Antireflection film, preparation method thereof and solar cell

Abstract

The invention discloses an antireflection film and a preparation method thereof, and a solar cell, and the antireflection film comprises at least one optical thin film layer, and the refractive index of the at least one optical thin film layer is gradually reduced from the light receiving surface to the top; wherein each optical thin film layer comprises at least three layers of thin film materials. Through alternate arrangement of thin film materials with specific high and low refractive indexes, the thin film can be equivalent to a single-layer film with specific effective refractive index and equivalent optical thickness, and optical parameters of the thin film can be flexibly adjusted by adjusting the thickness ratio of each layer and the total thickness of the thin film. The optimal value of the ideal multilayer film is obtained through theoretical calculation, and the reflectivity of the ideal multilayer film is minimized in a wide spectrum range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photoelectric device preparation, in particular to a reflection-reducing film, a preparation method thereof and a solar cell. BACKGROUND

[0002] Multi-junction solar cells, especially inverted multi-junction (IMM) triple-junction solar cells based on gallium arsenide (GaAs), are the core devices to achieve ultra-high photoelectric conversion efficiency (AM1.5G spectrum > 35%, AM0 spectrum > 30%). In order to maximize the photo-generated current, the light reflection loss on the surface of the cell must be minimized as much as possible.

[0003] Traditional anti-reflective coatings (ARCs) usually adopt a single-layer or conventional double-layer structure, and the overall reflectivity of these traditional ARCs and the high reflectivity band gradually cannot meet the requirements of higher conversion efficiency.

[0004] Therefore, how to overcome the problems of the traditional single-layer / double-layer anti-reflective film, such as narrow low reflectivity band, unable to effectively cover the wide spectral absorption range of the IMM triple-junction cell, resulting in large light reflection loss in some bands, and limiting the further improvement of the overall short-circuit current density of the cell, has become one of the problems to be solved by those skilled in the art. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a reflection-reducing film, a preparation method thereof and a solar cell, to solve the problem of the narrow low reflectivity band of the traditional single-layer / double-layer anti-reflective film, which cannot effectively cover the wide spectral absorption range of the IMM triple-junction cell.

[0006] The reflection-reducing film comprises:

[0007] at least one optical thin film layer, and the refractive index of the at least one optical thin film layer decreases in turn from the light-receiving surface upwards;

[0008] wherein each of the optical thin film layers comprises at least three thin film materials.

[0009] Optionally, the number of layers of the at least one optical thin film layer is any one of 1-4.

[0010] Optionally, each of the optical thin film layers comprises three thin film materials, and the refractive indices of the two outer layers of the three thin film materials are the same and different from the refractive index of the middle layer of the three thin film materials.

[0011] Optionally, the three thin film materials are MgF2-ZnS-MgF2, and the thicknesses of each layer are 40 nm, 20 nm and 40 nm, respectively.

[0012] or,

[0013] The three-layer thin film material is TiO2-SiO2-TiO2, and the thickness of each layer is 20 nm, 20 nm and 20 nm respectively.

[0014] Optionally, the refractive index of the single-layer thin film material of the adjacent two layers of the optical thin film layer is the same or different.

[0015] Optionally, the equivalent thickness γ and the equivalent refractive index N of the optical thin film layer satisfy:

[0016] M 11 = M 22 = cos γ

[0017] N = isin γ / M 12 = -iM 21 / sin γ

[0018] And:

[0019]

[0020] Wherein, φ p = 2πn p x p / λ0, φ q = 2πn q x q / λ0

[0021] n p is the refractive index of the outer layer material in the three-layer thin film material, x p is the thickness of the outer layer material in the three-layer thin film material;

[0022] n q is the refractive index of the intermediate material in the three-layer thin film material, x q is the thickness of the intermediate layer material in the three-layer thin film material;

[0023] λ0 is the wavelength of the incident light;

[0024] i is the imaginary unit.

[0025] Optionally, the thin film material includes any one of Ta2O5, Nb2O5, HFO2, SiO2, Na3AlF6, TiO2, MgF2, ZnS.

[0026] The preparation method of the antireflection film comprises the following steps:

[0027] Cleaning the impurities on the surface of the solar cell epitaxial wafer;

[0028] Removing the oxide layer and impurities on the surface of the solar cell epitaxial wafer by electron beam bombardment of the target material;

[0029] Depositing the thin film material and controlling the evaporation rate to be between 2.5-3.5 A / s.

[0030] Before depositing the thin film material, the vacuum chamber is evacuated to a vacuum degree below 4.0E-6 Torr.

[0031] The solar cell comprises, from the back surface to the light receiving surface, a metal reflector, a bottom cell, a lattice gradient layer, a middle cell, a top cell, and the antireflection film.

[0032] The present application can be optically equivalent to a single layer film with a specific equivalent refractive index and equivalent optical thickness by the specific high and low refractive index thin film material in the alternative arrangement, and the optical parameters can be flexibly adjusted by adjusting the thickness ratio of each layer and the total thickness, so as to be closer to the optimal value of the ideal multilayer film which minimizes the reflectivity in a wide spectral range obtained by theoretical calculation.

[0033] The present application uses the optical transfer matrix theory to establish an accurate optical model containing the antireflection film and the structure of each layer of the entire cell. Specifically, the internal quantum efficiency (IQE) data of each sub-junction of the cell is input, and the target spectral power distribution (AM1.5G or AM0) is combined, so as to obtain the short-circuit current of each sub-junction. The thickness and material selection of each layer of the antireflection film are taken as optimization variables, and the total short-circuit current of the three-junction cell is maximized as the target, while considering the current distribution constraints among the top, middle and bottom junctions. Through iterative calculation by a numerical optimization algorithm, the structure parameters of the antireflection film which can minimize the wide spectral weighted reflectivity and meet the three-junction current distribution target are finally determined, and the parameters specifically include the thickness and refractive index of the thin film material. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is a schematic diagram of the antireflection film in an embodiment;

[0035] Figure 2 It is a schematic diagram of the antireflection film in another embodiment;

[0036] Figure 3 It is a schematic diagram of a solar cell;

[0037] Figure 4 It is a comparison diagram of the reflectivity of the embodiment of the present application and the conventional double-layer antireflection film at different wavelengths;

[0038] Among them, Figure 3 The arrow direction is the light entering direction;

[0039] 1-metal reflector; 2-bottom cell; 3-lattice gradient layer; 4-middle cell; 5-top cell; 6-metal grid line; 7-antireflection film. DETAILED DESCRIPTION

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the example embodiments of the present application are described in detail below with reference to the drawings. In the drawings, the same or similar components are denoted by the same or similar reference signs. Also, some components that are not directly related to the concept of the present application can be omitted from the drawings. The drawings and the detailed description of the embodiments are only for better understanding of the present application, and the present application is not limited to the embodiments illustrated in the drawings and described in the specification.

[0041] The technical terms or scientific terms used in the present application should be understood as the common meanings understood by those skilled in the art to which the present application belongs. The similar words such as “include” or “contain” in the present application represent an open meaning, and do not exclude other elements, components, parts or items except the elements, components, parts or items explicitly listed. The similar words such as “connect” or “connected” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The words such as “upper”, “lower”, “left”, “right” are only used to represent relative positional relationships, and when the absolute positions of the described objects are changed, the relative positional relationships can also be changed accordingly. The words such as “first”, “second” are used for the purpose of distinguishing different components, and do not represent a specific order.

[0042] The low reflectivity band of the conventional single-layer or double-layer anti-reflective film is narrow (the low reflection band described in the present application is the wavelength range in which the reflectivity reaches a lower preset value), which cannot effectively cover the wide spectral absorption range of the IMM three-junction solar cell, resulting in a large light reflection loss in some bands, thus limiting the further improvement of the overall short-circuit current density of the cell. The design of the existing anti-reflective film is often disconnected from the internal quantum efficiency and sub-junction current of the cell, and the overall short-circuit current of the three-junction cell is not optimized from the system level, so the anti-reflective film of the present application is designed, specifically:

[0043] The anti-reflective film comprises:

[0044] At least one optical thin film layer, and the refractive index of the at least one optical thin film layer decreases in turn from the light-receiving surface upwards;

[0045] Each of the optical thin film layers comprises at least three thin film materials.

[0046] The cooperation between the setting of the multi-layer thin film materials with different refractive indexes and the thickness of the thin film materials can realize the effective absorption of sunlight in a wider spectral range; each optical thin film layer is composed of at least three thin film materials, the refractive indexes of the multi-layer optical thin film layer can be selected in multiple ways, and the refractive indexes of the optical thin film layer can be adjusted according to the design by adjusting the materials and thickness of each layer in real time, so that the process is simplified and convenient. In addition, the main point is that the light receiving surface is the surface of the solar cell epitaxial wafer receiving solar light, that is, the surface in contact with the anti-reflection film.

[0047] Optionally, the number of layers of the at least one optical thin film layer is any one of 1-4.

[0048] Optionally, each optical thin film layer includes three thin film materials, and the refractive indexes of the two groups of outer layer materials are the same and different from the refractive index of the middle layer of the three thin film materials.

[0049] Optionally, the three-layer thin film materials of "low-high-low" are MgF2-ZnS-MgF2, and the thicknesses of each layer are 40nm, 20nm and 40nm respectively.

[0050] Or,

[0051] The three-layer thin film materials of "high-low-high" are TiO2-SiO2-TiO2, and the thicknesses of each layer are 20nm, 20nm and 20nm respectively.

[0052] In some embodiments, the multiple layers of optical film layers all adopt the combination of "low-high-low", or all adopt the combination of "high-low-high", or a mixed combination of the two combinations, such as being provided with three layers of optical film layers, and sequentially selecting the combination 1 "high-low-high", the combination 2 "low-high-low", and the combination 3 "high-low-high" from the direction of incident light, or in combination with a single layer of optical film layer, such as the combination of single layer film material 1, high-low-high, and single layer film material 2. The specific selection of the combination form needs to be selected in combination with the design of the actual refractive index. It needs to be noted that the high or low refractive index described in the present application is a comparison between the refractive indices of adjacent film materials, and is not an absolute high or low value, and the specific high refractive index or low refractive index of the film material is not limited in the present application. For example, the "high-low-high" combination TiO2-SiO2-TiO2 described in the present application, wherein the refractive index of TiO2 is 2.3, and the refractive index of SiO2 is 1.46. Compared with each other, TiO2 is a high refractive index film material, and SiO2 is a low refractive index film material. It also needs to be noted that the high or low refractive index described in the present application only refers to the comparison between the refractive indices of adjacent film materials in the present layer of optical film layer. For example, the anti-reflective film includes MgF2-SiO2-MgF2, TiO2-SiO2-TiO2, and MgF2-SiO2-MgF2, wherein the refractive index of MgF2 is 1.38, the refractive index of SiO2 is 1.45, and the refractive index of TiO2 is 2.55. In the anti-reflective film structure, SiO2 is a high refractive index material in the combination MgF2-SiO2-MgF2, and is a low refractive index material in the combination TiO2-SiO2-TiO2.

[0053] Optionally, the refractive indices of the single layer film materials of the two adjacent layers of optical film layers are the same or different.

[0054] In some embodiments, the refractive indices of the single layer film materials of the two adjacent layers of optical film layers are different, such as Figure 1 As shown, the refractive indices of the single layer film materials of the two adjacent layers of optical film layers are the same, such as Figure 2 As shown, as long as the refractive indices of the multiple layers of optical film layers can be sequentially decreased from the light receiving surface upwards, for example, the letter A refers to the material with the maximum refractive index, and the refractive indices of the materials referred to by the letters in the order of A-Z are sequentially decreased. For example, with three layers of optical film layers, it can be ABA-ACA-ADA, or ABA-BCB-CDC, or ADA-BDB-CDC, or AEB-BFC-CGD.

[0055] Optionally, the equivalent thickness γ and the equivalent refractive index N of the optical film layer satisfy:

[0056] M 11 = M 22 = cosγ

[0057] N = isinγ / M 12 =-iM 21 / sinγ

[0058] and:

[0059]

[0060]

[0061] Where, φ p =2πn p x p / λ0,φ q =2πn q x q / λ0

[0062] n p x is the refractive index of the outer layer material in the three-layer thin film material. p The thickness of the outer layer material in the three-layer thin film material;

[0063] n q x is the refractive index of the intermediate layer material in the three-layer thin film material. q The thickness of the intermediate layer material in the three-layer thin film material;

[0064] λ0 is the wavelength of the incident light ray;

[0065] i is the imaginary unit.

[0066] Based on the optical transfer matrix theory, for monochromatic light incident perpendicularly into a series of thin-film media, its optical properties can be described by the electric field intensity E and magnetic field intensity H at each point. Assuming that the electric field and magnetic field intensity at any point are E and H respectively, then they satisfy the following relationship with the electric field intensity E' and magnetic field intensity H' at another point:

[0067]

[0068] Here, matrix (M) is the optical transmission matrix. For a uniform single-layer thin film medium with refractive index n and thickness x, its transmission matrix can be written as:

[0069]

[0070] Where λ0 is the vacuum wavelength of the incident light, also defined as the optical thickness of the thin film, in units of phase. The optical matrices of multiple thin film materials can be obtained by sequentially multiplying them. Therefore, at a single wavelength, any multilayer optical thin film can be equivalent to a single-layer thin film. Optionally, when the multilayer optical thin film structure is symmetrical, such as a three-layer thin film pqp, where the p-layer and q-layer are two different sets of thin film materials, and the two outer p-layers have the same refractive index and thickness, the optical properties of the three-layer thin film pqp can be equivalent to a single-layer film with an equivalent thickness of γ and an equivalent refractive index of N at different wavelengths. For the three-layer thin film pqp structure, its (M) matrix is:

[0071]

[0072] The equivalent thickness γ and equivalent refractive index N of the corresponding three-layer thin film pqp structure can be obtained by solving the following system of equations:

[0073] M 11 =M 22 =cosγ

[0074] N = isinγ / M 12 =-iM 21 / sinγ

[0075] Where, n p x is the refractive index of the outer layer material in the three-layer thin film material. p The thickness of the outer layer material in the three-layer thin film material;

[0076] n q x is the refractive index of the intermediate layer material in the three-layer thin film material. q The thickness of the intermediate layer material in the three-layer thin film material;

[0077] λ0 is the wavelength of the incident light ray;

[0078] i is the imaginary unit.

[0079] Because the equivalent thickness and equivalent refractive index of the three-layer thin film pqp structure can be flexibly adjusted by the thickness ratio of the p and q layers and the thickness of each layer, it breaks through the limitation of the inherent refractive index of the material. Therefore, it can be used to design new anti-reflection films for solar cells and improve their optical performance.

[0080] Optionally, the thin film material includes any one of Ta2O5, Nb2O5, HFO2, SiO2, Na3AlF6, TiO2, MgF2, and ZnS.

[0081] This invention discloses a method for preparing an antireflective film, comprising the following steps:

[0082] Clean impurities from the surface of the epitaxial wafer of a solar cell;

[0083] The oxide layer and impurities on the surface of the solar cell epitaxial wafer are removed by bombarding the target with an electron beam.

[0084] The thin film material is deposited, and the evaporation rate is controlled between 2.5 and 3.5 A / s.

[0085] Before depositing the thin film material, the vacuum chamber is evacuated to a vacuum level below 4.0E-6 Torr.

[0086] Optionally, cleaning impurities on the surface of solar cell epitaxial wafers includes ultrasonic cleaning and / or drying. Specifically, ultrasonic cleaning with acetone, isopropanol, and deionized water removes oil and particulate impurities from the epitaxial wafer surface; a spin dryer is then used, followed by nitrogen purging to prevent residual moisture. These cleaning methods effectively remove contaminants from the epitaxial wafer surface, preventing film defects.

[0087] This invention also discloses a solar cell, which, from the backlight surface to the light-receiving surface, sequentially includes a metal reflector, a bottom cell, a lattice gradient layer, a middle cell, a top cell, and the aforementioned antireflective film, as shown below. Figure 3 As shown.

[0088] To further illustrate the present invention and its beneficial effects, the following specific embodiments are disclosed. However, it should be noted that these are merely a set of preferred embodiments and should not be construed as limiting the present invention:

[0089] The antireflective coating of this invention adopts a combination of single-layer thin film material 1, high-low-high, and single-layer thin film material 2, specifically: a MgF2 layer with a thickness of 100nm, an optical thin film layer with a thickness of 45nm, and a ZnS layer with a thickness of 50nm, wherein the optical thin film layer includes: a ZnS layer with a thickness of 15nm, a MgF2 layer with a thickness of 15nm, and a ZnS layer with a thickness of 15nm, forming a novel antireflective coating.

[0090] By comparing the reflectivity with existing double-layer antireflection films (88 nm thick Al2O3 layer - 52 nm thick TiO2 layer), the reflectivity curves under incident light of different wavelengths are shown in the figure. Figure 4As shown, the novel antireflective film of this invention has a wider low reflectivity band. Specifically, the conventional double-layer antireflective film has a reflectivity band of less than 15% in the 380-1230nm range, while the novel antireflective film of this invention has a reflectivity band of less than 15% in the 300-1300nm range. Compared with the conventional double-layer antireflective film, the novel antireflective film of this invention adds the (300-380nm) and (1230-1300nm) bands to its low reflectivity band, thus having a wider low reflectivity band. Furthermore, the average reflectivity in the absorption band (300-1350nm) of the triple-junction solar cell is also significantly reduced.

[0091] In summary, the antireflective film of this invention, utilizing an equivalent film structure, exhibits a significantly lower average reflectivity in the absorption band (300-1350nm) of triple-junction solar cells compared to conventional single-layer or double-layer films, and its overall current is also significantly higher than that of cells using conventional single-layer or double-layer films, resulting in higher cell conversion efficiency.

[0092] The foregoing description of exemplary embodiments of this disclosure is for illustrative and descriptive purposes only and is not intended to be exhaustive or to limit this disclosure to the precise forms disclosed. Many modifications and variations are possible in accordance with the foregoing teachings. The embodiments were chosen and described to explain the principles of this disclosure and its practical application, so that others skilled in the art can utilize this disclosure and various embodiments with various modifications suitable for the particular purpose contemplated. Alternative embodiments will become apparent to those skilled in the art to which this disclosure pertains without departing from the spirit and scope of this disclosure. Therefore, the scope of this disclosure is defined by the appended claims rather than by the foregoing description and the exemplary embodiments described therein.

Claims

1. An antireflective coating, characterized in that, include: At least one optical thin film layer, wherein the refractive index of the at least one optical thin film layer decreases sequentially from the light-receiving surface upwards; Each of the optical thin film layers comprises at least three thin film materials.

2. The antireflective film according to claim 1, characterized in that, The number of layers of the at least one optical thin film layer is any one of 1 to 4.

3. The antireflective film according to claim 1, characterized in that, Each optical thin film layer comprises three thin film materials, and the refractive indices of the two outer layers of the three thin film materials are the same and different from the refractive index of the middle layer of the three thin film materials.

4. The antireflective film according to claim 3, characterized in that, The three thin film materials are MgF2-ZnS-MgF2, and the thicknesses of each layer are 40nm, 20nm, and 40nm, respectively. or, The three thin film materials are TiO2-SiO2-TiO2, and the thicknesses of each layer are 20nm, 20nm, and 20nm, respectively.

5. The antireflective film according to claim 3, characterized in that, The refractive indices of the single-layer thin film materials of two adjacent optical thin film layers may be the same or different.

6. The antireflective film according to claim 3, characterized in that, The equivalent thickness γ and equivalent refractive index N of the optical thin film layer satisfy: M 11 =M 22 =cosγ N=isinγ / M 12 =-iM 21 / sinγ and: in, n p x is the refractive index of the outer layer material in the three-layer thin film material. p The thickness of the outer layer material in the three-layer thin film material; n q x is the refractive index of the intermediate layer material in the three-layer thin film material. q The thickness of the intermediate layer material in the three-layer thin film material; λ0 is the wavelength of the incident light ray; i is the imaginary unit.

7. The antireflective film according to claim 1, characterized in that, The thin film material includes any one of Ta2O5, Nb2O5, HFO2, SiO2, Na3AlF6, TiO2, MgF2, and ZnS.

8. The method for preparing the antireflective film according to any one of claims 1-7, characterized in that, Includes the following steps: Clean impurities from the surface of the epitaxial wafer of a solar cell; The oxide layer and impurities on the surface of the solar cell epitaxial wafer are removed by bombarding the target with an electron beam. The thin film material is deposited, and the evaporation rate is controlled between 2.5 and 3.5 A / s.

9. The method for preparing the antireflective film according to claim 8, characterized in that, Before depositing the thin film material, the vacuum chamber is evacuated to a vacuum level below 4.0E-6 Torr.

10. A solar cell, characterized in that, The components, from the backlight surface to the light-receiving surface, are sequentially a metal reflector, a bottom cell, a lattice gradient layer, a middle cell, a top cell, and an anti-reflection film according to any one of claims 1-7.

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