Double-sided solar cell and preparation method thereof

Abstract

The invention relates to the technical field of photovoltaics, and particularly discloses a double-sided solar cell based on high-viscosity molten selenium (Se) and a preparation method of the double-sided solar cell. In order to solve the problems of complicated equipment, high cost, damage to functional layers and the like caused by dependence on a magnetron sputtering process in the prior art, the invention provides an innovative solution that high-viscosity molten selenium is used as a photovoltaic absorption layer and an adhesive, and two pre-deposited charge transport layers (CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL, CTL and CTL) comprising an electron transport layer ETL and a hole transport layer HTL is directly bonded to form a double-sided solar cell. According to the method, a traditional magnetron sputtering process is abandoned, the bottom functional layer is prevented from being damaged by plasmas, and meanwhile the material utilization rate and the process compatibility are improved. The efficiency of the double-sided cell reaches 10.01% under the reflection illumination of 0.5, and the indoor photovoltaic efficiency of the double-sided cell reaches 26.17% under the reflection illumination of 0.8. The method is suitable for the field of building integrated photovoltaic (BIPV), and has remarkable industrialization value.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic materials and the preparation of thin-film solar cells, and particularly relates to a bifacial solar cell and a preparation method thereof. Background Art

[0002] By adopting a double transparent electrode design, a bifacial solar cell can not only capture direct sunlight from the front, but also absorb reflected and scattered light from the back. This unique light-trapping mechanism enables it to effectively utilize more available solar energy resources, thereby providing a higher power output than traditional single-sided photovoltaic cells without significantly increasing the manufacturing cost. Therefore, bifacial photovoltaic technology dominates the current market and provides a new solution for improving the energy conversion efficiency of solar cells. At the same time, it also provides new impetus for promoting the sustainable development of the photovoltaic industry. The key to optimizing the performance of a bifacial solar cell lies in the effective control of its rear-side power conversion efficiency (PCE), which mainly depends on the design and selection of the transparent conductive back electrode. Different from the opaque metal back electrode used in single-sided photovoltaic cells, a bifacial cell needs to use a transparent conductive material to achieve back-side light absorption. Currently, the widely used transparent conductive back electrode materials mainly include transparent conductive oxides (TCOs), such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and indium zinc oxide (IZO), etc. TCOs not only have high transparency and excellent conductivity, but also exhibit good chemical stability and can be compatible with adjacent functional layers. TCO materials have significant advantages in the preparation of bifacial solar cells and are the preferred materials for preparing bifacial solar cells. Magnetron sputtering has become the preferred method for preparing TCO materials due to its excellent film quality, good uniformity, and controllability. Although the cost of magnetron sputtering equipment is high and it may cause damage to the underlying functional layer due to plasma effects, its importance in the preparation of high-efficiency bifacial solar cells makes it the most commonly used technology.

[0003] Prior arts such as CN114944435A, CN113611762A, CN112701182A, and CN104600146A have reported bifacial solar cells. However, the mainstream deposition method of the transparent conductive back electrode (TCO) of current bifacial solar cells, radio frequency magnetron sputtering, causes damage to the underlying functional layer due to plasma effects, thereby affecting the device performance; moreover, the high equipment cost of magnetron sputtering is also a factor restricting the development of bifacial solar cells. Therefore, developing an efficient, low-cost, and non-damaging transparent conductive electrode deposition technology remains an important research direction in the field of bifacial solar cells. Summary of the Invention

[0004] To solve the problems of complex magnetron sputtering technology, high cost, plasma effect damage to the underlying functional layer, and low material utilization rate during sputtering in current bifacial solar cells. The present invention provides a bifacial solar cell based on highly viscous molten selenium (Se) as a photovoltaic absorption layer and its preparation method. Using highly viscous molten selenium as both the photovoltaic absorption layer and the adhesive, two fluorine-doped tin oxide (FTO) glass substrates with pre-deposited charge transport layers (CTL, including electron transport layer ETL and hole transport layer HTL) are directly bonded to form a bifacial solar cell. This method abandons the traditional magnetron sputtering process, avoids plasma damage to the underlying functional layer, and simultaneously improves material utilization rate and process compatibility.

[0005] Selenium (Se), as an early discovered photovoltaic material, has advantages such as low price, green non-toxicity, and good stability. Its bandgap is approximately 1.8 eV. The low-temperature melting characteristic enables molten selenium to be directly used as the photovoltaic absorption layer adhesive of the bifacial selenium solar cell, thus simplifying the manufacturing process and reducing production costs. However, despite the significant potential of selenium in the photovoltaic field, its application in bifacial solar cells has been rarely reported. Therefore, exploring the unique advantages of selenium in the gluing strategy and its application in bifacial solar cells not only helps to develop efficient and low-cost photovoltaic devices but also may provide a new research direction for the revival of selenium-based photovoltaic technology. The high viscosity characteristic of molten selenium stems from the entanglement effect of disordered selenium chains. During the melting process, the trigonal selenium (t-Se) crystal structure dissociates, and the selenium chains change from long-range ordered arrangement to short-range disordered chain-like structure, which significantly increases the viscosity of molten selenium. Molten selenium exhibits a significant viscosity change (6.3 - 1.0 Pa·s) in the temperature range of 217°C to 300°C. Its viscosity value is much higher than that of traditional photovoltaic precursor slurries. This characteristic enables it to flow uniformly during the coating process, forming a film with uniform thickness and flat surface, and showing typical Newtonian fluid behavior with excellent rheological properties. The bifacial selenium solar cell prepared by this technology in the present invention has a bifacial factor as high as 90.1% (bifacial factor calculation formula: BF = PCE front / PCE rear ×100%), exceeding all traditional thin-film solar cell systems, and indicating that the cell can also generate electricity efficiently under backside illumination conditions, broadening its application scenarios. Under standard AM1.5G illumination conditions, the cell achieved a power conversion efficiency (PCE) of 10.01% when the albedo was 0.5, and under 1000 lux indoor illumination conditions with an albedo of 0.8, the PCE was further increased to 26.17%. This power conversion efficiency not only exceeds that of commercial amorphous silicon cells but also is superior to that of the currently best-performing lead-free perovskite cells, indicating that selenium-based photovoltaic devices have great potential in the field of indoor energy harvesting. The present invention provides a photovoltaic device technology with high efficiency, low cost, and strong stability, having broad application prospects.

[0006] The object of the present invention is achieved by the following technical solutions:

[0007] A double-sided solar cell, which is a multi-layer composite structure, includes a transparent conductive substrate deposited with an electron transport layer (ETL), a transparent conductive substrate deposited with a hole transport layer (HTL), and a selenium layer. The two substrates are bonded by molten selenium, and the selenium layer serves as both a light absorption layer and an adhesive.

[0008] On the other side surface of the transparent conductive substrate, there is also a transparent substrate, which is transparent glass with a thickness of 1 - 3 mm.

[0009] Further, the thickness of the molten selenium layer is 2.5 - 5.5 μm, preferably 2.5 - 3.0 μm; the thickness of the electron transport layer is 500 - 1000 nm, preferably 750 - 900 nm; the thickness of the hole transport layer is 10 - 50 nm, preferably 10 - 20 nm; the thickness of the conductive transparent substrate is 200 - 500 nm, preferably 300 - 400 nm.

[0010] Further, the transparent conductive substrate is selected from at least one of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and indium zinc oxide (IZO); the electron transport layer is selected from at least one of TiO 2 , Zn x , Mg 1-x O, SnO 2 ; the hole transport layer is selected from at least one of MoO x , CuSCN (copper cyanide), and Spiro-OMeTAD.

[0011] Preferably, the outer layer of the electron transport layer is a mesoporous TiO 2 layer, and the thickness of the mesoporous TiO 2 layer is 500 - 700 nm; the hole transport layer is MoO x . The core idea of the present invention is to use selenium as both a light absorption layer and an adhesive. Molten selenium exhibits a significant viscosity change in the temperature range of 217°C to 300°C, and its viscosity value is much higher than that of traditional photovoltaic precursor slurries. The inventor found that selenium Se in the battery serves as both an adhesive and a light absorption layer. In order to better achieve the bonding effect with the selenium intermediate layer, the inventor unexpectedly found that when the hole transport layer is MoO x , the performance of the obtained solar cell is the best. The possible reason is that the subsequent annealing treatment of the double-sided solar cell promotes the chemical interaction at the t-Se / MoOx interface, which leads to the formation of an interfacial MoSe x layer, thereby improving the valence band alignment and reducing the barrier and recombination losses.

[0012] The inventor's previous patent CN115084290A disclosed a polycrystalline selenium film and its preparation method, which utilized the low melting point of selenium and the lower processing temperature characteristics, and made the film by the selenium melting method. However, molten selenium has a great influence on the TiO 2 The material has poor wettability. In order to reduce the surface tension of molten selenium, a dense TiO 2 Surface modification with a layer of mesoporous TiO 2 , thereby improving the wettability of the electron transport layer to molten selenium. The present invention also operates in the same way. The significant difference between the present invention and the prior art is that in the prior art, the application of molten selenium is limited to its film-forming properties, while the present invention reveals for the first time the multifunctional potential of molten selenium in the photovoltaic field. Traditionally, molten selenium is used as a film layer forming material, and the technical breakthrough of the present invention is that the high viscosity characteristics of molten selenium are deeply excavated, and it is used as an adhesive while also having the function of a photovoltaic absorption layer. This novel design not only achieves efficient photovoltaic absorption, but also successfully achieves effective bonding to the substrate through the high viscosity characteristics of molten selenium. Molten selenium has a disordered long chain structure, similar to a polymer, and exhibits a high viscosity characteristic. Based on this characteristic, we found that by utilizing the high viscosity of molten selenium, not only good optoelectronic performance can be guaranteed, but also the preparation process of double-sided solar cells can be simplified. Specifically, molten selenium not only helps to form a uniform photovoltaic absorption layer, but also provides a strong bonding effect between the film layer and the substrate. This combination of dual functions of film formation and bonding is difficult to achieve in the prior art. The innovation of this invention lies not only in the expansion of the application of molten selenium, but also in the in-depth exploration and functional application of its high viscosity properties, which successfully maximizes the role of molten selenium in film formation and bonding. Through this unique method, the preparation efficiency and stability of bifacial solar cells are significantly improved, which has obvious technical advantages and broad commercial application prospects.

[0013] The present invention also provides a method for preparing the bifacial solar cell, comprising the following steps:

[0014] (S1) depositing an electron transport layer on a conductive transparent substrate in sequence, and the surface of the electron transport layer is mesoporous TiO 2 layer, annealing treatment to obtain a bottom substrate;

[0015] (S2) depositing a hole transport layer on a conductive transparent substrate, controlling the deposition thickness to 10-20 nm, to obtain a top substrate;

[0016] (S3) Selenium powder is evenly placed on the surface of the bottom substrate, heated to temperature T1, the top substrate preheated to temperature T2 is aligned with the bottom substrate, the two substrates are hot pressed at temperature T3, cooled to room temperature, and annealed to obtain a double-sided solar cell.

[0017] Further, in step (S1), the electron transport layer is a composite of a dense TiO 2 layer and a mesoporous TiO 2 layer. First, the TiO 2 layer is deposited, and then a layer of mesoporous TiO 2 is continuously deposited on the surface of the TiO 2 layer. The operation of depositing TiO 2 can refer to the previous patent CN115084290A. Specifically, first, the conductive transparent substrate is cleaned by ultrasonic cleaning with deionized water, acetone, and isopropanol for 20 - 60 minutes in sequence, then blown dry with high-purity nitrogen, and then cleaned with ultraviolet-ozone for 15 - 30 minutes. After that, a dense titanium dioxide precursor slurry is coated on the cleaned conductive transparent substrate and calcined for the first time to obtain a dense TiO 2 layer. A titanium dioxide slurry is coated on the surface of the dense TiO 2 layer and calcined for the second time to obtain a composite of a dense TiO 2 layer and a mesoporous TiO 2 layer.

[0018] Among them, the dense titanium dioxide precursor is at least one of bis(acetylacetonato)diisopropyl titanate and titanium tetrachloride. The slurry is prepared using an alcohol solvent, and the volume ratio of the dense titanium dioxide precursor to the alcohol is 1 - 2:10. The preparation of the dense titanium dioxide layer uses the spin coating method. The spin coating method is well-known in the art. In a specific embodiment of the present invention, the substrate is placed at the center position of the spin coater, and an appropriate amount of solution is sucked with a pipette and evenly coated on the substrate, and spin-coated at a certain rotation speed. After spin coating, the substrate is transferred to a hot stage, preheated at 150 - 200°C for 5 - 15 min, the temperature is raised to 400 - 500°C, and kept warm for 20 - 30 min to obtain a dense titanium dioxide layer

[0019] The average particle size of titanium dioxide in the titanium dioxide slurry is 30 - 40 nm, and the mass ratio of titanium dioxide paste to absolute ethanol is 1:2.5 - 5. The preparation of the titanium dioxide mesoporous layer uses the spin coating method. Preferably, the rotation speed during spin coating is 3000 - 5000 rpm, and the spin coating time is 20 - 40 seconds. After spin coating, it is transferred to a hot stage, preheated at 150 - 200°C for 5 - 15 min, the hot stage temperature is raised to 450 - 550°C, and kept warm for 30 - 60 min, and then cooled to obtain a titanium dioxide mesoporous layer

[0020] The thickness of the blank substrate (including glass and transparent conductive oxide) is 1 - 3 mm, the thickness of the dense titanium dioxide layer is 50 - 100 nm, and the thickness of the titanium dioxide mesoporous layer is 500 - 700 nm. Both the dense titanium dioxide layer and the titanium dioxide mesoporous layer are anatase phase. The average pore diameter of the titanium dioxide mesoporous layer is 30 - 40 nm.

[0021] Further, in step (S2), the deposition is carried out by thermal evaporation or magnetron sputtering, preferably thermal evaporation. Thermal evaporation is carried out under a vacuum pressure of 1×10 -4 Pa, and the current is controlled at 50 - 100 mA in the constant current mode for deposition.

[0022] Further, in step (S3), the temperatures T1, T2, and T3 are independently in the range of 260 - 300 °C, preferably 280 - 300 °C, more preferably 290 - 300 °C. The hot pressing is carried out at a pressure of 800 - 1500 kPa for 30 - 300 s. Preferably, the hot pressing is carried out at a pressure of 1000 - 1200 kPa for 60 - 120 s. The purity of the selenium powder is 99%, preferably ≥99.5%, more preferably ≥99.8%. In a preferred embodiment of the present invention, the purity of the selenium powder is ≥99.9%.

[0023] According to the present invention, in step (S3), the temperature of the annealing treatment is 190 - 220 °C; preferably 200 - 215 °C.

[0024] The molten selenium has a high viscosity and exhibits Newtonian fluid characteristics. The polycrystalline selenium thin film of the double-sided selenium solar cell obtained by the hot pressing method is dense, has good crystallinity, large grains, uniform thickness, and a flat surface. Therefore, the double-sided selenium solar cell has excellent performance, and the bifacial factor (bifacial factor calculation formula: BF = PCE front / PCE rear ×100%) is as high as 90%, and a double-sided selenium solar cell with excellent optoelectronic performance can be obtained.

[0025] The structure of the double-sided solar cell of the present invention is as Figure 1 shown, Figure 2 which is a schematic diagram of the method for preparing the double-sided selenium thin film solar cell of the present invention.

[0026] Advantages of the present invention:

[0027] 1. The present invention provides a method for fabricating a high-quality double-sided thin film solar cell, which uses a high-viscosity slurry as the photovoltaic absorption layer and is prepared by the hot pressing method, avoiding the use of the magnetron sputtering method, thereby effectively preventing potential damage to the functional layer. This method not only has low equipment requirements, a simple manufacturing process, but also high material utilization rate and good film-forming quality, and has a high process efficiency.

[0028] 2. The surface of the electron transport layer of the present invention is mesoporous titanium dioxide, and the hole transport layer is preferably MoO x . The surfaces of the electron transport layer and the hole transport layer are in direct contact with the molten selenium, and their surface properties directly determine the performance of the fabricated double-sided solar cell. The inventors found that the surface of the electron transport layer is mesoporous titanium dioxide, and the hole transport layer is preferably MoOx The resulting bifacial solar cell exhibits the best performance.

[0029] 3. In the high-bifacial selenium thin-film solar cell provided by the present invention, the Se element in the p-type absorption layer material of the solar cell is an element with a relatively high content in the earth's crust. It is a semiconductor optoelectronic material that is inexpensive, has stable performance, and is environmentally friendly. Its direct bandgap is 1.87 eV, covering most of the visible light spectrum, and its absorption coefficient is as high as 10 5 cm -1 . The specific advantages are as follows: a) High bifaciality: The bifaciality of the bifacial solar cell of the present invention can reach 90.1%, significantly higher than that of traditional thin-film solar cells; b) High efficiency: Under 1-sun illumination, the efficiency can reach 10.01% (reflectivity 0.5); under 1000-lux indoor illumination, the efficiency can reach 26.17% (reflectivity 0.8); c) High stability: After the unencapsulated device is stored under environmental conditions for 1000 hours, its performance does not show significant degradation; d) No need for magnetron sputtering: Avoids potential damage to the underlying functional layer by magnetron sputtering and simplifies the manufacturing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic structural diagram of the bifacial selenium thin-film solar cell prepared by the present invention.

[0031] Figure 2 It is a schematic diagram of the method for preparing the bifacial selenium thin-film solar cell of the present invention.

[0032] Figure 3 It is the thermogravimetric (TGA) graph of Se powder and the relationship between the vapor pressure of Se and temperature.

[0033] Figure 4 It is the Raman graph for characterizing the molecular structure of molten selenium of the present invention.

[0034] Figure 5 It is the graph of the viscosity of molten selenium of the present invention varying with temperature.

[0035] Figure 6 It is the graph of the Newtonian fluid characteristics of molten selenium of the present invention.

[0036] Figure 7 It is the cross-sectional electron scanning microscope image of the bifacial selenium thin-film solar cell prepared in Example 1 of the present invention.

[0037] Figure 8 It is the electron scanning microscope image of the selenium thin film in the bifacial solar cell prepared in Example 1 of the present invention.

[0038] Figure 9 It is the X-ray diffraction pattern of the selenium thin film in the bifacial solar cell prepared in Example 1 of the present invention.

[0039] Figure 10 This is the fitting graph of the absorption spectrum and band gap of the selenium thin film in the bifacial solar cell in Embodiment 1 of the present invention.

[0040] Figure 11 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 1 of the present invention.

[0041] Figure 12 This is the test graph of the I-V curve when light is incident on both the front and back sides of the bifacial solar cell simultaneously in Embodiment 1 of the present invention.

[0042] Figure 13 This is the cross-sectional image of the electron scanning microscope of the selenium thin films with different thicknesses prepared in Embodiment 2 of the present invention.

[0043] Figure 14 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 2 of the present invention

[0044] Figure 15 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 3 of the present invention.

[0045] Figure 16 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 4 of the present invention.

[0046] Figure 17 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 5 of the present invention.

[0047] Figure 18 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 6 of the present invention.

[0048] Figure 19 This is the test graph of the I-V curve when light is incident on the front and back sides of the bifacial solar cell in Embodiment 7 of the present invention.

[0049] Figure 20 This is the test graph of the stable power output of the bifacial selenium thin film solar cell in the present invention.

[0050] Figure 21 This is the stability test graph of the bifacial selenium thin film solar cell in the present invention.

[0051] Figure 22 This is the statistical graph for the analysis of the device performance of the bifacial selenium thin film solar cell in the present invention.

[0052] Figure 23 This is the test graph of the I-V curve when indoor photovoltaic (IPV) light is incident on the front and back sides of the bifacial solar cell in the invention.

[0053] Figure 24 It is a test chart of the I-V curve with simultaneous incident light on both the front and back sides of an indoor photovoltaic (IPV) double-sided solar cell in the invention.

[0054] Figure 25 It is a test chart of the I-V curve with incident light on the front and back sides of the double-sided solar cell respectively in Comparative Example 1 of the present invention. Detailed implementation manners

[0055] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content recorded in the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the present invention.

[0056] The double-sided thin-film solar cell prepared by the present invention with a polycrystalline selenium thin film as the absorption layer is as Figure 1 shown. The double-sided solar cell includes a conductive glass, an n-type window layer, a p-type absorption layer, a hole transport layer, and a conductive glass stacked in sequence.

[0057] The preparation method of the double-sided selenium thin-film solar cell prepared by the present invention is as Figure 2 shown, and includes a hot press, a conductive substrate, and selenium powder.

[0058] Preparation Example 1

[0059] In order to find the optimal melting processing temperature of Se, Figure 3 thermogravimetric analysis (TGA) was performed on Se powder to deeply understand the melting process of Se. Figure 3 It shows that the weight loss of Se starts at about 300 °C, which is higher than the melting point of Se (217 °C), indicating that when the heating temperature is in the range of 217 to 300 °C, the molten Se still exists in a liquid state instead of evaporating as Se vapor. Figure 3 It is a relationship diagram of the vapor pressure and temperature of Se. Se starts to exhibit a relatively high vapor pressure of 13 Pa at 300 °C. The above results can show that it is feasible to perform the melting processing of Se at a temperature of 300 °C and below.

[0060] The present invention uses molten selenium hot pressing to form a film at a specific temperature range (217 - 300 °C) to make a high-quality double-sided selenium thin-film solar cell. This is because within the melting processing temperature range of 217 - 300 °C for selenium, the molten selenium is convenient for processing and will not evaporate into a gaseous state.

[0061] Another key parameter for the melt processing of molten selenium using the gluing strategy is the viscosity characteristic of molten selenium. To find the viscosity advantage of the melt processing of molten selenium, we characterized the structural features and viscosity properties of molten selenium to confirm this. We performed Raman characterization on the molecular structure of molten selenium, as Figure 4 , molten selenium exhibits a prominent characteristic Raman vibration peak at 251 cm-1, which can be attributed to the stretching vibration mode of the Se-Se bond in the disordered selenium chain. The entanglement effect of the disordered chain is considered to be the key factor leading to the high viscosity of molten selenium. We characterized the viscosity properties of molten selenium, as Figure 5 , the variation of the viscosity of molten selenium with temperature. Molten selenium shows a significant viscosity change (6.3 - 1.0 Pa·s) in the temperature range of 217 - 300 °C, and its value is more than an order of magnitude higher than that of traditional photovoltaic precursor slurries (usually less than 1.0 Pa·s). The viscosity properties of molten selenium, such as Figure 6 , within the shear rate range of 0.1 - 100 s-1, the viscosity value of molten selenium remains basically constant (the fluctuation amplitude is less than 5%), showing typical Newtonian fluid behavior. This unique rheological property makes molten selenium suitable as an adhesive between two transparent conductive substrates and also serves as a light absorption layer, playing a dual role.

[0062] Example 1

[0063] a) n-type window layer deposition step: Using the spin-coating method, deposit the n-type window layer on the conductive glass substrate;

[0064] The substrate includes transparent glass (or white glass) and a transparent FTO (SnO 2 :F) coating (denoted as FTO conductive glass or FTO glass) covering the transparent glass. The thickness of the transparent glass is 2 mm, and the thickness of the FTO glass is 300 nm;

[0065] Ultrasonically clean the substrate with deionized water, acetone, and ethanol for 30 minutes respectively, then blow it dry with high-purity nitrogen, and then clean it with ultraviolet-ozone for 15 minutes;

[0066] The material of the n-type window layer 2 is TiO 2 , which is divided into a TiO 2 dense layer and a TiO 2 mesoporous layer, with thicknesses of 50 nm and 700 nm respectively;

[0067] Deposit the n-type window layer on the substrate by spin coating. The deposition steps are as follows: Take 1 mL of bis(acetylacetonato)diisopropyl titanate and mix it with 10 mL of absolute ethanol to form a solution. Filter this solution through a filter head with a pore size of 0.22 μm. Place the substrate at the center position of the spin coater, and use a pipette to suck an appropriate amount of the above solution and evenly coat it on the substrate. Spin coat for 30 s at a speed of 4000 rpm. Transfer the spin-coated substrate to a hot plate. At this time, the temperature of the hot plate is 150 °C. After 10 min, raise the temperature of the hot plate to 500 °C, cover the lid, keep warm for 30 min, and then remove the substrate after cooling to obtain TiO 2 dense layer.

[0068] Weigh 3 g of TiO 2 slurry (commercially available 30NR-D, average particle size 30 nm) and mix it with 7.5 g of absolute ethanol to form a solution. Place the substrate with the TiO 2 dense layer at the center position of the spin coater, and use a pipette to suck an appropriate amount of the above solution and evenly coat it on substrate 1. Spin coat for 30 s at a speed of 2000 rpm. Transfer the spin-coated substrate 1 to a hot plate. At this time, the temperature of the hot plate is 150 °C. After 10 min, raise the temperature of the hot plate to 500 °C, cover the lid, keep warm for 30 min, and then remove the substrate after cooling to obtain TiO 2 mesoporous layer.

[0069] b) Hole transport layer deposition steps: Deposit on the conductive glass substrate by thermal evaporation. The substrate includes transparent glass (or white glass) and a transparent FTO (SnO 2 :F) coating (denoted as FTO conductive glass or FTO glass). The thickness of the transparent glass is 2 mm, and the thickness of the FTO glass is 300 nm;

[0070] Ultrasonically clean the above substrates with deionized water, acetone, and ethanol for 30 minutes respectively, then blow them dry with high-purity nitrogen, and then clean them with ultraviolet-ozone for 15 minutes;

[0071] The hole transport layer material is MoO x Use high-purity MoO x powder, and under the condition of a vacuum pressure of 5×10-4 Pa, control the deposition thickness to be 15 nm.

[0072] c) Preparation of double-sided selenium solar cell:

[0073] The schematic diagram of the method for preparing a double-sided solar cell by hot pressing is as shown in Figure 2 Figure [Figure number not provided], and the production steps are as follows: Weigh 0.3 g of high-purity selenium powder, and use a 200-mesh sieve to evenly place it on the TiO of the bottom substrate 2On the layer; directly place the preheated top substrate at 300 °C directly opposite above the selenium powder; hot press the two substrates at 300 °C under a pressure of 1200 kPa for 60 seconds, and complete the hot pressing process using a self-made press. After hot pressing, the sample is naturally cooled to room temperature, and then annealed on a hot stage at 215 °C for 2 minutes to enhance the crystallinity of the selenium thin film.

[0074] Figure 7 Figure 4 is the cross-sectional electron microscopy image of the double-sided selenium thin film solar cell prepared in Example 1. The average thickness of the selenium thin film was measured to be about 2.5 μm, and the thickness distribution was uniform. The crystallinity of the selenium thin film in the double-sided solar cell prepared by the hot pressing method was very good.

[0075] Figure 8 Figure 8 is the electron microscopy image of the selenium thin film in the double-sided solar cell prepared in Example 1, which has very good uniformity and density, and has large grains in the micron scale.

[0076] Figure 9 Figure 12 is the X-ray diffraction pattern of the selenium thin film in the double-sided solar cell prepared in Example 1. It can be seen that the selenium thin film prepared under the natural cooling condition exhibits typical amorphous structural characteristics. After annealing treatment, all the diffraction peaks of the selenium thin film in the double-sided selenium thin film solar cell obtained in the present invention are in agreement with JCPDS 73-0465, indicating that the selenium thin film in the double-sided solar cell prepared by the hot pressing method in the present invention has a photovoltaic trigonal phase and is suitable as a photovoltaic material for solar cells.

[0077] Figure 10 Figure 16 is the absorption spectrum and band gap fitting diagram of the selenium thin film in the double-sided solar cell in Example 1. The band gap of the polycrystalline selenium thin film prepared by the hot pressing method in the present invention is 1.87 eV, and its absorption spectrum can cover most of the visible light.

[0078] Figure 11 Figure 20 is the I-V curve test diagram of the double-sided solar cell in Example 1 with light incident on the front and back sides respectively. When the device is illuminated from the front, its photoelectric conversion efficiency (PCE) reaches 6.50%, and when illuminated from the back, the device performance decreases slightly, and the PCE is 5.94%. Based on the calculation formula of the bifaciality factor (BF): BF = PCE front / PCE rear × 100%, the bifaciality factor of this device is calculated to be as high as 90.1%.

[0079] Figure 12 Figure 28 is the I-V curve test diagram of the double-sided solar cell in Example 1 with light incident on the front and back sides simultaneously. When the albedo is 0 (i.e., single-sided illumination), the PCE of the device is 6.50%. Introducing 10 mW / cm 2After the backside illumination (albedo = 0.1, equivalent to the reflection condition of the black soil surface), the PCE is increased to 7.16%. When the albedo is increased to 0.2, the PCE further increases to 7.85%, with an increase of more than 20% compared to the single-sided efficiency. As the albedo continues to increase to 0.3, 0.4, and 0.5, the PCE of the device reaches 8.61%, 9.28%, and 10.01% respectively.

[0080] Example 2

[0081] It is basically the same as that in Example 1, except that: in step c), the pressure of the hot press is reduced from 1200 kPa to 900 kPa, and the thickness of the selenium film shows an obvious increasing trend.

[0082] For the selenium film of the double-sided solar cell prepared in Example 2, direct measurement by cross-sectional scanning electron microscopy (SEM) shows that the film thickness is 3.3 μm, and the results are as Figure 13 shown.

[0083] Testing its I-V curve graph, the results are as Figure 14 shown. When 1 sun of sunlight is incident on the front and back sides of the device respectively, when the device receives light from the front, the device efficiency is 6.1%, and when receiving light from the back, the device performance slightly decreases, with a PCE of 5.28%.

[0084] It can be seen from Examples 1 and 2 that when the pressure of the hot press decreases, the film thickness increases and the device efficiency decreases. The increase in thickness will affect the electron transport efficiency, resulting in a decrease in the open voltage and a decline in the device performance.

[0085] Example 3

[0086] It is basically the same as that in Example 1, except that: in step c), the hot press is heated to 280 °C. After the temperature is stabilized, the titanium dioxide substrate is transferred to the hot stage, selenium powder is placed, and then the substrate preheated to 280 °C and deposited with MoO x is placed on it. The pressure of the hot press is 1200 kPa to fabricate a double-sided selenium film solar cell. Testing its I-V curve graph, the results are as Figure 15 shown. When 1 sun of sunlight is incident on the front and back sides of the device respectively, when the device receives light from the front, its photoelectric conversion efficiency (PCE) is 5.92%, and when receiving light from the back, the device performance slightly decreases, with a PCE of 4.66%.

[0087] It can be seen from Examples 1 and 3 that when the working temperature of hot-pressing selenium into a film decreases from 300 °C to 280 °C, the coverage of the film on the substrate decreases, the quality of the polycrystalline selenium film decreases, and the photoelectric conversion efficiency of the fabricated double-sided solar cell decreases. The selenium polycrystalline film prepared at a working temperature of 280 °C exhibits poor surface coverage, which is due to the poor wettability of molten selenium on the substrate at a relatively low temperature.

[0088] Example 4

[0089] It is basically the same as that in Example 1, except that: in step c), the further annealing treatment of the double-sided selenium solar cell is carried out on a hot stage at 200 °C, and its I-V curve graph is tested. The results are as Figure 16 shown. When the device receives light from the front side, its photoelectric conversion efficiency (PCE) reaches 6.15%, and when receiving light from the back side, the device performance slightly decreases, and the PCE is 5.42%.

[0090] It can be seen from Examples 1 and 4 that when the annealing treatment temperature of the double-sided selenium solar cell decreases from 215 °C to 200 °C, the crystallization quality of the polycrystalline selenium film slightly decreases, and the photoelectric conversion efficiency of the fabricated double-sided solar cell decreases.

[0091] Example 5

[0092] It is basically the same as that in Example 1, except that: in step b), the hole transport layer material is CuSCN, and the process parameters are controlled so that the thickness of the hole transport layer is also 15 nm. Its I-V curve graph is tested. The results are as Figure 17 shown. When the device receives light from the front side, its photoelectric conversion efficiency (PCE) reaches 5.61%, and when receiving light from the back side, the device performance slightly decreases, and the PCE is 4.87%.

[0093] Example 6

[0094] It is basically the same as that in Example 1, except that: in step b), the hole transport layer material is Spiro-OMeTAD, and the process parameters are controlled so that the thickness of the hole transport layer is also 15 nm. Its I-V curve graph is tested. The results are as Figure 18 shown. When the device receives light from the front side, its photoelectric conversion efficiency (PCE) reaches 4.67%, and when receiving light from the back side, the device performance slightly decreases, and the PCE is 3.56%.

[0095] Example 7

[0096] It is basically the same as that in Example 1, except that: in step b), the hole transport layer material is NiOx, and the process parameters are controlled so that the thickness of the hole transport layer is also 15 nm. Its I-V curve graph is tested. The results are as Figure 19As shown, when the device is illuminated from the front side, its power conversion efficiency (PCE) reaches 3.64%, and when illuminated from the back side, the device performance slightly decreases, with a PCE of 2.79%.

[0097] Example 8

[0098] We conducted stability and process repeatability tests on the double-sided selenium thin-film solar cells prepared in Example 1. The double-sided selenium thin-film solar cells obtained in the present invention were subjected to a stable power output (SPO) test. As Figure 20 shown, during the 300-second continuous test, the SPO PCEs of the device under front-side and back-side illumination conditions were stably around 6.5% and 5.8% respectively, and the fluctuation range was negligible (<1%). These SPO PCE values were highly consistent with the results obtained from the J-V characteristic test, indicating that the device has good working stability. A long-term stability test of up to 1000 hours was carried out on the unencapsulated device at room temperature (25°C) and a wide humidity range (25%-85% RH), and no obvious efficiency loss was shown ( Figure 21 ). We conducted a statistical performance analysis on 20 independently prepared devices. As Figure 22 shown, under the standard AM1.5G illumination conditions on the front side and the back side, the average PCEs of the device were 6.3±0.2% and 5.7±0.2% respectively, showing excellent performance consistency and process repeatability.

[0099] Example 9

[0100] We conducted performance tests on the double-sided selenium thin-film solar cells prepared in Example 1 in indoor photovoltaic (IPV) applications. An LED with a color temperature of 6500K was used as the simulated indoor light source in the experiment. Figure 23 The current density-voltage (J-V) characteristic curve of the double-sided selenium device with the optimal performance under indoor illumination conditions of 1000 lux is shown. The test results show that the PCEs of the device under front-side and back-side illumination reach 15.24% and 13.88% respectively, showing excellent indoor light conversion efficiency. The indoor photovoltaic (IPV) performance of the device under an albedo of 0.8 was further tested. As Figure 24 shown, the double-sided selenium solar cell achieved a record PCE of 26.17% under indoor illumination of 1000 lux, with a corresponding open-circuit voltage (Voc) of 0.769V, a short-circuit current density (Jsc) of 168.51 μA / cm 2 , and a fill factor (FF) of 63.9%.

[0101] Comparative Example 1

[0102] It is basically the same as that in Example 1, except that:

[0103] The n-type window layer material is TiO 2 The thickness of the dense layer is 300 nm.

[0104] The n-type window layer is deposited on the substrate by spin coating. The deposition steps are as follows: 1 mL of bis(acetylacetonato)diisopropyl titanate and 10 mL of absolute ethanol are mixed to form a solution, and the solution is filtered through a filter head with a pore size of 0.22 μm; the substrate is placed at the center of the spin coater, and an appropriate amount of the above solution is pipetted and uniformly coated on the substrate, and then spin-coated at a speed of 2000 rpm for 30 s; the spin-coated substrate is transferred to a hot stage, at this time the temperature of the hot stage is 150 °C, after 10 min, the temperature of the hot stage is raised to 500 °C, the lid is covered, and it is kept warm for 30 min, and after cooling, the substrate is taken down to obtain TiO 2 Dense layer. Then, the double-sided solar cell is prepared by hot pressing on the TiO 2 Dense layer, rather than TiO 2 Mesoporous layer.

[0105] For the double-sided selenium thin film solar cell prepared in Comparative Example 1, as Figure 25 shown, when the device is illuminated from the front, its photoelectric conversion efficiency (PCE) reaches 5.53%, and when illuminated from the back, the device performance drops significantly, and the PCE is 3.34%.

Claims

1. A double-sided solar cell, characterized in that: It is a multi-layer composite structure, including a transparent conductive substrate deposited with an electron transport layer (ETL), a transparent conductive substrate deposited with a hole transport layer (HTL) and a selenium layer. The two substrates are bonded by molten selenium, and the selenium layer serves as a light absorption layer and an adhesive at the same time.

2. The double-sided solar cell according to claim 1, characterized in that: The thickness of the molten selenium layer is 2.5-5.5 μm, preferably 2.5-3.0 μm; the thickness of the electron transport layer is 500-1000 nm, preferably 750-900 nm; the thickness of the hole transport layer is 10-50 nm, preferably 10-20 nm; the thickness of the conductive transparent substrate is 200-500 nm, preferably 300-400 nm.

3. The double-sided solar cell according to claim 1, characterized in that: The transparent conductive substrate is selected from at least one of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and indium zinc oxide (IZO); the electron transport layer is selected from TiO2, Zn x Mg 1-x At least one of O and SnO2; the hole transport layer is selected from MoO x , CuSCN (copper cyanide), and Spiro-OMeTAD.

4. The double-sided solar cell according to claim 1, characterized in that: The outer layer of the electron transport layer is a mesoporous TiO2 layer with a thickness of 500-700nm; the hole transport layer is MoO x .

5. The method for preparing a bifacial solar cell according to any one of claims 1 to 4, characterized in that: The steps include: (S1) depositing an electron transport layer on a conductive transparent substrate in sequence, wherein the surface of the electron transport layer is a mesoporous TiO2 layer, and annealing is performed to obtain a bottom substrate; (S2) depositing a hole transport layer on a conductive transparent substrate, controlling the deposition thickness to 10-20 nm, to obtain a top substrate; (S3) Selenium powder is evenly placed on the surface of the bottom substrate, heated to temperature T1, the top substrate preheated to temperature T2 is aligned with the bottom substrate, the two substrates are hot pressed at temperature T3, cooled to room temperature, and annealed to obtain a double-sided solar cell.

6. The preparation method according to claim 5, characterized in that: In step (S1), the electron transport layer is a composite of a dense TiO2 layer and a mesoporous TiO2 layer. The TiO2 layer is deposited first, and then a layer of mesoporous TiO2 is deposited on the surface of the TiO2 layer. The thickness of the dense titanium dioxide layer is 50-100nm, and the thickness of the mesoporous titanium dioxide layer is 500-700nm.

7. The preparation method according to claim 6, characterized in that: The operation of TiO2 deposition is to coat a dense titanium dioxide precursor slurry on a clean conductive transparent substrate, calcine it for the first time to obtain a dense TiO2 layer, coat the titanium dioxide slurry on the surface of the dense TiO2 layer, and calcine it for the second time to obtain a composite of a dense TiO2 layer and a mesoporous TiO2 layer; Furthermore, the dense titanium dioxide precursor is at least one of bis(acetylacetonato)diisopropyl titanate and titanium tetrachloride, the slurry is prepared using an alcohol solvent, and the volume ratio of the dense titanium dioxide precursor to the alcohol is 1-2:10; the preparation of the titanium dioxide dense layer is carried out by spin coating; after spin coating, the substrate is transferred to a hot stage, preheated at 150-200°C for 5-15min, the temperature is increased to 400-500°C, and kept warm for 20-30min to obtain a titanium dioxide dense layer; The average particle size of titanium dioxide in the titanium dioxide slurry is 30-40nm, and the mass ratio of titanium dioxide paste to anhydrous ethanol is 1:2.5-5; the titanium dioxide mesoporous layer is prepared by spin coating; after spin coating, it is transferred to a hot plate, preheated at 150-200°C for 5-15min, the temperature of the hot plate is increased to 450-550°C, kept warm for 30-60min, and the titanium dioxide mesoporous layer is obtained after cooling; the average pore size of the titanium dioxide mesoporous layer is 30-40nm.

8. The preparation method according to claim 5, characterized in that: In step (S2), the deposition is carried out by thermal evaporation or magnetron sputtering, preferably thermal evaporation, which is carried out at a vacuum pressure of 1×10 -4 Pa, deposited under the condition of controlling the current to 50-100 mA in constant current mode.

9. The preparation method according to claim 5, characterized in that: In step (S3), the temperatures T1, T2 and T3 are independently 260-300° C., preferably 280-300° C., more preferably 290-300° C. The hot pressing is performed at a pressure of 800-1500 kPa for 30-300 s, preferably at a pressure of 1000-1200 kPa for 60-120 s.

10. The preparation method according to claim 5, characterized in that: In step (S3), the annealing temperature is 190-220°C, preferably 200-215°C.

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