JP2010199418A - Substrate for thin film solar cell and thin film solar cell using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a substrate for a thin film solar cell, which has high light confinement effect and reflectivity and whose surface shape is flat to such a degree that it does not exert a bad influence on a power generation layer formed thereupon. <P>SOLUTION: On the substrate 1 for the thin film solar cell, the power generation layer 2, and a transparent conductive film 9 and a silver electrode 10 forming an upper electrode are formed. The substrate 1 for the thin film solar cell includes a high-reflectivity substrate 3 having a high-reflectivity reflecting surface on a surface and a transparent and conductive composite film formed thereupon. The transparent composite film is formed by dispersing and arranging, in a substrate in-plane direction, two or more kinds of materials, for example, a high-reflectivity material 4 and a low-reflectivity material 5 which are transparent and different in reflectivity to light in the visible range to the near-infrared range, and at least one of the high-reflectivity material 4 and low-reflectivity material 5 has conductivity. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thin film solar cell substrate for forming a semiconductor layer as a power generation layer thereon and a thin film solar cell using the same.

  Solar cells are attracting attention as a renewable energy source capable of obtaining electric power without using fossil fuels, and are being popularized. In particular, thin-film silicon solar cells use a silicon thin film with a thickness of about 0.2 to 5 μm as a light absorption layer, and have fewer resource restrictions than other solar cells. There are great expectations. By the way, improvement of the light conversion efficiency is mentioned as one of the subjects of a solar cell. As a means for improving the light conversion efficiency, it is widely adopted that the lower electrode is provided with a reflection function and an uneven structure is formed to use the light confinement effect (see, for example, Patent Document 1).

  Conventionally used light confinement structures include a concavo-convex structure that is self-formed when a transparent conductive film and a light absorbing layer are formed by chemical vapor deposition, and a concavo-convex structure that is formed when a transparent conductive film is chemically etched. There are, for example, a concavo-convex structure that is self-formed when a metal electrode is formed by a reactive sputtering method. In general, the mean square roughness (RMS) of these concavo-convex structures is 30 to 200 nm. Using a substrate having such a concavo-convex structure for a thin-film solar cell, the amount of light absorption is increased by using (i) the light scattering effect and (ii) the total reflection effect at the interface of the light absorption layer. The photocurrent can be increased. Therefore, by providing an appropriate uneven shape on the thin film solar cell substrate, the power generation efficiency of the solar cell can be improved and its value can be increased.

On the other hand, the solar cell substrate having a concavo-convex structure causes defects inside the semiconductor solar cell power generation layer formed on the solar cell substrate, which worsens the open-circuit voltage and the fill factor, in addition to the advantage of providing the light confinement effect. There are also drawbacks. Generally, in order to obtain a high light confinement effect, it is necessary to make the uneven shape large and deep. However, if the uneven shape is excessively large and deep, even if the photocurrent increases, the degradation of the open circuit voltage and the fill factor becomes more remarkable, and the resulting power generation efficiency decreases (for example, see Non-Patent Document 1). ). For this reason, a concavo-convex structure having a relatively gentle surface shape is generally used so that the open circuit voltage and the fill factor do not deteriorate so much (see, for example, Patent Document 2).
From the above, as long as the optical confinement structure using the conventional uneven structure is used, there is a trade-off relationship between the photocurrent and the open-circuit voltage / curve factor. It is extremely difficult to improve the power generation efficiency.

  As means for solving this problem, several solar cell substrates having a flat surface and an uneven light reflecting surface have been proposed (for example, see Patent Document 3). In what is described in Patent Document 3, a light-transmitting substrate having a polygonal pyramid-shaped concavo-convex structure on one surface and a flat shape on the other surface is formed, and a light reflecting layer is formed on the concavo-convex surface. A power generation layer sandwiched between transparent conductive films is formed on the flat surface side. According to this technique, light can be confined and a semiconductor film of good quality (in this case, a crystalline silicon film) can be formed.

JP 2001-15780 A JP 2000-340815 A JP 2002-299661 A

Journal of Non-Crystalline Solids Vol. 299-302 (2002), pp. 1152-1156

However, according to Patent Document 3, since the metal thin film serving as the light reflection layer is formed on the surface having an uneven shape, the metal thin film also has an uneven shape. In general, a metal film has light scattering properties due to its surface roughness, but at the same time, there is a tendency for light absorption to increase. As a whole, the reflectance decreases and the amount of light reflected by the power generation layer decreases. End up. In addition, since the light-transmitting conductive film is disposed on the lower layer side in contact with the power generation layer, and the refractive index of the light-transmitting conductive film is generally smaller than that of the power generation layer, the light scattering angle inside the power generation layer is transparent. It becomes smaller than the inside of the photoconductive film.
An object of the present invention is to solve the above-mentioned problems of the prior art, and its purpose is to have a high light confinement effect and reflectivity, and to adversely affect the power generation layer formed thereon. An object of the present invention is to provide a thin film solar cell substrate that is flat to an extent and a thin film solar cell using the same.

  In order to achieve the above object, according to the present invention, a substrate having a flat surface and a surface serving as a light reflecting surface, and transparent to visible to near-infrared light formed on the substrate and There is provided a thin film solar cell substrate comprising a transparent composite film made of two or more materials having different refractive indexes and having conductivity in the vertical direction.

  Moreover, in order to achieve said objective, according to this invention, it has a semiconductor layer which has a 1 or several semiconductor junction used as a power generation layer on the board | substrate for thin film solar cells, and has an upper electrode on it. There is provided a thin film solar cell characterized in that.

  According to the thin film solar cell substrate according to the present invention, the light reflecting surface is formed to be a flat surface, and thus a light reflecting surface having a high light reflectance can be used. Since the transparent composite film formed on the light reflecting surface has a plurality of transparent materials having different refractive indexes distributed, the light passing through the transparent composite film is reflected, refracted and diffracted, resulting in high light confinement. The effect can be expected. Furthermore, since the surface of the transparent composite film is flat, it is possible to improve the quality of the solar cell layer formed thereon compared to a substrate having a conventional uneven structure, and a highly efficient thin film solar A battery can be realized.

Sectional drawing which shows the thin film solar cell formed using the board | substrate for thin film solar cells which concerns on this invention. The perspective view which shows an example of the board | substrate for thin film solar cells which concerns on this invention. The perspective view which shows the other example of the board | substrate for thin film solar cells which concerns on this invention. Sectional drawing of order of a process which shows an example of the preparation methods of the board | substrate for thin film solar cells which concerns on this invention. Sectional drawing of the order of a process which shows the other example of the manufacturing method of the board | substrate for thin film solar cells concerning this invention. The figure which shows the result of having calculated | required the propagation behavior of light by the numerical calculation at the time of using the board | substrate for thin film solar cells which concerns on this invention.

Next, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a cross-sectional view showing a thin film silicon solar cell produced by using the thin film solar cell substrate according to the present invention. The thin film solar cell according to the present invention includes a solar cell substrate 1, a power generation layer 2, and surface electrodes (transparent conductive film 9, silver electrode 10). The solar cell substrate 1 is composed of a high-reflectance substrate 3 having a high-reflectivity reflecting surface and a transparent composite film including a high-refractive index material 4 and a low-refractive index material 5 formed thereon. . This transparent composite film forms a part of the lower electrode and needs to have conductivity, and at least one of the materials 4 and 5 is required to have conductivity. On top of this, a power generation layer 2 composed of an n-type, intrinsic and p-type crystalline thin film silicon layer is formed, and further, a transparent conductive film 9 and a silver electrode 10 whose thicknesses are adjusted so as to exhibit an antireflection effect are formed. ing. In the embodiment shown in FIG. 1, a crystalline thin film silicon layer is used as a semiconductor layer, but other semiconductor layers may be used as long as solar cell characteristics can be obtained. That is, it may be an amorphous semiconductor, a compound semiconductor (including SiC, SiGe) or an organic material as a material, or a so-called tandem structure in which a plurality of junctions are stacked. . Further, a heterojunction in which the band gap is discontinuous may be included.

  The high reflectivity substrate 3 has at least a surface formed of a highly reflective metal material (for example, Ag, AgPdCu alloy, Al, etc.) and a flat surface in order to achieve high reflectivity. The high reflectivity substrate 3 may be composed of a single metal material, or may be a metal material or non-metal material coated with a high reflectivity metal material. This high reflectance substrate 3 constitutes a lower electrode together with the transparent composite film thereon.

The transparent composite film formed on the high reflectivity substrate 3 may be any material as long as it contains two or more materials having different refractive indexes, and at least one of them is required to have conductivity. In the example shown in FIG. 1, the high refractive index material 4 and the low refractive index material 5 are both formed so as to be in contact with the high reflectance substrate 3 and the n-type silicon thin film 6, but at least a conductive material. Is required to be in contact with the high-reflectance substrate 3 and the n-type silicon thin film 6, and the non-conductive material may be either one of the high-reflectance substrate 3 or the n-type silicon thin film 6 or both. It is not necessary to touch. The interface with which the materials having different refractive indexes are in contact with each other may not be vertical as shown in FIG.
The surface of the transparent composite film is desired to be flat enough not to adversely affect the power generation semiconductor layer formed thereon. A preferred mean square surface roughness (RMS) is 25 nm or less.

  2 and 3 are perspective views showing examples of the thin film solar cell substrate according to the present invention. As shown in FIG. 2 and FIG. 3, a single film composed of a transparent high refractive index material 4 and a transparent low refractive index material 5, a transparent composite film, is formed on the high reflectance substrate 3. . In the example shown in FIG. 2, the high refractive index material 4 having a random size is dispersed and arranged in a random distribution in the low refractive index material 5. The interval between the dispersions and the size of the dispersed particles are preferably approximately the same as the wavelength that is the target of light confinement. When used for a solar cell substrate of a conversion device that photoelectrically converts sunlight, 0.1 to 2 μm. It is desirable to be in the range. Here, the dispersed particles may take a three-dimensional arrangement.

  On the other hand, in the example shown in FIG. 3, the high refractive index material 4 having a certain size is arranged with regularity in the low refractive index material 5 to constitute a diffraction grating. According to such an arrangement, the optical characteristics can be controlled more precisely. Also in this case, it is desirable that the size and the interval of the high refractive index material 4 are in the range of 0.1 to 2 μm. In the illustrated example, the diffraction grating has only one grating constant, but a diffraction grating having a plurality of grating constants with two or more sizes or intervals of the high refractive index material 4 may be used. Further, a plurality of diffraction gratings having different lattice constants may be laminated to form one transparent composite film. Thereby, a diffraction grating having a three-dimensional structure can be formed.

  In the example shown in FIGS. 2 and 3, the high refractive index material 4 is dispersedly arranged in the low refractive index material 5. On the contrary, the low refractive index material 5 is the high refractive index material 4. It may be distributed inside. In order to increase light scattering, it is desirable that the difference in refractive index of the plurality of transparent materials used in the substrate is as large as possible, and the difference is preferably 0.5 or more. Further, in order to reduce reflection at the interface between the power generation layer and the composite transparent film, one of the transparent materials is desired to have a refractive index that is the same as or close to that of the semiconductor constituting the power generation layer. Furthermore, it is desirable that these transparent materials have as small an absorption coefficient as possible with respect to sunlight that reaches the substrate through the power generation layer. As an example, amorphous silicon (refractive index ~ 3.5) is used as a high refractive index material, silicon oxide (refractive index ~ 1.5) or zinc oxide (refractive index ~ 2.0), oxide is used as a low refractive index material. Use of oxides such as tin (refractive index˜2.0) and ITO (refractive index˜2.1) can be mentioned. As the low refractive index material, in addition to the inorganic oxide material, a resin material or air can be used. In the case of air, it is distributed in the high refractive index material as bubbles.

  FIG. 4 is a cross-sectional view in order of steps showing an example of a method for producing a thin film solar cell substrate according to the present invention. First, a high reflectance substrate 3 having a reflective surface with a high reflectance on the surface is prepared [FIG. 4A]. A colloidal solution in which fine particles of the high refractive index material 4 are dispersed in an organic solvent is applied thereon, the organic solvent is evaporated, and the fine particles are dispersedly arranged on the substrate. The size of the fine particles is 0.1 to 2 μm, and the dispersion conditions are adjusted so that the intervals at which the fine particles are dispersed are similar [FIG. 4B]. The fine particles are embedded with a solution having an appropriate viscosity and fixed on the substrate surface. As this solution, a solution that is easily oxidized by heat treatment to become an oxide, such as a solution containing silicon or zinc, is used. One example is SOG (spin on glass). Alternatively, a low refractive index resin material may be used. As a result, a film in which fine particles having a high refractive index are dispersed in the layer of the low refractive index material 5 is obtained [FIG. 4C]. Finally, the film surface is flattened to expose the high refractive index layer (FIG. 4D). As a planarization method, a method of applying mechanical pressure, a method of polishing, a method of heat treatment, a method of using a chemical reaction, or a combination thereof, for example, CMP can be considered.

FIG. 5: is sectional drawing of the order of a process which shows the other example of the manufacturing method of the board | substrate for thin film solar cells which concerns on this invention. First, as in the case of FIG. 4, a high reflectance substrate 3 is prepared [FIG. 5A], and a thin film of a low refractive index material 5 is formed thereon [FIG. 5B]. A resist film 11 serving as an etching mask is formed on the surface by a method such as spin coating or photolithography [FIG. 5C]. Then, the pattern of the low refractive index material 5 is formed by etching the film of the low refractive index material 5 using the resist film 11 as a mask [FIG. 5D]. The period of this pattern is adjusted to be 0.1 to 2 μm. The resist film 11 is removed [FIG. 5E], and then a thin film of the high refractive index material 4 is formed [FIG. 5F]. Finally, a flattening process is performed in the same manner as in FIG. 4 (FIG. 5G). The mask removal process is not always necessary.
In the etching step shown in FIG. 5D, the surface of the underlying high-reflectance substrate 3 may be roughened, and the reflectance of the surface may be reduced. In order to cope with this, it is effective to form a transparent conductive film such as an ITO film on the surface of the high reflectance substrate 3. In such a case, even if the thin film of the high refractive index material 4 after the etching process is formed by a film forming method using a vacuum technique such as a CVD method, the surface of the high reflectance substrate 3 in the film forming process. Can be avoided.

  FIG. 6 is a graph specifically obtained by numerical calculation of the state of light scattering by the thin film solar cell substrate according to the present invention. The thin film solar cell substrate assumed in this calculation has a structure comprising a flat diffraction grating using silicon as a high refractive index material and silicon oxide as a low refractive index material, and a silver reflective layer on the back surface thereof. In addition, silicon is assumed as a power generation layer formed on the substrate. The horizontal axis in the figure represents the wavelength, and the vertical axis represents the reflection angle on the substrate surface. The darker the color in the figure, the stronger the reflection. FIG. 6 shows that strong reflected light is generated in the range of 20 to 40 degrees on the longer wavelength side than the wavelength of 0.9 μm where light confinement is important. From this result, it is understood that if a material having an appropriate refractive index and structural parameters are used, light is strongly scattered (diffracted) in an oblique direction even on a substrate having a flat surface. As a result, the optical path length increases and total reflection occurs at the power generation layer interface. As a result, it can be expected that light is confined inside the power generation layer and efficiently absorbed.

  As described above, by using the solar cell substrate according to the present invention, it is possible to obtain a very good light confinement effect in the thin film solar cell without deteriorating the quality of the power generation layer formed thereon. And by extension, high power generation efficiency can be expected. Further, since the surface shape is flat, it has good consistency with the wafer bonding technique and can be applied to a mechanical stack type solar cell. Further, by selecting a production method suitable for mass production such as a transfer method, the solar cell substrate and the solar cell element according to the present invention may be provided at low cost.

DESCRIPTION OF SYMBOLS 1 Substrate for solar cells 2 Power generation layer 3 High reflectance substrate 4 High refractive index material 5 Low refractive index material 6 n-type silicon thin film 7 intrinsic silicon thin film 8 p-type silicon thin film 9 transparent conductive film 10 silver electrode 11 resist film

Claims (12)

  A vertical surface consisting of a substrate having a flat surface and a light reflecting surface, and two or more materials formed on the substrate and transparent to visible to near-infrared light and having different refractive indexes. A thin film solar cell substrate comprising: a transparent composite film having conductivity.   2. The thin-film solar cell substrate according to claim 1, wherein an average square surface roughness of the surface is 25 nm or less.   The thin film solar cell substrate according to claim 1 or 2, wherein a refractive index difference is 0.5 or more in at least one combination among two or more kinds of materials constituting the transparent composite film.   4. The thin-film solar cell according to claim 1, wherein the size of the dispersed particles of the transparent material dispersed in the transparent composite film and the distance between the dispersed particles are in the range of 0.1 to 2 μm. Substrate.   The transparent material used for the said transparent composite film contains a silicon | silicone or a metal oxide, The board | substrate for thin film solar cells in any one of Claim 1 to 4 characterized by the above-mentioned.   The substrate is a simple metal, or a transparent conductive film formed on a single metal, or a metal film formed on a support, or a metal film and a transparent conductive film on a support. The thin film solar cell substrate according to any one of claims 1 to 5, wherein the thin film solar cell substrate is one having a laminated film formed thereon.   The thin film solar cell according to any one of claims 1 to 6, wherein in the transparent composite film, one of two or more kinds of transparent materials is arranged with regularity to form a diffraction grating. substrate.   The thin film solar cell substrate according to claim 7, wherein a period of the diffraction grating is in a range of 0.1 to 2 μm.   The thin-film solar cell substrate according to claim 7 or 8, wherein the diffraction grating has two or more different periods.   The substrate for a thin film solar cell according to any one of claims 7 to 9, wherein the diffraction grating has a two-dimensional or three-dimensional structure.   A thin film solar cell substrate according to any one of claims 1 to 10, comprising a semiconductor layer having one or more semiconductor junctions to be a power generation layer, and having an upper electrode thereon. Thin film solar cell.   The thin film solar cell according to claim 11, wherein the semiconductor layer is made of an amorphous body or a crystalline body of silicon or an alloy thereof, or a combination thereof.

Images