Methods and applications for reducing the surface roughness of FTO conductive films

Abstract

This invention discloses a method for reducing the surface roughness of FTO conductive films and its application. The method includes immersing the FTO conductive film and / or a substrate with the FTO conductive film in an aqueous sodium borohydride solution for a period of time, followed by removal, cleaning, and drying. This invention significantly reduces the surface roughness of FTO and optimizes its conductivity while perfectly preserving the underlying continuous conductive network. The treated FTO exhibits breakthrough improvements in open-circuit voltage, fill factor, and overall photoelectric conversion efficiency in perovskite solar cells.

Description

Technical Field

[0001] This invention relates to the field of transparent conductive thin films and solar cell materials, specifically to a method for controlling the surface roughness of fluorine-doped tin oxide (FTO) transparent conductive substrates using sodium borohydride solution, and the application of the FTO substrates obtained by this method in perovskite solar cells. Background Technology

[0002] In the research and industrial application of transparent conductive oxide conductive glasses, FTO thin films are widely used in solar cells, flat panel displays, and energy-saving glass due to their advantages such as high conductivity, high transmittance in the visible light region, and low cost. However, the current large-scale industrial fabrication of FTO thin films mainly adopts atmospheric pressure chemical vapor deposition (CVD). During the high-temperature deposition process, in order to pursue lower sheet resistance and higher carrier mobility, it is usually necessary to promote the full growth of SnO2 grains. This thermodynamically driven crystal growth process inevitably leads to the formation of polycrystalline columnar or pyramidal grain structures in FTO films. These grains with specific crystal orientations form sharp peak and valley morphologies on the film surface, resulting in a root mean square roughness (Ra) of commercial FTO that is typically as high as 15-30 nm. Due to the intrinsic physical trade-off between conductivity and flatness in the CVD process, it is extremely difficult to obtain a flat surface while maintaining high conductivity simply by adjusting the CVD process parameters.

[0003] FTO is the most widely used transparent electrode in thin-film solar cells, especially in the rapidly developing perovskite solar cells. However, the uneven, rough surface morphology of FTO negatively impacts the heterogeneous nucleation and growth of the overlying perovskite crystal. In sharp peak-valley regions, the perovskite film is prone to uneven stress distribution and grain boundary enrichment, introducing numerous defects at the FTO / transport layer / perovskite buried interface. Furthermore, according to the principle of electromagnetic tip discharge, sharp protrusions on the FTO surface form localized high-field regions under the built-in electric field, readily trapping photogenerated carriers. These factors combined lead to a sharp increase in non-radiative carrier recombination at the heterojunction interface, ultimately manifesting macroscopically as a significant loss in device open-circuit voltage (Voc) and fill factor (FF).

[0004] To address the issue of rough FTO surfaces, existing technologies have attempted methods such as mechanical polishing, plasma etching, or chemical etching with strong acids and alkalis. However, mechanical polishing is prone to leaving scratches on the surface and causing lattice damage (Kim S, Bark C W. Effect of surface treatment by chemical-mechanical polishing for transparent electrode of perovskite solar cells[J]. Energies, 2020, 13(3):585.); plasma etching is expensive and difficult to achieve large-area uniform treatment (Yu W, Lee JG, Joo YH, etal. Etching characteristics and surface properties of fluorine-doped tin oxide thin films under CF4-based plasma treatment[J]. Applied Physics A,2022, 128(10): 942.); while traditional strong chemical etching methods are usually too harsh, often severely damaging the conductive network inside FTO while reducing roughness, resulting in an exponential increase in sheet resistance, completely losing its application value as a transparent electrode (Triana SL, Kusumandari, Suryana R. Effect of wet etching process on the morphology and transmittance of fluorine doped tin oxide). (FTO)[J].Journal of Physics: Conference Series, 2016, 776: 012005.). Summary of the Invention

[0005] In view of the above-mentioned technical problems and the shortcomings in the field, the present invention provides a method for reducing the surface roughness of FTO conductive thin films and its application, which can solve the technical problems of high FTO surface roughness and the resulting poor interface contact and difficulty in temperature leveling of perovskite solar cell devices.

[0006] The specific technical solution is as follows: In a first aspect, the present invention provides a method for reducing the surface roughness of an FTO conductive film, comprising: immersing (preferably submerging) an FTO conductive film and / or a substrate with an FTO conductive film (e.g., an FTO conductive glass substrate) in an aqueous solution of sodium borohydride for a period of time, and then removing, cleaning, and drying the film.

[0007] In some preferred embodiments, the method for reducing the surface roughness of the FTO conductive film is carried out at room temperature.

[0008] In some preferred embodiments, the method for reducing the surface roughness of the FTO conductive film uses an aqueous solution of sodium borohydride with a sodium borohydride concentration of 0.5 M to 3.5 M, such as 1 M, 1.5 M, 2 M, 2.5 M, 3 M, etc., and more preferably 2 M.

[0009] In some preferred embodiments, the method for reducing the surface roughness of the FTO conductive film has a reaction time of 5 min to 120 min, such as 10 min, 30 min, 50 min, 90 min, etc.

[0010] The cleaning process may include common cleaning methods for materials such as glass, such as ultrasonic cleaning with deionized water.

[0011] The drying process may include common drying methods for materials such as glass, such as drying with a nitrogen gun.

[0012] In some preferred embodiments, the method for reducing the surface roughness of the FTO conductive film employs a static reaction.

[0013] In the method for reducing the surface roughness of the FTO conductive film, the absolute amount of sodium borohydride can be sufficient or excessive, as long as the reaction process can continue during the soaking process.

[0014] In a second aspect, the present invention provides an FTO conductive film or a substrate with an FTO conductive film obtained by the method described in the first aspect.

[0015] Thirdly, the present invention provides the application of the FTO conductive film or substrate with FTO conductive film described in the second aspect in perovskite solar cells.

[0016] Fourthly, the present invention provides a perovskite solar cell comprising sequentially stacked FTO conductive glass, an electron transport layer (such as SnO2), a perovskite light-absorbing layer, a passivation layer (such as phenylethylamine iodide (PEAI)), a hole transport layer (such as Spiro-OMeTAD), and a metal electrode (such as silver), wherein the FTO conductive glass is obtained by processing the method described in the first aspect.

[0017] As a general inventive concept, in a fifth aspect, the present invention provides the application of sodium borohydride aqueous solution for reducing the surface roughness of FTO conductive films.

[0018] In some preferred embodiments, the application described in the fifth aspect involves immersing (preferably submerging) an aqueous solution of sodium borohydride into the FTO conductive film and / or a substrate with an FTO conductive film (e.g., an FTO conductive glass substrate) to react, thereby reducing the surface roughness of the FTO conductive film.

[0019] The applications described in the fifth aspect can be further selected and optimized by referring to the methods described in the first aspect.

[0020] In some preferred embodiments, in the application described in the fifth aspect, the sodium borohydride concentration in the aqueous solution is from 0.5 M to 3.5 M, for example 1 M, 1.5 M, 2 M, 2.5 M, 3 M, etc., and more preferably 2 M.

[0021] In some preferred embodiments, the application described in the fifth aspect is carried out at room temperature.

[0022] In some preferred embodiments, the application described in the fifth aspect has a reaction time of 5 min to 120 min, such as 10 min, 30 min, 50 min, 90 min, etc.

[0023] In some preferred embodiments, the application described in the fifth aspect employs a static reaction.

[0024] This invention utilizes the reducing environment created by the slow release of hydrogen gas from the reaction of sodium borohydride with water, along with a gentle etching effect, to reduce the surface roughness of FTO conductive films.

[0025] Compared with the prior art, the beneficial effects of this invention are as follows: 1. This invention can minimize the surface roughness of FTO, reduce the number of rough and large particles on the FTO surface, and at the same time, it will not damage the internal conductive network. Under certain conditions, it can optimize the conductivity of FTO and avoid a significant increase in sheet resistance.

[0026] 2. Compared with untreated rough FTO, the optimized smooth interface significantly reduces the interface defect density of the device and suppresses non-radiative carrier recombination, making it more suitable for the fabrication of perovskite solar cells. Attached Figure Description

[0027] Figure 1 These are atomic force microscopy (AFM) morphology comparison images of FTO prepared in Examples 1-5 of this invention and untreated FTO. Figure 2The images show a comparison of the cross-sectional scanning electron microscope morphology of FTO prepared in Examples 1-5 of this invention with that of untreated FTO. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Operating methods not specifically specified in the following embodiments are generally performed under conventional conditions or as recommended by the manufacturer.

[0029] Example 1: (1) Weigh a certain amount of sodium borohydride powder and dissolve it in deionized water to prepare a sodium borohydride aqueous solution with a concentration of 0.5 M; (2) The FTO conductive glass substrate was completely immersed in the above sodium borohydride aqueous solution and allowed to stand at room temperature for 30 min to react; (3) Take out the FTO conductive glass substrate after soaking treatment, quickly rinse it with a large amount of deionized water to terminate the reaction, then ultrasonically clean it in deionized water for 15 min, and blow it dry with nitrogen to obtain an FTO conductive glass substrate with reduced roughness.

[0030] Example 2: The only difference from Example 1 is that the concentration of sodium borohydride aqueous solution is 1 M, and all other aspects are the same, resulting in an FTO conductive glass substrate with reduced roughness.

[0031] Example 3: The only difference from Example 1 is that the concentration of sodium borohydride aqueous solution is 2 M, and all other aspects are the same, resulting in an FTO conductive glass substrate with reduced roughness.

[0032] Example 4: The only difference from Example 1 is that the concentration of sodium borohydride aqueous solution is 3 M, and all other aspects are the same, resulting in an FTO conductive glass substrate with reduced roughness.

[0033] Example 5: The only difference from Example 1 is that the concentration of sodium borohydride aqueous solution is 3.5 M, and all other aspects are the same, resulting in an FTO conductive glass substrate with reduced roughness.

[0034] Application Example 1: A formal perovskite solar cell has the following device structure: FTO / SnO2 electron transport layer / perovskite light-absorbing layer / phenylethylamine iodide (PEAI) layer / hole transport layer (Spiro-OMeTAD) / silver electrode. This application example uses an FTO conductive glass substrate processed by the method in Example 3 of this invention or an untreated commercial FTO conductive glass substrate. The specific device fabrication steps are as follows: 1) First, the FTO conductive glass substrate undergoes surface treatment to ensure good adhesion and wettability. The specific steps are as follows: the substrate is ultrasonically cleaned with glass cleaning fluid, deionized water and isopropanol respectively, each step for 15 minutes, then the substrate is dried with nitrogen gas, and further treated with ultraviolet ozone for 15 minutes to improve the wettability of the substrate.

[0035] 2) Using a 3wt% SnO2 aqueous solution, spin-coat the FTO at 2500 rpm / s to 4000 rpm for 25 s, and then anneal at 150℃ for 30 min to form an electron transport layer.

[0036] 3) Dissolve a mixture of 880.5 mg lead iodide (PbI2), 309.5 mg formamidinium hydroiodate (FAI) and 42.5 mg methylammonium chloride (MACl) in a mixed solvent of 800 μL N,N-dimethylformamide (DMF) and 200 μL dimethyl sulfoxide (DMSO) and stir to prepare a perovskite precursor solution.

[0037] 4) Add 0.1 mL of perovskite precursor solution dropwise onto the electron transport layer, accelerate from 2000 rpm / s to 4000 rpm, and spin coat for 30 s. Add 100 μL of chlorobenzene (CB) dropwise in the last 10 s for pre-crystallization nucleation, and then anneal at 150 °C for 30 min in air to complete the preparation of the perovskite layer.

[0038] 5) Dissolve 9 mg of phenylethylamine iodide in 2 mL of isopropanol to prepare a passivation layer solution. Then drop the solution onto the perovskite layer, accelerate it from 1000 rpm / s to 3000 rpm, and spin coat for 30 s.

[0039] 6) Dissolve 180 mg Spiro-OMeTAD in 2 mL CB, add 40 μL Li-TFSI solution (500 mg / mL dissolved in acetonitrile) and 80 μL 4-tert-butylpyridine to prepare a hole transport layer precursor solution. Then drop the solution onto the passivation layer, accelerate it from 2000 rpm / s to 5000 rpm, and spin-coat for 30 s.

[0040] 7) Use a vacuum coating machine, at a temperature <5×10 -4 Under a vacuum of Pa, an 80 nm metallic silver electrode was deposited by vapor deposition at a rate of 0.15 nm / s, resulting in an effective area of ​​0.04 cm² for a single sub-cell. 2 .

[0041] The samples from Examples 1-5 and Application Example 1 were analyzed as follows: Figure 1This paper presents a three-dimensional morphological comparison of the untreated commercial FTO conductive glass substrate (C-FTO) and the FTO conductive glass substrates treated in Examples 1-5 of this invention, obtained using atomic force microscopy (AFM). Figure 1 As can be clearly observed, the untreated control group C-FTO surface exhibits an extremely rugged microstructure, covered with sharp particle protrusions, with a root mean square roughness (Ra) as high as 7.95 nm. This sharp peak-valley morphology easily penetrates the ultrathin electron transport layer in the device, leading to severe interfacial leakage. In contrast, the FTO conductive glass substrate treated with sodium borohydride according to this invention effectively etched and passivated the sharp protrusions on its surface, resulting in a smoother and more uniform overall morphology, with Ra successfully reduced to a minimum of 3.78 nm.

[0042] Figure 2 This further demonstrates the gentle etching effect of the method of the present invention. Compared with C-FTO, the thickness of FTO films treated by the method of the present invention is further reduced. In addition, the interface flatness is further improved, thus providing a more ideal flat substrate for subsequent high-quality, dense and pore-free electron transport layer deposition.

[0043] Table 1 shows a comparison of the conductivity of FTO prepared in Examples 1-5 of this invention with that of untreated FTO. It lists the conductivity parameters of FTO after treatment using the method of this invention. Because sodium borohydride reacts with water to release active hydrogen, the induced oxygen vacancies provide a large number of free electrons. Furthermore, the fusion and densification of grain boundaries significantly weaken the barrier scattering of charge carriers when crossing grain boundaries, resulting in a surge in the carrier mobility of the sample in Example 3 to approximately 43 cm⁻¹. 2 / V·s. This demonstrates that the method of the present invention can optimize conductivity without damaging the underlying conductive network structure, and gently and effectively smooth out local sharp morphologies.

[0044] Table 1 Table 2 presents the average performance statistics of 15 formal perovskite solar cells fabricated using commercial FTO and FTO treated in Example 3 of this invention, detailing the core photovoltaic performance parameters of devices fabricated on C-FTO and the substrates treated in Example 3 under standard test conditions. The data show that devices on substrates treated with the method of this invention achieve significant and comprehensive improvements in open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and final power conversion efficiency (PCE). Specifically, compared with the control group devices based on C-FTO, the Voc of the devices based on the FTO substrate treated with this invention significantly increased from 1.06 V to 1.13 V. This significant increase in Voc provides strong macroscopic evidence that the smooth surface of the modified FTO effectively improves the interfacial contact quality of the FTO / electron transport layer / perovskite heterojunction, significantly reducing the deep-level defect state density at the interface, thereby successfully and significantly suppressing non-radiative carrier recombination at the interface. Meanwhile, the FF of the modified device increased significantly from 70.91% in the control group to 79.91%, and Jsc also showed a slight upward trend (from 22.62 mA / cm). 2 Increased to 24.25 mA / cm 2 The significant improvement in FF further confirms that interface smoothing effectively eliminates local short-circuit leakage paths, increases the parallel resistance of the device, and greatly optimizes the extraction and transport efficiency of photogenerated charges at the interface. Its PCE increased from an average of 20.10% to an average of 21.92%, achieving an absolute efficiency improvement of up to 1.82%.

[0045] Table 2 In summary, this invention can significantly reduce the surface roughness of FTO and optimize its conductivity while perfectly preserving the underlying continuous conductive network. The treated FTO can achieve breakthrough improvements in open-circuit voltage, fill factor, and overall photoelectric conversion efficiency in perovskite solar cells.

[0046] Furthermore, it should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for reducing the surface roughness of an FTO conductive thin film, characterized in that, include: After immersing the FTO conductive film and / or the substrate with the FTO conductive film in an aqueous solution of sodium borohydride for a period of time, remove, clean, and dry.

2. The method according to claim 1, characterized in that, The reaction temperature was room temperature.

3. The method according to claim 1, characterized in that, The sodium borohydride concentration in the aqueous solution is from 0.5 M to 3.5 M, preferably 2 M.

4. The method according to claim 1, characterized in that, The reaction time ranges from 5 min to 120 min.

5. The method according to claim 1, characterized in that, A static reaction is adopted.

6. The FTO conductive film or substrate with an FTO conductive film obtained by the method according to any one of claims 1-5.

7. The application of the FTO conductive thin film or the substrate with the FTO conductive thin film according to claim 6 in perovskite solar cells.

8. A perovskite solar cell, characterized in that, The invention comprises sequentially stacked FTO conductive glass, an electron transport layer, a perovskite light-absorbing layer, a passivation layer, a hole transport layer, and a metal electrode, wherein the FTO conductive glass is obtained by the method described in any one of claims 1-5.

9. An application of sodium borohydride aqueous solution for reducing the surface roughness of FTO conductive films, characterized in that, The FTO conductive film and / or the substrate with the FTO conductive film are immersed in an aqueous solution of sodium borohydride to react, thereby reducing the surface roughness of the FTO conductive film.

10. The application according to claim 9, characterized in that, The concentration of sodium borohydride in the aqueous solution is from 0.5 M to 3.5 M, preferably 2 M; The reaction time ranges from 5 min to 120 min; The reaction temperature was room temperature; A static reaction is adopted.

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