WS2-based organic photovoltaic interface transport layer, and preparation method and application thereof

Abstract

The application discloses a kind of tungsten disulfide nanosheet film prepared by mechanical peeling and its application in organic photovoltaics.The application, by using low-temperature ultrasonic peeling method, combines water-alcohol mixed solution to peel the disulfide powder, reduces the aggregation in the WS2 peeling process, and improves the mechanical peeling effect.This method simplifies the preparation process requirements of WS2 nanosheet, and is conducive to being put into commercial production.The prepared tungsten disulfide nanosheet is spin-coated on an ITO substrate without plasma treatment, and rapid annealing is carried out at 60 DEG C, to obtain a film with high light transmission and high hole transport characteristics.The film of the application not only has high hole transport performance in organic solar cells, but also has higher short-circuit current and device efficiency than the traditional hole transport material PEDOT:PSS, and has great research value and application potential in semi-transparent organic photovoltaics.

Description

Technical Field

[0001] This invention relates to the field of nanomaterial preparation technology, specifically to a method for preparing WS2 nanosheets by low-temperature ultrasonic-assisted liquid phase exfoliation and the application of the obtained WS2 in organic solar cells. Background Technology

[0002] Organic solar cells, due to their low production cost, flexibility, light weight, and large-area processing capabilities, have the potential to become the next generation of photovoltaic new energy. Thanks to the efforts and innovations of researchers worldwide, the highest efficiency of single-junction small-area organic solar cells has recently been reported to exceed 18%. Although the device efficiency of organic solar cells is lower than that of inorganic solar cells, their high absorption coefficient, solution-processability, and bendability, when fabricated into semi-transparent devices, show broad application prospects in building-integrated photovoltaics (BIPV) applications such as semi-transparent insulation and greenhouses. In terms of device structure, the interface layer located between the photoactive layer and the electrodes plays a crucial role in regulating the work function, optimizing carrier mobility, and reducing carrier recombination. Therefore, research on the interface layer in organic photovoltaics has become a key focus for researchers worldwide.

[0003] In two-dimensional materials, transition metal sulfides, with their unique band gap properties, have the potential to be developed into next-generation nano-optoelectronic devices. When the number of layers in a transition metal sulfide changes from multilayer to monolayer, the corresponding band gap changes from an indirect band gap to a direct band gap. This transformation is highly beneficial for improving the optoelectronic performance of the device. Common transition metal sulfides MS2 (such as WS2, MoS2, TiS2, NbS2, TaS2, etc.) are stacked in an SMS pattern in monolayer structures and form covalent bonds, while adjacent sheets are stacked together by weak van der Waals forces to form a layered structure. Among them, WS2 is relatively easy to peel off from the bulk structure due to its weak interlayer interaction forces.

[0004] Chinese invention patent number 201410710313.6 discloses a top-down method for preparing tungsten disulfide nanosheet films. This method uses Li... + Intercalation technology involves reacting tungsten disulfide and n-butyllithium solution at high temperature to generate the precursor Li. x WS2. Then, the precursor is reacted with water to generate a monomolecular suspension, and impurities in the solution, such as Li, are removed by dialysis. + Tungsten disulfide nanosheets were obtained by vacuum drying. However, this intercalation method requires a long reaction time (usually 3-5 days), and the degree of intercalation is difficult to control, making it time-consuming and labor-intensive. More importantly, in the removal of Li... +The process can easily lead to the aggregation of tungsten disulfide nanosheets; ion intercalation can cause WS2 to transform from the semiconductor-type 2H crystal phase to the conductor-type 1T crystal phase, all of which will have an adverse effect on the performance of semiconductor optoelectronic devices.

[0005] Chinese invention patent No. 201610258405.4 discloses a bottom-up method for preparing tungsten disulfide nanosheet thin films. This method employs chemical vapor deposition (CVD), where MoO3 powder and sulfur powder are vaporized at a high temperature of 750–900°C. The reactants are then transferred to a silicon substrate by an argon gas flow, where a redox reaction occurs on the substrate surface to generate a monolayer or few-layer tungsten disulfide thin film. While this CVD method can be extended to the preparation of large-area tungsten disulfide thin films, it requires high-quality raw materials, is difficult to replicate, and generates crystal defects during film formation, making the operation difficult and posing significant safety risks.

[0006] Two-dimensional materials obtained through mechanical exfoliation possess a complete atomic layer structure and their electrical properties closely resemble their intrinsic characteristics. This method is simple to operate, and the nanosheet products can be uniformly dispersed in solution. In liquid-phase mechanical exfoliation, powder samples are often mixed with DMF or NMP, which have similar surface energies, to improve the exfoliation effect and solution stability. If monolayer nanosheet materials could be obtained without using these two solvents, and could remain stable in other low-boiling-point and environmentally friendly solutions while improving film purity, this would be highly desirable in industrial applications. Furthermore, combining the tunable bandgap and excellent carrier transport properties of WS2 with the advantages of the liquid-phase preparation method will expand the applications of this type of material in optoelectronic semiconductor devices. Summary of the Invention

[0007] To address the high time and cost of lithium-ion insertion / exfoliation and chemical vapor deposition methods, as well as the technical challenges of industrial application, this invention proposes a low-temperature mechanical exfoliation method for tungsten disulfide nanosheets and its application in organic photovoltaics. By using an environmentally friendly solvent, a homogeneous thin film is formed from tungsten disulfide nanosheets through rapid evaporation, while providing interfacial tension matching that of the disulfide. This film layer can be used as a hole transport layer in organic photovoltaics, exhibiting carrier transport performance comparable to the commercially available hole transport material PEDOT:PSS. Furthermore, the film's transmittance at 500-800 nm is even superior to that of PEDOT:PSS, demonstrating the potential for fabricating semi-transparent organic photovoltaic devices.

[0008] The technical solution of this invention is as follows.

[0009] A method for fabricating an organic photovoltaic interface transport layer based on WS2 includes the following steps:

[0010] (1) Add a mixture of deionized water and ethanol to tungsten disulfide powder to prepare a mixed solution with a concentration of 5-15 mg / mL;

[0011] (2) Ultrasonication was performed in an ultrasonic nanomaterial disperser with an ultrasonic probe at a power of 500-1000W, while the solution was kept at a low temperature of 0-5℃.

[0012] (3) The solution after ultrasonic exfoliation is centrifuged and its supernatant is retained to obtain tungsten disulfide nanosheet solution;

[0013] (4) The tungsten disulfide nanosheet solution is directly spin-coated to form a dense film on a clean indium tin oxide (ITO) substrate, and the ITO does not need to be treated with O2-plasma to obtain an organic photovoltaic interface transport layer based on WS2.

[0014] In the above method, the tungsten disulfide interface layer solution is prepared by low-temperature ultrasonic-assisted exfoliation. Step (4) specifically involves spin-coating the tungsten disulfide solution onto an untreated ITO substrate. The spin-coating speed is controlled at 900–3000 rpm, the spin-coating time is 30 seconds, and the substrate is rapidly annealed at 60°C for 5 minutes to obtain an organic photovoltaic interface transport layer based on WS2.

[0015] In the above method, the tungsten disulfide nanosheet solution obtained by mechanical exfoliation has a concentration of 0.5 to 1.5 mg / mL. It is then applied to an ITO substrate that has not been treated with O2-plasma by spin coating, blade coating or spray coating, and annealed at 60°C for 5 min. The film thickness is about 1 to 5 nm.

[0016] In the above method, the nanosheets are composed of monolayer (majority) and few-layer structures, wherein the thickness of the monolayer nanosheets is 1-5 nm and the length is 10-15 nm; the thickness of the few-layer nanosheets is 10-60 nm and the length is 40-300 nm.

[0017] In the above method, in step (1), the volume ratio of deionized water to ethanol in the mixed solution of deionized water and ethanol is 1:1.

[0018] In the above method, the ultrasound time in step (2) is 9 hours.

[0019] In the above method, in step (3), the centrifugation rate is 4000 to 10000 rpm.

[0020] An organic photovoltaic interface transport layer based on WS2 is applied to an organic photovoltaic device, and comprises the following structures from bottom to top:

[0021] The substrate consists of a bottom electrode, a hole transport layer, a photoactive layer, an electron transport layer, and a top electrode, wherein the hole transport layer is the organic photovoltaic interface transport layer of WS2.

[0022] Furthermore, in the photoactive layer, the donor is PM6, the acceptor is Y6, the donor-acceptor mass ratio is 1:1.2, the total concentration is 10-16 mg / mL, the solvent used is chloroform, and 0.5% volume fraction of 1-chloronaphthalene is added, and the thickness of the active layer is 100-120 nm.

[0023] Furthermore, the electron transport layer material is PFN-Br, prepared as a 0.5 mg / mL solution using methanol as a solvent, with a thickness of 5 nm to 10 nm.

[0024] Furthermore, the cathode material is Ag or Al, and the electrode is deposited onto the device by thermal evaporation, with the pressure inside the cavity during deposition being less than 4 × 10⁻⁶. -4 Pa, thermal evaporation rate is The cathode thickness is 100 nm.

[0025] In this application, the concentration of nanosheets in the WS2 transport layer and the work function of WS2 and the ITO substrate were controlled. By adjusting the initial concentration of WS2 and the ultrasonic power, ultrasonic time, and centrifugation time of the suspension during mechanical exfoliation, and using a mixed solution of ultrapure water and ethanol prepared in a specific ratio, a homogeneous film could be rapidly formed through thermal annealing at a lower temperature. Since water molecules may interact strongly with individual nanosheets, inducing them to stack into clusters, the mixed solvent of water and ethanol provides a more suitable interfacial tension, reducing cluster formation. In the water-ethanol mixed solvent, water molecules can be considered as a binder, enhancing the interaction between nanosheets. This WS2 film forms a suitable hole transfer barrier between the HOMO energy levels of ITO and the polymer donor PM6 (the work function of the WS2 film is 5.67 eV), which is beneficial for carrier injection and results in a higher hole mobility. The performance comparison with PEDOT:PSS is shown in Table 1.

[0026] Table 1

[0027]

[0028] This invention provides a method for preparing an interface transport layer for organic solar cells with high transmittance and high carrier mobility through low-temperature ultrasonic-assisted liquid-phase exfoliation. The beneficial effects of this invention are as follows:

[0029] 1. In this invention, the disulfide powder is exfoliated using a low-temperature ultrasonic exfoliation method combined with a water-alcohol mixture, reducing aggregation during the WS2 exfoliation process and improving the mechanical exfoliation effect. This method simplifies the preparation process requirements of WS2 nanosheets and is beneficial for commercial production.

[0030] 2. When the WS2 of the present invention is applied to organic photovoltaic devices, there is no need to perform O2-plasma treatment on the ITO substrate, which can produce better film formation effect and exhibit better device efficiency.

[0031] 3. The WS2 of the present invention has high light transmittance in the 500-800nm ​​range, and has broad application prospects in semi-transparent organic photovoltaic devices with selective light transmittance requirements. Attached Figure Description

[0032] Figure 1 A schematic diagram of the preparation of WS2 nanosheets by ultrasound-assisted liquid phase exfoliation.

[0033] Figure 2 This is a schematic diagram of an organic photovoltaic device structure based on the WS2 hole transport layer.

[0034] Figure 3 For ITO / WS2 and ITO plasma / PEODT: PSS transmission spectrum.

[0035] Figure 4 An atomic force microscope image of the WS2 film on an ITO substrate before O2-plasma treatment.

[0036] Figure 5 An atomic force microscope image of the WS2 film after O2-plasma treatment on an ITO substrate.

[0037] Figure 6 For ITO plasma / WS2, ITO / WS2 and ITO plasma / PEDOT:PSS's organic photovoltaic device current density-voltage curve under simulated sunlight.

[0038] Figure 2 The components are as follows:

[0039] 1 is an ITO substrate, 2 is a tungsten disulfide film, 3 is a photoactive layer, 4 is a PFN-Br electron transport layer, and 5 is an Ag or Al metal back electrode. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto. For process parameters not specifically noted, conventional techniques can be referred to.

[0041] To further clarify the preparation method of mechanically stripped disulfides and their application in organic photovoltaics, the following explanation is provided in conjunction with accompanying figures and examples.

[0042] The method in this embodiment is as follows.

[0043] The tungsten disulfide nanosheet solution suitable for hole transport layers in organic photovoltaics provided by this invention includes the following steps:

[0044] (1) Add a solvent made of deionized water and anhydrous ethanol in a 1:1 ratio to WS2 powder to prepare a mixed solution of 5-15 mg / mL, and seal the mouth of the beaker with plastic wrap.

[0045] (2) Immerse the beaker in a cooling device with circulating water at 0-5℃, and use an ultrasonic nanomaterial disperser with an ultrasonic probe to perform ultrasonic exfoliation on the WS2 solution. Adjust the ultrasonic power to 500-1000W and the ultrasonic time to 6-12h.

[0046] (3) Transfer the solution after WS2 stripping in step (2) to a centrifuge tube and centrifuge at 4000-10000 rpm to obtain WS2 solutions with different number of layers and nanosheet sizes. After centrifugation, the solution changes from dark brown to light yellow. Take 1 / 4 of the supernatant and store it under cold.

[0047] Organic solar cells using tungsten disulfide nanosheets of the present invention as hole transport layers can be prepared according to the following method:

[0048] (1) The transparent ITO substrate was ultrasonically cleaned sequentially with an oil cleaner, deionized water, and isopropanol, with each ultrasonic cleaning lasting 20 minutes. The cleaned ITO substrate was then placed in an oven at 70°C and dried overnight.

[0049] (2) The prepared WS2 nanosheet solution was spin-coated onto ITO in air. The spin-coating speed was controlled at 900–3000 rpm, the spin-coating time was 30 seconds, and then rapidly annealed at 60°C for 5 minutes. It should be noted that the ITO substrate does not need to be treated with O2-plasma: on the one hand, after plasma treatment, the energy levels are deepened, and excessively deep energy levels affect hole transport; on the other hand, atomic force microscopy shows that WS2 without plasma treatment can adhere to more fine small-diameter WS2 flakes, thereby improving the hole mobility of WS2. The nanosheets are composed of monolayer (majority) and few-layer structures, wherein the thickness of the monolayer nanosheets is 1–5 nm and the length is 10–15 nm; the thickness of the few-layer nanosheets is 10–60 nm and the length is 40–300 nm.

[0050] (3) Prepare a PM6:Y6 photoactive layer solution with a total concentration of 16 mg / mL at a ratio of 1:1.2. Dissolve the active layer material in chloroform and add 0.5% (v / v) of 1-chloronaphthalene. Stir overnight at 50°C. Spin-coat the above active layer solution onto the film from step (2). The resulting active layer film has a thickness of 100–120 nm.

[0051] (4) Prepare an electron transport layer PFN-Br solution with a concentration of 0.5 mg / mL. The PFN-Br is dissolved in methanol and stirred overnight at 60°C. Spin-coat a 5 nm to 15 nm layer of PFN-Br onto the film from step (3).

[0052] (5) A 100 nm metal cathode is deposited on the electron transport layer. The electrode material used is Ag or Al, and the pressure inside the deposition chamber is reduced to below 4 × 10⁻⁶. -4 Pa, controlling the thermal evaporation rate to be The evaporation rate needs to be increased slowly in a gradient.

[0053] Example 1

[0054] This example further illustrates the preparation method of WS2 nanosheets by low-temperature ultrasonic exfoliation. The specific steps are as follows:

[0055] Step 1:

[0056] Dissolve 0.364 g of WS2 powder in a 1:1 mixture of anhydrous ethanol and deionized water to obtain a 14 mg / mL solution. Seal the mouth of the beaker with a protective film and make a hole slightly smaller than the diameter of the ultrasonic nanodispersant probe. Immerse the bottom of the beaker in a constant temperature water bath at 0-5℃. Insert the ultrasonic probe (instrument amplitude rod) 1-1.5 cm below the liquid surface. Adjust the instrument power to 750W, adaptive amplitude, and the ultrasonic device as follows. Figure 1As shown. The solution was continuously sonicated for 9 hours under atmospheric conditions. The sonicated WS2 was transferred to a centrifuge tube and centrifuged at 8000 rpm for 30 minutes. The top 1 / 4 of the centrifuged solution was then collected and stored in a refrigerator.

[0057] Step Two:

[0058] The ITO substrate was ultrasonically cleaned sequentially using an oil-removing cleaner, deionized water, and isopropanol, with each ultrasonic cleaning lasting 20 minutes. The cleaned ITO substrate was then dried overnight in a 70°C oven. This structure corresponds to... Figure 2 The number 1.

[0059] Step 3:

[0060] The prepared WS2 nanosheet solution was spin-coated onto ITO in air. 50 μL of the solution was used, and the spin-coating speed was controlled at 900–3000 rpm for 30 seconds, followed by rapid annealing at 60°C for 5 minutes. This structure corresponds to… Figure 2 The number 2 is used in the middle section. Furthermore, the control sample, the PEDOT:PSS film, was prepared by spin-coating a PEDOT:PSS solution onto a plassa-treated ITO substrate at 3000 rpm for 30 seconds, followed by rapid annealing for 15 minutes. The transmittance spectra of the two hole transport layers are as follows: Figure 3 As shown.

[0061] It is important to note that in this example, the ITO substrate for WS2 does not require O2-plasma treatment: after plasma treatment, the oxygen vacancies are filled, reducing the number of dangling bonds on the ITO and weakening the chemisorption of WS2 onto the substrate. Atomic force microscopy observation shows that untreated WS2 can adhere to more finely sized WS2 flakes, which enhances interfacial conductivity and thus improves the hole migration performance of WS2. Figure 4 As shown.

[0062] Example 2

[0063] Based on Example 1, ITO was subjected to O2-plasma treatment before spin-coating the WS2 nanosheet solution. The spin-coating speed was controlled at 900–3000 rpm, the spin-coating time was 30 seconds, and then it was rapidly annealed at 60°C for 5 minutes. The atomic force microscopy image is shown below. Figure 5 As shown.

[0064] Example 3

[0065] Based on Example 1, a method for preparing an organic photovoltaic based on a WS2 hole transport layer includes the following steps:

[0066] (1) A PM6:Y6 photoactive layer solution with a total concentration of 16 mg / mL, prepared at a ratio of 1:1.2, was spin-coated onto ITO containing WS2 and PEDOT:PSS. The thickness of the active layer was 100–120 nm. This structure corresponds to… Figure 2 The middle number is 3. The active layer solution was prepared as follows: the receptor mixture was dissolved in chloroform and 0.5% (v / v) of 1-chloronaphthalene solution was added, and the mixture was stirred overnight at 50°C.

[0067] (2) A 5-15 nm thick 0.5 mg / mL electron transport layer PFN-Br is spin-coated onto the above active layer. This structure corresponds to... Figure 2 The middle number is 4. The preparation method of PFN-Br is as follows: Dissolve PFN-Br in methanol and stir overnight at 60°C.

[0068] (3) A 100 nm metal cathode is deposited on the aforementioned electron transport layer. This structure corresponds to... Figure 2 The number 5 is used. The electrode material is Ag or Al, and the pressure inside the vapor deposition chamber is reduced to below 4 × 10⁻⁶. -4 Pa, controlling the thermal evaporation rate to be The deposition rate needs to be increased slowly in a gradient. The current density-voltage curve of the device is as follows: Figure 6 As shown.

[0069] Example 4

[0070] Based on Example 2, a method for preparing an organic photovoltaic based on a tungsten disulfide hole transport layer includes the following steps:

[0071] (1) On ITO coated with WS2, a PM6:Y6 photoactive layer solution with a total concentration of 16 mg / mL prepared at a ratio of 1:1.2 was spin-coated, and the thickness of the active layer was 100-120 nm. The active layer solution was prepared as follows: the acceptor mixture was dissolved in chloroform and 0.5% by volume of 1-chloronaphthalene was added, and the mixture was stirred overnight at 50 °C.

[0072] (2) Spin-coat a 5-15 nm thick 0.5 mg / mL electron transport layer PFN-Br onto the above active layer. The preparation method of PFN-Br is as follows: dissolve PFN-Br in methanol and stir overnight at 60 °C.

[0073] (3) A 100 nm metal cathode is deposited on the electron transport layer described above. The electrode material used is Ag or Al, and the pressure inside the deposition chamber is reduced to below 4 × 10⁻⁶. -4 Pa, controlling the thermal evaporation rate to be The deposition rate needs to be increased slowly in a gradient. The current density-voltage curve of the device is as follows: Figure 6 As shown.

[0074] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing an organic photovoltaic interface transport layer based on WS2, comprising the following steps: (1) Add a mixture of deionized water and ethanol to tungsten disulfide powder to prepare a mixed solution with a concentration of 5~15 mg / mL; (2) Ultrasonication is performed in an ultrasonic nanomaterial disperser with an ultrasonic probe at a power of 500~1000W, while the solution is kept at a low temperature of 0~5℃. (3) The solution after ultrasonic exfoliation is centrifuged and its supernatant is retained to obtain tungsten disulfide nanosheet solution; (4) The tungsten disulfide nanosheet solution is directly spin-coated to form a dense film on a clean indium tin oxide (ITO) substrate, and the ITO does not need to be treated with O2-plasma to obtain an organic photovoltaic interface transport layer based on WS2. Tungsten disulfide nanosheet solution obtained by mechanical exfoliation, with a concentration of 0.5~1.5 mg / mL, was applied to an ITO substrate that had not been treated with O2-plasma by spin coating, blade coating or spray coating, and annealed at 60°C for 5 min, with a film thickness of about 1~5 nm. The nanosheets are composed of single-layer and few-layer structures, wherein, Single-layer nanosheets have a thickness of 1-5 nm and a length of 10-15 nm; few-layer nanosheets have a thickness of 10-60 nm and a length of 40-300 nm.

2. The method for preparing an organic photovoltaic interface transport layer based on WS2 according to claim 1, characterized in that: Step (4) specifically involves spin coating to form a film on an untreated ITO substrate. The spin coating speed is controlled at 900~3000 rpm, the spin coating time is 30 seconds, and the substrate is rapidly annealed at 60°C for 5 minutes to obtain an organic photovoltaic interface transport layer based on WS2.

3. The method for preparing an organic photovoltaic interface transport layer based on WS2 according to claim 1, characterized in that: In step (1), the volume ratio of deionized water to ethanol in the mixed solution is 1:1; in step (2), the ultrasonic time is 9 hours; in step (3), the centrifugation rate is 4000~10000 rpm.

4. An organic photovoltaic interface transport layer based on WS2 is prepared by the preparation method according to any one of claims 1 to 3.

5. The WS2-based organic photovoltaic interface transport layer of claim 4 is applied to an organic photovoltaic device, characterized in that, The following structures are included from bottom to top: The substrate consists of a bottom electrode, a hole transport layer, a photoactive layer, an electron transport layer, and a top electrode, wherein the hole transport layer is the organic photovoltaic interface transport layer of WS2.

6. The application according to claim 5, characterized in that: In the photoactive layer, the donor is PM6, the acceptor is Y6, the donor-acceptor mass ratio is 1:1.2, the total concentration is 10-16 mg / mL, the solvent used is chloroform, and 0.5% volume fraction of 1-chloronaphthalene is added. The thickness of the active layer is 100-120 nm.

7. The application according to claim 5, characterized in that: The electron transport layer material is PFN-Br, prepared as a 0.5 mg / mL solution using methanol as a solvent, with a thickness of 5 nm to 10 nm.

8. The application according to claim 5, characterized in that: The bottom or top electrode is made of Ag or Al, and is deposited onto the device via thermal evaporation, with the pressure inside the cavity during deposition being less than 4 × 10⁻⁶. -4 Pa, with a thermal evaporation rate of 0.1~2 Å / s.

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