SWITCHABLE ABSORBER ELEMENT AND SOLAR CELL - Patent application

Abstract

The present invention relates to a switchable absorbing element and a solar cell based thereon. The switchable absorbing element according to the present invention comprises an absorbing layer. The absorbing element further comprises at least one front-side reflective layer and at least one back-side reflective layer, the absorbing layer being disposed between the front-side reflective layer and the back-side reflective layer, and the optical path length between the front-side reflective layer and the back-side reflective layer is less than 400 nm, at least for light incident perpendicularly to the cell. The absorbing element according to the present invention is characterized in that at least one of the reflective layers has a switchable reflectivity. [Selected Figure] Figure 1

Description

[Technical field]

[0001] The present invention relates to a switchable absorption element and a solar cell based thereon. [Background technology]

[0002] Shading systems are used in many areas, for example in high-rise buildings with large window facades. For that purpose, known systems rely on mechanical shading, such as roller shutters or sun shades. However, due to the many moving parts, mechanical shading systems are prone to failure and are maintenance intensive. Windows already exist that shade independently in response to increasing temperature or illumination, or that can be variably darkened by the user, for example electrically, so that there is no need to rely on mechanical shading.

[0003] In the summer, this type of building typically gets very hot and therefore requires a lot of energy for environmental control at all times, as well as at times when the need for shading is high. It would be highly advantageous to generate very large amounts of energy, especially at times when the need for shading is high.

[0004] Electrical energy can be generated from the light incident on the cell by means of a solar cell, also called a photovoltaic cell. For this purpose, a solar cell has an absorbing element capable of generating free charge carriers by the supply of energy in the form of electromagnetic radiation.

[0005] Known shading systems, however, either block light by reflection or simple absorption, and do not use the energy of the light thus blocked. In general, there are two systems that generate heat or electricity in response to active shading, which are based on a combination of solar thermal or photovoltaic power generation and traditional mechanical shading components, with the drawbacks already mentioned above. The theoretically existing approach, in which solar cells are combined with electrically switchable absorbers or mirrors, has the drawback that only a small part of the total surface is equipped with solar cells, which also serves only the purpose of generating energy for the operation of the switchable shading and at the same time provides automatic switching in response to the emission of light. The surfaces on which the solar cells are provided are either light-tight or have a static shading, respectively. Furthermore, the energy from the dynamically blocked light is lost.

[0006] A solar cell is known from German patent application DE 10 2013 217 653 A1 and has a front electrode, a back electrode and a photoelectrically active absorber layer arranged between the front electrode and the back electrode, the photoelectrically active absorber layer containing at least 30 mass percent germanium. The absorber layer has a thickness of less than 40 nm, the solar cell has a front reflector layer and a back reflector layer, the absorber layer is arranged between the front reflector layer and the back reflector layer, and the optical path length between the front reflector layer and the back reflector layer is smaller than half the path length corresponding to the band gap of the absorber layer. The optical path length is thereby calculated from the product of the refractive index and the geometric path length. This solar cell has the advantage that the thickness of the absorber layer is reduced compared to known solar cells and the absorption capacity is only slightly reduced. Summary of the Invention

[0007] It is an object of the present invention to provide an absorbing element which can be reversibly switched for as long as possible and with the maximum possible stroke, the stroke being understood in the following as the difference between minimum absorptance / maximum transmittance and maximum absorptance / minimum transmittance. It is a further object of the present invention to provide a solar cell with an absorbing element which can be switched for as long as possible and in a reversible manner with the maximum possible stroke.

[0008] According to the invention, this object is solved by an absorber element having the features of independent claim 1. Advantageous further developments of the absorber element are obtained from dependent claims 2 to 10. This object is further solved by a solar cell according to claim 11. Advantageous embodiments of the solar cell are obtained from dependent claims 12 to 14.

[0009] The switchable absorbing element according to the invention comprises an absorbing layer. The absorbing element further comprises at least one front-side reflective layer and at least one back-side reflective layer, the absorbing layer being arranged between the front-side reflective layer and the back-side reflective layer, and the optical path length between the front-side reflective layer and the back-side reflective layer is smaller than 400 nm, at least for light perpendicularly incident on the cell. Hereinafter, the term "front side" thereby refers to the side of the absorbing element facing a light source, for example the sun, while hereinafter, the term "back side" refers to the side of the absorbing element facing away from the light source. The optical path length can be calculated from the product of the refractive index and the geometrical layer thickness. The absorbing element according to the invention is characterized in that at least one of the reflective layers has a switchable reflectivity.

[0010] A thin absorbing layer located between at least two partially or completely reflecting reflective layers absorbs significantly less than 50% of the incident light in one pass. Resonance or standing waves or multiple reflections, respectively, are generated between the reflective layers under certain conditions. Compared to a simple pass through the layers, this results in a multiplied absorption coefficient. This type of absorbing element can, for example, contain germanium. Provided that the absorbing layer has a thickness dimensioned such that the optical path length between the front-side reflective layer and the back-side reflective layer is smaller than 400 nm at least for light perpendicularly incident on the cell, a broadband optical resonance is formed between the front-side reflective layer and the back-side reflective layer, so that the absorption coefficient of the absorbing layer is significantly increased. If at least one of the reflective layers has a switchable reflectivity, the incident light can either be mostly transmitted or, in another switching state, be mostly absorbed by the resonance and can be used, for example, to generate heat and / or electrical energy. When the reflectivity of at least one of the reflecting layers is switched off, the resonance effect disappears almost completely, so that the light passes through the absorbing layer only once and is mostly transmitted. In contrast, when the reflectivity of the reflecting layers is switched on together, the absorbing layer is repeatedly passed due to the resonance or standing wave, respectively, which leads to high absorption, which leads to high stroke, and little light is transmitted, which leads to a light blocking effect. The inventive arrangement of the switchable absorbing element is further designed such that when the switchable absorbing element is integrated into a solar cell further having a charge carrier selective electrode, at least one front side electrode and at least one back side electrode, and an absorbing layer arranged between the front side electrode and the back side electrode, it leads to efficient charge carrier regeneration and high electric energy generation, and the absorbing layer is photoelectrically active.

[0011] These materials or material combinations have been found to be advantageous as materials or material combinations for the absorbing layer, respectively, since they already ensure an absorption of at least about 5%, preferably at least about 10%, and up to about 30%, preferably up to about 20%, for the light reflected in the reflection mode by at least one of the switchable reflecting layers after one pass. In order to achieve a high absorption of the absorbing layer due to multiple reflections formed between the front-side reflecting layer and the back-side reflecting layer in the reflection mode of the at least one switchable reflecting layer itself, the absorbing layer already requires a certain minimum absorption after one pass, provided that it is not itself switchable. This minimum absorption should be so small that only little light is absorbed in the transmission mode of the at least one switchable reflecting layer after one pass, while in the reflection mode of the at least one switchable reflecting layer a high absorption is achieved due to resonances as a result of, for example, effectively passing through the absorbing layer ten times. In a single pass, its value is a function of the material and thickness of the absorbing layer but not of the switching state of the reflective layer.

[0012] In order, on the one hand, to ensure as achromatic a color as possible in the transmission mode of the switchable absorbing element, and, on the other hand, to ensure the maximum possible broadband absorption in the blocking mode, the optical path length between the front-side reflective layer provided with the absorbing layer and the back-side reflective layer must be as small as possible. In a preferred embodiment, the optical path length between the front-side reflective layer and the back-side reflective layer is therefore smaller than 200 nm, at least for light perpendicularly incident on the cell.

[0013] In particularly preferred embodiments, the optical path length between the front-side reflective layer and the back-side reflective layer is less than 100 nm, at least for light normally incident on the cell.

[0014] In an advantageous embodiment, the interface between the absorbing layer and the front-side reflective layer has a reflectivity of at least 10%.

[0015] It has further proven to be advantageous when the absorber layer comprises amorphous germanium and / or an alloy based thereon.

[0016] In other embodiments, the absorber layer is made of transition metal dichalcogenides (TMDCs) and / or other metals, such as MoS 2 , W.S. 2 , MoSe 2 , WSe 2 , SnS x , MoO x and other metal-based semiconductor materials such as

[0017] In a further preferred embodiment, the absorbent layer itself has a switchable absorption rate, so that the switchable stroke is further increased.

[0018] The described advantageous embodiments result in an optimization of the energy production, especially when the switchable absorption element is implemented to be photoelectrically active.

[0019] The solar cell of the invention is characterized in that the solar cell comprises an absorber element of the invention, the solar cell further comprising a charge carrier selective electrode, at least one front electrode and at least one back electrode, the absorber layer designed to be photoelectrically active being arranged between the front electrode and the back electrode.

[0020] It has been found to be advantageous if the solar cell comprises an additional electrode which removes the generated current.

[0021] A further advantageous embodiment of the solar cell is characterized in that at least one of the layers of the absorber layer, the front-side reflector layer and / or the back-side reflector layer is structured in three dimensions. In the case of thin-layer solar cells, the structuring of the different layers can take place, for example, by means of a laser beam, in order to achieve an electrical series connection of the individual cell strips of the solar cell, i.e. to generate high voltages instead of high currents.

[0022] The at least one switchable reflective layer can be based on electrochromic, gasochromic, thermochromic or photochromic materials. Liquid crystal or chiral materials are also possible as main components. The at least one switchable reflective layer can in particular have a multilayer stack, layers containing one or several of the above-mentioned switchable materials, optionally complemented by additional catalyst layers and, in the case of electrochromic materials, by electrode layers, for example ITO layers. However, how exactly the at least one switchable reflective layer is constructed is generally not important.

[0023] In a preferred embodiment, at least one of the reflective layers is based on an electrochromic material and has a common electrode with the solar cell, thereby saving electrodes.

[0024] Further advantages, features and advantageous further developments of the invention emerge from the dependent claims and from the following description of preferred exemplary embodiments based on the drawings. [Brief description of the drawings]

[0025] [Figure 1] 1 shows a general configuration of an absorbing element according to the present invention. [Diagram 2] 1 illustrates an exemplary embodiment of an absorption element according to the present invention that includes a germanium absorption layer. [Diagram 3] 1 shows an exemplary embodiment of an absorbing element according to the present invention comprising a switchable back-side reflective layer that includes an n-layer as part of the front-side reflective layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] FIG. 1 shows a general configuration of an absorbent element 1 according to the invention. Light is substantially perpendicularly incident on the absorbent element 1, as indicated by the three broad arrows. The absorbent element 1 comprises a front-side reflective layer 5 and a back-side reflective layer 6. The absorbent layer 4 has a thickness dimensioned such that the optical path length between the front-side reflective layer 5 and the back-side reflective layer 6 is smaller than 400 nm, at least for light perpendicularly incident on the cell. A broadband resonance or standing wave or multiple reflections are then formed between the front-side reflective layer 5 and the back-side reflective layer 6, respectively. In comparison with a simple passage through the layers, this results in a multiplied absorption coefficient. The absorbent layer 4 thus forms a resonator 8. The absorbent layer 4 can thereby be composed of a single layer or a layer stack of different layers. At least one of the reflective layers (front-side reflective layer 5 and / or back-side reflective layer 6) has a switchable reflectivity.

[0027] 2 shows an exemplary embodiment of an absorber element 1 according to the invention, comprising an absorber layer 4 of germanium. The front-side reflective layer 5 is shaped as a metal grid structure, in particular of silver, the metal covering only a small part, for example 10%, of the surface of the absorber element 1, while a further layer of TCO (transparent conductive oxide), in particular one of the materials ZnO:Al (AZO), SnO2:F (FTO), In2O3:F (ITO), is shaped on the front-side of this metal grid, so that both layers together cover at least 80% of the absorber element 1. Due to the fact that both materials have a high reflectivity, together at the interface, for example to amorphous or microcrystalline silicon, they form the front-side electrode 2 and at the same time the front-side reflective layer 5, in this case for example 80 nm thick. This front-side reflective layer 5 has a refractive index mainly of about 2.

[0028] The absorbing layer 4 is formed as an amorphous or microcrystalline germanium layer. It is thereby composed of a layer stack comprising a first layer 4a on the light incidence side of an amorphous, n-doped silicon alloy with a thickness of approximately 10 nm. A second layer 4b of an amorphous intrinsic germanium alloy with a layer thickness of approximately 3 to 30 nm follows the first layer 4a. A final third layer 4c comprises a microcrystalline p-doped silicon alloy with a layer thickness of approximately 10 nm. This layer stack forms a resonator 8 with a refractive index of approximately 3.5 to 4.5.

[0029] The absorbing element 4 is completed by a switchable back-side reflective layer 6 which is formed flat.

[0030] In this embodiment, the n-th layer is formed as an internal part of the cavity 8, in that its refractive index is essentially similar to the refractive index of the adjacent second layer 4b of the absorbing layer.

[0031] FIG. 3 shows a switchable absorbing element 1 including an n-layer as part of the front-side reflective layer 5. The configuration thus corresponds to that of the exemplary embodiment shown previously, but with the difference that the n-th layer is not the first layer 4a of the absorbing layer 4, i.e. of the resonator 8, but the second layer 5b of the front-side reflective layer 5, the refractive index of the second layer 5b of the front-side reflective layer being significantly different from the refractive index of the second layer 4b of the absorbing layer. Here, the resonator 8 consists only of a second layer 4b of an amorphous intrinsic germanium alloy with a layer thickness of about 3 to 30 nm and a final layer 4c of a microcrystalline p-doped silicon alloy with a layer thickness of about 10 nm. This layer stack forms a resonator 8 with a refractive index of about 3.5 to 4.5. In this embodiment, the front-side reflective layer 5 consists of a first layer 5a of TCO (transparent conductive oxide), in particular of ITO. The front-side reflective layer 5 further comprises a second layer 5b of amorphous, n-doped silicon alloy having a thickness of about 10 nm. The refractive index of the front-side reflective layer 5 of about 2 and the refractive index of the cavity of about 3.5 to 4.5 are not affected by this modification.

[0032] The embodiments shown herein are merely examples of the present invention and should therefore not be understood as limiting. Other embodiments contemplated by those skilled in the art are equally included in the scope of protection of the present invention. [Explanation of symbols]

[0033] 1 Absorbing element 2 Surface side electrode 3 Back electrode 4 Absorbing Layer 4a 1st layer 4b 2nd layer 4c 3rd layer 5 Surface reflective layer 5a 1st layer 5b 2nd layer 6 Back side reflective layer 7. Solar Cells 8 resonator

Claims

1. An absorbing element (1) with switchable reflectivity, An absorbent layer (4), the switchable absorption element (1) has at least one front-side reflective layer (5) and at least one back-side reflective layer (6), the absorption layer (4) is disposed between the front-side reflective layer (5) and the back-side reflective layer (6), an optical path length between the front-side reflective layer (5) and the back-side reflective layer (6) is smaller than 400 nm, at least in the optical path length of light when incident perpendicularly on the absorption layer (4), and the optical path length is the product of the refractive index of the absorption layer (4) and the layer thickness of the absorption layer (4); 1. A switchable absorption element (1), characterized in that at least one of the front-side reflective layer (5) and the back-side reflective layer (6) has a switchable reflectivity, and the interface between the absorption layer (4) and the front-side reflective layer (5) has a reflectivity of at least 10%.

2. 2. The switchable absorption element (1) of claim 1, characterized in that the switchable reflectivity of at least one of the front-side reflective layer (5) and the back-side reflective layer (6) is designed such that it can be used to generate thermal and / or electrical energy, and incident light can either be transmitted in a first switching state or absorbed in a second switching state by resonance.

3. 3. A switchable absorbent element (1) as claimed in claim 1 or 2, characterized in that the switchable absorbent element (1) can be reversibly switched within a stroke, the stroke being the difference between the minimum absorption / maximum transmittance of the absorbent element (1) and the maximum absorption / minimum transmittance of the absorbent element (1).

4. 4. The switchable absorption element (1) of claim 3, characterized in that when the reflectivities of the front-side reflective layer (5) and the back-side reflective layer (6) are in a second switching state, the absorption layer (4) is repeatedly passed by a resonant or standing wave, thereby generating a light blocking effect.

5. 5. The switchable absorption element (1) according to claim 1, characterized in that an optical resonance is formed between the front-side reflective layer (5) and the back-side reflective layer (6), as a result of which the absorption coefficient of the absorption layer (4) is increased and at least one of the front-side reflective layer (5) and the back-side reflective layer (6) has a switchable reflectivity, and incident light can either be transmitted in a first switching state or absorbed by resonance in a second switching state and can be used to generate thermal and / or electrical energy.

6. a. the absorbing layer (4) absorbs less than 50% of the incident light when the light passes through the absorbing layer (4) once; b. Resonance or standing waves or multiple reflections are generated between the front-side reflective layer (5) and the back-side reflective layer (6), which results in a multiplied absorption coefficient compared to the single pass through the absorbing layer (4); c. the switchable reflectivity of at least one of the front-side reflective layer (5) and the back-side reflective layer (6) is designed such that the incident light can either be transmitted in a first switching state or absorbed by the resonance in a second switching state and can be used to generate thermal and / or electrical energy; and d) The switchable absorption element (1) according to any one of claims 1 to 5, characterized in that when the reflectivity of at least one of the front-side reflective layer (5) and the back-side reflective layer (6) is in a first switching state, the effect of the resonance disappears, so that the light passes through the absorption layer (4) only once and is transmitted, and in contrast, when the reflectivity of at least one of the front-side reflective layer (5) and the back-side reflective layer (6) is in a second switching state, a resonance of light is formed between the front-side reflective layer (5) and the back-side reflective layer (6), so that the absorption layer (4) is repeatedly passed by a resonance or a standing wave, thereby generating a light blocking effect.

7. A switchable absorption element (1) according to any one of claims 1 to 6, characterized in that the optical path length between the front-side reflective layer (5) and the back-side reflective layer (6) is less than 200 nm, at least in the optical path length of light when incident perpendicularly on the switchable absorption element (1).

8. 8. A switchable absorption element (1) according to claim 1, characterized in that the optical path length between the front-side reflective layer (5) and the back-side reflective layer (6) is less than 100 nm, at least in the optical path length of light when incident perpendicularly on the switchable absorption element (1).

9. 9. A switchable absorption element (1) according to any one of claims 1 to 8, characterized in that in the wavelength range of light reflected by at least one of the front-side reflective layer (5) and the back-side reflective layer (6), the absorption layer (4) has an absorption rate of at least 5% when the light passes through the absorption layer (4) once.

10. 10. A switchable absorption element (1) according to claim 1, characterized in that in the wavelength range of light reflected by at least one of the front-side reflective layer (5) and the back-side reflective layer (6), the absorption layer (4) has an absorption rate of at least 10% when the light passes through the absorption layer (4) once.

11. the absorption layer (4) has an absorption rate of up to 30% in the wavelength range of light reflected by at least one of the front-side reflective layer (5) and the back-side reflective layer (6) when the light passes through the absorption layer (4) once; A switchable absorption element (1) according to any one of the preceding claims.

12. The switchable absorbing element (1) according to any one of the preceding claims, characterized in that the absorbing layer (4) comprises amorphous germanium and / or alloys based thereon.

13. The switchable absorbing element (1) according to any one of the preceding claims, characterized in that the absorbing layer (4) comprises transition metal dichalcogenides (TMDCs) and / or other metal-based semiconducting materials.

14. The switchable absorption element (1) according to any one of the preceding claims, characterized in that the absorption layer (4) has a switchable absorption rate.

15. A solar cell (7), 15. A solar cell (7), comprising a switchable absorption element (1) according to any one of claims 1 to 14, characterized in that the solar cell (7) further comprises a charge carrier selective electrode, at least one front side electrode (2), at least one back side electrode (3) and the absorption layer (4) arranged between the front side electrode (2) and the back side electrode (3), the absorption layer (4) being designed to be photoelectrically active.

16. 16. The solar cell (7) according to claim 15, characterized in that the solar cell (7) has a further electrode for removing the generated current.

17. 17. The solar cell (7) according to claim 15 or 16, characterized in that at least one of the absorber layer (4), the front-side reflective layer (5) and the back-side reflective layer (6) is three-dimensionally structured.

18. The solar cell (7) according to any one of claims 15 to 17, characterized in that at least one of the front-side reflective layer (5) and the back-side reflective layer (6) is based on an electrochromic material, and the front-side reflective layer (5) and the back-side reflective layer (6) based on an electrochromic material have a common electrode with the solar cell (7).

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