Gradient perovskite heterojunction and preparation method and application thereof

Abstract

The invention relates to the technical field of X-ray detection, in particular to a gradient perovskite heterojunction and a preparation method and application thereof. The invention provides a preparation method of a gradient perovskite heterojunction, and the method comprises the following steps: employing an electric field driven ion deposition method, enabling ITO glass and ITO glass fixed with perovskite single crystals to be oppositely arranged in an AX ethanol solution under the condition of applying a reverse bias electric field, then carrying out the first crystallization, obtaining a perovskite gradient layer, and carrying out the second crystallization to obtain the gradient perovskite heterojunction. The reverse bias electric field is switched into a forward bias electric field for second crystallization, and the gradient perovskite heterojunction is obtained; a in the AX is an organic cation or an inorganic cation, and X in the AX is a halogen anion. According to the preparation method, the closely integrated gradient perovskite heterojunction can be prepared and constructed, and meanwhile the preparation method has the advantages of being rapid, high in universality and high in adhesive force.

Description

Technical Field

[0001] The present invention relates to the field of X-ray detection technology, and in particular to a gradient perovskite heterojunction and a preparation method and application thereof. Background Art

[0002] Perovskite semiconductors are excellent candidates for next-generation X-ray detectors due to their unique tunable composition, high carrier mobility and lifetime product (μτ), high radiation attenuation coefficient or light absorption coefficient, and low defect density. Three-dimensional (3D) perovskites, such as MAPbI 3 and FAPbI 3 , has been shown to have approximately 1×10 3 ~1×10 6 μC·Gy -1 cm -2 High sensitivity and excellent performance. However, under the action of electric field, high dark current, noise and significant ion migration cause stability problems. In contrast, two-dimensional (2D) perovskites not only have extremely high bulk resistance that can suppress dark current and noise, but also show inhibition of ion migration. However, due to the long chain spacer cations in the two-dimensional perovskite that hinder the charge, it shows lower sensitivity. Therefore, the heterogeneous engineering of three-dimensional / two-dimensional perovskites that organically combines the advantages of different components has quickly become a research hotspot this year.

[0003] Among the many studies exploring how to effectively form heterostructures, liquid phase epitaxy is one of the most effective and common methods. It allows two or more single crystal perovskites to be firmly combined to form a heterostructure. However, such methods still have limitations: 1) Although spatial confinement technology can be adjusted, it is still challenging to grow a specified perovskite at a specified location and has poor versatility; 2) The interface between different perovskites is too obvious to effectively form a gradient heterostructure, which is not conducive to building a built-in electric field with a wider depletion layer. It is worth noting that planar heterojunctions with a transition layer thickness of less than 1μm can still extract photogenerated charges within an effective distance, but planar heterojunctions with sharp interfaces are still not the best choice for X-ray detection. Therefore, it is still challenging to construct tightly integrated gradient perovskite heterojunctions to improve X-ray detection. Summary of the invention

[0004] The object of the present invention is to provide a gradient perovskite heterojunction and a preparation method and application thereof. The preparation method can prepare a gradient perovskite heterojunction that is tightly integrated and has the advantages of rapidity, strong universality and high adhesion.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing a gradient perovskite heterojunction, comprising the following steps:

[0007] Under the condition of applying a reverse bias electric field, the ITO glass and the ITO glass with the perovskite single crystal fixed thereon are placed relative to each other in an ethanol solution of AX and then subjected to a first crystallization to obtain a perovskite gradient layer, and then the reverse bias electric field is switched to a forward bias electric field to perform a second crystallization to obtain the gradient perovskite heterojunction;

[0008] A in the AX is an organic cation or an inorganic cation, and X in the AX is a halogen anion.

[0009] Preferably, the organic cation is MA + and / or FA + ;

[0010] The inorganic cation is Cs + .

[0011] Preferably, the perovskite single crystal in the ITO glass having the perovskite single crystal fixed thereon is a two-dimensional perovskite.

[0012] Preferably, the general formula of the two-dimensional perovskite single crystal is A' 2 B'X' 4 ;

[0013] Where A' is PEA + or FPEA + ; B' is Pb + Sn + Or Bi + ; X' is a halogen anion.

[0014] Preferably, the concentration of AX in the ethanol solution of AX is 0.02 to 1.5 mol / L.

[0015] Preferably, the voltage of the reverse bias electric field increases linearly and uniformly within a range of 1 to 15 V; and the time of the first crystallization is ≤ 60 min.

[0016] Preferably, the voltage of the forward bias electric field increases linearly and uniformly within a range of 1 to 15 V, and the time for applying the forward bias electric field is ≤10 min.

[0017] Preferably, the gradient perovskite heterojunction is a gradient perovskite heterojunction formed between a two-dimensional perovskite and a three-dimensional perovskite;

[0018] The general formula of the three-dimensional perovskite is ABX 3 ;

[0019] Wherein, A is an organic cation or an inorganic cation; B is Pb + Sn+ Or Bi + ; X is a halogen anion.

[0020] The present invention also provides a gradient perovskite heterojunction prepared by the preparation method described in the above technical solution.

[0021] The present invention also provides the application of the gradient perovskite heterojunction described in the above technical solution in the field of X-ray detection.

[0022] The present invention provides a method for preparing a gradient perovskite heterojunction, comprising the following steps: using an electric field driven ion deposition method, under the condition of applying a reverse bias electric field, placing an ITO glass and an ITO glass fixed with a perovskite single crystal relative to each other in an ethanol solution of AX, and then performing a first crystallization to obtain a perovskite gradient layer, and then switching the reverse bias electric field to a forward bias electric field for a second crystallization to obtain the gradient perovskite heterojunction. A in the AX is an organic cation or an inorganic cation, and X in the AX is a halogen anion. Since the ion exchange process usually occurs rapidly within a few seconds after the perovskite single crystal enters the ethanol solution of the AX, applying a reverse bias electric field first can repel or delay the ion exchange process. During the crystallization process, the perovskite single crystal carries a positive charge and can repel all cations in the solution (such as Figure 2 As shown), the cations in the solution will be enriched at the end far away from the perovskite single crystal, and gradually decrease at the end close to the perovskite single crystal, thereby forming a perovskite gradient layer with a gradually increasing cation content in the direction away from the perovskite single crystal; thereby avoiding the cations from combining too quickly with the inorganic skeleton (the anions remaining after the cations in the perovskite single crystal are dissolved by ethanol), resulting in the problem of poor crystal quality. In the present invention, during the first crystallization process, what is deposited on the surface of the perovskite single crystal is not a new pure perovskite layer (the new pure perovskite can be understood as a perovskite that is different from the perovskite single crystal), but a gradient layer of new pure perovskite and perovskite single crystal. At the same time, because the negative charge of the ITO glass itself will attract cations, the cations will be more inclined to penetrate into the interior of the perovskite, that is, the location of the ITO glass; because the cations will be enriched at one end close to the perovskite single crystal, the higher concentration of MA + The penetration ability of cations into perovskite will also be enhanced, which will not only increase the thickness of the gradient layer of new pure perovskite and perovskite single crystal on the surface of the perovskite monomer, but also make the new pure perovskite firmly attached to the two-dimensional perovskite. + It can penetrate deeper, resulting in the formation of a gradient perovskite heterojunction that is tightly integrated, rapid, and has high adhesion. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1It is a schematic diagram of the structure during the preparation process of the gradient perovskite heterojunction of the present invention;

[0024] Figure 2 Schematic diagram of the preparation principle of the gradient perovskite heterojunction of the present invention;

[0025] Figure 3 MAPbI 3 、(FPEA) 2 PbI 4 , XRD patterns of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1;

[0026] Figure 4 It is a schematic diagram of the structure of the tensile testing device;

[0027] Figure 5 The results of the firmness test of the gradient perovskite heterojunction described in Example 1;

[0028] Figure 6 The SEM images and EDS images of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1;

[0029] Figure 7 Schematic diagram of the internal structure of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1;

[0030] Figure 8 Schematic diagram of the energy level structure of the gradient perovskite heterojunction described in Example 1;

[0031] Fig. 9 For (FPEA) 2 PbI 4 Two-dimensional perovskite, MAPbI 3 The bulk resistivity of the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1;

[0032] Fig.10 For (FPEA) 2 PbI 4 Two-dimensional perovskite, MAPbI 3 1 / f noise of the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1;

[0033] Fig.11 For (FPEA) 2 PbI 4 Photoconductivity (μτ) test results of the two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1;

[0034] Fig.12 For (FPEA) 2 PbI 4The two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 at 500 V·cm -1 On / off signal current response under large electric fields;

[0035] Fig.13 MAPbI 3 SCLC test results of the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1;

[0036] Fig.14 For (FPEA) 2 PbI 4 The sensitivity of the two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 under different electric fields;

[0037] Fig.15 MAPbI 3 The three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 were tested at 28.2 nGy·s -1 The current response to low-dose rate X-rays;

[0038] Fig.16 The stability test of the gradient perovskite heterojunction described in Example 1;

[0039] Fig.17 A single pixel imaging system developed based on the gradient perovskite heterojunction described in Example 1;

[0040] Fig.18 The actual picture of the nut under light and the Fig.17 The single-pixel imaging system performs imaging results of different ray intensities;

[0041] Fig.19 For (FPEA) 2 PbI 4 Two-dimensional perovskite, MAPbI 3 Schematic diagram of the structure of a heterojunction device prepared by a three-dimensional perovskite and a gradient perovskite heterojunction as described in Example 1;

[0042] Fig. 20 This is a physical picture of the gradient perovskite heterojunction prepared in Examples 1 to 4. DETAILED DESCRIPTION

[0043] The present invention provides a method for preparing a gradient perovskite heterojunction, comprising the following steps:

[0044] By using an electric field driven ion deposition method (EFID), under the condition of applying a reverse bias electric field, the ITO glass and the ITO glass fixed with the perovskite single crystal are placed relative to each other in an ethanol solution of AX for the first crystallization to obtain a perovskite gradient layer, and then the reverse bias electric field is switched to a forward bias electric field for the second crystallization to obtain the gradient perovskite heterojunction (such as Figure 1 shown);

[0045] A in the AX is an organic cation or an inorganic cation, and X in the AX is a halogen anion.

[0046] In the present invention, unless otherwise specified, all preparation raw materials are commercially available products well known to those skilled in the art.

[0047] The present invention does not have any special limitation on the distance between the relatively arranged ITO glass and the ITO glass with the perovskite single crystal fixed thereon, and can be adjusted using a distance known to those skilled in the art. In an embodiment of the present invention, the distance between the ITO glass and the ITO glass with the perovskite single crystal fixed thereon can be 1 cm.

[0048] In the present invention, the perovskite single crystal in the ITO glass having the perovskite single crystal fixed thereon is preferably a two-dimensional perovskite.

[0049] In the present invention, the general formula of the two-dimensional perovskite single crystal is preferably A' 2 B'X' 4 ; wherein A' is preferably PEA + or FPEA + ; B' is preferably Pb + Sn + Or Bi + ; X 'is preferably a halogen anion, more preferably I - , Cl - or Br - .

[0050] In the present invention, the preparation method of the ITO glass fixed with a perovskite single crystal preferably includes: fixing the perovskite single crystal on the ITO glass using ultraviolet glue and sealing. The present invention does not have any special restrictions on the type of the ultraviolet glue, and the one familiar to those skilled in the art can be used. The present invention does not have any special restrictions on the sealing process, and the process familiar to those skilled in the art can be used. After the sealing is completed, the present invention also preferably includes using gallium to increase the electrical contact between the perovskite and the ITO glass to ensure that the perovskite crystal is in contact with the ITO, and to ensure that the crystal can be powered on in the subsequent power-on step. In the present invention, the role of the sealing is to prevent unnecessary contact between the solution and the edge of the crystal; the role of the gallium enhancement is to ensure that the electric field can be well applied to the entire perovskite crystal.

[0051] In the present invention, the concentration of AX in the ethanol solution of AX is preferably 0.02 to 1.5 mol / L, more preferably 0.3 to 1.2 mol / L. In an embodiment of the present invention, the concentration of AX in the ethanol solution of AX may be 0.4 mol / L.

[0052] In the present invention, A in AX is an organic cation or an inorganic cation, preferably an organic cation, more preferably MA + and / or FA + ; The inorganic cation is preferably Cs + ; X in the AX is a halogen anion, preferably 1 - , Cl - or Br - In the present invention, the organic cation A can better ensure the growth of the corresponding type of perovskite in the epitaxial layer during subsequent applications.

[0053] In the present invention, the voltage value can avoid the epitaxial layer and the surface of the perovskite single crystal being damaged due to excessive voltage.

[0054] In the present invention, since the ion exchange process usually occurs rapidly within a few seconds after the perovskite single crystal enters the ethanol solution of AX, applying a reverse bias electric field first can repel or delay the ion exchange process.

[0055] In the present invention, the first crystallization time is preferably ≤ 60 minutes, more preferably 30 to 50 minutes. In the present invention, the first crystallization process is preferably a uniform linear increase of the reverse bias electric field in the range of 1 to 15V. In an embodiment of the present invention, the first crystallization time can be 35 minutes, and the increasing rate of the reverse bias electric field can be 0.067V·s -1 .

[0056] In the present invention, during the first crystallization process, the two-dimensional perovskite is positively charged and can repel all cations in the solution (such as Figure 2 As shown), the cations in the solution will be enriched at the end far from the perovskite and gradually decrease at the end close to the perovskite; thereby avoiding the problem of too fast binding of the cations to the inorganic skeleton, resulting in poor crystal quality. In the present invention, during the crystallization process, what is deposited on the surface of the perovskite single crystal is not a new pure perovskite layer, but a gradient layer of a new pure perovskite layer and a perovskite single crystal (the gradient layer refers to the ratio of the two perovskites in a certain layer gradually changing along the thickness direction).

[0057] In the present invention, the voltage of the forward bias electric field is preferably uniformly linearly increased in the range of 1 to 15 V. The present invention does not have any special restrictions on the increasing rate of the reverse bias electric field. In the present invention, the time for applying the forward bias electric field is preferably ≤ 10 min. In an embodiment of the present invention, the time for applying the forward bias electric field can be 5 min, and the increasing rate of the forward bias electric field can be 0.467 V·s -1 .

[0058] In the present invention, during the application of the forward bias electric field, the cations in the solution will approach the perovskite single crystal substrate under the action of the electric field, and the forward bias electric field can further enhance the penetration of the cations into the lattice of the perovskite single crystal (e.g. Figure 2 shown).

[0059] In the present invention, the gradient perovskite heterojunction is preferably a gradient perovskite heterojunction formed between a two-dimensional perovskite and a three-dimensional perovskite; the general formula of the three-dimensional perovskite is preferably ABX 3 ; wherein A is an organic cation or an inorganic cation, preferably an organic cation, more preferably MA + and / or FA + ; B is Pb + Sn + Or Bi + ; X is a halogen anion, preferably I - , Cl - or Br - In the present invention, the general formula of the two-dimensional perovskite single crystal is preferably A' 2 B'X' 4 ; wherein A' is preferably PEA + or FPEA + ; B' is preferably Pb + Sn + Or Bi + ; X 'is preferably a halogen anion, more preferably I - , Cl - or Br - .

[0060] The present invention also provides a gradient perovskite heterojunction prepared by the preparation method described in the above technical solution.

[0061] The present invention also provides the application of the gradient perovskite heterojunction described in the above technical solution in the field of X-ray detection. The present invention does not have any special limitation on the method of the application, and the method well known to those skilled in the art can be used.

[0062] The gradient perovskite heterojunction provided by the present invention and its preparation method and application are described in detail below in conjunction with the embodiments, but they should not be understood as limiting the scope of protection of the present invention.

[0063] Comparative Example 1

[0064] Use UV glue to (FPEA) 2 PbI 4 The two-dimensional perovskite single crystal is fixed on the ITO glass and sealed, and then the gallium-enhanced (FPEA) is used. 2 PbI 4 Electrical contact between the two-dimensional perovskite single crystal and the ITO glass to obtain the ITO glass with the perovskite single crystal fixed thereon;

[0065] The ITO glass with the fixed perovskite single crystal was immersed in an ethanol solution of MAI (concentration of 0.4 mol / L) for 60 min to obtain a gradient perovskite heterojunction (the internal structure schematic diagram is shown in FIG. Figure 7 shown).

[0066] Example 1

[0067] Use UV glue to (FPEA) 2 PbI 4 The two-dimensional perovskite single crystal is fixed on the ITO glass and sealed, and then the gallium-enhanced (FPEA) is used. 2 PbI 4 Electrical contact between the two-dimensional perovskite single crystal and the ITO glass to obtain the ITO glass with the perovskite single crystal fixed thereon;

[0068] Under the condition of applying a reverse bias electric field, the ITO glass and the ITO glass with the perovskite single crystal fixed thereon are placed opposite to each other (with a spacing of 1 cm) in an ethanol solution of MAI (with a concentration of 0.4 mol / L) and then crystallized (the voltage of the reverse bias electric field is uniformly increased linearly in the range of 1 to 15 V, with an increasing rate of 0.067 V·s -1 ) After 35 minutes, the reverse bias electric field is switched to a forward bias electric field (the voltage of the forward bias electric field increases linearly and uniformly in the range of 1 to 15 V, and the increasing rate is 0.467 V·s -1 ) for 5 min to obtain the gradient perovskite heterojunction (the general formula of the three-dimensional perovskite in the gradient perovskite heterojunction formed between the two-dimensional perovskite and the three-dimensional perovskite is MAPbI 3 , the internal structure diagram is as follows Figure 7 As shown by Figure 7It can be seen that the dependence of the gradient perovskite heterojunction described in Comparative Example 1 on ion activity leads to a thinner gradient layer that is more prone to defects. In contrast, the gradient perovskite heterojunction described in Example 1 firmly combines the three-dimensional perovskite and the two-dimensional perovskite, thickens the gradient layer, and optimizes the energy band structure of the heterojunction; the energy level structure schematic diagram is shown in Figure 8 As shown by Figure 8 It can be seen that when irradiated, electron-hole pairs are generated and separated under the action of the electric field. Holes flow to the Au electrode through the valence band maximum (VBM) of the perovskite, while electrons flow to the Ga electrode through the conduction band minimum (CBM) of the 3D perovskite. The tilted valence band in the graded heterojunction enhances charge collection by promoting the flow of electrons and holes in the wide depletion layer, thereby improving charge separation and collection efficiency).

[0069] The physical picture of the gradient perovskite heterojunction and the physical picture under 532nm ultraviolet flashlight excitation are shown in Fig. 20 As shown in a, Fig. 20 As can be seen from a in the figure, the actual image of the gradient perovskite heterojunction and the actual image under 532nm ultraviolet flashlight excitation are shown in Fig. 20 As shown in a, Fig. 20 As can be seen from a in the figure, the gradient perovskite heterojunction emits green light under the excitation of a 532nm ultraviolet flashlight;

[0070] Figure 3 MAPbI 3 、(FPEA) 2 PbI 4 , the XRD patterns of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1, Figure 3 It can be seen that the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Document 1 both have MAPbI 3 The characteristic peaks of MAPbI 3 However, compared with the gradient perovskite heterojunction described in Example 1, the perovskite heterojunction described in Comparative Document 1 clearly shows some impurity peaks and exposes (FPEA) 2 PbI 4 The characteristic peaks (2θ≈21.8°, 27.2°) of + ion permeability and effectively increased the MAPbI 3 Furthermore, by controlling the direction of the electric field to tune the deposition rate, MAPbI 3 The crystal quality has also been improved, resulting in fewer defects;

[0071] We evaluate the robustness of the heterojunction by using tapes of different viscosities, where the tension of the tape is measured using a spring dynamometer. During the test, the crystal at one end is adhered to the tape, while the other end is firmly fixed to the lower end of the spring dynamometer. When the spring dynamometer is slowly lifted, the forces applied to the crystal are roughly balanced. Considering that the weight of the crystal is negligible, the tension of the tape on the bottom of the crystal will be balanced by the elastic force of the spring, and this force value will be displayed on the spring dynamometer (the schematic diagram of the device is shown in the figure). Figure 4 shown). Figure 5 The results of the test on the firmness of the gradient perovskite heterojunction described in Example 1 are as follows: Figure 5 It can be seen that the heterojunction obtained by the EFID method (Example 1) did not detach even under a tape pull of about 10N, indicating that it has high firmness. In contrast, the heterojunction obtained by the immersion method (Comparative Example 1) began to fall off under a tape pull of 3N and completely detached at about 10N. Therefore, the firmness of the heterojunction prepared by the electrodeposition method is better than that of the heterojunction prepared by the immersion method;

[0072] Figure 6 The SEM images and EDS images of the gradient perovskite heterojunction described in Example 1 and the perovskite heterojunction described in Comparative Example 1 are shown in FIG. Figure 6 It can be seen that through the SEM image, it can be directly observed that after treatment with the same concentration of MAI solution, the surface of the heterojunction prepared by the EFID strategy is more uniform and smooth, and the thickness of the 3D / 2D gradient layer is also larger. Fluorine (F) is a characteristic element of organic cations in two-dimensional perovskites, and the fluorine-lead ratio (F / Pb) and iodine-lead ratio (I / Pb) are used to jointly detect the diffusion of two-dimensional and three-dimensional structures. On the 3D / 2D cross-section, compared with the simple immersion method, the F / Pb ratio and I / Pb ratio under the EFID strategy decrease more slowly, showing the characteristics of a gradient heterojunction. This gradient heterojunction can effectively increase the thickness of the depletion layer, improve charge separation and transport, and inhibit ion migration under a larger working bias. EDS scanning further shows that the thickness of the pure 3D perovskite prepared by the immersion method is much smaller than the thickness obtained by the EFID strategy. This can be attributed to the limited ion diffusion capacity of the immersion method, which cannot provide sufficient driving force for ion exchange;

[0073] Fig. 9 For (FPEA) 2 PbI 4 Two-dimensional perovskite, MAPbI 3 The heterojunction device prepared by the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19, a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1 (including an ITO glass layer, a Ga layer, a gradient perovskite heterojunction layer (corresponding to the 2D perovskite layer and the 3D perovskite layer in the figure) and an Ag layer stacked in sequence), and b is (FPEA) 2 PbI 4 Two-dimensional perovskite or MAPbI 3 Heterojunction device prepared by three-dimensional perovskite (including ITO glass layer, Ga layer, (FPEA) stacked in sequence) 2 PbI 4 Two-dimensional perovskite layer or MAPbI 3 The volume resistivity of the three-dimensional perovskite layer (corresponding to the Controlperovskite layer in the figure) and the Ag layer) is given by Fig. 9 It can be seen that MAPbI 3 The volume resistivity of the three-dimensional perovskite is 1.60×1011Ω·cm; the volume resistivity of the gradient perovskite heterojunction described in Example 1 is 8.84×10 10 Ω·cm, slightly lower, but still lower than 3D perovskite (5.59×10 8 Ω·cm) is more than two orders of magnitude higher.

[0074] Fig.10 For (FPEA) 2 PbI 4 Two-dimensional perovskite, MAPbI 3 The heterojunction device prepared by the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is (FPEA) 2 PbI 4 Two-dimensional perovskite or MAPbI 3 The 1 / f noise of the heterojunction device prepared by three-dimensional perovskite is given by Fig.10 It can be seen that for X-ray detectors, higher volume resistivity means lower noise current, which improves the signal-to-noise ratio and provides a better detection limit, so its 1 / f noise is measured. Fig.10 It is confirmed that the ultra-low noise of the gradient perovskite heterojunction described in Example 1 is much lower than that of the three-dimensional perovskite;

[0075] Fig.11 For (FPEA) 2 PbI 4 The heterojunction device prepared by the two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19, a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is (FPEA) 2 PbI 4 The photoconductivity (μτ) test results of the heterojunction device prepared by two-dimensional perovskite are given by Fig.11 It can be seen that the μτ product of the gradient perovskite heterojunction described in Example 1 is 1.21×10 -4 cm 2 ·V - 1, higher than the control device's 2.67×10 -5 cm 2 ·V - 1;

[0076] Fig.12 For (FPEA) 2 PbI 4 The heterojunction device prepared by the two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is (FPEA) 2 PbI 4 Heterojunction devices made from two-dimensional perovskite) at 500V·cm -1 The on / off signal current response under large electric field is given by Fig.12 It can be seen that the signal current of the gradient perovskite heterojunction in Example 1 is significantly higher than that of pure (FPEA) 2 PbI 4 The signal current of the two-dimensional perovskite preliminarily confirms the results of the μτ product;

[0077] Defect density measurement is an important criterion for determining device noise in low-dose probing. We use the space charge limited current (SCLC) technique to evaluate the defect density of different devices. Fig.13 MAPbI 3 The heterojunction device prepared by the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is MAPbI 3 SCLC test results of heterojunction devices prepared from three-dimensional perovskites); Fig.13 It can be seen that Fig.13 The SCLC curve shows three different regions, including the Ohmic region, the TFL region, and the Child region, which determine the defect density and carrier mobility. The defect density of the gradient perovskite heterojunction device described in Example 1 is 1.18×10 9 cm -3 , while the defect density of 3D perovskite devices is 2.25×1010 cm -3 , indicating the superiority of heterojunction devices in terms of defect density. Although the carrier mobility of heterojunction single crystals is about 7.4 cm 2 ·V -1 ·s -1 , lower than the expected 21.7 cm for 3D perovskites 2 ·V -1 ·s -1 , heterojunction devices have higher signal-to-noise ratios than their 2D and 3D perovskite counterparts, and thus have the potential to achieve higher signal-to-noise ratios and lower detection limits;

[0078] Fig.14 For (FPEA) 2 PbI 4 The heterojunction device prepared by the two-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is (FPEA) 2 PbI 4 The sensitivity of heterojunction devices prepared by two-dimensional perovskite under different electric fields is determined by Fig.14 It can be seen that the net response current is highly sensitive to the dose rate change under different electric fields. The response of the gradient perovskite heterojunction in Example 1 is significantly higher than that of pure (FPEA) 2 PbI 4 The response of the two-dimensional perovskite. By fitting the slope of the response current density and the applied dose rate, it is found that the heterojunction single crystal detector is -1 The sensitivity under electric field strength is as high as 21225μC·Gy -1 cm -2 , compared with (FPEA) 2 PbI 4 Two-dimensional perovskite (6717μC·Gy -1 cm -2 ) increased by 243%. This excellent X-ray sensitivity can be attributed to the built-in electric field width enlarged by the gradient heterojunction. Due to its high bulk resistivity and low defect density, the perovskite heterojunction detector exhibits excellent detection limit and ion migration suppression capabilities;

[0079] Fig.15 MAPbI 3 The heterojunction device prepared by the three-dimensional perovskite and the gradient perovskite heterojunction described in Example 1 (the specific structural schematic diagram is shown in FIG. Fig.19 , a is the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1, and b is MAPbI 3 Heterojunction devices prepared from three-dimensional perovskite) at 28.2nGy·s-1 The current response to low-dose rate X-rays is given by Fig.15 It can be seen that even under a bias voltage of 50 V, MAPbI 3 The three-dimensional perovskite has almost no signal response, while the gradient perovskite heterojunction described in Example 1 can still generate significant signal current even at a high bias voltage of 200V;

[0080] Fig.16 The stability test of the heterojunction device prepared by the gradient perovskite heterojunction described in Example 1 is performed by Fig.16 It can be seen that the heterojunction device shows excellent stability in harsh external environments. -1 Under high electric field, irradiated with 120kV hard X-rays with a total dose of 2.5Gy, it can still output stable signal current, which is significantly better than the three-dimensional perovskite control group under the same conditions;

[0081] Evaluation Fig.17 The X-ray imaging capability of the single pixel imaging system (developed based on the gradient perovskite heterojunction described in Example 1), in which the gradient perovskite heterojunction described in Example 1 is used as a sensing pixel and can be continuously moved in the XY direction. After recording the current value of each pixel point, the computer generates and processes the image, Fig.18 The actual picture of the nut under light and the Fig.17 The imaging results of the single pixel imaging system at different ray intensities are as follows: Fig.18 It can be seen that at 578μGy·s -1 The device is able to obtain very clear images at higher dose rates. According to the definition of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the dose rate is less than 0.1mGy·min -1 (1.67μGy·s -1 ) is considered to be a low dose rate and will not cause substantial pathological harm to human health. Even if the dose rate is reduced to 8.57 μGy·s -1 and 1.31 μGy·s -1 , the outline of the nut can still be identified, indicating that this heterojunction device has potential in the field of low-dose imaging.

[0082] Example 2

[0083] Referring to Example 1, the difference is that the ethanol solution of MAI is replaced by the ethanol solution of FAI to obtain a gradient perovskite heterojunction; the (FPEA) 2 PbI 4 The physical picture of the two-dimensional perovskite single crystal, the gradient perovskite heterojunction and the physical picture of the gradient perovskite heterojunction after annealing at 150°C for 30 minutes are shown in FIG. Fig. 20 As shown in b, Fig. 20 From the b in the figure, we can see that at room temperature, that is, the newly generated FAPbI 3 The yellow δ-FAPbI 3 Phase, also known as yellow phase, is relatively stable, but has poor performance. After annealing at 150°C, the yellow δ phase transforms into the α phase, also known as the black phase, which is the phase truly used for perovskite X-ray detectors. This phase transition also indirectly proves that FAPbI is indeed generated. 3 .

[0084] Example 3

[0085] Reference Example 1, the difference is: (FPEA) 2 PbI 4 Two-dimensional perovskite single crystal replaced by MAPbBr 3 crystal, replacing the ethanol solution of MAI with the ethanol solution of FAI to obtain a gradient perovskite heterojunction; the MAPbBr 3 The physical picture of the crystal and the physical picture of the gradient perovskite heterojunction are as follows Fig. 20 As shown in c.

[0086] Example 4

[0087] Reference Example 1, the difference is: (FPEA) 2 PbI 4 Two-dimensional perovskite single crystal replaced by CsPbBr 3 crystal, replacing the ethanol solution of MAI with the ethanol solution of PEAI to obtain a gradient perovskite heterojunction; the CsPbBr 3 The physical picture of the crystal, the physical picture of the gradient heterojunction, and the physical picture of the gradient heterojunction under 532nm ultraviolet flashlight excitation are as follows: Fig. 20 As shown in d, by Fig. 20 From the d in, we can know that (PEA) 2 PbB 4 It emits blue light, while the base perovskite CsPbBr 3 It emits green light.

[0088] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a gradient perovskite heterojunction, characterized in that: The following steps are involved: Under the condition of applying a reverse bias electric field, the ITO glass and the ITO glass with the perovskite single crystal fixed thereon are placed relative to each other in an ethanol solution of AX and then subjected to a first crystallization to obtain a perovskite gradient layer, and then the reverse bias electric field is switched to a forward bias electric field to perform a second crystallization to obtain the gradient perovskite heterojunction; A in the AX is an organic cation or an inorganic cation, and X in the AX is a halogen anion.

2. The preparation method according to claim 1, characterized in that The organic cation is MA + and / or FA + ; The inorganic cation is Cs + .

3. The preparation method according to claim 2, characterized in that: The perovskite single crystal in the ITO glass fixed with the perovskite single crystal is a two-dimensional perovskite.

4. The preparation method according to claim 1 or 3, characterized in that: The general formula of the two-dimensional perovskite single crystal is A'2B'X'4; Where A' is PEA + or FPEA + ; B' is Pb + Sn + Or Bi + ; X' is a halogen anion.

5. The preparation method according to claim 1, characterized in that: The concentration of AX in the ethanol solution of AX is 0.02-1.5 mol / L.

6. The preparation method according to claim 1, characterized in that: The voltage of the reverse bias electric field increases linearly and uniformly within the range of 1 to 15 V; and the time of the first crystallization is ≤ 60 min.

7. The preparation method according to claim 1, characterized in that: The voltage of the forward bias electric field increases linearly and uniformly within the range of 1 to 15 V, and the time for applying the forward bias electric field is ≤10 min.

8. The preparation method according to any one of claims 3 to 7, characterized in that: The gradient perovskite heterojunction is a gradient perovskite heterojunction formed between a two-dimensional perovskite and a three-dimensional perovskite; The general formula of the three-dimensional perovskite is ABX3; Wherein, A is an organic cation or an inorganic cation; B is Pb + Sn + Or Bi + ; X is a halogen anion.

9. The gradient perovskite heterojunction prepared by the preparation method according to any one of claims 1 to 8.

10. Application of the gradient perovskite heterojunction according to claim 9 in the field of X-ray detection.

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