WO2020018456A1

WO2020018456A1 - 2d perovskite stabilized phase-pure formamidinium perovskite solar cells and light emitting diodes

Info

Publication number: WO2020018456A1
Application number: PCT/US2019/041874
Authority: WO
Inventors: Yang Yang; Jinwook Lee; Taehee Han
Original assignee: The Regents Of The University Of California
Priority date: 2018-07-16
Filing date: 2019-07-15
Publication date: 2020-01-23

Abstract

An optoelectronic device includes a first electrode, a second electrode spaced apart from the first electrode, and an active layer between said first and second electrodes. The active layer includes a combination of FA_1-xCs_xPbI₃perovskite and A₂PbI₄two-dimensional (2D) perovskite in a ratio r. FA is formamidinium. A is selected from the group consisting of phenethylammonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2-(4-Trifluoromethylphenyl)ethylamine (FMPEA), 4-Fluorophenethylamine (FPEA), 3,4-Difluorobenzylamine (DFPEA), and any alkyl amine groups. Ratio r is at least 0.1 mol% and less than 20 mol%, and x is within the range of 0.0 to 0.2.

Description

2D PEROVSKITE STABILIZED PHASE-PURE FORMAMIDINIUM PEROVSKITE SOLAR CELLS AND LIGHT EMITTING DIODES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present patent application claims priority benefit to U.S. Provisional Patent Application No. 62/698,689 filed on July 16, 2018, the entire content of which is incorporated herein by reference. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.

BACKGROUND

1. Technical Field

[0002] The field of the currently claimed embodiments of this invention relates to optoelectronic devices and methods of producing optoelectronic devices, and more particularly to optoelectronic devices that have a combined two-dimensional (2D) perovskite and formamidinium perovskite active layer and methods of production.

2. Discussion of Related Art

[0003] Typical perovskite absorbers employ 3D ABXs structures, where a monovalent‘A-site’ cation in the cubo-octahedral site bonds with the BX* oclahedra. Compositional engineering has been considered an important approach to enhance the stability and performance of perovskite solar cells. Important milestones have been achieved through compositional engineering. For example, incorporation of the formamidinium (FA) cation into the‘A-site’ has enabled the formation of a cubic FAPbb phase with a lower bandgap (E_g) of 1.48 eV, higher absorption coefficient and longer carrier diffusion lengths than methylamnioniuni (MA) based tetragonal MAPbb (E_e=L57 eV). However, FAPbb has poor ambient stability became its non-perovskite hexagonal phase is thermodynamically more favorable than the cubic phase at room temperature. Partial substitution of FA and I with MA and/or Br has enabled fabrication of phase-pure FAPfrh with improved performance and stability. Recently, incorporation of smaller inorganic‘A’ cations, such as Cs and Rb, has further improved the stability and PCE of the PVSK solar cells with the lowest Voc deficit of 0.39 V. As a result, typical high efficiency devices nowadays incorporate PVSK with FA, MA, Cs, Rb and Br having relatively large E_g >1.60 eV. However, such compositional engineering has enhanced the Voc and stability at the expense of Jsc due to increased E_g. Utilization of pure FAPbfa is desired in regards to its lower E_g, which is close to the optimum value for a single junction solar cell suggested by the detailed balance limit However, no efficient method has been developed so far to fabricate a high quality phase-pure FAPbh film and devices. Therefore, there remains a need for improved optoelectronic devices that have a combined two-dimensional (2D) perovskite and formamidinium perovskite active layer and methods of production.

SUMMARY

[0004] An aspect of the present invention is to provide an optoelectronic device including: a first electrode, a second electrode spaced apart from the first electrode, and an active layer between said first and second electrodes. The active layer includes a combination of FA Cs Pbl perovskite and A Pbl_< two-dimensional (2D) perovskite in a ratio r. FA is formamidinium. A is selected from the group consisting of phenethylammonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2 -(4-T rifluoromethylphenyl)ethylamine (FMPEA), 4-Fluorophenethylamine (FPEA), 3,4-Difluorobenzylamine (DFPEA), and any alkyl amine groups. Ratio r is at least 0.1 mol% and less than 20 mol%, and x is within the range of 0.0 to 0.2.

[0005] Another aspect of the present invention is to provide a method of producing an optoelectronic device. The method includes forming a first electrode, forming a second electrode spaced apart from said first electrode, and producing an active layer between said first and second electrodes. The active layer includes a combination of FA CsPbl perovskite and A Pbl₄ two-dimensional (2D) perovskite in a ratio r. FA is formamidinium. A is selected from the group consisting of phenethyiammonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2-(4- T rifluoromelhylphenyl)ethy lamine (FMPEA), 4-Fluorophenethyiamine (FPEA), 3,4- Difluorobenzylamine (DFPEA), and any alkyl amine groups. The ratio r is at least 0.1 mol% and less than 20 mol%, and x is within the range of 0.0 to 0.2.

[0006] Yet another aspect of the present invention is to provide a formamidinium perovskite film having grains of FA Cs Pbl perovskite, and A Pbl two-dimensional perovskite formed in grain boundaries of said grains so as to stabilize a phase of said FA Cs Pb^ perovskite within said grains. FA is formamidinium. A is selected from the group consisting of phenethyiammonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2-(4-Trifluoromethylphenyl)ethylamine (FMPEA), 4-Fluorophenethylamine (FPEA), 3,4-Difluorobenzylamine (DFPEA), and any alkyl amine groups, x is within the range of 0.0 to 0.2.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention.

[0008] FIG. I A shows photoluminescence spectra of FAPbh PVSK films with different amounts of added 2D PEA Pbfi perovskite, according to an embodiment of the present invention;

[0009] FIG. IB shows normalized photoluminescence spectra of FAPbb PVSK films with different amounts of added 2D PEA₂PM₄ perovskite, according to an embodiment of the present invention; [0010] FIG. 2A shows peak positions of photoluminescence (PL) spectra for steady- state phololuminescence (PL) and normalized power conversion efficiency (PCE) of the devices for FAPbfe perovskite with different amounts of added 2D PEAjPbL perovskite, according to an embodiment of the present invention;

[0011] FIG. 2B shows normalized short-circuit current density (Jsc) versus the 2D perovskite concentration, according to an embodiment of the present invention;

[0012] FIG. 2C is a graph of normalized voltage open circuit voltage (Foe) versus the concentration of 2D perovskite, according to an embodiment of the present invention;

[0013] FIG. 2D is a graph of normalized fill factor (FF) of planar FAPbJa perovskite solar cells with different amounts of the added 2D PEA2Pbl₄ perovskite, according to embodiment of the present invention;

[0014] FIGS. 3A and 3B are cross-sectional scanning electron microscopic (SEM) images of a perovski te device, according to an embodiment of the present invention;

[0015] FIG. 4 shows X-ray diffraction patterns (XRD) of bare FAPbb and FAPbli with 1.67 mol% PEA₂PM₄, according to an embodiment of the present invention;

[0016] FIGS. 5A-5F show X-ray diffraction patterns with different amounts of 2D perovskite, according to various embodiments of the present invention;

[0017] FIG. 6 shows peak area and full-width-half-maximum (FWHM) calculated from X-ray diffraction patterns with different amounts of PEAaPbh, according to an embodiment of the present invention;

[0018] FIGS. 7 A and 7B are Normalized X-ray diffraction (XRD) patterns of FAPbb perovskite films with different amounts of added 2D PEA₂PM₄ perovskite, according to embodiments of the present invention;

[0019] FIG. 8 is a graph of the absorbance spectra versus wavelength for FAPbb film and the pure phase PVSK film with 1.67 mol% PEA2PM4 according to an embodiment of the present invention; [0020] FIGS. 9A-9B show the Effect of added 2D perovskite on absorption spectra, according to an embodiment of the present invention;

[0021] FIGS. 10A-10B show the Photoluminescence properties and photovoltaic performance of the device, according to an embodiment of the present invention;

[0022] FIG. IOC is a power conversion efficiency (PCE) distribution of the devices incorporating the perovskites, according to an embodiment of the present invention;

[0023] FIG. 10D are current density-voltage (J-V) curves of the device, according to an embodiment of the present invention;

[0024] FIG. 10E are steady state PCE measurement as function of time of the device, according to an embodiment of the present invention;

[0025] FIG. 10F are external quantum efficiency (EQE) spectra of perovskite solar cells incorporating bare FAPbb (control) and FAo^CsooaPbb with 2D perovskite (target), according to an embodiment of the present invention;

[0026] FIG. 11 shows short-circuit current density (Jisc), open circuit voltage (Foe), fill factor (FF) and power conversion efficiency (PCE) of FAPbb perovskite solar cells incorporating 1.67 mol% PEAzPbL* and different Cs amount, according to embodiments of the present invention.;

[0027] FIGS. 12A and 12B show the effect of Cs on crystal structure and absorption, according to an embodiment of the present invention;

[0028] FIGS. 13A-13C are surface scanning electron microscopic (SEM) images showing the effect of Cs on morphology of perovskite film, according to an embodiment of the present invention;

[0029] FIGS. 14A-14C show humidity stability of FAPbb and FAo ggCsoozPbb perovskites, according to embodiments of the present invention; [0030] FIG. 15A and 15B show photoluminescence (PL) properties of a device, according to an embodiment of the present invention;

[0031] FIG. 16 shows Photoluminescence decay profile of quasi-3D perovskite, according to an embodiment of the present invention;

[0032] FIGS. 17A-17C show photovoltaic parameters with 2D perovskite and Cs, according to an embodiment of the present invention;

[0033] FIGS. 18A-18D show distribution of photovoltaic parameters for target devices, according to an embodiment of the present invention;

[0034] FIG. 19 shows a target device with the highest open-circuit voltage (Foe), according to an embodiment of the present invention;

[0035] FIG. 20 shows a Certified stabilized efficiency, according to an embodiment of the present invention;

[0036] FIGS 21 A and 2 IB are 7- F and EQE curves of the certified device, according to an embodiment of the present invention;

[0037] FIGS. 22A and 22B show the electroluminescence properties of control and target devices, according to an embodiment of the present invention;

[0038] FIG. 23A shows images of the PVSK film stored for different time, according to an embodiment of the present invention;

[0039] FIG. 23B shows the absorbance (at 600 nm) of the FAPbb films with and without 2D PVSK as a function of exposure time, according to an embodiment of the present invention;

[0040] FIGS. 24A-24C show the Evolution of absorption under high relative humidity (RH), according to an embodiment of the present invention;

[0041] FIGS. 25 A-25C show the evolution of X-ray diffraction patterns, according to an embodiment of the present in vention; [0042] FIGS. 26A-26F show the morphology of FAPfcb film with added 2D perovskite, according to an embodiment of the present invention;

[0043] FIGS. 27A-27E show improved moisture stability with 2D perovskite at grain boundaries, according to an embodiment of the present invention;

[0044] FIG. 28 shows an X-ray diffraction pattern of the PEA2PM4 perovskite film, according to an embodiment of the present invention;

[0045] FIGS. 29A and 29B show an energy dispersive X-ray spectroscopic (EDS) analysis, according to an embodiment of the present invention;

[0046] FIG. 30A is a schematic illustration of a device incorporating a polycrystalline 3D perovskite film with 2D perovskite at grain boundaries, according to an embodiment of the present invention;

[0047] FIG. 30B shows a band structure of each layer in device analyzed by ultraviolet photoelectron spectroscopy (UPS) and Tauc plots, according to an embodiment of the present invention;

[0048] FIGS. 30C and 30E are conductive atomic force microscopic (c-AFM) images of bare FAPbb according to an embodiment of the present invention;

[0049] FIGS. 30D and 30F are conductive atomic force microscopic (c-AFM) images of 2D perovskite films on Sn0₂ coated ITO glass, according to an embodiment of the present invention;

[0050] FIGS. 31A-31C show an ultraviolet photoelectron spectroscopic (UPS) analysis, according to an embodiment of the present invention;

[0051] FIG. 32A shows complete spectra and low binding energy onset of UPS analysis of FAo geCso ozPbE with 1.67 mol% PEAaPbU and pure PEA2PM4 perovskite films, according to an embodiment of the present invention; [0052] FIG. 32B shows complete spectra and low binding energy onset of UPS analysis of FAo.9itCso.o2Pbl3 with 1.67 mol% PEA₂PM₄, according to an embodiment of the present invention;

[0053] FIG. 33A shows an evolution of power conversion efficiency (PCE) of control and target devices, according to an embodiment of the present invention;

[0054] FIG. 33 B shows a maximum power point tracking of the devices under 1 sun illumination in ambient condition without encapsulation, according to an embodiment of the present invention;

[0055] FIG. 33C shows an evolution of the PCEs measured from the encapsulated control and target devices exposed to continuous light (90±5 mWcrn^"2) under open-circuit condition, according to an embodiment of the present invention;

[0056] FIG. 34A-34C shows ambient stability test of control and target devices, according to an embodiment of the present invention;

[0057] FIG. 35 shows a maximum power point tracking of the target device, according to an embodiment of the present invention;

[0058] FIGS. 36A-36D show temperature-dependent conductivity of (a) bare FAPbb film, (b) with 1.67 mol% 2D PEAaPbL, perovskite, (c) bare FAPbb film and (d) with 1.67 mol% 2D PEAaPbh perovskite, according to embodiments of the present invention; and

[0059] FIG. 37 is a schematic diagram of an optoelectronic device, according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0060] Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention.

[0061] FIG. 37 is a schematic diagram of an optoelectronic device 100, according to an embodiment of the present invention. The optoelectronic device 100 includes a first electrode 102 and a second electrode 104 spaced apart from the first electrode 102. The optoelectronic device 100 also includes an active layer 106 between the first electrode 102 and the second electrode 104. The active layer 106 comprises a combination of FA Cs Pbl perovskite and A Pbl^ two-dimensional (2D) perovskite in a ratio r. The value of x is within the range of 0.0 to 0.2. FA is formamidinium. A is selected from the group consisting of phenethylammonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2-(4-Trifluoromethylphenyl)ethylamme (FMPEA), 4- Fluorophenethylamine (FPEA), 3 ,4-Difluorobenzylam ine (DFPEA), and any alkyl amine groups. The ratio r is at least 0.1 mol% and less than 20 mol%.

[0062] In an embodiment, the active layer 106 comprises grains of said FA Cs Pbl perovskite with said A PbI 2D perovskite formed in grain boundaries. In an embodiment the ratio r is at least 0.8 mol% and less than 3 mol%. In an embodiment the ratio r is at least 1 mol% and less than 2 mol%. In an embodiment the ratio r is at about 1.67 mol%. In an embodiment, the value of x is about 0.02. In an embodiment, at least one of the first electrode 102 and the second electrode 104 is a transparent electrode.

[0063] The term“transparent” is used herein to mean transparent to radiation in the visible wavelength rang?;, the infrared wavelength range, or the ultraviolet wavelength range, or any combination thereof. For example, transparent to solar radiation. In addition, the term“transparent” is used herein to broadly mean a transmission of the radiation through the first electrode 102 or the second electrode 104 between about 50% and about 100% (for example, between about 70% and about 99%).

[0064] In an embodiment, the active layer 106 can be a formamidinium perovskite film. The film has grains of FA Cs Pbl perovskite; and A Pb^ two-dimensional perovskite formed in grain boundaries of the grains so as to stabilize a phase of the FA^CsJPbl^ perovskite within said grains. FA is formamidinium and A is selected from the group consisting of phcnethylammonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2-(4-Trifluoromethytphenyl)ethylamine (FMPEA), 4-Fluorophenethylamine (FPEA), 3,4-Difluorobenzylamine (DFPEA), and any alkyl amine groups. The value of x is within the range of 0.0 to 0.2. For example, in an embodiment, the formamidinium perovskite film can be used in the active layer 106 in the optoelectronic device 100.

[0065] In an embodiment, the FA Cs Pbl perovskite and A Pbl two-dimensional (2D) perovskite are in a ratio r that is at least 0.1 mol% and less than 20 mol%. In an embodiment, the ratio r is, for example, at least 0.8 mol% and less than 3 mol%. In an embodiment, the ratio r is at least 1 mo!% and less than 2 mol%. In an embodiment, the ratio r is at about 1.67 mol%. In embodiment, the value of x is about 0.02.

[0066] In an embodiment, the optoelectronic device 100 includes a first electrode 102 (e.g., top electrode) and a second electrode 104 (e.g., bottom electrode) spaced apart from the first electrode 102 (e.g., top electrode). The active layer 106 is disposed between the first electrode 102 (e.g., top electrode) and the second electrode 104 (e.g., bottom electrode). In an embodiment, the optoelectronic device 100 further includes a hole transporting layer (HTL) 108 between the active layer 106 and the first electrode (e.g., top electrode) 102. In an embodiment, the optoelectronic device further includes an electron transporting layer (ETL) 110 between the active layer 106 and the second electrode (c.g., bottom electrode) 104.

[0067] In an embodiment, when the optoelectronic device 100 is a solar cell, the second electrode (e.g., bottom electrode) 104 can include Indium doped tin oxide (GGO) glass, the ETL layer 110 can include tin oxide (SnCh), the HTL layer 108 can include spiro- MeOTAD, and the first electrode (e.g., top electrode) 102 can include silver or gold.

[0068] In an embodiment, when the optoelectronic device 100 is a light emitting diode (LED), the second electrode (e.g,, bottom electrode) 104 can include Indium doped tin oxide (ITO) glass, the ETL layer 110 can include Zinc oxide (ZnO), the HTL layer 108 can include Po!y-TPD and molybdenum oxide (MoOx), and the first electrode (e.g., the top electrode) 102 can include silver or gold. [0069] Another embodiment provides a method of producing the optoelectronic device 100. The method includes forming the first electrode 102 and forming the second electrode 104 spaced apart from the first electrode 102. The method also includes producing the active layer 106 between the first electrode 102 and the second electrode 104. In an embodiment, the active layer 106 comprises a combination of FA Cs Pbl perovskite and A Pbl two-dimensional (2D) perovskite in a ratio r. FA is formamidinium, where A is selected from the group consisting of phenethylammonium (PEA), pheny!amine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2-(4- Trifluoromethylphenyl)ethylamine (FMPEA), 4-Fluorophenethylamine (FPEA), 3,4- Difluorobenzylamine (DFPEA), and any alkyl amine groups. The ratio r is at least 0.1 mol% and less than 20 mol%. The value of x is within the range of 0.0 to 0.2.

[0070] An embodiment of the current invention provides an effective approach to fabricate phase-pure FAPbb films with excellent optoelectronic quality and stability by adding a 2D phenylethylammonium lead iodide (PEA2PblO. We also demonstrate highly efficient perovskite solar cells and light emitting diodes using the perovskite materials we developed. The following are some example findings from our study.

[0071] In an embodiment, incorporation of 1.67 mol% of 2D phenylethylammonium lead iodide (PEA2Fbl4) into the precursor solution has enabled the formation of phase-pure FAPbb films with improved crystallinity and an order of magnitude enhanced photoluminescence lifetime.

[0072] In an embodiment, the resulting perovskite film has an identical bandgap (1.48 eV) to that of FAPbb, as the added 2D PEA₂PW4 is spontaneously ibnned at grain boundaries. The 2D perovsldtes at grain boundaries protects the FAPbb from moisture, resulting in significantly enhanced moisture stability.

[0073] In an embodiment, the 2D perovskites at grain boundaries assist in photogenerated charge separation and collection. A power conversion efficiency (PCE) of 21.06% (stabilized PCE of 20.64%) is achieved. We certified a stabilized PCE of 19.77% (20.32% from IV scan) from Newport Corporation. This is believed to be the first report of certified stabilized efficiency for perovskite solar cells. The certified stabilized efficiency is a new standard that Newport recently adopted, and the present device has achieved the highest efficiency under this new standard.

[0074] In an embodiment, the perovskite solar cell shows a short-circuit current density exceeding 24.5 mA/cm² and open circuit voltage of 1.130 V, corresponding to the lowest loss-in- potential of 0.35 V versus 0.39 V for delicately engineered mixed-cation-halide perovskite solar cells (FAi-x-y._zMAxCsyRbzPbl3_HiBr_q).

[0075] In an embodiment, the perovskite device retains 98% of initial PCE for 1392 hours storage under ambient condition and shows enhanced operational stability under one sun illumination.

[0076] In an embodiment, for Light Emitting Diodes (LEDs), uniformly distributed 50-100 nm sized nanograins of perovskite film is made by incorporating 1.67-3.33 mol% of 2D phenylethylammonium lead iodide (PEAzPbL?) into formamidinium perovskite solution.

[0077] In an embodiment, incorporation of 2D phenylethylammonium lead iodide (PEAsPbh) dramatically improves crystallinity, photoluminescence, and carrier lifetime of the formamidinium perovskite.

[0078] hi an embodiment, incorporation of 1.67 mol% 2D phenylethylammonium lead iodide (PEAjPbU) into formamidinium perovskite showed 15-fold increase of electroluminescence efficiency from 0.5 to 7.5 ph/el %.

[0079] In an embodiment, surface-functionalized formamidinium perovskite by 1.67 mol% 2D phenylethylammonium lead iodide (PI^PbLt) have showed improved operational stability under successive voltage bias stresses (100 cycles).

[0080] Methods for forming perovskite films using the adduct approach can be found in the following publications. These methods can be used with some embodiments of the current invention. 1. Ahn, N. et al. Highly reproducible perovskite solar cells with average efficiency of 18.3% and best efficiency of 19.7% fabricated via Lewis base adduct of lead (II) iodide.7. Am. Chem. Soc. 137, 8696-8699 (2015).

2. Lee, J.-W. et al. Lewis Acid-Base Adduct Approach for High Efficiency Perovskite Solar Cells. Acc. Chem. Res. 49, 311-319 (2016).

3. Lee, J.-W. et al. Tuning Molecular Interactions for Highly Reproducible and Efficient Formamidinium Perovskite Solar Cells via Adduct Approach. J. Am. Chem. Soc. 140, 6317-6324 (2018).

[0081] The following publications describe perovskite solar cells that incorporate a mixture of 3D and 2D perovskites.

1. Fu, Y. et al. Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization. Nano Lett . 17, 4405-4414 (2017).

2. Li, N. et al. Mixed Cation FAxPEAi-*PbL with Enhanced Phase and Ambient Stability toward High- Performance Perovskite Solar Cells. Adv. Energy Mater : 7, 1601307 (2017).

3. Wang, F. et al Phenylalkylaminc Passivation of Organolead Halide Perovskites Enabling High- Efficiency and Air- Stable Photovoltaic Cells. Adv. Mater . 28, 9986- 9992 (2016).

4. Quan, L. N. et al. Ligand-stabilized reduced-dimensionality perovskites. J. Am. Chem. Soc . 138, 2649-2655 (2016).

5. Grancini, G. et al. One-Year stable perovskite solar cells by 2D/3D interface engineering. Nat. Commun. 8, 15684 (2017).

6. Zhang, T, et al. Bication lead iodide 2D perovskite component to stabilize inorganic a-CsPbD perovskite phase for high-efficiency solar cells. Sci. Adv. 3, el 700841 (2017)

[0082] The following publications describe perovskite light emitting diodes that incorporate mixtures of 3D and 2D perovskites. 1. M. Yuan et al. Perovskite energy funnels for efficient light-emitting diodes. Nat. Nanotech. 11, 872-877 (2016)

2. L. Zhang et al. Ultra-bright and highly efficient inorganic based perovskite light- emitting diodes. Nat. Commun. 8, 15640 (2017)

3. J. Byun et al. Efficient Visible Quasi-2D Perovskite Light-Emitting Diodes. Adv. Mater. 28, 7515-7520, (2016)

4. X. Yang et al. Efficient green light-emitting diodes based on quasi-two- dimensional composition and phase engineered perovskite with surface passivation. Nat. Commun. 9, 570 (2018)

[0083] An embodiment of the current invention provides a method to fabricate highly efficient and stable perovskite solar cells and light emitting diodes based on formamidinium (cesium) perovskite (FAj-_xCs_xPbb) mixed with 2D perovskites (AaPbLi), where A includes phenethylanimonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylencdiamine (EDA), 2-(4-Trifluoromethylphenyl)ethylamine (FMPEA), 4- Fluorophenethylamine (FPEA), 3,4-Difluorobenzyiamine (DFPEA), and any alkyl amine groups. The following describes an example of using PEA as an A site molecule. Detailed description of the data can be found in the examples below.

[0084] Synthesis of Phenylethylammonium iodide: In an example of a synthesis, 4.8 g of phenethylamme (39.6 mmol, Aldrich, >99%) is dissolved in 15 mL of ethanol and placed in iced bath. Under vigorous stirring, 10.8 g of hydroiodic acid (57 wt% in HzO, 48.1 mmol, Sigma-Aldrich, 99.99%, contains no stabilizer) is slowly added to the solution. The solution is stirred overnight to ensure complete reaction, which is fol lowed by removal of the solvent by a rotary evaporator. The resulting solid is washed with diethyl ether several times until the color is changed to white. The white solid is further purified by recrystallization in mixed solvent of methanol and diethyl ether. Finally, white plate-like solid is filtered and dried under vacuum (yield -90%).

[0085] Fabrication of perovskite solar cells: Indium doped tin oxide (ITO) glass is cleaned with successive sonication in detergent, deionized (DI) water, acetone and 2- propanol for 15 min reflectively. The cleaned substrates are further treated with UV-ozone to remove the organic residual and enhance the wettability. 30 mM SnCh ^RbO (Aldrich, >99.995%) solution is prepared in ethanol (anhydrous, Decon Laboratories Inc.), which is filtered by 0.2 pm syringe filter before use. To form a SnO? layer, the solution is spin-coated on the cleaned substrate at 3000 ipm for 30 s, which is heat-treated at 150 °C for 30 min. After cooling down to room temperature, the spin-coating process is repeated one more time, which is followed by annealing at 150 °C for 5 min and 180 °C for 1 hour. The SnOa coated GGO glass is further treated with UV-ozone before spin-coating of perovskite solution. The perovskite layer is prepared by the modified adduct method. The bare FAPbb layer is formed from the perovskite solution containing equimolar amount of H€(NHz)zΐ (FAI, Dyesol), Pbb (TCI, 99.99%) and N-Methyl-2-pyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%) in N,N-Dimethylformamide (DMF, Sigma-Aldrich, anhydrous, 99.8%). Typically, 172 mg of FAI, 461 mg of Pbla and 99 mg of NMP are added to 600 mg of DMF. For the 2D perovskite (PEAaPbL) and Cs incorporated perovskite, corresponding amount of FAI is replaced with PEAI and Csl. For example, FAPbb with 1.67 mol% PEAzPbL perovskite is formed from the precursor solution containing 166.4 mg of FAI, 8.2 mg of phenylethylammonium iodide (PEAI), 453.4 mg of Pbh and 97.4 mg of NMP in 600 mg of DMF. With 2 mol% of Cs, the precursor solution is prepared by mixing 163.0 mg of FAI, 8.2 mg of PEAI, 5.0 mg of Csl (Alfa Aesar, 99.999%), 453.4 mg of Pbb and 97.4 mg of NMP in 600 mg.

[0086] For the best performing devices in, the amount of DMF is adjusted to 550 mg.

Spin-coating of perovskite and hole transporting layer is performed in a glove box filled with dry air. The perovskite solution is spin-coated at 4000 rpm for 20 s where 0.15 ml, of diethyl ether (anhydrous, >99.0%, contains BHT as stabilizer, Sigma- Aldrich) is dropped after 10 s on the spinning substrate. The resulting transparent adduct film is heat-treated at 100 °C for 1 min followed by 150 °C for 10 min. (for the best performing target device, the annealing condition is adjusted to 80 °C 1 min followed by 150 °C for 20 min) The spiro-MeOTAD solution is prepared by dissolving 85.8 mg of spiro-MeOTAD (Lumtec) in 1 ml of chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich) which is doped by 33.8 mΐ of 4-tert- butylpyridine (96%, Aldrich) and 19.3 mΐ of Li-TFSl (99.95%, Aldrich, 520 mg/mL in acetonitrile) solution. The spiro-MeOTAD solution is spin-coated on the perovskite layer at 3000 tpm for 20 s by dropping 17 mΐ of the solution on the spinning substrate. On top of the spiro-MeOTAD layer, ca. 100 nm-thick silver or gold layer is thermally evaporated at 0.5 A/s to be used as an electrode.

[0087] Fabrication of light-emitting diodes: indium doped tin oxide (GGO) glass is cleaned with successive sonication in detergent, deionized (DI) water, acetone and 2- propanol tor 15 min respectively. The cleaned substrates arc further treated with UV-ozone to remove the organic residual and enhance the wettability for 15 min. Zinc oxide nanoparticle ink (2.5 wt. %, viscosity 3 cP, work function -4.3eV, Sigma-Aldrich) dissolved in 2-propanol solution is spin-coated on top of the UV-ozone treated GGO at 5000 rpm for 30 s, and then annealed at .100 °C for 10 min. The ZnO nanoparticle coated GGO glass is further treated with UV-ozone before spin-coating of perovskite solution. The perovskite layer is prepared by the modified adduct method. The bare FAPbb layer is formed from the perovskite solution containing equimolar amount of HC(NH2)zT (FAI, Dyesol), Pbb (TCI, 99.99%) and N-Methyl-2-pyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%) in N,N- Dimethylformamide (DMF, Sigma-Aldrich, anhydrous, 99.8%). Typically, 172 mg of FAI, 461 mg of Pbb and 99 mg of NMP are added to 600 mg of DMF. For the 2D perovskite (PEA2PM4) and Cs incorporated perovskite, corresponding amount of FAI is replaced with PEAI and Csl. For example, FAPbb with 1.67 mol% PEA2PM4 perovskite is formed from the precursor solution containing 166.4 mg of FAI, 8.2 mg of phenylethylammonium iodide (PEAI), 453.4 mg of Pbb and 97.4 mg of NMP in 600 mg of DMF. With 2 mol% of Cs, the precursor solution is prepared by mixing 163.0 mg of FAI, 8.2 mg of PEAI, 5.0 mg of Csl (Alfa Aesar, 99.999%), 453.4 mg of Pbb and 97.4 mg of NMP in 600 mg. Spin-coating of perovskite and hole transporting layer (HTL) is performed in a glove box filled with dry air. The perovskite solution is spin-coated at 4000 rpm for 20 s where 0.2 mL of chloroform

(anhydrous, anhydrous, ³99%, contains 0.5-1.0% ethanol as stabilizer, Sigma-Aldrich) is dropped after 10 s on the spinning substrate. The resulting transparent adduct film is heat- treated at 100 °C for 1 min followed by 150 °C for 1 min. A 0.5wt% of poly(4-butylphenyl- diphenyl-amine) (poly-TPD) solution dissolved in chlorobenzene (anhydrous, 99.8%, Sigma- Aldrich) is spin-coated on the perovskite layer at 4000 rpm for 30 s as hole transporting layer. On lop of the hole transporting layer, 10-nm-thick molybdenum trioxide and 100 urn- thick silver is thermally evaporated at 0.5 A/s to be used as an electrode.

[0088] In an embodiment, FAPbb Films are prepared by the modified adduct method, in which N-Mcthyl-2-pyrrolidone (NMP) is used as a Lewis base. To the PVSK precursor solution, 2D PVSK (PEA₂Pbb) precursors with different molar ratios ranging from 1.25 to 10 mo1% are added. The steady-state PL spectra of the films are measured and are shown in FIGS. 1 A and IB. FIG. 1A are photoluminescence spectra of FAPbb PVSK films with different amount of added 2D PEAaPbb perovskite, according to an embodiment of the present invention. FIG. IB are normalized photoluminescence spectra of FAPbb PVSK films with different amount of added 2D PEAaPbb perovskite, according to an embodiment of the present invention.

[0089] FIG. 2A is a peak position of photoluminescence (PL) spectrum for steady- state photoluminescence (PL) spectrum and normalized power conversion efficiency (PCE) of the devices for FAPbb perovskite with different amount of added 2D PEAaPbb perovskite, according to an embodiment of the present invention. The error bar of the normalized PCE indicates standard deviation of the PCEs. At least 10 devices are fabricated for each condition.

[0090] As shown in FIG. 1A and FIG. 2A, we observe no obvious changes in PL peak position until the amount of 2D PVSK reached 10 mol%. With 10 mol% PVSK, the PL peak is blue-shifted by 6 nm. The blue-shift of the PL peak may be due to formation of a quasi-3D PVSK, where charge carriers are confined by large potential barrier originated from the 2D PVSK. Based on this observation, we presume the added 2D PVSK does not result in the formation of the quasi-3D PVSK if it remains below a certain threshold. This threshold is found to be less than 10 mol%, where this quantity is then optimized based on photovoltaic performance (FIG. 2A).

[0091] FIG. 2B-2D show the effect of 2D perovskite on photovoltaic performance, according to an embodiment of the present invention. FIG. 2B is a Normalized short-circuit current density (/sc) versus the 2D perovskite concentration, according to an embodiment of the present invention. FIG. 2C is a graph of normalized voltage open circuit voltage (Foe) versus the concentration of 2D perovskite, according to an embodiment of the present invention. FIG. 2D is a normalized fill factor (FF) of planar FAPbb perovskite solar cells with different amount of the added 2D PEA₂PM₄ perovskite, according to embodiment of the present invention. The photovoltaic parameters are obtained from reverse scan (from Voc to Jsc) with scan rale of 0.1 Vs^'1. The error bar of die normalized PCE indicates standard deviation of the PCEs. At least 10 devices are fabricated for each condition.

[0092] FIGS. 3A and 3B are cross-sectional scanning electron microscopic (SEM) images of a perovskite device, according to an embodiment of the present invention. Cross- sectional SEM images of the perovskite device with 1.67 mol% of PEAaPbl* 2D perovskite are shown in FIGS. 3A and 3B. The structure of the device is ITO/compact- SnOz/perovskite/spiro-MeOTAD/Ag or Au. FIG. 3 A is a lower magnification SEM image. FIG. 3B is a higher magnification image of the device. A planar heterojunction architecture consisting of Indium doped Sn0₂ (1TO) glass/compact-SnOz/PVSK/spiro-MeOTAD/Ag or Au can be utilized for construction of PVSK solar cells in this study. The addition of 1.67 mol% 2D PVSK is found to be optimal for the photovoltaic performance (ca. 11% improvement in PCE). Notably, addition of 10 mol% 2D PVSK significantly degraded the PCE to less than 1%, which can result from formation of quasi-3D PVSK as the lat¾e potential barrier originating from 2D PVSK could hinder the charge transport.

[0093] FIG. 4 shows X-ray diffraction patterns (XRD) of bare FAPfrh and FAPbb with 1.67 mo!% PEAsPbU, according to an embodiment of the present invention. As shown in FIG. 4, the bare FAPbb film contains hexagonal non-PVSK phase (5-phase) while the PVSK film prepared with 1.67 mol% PEA₂PW₄ shows pure PVSK phase.

[0094] Even smaller amount of 2D PVSK (1.25 mol% PEA2PW4) effectively suppresses the formation of 5-phase (FIGS. 5A-5F). FIGS. 5A-5F show X-ray diffraction patterns with different amount of 2D perovskite, according to various embodiments of the present invention. FIG. 5A shows the diffraction pattern of bare FAPbb, according to an embodiment of the present invention. FIG. 5B shows the diffraction pattern of 1.25 mol%, according to an embodiment of the present invention. FIG. 5C shows the diffraction pattern for 1.67 mol%, according to an embodiment of the present invention. FIG. 6D shows the diffraction pattern for 2.50 mol%, according to an embodiment of the present invention. FIG. 5E shows the diffraction pattern for 5.00 mol%. FIG. 5F shows the diffraction pattern for 10.00 mol% 2D PEA2PM4 perovskite, according to an embodiment of the present invention. Peaks from cubic FAPbb phase are indexed in a. d and * indicate hexagonal FAPbb and Pbl₂ respectively.

[0095] Furthermore, the overall signal intensity and full-widlh-half-maximum (FWHM) are enhanced with the addition of the 2D PVSK, indicating improved crystallinity. We speculate that the added large phenyleihy!ammonium molecules from 2D PVSK precursors interact with FAPbb crystals to facilitate formation of the cubic PVSK phase during crystallization. Such a speculation is correlated with the observation in the XRD measurements in FIGS. 5A-5F, in which the signal intensity and FWHM of XRD peaks are systematically enhanced with increased amounts of the added 2D PVSK (FIGS. 5A-5F and

6).

[0096] FIG. 6 shows peak area and full-width-half-maximum (FWHM) calculated from X-ray difiraction patterns with different amount of PEA₂PM4, according to an embodiment of the present invention. The crystallinity depends on added 2D perovskite. The (002) peak (at around 28”) is used for the Gauss fit The enhancement of preferred orientation along the (001) plane with increased 2D PVSK also indicates the added precursors of the 2D PVSK functionalize the specific crystal facet to change the surface energy during the crystal growth. A closer inspection on the normalized X-ray diffraction (XRD) patterns of the PVSK films with different amounts of added 2D PVSK is taken to find any correlations between the added 2D PVSK and crystal structure of FAPbh.

[0097] FIGS. 7A and 7B are Normalized X-ray diffraction (XRD) patterns of FAPbb perovskite films with different amount of added 2D PEAaPbLi perovskite, according to embodiments of the present invention. FIG. 7A shows full spectra and FIG. 7B shows magnified (001) orientation peaks. Interestingly, a systematic change in peak position is observed with different amounts of 2D PVSK for which the XRD peaks are slightly shifted towards higher angles with the addition of relatively smaller amounts of 2D PVSK (1.25, 1.67, 2.50 and 5.0 mol%). This indicates that the lattice constant of FAPbh is reduced, likely due to compressive strain associated with the added 2D PVSK. We speculate that the reduction in lattice constant can be also related to the enhanced phase-purity of cubic FAPbb as it will have equivalent effects with incorporation of smaller‘A’ site cations on the tolerance factor and thus enthalpy of formation. Lower angle peaks at around 12° appear upon addition of 10 mol% 2D PVSK corresponding to the formation of quasi-3D PVSK (see, the inset of FIG. 5F).

[0098] FIG. 8 is a graph of the absorbance spectra versus wavelength for FAPbb film and the pure phase PVSK film with 1.67 mol% PEAzPbb according to an embodiment of the present invention. This graph shows enhanced absorption over ail wavelengths for PVSK film with 1.67 mol% PEAjPbU compared to the bare FAPbb film where the absorption onset is hardly changed (Inset of FIG. 8). The absorption onset is complemented by almost identical normalized PL spectra, which indicates that the Eg is maintained. The enhanced absorption as seen when the 2D PVSK is added is probably due to an enhanced phase purity of the FAPbb, with partial contribution from an enhanced light scattering owing to the improved crystallinity.

[0099] The absorption spectra with different amounts of 2D PVSK are shown in

FIGS. 9A and 9B. FIGS. 9A-9B show the Effect of added 2D perovskite on absorption spectra, according to an embodiment of the present invention. Absorption spectra of FAPbb perovskite films with different amount of added 2D PEAaPbb perovskite. FIG. 9A shows the full spectra and FIG. 9B shows the magnified onset region. While all the PVSK films with 2D PVSK show enhanced absorption compared to bare FAPbb films, a slight blue shift of the absorption onset with decreases in absorption over the whole wavelength region is identified with the addition of 10 mol% of 2D PVSK, which is correlated with the blue shift of the steady-state PL spectrum that can be associated with the formation of quasi-3D PVSK.

[00100] Photoluminescence properties and photovoltaic performance: FIGS. 10A-10B show the Photoluminescence properties and photovoltaic performance of the device, according to an embodiment of the present invention. FIG. 10A is a steady-state. FIG. 10B is a time resolved PL spectra of the perovskite films incorporating bare FAPbb, FAPbb with 2D perovskite and FAo.98C$o.o2Pbb with 2D perovskite. Gray solid lines in FIG. 10B are fitted lines for each curve. FIG. IOC is a power conversion efficiency (PCE) distribution of the devices incorporating the perovskites. All the devices are fabricated in same batch. FIG. 10D is a current density-voltage (J-V) curves. FIG. 10E are steady state PCE measurement. FIG. 10F are external quantum efficiency (EQE) spectra of perovskite solar cells incorporating bare FAPbb (control) and FAo.geCso.oaPbb with 2D perovskite (target). Photovoltaic parameters of the highest performing devices are summarized in the table in d, in which the values with and without parenthesis are from reverse (from Foe to Jsc) and forward scan (from Jsc to Foe) respectively.

[00101] Steady-state and time-resolved PL profiles are investigated in FIG. 10A and FIG. 10B. The steady-state PL intensity is largely enhanced more than five times from 4.3X10⁵ to 2.3X10⁶ with addition of 1.67 mol% PEA₂PbLi into FAPbb film (FIG. 10A). The large enhancement of PL intensity is attributed to a significantly elongated PL lifetime as shown in FIG. 10B. The time resolved PL profiles are fitted to exponential decay, in which bi- and tri-exponential decay models are used for the bare and 2D PVSK incorporated PVSK films, respectively (Table 1). Table 1| shows fitted parameters for time resolved photoluminescence decay, according to an embodiment of the present invention. Biexponential and triexponential decay models are used for bare FAPbh and FAPbb with 2D perovskite respectively. The values in parenthesis indicate proportion of the each decay component.

[00102] Table 1

[00103] The relatively fast decay component (n around 3 ns) is assigned to charge carrier trapping induced by trap states formed due to the structural disorder such as vacancy or interstitial defects while much slower components (r₂, t¾) are assigned to free carrier radiative recombination. With addition of 1.67 mol% 2D PVSK, proportion of the fast decay component (n) is decreased (from 51.8% to 46.5%) while ti significantly elongated from 78.5 ns to 148.7 ns, which indicates reduced defect density and enhanced charge carrier lifetime. We attributed such improvements to enhanced phase purity and crystallinity of FAPbh as observed from XRD measurements, which decreases the structural disorders at grain interiors and/or boundaries. Moreover, a new decay component (¾) with a significantly long lifetime (>1 ps) appeared after addition of the 2D PVSK, which is likely related to the added 2D PVSK. As a result, the average PL lifetime is enhanced by almost one order of magnitude from 39.4 ns to 376.9 ns with addition of 2D PVSK. During the optimization of the device, incorporation of 2 mol% Cs is found to further enhance the performance and reproducibility of the devices without a noticeable change in Eg.

[00104] FIG. 11 shows short-circuit current density (/sc), open circuit voltage (Foe), fill factor (FF) and power conversion efficiency (PCE) of FAPbb perovskite solar cells incorporating 1.67 mol% PEA₂Pbl4 and different Cs amount, according to embodiments of the present invention. Fig. 11 shows the effect of cesium (Cs) amount on photovoltaic performance, according to an embodiment of the present invention. The photovoltaic parameters are obtained from reverse scan (from Foe to Jsc) with scan rale of 0.1 Vs^"1.

[00105] FIGS. 12A and 12B show the effect of Cs on crystal structure and absorption, according to an embodiment of the present invention. FIG. 12A shows a comparison of X-ray diffraction pattern. FIG. 12B shows absorption spectra of FAPblj with 1.67 mol% PEAaPbk (FAREA) and FAo.98Cso.o2Pbb with 1.67 mol% PEA2PM4 (FAPEACs).

[00106] FIGS. 13A-13C are surface scanning electron microscopic (SEM) images showing the effect of Cs on morphology of perovskite film, according to an embodiment of the present invention. FIG. 13A shows the surface scanning electron microscopic (SEM) image of bare FAPbb (FA). FIG. 13B shows the SEM image of 1.67 mol% PEA₂Pbb (FAPEA). FIG. 13C shows an SEM image of FAovsCsooaPbb with 1.67 mol% PEAaPbb (FAPEACs).

[00107] FIGS. 14A-14C show humidity stability of FAPbb and FAo.<wCso.o2Pbb perovskites, according to embodiments of the present invention. FIG. 14A show images at lime intervals after exposure of perovskite, according to an embodiment of the present invention. FIG 14B and FIG. 14C show X-ray diffraction patterns of the FAPbb and FAo.98Cso.toPbb perovskite films with exposure to relative humidity (RH) of 70±5% at 20±2° for different time, according to an embodiment of the present invention. Peaks from cubic FAPbb phase are indexed by a whereas d and * indicate hexagonal FAPbb and Pbb, respectively.

[00108] FIG. 15A and 15B show photoluminescence (PL) properties of the device, according to an embodiment of the present invention. FIG. 15A shows steady-state PL measurements of the FAPbb and FAoveCso-tnPbb perovskite films, according to an embodiment of the present invention. FIG. 15B shows time-resolved PL measurements of the FAPbb and FAovsCso.rcPbb perovskite films, according to an embodiment of the present invention.

[00109] FIG. 16 shows Photoluminescence decay profile of quasi-3D perovskite, according to an embodiment of the present invention. Specifically, FIG. 17 shows Time- resolved phololuminescence decay profile of the FAPbb perovskite film with incorporation of 10 mol% of PEA2PW4 perovskite.

[00110] With additional 2 mol% Cs, the fraction of T_I can be further decreased, indicating a further decreased defect density, which can also be observed in previous studies. Consequently, the steady-state PL intensity and average PL lifetime is further enhanced, rationalizing the improved PCE with 2 mol% Cs (Table 1). It is worth noting that the PL lifetime is significantly reduced with 10 mol% of 2D PVSK due to formation of quasi-3D PVSK (FIG. 16). [00111] For optimization of the device, we first controlled the concentration and heat- treatment process for FAPbb with 1.67 mol% PEAiPbb perovskite. The data in FIGS. 2A- 2D and 5A-5F is obtained using low concentration (1 mmol of perovskite precursors in 600 mg of DMF) precursor solution with same annealing time (10 min at 150 °C) to avoid the effect of annealing time on crystallinity. With 2D perovskite, we are able to prolong die annealing time since it is thermally more stable compared to bare FAPbb. The optimized annealing time is 20 min at 150 °C with 2D perovskite (For bare FAPbb, the optimum annealing time is 10 min). Optimization of the heat-treatment and concentration (1 mmol of perovskite precursors in 560 mg of DMF) enhanced the average PCE up to 17.86±0.56% (FIG. 11 with 0 mol% Cs). In addition to this, we noticed that incorporation of small amount of Cs (2 mol%) further improve the average PCE up to 19.00±0.69% due to enhancement in Voc and FF (FIG. 11 ). The origin of the improvement might be passivation of defects.

[00112] Incorporation of relatively smaller Cs cation into FAPbb can increase the bandgap of the perovskite. In the previous studies, typically higher than 5 mol% of the Cs is used to stabilize the FAPbb while we incorporated only 2 mol% of Cs. We compare the X- ray diffraction (XRD) pattern and absorption spectrum of the perovskite films without and with 2 mol% Cs. As shown in FIG. 12A, XRD patterns are similar with and without 2 mol% Cs. The absorption spectra of the films with and without 2 mol% Cs are also almost identical as seen in FIG. 12B, which indicates bandgap of the perovskite is maintained after addition of 2 mo!% of Cs (The bandgap is determined to be 1.48 eV). As a result, external quantum efficiency (EQE) spectrum of our target device shows almost identical onset (rather slightly red shifted) with bare FAPbb device. Surface scanning electron microscopic (SEM) images of the films are compared in FIGS. 13A-13C. Relatively larger grains without specific shape are observed with bare FAPbb while relatively smaller granular grains are observed with 2D perovskite. The grain size is decreased with 2 mol% of Cs, which is correlated with slight decrease of peak intensity in XRD spectrum.

[00113] The PCE distribution of the devices incorporating corresponding the PVSKs is compared (distribution of photovoltaic parameters can be found in FIGS. 17A-17C). FIGS. 17A-17C show photovoltaic parameters with 2D perovskite and Cs, according to an embodiment of the present invention. FIG. 17A shows a Short-circuit current density (/sc) of the device incorporating bare FAPbb (FA), FAPbb with 1.67 mol% PEAaPbLt (FAPEA) and FA0.WCS0.02PM3 with 1.67 mol% PEAaPbt* (FAPEACs). FIG. 17B shows open circuit voltage (Foe) of the device incorporating bare FAPbb (FA), FAPbb with 1.67 mol% PEA2PM4 (FAPEA) and FAimCso.oaPbb with 1.67 mol% PEAjPbb (FAPEACs). FIG. 17C shows fill factor (FF) of the device incorporating bare FAPbb (FA), FAPbb with 1.67 moi% PEA2PbU (FAPEA) and FAoveCsoxdPbb with 1.67 mol% PEA2PW4 (FAPEACs). The photovoltaic parameters are obtained from reverse scan (from Foe to Jsc) with scan rate of 0.1 Vs^'1. All the devices are fabricated in same batch.

[00114] The average photovoltaic parameters are summarized in Table 2. Table 2 provides improved photovoltaic parameters with 2D perovskate and Cs. Photovoltaic parameters of the device incorporating bare FAPbb (FA), FAPbb with 1.67 mol% PEA2PW4 (FAPEA) and FAoseCso-ozPbb with 1.67 mol% PEA2Pbl4 (FAPEACs). The photovoltaic parameters are obtained from reverse scan (from Foe to Jsc) with scan rate of 0.1 Vs ¹. All the devices are fabricated in same batch.

[00115] TABLE 2

[00116] The average PCE of the bare FAPbb PVSK solar cells is significantly enhanced by 13% from 15.95*0.36% to 18.08*0.52% with addition of 1.67 mol% PEA₂PM₄.

The average PCE is further enhanced to 19.16*0.37% with 2 mol% of Cs (Hereafter, the devices based on bare FAPbb are denoted as control while the devices based on FAo.98Cso.o2Pbl3 with 1.67 mol% PEAiPbU are denoted as target for convenience). Current density and voltage U-V) curves of the optimized control and target devices are demonstrated in HG. 10D, in which the highest PCE of the target device reached 21.06% (/sc;: 24.44 mAcrn^"2, Fbc: LI 26 V, FF: 0.765) while a PCE of 16.41% is achieved with the control device (/sc;: 24.23 mAcrn^"2, Fix;: 1.048 V, FF: 0.646). A stabilized PCE of 20.64% is achieved with the target device while that of control device is 15.80% (FIG. 10E). External quantum efficiency (EQE) spectra of the devices are compared in FIG. 10F. An integrated /sc; of 23.9 mAcrn^"2 from the target device is well-matched with the value measured from the I-V scan (less than 5% discrepancy), while control device shows that of 21.2 mAcm^"2 with a relatively large discrepancy of 14%. The relatively large discrepancy from the control FAPbh device is probably due to a more pronounced hysteresis, as shown in FIG. 10D, which also results in a large discrepancy between the stabilized PCE and the PCE measured from the J-V scan. The performance of control device is highly reproducible. With optimized process parameters, the average PCE of 20.05±0.45% is demonstrated over 74 devices (FIG. 18A-18D). FIGS. 18A-18D show distribution of photovoltaic parameters for target devices, according to an embodiment of the present invention. FIG. 18A is a short-circuit current density (Jsc) of the device, according to an embodiment of the present invention. FIG. 18B is an open circuit voltage (Voc) of the device, according to an embodiment of the present invention. FIG. 18C is a fill factor (FF) of the device, according to an embodiment of the present invention. FIG. 18D is a power conversion efficiency (PCE) of the device incoiporating FAo.osCso-oaPbL, with 1.67 mol% PEAjPbL*. 74 devices are fabricated from two different batches.

[00117] FIG. 19 shows a target device with the highest open-circuit voltage (Foe;), according to an embodiment of the present invention. FIG. 19 is a graph of the current density-voltage (J-V) curve of the target device showing the highest Fix;. The J-V curve is obtained from reverse scan with scan rale of 0.1 Vs ¹.

[00118] FIG. 20 shows a Certified stabilized efficiency, according to an embodiment of the present invention. Certified stabilized efficiency for a target device. All the parameters are stabilized. A bias voltage-stabilized out power curve is included.

[00119] FIGS 21 A and 21B are J-V and EQE curves of the certified device, according to an embodiment of the present invention. FIG. 21A shows a current density-voltage (J-V) curve. FIG. 21B shows the normalized external quantum efficiency (EQE) curve for the certified device provided by Newport Corporation. The J-V curve is obtained from reverse scan (from Voc to Jse) with scan rate of 36 mVs^'1.

[00120] FIGS. 22A and 22 B show the] electroluminescence properties of control and target devices, according to an embodiment of the present invention. FIG. 22A shows a current density-voltage curve and measured radiance of the control and target devices. FIG. 22B shows a calculated electroluminescence external quantum efficiency (EL EQE) spectra for the control and target devices. Inset of B shows normalized EL spectra of the devices at bias voltage of 2 V.

[00121] We obtained the peak Fbc of 1.130 V with the target device (FIG. 19) corresponding to a loss-in-potential of 0.35 V considering a £_g of 1.48 eV, which is the lowest Voc deficit reported to date for PVSK solar cells. One of the target devices is sent out for measurement in an independent laboratory and achieved a certified stabilized PCE of 19.77% (FIG. 20). The current-voltage curve and EQE spectra matched well with those measured by our group (FIG. 21A and 21B). The enhanced device performance with 2D PVSK is mainly due to improved FF and Koc, which can be attributed to improved phase purity and elongated carrier lifetime with reduced defect density, facilitating carrier transport and reducing the charge recombination¹⁴. The reduced non-radialive recombination loss with 2D PVSK is also confirmed in devices by electroluminescence (EL) measurements in FIG. 22A-22B, in which maximum radiance (40.4 Wsr^'W²) and EL EQE (0.49%) of the target devices are significantly enhanced compared to those of the control devices (2.87 Wsr’cm^"2, 0.06%).

[00122] Moisture stability and TEM analysis: Under ambient conditions, a cubic FAPbb PVSK phase is subject to undergo conversion to a hexagonal non-PVSK phase, resulting in serious degradation in photovoltaic performance. The phase transformation is even accelerated under high relative humidity. To evaluate the effects of 2D PVSK incorporation on phase stability, we investigated changes in absorbance of the film under relative humidity (RH) of 80±5%. FIG. 23A shows images of the PVSK film stored for different time, according to an embodiment of the present invention. Bare FAPbl_? film is almost completely bleached within 24 h whereas no obvious change in color is observed from the films containing 2D PVSK both with and without Cs. FIG. 23B shows the absorbance (at 600 nm) of the FAPbb films with and without 2D PVSK as a function of exposure time (individual absorption spectra can be found in FIGS. 24A-24C), according to an embodiment of the present invention. The absorbance of the bare FAPbb rapidly degraded during 24 h, while FAPbb films with 2D PVSK did not show noticeable degradation within 24 h. With addition of 2 mol% Cs, the film also remained stable after 24 h. The color change of the bare FAPbb film under high RH is due to its transformation to the d-phase as can be seen in XRD spectra in FIG. 25 A whereas no detectable change in color for the films with 2D PVSK is correlated with their neat XRD spectra without the d-phase (FIGS. 25B and 25C). The enhanced phase stability under high RH implies that the possible ingression pathway of moisture in the PVSK film is passivated. Previously, we demonstrated grain boundary engineering techniques using the adduct approach, in which the additives had precipitated at grain boundaries if not incorporated into the lattice of PVSK. We supposed that grain boundaries within the film, which have been reported to be ingression pathways for moisture, might be passivated by the added 2D PVSK.

[00123] FIGS. 24A-24C show the Evolution of absorption under high relative humidity (RH), according to an embodiment of the present invention. FIG. 24A shows the evolution of absorption spectra under relative humidity of 80±5% of bare FAPbb. FIG. 24B shows the evolution of absorption spectra under relative humidity of 80±S% of FAPbb with 1.67 mol% PEAzPbb. FIG. 24C shows the Evolution of absorption spectra under relative humidity of 80±5% of FAo.98Cso.2Pbl3 with 1.67 mol% PEAzPbb. Three films are tested for each composition.

[00124] FIGS. 25A-25C show the evolution of X-ray diffraction patterns, according to an embodiment of the present invention. FIG. 25A shows the evolution of XRD patterns of the perovskite films under relative humidity (RH ) of 80±5% for bare FAPbb (FA). FIG. 25B shows evolution of XRD patterns of the perovskite films under relative humidity (RH) of 80±5% for FAPbb with 1.67 mol% PEAzPbb (FAPEA). FIG. 25C shows evolution of XRD patterns of the perovskite films under relative humidity (RH) of 80±5% for FAo.9eCso.2Pbb with 1.67 mol% PEAzPbb (FAPEACs). Indeed, the vertically aligned 2D PVSK is sparsely observed from SEM images in FIG. 13B and FIG. 13C with addition of 2D PVSK (see also FIGS. 26A-26F).

[00125] In FIGS. 13B and 13C, bright colored plates are sparsely observed with addition of 1.67 mol% of 2D perovskite into precursor solution. The morphology of plates in FIGS. 13B and 13C is clearly distinguished from one in FIG. 13 A, which might be the thermally induced Pbl₂. We speculated the plates arc crystallized 2D perovskite. Although it is not detected in XRD patterns, surface SEM images in FIGS. 26A-26F support this speculation. Compared to bare FAPbh (FIGS. 26A and 26D), the bright colored plates are formed between grains with addition of 5 mol% of 2D perovskite (FIGS. 26B and 26E). The amount and size of the plates are further enhanced with addition of 10 mol% of 2D perovskite (FIGS. 26C and 26F) although the amount of Pbb in XRD is not increased (FIGS. 5A-5F).

[00126] FIGS. 26A-26F show the morphology of FAPbh film with added 2D perovskite, according to an embodiment of the present invention. FIGS. 26A and 26D are surface scanning electron microscopic (SEM) images of bare FAPbh. FIGS. 26B and 26E are surface scanning electron microscopic (SEM) images of FAPbh with 1.67 mol% PEA₂FbU. FIGS. 26C and 26F are surface scanning electron microscopic (SEM) images of FAPbh with 10.00 mol% PEA2PM4. All the films are annealed at 150 °C for 10 min.

[00127] FIGS. 27A-27E show improved moisture stability with 2D perovskite at grain boundaries, according to an embodiment of the present invention. FIG. 27A are images of the perovskite films incorporating bare FAPbh, FAPbh with 2D perovskite and FAo.9aCso.o2Pbh with 2D perovskite exposed to relative humidity (RH) of 80±5% at 20±2 °C for different lime. FIG. 27B depicts the evolution of absorption of the films at 600 nm under RH 80±5% at 20±2 °C. The error bar indicates standard deviation of the absorbance measured from 3 films for each condition. FIGS. 27C and 28E are transmission electron microscopic (TEM) images of the FAo.9sCso.o2Pbh film with 1.67 mol% PEA2PM4. Inset of FIG. 28C is the lower magnification image showing the polyctystalline nature with grain boundaries. The highlighted area (1) and (2) are investigated in FIGS. 28D and 28E, respectively. Inset of FIG. 27D and FIG. 27E show Fast Fourier transform (FFT) analysis of the area within boxes, respectively.

[00128] The enhanced moisture stability throughout the whole film implies that the 2D PVSK probably exist along the grain boundaries. To confirm our assumption, transmission electron microscopic (TEM) images of the FAPbh film with 2D PVSK is analyzed in FIGS. 27C and 27E. The inset of FIG. 27C shows a chunk of the polycrystalline film scratched off from the substrate. Several hundreds of nanometer sized grains and their boundaries are clearly visible from the image, and from which one of the grains is magnified in FIG. 27C. The region (1) in FIG. 27C, which is the grain interior, is magnified and analyzed tying Fast Fourier transform (FFT) in FIG. 27D, in which an inter-planar spacing of 3.2 A is well matched with the (002) reflection of cubic FAPbh (Table 3). At region (2), which is grain boundary, the FFT analysis revealed an inter-planar distance of 8.0 A (FIG. 28E), correlating to a characteristic (002) reflection of 2D PEAaPbL* (FIG. 28 and Table 4).

[00129] FIG. 28 shows X-ray diffraction pattern of the PEAzPbU perovskite film, according to an embodiment of the present invention. The film is coated on GTO substrate. Table 3 lists d spacing values for FAo.98Cso.o2Pbh with 1.67 mol% PEA2PbL». The values are calculated from X-ray diffraction pattern in FIG. 11 using Bragg's law.

[00130] Table 4 lists Calculated d spacing values for PEA2PM4. The values are calculated from an X-ray diffraction pattern using Bragg's law.

TABLE 4

hkl d ( A)

001 16.0

002 8.10

003 5.42

004 4.07

005 3.26

006 2.72

[00131] This supports the presence of 2D PVSK at grain boundaries, which is further confirmed by elemental distribution (EDS) analysis (FIGS. 29A and 29B). At grain boundary regions, relatively larger amounts of carbon and nitrogen are detected, which could be due to presence of phenylethylammonium cation in the 2D PVSK.

[00132] FIGS. 29A and 29B show the energy dispersive X-ray spectroscopic (EDS) analysis, according to an embodiment of the present invention. FIG. 29A shows a Drift corrected scanning transmission electron microscopic (STEM) image of the FA0.98CS0.02PM3 perovskite with 1.67 mol% of PEA2PM4 perovskite. A red arrow indicates the profile where EDS line analysis is performed. FIG. 29B shows Elemental distribution profile obtained from the EDS line scan. Grain boundary region is indicated with dashed line and red arrows.

[00133] Band structure and electrical properties: FIGS. 30A-30F show band alignment and local conductivity with 2D perovskite, according to an embodiment of the present invention. FIG. 30A depicts schematics of the device incorporating polycrystalline 3D perovskite film with 2D perovskite at grain boundaries, according to an embodiment of the present invention. FIG. 30B shows a band structure of each layer in device analyzed by ultraviolet photoelectron spectroscopy (UPS) and Tauc plots, according to an embodiment of the present invention, FIGS. 30C and 30E are conductive atomic force microscopic (c-AFM) images of bare FAPbb, according to embodiment of the present invention. FIGS. 30D and 30D are conductive atomic force microscopic (c-AFM) images of 2D perovskile films on Sn0₂ coated 1TO glass, according to embodiment of the present invention. The measurement is carried out with bias voltage of 100 mV under (FIGS. 30C and 30D) room light or low intensity light illumination provided by the AFM setup. Inset of each image shows corresponding topology of the films. Scale bar at left side is for (FIG. 30C) and (FIG. 30D) while at right side is for (FIG. 30E) and (FIG. 30F).

[00134] The schematic FIG. 30A shows 2D PVSK formation at the grain boundaries of the 3D PVSK film. Since the 2D PEA2PM4 PVSK with aromatic rings and longer alkyl chains is expected to be more resistant to moisture, it protects the defective grain boundaries of 3D PVSK, resulting in significantly enhanced moisture stability of the film. Regardless of the improved stability, however, one can expect degraded electronic properties of the film due to the poor charge carrier mobility of the 2D PVSK

[00135] We investigated the band structure of FAo wCsoxaPbh (with 1.67 mol% of 2D PVSK) and PEA₂PbL· PVSK, which is illustrated in FIG. 30B. The valence band maximum is measured using ultraviolet photoeleclron spectroscopy (UPS, FIGS. 31 A-31C), while the Eg is determined from Tauc plots (FIG. 32A and 32B). As shown in FIG. 30B, FA0.98CS0.02PM3 and PEA₂Pbl4 PVSK shows type I band alignment. Such band alignment resembles the alignment between PVSK and Pbb formed at grain boundaries, which is found to reduce charge recombination and assist in charge separation/collection. Thus, analogous advantages of 2D PVSK at grain boundaries can be expected. Conductive atomic force microscopy (c-AFM) is performed in FIG. 30C-30F to see spatially resolved electrical properties of the films. Under ambient light conditions (FIG. 30C and FIG. 30D), current flow in the PVSK film with 2D PVSK is higher at/near the grain boundaries while relatively uniform current flow is observed in the bare FAPbb film. With light illumination (FIG. 30E and FIG. 30F), the current flow is further enhanced at/near the grain boundaries with 2D PVSK whereas current flow in bare FAPbb film is uniformly increased, which indicates charge separation and collection of photo-generated electrons is facilitated more so at grain boundaries with 2D PVSK As suggested for Pbl₂, thin 2D PVSK regions at grain boundaries might suffer downward band bending under illumination (dashed line in FIG. 30B) where photo-generated electrons are transferred from grain interiors. Due to the high potential barrier to the holes, charge recombination will be reduced, which might be the origin of the superior PL lifetime and photovoltaic performance with 2D PVSK.

[00136] FIGS. 31A-31C show an ultraviolet photoelectron spectroscopic (UPS) analysis, according to an embodiment of the present invention. FIG. 31 A shows complete spectra and low binding energy onset of UPS analysis of FAawCso-mPbb with 1.67 mol% PEA₂Pbl₄ and pure PEA?PbL· perovskite films, according to an embodiment of the present invention. FIG. 3 IB shows complete spectra and low binding energy onset of UPS analysis of FAo.98Cso.o2Pbl3 with 1.67 mol% PEAaPbl*, according to an embodiment of the present invention. FIG. 31C shows Complete spectra and low binding energy onset of UPS analysis of pure PEAzPbL perovskite films, according to an embodiment of the present invention.

[00137] FIGS. 32A and 32B show a determination of bandgap using a Tauc plot, according to an embodiment of the present invention. FIG. 32A is a Tauc plots for FAo.98Cso.o2Pbl3 perovskite with 1.67 mol% PEA2PM4, according to an embodiment of the present invention. FIG. 32B is a Tauc plot for pure PEAsPbU perovskite films, according to an embodiment of the present invention. Onset region is fitted to derive the optical bandgap.

[00138] Ambient and operational stability: Finally, the stability of the control and target devices is compared. FIGS. 33A-33C show improved stability with 2D perovskite, according to an embodiment of the present invention. FIG. 33A shows an evolution of power conversion efficiency (PCE) of control and target devices, according to an embodiment of the present invention. The devices are stored under dark with controlled humidity (relative humidity, RH lower than 30%). FIG. 33B shows a maximum power point tracking of the devices under 1 sun illumination in ambient condition without encapsulation, according to an embodiment of the present invention. FIG. 33C shows an evolution of the PCEs measured from the encapsulated control and target devices exposed to continuous light (90±5 mWcnV²) under open-circuit condition, according to an embodiment of the present invention. The stabilized PCEs are measured at each time. Initial stabilized PCEs for control and target devices are 14.5% and 17.5%, respectively. The broken lines are linear fit of the post bum-in region (after 48 h). FIG. 33A demonstrates the changes in PCE of the unencapsulated devices stored in a desiccator, according to an embodiment of the present invention.

[00139] A relative humidity lower than 30%, evolution of an individual photovoltaic parameter can be found in FIG. 34A-34C. FIG. 34A-34C shows ambient stability test of control and target devices, according to an embodiment of the present invention. FIG. 34A shows an evolution of short-circuit current density (/sc), according to an embodiment of the present invention. FIG. 34B shows an evolution of open circuit voltage (Foe), according to an embodiment of the present invention. FIG. 34C shows an evolution of a fill factor (FF) of the control and target device, according to an embodiment of the present invention.

[00140] While the control device degraded by 29% for 1392 hours, the target device maintained 98% of its initial efficiency during this time. The operational stability of the devices is also compared by maximum power point (MPP) tracking under 1 sun illumination in FIG. 33B. Without encapsulation, the PCE of the control device rapidly degraded by 68% during 450 min whereas that of target device is relatively less (20%) during the time. We performed 500 h of light exposure test with the encapsulated control (bare FAPbb device) and target devices (w/ 1.67 mol% 2D PVSK). The encapsulated devices are exposed to ca. 0.9 sun (90±l 0 mWcm^'2) under open-circuit condition, of which the steady-state PCE is periodically measured for different exposure time. As shown in FIG. 33C, both of the devices showed a rapid initial decay in PCE followed by slower decay with an almost linear profile, which is in agreement with previous reports. After 500 h of exposure, the control device degraded to ca. 52.3% of its initial PCE whereas the target device maintained 72.3% of the initial PCE, indicating enhanced stability with addition of 2D PVSK. We could extract tentative T80 (time at which PCE of the device decays to 80% of initial PCE) for the devices by fitting of the post-bum-in region in which the PCE of the device shows an almost linear decay profile (after 48 h). The T80 for control and target devices are calculated to be 592 h and 1362 hours, respectively. This indicates the stability of the device is significantly improved with addition of 2D PVSK.

[00141] We also performed MPP tracking of the encapsulated target device under 1 sun (100 mW/cm²) illumination in FIG. 35. FIG. 35 shows a maximum power point tracking of the target device, according to an embodiment of the present invention. The measurement is performed under 1 sun illumination in ambient condition with encapsulation. Inset of FIG. 35 shows the PCE without normalization. 18.7% of initial PCE is degraded for 130 h of operation, which is relatively slower compared to the device maintained at open-circuit condition. This is correlated with previous studies that attributed the faster degradation under open-circuit condition to larger number of photo-generated charge carriers recombining within the device. Under operational condition with abundant photo-generated charges and built-in electric field, the major factors causing the degradation of the devices might be the highly mobile and reactive charged defects (ions) and/or trapped charge carriers associated with it. We suppose that migration of the charged defects or ions is possibly suppressed by 2D PVSK at grain boundaries.

[00142] FIGS. 36A-36D show temperature-dependent conductivity of (a) bare FAPbh film, (b) with 1.67 mol% 2D PEAaPbLi perovskite, (c) bare FAPbh film and (d) with 1.67 mol% 2D PEA^PbLt perovskite, according to embodiments of the present invention. Red circles in (b) indicate the data measured under moderate light illumination (intensity lower than 10 mWcm ²). Current-voltage curves measured from the devices at 180 K. The temperature-dependent conductivity (ø) measurement of the lateral devices is performed to evaluate the activation energy for the ion migration (FIG. 36A). The activation energy (E*) for the migration can be determined according to the Nemst-Einstein relation,

where k is Boltzmann constant, ø& is a constant.

[00143] Inset of FIG. 36A describes the structure of the lateral devices. With bare

FAPbh PVSK, exponential enhancement in conductivity is clearly identified at around 130 K

(FIG. 36A), which is attributed to contribution of ions. The E* for bare FAPbh film is calculated to be 0.16 eV, indicating significant contribution of activated ions at room temperature, which might cause degradation of the material and device under operational condition with built-in electric field. The pronounced current-voltage hysteresis behavior is observed even at very low temperature (180 K, FIG. 36C). In case of the PVSK film with 2D PVSK, the film did not show noticeable enhancement in conductivity with increased temperature although the overall conductivity is relativity lower than the bare FAPbl_? film (FIG. 36C). Even with moderate light illumination, it does not show the indicative of activated ions. As a result, the current-voltage curve did not show any hysteresis behavior (FIG. 36D). As the grain boundaries of 3D perovskite are reported to be a major pathway for the migration of ions, passivating the grain boundaries by incorporation of the ion-migration- immune 2D PVSK likely suppressed overall ion migration in the target device. In addition, the improved phase purity of the film might also partially contribute to the suppressed ion migration because the secondary phase can generate defect sites that can act as an additional pathway for ion migration. We believe the suppressed ion migration contributes to enhanced operational stability of the target devi ce.

[00144] According to some embodiment of the present invention, we demonstrate a reproducible way to fabricate phase-pure formamidinium tri-iodide PVSK with high optoelectronic quality and stability by incorporating 2D PVSK. The large phenylethylammonium molecules from 2D PVSK precursors interact with FAPbh crystals to facilitate formation of the cubic PVSK phase during crystallization, which subsequently functionalize the grain boundaries after completion of the crystallization. The resulting phase-pure PVSK film has an identical E_g (1.48 eV) to that of pure FAPbh with an order of magnitude enhanced PL lifetime. Average PCE of 20.05^0.45% over 74 devices and the highest stabilized PCE of 20.64% (certified stabilized PCE of 19.77%) is achieved. Regardless of its low Eg, the PVSK solar cell showed the peak Voc of 1.130 V, corresponding to the lowest loss-in-potential of 0.35 V among all the reported PVSK solar cells. Owed to the functionalized grain boundaries by the 2D PVSK, the phase stability of the film under high RH significantly improved and migration of ions (or charged defects) is suppressed, resulting in significantly improved ambient and operational stability of the device. We believe our approach to utilize spontaneously formed gram boundary 2D PVSK will provide important insights for the research community to design PVSK materials to achieve record PCEs accompanied by high stability and longevity. [00145] Methods of Synthesis of Phenylethylammonium iodide: In a typical synthesis, 4.8 g of phenethylamine (39.6 mmol, Aldrich, >99%) is dissolved in 15 mL of ethanol and placed in iced bath. Under vigorous stirring, 10.8 g of hydroiodic acid (57 wt% in HbO, 48.1 mmol, Sigma-Aldrich, 99.99%, contains no stabilizer) is slowly added to the solution. The solution is stirred overnight to ensure complete reaction, which is followed by removal of die solvent by a rotary evaporator. The resulting solid is washed with diethyl ether several times until the color is changed to white. The white solid is further purified by recrystallization in mixed solvent of methanol and diethyl ether. Finally, white plate-like solid is filtered and dried under vacuum (yield around 90%).

[00146] Device fabrication: Indium doped tin oxide (GGO) glass is cleaned with successive sonication in detergent, deionized (Dl) water, acetone and 2-propanol for 15 min respectively. The cleaned substrates are further treated with UV-ozone to remove the organic residual and enhance the wettability. 30 mM SnCb^HaO (Aldrich, >99.995%) solution is prepared in ethanol (anhydrous, Decon Laboratories Inc.), which is filtered by 0.2 pm syringe filter before use. To form a Sn0₂ layer, the solution is spin-coated on the cleaned substrate at 3000 rpm for 30 s, which is heat-treated at .150 °C for 30 min. After cooling down to room temperature, the spin-coating process is repeated one more time, which is followed by annealing at 150 °C for 5 min and 180 °C for 1 h. The SnCh coated 1TO glass is further treated with UV-ozone before spin-coating of PVSK solution. The PVSK layer is prepared by the modified adduct method.²⁶ The bare FAPbh layer is formed from the PVSK. solution containing equimolar amount of HC(NH₂)2l (FAI, Dyesol), Pbh (TCI, 99.99%) and N- Methyl-2-pyrrolidone (NMP, Sigma-Aldrich, anhydrous, 99.5%) in N,N-Dimethylformamide (DMF, Sigma-Aldrich, anhydrous, 99.8%). Typically, 172 mg of FAI, 461 mg of Pbh and 99 mg of NMP are added to 600 mg of DMF. For the 2D PVSK (PEAaPbL) and Cs incorporated PVSK, corresponding amount of FAI is replaced with PEAI and Cs!. For example, FAPbh with 1.67 mol% PEAaPbLt PVSK is formed from the precursor solution containing 166.4 mg of FAI, 8.2 mg of phenylethylammonium iodide (PEAI), 453.4 mg of Pbh and 97.4 mg of NMP in 600 mg of DMF. With 2 mol% of Cs, the precursor solution is prepared by mixing 163.0 mg of FAI, 8.2 mg of PEAI, 5.0 mg of Csl (Alfa Aesar, 99.999%), 453.4 mg of Pbh and 97.4 mg of NMP in 600 mg. For the best performing devices in FIG. 2D, the amount of DMF is adjusted to 550 mg. Spin-coating of PVSK and hole transporting layer is performed in a glove box filled with dry air. The PVSK solution is spin-coated at 4000 rpm for 20 s where 0.15 mL of diethyl ether (anhydrous, >99.0%, contains BHT as stabilizer, Sigma- Aldrich) is dropped after 10 s on the spinning substrate. The resulting transparent adduct fi lm is heat-treated at 100 °C for 1 min followed by 150 °C for 10 min. (for the best performing target device, the annealing condition is adjusted to 80 °C 1 min followed by 150 °C for 20 min) The spiro-MeOTAD solution is prepared by dissolving 85.8 mg of spiro-MeOTAD (Lumtec) in 1 mL of chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich) which is doped by 33.8 mΐ of 4-tert-butylpyridine (96%, Aldrich) and 19.3 mΐ of Li-TFSl (99.95%, Aldrich, 520 mgmL^"1 in acetonitrile) solution. The spiro-MeOTAD solution is spin-coated on the PVSK layer at 3000 rpm for 20 s by dropping 17 mΐ of the solution on the spinning substrate. On top of the spiro-MeOTAD layer, ca. 100 nm-thick sil ver or gold layer is thermally evaporated at 0.5 As^"1 to be used as an electrode.

[00147] Material characterization: The PVSK layer is coated on a Sn0₂ coated GGO substrate for the measurements. UV-vis absorption spectra are recorded by U-4100 spectrophotometer (HITACHI) equipped with integrating sphere. The monochromatic light is incident to the substrate side. X-ray diffraction (XRD) patterns are obtained by X-ray diffractometer (PANalytical) with Cu ka radiation at a scan rate of 4 "min ¹. Surface and cross sectional morphology of the films and devices are investigated by scanning electron microscopy (SEM, Nova Nano 230). For the cross sectional image, cross sectional surface of the sample is coated with ca. 1 nm-thick gold using sputter to enhance the conductivity. Transmission electron microscopic (TEM) analysis is performed by Titan Krios (FE1), The PVSK film is scratched off from the substrate and dispersed in toluene by sonication for 10 min, which is dropped on an aluminum grid. Accelerating voltage of 300 kV is used for the measurement. Steady-state photoluminescence (PL) signal is analyzed by a Horiba Jobin Yvon system. A 640 nm monochromatic laser is used as an excitation fluorescence source. Time resolved PL decay profiles are obtained using a Picoharp 300 with time-correlated single-photon counting capabilities. The films are excited by a 640 nm pulse laser with a repetition frequency of 100 kHz provided by a picosecond laser diode head (PLD 800B, PicoQuant). The energy density of the excitation light is ca. 1.4 nJcmf², in which carrier annihilation and non-geminate recombination are negligible^1,1. Ultraviolet photoelectron spectroscopic (UPS) analysis is carried out using Kratos Ultraviolet photoelectron spectrometer. He I (21.22 eV) source is used as an excitation source. The PVSK films arc coated on ITO substrate and grounded using silver paste to avoid the charging during the measurement Conductive atomic force microscopic (AFM) measurement is performed by Bruker Dimension Icon Scanning Probe Microscope equipped with TUNA application module. The TUNA module provides ultra-high tunneling current sensitivity (<1 pA) with high lateral resolution. Antomony doped Si tip (0.01-0.025 Ohm-cm) coated with 20 nm Pt-lr is used as a probe. To avoid the electrically driven degradation during the measurement, low bias voltage (100 mV) is applied. The measurement is carried out under either room right or low intensity light illumination provided by AFM setup. The temperature-dependent conductivity measurement is carried out using a commercial probe station (Lakeshore, TTP4) in which temperature of the device is controlled by thermoelectric plate and flow of liquid nitrogen. The electrical measurement is conducted with a source/measurement unit (Agilent, B2902A).

[00148] Device characterization: Current density- voltage (J~V) curves of the devices are measured using KEITHLEY 2401 source meter under simulated one sun illumination (AM 1.5G, 100 mWcm^"2) in ambient atmosphere. The one sun illumination is generated from Oriel SoBA with class AAA solar simulator (Newport), in which light intensity is calibrated by NREL-certified Si photodiode equipped with KG-S filter. Typically, the J-V curves are recorded at 0.1 Vs^"1 (between 1.2 V and -0.1 V with 65 data points and 0.2 s of delay time per point). During the measurement, the device is covered with metal aperture (0.100 cm²) to define the active area. All the devices are measured without pre-conditioning such as light- soaking and applied bias voltage. Steady-state power conversion efficiency is calculated by measuring stabilized photocurrent density under constant bias voltage. The external quantum efficiency (EQE) is measured using specially designed system (Enli tech) under AC mode (frequency=133 Hz) without bias light. For electroluminescence measurement, a KEITHLEY 2400 source meter and silicon photodiode (HAMAMATSU SI 133-14, Japan) are used to measure Current-voltage-luminance characteristics of PVSK solar cells. Electroluminescence spectra are recorded by HORIBA JOB1N YVON system, and used to calculate radiance and external quantum efficiency of PVSK solar cells. All the devices are assumed as Lambertian emitter in the calculation.

[00149] Stability test: Moisture stability of the films is tested by exposing the PVSK films under relative humidity of 80±5% and room light. Absorbance of the films is measured every 2 h while XRD of the films are recorded every 12 h. For the devices, ex-situ test is conducted by storing the devices in desiccator (relative humidity, RH < 30%) under dark condition. The device is taken out and measured in ambient condition. For operational stability, maximum power point (MPP) tracking and continuous light exposure under open- circuit condition are performed in ambient condition (RH around 50%, T around 40 °C). For the MPP tracking, the photocurrent density is monitored while the devices are biased at MPP under 1 sun illumination. For light exposure under open-circuit condition, the encapsulated devices are exposed to ca. 0.9 sun (90±10 mWcm^"2) generated by halogen lamps under open- circuit condition, of which steady-state PCE is periodically measured with different exposure time under 1 sun illumination. The encapsulation of the device is performed inside the glove box filled with nitrogen by using an UV-curable adhesive and a piece of glass. The glass substrate is superimposed on active layer and fixed with the UV-curable adhesive.

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[00150] The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.

Claims

WE CLAIM:

1. An optoelectronic device, comprising:

a first electrode;

a second electrode spaced apart from said first electrode; and

an active layer between said first and second electrodes,

wherein said active layer comprises a combination of FA Cs Fbl perovskile and A Fbl two-dimensional (2D) perovskite in a ratio r,

wherein FA is formamidinium,

wherein A is selected from the group consisting of phenethylanunonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2- (4-Trifl uoromethylphenyl)ethylamine (FMPEA), 4-F1 uorophenethylamine (FPEA), 3,4- Difluorobenzylamine (DFPEA), and any alkyl amine groups,

wherein r is at least 0,1 mol% and less than 20 mol%, and

wherein x is within the range of 0,0 to 0.2.

2. The device according to claim 1 , wherein said active layer comprises grains of said FA Cs Pb^ perovskite with said A Pbl 2D perovskite formed in grain boundaries.

3. The device according to claim 1 or 2, wherein r is at least 0.8 mol% and less than 3 mol%.

4. The device according to claim 1 or 2, wherein r is at least 1 mol% and less than 2 mol%.

5. The device according to claim 1 or 2, wherein r is at about 1.67 mol%.

6. The device according to any one of claims 1-5, wherein x is about 0.02.

7. The device according to any one of claims 1 -6, wherein at least one of said first and second electrodes is a transparent electrode.

8. A method of producing an optoelectronic device, comprising:

forming a first electrode;

forming a second electrode spaced apart from said first electrode; and

producing an active layer between said first and second electrodes,

wherein said active layer comprises a combination of FA Cs Fbl perovskile and A Pbl two-dimensional (2D) perovskile in a ratio r,

wherein FA is formamidinium,

wherein A is selected from the group consisting of phenethylanunonium (PEA), phenylamine (PA) benzylammonium (BZA), butylammonium (BA), ethylenediamine (EDA), 2- (4-Trifl uoromethylphenyl)ethylamine (FMPEA), 4-F1 uorophcnethylamine (FPEA), 3,4- Difluorobenzylamine (DFPEA), and any alkyl amine groups,

wherein r is at least 0,1 mol% and less than 20 mol%, and

wherein x is within the range of 0.0 to 0.2.

9. The method according to claim 8, wherein said active layer comprises grains of said FA Cs Pb^ perovskile with said A Pbl^D perovskile formed in grain boundaries.

10. The method according to claim 8 or 9, wherein r is at least 0.8 mol% and less than 3 mol%.

11. The method according to claim 8 or 9, wherein r is at least 1 mol% and less than 2 mol%.

12. The method according to claim 8 or 9, wherein r is at about 1.67 mol%.

13. The method according to any one of claims 8-12, wherein x is about 0.02.

14. The method according to any one of claims 8-13, wherein at least one of said first and second electrodes is a transparent electrode.

15. A formamidinium perovskite film, comprising:

grains ofFA Cs Pbl perovskite; and

A Pbl two-dimensional pcrovskite formed in grain boundaries of said grains so as to stabilize a phase of said FA Cs Pbl pcrovskite within said grains,

wherein FA is formamidinium,

wherein A is selected from the group consisting of phenethylammonium (PEA), phenylamine (PA) benzylammonium (BZA), bulylammonium (BA), ethylenediamine (EDA), 2- (4-Trifl uoromethylphenyl)ethylamine (FMPEA), 4-F1 uorophenethylamine (FPEA), 3,4- Difluorobenzylamine (DFPEA), and any aikyl amine groups, and

wherein x is within the range of 0,0 to 0.2.

16. The formamidinium perovskite film of claim 15, wherein said FA. Cs Pbl^perovskite and A Pbl^ two-dimensional (2D) perovskite are in a ratio r, and

wherein r is at least 0.1 mol% and less than 20 mol%.

17. The formamidinium perovskite film of claim 16, wherein r is at least 0.8 mol% and less than 3 mol%.

18. The formamidinium perovskite film of claim 16, wherein r is at least 1 mol% and less than 2 mol%.

19. The formamidinium perovskite film of claim 16, wherein r is at about 1.67 mol%.

20. The formamidinium perovski te film according to any one of claims 15-19, wherein x is about 0.02.