Pure organic perovskite material and preparation method thereof

Abstract

The present application relates to the field of perovskite materials, and in particular to a pure organic perovskite material and its preparation method. The chemical structure of the pure organic perovskite material is ABX3; wherein A is a conjugated divalent organic cation containing a double bond and / or an aromatic ring, and B is NH4 + , X is a monovalent halogen anion and / or a quasi-halogen anion. The pure organic perovskite material of the present application replaces the previous organic cation with a divalent organic cation with a higher conjugation degree of A, and uses NH4 + By replacing the metal ions in traditional metal-based perovskite materials, the band gap of the pure organic perovskite material of the present application is greatly reduced, and the light absorption can be extended to the visible light region, thereby forming a pure organic perovskite material with a low band gap and light response in the visible light region.

Description

Technical Field

[0001] The present application belongs to the technical field of perovskite materials, and in particular relates to a pure organic perovskite material and a preparation method thereof. Background Art

[0002] Perovskites are a class of materials with a crystal structure similar to calcium titanate (CaTiO3). They were discovered by Gustav Rose in 1839 and later named by Russian mineralogist L.A. Perovski. The structural formula of perovskites is generally ABX3, where A is a divalent cation, B is a monovalent cation, and X is a monovalent anion.

[0003] As early as 2002, the Journal of the American Chemical Society first reported the synthesis and crystal structure characteristics of metal-free, purely organic perovskite materials. However, it was not until 2018 that this new type of purely organic perovskite material began to find specific applications, with its use as a ferroelectric material. Since then, its application has gradually expanded to dielectrics, X-ray imaging, and thermoelectrics. Although the application areas of this type of material continue to expand, the band gap of this type of material remains large, and therefore it often lacks photoresponse in the near-ultraviolet, visible light, and infrared ranges, making its application in new energy applications, especially solar cells, difficult to achieve. Summary of the Invention

[0004] The purpose of this application is to provide a pure organic perovskite material and a preparation method thereof, aiming to solve the technical problem of how to make the pure organic perovskite material have visible light response.

[0005] To achieve the above application objectives, the technical solutions adopted in this application are as follows:

[0006] In a first aspect, the present application provides a pure organic perovskite material, the general chemical structure of which is ABX3; wherein,

[0007] A is a conjugated divalent organic cation containing a double bond and / or an aromatic ring,

[0008] B is NH4 + ,

[0009] X is a monovalent halogen ion and / or a halogen-like ion.

[0010] The present application provides a new pure organic perovskite material with a general chemical structure of ABX3. Based on the molecular design of existing perovskite materials, a conjugated divalent organic cation containing a double bond and / or an aromatic ring is introduced into A, and the previous organic cation is replaced by the divalent organic cation with a higher degree of conjugation, and NH4 is used. +By replacing the metal ions in traditional metal-based (such as lead, tin-based) perovskite materials, the band gap of the pure organic perovskite material of the present application is greatly reduced, and the light absorption can be extended to the visible light region, thereby forming a pure organic perovskite material with a low band gap and light response in the visible light region; therefore, such a new pure organic perovskite can have good applications in the optoelectronic field.

[0011] In a second aspect, the present application provides a method for preparing the above-mentioned pure organic perovskite material, comprising the following steps:

[0012] Unprotonated A and BX are added to an acid solution and heated to react to obtain a pure organic perovskite material as shown in ABX3.

[0013] This application provides a method for preparing a pure organic perovskite material. The method involves adding raw materials, namely, unprotonated A and BX, to an acid solution for reaction, thereby generating a pure organic perovskite material represented by ABX3. Because this method can introduce A, namely, a conjugated divalent organic cation containing double bonds and / or aromatic rings, from the raw materials, a pure organic perovskite material with a low band gap and photosensitivity in the visible light region can be produced. This preparation method is simple and amenable to industrial production, opening up the possibility of optoelectronic applications for this novel type of pure organic perovskite. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0015] Figure 1 is the X-ray diffraction spectrum of the pure organic perovskite material obtained in Example 1 of the present application;

[0016] Figure 2 This is the UV-visible absorption spectrum of the pure organic perovskite material obtained in Example 2 of the present application;

[0017] Figure 3 is the X-ray diffraction spectrum of the pure organic perovskite material obtained in Example 2 of the present application;

[0018] Figure 4 is the ultraviolet-visible absorption spectrum of the pure organic perovskite material obtained in Example 3 of the present application;

[0019] Figure 5 is the X-ray diffraction spectrum of the pure organic perovskite material obtained in Example 3 of the present application;

[0020] Figure 6This is the ultraviolet-visible absorption spectrum of the pure organic perovskite material obtained in Comparative Example 1 of the present application;

[0021] Figure 7 This is the X-ray diffraction spectrum of the pure organic perovskite material obtained in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0022] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0023] In this application, the term "and / or" describes the relationship between associated objects, indicating that three possible relationships exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0024] In this application, "at least one" means one or more, "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items.

[0025] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0026] The terms used in the embodiments of the present application are for the purpose of describing specific embodiments only and are not intended to limit the present application. The singular forms "a", "an", "the" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise.

[0027] In a first aspect, the present invention provides a pure organic perovskite material, the general chemical structure of which is ABX3; wherein:

[0028] A is a conjugated divalent organic cation containing a double bond and / or an aromatic ring,

[0029] B is NH4 + ,

[0030] X is a halogen ion and / or a halogen-like ion.

[0031] The pure organic perovskite material provided in the embodiment of the present application has a general chemical structure of ABX3. Based on the molecular design of the existing perovskite material, a divalent organic cation with a conjugated structure containing a double bond and / or an aromatic ring is introduced into A, and the organic cation is replaced by the divalent organic cation with a higher degree of conjugation, and NH4 is used. + By replacing the metal ions in traditional metal-based (such as lead, tin-based) perovskite materials, the band gap of the pure organic perovskite material of the present application is greatly reduced, and the light absorption can be extended to the visible light region, thereby forming a metal-free pure organic perovskite material with a low band gap and light response in the visible light region; therefore, such a new pure organic perovskite can have good applications in the optoelectronic field.

[0032] In the pure organic perovskite material shown in ABX3, B has been determined to be NH4 + , and the selectable range of A and X needs to meet the tolerance factor α required for the perovskite crystal form.

[0033] That is, the tolerance factor:

[0034]

[0035] Among them, r A 、r B 、r X are the corresponding ionic radii.

[0036] In some embodiments, the selection of A and X results in a tolerance factor of the pure organic perovskite material of the present embodiment being 0.8 to 1.05. Within this tolerance factor range, the pure organic perovskite material provided by the present embodiment has better stability.

[0037] In some embodiments, A is a divalent organic cation having a conjugated structure containing a double bond, and the divalent organic cation having a conjugated structure containing a double bond is selected from at least one of the following Formulas 1 to 5.

[0038] Alternatively, in some embodiments, A is a divalent organic cation having a conjugated structure containing an aromatic ring, and the divalent organic cation having a conjugated structure containing an aromatic ring is selected from at least one of the following Formulas 6 to 13.

[0039]

[0040] in,

[0041] In the divalent organic cation represented by Formula 1, R1, R2, R3 and R4 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation represented by Formula 2, R5 and R6 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation represented by Formula 3, R7, R8 and R9 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation represented by Formula 4, R 10 、R 11 、R 12 and R 13 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation shown in Formula 5, R 14 、R 15 、R 16 and R 17 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation shown in Formula 6, R 18 and R 19 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation shown in Formula 7, R 20 and R 21 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation shown in Formula 8, R 22 、R 23 、R 24 and R 25 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation shown in Formula 9, R 26 、R 27 、R 28 and R 29 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation represented by formula 11, R 30 and R 31 are independently selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation represented by formula 12, R 32 is selected from H or an alkyl group containing 1-10 carbon atoms; in the divalent organic cation represented by formula 13, R 33 and R 34 Each is independently selected from H or an alkyl group containing 1 to 10 carbon atoms.

[0042] The above R1~R 34 In the formula (1), the cations (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48) (49) (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63) (6

[0043] In some embodiments, for A being a divalent organic cation having a conjugated structure containing a double bond, A may be a protonated dihydropyrazine (i.e., a divalent organic cation shown in Formula 1, wherein R1, R2, R3, and R4 are all H), or a protonated 5,6-dimethyl-2,3-dihydropyrazine (i.e., a divalent organic cation shown in Formula 1, wherein R1 and R2 are all H, and R3 and R4 are all methyl) or other divalent organic cations having a conjugated structure containing a double bond.

[0044] In some embodiments, for A being a divalent organic cation having a conjugated structure containing an aromatic ring, A can be a protonated pyrazine (i.e., a divalent organic cation as shown in Formula 8, wherein R 22 、R 23 、R 24 and R 25 are all H), or A may be a protonated 1,2,3,4-tetrahydroquinoxaline (i.e., a divalent organic cation as shown in Formula 6, wherein R 18 and R 19 are all H), or may be a protonated pyrazine derivative (i.e., a divalent organic cation as shown in Formula 8, wherein R 22 、R 23 、R 24 and R 25 At least one of them is an alkyl group containing 1 to 10 carbon atoms), or it may be a phenylenediamine (i.e., a divalent organic cation as shown in Formula 10, or a divalent organic cation as shown in Formula 9, wherein R 26 、R 27 、R 28 and R 29 are all H), or may be aminomethylpyridine (i.e., a divalent organic cation as shown in formula 11, wherein R 30 and R 31 are all divalent organic cations with a conjugated structure containing an aromatic ring, such as H).

[0045] In some embodiments, when X is a monovalent halogen anion, the halogen anion is selected from Cl - Br - and I - At least one of; or, when X is a monovalent halogen-like anion, the halogen-like anion is selected from ClO4 - and BF4 - At least one of .

[0046] The aforementioned selection of A and X enables the pure organic perovskite material of the present application to meet the tolerance factor requirements required for the perovskite crystal form while also exhibiting a low band gap and photoresponsiveness in the visible light region. Furthermore, A and X can be a single structure or a combination of any two or more structures within their respective selectable ranges.

[0047] In one embodiment, A in ABX3 is selected from protonated 5,6-dimethyl-2,3-dihydropyrazine (structure shown below), and X is BF4 - The tolerance factor of the formed organic perovskite material is 0.965, and the formed perovskite material is easily soluble in N,N-dimethylformamide.

[0048] Protonated 5,6-dimethyl-2,3-dihydropyrazine:

[0049] In another embodiment, A in ABX3 is protonated pyrazine (structure shown below), and X is BF4 - The tolerance factor of the formed organic perovskite material is 0.939.

[0050] Protonated pyrazine:

[0051] In another embodiment, A in ABX3 is protonated 1,2,3,4-tetrahydroquinoxaline (structure shown below), and X is BF4 - The tolerance factor of the formed organic perovskite material is 0.981.

[0052] Protonated 1,2,3,4-tetrahydroquinoxaline:

[0053] A second aspect of the present invention provides a method for preparing the above-mentioned pure organic perovskite material, comprising the following steps:

[0054] Unprotonated A and BX are added to an acid solution and heated to react to obtain a pure organic perovskite material as shown in ABX3.

[0055] The preparation method of a pure organic perovskite material provided in the embodiments of the present application involves adding raw materials, namely, unprotonated A and BX, to an acid solution for reaction, thereby generating a pure organic perovskite material represented by ABX3. Because this preparation method can introduce A, namely, a conjugated divalent organic cation containing double bonds and / or aromatic rings, from the raw materials, a pure organic perovskite material with a low band gap and photosensitivity in the visible light region can be produced. This preparation method is simple and amenable to industrial production, opening up the possibility of optoelectronic applications for this new type of pure organic perovskite.

[0056] The unprotonated starting material A is the corresponding starting material when a conjugated divalent organic cation containing a double bond and / or an aromatic ring is unprotonated. For example, when the divalent organic cation A in ABX3 is protonated 5,6-dimethyl-2,3-dihydropyrazine or protonated pyrazine, the corresponding unprotonated starting material A used in the preparation method is 5,6-dimethyl-2,3-dihydropyrazine or pyrazine. The starting material BX is the corresponding ammonium halide salt (e.g., NH4Cl, NH4Br, NH4I, etc.) or a quasi-ammonium halide salt (e.g., NH4ClO4, NH4BF4, etc.).

[0057] The acid solution can be a phosphoric acid solution, so that the unprotonated A can undergo a protonation reaction more effectively. The temperature of the heating reaction between the unprotonated A and BX in the phosphoric acid solution can be 100-160°C. In the embodiments of the present application, a lower material synthesis temperature can be used to obtain a pure organic perovskite material.

[0058] The following describes the details in conjunction with specific embodiments.

[0059] Example 1

[0060] A pure organic perovskite material with a general chemical structure of ABX3; where A is protonated pyrazine and B is NH4 + , X is BF4 - The tolerance factor of this pure organic perovskite material is 0.939.

[0061] The synthesis method of the pure organic perovskite material includes: adding 109 mg of pyrazine and 452 mg of ammonium fluoroborate to a 40 ml polytetrafluoroethylene reactor liner, and then adding 2 ml of deionized water and 0.4 ml of phosphoric acid (mass fraction is 85%). After stirring and dissolving, transfer to a 160°C oven for hydrothermal reaction. After 72 hours, take out the reactor, and when it cools to room temperature (25-27°C), filter to obtain a black filter residue. After washing the filter residue with ethanol, air-dry and collect the filter residue, which is the pure organic perovskite material.

[0062] Performance Characterization: The resulting pure organic perovskite material was dispersed in terpineol and, after uniform grinding, quickly formed into a film on quartz glass using a glass rod doctor blade. The film was then transferred to a glove box and heated at 100°C for 10 minutes to obtain a thin film suitable for UV-visible absorption spectroscopy measurements. For X-ray diffraction measurements, a thick perovskite film was prepared using a solid powder X-ray diffraction film deposition method.

[0063] The results show that: Figure 1 From the X-ray diffraction pattern of the powder, it can be seen that the powder has obvious diffraction peaks, indicating obvious crystalline characteristics.

[0064] Example 2

[0065] A pure organic perovskite material with a general chemical structure of ABX3; wherein A is protonated 5,6-dimethyl-2,3-dihydropyrazine and B is NH4 + , X is BF4 - The tolerance factor of this pure organic perovskite material is 0.965.

[0066] The synthesis method of the pure organic perovskite material includes: adding 300 mg of 5,6-dimethyl-2,3-dihydropyrazine and 906 mg of ammonium fluoroborate to a 40 ml polytetrafluoroethylene reactor liner, and then adding 4 ml of deionized water and 1 ml of phosphoric acid (mass fraction of 85%). After stirring and dissolving, transfer to a 120 ° C oven for hydrothermal reaction. After 48 hours, take out the reactor, and when cooled to room temperature, filter to obtain a black filter residue. After washing the filter residue with ethanol, air-dry and collect the filter residue to obtain the pure organic perovskite material.

[0067] Performance Characterization: The resulting pure organic perovskite material was dissolved in N,N-dimethylformamide to form a 4 mg / mL solution. A film was then rapidly formed on quartz glass at 4000 rpm. The film was then transferred to a glove box and heated at 100°C for 10 minutes to obtain a thin film suitable for UV-visible absorption spectroscopy measurements. For X-ray diffraction measurements, a thick perovskite film was prepared using a solid powder X-ray diffraction film deposition method.

[0068] The results show that the pure organic perovskite material has high solubility and can be easily formed into a film of suitable thickness by solution spin coating. Figure 2 This is the UV-visible absorption spectrum of the film. It can be seen from the figure that the absorption band edge of the film is 659 nanometers, and the corresponding band gap is 1.88 eV, which has achieved optical response in the visible light band. Figure 3 This is the X-ray diffraction pattern of the powder. It can be seen that the film has obvious diffraction peaks, indicating that the film has obvious crystalline characteristics.

[0069] Example 3

[0070] A pure organic perovskite material with a general chemical structure of ABX3; wherein A is protonated 1,2,3,4-tetrahydroquinoxaline and B is NH4 + , X is BF4 - The tolerance factor of this pure organic perovskite material is 0.981.

[0071] The synthesis method of the pure organic perovskite material includes: adding 365 mg of 1,2,3,4-tetrahydroquinoxaline and 906 mg of ammonium fluoroborate to a 40 ml polytetrafluoroethylene reactor liner, and then adding 4 ml of deionized water and 1 ml of phosphoric acid (mass fraction is 85%). After stirring and dissolving, transfer to a 160 ° C oven for hydrothermal reaction. After 48 hours, take out the reactor, wait until it cools to room temperature, and filter to obtain a black filter residue. After washing the filter residue with ethanol, air-dry and collect the filter residue, which is the pure organic perovskite material.

[0072] Performance Characterization: The resulting pure organic perovskite material was dissolved in N,N-dimethylformamide to form a 4 mg / mL solution. A film was then rapidly formed on quartz glass at 4000 rpm. The film was then transferred to a glove box and heated at 100°C for 10 minutes to obtain a thin film suitable for UV-visible absorption spectroscopy measurements. For X-ray diffraction measurements, a thick perovskite film was prepared using a solid powder X-ray diffraction film deposition method.

[0073] The results show that the pure organic perovskite material has high solubility and can be easily formed into a film of suitable thickness by solution spin coating. Figure 4 This is the UV-visible absorption spectrum of the film. It can be seen from the figure that the absorption band edge of the film is 676 nanometers, and the corresponding band gap is 1.83 eV, which has achieved optical response in the visible light band. Figure 5 This is the X-ray diffraction pattern of the powder. It can be seen that the film has obvious diffraction peaks, but the content of the non-crystalline part in the background is also not low, indicating that the powder is a mixture of crystals and amorphous materials.

[0074] Comparative Example 1

[0075] A pure organic perovskite material with a general chemical structure of ABX3; where A is protonated triethylenediamine (i.e., H2DABCO, as shown below), and B is NH4 + , X is BF4 - The tolerance factor of this pure organic perovskite material is 0.925.

[0076]

[0077] The synthesis method of the pure organic perovskite material includes: adding 312 mg of triethylenediamine and 906 mg of ammonium fluoroborate to a 40 ml polytetrafluoroethylene reactor liner, and then adding 4 ml of deionized water and 0.8 ml of phosphoric acid (mass fraction is 85%). After stirring and dissolving, transfer to a 160 ° C oven for hydrothermal reaction. After 72 hours, take out the reactor, and when cooled to room temperature, filter to obtain colorless particles (due to the influence of light scattering, the surface is white and translucent). After washing the filter residue with ethanol, air-dry and collect the filter residue, which is the pure organic perovskite material.

[0078] Performance Characterization: The resulting pure organic perovskite material was dissolved in N,N-dimethylformamide to form a 4 mg / mL solution. A film was then rapidly formed on quartz glass at 4000 rpm. The film was then transferred to a glove box and heated at 100°C for 10 minutes to obtain a thin film suitable for UV-visible absorption spectroscopy measurements. For X-ray diffraction measurements, a thick perovskite film was prepared using a solid powder X-ray diffraction film deposition method.

[0079] The results show: Figure 6 is the UV-visible absorption spectrum of the film. It can be seen from the figure that the film has no light response in the 300-800 nanometer band, which is consistent with the color of the pure organic perovskite material. Figure 7 This is the X-ray diffraction pattern of the powder. It can be seen that the film has obvious diffraction peaks, indicating that the film has obvious crystalline characteristics.

[0080] The above embodiments and comparative examples show that the pure organic perovskite material of the embodiment of the present application has the characteristics of low band gap and photoresponse in the visible light region by selecting A as a conjugated divalent organic cation containing double bonds and / or aromatic rings, while A in the comparative example is not a conjugated structural unit containing double bonds or aromatic rings, and thus does not have photoresponse in the visible light band.

[0081] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A pure organic perovskite material having a general chemical structure of ABX3, characterized in that: A is a conjugated divalent organic cation containing a double bond and / or an aromatic ring, B is NH4 + , X is a monovalent halogen anion and / or a halogen-like anion, wherein the halogen-like anion is selected from ClO4 - and BF4 - At least one of; The conjugated divalent organic cation containing a double bond in A is selected from at least one of the following formulas 1 to 5, and the conjugated divalent organic cation containing an aromatic ring is selected from at least one of the following formulas 6 to 8; in, R1, R2, R3 and R4 in Formula 1 are independently selected from H or an alkyl group containing 1 to 10 carbon atoms, R5 and R6 in Formula 2 are independently selected from H or an alkyl group containing 1 to 10 carbon atoms, R7, R8 and R9 in Formula 3 are independently selected from H or an alkyl group containing 1 to 10 carbon atoms, R in Formula 4 10 、R 11 、R 12 and R 13 are independently selected from H or an alkyl group containing 1 to 10 carbons, R in Formula 5 14 、R 15 、R 16 and R 17 are independently selected from H or an alkyl group containing 1 to 10 carbons, R in Formula 6 18 and R 19 are independently selected from H or an alkyl group containing 1 to 10 carbons, R in Formula 7 20 and R 21 are independently selected from H or an alkyl group containing 1 to 10 carbons, R in Formula 8 22 、R 23 、R 24 and R 25 Each is independently selected from H or an alkyl group containing 1 to 10 carbon atoms.

2. The pure organic perovskite material according to claim 1, characterized in that The selection of A and X makes the tolerance factor of the pure organic perovskite material be 0.8 to 1.

05.

3. The pure organic perovskite material according to claim 1, characterized in that A is protonated 5,6-dimethyl-2,3-dihydropyrazine.

4. The organic perovskite material according to claim 1, wherein A is protonated pyrazine or protonated 1,2,3,4-tetrahydroquinoxaline.

5. The organic perovskite material according to any one of claims 1 to 4, wherein X is a halogen anion selected from Cl - Br - and I - At least one of .

6. The organic perovskite material according to any one of claims 1 to 4, wherein The organic perovskite material has a photoresponse in the visible light band.

7. A method for preparing a pure organic perovskite material according to any one of claims 1 to 6, characterized in that: The following steps are involved: Unprotonated A and BX are added to an acid solution and heated to react to obtain a pure organic perovskite material as shown in ABX3.

8. The method for preparing a pure organic perovskite material according to claim 7, wherein: The temperature of the heating reaction is 100-160° C.; and / or the acid solution is a phosphoric acid solution.

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