A method for synthesizing perovskite thin films with continuously graded band gaps using thermal evaporation.

Abstract

This invention discloses a method for synthesizing perovskite thin films with continuously graded band gaps using thermal evaporation. The apparatus includes a thermal evaporation vacuum chamber, a substrate stage, an evaporation source, a thermal evaporation vacuum module, and a vacuum displacement module. By using thermal evaporation equipment and a vacuum displacement motor, a perovskite thin film with continuously graded band gaps can be synthesized in one step under high vacuum. This results in the overall ratio of Br to Cl continuously increasing and the ratio of Br to I continuously decreasing from one end to the other after evaporation, and the band gap continuously narrowing from left to right.

Description

Technical Field

[0001] This invention belongs to the field of thermal evaporation, specifically relating to a method for synthesizing perovskite thin films with continuously graded band gaps using thermal evaporation. Background Technology

[0002] Gradient bandgap perovskite materials have played a significant role in the optoelectronic field due to their advantages such as higher photoelectric conversion efficiency, wider spectral absorption range, and flexible optical control capabilities, including improving the efficiency of tandem silicon solar cells, designing tunable light-emitting diodes, and broadband detectors.

[0003] The band gap gradient of perovskite materials is mainly achieved by controlling the proportion of halogen elements. Therefore, it can be prepared by repeatedly mixing solutions of different concentrations of halogen elements. However, this preparation method is complicated and cannot synthesize perovskite materials with continuous band gap gradients in one go.

[0004] In recent years, to achieve the synthesis of perovskite materials with continuously gradient band gaps in a single step, scientists have proposed methods such as solid-gas exchange, solid-liquid exchange, gradient coating, and droplet self-diffusion. The solid-gas exchange method involves placing a pre-prepared perovskite film in a halide gas, utilizing the different degrees of ion exchange between the gas and the solid film at different temperatures to achieve a gradient. However, this method offers poor controllability and has a narrow range of applicable materials, lacking versatility. The solid-liquid exchange method involves immersing a pre-prepared perovskite film in a halide solution and then slowly pulling the film out, utilizing the varying lengths of solid-liquid ion exchange to prepare a gradient film. Compared to the previous method, this method offers increased controllability, but the presence of solvent in the solution often reduces the performance of subsequent optoelectronic devices. The gradient coating method prepares a gradient film by continuously changing the halogen element ratio during the coating process; similarly, the solvent in the solution can affect subsequent optoelectronic devices. The droplet self-diffusion method utilizes the self-diffusion and mutual diffusion between perovskite precursor droplets to prepare a gradient film at the interface. Therefore, the prepared film is small in size, and the gradient range is narrow.

[0005] In summary, due to limitations in experimental methods and equipment, the proposed synthesis methods currently available suffer from problems such as narrow applicability, poor controllability, and limited gradient range, and none of them can meet the requirements for homogenization and large-scale compatibility with industrial production. Summary of the Invention

[0006] To address the problems in the prior art, this invention proposes an apparatus for synthesizing perovskite thin films with continuously graded band gaps using thermal evaporation, comprising a thermal evaporation vacuum chamber, a substrate stage, an evaporation source, a thermal evaporation vacuum module, and a vacuum displacement module.

[0007] The thermal evaporation vacuum chamber provides a vacuum environment; the substrate stage is located at the top inside the thermal evaporation vacuum chamber and is used to fix the substrate; the evaporation source is located at the bottom inside the thermal evaporation vacuum chamber and is used to hold the precursor material; the thermal evaporation vacuum module is used to evacuate the thermal evaporation vacuum chamber; the vacuum displacement module is located inside the thermal evaporation vacuum chamber and is used to shield the material during the thermal evaporation process.

[0008] The vacuum displacement module includes a masking plate, a support rod, and a vacuum displacement motor; the masking plate is fixedly connected to one end of the support rod, and the other end of the support rod is fixed to the vacuum displacement motor; during the thermal evaporation process, the thickness of the evaporation material is adjusted by moving the vacuum displacement motor.

[0009] The present invention also provides a method for synthesizing perovskite thin films with continuously graded band gaps based on the above-described apparatus, comprising the following steps:

[0010] 1) Fix the substrate on the substrate stage, and place sufficient amounts of PbBr2 powder, CsI powder and CsCl powder in the three evaporation sources respectively;

[0011] 2) The hot-evaporation vacuum chamber is evacuated using a hot-evaporation vacuum module;

[0012] 3) The substrate stage is rotated at a constant speed, and PbBr2 is uniformly deposited on the substrate by thermal evaporation.

[0013] 4) After PbBr2 deposition is completed, stop the substrate stage rotation; deposit CsI or CsCl by thermal evaporation, and at the same time start the vacuum displacement motor to drive the mask plate to move, so that the deposited CsCl or CsI has a gradually changing thickness.

[0014] 5) Reset the vacuum displacement motor (10) and rotate the substrate stage 180°. Deposit the remaining precursor material by thermal evaporation. At the same time, start the vacuum displacement motor to drive the masking plate to move, so that the thickness of the last precursor material deposited is constantly changing.

[0015] 6) After CsCl and CsI deposition is completed, the substrate is removed and subjected to high-temperature annealing to obtain a perovskite thin film with a continuous band gap gradient.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] This invention is the first to introduce a vacuum displacement motor into a thermal evaporation deposition chamber, laying the foundation for large-scale industrial coating.

[0018] This invention enables the one-time synthesis of perovskite materials with continuously varying band gaps under high vacuum. The prepared thin film achieves continuous band gap variation through thickness gradient. The high vacuum environment ensures higher overall film quality, more uniform vapor-deposited film, and further reduction in roughness. The absence of solution also avoids the dissolution of crystals. This invention provides more precise control over film thickness, achieving nanometer-level precision.

[0019] The method of this invention can prepare films with a wider range of band gap gradients, and can realize all the band gaps that can be varied in perovskite materials. Moreover, the method is applicable to a wide range of materials and is also applicable to other materials, so it has high versatility. Attached Figure Description

[0020] Appendix Figure 1 This is a schematic diagram of an apparatus for synthesizing a continuously graded perovskite with a band gap using thermal evaporation, according to the present invention.

[0021] Appendix Figure 2 This is a detailed evaporation diagram of a method for synthesizing a perovskite thin film with a continuously gradient bandgap using thermal evaporation according to the present invention.

[0022] Appendix Figure 3 This is a fluorescence spectrum diagram of a perovskite thin film with a continuously graded bandgap synthesized by thermal evaporation according to the present invention. Detailed Implementation

[0023] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0024] This invention proposes a novel method that utilizes thermal evaporation equipment and a vacuum displacement motor to achieve the one-step synthesis of perovskite materials with continuously gradient band gaps under high vacuum. Compared to solution methods, the evaporation process produces films with precisely controllable thickness and a smoother, more even surface. Furthermore, because it eliminates the need for solvents such as DMF, methanol, or chlorobenzene, it avoids the dissolution of the prepared crystals in solution. Simultaneously, the film exhibits a wider range of band gap gradients, enabling the realization of all variable band gaps in perovskite materials. This method is also applicable to a broad range of materials, demonstrating high versatility.

[0025] Vacuum thermal evaporation deposition for perovskite thin films includes single-source evaporation, dual-source layer evaporation, and dual-source co-evaporation. Single-source evaporation involves using a single evaporation source to simultaneously mix the evaporation materials. For example, in preparing all-inorganic perovskite CsPbX3 (where X is a halogen element Cl, Br, or I) thin films, the precursor materials CsX and PbX2 are placed in one evaporation source and deposited onto the substrate simultaneously. After high-temperature annealing, a high-quality perovskite thin film is generated. However, for precursor materials with significant differences in melting and boiling points, evaporation can cause a ratio imbalance, often resulting in significant defects. Dual-source layer evaporation involves depositing the thin film step-by-step in layers. First, CsX (or PbX2) from one evaporation source is deposited onto the substrate, followed by PbX2 (or CsX) from another evaporation source. High-temperature annealing is also used to generate a high-quality thin film. The dual-source co-evaporation method involves simultaneously depositing CsX and PbX2 from two evaporation sources to prepare perovskite thin films through in-situ reactions during deposition. This method produces films of the highest quality, and some materials do not even require subsequent annealing processes. However, it has high requirements for equipment such as dual evaporation sources and dual film thickness detection devices. It also requires strict control of the material ratio of the two precursors reaching the substrate, which reduces the controllability of the experiment.

[0026] like Figure 1 As shown, the device of the present invention includes a thermal evaporation vacuum chamber 3, a substrate stage 1, an evaporation source, a thermal evaporation vacuum module, and a vacuum displacement module.

[0027] The thermal evaporation vacuum chamber 3 provides a vacuum environment; the substrate stage 1 is located at the top inside the thermal evaporation vacuum chamber 3 and is used to fix the substrate; the evaporation source is located at the bottom inside the thermal evaporation vacuum chamber 3 and is used to hold the precursor material; the thermal evaporation vacuum module 9 is used to evaporate the thermal evaporation vacuum chamber 3; the vacuum displacement module is located inside the thermal evaporation vacuum chamber 3 and is used to shield the evaporation material during the thermal evaporation process.

[0028] The vacuum displacement module includes a masking plate 6, a support rod 7, and a vacuum displacement motor 10; the masking plate 6 is fixedly connected to one end of the support rod 7, and the other end of the support rod 7 is fixed to the vacuum displacement motor 10; during the thermal evaporation process, the thickness of the evaporation material is adjusted by moving the vacuum displacement motor 10.

[0029] This invention improves the dual-source layer evaporation method, combining it with a vacuum displacement motor to achieve the one-time preparation of perovskite thin films with continuously gradient band gaps. Taking the preparation of an all-inorganic perovskite CsPbX3 thin film as an example, the precursor materials are selected as PbCl2 and CsCl, PbBr2 and CsBr, and PbI2 and CsI. Through layer evaporation, PbCl2 is first deposited, followed by CsCl. After subsequent high-temperature annealing, the material obtained is CsPbCl3. The same method can be used to prepare CsPbBr3 and CsPbCl3.

[0030] Furthermore, when PbBr2 is first deposited, followed by multiple deposits of CsI of varying thicknesses, the ratio of halogen elements Br and I changes, resulting in multiple CsPbBr2 sheets. (x) I (3-x) Different band gaps appeared in the thin film, and ultraviolet laser pumping yielded fluorescence ranging from green to red. Similarly, by first depositing PbBr2 and then repeatedly depositing CsCl of varying thicknesses, CsPbBr2 could be obtained. (x) Cl (3-x) Different fluorescence ranging from green to blue.

[0031] Furthermore, after pre-depositing the first layer of PbBr2, when depositing the second layer of CsCl (or CsI), the deposition rate is kept constant, and the mask plate, which is almost attached to the substrate, is moved at a constant speed. The mask plate is pulled by a vacuum displacement motor, and the deposited CsCl (or CsI) will have a continuously varying thickness. That is, after subsequent high-temperature annealing, the ratio of Br and Cl (or I) along the direction of motor movement is continuously changing. Therefore, a perovskite thin film with a continuously varying bandgap is synthesized in one step, and the film can achieve continuously varying fluorescence on a single substrate.

[0032] Taking inorganic perovskite CsPbX3 as an example, the specific vapor deposition process is shown in the attached figure. Figure 1 As shown, firstly, sufficient PbBr powder is placed in evaporation source one, and sufficient CsI or CsCl powder is placed in evaporation source two (in actual evaporation, three evaporation boats are used to hold precursor materials respectively). Then, the pressure inside the thermal evaporation vacuum chamber is brought to 5e-4 Pa using the thermal evaporation vacuum system. The power supply to the evaporation source is turned on, and the substrate stage is set to rotate at a constant speed to uniformly deposit PbBr onto the placed substrate. Then, the substrate stage rotation is stopped, and a vacuum displacement motor installed in the vacuum chamber is set to move the mask plate at a constant speed while simultaneously depositing CsI or CsCl at a constant speed, depositing a second layer of gradually varying thickness film, i.e., the ratio of Br to I (or Cl) gradually changes along the direction of motor movement. Afterward, the film is removed and subjected to high-temperature annealing to obtain a high-quality perovskite film with a continuously varying bandgap.

[0033] Details of the vapor-deposited thin film are attached. Figure 2As shown, the first deposition was of a uniform PbBr film. The second deposition used a vacuum displacement motor to assisted in the deposition of a CsCl film with varying thickness. The third deposition involved changing the direction of the vacuum displacement motor to deposit a CsI film with varying thickness. Therefore, from left to right, the ratio of Br to I in the deposited films continuously increases, while the ratio of Br to Cl continuously decreases, resulting in a continuously widening band gap. After high-temperature annealing to promote ion exchange, the fluorescence of the films was characterized as shown in the attached figure. Figure 3 As shown. However, at this point, the fluorescence continuous gradient range can only cover approximately 440-630 nm. Further improvements can be made by... Figure 2 PbI2 is deposited on the far left of the thin film structure, CsBr is deposited in the middle, and PbCl2 is deposited on the far right, thus achieving continuous fluorescence covering 418-704nm. The dashed lines in the figure represent the fluorescence corresponding to CsPbCl3, CsPbBr3, and CsPbI3, while the solid lines represent the fluorescence collected at 20nm intervals on the prepared perovskite thin film with a continuous bandgap gradient.

[0034] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for synthesizing a perovskite thin film with a continuously graded bandgap, the method being based on an apparatus for synthesizing a perovskite thin film with a continuously graded bandgap using thermal evaporation; the apparatus for synthesizing a perovskite thin film with a continuously graded bandgap using thermal evaporation includes a thermal evaporation vacuum chamber (3), a substrate stage (1), an evaporation source, a thermal evaporation vacuum module (9), and a vacuum displacement module. The thermal evaporation vacuum chamber (3) is used to provide a vacuum environment; the substrate stage (1) is located at the top inside the thermal evaporation vacuum chamber (3) and is used to fix the substrate; the evaporation source is located at the bottom inside the thermal evaporation vacuum chamber (3) and is used to hold the precursor material; the thermal evaporation vacuum module (9) is used to evaporate the thermal evaporation vacuum chamber (3); the vacuum displacement module is located inside the thermal evaporation vacuum chamber (3) and is used to shield the evaporation material during the thermal evaporation process. The vacuum displacement module includes a masking plate (6), a support rod (7), and a vacuum displacement motor (10); the masking plate (6) is fixedly connected to one end of the support rod (7), and the other end of the support rod (7) is fixed to the vacuum displacement motor (10); during the thermal evaporation process, the thickness of the evaporation material is adjusted by moving the vacuum displacement motor (10); Its features are, The method includes the following steps: 1) Fix the substrate on the substrate stage (1) and place sufficient amounts of PbBr2 powder, CsI powder and CsCl powder in the three evaporation sources respectively; 2) The hot evaporation vacuum chamber (3) is evacuated using the hot evaporation vacuum module (9); 3) Rotate the substrate stage (1) at a constant speed, and at the same time deposit PbBr2 uniformly on the substrate by thermal evaporation. 4) After PbBr2 deposition is completed, stop the substrate stage (1) from rotating; deposit one of CsI or CsCl by thermal evaporation, and at the same time start the vacuum displacement motor (10) to drive the masking plate to move, so that the deposited CsCl or CsI has a gradually changing thickness. 5) Reset the vacuum displacement motor (10) and rotate the substrate stage (1) 180°. Deposit the remaining precursor material by thermal evaporation. At the same time, start the vacuum displacement motor (10) to drive the masking plate to move, so that the thickness of the last material deposited is gradually changing. 6) After CsCl and CsI deposition is completed, the substrate is removed and subjected to high-temperature annealing to obtain a perovskite thin film with a continuous band gap gradient.

2. The method for synthesizing perovskite thin films with continuously graded band gaps according to claim 1, characterized in that, In step 2), after the hot evaporation vacuum chamber (3) is evacuated by the hot evaporation vacuum module (9), the vacuum level in the hot evaporation vacuum chamber (3) is better than 5e. -4 pa.

3. The method for synthesizing perovskite thin films with continuously graded band gaps according to claim 1, characterized in that, After evaporation, the substrate surface has a gradually thickening CsI layer on one side and a gradually thickening CsCl layer on the other side.

4. The method for synthesizing perovskite thin films with continuously graded band gaps according to claim 3, characterized in that, The ratio of Br to Cl on the surface of the perovskite thin film with a continuously gradient band gap after evaporation decreases from one side to the middle, and the ratio of Br to I decreases from the other side to the middle. Therefore, the band gap will continuously narrow from one side to the other.

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