WO2021095052A1

WO2021095052A1 - White light emission from single semiconductor material based on trivalent mixed halide double perovskites

Info

Publication number: WO2021095052A1
Application number: PCT/IN2020/050951
Authority: WO
Inventors: Aravind Kumar Chandiran; Tamilselvan APPADURAI; Poonam SIKARWAR
Original assignee: Indian Institute Of Technology Madras
Priority date: 2019-11-15
Filing date: 2020-11-12
Publication date: 2021-05-20

Abstract

The present invention relates to a new single semiconductor material based on trivalent mixed halide double perovskites that exhibits white light emission. An electrophoretic deposition method can be employed to deposit thin films of trivalent mixed double halide perovskites on conducting or semiconducting substrates. The synthesized semiconductor material exhibits high carrier life time, photoluminescence life time or fluorescence life time, which provides opportunities for the manufacture of white LED devices from single emitters. White light emitting devices can be fabricated by depositing the compounds of the present invention along with transparent polymers through a dip coating method.

Description

TITLE OF THE INVENTION

WHITE LIGHT EMISSION FROM SINGLE SEMICONDUCTOR MATERIAL BASED ON TRIVALENT MIXED HALIDE DOUBLE PEROVSKITES FIELD OF THE INVENTION

The present invention relates to a semiconductor material based on trivalent mixed halide double perovskites that exhibit white light emission, a method for thin fdm deposition of trivalent mixed halide double perovskites, and a method of employing the compounds of the present invention for the fabrication of white light emitting devices.

BACKGROUND OF THE INVENTION

Light emitting devices that convert electricity into light have evinced more interest among the researchers due to their potential application in solid state lighting (SSL). Solid state lighting is an energy saving lighting technology that reduces global energy consumption by about 50%. Compared with conventional lighting, SSL is highly luminescent, compact, has longer life time and good reliability. In addition, SSL has better energy efficiency and therefore would reduce heat generation, resulting in environment cleanliness. Particularly, white LED has been regarded as one of the reliable and energy efficient replacements for conventional white lighting sources. White LEDs are generally manufactured by two methods. One is by the combination of red, green and blue light emitting LEDs and the other is by means of coating blue or ultraviolet LEDs with yellow phosphor or multi-chromatic phosphors. In either case, production of white light requires at least two different materials as there is no single material that is effective in emitting entire visible spectrum of light i.e., white light. However, there is a possibility of reduction in device efficiency due to problems such as self-ab sorption, non-radioactive carrier losses, stability and reproducibility. Therefore, it is essential to develop a single material that can emit efficient and stable white light which is suitable for lighting applications. But achieving photon or light emission covering the entire visible spectrum (400nm to 700 nm) is very challenging.

Till date, very few materials could achieve the goal of producing entire visible spectrum from a single semiconductor. Some of the white light emitters are single layered perovskites, one- or two- dimensional perovskites and zero dimensional materials, which are specifically known as halide perovskites. Also, these materials are not stable in normal atmosphere and need special handling such as glove boxes with inert atmosphere. US20020185965A1 discloses a light emitting device comprising a stack of semiconductor layers including an n-type region, an active region, and a p-type region. However, white light emission is attained by covering the light emitting device with a luminescent material structure. Bin Yang et al. reported the synthesis of dual colour emitting lead-free double perovskite nanocrystals of Cs2AgIn_xBii-_x Ck wherein x has values of 0, 0.25, 0.5, 0.75 and 0.9. However, these compounds are nanosized, difficult to scale-up, and do not emit white light.

To address the aforementioned problems, the inventors of the present invention have developed a trivalent mixed halide double perovskite semiconductor material exhibiting white light emission. OBJECT OF THE INVENTION

The object of the present invention is to synthesize a white light emitting single semiconductor material based on trivalent mixed halide double perovskites using hydrothermal method. Another obj ect of the invention is to fabricate thin films of the trivalent mixed halide double perovskites by electrophoretic deposition method.

Another object of the invention is to employ the compounds of the present invention for the fabrication of white light emitting devices through dip coating method.

SUMMARY OF THE INVENTION

In the present invention, the inventors have developed a single semiconductor material based on trivalent mixed halide double perovskites that exhibits white light emission. The inventors have synthesized a new trivalent cation mixture which is based on halide double perovskites, Cs2AgMli-_xM2_xCl6, whereinMl and M2 are trivalent cations selected from the group consisting of Bi³⁺ and In³⁺ ions, and x value is in the range of 0 to 1. The compounds of the present invention are dip coated along with transparent polymer such as polymethyl methacrylate (PMMA) polymer for the fabrication of white light emitting devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a representative photographic image of Cs2AgBi0.10In0.90Q6 sample; Figure 2 shows the XRD pattern of CsiAgBi _lOmCF. wherein x value is in the range of 0 to 1 ;

Figure 3 shows the UV visible spectrum of Cs2AgBii-_xIn_xCl6, wherein x value is in the range of 0 to 1 ;

Figure 4 shows the steady state photoluminescence of (a) Cs2AgBi0.20In0.80O6, (b) Cs2AgBio.15Ino.85O6, and (c) Cs2AgBi0.10In0.90O6 in powder form;

Figure 5 shows the emission spectrum of single crystal Cs2AgBi0.10In0.90Q6;

Figure 6 shows the time-resolved correlated photoluminescence decay of (a) Cs2AgBi0.20In0.80O6, (b) Cs2AgBio.15Ino.85O6, and (c) Cs2AgBi0.10In0.90O6;

Figure 7 shows the time-resolved correlated photoluminescence decay of single crystal Cs2AgBi0.10In0.90O6;

Figure 8 shows photographs of the powder form (a) Cs2AgBi0.20In0.80O6, (b) Cs2AgBio.15Ino.85O6, and (c) Cs2AgBi0.10In0.90Q6 before and after excitation at a wavelength of 365 nm; Figure 9 shows photographic images of (a) powder, (b) single crystal, and (c) thin film of Cs2AgBi0.10In0._90Q6 compound before and after excitation at a wavelength of 365 nm;

Figure 10 shows (a) steady state photoluminescence and (b) time-resolved correlated photoluminescence decay of Cs2AgBi0.10In0._90Q6 thin film;

Figure 11 shows the photograph of the compounds Cs2AgBi0.10In0.90Q6 with PMMA (a) before excitation and (b) after excitation at a UV light; and

Figure 12 shows the (a) spectrum of UV light, and (b) emission spectrum of the devices after illumination, under UV light.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of skill in the art to which the disclosed disclosure belongs. The term “halide double perovskites”, refers to the materials described by the general formula A2BMX₆, wherein “A” is any organic cation or inorganic cation, typically monovalent cation of larger ionic radii, and “B” comprises of a monovalent cation silver (Ag⁺) or copper (Cu⁺) of smaller ionic radii. An example of organic cation includes methyl-ammonium (CH3NH3⁺), ethyl-ammonium (CH3CH₂NH3⁺), formamidinium (NH2CH=NH2⁺), and inorganic cation includes cesium (Cs⁺) or rubidium (Rb⁺). In the present disclosure, the element “A” comprises of inorganic cations namely cesium ions, and the element “B” comprises of inorganic monovalent cations namely silver ions. Element “M” refers to the trivalent cations. Generally, for halide double perovskites, the trivalent cations are usually elements selected from the group 13, 14 and 15 of the periodic table, that include B, Al, Ga, In, Tl, As, Sb and Bi. In the present disclosure, “M” comprises of an inorganic trivalent cation, namely bismuth and indium cations or combination thereof in various proportions. Element “X” of the present invention refers to an inorganic monovalent anion selected from a halide or mix of halides, especially chloride, bromide or iodide. In the present disclosure, the monovalent anion is a chloride.

The term “lifetime” is defined as the average time that 50% of photogenerated electrons spend in the excited state prior to return to the ground state, i.e., the average time spent by the electrons in the excited state of the material.

The term “substrate” refers to an electrode, which comprises any conducting or semiconducting material. Examples of conducting materials include fluorine doped tin oxide (FTO) or indium doped tin oxide (ITO) or titanium foil or platinum foil. Examples of semiconducting materials include silicon or doped silicon wafers. The term “potential” refers to the voltage that is applied between the two electrodes. The term “electrophoretic deposition” is defined as the process of depositing the materials in the form of thin layers onto a conducting or semiconducting substrate by means of applying electric potential between the two electrodes. Herein, two similar types of electrodes or substrates based on transparent conducting glasses, are immersed in a solution containing materials that are to be deposited on the substrate. The main aspect of the present invention relates to the synthesis of single semiconductor material that exhibits white light emission. The single semiconductor material is a trivalent mixed halide double perovskite having the general formula Cs2AgMli-_xM2_xCl6, wherein Ml and M2 are trivalent cations selected from the group consisting of Bi³⁺ and In³⁺ ions. The single semiconductor material includes Cs2AgBii-_xIn_xCl6 and Cs2AgIm-_xBi_xCl6, wherein x value is in the range of 0 to 1, and emit white light only when trivalent cations of Bi and In cations are mixed in specific ratios. Cs2AgIm-_xBi_xCl6 compounds with x value in the range of 0.05 to 0.8 and 0.1 to 1, and Cs2AgBii-_xIn_xCl6 compounds with x value in the range of 0.2 to 0.95, exhibit white light emission. Cs2AgBii-_xIn_xCl6 compounds with x value is in the range of 0.4 to 0.95 and 0.8 to 0.9, and Cs2AgIm-_xBi_xCl6 compounds with x value in the range of 0.05 to 0.6 and 0.1 to 0.2, emit intense white light upon irradiation with deep ultraviolet or ultraviolet or blue light. The single semiconductor material is in the form of microcrystalline powder, single crystal, or thin film. These materials were synthesized using hydrothermal method with various combinations of the trivalent cations, and their properties were studied. The trivalent mixed halide double perovskites of formula Cs2AgIm-_xBi_xCl6, where x is 0.1 to 1, and Cs2AgBii-_xIn_xCl6, where x is 0 to 0.9, exhibit white light emission with a carrier life time in the range of 48ns to 990ns.

The single semiconductor material of the present disclosure was synthesized based on a procedure reported previously (J. Mater. Chem. A, 2017, 5,15031). Cs2AgMli- _xM2_xCl6 microcrystalline powder was synthesized using the precursors CsCl, AgCl, InCb and BiCb salts in appropriate ratios and then dissolved in hydrochloric acid. The resultant solution was carefully transferred and sealed in 30 ml Teflon lined SS autoclave and kept in oven at 180°C for 12 hours. The powder samples were washed with isopropanol and dried under reduced pressure. For Cs2AgMli-_xM2_xCl6 single crystals, similar experimental procedures were followed, and the sample was subsequently allowed to cool to room temperature for 48 hours.

According to an embodiment of the present invention, single crystals of CsiAgMl i- _xM2_xCb compounds were grown by hydrothermal method in which trivalent cations Ml and M2 are used in various ratios.

According to another embodiment of the present invention, microcry stalline powder of Cs2AgBii-_xIn_xCl6 semiconductor double perovskites were synthesized with varying concentrations of x value which is in the range of 0 to 1. The synthesized compounds include Cs2AgInCk, Cs2AgBi0.05In0.95d6, Cs2AgBi0.10In0.90Ck, Cs2AgBio. i5lno.85Ck, Cs2AgBi0.20In0.80Ck,

Cs2AgBi0.30In0.70Ck, Cs2AgBi0.40In0.60Ck, Cs2AgBi0.60In0.40Ck,

Cs2AgBi0.80In0.20Ck, and Cs2AgBiCk. Cs2AgBi0.10In0.90Ck single crystal compound was synthesized. Figure 1 represents the single crystal image of Cs2AgBi0.10In0.90Ck compound. These broad white light emitting compounds have shown to exhibit very high photoluminescence or fluorescence or excited state carrier life time, and specifically Cs2AgBi0.10In0.90Ck compound displays higher lifetime of 990 ns. Table 1 shows the life time of Cs2AgBii-_xIn_xCl6 compound, wherein x value is in the range of 0 to 1.

Table 1: The life time of Cs2AgBii-_xIn_xCl6 compounds, wherein x value is in the range of 0 to 1.

It was observed that the lifetime of the compound CsiAgBiCh, was low compared to that of the trivalent halide double perovskites containing Indium.

As shown in Figure 2, all reflection peaks of XRD were exactly matching with that of a cubic crystalline structure of space group Fm3m. Further, notable peak shifts were observed as shown in the XRD diffractograms confirming the presence of different compounds.

Figure 3 illustrates the absorption spectra of Cs2AgBii-_xIn_xCl6 with varying concentrations of x value is in the range of 0 to 1. Figure 4 shows the steady state photoluminescence spectra of Cs2AgBi0.20In0.80d6, Cs2AgBio.15Ino.85d6, and Cs2AgBi0.10In0.90d6. These spectra clearly show that emission from 400 nm to 750 nm with the maximum peak at 565 nm, which indicates the broad emission of entire visible light i.e., white light.

Figure 5 displays the steady state photoluminescence spectra of single crystal compound Cs2AgBi0.10In0.90d6, which also has emission wavelength from 400 nm to 750 nm. The lifetime of the photogenerated charge carrier of Cs2AgBi0.20In0.80d6, Cs2AgBio.15Ino.85d6, and Cs2AgBi0.10In0.90d6 compounds was confirmed based on Time-correlated single photon counting (TCSPC) measurements. Figure 6 shows the photoluminescence decay of Cs2AgBi0.20In0.80d6 (a), Cs2AgBio.15Ino.85d6 (b), and Cs2AgBi0.10In0.90d6 (c), which indicates the slow decay process with a characteristic time of countable nanoseconds. As can be seen in Figure 7, the powder sample of Cs2AgBi0.10In0.90d6 (c) exhibits long half decay life time of 990 ns, which is reasonably higher than that of single crystal compound Cs2AgBi0.10In0.90d6 (713 ns). These can be attributed to the recombination of electron-hole pair. It is observed that photoluminescence is sustained for several microseconds. Such long half decay life time is not observed in other single or double perovskites. Figure 8 shows the photograph of the powder compounds Cs2AgBi0.20In0.80d₆ (a), Cs2AgBio.15Ino.85d₆ (b), and Cs2AgBi0.10In0.₉₀d₆ (c) before and after excitation at a wavelength of 365 nm, which indicates that these materials emit white light. Figure 9 shows the photograph of Cs2AgBi0.10In0.90d6 compound before and after excitation at a wavelength of 365 nm, in powder (a), single crystal (b), and thin film (c) forms. The broad white light emission from the samples is due to their intrinsic materials property. According to another embodiment of the present invention, thin films of the white light emitting Cs2AgBii-_xIn_xCl6 compounds can be synthesized by depositing the powder form of the said compounds, on any conducting or semiconducting substrate using an electrophoretic deposition process. Cs2AgBii-_xIn_xCl6 materials were suspended or dispersed in ethanol, and two electrodes were immersed in the solution. The materials were deposited on any of the electrodes by applying a DC power supply of 50 V. Said solution has the ability to withstand the applied voltage and the deposited particle adheres to the substrate without peeling off. During the deposition process, the film thickness on the substrate depends upon the applied voltage, distance between the two electrodes, and the area of the electrodes used. Deposition of the materials onto the respective substrate occurs due to the polarization of the particles as a result of the applied voltage. Figure 10 shows the steady state photoluminescence (a) exhibiting the white light emission from wavelength 400 nm to 750 nm and (b) photoluminescence decay of about 758 ns of Cs₂AgBi0_.10In0_.90Q6 thin fdm. The single semiconductor material of the present invention exhibits entire visible light emission i.e., broad Full Width Half Maximum (FWHM), which is advantageous over existing white light LED materials. Also, this material has long carrier lifetime. Hence, the single semiconductor material can be effectively used as white light LEDs.

Therefore, according to another embodiment of the present invention, the compounds of the present invention can be employed for the fabrication of white light emitting devices, through dip coating method. In accordance with the embodiments of the present invention, the white light emitting CsiAgBii-xInxCk compounds are deposited on a transparent substrate along with a transparent polymer through a dip coating method. In accordance with the embodiments of the present invention, the compounds of the present invention, such as Cs2AgBi0.10In0.90d6 compounds presented here as an example but not limited to, are deposited in the powder form on a transparent substrate such as a glass substrate using dip coating method. Cs2AgBi0.10In0.90Q6 compounds (30 mg to 60 mg) are added to dichloromethane (DCM) solution (10 ml to 50 ml) and subjected to ultrasonic treatment for 10 mins, to form dispersed solution. PMMA polymers (0.1 gm to 0.5 gm) are added to said dispersed solution and further subjected to ultrasonic treatment for 20 mins, and thus obtained viscous solution is allowed to settle for 5 mins. Then, washed transparent substrates such as glass slides, micro slides, or any transparent substrates are dipped into the above obtained Cs2AgBi0.10In0.90O6 compound comprising solution for 2 hours, removed from the solution after 2 hours, and kept at room temperature for 30 mins. The deposition of the material onto the transparent substrates depends upon the concentration of the PMMA, and concentration of the compounds used. According to some embodiments of the present invention, any transparent polymer may be used instead of PMMA polymers.

For example, in accordance with the embodiments of the present invention, 60 mg of Cs2AgBi0.10In0.90Q6 compounds are added to 30 ml of dichloromethane (DCM) solution, and subjected to ultrasonic treatment for 10 mins, to form dispersed solution. 0.3 gm of PMMA is added to said dispersed solution and further subjected to ultrasonic treatment for 20 mins, and thus obtained viscous solution is allowed to settle for 5 mins. Then, the washed transparent substrates are dipped into the above obtained Cs2AgBi0.10In0.90d6 compound comprising solution for 2 hours, removed from the solutions after 2 hours, and kept at room temperature for 30 mins. Figure 11 shows the photograph of the compound Cs2AgBi0.10In0.₉₀d₆ with PMMA (a) before excitation and (b) after excitation, at UV light, which clearly indicates that the materials exhibit white light emissions. Figure 12 (a) shows the spectrum of UV light (365 nm), and Figure 12 (b) shows the emission spectrum (365 nm and broad emission peaks at 575 nm wavelength) of the dip coated device after illumination under UV light. The broad emission peak visible at 575 nm emission correspond to white light emissions, which is observed in the dip coated devices.

It is to be understood, however, that the present invention would not be limited by any means to the methods, and approaches that are not specifically described, and any change and modifications to the methods and approaches can be made without departing from the spirit and scope described in the present invention.

Claims

CLAIMS We Claim:

1. A semiconductor compound comprising of trivalent mixed halide double perovskites for exhibiting white light emission, wherein the trivalent mixed halide double perovskites have the general formula Cs2AgMli-_xM2xCl₆, wherein Ml and M2 are trivalent cations selected from a group consisting of Bi³⁺, and In³⁺, and value of x is in the range of 0 to 1.

2. The semiconductor compound according to claim 1, wherein the trivalent mixed halide double perovskites are in the form of powder, single crystal or thin fdm.

3. The semiconductor compound according to claim 1, wherein the trivalent mixed halide double perovskites of formula Cs2AgIm-xBixC16, where x is 0.05 to 0.6, exhibits intense white light emission.

4. The semiconductor compound according to claim 1 and claim 3, wherein the trivalent mixed halide double perovskites of formula Cs2AgIni-_xBixCl6, where x is 0.1 to 0.2, exhibits intense white light emission.

5. The semiconductor compound according to claim 1, wherein the trivalent mixed halide double perovskites of formula Cs2AgBii-_xIn_xCl6, where x is 0.4 to 0.95, exhibits intense white light emission.

6. The semiconductor compound according to claim 1, wherein the trivalent mixed halide double perovskites of formula Cs2AgBii-_xIn_xCl6, where x is 0.8 to 0.9, exhibits intense white light emission.

7. The semiconductor compound according to claim 1, wherein the trivalent mixed halide double perovskites powder or single crystal of Cs2AgIm-_xBi_xCl6 compound, where x is 0.05 to 0.8, exhibits white light emission.

8. The semiconductor compound according to claim 7, wherein the trivalent mixed halide double perovskites Cs2AgIm-_xBi_xCl6, where x is 0.1 to 1, exhibits white light emission with a carrier life time in the range of 48ns to 990ns.

9. The semiconductor compound according to claim 1, wherein the trivalent mixed halide double perovskites powder or single crystal of Cs2AgBii-_xIn_xCl6 compound, where x is 0.2 to 0.95, exhibits white light emission.

10. The semiconductor compound according to claim 9, wherein the trivalent mixed halide double perovskites Cs2AgBii-_xIn_xCl6, where x is 0 to 0.9, exhibits white light emission with a carrier life time in the range of 48 ns to 990 ns.

11. A thin fdm formed by the deposition of the white light emitting semiconductor compound of claim 1 on a conducting or semiconducting substrate by an electrophoretic deposition process.

12. Fabrication of white light emitting devices through dip coating transparent substrates with white light emitting semiconductor compounds of claim 1, wherein said compounds are deposited along with transparent polymers.

13. The fabrication of white light emitting devices according to claim 12, wherein the amount of Cs2AgMli-_xM2_xCl6is between 30 mg and 60 mg, and the amount of transparent polymer is between 0.1 gm and 0.5 gm.