JP2012174735A - Thin film solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the amount of light that is incorporated into a photoelectric conversion unit by controlling the refraction index of an uneven layer using specific nanoparticles.SOLUTION: A thin film solar cell that includes: an uneven layer, a transparent electrode layer, at least one or more photoelectric conversion units, a back electrode on a base material in this order. The uneven layer includes nanoparticles of at least one metal oxide selected from the group consisting of zinc, niobium, titanium, zirconium, aluminum, cerium, and yttrium, and a matrix material.

Description

  The present invention enables high light capturing efficiency or light extracting efficiency mainly in transparent electrodes of non-single crystal silicon-based thin film solar cells.

  A thin-film solar cell typified by a non-single-crystal silicon system has a semiconductor silicon layer as a photoelectric conversion layer formed on a transparent electrode layer formed on a transparent substrate, and improves photoelectric conversion efficiency. Research is being conducted vigorously. One measure for improving the photoelectric conversion efficiency is “improvement of light confinement efficiency”. The light confinement efficiency is such that light incident from the transparent substrate is diffused in the transparent electrode layer or photoelectric conversion layer, and in order to increase the optical path length, at the interface between the substrate / transparent electrode layer and the transparent electrode layer / photoelectric conversion layer, Light incident perpendicularly to the substrate due to the refractive index difference of each layer is also refracted. Due to this light refraction, the traveling distance of the light passing through the photoelectric conversion unit becomes longer than the film thickness direction. As a result, the “apparent film thickness” increases, and as a result, the short-circuit current can be increased.

  As one of the above-mentioned means, a method of providing an uneven layer having an uneven shape on a transparent substrate is known. In Patent Document 1, a concavo-convex shape is formed by adding a scatterer made of a metal oxide material or the like to a light-transmitting conductive film made of a metal oxide or silicon oxide on the back electrode side of the thin film solar cell. It is described that the light scattering effect is enhanced.

  However, although the technique using the translucent conductive film as described above is effective on the back electrode side, it is considered difficult to use it on the light incident side from the viewpoint of absorption and the like as described below. That is, the metal oxide or silicon oxide used as the translucent conductive film has “reflection / absorption derived from free electrons” on the long wavelength side and “exciton absorption between bands” on the short wavelength side. Present and both reduce the transmittance. For this reason, for example, when a transparent conductive film having a thickness capable of forming an uneven layer is formed as a transparent electrode layer on the light incident side, it is difficult to use because the transparent electrode layer itself absorbs light. it is conceivable that.

  Since the incident light has the highest amount of light between the transparent substrate and the transparent electrode layer on the light incident side that reaches first, it is extremely important to provide an uneven shape on the light incident side. For this reason, intensive research has been conducted to increase the light confinement efficiency by forming an uneven layer on the light incident side.

For example, in Patent Document 2, a sol-gel material having silicon oxide as a main component and a refractive index of about 1.4 to 1.5 is used as an uneven layer on the light incident side of a thin film solar cell. This is considered to be because light can be taken in with almost no optical loss, as will be described later.
However, when the concavo-convex layer is formed only by the sol-gel material, the concavo-convex layer has a lower refractive index than the transparent substrate, which is optically disadvantageous. That is, reflection occurs at the interface between the transparent substrate and the concavo-convex layer, and a large reflection occurs at the interface between the concavo-convex layer and the transparent electrode layer, where a large refractive index difference occurs. is assumed.

  For this reason, Patent Document 2 describes a technique of adding a metal oxide such as silicon or titanium in order to adjust the refractive index of the concavo-convex layer. It is formed by reacting a titanium coupling agent.

  However, in Patent Document 2, particularly when a titanium coupling agent is used, oxidation does not occur sufficiently, and titanium tends to be in a low oxidation state, and absorption due to electron transfer transition derived from titanium occurs, which is optically disadvantageous. There is a concern that it is likely to become. In particular, when treating with plasma or the like, only the surface is oxidized, and the low-oxidized titanium inside cannot be treated, and there is a concern about absorption loss. Moreover, in order to fully oxidize, since it is necessary to make it high temperature near 1000 degreeC, the kind of board | substrate will be limited, and since generally used glass cannot be used, it becomes disadvantageous in cost.

JP 2003-179241 A JP 2006-168147 A

  In view of the above, the present invention provides a reflection layer for light from a substrate to a transparent electrode by providing a concavo-convex layer using metal oxide nanoparticles between the transparent substrate and the transparent electrode layer on the light incident side of the thin film solar cell. It aims at providing the thin film solar cell which can improve solar cell characteristics and can improve.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive studies and found that the use of a predetermined uneven layer can improve the solar cell characteristics. That is, the present invention relates to the following.

(1) A thin-film solar cell having an uneven layer, a transparent electrode layer, at least one photoelectric conversion unit, and a back electrode in this order on a substrate,
The thin-film solar cell, wherein the uneven layer includes at least one metal oxide nanoparticle selected from zinc, niobium, titanium, zirconium, aluminum, cerium, and yttrium, and a matrix material.
(2) The thin film solar cell according to (1), wherein the uneven layer includes a matrix material formed of a sol-gel material.
(3) The thin film solar cell according to any one of (1) to (2), wherein the uneven layer has a refractive index in a range of 1.55 to 2.25.
(4) The thin film solar cell according to any one of (1) to (3), wherein the nanoparticles have a particle size in a range of 10 to 60 nm.
(5) The thin film solar cell according to any one of (1) to (4), wherein the photoelectric conversion unit includes a non-single-crystal silicon layer.

  By using the concavo-convex layer of the present invention on the light incident side of the thin-film solar cell, reflection of light at each interface occurring from the transparent substrate to the transparent electrode layer can be suppressed, and a large amount of light is taken into the photoelectric conversion unit. As a result, the solar cell characteristics can be improved.

It is a section schematic diagram of a substrate with a transparent electrode in the present invention. It is a section schematic diagram of a thin film solar cell in the present invention.

  The present invention provides a thin film solar cell having a concavo-convex layer, a transparent electrode layer, at least one photoelectric conversion unit, and a back electrode in this order on a base material, wherein the concavo-convex layer comprises zinc, niobium, titanium, zirconium A thin film solar cell comprising at least one metal oxide nanoparticle selected from aluminum, cerium, and yttrium and a matrix material.

  In FIG. 1, the typical schematic diagram of the thin film solar cell of this invention is shown.

  In the thin film solar cell in FIG. 1, a transparent electrode layer 3, a photoelectric conversion unit 4, and a back electrode 5 are formed in this order on a transparent substrate 1, and the uneven layer 2 of the present invention has a transparent substrate and a transparent electrode. It is provided between the layers.

  A known transparent material can be used for the transparent substrate 1. Of these, glass, sapphire, etc. are preferably used. Specific examples of the glass include alkali glass, borosilicate glass, and non-alkali glass.

  The thickness of the transparent substrate using glass or sapphire can be arbitrarily selected depending on the purpose of use, but 0.5 to 10.0 mm can be exemplified as a preferable range in consideration of the balance between handling and weight. If it is too thin, the strength will be insufficient and it will be easily broken by impact. On the other hand, if the thickness is too large, the weight becomes heavy and the thickness of the device is affected. Therefore, it is difficult to use the portable device, and it is not preferable in terms of transparency and cost.

  The method for forming the uneven layer 2 on the transparent substrate 1 is not particularly limited, but the imprint technique is the simplest and can be used as a method with high patterning reproducibility. Imprint technology is a method of forming a reverse pattern of a desired pattern in a mold and transferring the pattern of the mold to the substrate to form a pattern on the substrate. Thus, it is possible to form a nano uneven structure. In addition, by setting the temperature of the transparent substrate and the mold, the concavo-convex structure can be formed even when a low melting point material such as a thermoplastic resin or a high melting point material such as glass is used as the transparent substrate.

  The material of the mold is not particularly limited, but a material that is less susceptible to deterioration and deformation due to heat and can withstand multiple imprints is preferable. For example, metal materials such as silicon, nickel, and molybdenum can be used. When curing with light, it is possible to cope with this by using a quartz mold. For example, the mold can be produced by subjecting a single crystal silicon substrate to an alkali treatment to form fine irregularities on the substrate. Surface treatment using a known release agent for the mold reduces burr defects during pattern formation, enables accurate transfer of the concavo-convex structure, and improves the durability of the mold when used multiple times. .

  As the mold, a mold having a general uneven structure can be used. For example, there are a pyramid type, an inverted pyramid type, a cylindrical type, a line & space type, etc., but any shape can be used in the present invention. .

  The uneven layer 2 in the present invention includes nanoparticles 2-1 and a matrix material 2-2. The refractive index of the uneven layer 2 is preferably 1.55 to 2.25 as a value measured at a wavelength of 500 nm, and more preferably 1.60 to 2.05.

  A sol-gel material is preferably used as the matrix material 2-2. The reason is that light can be taken in with almost no optical loss by using a transparent sol-gel material on the light incident side. Here, since the sol-gel material does not have conductivity, that is, is an insulating material, when it is provided on the back electrode side (that is, between the photoelectric conversion layer and the back electrode), the series resistance is significantly increased. Therefore, since all the conductive carriers generated in the photoelectric conversion layer are consumed as heat and cannot be taken out as electricity, it is difficult to use the sol-gel material on the back electrode side.

  On the other hand, when a sol-gel material is used as the uneven layer on the light incident side as in the present invention, it is preferable because photo-induced carriers can be taken out even if the uneven layer is insulating. In addition, since the sol-gel material has a high degree of freedom in designing the concavo-convex shape, it is preferable because the concavo-convex shape suitable for the material and band gap of the photoelectric conversion layer can be designed and realized, and the nanoparticles can be easily dispersed.

  As the sol-gel material, it is preferable to use an inorganic material from the viewpoint of heat resistance and transparency in a wide wavelength region. Among these, it is more preferable to have a material selected from one or more of aluminum oxide, magnesium oxide, silicon oxide, and titanium oxide. By using these materials for the uneven layer, a substrate with a transparent electrode having a high light transmittance can be produced. In particular, it is preferable to use silicon oxide in consideration of compatibility with nanoparticles.

  The refractive index of the matrix material in the present invention is preferably 1.35 to 1.55, and more preferably 1.40 to 1.50.

In the present invention, the nanoparticle 2-1 contained in the uneven layer 2 is a nanoparticle of at least one metal oxide selected from zinc, niobium, titanium, zirconium, aluminum, cerium, and yttrium. And By using the metal oxide nanoparticles, it is easy to disperse in the concavo-convex layer, and the refractive index when the concavo-convex layer is formed can be controlled. In addition, since the metal oxide is highly refractory and transparent, the optimum refractive index can be controlled with a minimum addition amount, and improvement of optical characteristics and accompanying solar cell characteristics can be expected.
Of these, zinc, niobium, titanium, zirconium, and aluminum are preferable from the viewpoints of transparency and productivity. In this case, the transmittance | permeability of a board | substrate with an uneven | corrugated layer improves, As a result, the improvement of the short circuit current of a thin film solar cell can be anticipated. Moreover, by using the above nanoparticles, the transparency of the concavo-convex layer can be increased as compared with that of Patent Document 2 using a coupling agent. This is presumably because there is no absorption derived from CT transition in the visible light region because there is no organic functional group necessarily contained in the coupling agent.

  The refractive index of the nanoparticles is preferably 1.60 to 2.50 at a wavelength of 550 nm in terms of bulk. By using the material having the above refractive index for the nanoparticles, the refractive index of the uneven layer 2 can be in the range of 1.55 to 2.25. By setting it as the refractive index of this range, reflection of the light in each interface of a transparent substrate-a transparent electrode layer can be suppressed, and more light can be permeate | transmitted.

Here, in order to transmit more light, the nanoparticles need to be kneaded and dispersed in the matrix material. When, for example, silicon oxide is used as the matrix material, the coating liquid can be prepared by mixing nanoparticles with the raw material before forming the uneven layer.
As a raw material of silicon oxide, for example, there is polydimethylsiloxane, which is a polar compound, and is preferably used because it can disperse oxide nanoparticles well. Even when a material other than silicon oxide is used as the matrix material, the coating liquid can be similarly produced.

  The added amount of the nanoparticles is preferably 50 to 200 parts by weight, more preferably 75 to 150 parts by weight, and particularly preferably 80 to 120 parts by weight with respect to 100 parts by weight of the matrix material (polydimethylsiloxane in the above). By mixing in this range, it becomes possible to achieve both workability and optical characteristics.

Although the particle diameter of a nanoparticle can be known, for example with a transmission electron microscope (TEM) etc., 10-60 nm is preferable and 20-50 nm is more preferable. By setting it in this range, it becomes possible to obtain good optical characteristics when uniformly dispersed in the uneven layer.
Furthermore, the shape of the nanoparticles is not particularly limited, and may be spherical or polygonal, and a satisfactory uneven layer can be formed as long as the particle diameter is within the above range. The particle diameter referred to here indicates an average ((Lmax + Lmin) / 2) of the longest diameter (Lmax) and the shortest diameter (Lmin). In addition, as the uneven | corrugated layer 2 in this invention, unless the function of this invention is impaired, you may contain other materials other than the said nanoparticle 2-1 and the matrix material 2-2.

  By providing the concavo-convex layer 2 having the above refractive index between the substrate 1 and the transparent electrode layer 3, light is reflected at each interface of the substrate 1 / concavo-convex layer 2 and the concavo-convex layer 2 / transparent electrode layer 3. It becomes possible to suppress. Furthermore, since the uneven shape can suppress total reflection of light at the interface, the light transmittance can be improved as a result. The measurement of the refractive index is not particularly limited, but for example, it can be measured using a spectroscopic ellipsometer.

  The incident angle here is an angle formed by a line segment perpendicular to the reflecting surface (substrate surface) and a line segment parallel to the incident light. Furthermore, by setting the refractive index of the concavo-convex layer to an intermediate value between the substrate and the transparent electrode layer, it is possible to suppress reflection loss at each interface and to use a lot of light for device characteristics.

  Examples of a method for forming the uneven layer 2 on the substrate 1 include application, and the application can be performed by any method such as spin coating, dipping, roll coating, spray coating, and the like. At this time, it is preferable to apply without solvent, but when a solvent is used, a higher alcohol or the like having low volatility at room temperature can be preferably used. Thereby, drying immediately after apply | coating a coating liquid can be prevented, and also a solvent can fully be removed by the heating after uneven | corrugated shape processing.

  After coating, preheating for removing the solvent is preferably performed at the boiling point of the solvent ± 20 ° C. If the temperature is too high, oxidation / curing of the matrix material is promoted and imprinting becomes impossible, which is not preferable. On the other hand, if the temperature is low, the solvent cannot be removed, and the solvent tends to flow after imprinting, which causes the disappearance of the pattern.

  After the coating liquid as described above is applied onto the substrate, the concavo-convex layer 2 can be formed by placing a mold on the substrate and heating and transferring as described above. After imprinting, it can be fired at 200 ° C. to 500 ° C. in the atmosphere. By firing in this way, it is possible to sufficiently cure the uneven layer and remove the solvent.

  As for the uneven | corrugated layer 2 formed as mentioned above, it is preferable that Ra (arithmetic mean roughness) is 100 nm or more and 250 nm or less, and 105 or more and 200 nm or less are more preferable. By setting it as this range, the improvement of the transmittance | permeability of a board | substrate with a transparent electrode can be anticipated. Moreover, in the thin film solar cell using this, it is expected that the amount of light taken into the photoelectric conversion unit is increased.

  The concavo-convex layer 2 preferably has an Rmax (maximum height difference) of 700 nm to 1300 nm, more preferably 750 nm to 1200 nm. By setting it as this range, when it uses for a thin film solar cell, suppression of the contact (short circuit) of a positive electrode and a negative electrode is anticipated.

Although there is Sds as an index representing the number of vertices per unit area, Sds of uneven layer 2 in the present invention is preferably 3.5 [mu] m ^-2 or 14.5 [mu] m ^-2 or less, 5.0 .mu.m ^-2 least 10. 0 μm ⁻² or less is more preferable. By setting it as this range, the improvement of the transmittance | permeability of a board | substrate with a transparent electrode can be anticipated. Moreover, in the thin film solar cell using this, it can be expected that the amount of light taken into the photoelectric conversion unit is increased.

  These indices representing the unevenness can be measured by, for example, AFM. Ra and Rmax can also be calculated by converting a cross-sectional image of SEM or the like into coordinates, and these values almost coincide with the measurement results by AFM.

  Here, the shape of the uneven | corrugated layer 2 in this invention is not specifically limited, For example, shapes, such as a random structure and a periodic structure, are preferable. Such a structure can be easily formed by designing the shape of the mold when formed by the nanoimprint method described above. Among these, a random structure is preferable because photoelectric conversion characteristics can be improved by causing scattering uniformly with respect to light having a wide wavelength.

  The film thickness of the concavo-convex layer is preferably larger than the film thickness that can maintain the concavo-convex shape (difference between the highest point and the lowest point of the concavo-convex shape), and is preferably about 0.5 to 5.0 microns from the optical viewpoint, Furthermore, 0.6 to 2.5 microns are preferable. By setting it as the film thickness of this range, the fault that uneven | corrugated shape cannot be formed when a film thickness is too thin can be eliminated, and the light absorption loss by being too thick can be suppressed. Here, the film thickness of the concavo-convex layer refers to the distance between the apex of the concavo-convex shape and the interface between the substrate and the concavo-convex layer.

  The transparent electrode layer 3 provided on the concavo-convex layer 2 can be used without limitation as long as it exhibits high transparency in the wavelength region of 350 to 1500 nm and is conductive. For example, as a solar cell or an organic EL device When used, it is preferable to use an oxide from the viewpoint of the thermal history during the production thereof. In particular, a transparent conductive oxide mainly composed of zinc oxide, indium oxide, indium-tin composite oxide, indium -Molybdenum complex oxide, indium-titanium complex oxide, etc. can be used. Here, “having zinc oxide as a main component” means that the transparent electrode layer 3 contains more than 50% of zinc oxide, preferably 70% or more. The transparent electrode layer 3 can be formed by forming these oxides with a film thickness of 100 to 2000 nm. By setting it as the film thickness of this range, the transparent conductive layer excellent in electroconductivity and transparency can be formed.

  As a method for forming the transparent electrode layer 3, a vapor deposition method is preferable from the viewpoint of conductivity. Vapor deposition is roughly classified into “chemical vapor deposition (CVD)” and “physical vapor deposition (PVD)”, and either method may be used. Specifically, in the case of CVD, there are organometallic CVD (MOCVD) and plasma CVD by reaction between a vaporized organometallic compound and water or oxygen. In the case of PVD, there are magnetron sputtering in which a transparent electrode material is sputtered with argon ions, pulse laser deposition, reactive ion deposition, and the like, but magnetron sputtering is preferable from the viewpoint of productivity.

  Further, as the transparent electrode layer 3, a plurality of layers having different crystallinity and orientation may be stacked to form finer unevenness on the uneven layer 2. Although it depends on the device design, it is expected that the optical characteristics will be improved in a wider wavelength region than that of the uneven layer 2 alone.

  As described above, by forming the concavo-convex layer and the transparent conductive layer 3 on the substrate 1, the substrate with a transparent electrode in the present invention can be formed.

The substrate with a transparent electrode of the present invention is characterized in that it is used as a transparent electrode layer on the light incident side of a thin film solar cell having at least one photoelectric conversion unit and a back electrode in this order.
The photoelectric conversion unit preferably has a crystalline silicon layer. Specifically, an amorphous silicon solar cell in which an amorphous and / or crystalline silicon semiconductor is laminated on a transparent electrode layer, a thin film polycrystalline silicon solar cell, or a multi-junction type in which both are connected in series When applied to a solar cell, the refractive index sequentially increases from the substrate toward the photoelectric conversion layer, so that reflection loss is reduced and light can be effectively taken into the power generation layer.

  For example, the photoelectric conversion unit 4 can be configured by arranging at least one silicon semiconductor laminated structure in which one unit is composed of a pin junction. The silicon structure used may be a polycrystalline structure or an amorphous structure, and the crystal structure may be different depending on p / i / n. Note that the amorphous or crystalline silicon-based material is not only a case where only silicon is used as a main element constituting a semiconductor, but also an alloy material including elements such as carbon, oxygen, nitrogen, and germanium. Good.

  Each semiconductor layer can be suitably manufactured by a plasma CVD method. The plasma CVD method is a method in which silane gas is used as a silicon material and silicon is formed using plasma energy, and an appropriate amount of gas such as diborane or phosphine is added to the formation of p-type and n-type layers, respectively. This is possible.

  Furthermore, power generation performance can be improved by stacking a plurality of the photoelectric conversion units. When a plurality of photoelectric conversion units are stacked, if a photoelectric conversion unit having a wide band gap in order from the light incident side is provided, incident light can be used effectively, so that an improvement in performance can be expected.

  For example, in the case of a thin-film silicon solar cell, if the first photoelectric conversion unit 4-1 with a wide band gap is disposed on the light incident side, and the second photoelectric conversion unit 4-3 with a narrow band gap is disposed thereon. Good. In this case, a photoelectric conversion unit made of amorphous silicon can be arranged as the first photoelectric conversion unit 4-1, and a photoelectric conversion unit made of microcrystalline silicon can be arranged as the second photoelectric conversion unit 4-3. In addition to the first photoelectric conversion unit 4-1 and the second photoelectric conversion unit 4-3, one or more photoelectric conversion units may be further arranged.

  A transparent conductive intermediate layer 4-2 can be formed between the plurality of photoelectric conversion units, and a layer that selectively reflects and transmits light can be provided. Thereby, in the above example, more light can be taken into the first photoelectric conversion unit 4-1, and the transmitted light can contribute to power generation of the second photoelectric conversion unit 4-3. .

  A layer intended to improve electrical contact can be provided between the transparent electrode layer 3 and the photoelectric conversion unit 4. When a semiconductor layer having a wider band gap than the photoelectric conversion unit is used as this layer, electron-hole recombination in the vicinity of the interface between the transparent electrode layer and the photoelectric conversion layer can be suppressed. As a result, the electron-hole generated in the photoelectric conversion layer can be efficiently taken out to the electrode, and as a result, the conversion efficiency can be improved, which is preferable. An example of such a semiconductor is p-type silicon carbide.

  A back electrode 5 is formed on the photoelectric conversion unit 4 thus provided. As the back electrode 5, for example, as shown in FIG. 2, two layers of a transparent conductive oxide layer 5-1 and a back metal electrode layer 5-2 can be provided. It can also be formed of a layer higher than the layer.

  The transparent conductive oxide layer 5-1 is used to suppress mutual diffusion of silicon forming the photoelectric conversion unit 4 and metal atoms forming the back surface metal electrode layer 5-2. Another reason is that light reflected by “silicon / transparent conductive oxide layer 5-1” and “transparent conductive oxide layer 5-1 / back metal electrode 5-2” causes interference. The transparent conductive oxide layer 5-1 is used to control the wavelength of interference by the film thickness of the transparent conductive oxide layer 5-1, thereby enhancing the light of any wavelength and improving the solar cell characteristics. .

  Here, the transparent conductive oxide layer 5-1 may be a transparent and conductive transparent conductive oxide. For example, a layer containing indium oxide, zinc oxide, titanium oxide, or the like can be used.

  The back metal electrode layer 5-2 is preferably highly conductive and has a high reflectivity in order to reflect light that has passed through the photoelectric conversion unit 4 and enter the photoelectric conversion unit 4 again. Examples of such a material include silver and aluminum.

  The transparent conductive oxide layer 5-1 is preferably provided with a film thickness in the range of 25 to 120 nm. Furthermore, the range of 30 to 85 nm is optically preferable. By setting the film thickness within this range, not only is it preferable in terms of optical effects, conductivity and cost, but also metal atoms used for the back surface metal electrode layer 5-2 and silicon atoms forming the photoelectric conversion unit 4 This is preferable because it can serve as a barrier layer that suppresses atomic diffusion.

  As described above, a thin film solar cell using the substrate with a transparent electrode in the present invention can be produced.

  As described above, by forming each layer by, for example, the CVD method, the unevenness difference of each layer is reduced each time the film is formed. That is, the unevenness difference after forming the back electrode can be made smaller than the unevenness difference of the uneven layer. Here, the “concavo-convex difference” means Ra and Rmax.

  The surface shape after the formation of the back metal electrode is preferably such that Ra is 5 to 150 nm. By setting it in this range, it is possible to efficiently reflect light on the back metal side and to incorporate light into the photoelectric conversion unit. Furthermore, it is preferable that it is 10-80 nm. As for the surface shape after forming the back surface metal electrode, Rmax is preferably 300 to 1000 nm, and more preferably 400 to 800 nm.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. In the present invention, a resistivity meter Loresta GP MCT-610 (manufactured by Mitsubishi Chemical Corporation) was used for the surface resistance measurement. The film thickness of the transparent conductive oxide layer was a spectroscopic ellipsometer VASE (manufactured by JA Woollam). Fitting was performed using the Chaucy model. The thickness of the concavo-convex layer was the distance from the interface between the substrate and the concavo-convex layer to the top.

Example 1
The mold for forming an uneven structure was produced according to the method described in “Yoichiro Nishimoto, Surface Technology, Vol. 56, No. 1 (2005)”. Specifically, 200 g of isopropyl alcohol was added to an aqueous solution obtained by dissolving 100 g of potassium hydroxide in 1700 g of pure water to prepare a wet etching solution. The liquid was heated to 70 ° C., and a single crystal silicon wafer (100 surface) was added while stirring with a magnetic stirrer and immersed for 30 seconds. After removal, the mold for forming an uneven structure was prepared by washing with pure water and drying. The uneven structure produced in this way was a random quadrangular pyramid structure. When this mold was observed by AFM, it was Ra: 110 nm and Rmax: 800 nm.

The coating liquid for forming the concavo-convex layer was prepared by adding 200 g of titanium oxide (TiO ₂ ) as nanoparticles to 200 g of polydimethylsiloxane as a matrix material and dispersing the nanoparticles with sufficient stirring.

  For the substrate, non-alkali glass (film thickness 0.7 mm, trade name OA-10, manufactured by Nippon Electric Glass Co., Ltd.) was applied, and the coating solution was applied onto the substrate and the mold was pressed against it. Pressure was applied, heated to 200 ° C. and held for 10 minutes. In this way, an uneven layer 2 (film thickness 600 nm) was formed on the substrate.

A transparent electrode layer 3 was formed on the uneven layer. As the transparent electrode layer 3, fluorinated tin oxide (F: SnO ₂ ) produced by a thermal CVD method was used. The film thickness of the transparent electrode layer 3 at this time was 800 nm, and the sheet resistance was 10Ω / □.

  On top of this, using a high frequency plasma CVD apparatus, a boron-doped p-type silicon carbide (SiC) layer is 10 nm, a non-doped amorphous silicon photoelectric conversion layer is 200 nm, and a phosphorus-doped n-type μc-Si layer is formed. The film was formed with a film thickness of 20 nanometers. As a result, a first photoelectric conversion unit 4-1 (top cell) made of amorphous silicon having a pin junction as a front photoelectric conversion unit was formed.

The transparent conductive intermediate layer 4-2 made of a conductive oxygenated silicon layer was formed by a high-frequency plasma CVD apparatus without taking the substrate on which the first photoelectric conversion unit 4-1 was formed into the atmosphere. Regarding the film forming conditions at this time, the plasma excitation frequency was 13.56 MHz, the substrate temperature was 150 ° C., and the reaction chamber pressure was 666 Pa. SiH ₄ , PH ₃ , CO ₂ , and H ₂ were used as source gases introduced into the plasma CVD reaction chamber. Under the above conditions, a 600 Å conductive oxygenated silicon layer 4-2 was formed.

  Further, a boron-doped p-type microcrystalline silicon layer having a thickness of 15 nanometers, a non-doped crystalline silicon photoelectric conversion layer having a thickness of 1500 nanometers, and a phosphorus-doped n-type microcrystalline silicon layer having a thickness of 20 nanometers are manufactured by plasma CVD. Filmed. Thereby, the 2nd photoelectric conversion unit 4-3 (bottom cell) which consists of crystalline silicon of the pin junction which is a back photoelectric conversion unit was formed.

  After the second photoelectric conversion unit 4-3 formed work in process is taken out from the high-frequency plasma CVD apparatus to the atmosphere, it is introduced into the film forming chamber of the high-frequency magnetron sputtering apparatus, and the second photoelectric conversion unit 4-3 A transparent conductive oxide layer 5-1 was formed thereon.

The transparent conductive oxide layer 5-1 was formed with a film forming pressure of 0.2 Pa, a substrate temperature of 150 ° C., and a substrate / target distance of 60 mm. The film thickness was 80 nm. Subsequently, an Ag film having a film thickness of 250 nanometers was formed as the metal electrode layer 5-2 using a vacuum deposition apparatus. The degree of vacuum during film ^formation was 1 × 10 ⁻⁴ Pa or less, and the film formation rate was 0.2 ± 0.02 nanometer / second.

  Thus, a thin film solar cell was produced. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 65 nm, Rmax: 500 nm.

(Example 2)
A thin-film solar cell was produced in the same manner as in Example 1 except that zirconium oxide (ZrO ₂ ) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 70 nm and Rmax: 500 nm.

(Example 3)
A thin-film solar cell was prepared in the same manner as in Example 1 except that niobium oxide (Nb ₂ O ₅ ) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 80 nm, Rmax: 600 nm.

Example 4
A thin-film solar cell was prepared in the same manner as in Example 1 except that aluminum oxide (Al ₂ O ₃ ) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 40 nm, Rmax: 550 nm.

(Example 5)
A thin-film solar cell was produced in the same manner as in Example 1 except that yttrium oxide (Y ₂ O ₃ ) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 40 nm, Rmax: 600 nm.

(Example 6)
A thin-film solar cell was prepared in the same manner as in Example 1 except that cerium oxide (CeO ₂ ) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 70 nm, Rmax: 600 nm.

(Example 7)
A thin-film solar cell was prepared in the same manner as in Example 1 except that zinc oxide (ZnO) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 60 nm, Rmax: 600 nm.

(Comparative Example 1)
A thin-film solar cell was prepared in the same manner as in Example 1 except that silicon oxide (SiO ₂ ) was used as the nanoparticles forming the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 50 nm and Rmax: 700 nm.

(Comparative Example 2)
A thin-film solar cell was produced in the same manner as in Example 1 except that the nanoparticles forming the uneven layer were not used, that is, the uneven layer was formed using only the matrix material. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, it was Ra: 40 nm, Rmax: 500 nm.

(Comparative Example 3)
A thin-film solar cell was produced in the same manner as in Example 1 except that the transparent electrode layer 2 was formed directly on the alkali-free glass substrate 1 without providing the uneven layer. When the surface shape of the thin film solar cell on the back electrode side was measured by AFM, Ra was 10 nm and Rmax was 400 nm.

  Table 1 shows the nanoparticles used, and the refractive index, film thickness, and surface shape of the uneven layer. Table 2 shows the photoelectric conversion characteristics of the thin film solar cell.

  From Table 2, when Example is compared with Comparative Examples 2 and 3, the current (Isc) is particularly high among the solar cell characteristics as compared with Comparative Example 2 that does not include nanoparticles and Comparative Example 3 that does not have an uneven layer. became. This is presumably because the reflection of light at each interface between the substrate and the concavo-convex layer and between the concavo-convex layer and the transparent electrode layer was reduced, and the amount of light taken into the photoelectric conversion unit was increased.

  Further, when Example and Comparative Example 1 were compared, a difference was observed in the short-circuit current (Isc) regardless of whether they contained nanoparticles. This is considered to be due to the difference in refractive index of the particles added to the uneven layer. That is, in each example in which a concave / convex layer having a refractive index intermediate to that of the transparent substrate and the transparent electrode layer is provided, reflection at each interface between the substrate / the concave / convex layer and the concave / convex layer / transparent electrode layer can be suppressed. In Comparative Example 1, the refractive index of the concavo-convex layer is as small as 1.5, that is, the refractive index difference between the concavo-convex layer and the transparent electrode layer is large, reflection at the interface is large, and the amount of light taken into the photoelectric conversion layer is small. This is considered to be because it is less than the embodiment.

  As described above, it was found that the light capturing efficiency of the thin-film solar cell is improved by using the uneven layer of the present invention.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Uneven layer 2-1 Nanoparticle 2-2 Matrix layer 3 Transparent electrode layer 4 Photoelectric conversion unit 4-1 First photoelectric conversion unit 4-2 Transparent conductive intermediate layer 4-3 Second photoelectric conversion unit 5 Back electrode 5-1 Transparent conductive oxide layer 5-2 Back metal electrode layer

Claims (5)

A thin-film solar cell having an uneven layer, a transparent electrode layer, at least one photoelectric conversion unit, and a back electrode in this order on a substrate,
The thin-film solar cell, wherein the uneven layer includes at least one metal oxide nanoparticle selected from zinc, niobium, titanium, zirconium, aluminum, cerium, and yttrium, and a matrix material.   The thin film solar cell according to claim 1, wherein the matrix material is formed of a sol-gel material.   The thin film solar cell according to claim 1, wherein the uneven layer has a refractive index in a range of 1.55 to 2.25.   The thin film solar cell according to any one of claims 1 to 3, wherein the nanoparticles have a particle size in a range of 10 to 60 nm.   The thin film solar cell according to claim 1, wherein the photoelectric conversion unit has a non-single crystal silicon layer.