Patent Application
20140102522
The present invention relates to a method for manufacturing a thin film solar cell as well as to a method for manufacturing a multi-junction thin film solar cell. Further, the present invention relates to a thin film solar cell.
Photovoltaic devices or solar cells are devices which convert light into electrical power. Thin film solar cells are of a particular importance for low-cost mass production since they allow for using inexpensive substrates, for example glass, and thin films of Si with the thickness in the range of 100 nm to 2 μm. One of the most used methods for the deposition of such Si layers is the plasma enhanced chemical vapour deposition (PECVD) method.
A prior art thin film solar cell in the so-called superstrate configuration is shown in
In the past years, a new concept of tandem cells 8 has been developed, as shown in prior art
Depending on the crystalline fraction, the crystallinity, of the i-type layer 14, 17 solar cells 8 or photoelectric conversion devices are characterized as an amorphous, a-Si, 14, or microcrystalline, μc-Si, 17, solar cells 9, 10, independent of the kind of crystallinity of the adjacent p- and n-layers 13, 15, 16, 18. Microcrystalline layers are being understood, as common in art, as layers comprising a significant fraction of the crystalline silicon, so-called micro-crystallites, in an amorphous matrix. Stacks of p-i-n junctions 9, 10 are called tandem or triple junction photovoltaic cells 8. The combination of an amorphous and microcrystalline p-i-n junction 9, 10, as shown in prior art
In order to achieve a high stabilized efficiency of single junction a-Si solar cells as well as of tandem junction solar cells one needs to optimize the most important cell parameters that account for the cell efficiency, which are current density Jsc, open circuit voltage Voc and fill factor FF, each for themselves known from prior art and also known for the man skilled in the art on how to measure. Additionally, the light induced degradation, LID, should be reduced as much as possible. For large area mass production of solar cells additional factors such as layer and cell uniformity or deposition time are also very important factors that have to be considered.
Usually, good stabilized efficiency values can be obtained through a complex optimization process of either the initial efficiency, e.g. by improving one or more of the before discussed cell parameters, or of the LID. Such an optimization process usually comprises a trade off between initial efficiency, stabilized efficiency and deposition rate. In turn, the production of photovoltaic modules can be realized either with superior performance and long processing times or with shorter processing times and lower performance in terms of power.
It is therefore an object of the present invention to overcome before described disadvantages of prior art, i.e. to provide a method for improving the performance of amorphous silicon single junction solar cells as well as of micromorph tandem solar cells by increasing the initial efficiency and simultaneously reducing the light induced degradation of the a-Si and micromorph tandem cells for large area mass production photovoltaic systems.
This object is achieved by the independent claims. Advantageous embodiments are detailed in the dependent claims.
Particularly, the object is achieved by a method for manufacturing a thin film solar cell, comprising the sequential steps of a) depositing a positively doped Si layer, b1) depositing a first intrinsic a-Si:H layer at a first deposition rate, b2) depositing a second intrinsic a-Si:H layer at a second deposition rate, and c) depositing a negatively doped Si layer, whereby the second deposition rate is greater than the first deposition rate.
Accordingly, the present invention is based on the idea of providing two different intrinsic absorber layers forming together the i-layer for a p-i-n-junction to result in a thin film photoelectric conversion unit, together with the positively doped Si layer and the negatively doped Si layer, when depositing the two different intrinsic layers with different deposition rates, i.e. having a second deposition rate that is greater than the first deposition rate. In turn, the thin film solar cell manufactured is characterized by an increased initial and stabilized efficiency while at the same time the overall deposition rate, even by depositing two different intrinsic layers, is kept at a reasonable and economic level. In detail, this is achieved by an intrinsic a-Si:H absorber layer for the photoelectric conversion unit of a solar cell comprising at least two intrinsic sublayers, whereby the first sublayer is of a so-called high quality a-Si:H deposited with respectively at the first deposition rate and the second sublayer is of a-Si:H deposited with respectively at a second, higher deposition rate. In such p-i-n configuration the high-quality sublayer, the first intrinsic a-Si:H layer, is arranged between the p-layer and the second sublayer, the second intrinsic a-Si:H layer. In sum, such high-quality a-Si:H layer, the first intrinsic a-Si:H layer respectively first sublayer, is achieved by controlling at least the deposition rate when depositioning the respective intrinsic layer. Preferably, the deposition of before described layers is carried out by a CVD process, most preferably within a CVD processing chamber.
The term processing in sense of the current invention comprises any chemical, physical and/or mechanical effect acting on a substrate.
The term substrate in sense of the current invention comprises a component, part or workpiece to be treated within a vacuum processing system. A substrate includes but is not limited to flat- and/or plate-shaped parts having rectangular, square or circular shapes. Preferably, the substrate is provided as an essentially, most preferably completely planar substrate having a planar surface of a size ≧1 m2, such as a thin glass plate.
The term vacuum processing or vacuum treatment system in sense of the current invention comprises at least an enclosure for the substrate to be treated under pressure lower than ambient atmospheric pressure.
The term CVD, chemical vapour deposition, and its flavours, comprises in sense of the current invention, a well-known technology allowing for the deposition of layers on heated substrates. A usually liquid or gaseous precursor material, a gas, is being fed to the process system where a thermal reaction of a precursor results in the deposition of the layer. Often, DEZ, diethyl zinc, is used as precursor material for the production of TCO layers in a vacuum processing system using low pressure CVD, LPCVD.
The term TCO stands for transparent conductive oxide, i.e. TCO layers are transparent conductive layers, whereby the terms layer, coating, deposit and film are interchangeably used within this invention for a film deposited in a vacuum process, be it CVD, LPCVD, plasma enhanced CVD (PECVD) or physical vapour deposition (PVD).
The term solar cell or photovoltaic cell, PV cell comprises in sense of the current invention an electrical component, capable of transforming light, essentially sun light, directly into electrical energy by means of the photovoltaic effect.
A thin film solar cell in a generic sense includes, on a supporting substrate, at least one p-i-n junction established by a thin film deposition of semiconductor compounds, sandwiched between two electrodes or electrode layers. A p-i-n junction or thin film photoelectric conversion unit includes an intrinsic semiconductor compound layer sandwiched between a p-doped and a n-doped semiconductor compound layer. The term thin film indicates that the layers mentioned are being deposited as thin layers or films by processing like, before-mentioned PECVD, CVD, PVD or a like. Thin layers essentially mean layers with a thickness of 10 μm or less, especially less than 2 μm.
In a further preferred embodiment, the first deposition rate is ≧40% and ≦75%, preferably ≧40% and ≦60% of the second deposition rate. More preferably, the first deposition rate is 2.1 Å/sec and the second deposition rate is 3.6 Å/sec.
In a further preferred embodiment, the deposition of the layers is carried out by a CVD process using RF power having during step b1) a level of ≧30% and ≦75%, preferably ≧30% and ≦50%, compared to the level during step b2). In a further preferred embodiment, the deposition of the layers is carried out by a CVD process using hydrogen and silane as precursor gas having a hydrogen to silane flow ratio during step b1) of ≧1 and ≦1.5 compared to the hydrogen to silane flow ratio during step b2). In another preferred embodiment the deposition of the layer is carried out by a CVD process having a process pressure during step b1) of ≧30% and ≦90% compared to the process pressure during step b2). Im sum, adjusting the deposition rate and/or the RF power and/or the hydrogen to silane flow ratio as outlined before results in two different intrinsic a-Si:H layers leading to an increased initial efficiency and simultaneously reduced light induced degradation of the so manufactured thin film solar cell. Exemplary, the first intrinsic a-Si:H layer can be achieved at 0.3 mbar pressure, 180 W RF power, 1.9 Å/sec deposition rate and H2/SiH4=1 hydrogen to silane flow ratio,
and the second intrinsic a-Si:H layer can be achieved at 1.0 mbar pressure, 600 W RF power, 4.6 Å/sec deposition rate and H2/SiH4=0.25 hydrogene to silane flow ratio.
According to another preferred embodiment, the first intrinsic a-Si:H layer and the second intrinsic a-Si:H layer are deposited such that the thickness of the first intrinsic a-Si:H layer is ≧15% and ≦45%, preferably ≧15% and ≦35%, of the overall intrinsic a-Si:H layer thickness of the first intrinsic a-Si:H layer and a second intrinsic a-Si:H layer. Preferably, the thickness of the overall intrinsic a-Si:H layer of the first intrinsic a-Si:H layer and a second intrinsic a-Si:H layer is 220 nm. In a further preferred embodiment, the positively doped Si layer and/or the negatively doped Si layer comprises a-Si:H or μC—Si:H.
In a further preferred embodiment, the method comprises the sequential steps of providing a substrate, depositing a first TCO layer on the substrate, depositing the layers as described before on the first TCO layer, i.e. the positively doped Si layer, the first intrinsic a-Si:H layer, the second intrinsic a-Si:H layer and the negatively doped Si layer, on the first TCO layer, and depositing a second TCO layer on the deposited layers as described before. Such way, the photoelectric conversion device as described before is further completed with the first and the second TCO layers for electrically contacting the thin film solar cell.
The object of the invention is further addressed by a method for manufacturing a multi-junction thin film solar cell, comprising the sequential steps of providing a substrate, depositing a first TCO layer on the substrate, depositing the layers as described before on the first TCO layer forming a top solar cell, i.e. the positively doped Si layer, the first intrinsic a-Si:H layer, the second intrinsic a-Si:H layer and the negatively doped Si layer, depositing at least one another solar cell comprising in sequential order a positively doped Si layer, an intrinsic a-Si:H or μC—Si:H layer and a negatively doped Si layer on the top solar cell, and depositing a second TCO layer on the backmost another solar cell. This way, a micromorph tandem cell can be realized that is characterized by reduced top cell degradation and higher top cell current, further resulting in a significantly lower LID and higher stabilized module power, both compared to prior art micromorph tandem cells.
The object of the invention is furthermore achieved by a thin film solar cell, comprising a positively doped Si layer, a first intrinsic a-Si:H layer deposited on the positively doped Si layer, a second intrinsic a-Si:H layer deposited on the first intrinsic a-Si:H layer, and a negatively doped Si layer deposited on the second intrinsic a-Si:H layer, whereby the thickness of the first intrinsic a-Si:H layer is ≧15% and ≦45%, preferably ≧15% and ≦35%, of the overall intrinsic a-Si:H layer thickness of the first intrinsic a-Si:H layer and the second intrinsic a-Si:H layer.
In a further preferred embodiment, the first intrinsic a-Si:H layer comprises a lower H-content CH incorporated in the intrinsic material than the second intrinsic a-Si:H layer, preferably the first intrinsic a-Si:H layer comprises a H-content cH of ≧9% and ≦11%, most preferably of 10.1%, and the second intrinsic a-Si:H layer comprises a H-content cH of ≧12% and ≦15%, most preferably of 13.7%. Preferably, the H-content cH is measured via mass spectroscopy, more preferably via SIMS (secondary ion mass spectrometry).
In a further preferred embodiment, the first intrinsic a-Si:H layer comprises a lower microstructure factor R than the second intrinsic a-Si:H layer, preferably the first intrinsic a-Si:H layer comprises a microstructure factor R of ≧2% and ≦6%, most preferably of 3.9% and the second intrinsic a-Si:H layer comprises a microstructure factor R of ≧8% and ≦13%, most preferably of 10.5%.
In another preferred embodiment, the first intrinsic a-Si:H layer comprises a denser intrinsic material characterised by less defects and/or less micro-voids incorporated in the intrinsic material than the second intrinsic a-Si:H layer. Preferably, material density respectively micro-voids are measured via TEM (transmission electron microscopy).
In a further embodiment, the first intrinsic a-Si:H layer comprises a lower band gap energy than the second intrinsic a-Si:H layer, preferably the first intrinsic a-Si:H layer comprises a band gap energy of ≧1810 eV and ≦1820 eV, most preferably of 1815 eV and the second intrinsic a-Si:H layer comprises a band gap energy of ≧1825 eV and ≦1835 eV, most preferably of 1830 eV. In another preferred embodiment the overall intrinsic a-Si:H layer thickness of the first intrinsic a-Si:H layer and the second intrinsic a-Si:H layer is ≧100 nm and ≦2 μm, preferably 220 nm. Preferably, the band gap energy is indirectly measured by irradiating the material with e.g. monochromatic light and observing the long wave dependent absorption of the material.
This way, i.e. with the embodiments described before, a higher stabilized efficiency for an a-Si single junction cell as well as for tandem or multi junction cell is achieved.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
In the drawings:
Within a solar cell 1-7, charge carriers generated in an absorber, i.e. in an intrinsic layer 4, are driven by an internal electric field to corresponding doped layers 3, 4, i.e., electrons are directed to an n-doped layer 5 while holes travel to an p-doped layer 3, as shown in prior art
However, a so-called high quality intrinsic a-Si:H layer, i.e. a first intrinsic a-Si:H layer 21, has higher hole mobility, as shown in
The hole density is significantly higher in the vicinity of the p-layer 3, compared to other regions of the i-layer 21, 22, and that is why a small fraction of the high quality absorber 21 close to the p-layer 3 is sufficient to improve the hole collection.
A modification in the cell design is employed in the following in order to improve the a-Si cell performance without a significant increase in the deposition time, as shown in
For improving cell performance, the required thickness of the low rate HQ i-layer 21 is significantly lower than the thickness of the Std i-layer 4. Therefore, the improved cell performance in combination with processing times suitable for mass production represents the advantage of this method for industrial applications.
An example of the above described concept is given in the following: The single layer properties of the Std i-layer 4, 22 and of the HQ i-layer 21 are shown in the table of
The graded a-Si:H i-layer 21, 22 employs a fraction of the HQ i-layer 21 which replaces the interface part of the Std i-layer. The table in
Single junction a-Si solar cells with graded absorber layers 21,022 according to the table shown in
The current density increases with the increasing amount of the HQ i-layer 21 in the graded i-layer 21, 22. Moreover, the increase of current density is stronger for the graded i-layers 21, 22 with an amount of HQ i-layer 21 up to 35% than for the graded i-layers 21, 22 with a higher amount of HQ i-layer 21. This is due to the fact that the charge carriers generated in the absorber layer are more efficiently collected when using the HQ i-layer 21 at the interface with the p-doped layer 3 due to a better hole mobility and collection. Since the open circuit voltage and the fill factor of the solar cells with the different graded i-layers 21, 22 do not change significantly with the fraction of the HQ i-layer 21, the cell efficiency for the different absorber layers follows a similar trend as the one of the current density. This is also shown in
The improved performance of the solar cells with the graded i-layers 21, 22 is even more evident after light induced degradation.
For the industrial mass production of solar modules the total deposition time of the absorber layer is a very important parameter with respect to the production throughput.
The strength of the improved performance due to graded a-Si:H i-layer 21, 22 concept might be dependent on the difference in the quality of the two absorber layers 21, 22 that constitute the graded layer 21, 22: the stronger the difference in the material quality of the two layers 21, 22 the stronger is the effect of the graded a-Si:H i-layer 21, 22.
The graded a-Si:H i-layer 21, 22 has also beneficial effects for the production of micromorph tandem modules. The lower light induced degradation and simultaneously higher current density for the top cell 9 are the key elements needed for a lower light induced degradation of micromorph modules. In
Hence, these two effects provide a significant improvement in the relative degradation of bottom-limited micromorph tandem modules.
The known PECVD process for the deposition of standard hydrogenated amorphous Si (a-Si:H) absorber layers 4 needs to be tuned in order to obtain a better material quality and higher current density. The common method of increasing the current density of an a-Si cell is to reduce the band gap energy of the absorber layer 4 by reducing the H-dilution of the SiH4 plasma. However, at least two negative effects can arise when applying this method: the Voc decreases and the LID increases. Contrary to the common method, a combination of reduced process pressure and RF power density is employed here in order to simultaneously increase the current density and reduce the light induced degradation. The deposition rate is the trade off factor of this method.
A state of the art a-Si:H absorber layer 4 for large area mass production a-Si and tandem solar cells is deposited by diluting the SiH4 gas by H2 in a ratio of 1:1. Typical deposition rates for such absorber layers 4 are about 3.2-3.6 Å/sec.
By reducing either the process pressure, e.g. down to 0.3 mbar, or the RF power density one can improve the material quality and slightly reduce the band gap energy of the a-Si:H absorber layer 4, resulting in the first s-Si:H intrinsic layer 21 i.e. the HQ i-layer 21. This is shown in the table in
The improved material quality and the reduced H-content incorporated in the absorber1 and absorber2 layers with respect to the standard a-Si:H absorber layer 4, 22 are two factors which are thought to contribute to a lower light induced degradation. The deposition rate of the absorber1 and absorber2 layers is slightly reduced. The layer non-uniformity over large areas, e.g. 1.4 m2, of the absorber2 layer is slightly higher than that of the standard absorber layer 4.
Significant improvement of material quality and reduction of band gap energy is given by the combination of reduced process pressure and RF power in the a-Si:H PECVD process. This is shown in the table of
Single junction a-Si solar cells with the above described absorber layers have been prepared on LPCVD ZnO FC. For all cells the thickness of the absorber layer was 265 nm and beside the different absorber layers the cell structure was the same for all cells.
The current density of the cells comprising the new absorber layers 21 is higher than that of the cells comprising the standard a-Si:H absorber layer 4, 22. The most significant increase in current density corresponds to the absorber3 layer for which the combination of reduced process pressure and RF power density was applied. The higher current density of the solar cells including the new absorber layers is due to a slightly lower band gap energy and improved material quality, as shown in the table of
The initial and stabilized performance of the solar cells with the different absorber layers strongly correlate with the single layer properties of the different absorber layers shown in the table of
The high quality a-Si:H absorber layer 21 was primarily optimized for being used in top cells 9 of micromorph tandem cells. However, they might be used in any single, double or triple junction cell concept when more current density and lower light induced degradation are needed.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/70082 | 11/14/2011 | WO | 00 | 11/7/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61414024 | Nov 2010 | US |
