WO2019017843A1

WO2019017843A1 - Composite film and method of forming the same

Info

Publication number: WO2019017843A1
Application number: PCT/SG2018/050354
Authority: WO
Inventors: Xin Yu Chin; Subodh Gautam Mhaisalkar; Nripan Mathews; Sjoerd Antonius Veldhuis; Natalia YANTARA; Annalisa Bruno; Bevita KALLUPALATHINKAL CHANDRAN
Original assignee: Nanyang Technological University
Priority date: 2017-07-18
Filing date: 2018-07-17
Publication date: 2019-01-24

Abstract

Various embodiments may provide a composite film comprising a plurality of first structures including a first halide perovskite material and a plurality of second structures including a second halide perovskite material different from the first halide perovskite material. An average size of the plurality of first structures may be smaller than an average size of the plurality of second structures. Preferably, the first structures are 3D nanoparticles comprising formamidinium lead bromide, and the second structures are 2D microplatelets comprising octylammonium lead bromide. More preferably, the two structures are formed by a ligand assisted reprecipitation method.

Description

COMPOSITE FILM AND METHOD OF FORMING THE SAME

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10201705871P filed on July 18, 2017, the contents of it being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[0002] Various aspects of this disclosure relate to a composite film. Various aspects of this disclosure relate to a method of forming a composite film.

BACKGROUND

[0003] Metal halide perovskites have emerged as the only solution-processable photovoltaic technology to outperform multi-crystalline silicon, by virtue of their intrinsic properties such as large absorption coefficient, balanced charge carrier transport, highly crystalline film formation, weak exciton binding energies, and slow bimolecular recombination.

[0004] Since 2014, perovskites have made strides in light-emitting applications with first demonstrations of amplified spontaneous emission. Light emitting devices (LEDs) including various perovskite compositions have been reported, with a vast majority describing LEDs with low efficiency and luminance. Low photoluminescence quantum yields (PLQYs) resulting from low exciton binding energies in three-dimensional (3D) perovskites (e.g. CH₃NH₃PbBr₃) and consequently slow electron-hole bimolecular recombination rates, especially at low carrier injection regimes where typical LEDs operate, is central to the below-par efficiency.

[0005] A high carrier density regime where PLQY could be higher would typically lead to material degradation. Further, under the high carrier density regime, the charge carrier concentrations (~10¹⁵ cm ³) are comparable to trap densities in 3D perovskites, and the large diffusion lengths (and charge carrier de-localization) lead to strong trap-mediated non-radiative recombination effects.

[0006] Strategies to counter these limitations include ultrathin emitter layers, nanostructured materials, grain size control through additives and process control, and formation of low- dimensionality layered-perovskites. Solvent engineering approaches led to methylammonium lead bromide (MAPbBr₃) nanograms (-100 nm grains) with maximum external quantum efficiency (EQE) of 8.5 % and efficiencies of -43 cd A ¹, with the enhanced performance attributed to increased radiative recombination rates in nanoparticles or small crystal domains, which are associated with the increased excitonic nature of the recombination. Long-chain ammonium halide surfactants provide surface passivation and limit the grain sizes to under 10 nm, resulting in enhanced EQE of 10.4%.

[0007] In quasi-two dimensional (quasi-2D) or Ruddlesden-Popper perovskites with PLQY of -70%, improved LED performances, and EQE of 11.7 %, have been attributed to an energy funneling mechanism related to the presence of domains exhibiting a range of energetic excited states, enabling spatial concentration and confinement of excitons and high electroluminescence.

[0008] These quasi-2D systems, despite showing promising performance, still have a lower concentration of emitting domains in the thin films. Furthermore, fabrication processes described thus far, involve precise control over surfactant addition and sensitive post-processing procedures, unfavorable to concerted commercialization efforts. Decoupling the synthesis of perovskite nanocrystals (NCs) from thin film processing step would allow for better control over crystal sizes and film thicknesses, thus improving fabrication reproducibility and advancing opportunities for scaling up device fabrication. Typically, long-chained capping ligands are employed during the nanocrystal synthesis to restrict the growth of the NCs and ensure colloidal stability of the solution. Despite providing good surface passivation, these long chain ligands may limit the electrical injection into the perovskite layer due to their insulating nature. However, promising results have been recently reported with methylammonium lead bromide (MAPbBr₃) nanocrystal LED, achieving external quantum efficiencies of approximately 3.8-5.1%.

SUMMARY

[0009] Various embodiments may provide a composite film. The composite film may include a plurality of first structures including a first halide perovskite material. The composite film may also include a plurality of second structures including a second halide perovskite material different from the first halide perovskite material. An average size of the plurality of first structures may be smaller than an average size of the plurality of second structures. [0010] Various embodiments may provide a method of forming a composite film according to various embodiments. The method may include forming a plurality of first structures including a first halide perovskite material. The method may also include forming a plurality of second structures including a second halide perovskite material different from the first halide perovskite material. An average size of the plurality of first structures may be smaller than an average size of the plurality of second structures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

FIG. 1 is a general illustration of a composite film according to various embodiments.

FIG. 2 shows a general illustration of a method of forming a composite film according to various embodiments.

FIG. 3A shows a plot of current density (in milliamperes per square centimeter or mA cm ²) / luminance (in candela per square meter or cd m^~2) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of light emitting devices (LEDs) according to various embodiments, with the inset showing a plot of the normalized electroluminescence (EL) intensity as a function of wavelength (in nanometer) illustrating the spectrum of the emitted light of the devices having nanocrystals formed with molar ratios of 5: 1 and 10: 1 of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments. FIG. 3B is a plot of energy (in electron volts or eV) as a function of position showing the schematic band diagram of the light emitting devices (LEDs) according to various embodiments.

FIG. 3C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m ²) illustrating the variation of characteristic current efficiency/ external quantum efficiency (EQE) with luminance for a device with device area of 3 mm² according to various embodiments.

FIG. 3D shows a plot of current efficiency (in candela per ampere or cd A ¹) / external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m ²) illustrating variation of current efficiency /EQE with luminance of a flexible (3 mm²) light emitting device (LED) and a large area (95.2 mm²) light emitting device (LED) according to various embodiments, with the insets showing images of the two devices according to various embodiments. Both devices may include NCs formed with 4 : 1 molar ratio of OA:PbBr₂.

FIG. 3E shows (top left) a box plot of luminance (x lO⁴ candela per square meter or cd m^~2) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂), (top right) a box plot of external quantum efficiency (EQE) (in percent or %) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂), (bottom left) a box plot of current efficiency (in candela per ampere or cd A ¹) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂), and (bottom right) a box plot of power efficiency (in lumens per watt or lm W^"1) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂) illustrating the characteristics of light emitting devices (LEDs) (n is the number of devices measured) based on different OA:PbBr₂ ratios according to various embodiments.

FIG. 4A is a plot of luminance (in candela per square meter or cd m ²) as a function of year showing the maximum luminance achieved for some conventional devices as a comparison to devices according to various embodiments.

FIG. 4B is a plot of peak external quantum efficiency (EQE) (in percent or %) as a function of year showing the peak EQE achieved for some conventional devices as a comparison to devices according to various embodiments.

FIG. 4C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of year showing the current efficiency achieved for some conventional devices as a comparison to devices according to various embodiments.

FIG. 5 is a table showing the different characteristics of some conventional devices fabricated to date compared to devices according to various embodiments.

FIG. 6 is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) illustrating the variation of power efficiency with luminance for a device with device area of 3 mm² according to various embodiments.

FIG. 7A is a plot of normalized electroluminescence intensity (EL) as a function of wavelength (in nanometer or nm) showing electroluminescence characteristics for samples prepared with different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments. FIG. 7B is a plot of normalized electroluminescence intensity (EL) as a function of wavelengths (in nanometer or nm) showing electroluminescence characteristics for samples at different bias according to various embodiments.

FIG. 7C is a plot of normalized electroluminescence intensity (EL) as a function of wavelengths (in nanometer or nm) showing electroluminescence characteristics for samples at different luminance intensities according to various embodiments.

FIG. 8 shows a plot of turn-on values (in volts or V) as a function of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) illustrating turn-on values shown as box plots for light emitting (LED) devices (n is number of devices measured) based on different OA:PbBr₂ ratios according to various embodiments.

FIG. 9 is a table showing device performance of devices according to various embodiments at various luminances.

FIG. 10 is a table showing a summary of the parameters of light emitting (LED) devices according to various embodiments used for constant current test.

FIG. 11 A shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of voltage (in volts or V) showing the current density-voltage (J-V) curves of electron only devices with different ratios of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments, with the inset showing the structure of an electron only device according to various embodiments.

FIG. 1 IB shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of ratios of n-octylamine (OA) to lead bromide (PbBr₂) showing box plots of current densities of electron only devices according to various embodiments at 4V.

FIG. l lC shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of voltage showing the current density-voltage (J-V) curves of hole only devices with different ratios of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments, with the inset showing the structure of a hole only device according to various embodiments.

FIG. 1 ID shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of ratios of n-octylamine (OA) to lead bromide (PbBr₂) showing box plots of current densities of hole only devices according to various embodiments at 2V. FIG. 12 is a plot of current (in amperes or A) as a function of voltage (in volts or V) showing the lateral conductivity measurements performed on composite films of different ratios of n- octylamine (OA) to lead bromide (PbBr₂) according to various embodiments.

FIG. 13 A shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of voltage (in volts or V) showing the current density - voltage characteristics of devices with different electron transport layers according to various embodiments, with the inset showing a plot of electroluminescence intensity (EL) as a function of wavelength illustrating the electroluminescence spectra at maximum luminance.

FIG. 13B is a plot of external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m^~2) showing the variation of EQE with luminance of devices with different electron transport layers according to various embodiments.

FIG. 13C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance

(in candela per square meter or cd m ²) showing the variation of current efficiency with luminance of devices with different electron transport layers according to various embodiments.

FIG. 13D is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance

(in candela per square meter or cd m ²) showing the variation of power efficiency with luminance of devices with different electron transport layers according to various embodiments.

FIG. 13E is a plot of energy (in electron volts or eV) showing band alignment of the device architecture.

FIG. 13F shows the molecular structures of hole and electron transporting materials used in devices according to various embodiments.

FIG. 14 is a table showing device characteristics of light emitting devices (LEDs) with different electron transporting layers according to various embodiments. The emissive layer of devices shown in FIGS. 13A-E, 14 may include the perovskite nanocrystals synthesized with ratio of OA:PbBr₂ of 4: 1.

FIG. 15 A is a plot of the square root of emission yield (in square root of counts per second or cps) as a function of energy (in electron volts or eV) showing the photoelectron spectroscopy in air (PESA) results of composite films according to various embodiments. FIG. 15B is a plot of energy (in electron volts or eV) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂) showing schematic representation of band levels of samples according to various embodiments prepared with different ratios of OA: PbBr₂.

FIG. 16A is an image of a light emitting device (LED) with active area of 3 mm² according to various embodiments operating at a voltage of 4.5 V.

FIG. 16B is an image of a light emitting device (LED) with active area of 9 mm² according to various embodiments operating at a voltage of 4.5 V.

FIG. 16C is an image of a light emitting device (LED) with active area of 15.2 mm² according to various embodiments operating at a voltage of 4.5 V.

FIG. 16D is an image of a light emitting device (LED) with active area of 35.2 mm² according to various embodiments operating at a voltage of 4.5 V.

FIG. 16E is an image of a light emitting device (LED) with active area of 95.2 mm² according to various embodiments operating at a voltage of 4.5 V. FIGS. 16A-E show a bright luminescence. FIG. 16F is an image of a light emitting device (LED) with active area of 3 mm² according to various embodiments operating at a voltage of 2.7 V.

FIG. 16G is an image of a light emitting device (LED) with active area of 9 mm² according to various embodiments operating at a voltage of 2.7 V.

FIG. 16H is an image of a light emitting device (LED) with active area of 15.2 mm² according to various embodiments operating at a voltage of 2.7 V.

FIG. 161 is an image of a light emitting device (LED) with active area of 35.2 mm² according to various embodiments operating at a voltage of 2.7 V.

FIG. 16J is an image of a light emitting device (LED) with active area of 95.2 mm² according to various embodiments operating at a voltage of 2.7 V.

FIG. 17A is a plot of current density (in milliamperes per centimeter square or mA cm ²) / luminance (in candela per square meter or cd m ²) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of three different devices according to various embodiments, with the inset showing a plot of electroluminescence intensity as a function of wavelength (in nanometers or nm) illustrating the electroluminescence spectra at maximum luminance. FIG. 17B is a plot of normalized electroluminescence as a function of time (in seconds or s) illustrating constant current stability of the devices according to various embodiments.

FIG. 17C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m^~2) illustrating the current efficiency of the devices according to various embodiments.

FIG. 17D is a plot of luminous power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m^~2) illustrating the luminous power density as a function of luminance of the devices according to various embodiments used for the constant current stability test.

FIG. 18A is a plot of current density (in milliamperes per centimeter square or mA cm ²) / luminance (in candela per square meter or cd m ²) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of flexible light emitting devices (LEDs) according to various embodiments.

FIG. 18B is a plot of external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m ²) showing the variation of EQE of flexible light emitting devices (LEDs) according to various embodiments.

FIG. 18C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m ²) illustrating the variation of current efficiency of flexible light emitting devices (LEDs) according to various embodiments.

FIG. 18D is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) showing the variation of luminous power efficiency of the flexible light emitting devices (LEDs) according to various embodiments.

FIG. 19 is a table showing device characteristic parameters of flexible devices according to various embodiments.

FIG. 20 is a table showing device performance of flexible light emitting devices according to various embodiments at various luminances.

FIG. 21 A is a plot of current density (in milliamperes per centimeter square or mA cm ²) / luminance (in candela per square meter or cd m ²) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of devices of different areas according to various embodiments. FIG. 2 IB is a plot of external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m^~2) showing the variation of EQE of flexible light emitting devices (LEDs) according to various embodiments.

FIG. 21C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m^~2) illustrating the variation of current efficiency of different devices according to various embodiments.

FIG. 2 ID is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) showing the variation of luminous power efficiency of the different devices according to various embodiments.

FIG. 22 is a table showing device characteristic parameters of large area devices according to various embodiments.

FIG. 23 is a table showing device characteristic performance of large area light emitting devices (LEDs) according to various embodiments at various luminances.

FIG. 24A shows cross-sectional microscopy images of composite films according to various embodiments.

FIG. 24B shows top-view images of the composite films according to various embodiments. FIG. 25A shows an image of a film according to various embodiments in which the octylammonium lead bromide ((OA)₂PbBr4) microplatelets are indicated as darker areas outlined with solid lines.

FIG. 25B is an image showing selected-area electron diffraction pattern of marked area of the film according to various embodiments. The diffraction signals show both two-dimensional (2D) and three-dimensional (3D) phases.

FIG. 25C is an image showing multiple formamidinium lead bromide (FAPbBr₃) nanocrystals according to various embodiments, with the inset showing a crystalline FAPbBr₃ nanocrystal of approximately 10 nm in diameter according to various embodiments.

FIG. 26A shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 10: 1 according to various embodiments.

FIG. 26B shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 7: 1 according to various embodiments. FIG. 26C shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 5: 1 according to various embodiments.

FIG. 26D shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 4: 1 according to various embodiments.

FIG. 26E shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 3: 1 according to various embodiments.

FIG. 27 shows (left) a top view field emission scanning electron microscopy image of a composite film according to various embodiments, and (right) a schematic of a portion of the composite film illustrating charge/energy transfer during light emitting device (LED) operation in the composite film according to various embodiments on the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) layer.

FIG. 28A is a plot of normalized photoluminescence (PL) intensity as a function of wavelength (in nanometers) showing the absorption and steady-state photoluminescence (excitation wavelength or λ«χ = 405 nm) spectra of (OA)₂PBr₄ layer according to various embodiments. FIG. 28B is a plot of normalized photoluminescence (PL) intensity as a function of wavelength showing the excitation spectra for the different (OA)₂PBr₄ layers according to various embodiments.

FIG. 28C is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) of an ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments, with the inset showing a plot of volume fraction as a function of nanocrystal diameter (in nanometer or nm) illustrating the particle size distribution of the ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

FIG. 28D is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) of an ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments, with the inset showing a plot of volume fraction as a function of nanocrystal diameter (in nanometer or nm) illustrating the particle size distribution of the ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments. FIG. 29 A is a plot of normalized absorbance as a function of wavelength (in nanometers or nm) showing the absorption spectra of different mixed-phase inks synthesized with different ratios of OA: PbBr₂ according to various embodiments.

FIG. 29B is a plot of normalized photoluminescence (PL) intensity as a function of wavelength (in nanometers or nm) showing the photoluminescence (PL) spectra of different mixed-phase inks synthesized with different ratios of OA: PbBr₂ according to various embodiments.

FIG. 29C is a plot of normalized intensity as a function of wavelength (in nanometers or nm) showing the excitation spectra of different mixed-phase inks synthesized with different ratios of OA: PbBr₂ according to various embodiments.

FIG. 30A is a plot of normalized intensity as a function of angle 2Θ (in degrees or deg) illustrating X-ray diffraction patterns of FAPbBr₃ and (OA)₂PbBr₄ thin films prepared with ratios of OA:PbBr₂ ranging 3: 1 to 10: 1 according to various embodiments.

FIG. 30B is a plot of intensity as a function of angle ω (in degrees or deg) illustrating rocking curves of self-assembled rocking curves of self-assembled mixed-phase FAPbBr₃ and (OA)₂PbBr₄ thin films prepared with OA:PbBr₂ ranging 3: 1 to 10: 1 according to various embodiments.

FIG. 31 A is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 3 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

FIG. 3 IB is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 4 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

FIG. 31C is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 5 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

FIG. 3 ID is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr4 ink prepared with a ratio of OA:PbBr₂ of 7 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

FIG. 3 IE is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 10 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

FIG. 3 IF is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments. FIG. 31G is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments. FIG. 31H is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments. FIG. 311 is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments. FIG. 31 J is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments. FIG. 32A is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self-assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 4 : 1 and left for a period of 1 minute to initiate nanocrystal self-assembly according to various embodiments.

FIG. 32B is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self-assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 4 : 1 and left for a period of 3 minutes to initiate nanocrystal self-assembly according to various embodiments.

FIG. 32C is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self-assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 4 : 1 and left for a period of 5 minutes to initiate nanocrystal self-assembly according to various embodiments.

FIG. 32D is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self-assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 10 : 1 and left for a period of 1 minute to initiate nanocrystal self-assembly according to various embodiments.

FIG. 32E is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self-assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 10 : 1 and left for a period of 3 minutes to initiate nanocrystal self-assembly according to various embodiments. FIG. 32F is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self-assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of 10 : 1 and left for a period of 5 minutes to initiate nanocrystal self-assembly according to various embodiments.

FIG. 33A shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

FIG. 33B shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

FIG. 33C shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

FIG. 33D shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

FIG. 33E shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

FIG. 34A shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 520 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

FIG. 34B shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 525 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments. FIG. 34C shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 520 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

FIG. 34D shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 520 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

FIG. 34E shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 525 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

FIG. 35 A is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

FIG. 35B is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

FIG. 35C is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

FIG. 35D is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

FIG. 35E is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

FIG. 35F is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) showing a magnified view of FIG. 35E at approximately 525 nm for the sample according to various embodiments.

FIG. 36 is a table showing the tabulated decay times for the global fitting results of samples according to various embodiments shown in FIGS. 35A - F.

FIG. 37A shows a cathodoluminescence image of a spin-coated film according to various embodiments on a silicon substrate, with the emission spectrum is centered at 525 nm.

FIG. 37B shows cathodoluminescence images of a spin-coated film according to various embodiments on a silicon substrate, with the emission spectrum centered at 440 nm for the center image and 525 nm for the right image.

FIG. 38A is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm).

FIG. 38B is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm).

FIG. 38C is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm).

FIG. 38D is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments with excitation wavelength of 405 nm.

FIG. 39 is a plot of normalized fluorescence as a function of time (in nanoseconds or ns) illustrating time -resolved fluorescence decays of a film prepared with a ratio of OA : PbBr₂ of 5 : 1 collected at different emission wavelengths from 440 nm to 540 nm at an excitation wavelength (Aex) of 405 nm.

FIG. 40A is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 5: 1 according to various embodiments at 405 nm excitation.

FIG. 40B is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 3: 1 according to various embodiments at 405 nm excitation.

FIG. 40C is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 4: 1 according to various embodiments at 405 nm excitation.

FIG. 40D is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 7: 1 according to various embodiments at 405 nm excitation.

FIG. 40E is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 10: 1 according to various embodiments at 405 nm excitation.

FIG. 41 A is a plot of time (in nanoseconds or ns) as a function of emission wavelength (in nanometers) showing transient photoluminescence (PL) spectra of a film prepared with a ratio of

OA : PbBr₂ of 5 : 1 according to various embodiments at time intervals from 0-4 picoseconds (ps).

FIG. 4 IB is a plot of normalized fluorescence as a function of wavelength (in nanometers or nm) illustrating normalized transient photoluminescence (PL) spectra of a film prepared with a ratio of

OA : PbBr₂ of 5 : 1 according to various embodiments at different time delays after excitation, as well as the steady-state PL spectrum of the film according to various embodiments.

FIG. 42A is a plot of photoluminescence quantum yield PLQY (in percent or %) as a function of power density (in milli-Watts per centimeter square or mW cm ²) illustrating variation of PLQY due to excitation fluence after continuous-wave (CW) laser excitation of the film according to various embodiments at wavelengths of 405 nm and 447 nm. PLQY in the low power density regime (up to 6 mW cm ²) recorded values of 80% consistently for both λβχ = 405 nm (i.e. 2D

MPLs are excited) and at λβχ = 447 nm (i.e. 2D MPLs are not excited).

FIG. 42B is a plot of the absolute photoluminescence quantum yield PLQY as a function of power density (in milli-Watts per centimeter square or mW cm ²) illustrating variation of PLQY due to excitation fluence of various films according to various embodiments. The measurement error may be approximately 5 - 10%. FIG. 43 shows a schematic diagram representing the energy cascade from two dimensional microplatelets (2D MPLs) to nanocrystals (NCs) of graded sizes in composite films according to various embodiments.

DETAILED DESCRIPTION

[0012] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0013] Embodiments described in the context of one of the methods or film are analogously valid for the other methods or films. Similarly, embodiments described in the context of a method are analogously valid for a film, and vice versa.

[0014] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0015] The word "over" used with regards to a deposited material formed "over" a side or surface, may be used herein to mean that the deposited material may be formed "directly on", e.g. in direct contact with, the implied side or surface. The word "over" used with regards to a deposited material formed "over" a side or surface, may also be used herein to mean that the deposited material may be formed "indirectly on" the implied side or surface with one or more additional layers being arranged between the implied side or surface and the deposited material. In other words, a first layer "over" a second layer may refer to the first layer directly on the second layer, or that the first layer and the second layer are separated by one or more intervening layers.

[0016] The film as described herein may be operable in various orientations, and thus it should be understood that the terms "top", "topmost", "bottom", "bottommost" etc., when used in the following description are used for convenience and to aid understanding of relative positions or directions, and not intended to limit the orientation of the film.

[0017] In the context of various embodiments, the articles "a", "an" and "the" as used with regard to a feature or element include a reference to one or more of the features or elements.

[0018] In the context of various embodiments, the term "about" or "approximately" as applied to a numeric value encompasses the exact value and a reasonable variance.

[0019] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0020] A methodology that would allow for precise nanoparticle synthetic control and assembly to form highly efficient emitter layers may be requisite for a breakthrough in device performance.

[0021] FIG. 1 is a general illustration of a composite film 100 according to various embodiments. The composite film 100 may include a plurality of first structures 102 including a first halide perovskite material. The composite film 100 may also include a plurality of second structures 104 including a second halide perovskite material different from the first halide perovskite material. An average size of the plurality of first structures 102 may be smaller than an average size of the plurality of second structures 104.

[0022] In other words, the film 100 may include different structures 102, 104 including different halide perovskite materials. A size of a typical first structure 102 may be smaller than a size of a typical second structure.

[0023] In various embodiments, the plurality of first structures 102 may be nanostructures. In various embodiments, the plurality of second structures 104 may be microstructures.

[0024] In the current context, a microstructure may be a structure in which all dimensions of the structure are less than 100 μπι. Correspondingly, a nanostructure may be a structure in which all dimensions of the structure are less than 100 nm.

[0025] The second structures 104 or the microstructures may be two-dimensional (2D) or layered structures. The second structures 104 or the microstructures may extend along a plane. In various embodiments, the second structures 104 or the microstructures may be microplatelets. In various embodiments, the plurality of microplatelets may have edge lengths from 0.1 μπι to 10 μηι, e.g. from 0.5 μπι to 2 μπι. In other words, the longest dimension of the microplatelets may range from 0.1 μπι to 10 μπι, e.g. from 0.5 μπι to 2 μπι.

[0026] In various other embodiments, the second structures 104 or microstructures may be, for instance, microplates, microsheets, microdisks, or microlayers.

[0027] The first structures 102 or the nanostructures may be three-dimensional (3D) structures. In various embodiments, the first structures 102 or the nanostructures may be nanoparticles. In various embodiments, the plurality of nanoparticles may have diameters from 1 nm to 50 nm, e.g. from 5 nm to 30 nm.

[0028] In various embodiments, the second halide perovskite material may be a two- dimensional perovskite material.

[0029] In various embodiments, the first halide perovskite material may be a three-dimensional perovskite material.

[0030] In various embodiments, the second halide perovskite material may include a ligand cation, such as an octylammonium cation.

[0031] In various embodiments, the second halide perovskite material may be an alkylammonium metal halide perovskite, such as octylammonium lead bromide ((OA)₂PbBr₄).

[0032] In various embodiments the first halide perovskite material may include a formamidinium cation.

[0033] In various embodiments, the first halide perovskite material may be formamidinium metal halide perovskite, such as formamidinium lead bromide.

[0034] In various embodiments, the composite film 100 may include FAPbBr₃ nanocrystals 102 and (OA)₂PbBr₄ microplatelets 104.

[0035] In various embodiments, a bandgap of the plurality of first structures 102 may be smaller than a bandgap of the plurality of second structures 104. A bandgap of a first structure of the plurality of first structures 102 may be smaller than a bandgap of a second structure of the plurality of second structures 104.

[0036] In various embodiments, each first structure of the plurality of first structures 102 may be in contact with a second structure of the plurality of second structures 104.

[0037] In various embodiments, the plurality of first structures 102 may be interspersed with the plurality of second structures 104. In various embodiments, a plurality of first structures 102, e.g. FAPbBr₃ nanocrystals may be between two second structures 104, e.g. (OA)₂PbBr4 microplatelets.

[0038] A first structure may be attached to or chemically bonded to a second structure.

[0039] In various embodiments, the composite film 100 may exhibit a first emission peak, and a second emission peak. The second emission peak may be at a longer wavelength than the first wavelength. In various embodiments, the second emission peak may be centered at any wavelength from a range of about 525 nm to about 530 nm. The first emission peak may be centered at a wavelength of about 440 nm. In various embodiments, the second emission peak may reach a maximum emission value after 500 ps. The second emission peak may be associated by the second structures 104, and the time delay may be due to the energy cascade from the first structures 102 to the second structures. The first emission may have an ultrafast build-up in the range of less than 10 ps, and may reach a maximum emission value before 500 ps, e.g. before 200 ps.

[0040] Various embodiments may provide a device including a composite film 100 as described herein.

[0041] The device may be a light-emitting device.

[0042] In various embodiments, the device may further include an electron transport layer in contact with a first surface of the composite film 100. The device may also include a hole transport layer having in contact with a second surface of the composite film 100. The device may additionally include a first electrode in contact with the electron transport layer. The device may also include a second electrode in contact with the hole transport layer.

[0043] The device may include the first electrode, the electron transport layer on the first electrode, the composite film 100 on the electron transport layer, the hole transport layer on the composite film 100, and the second electrode on the hole transport layer.

[0044] In various embodiments, the electron transport layer may include any suitable organic material or semiconductor (e.g. small molecules, conjugated polymer, macromolecules etc.), or any suitable inorganic material or semiconductor (e.g. metal oxides, graphene etc.). The electron transport layer may include one or more organic materials or semiconductors such as 2,4,6-Tris[3- (diphenylphosphinyl)phenyl]-l,3,5-triazine (PO-T2T), 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2- methylpyrimidine (B3PYMPM), l,3,5-Tris(l-phenyl-lHbenzimidazol- 2-yl)benzene (TPBi), 1,3- Bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB), 2,9-Dimethyl-4,7-diphenyl-l, 10- phenanthroline (BCP), 4,7-Diphenyl-l,10-phenanthroline (BPhen), and/or Tris(2,4,6-trimethyl-3- (pyridin-3-yl)phenyl)borane (3TPYMB). The electron transport layer may include one or more inorganic materials or semiconductors such as aluminum oxide (A10_x), titanium oxide (TiO_x), tin oxide (SnOx), graphene, graphene oxide, and/or molybdenum disulphide (M0S2).

[0045] In various embodiments, the hole transport layer may include any suitable organic material or semiconductor, or any suitable inorganic material or semiconductor. The hole transport layer may include one or more organic materials or semiconductors such as poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS), Dipyrazino[2,3-f :2',3'-h ]quinoxaline-2,3,6,7,10,l l-hexacarbonitrile (HAT-CN), Di-[4-(N,N -di-p -tolyl-amino)- phenyl]cyclohexane (TAPC), N4,N4' -Di(naphthalen-l-yl)-N4,N4' -bis(4-vinylphenyl)biphenyl- 4,4'-diamine (VNPB), and/or 9,9-Bis[4-[(4-ethenylphenyl)methoxy]phenyl]-N2,N7-di-l- naphthalenyl-N2,N7-diphenyl-9H-Fluorene-2,7-diamine (VB-FNPD). The hole transport layer may include one or more inorganic materials or semiconductors such as nickel oxide (NiO_x), tungsten oxide (WO_x), graphene, graphene oxide, and/or molybdenum disulphide (M0S2).

[0046] In various embodiments, the device may further include a substrate. The substrate may be in contact with the first electrode or the second electrode.

[0047] The first electrode, the electron transport layer, the composite film 100, the hole transport layer, and the second electrode may form a stacked arrangement. The stacked arrangement may be on the substrate.

[0048] The device may further include one or more passivation layers including one or more insulator materials such as lithium fluoride (LiF), silicon oxide (S1O2) etc. The one or more passivation layers may be over the stacked arrangement.

[0049] The substrate may include, for instance, a semiconductor material such as silicon, or a flexible material such as polyethylene terephthalate (PET).

[0050] In various embodiments, the device may achieve an external quantum efficiency (EQE) of above 10%. In various embodiments, the device may achieve a current efficiency (CE) of above 40 cd A ¹, e.g. above 50 cd A ¹. In various embodiments, the device may achieve a maximum luminance (L_max) of above 50,0000 cd m^~2, e.g. above 56,000 cd m^~2. [0051] In various embodiments, the device may have a leakage current of less than 1 μΑ cm ². The device may also have a sharp turn-on voltage (of both the current density and luminance) of any value in the range from 2.2V to 2.4 V.

[0052] In various embodiments, a surface area of the device may be about or more than 3 mm², or about or above 5 mm², or about or above 7 mm², or about or above 20 mm², or about or above 50 mm², or about or above 70 mm², or about or above 90 mm², or about or above 95 mm².

[0053] FIG. 2 shows a general illustration of a method of forming a composite film according to various embodiments. The method may include, in 202, forming a plurality of first structures including a first halide perovskite material. The method may also include, in 204, forming a plurality of second structures including a second halide perovskite material different from the first halide perovskite material. An average size of the plurality of first structures may be smaller than an average size of the plurality of second structures.

[0054] In other words, forming a composite film may include forming a plurality of first structures and a plurality of second structures bigger than the plurality of first structures. The first structures may include a first halide perovskite material, and the second structures may include a second halide perovskite material.

[0055] For avoidance of doubt, FIG. 2 does not mean that step 202 and step 204 are in sequence.

In various embodiments, step 202 and step 204 may occur simultaneously.

[0056] The method may be referred to as a ligand-assisted reprecipitation (LARP) method.

[0057] In various embodiments, forming the plurality of first structures and the plurality of second structures may include mixing a plurality of precursors. The plurality of precursors may, for instance, include lead bromide and formamidinium bromide. The plurality of precursors may be dissolved or suspended in a solvent such as Ν,Ν-dimethylformamide (DMF).

[0058] In various embodiments, forming the plurality of first structures and the plurality of second structures may further include adding the plurality of precursors to a ligand, such as octylamine, e.g. n-octylamine. The ligand may be required to passivate the plurality of first structures as well as to form the plurality of second structures. The ligand may be dissolved in a solvent such as toluene. The solvent may also include oleic acid and n-butanol.

[0059] Oleic acid may protonate the amines and may also work as ligands to the first structures, e.g. nanoparticles. Oleic acid may also separate the first structures, e.g. nanoparticles, after the formation of the first structures, e.g. nanoparticles, during synthesis. The carboxylate formed from oleic acid may bind to the surface of Pb sites of the first structures (various binding motifs possible: monodentate, bidentate, bidentate bridging, etc).

[0060] Oleic acid may also work as a ligand on the second structures, e.g. microplatelets. Oleic acid may also separate the second structures, e.g. microplatelets, after the formation of the second structures during synthesis. The carboxylate formed from oleic acid may bind to the surface of Pb sites of the second structures (various binding motifs possible: monodentate, bidentate, bidentate bridging, etc).

[0061] N-butanol may help in washing away of the excess ligands and may help in the precipitation of the first structures, e.g. nanoparticles, during the centrifugation.

[0062] Adding the plurality of precursor to the ligand may be carried out in a dropwise manner. A solution containing the plurality of first structures and the plurality of second structures may be formed when the plurality of precursor is added to the ligand. The plurality of first structures and the plurality of second structures may be washed using one or more centrifugation steps, e.g. in two centrifugation steps.

[0063] Various embodiments may relate to a composite film formed by any one method as described herein.

[0064] Various embodiments may relate to forming a device including a composite film according to various embodiments. Forming the device may include providing a substrate. The method may also include forming a stack arrangement on the substrate. The stacked arrangement may include a first electrode, an electron transport layer, the composite film, a hole transport layer, and a second electrode.

[0065] Various embodiments may provide a high-performance device fabricated from a synthetic protocol including judicious control over addition of ligands to PbBr₂ and formamidinium bromide (FABr), to form a hierarchical self-assembly of 2D (OA)₂PbBr4 microplatelets (MPLs) and FAPbBr₃ nanocrystals (NCs). The self-assembled mesoscopic thin film may include large plate-like (-0.5 - 2 μπι edge length) domains of (OA)₂PbBr₄. The thin film may also include FAPbBr₃ NCs. The (OA)₂PbBr₄ microplatelets and the FAPbBr₃ NCs may be sandwiched between suitable electron and hole transporting layers for yielding the highest combination of external quantum efficiency (EQE), luminance, power and current efficiency values (13.4% EQE, ~56k cd m^~2, 58.1 lm W^"1, 57.6 cd A ¹) demonstrated to-date in perovskite LEDs.

[0066] FIG. 3A shows a plot of current density (in milliamperes per square centimeter or mA cm ²) / luminance (in candela per square meter or cd m ²) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of light emitting devices (LEDs) according to various embodiments, with the inset showing a plot of the normalized electroluminescence (EL) intensity as a function of wavelength (in nanometer) illustrating the spectrum of the emitted light of the devices having nanocrystals formed with molar ratios of 5: 1 and 10: 1 of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments. The black, dark grey, grey, light grey, and ultralight grey curves show nanocrystals formed with molar ratios 3: 1, 4: 1, 5: 1, 7: 1, and 10: 1 of OA:PbBr₂, respectively.

[0067] FIG. 3B is a plot of energy (in electron volts or eV) as a function of position showing the schematic band diagram of the light emitting devices (LEDs) according to various embodiments.

[0068] FIG. 3C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m ²) illustrating the variation of characteristic current efficiency/ external quantum efficiency (EQE) with luminance for a device with device area of 3 mm² according to various embodiments. The black, dark grey, grey, light grey, and ultralight grey curves show nanocrystals formed with molar ratios 3: 1, 4: 1, 5: 1, 7: 1, and 10: 1 of OA:PbBr₂, respectively.

[0069] FIG. 3D shows a plot of current efficiency (in candela per ampere or cd A ¹) / external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m ²) illustrating variation of current efficiency /EQE with luminance of a flexible (3 mm²) light emitting device (LED) and a large area (95.2 mm²) light emitting device (LED) according to various embodiments, with the insets showing images of the two devices according to various embodiments. Both devices may include NCs formed with 4 : 1 molar ratio of OA:PbBr₂.

[0070] FIG. 3E shows (top left) a box plot of luminance (xlO⁴ candela per square meter or cd m ²) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂), (top right) a box plot of external quantum efficiency (EQE) (in percent or %) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂), (bottom left) a box plot of current efficiency (in candela per ampere or cd A ¹) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂), and (bottom right) a box plot of power efficiency (in lumens per watt or lm W^"1) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂) illustrating the characteristics of light emitting devices (LEDs) (n is the number of devices measured) based on different OA:PbBr₂ ratios according to various embodiments. The squares and crosses represent the median values and outliers respectively, whereas the error bars represent the minimum and maximum values. Lower and upper bars within the box, represent the first and third quartile (Ql and Q3), respectively.

[0071] FIGS. 4A-C illustrate the overview of best reported LED devices values for organic (OLED), semiconductor quantum dots (QLEDs), and perovskite (PeLED). TF and QD refer to thin film and quantum dot (or nanoparticle) respectively.

[0072] FIG. 4A is a plot of luminance (in candela per square meter or cd m ²) as a function of year showing the maximum luminance achieved for some conventional devices as a comparison to devices according to various embodiments.

[0073] FIG. 4B is a plot of peak external quantum efficiency (EQE) (in percent or %) as a function of year showing the peak EQE achieved for some conventional devices as a comparison to devices according to various embodiments.

[0074] FIG. 4C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of year showing the current efficiency achieved for some conventional devices as a comparison to devices according to various embodiments.

[0075] FIG. 5 is a table showing the different characteristics of some conventional devices fabricated to date compared to devices according to various embodiments. The following notations in FIG. 5 represent: (a) all perovskites display green emission unless stated differently; (b) Pe = perovskite; (c) ITO = In-doped Sn0₂; PEDOT:PSS = poly(3,4- ethylenedioxythiophene): polystyrene sulfonate; F8 = poly(9,9-dioctylfluorene); Buff-HIL = buffered hole-injection layer; TPBI = 2,2',2"-(l,3,5-benzinetriyl)-tris(l- 30 phenyl-l-H- benzimidazole); TPD = N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine); PIP = poly(imide) polymer; PEI = poly(ethylenimine), TFB = poly(9,9-dioctyl-fluorene-co-N-(4- butylphenyl)diphenylamine); EA = ethanolamine; SPB-02T = blue copolymer, Merck Co.; BCP = bathocuproine; PEO = poly(ethyleneoxide); PVK = poly(9-vinlycarbazole); PVK:PBD = (poly(9- vinylcarbazole):2-(4-biphenylyl)-5-phenyl-l,3,4-oxadiazole), BCP = bathocuproine, CBP = 4,40- Bis(N-carbazolyl)-l,10-biphenyl, and PO-T2T = 2,4,6-Tris[3- (diphenylphosphinyl)phenyl]- 1,3,5-triazine; and (d) not reported.

[0076] In addition, EQE represents external quantum efficiency, CE represents current efficiency, L_max represents maximum luminance. V_T represents the turn-on voltage.

[0077] The excellent quality of the diodes with the different OA:PbBr₂ ratios (FIGS. 3 A-E, FIGS. 4A-C, FIG. 5 ) is evidenced by the low leakage current (<1 μΑ cm ²) and the sharp turn-on of both the current density and luminance around 2.2-2.4 V.

[0078] FIG. 6 is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m^~2) illustrating the variation of power efficiency with luminance for a device with device area of 3 mm² according to various embodiments.

[0079] FIG. 7A is a plot of normalized electroluminescence intensity (EL) as a function of wavelength (in nanometer or nm) showing electroluminescence characteristics for samples prepared with different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments.

[0080] FIG. 7B is a plot of normalized electroluminescence intensity (EL) as a function of wavelengths (in nanometer or nm) showing electroluminescence characteristics for samples at different bias according to various embodiments.

[0081] FIG. 7C is a plot of normalized electroluminescence intensity (EL) as a function of wavelengths (in nanometer or nm) showing electroluminescence characteristics for samples at different luminance intensities according to various embodiments.

[0082] Devices in FIGS. 7B and 7C are prepared with OA:PbBr₂ ratio of 4: 1. The active device area is 3 mm².

[0083] FIG. 7A shows that EL spectra collected at maximum luminance for different OA:PbBr₂ ratios may be similar, except the slight blue-shift (3-5 nm) observed ratio 10: 1. FIG. 7B shows that normalized EL spectra, collected at sub-energy gap (2.2 V) and above energy gap bias (2.3-5 V), suggest that there may be no sub-gap, trap related EL emission at sub-energy-gap external voltage bias. FIG. 7C shows that EL spectra at different luminance levels scale proportionally to the emission intensity, suggesting that the emissive species at different luminance may be equal. [0084] FIG. 8 shows a plot of turn-on values (in volts or V) as a function of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) illustrating turn-on values shown as box plots for light emitting (LED) devices (n is number of devices measured) based on different OA:PbBr₂ ratios according to various embodiments. Each measured device may be represented by a filled circle and the devices show a normal distribution. The squares and crosses represent the median values and outliers, whereas the error bars represent the minimum and maximum values. Lower and upper bars within the box, represent the first and third quartile (Ql and Q3), respectively.

[0085] FIG. 9 is a table showing device performance of devices according to various embodiments at various luminances. The table summarizes the voltage applied, EQE, current efficiency, and luminous power efficiency at 100 and 1000 cd m^~2. The maximum values are listed, while the bracketed values show the average and standard deviation values.

[0086] FIG. 10 is a table showing a summary of the parameters of light emitting (LED) devices according to various embodiments used for constant current test. The devices were measured by sweeping voltage biases up to 2.9 V before the constant current stability test.

[0087] Device performance statistics (of 20-50 devices at each synthetic condition) show excellent reproducibility within the range of 4: 1 and 7: 1 OA : PbBr₂ ratios (FIGS. 5-10). This may validate the benefit of preparing and isolating the perovskite components prior to the integration into thin film devices.

[0088] FIG. 11 A shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of voltage (in volts or V) showing the current density-voltage (J-V) curves of electron only devices with different ratios of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments, with the inset showing the structure of an electron only device according to various embodiments.

[0089] FIG. 1 IB shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of ratios of n-octylamine (OA) to lead bromide (PbBr₂) showing box plots of current densities of electron only devices according to various embodiments at 4V.

[0090] FIG. l lC shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of voltage showing the current density-voltage (J-V) curves of hole only devices with different ratios of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments, with the inset showing the structure of a hole only device according to various embodiments.

[0091] FIG. 1 ID shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of ratios of n-octylamine (OA) to lead bromide (PbBr₂) showing box plots of current densities of hole only devices according to various embodiments at 2V.

[0092] The electron only devices and hole only devices may be collectively referred to as single carrier devices.

[0093] FIG. 12 is a plot of current (in amperes or A) as a function of voltage (in volts or V) showing the lateral conductivity measurements performed on composite films of different ratios of n-octylamine (OA) to lead bromide (PbBr₂) according to various embodiments.

[0094] The best performing devices were with OA:PbBr₂ ratios of 4: 1 and 5: 1, with degradation in performance at higher concentrations of OA, evidenced by the gradual increase of turn-on voltages (FIG. 3A and FIG. 8) due to reduction of electron transport (FIGS. 11A-D) and higher resistivity with higher concentration of (OA)₂PbBr₄ MPLs (FIG. 12).

[0095] FIG. 13 A shows a plot of current density (in milliamperes per centimeter square or mA cm ²) as a function of voltage (in volts or V) showing the current density - voltage characteristics of devices with different electron transport layers according to various embodiments, with the inset showing a plot of electroluminescence intensity (EL) as a function of wavelength illustrating the electroluminescence spectra at maximum luminance.

[0096] FIG. 13B is a plot of external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m ²) showing the variation of EQE with luminance of devices with different electron transport layers according to various embodiments.

[0097] FIG. 13C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m ²) showing the variation of current efficiency with luminance of devices with different electron transport layers according to various embodiments.

[0098] FIG. 13D is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) showing the variation of power efficiency with luminance of devices with different electron transport layers according to various embodiments. [0099] FIG. 13E is a plot of energy (in electron volts or eV) showing band alignment of the device architecture.

[00100] FIG. 13F shows the molecular structures of hole and electron transporting materials used in devices according to various embodiments.

[00101] FIG. 14 is a table showing device characteristics of light emitting devices (LEDs) with different electron transporting layers according to various embodiments. The emissive layer of devices shown in FIGS. 13A-E, 14 may include the perovskite nanocrystals synthesized with ratio of OA:PbBr₂ of 4: 1.

[00102] FIG. 15A is a plot of the square root of emission yield (in square root of counts per second or cps) as a function of energy (in electron volts or eV) showing the photoelectron spectroscopy in air (PESA) results of composite films according to various embodiments. FIG. 15B is a plot of energy (in electron volts or eV) as a function of ratio of n-octylamine (OA) to lead bromide (PbBr₂) showing schematic representation of band levels of samples according to various embodiments prepared with different ratios of OA: PbBr₂.

[00103] FIGS. 15A-B illustrate the valence band (VB) and conduction band (CB) level determination of FAPbBr₃ and (OA)₂PbBr₄ composite films (spin-coated 1 min at 1000 rpm) using photoelectron spectroscopy in air. In various embodiments, the bandgap of the composite film may be any value selected from a range from 2.31 eV to 2.34 eV.

[00104] The right choice of electron transport layer may be essential for charge balance and hence high efficiency (FIGS. 13A-F, FIG. 14, FIGS. 15A-B).

[00105] FIG. 16A is an image of a light emitting device (LED) with active area of 3 mm² according to various embodiments operating at a voltage of 4.5 V. FIG. 16B is an image of a light emitting device (LED) with active area of 9 mm² according to various embodiments operating at a voltage of 4.5 V. FIG. 16C is an image of a light emitting device (LED) with active area of 15.2 mm² according to various embodiments operating at a voltage of 4.5 V. FIG. 16D is an image of a light emitting device (LED) with active area of 35.2 mm² according to various embodiments operating at a voltage of 4.5 V. FIG. 16E is an image of a light emitting device (LED) with active area of 95.2 mm² according to various embodiments operating at a voltage of 4.5 V. FIGS. 16A- E show a bright luminescence. [00106] FIG. 16F is an image of a light emitting device (LED) with active area of 3 mm² according to various embodiments operating at a voltage of 2.7 V. FIG. 16G is an image of a light emitting device (LED) with active area of 9 mm² according to various embodiments operating at a voltage of 2.7 V. FIG. 16H is an image of a light emitting device (LED) with active area of 15.2 mm² according to various embodiments operating at a voltage of 2.7 V. FIG. 161 is an image of a light emitting device (LED) with active area of 35.2 mm² according to various embodiments operating at a voltage of 2.7 V. FIG. 16J is an image of a light emitting device (LED) with active area of 95.2 mm² according to various embodiments operating at a voltage of 2.7 V. FIGS. 16F- J show an uniform luminescence.

[00107] FIGS. 17 A - D show the device stability of three different devices under constant current bias. FIG. 17A is a plot of current density (in milliamperes per centimeter square or mA cm^"2) / luminance (in candela per square meter or cd m^"2) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of three different devices according to various embodiments, with the inset showing a plot of electroluminescence intensity as a function of wavelength (in nanometers or nm) illustrating the electroluminescence spectra at maximum luminance. FIG. 17B is a plot of normalized electroluminescence as a function of time (in seconds or s) illustrating constant current stability of the devices according to various embodiments. The legend indicates the Lo and the current density applied to each device during the stability test.

[00108] FIG. 17C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m ²) illustrating the current efficiency of the devices according to various embodiments.

[00109] FIG. 17D is a plot of luminous power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) illustrating the luminous power density as a function of luminance of the devices according to various embodiments used for the constant current stability test. The device parameters are summarized in FIG. 10. The devices were measured by sweeping voltage biases up to 2.9 V before the constant current stability test.

[00110] FIG. 18A is a plot of current density (in milliamperes per centimeter square or mA cm^" ²) / luminance (in candela per square meter or cd m ²) as a function of voltage (in volts or V) illustrating the current- voltage-luminance characteristics of flexible light emitting devices (LEDs) according to various embodiments. FIG. 18B is a plot of external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m^~2) showing the variation of EQE of flexible light emitting devices (LEDs) according to various embodiments.

[00111] FIG. 18C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m^~2) illustrating the variation of current efficiency of flexible light emitting devices (LEDs) according to various embodiments. FIG. 18D is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) showing the variation of luminous power efficiency of the flexible light emitting devices (LEDs) according to various embodiments. The active device area of the devices illustrated in FIGS. 18A-D is 3 mm². The lines in FIGS. 18A-D represent device data of a devices of the same batch (6 devices).

[00112] FIG. 19 is a table showing device characteristic parameters of flexible devices according to various embodiments. The table summarizes the maximum luminance, current efficiency, luminous power efficiency, and EQE of flexible LED devices. The emission area is 3mm².

[00113] FIG. 20 is a table showing device performance of flexible light emitting devices according to various embodiments at various luminances. The table summarizes the voltage applied, EQE, current efficiency, and luminous power efficiency at 100 and 1000 cd m ².

[00114] FIGS. 21 A-D show large area LED device characteristics. FIG. 21 A is a plot of current density (in milliamperes per centimeter square or mA cm ²) / luminance (in candela per square meter or cd m ²) as a function of voltage (in volts or V) illustrating the current-voltage-luminance characteristics of devices of different areas according to various embodiments.

[00115] FIG. 21B is a plot of external quantum efficiency (EQE) (in percent or %) as a function of luminance (in candela per square meter or cd m ²) showing the variation of EQE of flexible light emitting devices (LEDs) according to various embodiments.

[00116] FIG. 21C is a plot of current efficiency (in candela per ampere or cd A ¹) as a function of luminance (in candela per square meter or cd m ²) illustrating the variation of current efficiency of different devices according to various embodiments.

[00117] FIG. 21D is a plot of power efficiency (in lumens per watt or lm W^"1) as a function of luminance (in candela per square meter or cd m ²) showing the variation of luminous power efficiency of the different devices according to various embodiments. [00118] The solid lines in FIGS. 21B-D represent the as-measured device characteristics, while the dotted lines represent the characteristics after correcting for the saturation of the spectrometer. The corrections may be performed by scaling the luminance values linearly according to the spectral regions which may not saturate the spectrometer, under the assumption that the spectral shape may remain unchanged at any given injection density. The largest area device (95.2 mm²) is larger than the opening of the integrating sphere (78.5 mm²). The reported values are not corrected for the loss of photons (i.e. the photons not collected by the integrating sphere).

[00119] FIG. 22 is a table showing device characteristic parameters of large area devices according to various embodiments. At maximum luminance, LEDs with area > 15.2 mm² saturated the spectrometer. The bracketed number showed the characteristic values after correcting for the saturation, by scaling the luminance values linearly according to the spectral regions that does not saturate the spectrometer, based on the assumption that the spectral shape does not change at any injection density. The largest area device (95.2 mm²) is larger than the opening of the integrating sphere (78.5 mm²). The reported values are not corrected for the loss of photons (i.e. the photons not collected by the integrating sphere).

[00120] FIG. 23 is a table showing device characteristic performance of large area light emitting devices (LEDs) according to various embodiments at various luminances. The table summarizes the voltage applied, EQE, current efficiency, and luminous power efficiency at 100 and 1000 cd m ².

[00121] Indeed, the use of 2,4,6-Tris[3-(diphenylphosphinyl)phenyl]-l,3,5-triazine (PO-T2T), as opposed to 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM), as electron transporting layer of the LED devices lowered the turn-on voltage and improved the luminance, luminous power efficiency, current efficiency, and EQE. The overall improvement may be attributed to the better hole-blocking properties of PO-T2T, enabled by the deeper HOMO level compared to B3PYMPM (FIGS. 18A-D), while the reduction in turn-on voltage is likely related to the superior electron mobility of PO-T2T (~10 ³ cm² V ¹ s ¹) versus B3PYMPM (~10⁴ cm² V ¹ s ¹). Excellent device performance, with uniform and bright emission (FIGS. 16A-J) and fair temporal stability (FIGS. 17A-D), was also observed in flexible devices (3 mm2, 12.4% EQE, >13k cd m ²) and large devices (up to 95.2 mm2, 5.7% EQE, >13k cd m ²) as presented in FIG. 3D, FIGS. 18A-D, FIGS. 19-20, FIGS. 21A-D, FIGS. 22-23. [00122] FIGS. 24A-B show field emission scanning electron microscopy (FESEM) images of composite films prepared with different ratios of OA: PbBr₂. FIG. 24A shows cross-sectional microscopy images of composite films according to various embodiments. The composite films include FAPbBr₃ and (OA)₂PbBr4, and as shown in FIG. 24A are formed using different ratios of OA: PbBr₂. The FAPbBr₃ nanoparticle layers are indicated in the images.

[00123] FIG. 24B shows top-view images of the composite films according to various embodiments. The images showed that there is an increased number of microplatelets (dark areas) for samples prepared with OA:PbBr₂ ratios up to 7: 1. Increased ligand concentration may result in the formation of mostly microplatelets (MPLs).

[00124] FIGS. 25A-C show scanning transmission electron microscopy (STEM) images of an ink containing mixed-phase FAPbBr₃ and (OA)₂PbBr₄ drop-casted on a carbon-copper (Cu) grid in which the ink is prepared with the ratio of OA:PbBr₂ of 5: 1. FIG. 25 A shows an image of a film according to various embodiments in which the (OA)₂PbBr₄ microplatelets are indicated as darker areas outlined with solid lines.

[00125] FIG. 25B is an image showing selected-area electron diffraction pattern of marked area of the film according to various embodiments. The diffraction signals show both two-dimensional (2D) and three-dimensional (3D) phases.

[00126] FIG. 25C is an image showing multiple FAPbBr₃ nanocrystals according to various embodiments, with the inset showing a crystalline FAPbBr₃ nanocrystal of approximately 10 nm in diameter according to various embodiments.

[00127] FIGS. 26A-E show atomic force microscopy (AFM) images of composite films synthesized using different ratios of octylamine (OA) and lead bromide (PbBr₂). FIG. 26A shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 10: 1 according to various embodiments. FIG. 26B shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 7: 1 according to various embodiments. FIG. 26C shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 5: 1 according to various embodiments. FIG. 26D shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 4: 1 according to various embodiments. FIG. 26E shows an atomic force microscopy (AFM) image of a composite film prepared with a ratio of OA:PbBr₂ of 3: 1 according to various embodiments. [00128] FIG. 27 shows (left) a top view field emission scanning electron microscopy image of a composite film according to various embodiments, and (right) a schematic of a portion of the composite film illustrating charge/energy transfer during light emitting device (LED) operation in the composite film according to various embodiments on the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) layer. As shown in FIG. 27, the composite layer may include a first layer including FAPbBr₃ nanocrystals (NCs) and a second layer including (OA)₂PbBr₄ layer on the first layer.

[00129] As evidenced by FESEM, TEM, and cathodoluminescence studies, preferential deposition of the 2D MPLs may be observed on top of the nanocrystals, while the ratio between the 2D MPLs and 3D NCs may be conveniently modulated by varying the OA:PbBr₂ ratio. The self-assembly may arguably occur at the liquid-air interfaces, and may yield films where the rectangular (or polygonal) MPLs are intimately in contact with nanocrystals of graded particle sizes (~5 - 30 nm) to yield very smooth (root mean square or RMS ± 1 nm) mesoscopic films (FIGS. 24A-B, 25A-C, 26A-E, 27).

[00130] Morphological images of the films show the formation of uniform and compact films. Agglomerated structures may be observed only for thin films fabricated with OA:PbBr₂ ratio of 10: 1, indicating appropriate usage of surface ligands at lower ligand-to-precursor ratios. Dark patches, related to the existence of (OA)₂PbBr₄ MPLs, may be observed on the films' surface. The current distribution throughout the film was observed using conducting AFM at IV bias (FIGS. 26A-E).

[00131] Uniform current may be observed across the 2: 1, 3: 1, 5: 1, and 7: 1 films. This may indicate that the existence of 2D MPLs does not significantly hinder the current flow in the films. In case of the 10: 1 film, patches of higher resistance area were noticed, which may be related to the existence of 2D MPLs agglomeration, in agreement with the topographical images of the film (FIGS. 24B). The hierarchical architecture may thus produce a vectoral energy flow within the self-assembled mesoscopic structure that enables the diodes to operate as very efficient light emitters.

[00132] FIG. 28A is a plot of normalized photoluminescence (PL) intensity as a function of wavelength (in nanometers) showing the absorption and steady-state photoluminescence (excitation wavelength or λ«χ = 405 nm) spectra of (OA)₂PBr₄ layer according to various embodiments. 5 : 1 PL denotes the photoluminescence spectrum of the (OA)₂PBr₄ layer formed with a ratio of OA : PbBr₂ of 5 : 1, while 10 : 1 PL denotes the photoluminescence spectrum of the (OA)₂PBr4 layer formed with a ratio of OA : PbBr₂ of 10 : 1. On the other hand, 5 : 1 EA denotes the exitonic absorption spectrum of the (OA)₂PBr₄ layer formed with a ratio of OA : PbBr₂ of 5 : 1, while 10 : 1 EA denotes the exitonic absorption spectrum of the (OA)₂PBr₄ layer formed with a ratio of OA : PbBr₂ of 10 : 1. FIG. 28B is a plot of normalized photoluminescence (PL) intensity as a function of wavelength showing the excitation spectra for the different (OA)₂PBr₄ layers according to various embodiments. The spectrum labelled with 5 : 1 shows the excitation spectrum of the (OA)₂PBr₄ layer formed with a ratio of OA : PbBr₂ of 5 : 1 (emission wavelength or Aem = 533 nm), while the spectrum labelled with 10 : 1 shows the excitation spectrum of the (OA)₂PBr₄ layer formed with a ratio of OA : PbBr₂ of 10 : 1 (emission wavelength or Aem = 519 nm).

[00133] FIG. 28C is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) of an ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments, with the inset showing a plot of volume fraction as a function of nanocrystal diameter (in nanometer or nm) illustrating the particle size distribution of the ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

[00134] FIG. 28D is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) of an ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments, with the inset showing a plot of volume fraction as a function of nanocrystal diameter (in nanometer or nm) illustrating the particle size distribution of the ink including (OA)₂PBr₄ and FAPbBr₃ formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

[00135] Detailed characterization by means of absorption and photoluminescence spectroscopy and small-angle X-ray scattering (SAXS), and X-ray diffraction (FIGS. 28A-D) indicate the presence of 2D (OA)₂PbBr₄ MPLs and 3D FAPbBr₃ NCs with narrow size distribution. The MPLs may be aligned substantially parallel to the substrate's surface. [00136] FIG. 29 A is a plot of normalized absorbance as a function of wavelength (in nanometers or nm) showing the absorption spectra of different mixed-phase inks synthesized with different ratios of OA: PbBr₂ according to various embodiments.

[00137] FIG. 29B is a plot of normalized photoluminescence (PL) intensity as a function of wavelength (in nanometers or nm) showing the photoluminescence (PL) spectra of different mixed-phase inks synthesized with different ratios of OA: PbBr₂ according to various embodiments.

[00138] FIG. 29C is a plot of normalized intensity as a function of wavelength (in nanometers or nm) showing the excitation spectra of different mixed-phase inks synthesized with different ratios of OA: PbBr₂ according to various embodiments.

[00139] The steady state optical absorption spectra (FIGS . 29 A-C) of NC inks with 5 : 1 and 10 : 1 OA:PbBr₂ ratios indicate the presence of two main features, a peak at 440 nm (ca. 2.82 eV) that may correspond to the excitonic absorption of the 2D (OA)₂PbBr₄ MPLs and the band absorption of 3D FAPbBr₃ NCs at 520 nm (ca 2.38 eV). The peak intensity may consistently change with the OA content due to the increased presence of (OA)₂PbBr₄ MPLs with respect to FAPbBr₃ NCs. The PL spectra may show a pronounced emission from the MPLs at -440 nm (for the 10: 1 composition), however this emission may be markedly absent from the 5: 1 spectra. Excitation spectra (Aem = 533 and 519 nm, for 5: 1 and 10: 1, respectively) provide the first intimation of an energy cascade from the higher bandgap (OA)₂PbBr₄ MPLs to the lower bandgap FAPbBr₃ NCs through a PL contribution around 425-430 nm (FIG. 29C).

[00140] The presence of composite 2D and 3D films may be further confirmed by small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) patterns with Bragg reflections of 2D (OA)₂PbBr₄ MPLs at q -1.60 nm ¹ (d-spacing of - 3.93 nm) and 2Θ = 8-12°, respectively, at OA:PbBr₂ = 10: 1; and predominance of the 3D FAPbBr₃ NCs at lower OA:PbBr₂ ratios.

[00141] FIG. 30 A is a plot of normalized intensity as a function of angle 2Θ (in degrees or deg) illustrating X-ray diffraction patterns of FAPbBr₃ and (OA)₂PbBr₄ thin films prepared with ratios of OA:PbBr₂ ranging 3: 1 to 10: 1 according to various embodiments.

[00142] FIG. 30B is a plot of intensity as a function of angle ω (in degrees or deg) illustrating rocking curves of self-assembled rocking curves of self-assembled mixed-phase FAPbBr₃ and (OA)₂PbBr₄ thin films prepared with OA:PbBr₂ ranging 3: 1 to 10: 1 according to various embodiments. The left panel shows rocking curves of (001) reflection of (OA)₂PbBr₄ at 2Θ -9.6°, while the right panel shows rocking curves of (002) reflection of FAPbBr₃ at 2Θ -29.8°.

[00143] FIG. 31A is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr4 ink prepared with a ratio of OA:PbBr₂ of 3 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

[00144] FIG. 3 IB is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 4 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

[00145] FIG. 31C is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 5 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

[00146] FIG. 3 ID is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 7 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

[00147] FIG. 3 IE is a plot of scattering intensity as a function of magnitude of the scattering vector (in per nanometer or nm ¹) showing small-angle X-ray scattering (SAXS) curves of mixed FAPbBr₃ and (OA)₂PbBr₄ ink prepared with a ratio of OA:PbBr₂ of 10 : 1, while the inset shows a plot of volume fraction as a function of nanocrystal (NC) diameter (in nanometers or nm) illustrating the particle size distribution of the ink formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

[00148] FIGS. 31D-E show a Bragg reflection of 2D (OA)₂PbBr₄ platelets at q -1.60 nm ¹ (equivalent to a d-spacing of approximately 3.93 nm) at OA : PbBr₂ ratios equal to or greater than 7: 1.

[00149] FIG. 3 IF is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

[00150] FIG. 31G is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

[00151] FIG. 31H is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

[00152] FIG. 311 is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

[00153] FIG. 31J is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of the top surface of the composite film formed with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments. [00154] FIGS. 31F-J show that the films exhibit perpendicular alignment of 2D (OA)₂PbBr₄ platelets at q_x > 1.5 nm ¹ with respect to the surface of the substrate. The solid curves in FIGS. 311- J represent the integrated peak area for 1.5 < qx < 3 nm ¹.

[00155] FIG. 32A is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self- assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 4 : 1 and left for a period of 1 minute to initiate nanocrystal self-assembly according to various embodiments.

[00156] FIG. 32B is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self- assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 4 : 1 and left for a period of 3 minutes to initiate nanocrystal self-assembly according to various embodiments.

[00157] FIG. 32C is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self- assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 4 : 1 and left for a period of 5 minutes to initiate nanocrystal self-assembly according to various embodiments.

[00158] FIG. 32D is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self- assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 10 : 1 and left for a period of 1 minute to initiate nanocrystal self-assembly according to various embodiments.

[00159] FIG. 32E is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self- assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of OA : PbBr₂ of 10 : 1 and left for a period of 3 minutes to initiate nanocrystal self-assembly according to various embodiments.

[00160] FIG. 32F is a plot of magnitude of the scattering vector along the z-axis q_z (in per nanometer or nm ¹) as a function of magnitude of the scattering vector along the x-axis q_x (in per nanometer or nm ¹) illustrating grazing-incidence small-angle X-ray scattering (GISAXS) of self- assembled thin films of FAPbBr₃ and (OA)₂PbBr₄ prepared with a ratio of 10 : 1 and left for a period of 5 minutes to initiate nanocrystal self-assembly according to various embodiments.

[00161] The NC inks are drop-casted on top of the ITO/PEDOT:PSS substrate, and left for 1 to 5 min to initiate NC self-assembly, prior to spin-coating for 1 min at 1000 rpm. The 2D (OA)₂PbBr₄ platelets exhibit perpendicular alignment with respect to the surface of the substrate, as observed by the peak formation at q_x >1.5 nm ¹. From the changes in the integrated peak shape and position, represented by the solid curves in FIGS. 32D-F, it is evident that the 2D (OA)₂PbBr₄ platelets may self-assemble at longer waiting times.

[00162] Particle size distributions extracted from the SAXS scattering curves elucidate that the films are made up of nanocrystals with a median diameter around 10 nm; with a narrower distribution for samples with OA:PbBr₂ ratios of 5: 1 and with a much wider distribution at high OA ratios (refer to FIGS. 31A-J). Rocking curves of (001) reflection of (OA)₂PbBr₄ (2Θ: -9.6°) and (002) reflection of FAPbBr₃ (2Θ: -29.8°), display the occurrence of a peak, indicating a preferred orientation of both phases through self-assembly (refer to FIGS. 30A-B).

[00163] Grazing-incidence small-angle X-ray scattering (GISAXS) plots of mixed-phase thin films prepared with varying ratios of OA:PbBr₂ exhibit alignment of the 2D (OA)₂PbBr₄ MPLs parallel to the surface of the substrate, and the associated time-delay study provide evidence that the MPLs self-assemble at longer delay times between drop-casting and spin-coating (refer to FIGS. 31A-J, 32A-F for details).

[00164] Through a combination of transient optical spectroscopic techniques, it may be elucidated that an energy cascade mechanism takes place from high bandgap 2D (OA)₂PbBr₄ MPLs into progressively lower bandgap FAPbBr₃ NCs, giving rise to high luminescence efficiency in FAPbBr₃ NCs present in LEDs. Indeed, when charge carriers are injected (or photons are absorbed) into the 2D (OA)₂PbBr₄ MPLs, since the energy transfer is fast enough to overcome non-radiative recombination, the excitons may be preferentially transferred to energetically favored lower bandgap 3D FAPbBr₃ NCs, as in other multi-domain systems.

[00165] As a consequence, the photoexcited states may tend to concentrate in lower bandgap

NCs, where the higher exciton concentrations may enhance the bimolecular radiative recombination.

[00166] FIG. 33 A shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

[00167] FIG. 33B shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

[00168] FIG. 33C shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

[00169] FIG. 33D shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

[00170] FIG. 33E shows (top panel) a plot of absorbance as a function of wavelength (in nanometers or nm) illustrating the ultraviolet-visible absorption spectrum of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments; (middle panel) a plot of time (in picoseconds or ps) as a function of wavelength (in nanometers or nm) showing the transient absorption (TA) mapping of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments; and (bottom panel) a plot of relative optical density AOD as a function of wavelength (in nanometers or nm) illustrating the relative transient absorption (TA) of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

[00171] The TA mapping may be carried out with excitation at wavelength (λ) 375 nm, 150 fs, and 1 uJ cm^"2 fluency. The middle panels may show a range of 0 - 200 ps. The relative TA spectra may be up to 1000 ps.

[00172] The TA change following a λ = 375 nm pulsed excitation reveals two strong bleaching features at 440 and 520 nm for the 5: 1 sample, in agreement with the absorption spectra (FIG. 33C, middle and top panels). TA spectra of FIG. 33E panel is dominated by the photobleaching peak at 440 nm, indicating that the excitons are predominantly formed in the (OA)₂PbBr₄ MPLs. A secondary, albeit less intense peak at 520 nm, is detected and displays a rapid peak shift from 500 to 525 nm within 1 ps. The relative intensities of these two peaks are in agreement with the peaks in the absorption spectra for both samples. The 440 nm component is much shorter lived with respect to the 520 nm peak; the latter still observable even after 1000 ps (FIGS. 33C, 33E bottom panels).

[00173] FIG. 34A shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 520 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

[00174] FIG. 34B shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 525 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

[00175] FIG. 34C shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 520 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

[00176] FIG. 34D shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 520 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

[00177] FIG. 34E shows a plot of normalized relative optical density AOD as a function of time (in picoseconds or ps) showing the transient absorption (TA) signals at 440 nm and 525 nm of FAPbBr₃ nanocrystals prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

[00178] The kinetics of the TA signal at 440 nm and 520 nm / 525 nm for 5: 1 and 10: 1 samples (FIGS. 34C, 34E) may provide strong evidence of energy cascade between the (OA)₂PbBr₄ MPLs and FAPbBr₃ NCs. The 440 nm TA kinetics display very fast rising times, may be indistinguishable from the response function even after deconvolution (-150 fs), and may be compatible with photon absorption taking place in the (OA)₂PbBr₄ MPLs. In contrast, the 520 nm or 525 nm kinetics may display much slower (-400 fs) rise times, indicating a slower build-up of the absorption signal in FAPbBr₃ NCs, due an energy cascade, from higher bandgap 2D MPLs to the lower bandgap FAPbBr₃ NCs. Subsequently, the FAPbBr₃ exciton population may decay faster owing to radiative recombination than it grows from energy transfer. For all OA:PbBr₂ ratios, the decay kinetics at 520 nm or 525 nm may be slower than for 440 nm (refer to FIGS. 33A-E, FIGS. 34A-E, FIGS. 35A-F and FIG. 36 for details).

[00179] FIG. 35 A is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments.

[00180] FIG. 35B is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments.

[00181] FIG. 35C is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments.

[00182] FIG. 35D is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments.

[00183] FIG. 35E is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) illustrating the spectral contribution at the characteristics decay times, obtained by the global fitting procedure for a sample prepared with a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments.

[00184] FIG. 35F is a plot of relative optical density AOD as a function of wavelength (in nanometer or nm) showing a magnified view of FIG. 35E at approximately 525 nm for the sample according to various embodiments.

[00185] FIG. 36 is a table showing the tabulated decay times for the global fitting results of samples according to various embodiments shown in FIGS. 35A - F.

[00186] In order to visualize the emission from FAPbBr3 NCs and (OA)₂PbBr₄ MPLs locally, cathodoluminescence mapping may be used. [00187] FIG. 37 A shows a cathodoluminescence image of a spin-coated film according to various embodiments on a silicon substrate, with the emission spectrum is centered at 525 nm. The image is taken using an electron beam of 5 keV energy, current of ~1 In A, and exposure time of 10 ms.

[00188] FIG. 37B shows cathodoluminescence images of a spin-coated film according to various embodiments on a silicon substrate, with the emission spectrum centered at 440 nm for the center image and 525 nm for the right image. The images are taken using an electron beam of 5 keV energy, current of ~1 InA, and exposure time of 10 ms.

[00189] FIG. 37A shows a striking enhancement of green emission (at 525 nm) for FAPbBr₃ NCs existing in the vicinity of (OA)₂PbBr₄ MPLs. When the emission was filtered to exclude the green signal, the 440 nm emission from the MPLs could be discerned (see FIG. 37B, its relative intensity was very low due to the short life time of the emission signal).

[00190] FIGS. 38A-D illustrate time -resolved photoluminescence dynamics of NC with different OA:PbBr₂ ratios. FIG. 38A is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm).

[00191] FIG. 38B is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 4 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm).

[00192] FIG. 38C is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 7 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm).

[00193] FIG. 38D is a plot of normalized photoluminescence (PL) intensity as a function of time (in nanoseconds or ns) illustrating the photoluminescence (PL) dynamics of nanocrystals formed from a ratio of OA : PbBr₂ of 10 : 1 according to various embodiments with excitation wavelength of 405 nm. The legend indicates the emission wavelength in nanometers (nm). [00194] PL lifetimes probed between 440-540 nm; λβχ = 405 nm) showed a significant spread consistent with graded NC sizes (about 5-30 nm).

[00195] FIG. 39 is a plot of normalized fluorescence as a function of time (in nanoseconds or ns) illustrating time -resolved fluorescence decays of a film prepared with a ratio of OA : PbBr₂ of 5 : 1 collected at different emission wavelengths from 440 nm to 540 nm at an excitation wavelength (Aex) of 405 nm.

[00196] Moreover, when excitons form in the higher bandgap (i.e. smaller nanocrystals) and rapidly cascade into the lowest bandgap larger nanocrystals within the size distribution, a variation in relative weight of the components (of their corresponding PL life times) may be expected. At shorter probing wavelengths, the measured bi-exponential decay may be dominated by a fast component related to intraband relaxation pathways inducing carrier cascading, whereas the largest FAPbBr₃ NCs (i.e. probed at longer wavelengths) may show considerably longer lifetimes; where bimolecular recombination is the predominant process (FIGS. 40A-E).

[00197] FIG. 40 A is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments at 405 nm excitation. Signal collection is carried out at different emission wavelengths.

[00198] FIG. 40B is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 3 : 1 according to various embodiments at 405 nm excitation. Signal collection is carried out at different emission wavelengths.

[00199] FIG. 40C is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 4: 1 according to various embodiments at 405 nm excitation. Signal collection is carried out at different emission wavelengths.

[00200] FIG. 40D is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 7: 1 according to various embodiments at 405 nm excitation. Signal collection is carried out at different emission wavelengths.

[00201] FIG. 40E is a table showing characteristic fluorescence lifetimes for nanocrystals formed by a ratio of OA : PbBr₂ of 10: 1 according to various embodiments at 405 nm excitation. Signal collection is carried out at different emission wavelengths. [00202] Additionally, from transient photoluminescence spectra (FIG. 41A), the ultrafast buildup (within few picoseconds) of the emission at 440 nm and its rapid decay (within hundreds of picoseconds) may be discerned, which corroborates well with previous TA observations.

[00203] FIG. 41 A is a plot of time (in nanoseconds or ns) as a function of emission wavelength (in nanometers) showing transient photoluminescence (PL) spectra of a film prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments at time intervals from 0-4 picoseconds (ps).

[00204] Conversely, the emission associated with the NCs (about 525 nm) may only reach maximum emission after 500 ps, which confirms that the delayed built-up due to the energy cascade. This effect may be further highlighted by the progressive red-shift of the emission peak within 10 ns (from 525 to 530 nm) (FIG. 4 IB), and a steady-state emission at 532 nm.

[00205] FIG. 4 IB is a plot of normalized fluorescence as a function of wavelength (in nanometers or nm) illustrating normalized transient photoluminescence (PL) spectra of a film prepared with a ratio of OA : PbBr₂ of 5 : 1 according to various embodiments at different time delays after excitation, as well as the steady-state PL spectrum of the film according to various embodiments.

[00206] The concurrent dynamic changes in the spectral shape and decreasing full width at half maximum (FWHM) may thus be likely caused by the lower contribution of the smallest NCs (i.e. blue-shifted part of the spectra), and may be associated with the energy cascade from higher-to- lower bandgap. Consequently, this may result in a narrowing of the transient emission spectra at longer decay times, eventually culminating in an even narrower steady-state emission.

[00207] FIG. 42A is a plot of photoluminescence quantum yield PLQY (in percent or %) as a function of power density (in milli-Watts per centimeter square or mW cm ²) illustrating variation of PLQY due to excitation fluence after continuous-wave (CW) laser excitation of the film according to various embodiments at wavelengths of 405 nm and 447 nm. PLQY in the low power density regime (up to 6 mW cm ²) recorded values of 80% consistently for both λβχ = 405 nm (i.e. 2D MPLs are excited) and at λβχ = 447 nm (i.e. 2D MPLs are not excited). The invariance of PLQY as a function of power density, may indicate that owing to the energy cascade mechanism, the carrier densities in the nanocrystals may have increased to the point where the chances of non- radiative recombination are effectively suppressed, thus delivering a near unity value in photoluminescence yields.

[00208] Concomitantly, the similar high PLQY values obtained at different excitation wavelengths may further confirm that efficient energy transfer allows all absorbed photons (by the 2D MPLs) to be transferred as excitons in the graded FAPbBr₃ NCs.

[00209] FIG. 42B is a plot of the absolute photoluminescence quantum yield PLQY as a function of power density (in milli- Watts per centimeter square or mW cm ²) illustrating variation of PLQY due to excitation fluence of various films according to various embodiments. The measurement error may be approximately 5 - 10%.

[00210] FIG. 43 shows a schematic diagram representing the energy cascade from two dimensional microplatelets (2D MPLs) to nanocrystals (NCs) of graded sizes in composite films according to various embodiments. The deconvolution of the steady-state PL spectrum may clearly display the different PL contribution at varying NC sizes. The corresponding energy levels are extracted from PES A and optical absorption.

[00211] Deconvolution of the PL spectrum may identify the multiple contributions to emission expected from the size distribution identified by the SAXS measurements (approximately 5-30 nm) and confirmed by the considerable spread of characteristic lifetimes within the emission peak.

[00212] The LED performance may depend on the OA:PbBr₂ ratio, and in correlation with PLQY, may drop for films with ratios above 5: 1. As the amount of octylamine increases (to 10: 1), the fraction of 2D MPLs may increase, device efficiencies may degrade owing to factors including poor charge injection / transport and increased roughness of the films (FIGS. 26A-E). Emitters based on films formed with OA:PbBr₂ ratios of 3: 1, 4: 1 and 5: 1 may display good LED performances in line with the high PLQY observed for these films.

[00213] The 2D-3D self-assembled hierarchical composite films including large microplatelets (0.5 - 2 μπι) attached to FAPbBr3 nanocrystals (ca. 5-30 nm), may be ultra-smooth films (RMS ± 1 nm, for OA:PbBr₂ < 7: 1) and may play a critical role in the realization of these high performance films and devices.

[00214] Under electrically driven conditions, the 2D MPLs may function as an electron-injecting layer and as an intermediary to rapidly inject electrons into the NCs, while passivating the nanocrystals. The excitons may form in the higher bandgap (smaller nanocrystals) and may rapidly cascade into the lowest bandgap (largest) nanocrystals within the size distribution. In the largest nanocrystals, the carrier densities may increase substantially thereby reducing the chances of non- radiative recombination, delivering extremely high values of PLQY, EQE, luminance, power, and current efficiencies.

[00215] These hierarchical self-assemblies may be transferred to different substrates, may be scalable to larger active diode areas, and may present high feasibility of further development and continued improvements in performance. The levels of expansive high-performance luminescence metrics may be unparalleled in this field and may compare rather favorably with state of the art multi-layer green emitting OLEDs.

[00216] Nanocrystal synthesis & NC ink formation.

[00217] Ligand-assisted reprecipitation (LARP) method was used to synthesize the FAPbBr₃ nanocrystals at room temperature. Precursor solutions were prepared by mixing 0.2 mmol of FABr and 0.1 mmol of PbBr₂ in DMF (N,N-dimethylformamide), and subsequently 150 μL was added dropwise into a vigorously stirred solution containing 5 mL toluene, 5-50 n-octylamine (OA:PbBr₂ of 3: 1 to 10: 1 ; needed to passivate the as-formed NCs and concurrent formation of 2D layered perovskites), 0.3 mL oleic acid, and 2 mL n-butanol. Immediately after injection, a yellowish solution was formed, indicating the formation of FAPbBr₃ NCs. After reaction completion, the colloidal NC solution was washed using two centrifugation steps. In the first step, the NC solution was centrifuged at 14680 rpm, after which the supernatant phase was discarded and the precipitate re-dispersed in 1 mL of toluene. In the second centrifugation step, the re- dispersed NCs were centrifugated at 3750 rpm. The resultant supernatant phase was used as NC ink for the LED device fabrication.

[00218] Device fabrication

[00219] Pre-etched indium-tin oxide (ITO; sheet resistance -10 Ω cm ¹) glass substrates were sequentially washed in detergent solution, acetone, ethanol, and 2-propanol in an ultrasonication bath. Subsequently, the substrates were dried and treated for 20 min with UV-ozone. The hole transporting layer, PEDOT:PSS (Clevios 4083; filtered with 0.45 μπι PVDF filter) was then spin- coated for 1 min at 4000 rpm and thermally annealed for 10 min at 130 °C to remove any residual solvent. The NC inks were drop-casted on top of the PEDOT:PSS layer and left for 5 min to slowly evaporate (and initiate self-assembly), prior to spin-coating for 1 min at 1000 rpm, followed by thermal evaporation of 45 nm of electron transporting layer (either POT2T, 2,4,6-Tris[3- (diphenylphosphinyl)phenyl]-l,3,5-triazine, or B3PYMPM, 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)- 2-methylpyrimidine under high vacuum (10^~6 torr). Lastly, the cathode materials Ca (7 nm) and Al (80 nm) were subsequently thermally evaporated through a metal shadow mask, to define the device active area of 3 mm². Flexible devices (active area 3 mm2) were fabricated on ITO/glass substrates following a similar protocol, although thicker PEDOT:PSS layers (ca. 80 nm) were deposited. A filtered PEDOT:PSS solution was spincoated at 1000 rpm for 60s, followed by thermal annealing at 120 °C for 10 min. to reduce the surface roughness (i.e. due to the rough ITO layer on the flexible PET substrate). The ITO/PET substrates were etched with Zn powder and diluted hydrochloric acid, and subsequent sonication in soap water, acetone, ethanol, and 2- propanol for 10 minutes. The deposition of subsequent emissive layer, electron transporting layer, and cathode were the same as the standard devices. Large area devices are prepared similarly to the standard 3 mm² devices. For planar devices, FAPbBr₃ NCs were deposited on glass with the same spinning condition used for the LED device. Gold contacts (thickness -200 nm) have been evaporated on top of the films. The gap between the contacts is 100 μπι. The I-V measurements were performed by using a calibrated Agilent B2902A source-measure unit.

[00220] LED device stability

[00221] Before the stability test, the current-voltage-luminance characteristics of each device was recorded by sweeping voltages up to 2.9 V only to minimize the biasstress degradation on the device. A constant current density was applied according to the current-voltage-luminance characteristics. For initial luminance Lo -100 cd m ², the luminance of the device decayed to half of the initial luminance after -800 s (approximately 13 min).

[00222] X-Ray Diffraction

[00223] The crystal phase and lattice parameters of the synthesized NCs were determined using a Bruker D8 advance diffractometer with a 0D LynxEYE™ detector. Scans from 2Θ = 5-35° were recorded (step sizes of 0.05° and 10 s per step) of thin films spincoated on cleaned glass substrates coated with PEDOT:PSS (see device fabrication protocol).

[00224] Rocking curves of the (001) and (002) reflections of 2D (OA)₂PbBr₄ (2Θ -9.6°) and 3D FAPbBr₃ (2Θ -29.8°) were recorded from ω = 2-6° and ω = 13-17°, respectively, with step sizes of 0.05° and 10 s per step. [00225] Scanning Transmission electron microscopy

[00226] Measurements were performed with a Tecnai G2 F20 with a Schottky field emitter operated at 200kV. Selected samples were diluted in toluene, dropcasted on a carbon-copper grid, and mounted on a FEI Double Tilt Analytical Holder for examination. Tecnai G2 F20 STEM with an X-Twin lens objective lenses and field emission gun (Shottky field emitter) operates at a beam current of > 100 nA, providing high probe current (0.5 nA or more in 1 nm probe). The system is equipped with a fully embedded digital scan system; bright-field and annular dark-field modes are provided by ultra-high resolution high-angle annular dark-field (HAADF) detector.

[00227] Field-emission scanning electron microscopy

[00228] The morphological images of the films were recorded using field effect scanning electron microscope (FE-SEM, JEOL, J7600F).

[00229] Cathodoluminescence microscopy.

[00230] The measurements were performed in a scanning electron microscope equipped with a cathodoluminescence detection system, Attolight CL Allalin 4027 Chronos. A focused electron beam (electron energy 5 keV; beam current ~1 1 nA; dwell time 10-200 ms) scanned the samples while recording the light emission spectrum synchronously to produce hyperspectral images. The emitted light was collected by an achromatic reflective objective with a high numerical aperture (numerical aperture or NA 0.72) and sent to a ultraviolet-visible (UV-VIS) spectrometer (Horiba iHR320) equipped with a thermoelectrically cooled silicon charged-coupled device (CCD) array (Andor Newton).

[00231] Small Angle X-ray Scattering (SAXS)

[00232] With SAXS, local electron density inhomogeneities may be recorded at very small angles, which allows extraction of structural information on length scales typically < 200 nm.

[00233] These density fluctuations may arise from a homogeneous suspension of nanocrystals (with electron density p) in a solvent matrix of different electron density, po (or similarly from porosity within a particle). Conventionally, I(q) is plotted versus the magnitude of the scattering vector, q, and is related to the scattering angle (20) and the wavelength (λ) of the incident beam via:

q = ^ sin Θ ( 1)

A [00234] In the case of dispersed NCs, the recorded scattering intensity I(q) may be proportional to the square of electron density difference, (Δρ)², between the particles and the solvent matrix.

[00235] Here, the scattered intensity may arise from the internal electron interference of individual nanocrystals (intraparticular), or from the electrons in an assembly of particles (interparticular), and can be written as a function of a form factor, P(q), and/or a structure factor, S(q), respectively. The total scattered intensity is then:

I(q) = N. (Ap)². P(q). S(q) (2) where N is the number density of particles, and (Ap)² the scattering contrast. For dilute systems, the distance between individual particles may be substantial and no interparticular interference may be expected, i.e. S(q) = 1. The scattering intensity may then be only proportional to the shape (form) of the particles. For smooth, solid spheres, P can be written as:

(q.r₀)-q.r₀.cos(q.r₀)]²

Piq, r₀) = (3) wherein ro is the radius of the smallest scattering particles. The contribution of these smallest scatterers may be visible at high q values (q- ro »1), when the slope decreases asymptotically according to i(q) o q^~4 (Porod's law).

[00236] At low q values, the scattered intensity may be predominantly determined by the scattering of large particles or aggregates, and is described by the Guinier approximation: l{q) = I₀. exp [- ^!] (for q→0) (4)

[00237] The radius of gyration, R_g, may be defined as the root-mean square center-of-mass distances within a particle or an assembly of particles. It can be determined from the slope in a plot of q² vs In I(q); valid for q-Rg «1.

[00238] The Xenocs Nano-inXider, equipped with a Dectris Pilatus3 hybrid pixel detector was employed to record the combined small- and wide-angle X-ray scattering (SAXS/WAXS) patterns of NC inks. This allowed to measure an effective scattering vector magnitude in the range of 0.1 < q < 4 nm ¹ in SAXS, and up to 2Θ = 60° in WAXS. NC inks with OA:PbBr₂ ratios 3: 1, 4: 1, 5: 1 , 7: 1, and 10: 1 were measured in sealed glass capillaries (inner diameter 0.95 mm, length 100 mm) under vacuum at room temperature, with 15 min acquisition time. Thin film surfaces were investigated using grazing incidence small-angle X-ray scattering (GISAXS) recorded under a shallow angle of 0.2°.

[00239] Size distributions (without prior assumption of particle shape) were obtained from our scattering curves using the Monte Carlo based software package McSAS, using convergence criterion of 2, with 10 calculating repetitions and 500 contributions.

[00240] Photoluminescence

[00241] The photoluminescence (PL) spectra of the FAPbBr₃ NC films were measured using a Horiba Fluoromax-4 (slit width 0.4 nm and 0.1 s integration time), respectively. For the excitation spectra, the maximum PL emission peak was used (slit width 0.1-0.2 nm and 0.3-0.5 s integration time).

[00242] Absorbance

The absorbance spectra of the composite NC films were measured using a Shimadzu UV-2550 spectrophotometer with an integrating sphere attachment (20 nm slit width).

[00243] Time-resolved PL

[00244] The micro-PL setup is based on fiber coupled microscope system, where the excitation path and the emission collection from the side, using a visible-near infrared (VIS-NIR) microscope objective (lOx, NA= 0.65). The samples were excited with 5-MHz-repetition-rate, picosecond- pulse light sources at 405 nm (Picoquant P-C-405B) light-emitting diode. The beam spot size was about 10 mm. Time -resolved decay curves were collected using an Acton monochromator (SpectraPro 2300), fiber coupled to the microscope, to filter the desired wavelength, and detected by Micro Photon Devices single-photon avalanche photodiode. The signal was then acquired by a time-correlated single photon counting card. The temporal resolution is ~5 ps. The decay curves were fitted with double exponential function.

[00245] Photoluminescence quantum yield (PLQY)

[00246] PLQY dependencies on the excitation fluence were measured with a 2-inch integrating sphere (Thorlabs model IS200). Investigated samples were placed inside the sphere and excited using a semiconductor CW laser beam (200 mW full power) emitting at 405 and 447 nm. An optical fiber was attached to the sphere to direct the light to an Ocean Optics spectrometer. The excitation beam intensity was attenuated by means of calibrated neutral interference filters (Thorlabs). [00247] Transient Absorption

[00248] Visible pump visible probe transient absorption measurement was conducted using a Continuum IntegraC regenerative/multipass femtosecond amplifier system capable of generating <100 fs, 1 KHz and 2.5 mJ ultrashort pulse at 800 nm. Pump wavelength of 350 nm is generated by frequency doubled the 700 nm VIS2 output of a Continuum Pallitra optical parametric amplifier (OPA) pumped with 1 mJ of the laser output. Two dielectric mirrors designed for 3rd harmonic of Nd: YAG laser are used as filter to remove fundamental 700 nm and any other residue output from the OPA. White light continuum is generated by focusing part of the amplifier output onto a constantly rotating CaF₂ with appropriate beam size and power control. To prevent oversaturation of the CCD spectrometer, a 700 nm shortwave pass filter is used to remove the excessive 800 nm generation beam. As a result, a stable smooth broadband white light continuum spanning 370-650 nm is generated. On the sample, the probe white light is focused via a parabolic mirror to a spot size of -20 μπι. A f = 250 mm

[00249] UV fused silica (UVFS) lens is used to focus the pump beam onto the sample at its beam waist of -100 μπι diameter. To prevent sample degradation due to humidity and oxidation, the sample is taped onto a UVFS cuvette that has been filled with nitrogen. Longpass filters with cutoff wavelength of 375 nm were used to prevent scattered pump beam to enter the CCD spectrometer.

[00250] Global fitting

[00251] Global fitting is performed using the freely available Glotaran frontend of the R-based TIMP global and target analysis software package. The average of approximately -5 to - 1 ps regime data are used for the baseline correction to remove the contribution of long lived fluorescence signal around the photobleaching peak and coherent artifacts were removed numerically using the built-in instrument respond function (IRF) model.

[00252] Dispersion of the white-light probe and IRF were removed by numerical fitting using the default ParMu model for dispersion and Gaussian IRF.

[00253] Photoelectron spectroscopy in air (PESA)

[00254] Measurements on spincoated NC inks (1 min at 1000 rpm) were conducted using a Riken Keiki AC-2 spectrometer with a power setting of 800 nW (power number of 0.5). [00255] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A composite film comprising:

a plurality of first structures comprising a first halide perovskite material; and a plurality of second structures comprising a second halide perovskite material different from the first halide perovskite material;

wherein an average size of the plurality of first structures is smaller than an average size of the plurality of second structures.

2. The composite film according to claim 1 ,

wherein the plurality of first structures are nanostructures; and wherein the plurality of second structures are microstructures.

3. The composite film according to claim 2,

wherein the microstructures are microplatelets.

4. The composite film according to claim 3,

wherein the plurality of microplatelets has edge lengths from 0.5 μπι to 2 μπι.

5. The composite film according to any one of claims 2 to 4,

wherein the nanostructures are nanoparticles.

6. The composite according to claim 5,

wherein the plurality of nanoparticles has diameters from 5 nm to 30 nm.

7. The composite film according to any one of claims 1 to 6,

wherein the second halide perovskite material is a two-dimensional perovskite material; and wherein the first halide perovskite material is a three-dimensional perovskite material.

8. The composite film according to any one of claims 1 to 7,

wherein the second halide perovskite material comprises an octylammonium cation.

9. The composite film according to claim 8,

wherein the second halide perovskite material is octylammonium lead bromide.

10. The composite film according to any one of claims 1 to 9,

wherein the first halide perovskite material comprises a formamidinium cation.

11. The composite film according to claim 10,

wherein the first halide perovskite material is formamidinium lead bromide.

12. The composite film according to any one of claims 1 to 11,

wherein a bandgap of the plurality of first structures is smaller than a bandgap of the plurality of second structures.

13. The composite film according to any one of claims 1 to 12,

wherein each first structure of the plurality of first structures are in contact with a second structure of the plurality of second structures.

14. The composite film according to any of claims 1 to 13,

wherein the plurality of first structures are interspersed with the plurality of second structures.

15. A device comprising the composite film according to any one of claims 1 to 14.

16. The device according to claim 15, wherein the device is a light emitting device.

17. The device according to claim 15 or claim 16,

wherein the device further comprises:

an electron transport layer in contact with a first surface of the composite film; a hole transport layer having in contact with a second surface of the composite film;

a first electrode in contact with the electron transport layer; and

a second electrode in contact with the hole transport layer.

18. The device according to claim 17, wherein the electron transport layer comprises an

organic semiconductor.

19. The device according to claim 17, wherein the electron transport layer comprises an

inorganic semiconductor.

20. The device according to claim 17, wherein the hole transport layer comprises an organic semiconductor.

21. The device according to claim 17, wherein the hole transport layer comprises an

inorganic semiconductor.

22. A method of forming a composite film, the method comprising:

forming a plurality of first structures comprising a first halide perovskite material; and

forming a plurality of second structures comprising a second halide perovskite material different from the first halide perovskite material;

23. The method according to claim 22,

wherein forming the plurality of first structures and the plurality of second structures comprises mixing a plurality of precursors.

24. The method according to claim 23,

wherein the plurality of precursors comprises lead bromide and formamidinium bromide.

25. The method according to claim 23 or 24,

wherein forming the plurality of first structures and the plurality of second structures further comprises adding the plurality of precursors to a ligand.

26. The method according to claim 25,

wherein the ligand is n-octylamine.

27. A composite film formed according to any one of claims 22 to 26.