JP4105811B2 - Method for forming polycrystalline silicon film - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a polycrystalline silicon film, and more particularly to a method for forming a thin polycrystalline silicon film used for a solar cell or the like.
[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 both in Japan and overseas. Of these, thin-film polycrystalline silicon solar cells mainly made of silicon are cost-effective. Considering conversion efficiency, resource issues, environmental issues, etc., it is considered to be the most powerful next-generation solar cell.
[0003]
In general, the formation of a sufficiently high-quality photoactive layer is the most important requirement for forming a highly efficient thin-film polycrystalline silicon solar cell. In order to obtain conversion efficiency, it is necessary to improve the quality of various requirements in addition to the quality of the photoactive layer.
[0004]
Among the above requirements, the method of forming the BSF layer on the side opposite to the side where the pn junction of the photoactive layer is formed is generally the most effective and general method for obtaining high conversion efficiency in general solar cells. Widely used.
[0005]
In thin-film polycrystalline silicon solar cells, naturally, effective use of this BSF function is one of the important requirements for high efficiency, but this BSF function is formed to form a photoactive layer. It is common to have the underlying layer perform its function, and research and development of its formation method has been performed by various methods.
[0006]
As an example, a method is known in which amorphous or microcrystalline silicon previously formed on a substrate is irradiated with a laser or an energy beam to melt and recrystallize the amorphous or microcrystalline silicon (for example, Japanese Patent Laid-Open No. 9-31258). Etc.).
[0007]
The features of this method include that the crystal grain size of the underlying crystalline silicon can be made relatively large by laser annealing, and that the thin film polycrystalline silicon can be formed at a relatively low temperature. Therefore, when a practical large area substrate is used as a solar cell element, there is a problem in throughput.
[0008]
Also known is a method of polycrystallizing amorphous silicon formed on a SUS substrate by plasma CVD using SPC (solid phase crystallization) (for example, Proc. 1st. WCPEC (1994)). p.1315-1318, JP-A-2-28315, JP-A-6-204539, JP-A-7-135332, JP-A-7-335660, etc.).
[0009]
The feature of this method is that an inexpensive SUS substrate is used as the substrate and that the SPC process is a relatively low temperature process of about 600 ° C. On the other hand, the SPC process takes about 10 hours. It takes time, and it has a problem that it is difficult to sufficiently increase the grain size of the obtained crystal.
[0010]
In addition, there are known systems in which a polycrystalline silicon layer is formed on a carbon substrate by melting and injecting silicon grains by plasma spraying (for example, Japanese Patent Laid-Open Nos. 5-315258 and 5-315259). No. 5, JP-A-5-315260, JP-A-5-326414, JP-A-6-208960, JP-A-6-208961, etc.).
[0011]
The features of this method include the use of a carbon substrate that can be expected to be inexpensive as a substrate, the ability to deposit polycrystalline silicon at a very high speed, and the ability to increase the size of polycrystalline silicon grains. Since the film temperature needs to be a high temperature in the vicinity of the melting point of silicon, there is a problem that a large amount of heat energy is required.
[0012]
The present invention eliminates the problems of the above-mentioned various methods, and forms a base layer having a BSF function suitable for increasing the efficiency of a thin-film polycrystalline silicon solar cell at a low cost, thereby achieving a high efficiency and a low cost. It is an object of the present invention to provide a method for forming a polycrystalline silicon film capable of producing a thin film polycrystalline silicon solar cell.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, in the invention according to claim 1, a metal thin film layer made of a metal element that forms a eutectic system with silicon, and boron in a range of 1 × 10 ¹⁸ to 1 × 10 ²¹ atoms / cm on a substrate. A step of sequentially stacking a silicon layer containing ³ and a step of reacting the metal element and silicon in the silicon layer by heat treatment to form a polycrystalline silicon film having a BSF function on the substrate. And a step of forming a photoactive layer on the polycrystalline silicon film.
[0014]
Moreover, in the said invention, it is desirable for the metal thin film layer which consists of the said metal element to consist of any 1 type or multiple types of Al, Sn, and In.
[0015]
Moreover, in the said invention, the film thickness of the metal thin film layer which consists of said metal element is 0.3 micrometer or less, and the ratio of the film thickness of this metal thin film layer and the said silicon layer is 1: 0.7-1: 3. It is desirable.
[0016]
Furthermore, in the said invention, metal layers, such as Ti, Ni, W, Mo, Cu, Ag, Al, or its nitride layer, or its silicide layer are provided between the said board | substrate and the metal thin film layer which consists of the said metal element. It may be interposed.
[0017]
[Action]
With the above configuration, carrier recombination at the silicon-electrode interface can be suppressed sufficiently low, and at the same time, the sheet resistance value of the underlayer can be suppressed sufficiently small, and good ohmic characteristics between the silicon-electrodes Therefore, a base layer having a BSF function that can be expected to be highly efficient is obtained, and a low-cost and highly efficient thin-film polycrystalline silicon solar cell can be obtained.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the drawings.
FIG. 1 shows a photoelectric conversion device S1 as a whole, and a polycrystalline silicon layer 2 (a lower layer is 2a and an upper layer is 2b) containing one or more metal elements that form a eutectic system with silicon on a substrate 1, silicon A photoactive layer 3, a silicon layer 4 having a conductivity type opposite to that of the silicon photoactive layer 3, and an antireflection layer 6 which is a conductive antireflection film also serving as a light receiving surface electrode layer are sequentially laminated. In the figure, 7 is a front extraction electrode connected to the upper surface of the antireflection film 6, and 8 is a back extraction electrode connected to the upper surface of the polycrystalline silicon layer 2.
[0019]
As the substrate 1, an inexpensive and stable material such as glass, stainless steel, or carbon can be selected.
[0020]
In manufacturing the photoelectric conversion device, as shown in FIG. 2, first, a metal thin film layer 2a 'containing a metal element that forms a eutectic system with silicon is formed on a substrate 1 by vacuum such as electron beam evaporation or sputtering. A film is formed to a thickness of about 0.1 to 2 μm, preferably 0.1 to 0.3 μm by a film forming method (see FIG. 2A). Note that Al, Sn, In, or the like can be used as a metal element that forms a eutectic system with silicon.
[0021]
Next, a film thickness of 0.1 to 2 μm, preferably 0.1 to 0.9 μm, is formed on the substrate 1 on which the metal thin film layer 2a ′ is formed by a thin film forming technique such as plasma CVD or sputtering. An amorphous or microcrystalline silicon layer 2b 'containing B is formed.
[0022]
Next, the substrate is held at a relatively low temperature of 480 to 570 ° C. for about several minutes to one hour, and has a crystal grain size of 1 μm or more in a direction parallel to the substrate surface due to a reaction phenomenon between the metal element and silicon. A polycrystalline silicon layer 2 composed of a lower layer portion 2a and an upper layer portion 2b is formed (see FIG. 2B).
[0023]
That is, when the film thickness of the amorphous or microcrystalline silicon layer 2b ′ containing B is 0.1 μm or less, the silicon layer 2 tends not to be layered. Further, when the film thickness of the amorphous or microcrystalline silicon layer 2b ′ containing B is 2 μm or more, the silicon tends not to be crystallized. Furthermore, when the metal layer 2a ′ containing a eutectic metal element with silicon is 0.1 μm or less, the polycrystalline silicon tends to be island-like, and when it is 2 μm or more, the silicon becomes relatively small and a lot. Crystalline silicon tends to form islands.
[0024]
The film thicknesses of the upper layer 2b and the lower layer 2a of the polycrystalline silicon layer are desirably set in the range of 0.1 to 2 μm even for the following reason. In other words, if the heavily doped polycrystalline silicon layer 2 is too thick, impurity-derived recombination and Auger recombination in this layer cannot be ignored, and the open circuit voltage decreases, thereby reducing conversion efficiency. This is because the thinner the film thickness, the shorter the process time required for film formation, which is suitable for cost reduction.
[0025]
In particular, the film thickness of the thin film layer 2a ′ containing all layer elements is set to 0.3 μm or less, and the film thickness ratio of the thin film layer 2a ′ to the silicon layer 2b ′ is 1: 0.7 to 1: 3. It is desirable to be. By setting in this way, it is possible to control the coverage of the lower polycrystalline silicon layer 2a to the substrate to 90% or more, which is particularly effective. Thereby, the recombination loss of carriers on the back electrode 8 side shown in FIG. 1 can be reduced.
[0026]
At this time, the polycrystalline silicon layer 2 (2a, 2b) has a p-type content of about 1 × 10 ¹⁸ atoms / cm ³ of a metal element that forms a eutectic system with silicon, but further contains B. Increase the impurity concentration. When the doping concentration of B is outside the range of 1 × 10 ¹⁸ to 1 × 10 ²¹ atoms / cm ³ , the V _oc characteristic of the device is obtained in the case of a polycrystalline silicon layer (BSF layer) having a thickness of about 0.2 to 1 μm. Falls.
[0027]
On an # 1737 glass substrate, an Al layer is set to 0.2 μm by a vacuum deposition method, and a gas flow ratio is set to SiH ₄ : H ₂ : B ₂ H ₆ = 1: 146: 20, and an amorphous silicon layer is formed by a plasma CVD method. When 0.3 μm was laminated, heat-treated at 550 ° C. × 1 hour and treated with hydrochloric acid to measure various properties, the specific resistance was 3.4 to 3.5 × 10 ⁻³ Ω-cm, and the mobility was 33 to 36 cm ² / Vs, and the carrier concentration was 2 × 10 ¹⁹ cm ⁻³ .
[0028]
Further, on the ♯1737 glass board, and 0.3μm stacked 0.2μm the Al layer by vacuum deposition, the amorphous silicon layer not doped with B by the plasma CVD method, and heat-treated at 550 ° C. × 1 hour hydrochloride When various characteristics were measured after processing, the specific resistance was 0.012 Ω-cm, the mobility was 49 cm ² / Vs, and the carrier concentration was 4 × 10 ¹⁹ cm ⁻³ .
[0029]
It is desirable that the crystal plane orientation has orientation characteristics particularly with respect to the (100) and (111) planes. As a result, information of the same crystal orientation is easily transmitted to the crystalline silicon film 3 sequentially laminated on the polycrystalline silicon film 2 as an underlayer, and a fine and random uneven structure on the crystalline silicon surface described later. It is possible to make it easier to ensure in-plane uniformity of various processes performed in each process for manufacturing a solar cell element, such as formation of the substrate.
[0030]
Moreover, it is desirable that the crystal grain size is 1 μm or more with respect to the direction parallel to the substrate surface. As a result, it is possible to promote the formation of a stacked layer of the photoactive layer 3 having a larger crystal grain size, and to reduce the crystal grain boundary density, thereby forming a higher quality photoactive layer with few crystal defects resulting therefrom.
[0031]
Next, a polycrystalline or microcrystalline silicon layer 3 to be a silicon photoactive layer 3 of the same conductivity type (ie p-type) is formed on the polycrystalline silicon layer 2 to a thickness of about 2 to 30 μm (FIG. 2 (c). )reference). At this time, the polycrystalline silicon layer 2 functions as a base for forming the silicon photoactive layer 3 and promotes the crystal grain size expansion and crystal quality improvement of the crystalline silicon of the silicon photoactive layer 3. As a method for forming the silicon photoactive layer 3, a vacuum film forming method such as a plasma CVD method or a catalytic CVD method can be used. In particular, in the latter case, a polycrystalline silicon film is formed at a high quality and high speed at a relatively low temperature. Therefore, the manufacturing process can be shortened.
[0032]
Next, on the silicon photoactive layer 3, a non-single crystal silicon layer 4 containing an amorphous, polycrystalline or microcrystalline material having a conductivity type opposite to that of the same layer (that is, n-type) is applied to a vacuum such as plasma CVD or sputtering. The film is formed to a thickness of several hundred nm or less by a film forming method.
[0033]
Here, depending on the quality of the pn junction formed by the silicon photoactive layer 3 and the silicon layer 4, an intrinsic type (i-type) is provided between the silicon photoactive layer 3 and the silicon layer 4 as shown in FIG. The non-single-crystal silicon layer 5 may be interposed. In particular, when the same layer is formed of hydrogenated amorphous silicon, the film thickness is set to about 2 to 40 nm. When the silicon layer 5 is formed of crystalline silicon, the thickness is set to 1 μm or less. Further, when the silicon layer 4 and the silicon layer 5 are formed in an atmosphere containing hydrogen in particular, the defect level at the interface of each layer and its vicinity can be terminated and deactivated with hydrogen, and a higher quality pn junction can be obtained. Alternatively, a pin junction can be obtained.
[0034]
Further, an insulating antireflection film such as a silicon nitride film or a silicon oxide film or a conductive antireflection film 6 such as ITO or SnO ₂ is formed on the silicon layer 4 by a vacuum film formation method such as a plasma CVD method or a sputtering method. A film having a thickness of about 600 to 1000 nm is used (see FIG. 1).
[0035]
Next, the front extraction electrode 7 is formed on the antireflection film 6 by a vacuum film formation technique, a printing and baking technique, a plating technique, and the like. In the case where the insulating antireflection film 6 is coated on the second semiconductor layer 4, the region for forming the front extraction electrode 7 is removed by etching using an appropriate chemical solution such as buffered hydrofluoric acid. What is necessary is just to form so that the silicon | silicone surface of the layer 4 may be exposed and the front extraction electrode 7 may be connected here.
[0036]
For the back extraction electrode 8, as shown in FIG. 1, when the polycrystalline silicon layer 2 functions as a back electrode of the element, the polycrystalline silicon layer 2 is vacuum-deposited, printed and fired, and further plated. The connection may be formed by, for example. As shown in FIG. 2, when the back electrode layer 9 is used, the back extraction electrode 8 may be formed on the back electrode layer 9 by the same technique.
[0037]
As described above, it is possible to obtain the photoelectric conversion device S1 which is a low-cost and high-efficiency thin-film crystalline silicon solar cell. Further, the solar cell has been described as an example of the photoelectric conversion device, but the present invention is not limited to this, and can be applied to, for example, an optical sensor such as a position detection sensor, a luminance sensor, and a color sensor. Modifications can be made as appropriate without departing from the scope of the invention.
[0038]
FIG. 3 shows another photoelectric conversion device S2, with respect to the S1, an electrode layer 9 made of a metal film such as Ti or Ni or a nitride film or a silicide film thereof, and a silicon photoactive layer 3 and a silicon layer 4. The silicon layer 5 provided between the two is added.
[0039]
The electrode 9 is made of a metal layer such as Ti, Ni, W, Mo, Cu, Ag, Al, or a nitride film layer thereof or a silicide film thereof. These can serve as back electrodes suitable for increasing the efficiency of solar cells.
[0040]
In this case, the surface structure of the electrode 9 may be a fine and random uneven structure. Thereby, the reflection of the light reaching the back electrode 9 side is scattered and the light confinement inside the film is promoted, so that the light utilization efficiency is improved.
[0041]
In the present invention, a thin film polycrystal in which a polycrystalline silicon film 2a, 2b is used as a base layer on a substrate 1, and a silicon photoactive layer 3, a pn junction, an antireflection film 6, and electrodes 7, 8 are sequentially formed. In the silicon solar cell, it is desirable that the silicon surface on the light incident side outermost surface is formed in a fine and random uneven structure, and the reflectance of the bare silicon surface is 10% or less in the light wavelength range of 400 to 1000 nm. Thereby, the utilization efficiency of the incident light is further improved, and higher conversion efficiency can be obtained. As a method for reducing the reflectance on the surface of the element, it is possible to reduce the surface reflectance by forming a light trapping structure by making the surface of the element an appropriate uneven shape. As this method, RIE (Reactive Ion Etching) which is a dry etching technique has been shown to be effective (Technical Digest of the International PVSEC-9 (1996) 93-96, 109-110). If this method is used, the surface of the crystalline silicon can be made into random and fine irregularities independent of the crystal orientation of silicon, and low reflectance characteristics that are remarkably superior to various wet etching methods can be realized. That is, before forming the pn junction, the surface of the photoactive layer is made into a fine and random uneven shape (rough surface) that does not depend on the crystal orientation of silicon by the RIE method. Subsequently, the photoactive layer has a conductivity type. A different silicon layer is deposited to form a pn junction. Thereby, the surface reflectance can be suppressed to 10% or less in the light wavelength range of 400 to 1000 nm. In actual device formation, since the antireflection film is further formed, the actual surface reflectance can be further reduced. As described above, a light trapping structure in which the light utilization efficiency is remarkably improved can be realized, and the element conversion efficiency can be drastically improved.
[0042]
The remarkable improvement in light utilization efficiency enables the photoactive layer to be made thinner and the film formation time can be shortened, contributing to further cost reduction. .
[0043]
【The invention's effect】
According to the present invention, a metal thin film layer made of a metal element that forms a eutectic system with silicon and a silicon layer are laminated on a substrate, and then the metal element reacts with silicon by heat-treating it. Then, in the method for forming a polycrystalline silicon film for a photoelectric conversion device that forms a polycrystalline silicon film having two upper and lower layers on the substrate, boron is contained in the silicon layer at 1 × 10 ¹⁸ to 1 × 10 ²² toms / Since heat treatment is carried out with containing cm ^3, a polycrystalline silicon film suitable as a BSF layer of a solar cell can be obtained, and a highly efficient thin-film polycrystalline silicon solar cell can be obtained. Furthermore, higher conversion efficiency can be obtained by reducing the reflectance of the element surface. In addition, since the polycrystalline silicon underlayer film can be formed at a relatively low temperature and in a short time, a low-cost solar cell can be obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a photoelectric conversion device formed by a method for forming a polycrystalline silicon film according to claim 1;
FIG. 2 is a cross-sectional view showing a process of a method for forming a polycrystalline silicon film according to claim 1;
3 is a cross-sectional view showing a photoelectric conversion device formed by the method for forming a polycrystalline silicon film according to claim 3. FIG.
[Explanation of symbols]
1 ... Substrate, 2 ... Polycrystalline silicon layer, 3 ... Silicon photoactive layer, 4 ... Silicon layer with conductivity type opposite to silicon photoactive layer, 6 ... Antireflection layer , 7… .. Front extraction electrode, 8… Back extraction electrode

Claims (4)

On a substrate, a metal thin film layer made of a metal element forming the silicon and eutectic system, and a boron ^{1 × 10 18 ~1 × 10 21} atoms / cm3 comprising silicon layer, laminating in this order, the A step of reacting a metal element and silicon in the silicon layer by heat treatment to form a polycrystalline silicon film having a BSF function on the substrate; and a step of forming a photoactive layer on the polycrystalline silicon film; A method for forming a polycrystalline silicon film for a photoelectric conversion device. Forming a photovoltaic device for a polysilicon film according to claim 1, the metal thin film layer made of the metal element is Al, Sn, characterized in that it consists of any one or more of In. The thickness of the metal thin film layer made of a metal element at 0.3μm or less, the ratio of the thickness of the metal thin film layer and the silicon layer is 1: 0.7 to 1: claims, characterized in that the 3 Item 2. A method for forming a polycrystalline silicon film for a photoelectric conversion device according to Item 1. Between the metal thin film layer made of the metal element and the substrate, and wherein Ti, Ni, W, Mo, Cu, Ag, metal layer of Al, or a nitride layer, or that is interposed the silicide layer A method for forming a polycrystalline silicon film for a photoelectric conversion device according to claim 1.

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