A Method for Improving the Performance of Perovskite Solar Cells by Designing Hole-Transporting Layer Bonding Sites

Abstract

The present invention discloses a method for improving the performance of perovskite solar cells by designing bonding sites in the hole transport layer. By constructing a large number of stable hydroxyl groups on the surface of the hydroxylated transparent electrode as the bonding sites for subsequent self-assembled monolayers, the bonding density and stability of the self-assembled monolayers are improved; a large number of uncoordinated indiums can also be constructed on the ITO surface, enabling the self-assembled monolayer molecules to bond to the ITO surface through P=O bonds, creating a new bonding method. Based on the self-assembled monolayer constructed by the present invention as the hole transport layer, perovskite solar cells with higher efficiency and stability can be obtained. The present invention can also spontaneously form a nano anti-reflection structure on the ITO surface, which can greatly increase the photon transmission and thus improve the short-circuit current density of the perovskite solar cells. The present invention enables commercial ITO to be directly used after only one-step treatment according to the present invention, greatly saving the time cost and economic cost of device preparation, and is of great significance for promoting the industrial development of perovskite solar cells.

Description

Technical Field

[0001] The present invention relates to a method for improving the performance of perovskite solar cells by designing bonding sites of a hole transport layer, and belongs to the technical fields of semiconductor optoelectronic devices and solar cells. Background Art

[0002] As a new type of solar cell, perovskite solar cells have great development potential. Currently, the most efficient perovskite solar cells are mainly based on Figure 1 the structure shown, where the hole transport layer is a self-assembled monolayer molecule (such as MeO-2PACz, MeO-4PACz, Me-2PACz, Me-4PACz, etc.). The self-assembled monolayer molecule is very thin, only about 5 nm thick, and bonds to a transparent electrode (such as indium tin oxide ITO, fluorine-doped tin oxide FTO) to play the role of extracting photo-generated holes. Its bonding density directly determines whether it can effectively extract holes. In addition, if the distribution of the self-assembled monolayer molecule is not dense enough, it will also cause the upper perovskite layer and the lower ITO layer to come into direct contact, which will also cause the photo-generated carriers generated in the perovskite layer to be quenched. At the same time, the bonding stability of the self-assembled monolayer molecule is also very important. If it adsorbs from the ITO, it will directly lead to the attenuation of the photovoltaic performance of the device. Therefore, improving the bonding density and stability of the self-assembled monolayer molecule is crucial for the photovoltaic performance of perovskite solar cells.

[0003] Generally, the P-O bond on the assembled single molecule (taking MeO-2PACz as an example, Figure 2 Figure A therein) is bonded to the ITO through a condensation reaction with the hydroxyl (OH) group on the ITO surface ( Figure 2 Figures B and D therein). Therefore, as the bonding site, the density of the hydroxyl groups on the ITO determines the distribution density of the self-assembled monolayer. The stability of the hydroxyl groups on the ITO also affects the stability of the self-assembled monolayer. Constructing rich, dense and stable hydroxyl groups on the ITO is a prerequisite for obtaining a high-quality self-assembled monolayer. In addition, the self-assembled single molecule is mainly bonded through the condensation reaction between the P-O bond and the hydroxyl group on the ITO surface, and the bonding method is relatively single. Therefore, the present invention is proposed. Summary of the Invention

[0004] The object of the present invention is to provide a method for improving the performance of perovskite solar cells by designing bonding sites of a hole transport layer. Specifically, based on the solution method for hydroxylating the transparent electrode indium tin oxide, dense and stable hydroxyl groups and a large number of oxygen vacancies are constructed on its surface as the bonding sites for the subsequent self-assembled monolayer molecules, so as to improve the bonding density and stability of the self-assembled monolayer molecules on it, and improve the photovoltaic performance of perovskite solar cells.

[0005] The hydroxylation etching method of indium tin oxide for a transparent electrode provided by the present invention comprises the following steps:

[0006] Immerse ITO in a hydroxylation etching solution, and perform hydroxylation treatment under soaking or heating with ultrasonic waves.

[0007] The hydroxylation etching solution is a mixed solution of sulfuric acid, hydrogen peroxide and water.

[0008] The present invention directly soaks or ultrasonically treats commercial ITO with a solution having hydroxylation ability. The hydroxylation etching reaction formula is:

[0009]

[0010]

[0011]

[0012]

[0013] Since the surface of commercial ITO contains organic pollutants, it needs to be surface-cleaned before use. The commonly used method in the art is to ultrasonically treat ITO with a cleaner, deionized water, acetone and isopropyl alcohol respectively, or perform ultraviolet ozone treatment or oxygen plasma treatment to remove surface organic substances. However, the hydroxylation solution adopted by the present invention itself has strong oxidizing property, so it can naturally remove surface organic substances very thoroughly through oxidative dehydration during the hydroxylation treatment. That is, commercial ITO can be directly used after one-step hydroxylation by the present invention, and the traditional multi-step ITO pretreatment is omitted, greatly simplifying the preparation steps of perovskite solar cells and shortening the time cost. As Figure 12 shown.

[0014] Preferably, in the hydroxylation etching solution, the volume ratio of the sulfuric acid, the hydrogen peroxide and the water is 4 - 7:2 - 3:1 - 2, preferably 7:3:2.

[0015] Preferably, the mass concentration of the sulfuric acid is 80 - 98%, and the mass concentration of the hydrogen peroxide is 20 - 40%.

[0016] The hydroxylation solution can achieve sufficient hydroxylation of the ITO surface by simply soaking ITO in it for 15 seconds at the fastest, changing its surface from hydrophobic to hydrophilic. This ultra-fast hydroxylation can greatly shorten the preparation time of perovskite solar cells. The hydroxylated ITO surface has abundant hydroxyl groups, making the subsequent self-assembled monolayers bond more densely.

[0017] Preferably, the conditions for soaking are as follows:

[0018] The temperature is 60 - 90 °C and the time is not less than 15 s.

[0019] Since the ITO surface itself does not contain hydroxyl groups, the ITO hydroxylation commonly used in the art is usually ultraviolet ozone treatment or oxygen plasma treatment. However, the gas treatment method is not sufficient for the hydroxylation of the ITO surface, that is, a very dense hydroxyl distribution is not formed on the ITO surface. In addition, this traditional hydroxylation method will also introduce some unstable hydroxyl groups adsorbed physically. The hydroxylation method of the present invention can directly immerse the ITO in the hydroxylation solution. Compared with the traditional hydroxylation method, the method of the present invention can hydroxylate the ITO sufficiently, and the hydroxyl groups introduced on the ITO surface by the present invention are very stable, which is beneficial to the subsequent dense and stable self-assembled monolayer bonding.

[0020] Preferably, the conditions of the thermal ultrasound are as follows:

[0021] The temperature is 60 - 120 °C and the time is 10 - 35 min.

[0022] By heating the hydroxylation solution and performing ultrasonic treatment, a rich oxygen vacancy will be formed on the ITO surface. In the ITO lattice, since O and In are bonded. The oxygen vacancy is equivalent to uncoordinated indium. Therefore, a rich uncoordinated indium ion is constructed. Since the O atom on the P=O bond of the self-assembled monolayer has a lone pair of electrons, it can form a coordination bond with the uncoordinated indium ion ( Figure 2 Figure C in

[0023] That is, during the hydroxylation process, the present invention not only forms a large number of stable hydroxyl groups on the ITO surface for the subsequent P - O bond of the self-assembled monolayer to bond, but also constructs a rich uncoordinated indium for the subsequent P=O bond of the self-assembled monolayer to bond, creating a new bonding mode and thus providing more bonding sites. The present invention can change the bonding of the self-assembled monolayer on the ITO from the traditional monodentate to bidentate, which can further promote the subsequent self-assembled monolayer to bond more densely and stably.

[0024] Since the hydroxylation solution used in the present invention has strong corrosiveness, by heating and ultrasonic treatment, the weak crystalline phase on the ITO surface layer will be etched and removed during the hydroxylation process, which can improve the conductivity uniformity of the ITO, thereby improving the fill factor of the final perovskite solar cell.

[0025] The hydroxylated indium tin oxide electrode obtained by the method of the present invention also belongs to the protection scope of the present invention.

[0026] Based on the hydroxylated indium tin oxide electrode, the present invention further provides a perovskite solar cell device, the substrate of which is the transparent electrode indium tin oxide;

[0027] The self-assembled monolayer bonded on the transparent electrode indium tin oxide serves as a hole transport layer.

[0028] Based on the solution method to hydroxylate the surface of the transparent electrode, a large number of stable hydroxyl groups are constructed on its surface as the bonding sites for the subsequent self-assembled monolayer, improving the bonding density and stability of the self-assembled monolayer. Secondly, the method of the present invention can also construct a large number of uncoordinated indiums on the ITO surface, enabling the self-assembled monolayer molecules to also bond to the ITO surface through P=O bonds, creating a new bonding mode. The self-assembled monolayer constructed based on the method of the present invention as a hole transport layer can obtain a perovskite solar cell with higher efficiency and stability.

[0029] The method of the present invention can also spontaneously form a nano anti-reflection structure on the ITO surface, which can greatly increase the photon transmission and thus improve the short-circuit current density of the perovskite solar cell. In addition, in the traditional method, commercial ITO is default to require multiple time-consuming pre-treatments before use, while the present invention can enable commercial ITO to be directly used after only one-step treatment designed by the present invention, greatly saving the time cost and economic cost of device preparation, and having important significance for promoting the industrial development of perovskite solar cells. Brief Description of the Drawings

[0030] Figure 1 It is a schematic diagram of the structure of the perovskite solar cell device.

[0031] Figure 2 It is a schematic diagram of the self-assembled molecule and how it bonds to the ITO.

[0032] Figure 3 It is an XPS test result diagram of the ITO after being treated with different hydroxylation etching times.

[0033] Figure 4 It is the water contact angle test result of the ITO before and after hydroxylation etching.

[0034] Figure 5 It is a photo of the water drop experiment of the ITO before and after hydroxylation etching.

[0035] Figure 6 It is the water contact angle test result of the ITO with different soaking times.

[0036] Figure 7 It is the time-of-flight secondary ion mass spectrometer test result of the sample after preparing the self-assembled monolayer on the ITO treated by the traditional hydroxylation method and the hydroxylation method of the present invention.

[0037] Figure 8 Test results of conductive atomic force microscopy of samples after preparing self-assembled monolayers on ITOs treated by hydroxylation in the traditional manner and by the method of the present invention.

[0038] Figure 9 Test results of surface potential before and after aging the samples for 600 hours at 85 °C after preparing self-assembled monolayers on ITOs treated by hydroxylation in the traditional manner and by the method designed in the present invention.

[0039] Figure 10 Test results of atomic force microscopy of the ITO surface after treatment with different hydroxylation etching times.

[0040] Figure 11 Optical transmittance curves of ITOs after treatment with different hydroxylation etching times.

[0041] Figure 12 Schematic diagrams of pretreating ITOs in the traditional manner and by the method of the present invention.

[0042] Figure 13 Current-voltage scan curves of perovskite solar cells prepared on ITOs pretreated in the traditional manner and by the method of the present invention.

[0043] Figure 14 Test results of the stability of perovskite solar cells prepared on ITOs pretreated in the traditional manner and by the method of the present invention. Detailed implementation manners

[0044] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0045] Unless otherwise specified, the methods are all conventional methods. Unless otherwise specified, the raw materials can all be obtained from public commercial channels.

[0046] Example 1. Hydroxylation treatment of ITO substrates

[0047] 1) Preparation of hydroxylation solution: Take 700 ml of sulfuric acid (concentration 98%) and pour it into a beaker. Slowly pour 300 ml of hydrogen peroxide (concentration 30%) under stirring, and continue to add 200 ml of deionized water while stirring.

[0048] 2) Rapid hydroxylation of ITO: Directly immerse commercial ITO into the hydroxylation solution (temperature 80 °C) for 15 seconds and then take it out. Thoroughly rinse the surface residual hydroxylation solution with deionized water and dry the ITO with a nitrogen gun, and it can be directly used without any further treatment.

[0049] 3) ITO hydroxylation etching: Heat the hydroxylation solution to 60 °C, directly immerse the commercial ITO in the hydroxylation solution, and ultrasonicate it at 60 °C for 25 minutes. Then take out the ITO, thoroughly rinse the residual hydroxylation solution on the surface with deionized water, and dry the ITO with a nitrogen gun. It can be used directly without any further treatment.

[0050] Prepare a self-assembled monolayer as the hole transport layer on the ITO substrate obtained by the above method, and continue to complete the preparation of a conventional perovskite solar cell subsequently.

[0051] The preparation method of the subsequent perovskite solar cell device is as follows:

[0052] 1. Prepare the self-assembled monolayer: Take MeO-2PACz as an example for the self-assembled monolayer. Prepare a solution of 0.5 mg / ml in absolute ethanol. In the glove box, spin-coat it on the ITO substrate at a speed of 3000 revolutions per minute for 30 s. Subsequently, anneal it on a hot plate at 100 °C for 10 minutes.

[0053] 2. Prepare the perovskite layer: The composition of the perovskite is Rb 0.05 Cs 0.05 MA 0.05 FA 0.85 Pb(I 0.95 Br 0.05 )3. Its precursor is made by dissolving RbI: 15.9 mg, CsI: 19.3 mg, PbBr2: 27 mg, MABr: 9.4 mg, FAI: 219.3 mg, and PbI2: 657 mg in 1 ml of a mixed solvent of DMF:DMSO (v:v = 4:1). The spin-coating of the perovskite precursor is carried out in two consecutive steps. Spin-coat for 10 s at 1000 revolutions per minute, and then spin-coat for 35 s at 4000 revolutions per minute. The first step is to spread the solution, and the second step of high-speed rotation makes it spread more evenly. And at 25 s of the second step, add 150 μl of chlorobenzene to the precursor film as an anti-solvent. Then anneal the film at 100 °C for 10 minutes.

[0054] 3. Prepare the electron transport layer and the blocking layer: Spin-coat PCBM dissolved in chlorobenzene with a concentration of 20 mg / ml at 1500 revolutions per minute for 30 s, and then anneal it at 100 °C for 10 minutes as the electron transport layer. Then spin-coat BCP dissolved in isopropanol with a concentration of 0.5 mg / ml on the PCBM at 5000 revolutions per minute for 30 s as the blocking layer.

[0055] 4. Prepare the silver electrode: Use thermal evaporation to prepare silver with a thickness of about 80 nm on the above film as the electrode.

[0056] Figure 3Figure for XPS test results of ITO after being treated with different hydroxylation etching times. It can be found that with the extension of the treatment time, the surface hydroxyl groups gradually increase.

[0057] Figure 4 Water contact angles of ITO before and after treatment. As the most typical hydrophilic group, hydroxyl groups can directly reflect the number of hydroxyl groups on the surface through the water contact angle. The better the hydrophilicity, the smaller the water contact angle, and the more hydroxyl groups on the surface.

[0058] Figure 5 Photos of the water drop experiment of ITO before and after treatment. It can be seen that the water drops spread directly on the surface of the treated ITO, confirming its good water wettability, thus confirming that the ITO surface has abundant hydroxyl groups.

[0059] Figure 6 Test results of the water contact angles of ITO with different soaking times. In the present invention, the hydroxylation of ITO can be completed in as fast as 15 seconds. As can be seen from the figure, with the extension of the soaking time, the water contact angle gradually decreases and reaches 6 degrees at 15 seconds, confirming that abundant hydroxyl groups have been constructed on the ITO surface.

[0060] Figure 7 Test results of time-of-flight secondary ion mass spectrometer for the samples with self-assembled monolayers prepared on ITO treated by traditional hydroxylation method and the hydroxylation method of the present invention. Samples with self-assembled monolayers are prepared on ITO treated by traditional hydroxylation method and the hydroxylation method of the present invention, and then the samples are tested by time-of-flight secondary ion mass spectrometer. By observing the distribution of PO3 groups, the distribution of the self-assembled monolayer can be reflected. It can be found that the self-assembled monolayer prepared on ITO treated by the hydroxylation method of the present invention has a denser distribution because there are more bonding sites.

[0061] Figure 8 Test results of conductive atomic force microscope. Samples with self-assembled monolayers are prepared on ITO treated by traditional hydroxylation method and the hydroxylation method of the present invention, and then the samples are tested by conductive atomic force microscope. Since the underlying ITO has a conductivity two orders of magnitude higher than that of the self-assembled monolayer, if the self-assembled monolayer coverage is not dense, the probe will directly contact the underlying ITO during the test, resulting in a point of sudden increase in current. The coverage density of the self-assembled monolayer can be judged by the point of sudden increase in current. It can be found that the self-assembled monolayer prepared on ITO treated by the hydroxylation method of the present invention has a denser coverage.

[0062] Figure 9Surface potential test results before and after 600 hours of aging at 85°C. After preparing self-assembled monolayers on ITO treated by traditional hydroxylation and the method of the present invention, the surface potential test results before and after 600 hours of aging at 85°C were carried out on the samples. It can be found that the self-assembled monolayer prepared on the hydroxylated sample prepared by the present invention has very excellent stability.

[0063] Figure 10 Atomic force microscope test results of the ITO surface after different hydroxylation etching times. It can be found that its surface gradually becomes rough and gradually forms a pyramid-like structure, and these structures help to reduce the reflection loss of light.

[0064] Figure 11 Optical transmittance curves of ITO after different hydroxylation etching times. It can be found that the best light transmittance is obtained at 25 minutes.

[0065] Figure 12 Schematic diagrams of ITO pretreated by traditional methods and the method designed by the present invention.

[0066] Figure 13 Current-voltage scan curves of perovskite solar cells prepared on ITO pretreated by traditional methods and the method of the present invention. It can be found that the present invention can now improve the efficiency of perovskite solar cells to 26.3%.

[0067] Figure 14 Stability test of perovskite solar cells prepared on ITO pretreated by traditional methods and the method of the present invention. It can be found that under various aging conditions, the perovskite solar cells prepared on ITO pretreated by the method of the present invention have higher stability.

Claims

1. A hydroxylation etching method for indium tin oxide transparent electrode, comprising the following steps: Immerse ITO in a hydroxylation etching solution and perform hydroxylation treatment under heating and ultrasonic conditions; The hydroxylation etching solution is a mixed solution of sulfuric acid, hydrogen peroxide and water; In the hydroxylation etching solution, the volume ratio of the sulfuric acid, the hydrogen peroxide and the water is 4-7:2-3:1-2; The mass concentration of the sulfuric acid is 80-98%, and the mass concentration of the hydrogen peroxide is 20-40%; The conditions of the thermal ultrasound are as follows: The temperature is 60-120 °C and the time is 10-35 min.

2. Indium tin oxide transparent electrode obtained by the hydroxylation etching method according to claim 1.

3. Use of the indium tin oxide transparent electrode according to claim 2 in the preparation of perovskite solar cell devices.

4. A perovskite solar cell device, the substrate of which is the indium tin oxide transparent electrode according to claim 2; A self-assembled monolayer bonded on the indium tin oxide transparent electrode serves as a hole transport layer.

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