Patent Application
20130174899
The present invention relates to a novel method for improving the performance of amorphous Silicon (a-Si) 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 in large area mass production PV systems.
Photovoltaic devices or solar cells are devices which convert light into electrical power. The thin film solar cells are of a particular importance for low-cost mass production since they allow for using inexpensive substrates (e.g. glass) and thin films of Si with the thickness in the range of 100 nm-2 μm. One of the most used methods for the deposition of such Si layers is the PECVD method.
A known simple thin film solar cell in the so-called superstrate configuration is shown in
In the past years a new concept of tandem cells has been developed. The tandem cell allow for a better use of the solar spectra and for a reduced light induced degradation. It is based on two single junction cells deposited one on top of the other one. In the case of micromorph tandem cells the top cell is an a-Si cell and the bottom cell is a microcrystalline (mc-Si) silicon cell, see
Thereby
The a-Si cell absorbs preponderantly the blue part of the solar spectrum while the micro-crystalline cell absorbs mostly the red part of the solar spectra. The serial connection of the two junctions also helps to reduce the light induced degradation which is specific for the a-Si cells.
“Processing” in the sense of this invention includes any chemical, physical or mechanical effect acting on substrates.
“Substrates” in the sense of this invention are components, parts or workpieces to be treated in an inventive processing apparatus. Substrates include but are not limited to flat, plate shaped parts having rectangular, square or circular shape. In a preferred embodiment this invention addresses essentially planar substrates of a size >1 m2, such as thin glass plates.
A “vacuum processing” or “vacuum treatment system or apparatus” comprises at least an enclosure for substrates to be treated under pressures lower than ambient atmospheric pressure.
“CVD” Chemical Vapour Deposition is a well known technology allowing the deposition of layers on heated substrates. A usually liquid or gaseous precursor material is being fed to a process system where a thermal reaction of said precursor results in deposition of said layer. “LPCVD” is a common term for low pressure CVD.
“DEZ”—diethyl zinc is a precursor material for the production of TCO layers in vacuum processing equipment.
“TCO” stands for transparent conductive oxide, “TCO layers” consequently are transparent conductive layers.
The terms “layer”, “coating”, “deposit” and “film” are interchangeably used in this disclosure for a film deposited in vacuum processing equipment, be it CVD, LPCVD, plasma enhanced CVD (PECVD) or PVD (physical vapour deposition)
A “solar cell” or “photovoltaic cell” (PV cell) is an electrical component, capable of transforming light (essentially sun light) directly into electrical energy by means of the photoelectric effect.
A “thin-film solar cell” in a generic sense includes, on a supporting substrate, a 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 an n-doped semiconductor compound layer. The term “thin-film” indicates that the layers mentioned are being deposited as thin layers or films by processes like, PEVCD, CVD, PVD or alike. Thin layers essentially mean layers with a thickness of 10 μm or less, especially less than 2 μm.
Depending on the crystalline fraction (crystallinity) of the i-type layer 53, 45 solar cells or photoelectric (conversion) devices are characterized as amorphous (a-Si, 53) or microcrystalline (μc-Si, 45) solar cells, independent of the kind of crystallinity of the adjacent p and n-layers. “Microcrystalline” layers are being understood, as common in the art, as layers comprising of a significant fraction of crystalline silicon—so called micro-crystallites—in an amorphous matrix. Stacks of p-i-n junctions are called tandem or triple junction photovoltaic cells. The combination of an amorphous and microcrystalline p-i-n-junction, as shown in
In order to achieve a high stabilized efficiency of single junction a-Si solar cells as well as of tandem solar cells one needs to optimize the most important cell parameters that account for the cell efficiency: current density Jsc, open circuit voltage Voc, and the fill factor FF. Additionally, the light induced degradation (LID) should be reduced as much as possible. For large area mass production 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 could be obtained through a complex optimization process of either the initial efficiency (by improving one or more cell parameters) or of the LID. Such an optimization process usually comprises a trade off between initial efficiency, stabilized efficiency and deposition rate.
It is an object of the present invention to improve a thin film a-Si solar cell, be it in single or tandem or even higher order stapled configuration.
This is achieved by the method of manufacturing an absorber layer of a-Si:H of a thin film solar cell by plasma-enhanced-vapor-deposition, PECVD, in a RF-SiH4 plasma, comprising at least one of the steps of
The present invention thereby addresses a method for increasing the initial efficiency (by increasing the current density) and simultaneously reducing the LID of a-Si single junction cells. This is done by improving generically the material quality and adjusting the properties of the absorber layer of the a-Si cell. By applying this method a higher stabilized efficiency for a-Si single junction cells as well as for tandem cell is achieved. Moreover, for micromorph tandem cells the combination of reduced top cell degradation and higher top cell current leads to significantly lower LID and to significantly higher stabilized module power.
In one variant the method according to the invention comprises both steps a) and b).
In one variant of the method according to the invention the process pressure value is selected to be at least 0.3 mbar.
In one variant of the method according to the invention only step a) is performed at a pressure value of 0.45 mbar.
In one variant of the method according to the invention only step b) is performed at a power density value of 270 W/14000 cm2.
In one variant of the method according to the invention wherein steps a) and b) are performed, the process pressure is selected to 0.4 mbar and the power density to 230 W/14000 cm2.
The invention further addresses a photovoltaic absorber layer of a-Si:H comprising at least one of:
Thereby this absorber layer, in one embodiment, is manufactured according to one of the variants of the method according to the invention, thereby especially in which both steps a) and b) are performed and the pressure value is thereby selected to be at most 0.3 mbar or wherein the process pressure is 0.4 mbar and the power density 230 W/14000 cm2.
The invention further addresses a single junction a-Si solar cell comprising a low-pressure-chemical-vapor deposited (LPCVD) ZnO front contact layer the absorber layer comprising at least one of:
In the single junction a-Si solar cell according to the invention and in one of its embodiments the absorber layer has a thickness of 265 nm.
In the single junction a-Si solar cell according to the invention and in one of its embodiments there is valid, at least one of:
In one embodiment of the single junction a-Si solar cell according to the invention features I and II are valid.
In one embodiment the single junction a-Si cell has an absolute stabilized efficiency after 1000 h light soaking of at least 8.25% and a relative light induced degradation of less than 22%.
The invention further addresses a micromorph solar tandem cell comprising a top and a bottom cell, wherein the top cell comprises an a-Si absorber layer comprising both of:
The invention shall further be exemplified with the help of figures.
These figures show:
Please note, that throughout the figures “Std” stands for “prior art”.
Within the present invention, the PECVD process for the deposition of hydrogenated amorphous Si (a-Si:H) absorber layers is 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 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 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 are about 3.2-3.6 Å/sec.
By reducing according to the present invention either the process pressure (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. This is shown in the Table 1 where process parameters and single layer properties of the a-Si:H layers as discussed are presented also for two absorber layers for which either the process pressure (absorber1) or the RF power density (absorber2) was reduced. The material quality factor (or microstructure factor-R, derived from FTIR measurements), which is a measure of the micro-voids in the material is reduced for the absorber1 and absorber2, denoting a dense material with less Si—H2 and Si—H3 bonds. 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 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 area (1.4 m2) of the absorber2 layer is slightly higher than that of the standard absorber layer.
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 also shown in the Table 1 for the absorber3 layer. The material parameters of the absorber3 layer are significantly improved with respect to those of the standard absorber and the absorber1 and absorber2: much better microstructure factor, i.e. significantly less micro-voids and denser material as well as significantly lower H-content incorporated in the layer. The band gap energy E04 is also slightly reduced for the absorber3 layer. The deposition rate of the absorber3 layer is lower, but still above 2 Å/sec. Such a-Si:H absorber layers with excellent material quality at lower deposition rate are very interesting for large area mass production a-Si single junction and a-Si based tandem solar cells for which a lower light induced degradation and higher stabilized power are required.
a-Si Single Junction Results
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 Jsc and the cell efficiency of the cells with different absorber layers are shown in
Since the open circuit voltage and the fill factor of the above mentioned cells do not change significantly for the different absorber layers, the cell efficiency is mainly driven by the current density.
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 1. For instance, the highest current density, highest stabilized efficiency and lowest relative degradation for the absorber3 layer are the consequence of the best material quality of this absorber layer with respect to the other absorber layers.
The new a-Si:H absorber layers were primarily optimized for being used in top cells 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.
The external quantum efficiency curves in reversed bias corresponding to the two modules are shown in the
This leads to a larger difference between the top and bottom cell currents. Hence, the bottom cell current limitation in the module comprising the absorber3 layer is significantly stronger than that corresponding to the module with the standard a-Si:H absorber layer.
The top-bottom cell current limitation could be preserved when using the absorber3 in the top cell by e.g. increasing the thickness of the bottom cell absorber layer so that the bottom cell current is proportionally increased with the top cell current. For the case of bottom limited tandem modules this will result in a significantly increase in the module power in the initial state while the light induced degradation is expected to be also lower due to the lower degradation of the absorber3 layer.
This major difference in the relative degradation of the two micromorph modules leads to a significant difference in the stabilized power of the two modules. The stabilized power of the two micromorph modules is about 119 W for the micromorph module comprising the standard a-Si:H absorber layer and slightly above 127 W for the micromorph module comprising the a-Si:H absorber3 layer. Hence, a significantly higher stabilized power of micromorph modules is achieved when using the slower and better material quality a-Si:H absorber3 layer.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/065202 | 9/2/2011 | WO | 00 | 3/1/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61379861 | Sep 2010 | US |
