ES2957359B2 - PEROVSKITE NANOCRYSTALS WITH PASSIVATED SURFACE AND THEIR USE - Google Patents

Description

DESCRIPTION

Perovskite nanocrystals with passivated surface and their use

The present invention provides surface-passivated perovskite nanocrystals that exhibit excellent photoluminescent (PL) properties and stabilities. The invention also refers to its use, mainly, in the manufacture of printing inks with high emittance and stability.

Therefore, the present invention has an application in the field of optoelectronic devices.

STATE OF THE TECHNIQUE

After six years since their first synthesis, published by Kovalenko and his collaborators (Kovalenko, M. V.et al. Nano Letters2015,15(6), 3692-3696), perovskite nanocrystals (PNCs; also called dots quantum) remain the most attractive luminescent materials, thanks to their intrinsic properties (Dey. A. et alACS Nano2021, 15 (7), 10775-10981). Notable features that have been studied in optoelectronics and photovoltaics include a photoluminescence quantum yield (PLQY) of up to 100%, a narrow PL half-width, a structure with high defect tolerance, dynamic surface chemistry, easy manufacturing using different synthetic protocols and composition/dimension engineering to modulate the energy bandgap. What is most interesting of all is the nanoconfinement regime that PNCs present and that have been used to stabilize the photoactive crystalline phases in the ambient atmosphere. This is why PNCs have gained preference over mass-type perovskites, in which there is a greater tendency to form the inactive crystalline phase 5 (Masi, S.et al. ACS Energy Letters2020, 1974-1985). This is the case of iodine-based PNCs (l-PNCs), such as CsPbl<3> and FAPbl<3>, which show an optical energy bandgap of between —1.7 and 1.4eV, respectively, when the black a phase is obtained. By stabilizing this crystalline structure, a theoretical maximum photoconversion efficiency (PCE) of 32.3% (in the case of mass FAPbh) has been calculated (Shockley, W.et al. Journal of Applied Physics1961,32(3), 510-519), which is higher than the actual PCE record of -25.6% (Jeong, J.et al. Nature2021,592(7854), 381-385), close to the limit Shockley-Queisser theorist. Even the PCE of l-PNCs is still far from the previous value, for which a rapid increase of up to 18.1% with long-term stability has been achieved in the last two years. Strategies such as washing PNCs with low polarity antisolvents and fabrication of PNC films by layer-by-layer deposition have been found suitable for extracting some fractions of long-chain encapsulating ligands that act as insulators and reduce defect density. structural, thereby improving the operation of the devices (Han, R.et al. The Journal of Physical Chemistry C2021,125(16), 8469-8478; Sanehira, E. M.et al. Science Advances2017,3(10), eaao4204).

Although the stability in the black phase of iodine perovskites can be extended in the nanoconfinement regime, the aging process provides enough time for some encapsulation ligands, such as oleic acid (OA), to English) and oleylamine (OLA), are released from the surface of PNCs (Zhang, Y.et al. Chemistry of Materials2020,32(13), 5410-5423). This facilitates the formation of surface defects, especially halide deficiency, which deteriorate the PL properties of the materials. In most cases, the loss of ammonium species, such as oleylammonium iodide (OLAm-l), is the main reason for generating halide vacancies (uncoordinated Pb) that cause the octahedral tilt of the crystal lattice of the PNC and, in this way, accelerate the transformation from phase a to phase 5, as well as the appearance of a high density of non-radiative carrier traps. At this point, modification of synthetic processes, ligand engineering, metal doping and matrix encapsulation have been studied as possible alternatives for the manufacture of high-quality l-PNC with greater stability.

Mainly, the incorporation of a high OA/OLA content of the conventional protocol intervenes in the formation of sufficient OLAm-l species, dissolves the halide precursor and promotes the stabilization of the PNC (Hassanabadi, E.et al. Nanoscale2020,12 ,14194-14203). On the other hand, the replacement of Pb2+ by a reduced fraction of Bi3+ or Sr2+ cations has led to the suppression of non-radiative recombination dynamics, both in the PNCs of CsPbl3 and FAPbl3, focused on the compensation of Schottky defects (vacancy of Pb and I) (Gualdrón-Reyes, A.F.et al. Journal of Materials Chemistry C2021,9(5), 1555-1566). Meanwhile, employing encapsulating ligands with stronger binding ability to the surface of nanocrystals, for example, addition of trioctylphosphine (TOP) or treatment with sulfur-oleylamine, can reduce the density of halide defects in the freshly prepared material (Liu, F.et al. ACS Nano2017,11(10), 10373-10383). In the case of TOP, a PLQY of 100% can be obtained, with a stability of around one month. The most interesting thing is that this ligand has also been added to aged CsPbBr<3>-xlx PNC colloidal solutions to recover its PL properties (Wang, H.et al. The Journal of Physical Chemistry Letters2018,9(15), 4166 4173). In this case, the formation of Pb-P bonds is decisive to reduce the amount of uncoordinated Pb, thereby eliminating carrier electron traps to improve radiative recombination and prolong the life of the material. However, phosphines and most of the more recent encapsulation ligands that have been used in the stabilization of PNCs have inherent toxicity, which hinders their potential industrial applications.

Even the structural integrity of PNCs is favored by ligand engineering; their combination with certain solvents to prepare formulations suitable for the creation of free-form structures is a great challenge today. This is the objective of additive manufacturing, which uses some additives, such as surfactants and polymeric matrices, to modify the properties of inks based on nanostructured materials, such as surface tension, viscosity, the chemical composition of the solution and the quality of real 3D printed objects. This feature is well used in attractive applications such as inkjet printing, for the configuration of micro- and macrostructures, and lithographic ink, in which ink is deposited on both rigid and flexible substrates to fabricate a new generation of transistors and thin film circuits, which opens the doors to 3D optoelectronics. However, the use of solvents in the production of inks, such as dimethyl sulfoxide, y-butyrolactone and N-methyl-2-pyrrolidone, among others, usually dissolve the lead halide precursors, which can immediately deteriorate the PL properties of PNCs. Therefore, a versatile methodology for stabilizing red light-emitting PNCs without corrosive solvents in the ink formulation and preserving their PL properties after the printing process has not been widely exploited.

Therefore, there is a need to offer highly stable PNCs with optimal PL properties for use in the field of optoelectronic devices, such as the production of printing inks, among other applications. For this purpose, the present invention describes new perovskite nanocrystals with a passivated surface that have the necessary characteristics.

DESCRIPTION OF THE INVENTION

The authors of the present invention show for the first time the use of non-toxic and low-cost compounds that belong to the tocopherol (TCP) family, tocopherols (alpha, beta, gamma and delta) and to the of tocotrienols (alpha, beta, gamma, delta), to improve the photophysical properties and stability of perovskite nanocrystals (PNC) with and without lead, through post-synthetic surface passivation through ligands. These new strategies were valid in different types of PNC with and without lead. Furthermore, a new synthesis protocol has been developed for the preparation of double halide PNCs with ordered vacancies.

The use of different ratios of a single combination of acrylic polymers, cross-linkers and a UV photoinitiator has allowed the manufacture of photoluminescent devices through 3D printing and the development of inks for the inkjet printing process.

By adding vitamins, such as a-tocopherols (a-TCP, vitamin E), in a concentration range between 0.01 and 0.5 M, the PLQY in new and aged PNC, the highest values are obtained of -98% and 100%, respectively, and a reduction of this property of around 8% and 23%, respectively, after two months, under ambient conditions and a relative humidity of 60%. α-TCP passivates the surface of PNCs and mediates restoration of PL properties and reduction of non-radiative carrier traps, as well as improving radiative recombination dynamics with reduction of surface defects. The combination of TCP and PNC (referred to as: TCP@PNC or TCP-passivated PNC), facilitates the chemical formulation to manufacture PNC-acrylic polymer composite materials through 3D printing, with complex shapes and improved luminescent characteristics, long-term stability of more than 4 months, which is not achieved with unmodified PNCs. A PLQY of -92% was obtained in the composite material. In the case of red light-emitting materials, this is the highest value obtained in a 3D printed solid to date. The passivation cover offered by TCP ensures that PNC inks do not suffer any degradation process, avoiding contact with the environment, and preserves the properties after mixing with polar monomers during the manufacturing of the composite material. This contribution offers the possibility of stabilizing highly unstable PNCs and allows their processing through 3D printing, with a possible application in resizable and solid optoelectronic devices.

Furthermore, a first aspect of the present invention refers to surface passivated perovskite nanocrystals (TCP@PNC), which comprise:

- a halide-based perovskite nanocrystal (X-PNC) with the following selected formula: ABX<3>(undoped) and A<2>BX<6>doped with Bi3+ or Te4+, where ,

A is selected from: Cs, methylammonium (H<3>C(NH<2>), abbreviated as MA), formamidinium (HC(NH<2>)<2>, abbreviated as FA),

B is selected from Pb and Sn,

X is a halogen selected from chlorine, bromine or iodine; and

- a ligand attached to the surface of said halide-based perovskite nanocrystal, where the ligand is a tocopherol (TCP).

The term "perovskite nanocrystals" (PNCs, also called quantum dots (QDs) when the quantum confinement regime is reached) are semiconducting nanoparticles that have a perovskite structure with a particle size of 5- 100nm.

In the surface passivated PNCs of the present invention, there is a bond between the surface of the iodine-based PNC, chlorine-based PNC or bromine-based PNC and the vitamin TCP. The term "bond" as used herein is a concept that includes a covalent bond, an ionic bond, a coordinate bond or the like.

Preferably, the binding to the surface of the PNC is formed through the OH-phenolic ring of the TCP vitamin.

The term "tocopherol" as used herein includes any compound of the tocopherol family selected from alpha-, beta-, gamma-, and delta-tocopherol and alpha-, beta-, gamma-, and delta-tocotrienol.

In a preferred embodiment, the halide-based perovskite nanocrystal has the formula ABX<3> (this PNC is undoped). More preferably, it is an iodine-based PNC (X=I).

In a preferred embodiment, A is selected from FA and Cs; more preferably, A is Cs in the formula ABX3.

In another preferred embodiment, B is Pb in the formula ABX3.

In a more preferred embodiment, the halide-based perovskite nanocrystal has the formula CsPbÍ<3>.

In a preferred embodiment, the halide-based perovskite nanocrystal has the formula A<2>BX<6>doped with Bi3+ and/or Te4+. This formula corresponds to a double halide perovskite with ordered vacancies, where the BXs octahedra share the vertices with vacant virtual octahedra, in fact, each of the BXs are isolated from each other.

In a preferred embodiment, A is Cs in the formula A<2>BX<6>.

In another preferred embodiment, B is Sn in the formula A2BX5.

In another preferred embodiment, X is Cl in the formula A2BX5.

In a more preferred embodiment, the halide-based perovskite nanocrystal has the formula: Cs<2>SnCl<6>doped with Bi3+ (blue light-emitting perovskite), Cs<2>SnCl<6>doped with Te4+ (perovskite yellow light emitter).

In a preferred embodiment, the TCP is vitamin E or a-tocopherol (a-TCP).

In a preferred embodiment, the halide-based perovskite nanocrystal with the formula ABX3 has a particle size of between 10 and 11 nm, and the double perovskites (A2BX6 doped with Bi3+ and/or Te4+) have particle sizes of between 50 and 70nm. Particle sizes are quantified by transmission electron microscopy.

The halide-based perovskite nanocrystal of formula ABX3o A2BX6 doped with Bi3+ and/or Te4+ can be a new or aged nanocrystal. Fresh nanocrystal refers to the freshly prepared material obtained after synthesis, while aged nanocrystal refers to the same material previously aged, preferably, for 1 month under ambient conditions and a relative humidity of 60%.

A second aspect of the present invention refers to a process for the production of surface-passivated perovskite nanocrystals, which has already been described in the first aspect of the invention from halide-based PNC. This procedure includes:

The passivation of the surface of the halide-based PNC, by putting it in contact with TCP in an aqueous solution, where the concentration of the halide-based perovskite nanocrystals incorporated into the solution is 100 mg/ml, and the concentration of TCP in the solution is between 0.01 M and 0.5 M. The passivation of the nanocrystals takes place when the halide-based PNCs come into contact with the TCP solution.

In a preferred embodiment, the TCP concentration of the solution is 0.15 M.

The final PNC-TCP solution is stable for more than three months and, by centrifugation, the PNC precipitate can be separated from the solution.

On the other hand, halide-based PNCs used as starting material can be generated by following the following steps, as described in the literature (Kovalenko, M.V.et al. Nano Letters2015,15(6),3692-3696 ):

- The oleate solution with the A-position cation is synthesized by mixing a salt with an A-position cation (carbonate or acetate salt), oleic acid (OA) and 1-octadecene (1-ODE), with stirring strong and in vacuum/purge with N2. The oleate solution with the A-position cation is heated to a temperature range of 80-120 °C, whereupon the solution is ready for injection into the BX2 solution (see below).

- The stoichiometric content of BX2 is mixed with 1-ODE, with vigorous stirring and under vacuum/N2 purge. Next, a mixture of OA and oleylamine is incorporated into the mixture in a molar ratio of 1:1 and heated in a temperature range between 85 °C-1800C, where the oleate is rapidly injected with the cation in the position. A preheated to obtain PNC of ABX3. After cooling the reaction in an ice bath, luminescent PNCs are formed.

Lead-free perovskite PNCs doped with Bi3+ and Te4+ can be synthesized by following the following steps, as described in the literature (Mora-Seróet al. Nanoscale,2022, 14, 1468-1479):

- Stoichiometric amounts of acetate are mixed with the A position cation (I), acetate with the B position cation (IV) and different amounts of bismuth acetate (III) and/or tellurium (IV) (for doping with Bi3+ and/or Te4+) with oleic acid and 1-ODE in vacuum/N<2>purge and at 120 °C for 1 hour. The metal precursor solution is cooled to room temperature, and the solution is transferred to a beaker with HX (in aqueous medium). The temperature of the reaction medium is maintained between 25 °C-50 °C. Acetone is added to the beaker and placed in an ultrasonic bath for 10 minutes. In this way, doped PNCs are formed.

Freshly prepared (new) PNCs can be passivated with a TCP, such as vitamin E, immediately after preparation. Another option is to age the new PNCs and then use them in the passivation stage.

A third aspect of the invention refers to the use of PNCs with a passivated surface that have been described in the first aspect of the invention for the production of printing inks. More preferably, polymeric printing inks and, even more preferably, acrylic printing inks (inks produced from acrylic polymers or monomers).

In a preferred embodiment of the use of the invention, the printing inks refer to 3D printing inks.

In a preferred embodiment, the acrylic printing ink is obtained by polymerizing monomers selected from the list, comprising: butylacrylate (BA), isobornylacrylate (IBA), 1,4-butanedioldacrylate (DBA) or combinations of these, in presence of the PNC of the present invention.

Preferably, the polymerization takes place in the presence of a UV initiator, such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). More preferably, the UV initiator has a concentration of at most 3% by weight in the reaction mixture.

To prepare acrylic printing inks, it is preferable to use lead perovskites with the formula ABX<3>(B = Pb) and/or lead-free perovskite PNC doped with Bi3+ and Te4+ with the formula A<2>BX<6 >(B other than Pb) in combination with acrylic polymer. The mixture obtained after polymerization results in the formulation of a PNC composite material with stable and highly luminescent printing ink. Preferably, the mixture, once subjected to constant magnetic stirring for 30 min, is placed in a refrigerator for at least 15 min before UV treatment.

In a preferred embodiment, the printing inks contain a concentration of at most 6.0% by weight of passivated, lead perovskite-based PNCs with the formula ABX3 or at most 12.0% by weight of passivated perovskite-based lead-free doped with Bi3+ and Te4+ with the formula A<2>BX<6>.

In a preferred embodiment of the acrylic printing ink, obtained by polymerization of BA, IBA, DBA or combinations thereof, the percentage of the acrylic monomers BA IBA, in the ink, varies between 80 and 100% in moles, (it is possible to have only one type of these monomers), and the amount of DBA is 0-20 mol% in the polymer.

Cooled formulations of acrylic printing inks can be deposited in a print reservoir for MSLA (masked stereolithography apparatus, SLA) or digital light processing (DLP) printing. MSLA printing works on the concept of resins that are treated by exposure to ultraviolet light. To do this, MSLA printers use a large UV light source and then mask it with a liquid crystal display. They can be obtained by following the digital design that has been downloaded to the printer. Once the structure is 3D printed, the object is removed from the platform, washed and treated with UV light.

Devices with excellent optical properties and high stability can also be obtained by inkjet printing, coating process or film deposition processes.

To carry out inkjet printing, cooled formulations can be deposited on glass slides, acetate sheets or other substrates with an inkjet printer that is equipped with cartridges with high resolution inkjet heads without contact and a UV light treatment bridge.

To perform film deposition processes and coating processes, cooled formulations can be placed inside a mold, using gravity to fill the cavity, and allowing the polymeric formulation to harden, or on glass slides, sheets. of acetate, metallic films or other substrates with a wand-type drag applicator or regulating blade. The samples should then be exposed to UV radiation (Form Cure, Formlabs, USA), for further treatment. Multiple coating passes can be made until the desired thickness is obtained.

In summary, the present invention describes for the first time the use of non-toxic and low-cost vitamins as a very effective strategy to improve not only the intrinsic characteristics of PNC with synthesized halides, but also recover the lost properties of aged PNC. a-tocopherol (a-TCP) has been used, which is the most biologically active form of vitamin E and is most frequently used as an antioxidant. By passivating halide PNC surface ligands, the present invention, as demonstrated in the examples of the invention: 1) provides efficient passivation to achieve a photoluminescence PLQY close to unity; 2) recovers the PL properties of the materials and keeps the black phase stable for 2 months under ambient conditions; 3) recovers 28% of the PLQY of aged halide PNCs; and 4) stabilizes l-PNCs in polymeric films, especially those formulated for three-dimensional printing. Taking into account the theoretical calculations, the inventors propose the attachment of a-TCP to the surface of PNCs through the OH-phenolic ring, thereby compensating the halide vacancies through oxygen passivation. In this way, the phenolic-chromanol ring is capable of protecting PNC from environmental conditions and increasing its stability. Furthermore, vitamin-modified PNC inks retain their intrinsic properties when combined with acrylic monomers, such as isobornylacrylate and butylacrylate, forming three-dimensional red light-emitting polymer composite materials with exceptional PL characteristics. In this state, the two acrylate species polymerize with α-TCP, inducing the transport of charge carriers to the PNCs through the aromatic ring. This is believed to be the main reason for enhancing radiative recombination in the solid. Furthermore, the present invention offers a valuable alternative to (i) produce high-emitting, stable and compatible inks based on halide PNCs for inkjet printing processes; and (ii) facilitate the manufacturing of photoluminescent devices through 3D printing with different and more complex geometries.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly used and understood by any person skilled in the art to which this invention pertains. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and in the claims, the word "comprise" and its variants are not intended to exclude other technical characteristics, additives, components or steps. Other objects, advantages and features of the invention will become apparent to those skilled in the art upon evaluation of the description, or may be acquired through practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to limit the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Typical transmission electron microscopy (TEM) images of (a) 0-T@PNC-1 on day 0 and (b) 0.15-T@PNC-1 at 60 days (d). (c) Photoluminescence spectra of PNC samples at different aging time points under ambient conditions, with and without a-TCP. (d) X-ray diffraction (XRD) patterns of PNC-1 on day 0 and X-T@PNC-1 after 60 days, by adding different concentrations of a-TCP.

Figure 2. Selected area electron diffractometry (SAED) patterns obtained in samples of (a) 0-T@P1 on dayO and (b) 0.15-T@P1 after 60 days of aging.

Figure 3. Typical TEM image of 0-T@P1 after 20 days of aging. The inset of Figure 3 shows the photograph of the material with a partial transformation from phase a to Ia5.

Figure 4. Histograms for calculating the particle size of (a) 0-T@P1 on day 0 and (b) 0.15-T@P1 after 60 days of aging.

Figure 5. High-resolution X-ray photoelectron spectroscopy (HR-XPS) spectra of (a) Cs 3d, (b) I 3d, and (c) Pb 4f, and (d) calculated ratios of total oxygen to halide/ total oxygen with respect to lead of samples of 0T@P1 (P1) and X-T@P1 (X= 0.15M) at different aging time points (0 and 60 days).

Figure6. XPS survey spectrum of 0-T@P1 (P1) and X-T@P1 (X = 0.15M) samples at different aging time points (0 and 60 days).

Figure 7. PLQY values as a function of the days of aging of colloidal solutions of (a) X-T@P1 and (b) X-T@P2, in the absence or presence of different concentrations of a-TCP. Behavior of the decay constants of (c) radiative (kr) and non-radiative recombinations {km) and their corresponding (d) knr/kr ratios of the X-T@P1 samples, X = 0 (P1), 0.15 M (0.15M-1) and (b)X-T@ P2X= 0 (P2), 0.15 M (0.15 M-2) at 0 and 60 days, under ambient conditions, with and without the addition of the vitamin .

Figure 8. Comparison of the stability of the colloidal solutions of X-T@P1 (purple line) and X-T@P2 (pink line) (X=0.15 M), after 60 days of aging.

Figure9. PLQY of colloidal solutions of (a) X-T@P1 and (b) X-T@P2, by varying the concentration of a-TCP after 60 days of aging.

Figure 10. Time-resolved PL decay measurements of colloidal solutions of (a) X-T@P1 and (b) X-T@P2, in the absence and presence of 0.15 M a-TCP, on day 0 and after 60 days .

Figure 11. Representation of the organic ligands bound to the PNC surface: (a) oleic acid through carboxylate species, (b) a-TCP molecule through the oxygen of the phenolic ring (binding case I), ( c) a-TCP molecule through the oxygen of the epoxy ring (binding case II). Chemical contribution of atomic species of Pb (green), I (red), Cs (blue) and oxygen (black) to the molecular orbitals calculated in PNC showing up-spin (positive) and down-spin (negative) energy levels (d ) in the absence and (e) in the presence of a-TCP in the case of junction I. In the anatomical representation (a, b, c): the gray, purple and green dots represent Pb, I and Cs atoms, respectively, (f) Proton nuclear magnetic resonance (1H NMR) spectra (CDCI<3>, 400 MHz, 25 °C) of CsPbh PNCs, with and without a-TCP. The characteristics of the NMR resonances of the encapsulating ligand samples were also obtained for comparative purposes, (g) Molecular structure of a-TCP and the encapsulating ligands that stabilize PNCs.

Figure 12. (a) Atomic structures of a-TCP and oleic acid molecules after geometric relaxation carried out with CP2K, where the red dots represent the oxygen atoms. Atomic representation of (b) a cubic CsPbh nanocrystal (3.1 nm) with an oleic acid ligand on its surface and (c) the same crystal and the same molecule without contact, far from each other (12 Á away). to minimize interactions).

Figure 13. (a) 3D composite material of acrylic PNCs stabilized with CsPbl3. PL characteristics of (b) 3D acrylic composites of 0-T@P1 and (c) 0.15-T@P1 freshly prepared and exposed to environmental conditions at different time points: day 0 (black line), 20 and 120 days of the composite materials in the absence and presence of a-TCP, respectively (magenta line), (d) Chemical contribution of atomic species of Pb (green), I (red), Cs (blue) and oxygen (black) for the molecular orbitals calculated in a 0.15-T@P1-acrylate composite material, showing spin-up (positive) and spin-down (negative) energy levels, (e) 1H NMR spectra (CDCI<3>, 400 MHz, 25 °C) of the BA/IBA, BA/IBA-to-TCP and BA/IBA-0.15-T@P systems. (g) Typical structures of BA and IBA monomers.

Figure 14. Thermograms obtained by differential scanning calorimetry (DSC), of polyacrylic samples with 7% by weight of cross-linker, 1.5% by weight of UV initiator, 1.5% by weight of PNC of CsPbl32.5% in weight of a-TCP (0.15M in the nanocrystal solution), after treatment at 50 °C for 60 min.

Figure 15. Effect of a-TCP on the PL characteristics of freshly prepared 0-T@P1 and 0.15-T@P1 acrylic composites on day 0.

Figure 16. 1H NMR spectra (CDCI3, 400 MHz, 25 °C) of combinations based on (a) BA and (b) IBA with and without the presence of a-TCP and 0.15-T@P, respectively . The insets of Figure 16a,b show the characteristic NMR signals of CH3 bound to the phenolic ring of a-TCP (2” ,3” ,4” ).

EXAMPLES

Eiemplol: Synthesis of surface-passivated CsPbl3 perovskite nanocrystals

The CsPbl3 PNCs were obtained by a synthetic hot injection protocol through stoichiometric mixing of both Cs-oleate and PbL solutions, as described in other publications, with some modifications (Hassanabadi, E.et al .Nanoscale2020,12,14194-14203). To obtain the Cs-oleate solution, 0.61 g of Cs2C03 (202126, 99.9%, Sigma-Aldrich) and 2 ml of oleic acid (OA, 364525, 90%, Sigma-Aldrich) were mixed in 30 ml of 1-ODE (0806, 90%, Sigma-Aldrich) in a 50 ml three-necked flask, under vacuum conditions for 30 min at 120°C and maintained under vacuum conditions for 30 min. The mixture was heated to 150°C under N<2>purge conditions to dissolve the Cs2C03 completely. To avoid precipitating Cs-oleate to produce CS<2>O, the resulting solution was stored at 120°C under vacuum conditions.

To synthesize pure CsPbl3, 1 g of PbL (ABCR; AB111058, 99.999%) was mixed with 50 ml of 1-ODE inside a three-neck flask of 100 ml volume and then degassed at 120 °C under vigorous stirring conditions. for 1h. Next, 5 ml of preheated OA and oleylamine (HT-OA100, 98%, Sigma-Aldrich) were added to the flask and heated again to 170 °C. Simultaneously, 4 ml of Cs-oleate preheated to this temperature was rapidly injected into the yellow Pbl2 solution, resulting in a red precipitate in the colloidal solution. The flask was immediately immersed in an ice bath for 5 s to cool the reaction. In the case of the isolation procedure, the dispersed PNCs were centrifuged at 5000 r.p.m. for 5 min with methyl acetate (MeOAc, 296996, 99.5%, Sigma Aldrich) (all PNC liquors were washed with 120 ml of MeOAc). The precipitated PNCs were recovered from the supernatant and redispersed in 5 ml of hexane (CHROMASOLV, 34859, 99.7%, Honeywell). The PNC solution with CsPbl3 was dried and redispersed in hexane to obtain a final concentration of 100 mg/mL. In order to analyze the effect of tocopherol (a-TCP, 97%, Alfa Aesar) on the photophysical properties and stability of freshly prepared and aged CsPbl3 PNCs, different concentrations of the vitamin of around 0.04, 0.05, 0.08, 0.15, 0.35 and 0.50M at 0.3g of CsPbl3 at 100mg/ml. For the aging process, PNCs were exposed to ambient conditions for 1 month, with a relative humidity of 60%.

Example 2: Synthesis of Te:Cs<2>SnCls perovskite nanocrystals with passivated surface

Te4+-doped Cs<2>SnCÍ<6> nanocrystals: In a three-necked flask, 46 mg of tin (II) acetate and 76 mg of cesium acetate were added, along with 1 ml of oleic acid and 10 ml of 1-ODE, and was degassed under vacuum conditions at 120 °C for 1 h (Mora-Seróet al. Nanoscale,2022,14, 1468-1479). Once a clear solution of the metal precursor was observed, the reaction flask was charged with nitrogen and the temperature was lowered to room temperature. In a beaker, 4.8 mg of Te02 were dissolved in 1 ml of HCl, and the metal precursor solution was poured onto this mixture. The solution was sonicated for 10 min, followed by the addition of acetone, and stirred for 1 h under ambient conditions. The solution turned black just after the addition of acetone, and gradually changed the color to light yellow over one hour with gentle heating (~50 °C). The purification procedure and recovery of the nanocrystals followed the same procedure as described above.

Example 3: Preparation of films of CsPbl<3>PNC polymeric composite materials passivated with TCP, stable and highly luminescent

Butylacrylate (BA, Sigma-Aldrich) and isobornylacrylate (IBA, Sigma-Aldrich) monomers were used as the matrix of the composite specimens, and 1,4-butanedioldiacrylate (DBA, Sigma-Aldrich) as the cross-linking agent, as well as the UV initiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, Sigma-Aldrich), without further purification. A solution of toluene and CsPbl3 PNC was dried in a dark vial, under vacuum conditions and at room temperature, to extract the solvent. The percentages of a-TCP were incorporated followed by vigorous vortexing to obtain a homogeneous mixture. In order to perform UV light treatment, the acrylic composite of PNC with CsPbl3 was prepared according to the following procedure: after dried, the PNC with CsPbl3 were passivated with a-TCP, the mixture was dispersed in IBA at room temperature with constant and gentle stirring until a transparent solution was obtained. The BA monomer with the preselected weight was then added to the mixture, with a final gentle stirring. The acrylic monomers were mixed with the necessary amount of cross-linking agent and photoinitiator (Pl). The mixtures were subjected to constant magnetic stirring for 30 min. The dispersions obtained were then placed in the refrigerator for at least 15 min before treatment.

The cooled formulations were deposited in a printing reservoir for MSLA printing (Elegoo MARS 2P). The 3D structures were obtained from the digital design that had been downloaded to the printer. Once the structures were 3D printed, the object was removed from the platform, washed with isopropyl alcohol and treated with UV light at the treatment temperature (selected within the range of 30-60 °C), with a residence time selected (40 to 90min). The modified acrylic monomeric matrices containing 0-20 mol% of cross-linking agent and 0-5.0 wt% of passivated perovskite were mixed with 0.3-3.0 wt% of the initiator. UV.

The cooled formulations were also placed on glass slides with an inkjet printer equipped with a UV treatment bridge (Süsss Microtec). DMC-Dimatrix cartridges were used, and the working conditions were established at 300 dpi, 200 mm/s, X = 365 nm and 80% intensity. The formulations were deposited in a thin layer and finally treated with UV light.

The cooled solutions were also spread on glass slides with a stick applicator (film thickness: about 50 pm), and deposited inside 1 mm × 2 mm transparent rectangular molds. They were then exposed to UV radiation from a UV-Fore Cure unit, with 405 nm light, heating system treatment control (Formlabs, USA), under static working conditions.

In a UV cabinet, a set of transparent glass molds, each with 1 ml of the cooled mixture, was placed at the treatment temperature (selected within the range of 30 to 60 °C). After the selected residence time (40 min to 90 min), the molds were removed from the cabin. The obtained UV light-treated films were separated from the glass slides and mold and used in different characterizations. Flowchart 1 shows the schematic procedure of the preparation of stable and highly luminescent CsPbl3 PNC polymer composite films. The prepared polymer samples were labeled according to the percentage of each component of the mixture, with the labeling code (BA:IBA: DBA:FI:NCP:a-TCP).

The resulting polyacrylate films containing different percentages (wt%) of n-butylacrylate and isobornylacrylate and containing passivated perovskite (TCP@CsPbl<3>) or non-passivated perovskite (CsPbl<3>) were characterized. The best results were obtained for the polymeric material with 7% by weight of cross-linker, 1.5% by weight of UV initiator and passivated perovskite (1.5% by weight of PNC with CsPbl3 and 0.15 M of a-TCP (0.15M inside the nanocrystals).

Example 4: Preparation of films of PNC polymeric composite materials of Te:Cs<2>SnCl<6>passivated with stable and highly luminescent TCP

Butylacrylate (BA, Sigma-Aldrich) and isobornylacrylate (IBA, Sigma-Aldrich) monomers were used as the matrix of the composite specimens, and 1,4-butanedioldiacrylate (DBA, Sigma-Aldrich) as the cross-linking agent, as well as the UV initiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO, Sigma-Aldrich), without further purification. In a dark vial, the Te:Cs<2>SnCl<6>PNCs were dried under vacuum conditions and at room temperature to remove the solvent and were redispersed in methanol (MeOH) and subsequently dried again under vacuum conditions. to remove the solvent. Percentages of a-TCP were incorporated followed by vigorous vortexing to obtain a homogeneous mixture. In order to carry out the UV light treatment, the acrylic composite material was prepared following the following procedure: once the Te:Cs<2>SnCl<6>PNCs were dry and passivated with a-TCP, the mixture was dispersed in BA with gentle continuous stirring. Next, a preselected weight of IBA monomer was added to the mixture, and the new solution was kept under constant magnetic stirring for 30 min at 40 °C. The acrylic monomers were mixed with the required amount of cross-linking agent and photoinitiator (Pl) with the help of a magnetic stirrer for 30 min at room temperature. Next, the dispersions obtained were stored in the refrigerator for a minimum of 15 minutes before treatment.

The acrylic monomeric matrices containing 0-20.0 mol% of the cross-linking agent and 0.3-3.0 wt% of the UV initiator were mixed with 0-10.0 wt% of dry lead-free perovskite: Te:Cs <2>SnCl<6>(yellow light-emitting perovskite). The cooled solution was UV treated using different procedures: MSLA 3D printing, inkjet printing, and film deposition procedure.

The cooled formulations were deposited in a printing reservoir for MSLA printing (Elegoo MARS 2P). The modified acrylic monomeric matrices containing 0–20 mol% cross-linking agent and 0–5.0 wt% passivated perovskite were mixed with 0.3–3.0 wt% UV initiator.

The best results were obtained with the polymeric material containing 5% by weight of cross-linker, 2.0% by weight of UV initiator and 1.0% by weight of PNC with CsPbl<3>, in the presence of a 6, 0 wt% lead-free passivated perovskite.

The cooled formulations were also placed on glass slides with an inkjet printer equipped with a UV treatment bridge (Süsss Microtec). The formulations were deposited in a thin layer and finally treated with UV light. The best results were obtained from the polymeric material containing 2.5% by weight of cross-linker, 1.5% by weight of UV initiator and 2.5% by weight of PNC with CsPbh, in the presence of TCP (TCP @PNC).

The resulting polyacrylate films, which contained different percentages (wt%) of n-butylacrylate and isobornylacrylate, in the presence of passivated perovskite (TCP@Te:Cs<2>SnCl<6>) or non-passivated perovskite (Te:Cs< 2>SnCl<6>) were also treated at the selected temperature (ranging from 30 °C to 60 °C), with a selected residence time (40 min to 90 min) and characterized. The best results were obtained from the polymeric material containing 7 wt % cross-linker, 2.0 wt % UV initiator and 4.5 wt % passivated perovskite.

Theoretical calculations

The nanoparticles and ligands were studied theoretically with CP2k, a computer program designed to perform density functional theory (DFT) calculations, based on a combination of plane waves and Gaussian functions ( Hutter, J.et al. Wiley Interdisciplinary Reviews: ComputationalMolecular Science2014,4(1), 15-25; Giansante, C.et al. The Journal of Physical Chemistry Letters2017,8(20), 5209-5215). Specifically, to generate the pseudopotentials, the Perdew-Burke-Ernerhof (PBE) approach was used (Perdew, J. P.et al. Physical Review Letters1996,77(18), 3865-3868), a set of double-valence bases. zeta augmented with polarization (DZVP) and spin polarization functions. The atomic positions in all systems were geometrically relaxed with this code, until the forces of each nucleus were reduced to less than 0.003 Ha/yr, where 0 is the Bohr radius. Furthermore, the cubic nanocrystals were separated from each other with 12Á voids to minimize reciprocal interactions. The electronically balanced chemical formula of the net nanoparticle, 31 Á in size, is (CsPbl<3>) i<25>(Csl)<75>, which produces pure bandgaps without trapping states on the surface (Giansante, C.et al. The Journal of Physical Chemistry Letters2017,8(20), 5209-5215; Voznyy, O.;

Characterization of colloidal solutions of CsPbU PNCs passivated with TCP The morphology of the PNCs was analyzed by high resolution transmission electron microscopy (HR-TEM), using a LaB6 Jeol JEM 2100plus equipment, which applied a bias of 200 kV, while observing the crystalline structure of the PNCs using selected area electron diffractometry (SAED) patterns. The average particle size of PNCs was obtained from the TEM images using ImageJ software. The X-ray diffraction (XRD) profiles of the materials were obtained using a Bruker-AXS D4 Endeavor diffractometer, with a Cu Ka radiation source (A = 1.54056 Á). Steady-state and time-resolved photoluminescence (PL) measurements were carried out on PNC composite and colloidal solutions using a photoluminescence spectrophotometer (Fluorolog 3-11, Horiba). To obtain the PL in the equilibrium state, an excitation wavelength of 420 nm was used. The concentration of the samples was established at 2 mg/ml'1 in hexane, with a 10x10 mm quartz cuvette. Time-resolved PL measurements were carried out with a pulsed laser at 405 nm (NanoLED-405L, <100 ps pulse width, 1 MHz frequency). The absolute photoluminescence quantum yield (PLQY) of PNCs was calculated using the PLQY Absolute QY Measurement System C9920-02 from Hamamatsu, equipped with an integrating sphere, at an excitation wavelength of 400 nm. Before measurements were made, the absorbance was set to a range of around 0.4-0.5, where these values are adequate to achieve the maximum PLQY in the samples. The chemical composition and electronic state of the PQD surface were determined by X-ray photoelectron spectroscopy (XPS, ESCA-2R, Scienta-Omicron). Spectra were recorded with monochromatic Al Ka = 1486.6 eV. The following sequence of spectra was recorded: analysis spectra, C 1s, Pb 4f, I 3d, O 1s and N 1s. Analysis and high-resolution spectra were recorded with a pass energy of 150 and 20eV, respectively. The reference for the binding energy scale was adventitious carbon (284.8 eV). To analyze the data, the CasaXPS processing program (Casa software Ltd) was used, and the quantitative analysis was carried out with the sensitivity factors provided by the manufacturer. The PNC colloidal solutions were analyzed with nuclear magnetic resonance (NMR) spectroscopy, with the Bruker Avance III HD spectrometer at a 1H operating frequency of 400 MHz, at 298 K. The 1H spectra were analyzed with the MestReNova program.

Characterization of TCP-passivated CsPbU composites

The glass transition temperature (Tg) of the polymer samples was evaluated by differential scanning calorimetry (DSC). DSC measurements were carried out on a Mettler Toledo DSC2 device, and the results were analyzed with the STARe version 6.01 program. The calibration of the DSC equipment was carried out with high purity indium and zinc. Six to eight milligrams of the samples were weighed into DSC aluminum crucibles with hermetically sealed and perforated aluminum lids; three replicates of all experiments were analyzed. The experiments were performed under nitrogen flow of 50 ml/min. All samples were subjected to a dynamic DSC scan from -70 to 250 °C at 10 °C/min, to determine the glass transition temperature of the treated materials. The temperature corresponding to the beginning of the transition was taken as the Tg value.

Results and exhibition

In order to address how a-TCP affects the intrinsic properties and stability of PNCs, we added different molar concentrations of this vitamin to two different types of CsPbh PNC samples: (i) freshly prepared PNCs (P1) and (ii) the same material previously aged for 1 month (P2), in which the deterioration of its PL characteristics was expected (hereinafter referred to as X-T@P1 and X-T@P2, respectively, where X is the molar concentration of a-TCP used in the preparation of the passivated PNC). Figure 1a,b shows the typical transmission electron microscopy (TEM) images of the P1 sample in the absence of a-TCP (0-T@P1) on day 0 and 0.15-T@P1 after two months, respectively. The two materials show a nanocubic morphology, with an average particle size of around 11.7 and 10.1 nm, respectively. This data is indicative that the PNC samples are in a quantum confinement regime, taking into account that the Bohr diameter of CsPbl3 is 12 nm (Kovalenko, M. V. et al. Nano Letters2015,15(6), 3692- 3696). High-resolution (HR) TEM measurements reveal information on the crystal structure of PNCs after vitamin addition, specifically, selected area diffractometry (SAED) patterns (Figure 2 ). In both cases, the characteristic interplanar distances of the cubic phase (ICSD 161481) of the perovskite structure were identified, indicating that the presence of a-TCP does not promote any phase transformation. 0-T@P1 was observed transitioning from a to 5 phase in the absence of a-TCP after 20 days of exposure to ambient conditions (see Figure 3). Therefore, the inclusion of a-TCP efficiently stabilizes the photoactive cubic phase. Furthermore, a smaller particle size observed in the 0.15-T@P1 sample suggests that a-TCP facilitated the passivation of a surface ligand, which explains better monodispersity and a smaller size distribution, compared to the nanocrystals in the primary state (Figure 4). Furthermore, the photoluminescence (PL) spectrum of 0-T@P1 (on day 0) and 0.15-T@P1 exposed to ambient conditions for 2 months is shown in Figure 1C. In this case, the intensity of PL is higher in the case of PNC in the presence of the vitamin. This characteristic indicates that the radiative recombination of the carrier is favored, so it can be inferred that some surface defects caused by the extraction of the previous ligands can interact with the a-TCP and, in this way, enhance the photophysical characteristics of the nanocrystals.

Typical XRD patterns of X-T@P1 after 2 months of exposure to ambient conditions are shown in Figure 1D. The XRD profile of 0-T@P1 was also obtained for comparison purposes. All materials showed two main maximum peaks, associated with the (100) and (200) planes. These signals correspond to the cubic phase of the iodine perovskite, which coincides with the SAED patterns obtained by HR-TEM (figure 2). When comparing the yellow inactive phase of the material (see below), which shows orthorhombic phase 5 (ICSD 27979) without the addition of the vitamin, the presence of the black a phase in the aged modified samples is indicative that the a- TCP inhibits the rapid transformation from phase a to phase 5. Furthermore, the maximum XRD peaks are shifted towards higher Bragg angles in the presence of a-TCP, attributed to a lattice contraction. At this stage, we can conclude that the delay of the crystalline phase transition is favored by the hindrance of the octahedral tilt by the perovskite structure, mediated by the coating of the ligand by the α-TCP.

The influence of a-TCP on the properties, chemical medium, and surface composition of red light-emitting CsPbl3 PNCs was investigated by X-ray photoelectron spectroscopy (XPS). To this end, XPS measurements were carried out on samples of 0-T@PNC-1 and 0.15-T@PNC-1 at different aging time points (0 and 60 days). By obtaining the analysis spectra of the materials, the inventors were able to identify the presence of C, N, O, Cs, Pb and I (figure 6). Table 1 shows the corresponding chemical composition:

Table 1. Chemical-atomic composition of the 0-T@P1 and 0.15-T@P1 samples at different aging time points (0 and 60 days).

Figure 5a shows the 3d high-resolution (HR) XPS spectrum of Cs from the 0-T@P1 and 0.15-T@P1 samples, where two characteristic doublets of -724/738 eV are represented. These signals are representative of the core Cs 3ds/2 and Cs 3d3/2 levels of the Cs+ species found within the CsPbl3 structure. Furthermore, the Cs 3d doublets were shifted to higher binding energies (BEs) upon the addition of a-TCP. We suggest that this trend is related to the compensation of the halide deficiency by oxygen from a-TCP. Furthermore, the 0-T@P1 and 0.15-T@P1 samples with different aging times show a Cs+ deficiency, arising as a result of the reactions produced during the synthesis of the PNC and which results in an alteration of the stoichiometry of the PNC surface. This deficiency is complemented by the presence of oleylammonium iodide species generated in the mixture reaction, which mediates the stabilization of the PNCs. However, the creation of this type of defects could explain why 0-T@P1 has a tendency to undergo the transformation from phase a to phase 5 in a couple of days, thus losing its optical characteristics. It is worth highlighting the fact that 0.15-T@P1 continues to present the characteristics of red light emission after 2 months, which indicates that a-TCP improves stability against environmental conditions after the passivation process with the ligand. .

The 3d HR-XPS I spectra of the materials based on 0-T@P1 and 0.15-T@P1 are shown in Figure 5b. The presence of the typical cleavage of I 3ds/2 from the spin orbit and the maximum peaks of I 3d3/2 at -618.3/630 eV are observed, which correspond to the iodine species of the octahedral composition [PbÍ6] of the NCPs. Similar to Cs 3d doublets, a shift to higher BEs was also observed in the maximum peaks of I 3d in the presence of a-TCP. Since oxygen shows a greater electronegativity than iodine, this fact allows us to deduce again that the oxygen species from a-TCP occupy the halide vacancies of the PNC, creating new O-Pb-I bonds. Furthermore, the iodine content of 0.15-T@P1 after 2 months of aging decreases slightly compared to 0-T@P1 after the same period of time. This fact allows us to deduce that a-TCP prevents the cleavage of the iodine species of the PNCs, which is essential to provide stability to the nanocrystal structure.

Figure 7c shows the HR-XPS 4f spectra for the 0-T@P1 and 0.15-T@P1 samples, both on dayO and after 2 months of aging. It is demonstrated that the characteristic levels of the core of Pb 4f7/2 and Pb 4f5/2 reached -138/143 eV, respectively, which shows a variation of the BE towards higher energy values as a consequence of the incorporation within the PNC of more electronegative species. If we look at the chemical composition shown in Table 1, 0-T@P1 on day 0 shows an initial oxygen content, in addition to the absence of uncoordinated Pb. This has been related to the use of MeOAc as an antisolvent during the washing of the PNCs, which eliminates unreacted species after the synthesis and the passivation of some defects in the perovskite halides through the COO~ anions. Therefore, all PNC samples with or without a-TCP must present the same initial oxygen density, understood as the ratios of oxygen to total iodine and oxygen to total lead.

<( O tota i/ ( l+ O to ta i)>and <O tota i/ ( P b O to ta i),> respectively (figure 7d). However, the addition of a-TCP causes the increase of these ratios with respect to their corresponding initial values, and the highest in the case of 0.15-T@P1 is reached after 2 months. At this point, even the ratio<O t0ta i / ( P b O t 0tai)>. presents a slight variation after 60 days of aging, the ratio <(O to ta i/ ( l+ O to ta i)>increases widely, similar to what would happen to a degraded 0-T@P1 during the same aging period. aging time. However, it seems that a-TCP preserves the structural integrity of the PNC, delaying the release of iodine from the surface of the material. This trend allows us to confirm that the entire chemical environment of the PNC is modified by the aging. incorporation of an oxygenated group from a-TCP, so it is confirmed that this ligand is bound to the surface of the PNC during the postsynthetic treatment. As shown below, the phenolate anion of a-TCP is the preferred species with which this vitamin binds to the surface of the material.

Considering that long-chain encapsulating ligands are released from PNCs during the aging process, a highly defective material is expected to form, leading to structural distortion and possible loss of optical characteristics. Therefore, the addition of a potential ligand that provides adequate ligand passivation to the freshly prepared PNCs and restores the intrinsic properties of the aged ones is essential to produce samples of high quality and long-term stability. With this premise in mind, the influence of a-TCP on the photophysical properties of CsPbh PNCs was studied by adding different concentrations of this vitamin to freshly prepared and aged samples (previously named X-T@P1 and X-T@P2, respectively). ). Subsequently, their corresponding photoluminescence quantum yield (PLQY) was measured under ambient conditions and humidity of around 60%. As can be seen in Figures 3a, b, the PLQY samples of X-T@P1 and X-T@P2 reach the maximum value of ~98 and 100%, respectively, after 6 days in the presence of a-TCP. These values were obtained by increasing the content of this vitamin to a maximum of 0.15M in both cases. The PLQY then decreases upon addition of an excess of this organic compound. Taking into account the fact that the 0-T@P1 sample has a high initial PLQY of around 91%, it can be deduced that a-TCP not only restricts the transformation from the a to the 5 phase in the material, but also which, in addition, maximizes radiative recombination as the main route to guide carrier relaxation. This effect has been previously observed with the use of other organic ligands, such as bidentate zwitterionic didodecyldimethylammonium bromide (DDAB) ligands, which compensate or suppress structural defects on the surface of PNCs. However, these species do not have the ability to restore the intrinsic properties of aged red light-emitting PNCs in a postsynthetic treatment, apart from the existence of a certain degree of toxicity. Therefore, we propose that vitamin a-TCP, which can be isolated from natural sources such as fruits, vegetables, seeds, etc., is not only able to promote the passivation of surface defects in freshly prepared PNCs by creation of a coating of the material surface, but also restores the lifetime of PL properties of aged samples by compensating for structural defects (with the suppression of non-radiative recombination traps). The ligand passivation effect with a-TCP is more evident by highlighting the PLQY of the X-T@P2 samples, which was improved with respect to the 0-T@P2 material, which had a low initial value of around 48%. This fact allows us to infer again that a-TCP mediates the restoration of the surface of the PNC (we propose that the vitamin compensates for the iodine deficiency), resulting in a less defective structure from its own nucleus. However, excess vitamin could also alter the stoichiometry of the perovskite and, thus, affect its own structure and photophysical properties.

The stability of the materials is expected to be prolonged by ligand passivation of CsPbl<3> PNCs. In both cases, X-T@P1 and ). The trend of the estimated PLQY of the X-T@P1 and With the restriction of the appearance of halide vacancies (suppression of the non-radiative pathway), the octahedral tilt can be inhibited, preserving the black phase, unlike PNCs in the absence of a-TCP, where the transformation of the a phase to 5 is corroborated with the same aging time period due to a practically non-existent emission characteristic. These results agree favorably with the XRD patterns obtained in the PNCs in the absence and presence of a-TCP during this period of time, where the stabilization of the black phase was established in the PNCs modified with the vitamin. Furthermore, we analyzed the mechanism of the recombinant dynamics of X-T@P1 and X-T@P2 in the presence of a-TCP before and after 60 days under ambient conditions, using time-resolved PL (TRPL) measurements. We compiled their electron half-life times,Tavg,by fitting the PL decay spectra with a biexponential equation, and =y0 A1e~x/T ^+ A2e~x/T2.Upon addition of vitamin a the colloidal solution of PNCs and promote the aging process, the longer Tavg indicates the delay of carrier recombination (Figure 10, Table 2). Subsequently, given the estimated PLQY and the Tavg values, we calculate the non-radiative recombination decay constants, k and km, respectively. As detailed in Table 2, kr is reduced by the presence of a-TCP (Figure 7c). We propose that this behavior is caused by the compensation of iodine vacancies through the oxygen bonds of the vitamin. Thus, when oxygen species help the passivation of the trapping states formed by the appearance of iodine vacancies, the O 2p states are introduced in the vicinity of the maximum valence band. This may explain the decrease in radiative recombination. It should be noted that, compared to a 0-T@P2 sample aged on day 0 of the stability measurements (higher knr), the addition of the vitamin to this type of material improves its radiative channel. The latter is corroborated by the obtaining of a km (figure 7c) and a ratio k j k r (figure 7d) lower by 0.15-T@P2 at the same aging time point, which is close to the lowest knr demonstrated in the case of 0.15-T@P1. This fact indicates that a-TCP reduces the density of non-radiative carrier trapping through oxygen passivation. This result is largely consistent with the increase in relationships

<(O to ta i/ ( l+ O to ta i)>and<O to ta i/ (P b O to ta i)>after the addition of a-TCP (calculated using XPS), allowing compensation to be confirmed of halide deficiency by the introduction of oxygen Therefore, we conclude that the passivation of the surface of PNCs with a-TCP treatment is carried out through the oxygen bond, which is capable of fixing the tan structure. defective of the freshly prepared and aged PNC and that prevents the transformation from phase a to phase 5 and restore their intrinsic characteristics.

Table 2. Determination of radiative and nonradiative recombination decay rate constants (kr and knr, respectively) by fitting the time-resolved PL decays of freshly prepared and aged CsPbh PNCs, in the absence and presence of the a-TCP of Figure 8 for a biexponential function. Expressions used in the calculations: Tavg=( I A ítí<2/ I A ítí), Tavg>= 1/(kr+knr) and kr= (PLQY/Tavg). PLQY values were used in the range 0-1.

In order to propose a surface ligand passivation mechanism mediated by α-TCP, we performed theoretical calculations using density functional theory (DFT), based on a combination of plane waves and Gaussian functions. We have investigated the ability of a-TCP to passivate the surface of PNCs with CsPbh, compared to conventional oleic acid, by calculating their BEs on the crystal surface in the presence of an iodine vacancy. In the case of the a-TCP molecule shown in Figure 12a, we have considered two binding situations: in the first case, the ligand binds to a surface Pb cation by the terminal oxygen atom of the phenolic ring (labeled "I"), which releases its hydrogen atom and occupies the position of an iodine vacancy; in the second case, the ligand is approached to the surface Pb cation by the oxygen atom coming from the epoxy site (labeled “II”). In the case of the oleic acid structure, we propose that the oxygen atoms bond to the same surface Pb cation through the carboxylate anions after the release of its hydrogen atom. Furthermore, we have calculated two systems for each of the molecules and binding situations, composed of a nanocrystal with an iodine vacancy plus a ligand: in the first, the molecule is located by the quantum dot (figure 12b); while, in the second, the ligand is far from the surface of the dot (Figure 12c).

From the total energies of the two previous systems, E<contact>and E<no_contact,>respectively, we have calculated the BEs as Eb = Eno_contact-Econtact. In the case of the oleic acid molecule (figure 11a), Eb = 2.91 eV. Next, the α-TCP ligand of bonding case “I” showed Eb =1.96eV and a covalent bond was formed (Figure 11b). However, in the case of the α-TCP ligand of the “II” binding case, Eb is negative, and the passivation process is thermodynamically unfavorable (Figure 11c). The fact that the Eb of the oleic acid molecule attached to the surface of the PNC is higher than that of a-TCP in case I is predictable given its aliphatic nature, which is associated with less steric hindrance on the surface of the PNC, compared to vitamin. However, the calculated BEs suggest, on the one hand, that the CsPbh PNCs can be coated with the two organic ligands and, on the other, that the passivation process does not involve the exchange of oleic acid for the vitamin. This is deduced because oleic acid couples more strongly to the crystal than a-TCP and, therefore, the ligand exchange would be thermodynamically unfavorable. Once the corresponding chemical contribution of the PNCs has been achieved in the absence (Figure 11d) and presence of the a-TCP bound to the perovskite in case I (Figure 11e), we observe that the O 2p states are the new energy levels of the bands. intraband forbidden energies arising in nanocrystals. These states are found within 0.73 eV of the VB maximum, demonstrating that oxygen is introduced into the halide positions.

In order to contrast the above theoretical calculations, we carried out 1H nuclear magnetic resonance (NMR) spectroscopy, in which variations in the chemical environment of PNCs were observed after the introduction of a-TCP (the detailed information of the assignment of resonances is described in the supplementary information). As shown in Figure 11f, 1H NMR spectra were obtained from the OA/OLA mixture (black spectrum), the a-TCP sample (yellow spectrum), and the PNCs in the absence (purple spectrum) and presence of a- TCP (dark red spectrum). The molecular structure of the encapsulation ligands involved in the passivation process is shown in Figure 11 g, in which numbers are assigned to the protons involved in the spin–spin coupling. While the peak at 5 = 4.16 ppm assigned to the H attached to the O of the phenolic ring of a-TCP (1” ) disappears in the 0.15-T@P1 sample, in the 1H NMR signals characteristic of OA/ OLA in the PNC in the absence of the vitamin. A broad signal is then recorded at 5 = 3.32 ppm. The absence of this signal allows us to deduce the deprotonation of the phenolic OH of a-TCP, which induces the formation of phenolate species and is essential for the passivation of the PNC surface. On the other hand, taking into account that the other 1H NMR signals of a-TCP (especially those related to 2”, 3” and 4” (CH3) 5” (CH2)), present the same multiplicity and values of chemical shift, we claim that oxygen from the epoxy ring does not mediate the ligand passivation of PNCs, which is largely consistent with theoretical calculations. From these results, we can conclude that a-TCP is bound to the surface of CsPbl3 PNCs by oxygen from the phenolic ring component and thus compensates for the iodine deficiency, which is the reason for enhance the properties of PL and the stability of the nanocrystals. As for the oxygen shown in the epoxy ring, this is the active site that promotes the reaction with acrylate monomers, which is the fundamental factor for improving the PL properties of 3D polymeric composites, as described further forward.

Once the ability of a-TCP to stabilize unstable PNCs was established, the possibility of rapidly transferring the materials to solid materials was studied, with the ability to be printed by 3D printing in phases with complex geometries and layers of thin films. . Hence, 0.15-T@P1 inks were used to prepare polymeric acrylic compounds. It should be noted that the addition of a-TCP to acrylic monomers, such as butylacrylate (BA) and isobornylacrylate (IBA) species, allows the plasticization of the final polymer matrix, which supports the development of more versatile materials from the point of view. mechanic. This was demonstrated by the increase/decrease in Tg observed in the DSC analysis (Figure 14). Printing of vitamin-modified CsPbl3 PNC inks with MSLA technology was demonstrated by generating a polymeric composite material with red light-emitting CsPbl3 (Figure 13a), with improved PL properties, compared to control experiments without a-TCP (figure 15). According to the studies carried out in the solution, the presence of a-TCP increases the PLQY of the PNCs introduced into the polymeric matrices from 36% (without vitamin) to 59%. It is worth noting that, compared to the expected decrease in the PL characteristic of the PNCs integrated into the acrylic composite material in the absence of a-TCP observed at 20 days (PLQY ~3%) (Figure 13b, Table 3), the The resulting a-TCP modified composite material is extremely stable over time (4 months) and shows an increase in its initial PL characteristic (Figure 13c, Table 3) and reaches a PLQY of ~92%. This value is the highest value obtained to date for a red light-emitting 3D printed solid.

Table3. Comparison of PL parameters over time of acrylic composites 0-T@P1 and 0.15-T@P1 in the absence and presence of a-TCP. PLQY values were obtained within the range 0-1.

The fabrication of a 3D acrylic composite material with improved PL properties can be explained by DFT calculations (Figure 13d), where the filled O 2p energy states introduced by a-TCP are observed to shift towards the vicinity of the conduction band, while empty O 2p energy levels arise, both in the presence of acrylate species. Theoretically, the functional epoxy group of a-TCP can react with electron-rich species (e.g., oxygen with a negative formal charge) with ring opening. Ca epoxide (sp3 carbon) is a possible electrophilic reagent that is influenced by the high electronegativity of the oxygen inside the ring. The product of ring decomposition is the formation of alcoholate species. This hypothesis should explain the increase in energy in the filled O 2p energy states, which can be associated with the destabilization of the a-TCP structure through the ring opening reaction or the powerful interaction with acrylate, thus The electrons delocalize to form alcoholate anions or a partial negative charge. It should be noted that in the formation of alcoholate species, a great affinity for Cs+ on the surface of the PNCs is observed (observed in computational calculations) and the transfer of electrons to this metal. Thus, we propose that electrons empty the 0 2p orbitals and empty energy levels are generated. In this context, more electrons from the Cs-O interaction can accumulate within the PNCs, which facilitates electronic transmission to empty energy levels at a lower energy. This could be related to the increase in PL intensity (one order of magnitude) of the 3D solid with a-TCP and a higher PLQY (almost double) than that of the composite without a-TCP.

To evaluate the theory of epoxy ring decomposition, we also took 1H NMR spectra of the PNCs combined with BA (Figure 16a), IBA (Figure 16b), and the mixture of these acrylic monomers (Figure 13e). Detailed signal assignment data are described in the Supporting Information. NMR studies were also carried out on the BA/IBA mixture without PNC for comparative purposes. Taking into account that the PNCs are included within an acrylate matrix, it is expected to show the characteristic NMR signals of each monomer (Figure 13f), especially those coming from the protons attached to C=C, and multiplets coming from the protons contained in the butyl and isobornyl substituents of BA and IBA, respectively. However, variations were observed in the main signals of a-TCP. In particular, one of the singlets corresponding to the methyl components attached to the phenolic ring (3” , 4” ) (5 = 2.11 ppm) became a doublet, with an upward variation up to 5 = 2.05 and 2 .03 ppm, respectively. This fact indicates that the chemical environment of the two substituents has been modified (coupling of two types of protons). In addition, the CH<2> triplet in the 5” position also shifted upward, from 5 = 2.60 to 2.53 ppm. In this context, the results suggest that the epoxy ring can be opened under the specific condition discussed here. This could explain the change of spin-spin coupling in the CH<3>substitutionals attached to the phenolic ring.

By studying the monomeric mixture without (black spectra) and with a-TCP (purple spectra), the main posterior signals mentioned as doublet CH<3>(3”, 4” ) and triplet CH<2>(5') They are also detailed in the corresponding NMR characteristics. This allows us to infer that the disintegration of the epoxy ring takes place under the coexistence of the two acrylate species. It should be noted that, in the presence of PNC (pink spectra), the multiplicity of some of these signals increases, mainly those from the doublet CH<3>(3” ,4” ). This is a strong indication that the chemical environment of a-TCP is largely altered by the influence of PNCs, which could possibly be attributed to the interaction that occurs between the nanocrystal surface (metal cations or iodine species ) and the two monomers, where this system is an exceptional situation. We propose that PNCs act as a catalyst to induce the reaction between BA, IBA and a-TCP, a fact that does not occur when the photomaterial is combined separately with BA or IBA. To test this hypothesis, a more in-depth analysis will be carried out in the future. At this point, we conclude that a-TCP provides the preparation of highly stable and compatible CsPbl<3>PNC solutions for the fabrication of polymeric 3D printing materials with good mechanical and photophysical characteristics, which opens the way to other alternatives for obtaining different active layers for solid application in optoelectronics.

Claims (21)

1. A perovskite nanocrystal with a passivated surface comprising: - a halide-based perovskite nanocrystal with the following formula selected from: ABX3 and A2BX6 doped with Bi3+ and/or Te4+, where, A is selected from: Cs, methylammonium and formamidinium, B is selected from Pb and Sn, X is a halogen selected from chlorine, bromine or iodine; and - a ligand attached to the surface of said halogen-based perovskite nanocrystal, where the ligand is a tocopherol (TCP). 2. A passivated surface perovskite nanocrystal, according to claim 1, wherein the halide-based perovskite nanocrystal has the formula ABX3. 3. A passivated surface perovskite nanocrystal, according to claim 2, wherein A is selected from Cs and formamidinium. 4. A passivated surface perovskite nanocrystal, according to claim 2o3, where B is Pb. 5. A passivated surface perovskite nanocrystal, according to any of claims 2 to 4, where X is iodine. 6. A passivated surface perovskite nanocrystal, according to any of claims 2 to 5, the halogen-based perovskite nanocrystal has the formula CsPbl3. 7. A passivated surface perovskite nanocrystal, according to claim 1, wherein the halogen-based perovskite nanocrystal has the formula A2BX6, doped with Bi3+ and/or Te4+. 8. A passivated surface perovskite nanocrystal, according to claim 7, where B is Sn. 9. A passivated surface perovskite nanocrystal, according to claim 7u8, where X is Cl. 10. A passivated surface perovskite nanocrystal, according to claim 7u8, where A is Cs. 11. A passivated surface perovskite nanocrystal, according to any of claims 7 to 10, wherein the halogen-based perovskite nanocrystal has the formula: Cs<2>SnCl<6>doped with Bi3+ or Cs<2>SnCl< 6>doped with Te4+. 12. A passivated surface perovskite nanocrystal, according to any of the preceding claims, wherein the tocopherol is vitamin E or a-tocopherol. 13. A passivated surface perovskite nanocrystal, according to any of the preceding claims, wherein the halide-based perovskite nanocrystal of formula ABX<3>has a particle size between <1 0> and <11>nm, and the halide-based perovskite nanocrystal with the formula A<2>BX<6>doped with Bi3+ and/or Te4+ has particle sizes between 50 and 70 nm. 14. The procedure for the preparation of the perovskite nanocrystals with passivated surface described in any of claims 1 to 13, comprising: the passivation of the surface of the halide-based perovskite nanocrystals, by putting them in contact with TCP in an aqueous solution, where the concentration of the halide-based perovskite nanocrystals is 100 mg/ml, and the concentration of TCP in the solution is between 0.04 and 0.5 M. 15. Procedure, according to claim 14, wherein the TCP concentration of the solution is 0.15 M. 16. Method according to claim 14 or 15, wherein the halogen-based perovskite nanocrystals used in the passivation step are freshly prepared or have been previously aged for 1 month and 60% relative humidity before the passivation step. passivation. 17. Use of passivated surface perovskite nanocrystals described in any of claims 1 to 13 for the preparation of printing inks. 18. Use according to claim 17, wherein the printing ink is an ink for 3D printing. 19. Use according to claim 17 or 18, wherein the printing ink is an acrylic printing ink. 20. Use, according to claim 19, wherein the acrylic printing ink is obtained by polymerization of monomers selected from the list, which consists of: butylacrylate (BA), isobornylacrylate (IBA), 1,4-butanedioldacrylate (DBA) or combinations of them, in the presence of surface-passivated perovskite nanocrystals. 21. Use, according to claim 20, wherein the percentage of the acrylic monomers BA and IBA of the ink is between 80 and 100 mol%, and the amount of DBA is between 0-20 mol%.