DE102010006314A1 - Photovoltaic multiple thin-film solar cell - Google Patents

Abstract

The invention relates to a photovoltaic multiple thin - film solar cell, comprising a carrier substrate (1) and at least one upper and one lower subcell, each as a pin structure, which has a p-type layer (p-layer), an n-type layer (n Layer) and an intrinsic layer (i layer) arranged between the p- and n-layer, are formed, starting from a front side of the thin-film solar cell designed for the incidence of light, on the carrier substrate and / or on one or more further layers, First of all, the upper sub-cell (3) is arranged, in which the i-layer is made of hydrogenated amorphous silicon and furthermore the lower sub-cell (5) is arranged below the upper sub-cell, optionally on one or more intermediate layers, and in each sub-cell the p-type layer is arranged on the side facing the front. It is essential that the i-layer is formed from microcrystalline germanium in the lower subcell.

Description

The invention relates to a photovoltaic multiple thin-film solar cell according to the preamble of claim 1.

Photovoltaic thin-film solar cells have the advantage over wafer solar cells, such as solar cells based on silicon wafers, that due to the thinner compared to wafer semiconductor layers significantly lower costs incurred in the production of the solar cell for the semiconductor material.

The individual layers of a thin film solar cell are typically deposited by chemical vapor deposition (CVD). In particular, the plasma-enhanced PECVD (plasma enhanced chemical vapor deposition) method is used.

The layers can be deposited in different degrees of crystallization, for example as amorphous or microcrystalline layers. However, this typically has the disadvantage that defects with energy levels in the bandgap of the pn junction greatly affect the diffusion length of minority carriers, especially when compared to solar cell-based solar cells. For this reason, in the case of thin-film solar cells, the pn junction is typically formed as a pin junction: a p-doped layer (p-layer) is followed by an undoped (intrinsic) layer (i-layer), which in turn has an n-doped layer (n Layer) follows. This achieves a functional separation between doping layers and the i-layer formed primarily for absorbing the incident electromagnetic radiation, so that an optimization of the individual layers for the respective purpose is possible. In the case of a pin structure, the pn junction thus extends over the intrinsic layer, and correspondingly a charge carrier separation of the charge carrier pairs generated in the intrinsic layer by absorption of electromagnetic radiation takes place through the electric field established by the p and n layers.

To increase the efficiency of a thin-film solar cell, the stacking of several pin structures is known. For example, describes EP 2 017 895 A2 a silicon multiple solar cell, in which two sub-cells, which are each formed as a pin structure, are arranged one above the other.

In such tandem solar cells, which have two pin partial cells, the use of a microcrystalline silicon layer containing germanium, for increasing the light absorption and thus the overall efficiency of the thin-film solar cell is also known and, for example, in Takuya Matsui, Haijun Jia, Hiroyuki Fujiwara and Michio Kondo; Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST); 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568 Japan "MICROCRYSTALLINE Si1-xGex AS BOTTOM CELL ABSORBERS IN DOUBLE JUNCTION SOLAR CELLS" 23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain pp. 2113-2116 described.

Based on the previously known photovoltaic multiple thin-film solar cells, the present invention has the object to provide a photovoltaic multi-thin-film solar cell, in which the overall light output and thus the overall efficiency of the thin-film solar cell is increased.

This object is achieved by a thin-film solar cell according to claim 1. Advantageous embodiments of the thin-film solar cell according to the invention can be found in claims 2 to 18.

The thin-film solar cell according to the invention comprises a carrier substrate and at least one upper and one lower subcell. Upper and lower subcell are each formed as a pin structure comprising a p-type layer (p-type layer), an n-type (n-type) layer, and an intrinsic (i-type) layer. The i-layer is arranged between p and n-layer.

Starting from a formed for the incidence of light in the following description as above lying front side of the thin film solar cell, the upper part cell is initially disposed on the carrier substrate and / or on one or more further layers, wherein the i-layer of the upper part cell of hydrogenated amorphous silicon (a-Si: H) is formed. Below the upper part cell is further, optionally arranged on one or more further intermediate layers, the lower part cell. In each subcell, the p-type layer is at the top, d. H. arranged on the front side facing.

The term "hydrogenated" refers to material layers in which a high proportion of hydrogen (5-20 atomic percent) is present in the material due to the process. This is typically illustrated by ": H" in the material designation.

It is essential that in the lower part of the cell, the i-layer of microcrystalline germanium (μc-Ge) is formed.

In contrast to the prior art, the lower part cell in the thin-film solar cell according to the invention thus an absorber of microcrystalline germanium. On the other hand, has been dispensed with germanium in previous thin-film solar cells, since a germanium-containing layer in the manufacture compared to, for example, the use of silicon is relatively expensive and, moreover, according to current knowledge, there was no reason to turn away from the proven layer compositions, which are based essentially on silicon , Even in publications in which germanium-containing microcrystalline silicon layers were used, the basic aim was to keep the germanium content low due to the expected production costs.

In contrast, the i-layer of the lower subcell of the thin-film solar cell according to the invention has no silicon content. Investigations by the Applicant have shown that, surprisingly, the gain in luminous efficacy by designing the i-layer of the lower subcell of microcrystalline germanium causes such an increase in the overall efficiency of the solar cell that the additional costs caused by the use of germanium are more than compensated.

The term "microcrystalline" refers to material layers whose crystal sizes are substantially in the range of 1 nm to 100 nm in an amorphous matrix. By contrast, in the case of a purely "amorphous" layer, there are no crystallites.

When implementing the solar cell according to the invention with two partial cells, the i-layer of the upper partial cell is preferably formed with a thickness in the range between 200 nm and 400 nm and / or the i-layer of the lower partial cell ( 5 ) is formed with a thickness in the range between 50 nm and 500 nm. As a result, an optimization between light output and process time is achieved in the production.

A particularly high gain in the overall efficiency results in the preferred embodiment of the thin-film solar cell according to the invention with three sub-cells:
In the preferred embodiment, the thin-film solar cell additionally comprises an arranged between the upper and lower subcell middle subcell, which is also formed as a pin structure. The p-layer of the middle part cell is also arranged on the side facing the front side of the thin-film solar cell. It is essential that the i-layer of the middle subcell is formed of hydrogenated microcrystalline silicon (μc-Si: H).

In this so-called triple solar cell, there is a further increase in the overall efficiency compared to a multiple thin-film solar cell with two partial cells, since an additional light output is ensured by the further i-layer. Investigations by the Applicant have shown that, surprisingly, the use of amorphous silicon in the upper, of microcrystalline hydrogenated silicon in the middle and of microcrystalline germanium in the lower absorber ensures a particularly high luminous efficacy and thus a particularly high overall efficiency of the solar cell.

Investigations by the Applicant have furthermore shown that the following layer thicknesses are advantageous in the abovementioned preferred embodiment of the thin film solar cell according to the invention as a triple solar cell:
The i-layer of the upper part cell advantageously has a thickness between 200 nm and 400 nm, preferably about 300 nm.

The i-layer of the middle subcell preferably has a thickness of between 0.5 μm and 20 μm, preferably between 1.5 μm and 15 μm.

The thickness of the i-layer of the lower subcell is advantageously between 10 nm and 2000 nm, preferably between 100 nm and 1500 nm.

The ranges given for the thicknesses of the respective i-layers lead to an optimization between increasing the light absorption on the one hand and minimizing the manufacturing costs on the other hand.

Preferably, the p- and n-layers of the middle subcell are formed of microcrystalline silicon. This has the advantage that the flow of current to the upper part cell and thus to a lying above the upper part cell contacting is optimized. Likewise, however, the use of other materials for one or both of said layers in the invention, for example, amorphous silicon (a-Si) or amorphous silicon carbide (a-SiC) or microcrystalline silicon carbide (μc-SiC).

A further increase in the overall efficiency of the thin film solar cell according to the invention is achieved in the preferred embodiment as a triple solar cell by the light absorption in the central absorber is increased by additional reflection:
Preferably, an intermediate reflection layer is arranged between the middle and lower part cells. The intermediate reflection layer is designed in such a way that electromagnetic radiation impinging on the intermediate reflection layer from the middle subcell is reflected at least in the wavelength range from 500 nm to 700 nm.

This has the background that, especially in the said wavelength range, the luminous efficacy is to be ensured by the middle part cell, whereas at wavelengths below said lower limit, the luminous efficiency primarily by the upper part cell and at wavelengths above the upper limit of the wavelength range, the luminous efficacy primarily by the lower Sub-cell is guaranteed.

The intermediate reflection layer thus increases the luminous efficiency of the i-layer of the middle subcell in the wavelength range relevant to the middle subcell, without thereby reducing the luminous efficacy of the lower subcell, so that overall the overall efficiency of the solar cell is increased.

As a result of the intermediate reflection layer, or alternatively, the thickness of the i-layer of the middle subcell can be reduced, so that the production time and material costs are reduced while the luminous efficacy remains the same. In particular, in the arrangement of the said intermediate reflection layer, a layer thickness of the i-layer of the middle subcell of about 3 μm is advantageous.

In particular, it is advantageous to form the aforementioned intermediate reflection layer as a TCO layer (Transparent Conducting Oxide) or as silicon oxide. The intermediate reflection layer preferably has a layer thickness between 0.1 μm and 5 μm, preferably between approximately 1 μm and 2 μm.

In the aforementioned preferred embodiment of the thin-film solar cell according to the invention as a triple solar cell with intermediate reflection layer, the thickness of the i-layer of the lower part cell is preferably between 200 nm and 400 nm. This has the advantage that little germanium is needed to absorb the electromagnetic radiation , which results in a cost reduction.

Preferably, in the thin film solar cell according to the invention, between the upper part cell ( 3 ) and the middle subcell ( 4 ) arranged an intermediate reflection layer, which is designed such that, starting from the central part of the cell incident on the intermediate reflective layer electromagnetic radiation is reflected at least in the wavelength range of 300 nm to 600 nm, at least by a proportion of 10% in terms of intensity.

This increases the luminous efficiency in the upper part cell in the relevant wavelength range.

In the thin-film solar cell according to the invention, the p-type and n-type layers of the lower part cell are preferably formed from a material of the group amorphous silicon, microcrystalline silicon, amorphous germanium or microcrystalline germanium. This has the advantage that a deposition in a PECVD reactor and thus a cost-effective manufacturing process is possible.

Preferably, the p-layer of the upper part cell is formed of amorphous silicon carbide (a-SiC) or amorphous silicon oxide (a-SiO). This results in advantages with regard to the adaptation of the band gaps. These materials are also preferably used for the n-layer of the upper part cell. Likewise, the use of microcrystalline silicon (μc-Si) for the n-layer is within the scope of the invention.

The contacting of the p-layer of the upper part of the cell is preferably carried out in that above the upper part of the cell, a transparent transparent conductive upper contacting structure is arranged, which is electrically connected to the p-layer of the upper part cell.

The upper contacting structure is preferably formed as a TCO layer. In particular, it is advantageous that the upper contacting structure formed as a TCO layer has a doping with a doping concentration in the range between 0.1 × 10 ²⁰ cm ^-3 and 10 × 10 ²⁰ cm ^-3 . This has the advantage that the upper contacting structure has a sufficiently high transmission in the IR range, so that a high contribution to the overall light output of the thin-film solar cell is ensured, in particular by the i-layer of the lower part cell.

Preferably, a back-reflection layer is arranged below the lower part cell, optionally on further intermediate layers. The back-reflection layer reflects electromagnetic radiation, which passes through all the sub-cells of the thin-film solar cell unabsorbed, so that the reflected electromagnetic radiation is at least partially absorbed by again passing through the sub-cells and thus the overall light output of the thin-film solar cell and thus the overall efficiency is increased.

The back-reflection layer preferably has a thickness between 80 nm and 1 μm and is furthermore preferably formed from TCO and / or aluminum. Likewise, the use of a layer of white paint as a back-reflection layer is within the scope of the invention. When formed as a TCO layer is a thickness of 80 nm and when formed as an aluminum layer, a thickness of about 1 micron advantageous.

The layers formed as TCO preferably have a doping concentration in the range of 0.1 × 10 ²⁰ cm ^-3 to 10 × 10 ²⁰ cm ^-3 in order to ensure a sufficient transverse conductivity.

To further increase the luminous efficacy, it is advantageous that the solar cell comprises Lichteinfangsstrukturen, which are arranged above the upper part of the cell. Such Lichteinfangsstrukturen are known in the art, for example by being formed as pyramids or inverted pyramids. The function is primarily that a light beam once reflected at the Lichteinfangsstruktur incident at least a second time on a surface of the Lichteinfangsstruktur, so that the total coupling of incident on the front of the thin film solar cell electromagnetic radiation is increased.

Preferably, the carrier substrate of the thin-film solar cell according to the invention is transparent and the thin-film solar cell is constructed such that the carrier substrate is disposed above lying and below the carrier substrate, the upper, optionally the middle and the lower partial solar cell are arranged. Likewise, however, the arrangement of the carrier substrate at the bottom of the thin film solar cell within the scope of the invention, so that above the carrier substrate, first the lower part of the solar cell then optionally the middle part of the solar cell and the upper part of the solar cell are arranged. In this case, no formation of the carrier substrate as a transparent carrier substrate is necessary.

The layers of the thin-film solar cell according to the invention are preferably deposited in a manner known per se by means of chemical vapor deposition, in particular by means of PECVD. How such layers are deposited is known to the person skilled in the art.

Compared with wafer-based multiple solar cells, the thin-film solar cell according to the invention is additionally characterized in that exclusively elements from main group IV of the periodic table are used as essential component at least for the i-layers of all sub-cells, preferably for all layers and / or dopants of the thin-film solar cell according to the invention. preferably only silicon and germanium.

Further advantageous features and preferably embodiments are described below with reference to the figure and the embodiment. It shows 1 An embodiment of the photovoltaic multi-thin-film solar cell according to the invention as a triple solar cell.

The embodiment of the thin film solar cell according to the invention is in 1 shown schematically. In particular, the ratio of height to width is not to scale.

1 shows a thin-film solar cell, which is a transparent carrier substrate formed of glass 1 includes. The carrier substrate 1 forms the front of the thin-film solar cell designed for the coupling of light.

Starting from the carrier substrate 1 is a formed as a TCO layer upper contacting structure 2 arranged, which has a thickness of about 0.8 microns and a doping with a Dopant concentration of about 1 × 10 ²⁰ cm ^-3 . This ensures that, on the one hand, the charge carriers on the front side are able to be discharged through the upper contacting structure without appreciable transverse line resistance and, on the other hand, that electromagnetic radiation is not or only slightly absorbed in the upper contacting structure, in particular in the IR range.

This is followed by an upper part cell 3 , then a middle subcell 4 and then subsequently a lower subcell 5 ,

The sub-cells are each formed as pin structures with the top p-layer according to the following Table 1. Here, the more preferred materials or ranges of values are given in parentheses. It is within the scope of the invention to design the thin-film solar cell from combinations of the specified materials / value ranges. subcell layer material thickness endowment bandgap upper part cell p a-SiC: H, (a-SiO: H) 10 nm (2-30 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) i a-Si: H 300 nm 0 1.8 eV (1.7-1.9 eV) n a-Si: H (μc-Si: H) 10 nm (2-30 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) middle part cell p μc-Si: H, (μc-SiC: H or a-SiC: H or a-Si: H) 10nm (2-30nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) i c-Si: H 10 μm 0 1.2 eV (1.1-1.3 eV) n μc-Si: H, (a-Si: H) 10 nm (2-30 nm) 10 ¹⁹ cm ^-3 10 ¹⁸ -10 ²⁰ cm ^-3 lower part cell p a-Si: H (μc-Si: H or μc-SiC: H or μc-Ge: H or μc-SiGe: H or a-Ge: H) 10 nm (2-30 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) i μcGe: H 300 nm (100-500 nm) 0 0.7 eV (0.67-0.9 eV) n a-Si: H, (μc-Si: H or μc-Ge: H or μc-SiGe: H or a-Ge: H) 10 nm (2-30 nm) 10 ¹⁹ cm ^-3 (10 ¹⁸ -10 ²⁰ cm ^-3 ) Table 1

Below the n-layer of the lower part cell, a lower contacting structure is arranged, which is electrically conductively connected to the n-layer of the lower part cell.

The representation according to 1 shows only a partial section of the thin-film solar cell according to the invention. Advantageously, the thin film solar cell is formed in a conventional manner over a large area, wherein a series connection of individual portions of the thin film solar cell according to the invention is also carried out in a conventional manner. Such a construction of a large-area thin-film solar cell is known and already in Yamamoto, K. et al. Solar Energy 77, 939 described.

In the case of the thin-film solar cell according to the invention, it is also possible for substances other than those mentioned to be present in one layer, inter alia, since in the production processes the incorporation of Foreign matter can not be excluded. It is therefore within the scope of this description that a layer described as "intrinsic" or constructed from a particular element also comprises slightly different substances. Unless indicated otherwise, such details preferably indicate a substance content of at least 95 at% (atomic percent), ie that, for example, a layer described as being formed from germanium contains at least 95 at% germanium.

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

• EP 2017895 A2 [0005]

Cited non-patent literature

• Takuya Matsui, Haijun Jia, Hiroyuki Fujiwara and Michio Kondo; Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST); 1-1-1 Umezono, Tsukuba, Ibaraki, 305-8568 Japan "MICROCRYSTALLINE Si1-xGex AS BOTTOM CELL ABSORBERS IN DOUBLE JUNCTION SOLAR CELLS" 23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain pp. 2113-2116 [0006]
• Yamamoto, K. et al. Solar Energy 77, 939 [0050]

Claims (18)

Photovoltaic multiple thin-film solar cell, comprising a carrier substrate ( 1 ) as well as at least one upper and one lower part cell ( 5 ) each comprising a pin structure comprising a p-type layer (p-type layer), an n-type layer (n-type layer) and an intrinsic layer (i-type layer) disposed between p and n-type layer, are formed, starting from a trained for the light incidence front of the thin-film solar cell, on the carrier substrate ( 1 ) and / or on one or more further layers, first the upper part cell ( 3 ), in which the i-layer is formed from hydrogenated amorphous silicon and further below the upper part cell, optionally on one or more further intermediate layers, the lower part cell ( 5 ) and wherein in each subcell the p-conducting layer is respectively arranged on the side facing the front side, characterized in that in the lower subcell ( 5 ) the i-layer is formed of microcrystalline germanium. Thin-film solar cell according to claim 1, characterized in that the i-layer of the upper part cell ( 3 ) is formed with a thickness in the range between 200 nm and 400 nm and / or the i-layer of the lower part cell ( 5 ) is formed with a thickness in the range between 50 nm and 500 nm. Thin film solar cell according to claim 1, characterized in that the thin film solar cell additionally arranged between the upper and lower subcell middle subcell ( 4 ), which is formed as a pin structure and in which the p-layer is arranged on the front side of the thin-film solar cell side facing and wherein the i-layer of the middle part cell ( 4 ) is formed of hydrogenated microcrystalline silicon. Thin-film solar cell according to claim 3, characterized in that the p- and the n-layer of the middle subcell ( 4 ) are both formed of microcrystalline silicon. Thin-film solar cell according to one of claims 3 to 4, characterized in that the thickness of the i-layer of the upper part cell ( 3 ) is between 200 nm and 400 nm, preferably about 300 nm. Thin-film solar cell according to one of Claims 3 to 5, characterized in that the thickness of the i-layer of the middle subcell ( 4 ) is between 0.5 μm and 20 μm, preferably between 5 μm and 15 μm. Thin-film solar cell according to one of claims 3 to 6, characterized in that the thickness of the i-layer of the lower part cell ( 5 ) is between 10 nm and 2000 nm, preferably between 100 nm and 500 nm. Thin film solar cell according to one of claims 3 to 7, characterized in that between the middle and the lower part cell ( 5 ) is arranged, which is designed such that at least incident in the wavelength range of 500 nm to 700 nm, at least by a proportion of 10% in terms of intensity is reflected from the middle part of the cell incident on the intermediate reflective layer electromagnetic radiation. Thin-film solar cell according to claim 8, characterized in that the intermediate reflection layer is formed as TCO (Transparent Conducting Oxide) or as silicon dioxide, preferably, that the intermediate reflection layer has a layer thickness between 0.1 .mu.m and 5 .mu.m, preferably between 0.5 .mu.m and 2 .mu.m , Thin-film solar cell according to one of claims 8 to 9, characterized in that the thickness of the i-layer of the lower part cell ( 5 ) is between 200 nm and 400 nm. Thin-film solar cell according to one of Claims 8 to 10, characterized in that the thickness of the i-layer of the middle subcell ( 4 ) is between 1 μm and 4 μm. Thin-film solar cell according to one of the preceding claims, characterized in that the p- and the n-layer of the lower part cell ( 5 ) are both formed from a material of the group amorphous silicon, microcrystalline silicon, amorphous germanium or microcrystalline germanium. Thin-film solar cell according to one of the preceding claims, characterized in that the p- and the n-layer of the upper part cell ( 3 ) are both formed of amorphous silicon. Thin-film solar cell according to one of the preceding claims, characterized in that above the upper part cell ( 3 ) a transparent, transversely conductive upper contacting structure ( 2 ) is arranged for electrical contacting of the p-layer of the upper part of the cell. Thin-film solar cell according to claim 14, characterized in that the upper contacting structure ( 2 ) is formed as a TCO layer, which has a doping with a doping concentration between 0.1 × 10 ²⁰ cm ^-3 and 0.1 × 10 ²⁰ cm ^-3 . Thin-film solar cell according to one of the preceding claims, characterized in that below the lower part cell ( 5 ), optionally on further intermediate layers, a back-reflection layer is arranged, preferably that the back-reflection layer of TCO or silver or white color is formed. Thin-film solar cell according to one of the preceding claims, characterized in that the solar cell comprises Lichteinfangsstrukturen which above the upper part cell ( 3 ) are arranged, preferably that the Lichteinfangsstrukturen on and / or in the carrier substrate ( 1 ) are formed. Thin-film solar cell according to one of claims 3 to 17, characterized in that between the upper part cell ( 3 ) and the middle subcell ( 4 ) an intermediate reflection layer is arranged, which is designed such that, starting from the middle part cell incident on the intermediate reflection layer electromagnetic radiation is reflected at least in the wavelength range of 300 nm to 600 nm, at least by a proportion of 10% in terms of intensity.

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