JP4412766B2 - Thin film polycrystalline Si solar cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thin film polycrystalline Si solar cell .
[0002]
[Prior art and its problems]
In recent years, research and development of low-cost and high-efficiency next-generation solar cells have been actively promoted in Japan and overseas. When comprehensively considering cost, conversion efficiency, resource problems, environmental problems, etc., a thin-film polycrystalline Si solar cell using Si as a main material is considered to be the most promising next-generation solar cell.
[0003]
In order to form a highly efficient thin-film polycrystalline Si solar cell, it is most important to form a sufficiently high-quality photoactive layer. In order to make full use of the quality of this photoactive layer, it is necessary to improve the quality and characteristics of various other points. For example, the interface state density between the films must be reduced as much as possible.
[0004]
Since the thin-film polycrystalline Si solar cell has a multilayer structure, an interface necessarily exists between the films. In a thin film device such as a thin film polycrystalline Si solar cell, since the ratio of the area of the interface between the films to the volume of the film becomes high, the superiority or inferiority of the interface characteristics greatly affects the element characteristics.
[0005]
There are two main problems with the density of interface states between films: the generation of dark current at the interface and the disappearance due to recombination of photoexcited carriers in the thin film solar cell. The generation of dark current is directly related to the decrease in open circuit voltage V _oc , and the recombination loss of photoexcited carriers is directly related to the decrease in short circuit current density J _sc due to the decrease in photocurrent. In particular, this leads to deterioration of the overall device characteristics.
[0006]
As a method for solving this problem, there is a method in which the crystal orientation characteristics of each film are aligned to reduce the crystal structure mismatch at the film interface, thereby reducing the interface state density.
[0007]
In a conventional thin film polycrystalline Si solar cell, when a photoactive layer made of polycrystalline Si is deposited on a base layer made of polycrystalline Si, for example, a glass substrate or a film forming method represented by plasma CVD is used. Even in a low-temperature process of about 600 ° C. or lower where an SUS substrate can be used, it is desired to continuously deposit the underlayer and the photoactive layer with the same crystal orientation characteristics. A method for forming the active layer has not been known. For example, Proc. Of 1st WCPEC (1994), p.1575 states that a (110) oriented photoactive layer is formed by plasma CVD on a (111) oriented underlayer by laser annealing. ing.
[0008]
The present invention has been made based on such a background, and provides a thin - film polycrystalline Si solar cell that has solved the conventional problem that characteristics deteriorate due to the presence of a large interface state density between the films. For the purpose.
[0009]
[Means for Solving the Problems]
To achieve the above object, a thin - film polycrystalline Si solar cell according to claim 1 is the same as the first Si layer on the substrate, the first Si layer comprising a Ti film, a TiN film, and polycrystalline Si. conductivity type second Si layer of polycrystalline Si, and made by sequentially stacking the third Si layer made of non-single-crystal Si of a conductivity type different from that of the first Si layer, and the first Both the Si layer and the second Si layer have a (111) plane crystal orientation characteristic.
[0011]
[Action]
As described above, the second Si layer (photoactive layer) made of the same (111) -oriented polycrystalline or microcrystalline Si on the (111) -oriented polycrystalline Si (underlying layer). ), The interface state density between the underlayer and the photoactive layer is reduced, and the characteristics of a thin film polycrystalline Si element typified by a thin film polycrystalline Si solar cell can be improved.
[0012]
Specifically, the first Si layer (underlying layer) made of polycrystalline Si forms a film oriented mainly in the (111) plane by the flux method, and the second Si layer made of polycrystalline or microcrystalline Si. The (photoactive layer) forms a (111) oriented film by catalytic CVD. Since the alignment characteristics of the first Si layer (underlying layer) and the second Si layer (photoactive layer) match, the defect level density at the film interface is greatly reduced, and the device characteristics can be improved. . The growth of the second Si layer (photoactive layer) on the first Si layer (underlayer) is ideally epitaxial growth.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, taking a thin film polycrystalline Si solar cell as an example.
[0014]
In addition, although the example using glass as a board | substrate is demonstrated, you may replace with a SUS board | substrate.
[0015]
The thin film semiconductor device shown in FIG. 1 is formed on a glass substrate 1 with a metal film made of at least one of Ti, Ni, W, Mo, Cu, Ag, or Al, or a nitride film or silicide film 2 thereof. A back electrode, a first Si layer (underlayer) 3 made of polycrystalline Si of p ⁺ to p ⁺⁺ type containing one or more metal elements eutectic with Si, p-type or i-type Conductive antireflection film also serving as second Si layer (photoactive layer) 4 made of polycrystalline or microcrystalline Si, third Si layer 5 made of n-type non-single-crystal Si, and light-receiving surface electrode layer 6 are sequentially laminated. In the figure, 7 is a front extraction electrode formed on the upper surface of the antireflection film 6, and 8 is a back extraction electrode formed on the upper surface of the first Si layer (underlayer) 3 made of polycrystalline Si. .
[0016]
In manufacturing such a photoelectric conversion device, first, the back electrode 2 is appropriately formed on the glass substrate 1 by a vacuum film forming method such as an electron beam evaporation method or a sputtering method so that the sheet resistance is about 1 Ω / □ or less. Deposit to film thickness. Specifically, when a Ti film is formed to 1 μm and a TiN film is formed thereon to a thickness of 0.2 μm, a sheet resistance of 0.6Ω / □ is realized. The Ti film may be replaced with another metal as long as there is no problem in the following steps. The TiN film functions as a barrier layer that prevents reaction between a flux metal such as Al used in the flux method described below and the metal film and glass substrate under the TiN film, but has an equivalent function. Any other membrane can be substituted.
[0017]
Next, a first Si layer (underlayer) 3 made of polycrystalline Si is formed on the back electrode 2 by a flux method. Specifically, a metal thin film layer serving as a flux containing Al or the like is formed on the back electrode 2 to a film thickness of about 2 μm or less by a vacuum film forming method such as an electron beam evaporation method or a sputtering method. An amorphous or microcrystalline Si layer having a thickness of 2 μm or less is formed on the metal thin film layer by a thin film forming technique such as plasma CVD or sputtering. When this substrate is heat-treated at a relatively low temperature of 480 to 570 ° C. for several minutes to 1 hour, the metal flux reacts with Si, and a crystal grain size of 1 μm or more is formed on the back electrode 2 in a direction parallel to the substrate 1. Thus, a first Si layer (underlayer) 3 mainly made of polycrystalline Si selectively oriented in the (111) plane is obtained. The flux residue may be removed with an acid solution such as HCl or an alkali solution such as NaOH.
[0018]
In the formation of the first Si layer (underlayer) 3, if the temperature at the time of film formation is 480 to 570 ° C., the first Si layer (underlayer) 3 made of polycrystalline Si is formed simultaneously with the film formation. It is also possible to form it on the back electrode 2.
[0019]
Here, if Al is used as the flux metal, a first Si layer (underlayer) 3 made of p ⁺ type polycrystalline Si containing about 1 × 10 ¹⁸ to 10 ²⁰ / cm ³ of Al is automatically obtained. However, when it is desired to obtain the first Si layer (underlayer) 3 made of polycrystalline Si containing B of about 1 × 10 ¹⁸ to 10 ²² / cm ³ , B _{2 is} formed at the time of forming the Si film. A predetermined amount of H ₆ gas may be supplied. As a result, an underlayer having a BSF function essential for a high-efficiency solar cell can be obtained.
[0020]
Next, a polycrystalline or microcrystalline Si layer that becomes the second Si layer (photoactive layer) 4 of the same conductivity type (that is, p-type) as the same layer is formed on the first Si layer (underlayer) 3 as a catalyst. A thickness of about 1 μm to 30 μm is formed by CVD. At this time, for example, the substrate temperature is 100 to 500 ° C., the input voltage to the W (tungsten) catalyst body having a diameter of 0.5 mm is 30 to 45 V / m, the distance between the catalyst body and the substrate is about 5 cm, and the SiH ₄ gas flow rate. Is 5 to 20 sccm (preferably 10 sccm), the H ₂ gas flow rate is 100 sccm, and the deposition pressure is about 7 Pa, a (111) -oriented polycrystalline Si film can be obtained.
[0021]
Here, the first Si layer (underlying layer) 3 functions as the underside of the second Si layer (photoactive layer) 4, and the crystal grain size of the second Si layer (photoactive layer) 4 is increased. Improvement in quality can be promoted, and epitaxial growth can be achieved by optimizing the film formation conditions.
[0022]
As described above, the crystal orientations of the first Si layer (underlying layer) 3 and the second Si layer (photoactive layer) 4 are both aligned to (111), and an interface having a small defect level density can be formed. .
[0023]
Orientation characteristics when the deposition pressure is set to 7 Pa and the second Si layer (photoactive layer) 4 is formed by catalytic CVD by changing the flow rate of silane and hydrogen (SiH ₄ / H ₂ ) in three stages Is shown in FIG. The substrate temperature was set to 200 ° C., the input voltage to the catalyst body was set to 77 V, and the distance between the catalyst body and the substrate was set to 5 cm. It can be seen that the second Si layer 4 exhibits (111) orientation characteristics in any case where the gas flow rates of silane and hydrogen are 5/100 to 20/100.
[0024]
When the film forming pressure is 0.6 Pa, a (220) oriented film is obtained.
[0025]
Further, the first Si layer (underlayer) 3 is a p-type semiconductor layer having a concentration of Al (aluminum) or B (boron) in the range of 1 × 10 ¹⁸ to 10 ²² / cm ³ , respectively. When the Si layer (photoactive layer) 4 has the same conductivity type as the first Si layer (underlying layer) 3, the B concentration in the film is set in the range of 1 × 10 ¹⁶ to 10 ¹⁹ / cm ^3. . As a result, the first Si layer (underlayer) 3 can be an underlayer having a BSF function essential for high-efficiency solar cells, and the second Si layer (photoactive layer) 4 is suitable for high-efficiency solar cells. Thus, a photoactive layer with a small recombination current (dark current) can be obtained.
[0026]
Next, on the second Si layer (photoactive layer) 4, non-conductivity including amorphous, polycrystalline or microcrystalline of the conductivity type (that is, n-type) opposite to that of the first Si layer (underlayer) 3. A third Si layer 5 made of a single crystal Si layer is formed to a thickness of 1 μm or less by a vacuum film forming method such as a plasma CVD method or a sputtering method.
[0027]
Here, depending on the quality of the pn junction formed by the second Si layer (photoactive layer) 4 and the third Si layer 5 made of a non-single-crystal Si layer, the second Si layer (photoactive layer) An intrinsic (i-type) non-single-crystal Si layer 9 may be interposed between the fourth Si layer 5 and the third Si layer 5. In particular, when the same layer is formed of hydrogenated amorphous Si, the film thickness is set to about 2 to 40 nm. Further, when the third Si layer 5 and the non-single-crystal Si layer 9 are formed particularly in an atmosphere containing hydrogen, the interface between each layer and the defect level in the vicinity thereof can be deactivated by terminating with hydrogen. A high-quality pn junction or pin junction can be obtained.
[0028]
In the case where a fine and random uneven shape not depending on the crystal orientation of crystal Si is formed on the surface of the element by using the RIE method, and the light conversion efficiency is improved to improve the element conversion efficiency, before the pn junction is formed. In addition, the second Si layer (photoactive layer) 4 is subjected to processing by the RIE method, and then the third Si layer 5 is formed. By this RIE treatment, it is possible to reduce the reflectance of the bare Si surface to 10% or less at least in the light wavelength range of 400 nm to 1000 nm that contributes to power generation.
[0029]
The first Si layer (underlying layer) 3 has a thickness of 0.1 to 1 μm, the second Si layer (photoactive layer) 4 has a thickness of 1 to 30 μm, and the third Si layer 5 The film thickness is desirably 1 μm or less. The film thickness of the first Si layer (underlying layer) 3 is a value suitable for maximizing the BSF function, and the second Si layer (photoactive layer) 4 and the third Si layer 5 The film thickness is a value suitable for generating the maximum photocurrent.
[0030]
Next, a conductive antireflective film 6 such as ITO or SnO ₂ or an insulating antireflective film 6 such as a silicon nitride film or a silicon oxide film is formed on the third Si layer 5 by a vacuum such as plasma CVD or sputtering. A film is formed with a film thickness of about 600 to 1000 nm using a film method.
[0031]
Next, the front extraction electrode 7 is formed on the antireflection film 6 by using a vacuum film forming technique, a printing and baking technique, and a plating technique. When an insulating antireflection film is formed on the third Si layer 5, the insulating antireflection film is formed in a region where the front extraction electrode 7 is formed by an etching technique using an appropriate chemical such as buffered hydrofluoric acid. Is removed to expose the third Si layer 5, and the front extraction electrode 7 is brought into contact therewith.
[0032]
The back extraction electrode 8 can also be formed on the back electrode 2 by using a vacuum film forming technique, a printing and baking technique, and a plating technique.
[0033]
As described above, the interface state density existing at the interface between the first Si layer (underlying layer) 3 made of polycrystalline Si and the second Si layer (photoactive layer) 4 made of polycrystalline Si has a very low efficiency. Thin-film polycrystalline Si solar cells can be obtained.
[0034]
【The invention's effect】
As described above, according to the thin film polycrystalline Si solar cell according to claim 1, the interface state density between the base layer made of polycrystalline Si and the polycrystalline Si photoactive layer can be reduced, and the highly efficient thin film polycrystalline Si solar cell can be reduced. A solar cell can be manufactured .
[Brief description of the drawings]
FIG. 1 is a diagram showing an embodiment of a thin film semiconductor device according to the present invention.
FIG. 2 is a diagram showing alignment characteristics when a second Si layer (photoactive layer) is formed with a deposition pressure set to 7 Pa.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Metal film which consists of at least 1 sort (s) of Ti, Ni, W, Mo, Cu, Ag, or Al, its nitride film, or silicide film, 3 ... 1st Si layer, 4 ... 2nd Si layer, 5 ... 3rd Si layer

Claims (4)

On the substrate, a Ti film, a TiN film, a first Si layer made of polycrystalline Si, a second Si layer made of polycrystalline Si having the same conductivity type as the first Si layer, and the first Si layer And a third Si layer made of non-single-crystal Si having a different conductivity type from each other, and both the first Si layer and the second Si layer have (111) plane crystal orientation characteristics. A thin - film polycrystalline Si solar cell characterized by the above. 2. The thin - film polycrystalline Si solar cell according to claim 1, wherein the first Si layer is formed by reacting a metal thin film layer containing Al with Si. The first Si layer is a p-type semiconductor layer containing Al or B of 1 × 10 ¹⁸ to 10 ²² atoms / cm ³ , and the second Si layer is B of 1 × 10 ¹⁶ to 10 ¹⁹ atoms / cm 3. The thin film polycrystalline Si solar cell according to claim 1, which is a p-type semiconductor layer containing ³ . The thickness of the first Si layer is 0.1 to 1 μm, the thickness of the second Si layer is 1 to 30 μm, and the thickness of the third Si layer is 1 μm or less. The thin film polycrystalline Si solar cell according to claim 1, wherein

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