JP4193961B2 - Multi-junction thin film solar cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multi-junction thin film solar cell having a stable and high photoelectric conversion efficiency.
[0002]
[Prior art]
Petroleum and other fossil fuels have a problem of carbon dioxide emissions, which is worried about future supply and demand and causes global warming. In recent years, solar cells have attracted attention as an alternative energy source for fossil fuels such as petroleum.
[0003]
This solar cell includes a semiconductor having a pn junction in a photoelectric conversion layer that converts light energy into electric power. In general, silicon is most often used as a semiconductor constituting the pn junction. From the viewpoint of photoelectric conversion efficiency, it is preferable to use single crystal silicon, but there are problems of raw material supply, area increase, and cost reduction.
[0004]
On the other hand, thin-film solar cells using amorphous amorphous silicon as a photoelectric conversion layer have been put to practical use as an advantageous material for realizing large area and low cost. Furthermore, in order to realize a solar cell that combines high and stable photoelectric conversion efficiency at the single crystal silicon solar cell level with large area and low cost at the amorphous silicon solar cell level, a crystalline silicon photoelectric conversion layer Use in is being considered. 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.
[0005]
JP-A-11-289173 discloses a multijunction formed by laminating a photoelectric conversion element using amorphous silicon as an active layer and a photoelectric conversion element using crystalline silicon having an energy gap smaller than that of amorphous silicon as an active layer. A type thin film solar cell is disclosed. This multi-junction thin-film solar cell can utilize solar energy more efficiently than a single-junction type by adopting a structure in which sunlight is incident from the photoelectric conversion element side using amorphous silicon as an active layer. There is an advantage that you can. Further, since a plurality of photoelectric conversion elements are connected in series, a high open-circuit voltage can be obtained, and an amorphous silicon layer as an active layer can be thinned, so that deterioration in photoelectric conversion efficiency due to the Staebler-Wronski effect can be suppressed over time. Furthermore, there is an advantage that an amorphous silicon layer and a crystalline silicon layer can be manufactured by the same apparatus, and research and development are actively conducted as a means for achieving both high efficiency and low cost.
[0006]
In the following description, unless otherwise noted, the state of the term “crystalline” means not only a state consisting essentially of crystals such as “single crystal” and “polycrystal”, This includes a state in which a crystal component called “microcrystal” or “microcrystal” is mixed with an amorphous component.
[0007]
In realizing a thin film solar cell having high photoelectric conversion efficiency, light confinement is one of important elemental technologies. Light confinement means that the surface of a transparent conductive layer or metal layer in contact with the photoelectric conversion layer is made uneven, and light is scattered at the interface, thereby extending the optical path length and increasing the amount of light absorption in the photoelectric conversion layer. Is.
[0008]
For example, Japanese Patent No. 1681183 or Japanese Patent No. 2862174, which is a patent publication, discloses an example of a solar cell substrate in which the particle size and the unevenness size of a transparent conductive layer formed on a glass substrate are defined. .
[0009]
The improvement of the photoelectric conversion efficiency by this light confinement brings about the effect of reducing the film thickness of the photoelectric conversion layer. Thereby, as described above, in the case of an amorphous silicon solar cell, light degradation is suppressed.
[0010]
Furthermore, the crystalline silicon thin film solar cell is required to have a thickness on the order of several μm, which is several to tens of times that of amorphous silicon due to light absorption characteristics. However, when the photoelectric conversion efficiency is increased by utilizing the light confinement effect, even a crystalline silicon thin film solar cell can bring about a significant decrease in film formation time.
[0011]
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 application of thin film solar cells.
[0012]
[Problems to be solved by the invention]
However, the photoelectric conversion efficiency of the conventional crystalline silicon thin film solar cell as described above is compared with the photoelectric conversion efficiency of the amorphous silicon solar cell, despite the intense research and development up to now. It has only reached the same level.
[0013]
H. In Yamamoto et al, PVSEC-11, Sapporo, Japan, 1999, the following reports are made.
[0014]
When microcrystalline silicon is formed by plasma CVD on an Asahi-U substrate in which tin oxide having microscopic surface irregularities is laminated on the glass surface, silicon crystal grains are given priority in the direction perpendicular to the tin oxide surface. Grow up. And it was reported that a large number of defects occur when crystal grains with different crystal orientations grown from different uneven surfaces collide with each other. Since such a defect becomes a recombination center of carriers, the photoelectric conversion efficiency is remarkably deteriorated, and thus must be eliminated as much as possible.
[0015]
H. Yamamoto et al. Simultaneously reports as follows.
[0016]
When the size of the unevenness is reduced by laminating thicker zinc oxide on the tin oxide having unevenness on the surface, silicon crystal grains are perpendicular to the surface of the zinc oxide as in the case of only tin oxide. grow up. However, crystal grains grown from different uneven surfaces collide with each other but their orientation difference is small, so that fewer defects are generated.
[0017]
However, it is clear that the surface roughness of the substrate should be as small as possible in order to reduce defects in the crystalline silicon thin film. However, as described above, light confinement is an essential technique for thin film solar cells, and when practical use is considered, it should be avoided to eliminate or reduce the surface roughness.
[0018]
In JP-A-10-117006, JP-A-10-294448, JP-A-11-214728, JP-A-11-266027, and JP-A-2000-58892, publications relating to multi-junction solar cells are disclosed. It is disclosed.
[0019]
In particular, in the case of a multi-junction solar cell, a lower photoelectric conversion element having a photoelectric conversion layer made of a crystalline silicon layer is formed on a back electrode having an uneven surface, and the crystalline silicon layer is formed on the substrate surface. A structure of a thin film solar cell having parallel (110) preferential crystal orientation planes is disclosed.
[0020]
However, these all have a substrate type element structure in which light is incident from the photoelectric conversion element side. In a super straight type element structure in which light is incident from the substrate side using a transparent substrate, a crystalline silicon thin film is used. No suitable concavo-convex structure has been found that achieves both a defect density reduction and a light confinement effect.
[0021]
The object of the present invention has been made in view of the above-mentioned problems, and the object is to provide a photoelectric device having a crystalline silicon (semiconductor) layer with a reduced defect density while having a good light confinement effect. The object is to provide a multi-junction thin-film solar cell with high conversion efficiency.
[0022]
[Means for Solving the Problems]
In order to solve the above-described problems, the multi-junction thin film solar cell of the present invention is provided with a plurality of photoelectric conversion elements on the side opposite to the direction in which light is incident on the substrate, and between adjacent photoelectric conversion elements. At least one of the above is provided with an intermediate layer having an uneven surface, and the height of the unevenness of the intermediate layer is such that the mean square value is set in the range of 25 to 600 nm, and the intermediate layer The characteristic feature is that tan Θ is set in the range of 0.07 to 0.20, where Θ is the inclination angle of the concavo-convex surface with respect to the average line of the concavo-convex surface of the layer surface.
[0023]
According to said invention, the intermediate | middle layer is provided between adjacent photoelectric conversion elements. When adjacent photoelectric conversion elements are directly bonded, layers having different conductivity types are bonded to each other. As a result, defects such as a bonding failure due to mixing of impurities generated in the reverse bonding formation are caused. The intermediate layer is provided to overcome this problem.
[0024]
In the intermediate layer, at least the surface opposite to the side facing the substrate is roughened. Light is scattered at the interface between the uneven surface of the intermediate layer and the photoelectric conversion layer. This light scattering extends the optical path length, and as a result, the amount of light absorption in the photoelectric conversion layer increases. As described above, the light is confined, so that the photoelectric conversion efficiency can be improved. The film thickness of a photoelectric conversion layer becomes thin by the improvement in photoelectric conversion efficiency. Thereby, it is possible to significantly reduce the film forming time and manufacturing cost required for the photoelectric conversion layer.
[0025]
It is preferable that both the front surface and the back surface of the intermediate layer are roughened. In this case, light confinement between two adjacent photoelectric conversion elements can be performed via the intermediate layer. The photoelectric conversion efficiency is improved accordingly.
[0026]
By the way, if the crystal orientation of the unevenness of the intermediate layer is different, crystals may collide with each other depending on the height, thereby causing defects. Such a defect becomes the center of carrier recombination and significantly reduces the photoelectric conversion efficiency. In addition, the conventional multi-junction thin film solar cell only discloses an element structure in which light is incident from the side where the photoelectric conversion element is provided, and has a structure in which light is incident from the substrate side. However, it does not disclose an element structure that can achieve both the reduction of defects and light confinement.
[0027]
Therefore, according to the present invention, the height of the unevenness is set so that the mean square value is in the range of 25 to 600 nm, and the inclination angle of the uneven surface with respect to the average line of the unevenness is defined as Θ. Since tan Θ is set in the range of 0.07 to 0.20, it is possible to reliably avoid the crystals from colliding with each other. Therefore, it is possible to reliably prevent the photoelectric conversion efficiency from being deteriorated due to the defect.
[0028]
Moreover, in the said board | substrate, it is more preferable that the surface on the opposite side to the light incident side is uneven | corrugated. Thereby, since two different unevenness | corrugations exist, a light confinement effect can be improved more effectively and a multijunction thin film solar cell with high photoelectric conversion efficiency can be obtained.
[0029]
More preferably, the irregularities are made of a transparent conductive material. In this case, since incident light is scattered at the interface with the photoelectric conversion layer, the optical path length becomes long, and the light confinement effect can be enhanced.
[0030]
It is more preferable that the transparent conductive material is mainly composed of zinc oxide. As a result, this is a multi-junction thin film that has the advantages of being cheaper, more resistant to plasma and less susceptible to alteration than using materials such as tin oxide, indium oxide, or ITO (Indium Tin Oxide) that are widely used for transparent conductive films. A solar cell can be obtained.
[0031]
More preferably, the irregularities are formed by etching the material made of the transparent conductive material. Thereby, the surface shape of the transparent conductive material can be easily controlled by appropriately changing the type, concentration, etching time, etc. of the etchant, so that desired irregularities can be easily obtained.
[0032]
Another multi-junction thin-film solar cell of the present invention is provided with a plurality of photoelectric conversion elements provided on the opposite side of the substrate from the direction in which light is incident, and adjacent photoelectric conversion elements At least one of the intermediate layers is provided with an intermediate layer having an uneven surface, and the hole that is a part of the unevenness has a substantially hemispherical or conical shape with a diameter in the range of 200 to 2000 nm. It is characterized by having. The diameter of the hole is more preferably in the range of 400 to 1200 nm.
[0033]
In this case, the height of the concavo-convex has a mean square value in the range of 25 to 600 nm, and a tan Θ of 0.07 to 0.00 when the inclination angle of the concavo-convex surface with respect to the average line of the concavo-convex is Θ. It is within the range of 20, and it can be surely avoided that the crystals collide with each other. Therefore, a stable and high photoelectric conversion efficiency multi-junction thin film solar cell (a multi-junction thin film solar cell of the type in which light is incident from the substrate side) that can achieve both reduction of defects and light confinement is ensured. Can be provided.
[0034]
More preferably, the active layer of at least one photoelectric conversion element on the surface of the intermediate layer is made of crystalline silicon or a silicon alloy.
[0035]
As a result, long wavelength light having a wavelength of 700 nm or more that cannot be used for photoelectric conversion with amorphous silicon can be used for photoelectric conversion, so that high photoelectric conversion efficiency can be obtained, and a stable solar cell in which light degradation is suppressed can be obtained. .
[0036]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment]
One embodiment of the multi-junction thin film solar cell of the present invention will be described below with reference to FIG.
[0037]
As shown in FIG. 1, the multi-junction thin film solar cell 20 of the present invention includes a glass substrate 11a, an uneven surface layer 11b, an amorphous silicon photoelectric conversion layer 12, an intermediate layer 13, a crystalline silicon photoelectric conversion layer 14, and a back surface. The reflection layer 15 and the back electrode 16 are laminated in this order.
[0038]
The multi-junction thin film solar cell 20 has a plurality of different types of photoelectric conversion layers of an amorphous silicon photoelectric conversion layer 12 and a crystalline silicon photoelectric conversion layer 14, and light enters from the glass substrate 11a side. It is a super straight type multi-junction thin film solar cell.
[0039]
The solar cell substrate 11 is composed of a glass substrate 11a and an uneven surface layer 11b.
[0040]
The glass substrate 11a is a transparent substrate constituting the solar cell substrate 11. Although the thickness of a board | substrate is not specifically limited, For example, what is necessary is just about 0.1-30 mm so that it may have the suitable intensity | strength and weight which can support a structure.
[0041]
In the present embodiment, glass is used as the material of the transparent substrate, but various materials such as a resin having heat resistance of about 200 ° C. such as polyimide and polyvinyl, and those laminated thereon can be used. Furthermore, those whose surfaces are coated with a metal film, a transparent conductive film, an insulating film, or the like may be used.
[0042]
The uneven surface layer 11b is made of a transparent conductive material, and is formed by etching zinc oxide, which is a transparent conductive material. By using a transparent conductive material such as tin oxide, indium oxide, ITO, or zinc oxide, incident light is scattered at the interface between the amorphous silicon photoelectric conversion layer 12 and the uneven surface layer 11b, and the optical path length is long. Thus, it becomes possible to enhance the light confinement effect together with the effect of the unevenness of the intermediate layer described later. Further, by using zinc oxide as a material, it is possible to obtain the solar cell substrate 11 having the advantages of being inexpensive, having high plasma resistance and being hardly deteriorated.
[0043]
Further, an amorphous silicon photoelectric conversion layer (photoelectric conversion element) 12 that converts light incident on the multi-junction thin film solar cell 20 into electricity includes a p-type amorphous silicon layer 12a, an i-type amorphous silicon layer. 12b and the n-type silicon layer 12c.
[0044]
Similarly, the crystalline silicon photoelectric conversion layer (photoelectric conversion element) 14 includes a p-type crystalline silicon layer 14a, an i-type crystalline silicon layer 14b, and an n-type silicon layer 14c.
[0045]
The intermediate layer 13 is a layer provided between layers having different conductivity types (amorphous silicon photoelectric conversion layer 12 and crystalline silicon photoelectric conversion layer 14) of adjacent photoelectric conversion elements. The intermediate layer 13 is provided in order to prevent a bonding failure due to the mixing of impurities generated in the reverse direction when the layers having different conductivity types are directly bonded.
[0046]
The intermediate layer 13 is formed of a transparent conductive material made of zinc oxide.
[0047]
In addition, tin oxide, indium oxide, ITO, etc. can be used as the transparent conductive material, but by using zinc oxide as the material of the transparent conductive film, it is cheaper than using other materials, A stable multi-junction thin film solar cell 20 having high plasma resistance can be obtained.
[0048]
Further, the intermediate layer 13 may be made of these single materials, or may be a layer in which layers containing these materials are laminated a plurality of times. However, it is more preferable that both surfaces of the intermediate layer 13, particularly the surface not facing the glass substrate 11 a, are made of a transparent conductive film. These transparent conductive films can be produced by known methods such as sputtering, atmospheric pressure CVD, reduced pressure CVD, electron beam evaporation, sol-gel, and electrodeposition. Among these, it is particularly preferable to fabricate by a sputtering method because it is easy to control the transmittance and resistivity of the transparent conductive film to those suitable for the multi-junction thin film solar cell 20.
[0049]
A trace amount of impurities may be added to the transparent conductive film forming the intermediate layer 13. For example, when zinc oxide is added, 5 × 10 ²⁰ ~ 5x10 ^{twenty one} cm ^-3 Since the resistivity is reduced by containing a Group IIIB element such as gallium or aluminum or a Group IB element such as copper, it is suitable for use as the intermediate layer 13.
[0050]
Further, if the thickness of these transparent conductive films is too thin, there will be a problem in uniformity of characteristics, and if it is too thick, it will cause a decrease in transmittance and a decrease in photoelectric conversion efficiency due to an increase in series resistance and an increase in cost. The thickness is preferably about ˜50 nm.
[0051]
In the present embodiment, chemical treatment by etching is used as means for providing irregularities on the surface of the intermediate layer 13. Thereby, when the surface of the intermediate layer 13 is made of a transparent conductive film, the surface shape of the transparent conductive film can be easily controlled by appropriately changing the type, concentration, etching time or the like of the etchant.
[0052]
Using an acid or alkali solution as the etchant can be manufactured at a lower cost. In this case, examples of the acid solution that can be used include one or a mixture of two or more of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, acetic acid, formic acid, perchloric acid, and the like. Among these, it is particularly preferable to use hydrochloric acid or acetic acid. These acid solutions can be used at a concentration of about 0.05 to 5% by weight, for example, but in the case of a relatively weak acid such as acetic acid, it is used at a concentration of about 0.1 to 5% by weight. Is good. Examples of the alkaline solution include sodium hydroxide, ammonia, potassium hydroxide, calcium hydroxide, aluminum hydroxide and the like, and among them, sodium hydroxide is particularly preferable. . These alkaline solutions are preferably used at a concentration of about 1 to 10% by weight.
[0053]
In addition to etching, as a means for providing irregularities on the surface of the intermediate layer 13, for example, the intermediate layer 13 may be formed under conditions such that irregularities are formed on the surface simultaneously with the deposition. At this time, the formation condition of the intermediate layer 13 may be determined in consideration of the uneven shape of the surface of the intermediate layer 13 being affected by the uneven shape of the amorphous silicon photoelectric conversion layer 12 serving as a base. In addition, even if machining such as sand blasting is performed on the surface of the intermediate layer 13, it is possible to form an uneven shape.
[0054]
In the present embodiment, the surface of the intermediate layer 13 after etching is uneven. Here, the root mean square value indicating the uneven height of the intermediate layer 13 is set in a range of 25 to 600 nm, and the inclination angle of the uneven portion surface with respect to the average line of the uneven portion of the intermediate layer 13 surface is Θ. Is set in the range of 0.07 to 0.20.
[0055]
Thereby, since the unevenness | corrugation of the intermediate | middle layer 13 surface is provided so that the crystalline silicon photoelectric converting layer 14 (p-type crystalline silicon layer 14a) may be contacted, light is scattered in the interface. This light scattering extends the optical path length, and as a result, the amount of light absorption in the crystalline silicon photoelectric conversion layer 14 increases. Due to this light confinement effect, the photoelectric conversion efficiency is improved, the film thickness of the crystalline silicon photoelectric conversion layer 14 can be reduced, and the film formation time and the manufacturing cost required for forming the crystalline silicon photoelectric conversion layer 14 are greatly reduced. be able to.
[0056]
Further, according to the present invention, the height of the unevenness is set so that the mean square value thereof is in the range of 25 to 600 nm, and the inclination angle of the uneven surface with respect to the average line of the unevenness is defined as Θ. Since tan Θ is set in the range of 0.07 to 0.20, it is possible to reliably avoid the crystals from colliding with each other. Therefore, it is possible to reliably prevent the photoelectric conversion efficiency from being deteriorated due to the defect, and thus it is possible to obtain the multijunction thin film solar cell 20 having a stable and high photoelectric conversion efficiency.
[0057]
In order to obtain the effect of the present invention more effectively, the mean square value is in the range of 25 to 400 nm and the tan Θ is in the range of 0.07 to 0.15. The intermediate layer 13 having a large confinement effect and a small number of defects can be obtained.
[0058]
Further, the surface of the intermediate layer 13 by etching has a substantially hemispherical or conical hole. The diameter of the hole is controlled by etching so as to be in the range of 200 to 2000 nm.
[0059]
As a result, with regard to the mean square value of the height and tan Θ when the inclination angle is Θ, the above-described preferred range having a high light confinement effect (the mean square value = 25 to 600 nm and tan Θ = 0.07 to 0). .20) can be formed with good reproducibility. Furthermore, if the diameter of the hole is in the range of 400 to 1200 nm, an intermediate layer having irregularities in a suitable range can be reliably formed with good reproducibility.
[0060]
The back surface reflection layer 15 is a thin film made of zinc oxide having a thickness of 50 nm formed by a magnetron sputtering method.
[0061]
The back electrode 16 is formed of silver to a thickness of 500 nm by electron beam evaporation. A super-straight type multi-junction thin-film solar cell 20 is constructed in which the electrodes 17 are drawn out from the back electrode 16 and the concavo-convex surface layer 11b, and light is incident from the glass substrate 11a side.
[0062]
With the above configuration, a multi-junction thin film solar cell in which the crystalline silicon photoelectric conversion layer 14 with few defects is formed on the uneven surface of the intermediate layer 13 provided in order to increase the photoelectric conversion rate. 20 can be obtained.
[0063]
In the present embodiment, the crystalline silicon photoelectric conversion layer 14 has been described as an example including one photoelectric conversion element. However, the crystalline silicon photoelectric conversion layer 14 may be formed of a plurality of photoelectric conversion elements, and an active layer (I-type layer) in at least one of the photoelectric conversion elements is formed of crystalline silicon or a silicon alloy. It only has to be. Then, since long wavelength light with a wavelength of 700 nm or more which cannot be used for photoelectric conversion can be sufficiently utilized with amorphous silicon, a stable multi-junction thin film solar cell 20 in which high photoelectric conversion efficiency and light deterioration are suppressed can be obtained. Obtainable.
[0064]
In particular, when a photoelectric conversion element having an active layer made of crystalline silicon or a silicon alloy is formed on the surface of the intermediate layer 13 on the side not facing the glass substrate 11a, a sufficient light confinement effect and defect This is preferable because a multi-junction thin-film solar cell 20 having a stable and high photoelectric conversion efficiency in which the increase is suppressed can be obtained. The silicon alloy is, for example, Si in which tin is added to silicon. _x Sn _1-x , And Si doped with germanium _x Ge _1-x Etc. material.
[0065]
In the present embodiment, a glass substrate 11a on which a transparent conductive film having a smooth surface and an uneven layer is formed is used, and zinc oxide is interposed between the amorphous silicon photoelectric conversion layer 12 and the crystalline silicon photoelectric conversion layer 14. The super straight type multi-junction thin film solar cell 20 on which the light is incident from the glass substrate 11a side on which the intermediate layer 13 made of is formed has been described.
[0066]
However, the present invention is not limited to this embodiment, and can be applied to a multi-junction thin film solar cell called a substrate type, for example, with the following configuration. This substrate-type multi-junction thin-film solar cell has a structure in which a back surface reflection layer and an uneven surface layer are laminated on a substrate and the photoelectric conversion layer is turned upside down, and the multi-junction type in which light enters from the back electrode side It is a thin film solar cell.
[0067]
Furthermore, in order to make the effect of this invention concrete, Examples 1-3 and a comparative example are described below.
[0068]
(Example 1)
The production procedure of the multi-junction thin film solar cell 20 will be described with reference to FIG. For convenience of explanation, members having the same functions as those in the drawings described in the above embodiment are given the same reference numerals, and descriptions thereof are omitted.
[0069]
On one main surface of a glass substrate 11a having a smooth surface shown in FIG. 1, zinc oxide was formed to a thickness of 500 nm at a substrate temperature of 150 ° C. by a magnetron sputtering method. This zinc oxide has 1 × 10 ^{twenty one} cm ^-3 About gallium is added. As a result, the sheet resistance of the obtained zinc oxide was 10Ω / □, and the transmittance for light having a wavelength of 800 nm was 80%.
[0070]
Subsequently, the surface of the zinc oxide was etched. The glass substrate coated with zinc oxide is immersed in a 0.5 wt% hydrochloric acid aqueous solution having a liquid temperature of 25 ° C. for 15 seconds, and then the surface is sufficiently washed with pure water to have a concavo-convex surface layer 11b. Got.
[0071]
On the solar cell substrate 11 thus obtained, a 13.56 MHz p-type amorphous silicon layer 12a, an i-type amorphous silicon layer 12b, and an n-type silicon layer 12c are sequentially laminated to form an amorphous silicon photoelectric conversion layer. 12 was formed. The p-type amorphous silicon layer 12a is made of SiH. _Four Gas 12SCCM, H ₂ Gas 30SCCM, H ₂ B adjusted to 5000ppm by gas ₂ H ₆ The film was formed under the conditions of gas 1 SCCM, film forming chamber pressure 20 Pa, discharge power 25 W, substrate temperature 180 ° C., and the thickness was 15 nm. The i-type amorphous silicon layer 12b is made of SiH. _Four Gas 30SCCM, H ₂ A film was formed under the conditions of gas 70 SCCM, film forming chamber pressure 30 Pa, discharge power 30 W, and substrate temperature 180 ° C. to a thickness of 350 nm. The n-type silicon layer 12c is made of SiH _Four Gas 10SCCM, H ₂ PH adjusted to 1000ppm by gas _Three The film was formed under the conditions of gas 100 SCCM, film forming chamber pressure 27 Pa, discharge power 30 W, substrate temperature 180 ° C., and the thickness was 30 nm.
[0072]
It was once taken out from a plasma CVD apparatus (not shown), and zinc oxide was produced again as the intermediate layer 13 by the magnetron sputtering method under the same conditions as those for producing the substrate. However, the thickness of the intermediate layer 13 was set to 250 nm in consideration of the reduction of the film thickness due to etching. When the surface shape of the intermediate layer 13 was observed with a scanning electron microscope, it was found that a substantially hemispherical hole having a diameter of about 50 to 200 nm on the surface was formed.
[0073]
In order to examine the surface shape of the intermediate layer 13 in detail, the surface shape was measured with an atomic force microscope. From the shape of the hole in the depth direction, it was found that the shape of the hole was substantially hemispherical or conical. In order to express the feature of the surface shape numerically, the mean square value of the unevenness height was used as an index indicating the unevenness height. Furthermore, when the most frequent wavelength W of a 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, tan Θ = 2RMS / ( W / 2) = 4 RMS / W, and the values of RMS and tan Θ were used as indices of the concavo-convex shape. The unevenness height RMS at this time was 12 nm, and tan Θ was 0.05.
[0074]
Thereafter, the surface of the intermediate layer 13 was etched by dipping in a 0.5 wt% hydrochloric acid aqueous solution at a liquid temperature of 25 ° C. for 15 seconds. After the surface was sufficiently washed with pure water, the surface shape of the intermediate layer 13 was observed with a scanning electron microscope. As a result, many substantially hemispherical holes having a diameter of about 400 to 1200 nm on the surface were formed.
[0075]
In order to examine the surface shape of the intermediate layer 13 in detail, the surface shape was measured with an atomic force microscope. The shape of the hole formed on the surface of the concavo-convex surface layer in this example was substantially hemispherical or conical as before the etching, but the concavo-convex height RMS in this example was 32 nm and tan Θ was 0. .10.
[0076]
The p-type crystalline silicon layer 14a, the i-type crystalline silicon layer 14b, and the n-type silicon layer 14c are sequentially laminated on the intermediate layer 13 by the high-frequency plasma CVD method again by the plasma CVD method using a high frequency of 13.56 MHz. A crystalline silicon photoelectric conversion layer 14 was formed. The p-type crystalline silicon layer 14a is made of SiH. _Four Gas 3SCCM, H ₂ Gas 600SCCM, H ₂ B adjusted to 5000ppm by gas ₂ H ₆ A film was formed under the conditions of gas 1 SCCM, film forming chamber pressure 200 Pa, discharge power 25 W, and substrate temperature 140 ° C. to a thickness of 30 nm. The i-type crystalline silicon layer 14b is made of SiH. _Four Gas 11SCCM, H ₂ A film was formed under the conditions of gas 350 SCCM, film forming chamber pressure 200 Pa, discharge power 20 W, and substrate temperature 140 ° C. to a thickness of 2500 nm. The n-type silicon layer 14c is made of SiH. _Four Gas 10SCCM, H ₂ PH adjusted to 1000ppm by gas _Three The film was formed under the conditions of gas 100 SCCM, film forming chamber pressure 27 Pa, discharge power 30 W, substrate temperature 180 ° C., and the thickness was 30 nm.
[0077]
After taking out from the plasma CVD apparatus (not shown), when the X-ray diffraction method was performed with respect to the obtained photoelectric converting layer, the integrated intensity I of the (220) X-ray diffraction peak was obtained. ₂₂₀ And the integrated intensity I of the (111) X-ray diffraction peak I ₁₁₁ Ratio I ₂₂₀ / I ₁₁₁ Was 2.9. Here, the actually obtained X-ray diffraction peaks are not information of the i-type layer alone in the crystalline silicon photoelectric conversion layer, but the film thicknesses of the p-type layer and the n-type layer are much higher than those of the i-type layer. Since it is thin, it can be assumed that it reflects the crystal orientation of the i-type layer.
[0078]
Thereafter, a super-straight type multi-layer in which zinc oxide is formed as a back reflection layer 15 with a thickness of 50 nm by magnetron sputtering and silver is formed as a back electrode 16 with a thickness of 500 nm by an electron beam evaporation method, and light is incident from the glass substrate 11a side. A junction-type thin film solar cell 20 was obtained. The cell area is 1 cm. ² Met.
[0079]
AM1.5 (100 mW / cm) of the multi-junction thin film solar cell 20 ² ) When the current-voltage characteristics under irradiation conditions were measured, the short-circuit current was 12.8 mA / cm. ² The open circuit voltage was 1.205 V, the shape factor was 0.695, and the photoelectric conversion efficiency was 10.72%.
[0080]
From the above results, it was found that the surface shape of the intermediate layer 13 in which the mean square value of the intermediate layer 13 was 32 nm and tan Θ was 0.10 matched the conditions for obtaining a good photoelectric conversion rate. Similarly to the above, it was found that when the diameter of the hole of the intermediate layer 13 was 400 to 1200 nm as in this example, the conditions for obtaining a good photoelectric conversion rate were met.
[0081]
(Example 2)
Another embodiment of the multi-junction thin film solar cell of the present invention will be described as follows. For convenience of explanation, members having the same functions as those in the drawings described in the first embodiment are given the same reference numerals, and descriptions thereof are omitted.
[0082]
This example shows an example in which the uneven shape on the surface of the intermediate layer 13 is controlled without performing etching on the surface of the intermediate layer 13 by controlling the height of the unevenness of the uneven surface layer 11b as a base. Yes. It was produced in the same manner as in Example 1 except that the etching time of the zinc oxide surface for forming the uneven surface layer 11b was 45 seconds. As a result, the uneven height of the uneven surface layer 11b is increased, and the uneven height of the surface of the intermediate layer 13 is expected to be affected by an increase in unevenness of the uneven surface layer 11b serving as a base.
[0083]
An amorphous silicon photoelectric conversion layer 12 and an intermediate layer 13 were formed on the solar cell substrate 11 in the same manner as in Example 1. At this time, when the surface shape of the intermediate layer 13 was observed with a scanning electron microscope, it was found that many substantially hemispherical holes having a diameter of about 200 to 1400 nm on the surface were formed.
[0084]
In order to examine the surface shape of the intermediate layer 13 in detail, the surface shape was measured with an atomic force microscope. The shape of the concave / convex hole formed on the surface of the intermediate layer 13 of this example was substantially hemispherical or conical as in Example 1, but the concave / convex height RMS in this example was 28 nm. Tan Θ was 0.08.
[0085]
When the X-ray diffraction method was performed after forming the crystalline silicon photoelectric conversion layer 14 without etching the surface of the intermediate layer 13, the integrated intensity I of the (220) X-ray diffraction peak was obtained. ₂₂₀ And the integrated intensity I of the (111) X-ray diffraction peak I ₁₁₁ Ratio I ₂₂₀ / I ₁₁₁ Was 3.0, which was almost the same as that in Example 1.
[0086]
From this result, it can be seen that the surface of the intermediate layer 13 can be provided with unevenness that can obtain a light confinement effect substantially equivalent to the uneven shape obtained in Example 1 without etching the intermediate layer 13.
[0087]
AM1.5 (100 mW / cm of this multi-junction thin film solar cell 20 ² ) When the current-voltage characteristics under irradiation conditions were measured, the short-circuit current was 13.6 mA / cm. ² The open circuit voltage was 1.204 V, the shape factor was 0.694, and the photoelectric conversion efficiency was 11.36%.
[0088]
Compared with the case of Example 1, the values of the open circuit voltage and the form factor are hardly changed, but it can be seen that the value of the short circuit current is increased. This is combined with the light confinement effect by the uneven surface layer 11b provided on the solar cell substrate 11 side, and the structure of the surface of the intermediate layer 13 is suitably set, so that the crystalline silicon photoelectric conversion layer 14 It is considered that an effect that a large amount of defects were not introduced during the formation of.
[0089]
From this result, it was found that the shape of the intermediate layer 13 having a mean square value of 28 nm and tan Θ = 0.08 can obtain a higher photoelectric conversion rate than the uneven shape of Example 1. Similarly to the above, it was found that when the diameter of the hole of the intermediate layer 13 was 200 to 1400 nm as in this example, the conditions for obtaining a good photoelectric conversion rate were met.
[0090]
(Example 3)
The following will describe still another embodiment of the multi-junction thin film solar cell of the present invention. For convenience of explanation, members having the same functions as those in the drawings described in the above embodiment are given the same reference numerals, and descriptions thereof are omitted.
[0091]
This example is another example implemented in the same manner as in Example 2 except that the uneven shape on the surface of the intermediate layer 13 was controlled by etching the surface of the intermediate layer 13. Thereby, it is expected that the unevenness of the surface of the intermediate layer 13 becomes larger than that in the first embodiment.
[0092]
An amorphous silicon photoelectric conversion layer 12 and an intermediate layer 13 were formed on the solar cell substrate 11 produced in the same manner as in Example 2 in the same manner as in Example 2. However, the thickness of the intermediate layer 13 was set to 50 nm in consideration of a decrease in film thickness due to etching. Thereafter, the surface of the intermediate layer 13 was etched by dipping in a 0.5 wt% hydrochloric acid aqueous solution at a liquid temperature of 25 ° C. for 15 seconds. After the surface was sufficiently washed with pure water, the surface shape of the intermediate layer 13 was observed with a scanning electron microscope. As a result, many substantially hemispherical holes having a diameter of about 400 to 1200 nm on the surface were formed.
[0093]
In order to examine the surface shape of the intermediate layer 13 in detail, the surface shape was measured with an atomic force microscope. The shape of the hole formed on the surface of the concavo-convex surface layer of this example was substantially hemispherical or conical as in Example 2, but the concavo-convex height RMS in this example was 40 nm. tan Θ was 0.13.
[0094]
Furthermore, when the X-ray diffraction method was performed after forming the crystalline silicon photoelectric conversion layer 14, the integrated intensity I of the (220) X-ray diffraction peak was obtained. ₂₂₀ And (111) X-ray diffraction peak integrated intensity I ₁₁₁ Ratio I ₂₂₀ / I ₁₁₁ Was 2.8, which was almost the same as that in Example 1.
[0095]
AM1.5 (100 mW / cm of this multi-junction thin film solar cell 20 ² ) When the current-voltage characteristics under irradiation conditions were measured, the short-circuit current was 14.4 mA / cm. ² The open circuit voltage was 1.207 V, the shape factor was 0.693, and the photoelectric conversion efficiency was 12.05%.
[0096]
As compared with the case of Example 2, the values of the open circuit voltage and the shape factor are hardly changed, but it is understood that the value of the short circuit current is increased. It is considered that a more preferable uneven shape was obtained by etching the surface of the intermediate layer 13.
[0097]
From this result, it was found that the photoelectric conversion efficiency higher than that of Example 2 was obtained by the shape of the intermediate layer 13 such that the mean square value = 40 nm and tan Θ = 0.13. Similarly to the above, it was found that when the diameter of the hole in the intermediate layer 13 is 400 to 1200 nm as in this example, a higher photoelectric conversion rate than that in Example 2 can be obtained.
[0098]
(Comparative example)
It will be as follows if the comparative example of the thin film solar cell of this invention is demonstrated.
[0099]
In this comparative example, a multi-junction thin film solar cell was fabricated in the same manner as in Example 1 except that the thickness of the intermediate layer was 5 nm and etching was not performed. Thereby, compared with the said Examples 1-3, it is anticipated that the uneven | corrugated height of the intermediate | middle layer surface becomes small.
[0100]
When the surface shape of the intermediate layer was observed with a scanning electron microscope, it was found that many substantially hemispherical holes having a diameter of about 50 to 200 nm on the surface were formed.
[0101]
In order to examine the surface shape of the intermediate layer in detail, the surface shape was measured with an atomic force microscope. The shape of the hole formed in the surface of the intermediate layer of this comparative example was substantially hemispherical or conical as in Example 1, but the unevenness height RMS in this comparative example was 14 nm, and tanΘ Was 0.05, and the height of the unevenness was smaller than the results of Examples 1 to 3 above.
[0102]
Furthermore, when the X-ray diffraction method was performed after forming the crystalline silicon photoelectric conversion layer, the integrated intensity I of the (220) X-ray diffraction peak was obtained. ₂₂₀ And (111) X-ray diffraction peak integrated intensity I ₁₁₁ Ratio I ₂₂₀ / I ₁₁₁ Is 3.0, which is almost the same as in the second embodiment.
[0103]
AM1.5 (100 mW / cm of this multi-junction thin film solar cell ² ) When the current-voltage characteristics under irradiation conditions were measured, the short-circuit current was 10.7 mA / cm. ² The open circuit voltage was 1.205 V, the shape factor was 0.698, and the photoelectric conversion efficiency was 9.00%. Compared with the case of Example 1, although the value of an open circuit voltage and a form factor has hardly changed, it turns out that the value of a short circuit current has decreased. That is, since the etching time is insufficient, it is considered that the uneven structure on the surface of the intermediate layer is insufficient for producing the light confinement effect.
[0104]
From this result, it was found that when the mean square value of the intermediate layer was 14 nm and tan Θ was 0.05, an uneven surface layer having a good light confinement effect could not be obtained. In addition, it was found that when the diameter of the concave / convex hole in the concave / convex surface layer was 50 to 200 nm, an intermediate layer having a good light confinement effect could not be obtained as described above.
[0105]
From the results of Examples 1 to 3 and Comparative Example above, in order to use a good light confinement effect, the mean square value of the uneven height is in the range of 25 to 600 nm, and tan Θ is 0. It turned out that it may be in the range of 07-0.20.
[0106]
Therefore, it is apparent that the multi-junction thin film solar cell having high photoelectric conversion efficiency can be manufactured without introducing a large amount of defects during the formation of the crystalline silicon layer by the uneven layer structure on the surface of the intermediate layer of the present invention. became.
[0107]
Note that the substrate has a photoelectric conversion layer formed by stacking a plurality of photoelectric conversion elements, and an intermediate layer is provided in at least one of the adjacent photoelectric conversion elements. At least the surface not facing the substrate has a root mean square value in the range of 25 to 600 nm, and tan Θ in the range of 0.07 to 0.20 when the tilt angle is Θ. It may be a multi-junction thin film solar cell having certain irregularities.
[0108]
The intermediate layer may be a multi-junction thin film solar cell characterized in that at least the surface not facing the substrate is a surface made of a transparent conductive film.
[0109]
Further, in the multi-junction thin film solar cell, the surface unevenness on the side of the intermediate layer not facing the substrate is produced by etching a surface covered with a transparent conductive film. Also good.
[0110]
Moreover, the said unevenness | corrugation is a substantially hemispherical or conical hole, The diameter of this hole may exist in the range of 200-2000 nm, The multijunction thin film solar cell characterized by the above-mentioned may be sufficient.
[0111]
Further, the transparent conductive film may be a multi-junction thin film solar cell mainly composed of zinc oxide.
[0112]
A multi-junction thin-film solar cell may be characterized in that a photoelectric conversion element whose active layer is made of crystalline silicon or a silicon alloy is formed on at least the surface of the intermediate layer not facing the substrate. .
[0113]
By making the surface of the intermediate layer have a concavo-convex structure in which the mean square value of the height is in the range of 25 to 600 nm and the tan Θ is in the range of 0.07 to 0.20 when the inclination angle is Θ, Without increasing defects in the conversion layer, it is possible to increase the amount of light absorption due to the light confinement effect, and a multi-junction thin film solar cell having a stable and high photoelectric conversion efficiency can be manufactured at low cost.
[0114]
【The invention's effect】
As described above, the multi-junction thin film solar cell of the present invention is provided with a plurality of photoelectric conversion elements on the side opposite to the direction in which light is incident on the substrate, and at least one between adjacent photoelectric conversion elements. Is provided with an intermediate layer having an uneven surface, the root mean square value indicating the height of the unevenness of the intermediate layer is set in the range of 25 to 600 nm, and the unevenness of the intermediate layer surface is The tan Θ is set in the range of 0.07 to 0.20, where Θ is the inclination angle of the uneven surface with respect to the average line.
[0115]
With the above-described configuration, it is possible to prevent defects such as bonding failure due to the mixing of impurities generated in the reverse bonding formation that occurs between adjacent photoelectric conversion elements, and contact between the uneven surface of the intermediate layer and the photoelectric conversion layer At the interface, light is scattered and the optical path length is extended, and the photoelectric conversion efficiency can be improved. The film thickness of a photoelectric conversion layer becomes thin by the improvement in photoelectric conversion efficiency. Thereby, it is possible to significantly reduce the film forming time and manufacturing cost required for the photoelectric conversion layer.
[0116]
Further, according to the present invention, the height of the unevenness is set so that the mean square value is in the range of 25 to 600 nm, and the inclination angle of the uneven surface with respect to the average line of the unevenness is defined as Θ. Since tan Θ is set in the range of 0.07 to 0.20, it is possible to reliably avoid the crystals from colliding with each other. Therefore, it is possible to reliably prevent the photoelectric conversion efficiency from being deteriorated due to the defect.
[0117]
Moreover, in the said board | substrate, it is more preferable that the surface on the opposite side to the light incident side is uneven | corrugated. Thereby, two different unevenness | corrugations exist, and there exists an effect that a light confinement effect can be improved more effectively and a multijunction thin film solar cell with high photoelectric conversion efficiency can be obtained.
[0118]
Further, the unevenness is more preferably made of a transparent conductive material, and incident light is scattered at the interface with the photoelectric conversion layer, so that the optical path length becomes long and the light confinement effect is enhanced. There is an effect that it becomes possible.
[0119]
It is more preferable that the transparent conductive material is mainly composed of zinc oxide, and it is possible to obtain a multi-junction thin film solar cell that has the advantages of being inexpensive, having high plasma resistance and being hardly altered. Play.
[0120]
The unevenness is more preferably formed by etching the material made of the transparent conductive material, and the transparent conductive material can be changed by appropriately changing the type, concentration, etching time, or the like of the etchant. Since the surface shape of the conductive material can be easily controlled, the desired unevenness can be easily obtained.
[0121]
As described above, the multi-junction thin-film solar cell of the present invention has a configuration in which the hole that is a part of the unevenness has a substantially hemispherical or conical shape with a diameter in the range of 200 to 2000 nm. . The diameter of the hole is more preferably in the range of 400 to 1200 nm.
[0122]
With the above-described configuration, the height of the unevenness is in the range of the mean square value of 25 to 600 nm, and the tan Θ is 0.07 to 0 when the inclination angle of the uneven surface with respect to the average line of the unevenness is Θ. It becomes the range of 0.20, and it can avoid reliably that crystals collide. Therefore, a stable and high photoelectric conversion efficiency multi-junction thin film solar cell (a multi-junction thin film solar cell of the type in which light is incident from the substrate side) that can achieve both reduction of defects and light confinement is ensured. There is an effect that can be provided.
[0123]
More preferably, the active layer of at least one photoelectric conversion element on the surface of the intermediate layer is made of crystalline silicon or a silicon alloy, and long wavelength light having a wavelength of 700 nm or more that cannot be used for photoelectric conversion with amorphous silicon is also used for photoelectric conversion. Since it can be utilized, it is possible to obtain a stable solar cell in which high photoelectric conversion efficiency is obtained and light degradation is suppressed.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a multi-junction thin film solar cell structure of the present invention.
[Explanation of symbols]
11 Substrate for solar cell
11a glass substrate
11b Uneven surface layer
12 Amorphous silicon photoelectric conversion layer (photoelectric conversion element)
12a p-type amorphous silicon layer
12b i-type amorphous silicon layer
12c n-type silicon layer
13 Middle layer
14 Crystalline silicon photoelectric conversion layer (photoelectric conversion element)
14a p-type crystalline silicon layer
14b i-type crystalline silicon layer
14c n-type silicon layer
15 Back reflective layer
16 Back electrode
17 electrodes
20 Multi-junction thin-film solar cells

Claims (7)

There is a plurality of photoelectric conversion elements, and at least one of the adjacent photoelectric conversion elements is provided with an intermediate layer with an uneven surface,
When the root mean square value RMS of the unevenness obtained by measuring the unevenness with an atomic force microscope, Fourier transform of the surface shape waveform curve of the uneven surface obtained by measuring the unevenness with an atomic force microscope Is the most frequent wavelength W of the sinusoidal curve obtained, and the angle between the average line of unevenness measured with an atomic force microscope and the uneven surface of the average unevenness measured with an atomic force microscope is Θ The tan Θ in the case is:
tan Θ = 2RMS / (W / 2) = 4RMS / W
Represented by
The RMS is set in the range of 28 nm to 40 nm, and the tan Θ is set in the range of 0.08 to 0.13.
The multi-junction thin film solar cell according to claim 1, wherein the hole, which is a part of the unevenness, has a substantially hemispherical or conical shape with a diameter in the range of 200 to 1400 nm . The multi-junction thin-film solar cell according to claim 1, wherein the hole has a substantially hemispherical or conical shape having a diameter in the range of 400 to 1200 nm . A substrate in contact with the photoelectric conversion element;
The multi-junction thin-film solar cell according to claim 1 or 2 , wherein a surface of the substrate in contact with the photoelectric conversion element is uneven . 4. The multi-junction thin film solar cell according to claim 1, wherein the unevenness of the substrate and the unevenness of the intermediate layer are made of a transparent conductive material . 5. The multi-junction thin film solar cell according to claim 4 , wherein the transparent conductive material is mainly composed of zinc oxide . 6. The multi-junction type according to claim 4, wherein the unevenness of the substrate and the unevenness of the intermediate layer are formed by performing etching on the transparent conductive material. Thin film solar cell. The multi-junction thin film solar cell according to claim 1, wherein the active layer in the at least one photoelectric conversion element is made of crystalline silicon or a silicon alloy .

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