Perovskite thin film, preparation method thereof and perovskite solar cell

Abstract

The invention relates to the technical field of perovskite, and discloses a perovskite thin film, a preparation method thereof and a perovskite solar cell, the chemical formula of the perovskite thin film is ABX3, A is selected from at least two mixed cations of cesium, formamidine and methyl ammonium, B is lead, and X is mixed halogen of iodine and bromine; the thickness of the film is 300-600 nm, the grain size is 100-500 nm, the surface roughness Ra is smaller than or equal to 10 nm, guanidine thiocyanate is doped, the doping amount is 0.5-2.5% of the total mass of perovskite, and the molar ratio of iodine to bromine in mixed halogen changes in a gradient mode in the thickness direction of the film. By adopting the design of mixed cations and gradient halogen, the grain size of the prepared perovskite thin film is obviously increased, the surface roughness is obviously reduced, and the PL fluorescence lifetime is greatly prolonged, so that the service life of a carrier is obviously prolonged, and the effects of increasing the grain size of the perovskite thin film, reducing surface defects and enhancing thermal stability are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of perovskite, in particular to a perovskite thin film, a preparation method thereof and a perovskite solar cell. BACKGROUND

[0002] The perovskite thin film is a kind of thin film material with perovskite structure, and the perovskite structure is a specific crystal structure with unique photoelectric performance, which is widely used in photovoltaic, light-emitting diode, light detector and other fields. The perovskite thin film can be used as a light absorption layer in a photovoltaic cell to efficiently convert sunlight into electrical energy; in a light-emitting diode, it can realize efficient light emission for display technology; in a light detector, it can sensitively detect light signals.

[0003] However, the traditional perovskite thin film has problems such as crystal defects, poor interface contact and insufficient stability, which seriously restricts its performance and service life in practical applications. In addition, the previous products are prone to phase change and decomposition under high temperature and high humidity conditions, resulting in rapid degradation of device performance, which is difficult to meet the demand for long-term stable operation. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application provides a perovskite thin film, a preparation method thereof and a perovskite solar cell, which solves the problems of crystal defects, poor interface contact and insufficient stability of the traditional perovskite thin film.

[0005] To achieve the above purpose, the present application realizes the following technical scheme:

[0006] A perovskite thin film, the chemical formula of the perovskite thin film is ABX3, wherein A is at least two mixed cations selected from cesium, formamidinium and methylammonium, B is lead, and X is a mixed halogen of iodine and bromine; the thickness of the thin film is 300-600nm, the grain size is 100-500nm, the surface roughness Ra is ≤10nm, and the thin film is doped with guanidine thiocyanate, and the doping amount is 0.5-2.5% of the total mass of the perovskite; the molar ratio of iodine to bromine in the mixed halogen changes gradiently along the thickness direction of the thin film, and the bromine content of the surface layer is 5-10% higher than that of the bottom layer.

[0007] By adopting the above technical scheme: by adopting mixed cations (formamidinium / cesium) and gradient halogens (iodine / bromine) design, the grain size of the prepared perovskite thin film is significantly increased, the surface roughness is obviously reduced, and the PL fluorescence lifetime is greatly prolonged, which indicates that the crystal boundary defects are effectively inhibited, thereby significantly improving the lifetime of the carrier, and achieving the effects of increasing the grain size of the perovskite thin film, reducing the surface defects and enhancing the thermal stability.

[0008] Preferably, the molar ratio of formamidinium ion to cesium ion in the cation at position A is 0.8-1.2:0.1-0.3, and the molar ratio of iodine to bromine in the anion at position X is 1:0.15-0.25.

[0009] Preferably, a method for preparing a perovskite thin film, for use in the perovskite thin film, comprises the following steps:

[0010] S1, precursor solution preparation: lead iodide, formamidinium iodide, and cesium bromide are dissolved in a mixed solvent of γ-butyrolactone and dimethyl sulfoxide at a molar ratio of 1:0.8-1.2:0.1-0.3, the volume ratio of the mixed solvent is 7:3-9:1, and the total concentration is 1.2-1.6 M; guanidine thiocyanate and 0.1-0.5 vol% of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid are added, and 0.05-0.15 M methylamine hydrochloride is added at the same time, and stirring is carried out at 40-60°C for 6-12 hours;

[0011] S2, substrate pretreatment: the conductive substrate is sequentially subjected to ultraviolet ozone cleaning for 10-20 minutes, and then argon plasma treatment at a power of 50-100 W for 3-8 minutes;

[0012] S3, thin film coating: the pretreated substrate is spin-coated with the precursor solution at 2000-5000 rpm for 30-60 seconds, the environmental humidity is controlled to be ≤30% during the spin-coating process, 200-400 μL of diethyl ether anti-solvent is added at the 8th-12th second after the start of spin-coating, and ultrasonic treatment is applied for 10-30 seconds at a frequency of 40-60 kHz and a power of 50-100 W;

[0013] S4, gradient annealing: the spin-coated substrate is preheated at 80-100°C for 1-3 minutes, then heated to 120-150°C at a heating rate of 5-10°C / min, and annealed in a mixed gas atmosphere with a volume ratio of nitrogen to oxygen of 95:5-98:2 for 10-20 minutes, and then cooled to room temperature at a cooling rate of 3-8°C / min;

[0014] S5, post-treatment: the annealed thin film is immersed in a 5-15 mg / mL solution of phenethylammonium iodide in isopropanol for 10-30 seconds, spin-dried at 2000-4000 rpm, and annealed at 80-100°C for 2-5 minutes.

[0015] Preferably, the substrate is FTO glass with a microgroove array etched on the surface, the groove width is 10-30 μm, the depth is 200-500 nm, and the distance between adjacent grooves is 50-100 μm.

[0016] Preferably, the oxygen content in the annealing atmosphere of S4 is 2-5 vol%.

[0017] Preferably, a perovskite solar cell, for the perovskite thin film, comprises a transparent conductive substrate, an electron transport layer, a perovskite thin film, a hole transport layer and a metal electrode which are sequentially stacked; the electron transport layer is a double-layer structure comprising a SnO2 nanoparticle layer with a thickness of 20-50 nm and a fullerene derivative PCBM layer with a thickness of 5-15 nm, and the mass ratio of SnO2 to PCBM is 1:0.2-0.5; the hole transport layer is Spiro-OMeTAD doped with 15-25% lithium bis-trifluoromethanesulfonimide and 3-6% 4-tert-butylpyridine, and the thickness of the hole transport layer is 150-250 nm.

[0018] Preferably, the metal electrode is a composite layer of Ag and Au, wherein the thickness of the Ag layer is 80-120 nm and the Ag layer is doped with 0.5-1.5 at% Mg, and the thickness of the Au layer is 5-15 nm.

[0019] Preferably, the transparent conductive substrate is FTO glass with a surface etched micro-groove array, the grooves are filled with a conductive polymer layer with a thickness of 50-100 nm, the inner wall of the grooves has an inclination angle of 10°-30°, the groove width is 10-30 μm, the groove depth is 200-500 nm, and the adjacent pitch is 50-100 μm.

[0020] Preferably, the particle size of the SnO2 nanoparticles in the electron transport layer is 10-30 nm, and the surface is coated with an aluminum oxide layer with a thickness of 1-3 nm, and the specific surface area is 50-80 m 2 / g, and the purity of the PCBM layer is ≥99.5%.

[0021] Preferably, a lithium fluoride interface modification layer with a thickness of 2-5 nm is provided between the hole transport layer and the metal electrode.

[0022] The present application provides a perovskite thin film, a preparation method thereof and a perovskite solar cell.

[0023] Advantages:

[0024] 1. The present application significantly increases the grain size of the prepared perovskite thin film, significantly reduces the surface roughness, and greatly prolongs the PL fluorescence lifetime, which indicates that the grain boundary defects are effectively inhibited, thereby significantly improving the lifetime of the carriers, and achieving the effects of increasing the grain size of the perovskite thin film, reducing the surface defects and enhancing the thermal stability.

[0025] 2、The present application increases the contact area of the substrate and the thin film through the micro-groove etching and the conductive polymer filling, improves the interaction, and significantly improves the adhesion between the thin film and the substrate, and the conductive polymer filling effectively enhances the collection efficiency of the carriers at the interface, reduces the loss of the carriers in the transmission process, improves the photoelectric conversion efficiency of the whole device, and achieves the effects of enhancing the thin film adhesion, reducing the interface resistance and improving the carrier collection efficiency.

[0026] 3、The present application provides a SnO2 / aluminum oxide composite electron transport layer and a lithium fluoride interface modification layer, wherein the aluminum oxide coating layer effectively passivates the defect states on the surface of SnO2, reduces the occurrence of non-radiative recombination, significantly reduces the dark current density, and significantly improves the electron mobility, thereby improving the electron transport efficiency, and achieving the effects of inhibiting non-radiative recombination, optimizing energy level matching and improving the device filling factor. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 A flow chart of the method for preparing the perovskite thin film. DETAILED DESCRIPTION

[0028] The technical solutions of the present application will be described clearly and completely below with reference to the drawings of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0029] The present application provides a perovskite thin film, the chemical formula of the perovskite thin film is ABX3, wherein A is selected from at least two mixed cations of cesium, formamidinium and methylammonium, B is lead, and X is a mixed halogen of iodine and bromine; the thickness of the thin film is 300-600nm, the grain size is 100-500nm, the surface roughness Ra is less than or equal to 10nm, and the thin film is doped with guanidine thiocyanate, and the doping amount is 0.5-2.5% of the total mass of the perovskite; the molar ratio of iodine to bromine in the mixed halogen changes gradiently along the thickness direction of the thin film, and the bromine content of the surface layer is 5-10% higher than that of the bottom layer.

[0030] Specifically, in perovskite thin films, the change in the molar ratio of iodine to bromine affects the light absorption bandgap of the material. A high bromine content in the surface layer reduces the bandgap, enhancing the absorption of short-wavelength light in this layer, while a low bromine content in the bottom layer increases the bandgap, which is beneficial for the absorption of long-wavelength light. This gradient structure can expand the light absorption range, improve the film's utilization efficiency of sunlight, and thus enhance the light absorption performance, thereby improving the photoelectric conversion efficiency of perovskite thin films in photovoltaic and other fields. The surface layer with a higher bromine content can enhance the thermal and chemical stability of the film, resist high temperature and environmental corrosion, reduce defect state density, and reduce carrier recombination, while the bottom layer with a higher iodine content maintains the good photoelectric properties of perovskite. The gradient change in halogen composition forms an energy level structure and transport path that is conducive to carrier transport.

[0031] The molar ratio of formamidine to cesium ions in the A-position cation is 0.8-1.2:0.1-0.3, and the molar ratio of iodine to bromine in the X-position anion is 1:0.15-0.25.

[0032] Specifically, within this molar ratio range, cesium ions (Cs) + The introduction can fill the gap in formamidinium ion FA + The presence of voids in the crystal structure reduces lattice distortion caused by cation size mismatch; formamidinium ion (FA) + With a large ionic radius, it can provide strong organic-inorganic interactions, while the cesium ion (Cs) + Smaller ionic radii can enhance crystal compactness, thereby improving the overall stability of perovskite films and reducing phase transitions caused by temperature changes or environmental factors; a molar ratio of iodine to bromine of 1:0.15-0.25 allows for effective control of the band structure of perovskite films. - The introduction of halogens can reduce the optical band gap of perovskite, enabling the film to absorb shorter wavelengths of light. This mixed halogen system can expand the light absorption range of perovskite films, achieving a wider spectral absorption from long wavelengths to short wavelengths and improving light absorption efficiency.

[0033] Please see the appendix Figure 1 A method for preparing a perovskite thin film, for use in the aforementioned perovskite thin film, includes the following steps:

[0034] S1. Preparation of precursor solution: Dissolve lead iodide, formamidine iodoformide, and cesium bromide in a molar ratio of 1:0.8-1.2:0.1-0.3 in a mixed solvent of γ-butyrolactone and dimethyl sulfoxide, with a volume ratio of 7:3-9:1 and a total concentration of 1.2-1.6M; add guanidine thiocyanate and 0.1-0.5 vol% of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, and simultaneously add 0.05-0.15M methylamine hydrochloride, and stir at 40-60℃ for 6-12 hours;

[0035] S2, substrate pretreatment: the conductive substrate was sequentially subjected to ultraviolet ozone cleaning for 10-20 minutes, and then argon plasma treatment at a power of 50-100 W for 3-8 minutes;

[0036] S3, thin film coating: the pretreated substrate was spin-coated with a precursor solution at 2000-5000 rpm for 30-60 seconds, the environmental humidity was controlled to be ≤30% during the spin-coating process, 200-400 μL of diethyl ether anti-solvent was added at the 8th-12th second after the start of spin-coating, and ultrasonic treatment was applied for 10-30 seconds at a frequency of 40-60 kHz and a power of 50-100 W;

[0037] S4, gradient annealing: the spin-coated substrate was preheated at 80-100 °C for 1-3 minutes, then heated at a rate of 5-10 °C / min to 120-150 °C, and annealed in a mixed gas atmosphere with a nitrogen to oxygen volume ratio of 95:5-98:2 for 10-20 minutes, and then cooled to room temperature at a rate of 3-8 °C / min;

[0038] S5, post-treatment: the annealed thin film was immersed in a 5-15 mg / mL solution of phenethylammonium iodide in isopropanol for 10-30 seconds, spin-dried at 2000-4000 rpm, and annealed at 80-100 °C for 2-5 minutes.

[0039] Please refer to the attached Figure 1 The substrate is FTO glass with a microgroove array etched on the surface, the groove width is 10-30 μm, the depth is 200-500 nm, and the distance between adjacent grooves is 50-100 μm.

[0040] Specifically, in terms of optical performance, grooves with a width of 10-30 μm can achieve light scattering and trapping, enhancing the residence time of light in the perovskite thin film, thereby improving the light absorption efficiency. In terms of thin film formation and quality, grooves with a depth of 200-500 nm are beneficial to the filling and crystallization of the perovskite precursor solution, promoting the uniform growth of the perovskite thin film and reducing film defects. When the distance between adjacent grooves is 50-100 μm, an effective conductive channel can be formed, improving the collection efficiency of carriers.

[0041] Please refer to the attached Figure 1 The oxygen content in the annealing atmosphere of S4 is 2-5 vol%.

[0042] Specifically, during the annealing process, if the oxygen content is too low, some organic components in the perovskite precursor solution are prone to incomplete oxidation, producing impurities such as carbon defects, affecting the purity of the crystal, and maintaining the oxygen content at 2-5 vol% can effectively promote the moderate oxidation of organic components, reduce the formation of impurities, thereby improving the quality of the perovskite crystal, making it more pure, and conducive to achieving efficient photoelectric conversion; the appropriate amount of oxygen can participate in the reconstruction process of the perovskite lattice, and within this oxygen concentration range, oxygen atoms can interact with lead, iodine and other elements in the perovskite lattice, optimizing the lattice parameters and making the lattice more regular. The regular lattice structure helps to improve the carrier mobility, reduce the scattering and recombination probability of carriers during transmission, and thus improve the photoelectric performance of the perovskite thin film.

[0043] A perovskite solar cell for the perovskite thin film described above, comprising a transparent conductive substrate, an electron transport layer, a perovskite thin film, a hole transport layer and a metal electrode stacked in sequence; the electron transport layer is a double-layer structure, comprising a SnO2 nanoparticle layer with a thickness of 20-50 nm and a fullerene derivative PCBM layer with a thickness of 5-15 nm, and the mass ratio of SnO2 to PCBM is 1:0.2-0.5; the hole transport layer is Spiro-OMeTAD doped with 15-25% lithium bis-trifluoromethanesulfonimide and 3-6% 4-tert-butylpyridine, and the thickness of the hole transport layer is 150-250 nm.

[0044] Specifically, the transparent conductive substrate usually adopts FTO (fluorine-doped tin oxide) or ITO (indium tin oxide) glass, which provides a transparent and conductive support platform for the entire solar cell structure, allowing sunlight to pass through and irradiate the active layer such as the perovskite film, and also plays a role in collecting and transmitting photoelectric current, ensuring that the generated electrons can be effectively led out to the external circuit; the SnO2 nanoparticle layer has good electron transport performance and appropriate energy level structure, can effectively collect the electrons generated in the perovskite film, and transmit them to the transparent conductive substrate, PCBM is a fullerene derivative with high electron affinity and good electron transport performance, which can work together with SnO2 to form a more efficient electron transport channel; the perovskite film is the core light absorption layer of the solar cell, which can effectively absorb sunlight and convert it into electron-hole pairs; Spiro-OMeTAD is a commonly used organic hole transport material with good hole transport performance and stability, doping with bis-trifluoromethanesulfonimide lithium can improve its conductivity and hole injection capacity, and doping with 4-tert-butylpyridine can optimize its energy level structure and film morphology, further improving the hole transport efficiency; the doped and optimized hole transport layer can effectively collect and transport the holes generated in the perovskite film, reduce the recombination of holes and electrons, and improve the photoelectric conversion efficiency; the metal electrode usually adopts metal materials with low work function and high conductivity such as silver (Ag) and aluminum (Al), which forms a good ohmic contact with the hole transport layer, can effectively collect holes and lead them out to the external circuit to complete the output of current.

[0045] The metal electrode is a composite layer of Ag and Au, wherein the Ag layer has a thickness of 80-120 nm and is doped with 0.5-1.5 at% Mg, and the Au layer has a thickness of 5-15 nm.

[0046] Specifically, appropriate Mg doping can further optimize the conductivity of the Ag electrode, improve its stability, prevent Ag from migrating or aggregating during use, thereby prolonging the service life of the electrode, in addition, Mg doping can also improve the contact performance between Ag and the hole transport layer, reduce the contact resistance at the interface, and improve the injection and transport efficiency of carriers; Au has good chemical stability and oxidation resistance, which can effectively prolong the service life of the electrode, at the same time, the Au layer can also work together with the Ag layer to further improve the reflectivity and conductivity of the electrode, and enhance the light collection efficiency of the entire solar cell.

[0047] The transparent conductive substrate is FTO glass with a surface etched micro-groove array, the groove is filled with a conductive polymer layer with a thickness of 50-100 nm, the groove inner wall has an inclination angle of 10°-30°, the groove width is 10-30 μm, the groove depth is 200-500 nm, and the adjacent pitch is 50-100 μm.

[0048] Specifically, the micro-groove structure after etching increases the contact area of the FTO glass with the perovskite film and the conductive polymer layer, making the combination between the layers more compact, significantly enhancing the adhesion of the film to the substrate, which helps to prevent the film from peeling under environmental changes or mechanical stress, improving the stability and durability of the device; the inner wall of the groove has an inclination angle of 10-30°, and this inclined structure is conducive to guiding the transmission direction of the carriers, reducing the reflection and scattering loss of the carriers at the interface, so that more carriers can be smoothly collected by the transparent conductive substrate, thereby improving the collection efficiency of the carriers and reducing the recombination probability at the interface; the conductive polymer can well fill the voids in the micro-groove, forming a smooth and uniform surface, further optimizing the interface quality between the FTO glass and the perovskite film, reducing the defects and trap states at the interface, reducing the recombination probability of the carriers at the interface, and improving the transmission efficiency of the carriers.

[0049] The particle size of the SnO2 nanoparticles in the electron transport layer is 10-30nm, the surface is coated with an aluminum oxide layer with a thickness of 1-3nm, the specific surface area is 50-80m 2 / g, and the purity of the PCBM layer is ≥99.5%.

[0050] Specifically, the particle size of the SnO2 nanoparticles is between 10-30nm, which makes the SnO2 have a larger specific surface area (50-80m 2 / g). A larger specific surface area means that more contact sites can be provided when in contact with the perovskite film, which is conducive to the collection and transmission of electrons; SnO2 nanoparticles may have some defect sites such as dangling bonds on the surface during preparation and use, and these defects can capture electrons, increasing the probability of electron recombination; the aluminum oxide coating layer can fill these defect sites, reducing non-radiative recombination of electrons, thereby improving the transmission efficiency and lifetime of electrons; high-purity PCBM can provide a clean electron transport channel, allowing electrons to transmit more smoothly and reducing energy loss, thereby ensuring excellent electron transport performance.

[0051] A lithium fluoride interface modification layer with a thickness of 2-5nm is provided between the hole transport layer and the metal electrode.

[0052] Specifically, the lithium fluoride interface modification layer can adjust the energy level structure between the hole transport layer and the metal electrode, and also can fill the defects and trap states on the surface of the hole transport layer, reducing the defect density at the interface; lithium fluoride has good chemical stability and affinity, and can form a stable interface contact with the hole transport layer and the metal electrode, which improves the conductivity and reliability of the interface, enhances the adhesion between the metal electrode and the hole transport layer, prevents the metal electrode from migrating or falling off during use, and prolongs the service life of the device.

[0053] Example 1: Preparation and performance verification of mixed cation and gradient halide perovskite thin film

[0054] I. Technical solution:

[0055] 1. Preparation of precursor solution: Lead iodide (1 mol), formamidinium iodide (1.0 mol), cesium bromide (0.2 mol) were dissolved in a mixed solvent of γ-butyrolactone and dimethyl sulfoxide (volume ratio 8:2) with a total concentration of 1.4 M. Guanidine thiocyanate (1.5% of the total mass of perovskite) and 0.3 vol% of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid were added. Stirring at 50°C for 8 hours. The introduction of mixed cations (formamidinium and cesium) helps to optimize the crystal structure, and the addition of ionic liquid improves the stability of the solution and the morphology of the thin film.

[0056] 2. Substrate pretreatment: FTO glass was sequentially cleaned with ultraviolet ozone for 15 minutes and argon plasma treatment (power 80 W, 5 minutes). Ultraviolet ozone cleaning can remove organic impurities on the surface of the substrate, and argon plasma treatment can enhance the hydrophilicity of the substrate and improve the adhesion of the thin film.

[0057] 3. Thin film coating: spin coating at 3500 rpm for 30 seconds, adding 300 μL of ether anti-solvent at the 10th second, and simultaneously applying 50 kHz ultrasonic treatment (power 80 W, 20 seconds), with the ambient humidity controlled at 25%. The ether anti-solvent can accelerate the crystallization of the precursor solution, the ultrasonic treatment helps to eliminate internal bubbles in the thin film, and the humidity control is beneficial to obtain a uniform and dense thin film.

[0058] 4. Gradient annealing: preheating at 95°C for 2 minutes, increasing the temperature to 135°C at a rate of 8°C / min, annealing in a nitrogen-oxygen mixed gas atmosphere (volume ratio 97:3) for 15 minutes, and cooling at a rate of 5°C / min. The gradient annealing process simulates the natural growth environment of the crystal, which is beneficial to the formation of high-quality perovskite phase.

[0059] 5. Post-processing: immerse in a 10 mg / mL solution of phenethylammonium iodide in isopropanol for 20 seconds, spin dry at 3000 rpm, and then anneal at 90°C for 3 minutes. Phenethylammonium iodide post-processing can further passivate grain boundary defects and improve the performance of the thin film.

[0060] II. Verification of technical effects:

[0061] · Comparative Experiment 1 (optimization of cation ratio):

[0062] 1. Control group technical solution:

[0063] Preparation of precursor solution: lead iodide (1 mol), formamidinium iodide (1.2 mol), cesium bromide (0 mol), solvent and process parameters are the same as in Example 1.

[0064] Other steps (substrate processing, spin coating, annealing, etc.) are exactly the same as Example 1.

[0065] 2. Test results:

[0066] The control group has smaller grain size (80-150 nm), surface roughness Ra = 15 nm, and PL fluorescence lifetime 80 ns;

[0067] Example 1 has grain size 200-400 nm, Ra = 8 nm, and PL lifetime 120 ns.

[0068] Test standards: XRD crystal orientation analysis (JCPDS 54-0752), PL fluorescence lifetime test (Horiba FluoroMax).

[0069] Table 1:

[0070] Parameters Control (no cesium ions) Example 1 Test standards Precursor cation ratio Formamidinium: cesium = 1.2:0 Formamidinium: cesium = 1.0:0.2 XRD (JCPDS 54-0752) Grain size (nm) 80-150 200-400 SEM image analysis Surface roughness Ra (nm) 15 8 Atomic force microscopy (AFM) PL fluorescence lifetime (ns) 80 120 Time-resolved fluorescence spectroscopy

[0071] Summary: Cesium ion doping improves grain size and carrier lifetime by suppressing grain boundary defects, verifying the necessity of mixed cations.

[0072] · Comparative Experiment 2 (gradient halogen design):

[0073] Test standards: TOF-SIMS depth profiling (IONTOF GmbH).

[0074] 2. Control group technical solution:

[0075] Preparation of precursor solution: lead iodide (1 mol), iodomethanimine (1.0 mol), cesium bromide (0.2 mol), but the cesium bromide is uniformly distributed in the solution (no gradient design).

[0076] No ultrasonic assisted crystallization is used in the spin coating process, and there is no gradient heating in the annealing process (directly heated to 135℃ at a rate of 10℃ / min and annealed at a constant temperature).

[0077] 2. Test results:

[0078] TOF-SIMS shows that the bromine content of the control group is uniform (9%), and the bromine content of Example 1 is 12% on the surface and 7% on the bottom;

[0079] After aging at 85℃ for 1000 hours, the efficiency of the control group decays by 18%, and the efficiency of Example 1 decays by 8%.

[0080] Table 2:

[0081]

[0082] Summary: Gradient halogen combined with ultrasonic assisted crystallization process significantly improves the thermal stability of the film.

[0083] Example 2: Interface optimization of microgroove substrate and conductive polymer filling

[0084] I. Technical solution:

[0085] 1. Substrate etching: Laser etching microgrooves (width 20 pm, depth 300 nm, pitch 80 pm, inner wall inclination angle 20°) on the surface of FTO glass, filling PEDOT:PSS conductive polymer (thickness 70 nm) in the grooves. The microgroove structure can increase the contact area of the substrate and the thin film, and the conductive polymer filling is conducive to improving the interface charge collection efficiency.

[0086] 2. Thin film preparation: According to the steps of Example 1, prepare perovskite thin film using etched substrate.

[0087] II. Technical effect verification:

[0088] · Comparative experiment 3 (substrate structure influence):

[0089] Test standard: Adhesion test (JIS K5600-5-7 grid method), EQE external quantum efficiency test (Enlitech QE-R)

[0090] 1. Control group technical solution:

[0091] Use unetched flat FTO substrate without conductive polymer filling.

[0092] Other thin film preparation steps are exactly the same as Example 2.

[0093] 2. Test results:

[0094] The adhesion level of the control group is 3B (Example 2 is 5B);

[0095] The average value of the control group EQE in the 450-750 nm waveband is 75%, and Example 2 is improved to 87%.

[0096] Table 3:

[0097]

[0098] Summary: Microgroove structure increases the contact area of the thin film and the substrate, conductive polymer filling reduces the interface resistance, and synergistically improves the device efficiency.

[0099] Example 3: Interface passivation of SnO2 / aluminum oxide composite electron transport layer

[0100] I. Technical solution:

[0101] 1. Electron transport layer preparation:

[0102] SnO2 nanoparticles (particle size 20 nm, specific surface area 65 m 2 / g) coated with 1.5 nm aluminum oxide layer by atomic layer deposition (ALD); SnO2 dispersion solution (20 nm thick) was spin-coated, and then a PCBM layer (10 nm, purity 99.7%) was deposited. The aluminum oxide coating layer can passivate the surface defects of SnO2 and inhibit non-radiative recombination, and the PCBM layer can help improve the electron transport efficiency.

[0103] 2. Optimization of hole transport layer:

[0104] Spiro-OMeTAD doped with 20% lithium bis-trifluoromethanesulfonimide and 5% 4-tert-butylpyridine, thickness 200 nm. Doping optimization can improve the conductivity and stability of the hole transport layer.

[0105] 3. Interface modification: 3 nm lithium fluoride layer is evaporated between the hole transport layer and the Ag / Mg electrode. The lithium fluoride interface layer can optimize the energy matching level and reduce the contact resistance.

[0106] II. Verification of technical effects:

[0107] · Comparative Experiment 4 (electron transport layer passivation):

[0108] Test standards: dark current test (Keithley 2400), TRPL time-resolved fluorescence (PicoQuant FluoTime 300).

[0109] 1. Control group technical scheme:

[0110] The electron transport layer uses SnO2 nanoparticles (particle size 20 nm) without aluminum oxide coating, which is directly spin-coated into a film.

[0111] Other steps (PCBM layer, hole transport layer, etc.) are the same as Example 3.

[0112] 2. Test results:

[0113] The dark current density of the control group is 5.6 x 10 -6 mA / cm 2 , and that of Example 3 is reduced to 1.2 x 10 -6 mA / cm 2 ;

[0114] The electron mobility of the control group is 0.8 cm 2 / (V·s), and that of Example 3 is improved to 1.5 cm 2 / (V·s).

[0115] Table 4:

[0116]

[0117]

[0118] Summary: Al2O3 passivation layer passivates SnO2 surface oxygen vacancy defects, suppresses non-radiative recombination, and improves electron transport efficiency.

[0119] • Comparative Experiment 5 (lithium fluoride interface layer):

[0120] Test standard: Energy level matching test (UPS ultraviolet photoelectron spectroscopy, Thermo Scientific K-Alpha).

[0121] 1. Control group technical solution:

[0122] No lithium fluoride layer is deposited between the hole transport layer and the Ag electrode.

[0123] Other process parameters are exactly the same as in Example 3.

[0124] 2. Test results:

[0125] The fill factor of the control group is 78%, which is improved to 82% in Example 3;

[0126] UPS test shows that when there is no lithium fluoride layer, the work function of the hole transport layer is 4.9 eV, which has a potential barrier with the Ag electrode (4.3 eV).

[0127] Table 5:

[0128]

[0129] Summary: The lithium fluoride interface layer optimizes the energy level matching, reduces the hole extraction potential barrier, and improves the device fill factor.

[0130] Although embodiments of the present application have been shown and described, it is to be understood that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the present application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A perovskite thin film, characterized by, The chemical formula of the perovskite film is ABX3, wherein A is at least two mixed cations selected from cesium, formamidinium and methylammonium, B is lead, and X is mixed halogen of iodine and bromine; the thickness of the film is 300-600 nm, the grain size is 100-500 nm, the surface roughness Ra is ≤10 nm, and the film is doped with guanidinium thiocyanate, and the doping amount is 0.5-2.5% of the total mass of the perovskite; the molar ratio of iodine to bromine in the mixed halogen changes gradiently along the thickness direction of the film, and the bromine content of the surface layer is 5-10% higher than that of the bottom layer.

2. The perovskite thin film according to claim 1, characterized in that: The molar ratio of formamidinium ion to cesium ion in the A-site cation is 0.8-1.2:0.1-0.3, and the molar ratio of iodine to bromine in the X-site anion is 1:0.15-0.

25.

3. A method for preparing a perovskite thin film, characterized by, A perovskite film according to any one of claims 1-2, comprising the following steps: S1, precursor solution preparation: dissolve lead iodide, iodine formamidinium and cesium bromide in a mixed solvent of γ-butyrolactone and dimethyl sulfoxide at a molar ratio of 1:0.8-1.2:0.1-0.3, the volume ratio of the mixed solvent is 7:3-9:1, and the total concentration is 1.2-1.6 M; add guanidinium thiocyanate and 0.1-0.5 vol% of 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid, and simultaneously add 0.05-0.15 M methylamine hydrochloride, and stir at 40-60°C for 6-12 hours; S2, substrate pretreatment: sequentially perform ultraviolet ozone cleaning on the conductive substrate for 10-20 minutes, and then perform argon plasma treatment at a power of 50-100 W for 3-8 minutes; S3, film coating: spin-coat the precursor solution on the pretreated substrate at 2000-5000 rpm for 30-60 seconds, control the environmental humidity to ≤30% during the spin-coating process, drop 200-400 μL of ether anti-solvent at the 8th-12th second after the start of spin-coating, and apply ultrasonic treatment at 50-100 W for 10-30 seconds; S4, gradient annealing: preheat the spin-coated substrate at 80-100°C for 1-3 minutes, then heat it to 120-150°C at a heating rate of 5-10°C / min, and anneal it in a mixed gas atmosphere with a volume ratio of nitrogen to oxygen of 95:5-98:2 for 10-20 minutes, and then cool it to room temperature at a cooling rate of 3-8°C / min; S5, post-treatment: immerse the annealed film in a 5-15 mg / mL solution of phenethylammonium iodide in isopropanol for 10-30 seconds, spin-dry it at 2000-4000 rpm, and then anneal it at 80-100°C for 2-5 minutes.

4. The method of claim 3, wherein the perovskite thin film is prepared by a method comprising: The substrate is FTO glass with a micro-groove array etched on the surface, the groove width is 10-30 μm, the depth is 200-500 nm, and the distance between adjacent grooves is 50-100 μm.

5. The method for preparing a perovskite thin film according to claim 3, characterized in that: The oxygen content in the annealing atmosphere of S4 is 2-5 vol%.

6. A perovskite solar cell, characterized by, A perovskite thin film for use in any one of claims 1-2, comprising a transparent conductive substrate, an electron transport layer, a perovskite thin film, a hole transport layer and a metal electrode stacked in sequence; the electron transport layer is a double-layer structure comprising a SnO2 nanoparticle layer with a thickness of 20-50 nm and a fullerene derivative PCBM layer with a thickness of 5-15 nm, the mass ratio of SnO2 to PCBM being 1:0.2-0.5; the hole transport layer is Spiro-OMeTAD doped with 15-25% lithium bis-trifluoromethanesulfonimide and 3-6% 4-tert-butylpyridine, the thickness of the hole transport layer being 150-250 nm.

7. The perovskite solar cell according to claim 6, characterized in that: The metal electrode is a composite layer of Ag and Au, wherein the Ag layer has a thickness of 80-120 nm and is doped with 0.5-1.5 at% Mg, and the Au layer has a thickness of 5-15 nm.

8. The perovskite solar cell according to claim 6, characterized in that: The transparent conductive substrate is FTO glass with a surface etched microgroove array, the grooves being filled with a conductive polymer layer with a thickness of 50-100 nm, the inner wall of the grooves having an inclination angle of 10°-30°, the grooves having a width of 10-30 μm, a depth of 200-500 nm and a pitch of 50-100 μm. 9.The perovskite solar cell of claim 6, wherein: The particle size of the SnO2 nanoparticles in the electron transport layer is 10-30 nm, the surface is coated with an alumina layer with a thickness of 1-3 nm, the specific surface area is 50-80 m 2 / g, and the purity of the PCBM layer is ≥ 99.5%.

10. The perovskite solar cell of claim 6, wherein: A lithium fluoride interface modification layer with a thickness of 2-5 nm is provided between the hole transport layer and the metal electrode.

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