A method for preparing large-sized formamidinium lead iodide perovskite thin films by co-evaporation

Abstract

The present invention belongs to the technical field of perovskite materials, and discloses a method for preparing large-sized formamidinium lead iodide perovskite thin films by co-evaporation. The preparation method includes the following steps: co-evaporating PbI2 and FAI on a substrate to form a perovskite thin film containing a mixed phase of α-FAPbI3 and δ-FAPbI3; then performing vacuum treatment to form a perovskite thin film of pure δ-FAPbI3 phase; and then performing annealing treatment to form a formamidinium lead iodide perovskite thin film of pure α-FAPbI3 phase. The present invention uses vacuum environment to treat the crystal phase transformation, improves the perovskite grain size problem, and transforms it from an unstable mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase; through annealing treatment, it promotes the transformation of larger-sized crystals from δ-FAPbI3 to α-FAPbI3, which is beneficial to improving the efficiency of transporting and collecting carriers and enhancing the energy conversion efficiency of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of perovskite materials, and particularly relates to a method for preparing large-sized formamidinium lead iodide perovskite thin films by a co-evaporation method. Background Art

[0002] Metal halide perovskite solar cells have become a research hotspot among researchers due to their excellent optoelectronic properties and low cost. Among them, the "black photoactive" cubic phase formamidinium lead iodide (α-FAPbI3) is often used in the preparation of light-absorbing materials in perovskite solar cells because of its narrow optical bandgap (Eg) and good thermal stability. However, the lattice distortion caused by the large ionic size of FA + will cause α-FAPbI3 to easily transform into the "yellow non-photoactive" δ-FAPbI3, thereby damaging the photoelectric conversion efficiency of the solar cell. Therefore, the preparation of high-quality FAPbI3 (formamidinium lead iodide) perovskite thin films should avoid the formation of the δ-FAPbI3 phase. However, in the process of preparing FAPbI3 thin films by the co-evaporation method, if the thin film after evaporation is directly transformed from an unstable mixed phase of α-FAPbI3 and δ-FAPbI3 into a more stable α-FAPbI3 phase, the prepared thin film has a smaller grain size, and there is more δ-FAPbI3 in the thin film that has not been completely transformed into α-FAPbI3. To improve this problem, Xie Weiguang et al. found that the addition of an appropriate amount of water molecules can effectively reduce the energy barrier between the α-FAPbI3 phase and the δ-FAPbI3 phase, thereby promoting the formation of α-FAPbI3 (Advanced Functional Materials, 2022: 2208392). However, this method will also be accompanied by the formation of an intermediate phase of 6H-FAPbI3 and 4H-FAPbI3, thereby reducing the formation rate of α-FAPbI3, making the perovskite thin film exhibit poor crystallinity, and further affecting the photoelectric conversion efficiency of the perovskite solar cell.

[0003] Therefore, in the preparation process of high-efficiency FAPbI3-based perovskite solar cells, the formation of the intermediate phase of the FAPbI3 perovskite thin film should be avoided, the crystallinity of the perovskite thin film should be improved, and further the performance of the perovskite solar cell should be improved. Summary of the Invention

[0004] In view of the technical problem that in the process of preparing FAPbI3 perovskite thin films by the co-evaporation method at present, it is difficult to control the purity of the perovskite phase, resulting in poor crystallinity of the thin films, the present invention provides a method for preparing large-size formamidinium lead iodide perovskite thin films by the co-evaporation method. This preparation method adopts a one-step co-evaporation method and combines vacuum treatment and annealing treatment processes to prepare a stable high-crystallinity α-FAPbI3 thin film. When the α-FAPbI3 thin film of the present invention is applied to a solar cell, it has excellent photovoltaic performance.

[0005] The inventive concept of the present invention is as follows: First, based on the one-step co-evaporation method, PbI2 and FAI exist in the gas phase simultaneously and are uniformly mixed and reacted to prepare a FAPbI3 perovskite thin film with a mixed phase containing α-FAPbI3 and δ-FAPbI3. Then, deposition is carried out for a sufficient time under vacuum conditions. On the one hand, there are not too many water molecules in the FAPbI3 perovskite thin film under vacuum conditions to affect the phase transition path of α-FAPbI3, which can effectively avoid the problem of impure phase of the FAPbI3 perovskite thin film. On the other hand, by appropriately extending the vacuum treatment time, the perovskite is completely transformed from an unstable mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase, ensuring that there is no residue of the δ-FAPbI3 phase. Finally, annealing treatment is carried out to promote the transformation of larger crystals from the δ-FAPbI3 phase to the α-FAPbI3 phase. The prepared FAPbI3 perovskite thin film has the characteristics of large size and high crystallinity.

[0006] To solve the above technical problem, the first aspect of the present invention provides a method for preparing a formamidinium lead iodide perovskite thin film, comprising the following steps:

[0007] (1) Co-evaporate PbI2 and FAI on a substrate to form a perovskite thin film with a mixed phase containing α-FAPbI3 and δ-FAPbI3;

[0008] (2) Subject the perovskite thin film with the mixed phase containing α-FAPbI3 and δ-FAPbI3 to vacuum treatment to form a perovskite thin film with a pure δ-FAPbI3 phase;

[0009] (3) Subject the perovskite thin film with the pure δ-FAPbI3 phase to annealing treatment to form the formamidinium lead iodide perovskite thin film with a pure α-FAPbI3 phase.

[0010] Preferably, in step (1), the evaporation temperature of PbI2 is 305 - 315 °C.

[0011] Preferably, in step (1), the evaporation temperature of FAI is 155 - 165 °C.

[0012] Preferably, in step (1), the evaporation rate ratio of PbI2 and FAI is (1 - 2.2):1. It has been found that the evaporation rate ratio of PbI2 and FAI has a great influence on the performance of the formamidinium lead iodide perovskite thin film. As the evaporation rates of PbI2 and FAI decrease, the open-circuit voltage of the perovskite solar cell will decrease significantly, and the non-radiative recombination of carriers in the perovskite solar cell will gradually intensify, resulting in a decrease in the photoelectric conversion efficiency.

[0013] Preferably, in step (2), the air pressure of the vacuum treatment is lower than 1×10 -3 Torr.

[0014] Preferably, in step (2), the time of the vacuum treatment is 10 - 14 hours. After the "brown" perovskite thin film containing the α-FAPbI3 and δ-FAPbI3 mixed phases is subjected to vacuum treatment, it becomes a "yellow" pure δ-FAPbI3 phase perovskite thin film.

[0015] Preferably, in step (3), the temperature of the annealing treatment is 150 - 160 °C.

[0016] Preferably, in step (3), the time of the annealing treatment is 18 - 23 min.

[0017] Preferably, in step (3), the environmental humidity of the annealing treatment is lower than 25%. By reducing the environmental humidity, the influence of water molecules on the phase transition path of α-FAPbI3 can be reduced, thereby improving the purity of the α-FAPbI3 phase.

[0018] Preferably, in step (1), the substrate is FTO conductive glass coated with an electron transport layer on its surface.

[0019] Preferably, the electron transport layer is a metal oxide.

[0020] Preferably, the metal oxide thin film is a SnO2 layer, and the SnO2 layer is deposited on the surface of the FTO conductive glass by chemical deposition.

[0021] The second aspect of the present invention provides a formamidinium lead iodide perovskite thin film, which is prepared by the above preparation method, and the perovskite grain size in the formamidinium lead iodide perovskite thin film is distributed between 0.4 - 1 μm. The perovskite crystals in the formamidinium lead iodide perovskite thin film prepared by the preparation method of the present invention are larger in size and have the characteristics of high crystallinity.

[0022] The third aspect of the present invention provides a solar cell, which includes the above formamidinium lead iodide perovskite thin film.

[0023] The above technical solution of the present invention has at least the following technical effects or advantages over the prior art:

[0024] (1) During the preparation of the formamidinium lead iodide calcium titanate film of the present invention, a one-step co-evaporation method is first adopted, so that PbI2 and FAI exist in the gas phase at the same time and are uniformly mixed and reacted to obtain a FAPbI3 perovskite film with a mixed phase containing α-FAPbI3 and δ-FAPbI3; then it is deposited under vacuum conditions for a period of time to completely transform the perovskite from an unstable mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase; finally, annealing treatment is carried out to promote the transformation of larger crystals from the δ-FAPbI3 phase to the α-FAPbI3 phase. The prepared formamidinium lead iodide calcium titanate film has the characteristics of large size and high crystallinity, and good repeatability, and is suitable for the preparation of large-size films.

[0025] (2) The present invention uses a vacuum environment to treat the crystal form transformation, improves the perovskite grain size problem, and transforms it from an unstable mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase; at the same time, annealing treatment is carried out to promote the transformation of larger crystals from δ-FAPbI3 to α-FAPbI3, which is beneficial to improving the efficiency of transporting and collecting carriers and enhancing the energy conversion efficiency of the battery.

[0026] (3) The perovskite film prepared by the present invention has no holes and high phase purity. When it is applied to a solar cell, the open-circuit voltage can reach 1116 mV, the short-circuit current density can reach 23.69 mA / cm 2 , the fill factor can reach 0.753, and the forward-scan photoelectric conversion efficiency can reach 19.9%. Description of the Drawings

[0027] Figure 1 SEM images and particle size statistical charts of the FAPbI3 perovskite films prepared in Example 1 and Comparative Example 1;

[0028] Figure 2 XRD patterns of the FAPbI3 perovskite films prepared in Example 1 and Comparative Examples 1-2;

[0029] Figure 3 Current-voltage characteristic curves of the perovskite solar cells prepared in Example 1 and Comparative Examples 1-2;

[0030] Figure 4 Current-voltage characteristic curve of the perovskite solar cell prepared in Comparative Example 3;

[0031] Figure 5 Current-voltage characteristic curve of the perovskite solar cell prepared in Comparative Example 4;

[0032] Figure 6 Performance evolution diagrams of the perovskite solar cells prepared in Example 1 and Comparative Examples 1-2. Detailed implementation manners

[0033] The present invention will be specifically described below in conjunction with examples to facilitate the understanding of the present invention by those skilled in the art. It is necessary to specifically point out here that the examples are only used to further illustrate the present invention and cannot be construed as limiting the protection scope of the present invention. Those skilled in the art, based on the above-mentioned inventive content, make non-essential improvements and adjustments to the present invention, which should still fall within the protection scope of the present invention. At the same time, for the raw materials not specifically described below, they are all commercially available products; for the process steps or preparation methods not specifically mentioned, they are all process steps or preparation methods known to those skilled in the art.

[0034] Example 1

[0035] A method for preparing a formamidinium lead iodide perovskite thin film, comprising the following steps:

[0036] (1) Take 1.096 g of SnO2·2H2O solid, 5 g of urea, 5 mL of concentrated hydrochloric acid, 100 μL of mercaptoacetic acid and 400 mL of deionized water in a wide-mouth Shuni bottle, fully oscillate and mix evenly to obtain a SnO2 stock solution, and store it in a refrigerator for refrigeration. After aging for 12 hours, it can be used;

[0037] (2) Place the cleaned FTO conductive glass in a UV cleaning machine and irradiate it with ultraviolet light for 15 min; subsequently, take 20 mL of the SnO2 stock solution and 100 mL of deionized water; stir evenly, and then place it in an ultrasonic cleaning machine for ultrasonic treatment for 2 min to obtain a diluted SnO2 solution; Place the FTO conductive glass after UV cleaning face up in a glass box, and add the diluted SnO2 solution; wrap the glass box with plastic wrap and place it in an oven at 90 °C for 3 hours; then, wash the surface of the FTO conductive glass with deionized water and dry the glass surface with nitrogen; finally, place the FTO conductive glass on a hot plate at 180 °C and keep it warm for 1 hour. After the FTO conductive glass cools to room temperature, place it in a clean box and store it for later use;

[0038] (3) Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the SnO2-coated substrate prepared in step (2) at an evaporation rate ratio of 2:1 to form a perovskite thin film containing a mixed phase of α-FAPbI3 and δ-FAPbI3; then place the perovskite thin film containing a mixed phase of α-FAPbI3 and δ-FAPbI3 under a pressure lower than 1×10 -3Torr evaporator for 12 hours; in a vacuum environment, the mixed phase of α-FAPbI3 and δ-FAPbI3 is transformed into a stable pure δ-FAPbI3 phase perovskite film;

[0039] (4) The pure δ-FAPbI3 phase perovskite film obtained in step (3) is placed on a heating platform at 155° C. and annealed for 20 min to form a pure α-FAPbI3 phase formamidine lead iodide perovskite film.

[0040] A method for preparing a perovskite solar cell comprises the following steps:

[0041] (1) Weigh 500 mg of Li-TFSI, dissolve it in 1 mL of acetonitrile, shake it thoroughly to mix it evenly, and prepare it as lithium salt for later use; weigh 520 mg of trivalent cobalt-based complex FK209, dissolve it in 1 mL of acetonitrile, shake it thoroughly to mix it, and prepare it as cobalt salt for later use; weigh 73 mg of Spiro-OMeTAD powder and dissolve it in 1 mL of chlorobenzene, add 18 μL of lithium salt, 29 μL of cobalt salt and 30 μL of 4-tert-butylpyridine (TBP) respectively, and shake the solution on a shaker for 30 min;

[0042] (2) Using a pipette, 20 μL of the prepared Spiro-OMeTAD solution was drawn and evenly coated on the surface of the sample obtained in step (1); then, spin-coated at a speed of 3000 rpm / s for 30 seconds to prepare a Spiro-OMeTAD hole transport layer;

[0043] (3) Place the sample in step (2) into the mask plate and use a vacuum coating machine to evacuate the chamber to a vacuum of 1×10 - 3 Torr, and An 80nm gold electrode was evaporated at an evaporation rate to obtain a perovskite solar cell.

[0044] Example 2

[0045] A method for preparing a formamidinium lead iodine perovskite film comprises the following steps:

[0046] (1) 1.096 g of SnO2·2H2O solid, 5 g of urea, 5 mL of concentrated hydrochloric acid, 100 μL of thioglycolic acid and 400 mL of deionized water were placed in a wide-mouthed Shu Niu bottle, shaken and mixed thoroughly to obtain a SnO2 stock solution, which was then stored in a refrigerator and aged for 12 hours before use;

[0047] (2) Place the cleaned FTO conductive glass in a UV cleaning machine and irradiate it with ultraviolet light for 15 min. Subsequently, take 20 mL of the SnO2 stock solution and 100 mL of deionized water. After stirring evenly, place it in an ultrasonic cleaning machine and ultrasonically treat it for 2 min to obtain a diluted SnO2 solution. Place the FTO conductive glass after UV cleaning face up in a glass box, and add the diluted SnO2 solution. Wrap the glass box with plastic wrap and place it in an oven at 90 °C for 3 hours. Then, wash the surface of the FTO conductive glass with deionized water and dry the glass surface with nitrogen. Finally, place the FTO conductive glass on a hot plate at 180 °C and keep it warm for 1 hour. After the FTO conductive glass cools to room temperature, place it in a clean box for storage and standby;

[0048] (3) Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the substrate coated with SnO2 prepared in step (2) at an evaporation rate ratio of 1:1 to form a perovskite film containing a mixed phase of α-FAPbI3 and δ-FAPbI3. Then place the perovskite film containing the mixed phase of α-FAPbI3 and δ-FAPbI3 in an evaporator with a pressure lower than 1×10 -3 Torr for 10 hours; in a vacuum environment, promote the transformation of the mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase perovskite film;

[0049] (4) Place the pure δ-FAPbI3 phase perovskite film prepared in step (3) on a heating plate at 150 °C and anneal it for 18 min to form a pure α-FAPbI3 phase formamidinium lead iodide perovskite film.

[0050] The preparation method of the perovskite solar cell in Example 2 is the same as that in Example 1.

[0051] Example 3

[0052] A preparation method of a formamidinium lead iodide perovskite film, comprising the following steps:

[0053] (1) Take 1.096 g of SnO2·2H2O solid, 5 g of urea, 5 mL of concentrated hydrochloric acid, 100 μL of mercaptoacetic acid, and 400 mL of deionized water in a wide-mouth Shuni bottle. After fully shaking and mixing evenly, obtain the SnO2 stock solution, and store it in a refrigerator for refrigeration. After aging for 12 hours, use it again;

[0054] (2) Place the cleaned FTO conductive glass in a UV cleaning machine and irradiate it with ultraviolet light for 15 minutes. Subsequently, take 20 mL of the original SnO2 solution and 100 mL of deionized water. After stirring evenly, place it in an ultrasonic cleaning machine and ultrasonically treat it for 2 minutes to obtain a diluted SnO2 solution. Place the FTO conductive glass after UV cleaning face up in a glass box, and add the SnO2 diluted solution. Wrap the glass box with plastic wrap and place it in an oven at 90 °C for 3 hours. Then, wash the surface of the FTO conductive glass with deionized water and dry the glass surface with nitrogen. Finally, place the FTO conductive glass on a hot plate at 180 °C and keep it warm for 1 hour. After the FTO conductive glass cools to room temperature, place it in a clean box for storage and standby.

[0055] (3) Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the SnO2-coated substrate prepared in step (2) at an evaporation rate ratio of 2.2:1 to form a perovskite film containing a mixed phase of α-FAPbI3 and δ-FAPbI3. Then place the perovskite film containing a mixed phase of α-FAPbI3 and δ-FAPbI3 in an evaporator with a pressure lower than 1×10 -3 Torr for 14 hours. In a vacuum environment, promote the transformation of the mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase perovskite film.

[0056] (4) Place the pure δ-FAPbI3 phase perovskite film prepared in step (3) on a heating plate at 160 °C and anneal it for 23 minutes to form a pure α-FAPbI3 phase formamidinium lead iodide perovskite film.

[0057] The preparation method of the perovskite solar cell in Example 3 is the same as that in Example 1.

[0058] Comparative Example 1

[0059] The difference between Comparative Example 1 and Example 1 is only that in the preparation process of the formamidinium lead iodide perovskite film, the step of vacuum treatment is not included in step (3). Specifically: Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the SnO2-coated substrate prepared in step (2) at an evaporation rate ratio of 2:1 to form a perovskite film containing a mixed phase of α-FAPbI3 and δ-FAPbI3.

[0060] Comparative Example 2

[0061] The difference between Comparative Example 2 and Example 1 is only that in the preparation process of the formamidinium lead iodide calcium titanate thin film, in step (3), the vacuum treatment is replaced by a high-humidity treatment, specifically: Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the SnO2-coated substrate prepared in step (2) at an evaporation rate ratio of 2:1 to form a perovskite thin film containing a mixed phase of α-FAPbI3 and δ-FAPbI3; then transfer the perovskite thin film containing the mixed phase of α-FAPbI3 and δ-FAPbI3 to an acrylic glove box with a humidity of 80%, and wait for 5 - 10 min until the color of the perovskite thin film turns "yellow".

[0062] Comparative Example 3

[0063] The difference between Comparative Example 3 and Example 1 is only that in the preparation process of the formamidinium lead iodide calcium titanate thin film, the evaporation rate ratio in step (3) is smaller, specifically: Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the SnO2-coated substrate prepared in step (2) at an evaporation rate ratio of 0.75:1 to form a perovskite thin film containing a mixed phase of α-FAPbI3 and δ-FAPbI3; then place the perovskite thin film containing the mixed phase of α-FAPbI3 and δ-FAPbI3 in an evaporation apparatus with a pressure lower than 1×10 -3 Torr for 12 hours; in a vacuum environment, promote the transformation of the mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable pure δ-FAPbI3 phase perovskite thin film.

[0064] Comparative Example 4

[0065] The difference between Comparative Example 4 and Example 1 is only that in the preparation process of the formamidinium lead iodide calcium titanate thin film, the vacuum treatment time in step (3) is shorter, specifically: Weigh an appropriate amount of PbI2 and FAI (the mass of PbI2 is 1.2 g), and co-evaporate them on the SnO2-coated substrate prepared in step (2) at an evaporation rate ratio of 2:1 to form a perovskite thin film containing a mixed phase of α-FAPbI3 and δ-FAPbI3; then place the perovskite thin film containing the mixed phase of α-FAPbI3 and δ-FAPbI3 in an evaporation apparatus with a pressure lower than 1×10 -3 Torr for 6 hours; in a vacuum environment, promote the transformation of the mixed phase of α-FAPbI3 and δ-FAPbI3 into a stable δ-FAPbI3 phase perovskite thin film.

[0066] Performance Test

[0067] 1. Microstructure

[0068] Figure 1 SEM images and particle size distribution diagrams of the FAPbI3 perovskite thin films prepared for Example 1 and Comparative Example 1, fromFigure 1 As can be seen from the comparison, the crystal size of the perovskite Figure 1 c) treated by vacuum is significantly larger than that of the perovskite Figure 1 a) without vacuum treatment, and the compactness of the film is also significantly improved. The average grain size of Example 1 is 615nm( Figure 1 d), which is also significantly larger than the average grain size of 333nm( Figure 1 b) of Comparative Example 1.

[0069] 2. XRD Test

[0070] Figure 2 The XRD patterns of the FAPbI3 perovskite films prepared in Example 1 and Comparative Examples 1-2 are shown in Figure 2 where the abscissa 2Theta represents the 2θ diffraction angle, the ordinate Intensity represents the intensity of the diffraction peak, and Ref, 80%RH, and vacuum represent the XRD patterns of the FAPbI3 perovskite films prepared in Comparative Example 1, Comparative Example 2, and Example 1, respectively. From Figure 2 it can be seen that the full width at half maximum (FWHM) value of Comparative Example 1 under atmospheric pressure conditions is the largest, indicating the worst crystallinity of the film. Comparative Example 2 with high humidity has the lowest FWHM value, but there are small amounts of 4H-FAPbI3, 6H-FAPbI3, and PbI2 in the film, indicating that the purity of the perovskite phase is not high. Example 1 has a lower FWHM value than Comparative Example 1, indicating that the vacuum condition is beneficial to improving the crystallinity of the film, which further confirms the Figure 1 results. In addition, no diffraction peaks other than α-FAPbI3 were observed in Example 1, indicating that there are no other secondary phases in the perovskite.

[0071] 3. Photovoltaic Performance

[0072] Figure 3 The current-voltage characteristic curves of the perovskite solar cells prepared in Example 1 and Comparative Examples 1-2 are shown in Figure 3 a, Figure 3 b, and Figure 3 c represent the current-voltage characteristic curves of the perovskite solar cells prepared in Comparative Example 1, Comparative Example 2, and Example 1, respectively. The specific results are shown in Table 1. Among them: Voc represents the open-circuit voltage, Jsc represents the short-circuit current density, Fill Factor represents the fill factor, Efficienty represents the photoelectric conversion efficiency, Reverse Scan (RS) represents the forward scan, and ForwardScan (FS) represents the reverse scan.

[0073] Table 1:

[0074]

[0075] As can be seen from Figure 3 Table 1, all photovoltaic properties of the perovskite solar cells prepared in Example 1 are superior to those in Comparative Examples 1-2. The photoelectric conversion efficiency can reach 19.9% during the forward scan. However, under non-vacuum conditions or high humidity conditions, the photoelectric conversion efficiency decreases. Meanwhile, all photovoltaic properties of the perovskite solar cells prepared in Examples 2-3 are comparable to those in Example 1.

[0076] Figures 4 - 5 They are the current-voltage characteristic curves (forward scan) of the perovskite solar cells prepared in Comparative Examples 3-4, and the specific results are shown in Table 2.

[0077] Table 2:

[0078]

[0079] As can be seen from Figures 4 - 5 Table 2, as the evaporation rates of PbI2 and FAI decrease, the open-circuit voltage (V OC oc) of the perovskite solar cells decreases significantly, indicating that the non-radiative recombination of carriers in the perovskite solar cells gradually intensifies. Therefore, the photoelectric conversion efficiency of the perovskite solar cells continuously decreases as the evaporation rates of PbI2 and FAI decrease. When the vacuum treatment time is shortened, the "brown" perovskite film containing the mixed phase of α-FAPbI3 and δ-FAPbI3 fails to be completely transformed into the "yellow" pure δ-FAPbI3 phase perovskite film, which limits the photoelectric conversion efficiency of the perovskite solar cells. This shows that the performance of the formamidinium lead iodide perovskite film of the present invention is closely related to each preparation parameter.

[0080] 4. Evolution of cell performance

[0081] Figure 6 It is the performance evolution diagram of the perovskite solar cells prepared in Example 1 and Comparative Examples 1-2. Figure 6 It shows that the solar cell prepared in Example 1 (vacuum group) has the highest open-circuit voltage (VOC), indicating that the perovskite solar cells prepared by the present invention have the lowest non-radiative recombination of carriers. In addition, the solar cell has the lowest hysteresis index (H-index). The decrease of this value indicates that the ion migration inside the perovskite film in the vacuum group is alleviated and the density of defect states is reduced. This shows that the photoelectric conversion efficiency (PCE) of the solar cells in the vacuum group is the highest.

[0082] For those of ordinary skill in the art to which the present invention pertains, without departing from the concept of the present invention, several simple deductions or substitutions can be made without the need for creative labor. Therefore, any simple improvements made by those skilled in the art based on the disclosure of the present invention should fall within the protection scope of the present invention. The above embodiments are the preferred embodiments of the present invention, and all processes similar to the present invention and equivalent changes made thereto shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a formamidinium lead iodide calcium titanate thin film, characterized in that, It includes the following steps: (1) Co-evaporate PbI2 and FAI on a substrate to form a perovskite film containing a mixed phase of α-FAPbI3 and δ-FAPbI3; (2) Vacuum-treat the perovskite film containing the α-FAPbI3 and δ-FAPbI3 mixed phase to form a pure δ-FAPbI3 phase perovskite film; the air pressure of the vacuum treatment is lower than 1×10 -3 Torr, and the time of the vacuum treatment is 10 - 14 hours; (3) Anneal the pure δ-FAPbI3 phase perovskite film to form the formamidinium lead iodide perovskite film of pure α-FAPbI3 phase.

2. The preparation method of the formamidinium lead iodide calcium titanate thin film according to claim 1, wherein, In step (1), the evaporation temperature of the PbI2 is 305 - 315 °C; and / or, the evaporation temperature of the FAI is 155 - 165 °C.

3. The preparation method of the formamidinium lead iodide calcium titanate thin film according to claim 2, wherein, The evaporation rate ratio of the PbI2 and FAI is (1 - 2.2):

1.

4. The preparation method of the formamidinium lead iodide calcium titanate thin film according to claim 1, characterized in that, In step (3), the temperature of the annealing treatment is 150 - 160 °C; and / or, the time of the annealing treatment is 18 - 23 min.

5. The preparation method of the formamidinium lead iodide calcium titanate thin film according to claim 1, wherein In step (3), the environmental humidity of the annealing treatment is lower than 25%.

6. The preparation method of the formamidinium lead iodide calcium titanate thin film according to claim 1, wherein In step (1), the substrate is FTO conductive glass coated with an electron transport layer on its surface.

7. The preparation method of the formamidinium lead iodide calcium titanate thin film according to claim 6, characterized in that, The electron transport layer is a metal oxide film.

8. A formamidinium lead iodide calcium titanate thin film, characterized in that, The formamidinium lead iodide perovskite film is prepared by the preparation method according to any one of claims 1 - 7, and the perovskite grain size in the formamidinium lead iodide perovskite film is distributed between 0.4 - 1 μm.

9. A perovskite solar cell, characterized in that, The perovskite solar cell includes the formamidinium lead iodide perovskite film according to claim 8.

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