A perovskite thin film with a three-layer heterostructure, a photovoltaic device and a preparation method thereof

Abstract

This invention relates to a perovskite thin film with a three-layer heterostructure, a photovoltaic device, and a method for fabricating the same. The thin film has a 2D / 2D-3D / 2D three-layer heterostructure, consisting of a bottom 2D layer, a middle 2D-3D composite layer, and a top 2D layer, arranged from bottom to top. The bottom and top 2D perovskites are made of at least one of DJ phase and RP phase. The middle 2D-3D composite layer is a bulk 2D-3D continuous structure with the chemical formula C2A. m‑1 B m X 3m+1 and DA n‑ 1B n X 3n+1 At least one of the following: The preparation process employs reduced-pressure drying combined with segmented annealing to achieve interlayer energy level gradient matching and interface densification; in the photovoltaic device, the obtained perovskite thin film is used as the core photoelectric active layer (light-absorbing layer) to achieve efficient generation and bidirectional non-destructive extraction of photogenerated carriers. This invention achieves a dual improvement in thin film stability and photoelectric performance, making it suitable for large-scale mass production.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor optoelectronic materials and devices, specifically relating to a method for fabricating a perovskite thin film with a 2D / 2D-3D / 2D three-layer heterostructure and its inverted photovoltaic device. It is particularly suitable for the fabrication of high-efficiency inverted (pin) perovskite solar cells, and can also be extended to the field of perovskite light-emitting diodes and other optoelectronic devices. Background Technology

[0002] Perovskite solar cells have become a research hotspot in next-generation photovoltaic technology due to their high photoelectric conversion efficiency and low fabrication cost. Especially in inverted (pin) structure devices, their ease of fabrication with crystalline silicon in tandem cells demonstrates broad commercial application prospects. However, perovskite thin films contain numerous uncoordinated defects within and at their interfaces, making them susceptible to ion migration and component decomposition caused by external factors such as moisture, oxygen, and heat. This leads to severe non-radiative recombination and rapid performance degradation in the devices.

[0003] In existing technologies, methods to improve the stability and efficiency of perovskite thin films mainly include introducing two-dimensional (2D) perovskite for dimensional engineering modification. Although there are existing solutions that utilize dimensional engineering or interface modification to improve performance, for example, patent CN114566603B discloses a structure using a single RP-phase two-dimensional perovskite as the top passivation layer for unilateral modification. Its main purpose is to seal surface defects and block water and oxygen to improve environmental stability. However, this solution cannot simultaneously meet the requirements of top interface passivation and bottom buried interface recombination suppression. Simply increasing the thickness of the two-dimensional layer will lead to obstructed vertical carrier transport and a surge in interface contact resistance due to the van der Waals gaps between layers. Patent CN111525034B discloses a structure that directly blends 2D and 3D perovskite materials to form a mixed dimensional bulk phase, aiming to simultaneously achieve the high stability of two-dimensional materials and the high absorbance of three-dimensional materials. However, due to the differences in crystallization kinetics of cations of different sizes in the precursor solution, and the lack of spatial gradient confinement guidance, the phase distribution inside this randomly mixed material structure is disordered, resulting in weak energy level matching and binding force with the underlying transport layer. Under long-term photothermal stress, phase separation and rapid degradation of device performance are highly likely to occur. None of the above methods can fundamentally overcome the technical bottlenecks of the mutual constraints between "double-sided full passivation," "high structural resistance," and "efficient vertical charge extraction," nor can they meet the stringent process requirements for lossless interfacial transport in inverted structure devices. Summary of the Invention

[0004] This invention addresses the problems of numerous interface defects, hindered unilateral passivation transport, and poor environmental stability in existing inverted perovskite solar cells by providing a perovskite thin film with a three-layer heterostructure, a photovoltaic device, and its fabrication method. The thin film employs a specific dimensional 2D / 2D-3D / 2D three-layer gradient heterojunction stack design, specifically limiting the bottom layer to a double-sided physical encapsulation formed on a DJ-phase 2D perovskite structure without interlayer van der Waals gaps. The fabrication process utilizes reduced-pressure drying (1-10 Pa) combined with segmented annealing to achieve interlayer energy level gradient matching and interface densification. In the photovoltaic device, the obtained perovskite thin film serves as the core photoelectric active layer (light-absorbing layer) to achieve efficient generation and bidirectional non-destructive extraction of photogenerated carriers. This invention achieves a dual improvement in thin film stability and photoelectric performance, making it suitable for large-scale mass production.

[0005] The technical solution of this invention is as follows: A perovskite thin film, wherein the structure of the thin film is a 2D / 2D-3D / 2D three-layer heterostructure, consisting of a bottom 2D layer, a middle 2D-3D composite layer, and a top 2D layer from bottom to top; Among them, the bottom and top 2D perovskites are made of the same or different materials, and are at least one of Dion-Jacobson (DJ) phase and Ruddlesden-Popper (RP) phase; Its general chemical formula can be represented as C2BX4 (RP phase) or DBX4 (DJ phase); C is a monoamino organic cation; D is a diamino organic cation, which is bonded to the inorganic layer through hydrogen bonds at both ends, without interlayer van der Waals gaps; B is a divalent metal cation, selected from lead ions (Pb). 2+ ) and / or tin ions (Sn 2+ ); X is a halide anion, specifically an iodide ion (I). - ), bromide ions (Br) - ), chloride ions (Cl) - One or more combinations of ).

[0006] The intermediate 2D-3D composite layer has a bulk 2D-3D continuous structure with the chemical formula C2A. m-1 B m X 3m+1 and DA n-1 B n X 3n+1 At least one of the following (n≥2, m≥2 integers) is formed by organic cations and inorganic perovskite layers connected by chemical bonds; Wherein, C is a monoamino organic cation selected from phenylethylammonium cation (PEA). + ) or butylammonium cation (BA) + At least one of the following; D is a divalent diamino organic cation, selected from 1,3-propanediamine cation (PDA). 2+ ) or 3-aminomethylpyridine cation (3AMPY) 2+ At least one of the following; A is a small-sized monovalent cation selected from methylamine cations (MA). + ), formamidinium cation (FA) + ) or cesium cation (Cs + At least one of the following; B is a divalent metal cation, selected from lead ions (Pb). 2+ ) and / or tin ions (Sn 2+ ); X is a halide anion, selected from iodide ions (I₂). - ), bromide ions (Br) - ), chloride ions (Cl) - One or more combinations of ).

[0007] The thickness of the bottom 2D layer is 2~20 nm; the thickness of the middle 2D-3D composite layer is 300~800 nm; and the thickness of the top 2D layer is 2~20 nm.

[0008] The method for preparing the perovskite thin film includes the following steps: (1) Fabrication of the bottom 2D layer: After spin-coating the 2D perovskite precursor solution onto the substrate, the substrate is annealed at 40~120℃ for 10~20 min to form the bottom 2D thin film. In the D perovskite precursor solution, the solute is a C or D halide and a divalent metal halide (BX2), and the total solute concentration is 0.05 ~ 0.5 mol / L; the solvent is a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). In step (1), the spin coating speed is 1000~5000 rpm and the time is 10~60s.

[0009] The substrate is a conductive glass substrate, a flexible polymer substrate, or the aforementioned substrate with a hole transport layer deposited on its surface; (2) Preparation of the intermediate 2D-3D composite layer: The 2D-3D perovskite precursor solution was spin-coated (using a two-step spin-coating method: the first step was at a speed of 500-2000 rpm for 5-15 s; the second step was at a speed of 2000-6000 rpm for 20-60 s) onto the surface of the underlying 2D film obtained in the previous step. Immediately after spin-coating, the film was transferred to a vacuum drying chamber, and the pressure was controlled at 1-10 Pa for 30-200 s to accelerate solvent evaporation and induce initial crystallization of perovskite. Subsequently, it was placed on a heating stage and annealed at 60-170℃ for 5-30 min to form a continuous intermediate bulk phase structure, thus obtaining the intermediate 2D-3D composite layer. The 2D-3D perovskite precursor solution contains a halide of A, BX2, and a halide of 2D cations; the 2D cation is C or D; the concentration of B is 1.0 ~ 1.5 mol / L, and the molar amount of 2D cations added is 1% ~ 10% of the B ions. (3) Fabrication of the top 2D layer: A solution of long-chain organic ammonium salt in isopropanol was coated on the surface of the intermediate 2D-3D composite layer. The long-chain cations underwent an in-situ ion exchange reaction with the three-dimensional perovskite on the surface of the intermediate layer. After annealing at 40-120℃ for 10-20 min, a dense top 2D film was obtained in situ, which is the 2D / 2D-3D / 2D three-layer heterostructure perovskite film. The isopropanol solution for preparing the long-chain organic ammonium salt has a concentration of 0.004 ~ 0.02 mol / L; the long-chain organic ammonium salt is an ammonium halide salt of an organic cation C or D; In step (3), the spin coating speed is 2000~4000 rpm and the time is 20~40 s; A photovoltaic device with an inverted structure, comprising, from bottom to top, a transparent conductive substrate, a hole transport layer, the aforementioned three-layer heterostructure perovskite thin film, an electron transport layer, a cathode interface layer, and a top electrode; The transparent conductive substrate is ITO or FTO glass and a flexible polymer substrate; The hole transport layer is at least one of NiOx, self-assembled monolayer (SAM), or PTAA, with a thickness of 0.5~50 nm; The electron transport layer is at least one of C60, SnO2, PCBM or TPBI, and has a thickness of 10~50 nm. The cathode interface layer is a BCP with a thickness of 2~10 nm; The top electrode is a metal (at least one of copper, gold, and aluminum) or a transparent conductive oxide (at least one of ITO, IZO, and AZO) with a thickness of 50~200 nm.

[0010] A method for fabricating a photovoltaic device based on the above-mentioned perovskite thin film, the method comprising the following steps: Step 1, substrate and hole transport layer preparation: The transparent conductive substrate is cleaned and pretreated, and then a hole transport layer is prepared on its surface. Step 2, Preparation of the intermediate 2D-3D composite layer: On the surface of the hole transport layer obtained in the previous step, the intermediate 2D-3D composite layer is prepared according to the perovskite thin film preparation method described above. Step 3: Fabrication of the electron transport layer; Step four: Preparation of the cathode interface layer; Step 5: Electrode preparation; The inverted structure device was finally obtained.

[0011] The essential features of this invention are: This invention aims to solve the core problem of severe interfacial recombination in perovskite thin films, where conductivity and protection are difficult to achieve simultaneously. It provides a specific three-layer vertical heterogeneous structure: a 2D interfacial passivation layer, a bulk 2D / 3D light-absorbing layer, and a 2D interfacial passivation layer. The inventors discovered that introducing a DJ phase without van der Waals gaps into the bottom layer can produce multiple synergistic effects with the intermediate bulk phase structure and the top RP / DJ phase. 1. Breaking the constraints of double-sided passivation and charge extraction: The bottom DJ phase is connected at both ends of the double amino groups, eliminating the insulation gap of traditional 2D materials. While achieving strong adhesion and full passivation at the bottom buried interface, it opens up the vertical transport channel of holes.

[0012] 2. Synergistic effect of bulk structure and gradient annealing: The intermediate layer is not a simple physical mixture, but a bulk 2D-3D structure connected by chemical bonds. Combined with the segmented annealing process (high temperature for the intermediate layer, low temperature for the upper and lower layers), it ensures that the boundaries between the layers are clear and the connections are tight, forming a progressive energy level arrangement.

[0013] In this invention, the intermediate layer, by controlling the value of n (or m), allows the 2D and 3D phases to form a gradient distribution or alternate growth at the molecular level, thereby constructing a 2D-3D continuous heterojunction system that maintains high light absorption efficiency while suppressing ion migration. The control principle is as follows: it is achieved by adjusting the molar ratio of long-chain organic cations (C or D) to divalent metal cations (B) in the intermediate layer precursor solution; the lower the molar ratio of 2D cations, the larger the value of n (or m) of the continuous inorganic framework layer generated during film crystallization, and the more significant the bulk 3D characteristics; conversely, the higher the addition ratio, the smaller the value of n (or m), and the more significant the 2D layered characteristics.

[0014] Compared with the prior art, the present invention has the following significant advantages: 1. Extremely low interface recombination and significantly improved efficiency: The unique double-sided 2D wrapping achieves simultaneous passivation of the hole and electron bidirectional transport interface, significantly reducing nonradiative recombination losses. The open-circuit voltage (Voc) and fill factor (FF) of the device are greatly improved. Compared with the single-sided passivation technology (Comparative Example 2), the optimal structure of this invention (Example 1) increases the photoelectric conversion efficiency (PCE) from 24.5% to a maximum of 26.6% (an absolute increase of 2.1%); the fill factor (FF) increases from 78.5% to 84.5%; and the open-circuit voltage (Voc) increases from 1.16V to 1.20V (in the wide-bandgap system of Example 4, Voc can reach up to 1.28V). 2. Enhanced intrinsic stability: The upper and lower interface layers composed of DJ / RP phases act as a dense physical barrier, effectively blocking external water and oxygen erosion while inhibiting the migration of internal halide ions; the intermediate phase 2D-3D structure further enhances the intrinsic heat resistance and light decay resistance of the material. For example, the efficiency retention rate of the existing single-sided passivation technology (Comparative Example 2) is only 72%, and the pure 3D unprotected structure (Comparative Example 1) even decays to 55%; the efficiency retention rate of the three-layer encapsulation structure device of the present invention is improved to 94%.

[0015] 3. Reduced contact resistance: By utilizing the unique crystal structure of the DJ phase without gaps, the problem of increased series resistance caused by increasing the thickness of the interface protective layer has been solved, and lossless charge transfer at the interface has been achieved.

[0016] 4. Strong controllability of the preparation process: The step-by-step coating and well-defined gradient annealing process avoids severe interlayer mixing, resulting in good process reproducibility and easy compatibility with existing large-area coating processes. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the physical structure of the 2D / 2D-3D / 2D three-layer heterostructure perovskite photovoltaic device of the present invention; Figure 2 The JV characteristic curves of the inverted perovskite solar cells prepared in the examples and comparative examples are shown.

[0018] Figure 3 This is a comparison chart showing the long-term operational stability of inverted perovskite solar cells prepared in the examples and comparative examples. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0020] Example 1: Bottom DJ phase - intermediate phase - top RP phase (optimal structure) An inverted perovskite photovoltaic device has the following structure: ITO conductive glass / hole transport layer / bottom 2D perovskite (DJ phase) / middle 2D-3D perovskite / top 2D perovskite (RP phase) / C60 electron transport layer / BCP interface layer / metal electrode.

[0021] Its preparation method includes the following steps: 1. Pretreatment: Clean the ITO glass (2cm*2cm) and prepare the SAM hole transport layer (MeO-2PACz composition, approximately 2 nm thick).

[0022] 2. Substrate Preparation: Substrate 2D Layer (DJ Phase) Preparation: Preparation of PDA containing diamino cationic PDA. 2+ A DJ-phase precursor solution of (1,3-propanediamine cation) and PbI2 was prepared, with the total solute concentration controlled at 0.1 mol / L (the anion of PDA is iodide ion I). - The method involves using PDAI2 salt (PDAI2 to PbI2 molar ratio of 1:1) and a DMF:DMSO mixed solvent with a volume ratio of 4:1. This solution is spin-coated onto the surface of the SAM layer (4000 rpm, 30 s) and annealed at 80°C for 15 min to form a tightly bonded, van der Waals-free bottom DJ phase 2D film (the composition of this layer is (PDA)PbI4, with a thickness of approximately 10 nm).

[0023] 3. Preparation of the intermediate layer (bulk 2D-3D): A bulk 2D-3D precursor solution containing FAI, PbI2, and a small amount of PEAI (phenylethyl ammonium iodide) was prepared, wherein the PbI2 concentration was 1.2 mol / L, and the molar addition of PEAI was 5% of PbI2 (FAI concentration 1.2 mol / L, i.e., a molar ratio of 1:1 with PbI2, and the solvent was a DMF:DMSO mixed solvent with a volume ratio of 4:1). This solution was spin-coated onto the surface of the underlying DJ phase film (in two steps: 1000 rpm for 10 s, followed by 4000 rpm for 30 s). Immediately after spin-coating, the film was transferred to a vacuum drying chamber, where the pressure was controlled at 5 Pa and maintained for 100 seconds to accelerate solvent evaporation and induce initial perovskite crystallization. Subsequently, it was placed on a heating stage and annealed at 100℃ for 20 min to form a continuous bulk 2D-3D light-absorbing layer (due to the small amount of PEA). + Due to the confinement effect, this layer forms a continuous structure with large n values, and the average component structure can be expressed as PEA2FA. n-1 Pb n I 3n+1 (n≈20, thickness approximately 500 nm).

[0024] 4. Preparation of the top 2D layer (RP phase): A solution of the long-chain monoamino organic ammonium salt PEAI in isopropanol (concentration of 2 mg / mL) was spin-coated onto the surface of the intermediate layer (4000 rpm, 30 s), followed by annealing at 80 °C for 15 min to obtain the top RP phase 2D film in situ (the main component of this layer is PEA2PbI4, with a thickness of about 10 nm), thus completing the construction of the 2D / 2D-3D / 2D three-layer heterostructure.

[0025] 5. Back-end device fabrication: Vacuum thermal evaporation technology was used to sequentially deposit a 30 nm thick C60 layer, a 5 nm thick BCP buffer layer, and finally a 100 nm thick copper (Cu) metal top electrode on the perovskite thin film surface.

[0026] Example 2: Full DJ Phase Interface Structure Verification The other steps and device structure are the same as in Example 1, except that: 1. Both the bottom and top 2D layers use 3AMPY. 2+ The DJ phase material (3-aminomethylpyridine cation) replaces the original bottom layer PDAI2 (1,3-propanediamine iodide) and top layer PEAI (phenylethyl ammonium iodide). 2. The annealing conditions for the bottom and top layers are adjusted to 100℃ for 10 min; the annealing temperature for the intermediate 2D-3D perovskite layer is increased to 150℃ for 10 min.

[0027] Example 3: Verification of the full RP phase interface structure The device structure is the same as in Example 1, except that: 1. Both the bottom and top 2D layers use BA (Balanced Elements) + The RP phase monoamino material (butylammonium cation) replaces the original PDAI2 (1,3-propanediamine iodide) and the top layer PEAI (phenylethyl ammonium iodide). 2. The annealing conditions for the bottom and top layers are adjusted to 60℃ for 20 min; the annealing temperature for the middle layer is 120℃ for 15 min.

[0028] Example 4: Verification of wide bandgap, mixed halogen systems and semi-transparent devices The device structure is the same as in Example 1, except that: 1. The halogen source in the intermediate layer bulk 2D-3D precursor is replaced with a mixture of I and Br (I 0.8 Br 0.2 This forms a wide-bandgap perovskite light-absorbing layer with a bandgap of approximately 1.65 eV, suitable for top-cell applications in perovskite-crystalline silicon tandem solar cells.

[0029] 2. In step 5, the back-end device fabrication uses an ITO transparent conductive electrode deposited by magnetron sputtering to prepare a semi-transparent perovskite photovoltaic device, which is perfectly suited for the application of perovskite-crystalline silicon tandem solar cells.

[0030] Example 5: Narrow bandgap, lead-tin hybrid system verification The device structure is the same as in Example 1, except that: In the intermediate bulk 2D-3D precursor, the metal cation B site is replaced with a mixture of Pb and Sn (Pb 0.5 Sn 0.5 This forms a narrow bandgap perovskite light-absorbing layer with a bandgap of approximately 1.25 eV.

[0031] Example 6: Low-temperature / low-pressure crystallization process boundary verification The device structure is the same as in Example 1, except that: Strictly adhere to the process limits specified in the claims. The annealing temperature for the bottom and top 2D films was set at 40°C for 20 min. After spin-coating the intermediate 2D-3D perovskite precursor, the reduced pressure drying conditions were controlled at the lower limit pressure of 1 Pa and the upper limit time of 200 seconds. Subsequently, the annealing temperature was set at 60°C for 30 min.

[0032] Example 7: High-temperature / high-pressure crystallization process boundary verification The device structure is the same as in Example 1, except that: Strictly adhere to the process limits specified in the claims. The annealing temperature for the bottom and top 2D films was set at 120°C for 10 min. After spin-coating the intermediate 2D-3D perovskite precursor, the reduced pressure drying conditions were controlled at the upper limit pressure of 10 Pa and the lower limit holding time of 30 seconds. Subsequently, the annealing temperature was set at 170°C for 5 min.

[0033] Comparative Example 1: Pure 3D conventional structure, without upper and lower 2D layers The difference from Example 1 is that steps 2 and 4 are omitted. A pure 3D phase FAPbI3 precursor solution (without 2D cations such as PEAI) is directly spin-coated onto the SAM hole transport layer and annealed at 100°C for 20 min to form a light-absorbing layer. There are no 2D interfaces encapsulating the device.

[0034] Comparative Example 2: Single-sided 2D passivation structure The difference from Example 1 is that step 2 is omitted (no underlying DJ phase 2D layer). The host perovskite is prepared directly on the hole layer, and then PEAI treatment is performed on its surface to form the top RP phase 2D film (i.e., the traditional 3D / 2D structure).

[0035] Comparative Example 3: Mixed-dimensional volumetric structure with no hierarchical distribution The difference from Example 1 is that all cationic precursors (PDA) are used. 2+ PEA + FA + Pb 2+ The components are mixed in the same solution and spin-coated in one step, then annealed at 100°C for 20 min to form a mixed-dimensional bulk thin film with randomly distributed components.

[0036] Performance testing: The perovskite solar cells prepared in the above embodiments and comparative examples were tested using an AM 1.5G solar simulator to provide 100 mW / cm². 2 The photoelectric conversion efficiency (PCE), open-circuit voltage (Voc), and fill factor (FF) of the device were measured under illumination. Simultaneously, the unpackaged device was placed in a damp aging chamber at 85℃ / 85% RH for 500 hours to test its PCE retention rate. The test results are shown in Table 1.

[0037] Photovoltaic performance and stability test data of the examples and comparative examples.

[0038] Table 1: Test data for the examples and comparative examples

[0039] The photovoltaic performance data in Table 1 show that, compared to the randomly blended Comparative Example 3, the significantly improved fill factor (FF) of Example 1 confirms that the bottom DJ phase has opened up vertical charge extraction channels. Simultaneously, its excellent stability under harsh conditions of 85℃ / 85% RH confirms that a dense 2D physically encapsulated phase has formed on both the film surface and bottom. This 2D / 2D-3D / 2D structure, driven by a specific reduced-pressure drying (1-10 Pa) kinetic process and a segmented annealing thermodynamic process, is the physical basis for achieving the superior photoelectric performance described in this invention.

[0040] Data Analysis and Conclusions: 1. Core Advantages of the Three-Layer Heterojunction and the Underlying DJ Phase: Comparing Examples 1-3 and Comparative Examples 1-2, the three-layer encapsulation structure of this invention significantly improves the long-term damp-heat stability of the device. In particular, the introduction of the DJ phase without van der Waals gaps at the bottom layer (Examples 1 and 2) results in a significantly better fill factor (FF > 83%) than the pure RP phase interface (Example 3, FF = 79.2%). This data conclusively proves that the DJ phase achieves efficient vertical lossless hole transport while ensuring excellent adhesion to the bottom surface, with the highest photoelectric conversion efficiency reaching 26.6%, completely overcoming the defect of excessive contact resistance when the traditional RP phase is used as the bottom transport layer.

[0041] 2. Broad applicability to material systems and device structures: Example 4 (wide bandgap mixed halogen) successfully fabricated a semi-transparent device using an ITO transparent electrode, achieving an open-circuit voltage as high as 1.28 V without reflection from a metal back electrode; Example 5 (low-toxicity lead-tin mixture) also demonstrated excellent film quality and relatively high stability. This fully demonstrates that the "2D / 2D-3D / 2D" gradient heterostructure constructed in this invention is not limited to a specific lead-iodine system, but is applicable to the modulation of perovskite materials with various halogens and metal centers. Combined with its excellent compatibility with transparent electrodes, this greatly broadens the application prospects of this technology in tandem solar cells.

[0042] 3. Wide and controllable process window: Both Example 6 (low temperature / low pressure boundary) and Example 7 (high temperature / high pressure boundary) achieved high efficiency of over 24.8% and excellent stability. All core indicators comprehensively suppressed Comparative Example 3 (PCE=24.0%), which used a traditional random mixing process. This strongly verifies that the present invention has extremely high fault tolerance and industrial operability within the annealing temperature range of 40-170℃ and the reduced pressure drying range of 1-10 Pa. Through controllable vacuum desolvation kinetics and temperature gradient control, it can effectively avoid severe interlayer mixing and construct an excellent and continuous energy level arrangement.

[0043] Expansion of implementation methods The preparation method of this invention is not only applicable to rigid ITO / FTO glass substrates, but can also be extended to flexible PI / PET substrates. In addition to being used in single-junction inverted solar cells, the thin film of this invention can also be directly integrated as a top cell in perovskite-crystalline silicon tandem photovoltaic devices.

[0044] Matters not covered in this invention are common knowledge.

Claims

1. A perovskite thin film, characterized in that, The thin film has a 2D / 2D-3D / 2D three-layer heterostructure, consisting of a bottom 2D layer, a middle 2D-3D composite layer, and a top 2D layer from bottom to top. Among them: the bottom and top 2D perovskites are made of the same or different materials, and their general chemical formula is C2BX4 or DBX4; C is a monoamino organic cation; D is a diamino organic cation; B is a divalent metal cation; X is a halide anion; The intermediate 2D-3D composite layer has a bulk 2D-3D continuous structure with the chemical formula C2A. m-1 B m X 3m+1 and DA n- 1B n X 3n+1 At least one of the following, where n≥2 and m≥2 integers; Wherein, C is a monoamino organic cation; D is a divalent diamino organic cation; A is a small-sized monovalent cation; B is a divalent metal cation; X is a halide anion.

2. The perovskite thin film as described in claim 1, characterized in that, In the intermediate 2D-3D composite layer: A is at least one of methylamine cation, formamidinium cation, or cesium cation; In a three-layered heterostructure: B is Pb 2+ and Sn 2+ One or two of them; C is at least one of phenethylammonium cation or butylammonium cation; D is at least one of 1,3-propanediamine cation or 3-aminomethylpyridine cation; X is I - ,Br - Cl - One or more combinations thereof.

3. The perovskite thin film as described in claim 1, characterized in that, The thickness of the bottom 2D layer is 2~20 nm; the thickness of the middle 2D-3D composite layer is 300~800 nm; and the thickness of the top 2D layer is 2~20 nm.

4. The method for preparing perovskite thin films according to claim 1, characterized in that, Includes the following steps: (1) Fabrication of the bottom 2D layer: After spin-coating the 2D perovskite precursor solution onto the substrate, the substrate is annealed at 40~120℃ for 10~20 min to form the bottom 2D thin film. In the D perovskite precursor solution, the solute is a C or D halide and a divalent metal halide (BX2), and the total solute concentration is 0.05 ~ 0.5 mol / L; the solvent is a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). (2) Preparation of the intermediate 2D-3D composite layer: The 2D-3D perovskite precursor solution was spin-coated onto the surface of the underlying 2D film obtained in the previous step. Immediately after spin-coating, the film was transferred to a vacuum drying chamber, and the pressure was controlled at 1~10 Pa for 30~200 s. Then it was placed on a heating stage and annealed at 60~170℃ for 5~30 min to form a continuous intermediate bulk phase structure, thus obtaining the intermediate 2D-3D composite layer. The 2D-3D perovskite precursor solution contains a halide of A, BX2, and a halide of 2D cations; the 2D cation is C or D; the concentration of B is 1.0 ~ 1.5 mol / L, and the molar amount of 2D cations added is 1% ~ 10% of the B ions. (3) Fabrication of the top 2D layer: A solution of long-chain organic ammonium salt in isopropanol was coated on the surface of the intermediate 2D-3D composite layer. The long-chain cations underwent an in-situ ion exchange reaction with the three-dimensional perovskite on the surface of the intermediate layer. After annealing at 40-120℃ for 10-20 min, a dense top 2D film was obtained in situ, which is the 2D / 2D-3D / 2D three-layer heterostructure perovskite film. The isopropanol solution of the long-chain organic ammonium salt is prepared with a concentration of 0.004 ~ 0.02 mol / L; the long-chain organic ammonium salt is a halide ammonium salt of organic cation C or D.

5. The method for preparing perovskite thin films according to claim 4, characterized in that, In step (1), the spin coating speed is 1000~5000 rpm and the time is 10~60s; In step (2), the spin coating adopts a two-step spin coating method: the first step has a rotation speed of 500~2000 rpm and lasts for 5~15 s; the second step has a rotation speed of 2000~6000 rpm and lasts for 20~60 s. In step (3), the spin coating speed is 2000~4000 rpm and the time is 20~40 s.

6. The method for preparing perovskite thin films according to claim 4, characterized in that... The substrate is a conductive glass substrate, a flexible polymer substrate, or the aforementioned substrate with a hole transport layer deposited on its surface.

7. A photovoltaic device, characterized in that, The device has an inverted structure, consisting of, from bottom to top, a transparent conductive substrate, a hole transport layer, a three-layer heterostructure perovskite thin film as described in claim 1, an electron transport layer, a cathode interface layer, and a top electrode.

8. The photovoltaic device as described in claim 7, characterized in that, The transparent conductive substrate is ITO or FTO glass and a flexible polymer substrate; The hole transport layer is at least one of NiOx, self-assembled monolayer (SAM), or PTAA, with a thickness of 0.5~50 nm; The electron transport layer is at least one of C60, SnO2, PCBM or TPBI, and has a thickness of 10~50 nm. The cathode interface layer is a BCP with a thickness of 2~10 nm; The top electrode is a metal (at least one of copper, gold, and aluminum) or a transparent conductive oxide (at least one of ITO, IZO, and AZO) with a thickness of 50~200 nm.

9. The method for fabricating a photovoltaic device as described in claim 7, characterized in that, The method includes the following steps: Step 1, substrate and hole transport layer preparation: The transparent conductive substrate is cleaned and pretreated, and then a hole transport layer is prepared on its surface. Step 2, Preparation of the intermediate 2D-3D composite layer: On the surface of the hole transport layer obtained in the previous step, the intermediate 2D-3D composite layer is prepared according to the perovskite thin film preparation method described in claim 4. Step 3: Fabrication of the electron transport layer; Step four: Preparation of the cathode interface layer; Step 5: Electrode preparation; The inverted structure device was finally obtained.

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