Solution-processed perovskite heterostructure

Abstract

The solution-processed perovskite heterostructure includes a three-dimensional (3D) perovskite layer and a two-dimensional (2D) perovskite layer, and the perovskite solar cell includes the solution-processed perovskite heterostructure. A 2D perovskite seed solution containing a two-dimensional (2D) perovskite and an aprotic polar solvent is provided, the 2D perovskite seed solution is laminated on the three-dimensional (3D) perovskite layer to form a 3D / 2D bilayer, and the 3D / 2D bilayer is annealed such that the aprotic polar solvent evaporates to form a perovskite heterostructure film.

Description

Technical Field

[0001] This invention was made with government support under grant number DE-EE0008843 awarded by the Department of Energy. The government has certain rights in this invention.

[0002] <Cross - Reference to Related Applications> This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 365,822, filed on June 3, 2022, which is incorporated herein by reference.

Summary of the Invention

Means for Solving the Problems

[0003] This summary is provided to introduce some concepts that are further described in the following detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0004] In one aspect, embodiments disclosed herein relate to a method that includes providing a 2D perovskite seed solution comprising a 2D perovskite and a polar aprotic solvent, laminating the 2D perovskite seed solution onto a 3D perovskite layer to form a 3D / 2D bilayer, and annealing the 3D / 2D bilayer such that the polar aprotic solvent evaporates to form a perovskite heterostructure film.

[0005] In another aspect, embodiments disclosed herein relate to a perovskite solar cell comprising a solution - processed perovskite heterostructure comprising a 3D perovskite layer and a 2D perovskite layer, wherein the phase purity of the 2D perovskite layer ranges from 90% to 95%.

[0006] Other aspects and advantages of the claimed subject matter will become apparent from the following description and the appended claims.

Brief Description of the Drawings

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BRIEF DESCRIPTION OF THE EMBODIMENTS

[0026] Heterostructures are components of advanced semiconductor devices that are being developed and produced. General techniques used to develop these heterostructures include molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and atomic layer deposition (ALD). However, the use of these techniques in the development of halide perovskite heterostructures is still in development because strict temperature and atmosphere control are required for the growth of the heterostructures. In recent years, attempts have been made to fabricate small-scale heterostructures including halide perovskites using mechanical exfoliation and transfer methods. However, the use of solution processing techniques for fabricating perovskite / perovskite heterostructures has not been successful due to solvent incompatibility issues.

[0027] Embodiments of the present disclosure generally relate to two-dimensional (2D) perovskite heterostructures. The 2D perovskite heterostructures described herein may include 2D perovskites laminated on a suitable substrate. Such 2D perovskite heterostructures can have sharp interfaces and thus can be used in optoelectronic devices and applications in advanced electronic / spectroscopic research. Here, a sharp interface may refer to a transition region from a 2D region to a 3D region (or vice versa). Such an interface can have a size obtained by forming a transition region by hard fabrication using solution processing.

[0028] In one aspect, embodiments disclosed herein relate to a method of fabricating a solution-processed perovskite heterostructure comprising a substrate layer and a 2D perovskite layer. The 2D perovskite layer can have a crystalline structure and high phase purity. The method can enable precise control of the phase and composition of the substrate layer and the 2D perovskite layer at any thickness that has not been achieved previously. In certain embodiments, the substrate is a 3D perovskite. 3D perovskites generally have the formula: A x B y X z (wherein A is a small monovalent cation, B is a divalent metal, and X is a monovalent anion).

[0029] In another aspect, embodiments disclosed herein relate to a perovskite solar cell comprising a solution-processed perovskite heterostructure. The solution-processed perovskite heterostructure can include a phase-pure 2D perovskite laminated on a 3D perovskite. Here, "phase-pure 2D perovskite" refers to a 2D perovskite having at least 90% of a single n value. Such a perovskite heterostructure can have improved stability compared to conventional heterostructures known in the art.

[0030] In one or more embodiments, the method includes a novel solvent design principle for fabricating a solution-processed heterostructure of a 3D perovskite layer and a 2D perovskite layer having high-quality interfaces and any film thickness. The disclosed solvent design principle enables control of the n value and phase of the 2D perovskite layer. In contrast to using organic cations to synthesize mixed-layer perovskites (mainly with n ≤ 2) in situ on the 3D layer, the method uses high-purity 2D perovskite powder to fabricate a mesoscopic heterostructure. The method can utilize two important solvent properties of the processing solvent, the dielectric constant (ε) and the Gutmann number (DN), which control the coordination of the precursor ions with the solvent used.

[0031] The method according to the present disclosure includes providing a 2D perovskite seed solution. The seed solution may include a 2D perovskite and a processing solvent. In one or more embodiments, the 2D perovskite layer is L’A n-1 B n X 3n+1 and includes a 2D perovskite having the general formula L’A x B y X z wherein L’ is a long-chain organic cation, A is a small monovalent cation, B is a divalent metal, X is a monovalent anion, and n is the number of octahedra in the quantum well (which can also be called the layer thickness). In one or more embodiments, n has a value in the range of 1 to 7. In certain embodiments, n is 4 or less. In one or more embodiments, a phase-pure film is represented by the above general formula for a single n value. Suitable 2D perovskites can be halide perovskites such as, among others, Ruddlesden-Popper 2D perovskites, Dion-Jacobson 2D perovskites, alternating cation 2D perovskites, and combinations thereof. For example, suitable 2D perovskites can include the compounds listed in Table 1.

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[0032] In one or more embodiments, the 2D perovskite layer may comprise 2D perovskite formed from single crystal powder. The powder may include crystals having a single n value and a size range of micrometers to millimeters. Here, the single crystal powder is also called the parent crystal. In one or more embodiments, the parent crystal having a desired n value is crystallized from a set of precursor materials. Suitable precursor materials include lead iodide (PbI2), butylammonium iodide (BAI), methylammonium iodide (MAI), butylamine (BA), methylamine (MA), and combinations thereof. In one or more embodiments, the parent crystal has a high purity phase of a single n value in the range of 90-95% as measured by X-ray diffraction.

[0033] In one or more embodiments, 2D perovskite films are fabricated using 2D single-crystalline powders. In this embodiment, phase-pure parent crystals can then be dissolved in a suitable solvent at high temperature. In one or more embodiments, the high temperature ranges from 60 to 100 °C, and the suitable solvent is a polar aprotic solvent. In certain embodiments, the high temperature is 70 °C and the suitable solvent is dimethylformamide (DMF). The solution containing the 2D single-crystalline powder can be processed using techniques such as spin-casting, doctor-blading, drop-casting, and drop-die coating, and then annealed to provide a 2D perovskite film.

[0034] In one or more embodiments, heterostructures are fabricated using phase-pure 2D parent crystals. In this embodiment, the single crystals are dissolved in a processing solvent to provide a 2D perovskite seed solution. The dielectric constant (ε) of the solvent is a strong selection criterion for choosing a processing solvent for ionic solids. Additionally, recent reports have shown that Lewis acid-base interactions play a major role in selecting a processing solvent for halide perovskites. These are two different solvent properties, but the coordinating ability and dielectric constant of the solvent are related. The Lewis basicity of the processing solvent, defined by the Gutmann donor number (DN), competes with I - ions to prevent the formation of lead iodide salts (PbI n 2-n (n is 2 - 7)) and represents the strength of the solvent that coordinates with divalent metals (e.g., Pb 2+ or Sn 2+ ). In the case of crystalline perovskite compounds or powders (3D or 2D), the strength of the solvent coordination causes the structure to decompose and soluble complexes to form in the solution.

[0035] Figure 1A classifies and summarizes the ε and DN values of various processing solvents according to their ability to form a stable 2D perovskite dispersion while leaving the 3D perovskite intact. Non-polar solvents such as ether, chloroform (CHF), and chlorobenzene (CBZ) do not dissolve either 3D or 2D perovskites. Hydrogen-bonding polar protic solvents such as ethanol, isopropanol, and water dissolve perovskites slowly due to the difference in solubility of organic cations and metal halides. Therefore, stable perovskite solutions may be difficult to form with these solvents.

[0036] Generally, in agreement with previous reports, most polar aprotic solvents with ε > 30 such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) form stable 2D and 3D perovskite solutions, except for some exceptions in solubility that seem reasonable considering the DN of the solvent. Polar aprotic solvents with ε > 30 such as acetonitrile (ACN), tetramethyl sulfone (TMS), propylene carbonate (PC), and ethylene carbonate (EC) do not dissolve 3D perovskite compounds, probably due to weak Lewis acid-base interactions corresponding to a DN value of less than 18 kcal / mol. However, these solvents form stable 2D perovskite solutions containing 2D-phase seeds. An explanation is that the solvent extracts the interlayer organic molecules, partially destroying the 2D lattice.

[0037] Therefore, suitable processing solvents can have a dielectric constant ε > 30 and a Gutmann number 5 < DN < 18, because such properties enable effective dissolution of 2D perovskite powder without dissolving substrates such as 3D perovskites on which 2D perovskites can be stacked. In one or more embodiments, the 2D seed solution contains ACN, TMS, PC, EC, or a combination thereof as the processing solvent.

[0038] In one or more embodiments, the processing solvent also has a low boiling point, enabling uniform growth and rapid evaporation of the 2D perovskite layer, thereby avoiding diffusion or decomposition of the underlying 3D perovskite layer. For example, a suitable processing solvent may have a boiling point of 100 °C or lower. In certain embodiments, the processing solvent can be ACN.

[0039] In one or more embodiments, a method of fabricating a mesoscopic 3D / 2D heterostructure includes growing seed crystals from a precursor material. The seed crystals are dissolved in a suitable solvent to prepare a 2D perovskite seed solution. A 2D perovskite seed solution is used to fabricate a 2D heterostructure by spin-casting and annealing.

[0040] Figure 1B shows a method of fabricating a mesoscopic 3D / 2D heterostructure according to one or more embodiments. As shown in Figure 1B, a 2D perovskite seed solution 103 is dispersed in ACN and then dynamically spin-cast onto a control 3D perovskite layer 102 to form a 3D / 2D bilayer including a 3D perovskite layer 102 and a 2D perovskite layer 104. The 3D perovskite layer can be disposed on a substrate layer 101. Thereafter, the bilayer can be annealed at a high temperature for a certain period of time. For example, the bilayer can be annealed at a high temperature in the range of 60 °C to 100 °C for a certain period of time in the range of 1 to 10 minutes. In certain embodiments, the bilayer is annealed at 80 °C for 5 minutes to provide a perovskite heterostructure according to the present disclosure. This process can be compatible with other large-scale thin-film processing techniques such as the doctor blade method, the drop-cast method, and the slot-die coating method. The method of the present disclosure can enable control of the thickness and phase purity of the 2D perovskite layer in the perovskite heterostructure of one or more embodiments. For example, a perovskite heterostructure fabricated according to the foregoing method may include a 2D perovskite layer having a single n-value phase purity of at least 90%. Further, the thickness of the 2D perovskite layer in the perovskite heterostructure of one or more embodiments can be in the range of 1 nm to 1 μm.

[0041] In one or more embodiments, the perovskite heterostructure may have an interfacial transition between the 3D perovskite layer and the 2D perovskite layer in the range of 15 - 25 nm.

[0042] In one or more embodiments, the perovskite heterostructure may exhibit properties desirable for use in solar cells. In one or more embodiments, the solar cell device includes a substrate, an electron transport layer, a 2D perovskite film, a hole transport layer, and an indium-doped tin oxide layer. The film may be a Rudolphsen-Popper film. The solar cell device may be a Dion-Jacobson device using a 2D perovskite film.

[0043] For example, a perovskite solar cell including the disclosed solvent-treated perovskite heterostructure film has a 2D BA2MA2Pb3I with a thickness of 50 nm 10 In a normal n-i-p device with a glass / ITO / SnO2 / 3D-2D perovskite / Spiro-MeOTAD / Au covered by perovskite, it may have an efficiency of 24.5% and exhibit a high open-circuit voltage (VOC) of 1.2 V. In certain embodiments, the presence of the 2D heterostructure layer improves charge transport properties due to the presence of appropriate band alignment, and energy transfer is improved by the luminescence of the 2D perovskite layer, resulting in an overall improvement in efficiency. Additionally, in a perovskite solar cell including the exemplary perovskite heterostructure described above, T 99 > A high ISOS-L1 optical stability of > 2000 hours can be observed, which means that this 3D / 2D heterostructure film exceeds the stability of 2D perovskite, while a standard 3D / 2D classical passivated PSC retains only 90% of the initial PCE after 1000 hours. T 99 refers to the percentage of the efficiency of the solar cell retained after a durability test for a certain period of time. Here, T 99 > 2000 hours means that under continuous irradiation using simulated sunlight, the photovoltaic device retained more than 99% of its original efficiency after 2000 hours.

[0044] The perovskite heterostructures disclosed herein can also be used in applications such as bifacial tandem solar cells, light-emitting diodes, and photocatalysts.

[0045] Embodiments of the present disclosure can provide at least one of the following advantages. Existing techniques for fabricating heterostructures of halide perovskites are under development due to stringent temperature and processing requirements and solvent incompatibility. Existing techniques use organic cations to directly synthesize mixed layered perovskites in situ on 3D layers. In contrast, the present disclosure fabricates heterostructures using high-purity 2D perovskite powders by selecting an optimized solvent that effectively dissolves the 2D perovskite but not the underlying 3D layer. This technique enables the growth of various phases of 2D perovskite layers with different stacking thicknesses (n = 1 - 4), resulting in controlled band alignment from type I to type II heterojunctions and effective charge transport properties. The 2D perovskite layer exhibits high crystallinity comparable to films or crystals grown directly on glass / quartz substrates. The disclosed method controls the thickness of the underlying 3D layer and the overlying 2D layer by maintaining high interface quality. The 2D perovskite layer can be deposited on various different 3D perovskite materials such as methylammonium lead iodide and formamidinium lead iodide. This technique is compatible with other large-scale thin-film processing techniques such as doctor blade method, drop casting method, and slot die coating method, and can demonstrate scalability of the approach.

[0046] Here, “includes” is an open term that includes, but is not limited to, ~, and the same applies to its variants such as “including”.

Example

[0047] Example 1: Synthesis of Ruddlesden-Popper 2D Perovskite (n = 3) Lead oxide, methylamine hydrochloride (MACl), and butylamine (BA) were dissolved in a hydroiodic acid / hypophosphorous acid aqueous solution at 190 °C in an appropriate ratio and boiled. The solution was allowed to cool, and 2D perovskite crystals of BA2MA2Pb3I 10 were obtained.

[0048] Example 2: Synthesis of Ruddlesden-Popper 2D perovskite (n = 4) Lead oxide, methylamine hydrochloride, and butylamine were dissolved in a hydroiodic acid / hypophosphorous acid aqueous solution at 190 °C in an appropriate ratio and boiled. The solution was allowed to cool, and 2D perovskite crystals of BA2MA3Pb4I 13 were obtained.

[0049] Example 3: Synthesis of Dion-Jacobson 2D perovskite (n = 4) Lead oxide, methylammonium iodide, and 4-aminomethylpiperidine (4AMP) were dissolved in a hydroiodic acid / hypophosphorous acid aqueous solution at 240 °C and boiled. The solution was allowed to cool, and 2D perovskite crystals of (4AMP)-MA2Pb3I 10 were obtained.

[0050] Example 4: Synthesis of perovskite heterostructure film One of the 2D perovskites of Examples 1 to 3 was dissolved in an appropriate solvent or solvent solution at 70 °C for 6 hours to prepare a solution. The solutions of each 2D perovskite were prepared at 0.4 M using DMF, DMSO, DMF:DMSO (1:1), DMF:DMSO (1:1) with 1 μL of HI added thereto, and DMF-5 wt% MACl, respectively. 100 μL of the prepared 2D perovskite solution was dropped onto a substrate rotating at 4000 rpm, rotated for 30 seconds, and then heated at 100 °C to fabricate a thin film.

[0051] Example 5: Synthesis and testing of BA2MA2Pb3I 10 2D perovskite film Figure 2A shows the method used to fabricate a test mesoscopic 2D heterostructure according to the procedures of Examples 1 and 4. Figure 2A further shows a comparative conventional method (also called the "classical method") for fabricating the film. In this method, as shown in Figure 2A, seed crystals were grown from the precursor materials PbO, BAI, and MAI. The 2D perovskite seed solution was prepared by dissolving the seed crystals of BA2MA2Pb3I 10 in a suitable solvent. Using the 2D perovskite seed solution, a 2D heterostructure was fabricated by spin-casting and annealing. The optical absorption measurements of the 2D perovskite seed solution showed absorption edges of 2.1 eV at n = 2, 2.0 eV at n = 3, and 1.85 eV at n = 4, and the phase purity was 90% or even 95%. In the comparative classical method for fabricating the 2D perovskite thin film, the precursor materials (e.g., PbI2, BAI, and MAI) were dissolved in an appropriate amount for the desired n value for 12 hours, spin-cast onto a substrate, and annealed at 100 °C.

[0052] Figure 2B shows a schematic diagram of the formation of a 2D perovskite layer on a substrate consisting of a controlled regular 2D perovskite seed growth layer formed on the substrate by this method and the formation of a disordered layer formed by the comparative conventional method. The 2D perovskite film of BA2MA2Pb3I 10 was characterized by scanning electron microscopy and atomic force microscopy. From the results, it became clear that this method produced micrometer-sized regular particles, while the comparative conventional method produced a disordered wire-like morphology.

[0053] The change in the phase-pure film was monitored over time by X-ray diffraction. The phase ratio can be obtained by integrating each phase with respect to the total integrated diffraction as shown in Figures 2C and 2D. Figure 2C shows evidence of controlled growth compared to the formation of a disordered layer by the conventional method shown and evident in Figure 2D. The integration of the diffraction pattern was measured at room temperature for up to 80 minutes to slow down the nucleation and film formation rates.

[0054] By grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements, BA2MA2Pb3I10 It was found that the phase purity in the film was about 90%. In the comparative film produced by the conventional method, it was found that the distribution of the desired phase and phase impurities was estimated to be equal. In the film according to one or more embodiments, peaks corresponding to the 010, 101, 100 and / or 001 crystal planes were shown.

[0055] The average particle size or correlation length shown in FIGS. 3A and 3B was obtained by diffraction techniques. Dynamic light scattering measurements of the particle size in the 2D perovskite seed solution were performed, whereby the particle size was confirmed. The particle size distribution in the phase selection method showed a particle size of about 1 nm or less and about 100 - 400 nm, or a bimodal distribution of about 200 nm. In the classical method of film production, a unimodal particle size distribution of about 1 nm or less was obtained.

[0056] Example 6: NiO x Manufacture of perovskite solar cells The indium-doped tin oxide substrate was ultrasonically cleaned for more than 15 minutes each with water, acetone, acetone / ethanol (50:50) and isopropyl alcohol. Then, the substrate was dried under argon and UV-treated for 30 minutes. A solution prepared by adding monoethylamine to an ethanol solution of nickel(II) acetate tetrahydrate was spin-coated at 5000 rpm for 30 seconds to deposit the NiO x layer. Under argon, the solution according to Example 4 was spin-coated onto a substrate rotating at 4000 rpm, rotated for 30 seconds, and then heated at 100 °C to deposit the 2D perovskite layer. Then, the PCBM solution was spin-coated and deposited at 1000 rpm for 45 seconds. An aluminum layer was vapor-deposited using a shadow mask to obtain eight cells.

[0057] Example 7: Manufacture of PEDOT:PSS perovskite solar cells The indium-doped tin oxide substrate was ultrasonically cleaned in water, acetone, acetone / ethanol (50:50), and isopropyl alcohol for more than 15 minutes each. Then, the substrate was dried under argon and UV-treated for 30 minutes. The PEDOT:PSS layer was deposited by spin-coating at 5000 rpm for 30 seconds. Under argon, the solution according to Example 4 was spin-coated onto a substrate rotating at 4000 rpm, rotated for 30 seconds, and then heated at 100 °C to deposit the 2D perovskite layer. Thereafter, the PCBM solution was spin-coated and deposited at 1000 rpm for 45 seconds. An aluminum layer was vapor-deposited using a shadow mask to obtain eight cells.

[0058] Example 8: Fabrication and testing of perovskite solar cells Figure 4A shows a solar cell device prepared for testing using the procedure of Example 6, and includes an aluminum substrate 401, a PCBM electron transport layer 402, a 2D perovskite film 403 of BA2MA2Pb3I 10 as a device including a NiO hole transport layer 404 and an indium-doped tin oxide layer 405. A Dion-Jacobson device using a 2D perovskite film 403 of 4AMP-MA3Pb4I 13 was also fabricated. The current-voltage characteristics of the solar cell device were measured using a Newport ABB solar simulator (Figures 4B - E). In Figure 4B, the device uses a Rudolph-Den-Popper (BA) and Dion-Jacobson (3AMP) n = 3 thin film prepared by the phase selection method and a PEDOT:PSS HTL layer. The solar cell device described in 4A has advantageously high current density, J SC , V OC , fill factor, and peak current efficiency as shown in Figures 4C - E. Figure 4E shows the average stability of four devices of a Rudolph-Den-Popper (BA) solar cell with n = 4 under continuous 1 sun illumination and 60 ± 5% RH for devices with an average efficiency (16.0 ± 0.98%) by the phase selection method compared to the stability measured for a comparative thin-film solar cell with an average efficiency (10.58 ± 1.4%) fabricated by the classical method. Figure 5 has the structure described above with respect to Figure 4A and BA2MA3Pb4I13 The Rudolphusden-Popper (n = 4) champion device using a 2D perovskite layer and NiO as the HTL layer shows a stabilized efficiency of 17.0% measured at the maximum power point of 0.99. The champion device has a J SC = 17.56 mA / cm² -2 , V OC = 1.20 V, a fill factor of 81.1%, and a power conversion efficiency of 17.1%. The external quantum efficiency of the solar cell device was measured using a 2 kHz quartz-tungsten-halogen source (Figs. 6A - B). The external quantum efficiency of the solar cell device is consistent with the J-V characteristics of the Rudolphusden-Popper n = 3 BA₂MA₂Pb₃I 10 device and the n = 4 BA₂MA₃Pb₄I 13 device.

[0059] The photovoltaic parameters of the solar cell devices fabricated by the present phase selection method and the comparative conventional method are shown in Table 2.

Table 2

[0060] Example 9: Synthesis of Perovskite Heterostructure Films Any one of the 2D perovskites of Examples 1 - 3 was dissolved in acetonitrile to prepare a solution. 70 μL of the prepared 2D perovskite solution was dropped onto a 3D perovskite substrate rotating at 4000 rpm, rotated for 200 seconds, and then heated at 80 °C for 3 - 5 minutes to fabricate a thin film. The stability of the 2D perovskite film on the 3D perovskite substrate in the ITO / SnO₂ / 3D / 2D laminate was studied for the 2D perovskite film of BA₂MA₂Pb₃I 10 and Cs₅(Ma₀.₁₀FA 0.90 ) 95 Pb(I₀.₉₀Br 0.10)For devices containing a 3D perovskite layer of 3, it is shown in Figure 7 in comparison with the passivated 3D / 2D perovskite heterostructure and control samples of 2D and 3D perovskites. Stability was measured at the maximum power point at 55 °C under ambient conditions and continuous 1 sun illumination for epoxy-encapsulated solar cell devices. The initial PCE was 21% for the control, 22.93 for the passivated 3D / 2D perovskite heterostructure, 23.75% for the 3D / 2D perovskite bilayer device, and 16.3% for the 2D perovskite. The thickness of the 2D perovskite film on the 3D perovskite substrate was varied by changing the concentration of the 2D perovskite seed solution of BA2MA2Pb3I 10 When the thickness of the 2D perovskite layer increased from 0 to 50 nm, VOC increased from 1.09 to 1.2 V, the fill factor increased from 0.80 to 0.84, JSC increased from 23.54 to 24.34 mA.cm-2, and as a result, the PCE was 24.5% at a 2D perovskite thickness of 50 nm. As shown in Figure 9, the increase in the thickness of the 2D layer corresponded to an increase in the surface photovoltage measurements taken using scanning Kelvin probe microscopy for devices containing the 2D perovskite film of BA2MA2Pb3I 10 .

[0061] The phase purity of the 3D / 2D perovskite heterostructure was characterized by X-ray diffraction, optical absorption, and photoluminescence. Optical absorption measurements yielded exciton peaks from 2.4 eV (n = 1) to 1.9 eV (n = 4). Photoluminescence measurements showed uniform emission from both the 2D perovskite layer and the 3D perovskite substrate.

[0062] Grazing incidence wide-angle X-ray spectroscopy confirmed the uniform layer growth of the 2D perovskite film on the 3D perovskite substrate layer. The diffraction pattern showed the oriented 2D perovskite diffraction pattern.

[0063] Example 10: Fabrication of Perovskite Solar Cells The glass / FTO substrate was ultrasonically cleaned with soap, water, acetone, and acetone / ethanol (50:50) for more than 15 minutes each. Then, the substrate was air-dried and UV-treated for 30 minutes. After spin-coating with SnO2 colloidal solution at 5000 rpm for 30 seconds, it was heated at 150 °C for 30 minutes to deposit the SnO2 film. The substrate was UV-ozone treated for 15 minutes. A DMF solution of lead iodide, formamidinium iodide, lead bromide, and methylammonium bromide was mixed with a DMSO solution of cesium iodide to prepare a triple-cation perovskite solution (TC) (Cs5(Ma0.10FA 0.90 ) 95 Pb(I0.90Br 0.10 )3). The TC solution was spin-coated and annealed at 100 °C for 30 - 40 minutes to deposit it on the substrate. Any of the 2D perovskites of Examples 1 - 3 was dissolved in acetonitrile and spin-coated on the substrate. Then, Spiro-MeOTAD was spin-coated on the substrate using a chlorobenzene solution containing Li-TFSI / acetonitrile and tBP. An Au layer was deposited.

[0064] Example 11: Fabrication of Inverted Planar Perovskite Solar Cells The glass / FTO substrate was ultrasonically cleaned with soap, water, acetone, and acetone / ethanol (50:50) for 15 minutes each. Then, the substrate was air-dried and UV-treated for 30 minutes. A poly(triarylamine) layer was spin-coated using chlorobenzene and annealed at 150 °C for 10 minutes. A solution prepared by dissolving PbI2, FAI, MABr, PbBr2, and CsI in DMF:DMSO (4:1), namely Cs5(Ma0.10FA 0.90 ) 95 Pb(I0.90Br 0.10 )3 was spin-coated and annealed at 100 °C for 30 - 40 minutes to deposit the TC layer. Any of the 2D perovskites of Examples 1 - 3 was dissolved in acetonitrile and spin-coated on the substrate. Layers of C60, BCP, and copper were thermally deposited.

[0065] Although the above has described in detail only a few exemplary embodiments, those skilled in the art will readily understand that many modifications can be made to the exemplary embodiments without substantially departing from the present invention. Thus, all such modifications are intended to be included within the scope of the present disclosure as defined by the following claims. In the claims, the means-plus-function clauses are intended to cover not only structures described herein as performing the recited functions but also structural equivalents as well as equivalent structures. Thus, although a nail and a screw are not structural equivalents in that a nail uses a cylindrical surface to fasten wooden parts together while a screw uses a helical surface, in the context of fastening wooden parts, a nail and a screw are equivalent structures. As an express intention of the applicant, 35 U.S.C. § 112(f) is not invoked for any limitation in the claims of this patent, except when the claim explicitly uses the term "means for" in connection with the associated function.

Claims

**Claim 1** Providing a 2D perovskite seed solution comprising a 2D perovskite and a polar aprotic solvent; Laminating the 2D perovskite seed solution onto a 3D perovskite layer to form a 3D / 2D bilayer; and Annealing the 3D / 2D bilayer such that the polar aprotic solvent evaporates to form a perovskite heterostructure film A method comprising. **Claim 2** The above 2D perovskite has the general formula L’A ｎ＋１ B ｎ X ３ｎ＋１ wherein L’ is a long-chain organic cation, A is a small monovalent cation, B is a divalent metal, X is a monovalent anion, and n is the number of octahedra in the quantum well, the method according to claim 1. **Claim 3** The method according to claim 2, wherein n is 4 or less. **Claim 4** The method according to claim 1, wherein the 2D perovskite is a 2D halide perovskite selected from the group consisting of a Rudorff-Popper 2D perovskite, a Dion-Jacobson 2D perovskite, an alternating cation 2D perovskite, and combinations thereof. **Claim 5** The method according to claim 1, wherein the 2D perovskite is a single crystal powder having a crystal size in the range of micrometers to millimeters. **Claim 6** The method according to claim 5, further comprising crystallizing the single crystal powder such that the phase purity of the desired n value of the single crystal powder is at least 90% as measured by one or more of X-ray diffraction and light absorption before providing the 2D perovskite seed solution. **Claim 7** The method according to claim 1, wherein the polar aprotic solvent has a dielectric constant (ε) of 30 or more. **Claim 8** The method according to claim 1, wherein the polar aprotic solvent has a Gutmann donor number (DN) in the range of 5 to 18 kcal / mol. **Claim 9** The method according to claim 1, wherein the polar aprotic solvent has a boiling point of 100° C. or less. **Claim 10** The method according to claim 1, wherein the 2D perovskite is soluble in the polar aprotic solvent and the 3D perovskite is insoluble in the polar aprotic solvent. **Claim 11** The method according to claim 1, wherein the polar aprotic solvent is selected from the group consisting of acetonitrile, tetramethylene sulfone, polypropylene carbonate, ethylene carbonate, and combinations thereof. **Claim 12** The method according to claim 8, wherein the polar aprotic solvent is acetonitrile. **Claim 13** The method according to claim 1, wherein laminating the 2D perovskite seed solution includes implementing a technique selected from the group consisting of a spin-casting method, a doctor-blade method, a drop-casting method, a drop-die coating method, and combinations thereof.

14. The method according to claim 1, wherein the perovskite heterostructure film includes a 2D perovskite layer having a desired n-value phase purity in the range of 90 to 95%.

15. The perovskite heterostructure film is T ９９ > The method according to claim 1, having a stability of more than 2000 hours.

16. A perovskite solar cell including a solution-processed perovskite heterostructure including a three-dimensional (3D) perovskite layer and a two-dimensional (2D) perovskite layer, wherein the phase purity of the 2D perovskite layer is in the range of 90 to 95%.

17. The perovskite solar cell according to claim 16, wherein the solution-processed perovskite heterostructure has an interfacial transition between the 3D perovskite layer and the 2D perovskite layer in the range of 15 to 25 nm.

18. The perovskite solar cell according to claim 16, wherein the thickness of the 2D perovskite layer is in the range of 1 nm to 1 μm.

19. The perovskite solar cell according to claim 16, wherein the 2D perovskite layer is highly crystalline.

20. The above perovskite solar cell is T ９９ > The perovskite solar cell according to claim 16, which has a stability of more than 1500 hours.

21. Providing a 2D perovskite seed solution including a two-dimensional (2D) perovskite and an aprotic polar solvent; Laminating the 2D perovskite seed solution onto a substrate layer to form a 2D perovskite / substrate bilayer; and Annealing the 2D perovskite / substrate bilayer such that the aprotic polar solvent evaporates to form a heterostructure film A method comprising.

22. The substrate described above has the general formula A ｘ B ｙ X ｚ and includes a 3D perovskite having the formula, where A is a small monovalent cation, B is a divalent metal, and X is a monovalent anion, the method according to claim 21.

23. A method for producing a phase-pure 2D perovskite, comprising: Lead iodide (PbI ２ ), crystallizing a parent crystal from a precursor material selected from the group consisting of butylammonium iodide (BAI), methylammonium iodide (MAI), butylamine (BA), methylamine (MA) and combinations thereof, wherein the parent crystal has a single n value; Dissolving the parent crystal in a solvent at a high temperature to form a parent crystal solution; and Processing the parent crystal solution such that a phase-pure 2D perovskite is formed A method comprising.

24. The method according to claim 23, wherein the processing includes annealing after implementing a lamination technique selected from the group consisting of a spin-casting method, a doctor-blade method, a drop-casting method, a drop-die coating method, and combinations thereof.

25. The method according to claim 23, wherein the high temperature is 70°C.

26. The method according to claim 23, wherein the single n-value is in the range of 1 to 4.

27. The method according to claim 23, wherein the solvent is dimethylformamide (DMF).

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