DE102018206515A1 - Absorber element and photovoltaic cell - Google Patents

Abstract

The invention relates to an absorber element and a photovoltaic cell based thereon. An absorber element according to the invention has a photoelectrically active absorber layer, wherein the absorber element has at least one front-side reflection layer and at least one rear-side reflection layer, wherein the absorber layer is arranged between the front-side reflection layer and the back-reflection layer, wherein the optical path length between the front-side reflection layer and the back-reflection layer is at least 400 for incident light perpendicular to the cell nm is. The absorber element according to the invention is characterized in that at least one of the reflection layers has a spectrally selective reflectivity, wherein substantially usable by plants for photosynthesis light is substantially transmissive and light of other wavelengths is substantially reflectable.

Description

The invention relates to an absorber element and a photovoltaic cell based thereon.

By means of a photovoltaic cell, also called a solar cell, electrical energy can be obtained from the light incident on the cell. For this purpose, solar cells have an absorber element in which free charge carriers can be generated by the supply of energy in the form of electromagnetic radiation. Known photovoltaic cells aim to absorb as much as possible of the entire wavelength spectrum of the light incident on them.

From the German patent application DE 10 2013 217 653 A1 For example, a solar cell is known which has a front-side electrode and a rear-side electrode and a photoelectrically active absorber layer arranged between front and rear electrodes, which contains at least 30 percent by mass of germanium. The absorber layer has a thickness of less than 40 nm, wherein the solar cell has a front-side reflection layer and a rear-side reflection layer, wherein the absorber layer is arranged between the front and back reflection layer and the optical path length between the front and back reflection layer is less than half of the band gap at one of the band gap of the absorber layer corresponding wavelength. The solar cell has the advantage that the thickness of the absorber layer is reduced compared to previously known solar cells, wherein the absorption capacity is only slightly reduced.

For example, greenhouses that have large glass surfaces through which the light necessary for plant growth falls on the cultivated plants or the biomass are required for biomass production and cultivation. For plant growth, a photosynthesis process is required in which energy-rich biomolecules are produced from lower-energy substances with the aid of light energy. Plants, including, for example, algae, normally require light of specific wavelengths in the blue and red wavelengths. Green and infrared light is only weakly absorbed by the plants and therefore mostly transmitted or reflected. Where photosynthetic species are grown using sunlight or other broadband light sources, it is desirable to convert the energy of light that is unusable for photosynthesis into heat or electricity.

This requires a device that transmits light of those wavelengths that are usable by the plants, while absorbing the remaining wavelengths of light as broadband as possible. For this purpose, a concept is known in which a luminescent absorber material on a highly transparent glass pane absorbs green light and reemit in the form of red light, which then partially propagates in the glass and of highly efficient strip-shaped wafer solar cells, which are distributed on the surface of the disc are, absorbed and used. Disadvantage here is on the one hand that the solar cells themselves are opaque and already cover 14% of the window area. In addition, there are high costs for high-purity glass in order to enable the transmission of the light emitted by the dye to the solar cells. Moreover, the process by which the solar cells are cut into pieces, placed on the disks and wired is expensive.

The object of the invention is to provide an absorber element and a photovoltaic cell, the / the light of those wavelengths that can be used by plants, transmits, while it / the remaining wavelengths of light as broadband absorbed, the efficiency over prior art devices increases and the manufacturing cost is minimized.

According to the invention, this object is achieved by an absorber element having the features of independent claim 1. Advantageous developments of the absorber element emerge from the dependent claims 2 to 16 , The object is further achieved by a photovoltaic cell according to claim 17. Advantageous embodiments of the photovoltaic cell will be apparent from the dependent claims 18 and 19 ,

An absorber element according to the invention has an absorber layer, wherein the absorber element has at least one front-side reflection layer and at least one backside reflection layer, wherein the absorber layer is arranged between the front-side reflection layer and the back-reflection layer, wherein the optical path length between the front-side reflection layer and the back-reflection layer is at least 400 nm for incident light perpendicular to the cell , Here, and hereinafter, the term "front side" refers to the light source, for example the sun, facing side of the absorber element, while the term "back" here and below refers to the light source side facing away from the absorber element. The optical path length is calculated from the product of the refractive index with the geometric path, ie the geometric thickness of the layer stack. The absorber element according to the invention is characterized in that at least one of the reflection layers has a spectrally selective reflectivity, wherein light which is essentially usable by plants for photosynthesis is Substantially transmissive and light of other wavelengths is substantially reflective.

The thin absorber located between the at least two at least partially reflecting reflection layers absorbs significantly less than 50% of the incident light when it is passed through once. Resonances or standing waves or multiple reflections develop under certain conditions between the reflective layers. This results in a multiplication of the absorption compared to simply passing through the layer. Such an absorber element may include, for example, germanium. If the absorber layer has a thickness dimensioned such that the optical path length between front reflection layer and backside reflection layer is at least 400 nm for at least vertical light incident on the cell, a broadband optical resonance forms between the front and back reflection layer, whereby the absorption is significantly increased in the absorber layer. If at least one of the reflection layers has a spectrally selective reflectivity, essentially photosynthetic light being essentially transmissible by plants for photosynthesis and light of other wavelengths being substantially reflectable, the incident light can be used both for generating electrical energy and for plant growth. At wavelengths with reduced reflectivity then disappear the resonance effects almost completely, so that this light passes through the absorber layer only simply and is substantially transmitted. At wavelengths of high reflectivity, however, the absorber layer is traversed many times, resulting in a high absorption. At the same time this leads to suitable light conditions for the growth of photosynthetically active species as well as to a large total absorption of the remaining light. In addition, the inventive design of the spectrally selective absorber element results in an efficient charge carrier generation and high electrical energy yield when the spectrally selective absorber element is integrated into a photovoltaic solar cell, the charge carrier selective electrodes and at least one front side electrode and at least one rear side electrode and between front side electrode and Rear side electrode arranged absorber layer, wherein the absorber layer is configured photoelectrically active.

It has proved to be advantageous if at least one of the reflection layers has a spectrally selective reflectivity with a change in reflectivity of at least 50% for a limited wavelength range. The wavelength of the light which can be used for photosynthesis is about 400 nm to 500 nm in the blue region and about 630-730 nm in the red region. The wavelengths between about 500 nm and about 630 nm fall in the green and yellow areas and are generally not or only weakly used by plants. Even light in the UV range and above 630 nm contribute little to photosynthesis. These wavelengths can therefore be used for the production of electrical energy.

As materials or material combinations for the absorber layer, it has proven advantageous to absorb at least about 5%, preferably at least about 10%, and at most about a maximum absorption at an optical thickness of 400 nm or less in the spectral range reflected by the backside reflective layer ensure about 30%, preferably of at most about 20% even with a single pass. The optical thickness is calculated from the product of the refractive index with the geometric thickness.

In order to ensure as broadband absorption as possible in the wavelength ranges reflected by the rear-side reflection layer, the optical path length between the front-side reflection layer and the rear-side reflection layer given by the absorber layer must be as low as possible. Therefore, in one preferred embodiment, the optical path length between the front-side reflection layer and the back-side reflection layer, at least for light incident perpendicular to the cell, is less than 200 nm.

In a particularly preferred embodiment, the optical path length between the front-side reflection layer and the back-reflection layer is at least less than 100 nm, at least for light incident perpendicular to the cell.

The spectral selectivity of the absorber element can be supported by the absorber layer itself additionally having a spectrally selective absorption coefficient. In an advantageous embodiment, therefore, the absorber layer has a spectrally selective absorption, wherein light usable by plants for photosynthesis is substantially transmissive and light of other wavelengths is substantially absorbable.

In an advantageous embodiment, the boundary surface between absorber layer and front side reflection layer has a reflectivity of at least 10%.

Furthermore, it has proved to be advantageous if the absorber layer comprises amorphous germanium and / or an alloy based thereon.

In an advantageous embodiment, the absorber layer has transition metal Dichalcogenides (TMDCs) and / or other metal-based semiconductor materials, such as MoS ₂ , WS ₂ , MoSe ₂ , WSe ₂ , SnS _x and MoO _x .

In a further advantageous embodiment, the absorber layer comprises organic semiconductor materials or one or more organic dyes, in particular those having a spectrally selective absorption coefficient.

In a further advantageous embodiment, at least one of the reflection layers has a metal / oxide / metal multiple layer stack. In this case, a layer thickness of the inner oxide layer of 250 nm to 550 nm has proven to be advantageous.

The term "oxide" is to be understood here and below as a placeholder for materials which are substantially transparent in the visible wavelength range and have a high band gap of 2 eV or higher, as has become customary in the art. Besides the oxides (e.g., zinc oxide, silica, indium-tin oxide, alumina, titania, magnesia), e.g. Also, nitrides (e.g., silicon nitride), oxynitrides (e.g., silicon oxynitride) or carbides (e.g., silicon carbide) can be used.

In an alternative embodiment, at least one of the reflective layers comprises a metal / oxide / metal / oxide multiple layer stack.

In particular, it has proved to be advantageous if at least one of the reflection layers has a zinc oxide / silver / zinc oxide / silver multiple layer stack, the silver layer thicknesses being between 5 and 25 nm.

In particular, it has proved to be advantageous if the layer thickness of at least one of the oxide layers is between about 250 nm and 550 nm.

The described advantageous embodiments cause an optimization of the energy yield.

An inventive photovoltaic cell is characterized in that the photovoltaic cell has an inventive absorber element, wherein the solar cell further comprises charge carrier selective electrodes and at least one front-side electrode and at least one rear-side electrode. Between the front side electrode and the rear side electrode, the absorber layer is arranged, which is configured photoelectrically active.

It has proved to be advantageous if the photovoltaic cell has further electrodes for the removal of the generated electrical current. This increases the efficiency of the photovoltaic cell.

A further advantageous embodiment of the photovoltaic cell is characterized in that at least one of the layers of the absorber layer, the front-side reflection layer and / or the rear-side reflection layer is structured in three dimensions. For example, in the case of thin-film solar cells, a structuring of the different layers can be carried out by means of laser cuts in order to achieve an electrical series connection of individual cell strips of the photovoltaic cell and thus to generate a higher module voltage instead of a high current.

Further advantages, features and expedient developments of the invention will become apparent from the dependent claims and the following description of preferred embodiments with reference to the drawings.

• 1 den allgemeinen Aufbau eines erfindungsgemäßen Absorberelements.
• 2 ein Ausführungsbeispiel eines erfindungsgemäßen Absorberelements mit einer Germanium-Absorberschicht.
• 3 ein Ausführungsbeispiel eines erfindungsgemäßen Absorberelements mit spektral selektiver Transparenz mit n-Schicht als Teil der Vorderseitenreflexionsschicht

From the pictures shows:

• 1 the general structure of an absorber element according to the invention.
• 2 An embodiment of an absorber element according to the invention with a germanium absorber layer.
• 3 An embodiment of an absorber element according to the invention with spectrally selective transparency with n-layer as part of the front-side reflection layer

1 shows the general structure of an absorber element according to the invention 1 , Light falls almost perpendicular to the absorber element 1 as indicated by the three broad arrows. The absorber element 1 has a front side reflection layer 5 and a backside reflective layer 6 on. An absorber layer 4 is between the front side reflection layer 5 and the backside reflection layer 6 arranged. The absorber layer 4 has a thickness dimensioned such that the optical path length between front side reflection layer 5 and backside reflective layer 6 at least for incident perpendicular to the cell light is less than 400 nm. Between the reflective front side reflection layer 5 and the backside reflection layer 6 then arise broadband resonances or standing waves or multiple reflections. This results in a multiplication of the absorption compared to simply passing through the layer. The absorber layer 4 thus forms a resonator 8th , The absorber layer 4 can consist of a single layer or a layer stack of different layers. At least one of the reflection layers (front reflection layer 5 and / or backside reflective layer 6 ) has a spectrally selective reflectivity, wherein the Reflectance, for example, changed by at least 50% for a limited wavelength range.

2 shows an embodiment of an absorber element according to the invention 1 with a germanium absorber layer 4 , The front side reflection layer 5 is formed as a metal lattice structure, in particular of silver, in which the metal is the surface of the absorber element 1 only to a small extent, for example 10%, covered, while on the front side of this metal grid another layer of a TCO (Transparent Conductive Oxide), in particular from one of the materials ZnO: Al (AZO), SnO2: F (FTO), In2O3: F (ITO) is formed, such that both layers together a covering of the absorbent element 1 of at least 80%. This front side reflection layer 5 then has a refractive index of about 2 or less. Therefore, since both materials have high reflectivity at the interface with amorphous or microcrystalline silicon, for example, they together constitute a front-side electrode 2 and at the same time a front side reflection layer 5 of for example 80 nm thickness.

The front side reflection layer 5 may be formed as a known Bragg reflector with the aim of achieving wavelength-dependent transmission and reflection properties of the same. A Bragg reflector consists of alternating, thin layers, for example of a dielectric of different refractive indices. At each boundary layer, part of the electromagnetic wave of the light is reflected according to Fresnel's formulas. If the wavelength is close to four times the optical path length of the layers, then the reflected rays interfere constructively and a high quality reflector is formed.

The absorber layer 4 is formed as an amorphous or microcrystalline germanium layer. The absorber layer 4 consists of a layer stack with a light incident side first layer 4a of an amorphous, n-doped silicon alloy with a thickness of about 10 nm. On the first layer 4a follows a second layer 4b of an amorphous intrinsic germanium alloy with a layer thickness of about 3 to 30 nm. A last, third layer 4c has a microcrystalline p-doped silicon alloy with a layer thickness of about 10 nm. This layer stack forms the resonator 8th with a refractive index of about 3.5 to 4.5.

The absorber element 4 is a surface-formed spectrally selective backside reflection layer 6 completed. The backside reflection layer may also be formed as a Bragg reflector with the aim of achieving wavelength-selective transmission and reflection properties of the same.

In this embodiment, the n-layer is part of the interior of the resonator 8th formed by their refractive index of that of the adjacent second absorber layer 4b similar.

3 shows an absorber element 1 with spectrally selective transparency with n-layer as part of the front-side reflection layer 5 , The structure corresponds to that of the previously shown embodiment with the difference that the n-layer is not the first layer 4a the absorber layer 4 and thus the resonator 8th is, but second layer 5b the front side reflection layer 5 is, by their refractive index substantially different from that of the second layer 4b the absorber layer deviates. The resonator 8th here consists only of the second layer 4b of an amorphous intrinsic germanium alloy with a layer thickness of about 3 to 30 nm and a last, third layer 4c , which has a microcrystalline p-doped silicon alloy with a layer thickness of about 10 nm. This layer stack forms the resonator 8th with a refractive index of about 3.5 to 4.5. The front side reflection layer 5 in this embodiment consists of a first layer 5a from a TCO (Transparent Conductive Oxide), in particular from an ITO. Furthermore, the front side reflection layer includes 5 a second layer 5b of an amorphous, n-doped silicon alloy having a thickness of about 10 nm. The refractive indices of about 2 of the front-side reflection layer 5 and from about 3.5 to 4.5 of the resonator are not affected by this change.

The embodiments shown herein are only examples of the present invention and therefore should not be considered as limiting. Alternative embodiments contemplated by one skilled in the art are equally within the scope of the present invention.

LIST OF REFERENCE NUMBERS

1

absorber element

2

Front side electrode

3

Back electrode

4

absorber layer

4a

first shift

4b

second layer

4c

third layer

5

Front reflection layer

5a

first shift

5b

second layer

6

Back reflection layer

7

Photovoltaic cell

8th

resonator

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102013217653 A1 [0003]

Claims (19)

An absorber element (1) having an absorber layer (4), wherein the absorber element (1) has at least one front side reflection layer (5) and at least one rear side reflection layer (6), wherein the absorber layer (4) is arranged between front side reflection layer (5) and back side reflection layer (6) in that the optical path length between the front-side reflection layer (5) and the back-reflection layer (6) is at least 400 nm for incident light perpendicular to the cell, characterized in that at least one of the reflection layers (5, 6) has a spectrally-selective reflectivity Photosynthesis substantially usable light substantially transmissive and light of other wavelengths is substantially reflective. Absorber element (1) according to Claim 1 , characterized in that the optical path length between the front-side reflection layer (5) and the back-reflection layer (6) is less than 200 nm at least for incident light perpendicular to the cell. Absorber element (1) according to Claim 1 , characterized in that the optical path length between the front side reflection layer (5) and the back side reflection layer (6) is smaller than 100 nm at least for light incident perpendicular to the cell. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer (4) in the wavelength ranges reflected by the backside reflection layer (6) has an absorption of at least 5% in a single pass. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer (4) has an absorption of at least 10% in one pass through the wavelength ranges reflected by the backside reflection layer (6). Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer (4) has an absorption of not more than 30%, preferably not more than 20%, in a single pass through in the wavelength ranges reflected by the backside reflection layer (6). Absorber element (1) according to one of the preceding claims, characterized in that the boundary surface between absorber layer (4) and front side reflection layer (5) has a reflectivity of at least 10%. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer (4) comprises amorphous germanium and / or an alloy based thereon. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer (4) has transition metal dichalcogenides (TMDCs) and / or other metal-based semiconductor materials. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer comprises organic semiconductor materials. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer comprises at least one organic dye. Absorber element (1) according to one of the preceding claims, characterized in that the absorber layer (4) has a spectrally selective absorption, wherein light usable by plants for photosynthesis is substantially transmissive and light of other wavelengths is substantially absorbable. Absorber element (1) according to one of the preceding claims, characterized in that at least one of the reflection layers (5, 6) has a metal / oxide / metal multiple layer stack. Absorber element (1) according to one of Claims 1 to 12 , characterized in that at least one of the reflective layers (5, 6) comprises a metal / oxide / metal / oxide multiple layer stack. Absorber element (1) according to Claim 14 , characterized in that at least one of the reflective layers (5, 6) comprises a zinc oxide / silver / zinc oxide / silver multiple layer stack, wherein the silver layer thicknesses are between 5 and 25 nm. Absorber element (1) according to one of Claims 11 to 15 , characterized in that the layer thickness of the at least one oxide layer is between about 250 nm and about 550 nm. Photovoltaic cell (7), characterized in that the photovoltaic cell (7) comprises an absorber element (1) according to one of the preceding claims, wherein the photovoltaic cell (7) further comprises charge carrier selective electrodes and at least one front side electrode (2) and at least one rear side electrode ( 3) and between the front side electrode (2) and Rear side electrode (3) arranged absorber layer (4), wherein the absorber layer (4) is configured photoelectrically active. Photovoltaic cell (7) according to Claim 17 , characterized in that the photovoltaic cell (7) has further electrodes for the removal of the generated electric current. Photovoltaic cell (7) according to one of Claims 17 or 18 , characterized in that at least one of the layers (4, 5, 6) is structured in three dimensions.

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