A method for preparing a pure gamma-CsPbI3 thin film on an ITO surface

Abstract

The application discloses a method for preparing a pure gamma-CsPbI3 thin film on an ITO surface, which comprises the following steps: taking ITO as a substrate, preparing a SnO2 thin film on the surface of the substrate by spin coating SnO2 aqueous solution, spin coating a perovskite solution, and performing annealing treatment, wherein the volume ratio of SnO2 colloid to deionized water in the SnO2 aqueous solution is 3-7:1. The SnO2 thin film prepared by adding a layer of SnO2 aqueous solution with a specific concentration between the ITO and the CsPbI3 can (1) effectively block the high-temperature chemical reaction between the ITO and DMAI in the perovskite spin coating solution during the preparation process, inhibit the generation of holes of the CsPbI3 on the ITO substrate, and ensure the compactness of the thin film; and (2) inhibit the induced phase change of the ITO to the CsPbI3, prevent the generation of an intermediate phase in the thin film during the preparation process, and prevent the rapid transformation of the intermediate phase into a yellow delta phase without photoactivity; so that the photoelectric conversion efficiency of the solar cell prepared by taking the ITO as the substrate is improved to 14.02%.

Description

Technical Field

[0001] This invention relates to the field of perovskite thin film preparation technology, and specifically to a method for preparing pure γ-CsPbI3 thin films on ITO surfaces. Background Technology

[0002] Compared to organic-inorganic hybrid perovskite solar cells, all-inorganic CsPbI3 perovskite solar cells exhibit better thermal stability and lower ion mobility. However, the black α phase of CsPbI3 is unstable at room temperature, spontaneously transforming into the non-photoactive yellow δ phase. This phase instability is essentially due to the presence of cesium ions (CsPbI3). + The size is too small to support three-dimensional PbI. 3- Framework. Large lattice strain caused by ion size mismatch will transform the CsPbI3 lattice structure from a three-dimensional (3D) perovskite phase to a one-dimensional (1D) non-perovskite phase. Therefore, improving lattice symmetry and controlling lattice strain are crucial for addressing phase instability. Currently, the most widely used and effective strategy is to introduce the volatile additive DMAI into the precursor. Existing research reports that DMAI functions by lowering the crystallization energy barrier and slowing down the rapid crystallization process, thereby obtaining high-quality CsPbI3 films. Furthermore, the addition of DMAI can effectively reduce energy consumption (by significantly lowering the crystallization temperature).

[0003] Compared to perovskite photovoltaic devices based on FTO substrates, ITO-based perovskite photovoltaic devices have the significant advantage of being able to be fabricated at low temperatures on flexible substrates, thus ITO plays an irreplaceable role in flexible electronic devices. However, a rapid phase transition phenomenon in perovskite occurred during the fabrication of CsPbI3 solar cells on ITO substrates. Experiments confirmed that this phenomenon is due to a high-temperature chemical reaction between the additive DMAI and ITO. This reaction causes CsPbI3 on ​​the ITO substrate to exhibit a porous morphology and generate an intermediate phase, which then rapidly transforms into the yellow δ phase. This problem greatly limits the film formation and device fabrication of CsPbI3 on ​​ITO substrates, making its application on flexible substrates highly unfavorable. To better apply CsPbI3 in perovskite solar cells on ITO and flexible substrates, exploring its high-temperature phase transition mechanism and finding suitable blocking strategy layers have significant theoretical and practical value. Summary of the Invention

[0004] The present invention aims to provide a method for preparing pure γ-CsPbI3 thin films on ITO surfaces. This method effectively prevents ITO-induced phase transitions in CsPbI3, suppresses the formation of intermediate phases or conversion to non-photoactive δ phases in the film, thereby ensuring the excellent performance of CsPbI3 thin films prepared on ITO substrates.

[0005] Another objective of this invention is to provide a method for fabricating a perovskite solar cell with ITO as a substrate.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for preparing pure γ-CsPbI3 thin films on ITO surfaces, characterized in that: an ITO substrate is used as the substrate, a SnO2 thin film is first prepared by spin-coating an aqueous SnO2 solution onto its surface, then a perovskite solution is spin-coated, and then annealing is performed, wherein the volume ratio of SnO2 colloid to deionized water in the SnO2 aqueous solution is 3 to 7:1.

[0008] During the preparation of CsPbI3 perovskite films, it was found that the expected pure γ-CsPbI3 phase could not be obtained when preparing CsPbI3 films on ITO surfaces suitable for flexible substrates. An intermediate phase was present during the annealing process. Due to the presence of the intermediate phase, the film was subsequently transformed into a yellow δ phase without photoactivity.

[0009] This invention prepares a SnO2 film on the ITO surface using a SnO2 aqueous solution of a specific concentration before preparing the CsPbI3 film. This prevents the diffusion of DMAI generated during the preparation process into the ITO substrate, thus preventing the high-temperature reaction between DMAI and ITO during annealing, which would otherwise result in voids and reduced film density. These steps prevent the ITO-induced formation of an intermediate phase in CsPbI3, thus preventing a phase transition in CsPbI3 and ultimately producing a pure γ-CsPbI3 film.

[0010] Preferably, the volume ratio of SnO2 colloid to deionized water in the above SnO2 aqueous solution is 5 to 7:1.

[0011] Furthermore, the above-mentioned spin-coated SnO2 aqueous solution is prepared by dropping 100 μL of SnO2 aqueous solution onto the ITO surface, spin-coating at 4000 rpm for 30 s, and then annealing at 150 °C for 30 min.

[0012] Furthermore, the annealing temperature is 210℃ and the annealing time is 3 to 5 minutes.

[0013] Furthermore, the perovskite spin-coating solution is prepared by mixing DMAPbI3 and CsI in a molar ratio of 1:1 and dissolving them in DMF to form a 0.8M perovskite solution.

[0014] Furthermore, the above-mentioned DMAPbI3 is prepared by mixing PbI2 powder with DMF, stirring slowly at 80°C for 1 hour, then adding HI, heating to 100°C and reacting for 8 hours. After the reaction is completed, the solid material is collected, washed and dried.

[0015] Furthermore, the mass-to-volume ratio of the above-mentioned PbI2 powder, DMF, and HI is 4.6–5 g: 10 mL: 4 mL.

[0016] Furthermore, the drying temperature is 70℃ and the drying time is 24 hours.

[0017] Most specifically, a method for preparing a pure γ-CsPbI3 thin film on an ITO surface is characterized by the following steps:

[0018] (1) SnO2 colloid and deionized water were mixed in a volume ratio of 3 to 7:1 to prepare SnO2 aqueous solution and stirred at room temperature for 1 h; 100 μL of SnO2 aqueous solution was dropped onto ITO, spin-coated at 4000 rpm for 30 s, and annealed at 150 °C for 30 min.

[0019] (2) PbI2 powder and DMF are mixed and stirred at 80°C for 1 hour until completely dissolved. HI is added and reacted at 100°C for 8 hours. After cooling, the solid material is collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals are then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI is 4.6-5 g:10 mL:4 mL.

[0020] (3) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and annealed at 210 °C for 3 to 5 min.

[0021] A method for fabricating a perovskite solar cell with ITO as the substrate, characterized in that: the solar cell device structure is ITO / SnO2 / CsPbI3 / P3HT / Ag, and the specific fabrication steps are as follows:

[0022] Step 1: Prepare a SnO2 layer on ITO:

[0023] SnO2 colloid and deionized water were mixed at a volume ratio of 3 to 7:1 to prepare an aqueous SnO2 solution, which was stirred at room temperature for 1 hour; 100 μL of the SnO2 aqueous solution was dropped onto ITO, spin-coated at 4000 rpm for 30 seconds, and annealed at 150 °C for 30 minutes.

[0024] Step 2: Prepare perovskite thin films on SnO2:

[0025] (1) PbI2 powder and DMF are mixed and stirred at 80°C for 1 hour until completely dissolved. HI is added and reacted at 100°C for 8 hours. After cooling, the solid material is collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals are then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI is 4.6-5 g:10 mL:4 mL.

[0026] (2) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and annealed at 210℃ for 3-5 min.

[0027] Step 3: Prepare a P3HT layer on the perovskite film:

[0028] Dissolve 15 mg P3HT in 1 mL of chlorobenzene and heat at 60 °C for 8 h; coat 70 μL of P3HT solution onto perovskite and anneal at 100 °C for 5 min.

[0029] Step 4: Deposit Ag electrodes onto P3HT:

[0030] by 100 nm Ag was thermally evaporated at a rate of 10 ppm under a vacuum of 10 ppm. -4 ~10 -5 Pa.

[0031] The present invention has the following technical effects:

[0032] This invention prepares a SnO2 thin film by adding a SnO2 aqueous solution of a specific concentration between ITO and CsPbI3. (1) It effectively blocks the high-temperature chemical reaction between ITO and DMAI in the perovskite spin coating solution during the preparation process, inhibits the formation of pores in CsPbI3 on ​​the ITO substrate, and ensures the compactness of the film; (2) It inhibits the phase transition induced by ITO on CsPbI3, and prevents the formation of an intermediate phase in the film during the preparation process and its rapid transformation into a non-photoactive yellow δ phase; thereby increasing the photoelectric conversion efficiency of the solar cell prepared with ITO as the substrate to 14.02%. Attached Figure Description

[0033] Figure 1 CsPbI3 thin films prepared on different substrates.

[0034] Figure 2 Optical images of samples during the annealing process of CsPbI3 prepared on ITO and FTO.

[0035] Figure 3XRD patterns of samples during the annealing process of CsPbI3 thin films prepared on ITO and FTO.

[0036] Figure 4 SEM images of CsPbI3 prepared on ITO (annealed at 210℃ for 1 min) and CsPbI3 prepared on FTO (annealed at 210℃ for 5 min).

[0037] Figure 5 UV-Vis spectra of CsPbI3 prepared on ITO (annealed at 210℃ for 1 min) and CsPbI3 prepared on FTO (annealed at 210℃ for 5 min).

[0038] Figure 6 XRD patterns of DMAI films deposited on ITO and FTO during the annealing process.

[0039] Figure 7 (a) Optical images of CsPbI3 films deposited on ITO with different concentrations of SnO2 and then annealed for different times; (b) XRD pattern corresponding to annealing for 5 min.

[0040] Figure 8 Performance curves of solar cells composed of CsPbI3 thin films prepared on SnO2 with different concentrations. Detailed Implementation

[0041] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0042] Example 1

[0043] A method for fabricating a perovskite solar cell with ITO as the substrate, wherein the solar cell device structure is ITO / SnO2 / CsPbI3 / P3HT / Ag, and the specific fabrication steps are as follows:

[0044] Step 1: Prepare a SnO2 layer on ITO:

[0045] SnO2 colloid and deionized water were mixed at a volume ratio of 7:1 to prepare an aqueous SnO2 solution, which was stirred at room temperature for 1 h; 100 μL of the SnO2 aqueous solution was dropped onto ITO, spin-coated at 4000 rpm for 30 s, and annealed at 150 °C for 30 min.

[0046] Step 2: Prepare perovskite thin films on SnO2:

[0047] (1) PbI2 powder was mixed with DMF and stirred at 80°C for 1 hour until completely dissolved. HI was added and reacted at 100°C for 8 hours. After cooling, the solid material was collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals were then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI was 4.61 g: 10 mL: 4 mL.

[0048] (2) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of the perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and then annealed at 210 °C for 5 min.

[0049] Step 3: Prepare a P3HT layer on the perovskite film:

[0050] Dissolve 15 mg P3HT in 1 mL of chlorobenzene and heat at 60 °C for 8 h; coat 70 μL of P3HT solution onto perovskite and anneal at 100 °C for 5 min.

[0051] Step 4: Deposit Ag electrodes onto P3HT:

[0052] by 100 nm Ag was thermally evaporated at a rate of 10 ppm under a vacuum of 10 ppm. -4 ~10 -5 Pa.

[0053] Example 2

[0054] A method for fabricating a perovskite solar cell with ITO as the substrate, wherein the solar cell device structure is ITO / SnO2 / CsPbI3 / P3HT / Ag, and the specific fabrication steps are as follows:

[0055] Step 1: Prepare a SnO2 layer on ITO:

[0056] SnO2 colloid and deionized water were mixed at a volume ratio of 5:1 to prepare an aqueous SnO2 solution, which was stirred at room temperature for 1 h; 100 μL of the aqueous SnO2 solution was dropped onto ITO, spin-coated at 4000 rpm for 30 s, and annealed at 150 °C for 30 min.

[0057] Step 2: Prepare perovskite thin films on SnO2:

[0058] (1) PbI2 powder was mixed with DMF and stirred at 80°C for 1 hour until completely dissolved. HI was added and reacted at 100°C for 8 hours. After cooling, the solid material was collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals were then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI was 4.61 g: 10 mL: 4 mL.

[0059] (2) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of the perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and then annealed at 210 °C for 5 min.

[0060] Step 3: Prepare a P3HT layer on the perovskite film:

[0061] Dissolve 15 mg P3HT in 1 mL of chlorobenzene and heat at 60 °C for 8 h; coat 70 μL of P3HT solution onto perovskite and anneal at 100 °C for 5 min.

[0062] Step 4: Deposit Ag electrodes onto P3HT:

[0063] by 100 nm Ag was thermally evaporated at a rate of 10 ppm under a vacuum of 10 ppm. -4 ~10 -5 Pa.

[0064] Example 3

[0065] A method for fabricating a perovskite solar cell with ITO as the substrate, wherein the solar cell device structure is ITO / SnO2 / CsPbI3 / P3HT / Ag, and the specific fabrication steps are as follows:

[0066] Step 1: Prepare a SnO2 layer on ITO:

[0067] SnO2 colloid and deionized water were mixed at a volume ratio of 3:1 to prepare an aqueous SnO2 solution, which was stirred at room temperature for 1 h; 100 μL of the SnO2 aqueous solution was dropped onto ITO, spin-coated at 4000 rpm for 30 s, and annealed at 150 °C for 30 min.

[0068] Step 2: Prepare perovskite thin films on SnO2:

[0069] (1) PbI2 powder was mixed with DMF and stirred at 80°C for 1 hour until completely dissolved. HI was added and reacted at 100°C for 8 hours. After cooling, the solid material was collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals were then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI was 4.61 g: 10 mL: 4 mL.

[0070] (2) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of the perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and then annealed at 210 °C for 5 min.

[0071] Step 3: Prepare a P3HT layer on the perovskite film:

[0072] Dissolve 15 mg P3HT in 1 mL of chlorobenzene and heat at 60 °C for 8 h; coat 70 μL of P3HT solution onto perovskite and anneal at 100 °C for 5 min.

[0073] Step 4: Deposit Ag electrodes onto P3HT:

[0074] by 100 nm Ag was thermally evaporated at a rate of 10 ppm under a vacuum of 10 ppm. -4 ~10 -5 Pa.

[0075] Example 4

[0076] A method for fabricating a perovskite solar cell with ITO as the substrate, wherein the solar cell device structure is ITO / SnO2 / CsPbI3 / P3HT / Ag, and the specific fabrication steps are as follows:

[0077] Step 1: Prepare a SnO2 layer on ITO:

[0078] SnO2 colloid and deionized water were mixed at a volume ratio of 7:1 to prepare an aqueous SnO2 solution, which was stirred at room temperature for 1 h; 100 μL of the SnO2 aqueous solution was dropped onto ITO, spin-coated at 4000 rpm for 30 s, and annealed at 150 °C for 30 min.

[0079] Step 2: Prepare perovskite thin films on SnO2:

[0080] (1) PbI2 powder and DMF are mixed and stirred at 80°C for 1 hour until completely dissolved. HI is added and reacted at 100°C for 8 hours. After cooling, the solid material is collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals are then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI is 5g:10mL:4mL.

[0081] (2) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of the perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and then annealed at 210 °C for 3 min.

[0082] Step 3: Prepare a P3HT layer on the perovskite film:

[0083] Dissolve 15 mg P3HT in 1 mL of chlorobenzene and heat at 60 °C for 8 h; coat 70 μL of P3HT solution onto perovskite and anneal at 100 °C for 5 min.

[0084] Step 4: Deposit Ag electrodes onto P3HT:

[0085] by 100 nm Ag was thermally evaporated at a rate of 10 ppm under a vacuum of 10 ppm. -4 ~10 -5 Pa.

[0086] Example 5

[0087] A method for fabricating a perovskite solar cell with ITO as the substrate, wherein the solar cell device structure is ITO / SnO2 / CsPbI3 / P3HT / Ag, and the specific fabrication steps are as follows:

[0088] Step 1: Prepare a SnO2 layer on ITO:

[0089] SnO2 colloid and deionized water were mixed at a volume ratio of 7:1 to prepare an aqueous SnO2 solution, which was stirred at room temperature for 1 h; 100 μL of the SnO2 aqueous solution was dropped onto ITO, spin-coated at 4000 rpm for 30 s, and annealed at 150 °C for 30 min.

[0090] Step 2: Prepare perovskite thin films on SnO2:

[0091] (1) PbI2 powder was mixed with DMF and stirred at 80°C for 1 hour until completely dissolved. HI was added and reacted at 100°C for 8 hours. After cooling, the solid material was collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals were then placed in an oven at 70°C for 24 hours to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI was 4.8 g: 10 mL: 4 mL.

[0092] (2) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8M perovskite solution; 50 μL of the perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and then annealed at 210 °C for 4 min.

[0093] Step 3: Prepare a P3HT layer on the perovskite film:

[0094] Dissolve 15 mg P3HT in 1 mL of chlorobenzene and heat at 60 °C for 8 h; coat 70 μL of P3HT solution onto perovskite and anneal at 100 °C for 5 min.

[0095] Step 4: Deposit Ag electrodes onto P3HT:

[0096] by 100 nm Ag was thermally evaporated at a rate of 10 ppm under a vacuum of 10 ppm. -4 ~10 -5 Pa.

[0097] Comparative Example 1:

[0098] CsPbI3 thin films were prepared on ITO, FTO and glass substrates respectively:

[0099] The difference compared to Example 1 is that a CsPbI3 film was prepared directly on the ITO surface. The preparation process of the CsPbI3 film was the same as in Example 1, and an ITO / CsPbI3 film was prepared.

[0100] FTO and glass were used to prepare FTO / CsPbI3 thin films and glass / CsPbI3 thin films, respectively, by replacing ITO with FTO and glass.

[0101] Comparative Example 2:

[0102] CsPbI3 thin films were prepared on ITO, FTO and glass substrates respectively:

[0103] Compared with Example 1, the difference is that when preparing the SnO2 film, the volume ratio of SnO2 colloid to deionized water in the SnO2 aqueous solution is 1:5, and the steps are the same as those for preparing the CsPbI3 film in Example 1, thus preparing the ITO / SnO2 / CsPbI3 film.

[0104] Then, the ITO substrate was replaced with FTO and glass, respectively, to prepare FTO / SnO2 / CsPbI3 thin films and glass / SnO2 / CsPbI3 thin films.

[0105] The perovskite solution was spin-coated onto ITO, FTO, and glass substrates, respectively. After spin-coating, the films were annealed at 210°C for 5 minutes. The results showed that the films on FTO and glass were black, while a pale yellow film was formed on ITO (e.g., ...). Figure 1 (As shown in a). SnO2 solutions (SnO2:H2O = 1:5) were first spin-coated onto each substrate and annealed at 150℃ for 30 min. Then, CsPbI3 perovskite films were prepared on FTO / SnO2, ITO / SnO2, and glass / SnO2 substrates under the same conditions (annealing at 210℃ for 5 min). The results showed that black films were observed on FTO and glass, while yellow films were observed on ITO (as shown in a). Figure 1 (As shown in b). The above experimental phenomena indicate that there are significant differences between the CsPbI3 films prepared on ITO and FTO films, and this phenomenon still exists even after a conventional electron transport layer is inserted between the substrate and the perovskite, indicating that ITO induces a rapid transformation of the CsPbI3 film into the yellow phase. To further elucidate the specific process and reaction mechanism, we annealed the films prepared on ITO and FTO at 210℃ for different times (0–5 min).

[0106] Optical photographs and XRD patterns of the samples during the annealing process are shown in [link to image]. Figure 2 , Figure 3 As shown. The results show that the macroscopic morphology and phase composition of the samples differ significantly on different substrates. CsPbI3 on ​​ITO appears grayish-black at 1 min, as can be seen from the XRD test results. Figure 3 a) The grayish-black sample consisted of an intermediate phase and a small amount of γ-CsPbI3; subsequently, with increasing annealing time, the sample gradually turned yellow, and the entire sample became yellow after 5 minutes. XRD results showed that the film was a pure δ phase. CsPbI3 on ​​FTO appeared slightly brown after 1 minute, and XRD results showed that the film consisted of DMAPbI3 and γ-CsPbI3 phases. Figure 3 b) At 5 min, it turned completely brownish-black, indicating a pure γ phase. This suggests that during the annealing process of CsPbI3 films prepared on ITO, an intermediate phase forms in the sample. Due to the presence of this intermediate phase, the sample rapidly transforms into a yellow δ phase. We selected samples that appeared optically grayish-black / brownish-black and were prepared on ITO (annealed at 210℃ for 1 min) and FTO (annealed at 210℃ for 5 min) for scanning electron microscopy (SEM) testing. Figure 4As shown, the thin film on ITO exhibits grains of approximately 1 μm with relatively large pores between the grains; the thin film on FTO exhibits grains of 300 nm to 400 nm with no pores between the grains, and is very dense. The optical properties of the black thin films on the two substrates were investigated using UV-Vis spectroscopy, such as... Figure 5 As shown, the thin film on ITO has weak light absorption performance, with a corresponding optical band gap of 1.68 eV, while the thin film on FTO has strong light absorption performance, with a corresponding optical band gap of 1.693 eV.

[0107] The results above indicate that ITO reacted with substances in the perovskite or precursor, resulting in an ITO-induced perovskite phase transition. Since the CsPbI3 precursor contains CsI, PbI2, and DMAI, we first prepared a CsPbI3 solution with CsI and PbI2 as precursors (i.e., without DMAI) and compared the XRD patterns on ITO, FTO, and glass substrates after annealing at 210℃. The results showed that all three substrates exhibited a yellow δ-CsPbI3 phase before and after annealing (the color was yellow from 0-5 min annealing). This indicates that ITO did not directly react with CsPbI3, and without DMAI, no γ-phase CsPbI3 film was formed on ITO, FTO, or glass substrates before and after annealing.

[0108] After ruling out the possibility of ITO reacting with CsPbI3, we inferred that ITO might react with DMAI, a substance in the CsPbI3 precursor. We spin-coated a layer of DMAI directly onto ITO and FTO, and compared the XRD patterns after annealing at 210℃ (0–2 min). Figure 6 As can be seen, after annealing on FTO for 15 s, the 28.25° peak disappeared, while the 19.67° peak increased, indicating that DMAI changed its crystal orientation after annealing. After annealing for 30 s, DMAI had completely evaporated, leaving only peaks on the FTO substrate. After annealing on ITO for 15 s, the DMAI peaks at 17.15°, 24.82°, and 28.27° all disappeared, and new peaks at 14.45°, 15.48°, 24.23°, and 25.16° appeared. These peaks are attributed to the reaction products of ITO and DMAI at 210°C, possibly InI. x The compound; after annealing for 2 minutes, the reaction product peak disappeared, leaving only the ITO substrate peak. The possible reaction process involves the rapid decomposition of DMAI on ITO into DMA and I... - I - At high temperatures, it rapidly diffuses to the ITO surface and reacts, forming bonds with uncoordinated In to form an In-I compound, namely:

[0109]

[0110] As discussed above, the induced reaction between ITO and DMAI still occurs when a SnO2 layer is inserted between the substrate and the perovskite. Figure 1 We attempted to block the diffusion of DMAI using different concentrations of SnO2 (SnO2:H2O volume ratios of 1:5, 1:3, 1:1, 2:1, 3:1, 5:1, and 7:1) to inhibit the inductive effect of ITO. Figure 7 As can be seen from the XRD pattern, at concentrations of 1:5 and 1:3, a gray-black film is formed after 1 minute, consisting of an intermediate phase and a small amount of γ-CsPbI3. After 3 minutes, some of it transforms into a yellow phase; after 5 minutes, a large amount of yellow phase is formed. XRD analysis shows that the film annealed for 5 minutes exhibits the coexistence of δ, γ, and intermediate phases, indicating poor blocking effect. At concentrations of 1:1 and 2:1, only a small area forms a gray-black film after 1 minute, suggesting that this area is not completely blocked by SnO2. After 3 minutes, a slightly brown film forms in areas other than the gray-black film. After 5 minutes, part of the gray-black film turns yellow, and the slightly brown film turns brownish-black. XRD analysis shows that the film annealed for 5 minutes exhibits the coexistence of δ and γ phases, indicating incomplete blocking. Previous literature reports that when spin-coating high-performance SnO2 films and fabricating CsPbI3 devices on them, the maximum SnO2 to H2O volume ratio is 2:1. Further increasing the SnO2 concentration leads to an excessively thick electron transport layer, which is detrimental to improving the performance of solar cell devices. In this invention, only when the SnO2 concentration is 3:1, 5:1, and 7:1 does the film appear slightly brownish after 1 min, and brownish-black after 3 min and 5 min. XRD analysis shows that the film annealed for 5 min is a pure γ phase, indicating a good blocking effect, suitable for device fabrication.

[0111] Identical CsPbI3 thin films were spin-coated onto electron transport layers prepared with different concentrations of SnO2 to fabricate solar cell devices. Device performance was as follows: Figure 8 As shown in Table 1 (corresponding photovoltaic parameters of the device): when using thin SnO2 (1:5), the device efficiency is only 1.78%; when using the highest reported SnO2 concentration (2:1), the device efficiency is only 8.8%; when the concentration is greater than 3:1, the device efficiency is significantly improved, reaching 13.61%; further increasing the concentration from 3:1 to 7:1, the efficiency continues to improve; when the concentration is 7:1, the device efficiency increases to 14.02%. This illustrates the device performance and... Figure 7 The blocking effect is directly related to the concentration of SnO2 aqueous solution (3-7:1), that is, the higher the concentration of SnO2 aqueous solution, the better the blocking effect and the better the device performance.

[0112] Table 1:

[0113]

[0114] It can be seen that by increasing the SnO2 concentration to 3-7:1, the open-circuit voltage, short-circuit current and fill factor of the prepared device are significantly improved, thereby significantly improving the photoelectric conversion efficiency. In the preparation of SnO2 thin film, when the volume ratio of SnO2 to deionized water is 7:1, the corresponding device PCE reaches 14.02%.

Claims

1. A method for preparing pure γ-CsPbI3 thin films on ITO surfaces, characterized in that: Using ITO as a substrate, a SnO2 thin film is first prepared by spin-coating an aqueous SnO2 solution onto its surface, followed by spin-coating a perovskite solution and then annealing. The volume ratio of SnO2 colloid to deionized water in the aqueous SnO2 solution is 3~7:

1. The perovskite solution is prepared by mixing DMAPbI3 and CsI in DMF at a molar ratio of 1:1 to prepare a 0.8 M perovskite solution. The DMAPbI3 is prepared by mixing PbI2 powder with DMF, stirring slowly at 80℃ for 1 h, then adding HI, and heating to 100℃ for 8 h. After the reaction is completed, the solid material is collected, washed, and dried. The mass-volume ratio of PbI2 powder, DMF, and HI is 4.6~5 g:10 mL:4 mL.

2. The method for preparing a pure γ-CsPbI3 thin film on an ITO surface as described in claim 1, characterized in that: The spin-coated SnO2 aqueous solution is prepared by dropping 100 μL of SnO2 aqueous solution onto ITO, spin-coating at 4000 rpm for 30 s, and then annealing at 150 ℃ for 30 min.

3. A method for preparing a pure γ-CsPbI3 thin film on an ITO surface as described in claim 1 or 2, characterized in that: The annealing temperature is 210 °C, and the annealing time is 3~5 min.

4. A method for preparing pure γ-CsPbI3 thin films on ITO surfaces, characterized in that, Follow these steps: (1) Prepare an aqueous SnO2 solution by mixing SnO2 colloid with deionized water at a volume ratio of 3~7:1 and stirring at room temperature for 1 h; drop 100 μL of SnO2 aqueous solution onto ITO, spin coat at 4000 rpm for 30 s, and anneal at 150 ℃ for 30 min. (2) PbI2 powder and DMF are mixed and stirred at 80°C for 1 h until completely dissolved. HI is added and reacted at 100°C for 8 h. After cooling, the solid material is collected and washed three times with anhydrous ethanol to obtain yellow needle-like crystals. The crystals are then placed in an oven at 70°C for 24 h to obtain dry DMAPbI3 crystals. The mass-volume ratio of PbI2 powder, DMF and HI is 4.6~5 g:10 mL:4 mL. (3) CsI and DMAPbI3 crystals were mixed in DMF at a 1:1 molar ratio to prepare a 0.8 M perovskite solution; 50 μL of perovskite solution was dropped onto the surface of the SnO2 film prepared in step 2, and spin-coated at 3000 rpm for 30 s, and annealed at 210 °C for 3~5 min.

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