FROM SOLUTION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. provisional patent application number 63/166,480 that was filed March 26, 2021, the entire contents of which are incorporated herein by reference.

BACKGROUND

[0002] Highly sensitive hard X-ray and g-ray radiation detectors with large volume and high energy resolution (ER) based on scintillators and semiconductors play a critical role in homeland security, with broad industrial and biomedical applications. The commercially viable semiconductors for g -ray detection are high-purity Ge (HPGe) and cadmium telluride (CdTe)-based materials, which have been known since the 1950s. The HPGe detector is generally recognized as the gold standard for radiation detection due to its superior energy resolution, but its narrow intrinsic bandgap limits it to operation only at liquid nitrogen temperatures. Competitive semiconductor materials that can detect X-rays with high sensitivity at room temperature over long periods have been long pursued but with limited success. The alloy Cdi- _Y Zn _Y Te (x~0.1, CZT) has achieved commercialization but presents high manufacturing cost and unresolved growth issues.

[0003] The advent of halide perovskite materials, which have the formula ABX3 (where A is an inorganic or small organic cation, B is a metallic cation, and X is a halogen) has impacted the field of g -rays detection, which is a highly demanding application for a semiconductor because of the very strict criteria and severe quality constraints placed on them. An advantage of the halide perovskites is their ability to provide high charge transport at purity levels that are orders of magnitude lower than those required for more established hard radiation detector materials, due to their inherent defect tolerant nature.

[0004] The development of halide perovskite based electronic devices has focused heavily on polycrystalline films due to the relative ease for film formation. However, thin halide perovskite films are typically characterized by a high density of defects, such as point defects, impurities, grain boundaries, and residual inclusions, which degrade the properties of the devices made therefrom. Halide perovskite single crystals generally exhibit a much lower density of defects due to their ordered lattice arrangement. Therefore, free-standing perovskite single crystals are superior to thin films for many applications. However, in order to be useful single crystals of halide perovskites must be large and preferably square or rectangular with a low rectangle aspect ratio.

[0005] Large all-inorganic halide perovskite crystals can be grown from a melt by cooling the molten compound through its crystallization point using a Bridgman method or a melting cooling method. However, solution growth of halide perovskite crystals is more appealing because it can be done at relatively low temperatures, can be scaled up, and does not require complicated or expensive equipment. Unfortunately, while a few halide perovskites, such as the hybrid organic-inorganic perovskite materials MAPbX ₃ and FAPbX ₃ (where X= Cl, Br, I, MA, or FA; where FA is formamidinium (HC(NH2)2 ⁺ , and MA is methylammonium (CH ₃ NH ₃ ⁺ )), have been grown as large (e.g., ~ 1 cm size) crystals in solution, most halide perovskites, and particular those in which A includes a metal cation, have only be grown in solution as small crystals, typically having sizes of a few millimeters or smaller.

[0006] Attempts have been made to increase the size of halide perovskite crystals grown in solution using various additives. However, no additive has been found to be effective across different halide perovskites and the mechanisms by which the various additives that have been tried is not well understood. Therefore, the a priori identification of suitable additives for particular halide perovskites has not been possible.

[0007] The all-inorganic perovskite CsPbBn and hybrid perovskite (FA,Cs)PbBr3 are two hard radiation (e.g., X- and g -ray) detector materials with a lot of potential; however, they have met with limited success due to the inability to grow sufficiently large free-standing single crystals of these perovskites.

SUMMARY

[0008] Methods of growing large, free-standing single crystals of (FA _x Csi- _x )PbBr3 perovskites, where 0 < x < 1, in solution using alkyl ammonium salt additives, weak organic acid additives, or a combination thereof are provided.

[0009] One embodiment of a method for growing perovskite single-crystals having the formula (FA _x Csi- _x )PbBr3, where 0 < x < 1, includes the steps of: (a) forming a crystallization solution comprising: (i) one or more (FA _x Csi- _x )PbBr3 perovskite precursors; and (ii) one or more alkyl ammonium salts having the formula R _(4-x) H _x NX, where the Rs are alkyl groups selected from methyl groups, ethyl groups, propyl groups, butyl groups, and combinations thereof, x has a value of 0 or 1, and X is a halide anion, nitrate anion, acetate anion, perchlorate anion, or sulfate anion dissolved in an organic solvent; and (b) maintaining or ramping up the temperature of the crystallization solution for a time sufficient for solvent to evaporate and the perovskite precursors to react and form a (FA _x Csi- _x )PbBr ₃ perovskite single crystal. The method may optionally further include the step of transferring the (FA _x Csi- _x )PbBr ₃ perovskite single crystal to a fresh batch of crystallization solution and maintaining or ramping up the temperature of the fresh crystallization solution for a time sufficient for solvent to evaporate and the perovskite precursors to react and increase the size of the (FA _x Csi- _x )PbBr ₃ perovskite single crystal.

[0010] Another embodiment of a method for growing perovskite single-crystals having the formula (FA _x Csi- _x )PbBr ₃ , where 0 < x < 1, includes the steps of: (a) forming a crystallization solution comprising: (i) one or more (FA _x Csi- _x )PbBr ₃ perovskite precursors; and (ii) one or more weak organic acids dissolved in an organic solvent; and (b) maintaining or ramping up the temperature of the crystallization solution for a time sufficient for solvent to evaporate and the perovskite precursors to react and form a (FA _x Csi- _x )PbBr ₃ perovskite single crystal. The method may optionally include the additional step of transferring the (FA _x Csi- _x )PbBr ₃ perovskite single crystal to a fresh batch of crystallization solution and maintaining or ramping up the temperature of the fresh crystallization solution for a time sufficient for solvent to evaporate and the perovskite precursors to react and increase the size of the (FA _x Csi- _x )PbBr ₃ perovskite single crystal.

[0011] The (FA _x Csi- _x )PbBr ₃ perovskite single crystals grown using the methods described herein are large and have low aspect ratios - having, for example, at least one lateral dimension of 6 mm or greater and a square shape or a rectangular shape with a rectangle aspect ratio of 3 : 1 or less.

[0012] Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings.

[0014] FIG. 1 A is a schematic representation of a square single crystal.

[0015] FIG. IB is a schematic representation of a rectangular single crystal.

[0016] FIG. 2 shows a current-voltage (I-V) curve for detector LP7-B.

[0017] FIG. 3 shows detector LP7-B current response under lab X-ray.

[0018] FIG. 4 shows LP7-B Photocurrent as function of X-ray dose rate.

[0019] FIG. 5 shows LP7-B Sensitivity as function of detector reverse bias voltage.

[0020] FIG. 6 shows detector current response to different applied bias voltage at a fixed synchrotron X-ray flux 3.4e9 photons/s/mm ² (X-ray energy 58.6 keV).

[0021] FIG. 7 shows photocurrent as a function of applied bias voltage at a fixed synchrotron X-ray flux 3.4e9 photons/s/mm ² (X-ray energy 58.6 keV).

[0022] FIG. 8 shows detector LP13-3b photocurrent under synchrotron X-rays with different flux.

DETAILED DESCRIPTION

[0023] Methods of growing large, free-standing single crystals of (FA _x Csi- _x )PbBr3 perovskites, where 0 < x < 1, in solution using alkyl ammonium salt additives, weak organic acid additives, or a combination thereof are provided. By including the additives in a crystallization solution with perovskite precursors, larger single crystals can be grown by slow evaporation and/or inverse temperature crystallization than would be possible in the absence of the additives under the same growth conditions. With the aid of the additives, single crystals of (FA _x Csi- _x )PbBr3 perovskites having at least one lateral dimension of 6 mm or larger and a low aspect ratio can be grown in solution.

[0024] The crystals are grown in a crystallization solution that includes one or more (FA _x Csi- _x )PbBr3 perovskite precursors and one or more of the additives dissolved in an organic solvent. The preparation of the crystallization solution may be carried out in two steps, whereby a precursor solution that includes the perovskite precursors dissolved in the organic solvent is formed initially and the one or more additives are subsequently added to the precursor solution and dissolved to form the crystallization solution. The crystallization solution is then maintained at an elevated temperature or has its temperature ramped up across a range of elevated temperatures over a time period sufficient to allow for (FA _x Csi- _x )PbBr3 perovskite single crystal formation from the (FA _x Csi- _x )PbBr3 perovskite precursors in the presence of the additives.

[0025] In some embodiments of the methods, the additives are tertiary or ternary alkyl ammonium salts having the formula R ₍ 4- _X) H _x NX, where the Rs are alkyl groups selected from methyl groups, ethyl groups, propyl groups, butyl groups, and combinations thereof, x has a value of 0 or 1, and X is a halide anion, such as Br, Cl, or I, a nitrate anion, an acetate anion, a perchlorate anion, and/or a sulfate anion. Because crystal size tends to decrease for larger alkyl groups, the Rs are more desirably selected from methyl groups, ethyl groups, propyl groups, and/or combinations thereof.

[0026] In other embodiments of the methods, the additives are weak organic acids (that is, organic acids having a K _a of less than 1), examples of which include formic acid, acetic acid, propionic acid, and oxalic acid.

[0027] The concentration of the additives in the crystallization solution is a concentration at which they are able to carry out the function of enhanced single crystal growth. Typical concentrations are in the range from 0.001 mmol/mL to 0.1 mmol/mL. However, concentrations outside of this range can be used.

[0028] The (FA _x Csi- _x )PbBr3 perovskite precursors are two or more molecules that collectively contain all of the elements of the (FA _x Csi- _x )PbBr3 perovskite and that react in the crystallization solution to form a single crystal of the perovskite. In some embodiments of the methods, the precursors are selected such that they collectively contain only elements of the final (FA _x Csi- _x )PbBr3 perovskite. For the all-inorganic perovskite (x = 0), the precursors will typically take the form of inorganic salts, such as CsBr, PbBr2, and/or PbBr, while the precursors for the hybrid (FA _x Csi- _x )PbBr3 perovskite will typically include a formamidine salt, such as formamidine bromide, in addition to inorganic precursors, such as CsBr, PbBr2, and/or PbBr.

[0029] The organic solvent may be a single solvent or a solvent mixture, provided that the perovskite precursors and the additives are sufficiently soluble therein and, for ITC growth, further provided that the solubility of the (FA _x Csi- _x )PbBr3 perovskite varies sufficiently as a function of temperature in that organic solvent. Suitable solvents for the growth of single crystal (FA _x Csi- _x )PbBr3 perovskites include dimethylsulfoxide (DMSO), g- butyrolactone (GBL), dimethylformamide (DMF), and mixtures of two or more thereof. Generally, CsPbBr3 crystals can be grown using DMSO as the organic solvent, while a mixture of two or more of these may be used as the organic solvent for the growth of (FA _x Csi- _x )PbBr3 (0 < x < 1) crystals.

[0030] The growth of the final single crystal (FA _x Csi- _x )PbBr3 perovskite can be carried out in a single growth step in the original crystallization solution. Alternatively, the original crystallization solution can be used to grow seed crystals which are transferred from the original crystallization solution to a fresh crystallization solution in which the final single crystal (FA _x Csi- _x )PbBr3 perovskite is grown from the seed crystal by slow evaporation or inverse temperature crystallization (ITC). Like the original crystallization solution, the fresh crystallization solution comprises (FA _x Csi- _x )PbBr3 perovskite precursors and the additives are dissolved in an organic solvent. The seeded growth step can be carried out once or more than once iteratively to increase the size of the single crystal.

[0031] Crystal growth by solvent evaporation is a simple method to obtain large single crystals of the (FA _x Csi- _x )PbBr3 perovskites. During growth by slow evaporation, the organic solvent is allowed to evaporate under controlled heating, until crystallization takes place. The crystallization solution may begin as a saturated solution in order to enable crystal growth, or may become saturated during the initial stages of solvent evaporation.

[0032] During ITC growth, the solubility of the perovskite decreases in the selected organic solvent as the temperature of the solution increases, such that the crystallization temperature solution reaches supersaturation as a result of heating, enabling single crystal growth.

[0033] Simple evaporative crystallization and ITC are generally carried out at elevated temperatures, where an elevated temperature is a temperature attained by heating the precursor solution or crystallization solution above standard room temperature (23 °C). Typical elevated temperatures for the dissolution of the precursors and/or additives in a precursor solution and/or for carrying out non-seeded and/or seeded single crystal growth are in the range from 45 °C to 130 °C, and more typically in the range from 50 °C to 110 °C. However, temperatures outside of these ranges can be used, keeping in mind that lower temperatures will generally increase the time for crystal formation. The crystallization process is carried out for a time sufficient to form a large single crystal - generally a period of at least one day and more commonly at least four days. During simple evaporative crystallization, the crystallization solution can be maintained at a steady temperature, while the temperature is ramped up across a range of elevated temperatures during ITC. The temperature ramp up may be conducted at a steady rate or be step-wise. By way of illustration only, the temperature of a crystallization solution could be increased by an amount of 20 °C to 60 °C, starting at a temperature in the range from 50 °C to 60 °C, over the course of a time period lasting in the range from 5 to 15 days. For a single step (non-seeded) single crystal growth process, the single growth step may be carried out at the temperatures and time period disclosed herein. For a seeded growth process, each of the steps may be carried out at the temperatures and time periods disclosed herein. However, these times and temperatures are provided only as illustration.

[0034] One illustrative embodiment of a method for the growth of a large, low aspect ratio (FA _x Csi- _x )PbBr3 perovskite single crystal includes the following steps. A stoichiometric molar ratio of the perovskite precursors is dissolved in an organic solvent to prepare a precursor solution in a container. The container may be sealed using, for example, plastic wrap, to exclude dust, particulates, and other contaminants from the environment. Heating and/or stirring may be used to promote dissolution of the precursors. Once the precursor solution has been prepared, one or more additives are added to the precursor solution to form a crystallization solution. Heating and/or stirring may be used to promote the dissolution of the additives. Optionally, the crystallization solution may be filtered to remove insoluble impurities. The crystallization solution is then transferred to an oven to maintain the crystallization solution at a suitable crystallization temperature or to ramp the crystallization solution over a suitable temperature range to promote single crystal growth. The growth may continue until a crystal of a desired size is achieved. Alternatively, crystal growth in the initial crystallization solution can be interrupted and one or more of the resulting crystals can be transferred to a fresh crystallization solution that is transferred to the oven to maintain the fresh crystallization solution at a suitable crystallization temperature or to ramp the crystallization solution over a suitable temperature range to promote further growth of the (FA _x Csi- _x )PbBr3 perovskite single crystals. The final single crystals can then be removed from the solution and washed.

[0035] Using either the unseeded or seeded crystal growth methods described herein, (FA _x Csi- _x )PbBr3 (0 < x < 1) free-standing single crystals having at least one lateral dimension of 6 mm and a low aspect ratio can be formed, where the lateral dimensions are the length (1) and width (w) of a single crystal, as shown schematically in FIGS. 1 A and IB. This includes (FA _x Csi- _x )PbBr3 (0 < x < 1) single crystals having at least one lateral dimension of at least 8 mm or at least 10 mm. For example, single crystals having at least one lateral dimension in the range from 6 mm to 20 mm or larger can be grown. The aspect ratio (l:w) may be 1 : 1 in the case of a square crystal (FIG. 1 A) or greater than 1 in the case of a rectangular crystal (FIG. IB). However, even in the case of rectangular crystals, the present methods are able to produce crystals with low rectangle aspect ratios (l:w), including aspect ratios of 3 : 1 or lower, 2: 1 or lower, 1.5:1 or lower, and 1.2:1 or lower. The thickness of the single crystals may be smaller than the length and width dimensions but is still on the mm scale. Thus, the single crystals may have thicknesses of 1 mm or larger, including thicknesses of 2 mm or larger, and 3 mm or larger. By way of illustration, single crystals having a thickness in the range from 1 mm to 6 mm or greater can be grown.

EXAMPLES

[0036] Example 1: Large CsPbBn Single Crystals Made with an Alkyl Ammonium Halide Additive.

[0037] Preparation of Precursor Solution

[0038] A total of 0.05 mol of unpurified CsBr (>99%, Alfa Aesar) precursor, together with 0.1 mol of unpurified PbBr2 (>98%, Alfa Aesar) precursor was dissolved in 48.5 mL of dimethyl sulfoxide (DMSO, >99.5%) solvent with continuous stirring for 24 h at 60 °C on a hot plate. A mole ratio of PbBr2 and CsBr of around 2: 1 was used to form pure CsPbBn crystals. After all the raw materials were dissolved, 4.85 mmol of tetramethyl ammonium bromide (TMAB) was added into the 48.5 mL solution with continuous stirring for 24 h at 60 °C on a hot plate to form a crystallization solution. The solution was then filtered using a membrane with a 5 pm and 0.22 pm pore size subsequently to remove any insoluble impurities.

[0039] Unseeded Growth of square-shaped and rectangular crystals of CsPbBn Single Crystals of at least 8 mm in length

[0040] CsPbBn single crystals were grown by evaporative precipitation from the crystallization solution. While still at 60 °C, 40 mL of clear crystallization solution was put in either a 100 mm or 25 mm diameter crystallization dish. Each container was sealed with plastic wrap and a rubber band, which was then put into an FO Series 110V Oven (Across

International) for crystal growth. For the 100 mm crystallization dishes, the initial temperature of the oven at solution input was 60 °C before subsequent ramp up to 85 °C at a steady rate over 7 days. For the 25 mm crystallization dishes, the initial temperature of the oven at solution input was 85 °C, which was held constant.

[0041] This single crystal growing process lasted a maximum of 7 days for the 100 mm dishes and a maximum of 10 days for the 25 mm dishes. To remove crystals for seeded growth, growth was interrupted early at 5 days before continuing. Crystallization dishes were taken out at oven temperature and quickly began cooling. Upon removal, individual crystals were picked out and wiped of solution. Mainly square-shaped CsPbBn crystals with a size ranging from 5 to 10 mm in lateral length and 2-4 mm in thickness were obtained. The final crystals were washed with hexane solvent at room temperature. The area and thickness of the crystals are provided in Table 1.

[0042] Table 1. CsPbBn single crystals dimensions

[0043] Seeded growth of square-shaped and rectangular crystals of CsPbBn Single Crystals

[0044] A quantity 3 mL of fresh crystallization solution at 60 °C was put in a scintillator vial. One small and square seed crystal with 1-2 mm lateral length was quickly transferred from the 100 mm crystallization dish to the vial. The vial was capped only halfway to allow slow solvent evaporation and was then put into the oven with the same growth procedure as for non-seeded growth. The areas and thicknesses of the crystals are provided in Table 2.

[0045] Table 2. CsPbBn single crystals dimensions

[0046] CsPbBn crystal LP7-B was fabricated into a hard radiation detector. The detector has gallium indium eutectic alloy (Gain) as a top metal electrode and Au as a bottom electrode, so the device has a structure Galn/CsPbBn/Au.

[0047] The I-V curve of detector LP7-B did not show junction breakdown when reverse bias voltage was smaller than 300 V, which is shown in FIG. 2. The detector LP7-B current response under lab X-ray tube was stable, as shown in FIG. 3.

[0048] Detector LP7-B photocurrent as a function of X-ray dose rate is plotted in FIG. 4. The sensitivity of Detector LP7-B increased as a function of detector reverse bias voltage and approached a saturation value, which is shown in FIG. 5. A high sensitivity -5386 pC/Gy/cm ² at reverse 300 V was achieved, and the detector dark current and signal current were stable. It should be noted that this detector was not optimized and that better results can be achieved using crystals made with the present methods. The data presented here is merely to show that the crystals are suitable for use in hard radiation detection.

[0049] Another detector LP13-3a was fabricated using crystal LP13-3a and the same electrodes as previously. A low detector dark current was achieved, that is, 14 nA at -200V. The detector LP13-3a current response under lab X-ray tube was very stable up to 1000 V, the limit of the testing instrument. Detector LP13-3a exhibited a photocurrent as a function of X-ray dose rate and the sensitivity of Detector LP13-3a increased as a function of detector reverse bias voltage and approached a saturation value. A high sensitivity -10833 pC/Gy/cm ² at reverse 1000 V was achieved, and the detector dark current and signal current were stable.

[0050] A third detector LP13-3d was fabricated using crystal LP13-3d and the same electrodes as before. A low detector dark current was achieved, that is, 13 nA at -200 V. The detector LP13-3d current response under lab X-ray tube was very stable up to 1000 V, the limit of the testing instrument. Detector LP13-3d exhibited photocurrent as a function of X- ray dose rate and the sensitivity of Detector LP13-3d increased as a function of detector reverse bias voltage and approached a saturation value. A high sensitivity -10238 pC/Gy/cm ² at reverse 1000 V was achieved, and the detector dark current and signal current were stable.

[0051] Detection of synchrotron X-rays

[0052] Detector LP13-3b was fabricated and was tested under synchrotron X-rays. The detector current response to different applied bias voltage at a fixed synchrotron X-ray flux is shown in FIG. 6. The detector dark current and signal current were stable. The photocurrent showed good saturation behavior as a function of applied bias voltage, which is shown in FIG. 7.

[0053] Detector LP13-3b showed stable and reproducible photocurrent under synchrotron X-rays with different flux, which is shown in FIG. 8. The plotted photocurrent shows an excellent linearity as a function of synchrotron X-ray flux up to 4.3el0 photons/s/mm ² with X-ray energy 58.6 keV.

[0054] Example 2: Centimeter Sized Crystals of CsPbBn Made with a Weak Organic Acid Additive.

[0055] This example illustrates methods for growing large crystals of CsPbBn (at least 1 cm x 1 cm wide and at least 1 mm thick) and related derivatives (e.g., CsPbX ₃ X=combination of Cl, Br, I) using a weak organic acid (formic acid) additive.

[0056] Illustrative experimental process for large solution grown CsPbBn crystals: The CsPbBn precursor solution for crystal growth was obtained by dissolving CsBr (50 mmol) and PbBn (100 mmol) as the precursors with a molar ratio of 1 :2 in 48.5 mL dimethylsulfoxide (DMSO) solution. The solution was heated to 50 °C to accelerate the dissolution of these raw materials. After 5 hours, a transparent solution was obtained. Then 1.5 mL of formic acid was added to the transparent solution to obtain the final CsPbBn crystallization solution. After stirring at 50 °C for 10 minutes, the crystallization solution was filtered into a new and clear crystallization dish using a 0.2 pm pore size polytetrafluoroethylene (PTFE) filter to remove impurities and small CsPbBn crystals. Then, the crystallization dish was sealed with plastic wrap and transferred to an oven which was heated to 48 °C. To grow CsPbBn single crystals, the oven was heated from 48 °C to 120 °C at a heating rate of 5 °C per day. After about 15 days growth, several square-shaped CsPbBn single crystals formed at the bottom of the crystallization dish. The crystals were harvested, dried, and sealed inside suitable storage media. The crystals of CsPbBn had edge lengths of 1 cm.

[0057] The word "illustrative" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "illustrative" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, "a" or "an" can mean or only or can mean "one or more.” Embodiments of the inventions consistent with either construction are covered.

[0058] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.

The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.