The present invention relates to specific perovskite nanocrystals , to components and devices comprising the same, particularly to green light emitting diodes . The invention further relates to manufacturing of such nanocrystals , components and devices .

Pure green light emitting diodes (LEDs ) are essential to realize an ultra-wide color gamut in the next-generation displays , as is defined by the Rec . 2020 standard . However, because the human eye is more sensitive to the green spectral region, it is not yet possible to achieve an ultra-pure green electroluminescence (EL) with sufficiently narrow bandwidth that covers >95% of the Rec . 2020 standard in the CIE 1931 color space .

Protesescu et al ( JACS 2016, 138 , 14202) describe FAPbBr ₃ nanocrystals with bright and stable green photoluminescence . The crystals are monodisperse and cubic-shaped . Although suitable, the films described in that document do not met with the Rec . 2020 standard . Further, the synthesis described in that document ( "hot in ection" ) is considered sensitive, difficult to implement in large scale production and results in unwanted side-products .

Kumar et al (ACS Nano 2016, 10, 9720) describe MAPbBr ₃ nanocrystals showing green to blue tunable electroluminescence . The crystals are monodisperse and 2D shaped. Although suitable, the films described in that document do not met with the Rec . 2020 standard colour co-ordinates . Atanu et al (Chem. Comm, 2017, 53 , 3046) describe a solvent free mechanochemical synthesis of nanopartiulcate perovsiktes of formula APbBr3 (A= Cs , MA, or FA) and their optical properties . The geometry and size of these particles varies over a broad range, when followed the GP173920PC00_description

Dr . SG synthetic approach described. For FAPBBr ₃ , nanoparticles display a parallelogram shape of 30-40 nm x 60-70nm. Also described in that document are moderate electrolumineszent properties of the materials obtained. For example, the PLQY is below 40% and emission peaks around 552 nm. As a consequence of these properties, Rec. 2020 standard cannot be met.

Pathak et al (WO2017/ 017441) describe a specific matrix- incorporated perovskite nano-particles of formula AMX ₃ as luminescent material . Different to the present invention, the authors aim for identifying material to improve quality of white light emitting diodes . To that end, the document discusses emission tunability through anion exchange and mixed anion based perovskite NCs . As cation A, methylammonium is disclosed and the use of a number of other cations is speculated . The document neither discloses or speculates about the use of A = formamidinium cation .

In consequence, there is a need for pure green LEDs meeting with the requirements of Rec .2020 standard colour co-ordinates . Thus, it is an object of the present invention is to mitigate at least some of these drawbacks of the state of the art . In particular, it is an aim of the present invention to provide improved manufacturing methods for luminescent materials and to provide further, particularly improved, luminescent materials .

These ob ectives are achieved by an assembly of nanoparticles as defined in claim 1 and by the synthesis thereof as defined in claim 7. Further aspects of the invention are disclosed in the specification and independent claims , preferred embodiments are disclosed in the specification and the dependent claims . The present invention will be described in more detail below. It is understood that the various embodiments, preferences and ranges as provided / disclosed in this specification may be combined at will . Further, depending of the specific embodiment , selected definitions , embodiments or ranges may not apply . The invention relates,

• i n a 1 ^st aspect to novel assemblies of perovskite- type nanocrystals having 2D structure and narrow size distribution;

• i n a 2 ^ncl aspect to the manufacturing of such assemblies ;

• in a 3 ^rQ aspect to components comprising such novel assemblies, typically in the form of a thin film, and to the manufacturing of such components ;

• in a 4 ^Ln aspect to devices comprising such components and to the manufacturing thereof .

The present invention will be better understood by reference to the figures which show results obtained by using the inventive nanocrystals as described herein;

Fig . la shows a TE image of the inventive assembly of nanoparticles according to example 1.3 (pure FAPbBr ₃ which shows average horizontal dimension approx . 14 nm and vertical approx . 4.5 nm) . The nanoparticles are vertically and horizontally aligned and clearly show the two dimensional morphology as well as the narrow size distribution .

Fig . lb shows distributions of the intensity of light scattered by the inventive assembly of fig . la . Y-axis number PSD [%], X-axis [nm in logarithmic scale] ; FWHM is indicated ( approx . 10 nm) .

Fig . 2a Absorption and photoluminescence (PL) spectra of two-dimensional FAPbBr ₃ perovskite nano crystals according to example 1.3 with peak emission at 531 nm and FWHM less than 22 nm. Y-axis : Absorbance (black) / Photoluminescence (grey) [ a . u . ] ; X-axis : wavelength [nm] . Fig . 2b Photoluminescence spectra of mixed cations (FA, MA) PbBr _. 3 according to example the FA _x MA _y PbBr ₃ ( where x = 100% = 1 and y = 20% = 0.2) emission peaks ranging from 531 nm to 524 nm with FWHM less than 25 nm in the colloidal solutions by decreasing the FA concentration form 100 to 20% . Y-axis : normalized Photoluminescence intensity; X-axis : wavelength [ nm] .

Fig . 2c Photoluminescence spectra of mixed cations (FA, MA) PbBr _. 3 according to example FA _x MA _y PbBr ₃ ( where x = 100% = 1 and y = 20% = 0.2) emission peaks ranging from 530 to 523 nm with FWHM less than 25 nm in the spin- coated films by decreasing the FA concentration form 100 to 20% . Y-axis : normali zed Photoluminescence intensity; X-axis : wavelength [nm] .

Fig . 3a shows a schematic device architecture of active- emissive layer based PeLEDs . Including (1) substrate (glass , polymer sheet , and metal sheets ) , (2) anode layer (transparent conducting oxides (TCO) : ITO and FTO, (3) hole injection layer (e.g. PEDOT : PSS ) , (4) hole transporting layer (e.g. Poly-TPD, PVK, TFB, TPyPA, NPB, and NPD) , (5) Emissive layer (EML) (inventive assembly of nanoparticles optionally with polymer (PMMA or PS or PMMA-PS copolymer) , (6) Electron transporting layer (e.g. organic electron transporting materials (TPBi, BCP , BPhen, Alq ₃ , B3PYMPM, 3TPYMB, TmPyPB, and BmPyPhB) , (7) Electron in ection layer e.g. LiF, CsF, CsC0 ₃ , and Liq, (8) Cathode layer (Al, Ag, Au, Ca, and Mg) .

Fig . 3b shows the energy levels diagram for the materials used in the PeLEDs of fig . 3a.

Fig . 3c shows a schematic device architecture of passive- emissive layer PeLEDs .

Fig . 4a shows the electroluminescence (EL) spectrum of an LED comprising as active layer the inventive two- dimensional FAPbBr ₃ perovskite nano crystals . Y-axis : normali zed EL [ a . u . ] , X-axis : wavelength [nm] .

Fig . 4b shows the electroluminescence (EL) spectrum of an LED comprising as active layer the inventive two- dimensional FA ₀ . ₅ A ₀ . ₅ P Br ₃ perovskite nanocrystals . Y- axis: normalized EL [ a . u . ] , X-axis : wavelength [nm]

Fig . 4c shows Photoluminescence (PL) spectrum of passive down conversion (DC) -LEDs as outlined in fig . 3c,

comprising as active layer the inventive two-dimensional FAPbBr ₃ perovskite nanocrystals (grey lines ) and

FAo.sMAo.sPbBrs perovskite nanocrystals (black lines ) . Y- axis : normalized PL [ a . u . ] , X-axis : wavelength [nm] .

Fig. 5a AF image of pure two dimensional FAPbBr ₃ perovskite thin film shows a roughness rms less than 8 nm. The scale bar is 500 nm.

Fig . 5b AFM image of two dimensional FAPbBr ₃ perovskite and PMMA complex on ITO/PEDOT : PSS coated glass substrate shows a roughness rms less than 1 nm. The scale bar is 500 nm.

Fig . 6a SEM surface morphology of spin-coated thin film of pure two dimensional FAPbBr ₃ perovskites on plasma treated SiOx substrates . Scale bar is 200 nm.

Fig . 6b SEM surface morphology of spin-coated thin film of two dimensional FAPbBr ₃ perovskites and PMMA complex on plasma treated SiOx substrate . Scale bar is 200 nm. Fig . 6c SEM surface morphology of spin-coated thin film of two dimensional FAPbBr ₃ perovskites and PMMA complex on PEDOT : PSS coated SiOx substrate .

Unless otherwise stated, the following definitions shall apply in this specification :

As used herein, the term " a " , "an" , "the" and similar terms used in the context of the present invention (especially in the context of the claims ) are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context . As used herein, the terms "including" , "containing" and "comprising" are used herein in their open, non-limiting sense . In more general terms , in a first aspect, the invention relates to an assembly of nanopart icles , said assembly being characterized by the nanopart icles ^" perovskite- structure, the nanopart icles ^" crystal shape and the nanopart icles ^" size distribution . This aspect of the invention shall be explained in further detail below :

In one embodiment , the invention relates to an assembly of nanopart icles , wherein said nanopart icles are of perovskite-structure according to formula (I),

[FA _x MA _y ] PbBr _. 3 (I) , wherein

FA represents formamidinium

MA represents methylammonium

x is 1.0 - 0.33

y is 0.0 - 0.67

x+y is 1.0;

and wherein said nanopart icles having a two-dimensional crystal morphology; and wherein the size distribution of said nanopart icles within said assembly is narrow .

Nanopart icles of formula (I) are known per se. However, an assembly of nanopart icles of formula (I) with narrow size-distribution and 2D crystal morphology is not known .

Such specific assembly of nanocrystals is available according to the method described herein, 2 ^ηα aspect of the invention . Further, such assembly of nanocrystals shows beneficial optical properties as described herein, 3 ^ra and 4 ^th aspect of the invention . Two dimensional hybrid perovskites are described herein showing efficient , ultra-pure green electroluminescence (EL) . Through the dielectric-quantum-well (DQW) engineering, these perovskites exhibit a high exciton binding energy of 162 meV, resulting in a high photoluminescence quantum yield (PLQY) of -92% in the spin-coated films . Perovskites : The term perovskite is known in the field and relates to crystals having the general formula ABX ₃ .

In an advantageous embodiment , the nanopart icles are formula ( la) , [FA] PbBr ₃ (la), wherein the subst ituent s are as defined herein .

In an advantageous embodiment , the nanopart icles are formula ( lb) , [FA ₁ / ₂ MA ₁ / ₂ ] PbBr ₃ ( lb) , wherein the subst ituent s are as defined herein .

In an advantageous embodiment , the nanopart icles are formula ( Ic) , [FA _1/3 MA ₂ /3 ] PBBr ₃ ( Ic) , wherein the subst ituent s are as defined herein .

In an advantageous embodiment , the nanopart icles are formula ( Id) , [FA ₂ / ₃ MA _1/3 ] PbBr ₃ ( Id) , wherein the subst ituent s are as defined herein . Two-dimensional Morphology : As discussed herein the nanoparticles are of a specific shape, termed two- dimensional or platelet-like . These crystal-morphologies are known to the skilled person and distinguish from needles ( 1 -dimensional ) and cubic ( 3-dimensional ) shapes . Typically, the ratio height : length is in the range of 5:100 to 20:100 and length : width is in the range of 20:100 to 100:100. Preferably, the ratios height : length is 8:100 to 12:100 and length : width is 40:100 to 100:100.

As the term nano-part icle implies , the particle size of compounds of formula (I) is in the nanoscale range . Typically parameters include a length of 5-100 nm, preferably 10-50 nm; a width of 5-100 nm, preferably 10- 50 nm; and a height of 2-10 nm, preferably 2-5 nm.

Typically, more than 90% (n/n) , preferably more than 95%, much preferably more than 99% of the nanopart icles of the inventive assembly show the morphology as described herein .

Size Distribution : As outlined herein, the nanocrystals of the inventive assembly show uniform appearance . This uniform appearance is characterized by its 2D morphology and by the narrow size distribution . The particles of the assembly show a statistical size distribution, characterized by a FWH of less than 30 nm, preferably less than 25 nm, most preferably less than 15 nm. Particle size and size distribution may be determined by standard methods , such as dynamic light scattering (DLS ) or transmission electron microscopy (TE ) . Capping Agents : The assembly according to any of claim 1 4 , may additionally comprise capping agents on its surface . Such capping agents are believed to facilitate and influence crystal growth and crystal morphology .

Suitable capping agents are known in the field and include aliphatic amines and aliphatic carboxylic acids . The term aliphatic amines includes saturated (linear and branched) and unsaturated (linear and branched) amines . Suitable chain lengths for amines include C4-30, preferably Ce-is.,' specific examples include Octylamine, Octadecylamine, Oleylamine .

The term aliphatic carboxylic acids includes saturated (linear and branched) and nonsaturated (linear and branched) carboxylic acids . Suitable chain lengths for carboxylic acids include C4-30, preferably e-is, specific examples include Octanoic acid and Oleic acid .

In a further embodiment , the invention relates to the use of an assembly of nanocrystals as described herein for manufacturing a component (3 ^rcl aspect of the invention) , particularly for manufacturing a LED .

In a second aspect, the invention relates to a process for manufacturing an assembly of nanopart icles as described herein. This aspect of the invention shall be explained in further detail below:

The inventive method comprises the steps of : (a) providing of starting materials ; (b) nanocrystal formation; optionally (c) nanocrystal separation; and optionally (d) post - treatment steps .

Typically, at least step (b) , preferably all steps , are performed at temperatures between 10 - 80 °C, preferably 15 - 50 °C, most preferably 20 - 30 °C. It is considered a significant advantage over the prior art to provide a method allowing crystal formation at low temperatures . This greatly facilitates up-scaling compared to the known methods ( "hot in ections" , as discussed above) . Further, it is known that the nanocrystals of formula (I) are temperature-sensitive, resulting in degradation and formation of unwanted by-products . Such by-products are known to adversely affect product properties . Also, this negative effect is avoided using the inventive process .

Step (a) , providing of starting materials : Starting materials are provided in the form of solutions . Typically, one solution comprises capping agent ( s ) as defined herein, one solution comprised a Pb precursor and one solution comprised FA and MA, typically in the form of a halide (particularly the bromide) .

1 ^st solution comprising capping agents and an apolar solvent , said capping agent preferably selected from organic amines and a carboxylic acids as discussed above . 2 ^nd solution comprising PbBr ₂ and polar solvent .

3 ^rd solution comprising FABr and optionally MABr and polar solvent . Step (b) , nanocrystal formation : Nanocrystals are formed upon contact of 1 ^st , 2 ^ηα and 3 ^ra solution . This may be achieved by known methods , such as subsequent addition of each solution while mixing . In one embodiment, said 2 ^nc solution is added to said 1 ^st solution; followed by addition of said 3 ^rd solution to thereby obtain the assembly of nanoparticles and a supernatant solution .

In one alternative embodiment, said 3 ^ra solution is added to said 1 ^st solution, followed by addition of said 2 ^nci solution to thereby obtain the assembly of nanoparticles and a supernatant solution. Step ( c) , Separation : The assembly of nanocrystals obtained in step (b) may be directly used in further processing steps . Alternatively, said assembly may be separated from by-products using work-up procedures known in the field . The by-products are typically dissolved and present in a supernatant solution . Accordingly, a separation of the assembly of nanocrystals may be effected by filtration or centrifugation .

Step (d) , Post-treatment : The assembly of nanoparticles obtained in step (c) may be sub ect to further post- treatment steps . These steps are known in the field and include purification and dispersion . Purification may be performed by washing with solvents , e.g. to remove / reduce starting materials or capping agents . Re- dispersion may be performed to obtain an assembly of nanoparticles in a more suitable diluent for further manufacturing steps or to corn-bine the assembly of nanoparticles with further components . Capping agents : The term capping agents is discussed above, 1 ^st aspect of the invention . The term shall include one capping agent and a combination of two or more capping agents . Apolar solvent : A broad range of known apolar organic solvents may be used . Suitable solvents have a dielectric constant of 1-10; preferably 1.5-3. Examples include C ₆ -io aromatics , optionally substituted by C1-4 alkyl or halogen, C ₄ - ₂ o alkanes , optionally substituted with halogen . Specific examples include Toluene, Hexane, Octadecene, Chlorobenzene . The term shall include one apolar solvent and a combination of two or more apolar solvents .

Polar solvent : A broad range of known polar organic solvents may be used . Suitable solvents have a dielectric constant of 5-80; preferably 15-50. Suitable solvents are non- or low- prot ic . Examples include Ci- ₄ amides , Ci- ₄ sulfoxides , Ci- ₄ alcohols C ₃ - ₅ lactones . Specific examples include DMF, DMSO, gamma-butyrolactone , ethanol , isopropanol . The term shall include one polar solvent and a combination of two or more polar solvents .

Starting materials : The starting materials MABr, FABr and PbBr2 are known compounds and may be prepared according to known methods . These starting materials are soluble in the solvents discussed herein; making them suitable for upscaling and also allowing simple control of the synthesis .

In a third aspect, the invention relates to a component with an active layer, said layer comprises an assembly of nanopart icles as described herein and to the manufacture of such component . This aspect of the invention shall be explained in further detail below :

In one embodiment , the invention provides for a component comprising a substrate, an active layer and optionally one or more covering layers . Such components include LEDs .

In one further embodiment , the invention provides for a component comprising a composite material wherein an assemb1y of nanopart icles as described herein are homogeneously distributed within a polymer matrix . Such component is typically a thin fluorescent film. Such films find applications as Quantum Dot Enhancement Film (QDEF) and Quantum Dot Colour Filter (QDCF) in rigid, stretchable, bendable, and flexible displays. Moreover, these materials can also be applied as a phosphor for highly efficient down-conversion solid state lightings and flat panel displays.

In a preferred embodiment , the component is an LED . The term LED includes active LEDs , where the perovskite of formula (I) emits light , i.e. acts as an electroluminescent active material . The term LED further includes passive LEDs , where the perovskite of formula (I) down-coverts blue light , i.e. acts as a phosphor . LEDs comprising these perovskites , such as pure FAPbBr ₃ , show a maximum current efficiency of 13.04 cd A ^-1 . More importantly, the color coordinates of (0.168, 0.773) , covers 97% of the Rec . 2020 standard in the CIE 1931 color space . Inventive LEDs are available with large-area ( such as 3 cm ² ) and with small area ( such as 2 mm ) . In addition, the FAo. ₅ MAo. ₅ PbBr ₃ perovskites show a maximum current efficiency over 25 cd A ^-1 and a maximum power efficiency of -24 lm ΙΛΓ ¹ , the highest values ever reported in the colloidal perovskites LEDs . Notably, the color coordinates of (0.170, 0.780) , covers -98% and over 99% of the Rec .2020 standard in the CIE 1931 and CIE 1976 color space, respectively, and represents the "greenest" LEDs ever reported .

Inventive components also include ultra-flexible components ( such as a bending radius of 2 mm) .

Passive emissive layer-LED shows a luminous efficiency up to 90 lm W ^-1" with an emission peaking 528 ±3 nm. The passive emissive films of inventive materials absorb almost 100% emission of blue LEDs , and can be applied as a quantum dot enhancement film (QDEF) , quantum dot color filter (QDCF) .

Substrate ( i ) : Suitable substrates are known in the field . Such substrate may be selected from metals , glasses, polymers, in each case optionally coated with one or more functional layers . It is considered beneficial that the inventive nanoparticles , and the corresponding active layers , are compatible with known technology and thus also compatible with known coated or uncoated substrates .

Active layer ( ii ) : The active layer comprises an assembly of nanoparticles as described herein . The active layer may contain further components , particularly polymers . It is considered beneficial that the inventive nanoparticles , are compatible with known technology and thus also compatible with known polymers used in active layers .

Covering layer ( iii ) : Suitable covering layers , if present , are known in the field . It is considered beneficial that the inventive nanoparticles , and the corresponding active layers , are compatible with known technology and thus also compatible with known covering layers .

Polymers : Suitable polymers as part of the active layer are known in the field and may be selected by the skilled person depending on the intended use . Exemplary embodiments of polymers include acrylate polymers , such as PM A, and styrene polymers , such as PS . passive LED : In passive LEDs , nanoparticles are as defined herein, preferably where x represents 1.0 - 0.67 and y represents 0.0 - 0.33, particularly preferably where x represents 1 and y represents 0.

Further, in passive LEDs , preferably no covering layer ( iii ) is present . Accordingly, the top layer of a passive LED is the active layer as defined herein . Further, in passive LEDs , the active layer ( ii ) comprises nanoparticles as described herein and optionally a polymer . The amount of polymer, if present , may vary over a broad range but preferably is at least 15 wt% Suitable polymers are known in the field and include the polymers discussed herein . active LED : In active LEDs , nanoparticles are as defined herein, preferably where x represents 0.66 - 0.33 and y represents 0.34 - 0.67, particularly preferably where x represents 0.6-0.5 and y represents 0.4-0.5. Further, in active LEDs , preferably a covering layer ( iii ) is present .

Further, in active LEDs , the active layer ( ii ) comprises nanoparticles as described herein and optionally a polymer . The amount of polymer, if present , may vary over a broad range but preferably is at most 15 wt% . Suitable polymers are known in the field and include the polymers discussed herein .

The invention further relates to a method for manufacturing a component as described herein . Generally speaking, known methods for manufacturing components , such as LEDs , may be applied . Typically, such method comprise the steps of :

(a) providing an optionally coated substrate ( i ) and a first suspension, said first suspension comprising an assembly of nanoparticles as defined herein, an apolar diluent and optionally a polymer;

(b) coating said first suspension on said optionally coated substrate ( i ) ;

(c) optionally further coating and / or post-treatment steps .

It was found beneficial to use spin-coating in step (b) to obtain a component as described herein . Alternative coating methods , such as dip-coating or screen-printing or doctor-blading or gravure printing, or ink-jet printing were also found suitable . In a forth aspect, the invention relates to a device with a component as described herein and to the manufacture of such device . This aspect of the invention shall be explained in further detail below :

Devices comprising the components described herein are known per se and include flat panel displays and solid state lightings . Accordingly, the device comprises one or more components as described herein, a housing and optionally further parts .

The inventive components may be integrated into existing production facilities , thereby replacing known components . Accordingly, the invention also relates to a method for manufacturing a device as described herein, said method comprising the step of providing one or more components as described herein and combining them with a housing and optional further parts of the device . To further illustrate the invention, the following examples are provided . These examples are provided with no intend to limit the scope of the invention .

1. Synthetic procedures

1.1 Chemicals

Toluene (99.8%, Fisher Chemical) , Oleic acid (OLA, 90% technical grade, Aldrich) , Octylamine (OA, 99%, Aldrich) ,

Lead (II) bromide (PbBr ₂ , 98+%, Acros Organics) , Tert ^™ butyl alcohol (t-BuOH, for analysis , Fisher Chemical ) ,

N, N-Dimethylformamide (D F, >99.8%, Aldrich) , Ethanol (EtOH, absolute for analysis , Merck) , Hydrobromic acid (HBr , 48% in water, Sigma-Aldrich) , Methylamine (33% in absolute Ethanol , Acros Organics ) , Formamidine acetate (99%, Acros Organics) , Diethyl ether (>99.8%, Thommen-

Furler AG) . All chemicals listed above were used as received . 1.2 Synthesis of MABr and FABr

Methylammonium bromide (MABr) was synthesized by mixing 10 mL of methylamine (33% in EtOH) with 7.5 mL of HBr (48% in H ₂ 0) in 100 mL EtOH . The reaction mixture was stirred for 60 min under ambient conditions and followed by removal of the solvent at 60 °C by means of rotary evaporator . The resulting solid was washed several times with diethylether and recrystallized with EtOH . Finally the purified powder was dried overnight in vacuum oven at 60 °C.

FABr was synthetizied by dissolving Formamidine acetate was in 2 molar equivalents of HBr (48% in H ₂ 0) and left under stirring for 10 min at 50 °C . The solvent was removed at 100 °C by means of rotary evaporator . The resulting solid was washed several times with diethylether and recrystallized with EtOH followed by drying in vacuum oven at 60 °C.

1.3 Synthesis of colloidal 2D perovskite crystals

The 2D perovskites , FAPbBr ₃ and MAPbBr ₃ were synthesized as follows :

(a) The perovskite precursors , FABr or MABr (0.53 M) and PbBr2 (0.4 M) were distinctly dissolved in polar N, N- dimethylformamide (DMF) solvent . OLA ( 625 μL) and OA (25 μL) were mixed with non-polar solvent (Toluene, 12.5 mL) to thereby obtain the above identified 1 ^st , 2 ^ncl and 3 ^rci solution .

(b) Subsequently, the precursor solutions mentioned above were added dropwise (375 μL of MABr or FABr and 625 μL of PbBr ₂ ) to a non-polar toluene solution containing OA as long chain ligand and OLA as stabilizer under constant stirring . An instantaneous colloidal crystallization is triggered due to poor solubility of perovskite precursors in nonpolar toluene . A precipitate is formed immediately . (c) The precipitate is separated from the reaction mixture by means of centrifugation . (d) The resultant supernatant is discarded and the precipitate containing FAPbBr ₃ nanoplatelet s is dispersed in 2.5 mL of fresh toluene, which yields the final product .

However, contrarily to the case of MA-based perovskite nanoplatelet s , it was observed that the FA counterpart exhibits lower stability and therefore lower final concentration, when DMF (highly polar, aprot ic) is used to solubilize FABr. Presumably, due to the larger ionic radius of FA ⁺ , it is energetically less favorable to form the perovskite structure, when Br ^~ is used as an anion . Therefore, even small amounts of highly polar, aprot ic solvent (like DMF) can cause a significant degree of ligand desorpt ion from the nanocrystal surface . As a result , it leads to a formation of less stable colloidal dispersion . In order to solve this issue, instead of DMF, ethanol (less polar and prot ic) was used to dissolve FABr (0.53 M in EtOH) . The same volume (375 ]i ) was added to the reaction mixture as in case of DMF solution . This modification allowed to achieve more stable colloidal solutions of FAPbBr ₃ nanoplatelet s with significantly higher concentrations .

2. Characterizations

2.1 Ultraviolet-visible (UV-vis) spectra were collected using a JASCO V670 spectrometer . Results are provided in Fig .2a . 2.2 Photoluminescence (PL) spectra were recorded with Hamamat su CCD spectrometer . The absolute PLQYs in solid thin films and colloidal solutions (in toluene) were determined using the Quantaurus QY (C11347-11) from Hamamatsu . Results are provided in Fig 2. 2.3 Temperature dependent steady-state PL spectra were measured using a JASCO FP-8300 spectroiluorometer. Results are provided in Fig 2d. 2.4 Time resolved (TR) -PL measurements : TR-PL spectra were collected using a time-correlated single photon counting (TCSPC) measurement system equipped with a picosecond pulse laser head (LDH-P-C-405B, PicoQuant ) with 405 nm excitation wavelength, monochromator ( SP- 2155, Acton) , and ultrafast detection (MCP-PMT (R3809U- 50 , Hamamatsu) ) .

2.5 X-rays diffraction (XRD) patterns of 2D perovskite crystals (FAPbBr ₃ and MAPbBr ₃ ) were measured using a PANalytical X' Pert PRO-MPD diffractometer with Cu-K radiation . The data were recorded in the range of 10-60 ⁰ 2Θ at room temperature with an angular step size of with an angular step size of 0.017° and a counting time of 0.26 seconds per step . In the XRD spectra for both compounds , the three dominant peaks at around 15°, 30°, and 45° are assigned as the (100) , (200) , and (300) planes , for the cubic perovskite structure . The lattice constant is therefore determined to be 6.0 A for 2D FAPbBr ₃ consistent with those reported in the bulk counterparts . Nevertheless , the peak intensity associated with other planes , such as (110) at -21° and (210) at -34°, is very weak, confirming that the crystalline symmetry is not three dimensional . Results are presented in Table SI .

2.6 Grazing incidence X-ray diffraction (GIXD) patterns were measured on beamline BL13A at the National Synchrotron Radiation Research Center (NSRRC) , Taiwan . A monochromatic beam of λ = 1.0205 A was used, and the incident angle was 0.12°. In the GIWAXS pattern, the Debye-Scherrer ring for the (100) plane , corresponding to q = 10.5 nm-1 , becomes the strongest on the q _z axis, consistent with our previous observations in the spin- coated solids of two-dimensional perovskite dispersions.

2.7 High resolution scanning transmission electron microscopy (HRTEM) image was captured using HRTE operated at 200 kV . The solution sample was deposited on a copper grid and dried for 2 hours under vacuum. The TEM image shows each single crystallites either vertically or horizontally oriented on the copper grid (Fig . la) . As shown in Fig . la, each horizontally oriented two dimensional FAPbBr ₃ crystallite shows a dimension length of 13.40±1.20 nm, while vertically aligned crystallites show a thickness of 4.85±0.25 nm for two dimensional perovskites . Since the capping ligand layer is relatively soft, we suppose that there is a degree of underestimation here . To gain more insights into the thickness , the samp1e was subsequently analyzed by dynamic light scattering (DLS) , as shown in Fig . 2b. The number particle size distribution (PSD) shows a single peak confirming the monodisperse distribution with an average size of 18.2 nm. The mentioned discrepancies in the crystal size is reflected because of the measurement limitations in each method . 2.8 Ultraviolet photoelectron spectroscopy (UPS) was used to evaluate the valance band maximum (VBM) energy levels of 2D perovskites from the valance band edge and secondary electron cut-off, while the conduction band minimum ( CBM) energy levels were estimated from the PL emission peaks . Results are presented in Fig . 3b.

2.9 Atomic Force Microscopy (AFM) was used to evaluate the surface morphology of spin coated thin film of pure 2D FAPbBr ₃ perovskite and its complex with PMMA deposited on Poly-TPD layer with a layer sequence of ITO/PEDOT :PSS/Poly-TPD using Asylum Cypher S AFM operated in the tapping mode in the ambient conditions . The AFM images of pure two-dimensional FAPbBr ₃ perovskites (rms = 7.91 nm) and its complex PMMA (rms = 0.63 nm) , films were deposited on ITO/PEDOT : PSS substrates (Fig . 5) . 2.10 Scanning Electron Microscopy (SEM) was also use to further evaluate the surface morphologies of spin coated thin emissive layer . Moreover, Fig . 6 shows the SEM surface morphologies of pristine two-dimensional FAPbBr ₃ and its complex with PMMA on plasma treated silicon oxide ( SiO _x ) substrate using three different compositions , such as, (1) SiO _x /FAPbBr ₃ , (2) SiO _x /FAPbBr ₃ + PMMA, and (3) SiO _x /PEDOT :PSS/ FAPbBr ₃ + PMMA. Fig. 6a shows rough surface morphology with large uncovered patches when pristine colloidal two-dimensional FAPbBr ₃ perovskites layer was spin-coated on the plasma treated SiO _x substrate . On the contrary, a relatively smooth morphology with entire surface coverage when two- dimensional FAPbBr ₃ and PMMA complex emissive layer was spin coated (Fig . 6b) . A higher surface coverage and pin holes free surface morphology have been accomplish as complex emissive layer was spin-coated on the PEDOT : PSS /SiO _x substrate (Fig. 6c) .

3. Materials , fabrication , and characterization of perovskite LED components

3.1 Materials : Patterned indium tin oxide (ITO) coated glass substrates with a sheet resistance of 15 Ω /□ are purchased from Lumtech Corp . The hole in ection material poly (3, 4-ethylene-dioxythiophene) -poly (styrene sulfonate) (PEDOT : PSS ) is purchased from Heraeus (Clevios AI 4083) . Highly pure sublimed grade hole transporting materials , poly [N, N '-bis ( 4-butylphenyl ) -N, A/ ^" '-bis (phenyl ) -benzidine ] (Poly-TPD) , poly ( -vinylcarbazole) (PVK) , and poly [ (9,9- dioctylfluorenyl-2 , 7-diyl ) -co- (4, 4 '- (N- ( 4-sec- butylphenyl ) diphenylamine) ] (TFB) , are supplied by Lumtech Corp . A neutral host matrix poly methyl methyaerylate ( PMMA ) with an average M . W . 350, 000 is purchased from Sigma-Aldrich . The electron transporting materials 2, 2', 2 "-(1,3, 5-benzinetriyl) -tris ( 1-phenyl-1 -H- benz imidazole) (TPBi) , 4, 7-Diphenyl-l , 10-phenanthroline (BPhen) , 4, 6-bis (3, 5-di (pyridine-3-yl) phenyl ) -2- methylpyrimidine (B3PYMPM) , and tris (2,4, 6-trimethyl-3- (pyridin-3-yl ) phenyl) borane ( 3TPyMB) are procured from Lumtech Corp . The electron injection materials lithium fluoride (LiF) ( 99.98%) is purchased from Acros Organics . Aluminum (Al ) pellets ( 99.999%) were purchased from Kurt J . Lesker Co . Ltd . All sublimed grade molecular hole transporting, electron transporting and host materials and the GPC grade polymer compounds were used without any further purification .

3.2 Perovskite LEDs fabrication : Patterned ITO coated glass substrates were rinsed in the Extran MAO2 neutral detergent and deionized (DI) water mixture (1:3) . Afterward, these substrates were sequentially sonicated in DI water, acetone, and isopropanol , each for 10 minutes . The substrates were then exposed to oxygen plasma for 10 min . in diener plasma cleaner . Thereafter, the aqueous PEDOT : PSS solution was spin-coated on the pre-cleaned ITO glass at a speed of 4000 rpm for 20 s then all the substrates were then transported into a nitrogen atmosphere glove box for annealed at 130 °C for 0.5 h in the ambient conditions . All the annealed substrates were then transported into a nitrogen atmosphere glove box for the deposition of successive layers . Consequently, a 20±2 nm hole transporting layer (HTL) of Poly-TPD, TFB, or PVK (in chlorobenzene ) was spin-coated on PEDOT : PSS layer at 3000 rpm for 40 s . The HTL was then annealed at 130 °C for 0.5 h . Before spin- coating, the monodisperse 2D FAPbBr ₃ perovskites (in toluene) were mixed with the non-emissive PMMA host , while 2D MAPbBr ₃ were mixed with a low-A: host , TFB, to obtain the complex emissive layer (EML) . The resultant EML was then spin-coated at 2500 rpm for 40 s. All substrates were transferred in the ultrahigh vacuum evaporation chamber ( 8 10 ^~d mbar ) . Subsequently, a 35 nm ETL was deposited on the EML by the thermal evaporation. Finally, a 1 nm LiF electron injection layer and a 100 nm Al cathode layer were also deposited in a high vacuum chamber ( 8 10 ^~ ° mbar) by using a shadow mask . Each substrate is patterned to realize four devices , each with an active area of 37.5 mm as defined by the overlapping area of the bottom ITO anode and top Al cathode layers . All the devices were stored in the glove box and characterized under the ambient atmosphere .

A 50 μιτι thin polyimide (PI ) foil was used as a flexible substrates for the perovskite LED . First of all , 50 nm SiN _x layers was deposited on both side of PI substrates to reduce the surface roughness and enhance the chemical and thermal resistivity . Subsequently, a 120 nm ITO layer was deposited (at room temperature) by radio frequency (R.F.) magnetron sputtering using a standard anode metal mask. Then ITO coated substrates were cleaned in oxygen plasma chamber for 10 min . Afterward, PEDOT : PSS and Poly ^™ TPD were sequentially spin-coated on the flexible substrates then annealed at 130 °C for 0.5 h after each step . Subsequently, 2D FAPbBr ₃ and PMMA complex emissive was spin coated on the Poly-TPD layer . Then substrates were moved in the high vacuum chamber for the successive TPBi, LiF and Al layers deposition via thermal evaporation . Similar to glass substrates , each substrate is patterned to realize for device with an active area of 25 mm , which is defined by the overlapping between the ITO anode and Al cathode layers .

3.3 Perovskite LEDs characterization : Current density- voltage-luminance ( J-V-L) characteristics of the perovskite LEDs were measured and a Keithley 2400 source meter . The electroluminance (EL) spectra and colour coordinates of all the devices were recorded by using a Photo Research PR 655 spectrometer. The EQE was calculated as the total number of emitted photons divided by the total number of injected electrons by assuming a Lambertian-type emission pattern . The Commission Internationale de I ' eclairage (CIE) coordinates of 2D perovskite LEDs were calculated using a calibrated ASEQ LRl-Tv .2 (CCD) spectrometer .

Table SI. XRD peaks and corresponding planes of 2D perovskite crystals .

FA I'llBr,

20 I I Plane

15.325 001

21.575 Oil

30.225 002

33.775 012

45.775 003

62.175 004