In some embodiments, the article comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3; and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a ratio of the area under the largest x-ray diffraction (XRD) peak by area associated with the 2D perovskite to the area under the largest XRD peak by area associated with the 3D perovskite is greater than or equal to 50%. In some embodiments, the article comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal 13920912.1 to 150 µm3; and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a photoluminescence carrier lifetime of the article is greater than or equal to 1000 nanoseconds. Some aspects are related to solar cells. In some embodiments, the solar cell comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3; a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite; a hole transporting layer; and an electron transporting layer, wherein the 2D and 3D perovskites, the hole transporting layer, and the electron transporting layer are arranged in a regular n-i-p structure, the solar cell has an initial power conversion efficiency (PCE) of at least 18% when operated at 85 degrees C, and after exposure to full-spectrum 1-sun AM 1.5G illumination for 1000 hours at 85 degrees C, the solar cell has a secondary PCE that is at least 89% of the initial PCE. Some aspects are related to methods. In some embodiments, the method comprises exposing a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3to a solution comprising a first salt, a second salt, a first solvent, and a second solvent, wherein the first and second salts are soluble in the first solvent; and at least a portion of the 3D perovskite is dissolved by the second solvent such that a cation of the first and / or second salt and at least a portion of the dissolved 3D perovskite form a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite. In some embodiments, an article (e.g., a solar cell) is described. In certain embodiments, the article comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3); and a two- dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a ratio of an first area under an XRD peak (e.g., between 12.5 and 12.8 degrees corresponding to a PbI2 peak) associated with impurities (e.g., PbI2) to a second area under an XRD peak associated with the 2D perovskite is less than or equal to 25% (or less than or equal to 10%, less than or equal to 5%, less than or equal to 2%, less than or equal to 1%, less than or equal to 0.5%, less than or equal to 0.1%, or less than or equal to 0.01%). 13920912.1 In some embodiments, the article comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3); and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a photoluminescence carrier lifetime of the article is greater than or equal to 100 nanoseconds. In certain embodiments, the article comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3); and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein the article has an initial power conversion efficiency (PCE) and, after exposure to full-spectrum 1-sun AM 1.5G illumination for 500 hours, the article has a secondary PCE that is at least 89% (or at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, and / or up to 96, up to 97%, up to 98%, or up to 99%) of the initial PCE. Certain aspects are related to methods. In some embodiments, the method comprises exposing a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3) to a solution comprising a salt and a mixed solvent comprising a first solvent and a second solvent; wherein: the salt is soluble in the first solvent; and at least a portion of the 3D perovskite is dissolved by the second solvent such that a cation of the salt and at least a portion of the dissolved 3D perovskite to form a two- dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite. Other advantages and novel features of the present disclosure will become apparent from the following detailed description of various non-limiting embodiments of the disclosure when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and / or inconsistent disclosure, the present specification shall control. BRIEF DESCRIPTION OF THE DRAWINGS Non-limiting embodiments of the present disclosure will be described by way of example with reference to the accompanying figures, which are schematic and are not 13920912.1 intended to be drawn to scale unless otherwise indicated. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the disclosure shown where illustration is not necessary to allow those of ordinary skill in the art to understand the disclosure. In the figures: FIGS.1A-1C show schematic diagrams of an article and 2D perovskites, according to some embodiments. FIGS.1D-1E show schematic diagrams for analyzing peaks in an x-ray diffraction spectrum, according to some embodiments. FIGS.2A-2H, according to some embodiments, show 2D perovskite formation and crystallinity. FIGs.2A-2H show in situ GIWAXS measurements, and initial phase GIWAXS patterns of the Oc (FIGs.2A and 2B), OcMA (FIGs.2C and 2D), OcMA:F (FIGs.2E and 2F), and OcMA:S (FIGs.2G and 2H) 2D / 3D perovskites. Labels “D” and “H” denote the deposition time and annealing start time, respectively. FIG.2I shows azimuthal integrated GIWAXS patterns of the 2D / 3D perovskites. Top and bottom panels respectively show the initial (top) and final (bottom) 2D phases. FIG.2J shows time-resolved PL spectra of the 2D / 3D perovskites on glass. FIG.2K shows plot of carrier lifetime (dotted line) and 2D:3D integrated area ratio (solid line) of the 2D / 3D perovskites. The 2D:3D integrated ratio was calculated as the ratio of the 2D(n=2)(060) to 3D(001) integrated peak areas for the final 2D / 3D perovskites. FIGS.3A-3I, according to some embodiments, show expansion to different 2D fabrication systems, potential uniformity, and carrier dynamics. FIG.3A shows plot of the 2D crystallinity (vertical axis) and the initial phase species (see key). The 2D crystallinity was quantified by the 2D(n=2):3D integrated area ratio. FIG.3B shows interaction energy per solvent molecule with the 3D perovskite surface or 3D perovskite components. FIGs. 3C-3E show Slab models of the 3D perovskite surface bonded with acetonitrile (FIG.3C), propylene carbonate (FIG.3D), and dimethyl sulfoxide (FIG.3E). FIGs.3F-3G show AFM topography (FIG.3F) and KPFM potential mapping (FIG.3G) of the 2D / 3D perovskites. All scale bars are 1 μm. FIG.3F shows TRS carrier dynamics of the control Oc (left) and target OcMA:S (right) 2D / 3D perovskites on glass. The probe energy was 1.53 eV. Solid lines are fits using a diffusion model. Any decay in the TRS dynamics represents surface carrier dynamics and not recombination, i.e., transient absorption dynamics show no decay in 1,000 13920912.1 ps. FIG.3G shows TRMC transients of the control Oc and target OcMA:S 2D / 3D perovskites on quartz. FIGS.4A-4G, according to some embodiments, show device performance and stability. FIG.4A shows current density-voltage curves of the best devices based on CBD- SnO2 for the different 2D / 3D perovskites. FIG.4B shows a histogram of the device PCE distribution based on CBD-SnO2. FIG.4C shows current density-voltage curves of the champion OcMA:S device based on CBD-SnO2. FIG.4D shows MPP tracking under 1-sun AM1.5G illumination (UV included) of the devices at 40-45°C in a nitrogen atmosphere. FIG.4E shows thermal stability test of the devices in the dark. Error bars represent the standard deviation of 8 devices. FIG.4F shows 65°C MPP tracking of the OcMA:S device under 1-sun AM1.5G illumination (UV included) in a nitrogen atmosphere. FIG.4G shows 85°C MPP tracking of the OcMA:S device under 1-sun AM1.5G illumination (UV included) in a nitrogen atmosphere. FIGS.5A-5C, according to some embodiments, show evolution of 2D perovskite interlayers. FIG.5A shows XRD patterns of the target OcMA:S 2D / 3D perovskite aged under 1-sun AM 1.5G illumination under open-circuit condition in a nitrogen glovebox. The XRD was performed on devices of structure spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The 2D perovskite was thus buried under the spiro-MeOTAD layer for these measurements. FIG.5B shows post-mortem ToF-SIMS elemental depth profiling of the devices of structure Au / spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The devices were aged simultaneously together for 528 hours under 1-sun AM 1.5G illumination under open-circuit condition in a nitrogen glovebox. Note that the fluorine signal marks either the spiro-MeOTAD layer or FTO substrate layer. The former corresponds to the TFSI- (bis(trifluoromethanesulfonyl)imide) counter-anion in spiro(TFSI)2. FIG.5C shows a schematic illustrating the formation and transformation of 2D perovskite interlayers, representing the Oc (top) and OcMA:S (bottom) 2D / 3D perovskites. FIG.6, according to some embodiments, shows MPP tracking under 1-sun AM1.5G illumination (UV included) of the devices at 40-45°C in a nitrogen atmosphere. FIGS.7A-7D show, according to some embodiments, the following: FIG.7A shows XRD patterns of the control Oc 2D / 3D perovskite aged under 1-sun AM 1.5G illumination in open-circuit condition in a nitrogen glovebox. The XRD was performed on devices of structure spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The 2D perovskite was 13920912.1 thus buried under the spiro-MeOTAD layer for these measurements. The ω-2θ XRD has sufficient penetration depth to probe through the spiro-MeOTAD to investigate the buried 2D interlayer and 3D bulk perovskite. Evolution of the (FIG.7B) 2D(n=2) perovskite, (FIG. 7C) 2D(n=1) perovskite, and (FIG.7D) PbI2integrated peak areas. The time elapsed refers only to the illumination time. FIG.8, according to some embodiments, shows time-resolved PL spectra of the same Oc 2D / 3D perovskite before (A) and after (B) aging. The film was aged under 1-sun AM 1.5G illumination in a nitrogen glovebox. The fresh bare 3D perovskite (no aging) is also shown for comparison. All films were deposited on glass substrates. FIGS.9A-9D, according to some embodiments, show longer timescale (360 seconds) in situ GIWAXS measurements of the (FIG.9A) Oc, (FIG.9B) OcMA, (FIG.9C) OcMA:F, and (FIG.9D) OcMA:S 2D / 3D perovskites. Labels “D” and “H” denote the deposition time and annealing start time, respectively. FIGS.10A-10D, according to some embodiments, show the following: FIG.10A shows XRD spectra of the 3D and Oc 2D / 3D perovskites, with excess PbI2 composition. FIG.10B shows XRD spectra of the 3D perovskite, no excess composition, before and after being spincoated with pure IPA. FIG.10C shows XRD spectra of the 3D perovskite, with excess PbI2 composition, before and after being spincoated with pure IPA. FIG.10D shows XRD spectra of the 3D and Oc 2D / 3D perovskites, no excess composition. FIGS.11A-11D, according to some embodiments, show box plots showing the distribution of the photovoltaic parameters of (FIG.11A) VOC, (FIG.11B) JSC, (FIG.11C) FF, and (FIG.11D) PCE of the 2D / 3D and bare 3D devices with or without excess PbI2. The devices use NP-SnO2 as the electron-transporting layer. FIGS.12A-12D, according to some embodiments, show GIWAXS spectra of the 2D initial phase of the (FIG.12A) Oc, (FIG.12B) OcMA, (FIG.12C) OcMA:F, and (FIG.12D) OcMA:S 2D / 3D perovskites. FIGS.13A-13B, according to some embodiments, show (FIG.13A) Ratio of the 2D(n=2) to 3D(001) integrated peak areas, and (FIG.13B) FWHM of the 2D(n=2) peak, for the final 2D / 3D perovskites obtained from the GIWAXS spectra. Based on the Debye- Scherrer law, crystallite size is inversely proportional to the FWHM. Therefore, the OcMA:S 2D / 3D perovskite has the highest crystallinity with the largest crystallite size. 13920912.1 FIGS.14A-14G, according to some embodiments, show lab-based static high- resolution grazing-incidence XRD spectra of the (FIG.14A) Oc, (FIG.14B) OcMA:F, and (FIG.14C) OcMA:S 2D / 3D perovskites. Halder-Wagner plot of the (FIG.14D) Oc, (FIG. 14E) OcMA:F, and (FIG.14F) OcMA:S 2D(n=2) phase. (FIG.14G) Halder-Wagner analysis of the coherence length and microstrain of the 2D(n=2) phase. FIGS.15A-15L, according to some embodiments, show in situ GIWAXS measurements of the (FIG.15A) OcMA:PC, (FIG.15B) OcMA:A, (FIG.15C) OcMA:N, and (FIG.15D) OcMA:MG 2D / 3D perovskites. GIWAXS spectra of the 2D initial phase of the (FIG.15E) OcMA:PC, (FIG.15F) OcMA:A, (FIG.15G) OcMA:N, and (FIG.15H) OcMA:MG 2D / 3D perovskites. GIWAXS spectra of the final 2D phase of the (FIG.15I) OcMA:PC, (FIG.15J) OcMA:A, (FIG.15K) OcMA:N, and (FIG.15L) OcMA:MG 2D / 3D perovskites. FIGS.16A-16B, according to some embodiments, show the following: FIG.16A shows time-resolved PL spectra of the OcMA:PC 2D / 3D perovskite, in comparison to the bare 3D and Oc 2D / 3D perovskite. All films were deposited on glass substrates. FIG.16B shows carrier lifetime (left axis) and 2D:3D integrated area ratio (right axis) of the 2D / 3D perovskites. The 2D:3D integrated ratio is calculated as the ratio of the 2D(n=2)(060) to 3D(001) integrated peak areas for the final 2D / 3D perovskites. FIGS.17A-17C, according to some embodiments, show the following: FIG.17A shows in situ GIWAXS measurements of the Oc:S 2D / 3D perovskite. FIG.17B shows GIWAXS spectra of the 2D initial phase of the Oc:S 2D / 3D perovskite. FIG.17C shows GIWAXS spectra of the final 2D phase of the Oc:S 2D / 3D perovskite. FIGS.18A-18I, according to some embodiments, show in situ GIWAXS measurements of the (FIG.18A) OcMA-Cl, (FIG.18B) OcMA-Br, and (FIG.18C) OcMA 2D / 3D perovskites. GIWAXS spectra of the 2D initial phase of the (FIG.18D) OcMA-Cl, (FIG.18E) OcMA-Br, and (FIG.18F) OcMA 2D / 3D perovskites. GIWAXS spectra of the final 2D phase of the (FIG.18G) OcMA-Cl, (FIG.18G) OcMA-Br, and (FIG.18I) OcMA 2D / 3D perovskites. FIGS.19A-19C, according to some embodiments, show the following: FIG.19A shows in situ GIWAXS measurement of the OcMA:S 2D / 3D perovskite, but using a higher concentration of methylammonium iodide. In this case, the Oc:MA:S ratio was 1:1:2.5. GIWAXS spectra of the (FIG.19B) 2D initial phase, and (FIG.19C) final 2D phase. 13920912.1 FIGS.20A-20C, according to some embodiments, show slab models of the 2D(n=2) perovskites with different A-site cation compositions of (FIG.20A) MA, and (FIG.20B) FA. FIG.20C shows formation energy per molecule of the 2D(n=2) perovskites. FIGS.21A-21D, according to some embodiments, show XRD spectra of the 2D perovskites of (FIG.21A) OA2MAPb2Br2I5 and (FIG.21B) OA2FAPb2Br2I5. (FIG.21C) XRD spectra of the OcMA:S 2D perovskite on a baseline 3D perovskite. This 3D perovskite contained excess PbI2, MACl, and MAPbBr3. FIG.21D shows XRD spectra of the Oc 2D perovskite on 3D FAPbI3. This 3D perovskite was a pure FAPbI3 composition with only excess PbI2. FIG.22, according to some embodiments, shows the band alignments constructed from the UPS results. The highest occupied molecular orbital (HOMO) level of the spiro- MeOTAD hole-transporting layer is also included. FIGS.23A-23D, according to some embodiments, show surface FE-SEM images of the (FIG.23A) bare 3D, (FIG.23B) Oc, (FIG.23C) OcMA, and (FIG.23D) OcMA:S 2D / 3D perovskites. All scale bars are 600 nm. FIGS.24A-24B, according to some embodiments, show cross-sectional FE-SEM images of the (FIG.24A) Oc and (FIG.24B) OcMA:S 2D / 3D perovskites. All scale bars are 200 nm. FIGS.25A-25D, according to some embodiments, show box plots showing the distribution of the photovoltaic parameters of (FIG.25A) VOC, (FIG.25B) JSC, (FIG.25C) FF, and (FIG.25D) PCE of the devices. The devices use CBD-SnO2as the electron- transporting layer. FIG.26, according to some embodiments, shows MPP tracking under 1-sun AM1.5G illumination (UV included) of the devices with or without excess PbI2at 40-45°C in a nitrogen atmosphere. FIG.27, according to some embodiments, shows MPP tracking under 1-sun AM1.5G illumination (UV included) of the devices at 40-45°C in a nitrogen atmosphere. The devices use NP-SnO2 as the electron-transporting layer. FIG.28, according to some embodiments, shows MPP tracking under 1-sun AM1.5G illumination (UV included) of the devices at 40-45°C in a nitrogen atmosphere. For the “3D excess PbI2– S solvent wash” device, the solvent S (i.e. dimethylsulfoxide and IPA mixed solvent) was spincoated by itself without 2D perovskite on to the bare 3D perovskite. 13920912.1 FIG.29, according to some embodiments, shows XRD patterns of the bare 3D (no 2D) perovskite aged under 1-sun AM 1.5G illumination in a N2 glovebox. The XRD was performed on devices of structure spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The time elapsed refers only to the time under illumination. FIG.30, according to some embodiments, shows XRD patterns of the OcMA:S 2D / 3D perovskite aged under 1-sun AM 1.5G illumination in a N2 glovebox. The XRD was performed on devices of structure spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The time elapsed refers only to the time under illumination. FIG.31, according to some embodiments, shows XRD patterns of the OcMA:F 2D / 3D perovskite aged under 1-sun AM 1.5G illumination in a N2glovebox. The XRD was performed on devices of structure spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The time elapsed refers only to the time under illumination. FIG.32, according to some embodiments, shows XRD patterns of the “Oc 2D / 3D no excess” 2D / 3D perovskite aged under 1-sun AM 1.5G illumination in a N2glovebox. This uses the control Oc 2D perovskite, but applied to a 3D perovskite composition but with no excess / residual PbI2 at all. The XRD was performed on devices of structure spiro- MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The time elapsed refers only to the time under illumination. FIG.33, according to some embodiments, shows XRD patterns of the “OcMA:S 2D / 3D no excess” 2D / 3D perovskite aged under 1-sun AM 1.5G illumination in a N2glovebox. This uses the target OcMA:S 2D perovskite, but applied to a 3D perovskite composition but with no excess / residual PbI2 at all. The XRD was performed on devices of structure spiro-MeOTAD / 2D perovskite / 3D perovskite / SnO2 / FTO. The time elapsed refers only to the time under illumination. FIGS.34A-34B, according to some embodiments, show XRD patterns of the 2D / 3D perovskite with (FIG.34A) PEAI and (FIG.34B) PEAI-MA:S aged under 1-sun AM 1.5G illumination in a N2glovebox. For the PEAI-MA:S system, the PEAI:MAI:S ratio was 1:0.6:2.5. The time elapsed refers only to the time under illumination. FIGS.35A-35B, according to some embodiments, show XRD patterns of the 2D / 3D perovskite with (FIG.35A) BA and (FIG.35B) BA-MA:S aged under 1-sun AM 1.5G illumination in a N2glovebox. The time elapsed refers only to the time under illumination. 13920912.1 DETAILED DESCRIPTION Some aspects are generally related to articles including two-dimensional (2D) and three-dimensional (3D) perovskites. In some embodiments, the articles comprise a solar cell. Certain aspects of this disclosure relate to articles having 2D perovskites with a certain crystallinity and / or phase purity. In some embodiments, the 2D perovskite can be relatively crystalline, relative to the crystallinity of the underlying 3D perovskite material. For instance, in some embodiments, the ratio of the area under the largest XRD peak associated with the 2D perovskite to the area under the largest XRD peak associated with the 3D perovskite is greater than or equal to 50%. In some embodiments, the 2D perovskite material can be free of PbI2and / or other impurities. In certain embodiments, the phase of the 2D perovskite is generally pure. For instance, in some embodiments, the 2D perovskite consists essentially of an n =1, an n = 2, an n = 3, or an n = 4 phase pure material. The crystallinity and / or phase purity of the 2D perovskite within the article, in certain embodiments, provides improved articles for converting light into energy. Still other aspects are related to compositions, systems, methods, and the like. Semiconductor heterostructures form the backbone of modern optoelectronic devices. For perovskite solar cells (PSCs), the two-dimensional on three-dimensional (2D / 3D) perovskite bilayer heterostructure is typically used. The 2D / 3D perovskite structure has myriad benefits associated with the 2D interlayer. However, typical devices assume that the 2D interlayer does not change with time. In this disclosure, it was determined that 2D / 3D perovskites in a device stack evolve dynamically during its end-of-life decomposition. Moreover, counterintuitively, seemingly identical 2D interlayers may evolve differently, resulting in vastly different PSC stabilities. Accordingly, some aspects of the present disclosure are related to controlling the equilibrium state to create an unchanging 2D interlayer to improve PSC stability. For instance, two devices each having a phase-pure 2D perovskite may evolve differently during operation due to a difference in the crystallinity of the 2D perovskites. Advantageously, some aspects are generally related to 2D perovskites in articles, systems, and methods having a certain level of crystallinity. Recently, the development of 2D / 3D perovskite bilayer heterostructures has been instrumental in boosting the performance and stability of PSCs. The beneficial effects conferred by the 2D interlayer include defect passivation, selective charge transport, creating a built-in potential, preventing external ingression from the environment, and blocking ion 13920912.1 migration. However, these benefits are generally retained only if the 2D interlayer remains constant. These beneficial effects may degrade and / or be completely lost as the 2D interlayer evolves (e.g. structurally, compositionally, etc) into a different product. Some aspects of this disclosure are accordingly related to controlling the 2D perovskite phase distribution, for instance, to attain phase purity, which may result in long-term stabilities for the full PSC. Additionally, some aspects are related to crystallinity of a phase pure 2D perovskite, e.g., controlling the crystallinity and / or 2D perovskites having a certain level of crystallinity. Some aspects are generally related to articles. In some embodiments, the article comprises a perovskite, for instance, a three-dimensional (3D) perovskite and / or a two- dimensional (2D) perovskite. For example, FIG.1A is a schematic diagram of an article 100. In some embodiments, the article comprises a 3D perovskite and 2D perovskite. For example, in FIG.1A, article 100 comprises 3D perovskite 113 and 2D perovskite 114. Additional components may also be present, for example, when the article is or comprises a solar cell. To illustrate, referring to FIG.1A, article 100 includes a substrate 110, electrode 111, electron transporting layer 112, 3D perovskite 113, 2D perovskite 114, hole transporting layer 115, and electrode 116. In the depicted embodiment, the article 100 is arranged as a solar cell, and is exposed to incident light 122 from the sun 120. As described above, in some embodiments, articles are described that include a 3D perovskite. The 3D perovskite may be present as a portion of an article, in accordance with some embodiments. For instance, in some embodiments, the 3D perovskite may form a portion of an active layer of an article comprising a solar cell. A 3D perovskite generally has an ABX3 structure. In some embodiments, A is a cation comprising formamidinium, cesium (Cs), and / or methylammonium, B is a cation comprising lead (Pb), tin (Sn), and / or germanium (Ge), and X is a halide comprising fluoride, chloride, bromide, iodide, and / or astatide. In some embodiments, the A cation comprises formamidinium and / or methylammonium, the B cation comprises lead (Pb), and the X halide comprises chloride, bromide, and / or iodide. In some embodiments, the A cation comprises formamidinium and / or methylammonium. In some embodiments, the B cation comprises lead (Pb). In some embodiments, the X halide comprises chloride, bromide, and / or iodide. In accordance with some embodiments, the 3D perovskite comprises a formamidinium lead iodide composition. In accordance with some embodiments, the 3D 13920912.1 perovskite is formamidinium lead iodide. The structure of the 3D perovskite may be determined using x-ray diffraction (XRD). The 3D perovskite may have any suitable thickness, in accordance with some embodiments. In some embodiments, a thickness of a 3D perovskite is selected to be sufficiently thick to function within a solar cell, e.g., by converting certain wavelengths of light incident on the perovskite to electron-hole pairs that may be extracted from the 3D perovskite within the solar cell. For instance, in some embodiments, the 3D perovskite has a thickness of greater than or equal to 200 nm, greater than or equal to 300 nm, greater than or equal to 400 nm, greater than or equal to 500 nm, greater than or equal to 600 nm, greater than or equal to 700 nm, greater than or equal to 800 nm, or greater than or equal to 900 nm. In some embodiments, the 3D perovskite has a thickness of less than or equal to 1000 nm, less than or equal to 900 nm, less than or equal to 800 nm, less than or equal to 700 nm, less than or equal to 600 nm, less than or equal to 500 nm, less than or equal to 400 nm, or less than or equal to 300 nm. Combinations of the foregoing ranges are possible (e.g., greater than or equal to 200 nm and less than or equal to 1000 nm or greater than or equal to 300 nm and less than or equal to 500 nm). Other ranges are also possible. The 3D perovskite may have any suitable volume, in accordance with some embodiments. In some embodiments, a volume of a 3D perovskite is selected based on the desired thickness and area of the article comprising the 3D perovskite. For example, when the article is a solar cell, the area of the 3D perovskite may correspond to an area of a surface to be exposed to an incident light source. For instance, in some embodiments, the 3D perovskite has a volume of greater than or equal to 150 µm3, greater than or equal to 200 µm3, greater than or equal to 300 µm3, greater than or equal to 400 µm3, greater than or equal to 500 µm3, greater than or equal to 600 µm3, greater than or equal to 700 µm3, greater than or equal to 800 µm3, greater than or equal to 900 µm3, greater than or equal to 1000 µm3, greater than or equal to 5000 µm3, greater than or equal to 10,000 µm3, greater than or equal to 50,000 µm3, greater than or equal to 105µm3, greater than or equal to 106µm3, greater than or equal to 107µm10, greater than or equal to 1011µm3, or greater than or equal to 1012µm3. In some embodiments, the 3D perovskite has a volume of less than or equal to 1013µm3, less than or equal to 1012µm3, less than or equal to 1011µm3, less than or equal to 1010µm3, less than or equal to 109µm3, less than or equal to 108µm3, less than or equal to 108µm3, less than or equal to 107µm3, or less than or equal to 106µm3. Combinations of the 13920912.1 foregoing ranges are possible (e.g., greater than or equal to 150 µm3and less than or equal to 1013µm3). Other ranges are also possible. In some embodiments, articles are described that include a 2D perovskite. The 2D perovskite may be present as a portion of an article, in accordance with some embodiments. For instance, in some embodiments, the 2D perovskite may form a portion of an active layer of an article comprising a solar cell. In accordance with some embodiments, the 2D perovskite may be positioned adjacent to at least a portion of the 3D perovskite in the article. In some embodiments, when a 2D perovskite is positioned adjacent to the 3D perovskite, there may be one or more intervening materials between the 2D and 3D perovskites. Alternatively, in some instances, the 2D perovskite positioned adjacent to the 3D perovskite is in direct contact with the 3D perovskite. A 2D perovskite may comprise a capping ligand L, a first cation A, a second cation B, and / or a halide X, in any of a variety of suitable structures, in accordance with some embodiments. The A cation, B cation, and X halide, when present, may be selected from the same options listed elsewhere herein for the 3D perovskite. The capping ligand L may be selected from any of a variety of suitable ligands, in some embodiments. In some embodiments, the capping ligand forms a layer on either side (e.g., a top side and a bottom side) of the 2D perovskite, for instance, as described in more detail below in the context of FIGS.1B-1C. Those of ordinary skill in the art, given the insight provided by the present disclosure, would be capable of selecting a suitable capping ligand for the 2D perovskite. In some embodiments, the capping ligand comprises an aliphatic alkyl ammonium cation or aromatic ammonium cation. In some embodiments, the capping ligand comprises a primary alkylammonium, a secondary alkylammonium, a tertiary alkylammonium, and / or a quaternary alkylammonium, where the alkyl chain length has greater than 3 carbons and / or less than or equal to 20 carbons. In some embodiments, the capping ligand comprises benzylammonium, phenethylammonium, phenpropylammonium, and / or derivatives thereof. Derivatives of the foregoing capping ligands include capping ligands having similar structures with functional group substitutions on the benzene ring such as a halide, an aliphatic group (e.g., saturated, unsaturated, linear, and / or cyclic), an ether, an ester, an amine, an amide, a carbonyl, or the like. Those of ordinary skill in the art, given the insight provided by the present disclosure, will be capable of selecting derivatives of benzylammonium, phenethylammonium, and phenpropylammonium. In some embodiments, 13920912.1 the capping ligand comprises an alkane diammonium. In some embodiments, the capping ligand comprises n-octylammonium, phenethylammonium, and / or n-butylammonium. In some embodiments, the capping ligand comprises 4-fluoro-benzylammonium, 4-fluoro- phenethylammonium, and / or 4-fluoro-phenpropylammonium. In some embodiments, the capping ligand comprises ethane-1,2-diammonium, propane-1,3-diammonium, butane-1,4- diammonium, and / or Pentane-1,5-diammonium. In some embodiments, as described in more detail elsewhere herein, a portion of the 3D perovskite may be dissolved to facilitate formation of the 2D perovskite. Accordingly, in some embodiments, the 2D and 3D perovskites may share one or more common elements and / or components. For instance, in some embodiments, the 2D and 3D perovskites share a common “A” cation, “B” cation, and / or “X” halide. It will be understood, however, that is some embodiments, the “A” cation, “B” cation, and / or “X” halide of the 2D and 3D perovskites may be different. In some instances, the components (i.e., L, A, B, X) and stoichiometric ratios thereof present within the 2D perovskite may be related to the structure of the 2D perovskite. Generally, the structure of a 2D perovskite includes a layer of octahedra, where each of the octahedra comprise a B cation and an X halide, and the layer of octahedra is capped with the capping ligand. FIG.1B shows a schematic diagram of an example 2D perovskite structure, where octahedra 150 form a layer 152 that is capped with capping ligands 154. The presence of a single layer of octahedra correspond to a 2D perovskite having an n = 1 phase and a chemical formula of L2BX4. FIG.1C shows an additional schematic diagram of a 2D perovskite structure with two layers 152 of octahedra between capping ligands 154, and thus the 2D perovskite has an n = 2 phase. In this arrangement, the 2D perovskite further includes an A cation 156 present between the layers of octahedra, and the 2D perovskite has a chemical formula L2AB2X7. The 2D perovskites of this disclosure may generally have 1 to 4 layers of octahedra present between the capping ligand, which correspond to 2D perovskites having an n = 1 to an n = 4 phase, respectively. In certain embodiments, the 2D perovskite has 3 layers of octahedra between the capping ligands, corresponding to an n = 3 phase having a chemical formula comprising L2A2B3X10. In some embodiments, the 2D perovskite has 4 layers of octahedra between the capping ligands, corresponding to an n = 4 phase having a chemical formula comprising L2A3B4X13. 13920912.1 Accordingly, in certain cases, the 2D perovskite comprises an n =1 phase, an n = 2 phase, an n = 3 phase, and / or an n = 4 phase. Without wishing to be bound by any particular theory, it is believed that the presence of multiple phases (e.g., an n = 1 phase and an n = 2 phase) in the 2D perovskite may lead to degradation of the 2D perovskite within an article over time, e.g., after operating the article a certain amount of time. For example, as described elsewhere herein in more detail, in some embodiments, the 2D perovskite is included within a solar cell and comprises an n = 1 phase and an n = 2 phase. In some such embodiments, the 2D perovskite may degrade and result in a decrease in power conversion efficiency of the solar cell over time, e.g., after at least 500 hours and / or after at least 1,000 hours of operation due to the presence of both the n = 1 phase and the n = 2 phase. Accordingly, in some embodiments, the 2D perovskite consists essentially of a single phase. In some embodiments, the 2D perovskite consists essentially of an n = 1 phase, an n =2 phase, an n= 3 phase, and / or an n = 4 phase. In some embodiments, the 2D perovskite consists essentially of an n = 1 phase, an n =2 phase, an n= 3 phase, or an n = 4 phase. In some embodiments, the 2D perovskite comprises an n = 2 phase. In some embodiments, the 2D perovskite consists essentially of an n = 2 phase. The structure of the 2D perovskite may be determined using XRD. In some embodiments, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt%, at least 99.999 wt%, at least 99.9999 wt%, at least 99.99999 wt%, or essentially 100 wt% of the 2D perovskite is made up of a single type of phase, for example, the n = 1 phase, n = 2 phase, n = 3 phase, or n = 4 phase. In some embodiments, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt%, at least 99.999 wt%, at least 99.9999 wt%, at least 99.99999 wt%, or essentially 100 wt% of the 2D perovskite is made up of the n = 1 phase. In some embodiments, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt%, at least 99.999 wt%, at least 99.9999 wt%, at least 99.99999 wt%, or essentially 100 wt% of the 2D perovskite is made up of the n = 2 phase. In some embodiments, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt%, at least 99.999 wt%, at least 99.9999 wt%, at least 99.99999 wt%, or essentially 100 wt% of the 2D perovskite is made up of the n = 3 phase. In some embodiments, at least 90 wt%, at least 95 wt%, at least 98 wt%, at least 99 wt%, at least 99.9 wt%, at least 99.99 wt%, at least 99.999 wt%, at least 99.9999 wt%, at 13920912.1 least 99.99999 wt%, or essentially 100 wt% of the 2D perovskite is made up of the n = 4 phase. As noted above, the structure of the 2D perovskite and / or the 3D perovskite may be determined using XRD. In some embodiments, the 2D perovskite is formed on at least a portion of the 3D perovskite, and the crystallinity of the 2D perovskite may be determined from the XRD. Those of ordinary skill in the art will be able to assign certain peaks in an XRD to certain components, e.g., crystallographic planes associated with the 2D perovskite, the 3D perovskite, and / or impurities. Those of ordinary skill in the art, when acquiring an XRD spectrum, will generally obtain the spectrum over a range of angles that includes the angles 2θ from 0 degrees to 50 degrees. XRD peaks may be analyzed using the OriginLab software and the “Peak Analyzer” function. The “Peak Analyzer” function is an automated function that outputs fitted parameters. FIG.1D is a schematic diagram illustrating how to evaluate the peak area, full width at half max of a peak, and peak height. Specifically, a baseline 162 may be determined (e.g., using OriginLab software), and the peak area of peak 160 may be integrated. Similarly, the full width at half max 164 and peak height 166 of peak 160 may be determined using OriginLab software. As a non-limiting example, FIG.2K compares the 2D:3D integrated ratio as calculated by the ratio of the 2D(n=2)(060) to 3D(001) integrated peak areas for the 2D / 3D perovskites in an article. The crystallinity of the 2D perovskite may be determined by analyzing an XRD spectrum of the article. For instance, in some embodiments, the crystallinity of the 2D perovskite may be determined by comparison to peaks associated with an underlying 3D perovskite. In some embodiments, the crystallinity of the 2D perovskite may be determined from a ratio of the area under the largest XRD peak by area associated with the 2D perovskite to the area under the largest XRD peak by area associated with the 3D perovskite. In some embodiments, the ratio of the area under the largest XRD peak by area associated with the 2D perovskite to the area under the largest XRD peak by area associated with the 3D perovskite is greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95%, greater than or equal to 100%, greater than or equal to 150%, greater than or equal to 200%, greater than or equal to 300%, greater than or 13920912.1 equal to 400%, greater than or equal to 500%, greater than or equal to 600%, greater than or equal to 700%, greater than or equal to 800%, or greater than or equal to 900%. In some embodiments, the ratio of the area under the largest XRD peak by area associated with the 2D perovskite to the area under the largest XRD peak by area associated with the 3D perovskite is less than or equal to 1000%, less than or equal to 900%, less than or equal to 800%, less than or equal to 700%, less than or equal to 600%, less than or equal to 500%, less than or equal to 400%, less than or equal to 300%, less than or equal to 200%, less than or equal to 150%, less than or equal to 100%, less than or equal to 95%, less than or equal to 90%, less than or equal to 85%, less than or equal to 80%, less than or equal to 75%, less than or equal to 70%, less than or equal to 65%, less than or equal to 60%, or less than or equal to 55%. Combinations of the foregoing ranges are possible (e.g., greater than or equal to 50% and less than or equal to 1000%). Other ranges are also possible. In some embodiments, the crystallinity of the 2D perovskite may be determined by analyzing the full width at half max (FWHM) of certain peaks within an XRD spectrum of the article. For instance, in some embodiments, the crystallinity of the 2D perovskite may be determined from a ratio of the FWHM of the largest XRD peak by FWHM associated with the 3D perovskite to the FWHM of the largest XRD peak by FWHM associated with the 2D perovskite. In some embodiments, the ratio of the FWHM of the largest XRD peak by FWHM associated with the 3D perovskite to the FWHM of the largest XRD peak by FWHM associated with the 2D perovskite is greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95%, greater than or equal to 100%, greater than or equal to 150%, greater than or equal to 200%, greater than or equal to 300%, greater than or equal to 400%, greater than or equal to 500%, greater than or equal to 600%, greater than or equal to 700%, greater than or equal to 800%, or greater than or equal to 900%. In some embodiments, the ratio of the FWHM of the largest XRD peak by FWHM associated with the 3D perovskite to the FWHM of the largest XRD peak by FWHM associated with the 2D perovskite is less than or equal to 1000%, less than or equal to 900%, less than or equal to 800%, less than or equal to 700%, less than or equal to 600%, less than or equal to 500%, less than or equal to 400%, less than or equal to 300%, less than or equal to 200%, less than or equal to 150%, less than or equal to 100%, less than or equal 13920912.1 to 95%, less than or equal to 90%, less than or equal to 85%, less than or equal to 80%, less than or equal to 75%, less than or equal to 70%, less than or equal to 65%, less than or equal to 60%, or less than or equal to 55%. Combinations of the foregoing ranges are possible (e.g., greater than or equal to 50% and less than or equal to 1000%). Other ranges are also possible. In some embodiments, the crystallinity of the 2D perovskite may be determined by analyzing the peak height of certain peaks within an XRD spectrum of the article. For instance, in some embodiments, the crystallinity of the 2D perovskite may be determined from a ratio of the peak height of the tallest XRD peak associated with the 2D perovskite to the peak height of the tallest XRD peak associated with the 3D perovskite. In some embodiments, the ratio of the peak height of the tallest XRD peak associated with the 2D perovskite to the peak height of the tallest XRD peak associated with the 3D perovskite is greater than or equal to 50%, greater than or equal to 55%, greater than or equal to 60%, greater than or equal to 65%, greater than or equal to 70%, greater than or equal to 75%, greater than or equal to 80%, greater than or equal to 85%, greater than or equal to 90%, greater than or equal to 95%, greater than or equal to 100%, greater than or equal to 150%, greater than or equal to 200%, greater than or equal to 300%, greater than or equal to 400%, greater than or equal to 500%, greater than or equal to 600%, greater than or equal to 700%, greater than or equal to 800%, or greater than or equal to 900%. In some embodiments, the ratio of the peak height of the tallest XRD peak associated with the 2D perovskite to the peak height of the tallest XRD peak associated with the 3D perovskite is less than or equal to 1000%, less than or equal to 900%, less than or equal to 800%, less than or equal to 700%, less than or equal to 600%, less than or equal to 500%, less than or equal to 400%, less than or equal to 300%, less than or equal to 200%, less than or equal to 150%, less than or equal to 100%, less than or equal to 95%, less than or equal to 90%, less than or equal to 85%, less than or equal to 80%, less than or equal to 75%, less than or equal to 70%, less than or equal to 65%, less than or equal to 60%, or less than or equal to 55%. Combinations of the foregoing ranges are possible (e.g., greater than or equal to 50% and less than or equal to 1000%). Other ranges are also possible. Similarly, in some embodiments, the XRD spectrum of the article may be analyzed to determine an amount of impurities associated with the 2D perovskite. In some embodiments, the purity of the 2D perovskite may be determined using a ratio of the area 13920912.1 under XRD peaks associated with impurities to the area under the largest XRD peak by area associated with the 2D perovskite. FIG.1E is a schematic diagram illustrating how to evaluate the peak area, full width at half max of a peak, and peak height of a 2D perovskite peak when multiple impurity peaks are present. In this example, peak 160a is associated with the 2D perovskite, and has corresponding area determined using base line 162a, FWHM 164a, and peak height 166a. Peaks 160b and 160c are associated with impurities. Peak 160b has a corresponding area determined using base line 162b, FWHM 164b, and peak height 166b, whereas Peak 160c has a corresponding area determined using base line 162c, FWHM 164c, and peak height 166c. As peak 160c has a larger area, FWHM, and peak height compared to peak 160b, the corresponding parameters from peak 160c would be used when comparing to those of the 2D perovskite when analyzing the largest XRD peak associated with an impurity. In some embodiments, a ratio of the area under the largest XRD peak by area associated with an impurity to the area under the largest XRD peak by area associated with the 2D perovskite is less than or equal to 25%, less than or equal to 20%, less than or equal to 15%, less than or equal to 10%, less than or equal to 5%, less than or equal to 1%, less than or equal to 0.1%, less than or equal to 0.01%, or less. For example, in some embodiments, there may be no XRD peak (i.e., detectable peak) associated with impurities of the 2D perovskite. In some embodiments, again referring to FIG.1E, the total area under all XRD peaks associated with impurities are considered, and thus the area under both peaks 160b and 160c may be considered when compared to the largest XRD peak by area associated with the 2D perovskite. In some embodiments, the ratio of the total area under all XRD peaks associated with impurities to the area under the largest XRD peak by area associated with the 2D perovskite is less than or equal to 25%, less than or equal to 20%, less than or equal to 15%, less than or equal to 10%, less than or equal to 5%, less than or equal to 1%, less than or equal to 0.1%, less than or equal to 0.01%, or less. In some embodiments, the amount of impurities associated with the 2D perovskite may be determined by analyzing the full width at half max (FWHM) of certain peaks within an XRD spectrum of the article. For instance, in some embodiments, the amount of impurities associated with the 2D perovskite may be determined from a ratio of the FWHM of XRD peaks associated with the 2D perovskite to the FWHM of the largest XRD peak by FWHM associated with impurities. In some embodiments, the ratio of the FWHM of the largest XRD peak (as determined by FWHM) associated with the 2D perovskite to the 13920912.1 FWHM of the largest XRD peak (as determined by FWHM) associated with an impurity is less than or equal to 25%, less than or equal to 20%, less than or equal to 15%, less than or equal to 10%, less than or equal to 5%, less than or equal to 1%, less than or equal to 0.1%, less than or equal to 0.01%, or less. It will be understood that, in instances where there are no measurable impurities, there is no largest peak associated with an impurity and the ratio cannot be computed. Accordingly, in some embodiments, there are no measurable impurities associated with the 2D perovskite. In some embodiments, the amount of impurities associated with the 2D perovskite may be determined by analyzing the peak height of certain peaks within an XRD spectrum of the article. For instance, in some embodiments, the amount of impurities associated with the 2D perovskite may be determined from a ratio of the peak height of XRD peaks associated impurities to the peak height of the tallest XRD peak associated with the 2D perovskite. In some embodiments, the ratio of the peak height of the tallest XRD peak associated with an impurity to the peak height of the tallest XRD peak associated with the 2D perovskite is less than or equal to 25%, less than or equal to 20%, less than or equal to 15%, less than or equal to 10%, less than or equal to 5%, less than or equal to 1%, less than or equal to 0.1%, less than or equal to 0.01%, or less. In some embodiments, the ratio of the cumulative height of all peaks associated with impurities to the peak height of the tallest XRD peak associated with the 2D perovskite is less than or equal to 25%, less than or equal to 20%, less than or equal to 15%, less than or equal to 10%, less than or equal to 5%, less than or equal to 1%, less than or equal to 0.1%, less than or equal to 0.01%, or less. In this context, the cumulative height of all peaks associated with impurities is determined by adding the heights of all peaks associated with impurities. In some embodiments, the articles comprising 2D and 3D perovskites described herein have a relatively long photoluminescence carrier lifetime. For example, in some embodiments, the photoluminescence carrier lifetime is greater than 100 ns, greater than or equal to 200 ns, greater than or equal to 300 ns, greater than or equal to 400 ns, greater than or equal to 600 ns, greater than or equal to 800 ns, greater than or equal to 1,000 ns, greater than or equal to 1,200 ns, greater than or equal to 1,400 ns, greater than or equal to 1,600 ns, greater than or equal to 1,800 ns, greater than or equal to 2,000 ns, greater than or equal to 2,500 ns, greater than or equal to 3,000 ns, greater than or equal to 3,500 ns, greater than or equal to 4,000 ns, greater than or equal to 5,000 ns, greater than or equal to 7,500 ns, greater 13920912.1 than or equal to 10,000 ns, greater than or equal to 20,000 ns, greater than or equal to 30,000 ns, or greater than or equal to 40,000 ns. In some embodiments, the photoluminescence carrier lifetime is less than or equal to 50,000 ns, less than or equal to 40,000 ns, less than or equal to 30,000 ns, less than or equal to 20,000 ns, less than or equal to 10,000 ns, less than or equal to 7,500 ns, less than or equal to 5,000 ns, less than or equal to 4,000 ns, less than or equal to 3,500 ns, less than or equal to 3,000 ns, less than or equal to 2,500 ns, less than or equal to 2,000 ns, less than or equal to 1,800 ns, less than or equal to 1,600 ns, less than or equal to 1,400 ns, less than or equal to 1,200 ns, less than or equal to 1,000 ns, less than or equal to 800 ns, less than or equal to 600 ns, less than or equal to 400 ns, less than or equal to 300 ns, or less than or equal to 200 ns. Combinations of the foregoing ranges are possible (e.g., greater than or equal to 100 ns and less than or equal to 50,000 ns, or greater than or equal to 1,000 ns and less than or equal to 50,000 ns). Total photoluminescence carrier lifetime of an article can be measured by obtaining time-resolved photoluminescence spectra using a time-correlated single-photon counting card (Picoquant; PicoHarp 300) using a picosecond pulsed diode laser (Picoquant; LDH-P-FA-530-B) having an excitation wavelength of 532 nm, an excitation power of 100 nW, a spot size of 100 micrometers, and a repetition rate of 30 kHz adjusted using a pulse generator (Stanford Research; DG535). When determining the total photoluminescence lifetime, one generally arranges a 3D perovskite in contact with a substrate, and a 2D perovskite over the 3D perovskite, with the 2D perovskite positioned on an opposite side of the 3D perovskite than the substrate. The laser should be directed towards the article such that it is incident on the 2D perovskite before the substrate. In some embodiments, the 2D perovskite material and the 3D perovskite material in combination, without any other materials present, have a photoluminescence carrier lifetime falling within any of the ranges outlined above. In some embodiments, as noted above, an article includes a 2D perovskite that has a high level of crystallinity. In some such embodiments, the article having the highly crystalline 2D perovskite layer has a photoluminescence carrier lifetime that is relatively long. For instance, in some embodiments, the ratio of the area under the largest XRD peak associated with the 2D perovskite to the area under the largest XRD peak associated with the 3D perovskite is greater than or equal to 50% (and / or greater as described above), and the photoluminescence carrier lifetime is greater than or equal to 100 ns. 13920912.1 Some aspects are related to articles comprising a solar cell. In some embodiments, the article is included within a solar cell. In some embodiments, the article is a solar cell. The term “solar cell,” as used herein, is interchangeable with the term “photovoltaic cell.” Solar cells can be used to generate electricity from energy produced by the sun or other sources of energy. In some embodiments, the solar cell is part of a system that uses the sun as a source of energy that generates electricity from the solar cell. The solar cell could also be used in a system that employs other sources of energy to generate electricity from the solar cell. In some embodiments, the solar cell comprises a 2D and 3D perovskite as described in more detail elsewhere herein. In certain embodiments, in addition to the 2D and 3D perovskites, the solar cell may include various other components. For example, in some embodiments, the solar cell further comprises a substrate, an electron transporting layer, and / or a hole transporting layer. In some such embodiments, the 2D perovskite is positioned between the 3D perovskite and a hole transporting layer. In some embodiments, the 2D perovskite and the 3D perovskite are both positioned between the hole transporting layer and the electron transporting layers. In some embodiments, the solar cell may further include electrodes, e.g., for extracting charge carriers from the perovskite layers. Any of a variety of substrates, electrodes, electron transporting layers, and / or hole transporting layers are suitable for use in the solar cells described herein, and those of ordinary skill in the art are capable of selecting suitable substrates, electrodes, electron transporting layers, and / or hole transporting layers. In some embodiments, suitable substrates may facilitate deposition and / or growth of one or more layers thereon. In some embodiments, the substrate may also be suitably transparent to electromagnetic radiation having wavelength(s) within certain wavelength ranges (e.g., visible light, near IR). This can allow, for example, for light to pass through the substrate and to impinge upon other layers of the solar cell. In some embodiments, the substrate comprises glass (e.g., silica- based glass). In some instances, electrodes may be present to facilitate extraction of charge carriers from the 3D perovskite and / or the 2D perovskite following exposure to a light source (i.e., the sun in the case of a solar cell). In some instances, the electrode may be transparent, for example, in cases where it is advantageous for electromagnetic radiation (e.g., visible light, near IR) to pass therethrough. For instance, in some embodiments, it may be desirable for a first electrode (e.g., a cathode) to be transparent whereas a second electrode (e.g., an anode) might or might not be transparent. Other configurations where the 13920912.1 second electrode is transparent and the first electrode is not, and / or where both electrodes are transparent are also possible. Those of ordinary skill in the art are capable of selecting appropriate electrode materials based on the desired solar cell configuration. In some embodiments, the electrode comprises copper, gold, fluoride-doped tin oxide, and / or indium-doped tin oxide. Any of a variety of suitable materials for the electron transporting layer and / or the hole transporting layer are possible, and those of ordinary skill in the art may select them based on energy alignment between the electron transporting layer and the 3D perovskite or the hole transporting layer and the 2D perovskite, the transparency of the material of the electron transporting layer and / or the hole transporting layer, and / or the stability of the materials (e.g., during operating conditions and / or after exposure to a light source as described in more detail elsewhere herein). For instance, in some embodiments, the electron transporting layer comprises SnO2, TiO2, and / or ZnO. In some embodiments, the electron transporting layer comprises nanoparticles comprising SnO2, TiO2, and / or ZnO. In some embodiments, the hole transporting layer comprises 2,2′,7,7′-Tetrakis(N,N-di-p- methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), poly(triaryl amine) (PTAA), and / or poly(3-hexylthiophene-2,5-diyl) (P3HT). In some embodiments, the solar cells described herein are arranged in a regular n-i-p structure. A “n-i-p” structure is one that includes a hole transport region (HTR), an electron transport region (ETR), and a perovskite absorber region (PAR) between the HTR and the ETR, with the ETR being closer to the energy source than the HTR and the PAR. The HTR, PAR, and / or ETR may be in the form of a layer, in certain embodiments. FIG.1A shows a schematic diagram of an example of a solar cell in an n-i-p configuration, where the device is configured such that incident light 122 from the sun 120 first passes through the electron transporting layer 112, 3D perovskite 113, 2D perovskite 114, and hole transporting layer 115, in that order. In some embodiments, the solar cells described herein desirably operate at high power conversion efficiencies (PCEs) and / or maintain their PCE for a long time. Without wishing to be bound by any particular theory, it is believed that high crystallinity of the 2D perovskites described herein do not degrade significantly during device operation, and thus facilitate and / or high PCE during operation. In some embodiments, the solar cell has an initial PCE when operated at approximately 85 degrees C of at least 18%, at least 19%, at 13920912.1 least 20%, at least 21%, or at least 22% and / or no more than 23%, no more than 24%, no more than 25%, or no more than 26%. In some embodiments, the solar cell has an initial PCE of at least 18% when operated at 85 degrees C. In some embodiments, after exposure to full-spectrum 1-sun AM 1.5G illumination for 1000 hours at 85 degrees C, the solar cell has a secondary some embodiments, the secondary PCE does not decrease much or at all relative to the initial PCE. For instance, in some embodiments, the secondary PCE is at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, or at least 95% and / or up to 96, up to 97%, up to 98%, or up to 99% of the initial PCE. It will be understood that, in some embodiments, the PCE (e.g., the initial PCE and / or the secondary PCE) is provided by the 3D perovskite and the 2D perovskite together and alone. That is, in certain embodiments, the initial PCE of at least 18% is provided by the 3D and 2D perovskite of the solar cell, and no other absorbing layers (i.e., another absorber in a tandem solar cell) is considered as contributing to the PCE of at least 18%. Some aspects are related to methods. For example, in some embodiments, methods for forming a 2D perovskite are described. In certain embodiments, a method includes providing a 3D perovskite. In some instances, the 3D perovskite may be synthesized, purchased commercially, or obtained through any other suitable method. In some instances, the 3D perovskite further comprises an excess of PbI2, or similar salt comprising the B cation and the X halide from the 3D perovskite. In some embodiments, the method includes exposing a 3D perovskite having a thickness of greater than or equal to 200 nm (e.g., or greater, as described elsewhere herein) and a volume of greater than or equal to 150 µm3(e.g., or greater, as described elsewhere herein) to a solution comprising a first salt, salt, a first solvent, and a second solvent. In some embodiments, it is desirable to expose the solution to a 3D perovskite that includes a slight excess of PbI2 (and / or an equivalent salt from which it is desirable to form the 2D perovskite). The presence of PbI2 may facilitate the formation of the 2D perovskite, in some embodiments. In some such embodiments, the amount of excess PbI2is an amount sufficient to form the 2D perovskite layer, but small enough such that little to no PbI2 remains after forming the 2D perovskite layer. Advantageously, the presence of the PbI2 initially facilitates the 2D perovskite formation, but its presence in the 2D / 3D perovskite may disadvantageously degrade performance (e.g., PCE and / or stability) 13920912.1 Those of ordinary skill in the art, given the insight provided by the present disclosure, would be capable of selecting a suitable first salt for forming the 2D perovskite. In some embodiments, the first salt includes a capping ligand and / or a halide, and may be the source of the capping ligand and / or the halide when forming the 2D perovskite. In some embodiments, the first salt comprises a capping ligand L and a X halide as described elsewhere herein. For example, in some embodiments, the first salt comprises n- octylammonium bromide, phenethylammonium iodide, and / or n-butylammonium bromide. Those of ordinary skill in the art, given the insight provided by the present disclosure, are capable of selecting a suitable second salt for forming the 2D perovskite. In accordance with some embodiments, the second salt aids in forming the 2D perovskite, for instance, by providing an A cation and / or an X halide of the 2D perovskite. In some embodiments, the second salt comprises an A cation and an X halide as described elsewhere herein. In some instances, the second salt comprises a methylammonium halide and / or formamidinium halide, where the halide may be selected from chloride, bromide, and / or iodide. Those of ordinary skill in the art, given the insight provided by the present disclosure, are capable of selecting a suitable first solvent for forming the 2D perovskite. In accordance with some embodiments, the first solvent is selected such that the first salt is soluble therein. In some embodiments, the first salt may be considered soluble in the first solvent when at least 0.1 mmol, at least 1 mmol, at least 10 mmol, or at least 100 mmol and / or up to 1 mol, up to 10 mol, up to 100 mol of the first salt dissolves in 1 L of the first solvent at 20 degrees C and 1 atm pressure. In some embodiments, the first solvent is generally nonpolar or weakly polar, having a dielectric constant (ε) of less than or equal to 18. In some embodiments, the first solvent, when exposed to a 3D perovskite, is capable of dissolving an “A” cation from the 3D perovskite. In some instances, the first solvent comprises ethanol, isopropanol, butanol, ethyl acetate, chlorobenzene, and / or chloroform. In some instances, the first solvent comprises isopropanol. Those of ordinary skill in the art, given the insight provided by the present disclosure, are capable of selecting a suitable second solvent for forming the 2D perovskite. In some embodiments, the second solvent is capable of dissolving at least a portion of the 3D perovskite. Accordingly, in some embodiments, the method includes allowing at least a portion of the 3D perovskite to be dissolved by the second solvent. In some embodiments, 13920912.1 the method includes dissolving at least a portion of the 3D perovskite by the second solvent. In some embodiments, the second solvent desirably has a strong interaction and effectively dissolves the 3D perovskite, the second salt, and PbI2 (e.g., or other equivalent precursor salts containing the B cation and an X halide as described in more detail elsewhere herein). The ability of a particular solvent to dissolve such materials may be determined using density functional theory, once the materials that are being used for the 3D perovskite and the second salt. In some embodiments, the second solvent is generally polar, having a dielectric constant (ε) of greater than 18. In some embodiments, the second solvent is a polar aprotic solvent. For instance, in some embodiments, the second solvent comprises propylene carbonate, acetonitrile, methyl glycol, dimethylformamide, 1-methyl-2- pyrrolidone, γ-butyrolactone, dimethylacetamide, acetone, and / or dimethyl sulfoxide. In some embodiments, it may be advantageous for the second solvent to comprise dimethyl sulfoxide due to its particularly strong interactions with certain 3D perovskites, certain second salts, and PbI2(e.g., or other equivalent salts containing the B cation and X halide of the 3D perovskite). In some embodiments, the second solvent comprises water. In some instances, the method includes dissolving at least a portion of the 3D perovskite using the second solvent, whereafter a cation of the first and / or second salt and at least a portion of the dissolved 3D perovskite form a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite. In some embodiments, the method further includes heating the 3D perovskite. In some embodiments, heating the 3D perovskite may heat the solution exposed to the 3D perovskite. In some such instances, this may evaporate the first and / or second solvent, and may facilitate the formation of the 2D perovskite on at least a portion of the 3D perovskite. In some embodiments, the 2D perovskite forms on the portion of the 3D perovskite which was exposed to the solution. Heating the 3D perovskite may comprise forming a 2D perovskite on at least a portion of the 3D perovskite, wherein the 2D perovskite comprises and / or consists essentially of an n = 2 phase. Some aspects are related to using the 2D perovskite on 3D perovskite structures described herein. For instance, in some embodiments, the 2D and 3D perovskites are included within an article such as a solar cell or other photovoltaic cell. Accordingly, in some embodiments, methods may include exposing an article comprising the 2D and 3D 13920912.1 perovskites to a light source (e.g., the sun) and converting light from the light source into electricity. In some embodiments, an article (e.g., a solar cell) comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3); and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a ratio of an first area under an XRD peak (e.g., between 12.5 and 12.8 degrees corresponding to a PbI2 peak) associated with impurities (e.g., PbI2) to a second area under an XRD peak associated with the 2D perovskite is less than or equal to 25% (or less than or equal to 10%, less than or equal to 5%, less than or equal to 2%, less than or equal to 1%, less than or equal to 0.5%, less than or equal to 0.1%, or less than or equal to 0.01%). In some embodiments, an article (e.g., a solar cell) comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3); and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a photoluminescence carrier lifetime of the article is greater than or equal to 100 nanoseconds. In some embodiments, an article (e.g., a solar cell) comprises a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3); and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein the article has an initial power conversion efficiency (PCE) and, after exposure to full-spectrum 1-sun AM 1.5G illumination for 500 hours, the article has a secondary PCE that is at least 89% (or at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, and / or up to 96, up to 97%, up to 98%, or up to 99%) of the initial PCE. In some embodiments, a method comprises exposing a three-dimensional (3D) perovskite having a thickness of greater than or equal to 300 nm (or greater than or equal to 500 nm) and a volume of greater than or equal to 150 µm3(or greater than or equal to 300 µm3) to a solution comprising a salt and a mixed solvent comprising a first solvent and a second solvent; wherein: the salt is soluble in the first solvent; and at least a portion of the 13920912.1 3D perovskite is dissolved by the second solvent such that a cation of the salt and at least a portion of the dissolved 3D perovskite to form a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite. In some embodiments, the method further includes heating the 3D perovskite. In some embodiments, the first solvent of the method comprises propylene carbonate, acetonitrile, methyl glycol, dimethylformamide, 1-methyl-2-pyrrolidone, and / or dimethyl sulfoxide. In some embodiments, the second solvent of the method comprises isopropanol. In some embodiments, the salt of the method comprises formamidinium iodide, formamidinium bromide, formamidinium chloride, methylammonium iodide, methylammonium bromide, and / or methylammonium chloride. In some embodiments, the 3D perovskite comprises a formamidinium lead iodide composition. In some embodiments of the method or articles disclosed herein, the 2D perovskite comprises n-octylammonium methylammonium lead halide. In some embodiments, the article is included within a solar cell. In some such embodiments, the solar cell further includes a substrate, an electron transporting layer, and / or a hole transport layer. In certain embodiments, the 3D perovskite is in the form of a layer. In certain embodiments, the 2D perovskite is in the form of a layer. The term “layer” is generally used herein to refer to a form factor having a thickness dimension, a first lateral dimension that is perpendicular to and larger than the thickness dimension, and a second lateral dimension that is perpendicular to and larger than the thickness dimension and perpendicular to the first lateral dimension. In some embodiments, the length of first dimension of the layer is at least 3 times, at least 5 times, at least 10 times, at least 100 times, at least 1,000 times, or more times the thickness of the layer. In some embodiments, the length of second dimension of the layer is at least 3 times, at least 5 times, at least 10 times, at least 100 times, at least 1,000 times, or more times the thickness of the layer. The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention. EXAMPLE 1 In this example, articles including a 2D on 3D perovskite bilayer heterostructure are described. The two-dimensional on three-dimensional (2D / 3D) perovskite bilayer heterostructure can improve the stability and performance of perovskite solar cells. In this 13920912.1 example, it is shown that the 2D / 3D perovskite stack in a device evolves dynamically during its end-of-life decomposition. Initially phase-pure 2D interlayers can evolve differently, resulting in different device stabilities. It is shown that a robust 2D interlayer can be formed using mixed solvents to regulate its crystallinity and phase purity. The resulting 2D / 3D devices achieved 25.9% efficiency and had good durability, retaining 91% of their initial performance after 1,074 hours at 85 °C using maximum power point tracking. The development of 2D / 3D perovskite bilayer heterostructures has helped boost the performance and durability of perovskite solar cells (PSCs). The 2D interlayer can passivate defects, control charge transport, create a built-in potential, prevent external ingression from the environment, and block ion migration. However, for these advantages to be retained, the 2D interlayer needs to be stable; the 2D interlayer must not evolve structurally or compositionally to become non-functional. In this example, it is shown that 2D interlayers can evolve dynamically during device aging, and that phase-pure 2D interlayers can undergo different evolution pathways. It is also demonstrated how excess PbI2can facilitate 2D formation to promote phase-purity and crystallinity. The changes that can occur to 2D perovskite interlayers during long-term illumination were investigated. A baseline control 2D / 3D device was aged under full- spectrum 1-sun illumination with ultraviolet (UV) included in a nitrogen atmosphere. The 3D perovskite was a formamidium (FA)- lead(Pb)-iodide (FAPbI3)-based composition with added MACl and MAPbBr3for improved crystallization and excess PbI2. The excess PbI2has several roles that will be discussed throughout this example. All devices are based on the n-i-p structure with SnO2 and spiro-MeOTAD (2,2',7,7'-Tetrakis[N,N-di(4- methoxyphenyl)amino]-9,9'-spirobifluorene) as the charge transporting layers (i.e., electron and hole transporting layers, respectively). Under maximum power point (MPP) tracking, the control 2D / 3D device had poor photostability (FIG.6), where 74% of its initial performance was maintained after 500 hours. In this example, coupled-geometry x-ray diffraction (XRD) with sufficient penetration depth was used to probe the buried 2D interlayer in a device stack. Specifically for the control 2D / 3D device (FIGS.7A-7D), the initial n=2 quasi-2D perovskite phase [L2APb2X7, hereafter referred to as 2D(n=2), where L, A, and X refer to the spacer organic ligand octylammonium (Oc), the A-site organic cation, and halide anion, respectively] gradually disappeared, with partial transformation into PbI2 and the n=12D phase [L2PbX4, 13920912.1 referred to as 2D(n=1)]. It was postulated that the poor device stability under MPP tracking was related to this disappearance of most of the 2D interlayer. The bare 3D device had low power conversion efficiency (PCE) because of abundant unpassivated defects. With loss of most of the 2D interlayer, the 2D / 3D device lost its defect passivation and essentially reverted back to behave like a bare 3D device. The PCE decreased through degradation because the 2D / 3D device slowly turned into a bare 3D device. This degradation process was also evident in the changes of their carrier lifetimes before and after illumination (FIG. 8). Additionally, the bare 3D device underwent severe initial decay. This so-called burn-in decay was caused by rapid defect migration. As the 2D interlayer disappeared with time, defect migration occurred over a longer duration. The burn-in was not rapid but became an extended period degradation of the 2D / 3D device. Overall, the control 2D / 3D device was not stable, and most of the 2D perovskite layer disappeared under illumination (discussion in section 1.1). These findings motivated the creation of the durable 2D interlayer described in this example. 2D interlayer formation studies Time-resolved, in situ grazing-incidence wide-angle x-ray scattering (in situ GIWAXS) was used to study the formation dynamics of 2D / 3D perovskites. Hereafter, the phases present initially upon deposition, excluding the 3D perovskite, are referred to as the initial phase. The previously discussed control 2D / 3D perovskites are labelled as the Oc films. The initial phase consisted of the 2D(n=1) perovskite and unreacted PbI2(FIGS.2A, 2B, and 9A-9D). The PbI2peak subsequently weakened when annealing of the film began, but this PbI2 was not fully consumed and was residual in the final film. The 2D(n=1) perovskite only partially converted into the 2D(n=2) phase, so the final film contained an impure mixture of 2D(n=1), 2D(n=2), and PbI2phases. Further experiments indicate that the excess PbI2 in the 3D perovskite composition facilitated 2D formation by providing a readily available PbI2 source (FIGS.10A-10D), but with the disadvantage that unreacted PbI2 would remain. For the 3D phase with no excess composition, although no unreacted PbI2remained, the 2D perovskite phase had low crystallinity and the PCE was poor (FIGS.11A-11D, discussion in section 1.2). This trade-off presented an opportunity to assess the interplay between 2D perovskite crystallinity and residual PbI2. Unless otherwise stated, all 2D / 3D perovskites in this example were based on the 3D with excess PbI2 composition. Utilizing mixed solvent systems with methylammonium 13920912.1 (MA) led to a stepwise progression to promote formation of the 2D(n=2) phase. First, OcMA (Oc ligand with MA additive in isopropanol (IPA)) formed a 2D(n=1) initial phase (FIG.2C and 2D), which converted into the 2D(n=2) phase upon annealing. PbI2 and 2D(n=1) were fully reacted, unlike in the control Oc film. Next, the initial phase formed with OcMA:F (OcMA with mixed dimethylformamide and IPA) consisted of a multi-phase mixture of 2D(n=1) and 2D(n=2), and the 2D(n=1) fully converted into 2D(n=2) in the final film (FIG.2E and 2F). Finally, with OcMA:S (OcMA with mixed dimethylsulfoxide and IPA), the 2D(n=2) perovskite now became the equilibrium phase, forming spontaneously (FIGS.2G, 2H, and 12A-12D); neither PbI2, 2D(n=1), nor any other secondary by-products were present. Despite the excess PbI2in the 3D perovskite composition, there was no unreacted PbI2left after 2D formation, and a phase-pure 2D perovskite formed with high crystallinity (FIG.2I). OcMA, OcMA:F, and OcMA:S all attained a final phase-pure 2D(n=2) perovskite. This allowed one to exclude phase purity as a contributing factor when comparing their materials properties. The final crystallinity of the 2D(n=2) phase increased in the order OcMA:S > OcMA:F > OcMA > Oc. This trend was consistent when evaluating the GIWAXS integrated peak area (FIGS.2I and 13A-13B), FWHM peak broadening, and Halder-Wagner analysis from lab-based grazing-incidence XRD of the 2D / 3D perovskites (FIGS.14A-14G). A correlation between 2D crystallinity and carrier lifetime was observed (FIGS.2J and 2K). The bare 3D film had a carrier lifetime of 1,067 ns, which increased by 244% to 2,608 ns for the Oc film due to defect passivation by the 2D interlayer. The unreacted PbI2 left over likely contributed to the increased lifetime, convoluting with the 2D interlayer contribution. Regardless, the OcMA:S film, even with no unreacted PbI2, showed the longest carrier lifetime of 4,205 ns (394% increase over the bare 3D film), demonstrating effective suppression of non-radiative recombination. The study was extended to understand the 2D formation mechanisms (FIG.3A and table 1). Different mixed solvents had varying consequences (FIGS.15A-15L). With OcMA:MG (methyl glycol and IPA) and OcMA:A (acetonitrile and IPA), 2D crystallinity and formation were similar compared to OcMA (IPA only). Meanwhile, OcMA:F (FIG.2E), OcMA:N (1-methyl-2-pyrrolidone and IPA), and OcMA:PC (propylene carbonate and IPA) all formed a mixed 2D(n=1) and 2D(n=2) initial phase, with final 2D(n=2) crystallinity improved compared to OcMA. This also suggested OcMA:PC as a promising candidate, and 13920912.1 the measured carrier lifetime of 3,842 ns trended with 2D crystallinity (FIGS.16A-16B). The mixed solvent and MA together synergistically regulated 2D formation. Without MA, Oc:S (Oc ligand with dimethylsulfoxide and IPA) showed comparable crystallinity and formation relative to the control Oc (FIGS.17A-17C). Without the mixed solvent, and regardless of the MA halide choice, the final 2D(n=2) crystallinity was only slightly improved over the control Oc (FIGS.18A-18I). In this example, only OcMA:S (FIG.2G) spontaneously formed a 2D(n=2) initial phase and had the highest final 2D crystallinity. Lastly, increasing the MA concentration in OcMA:S showed the possibility of forming the higher-order 2D(n=3) phase (FIGS.19A-19C). Table 1. List of naming abbreviations. Label Ligand A cation Solvent Oc 10 mM - isopropanol n-octylammonium bromide Oc:S 10 mM - 25 mM dimethylsulfoxide n-octylammonium bromide in isopropanol OcMA 10 mM 3 mM isopropanol n-octylammonium methylammonium bromide iodide OcMA-Br 10 mM 3 mM isopropanol n-octylammonium methylammonium bromide bromide OcMA-Cl 10 mM 3 mM isopropanol n-octylammonium methylammonium bromide chloride OcMA:PC 10 mM 3 mM 25 mM propylene carbonate in n-octylammonium methylammonium isopropanol bromide iodide 13920912.1 OcMA:A 10 mM 3 mM 25 mM acetonitrile in isopropanol n-octylammonium methylammonium bromide iodide OcMA:MG 10 mM 3 mM 25 mM methyl glycol in isopropanol n-octylammonium methylammonium bromide iodide OcMA:F 10 mM 3 mM 25 mM dimethylformamide in n-octylammonium methylammonium isopropanol bromide iodide OcMA:N 10 mM 3 mM 25 mM 1-methyl-2- pyrrolidone in n-octylammonium methylammonium isopropanol bromide iodide OcMA:S 10 mM 3 mM 25 mM dimethylsulfoxide in n-octylammonium methylammonium isopropanol bromide iodide PEAI 10 mM - isopropanol Phenethylammonium iodide PEAI- 10 mM 6 mM 25 mM MA:S dimethylsulfoxide in Phenethylammonium methylammonium isopropanol iodide iodide BA 10 mM isopropanol n-butylammonium bromide BA-MA:S 10 mM 3 mM 25 mM dimethylsulfoxide in n-butylammonium methylammonium isopropanol bromide iodide Preliminary theoretical calculations helped shed light on the experimental observations (section 1.3).2D formation involved partial dissolution of the 3D perovskite 13920912.1 surface, followed by secondary recrystallization into the 2D phase. A stronger solvent interaction assisted with this dissolution (FIGS.3B-3E), which was necessary to source the precursor components (PbI2 and FA) from the 3D surface used for 2D formation. Sourcing of FA also led to the formation of a FA-based 2D(n=2) perovskite. The MA additive functioned to thermodynamically promote 2D(n=2) formation during its initial stage (FIGS. 20A-20C), followed by MA volatilization and FA substitution, resulting in a high crystallinity FA-based 2D(n=2) phase (FIGS.21A-21D, section 1.4, and table 2). This is akin to the role of MA to promote 3D perovskite intermediate phase crystallization. Furthermore, a stronger solvent interaction also slowed down drying and removal, which reduced the nucleation rate during recrystallization to promote higher crystallinity. Table 2. Fitted XRD peak positions. 2D 2D / 3D I3I2 Ultraviolet photoelectron spectroscopy (UPS) measurements showed that the Oc 2D / 3D perovskite created a type I band alignment relative to the bare 3D perovskite (FIG. 22). This unfavorable energy offset impeded hole extraction toward spiro-MeOTAD. In contrast, the work function and valence band maximum shifts netted a type II alignment for the OcMA:S 2D / 3D perovskite and created an energy cascade to bridge hole extraction. OcMA:S and OcMA had similar energetic shifts, consistent with both films having phase- pure 2D(n=2) interlayers, as shown in GIWAXS results. In terms of topographical morphology, no obvious difference was observed between the 2D / 3D perovskites in FIGS. 3F and 23A-23D from atomic force microscopy (AFM). The surface morphology of the 2D 13920912.1 interlayer was not obviously visible in FIG.3F because the AFM measurement setup was optimized for electrical sensitivity, which sacrificed topography resolution (section 1.5). Potential maps measured with Kelvin probe force microscopy (KPFM) (FIG.3G) showed that the bare 3D film had large intra-grain and grain-to-grain potential variations caused by abundant surface defects that created localized charging effects. The potential profile transformed into a different pattern for the Oc film as a result of 2D interlayer formation, but individual grains were not resolvable because of intragrain potential inhomogeneity. In this case, the potential fluctuations were caused by the co-existence of multiple phases – unreacted PbI2, 2D(n=1), and 2D(n=2). For the OcMA:S film, the intra-grain potential uniformity was smoothened out (rightmost in FIG.3G) by the phase-pure 2D(n=2) interlayer and its long-range structural ordering (i.e., high crystallinity). The AFM morphological grain boundary now corresponded spatially with the KPFM potential boundary, and individual grains were distinguishable because of the homogenized intra-grain potential. Transient reflection spectroscopy (TRS) allowed the decoupling of the surface versus bulk carrier dynamics in the 3D perovskite by optically generating carriers with short and long wavelengths and modelling the carrier diffusion from the surface (FIG.3H). For the OcMA:S film, the carriers diffused away from the 2D / 3D interface more efficiently. For example, with 3.54 eV excitation light, 50% of photocarriers moved away from the surface within 50 ps for the OcMA:S film, whereas it took 100 ps for the Oc film, consistent with the homogenized surface potential profile and reduced non-radiative recombination. Conversely, using time-resolved microwave conductivity (TRMC), it was found that the carrier lifetime for the OcMA:S film was marginally improved over the Oc film (17.7 to 19.0 µs, 7.3% increase) (FIG.3I). Because TRMC is a bulk technique lacking surface sensitivity, these results implied that the 2D perovskite had minimal impact on the bulk 3D perovskite. Direct visualization was obtained by cross-sectional SEM imaging, where the 2D perovskite was seen as a conformal layer formed on top of the 3D perovskite (FIGS.24A- 24B). This result was also consistent with time-of-flight secondary ion mass spectrometry (ToF-SIMS) profiling of the elemental distributions. Moreover, the optical bandgap of the 2D / 3D perovskites was also comparable. These results suggest that the 2D perovskite was mostly confined to the 3D perovskite surface with no appreciable bulk diffusion. Device characterization 13920912.1 The performance of devices based on the 2D / 3D perovskites that had the longest carrier lifetimes were compared and it was found that the PCE increased as OcMA:S > OcMA:PC > OcMA:F > Oc > bare 3D devices (FIGS.4A-4B, FIGS.25A-25D, and Table 3). This trend was primarily due to an increasing open-circuit voltage and fill factor, consistent with the increasing carrier lifetime.2D phase purity contributed to the improvement when comparing between the Oc device versus the other 2D / 3D devices, whereas 2D crystallinity accounted for the differences between the OcMA:S, OcMA:PC, and OcMA:F devices, which were all phase-pure and differed only in crystallinity. Generally, the chemical bath deposition (CBD)-SnO2 devices had higher PCE than the nanoparticle (NP)-SnO2devices. The OcMA:S devices had the best performance overall, with the champion device based on CBD-SnO2reaching a PCE of 25.9% with minimal hysteresis (FIG.4C and table 4). Table 3. Average device photovoltaic parameters. VOC (V) JSC (mA cm−2) FF (%) PCE (%) VOC(V) JSC (mAcm−2) FF (%) PCE (%) 13920912.1 The device stability was assessed through MPP tracking under full-spectrum 1-sun AM 1.5G illumination (UV included). All devices for all stability tests that were tested were of the n-i-p structure (e.g., see FIG.1A) based on spiro-MeOTAD. No active cooling or heating was applied, and the actual device temperature was measured to be 40-45°C. The target OcMA:S device (FIG.4D), with a peak PCE of 24.3%, sustained its performance with minimal degradation, and retained 96.4% of its initial PCE after >1,700 hours. The control Oc device degraded the most rapidly, dropping to 74.1% of its initial performance after 500 hours. The OcMA:PC and OcMA:F devices had stability intermediate to those of the OcMA:S and Oc devices. Both the OcMA:PC and OcMA:F devices showed the same decay behavior (linear decrease after initial rise for approximately 75 hours), but was distinct from the OcMA:S device. Comparing the devices with or without excess PbI2 demonstrated the role of the 2D interlayer. The “Oc no excess” device had no excess PbI2 in the 3D composition, but its stability and PCE were still far short of the target OcMA:S device (Fig.4D). The “OcMA:S no excess” device was more durable than the “Oc no excess” device (FIG.26). Yet, the target OcMA:S device (excess PbI2 composition but no residual PbI2 after 2D formation) showed the greatest durability improvement. These comparisons demonstrate the interplay between the 2D interlayer and excess PbI2 to stabilize the device and simultaneously maintaining a high PCE. None of the 2D / 3D devices underwent the rapid initial burn-in degradation seen for all of the bare 3D devices (FIG.27), so burn-in was not caused by residual PbI2(section 1.6). The 2D interlayer passivation of defects accounted for the absence of burn-in, which was caused by rapid defect migration. Additionally, the solvent by itself without 2D interlayer formation did not improve stability (FIG.28). Conventional spiro-OMeTAD is doped with tBP (4-tert-butylpyridine) and LiTFSI (lithium bis(trifluoromethane)sulfonimide). Such dopants are hygroscopic, highly diffusible, and lower the glass transition temperature of spiro-OMeTAD, causing a variety of device degradation problems especially at higher temperatures. The spiro-OMeTAD used in this work did not contain tBP nor LiTFSI, which motivated the assessment of the devices at elevated temperatures. The thermal stability was first tested in the dark, progressively increasing the temperature from 65-75°C (FIG.4E). After >2,100 hours, on average (8 devices), the OcMA:S devices preserved 86% of their initial PCE. The most stable OcMA:S device retained 91% of its initial performance. MPP tracking was then performed of the 13920912.1 OcMA:S device actively heated at 65°C (FIG.4F). The lower initial PCE was caused by the negative temperature coefficient of PSCs. The device retained 95% of its initial PCE after >800 hours. The heating temperature was further increased to 85°C under 1-sun AM 1.5G illumination (UV included). This n-i-p device based on the spiro-MeOTAD retained 91% of its initial performance after 1,074 hours illumination (FIG.4G). All devices in this example were of the n-i-p structure based on spiro-MeOTAD, which generally have poorer stability than their inverted p-i-n counterparts. Especially at higher temperatures, fewer n-i-p devices have been reported. This example demonstrates competitive and durable n-i-p devices. A robust 2D interlayer The control Oc device lost most of its 2D interlayer under illumination, whereas the bare 3D perovskite under illumination did not undergo any sort of decomposition (FIG.29). Thus, problems with the 2D interlayer limited the overall durability of the 2D / 3D Oc device. For the target OcMA:S device, the 2D(n=2) phase was in a stable state both structurally (crystalline long-range ordering) and thermodynamically (spontaneously-formed single- phase). The 2D interlayer was still intact after >2,500 hours illumination without any sort of phase transformation (FIGS.5A and 30). Long-term retention of this robust 2D interlayer was important to stabilizing the device. During aging, the 2D interlayer passivated defects and suppressed ion migration, functioning like a blocking barrier. When the 2D perovskite degraded, ions penetrated into the transport layer unimpeded, as observed through post- mortem analysis of the aged devices (FIG.5B). Initially phase-pure 2D interlayers could degrade differently. For both the OcMA:PC and OcMA:F devices, although the pristine 2D perovskite was phase-pure 2D(n=2), they transformed into the 2D(n=1) phase (FIG.31). This transformation was due to low crystallinity of the OcMA:PC and OcMA:F 2D perovskites, compared to that of the OcMA:S 2D perovskite. Nevertheless, further decomposition into PbI2 did not occur, unlike the control Oc device. Generalization of these results was attempted. For the “Oc no excess” device (FIG.32), a phase-pure 2D(n=2) perovskite transformed into 2D(n=1) under illumination, so the 2D interlayer transformation was not caused by excess PbI2. In contrast, the “OcMA:S no excess” device (FIG.33) was durable under the same conditions, so the method worked without excess PbI2. The strategy also worked on alternative 2D interlayers 13920912.1 based on different spacer organic ligands (FIG.34A-34B and 35A-35B). Overall, this example showcases the importance of attaining a robust 2D interlayer (FIG.5C). Materials and Methods Materials All materials used in this example were purchased from Sigma-Aldrich, unless otherwise stated. Formamidinium iodide, methylammonium chloride, methylammonium bromide, methylammonium iodide, n-octylammonium bromide, and phenethylammonium iodide were purchased from Greatcell Solar Materials. SnO2 colloidal solution was purchased from Alfa Aesar Chemicals. 2,2',7,7'-tetrakis(N,N -di-p methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) was purchased from Luminescence Technology Corp. N-Propyl-3- methylpyridinium bis(trifluoromethylsulfonyl)imide was purchased from Strem Chemicals. All chemicals were used directly as received without further purification. Synthesis of MAPbBr3 single crystals MAPbBr3single crystals were synthesized using the inverse temperature crystallization method.1.34 g of MABr was added to 10 mL of DMF and stirred at room temperature. After fully dissolved, 4.40 g of PbBr2 was added to the MABr / DMF solution. This solution was filtered and then heated at 90 °C in an oil bath with continuous stirring for 3 hours. Finally, the orange MAPbBr3single crystals were washed with diethyl ether and dried overnight. Synthesis of spiro2+(TFSI-)2 single crystals Spiro2+(TFSI-)2was synthesized by oxidation of Spiro-OMeTAD using Silver(I) bis(trifluoromethanesulfonyl)imide (Ag-TFSI). Spiro-OMeTAD (0.5 g) and 10 mL anhydrous dichloromethane were added to a flask with continuous stirring. After fully dissolved, Ag-TFSI (0.3166 g) was added. The flask was evacuated and backfilled with nitrogen. The resultant mixture was stirred at room temperature for 5 hours. The reaction was then diluted by adding dichloromethane. A gray precipitate of silver was filtered off, and the crude product was obtained by rotary evaporator. The solid was dissolved in a minimal amount of dichloromethane and precipitated in dry diethyl ether. The resulting powder of spiro2+(TFSI-)2was collected via vacuum filtration. Deposition of SnO2 electron transport layers FTO substrates were sequentially cleaned by ultrasonication for 10 min each in Hellmanex, deionized water, acetone, and isopropanol. 13920912.1 For chemical bath deposition (CBD) of SnO2, a solution was prepared by mixing 137.5 mg of SnCl2·2H2O, 625 mg of urea, 625 μL of hydrochloric acid, and 12.5 μL of thioglycolic acid in 50 mL deionized water. The front electrode of the FTO substrates was taped using Kapton tape before SnO2deposition. The substrates and solution were then loaded into a Hellendahl glass reaction vessel (volume of approximately 170 mL) and heated at 65 °C for 14 hours in a water bath. After reaction completion, the SnO2 / FTO substrates were sequentially cleaned by ultrasonication for 5 min each in deionized water and isopropanol. Finally, the SnO2 / FTO substrates were then annealed at 170 °C for 1 hour in an ambient environment. For nanoparticle SnO2 (NP-SnO2), the SnO2 colloidal solution was diluted in deionized water in 1:6.5 ratio. The FTO substrates were first treated by oxygen plasma for 10 mins. The diluted SnO2colloidal solution was then spun at 3000 rpm for 30 secs. The SnO2 / FTO substrates were then annealed at 150 °C for 30 mins in an ambient environment. Perovskite solar cell fabrication For passivation of the buried interface, the SnO2 / FTO substrates were first treated by oxygen plasma for 10 mins. Subsequently, 10 mM potassium chloride in deionized water was spun at 3000 rpm for 30 secs, followed by annealing at 100 °C for 10 mins. The FTO / SnO2 substrates were treated again by oxygen plasma for 10 mins before perovskite deposition. The perovskite precursor solution was prepared by mixing 1.4 M FAI, 0.5 M MACl, 0.013 M MAPbBr3, and 1.53 M PbI2 in a mixed DMF / DMSO solvent with volume ratio 8:1. The solution was stirred for at least 3 hours at room temperature with a magnetic stirrer before use. Fresh solutions were prepared and used within the same day for perovskite fabrication. The solution was filtered with a 0.22 μm PTFE filter before use. The perovskite films were deposited via spin coating at 500 rpm for 7 secs, then 1000 rpm for 8 secs, then 5000 rpm for 30 secs.10 secs into the 5000 rpm stage, 700 μL of diethyl ether antisolvent was dropped on the wet film. The films were then annealed successively first at 100 °C for 40 mins, then 150 °C for 5 mins. All processing steps for the perovskite films are conducted in a nitrogen glovebox at a temperature of 25 °C. For 2D perovskite deposition, the control treatment consists of 10mM of octylammonium bromide dissolved in isopropanol and deposited at 4000rpm for 30 secs. This was then annealed at 100 °C for 5 mins. For the mixed methylammonium halide and solvent systems, 13920912.1 a molar ratio of octylammonium bromide:methylammonium halide:solvent additive of 1:0.3:2.5 was used, all dissolved in isopropanol for all 2D fabrication systems. For example, for the target OcMA:S treatment, 10mM octylammonium bromide, 3mM methylammonium iodide, and 25mM dimethyl sulfoxide dissolved in isopropanol. This was then deposited at 4000rpm for 30 secs, and annealed at 100 °C for 5 mins. The spiro-OMeTAD hole transporting layer solution was prepared by mixing 90.0 mg spiro- OMeTAD, 9.0 mg of spiro2+(TFSI-)2, and 3.17 µL of N-Propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide in 1 mL of chlorobenzene. The solution was filtered with a 0.22 μm PTFE filter before use. This was then deposited via spin coating at 4000 rpm for 20 secs. Lastly, 100 nm gold electrode was deposited via thermal vacuum evaporation at a rate of 0.5 A s-1for the first 10 nm, then 1.0 A s-1for the rest 90 nm. Device stability testing Photostability tests were performed by tracking the maximum power point (MPP) under full-spectrum 1-sun AM 1.5G illumination using a G2V Optics Pico LED solar simulator. No wavelength filters (e.g., ultraviolet) were used. Without active heating or cooling, the measured device temperature was 40-45°C. On the other hand, for MPP tracking at 65°C or 85°C, the device was actively heated using a hotplate. All tests were performed in N2. All devices for all photostability tests used nanoparticle SnO2as the electron transport layer. For only the 85°C MPP photostability test, the MACl amount in the 3D perovskite composition was reduced to 0.3 M, and everything else about the fabrication was kept the same as described in the previous section. Thermal stability tests in the dark were performed by heating the devices on a hotplate in N2 at a set temperature progressively increasing from 65°C, to 70°C, to 75 °C. Periodically, the devices were transferred to a simulated 1-sun AM 1.5G illumination solar simulator to measure their efficiency. All devices for the dark thermal stability tests used chemical bath deposition SnO2 as the electron transport layer. In situ wide-angle X-ray scattering measurements The in-situ GIWAXS measurements were carried out at the 12.3.2 microdiffraction beamline at the Advanced Light Source (ALS). A custom-designed analytical chamber allowed for processing of the perovskite film during simultaneous GIWAXS measurements. The glass / ITO substrates were placed onto the integrated spin coating and puck-heater and adhered 13920912.1 on by a heat transfer paste. The analytical chamber was sealed off from the external environment by being held under nitrogen flow before the precursor solution was applied to the substrate. The incident angle of the incoming X-ray beam for GIWAXS measurements was 1° with a beam energy of 10 keV. The sample detector distance (SDD) was approximately 155 mm, and the detector was positioned at an angle of 35° from the sample plane. The GIWAXS data were continuously recorded with an integration time of one second using a 2D Pilatus 1 M detector (Dectris Ltd.). After closing the beamline hutch and starting the measurement, the spin coating and annealing procedure was remotely initialized including a remote-controlled dispense. At the end of the spin coating protocol, the puck was heated up to the respective annealing temperature while continuously spinning at 200 rpm in order to ensure uniform measurement conditions during the annealing step. The resulting GIWAXS patterns were calibrated using an Al2O3 reference sample. AFM and KPFM To investigate the surface potential homogeneity, Kelvin Probe Force Microscopy (KPFM) measurements were conducted using an Atomic Force Microscopy (AFM) measurement system (NX-10, Park Systems) installed inside a glovebox filled with N2. The surface potential value of each sample was determined by obtaining the difference in work function between the sample surface and the tip apex. To accurately measure the contact potential difference (CPD) of the sample, the biased tip was calibrated at a scan rate of 0.30 Hz before and after each measurement using highly ordered pyrolytic graphite, which is inert and exhibits a reliable work function of 4.65 eV. An NSC36 chromium-gold tip was used to maximize the image resolution of the CPD in consideration of its high electrical sensitivity. Transient Reflection Spectroscopy Transient reflectance measurements were collected in a pump–probe set up. The fundamental laser pulse (1030 nm, 190 fs, 1 kHz) is generated from a femtosecond ytterbium-doped potassium gadolinium tungstate (Yb:KGW) laser (PHAROS, Light Conversion). The fundamental pulse is split into two parts by a beam splitter. One part is sent to an optical parametric amplifier (ORPHEUS) for tunable pump generation where the pump is modulated at a frequency of 500 Hz. The other part of the fundamental pulse is focused into a sapphire crystal to generate a white-light continuum that is used as the probe. The pump and probe are spatially overlapped on the surface of the sample. 13920912.1 Time-resolved Microwave Conductivity Perovskite films were prepared in an identical manner to those used in devices, except onto precleaned 25×11×1-mm quartz plates (Technical Glass Products, Inc.). Microwave conductivity measurements were conducted using typical protocols. In brief, a 5-ns pulse width, 10-Hz laser at 640 nm was coupled into an X band resonant cavity to photo-generate carriers in each perovskite film, the power from which was measured before and after each measurement using a photothermal detector placed at the sample position and masked by the optical window and waveguide sections when the sample was present. Each sample was positioned inside the microwave cavity such that excitation was always incident to the quartz side, and continuous nitrogen purge was applied to the cavity during all measurements. Neutral density filters were used to attenuate the beam power over one order of magnitude to below 1010 cm-2absorbed photon flux. For analysis, each sample’s fraction of absorbed light was measured inside an integrating sphere diffuse reflectance accessory (Cary 7000i), whereas the beam attenuation profiles of the filter combinations were extracted from the measured specular transmission data for each neutral density filter at the excitation wavelength. Material and device characterizations Time-resolved photoluminescence (PL) spectra were measured using a time-correlated single-photon counting card (Picoquant; PicoHarp 300). The excitation was a 532 nm picosecond pulsed diode laser (Picoquant; LDH-P-FA-530-B) with a repetition rate of 30 kHz adjusted using a pulse generator (Stanford Research; DG535). UV-Vis spectra were measured using a Cary UV-Vis-NIR spectrophotometer. Static XRD and grazing-incidence XRD spectra were measured using a Rigaku SmartLab and a Bruker D8 Discovery Diffractometer with a General Area Detector Diffraction System. ToF-SIMS measurements were carried out using a TOF-SIMS-5 (ION-TOF) equipped with oxygen-ion beam sputter gun and bismuth primary ion gun. The area of 150 × 150 μm2was sputtered by the oxygen ion beam with energy of 1 keV and 25 keV bismuth ion beam was subsequently used to collect positive ion species in 40 × 40 μm2within the sputtered area. Ultraviolet photoelectron spectroscopy (UPS) spectra were obtained from NEXSA (ThermoFisher Scientific) equipped with a He Ⅰ discharge UV lamp with photon energy of 21.22 eV. FE- SEM measurements were carried out using a Zeiss Gemini 450 field emission scanning electron microscope. 13920912.1 Current density-voltage (J-V) curves were recorded using a Newport Oriel Class AAA, 91195A solar simulator and a Keithley 2420 source meter unit. All J-V measurements were recorded under AM 1.5G illumination, calibrated to 100 mW / cm2using a calibrated silicon reference cell. J-V measurements were performed with a step voltage of 10 mV and a delay time of 50 ms. A dark metal aperture was used to precisely define the device active area during measurements. Section 1.1: Disappearing 2D interlayer and its effects on device efficiency and stability From the results in FIG.6-8, for only the control 2D / 3D Oc device specifically, that the 2D(n=2) perovskite was observed to gradually disappeared with partial transformation into the 2D(n=1) phase and PbI2. The Oc device lost the majority of its 2D interlayer under illumination. This has negative consequences on efficiency and stability for several reasons. (1) The 2D perovskite is important because of its defect passivation effects. With disappearance of the 2D perovskite, defect passivation is lost, and the device basically reverted back into the bare 3D device. In simple words, the PCE decreased because the 2D / 3D Oc device is going back to reach the low PCE of the bare 3D device. (2) Defect passivation is also important to suppress defect migration. The 2D perovskite functions as a blocking barrier in-between the 3D perovskite and transporting layer. This barrier effect is lost if the 2D perovskite disappears. Left unimpeded, halide migration into spiro-MeOTAD can cause irreversible redox de-doping reactions to degrade its hole-transport capability. Loss of the 2D interlayer will also worsen extrinsic species from entering into and corroding the 3D perovskite. Penetration of extrinsic material (e.g. electrode material, moisture, oxygen, etc) negatively impacts the device stability. (3) PbI2 generated is unstable to light and undergoes photolysis into metallic Pb and I2gas. Metallic Pb constitutes a deep-level trap state, while I2gas corrodes the perovskite and aggravates decomposition into the yellow δ-FAPbI3 phase. (4) Unbounded spacer organic ligands are insulating, and form defect states (e.g., interstitial defects) to trap charges and cause non-radiative recombination. Moreover, unbounded spacer organic ligands are also more susceptible to defect migration. The above degradation mechanisms will increasingly activate as the 2D perovskite disappears. It is not possible to isolate and assign which slope in FIG.6 corresponds to which specific degradation mechanism. This is because the overall degradation slope is convoluted by every degradation mechanism simultaneously happening at the same time. As such, it is 13920912.1 more important to focus on the overall device stability and efficiency. The control 2D / 3D Oc device was not stable, and its 2D perovskite was observed to disappear under illumination. Section 1.2: Role of excess PbI2 and 2D perovskite formation The 2D / 3D and bare 3D perovskites, with or without excess PbI2composition, were studied: 1) 3D excess PbI2 2) 3D no excess 3) 3D excess PbI2– spincoated with isopropanol (no 2D) 4) 3D no excess – spincoated with isopropanol (no 2D) 5) Oc 2D / 3D excess PbI2 6) Oc 2D / 3D no excess For clarification, other definitions in the context of this example include: • Excess PbI2 = Extra PbI2 added to the 3D perovskite composition. In other words, the FAI:PbI2ratio is not 1:1 in the perovskite precursor solution. • Unreacted PbI2 = Unconsumed PbI2 leftover after 3D or 2D / 3D perovskite formation. • Residual PbI2= Same as unreacted PbI2. Analyzing the XRD results together help clarify the role and function of the excess PbI2: 1) To form 2D perovskites, a source of PbI2 is used. For the “3D excess PbI2” film (“A” trace, FIG.10A), the PbI2is readily available, leftover as residual PbI2after 3D perovskite fabrication. This PbI2 is consumed to form the 2D perovskite, where the PbI2 peak at 2θ~12.6° decreased concurrently to the appearance of the 2D peaks (“B” trace, FIG.10A). 2) The stochiometric 3D perovskite is labelled “3D no excess” (“A” trace, FIG.10B). No excess PbI2 is added to the 3D perovskite precursor solution. To form 2D perovskites, the PbI2source can be obtained in two ways. In the first way (minor source), there is a tiny amount of residual PbI2 leftover after 3D perovskite fabrication. The PbI2 peak is almost negligible and only barely visible when magnified (“A” trace inset, FIG.10B). This is due to the volatilization of a tiny amount of organic cations during the 3D perovskite thermal annealing: 13920912.1 ^^ ^^^^^^^^^ APbX^^3D perovskite^^¾¾¾¾¾¾¾¾¾¾¾¾^ PbX^ ^solid^ + AX ^gas^ ^volatization^In the second way (major source), isopropanol (IPA), which is used to deposit the organic spacer ligands, has an additional dual-function to form PbI2 by dissolving the 3D perovskite organic ‘A’ cations (“B” trace, FIG.10B): &'( )^**+^,^ APbX^^3D perovskite^ ^¾¾¾¾¾¾¾¾¾¾¾^ PbX^^solid^ +AX ^in IPA^ ^IPA dissolution^Formamidinium is highly soluble in IPA, but PbI2is weakly soluble. Solid PbI2is left behind for the organic spacer ligands to react with to form 2D perovskites. Note that IPA dissolution also happens for the “3D excess PbI2” film, as seen in FIG.10C. 3) For the “3D no excess” perovskite, annealing (minor source) and IPA dissolution (major source) combined to generate the PbI2 source to form 2D perovskites (“B” trace, FIG. 10D). Although no unreacted PbI2is leftover, the Oc 2D perovskite has low crystallinity (i.e., broad FWHM and low peak intensity). 4) Comparing “Oc 2D / 3D excess PbI2” and “Oc 2D / 3D no excess”, there is a trade-off between unreacted PbI2and 2D formation. In other words, excess PbI2facilitated 2D formation by providing a readily available PbI2 source, but leftover unreacted PbI2 is a problem. Now it becomes important to compare their stability and efficiency. Firstly, in terms of efficiency (FIGS.11A-11D), the “Oc 2D / 3D excess PbI2” devices have higher FF, VOC, and PCE than the “Oc 2D / 3D no excess” devices. Higher 2D crystallinity is the primary reason for the improved FF and VOC, while the residual PbI2 contributed to some of the VOCincrease. Particularly, the improved FF of the “Oc 2D / 3D excess PbI2” devices can be mainly attributed to the higher 2D crystallinity. The FF of the “3D excess PbI2” devices is similar or slightly lower than the “3D no excess” devices, meaning that residual PbI2alone does not improve FF. Overall, the “Oc 2D / 3D excess PbI2” devices have the highest PCE in FIGS.11A-11D, but it is important to consider their device stability. Comparing these devices presents an opportunity to assess how the trade-off between unreacted PbI2 and 2D formation affects device stability, which will be discussed later in the example. Section 1.3: Preliminary theoretical studies 13920912.1 First-principles calculations within the framework of density functional theory (DFT) were applied, employing a plane-wave basis set and the projected augmented wave method, as implemented in the VASP package. For describing the exchange-correlation functionals in DFT, the SCAN+rvv10 type meta-GGA functional with a van der Waals (vdW) functional was opted for. All starting geometries are generated based on the experimental lattice parameters of FAPbI3. Self-consistent field calculations and geometry optimizations were used, utilizing a 400 eV cutoff energy for the plane-wave basis sets and a 4 × 4 × 1 gamma- centered k-point mesh was chosen for Brillouin-zones sampling. During the geometry optimizations, the relaxation of ionic positions and cell dimensions was permitted using a conjugate gradient algorithm, enforcing that all residual forces were reduced to less than 0.02 eV / Å. For the calculations involving surfaces and 2D structures, periodic slabs, with a minimum separation of 15-20 Å of vacuum in the direction perpendicular to the surfaces, were used to model the surfaces and 2D perovskites. A (100) direction was used in all surface calculations. Formation energy of 2D perovskites were calculated from: Δ0 = 2^3456 − [92:;<= + 9 / 22@< + 9256<]for BC or DC of units to generate the 2Dperovskite having an Oc unit for each octahedral gap, 2^3456is the energy of the whole complex, 2:;<=is the energy of the FGH^in the hexagonal phase and 2I@<, 2J@<are the energies of DCH and BCH crystals in the rock-salt phase, respectively, and 256<is the energy of KLH in the gas phase. Molecular binding energies to the (100) surface are calculated from: Δ2;MNO = 2PQRS4TUV − [2PQRS + 2TUV]where 2PQRS4TUVis the complex, 2PQRSis the energy of the perovskite slab only and 2TUVis the energy of the molecule only. Solvent molecule and FGH^interactions in the gas phase are calculated from: Δ2;MNO = 2:;<=4TUV − [2:;<= + 2TUV]where 2:;<=4TUVis the complex, 2:;<=and 2TUVare the gas-phase energies of FGH^and solvent molecule. Charge-density difference (CDD) distribution, ΔW, is calculated from: ΔW = WPQRS4TUV − [WPQRS + WTUV] 13920912.1where W = W^X⃗^ is the charge-density distribution; WPQRS4TUV is the charge-density of theperovskite slab and the molecule complex, WPQRSis the charge-density of the perovskite slab only and WTUVis the charge-density of the molecule only. Section 1.4: The role of MA in 2D formation The primary role of the MA additive was to thermodynamically promote 2D(n=2) formation during its initial stage. During crystallization, MA vaporization occurred, following by FA substitution, which finally formed a FA-based 2D(n=2) phase. Looking at the “OcMA:S 2D / 3D” perovskite (FIG.21C), the 2D peak positions closely matched that of 2D “OA2FAPb2X7” (FIG.21B). Note that the measurement uncertainty was ±0.02°. In contrast, the 2D(n=2) peak positions of “OcMA:S 2D / 3D” differed significantly from that of 2D “OA2MAPb2X7” (FIG.21A). For the 2D interlayer, vaporization occurred readily since it is a small amount of MA (3 mM) in the 2D precursor solution. The precursor solution was also deposited to the top film surface, allowing it to escape easily. If any MA is leftover, it is at most trace amounts. For additional evidence, next the “Oc on FAPbI3” 2D / 3D perovskite (FIG.21D) was fabricated. Here, the control Oc 2D perovskite (only Oc ligand in IPA solvent) was deposited on the pure FAPbI33D perovskite. This FAPbI3 contained neither MACl nor MAPbBr3. There was no MA at all in both the 2D and 3D perovskite precursor solutions. The 2D(n=2) perovskite still formed, which demonstrated that FA substitution must have occurred. And this must only be a pure FA-based 2D(n=2) perovskite. Comparing their 2D peaks, the peak positions matched for “Oc on FAPbI3” and “OcMA:S 2D / 3D”. Section 1.5: Image resolution in AFM and KPFM The AFM images in FIG.3F cannot properly resolve the 2D perovskite morphology. This is because the NSC36 cantilever tip was used in non-contact mode imaging to maximize and obtain the best electrical resolution. There is a trade-off between electrical resolution versus morphology / topography resolution. The NSC36 tip has high electrical sensitivity, but low resonance frequency (90 kHz) and low spring constant (1 N m-1), which also means it is not suitable for contact mode imaging. Contact mode imaging (i.e., dynamic force tapping mode) would have given the best morphology resolution. Overall, the goal of the experiments in FIG.3F-3G is to study the electrical potential uniformity, so electrical resolution was maximized. 13920912.1 Section 1.6: Decoupling 2D perovskite versus residual PbI2To study the contributions of the 2D perovskite and residual PbI2, MPP stability tests on 2D / 3D and bare 3D devices were performed, with or without excess PbI2: 1) Oc 2D / 3D excess PbI22) Oc 2D / 3D no excess 3) 3D excess PbI2 4) 3D no excess Firstly, looking at the 2D / 3D devices (FIG.27), the “Oc 2D / 3D no excess” device had lower PCE, but was more stable (T85 = 500 hours) than the “Oc 2D / 3D excess PbI2” device (T74 = 500 hours). This is well-known, where the presence of residual PbI2for the “Oc 2D / 3D excess PbI2” device improved PCE but sacrificially worsened degradation. More importantly, now contributions of the 2D perovskite versus the absence of residual PbI2 on device stability were decoupled. The target “OcMA:S” device (FIG.4D) also had no residual PbI2. Even though its 3D perovskite used the excess PbI2 composition, no unreacted PbI2 was leftover after 2D formation. The target “OcMA:S” device (T96 > 1,700 hours) was by far more stable (and higher PCE) than the “Oc 2D / 3D no excess” device. Their huge contrast is obvious when the devices are plotted together (FIG.4D). This implies that the absence of residual PbI2 only contributed minorly. If the absence of residual PbI2 was the major reason, then the “Oc 2D / 3D no excess” and “OcMA:S” devices would be expected to have similar stability, which is certainly not the case. In other words, the 2D perovskite was still the dominant reason that stabilized the OcMA:S device, while the absence of residual PbI2 only contributed partially. All 2D / 3D devices were more stable (and higher PCE) than all bare 3D devices (FIG.27). The “3D excess PbI2” device was the most unstable among all devices, with a severe initial burn-in, where the PCE rapidly dropped to 73% of its initial PCE in the first 30 hours. The “3D no excess” device – even with no excess PbI2at all – only improved barely. The initial burn-in was still severe (82% retained in first 30 hours) – showing that the burn-in is not caused by the residual PbI2 – and the device was still unstable overall (T67 = 500 h). This again shows that although the absence of residual PbI2did improve stability, the 2D perovskite had a much more significant contribution. 13920912.1 Rapid defect migration is responsible for the burn-in effect. Especially, none of the 2D / 3D devices experienced the rapid initial burn-in (FIG.27). This shows the effectiveness of the 2D interlayer to passivate defects and suppress defect migration, functioning like a blocking barrier. But if the 2D interlayer is disappearing, defect passivation is slowly lost and defect migration occurred over a longer duration – essentially, the “burn-in” is no longer rapid, but became stretched out and turned into the long-term degradation experienced by the 2D / 3D device. For the target OcMA:S device, its unchanging 2D interlayer was robust and structurally intact during aging. This was important to stabilize the OcMA:S device. This is why it is critically important to make sure that the 2D interlayer is not lost during aging. While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and / or structures for performing the functions and / or obtaining the results and / or one or more of the advantages described herein, and each of such variations and / or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and / or configurations will depend upon the specific application or applications for which the teachings of the present invention is / are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and / or methods, if such features, systems, articles, materials, and / or methods are not mutually inconsistent, is included within the scope of the present invention. The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements 13920912.1 may optionally be present other than the elements specifically identified by the “and / or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and / or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non- limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and / or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another 13920912.1 embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. As used herein, “wt%” is an abbreviation of weight percentage. As used herein, “at%” is an abbreviation of atomic percentage. Some embodiments may be embodied as a method, of which various examples have been described. The acts performed as part of the methods may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include different (e.g., more or less) acts than those that are described, and / or that may involve performing some acts simultaneously, even though the acts are shown as being performed sequentially in the embodiments specifically described above. Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03. 13920912.1

Claims

CLAIMS What is claimed is:

1. An article, comprising: a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3; and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a ratio of the area under the largest x-ray diffraction (XRD) peak by area associated with the 2D perovskite to the area under the largest XRD peak by area associated with the 3D perovskite is greater than or equal to 50%.

2. An article, comprising: a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3; and a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite, wherein a photoluminescence carrier lifetime of the article is greater than or equal to 1000 nanoseconds.

3. The article of claim 1 or 2, wherein the article is included within a solar cell.

4. The article of claim 3, wherein the solar cell further includes a substrate, an electron transporting layer, and / or a hole transporting layer.

5. The article of claim 4, wherein solar cell comprises the hole transporting layer and the 2D perovskite is positioned between the 3D perovskite and a hole transporting layer.

6. A solar cell, comprising: a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3; 13920912.1a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite; a hole transporting layer; and an electron transporting layer, wherein: the 2D and 3D perovskites, the hole transporting layer, and the electron transporting layer are arranged in a regular n-i-p structure, the solar cell has an initial power conversion efficiency (PCE) of at least 18% when operated at 85 degrees C, and after exposure to full-spectrum 1-sun AM 1.5G illumination for 1000 hours at 85 degrees C, the solar cell has a secondary PCE that is at least 89% of the initial PCE.

7. The solar cell of claim 6, wherein the 3D perovskite and the 2D perovskite together and alone provide the PCE.

8. The article or solar cell of any one of claims 2-7, wherein a ratio of the area under the largest x-ray diffraction (XRD) peak by area associated with the 2D perovskite to the area under the largest XRD peak by area associated with the 3D perovskite is greater than or equal to 50%.

9. A method, comprising: exposing a three-dimensional (3D) perovskite having a thickness of greater than or equal to 200 nm and a volume of greater than or equal to 150 µm3to a solution comprising a first salt, a second salt, a first solvent, and a second solvent, wherein: the first and second salts are soluble in the first solvent; and at least a portion of the 3D perovskite is dissolved by the second solvent such that a cation of the first and / or second salt and at least a portion of the dissolved 3D perovskite form a two-dimensional (2D) perovskite positioned adjacent to at least a portion of the 3D perovskite. 13920912.

110. The method of claim 9, further comprising heating the 3D perovskite.

11. The method of claim 9 or 10, wherein the second solvent comprises propylene carbonate, acetonitrile, methyl glycol, dimethylformamide, 1-methyl-2-pyrrolidone, and / or dimethyl sulfoxide.

12. The method of any one of claims 9-11, wherein the first solvent comprises isopropanol.

13. The method of any one of claims 9-12, wherein the second salt comprises formamidinium iodide, formamidinium bromide, formamidinium chloride, methylammonium iodide, methylammonium bromide, and / or methylammonium chloride.

14. The method of any one of claims 9-13, wherein the first salt comprises n- octylammonium bromide, phenethylammonium iodide, and / or n-butylammonium bromide.

15. The article, solar cell, or method of any one of the preceding claims, wherein the 3D perovskite comprises a formamidinium lead iodide composition.

16. The article, solar cell, or method of any one of the preceding claims, wherein the 2D perovskite comprises n-octylammonium methylammonium lead halide.

17. The article, solar cell, or method of any one of the preceding claims, wherein the thickness of the 3D perovskite is greater than or equal to 500 nm.

18. The article, solar cell, or method of any one of the preceding claims, wherein the volume of the 3D perovskite is greater than or equal to 300 µm3.

19. The article, solar cell, or method of any one of the preceding claims, wherein a ratio of the area under the largest XRD peak by area associated with an impurity to the area under the largest XRD peak by area associated with the 2D perovskite is less than or equal to 25%. 13920912.

120. The article, solar cell, or method of any one of the preceding claims, wherein a ratio of the area under the largest XRD peak by area associated with an impurity to the area under the largest XRD peak by area associated with the 2D perovskite is less than or equal to 0.01%.

21. The article, solar cell, or method of any one of the preceding claims, wherein the 2D perovskite comprises an n = 2 phase.

22. The article, solar cell, or method of any one of the preceding claims, wherein the 2D perovskite consists essentially of an n = 2 phase.

23. The article, solar cell, or method of any one of the preceding claims, wherein the 2D perovskite consists essentially of an n = 1 phase, an n = 2 phase, an n = 3 phase, and / or an n = 4 phase.

24. The article, solar cell, or method of any one of the preceding claims, wherein the 3D perovskite has an ABX3structure, wherein: A is a cation comprising formamidinium, cesium, and / or methylammonium; B is a cation comprising lead (Pb), tin (Sn), and / or germanium (Ge); and X is a halide.

25. The article, solar cell, or method of any one of the preceding claims, wherein the 2D perovskite comprises an n = 2 phase that has an L2AB2X7, wherein: L is a capping ligand; A is a cation comprising formamidinium, cesium, and / or methylammonium; B is a cation comprising lead (Pb), tin (Sn), and / or germanium (Ge); and X is a halide.

26. The article, solar cell, or method of claim 24 or 25, wherein the halide comprises chloride, bromide, and / or iodide. 13920912.

127. The article, solar cell, or method of claim 25 or 26, wherein the capping ligand comprises n-octylammonium, phenethylammonium, and / or n-butylammonium. 13920912.1

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