Ultrabroad near-infrared emitting perovskite materials

Abstract

The present invention further relates to a perovskite material having broad NIR emissions with high full width at half maxima (FWHM) and enhanced photoluminescence (PL) intensity. The present invention relates to a perovskite material that enables its use in NIR pc- LED devices. There is provided a perovskite material of general formula (I): Cs2AxA'1- xBiyB1-yCl6 wherein A is selected from sodium (Na) or potassium (K); A' is selected from silver (Ag) or copper (Cu); and B is selected from tungsten (W) or molybdenum (Mo); and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99.

Description

ULTRABROAD NEAR-INFRARED EMITTING PEROVSKITE MATERIALSFIELD OF THE INVENTION

[0001] The present invention relates to a perovskite material that has broad near-infrared (NIR) emissions. The present invention further relates to a perovskite material having broad NIR emissions with high full width at half maxima (FWHM) and enhanced photoluminescence (PL) intensity. The present invention relates to a perovskite material that enables its use in NIR phosphor-converted light emitting diode (pc-LED) devices.BACKGROUND OF THE INVENTION

[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Ultrabroad radiation sources, such as incandescent lamps (e.g., tungsten and tungstenhalogen lamps), emit across the visible and NIR regions but are inefficient in terms of energy usage. These inefficient lamps have been replaced by more efficient sources like pc-LEDs, which emit broad-spectrum white light for indoor and outdoor lighting. However, designing broadband NIR-emitting phosphors remains a challenge compared to their white-light counterparts, despite the growing demand for NIR sources in applications such as food and fabric analysis, night vision, security surveillance, and gesture recognition.

[0004] Efforts to achieve broad NIR emissions through lanthanide doping or quantum dots like PbS have resulted in narrow emission bands, while combining multiple narrow emissions often leads to incomplete spectral coverage. The challenge lies in achieving a high FWHM with continuous spectral coverage. Previous studies, such as Cr3doping in LasGai.'wGcOu and Cr37Ni2co-doping in LaZnGanOip, have yielded FWHMs of 330 nm and 137-239 nm, respectively, yet failed to provide the desired ultrabroad NIR emission.

[0005] Thus, there is a need in the art to develop an efficient, ultrabroad NIR emitting phosphor material that overcomes the limitations of narrow spectral emissions and discontinuous coverage seen in contemporary technologies. Specifically, the solution must provide continuous, broad NIR emissions with high FWHM and enhanced PL intensity, enabling reliable and energy-efficient performance for applications such as food and fabric analysis, night vision, security surveillance, and gesture recognition. Moreover, the materialshould be compatible with scalable manufacturing techniques, such as 3D printing, to enable practical integration into NIR pc-LED devices.OBJECTS OF THE INVENTION

[0006] The principal object of the present invention is to overcome the disadvantages of the prior art.

[0007] An object of the present invention to provide a perovskite material exhibiting ultrabroad NIR emission with high quantum yield.

[0008] Another object of the present invention is to provide a perovskite material with a tunable emission spectrum in the NIR region.

[0009] A further object of the present invention is to provide a perovskite material that is thermally stable and resistant to degradation.

[0010] Yet another object of the present invention is to provide a perovskite material that can be easily integrated into various optoelectronic devices or light emitting devices.

[0011] An additional object of the present invention is to provide a method for synthesizing the perovskite material that is efficient and scalable.SUMMARY OF THE INVENTION

[0012] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0013] The present invention relates to a perovskite material that has an efficient, ultrabroad NIR emitting capability. In particular, present invention relates to a perovskite material that has broad NIR emissions with high FWHM and enhanced PL intensity. The inventors of the present invention have developed a perovskite material that emit ultrabroad NIR radiation with spectral widths of 434 nm (898 meV) and 468 nm (983 meV). The inventors have also developed methods for producing such perovskites.

[0014] The present inventors have surprisingly found that the introduction of Mo4+and W4+as doping agents substitute the Bi3+site in the lattice. Replacing Bi3+with heterovalent Mo4+or W4+requires charge compensation. This may involve the creation of vacancies or the introduction of additional ions, leading to local structural changes.

[0015] According to an aspect there is provided a perovskite material of general formula (I): CszAxA’ xBiyB yClg . (I)wherein A is selected from sodium (Na) or potassium (K);A’ is selected from silver (Ag) or copper (Cu); andB is selected from tungsten (W) or molybdenum (Mo); and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99.

[0016] According to another aspect there is provided a method of producing a perovskite material of general formula (I):Cs2AxA’i.xBiyBi.yCl6. (I) wherein A is selected from sodium (Na) or potassium (K);A’ is selected from silver (Ag) or copper (Cu); andB is selected from tungsten (W) or molybdenum (Mo); and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99.

[0017] The step of producing a perovskite material of general formula (I) comprises a) forming a precursor solution comprising precursors of the perovskite material dissolved in a solvent; b) heating said precursor solution for about 10 to 30 hours at a temperature ranging between 120 to 200°C; and c) removing the solvent to produce solid crystals of the perovskite material.

[0018] Other aspects of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learnt by the practice of the invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The following drawings form part of the present specification and are included to further illustrate aspects of the present disclosure. The disclosure may be better understood by reference to the drawings in combination with the detailed description of the specific embodiments presented herein.

[0020] Figure 1 shows (a) undoped, (b) 2.5% W4+-doped, and (c) 2.8% Mo4+-doped crystals of the perovskite material under room light in accordance to the present invention.

[0021] Figure 2a-d illustrates the structure of the perovskite material in accordance to the present invention.

[0022] Figure 3a illustrates the thermal stability of the perovskite material by showing XRD (X-ray diffraction) patterns at different temperatures in accordance to the present invention.

[0023] Figure 3b illustrates the stability of the perovskite material in ambient conditions by comparing XRD patterns in accordance to the present invention.

[0024] Figure 4 illustrates the PL properties and Reflectance Spectra properties of the perovskite material in accordance to the present invention.

[0025] Figure 5 illustrates the PL quantum yield (QY) of the perovskite material in accordance to the present invention.

[0026] Figure 6 illustrates the Temperature-Dependent Structure and PL of the perovskite material in accordance to the present invention.

[0027] Figure 7 illustrates the applications in Ultrabroad NIR pc -LEDs and Films of the perovskite material in accordance to the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0028] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[0029] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0030] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0031] In some embodiments, numbers have been used for quantifying area, weights, percentages, and so forth, to describe and claim certain embodiments of the invention and are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinaryrounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0032] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0033] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0034] Unless the context requires otherwise, throughout the specification which follows, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0035] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.

[0036] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0037] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified.

[0038] The description that follows, and the embodiments described therein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.

[0039] It should also be appreciated that the present disclosure can be implemented in numerous ways. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes or systems. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.

[0040] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0041] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0042] The present invention relates to a perovskite material that has an efficient, ultrabroad NIR emitting capability. In particular, present invention relates to a perovskite material that has broad NIR emissions with high FWHM and enhanced photoluminescence intensity. The inventors of the present invention have developed a perovskite material that emit ultrabroad NIR radiation with spectral widths of 434 nm (898 meV) and 468 nm (983 meV). The inventors have also developed methods for producing such perovskites.

[0043] The present inventors have surprisingly found that the introduction of Mo4+and W4+as doping agents substitute the Bi3+site in the lattice. Replacing Bi3+with heterovalent Mo4+or W4+requires charge compensation. This may involve the creation of vacancies or the introduction of additional ions, leading to local structural changes.

[0044] According to an aspect there is provided a perovskite material of general formula (I): CszAxA’ xBiyB yClg . (I) wherein A is selected from sodium (Na) or potassium (K);A’ is selected from silver (Ag) or copper (Cu); andB is selected from tungsten (W) or molybdenum (Mo); and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99.

[0045] According to another aspect there is provided a method of producing a perovskite material of general formula (I):CszAxA’ xBiyB yClg . (I) wherein A is selected from sodium (Na) or potassium (K);A’ is selected from silver (Ag) or copper (Cu); andB is selected from tungsten (W) or molybdenum (Mo); and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99.

[0046] The step of producing a perovskite material of general formula (I) comprises a) forming a precursor solution comprising precursors of the perovskite material dissolved in a solvent; b) heating said precursor solution for about 10 to 30 hours at a temperature ranging between 120 to 200°C; and c) removing the solvent to produce solid crystals of the perovskite material.

[0047] In an embodiment, the precursor solution comprises chloride salts of one or more of cesium (Cs), sodium (Na), potassium (K), silver (Ag), copper (Cu), bismuth (Bi), tungsten (W), molybdenum (Mo), or any combination thereof.

[0048] In an embodiment, the precursor solution comprises one or more of CsCl, NaCl, KC1, AgCl, CuCh. Bids, WCI4, M0CI5 or any combination thereof.

[0049] In an embodiment, the solvent may be selected from hydrochloric acid, hydrofluoric acid, hydroiodic acid, acetic acid, formic acid or any combination thereof.

[0050] In an embodiment, the precursor solution comprises CsCl : NaCl : AgCl : BiCh in the ratio of 2 : 0.95 : 0.05 : 1.

[0051] In an embodiment, the precursor solution comprises WCI4 or M0CI5 individually in the concentration range of 1 to 15 mol %, preferably WCI4 is present in the concentration range of 0.1 to 5.3 mol%; and preferably M0CI5 is present in the concentration range of 2.2 to 6.5 mol %.

[0052] In an embodiment, the solvent is preferably hydrochloric acid or HC1.

[0053] In an embodiment, the precursor solution is heated in an autoclave.

[0054] In an embodiment, the solid crystals of the perovskite material are separated from the solution by decantation and washed in iso-propanol.

[0055] In an embodiment, the solid crystals of the perovskite material are vacuum dried.

[0056] In an embodiment, the solid crystals of the perovskite material are dried for approximately 2-5 hours. In an embodiment, the solid crystals of the perovskite material are dried inside a desiccator.

[0057] In an embodiment, the present invention provides a composite material comprising the perovskite material as defined in the present invention and different polymers including a biodegradable polymer.

[0058] In an embodiment, the present invention provides a light-emitting device comprising the perovskite material as defined in the present invention. In an embodiment, the lightemitting device is a pc-LED.

[0059] In a preferred embodiment, the present invention provides a perovskite material comprising Cs2Nao.95Ago.o5BiC16 doped with W4+or Mo4+wherein when Bi3+is replaced with a heterovalent Mo4+or W4+it may require charge compensation.

[0060] In an embodiment, the introduction of dopant ions may lead to new PL emission bands. These new emissions may arise from transitions involving the dopant energy levels or from energy transfer processes between the host and dopant ions.

[0061] In an embodiment, the PL quantum yield may be affected by the doping.

[0062] In an embodiment, the activation energy for thermal quenching of PL can be influenced by doping. The introduction of dopant ions can modify the potential energy landscape and affect the stability of the excited states, leading to changes in the thermal quenching behavior.

[0063] In an embodiment, the present invention provides a method of fabricating a lightemitting device, method comprising the steps of preparing a slurry of the perovskite material as described in the present invention and a biodegradable polymer; applying the slurry to a UV LED chip; and curing the slurry to form the light-emitting device.

[0064] In an embodiment, the present invention provides a system for NIR imaging comprising the light-emitting device of the present invention and a NIR camera.EXAMPLES

[0065] Materials: Cesium chloride (CsCl, 99.9%, Sigma Aldrich), bismuth chloride (BiC13, 99.9%, Sigma Aldrich), sodium chloride (NaCl, 99.5%, Sigma Aldrich), silver chloride (AgCl, 99.99%, Sigma Aldrich); tungsten chloride (WC14, 95%, Sigma Aldrich), molybdenum chloride (MoC15, 99.6%, metal basis, Thermoscientific) and hydrochloride acid (HC1, 37 wt. %, 99.99%, Sigma Aldrich), barium sulphate (BaSO4, 98%), iso-propanol (Rankem), chloroform (CC14, Thermoscientific), Pet ether (Leonid chemicals pvt. Ltd.). Polylactic acid (Ingeo biopolymer 3D850, Nature works, Minnesota US) having melt flow rate (MRF) of 0.7-0.9 g / min was purchased from the Refurbin Laboratories, India. Chemicals are used directly without further purification.

[0066] The perovskites samples of Cs2Nao.95Ago.o5BiC16, both undoped and doped with W4+and / or Mo4+, are prepared using a hydrothermal synthesis method. High-purity reagents from Sigma Aldrich, including CsCl, BiCT. NaCl, AgCl, WCL. HC1, and MoCL. are used. To synthesize W4+-doped samples with varying dopant concentrations, different amounts of WCL are added to a reaction mixture containing CsCl, NaCl, AgCl, and BiCE in HC1. The mixture was then heated in an autoclave at 180°C for 12 hours. The resulting crystals are washed and dried. A similar procedure was followed for Mo4+-doped samples, using MoCL instead of WCL. The actual dopant percentages in the final products are determined and are listed in Tables 1 & 2. Figure 1 shows (a) undoped, (b) 2.5% W4+-doped, and (c) 2.8% Mo4+-doped Cs2Nao.95Ago.o5BiC16 crystals under room light.

[0067] Table 1 : Undoped and W4+-doped Cs2Nao .gsAgo.osBiCk Comparison of concentration of Na, Ag and dopant, W used in precursor (feeding molar concentration) with that of in the product obtained from ICP-AES measurements:

[0068] Table 2: Mo4+-doped Cs2Nao .gsAgo.osBiCk Comparison of concentration of Na, Ag and dopant, Mo used in precursor (feeding molar concentration) with that of in the product obtained from ICP-AES measurements:

[0069] Inventors noted that Cs2Nao.95Ago.o5BiC16 double perovskites, exhibit self-trapped exciton (STE) NIR emission at 700 nm with a FWHM of 270 nm. By doping these perovskites with Mo4+or W4+, their low d-d transition energy and spin-orbit coupling relax spin-selection rules and enhance photoluminescence (PL) intensity. Furthermore, the distortion of the [ MCle (M: Mo, W) octahedra due to Jahn-Teller effects and heterovalent doping increases PL intensity. Structural investigations confirm that Mo4+and W4+substitute Bi3+in the lattice, introducing broad NIR-II emissions and overlapping with NIR-I emissions of the host to achieve ultrabroad NIR emission with FWHMs of 434 and 468 nm, and PL quantum yields of 42% and 31%, respectively.

[0070] The data from Rietveld refinement of powder X-ray diffraction (XRD), of Cs2Nao.95Ago.o5BiC16, using the reference pattern reported by Zhang et al. (CCDC No. 2177254), which is shown in Figure 2a confirms that the Cs2Nao.95Ago.o5BiC16 crystalizes in Fm3m (cubic, a = b = c = 10.839 A). Figure 2b shows powder XRD patterns of undoped, 2.5% W4+-doped, and 2.8% Mo4+-doped Cs2Nao.95Ago.o5BiC16. Figure 2c shows magnified view of the XRD patterns in the 20 range of 22-24 degrees. Figure 2d is a schematic illustration of the incorporation of dopants (W4+and Mo4+) into the Cs2Nao.95Ago.o5BiC16 double perovskite host. Powder XRD patterns in Figure 2b do not show any impurity peak after doping. Interestingly, a magnified view of the patterns in Figure 2c shows a small shift in XRD peak towards larger 20 values with both Mo4+and W4+doping. This shift to higher 20 values suggests the incorporation of the dopant ions in the host lattice (Figure 2d), since the ionic radii of both W4+(66 pm) and Mo4+(65 pm) are smaller than the Bi3+(103 pm) in octahedral coordination.

[0071] Field emission scanning electron microscopy (FESEM) reveals a few 100 micron sized crystals. Cs2Nao.95Ag0.o5BiC16 is thermally stable till 400 °C (673 K) as shown by XRD patterns in Figure 3a. Peaks indicated with down headed arrow are due to Pt-substrate and corresponds to (111), (200) and (220) diffraction from cubic phase of Pt. Figure 3b shows comparison of powder XRD data of freshly prepared and 403 days aged Cs2Nao.95Ago.o5BiC16. The sample was stored in ambient environment and stable for more than a year.

[0072] Figures 4a-4e illustrate photoluminescence (PL) properties and reflectance spectra. Figure 4a compares the PL emission spectra of undoped and doped Cs2Nao.95Ago.o5BiC16 in the visible and near-infrared (NIR) regions (450 - 1500 nm). The undoped sample exhibits abroad NIR emission with a peak at 680 nm and an average lifetime of 1.8 ps, attributed to defect-mediated self-trapped exciton (STE) emission (probably originating from Bi3s^p1to 6s2transitions). Upon doping with W4+or Mo4+, an additional emission peak appears at 950 nm or 940 nm, respectively. The overall PL quantum yield (QY) for the doped samples is higher than the undoped sample. The PL QY (see Figure 5 for calculation) of 42% and 31% are obtained for the combined emission (450 - 1500 nm) of 2.5% W4+- and 2.8% Mo4+-doped samples, respectively. Figure 5 shows that the total PLQY calculation method for 2.5% W4+- doped and 2.8% Mo4+-doped Cs2Nao.95Ago.o5BiC16: For PLQY the PL has been divided at 720 nm as Area-1 and Area-2. PLQY of Area-1 is absolute, 10.1%. PLQY of the Area-2 is (relative to Area-1), 31.7% as shown in Figure 5. It results total of 42% PLQY for 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16. Similarly, for 2.8% Mo4+-doped Cs2Nao.95Ago.o5BiC16 PLQY is 31%. Figure 4b shows the PL decay curves for the doped samples. The longer lifetimes of 27 and 32 ps observed for the W4+and Mo4+emissions, respectively, are indicative of partially forbidden d-d transitions. Additionally, the faster PL decay of the host emission in the doped samples suggests energy transfer from the host to the dopants. Figure 4c compares the spectral broadening of the present invention with previously reported compositions. The emission spectra consist of host and dopant emissions. The FWHM of the combined emission is significantly larger for the doped composition of present invention compared to previous reported compositions / products. For 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16, the FWHM of host and dopant emissions are 265 and 169 nm, respectively, resulting to FWHM of combined emission as 434 nm. Likewise, the FWHM of the overall emission of 2.8% Mo4+- doped Cs2Nao.95Ago.o5BiC16 is 468 nm. Clearly, a significant jump in FWHM, an increase by >130 nm, is observed for our sample compared to the highest NIR FWHM reported earlier. Figure 4d shows the UV-visible-NIR reflectance spectra of WCL and 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16. Three d-d transitions are observed from the dopants. Figure 4e presents the PL excitation (PLE) spectra of 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16. The PLE spectra for the dopant emission show the same 3 transitions as the reflectance spectrum. The observed transitions in the PLE spectra can be attributed to specific electronic transitions within the [WCL]2-octahedron using the established Tanabe-Sugano diagram for d electronic configuration. The large spin-orbit coupling of W4+and Mo4+relaxes the selection rules for these transitions, making them partially allowed. The de-excitation process ^^Ti ->3TI, leads to the PL emission around 950 nm. The 2.8% Mo4+-doped Cs2Nao.95Ag0.o5BiC16 also shows similar d — d transitions in its reflectance and PLE spectra.

[0073] Figures 6a-6f illustrate the temperature-dependent structure and PL. Figure 6a shows a pseudo-color map representing the temperature-dependent powder XRD patterns of Cs2Nao.95Ago.o5BiC16 whose spectral features are shown at 300 and 15 K in the Figure 6b. The (400) peak shifts to higher 20 values at lower temperatures due to unit cell compression. Below 100 K, the peak splits into two peaks, indicating a phase transition from cubic to tetragonal. Figure 6c shows the temperature-dependent PL spectra of 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16. The STE emission remains broad even at low temperatures, indicating a localized origin. The variation of FWHM with temperature was fitted to determine the Huang-Rhys parameter, which is found to be high (S = 57), suggesting strong electronphonon coupling. The undoped sample also shows high S = 52 indicating strong electronphonon coupling responsible for the broad STE emission. Figure 6d shows the temperaturedependent PL spectra of the [1E / 1Ti (JD) ->3TI (3F)] dopant emission in 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16. At low temperatures, the emission splits into multiple vibrationally coupled peaks. Above 175 K, a broad and asymmetric peak is observed with peak at 950 nm. But at lower temperatures till 7 K, the emission splits into multiple vibrationally coupled peaks, with separation around 853-1088 cm'1. Figure 6e shows the PL excitation [3TI (3F) -> ^^Ti (3D)] spectrum for the dopant emission, which also exhibits vibronically coupled states. Similar behavior is observed in 2.8% Mo4+-doped Cs2Nao.95Ago.o5BiC16. Such vibronically coupled d — d transitions for Mo4+and W4+has been also reported earlier from different molecular complexes and crystals at cryogenic temperature. NIR PL in such systems often quenches at room temperature via radially available C-N, C-H O-H overtone vibrations. However, the broad d — d transition intense PL observed at room temperature in the present invention is because of overlap of different vibronically coupled emissions. Figure 6f summarizes the excitation and emission mechanisms. The 350 nm excitation wavelength can excite both the host band gap and the dopant. This leads to both broad STE emission in the NIR-I region and dopant emission in the NIR-II region. The strong electron-phonon coupling broadens both emissions, and the strong spin-orbit coupling and distortion of the | MCL> |2octahedra make the dopant emission partially allowed.

[0074] Figures 7a- 7j illustrate the applications in ultrabroad NIR pc-LEDs. Figure 7a shows the luminescence spectra of a pc -LED [under different operating current (0-800 mA)] fabricated by drop-casting a slurry of 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16 prepared in chloroform and polylactic acid (PLA) onto a UV LED chip emitting at 367 nm. The device exhibits ultrabroad NIR emission under various operating currents. Ultrabroad NIR emissionis observed along with the residual UV LED emission. Figure 7b presents the device output power as a function of operating current. The maximum output power is 6.2 mW at 475 mA. Figure 7c shows a digital image of UV-LED panel that used to convert to pc-LED panel of dimension: 170 X 170 mm2using 2.5 % W4+-doped Cs2Nao.95Ag0.o5BiC16 with PLA. Using this panel as NIR radiation source we demonstrate NIR imaging in Figure 7d-7g. The panel can penetrate non-transparent materials and enables to image a toy using NIR camera as shown in Figure 7g. Such NIR imaging is desired for security camera.

[0075] The rational doping of W4+and Mo4+into Cs2Nao.95Ago.o5BiC16 double perovskites resulted in ultrabroad NIR phosphors with unprecedented spectral widths of 434 and 468 nm, respectively, for 2.5% W4+-doped and 2.8% Mo4+-doped samples. These phosphors also exhibited high PL QYs of 42% and 31%, respectively.

[0076] A composite of 2.5% W4+-doped Cs2Nao.95Ago.o5BiC16 phosphor with different polymers including biodegradable polymer PLA formed NIR-emitting composites, pc -LEDs fabricated from this composite exhibited broad NIR emission with an output power of 6.2 mW at 475 mA current.

[0077] While the foregoing describes various embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

Claims

We Claim:

1. A perovskite material having general formula (I):Cs2AxA’i.xBiyBi.yCl6. (I) wherein A is selected from sodium (Na) or potassium (K); A’ is selected from silver (Ag) or copper (Cu); and B is selected from tungsten (W) or molybdenum (Mo); and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99.

2. The perovskite material, as claimed in claim 1, wherein the perovskite material comprises Cs2Nao.95Ago.o5BiC16 doped with W4+or Mo4+.

3. The perovskite material, as claimed in claim 2, wherein doping results in heterovalent substitution of Bi3+with Mo4+or W4+requiring charge compensation.

4. The perovskite material, as claimed in claim 2, wherein the dopant is WCL or MoCL present in a concentration range of 1 to 15 mol%.

5. A method of producing a perovskite material having general formula (I):CszAxA’ xBiyB yClg . (I) wherein A is selected from sodium (Na) or potassium (K), A’ is selected from silver (Ag) or copper (Cu), and B is selected from tungsten (W) or molybdenum (Mo), and wherein X may range from 0.93 to 0.97 and Y may range from 0.93 to 0.99; said method comprising the steps of a) forming a precursor solution comprising precursors of the perovskite material dissolved in a solvent; b) heating said precursor solution for about 10 to 30 hours at a temperature ranging between 120 to 200°C; and c) removing the solvent to produce solid crystals of the perovskite material.

6. The method as claimed in claim 5, wherein the precursor solution comprises chloride salts of one or more of cesium (Cs), sodium (Na), potassium (K), silver (Ag), copper (Cu), bismuth (Bi), tungsten (W), molybdenum (Mo), or any combination thereof.

7. The method as claimed in claim 5, wherein the precursor solution comprises one or more of CsCl, NaCl, KC1, AgCl, CuCb. BiCh, WCI4, M0CI5 or any combination thereof.

8. The method as claimed in claim 5, wherein the solvent may be selected from hydrochloric acid, hydrofluoric acid, hydroiodic acid, acetic acid, formic acid or any combination thereof.

9. The method as claimed in claim 5, wherein the precursor solution comprises CsCl: NaCl: AgCl: BiCh in the ratio of 2: 0.95: 0.05: 1.

10. The method as claimed in claim 5, wherein the solid crystals of the perovskite material are separated from the solution by decantation and washed in iso-propanol.

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