Application of a Method of Continuous Molecular Injection in Perovskite Solar Cells

Abstract

Application of a method of continuous molecular injection in perovskite solar cells. Through the MISD process, the present invention achieves higher-efficiency molecular doping, thereby greatly improving the conductivity and hole mobility of the thin film, optimizing the surface potential of the thin film, and reducing the HOMO energy level of the thin film. These improvements significantly reduce the non-radiative recombination of the device, reduce the loss of carriers during the transmission process, and ultimately greatly improve the device efficiency. Due to the absence of the influence of dopants such as LiTFSI and tBP, the MISD device also exhibits excellent stability and can still maintain more than 90% of the initial efficiency after being stored in the environment for 5200 h. This shows that the present invention can improve both the battery efficiency and stability, providing inspiration and reference for the commercial development of perovskite solar cells.

Description

Technical Field

[0001] The present invention relates to the field of preparing organic-inorganic metal halide perovskite solar cells, and particularly relates to the application of a continuous molecular injection method in perovskite solar cells. Background Art

[0002] Converting light energy into electrical energy using solar cells is an important way to alleviate the energy crisis. Currently, compared with the mainstream silicon-based solar cells in the market, organic-inorganic metal halide perovskite solar cells have become one of the most promising photovoltaic candidate materials due to their excellent optoelectronic properties, low cost, and ease of fabrication. In the past decade, solar cells with perovskite as the light absorption layer have developed rapidly, and the certified efficiency of single-junction has exceeded 25%.

[0003] In perovskite solar cells (PSCs), it usually consists of a conductive glass substrate (ITO), a hole transport layer (HTL), a perovskite layer, an electron transport layer, and a metal electrode. Among them, the hole transport layer (HTL) plays an important role in PSCs because it allows the removal of positive charges from the perovskite layer, which is crucial for reducing non-radiative recombination losses and improving charge transfer efficiency. Currently, Spiro-OMeTAD is the most commonly used hole transporting material (HTM), but due to the weak intermolecular interaction force, the conductivity and hole mobility of pure-phase Spiro-OMeTAD are relatively low. Therefore, chemical doping is considered the most effective way to improve the film conductivity and hole mobility.

[0004] Currently, the most commonly used chemical dopants are Li-TFSI and tBP. However, Li salts have strong water absorption, which will inevitably absorb moisture in the surrounding environment, thus exacerbating the degradation of the perovskite layer and HTL, and then seriously affecting the long-term stability of the device. In addition, during the device testing process, due to the influence of the applied bias voltage, Li+ is very easy to migrate from the HTL to the perovskite layer, and Li+ is also prone to aggregation in the HTL. In addition to Li-TFSI, due to its low boiling point, tBP is prone to volatilization during the long-term storage of perovskite solar cell devices, forming holes in the hole transport layer, increasing the probability of reaction between the metal electrode and the perovskite layer, and also seriously affecting the long-term stability of the device. At the same time, during the testing process, halogen ions (I-, Br-) in the perovskite layer are prone to migrate from the perovskite layer to the hole transport layer due to the influence of light and applied bias voltage, thus forming an over-doping phenomenon, which will also affect the conductivity of the HTL, thereby having an adverse impact on the performance of the device.

[0005] After the HTL is prepared by the spin coating method, Li-TFSI reacts with Spiro-OMeTAD to form a certain intermediate phase. This type of oxidation reaction must be carried out in an air atmosphere, and the highest efficiency can only be obtained after reacting for a period of time. This phenomenon affects the stable output of the device power.

[0006] In addition, the current method for preparing HTL based on Spiro-OMeTAD is usually the spin coating method. Although this method is relatively simple, there are many problems, such as being unable to perform large-area preparation, having a high selectivity for the substrate, being unable to precisely control the film formation thickness, and a relatively serious waste rate of the precursor solution. These problems restrict the commercialization process of perovskite solar cells. Compared with the spin coating method, the thermal evaporation method for preparing HTL can effectively avoid this type of problem. The thermal evaporation method can perform large-area preparation of the thin film, has a low selectivity for the substrate, can deposit and form a film on a flexible substrate or a substrate with a large roughness, and can precisely control the film thickness and has a high utilization rate of the precursor. Therefore, the thermal evaporation method for preparing HTL has many potentials in the actual application process. However, at present, the thermal evaporation method for preparing HTL still faces the problem of low device efficiency.

[0007] We found from previous studies that the efficiency of preparing undoped Spiro-OMeTAD HTL by the thermal evaporation method is low because the spirofluorene of Spiro-OMeTAD is twisted, which affects the stacking state of Spiro-OMeTAD molecules. The molecular stacking state is relatively fluffy, there are many voids between molecules, and the density of the thin film is low, which greatly increases the carrier transport distance and the thin film resistance. In addition, there is a strong Light soaking phenomenon in the device prepared by the evaporation method for Spiro-OMeTAD HTL. The initial efficiency of the device is low, and it takes a long time of light irradiation test to reach its highest efficiency, which also poses a challenge to the stable power output of the device. Another problem with pure-phase Spiro-OMeTAD is that the HOMO of its molecules is relatively high, and there is a large energy level difference with the HOMO of the perovskite layer, which will cause carriers to easily recombine during the transport process. Compared with plasma doping such as Li-TFSI, molecular doping shows its great advantages. However, traditional molecular doping is only at the interface and the doping efficiency is low. Summary of the Invention

[0008] The object of the present invention is to solve the above problems in the prior art, and provide an application of a continuous molecular injection method in perovskite solar cells. By means of post-treatment, continuous molecular injection is carried out to improve the efficiency and stability of perovskite solar cells. Specifically, after the perovskite layer is prepared, Spiro-OMeTAD thin film and F4TCNQ layer are prepared by thermal evaporation method. After the two thin films are prepared, the prepared thin films are first transferred to the solvent atmosphere of isopropanol to realize the downward penetration of F4TCNQ molecules, and the step of molecular injection doping is completed. Then the thin films are transferred to the solvent atmosphere of chlorobenzene, and the compression of Spiro-OMeTAD thin film is realized through the post-treatment process of chlorobenzene, changing the stacking state of Spiro-OMeTAD molecules, reducing the carrier transport distance, and improving the carrier transport efficiency. The present invention proposes a process of molecular implantation sequential doping (MISD) to improve the doping efficiency of molecular doping.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] An application of a continuous molecular injection method in perovskite solar cells, comprising the following steps:

[0011] 1) Treat the cleaned ITO glass with ultraviolet-ozone to improve wettability;

[0012] 2) Mix the SnO2 (15wt%) solution and deionized water in a volume ratio and stir, directly spin-coat the prepared SnO2 solution on the ITO substrate, and anneal on a hot stage to prepare an electron transport layer. After cooling to room temperature, treat with ultraviolet-ozone again;

[0013] 3) Prepare a perovskite Cs 0.07 FA 0.9 MA 0.03 Pb(I 0.92 Br 0.08 )3 precursor solution, and spin-coat the prepared perovskite precursor solution on the electron transport layer and anneal;

[0014] 4) Put the sample prepared in step 3) into an evaporation chamber, and then thermally evaporate Spiro-OMeTAD and F4TCNQ on the perovskite thin film in sequence;

[0015] 5) First transfer the sample prepared in step 4) to an isopropanol solvent to realize the downward penetration of F4TCNQ molecules; then transfer the thin film to a chlorobenzene solvent, and realize the compression of Spiro-OMeTAD through the post-treatment of chlorobenzene, changing the stacking state of Spiro-OMeTAD molecules;

[0016] 6) Place the sample prepared in step 5) into the evaporation chamber for the thermal evaporation preparation of the Ag electrode.

[0017] In steps 1) and 2), the time for ultraviolet-ozone treatment is 5 - 30 min, preferably 10 - 20 min.

[0018] In step 2), the volume ratio is 1:1 - 1:5, and the stirring time is 1 - 30 min. Preferably, the volume ratio is 1:3 and the stirring time is 5 min.

[0019] In step 2), the spin coating speed is 1000 - 5000 r / min, and the duration is 15 - 50 s. Preferably, the speed is 3000 r / min and the time is 30 s.

[0020] In step 2), the annealing temperature is 50 - 350 °C and the time is 10 - 100 min. Preferably, the annealing temperature is 200 °C and the time is 40 min.

[0021] In step 3), the perovskite Cs 0.07 FA 0.9 MA 0.03 Pb(I 0.92 Br 0.08 )3 precursor solution uses a mixed solution of DMF and DMSO as the solvent.

[0022] In step 3), the spin coating speed is 1000 - 8000 r / min, and the duration is 15 - 50 s.

[0023] In step 3), the annealing temperature is 50 - 500 °C and the time is 10 - 100 min. Preferably, the annealing temperature is 100 °C and the time is 40 min.

[0024] In step 4), the thickness of Spiro-OMeTAD is 10 - 150 nm; the thickness of F4TCNQ is 5 - 30 nm. Preferably, the thickness of Spiro-OMeTAD is 60 - 80 nm; the thickness of F4TCNQ is 15 - 30 nm.

[0025] In step 5), the time in isopropanol and chlorobenzene solvents is not more than 10 min. Preferably, the time in isopropanol and chlorobenzene solvents is 4 - 6 min.

[0026] In step 6), the thickness of Ag is 10 - 120 nm. Preferably, the thickness is 60 - 90 nm.

[0027] Compared with the prior art, the beneficial effects achieved by the technical solution of the present invention are:

[0028] The present invention uses a thermal evaporation method to prepare a hole transport layer, which can fabricate the thin film on a large area. Meanwhile, it has a low selectivity to the substrate and can deposit a film on a flexible substrate or a substrate with a large roughness. Moreover, the film thickness can be precisely controlled and the utilization rate of the precursor is also high. In addition, without the influence of dopants such as LiTFSI and tBP in the hole transport material, the devices of MISD also exhibit excellent stability and can still maintain more than 90% of the initial efficiency after being stored in the environment for 5200 h. Furthermore, in the present invention, after the prepared thin film is first transferred to the solvent atmosphere of isopropanol, the downward penetration of F4TCNQ molecules is realized to complete the step of molecular injection doping. Then the thin film is transferred to the solvent atmosphere of chlorobenzene, and the compression of the Spiro-OMeTAD thin film is realized through the post-treatment process of chlorobenzene to change the stacking state of Spiro-OMeTAD molecules. Through the process of MISD, more efficient molecular doping is achieved, thereby greatly improving the conductivity and hole mobility of the thin film, optimizing the surface potential of the thin film, and reducing the HOMO energy level of the thin film. These improvements greatly reduce the non-radiative recombination of the device, reduce the loss of carriers during the transmission process, and ultimately greatly improve the device efficiency (>22.3%). Description of the Drawings

[0029] Figure 1 is a schematic structural diagram of the organic-inorganic lead halide perovskite solar cell of the present invention;

[0030] Figure 2 are the J-V characteristic curves of the Control, SD, and MISD devices based on a glass substrate;

[0031] Figure 3 are the J-V characteristic curves of the Control, SD, and MISD devices based on a flexible substrate;

[0032] Figure 4 are the PCE histograms of different HTL devices based on a flexible substrate and B) a photo of the MISD device;

[0033] Figure 5 are the long-term stability test results of the unsealed devices of SD, Control, and MISD, where the devices are stored in a drying cabinet with a relative humidity of about 20% - 30%. Detailed Description of the Invention

[0034] In order to make the technical problems, technical solutions, and beneficial effects to be solved by the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the drawings and embodiments.

[0035] Example 1

[0036] SeeFigure 1 , in this embodiment, three types of devices with different structures were fabricated. Control represents the device with only thermally evaporated Spiro-OMeTAD, SD represents the device with continuously thermally evaporated Spiro-OMeTAD and F4TCNQ, and MISD represents the device after post-treatment in an isopropanol and chlorobenzene atmosphere successively after continuously thermally evaporating Spiro-OMeTAD and F4TCNQ.

[0037] Fabrication was carried out according to Figure 1 the device structure, and then device characterization was performed. The fabrication steps of the MISD device are as follows:

[0038] 1. The ITO glass substrate was ultrasonically treated successively in deionized water, glass cleaning solution, acetone, and isopropanol, and then dried with dry compressed air. The cleaned ITO glass was treated with ultraviolet-ozone for 15 min to improve wettability.

[0039] 2. The SnO2 (15 wt%) solution was mixed with deionized water at a volume ratio of 1:3 and stirred for 5 min. The prepared SnO2 solution was directly spin-coated on the ITO substrate at 3000 r / min for 30 s and annealed at 200 °C on a hot stage for 40 min to prepare the electron transport layer. After cooling to room temperature, it was treated with ultraviolet-ozone again for 15 min.

[0040] 3. Prepare the Cs 0.07 FA 0.9 MA 0.03 Pb(I 0.92 Br 0.08 )3 perovskite precursor solution: 191.4 mg of FAI, 13.5 mg of FABr, 24.6 mg of CsI, 31.9 mg of MACl, 39.6 mg of PbBr2, 0.64 mg of MAI, and 591.2 mg of PbI2 were added to 1 mL of DMF / DMSO (v / v, 1:4) solution; 40 μL of the perovskite precursor solution was directly spin-coated on the ITO substrate, first at a rotation speed of 1500 r / min for 10 s, and then at 4000 rpm / min for 30 s; then the obtained film was transferred to a hot stage and heated at 100 °C for 40 min.

[0041] 4. The prepared perovskite thin film was transferred into a thermal evaporation chamber, and 80 nm thick and 30 nm thick Spiro-OMeTAD and F4TCNQ were successively thermally evaporated on the prepared perovskite thin film.

[0042] 5. After the films of Spiro-OMeTAD and F4TCNQ were fabricated, the films were first transferred to an isopropanol solvent atmosphere for post-treatment for 5 min, and then the films were transferred to a chlorobenzene solvent atmosphere for treatment for 5 min.

[0043] 6. An 80-nm-thick Ag electrode was thermally evaporated onto the thin film to complete the preparation of the device.

[0044] Using a xenon lamp solar simulator, the test light source intensity was AM 1.5G, 100 mW / cm². -2 The open-circuit voltage, short-circuit current, and fill factor of the prepared battery device were tested. Figures 2 - 3 are the J-V curves of the devices prepared on glass substrates and flexible substrates, respectively. Figure 4 are the PCE histograms of different HTL devices based on flexible substrates and B) photos of MISD devices. Figure 5 is the long-term stability test chart of the device.

[0045] Figure 2 shows the open-circuit voltage V of the MISD device. oc is 1.125 V, and the short-circuit current J sc is 24.28 mA / cm². 2 , the fill factor FF is 81.64%, the photoelectric conversion efficiency is 22.31%, and the prepared device still maintains 90% of the initial efficiency after being stored in the glove box for 5200 h ( Figure 5 ). In comparison, for the SD device, its V oc is 0.945 V, the short-circuit current J sc is 24.11 mA / cm². 2 , the fill factor FF is 75.06%, the photoelectric conversion efficiency is 17.1%, and the efficiency drops to 50% of the initial efficiency after being stored under the same conditions for 2000 h. The above results show that the devices prepared by the method of the present invention have simultaneously improved efficiency and stability.

[0046] Figure 3 shows the synthesized device with a flexible substrate, indicating that the device synthesized by the present invention can not only be applied to rigid substrates (such as glass), but also be synthesized on flexible substrates, providing a basis for the subsequent application of perovskite solar cells in life. Figure 3 The efficiency parameter trend of the flexible substrate device in is similar to that of the glass substrate device, but the parameters of the flexible substrate device are lower than those of the glass substrate device. Figure 4 A) The photoelectric conversion efficiency of the flexible substrate device of MISD is 19.51%, and the efficiency of the corresponding SD device is only 14.76%.

[0047] Through the MISD process, the present invention achieves more efficient molecular doping, thereby greatly improving the conductivity and hole mobility of the thin film, optimizing the surface potential of the thin film, and reducing the HOMO energy level of the thin film. These improvements significantly reduce the non-radiative recombination of the device, reduce the loss of carriers during the transmission process, and ultimately greatly improve the device efficiency (>22.3%). Due to the absence of the influence of dopants such as LiTFSI and tBP, the MISD device also exhibits excellent stability and can still maintain more than 90% of the initial efficiency after being stored in the environment for 5200 h. This shows that the present invention can improve both the battery efficiency and stability, providing inspiration and reference for the commercial development of perovskite solar cells.

Claims

1. Application of a method for continuous molecular injection in a perovskite solar cell, characterized in that Including the following steps: 1) Treat the cleaned ITO glass substrate with ultraviolet-ozone to improve wettability; 2) Spin-coat the SnO2 solution on the ITO glass substrate and anneal it on a hot stage to prepare the electron transport layer. After cooling to room temperature, treat it with ultraviolet-ozone again; 3) Prepare the perovskite Cs 0.07 FA 0.9 MA 0.03 Pb(I 0.92 Br 0.08 )3 precursor solution, and spin-coat the prepared perovskite precursor solution on the electron transport layer and anneal it to form a perovskite thin film; 4) Place the sample prepared in step 3) into the evaporation chamber, and then thermally evaporate Spiro-OMeTAD and F4TCNQ onto the perovskite film in sequence; 5) First, transfer the sample prepared in step 4) to an isopropanol solvent to achieve the downward penetration of F4TCNQ molecules; then transfer the sample to a chlorobenzene solvent, and compress Spiro-OMeTAD through the post-treatment of chlorobenzene to change the stacking state of Spiro-OMeTAD molecules; 6) Place the sample prepared in step 5) into the evaporation chamber for the thermal evaporation preparation of the Ag electrode; In steps 1) and 2), the time of ultraviolet-ozone treatment is 5 - 30 min.

2. The application of a continuous molecular injection method as described in claim 1 to a perovskite solar cell, characterized in that: In step 2), the spin-coating speed is 1000 - 5000 r / min, and the duration is 15 - 50 s; the annealing temperature is 50 - 350 °C, and the time is 10 - 100 min.

3. Application of a continuous molecular injection method in a perovskite solar cell as described in claim 1, characterized in that: In step 3), the solvent used for the perovskite Cs 0.07 FA 0.9 MA 0.03 Pb(I 0.92 Br 0.08 )3 precursor solution is a mixed solution of DMF and DMSO.

4. Application of a method of continuous molecular injection in a perovskite solar cell according to claim 1, characterized in that: In step 3), the spin-coating speed is 1000 - 8000 r / min, and the duration is 15 - 50 s.

5. Use of the method of continuous molecular injection as described in claim 1 in a perovskite solar cell, characterized in that: In step 3), the annealing temperature is 50 - 500 °C, and the time is 10 - 100 min.

6. The application of a method of continuous molecular injection in a perovskite solar cell according to claim 1, wherein: In step 4), the thickness of Spiro-OMeTAD is 10 - 150 nm; the thickness of F4TCNQ is 5 - 30 nm.

7. Use of the method of continuous molecular injection in a perovskite solar cell according to claim 6, characterized in that: In step 4), the thickness of Spiro-OMeTAD is 60 - 80 nm; the thickness of F4TCNQ is 15 - 30 nm.

8. Use of the method of continuous molecular injection according to claim 1 in a perovskite solar cell, characterized in that: In step 5), the time in both isopropanol and chlorobenzene solvents is not more than 10 min.

9. Application of a continuous molecular injection method in a perovskite solar cell as described in claim 1, characterized in that: In step 6), the thickness of Ag is 10 - 120 nm.

Images