JP4756820B2 - Solar cell - Google Patents

Description

  The present invention relates to a high-efficiency solar cell, and more particularly, to a thin-film solar cell using a crystalline silicon intermediate layer for a photoelectric conversion portion.

  Since fossil fuels such as oil are concerned about future supply and demand, and there is a problem of carbon dioxide emission that causes a global warming phenomenon, solar cells are attracting attention as an alternative energy source for the fossil fuels.

  This solar cell uses a semiconductor having a pn junction in a photoelectric conversion portion that converts light energy into electric power, and silicon is most commonly used as a semiconductor constituting the pn junction. From the viewpoint of photoelectric conversion efficiency, it is preferable to use single crystal silicon. However, when supplying the silicon raw material and obtaining a single crystal having a large area, a large cost is required.

  On the other hand, there is a thin-film solar cell in which a photoelectric conversion portion is formed using amorphous silicon as a material useful for realizing a large area and cost reduction. Furthermore, in order to realize a solar cell having both high and stable photoelectric conversion efficiency, which is a level of a single crystal silicon solar cell, and a large area and a low cost of an amorphous silicon solar cell. The use of crystalline silicon for photoelectric conversion parts has been studied. In particular, a thin film solar cell (hereinafter referred to as a crystalline silicon thin film solar cell) in which a crystalline silicon thin film is formed using a thin film formation technique by chemical vapor deposition (hereinafter referred to as a CVD method) similar to the case of amorphous silicon. Is attracting attention.

  In the present specification, unless otherwise specified, the term “crystalline” means not only a state consisting essentially of crystals such as single crystals and polycrystals, but also a crystal component called microcrystal or microcrystal. It also means that an amorphous component is included.

  One of the important technologies for achieving a highly efficient crystalline silicon thin film solar cell is light confinement. Light confinement means that the surface of a transparent conductive film or metal layer in contact with the photoelectric conversion part is made uneven, and light is scattered at the interface, thereby extending the optical path length and increasing the amount of light absorbed by the photoelectric conversion part. It is something to be made. For example, Patent Document 1 below discloses an example of a solar cell substrate in which the unevenness size of a transparent conductive film produced on a glass substrate and the inclination angle thereof are defined.

  By improving the photoelectric conversion efficiency by this light confinement, the film thickness of the photoelectric conversion part can be reduced. In particular, in the case of an amorphous silicon solar cell, it is possible to suppress a decrease in photoelectric conversion efficiency due to light degradation caused by the Staebler-Wronski effect. Furthermore, in the case of a crystalline silicon solar cell, a film thickness of several μm order, which is several to ten times as large as that of amorphous silicon, is usually required for light absorption characteristics. The film thickness can be reduced as compared with the case of using, and the film forming time can be greatly shortened. That is, light confinement is an indispensable technique as a means for solving all of high efficiency, stabilization, and cost reduction, which are major issues for practical use of crystalline silicon thin film solar cells.

  By the way, in order to realize a highly efficient crystalline silicon thin film solar cell, it is necessary to improve the film quality of the photoelectric conversion portion. According to the following Patent Document 2, it is suggested that it is necessary for enhancing the (110) crystal orientation of the i-type crystalline silicon layer with high efficiency. This is because the crystal oriented in (110) has few grain boundaries and can efficiently collect carriers generated by light irradiation.

  However, even if the conditions for forming a high-quality i-type crystalline silicon layer are applied in the formation of the i-layer, it is not possible to form a high-quality film from the beginning of crystal growth. This is because a crystal growth initial film having many defects is formed first. Since this causes a reduction in the open circuit voltage and fill factor, this must be eliminated as much as possible.

  On the other hand, in Patent Document 2 described below, as a method of obtaining a high-quality i-type crystalline silicon layer from the initial stage of growth, by forming a thin amorphous silicon film after forming a conductive layer, It is disclosed that this is used as a seed layer or a crystallization control layer to enhance the (110) plane orientation of the i-type crystalline silicon layer.

  However, the amorphous silicon film has lower electron mobility than crystalline silicon, and even if the film quality of the i-type crystalline silicon layer can be improved, the thin amorphous silicon film Conversion efficiency decreases. The desired film thickness of the amorphous silicon film to be inserted is 10 to 50 mm, and a desired result cannot be obtained even if it is thinner or thicker than this range. Therefore, it is technically difficult to control the film thickness delicately.

Patent Document 3 below discloses a silicon-based thin film photoelectric conversion device in which an i-type crystalline silicon layer, an i-type microcrystalline intermediate layer, and a conductive crystalline silicon layer are sequentially stacked. However, the invention described in the above document aims to increase the band gap by reducing the crystallization rate of the conductive crystalline silicon layer as the window layer. That is, this technique is not intended to obtain a high-quality i-type crystalline silicon layer. Considering that the intermediate layer is formed after the i-type crystalline silicon layer is formed, it is impossible to obtain a practically high-quality i-type crystalline silicon layer.
JP 2002-141525 A JP-A-11-87742 JP 2000-188413 A

  The present invention has been made in order to solve the above-mentioned problems of the prior art, and the purpose of the present invention is to improve the quality of the initial formation of the i-type crystalline silicon layer as a photoelectric conversion part. It is to provide a crystalline silicon thin film solar cell having high photoelectric conversion efficiency by reducing the bad initial film and adjusting the crystallization rate of the crystalline silicon intermediate layer and the i-type crystalline silicon layer.

  The present invention relates to a solar cell including a substrate and a photoelectric conversion unit formed on the substrate, wherein the photoelectric conversion unit includes a first conductive silicon layer, a crystalline silicon intermediate film, and an i-type crystalline silicon. The crystalline silicon intermediate layer has a thickness in the range of 5 to 100 nm, and the crystalline silicon intermediate layer has a crystallization ratio Xca in the range of 0.6 to 3. The i-type crystalline silicon layer has a thickness in the range of 1 to 5 μm, and the i-type crystalline silicon layer has a crystallization ratio Xcb in the range of 2 to 8. A solar cell is provided.

  According to this, the initial growth film of the low-quality i-type crystalline silicon layer is reduced by inserting the crystalline silicon intermediate layer between the first conductive silicon layer and the i-type crystalline silicon layer. , Conversion efficiency can be improved. Moreover, a highly efficient solar cell can be produced by setting the crystallization ratio Xcb and the film thickness of the i-type silicon crystalline layer within suitable ranges.

  Preferably, the root mean square value RMS of the unevenness on one main surface of the substrate is in the range of 25 to 600 nm, and the inclination angle of the uneven surface with respect to the average line of unevenness on the substrate surface is θ. Tan θ is in the range of 0.07 to 0.20.

  Preferably, the main surface having the unevenness of the substrate is a surface coated with a transparent conductive film, and the unevenness of the substrate is formed by etching the transparent conductive film.

  Preferably, at least a part of the irregularities of the substrate is a substantially spherical or conical hole, the diameter of the hole is in the range of 200 to 2000 nm, and the transparent conductive film is mainly formed of zinc oxide. ing.

  Between the first conductive silicon layer 12 and the i-type crystalline silicon layer 13b, the crystalline silicon intermediate layer 13a is set in a thickness range of 5 to 100 nm and a crystallization rate Xca is in a range of 0.6 to 3. By inserting it in, it is possible to reduce the initial growth film of the low-quality i-type crystalline silicon layer and improve the conversion efficiency. At that time, a highly efficient solar cell can be produced when the crystallization rate Xcb of the i-type silicon crystalline layer is in the range of 2 to 8 and the film thickness is 1 to 5 μm. The present invention is also effective for a solar cell manufactured using a substrate provided with irregularities, and in that case, a more efficient solar cell can be manufactured by light confinement. The effective unevenness range is such that the root mean square value RMS of the unevenness height is 25 to 600 nm, and tan θ is 0.07 to 0.20 when the inclination angle of the unevenness is θ.

  Hereinafter, the present invention will be described in detail with reference to FIG.

(Solar cell structure)
FIG. 1 is a schematic cross-sectional view showing the structure of the solar cell of the present invention. In FIG. 1, a solar cell 20 of the present invention includes a substrate 11 and at least a first conductive silicon layer 12, a crystalline silicon intermediate film 13a, and an i-type crystalline silicon layer 13b stacked in this order. Thus, the photoelectric conversion unit 21 is formed. Further, the substrate 11 includes a base substrate 11a and an uneven surface layer 11b. Preferably, the second conductive silicon layer 14 and the back electrode are laminated in this order on the i-type crystalline silicon layer 13b. A back reflective layer 15 may be formed between the second conductivity type silicon layer 14 and the back electrode 16. Here, the second conductivity type silicon layer 14 is formed of a material different from that of the first conductivity type silicon layer 12.

In the present invention, when the crystallization rate of the crystalline silicon intermediate layer 13a is Xca, Xca is in the range of 0.6 to 3, and the crystallization rate of the i-type crystalline silicon layer is Xcb. , Xcb is in the range of 2-8. Note that the i-type crystalline silicon layer includes a p ^- type or n ^- type substantially intrinsic semiconductor.

Here, the crystallization rate refers to the peak height of crystalline silicon at 520 cm ⁻¹ in the Raman spectrum of a single layer film formed on a glass substrate under the same film forming conditions as those for solar cell production, Ic, 480 cm. ^When the peak height of amorphous silicon at ⁻¹ is Ia, the value of the ratio of Ic to Ia, that is, Ic / Ia is defined as the crystallization rate in this specification. This value is not a value representing the absolute value of the crystal volume fraction. However, since the ratio value has a positive correlation with the crystal volume fraction, it can be used as an index for evaluating the tendency.

  The solar cell 20 of the present invention configured as described above includes a crystalline silicon intermediate layer 13a having a controlled film thickness and crystallization rate between the first conductive silicon layer 12 and the i-type crystalline silicon layer 13b. By providing, the film in the initial stage of growth of the i-type crystalline silicon layer is amorphous and has a large number of defects, and functions as a base layer for depositing a high-quality i-type crystalline silicon layer. Is. Hereinafter, the configuration of the solar cell of the present invention will be described in more detail.

(substrate)
In the present invention, the substrate 11 has a structure in which an uneven surface layer 11b is formed on a base substrate 11a. As the base substrate 11a, a transparent substrate such as glass, a resin having heat resistance of about 200 ° C. such as polyimide and polyvinyl, and a laminate of these can be used. Furthermore, these substrate surfaces may be coated with a transparent conductive film or an insulating film. In addition, the thickness of the base substrate 11a is not particularly limited, but may be 1 to 30 mm, for example, so as to have a strength that can support the solar cell 20.

The uneven surface layer 11b is preferably made of a transparent conductive film. A material known in the art can be used for the transparent conductive film as the concavo-convex surface layer 11b. Specific examples include tin oxide, indium oxide, and ITO. In particular, zinc oxide or this as a main component is used. It is preferable in that it has high plasma resistance and is not easily altered. The transparent conductive film may contain impurities. For example, when zinc oxide is the main component, a group III element such as gallium or aluminum, or an IB element such as copper may be contained within a range of about 5 × 10 ^{20 to} 5 × 10 ²¹ cm ^−3. Good. When these elements are mixed, the resistivity of the transparent conductive film is lowered, so that it can be preferably used as an electrode.

  In the present invention, the unevenness in the uneven surface layer 11b can be formed by etching. When forming irregularities in the transparent conductive film of the irregular surface layer 11b using etching, the surface shape of the irregular surface layer can be easily controlled by appropriately adjusting the type, concentration or etching time of the etchant. As the etchant, an acid or alkali solution can be used. Specifically, as the usable acid solution, one or two kinds such as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, acetic acid, formic acid and perchloric acid are used. Examples of the mixture include hydrochloric acid and / or acetic acid. These acid solutions can be used at a concentration of, for example, about 0.05 to 5% by mass. However, particularly in the case of a relatively weak acid such as acetic acid, the acid solution may be used at a concentration of about 0.1 to 5% by mass. . Moreover, as an alkaline solution, 1 type, or 2 or more types of mixtures, such as sodium hydroxide, ammonia, potassium hydroxide, calcium hydroxide, and aluminum hydroxide, are mentioned. Of these, sodium hydroxide is preferable. These alkaline solutions may be used at a concentration of about 1 to 10% by mass.

In the present invention, the shape of the concavo-convex surface layer 11b was measured using an AFM (atomic force microscope), and based on the result, the root mean square value RMS of the concavo-convex structure was used as an index of the concavo-convex height. In addition, the frequent wavelength λ of the sinusoidal curve appearing in the Fourier transform spectrum of the AFM image was set as the average size in the horizontal direction. Using these, the average angle θ is given by
tan θ = 4 × RMS / λ
Defined by

  Tan θ obtained by the above equation is a numerical value for evaluating the inclination of the unevenness, and θ obtained using the above equation does not directly represent the inclination of the unevenness. This numerical value can be used as an index for relative evaluation. However, since this is a numerical value uniquely determined from one AFM image by mathematical processing, it is highly objective and useful as an index for relative evaluation.

  In the present invention, RMS is preferably in the range of 25 to 600 nm, and tan θ is preferably in the range of 0.07 to 0.20. When RMS is smaller than 25 nm or tan θ is smaller than 0.07, the unevenness of the transparent conductive film is small and gentle, so that the light scattering effect is small and an effective light confinement effect in the photoelectric conversion unit is obtained. Is difficult. Further, when RMS is larger than 600 nm or tan θ is larger than 0.20, the unevenness of the transparent conductive film is large and the inclination is steep, so that it grows from the inclined surface opposite to the unevenness of the transparent conductive film. When the crystals collide with each other, a crystal grain boundary is generated and defects generated are increased, so that the conversion efficiency is lowered. RMS and tan θ in the above range can be easily achieved by etching the transparent conductive film as described above.

  In the present invention, the thickness of the transparent conductive film as the uneven surface layer 11b is preferably in the range of about 0.1 to 2 μm. If the thickness of the transparent conductive film is too thin, there will be a problem in the uniformity of characteristics, and if it is too thick, the photoelectric conversion efficiency will decrease due to the decrease in transmittance and the cost will increase due to the increase in film forming time. Examples of the method for producing a transparent conductive film in the present invention include known methods such as a sputtering method, an atmospheric pressure CVD method, a low pressure CVD method, an electron beam evaporation method, a sol-gel method, and an electrodeposition method. In particular, the sputtering method is preferable in that it is easy to control the transmittance and low efficiency of the transparent conductive film to those suitable for the thin film solar cell.

  In addition, as a means for providing unevenness on the surface of the substrate, for example, a film that forms unevenness on the surface at the same time as deposition may be formed on a substrate having a smooth surface. The film on which the unevenness is formed on the surface may be the same material as the substrate or a different material. It is also possible to form irregularities on the substrate surface by machining such as sandblasting.

  At least a part of the unevenness provided on the substrate surface by etching is a substantially spherical or conical hole. Since the preferred range described above for the mean square RMS of the height and the tan θ when the inclination angle is θ has good reproducibility of the unevenness, the diameter of the hole is preferably in the range of 200 to 2000 nm. . More preferably, the diameter of the hole is in the range of about 400 to 1200 nm.

  In the present invention, the surface of the substrate 11 may be coated with a conductive film for use as an electrode. Further, the conductive film is preferably transparent. Thus, by forming a conductive film on the surface of the substrate 11, it is possible to improve the light confinement effect, and further, the impurity contained in the substrate 11 is a photoelectric conversion unit made of a silicon layer. Can be prevented from being taken into the photoelectric conversion part.

(Photoelectric converter)
In the present invention, as described above with reference to FIG. 1, the photoelectric conversion unit 21 is provided on the substrate 11. The photoelectric conversion unit 21 includes a first conductive silicon layer 12, a crystalline silicon intermediate layer 13a laminated thereon, an i-type crystalline silicon layer 13b laminated thereon, and a laminated layer thereon. The second conductive type silicon layer 14 is formed. Here, the first conductive silicon layer 12 and the second conductive silicon layer 14 are not the same. In other words, one may be an n-type semiconductor and the other may be a p-type semiconductor, and the materials forming the semiconductor may be different.

  In the present invention, the thickness of the crystalline silicon intermediate layer 13a is preferably in the range of 5 to 100 nm. If the film thickness is less than 5 nm, a sufficient effect as an underlayer for depositing a high-quality i-type crystalline silicon layer cannot be obtained. When the film thickness is larger than 100 nm, the crystallization rate of the film increases with the film thickness, so that the crystallization rate of the crystalline silicon intermediate layer 13a in the vicinity of the first conductive silicon layer 12 and the crystalline silicon intermediate layer 13a The crystallization rate on the surface is different. Therefore, even if the crystalline silicon intermediate layer 13a has a crystallization rate in the range described later, the influence of defects and the like accompanying the initial growth layer of the crystalline silicon intermediate layer 13a itself on the conversion efficiency can be ignored. Disappear.

  The crystallization ratio Xca of the crystalline silicon intermediate layer 13a is preferably in the range of 0.6-3. Within this range, an effect as an underlayer for depositing the above-described high-quality i-type crystalline silicon layer 13b can be obtained. When the crystallization ratio Xca is less than 0.6, the film is almost amorphous, and the electric field is reduced due to a decrease in electron mobility in this part and a higher resistance in the amorphous part than in the crystalline part. Since it becomes difficult to apply a sufficient electric field to the i-type crystalline silicon layer 13b that concentrates in the amorphous portion and contributes to power generation, the conversion efficiency is lowered. On the other hand, if the crystallization ratio Xca is larger than 3, the crystal volume fraction in the film is high, that is, the number of crystal grains increases, and the crystal grain boundaries that can become defects increase accordingly, resulting in a decrease in open circuit voltage and fill factor. It causes a decrease in conversion efficiency.

  The i-type crystalline silicon layer 13b needs to have a thickness of several μm in consideration of its light absorption characteristics, but it is desirable that the i-type crystalline silicon layer 13b be as thin as possible in consideration of productivity. Good conversion efficiency can be obtained by controlling the crystallization ratio Xcb of the i-type crystalline silicon layer 13b within the range of 1 to 5 μm. When the crystallization rate Xcb is less than 2, the short-circuit current due to crystalline silicon is reduced. On the other hand, when Xcb is greater than 8, the crystal volume fraction in the film is high, that is, the number of crystal grains is increased, resulting in defects. The increase in the grain boundaries to be obtained causes a decrease in conversion efficiency due to a decrease in open circuit voltage and fill factor.

In the crystalline silicon intermediate layer 13a and the i-type crystalline silicon layer 13b, as means for forming a film so as to satisfy the above-mentioned range of the crystallization rate, for example, plasma CVD is used, and SiH ₄ or Si ₂ H ₆ is used. One or a combination of two or more of the method for changing the hydrogen dilution rate of the silane-based source gas, the method for changing the power output for plasma generation, the method for changing the substrate temperature, the method for changing the pressure in the deposition chamber, etc. In particular, it is preferable to use a method of changing the hydrogen dilution rate of the silane-based source gas and a method of changing the power output for generating plasma.

  By the crystalline silicon intermediate layer 13a obtained as described above, an i-type crystalline silicon layer 13b with few defects is obtained from the initial stage of film formation, and the photoelectric conversion efficiency is improved.

  In the present invention, the first and second conductive silicon layers may be formed by any method, and a general method for forming the silicon layer is a CVD method. Here, this CVD method may be any of atmospheric pressure CVD, low pressure CVD, plasma CVD, ECR plasma CVD, high temperature CVD, low temperature CVD, etc., but is based on a high frequency in the frequency band from RF to VHF, ECR plasma. It is preferable to form the film by a CVD method or a combination thereof. For example, when using the plasma CVD method, the conditions are a frequency of about 10 to 200 MHz, a power of several W to several kW, a chamber internal pressure of about 13.3 to 2660 Pa (0.1 to 20 Torr), and the substrate temperature is room temperature. It should just be about -600 degreeC.

FIG. 12 is a conceptual diagram showing the relationship between the silane concentration [SiH ₄ / (SiH ₄ + H ₂ )] and the crystallization rate when a crystalline silicon thin film is formed by plasma CVD, using a graph. . According to FIG. 12, the crystallization rate increases as the hydrogen flow rate increases and the silane concentration decreases. However, the tendency depends on the film thickness, and the crystallization becomes easier as the film thickness increases.

  In the present invention, it is particularly preferable that at least one i-type crystalline silicon layer in the photoelectric conversion portion 21 shown in FIG. 1 is made of crystalline silicon or a silicon alloy. The back surface reflection layer 15 is a thin film electrode made of zinc oxide having a thickness of about 40 to 60 nm, formed by magnetron sputtering.

  Further, in the present invention, a substrate type solar cell in which light is incident from a photoelectric conversion unit in addition to the above-described super straight type solar cell will be described with reference to FIG.

  As the base substrate 11a, a transparent substrate such as glass, a resin having heat resistance of about 200 ° C. such as polyimide and polyvinyl, a metal such as stainless steel, and a laminate of these can be used.

  In addition, as in the case of the above-described superstrate type solar cell, the uneven surface layer 11b is formed with a transparent conductive film using a material known in the art and etched with an acid or alkali solution to form unevenness. To do. Needless to say, as means for providing unevenness, it is possible to form a film in which unevenness is formed at the same time as depositing, or to use machining such as sand blasting.

  On the surface of the substrate 11 or the surface of the uneven surface layer 11b, a metal film having a high visible light reflectivity such as Ag, Al, Ti and Pd as a back surface reflecting electrode film, and alloys thereof are used. Formed by means known in the art.

  A substrate type solar cell can be obtained by forming the above-described photoelectric conversion portion 21 on the substrate 11 thus configured. In the substrate type solar cell, the transparent conductive film 17 is formed on the photoelectric conversion unit 21 as a light incident side electrode. Further, a comb-shaped electrode 18 is formed as an electrode for collecting current.

(electrode)
The back electrode 16 is formed by forming silver into a thickness of about 400 to 600 nm by an electron beam vapor deposition method. From the back electrode 16 and the rugged surface layer 11b, the electrodes 17 are respectively drawn out to form a super straight type. The thin film solar cell 20 is configured.

  With the above configuration, the high-efficiency thin-film solar cell 20 can be obtained by forming the high-quality photoelectric conversion portion 21 with few defects on the substrate 11.

  In addition, in this embodiment, the photoelectric conversion part 21 demonstrated the example which consists of one crystalline silicon thin film photovoltaic cell. However, the photoelectric conversion unit 21 may be formed of a plurality of crystalline silicon thin-film solar cells connected in series by being sequentially stacked, and among them, an i-type crystalline silicon layer in at least one photoelectric conversion element. A crystalline silicon intermediate layer may be formed between the first conductive crystalline silicon layer and the first conductive crystalline silicon layer, and the i-type crystalline silicon layer may be formed of crystalline silicon or a silicon alloy.

Among the photoelectric conversion parts, by using an i-type crystalline silicon layer of at least one crystalline silicon thin film solar cell as a material made of crystalline silicon or a silicon alloy, amorphous silicon cannot be used for photoelectric conversion and has a wavelength of 700 nm or more. Long wavelength light can be fully utilized. The silicon alloy is a material such as Si _x Sn _1-x in which tin is added to silicon and Si _x Ge _1-x in which germanium is added.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to these.

Example 1
A procedure for manufacturing the solar cell 20 shown in FIG. 1 will be described. For convenience of explanation, members having the same functions as those in the drawings described in the above embodiments are given the same reference numerals, and descriptions thereof are omitted. In this example, a thin film silicon solar cell 20 having a crystalline silicon intermediate layer 13a having different crystallization ratios and film thicknesses as shown in Table 1 will be particularly described.

In order to measure the crystallization ratios (xca, Xcb) of the crystalline silicon intermediate layer 13a and the i-type crystalline silicon layer 13b, a single layer deposited on a glass substrate is measured by Raman spectroscopy. The crystallization rate was determined. The wavelength of the laser beam used for the measurement is 514.5 nm. Generally the Raman spectrum, the peak of the crystalline silicon to 520 cm ^-1, the peak of the amorphous silicon appears in 480 cm ^-1. The peak height of 520 cm ⁻¹ is Ic, the peak height of 480 cm ⁻¹ is Ia, and Ic and Ia obtained from the Raman spectrum of the film formed as a single layer on the glass using the same film forming conditions The ratio value (Ic / Ia) is defined as the crystallization ratio in the present specification as described above. This value is not a value representing the absolute value of the crystal volume fraction. However, since the ratio value has a positive correlation with the crystal volume fraction, it can be used as an index for evaluating the tendency.

On one main surface of the glass plate 11a having a smooth surface, zinc oxide was formed to a thickness of 500 nm as a transparent conductive film as the uneven surface layer 11b at a substrate temperature of 150 ° C. by a magnetron sputtering method. To this zinc oxide, gallium of about 1 × 10 ²¹ cm ⁻³ is added. As a result, the sheet resistance of the obtained uneven surface layer 11b was 10Ω / □, and the transmittance for light with a wavelength of 800 nm was 80%. In order to investigate the surface shape of the uneven surface layer 11b in detail, the surface shape was measured with an atomic force microscope. From the shape of the hole in the depth direction, the shape of the hole was found to be approximately spherical or conical. Then, in order to express the feature of the surface shape with a numerical value, the root mean square value RMS was used as an index of the unevenness height. Furthermore, when the most frequent wavelength λ of the sinusoidal curve obtained when the curve representing the surface shape is Fourier transformed is used as an index of the uneven pitch, and the inclination of the uneven surface with respect to the average line of the surface unevenness is θ, as described above, There is a relationship of tan θ = 4 × RMS / λ. From this equation, RMS = 4 nm and tan θ = 0.005 were obtained.

  On the substrate 11 thus obtained, a p-type crystalline silicon layer, a crystalline silicon intermediate layer 13a, and an i-type crystalline silicon layer 13b are formed as the first conductive silicon layer 12 by plasma CVD using a high frequency of 13.56 MHz. , N-type amorphous silicon layers are sequentially stacked as the second conductive silicon layer 14, and sample Nos. 1-1-1 to 1-5-5 solar cells were obtained.

The p-type crystalline silicon layer is composed of SiH ₄ gas ₂ sccm, H ₂ gas 300 sccm, B ₂ H ₆ gas flow rate ₆ sccm adjusted to 5000 ppm with H ₂ gas, film forming chamber pressure 668 Pa, discharge power 30 W, substrate temperature 140 ° C. The film was formed under the conditions of 30 nm. In the crystalline silicon intermediate layer 13a, the SiH ₄ gas 8 sccm, the H ₂ gas flow rate are appropriately changed in each sample in consideration of the relationship shown in FIG. 11 in order to control the crystallization rate, the film forming chamber pressure 668 Pa, and the discharge power 20 W. A film was formed under the conditions of a substrate temperature of 170 ° C., and the film thickness shown in Table 1 was obtained by adjusting the film formation time. The i-type crystalline silicon layer 13b was formed under the conditions of SiH ₄ gas 9 sccm, H ₂ gas flow rate 400 sccm, film forming chamber pressure 668 Pa, discharge power 35 W, and substrate temperature 170 ° C. to a thickness of 1 μm. The n-type amorphous silicon layer is composed of SiH ₄ gas 8 sccm, H ₂ gas 40 sccm, PH ₃ gas 50 sccm diluted to 1000 ppm with H ₂ gas, film forming chamber pressure 113 Pa, discharge power 10 W, and substrate temperature 180 ° C. To a thickness of 30 nm.

  In addition, when the i-type crystalline silicon layer single layer was deposited on the glass substrate on the same film forming conditions as the above and the crystallization rate Xcb was calculated | required by the Raman spectroscopy, it was Xcb = 2.0.

  Thereafter, a super straight type crystal in which zinc oxide is formed as a backside reflection layer 15 by a magnetron sputtering method with a thickness of 50 nm, silver is formed as a backside electrode 16 by an electron beam evaporation method with a thickness of 500 nm, and light is incident from the glass plate 11a side. A silicon thin film solar cell 20 was obtained.

The photoelectric conversion efficiency of the crystalline silicon thin film solar cell 20 thus obtained was measured. The measurement was performed using a solar simulator capable of irradiating light having a spectrum AM of 1.5 and a light intensity of 100 mW / cm ² while maintaining the temperature of the sample at 25 ° C. The current-voltage characteristics of this crystalline silicon thin film solar cell 20 under the AM1.5 (100 mW / cm ² ) irradiation condition were measured. The results are shown in Table 1 and FIG. FIG. 3 is a graph showing the conversion efficiency when the film thickness of the crystalline silicon intermediate layer is changed in the solar cell according to the example. In FIG. 3, the horizontal axis is the crystalline silicon intermediate film thickness (nm), the vertical axis is the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Example 2)
A crystalline silicon solar cell 20 was fabricated in the same manner as in Example 1 except that the hydrogen flow rate was changed to 450 sccm among the film forming conditions for the i-type crystalline silicon layer 13b. A single i-type crystalline silicon layer was deposited on a glass substrate using the conditions described in Example 1, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 4.0. The obtained results are shown in Table 2 and FIG. FIG. 4 is a graph showing the conversion efficiency when the film thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 4, the horizontal axis is the crystalline silicon intermediate film thickness (nm), the vertical axis is the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Example 3)
A crystalline silicon solar cell 20 was produced in the same manner as in Example 1 except that the hydrogen flow rate was changed to 500 sccm among the film forming conditions of the i-type crystalline silicon layer 13b. A single i-type crystalline silicon layer was deposited on the glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 8.0. The obtained results are shown in Table 3 and FIG. FIG. 5 is a graph showing the conversion efficiency when the film thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 5, the horizontal axis is the crystalline silicon intermediate film thickness (nm), the vertical axis is the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

Example 4
A crystalline silicon solar cell 20 was produced in the same manner as in Example 1 except that the film thickness of the i-type crystalline silicon layer 13b was changed to 3 μm. A single i-type crystalline silicon layer was deposited on a glass substrate using the same conditions as described in Example 1, and the crystallization rate Xcb was determined by Raman spectroscopy. Xcb = 2.0. The obtained results are shown in Table 4 and FIG. FIG. 6 is a graph showing the conversion efficiency when the thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 6, the horizontal axis represents the crystalline silicon intermediate film thickness (nm), the vertical axis represents the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Example 5)
A crystalline silicon solar cell 20 was fabricated in the same manner as in Example 4 except that the hydrogen flow rate was changed to 450 sccm among the film forming conditions for the i-type crystalline silicon layer 13b. A single i-type crystalline silicon layer was deposited on a glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 4.0. The obtained results are shown in Table 5 and FIG. FIG. 7 is a graph showing the conversion efficiency when the film thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 7, the horizontal axis is the crystalline silicon intermediate film thickness (nm), the vertical axis is the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Example 6)
A crystalline silicon solar cell 20 was fabricated in the same manner as in Example 4 except that the hydrogen flow rate was changed to 500 sccm among the film forming conditions for the i-type crystalline silicon layer 13b. A single i-type crystalline silicon layer was deposited on the glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 8.0. The obtained results are shown in Table 6 and FIG. FIG. 8 is a graph showing the conversion efficiency when the thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 8, the horizontal axis represents the crystalline silicon intermediate film thickness (nm), the vertical axis represents the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Example 7)
A crystalline silicon solar cell 20 was produced in the same manner as in Example 1 except that the film thickness of the i-type crystalline silicon layer 13b was changed to 5 μm. A single i-type crystalline silicon layer was deposited using the same conditions as in this example, and the crystallization rate Xcb was determined by Raman spectroscopy. Xcb = 2.0. The obtained results are shown in Table 7 and FIG. FIG. 9 is a graph showing the conversion efficiency when the film thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 9, the horizontal axis is the crystalline silicon intermediate film thickness (nm), the vertical axis is the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Example 8)
A crystalline silicon solar cell 20 was fabricated in the same manner as in Example 7 except that the hydrogen flow rate was changed to 450 sccm among the film forming conditions of the i-type crystalline silicon layer 13b. A single i-type crystalline silicon layer was deposited on a glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 4.0. The obtained results are shown in Table 8 and FIG. FIG. 10 is a graph showing the conversion efficiency when the film thickness of the crystalline silicon intermediate layer is changed in the solar cell according to this example. In FIG. 10, the horizontal axis represents the crystalline silicon intermediate film thickness (nm), the vertical axis represents the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

Example 9
A crystalline silicon solar cell 20 was fabricated in the same manner as in Example 7 except that the hydrogen flow rate was changed to 500 sccm among the film forming conditions of the i-type crystalline silicon layer 13b. A single i-type crystalline silicon layer was deposited on the glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 8.0. The obtained results are shown in Table 9 and FIG. FIG. 11 is a graph showing the conversion efficiency when the thickness of the crystalline silicon intermediate layer is changed in the solar cell according to the present example. In FIG. 11, the horizontal axis represents the crystalline silicon intermediate film thickness (nm), the vertical axis represents the conversion efficiency (%), and the symbols in the graph mean as shown in the figure.

(Consideration about Examples 1-9)
Sample No. of Examples 1-9 above. When a crystalline silicon intermediate layer having a low crystallization rate was inserted as in n-1-1-1 to n-1-5 (n = 1, 2,..., 9), the conversion efficiency was low. Moreover, the conversion efficiency continued to decrease as the film thickness increased. This is thought to be due to the low mobility of electrons in the amorphous component often present in the crystalline silicon intermediate layer having a low crystallization rate, and the fact that the amorphous component becomes a series resistance component. The effect appeared with increasing film thickness.

  Sample No. Even when a crystalline silicon intermediate layer having a high crystallization rate was inserted as in n-5-1 to n-5-5, the conversion efficiency was a relatively low value. This is because the crystal volume fraction in the film is high in the crystalline silicon intermediate layer having a high crystallization rate, that is, the number of crystal grains increases, and the grain boundaries that can become defects increase accordingly, thereby increasing the open-circuit voltage and the fill factor. This is thought to be due to a decrease in conversion efficiency that accompanies this decrease.

  Moreover, the sample No. of the said Examples 1-9. When a crystalline silicon intermediate layer having a thickness of 1 nm was inserted as in n-1-1-1 to n-5-1, the conversion efficiency was low. This is because the film thickness of less than 5 nm cannot sufficiently cover the unevenness of the surface of the first conductive type silicon layer having fine unevenness due to the substrate and the first conductive type silicon layer itself. This is presumably because a sufficient effect cannot be obtained as an underlayer for depositing the type crystalline silicon layer.

  Moreover, the sample No. of the said Examples 1-9. When a crystalline silicon intermediate layer having a film thickness of 500 nm was inserted as in n-1-5 to n-5-5, the conversion efficiency was low. This is because the crystallization rate of the film increases with the film thickness, and even if the film thickness is 100 nm or more, even if the crystalline silicon intermediate layer has a crystallization ratio in the range described later, This is thought to be because the influence of the initial growth layer of the layer cannot be ignored.

  From the above, the range of the crystallization ratio Xca in which the effect of reducing the low-quality initial film by the crystalline silicon intermediate layer works well and the adverse effect accompanying the insertion of the crystalline silicon intermediate layer does not appear As shown in Examples n-2-2 to n-4-4, it is 0.6 to 3.0 and the film thickness is 5 to 100 nm.

(Example 10)
In Example 1, after forming a transparent conductive film using zinc oxide as the concavo-convex surface film 11b, using the substrate 11 on which the zinc oxide surface was etched, a crystalline silicon solar cell was obtained. 20 was produced. A method for manufacturing the substrate is described below.

  The glass plate coated with zinc oxide was immersed in a 0.5 mass% hydrochloric acid aqueous solution at a liquid temperature of 25 ° C. for 45 seconds, and then the surface was sufficiently washed with pure water to form an uneven surface layer 11b. A solar cell substrate 11 having a transparent conductive film on the surface was obtained. When the surface shape of the transparent conductive film as the uneven surface layer 11b was observed with a scanning electron microscope, a large number of substantially circular holes having a diameter of about 200 to 1400 nm on the surface were formed.

  In order to examine the surface shape of the transparent conductive film as the uneven surface layer 11b in detail, the surface shape was measured with an atomic force microscope. From the shape of the hole in the depth direction, the shape of the hole was found to be approximately spherical or conical. Then, in order to express the feature of the surface shape with a numerical value, the root mean square value RMS was used as an index of the unevenness height. Furthermore, tan θ = 4 × RMS, where the most frequent wavelength λ of the sinusoidal curve obtained when Fourier transforming the curve representing the surface shape is an index of the uneven pitch, and the inclination of the uneven surface with respect to the average line of the surface unevenness is θ. / Λ, RMS = 26 nm, and tan θ = 0.09.

  Next, a crystalline silicon solar cell 20 was produced in the same manner as in Example 5-3-3 except that the substrate thus obtained was used. The conversion efficiency of this crystalline silicon solar cell 20 was 11.9%. In this way, by forming irregularities on the substrate, incident light can be confined by multiple reflection and used effectively, and conversion efficiency is improved.

(Comparative Example 1)
Sample No. 2 in Example 2 was used except that the film thickness of the i-type crystalline silicon layer 13b was 0.5 μm. A crystalline silicon solar cell 20 was fabricated in the same manner as in 2-3-3. A single i-type crystalline silicon layer was deposited on a glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 4.0. The conversion efficiency of this crystalline silicon solar cell was 6.55%.

(Comparative Example 2)
Except for the film thickness of the i-type crystalline silicon layer 13b being 6 μm, all the sample Nos. A crystalline silicon solar cell 20 was produced in the same manner as in 2-3-3. A single i-type crystalline silicon layer was deposited on a glass substrate using the above conditions, and the crystallization ratio Xcb was determined by Raman spectroscopy. Xcb = 4.0. The conversion efficiency of this crystalline silicon solar cell 20 was 6.28%.

(Consideration about Example 2 and Comparative Examples 1 and 2)
As can be seen from Example 2 and Comparative Examples 1 and 2, there is an optimum range for the film thickness of the i-type silicon crystalline layer 13b, which is 1 to 5 μm. If the film thickness of the i-type silicon crystalline layer 13b is smaller than 1 μm, the electric field applied to the i-type silicon crystalline layer 13b increases, but the thin film cannot significantly absorb incident light, and thus the short-circuit current value decreases greatly. It is thought that the conversion efficiency became low. Further, when the film thickness is larger than 5 μm, incident light can be sufficiently absorbed, but the distance between the electrodes in which the carriers generated by the light must travel is large, and the carriers are deactivated by recombination before reaching the electrodes. As a result, the conversion efficiency is considered to be a low value. From the above, even if the initial growth film of the i-type silicon crystalline layer is reduced, if the film thickness of the i-type silicon crystalline layer is not in an appropriate range, it cannot be a practical solar cell. . Therefore, in order to obtain a highly efficient crystalline silicon solar cell 20, the thickness of the i-type silicon crystalline layer 13b is preferably 1 to 5 μm.

(Comparative Example 3)
Of the film forming conditions for the i-type crystalline silicon layer 13b, all the sample Nos. in Example 4 were used except that the H ₂ gas flow rate was 350 sccm. A crystalline silicon solar cell 20 was produced in the same manner as 4-3-3. A single i-type crystalline silicon layer was deposited on a glass substrate using the above conditions, and the crystallization rate Xcb was determined by Raman spectroscopy. Xcb = 0.5. The conversion efficiency of this crystalline silicon solar cell 20 was 5.85%.

(Comparative Example 4)
Of the film forming conditions for the i-type crystalline silicon layer 13b, all of the sample No. 1 in Example 4 except that the H ₂ gas flow rate was 600 sccm. A crystalline silicon solar cell 20 was produced in the same manner as 4-3-3. A single i-type crystalline silicon layer was deposited on the glass substrate using the above conditions, and the crystallization rate Xcb was determined by Raman spectroscopy. Xcb = 10.0. The conversion efficiency of this crystalline silicon solar cell was 6.13%.

(Consideration about Example 4 and Comparative Examples 3 and 4)
Further, as shown in Example 4 and Comparative Examples 3 and 4, there is an optimum range for the crystallization ratio Xcb of the i-type silicon crystalline layer 13b, which is 2-8. If the crystallization rate of the i-type silicon crystalline layer 13b is smaller than 2, it is considered that the absorption of long wavelength light by the crystal component becomes insufficient, so that the short-circuit current value is lowered and the conversion efficiency is lowered. Further, when the crystallization rate is higher than 8, the crystal volume fraction in the film is high, that is, the number of crystal grains increases, and the crystal grain boundaries that can become defects increase accordingly, thereby reducing the open-circuit voltage and the fill factor. This is thought to cause a reduction in conversion efficiency. From the above, even if the initial growth film of the i-type silicon crystalline layer is reduced, if the crystallization rate of the i-type silicon crystalline layer is not within an appropriate range, it cannot be a practical solar cell. . Therefore, in order to obtain a highly efficient crystalline silicon solar cell 20, the crystallization rate Xcb of the i-type silicon crystalline layer 13b is preferably 2-8.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a schematic sectional drawing of the structure of the solar cell according to this invention. It is a schematic sectional drawing of the structure of the substrate type solar cell according to this invention. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. In the solar cell according to a present Example, it is the figure which represented the conversion efficiency when changing a crystalline silicon intermediate | middle layer film thickness using a graph. It is a conceptual diagram which uses a graph to show the relationship between the silane concentration [SiH ₄ / (SiH ₄ + H ₂ )] and the crystallization rate when a crystalline silicon thin film is formed using the plasma CVD method.

Explanation of symbols

  11 substrate, 11a base substrate, 11b transparent, 12 first conductive silicon layer, 13a crystalline silicon intermediate film, 13b i-type crystalline silicon layer, 14 second conductive silicon layer, 15 back reflective layer, 16 back surface Electrode, 18 comb-shaped electrode, 20 solar cell, 21 photoelectric conversion part.

Claims (5)

In a solar cell including a substrate and a photoelectric conversion part formed on the substrate,
In the photoelectric conversion unit, a first conductive silicon layer, a crystalline silicon intermediate layer, and an i-type crystalline silicon layer are laminated in this order,
The film thickness of the crystalline silicon intermediate layer is in the range of 10 to 100 nm,
The crystalline silicon intermediate layer is formed under the same conditions as those for forming a single silicon layer on the glass substrate with a crystallization ratio Xca measured by Raman spectroscopy in the range of 1.5 to 3. And
The i-type crystalline silicon layer has a thickness in the range of 1 to 5 μm,
The i-type crystalline silicon layer was formed under the same conditions as those for forming a single-layer silicon layer on the glass substrate with a crystallization rate Xcb measured by Raman spectroscopy in the range of 4 to 8. Is,
The root mean square value RMS of the concavo-convex height existing on one principal surface of the substrate is in the range of 25 to 600 nm, and tan θ when the inclination angle of the concavo-convex surface with respect to the average line of the concavo-convex surface of the substrate surface is θ. Is in the range of 0.07 to 0.20.   2. The solar cell according to claim 1, wherein one main surface having the unevenness of the substrate is a surface coated with a transparent conductive film.   The solar cell according to claim 1, wherein the unevenness of the substrate is formed by etching the transparent conductive film.   The at least part of the unevenness of the substrate is a substantially spherical or conical hole, and the diameter of the hole is in a range of 200 to 2000 nm. Solar cell.   The solar cell according to claim 2, wherein the transparent conductive film is mainly made of zinc oxide.

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