JPWO2011158724A1 - Thin film solar cell - Google Patents

Abstract

It is an object to provide a thin film solar cell with further improved power generation characteristics. The thin film solar cell 10 includes a transparent insulating substrate 1, a transparent electrode layer 2, a photoelectric conversion unit 3, a transparent conductive oxide layer (A) 4 mainly composed of zinc oxide, and a back electrode 5 from the light incident side. It has a structure arranged in this order. The transparent conductive oxide layer (A) 4 is composed of a transparent conductive oxide layer (B) containing conductive impurities mainly composed of silicon, and a conductive material mainly composed of elements other than silicon (such as aluminum). It has at least two layers of the transparent conductive oxide layer (C) containing impurities. The film thickness of the transparent conductive oxide layer (B) accounts for 30% to 80% of the film thickness of the transparent conductive oxide layer (A).

Description

  The present invention relates to a thin film solar cell. The present invention improves the characteristics of a thin-film solar cell by using a transparent conductive oxide layer suitable for a thin-film solar cell, particularly a transparent conductive oxide layer that provides good electrical bonding between a semiconductor layer and a back electrode. To achieve.

  As a thin film solar cell, a thin film silicon formed in a pin structure having an intrinsic silicon (i-type) semiconductor sandwiched between a p-type or n-type doped silicon semiconductor such as amorphous silicon or thin-film polycrystalline silicon. Solar cells and chalcopyrite thin film solar cells using compound semiconductors such as copper-indium-selenium (CIS type) and copper-indium-gallium-selenium (CIGS type) are known and widely researched and developed. ing. Research on solar cells using wide-gap semiconductors, organic thin-film solar cells, and the like has also been vigorously conducted.

  In such a thin film solar cell, in order to use light incident on the photoelectric conversion unit more effectively, a back electrode made of a metal material having a high light reflectance is formed. Most of the light that is transmitted without being absorbed by the photoelectric conversion unit is reflected by the back electrode, and is re-incident on the photoelectric conversion unit, whereby photoelectric conversion is performed.

  The back electrode is formed of a metal material having a high light reflectance such as silver or aluminum, whereby the amount of light taken into the photoelectric conversion unit can be improved. Generally, by providing a metal oxide layer between the back electrode and the photoelectric conversion unit, it is possible to reduce parasitic absorption derived from the nanostructure of the back electrode when the back electrode is directly formed on the photoelectric conversion unit. Known to.

  In addition, there is a concern about performance deterioration caused by diffusion of atoms constituting the back electrode material into the photoelectric conversion unit. In order to solve these problems, a technique of arranging a metal oxide layer or the like between the photoelectric conversion unit and the back electrode has been proposed (for example, Patent Document 1).

  As the metal oxide, a transparent conductive oxide such as ITO or an inorganic dielectric compound such as silicon oxide is used.

  Such a transparent conductive oxide layer is formed by a chemical vapor deposition method (MOCVD method) using an organic metal or a process using atomic diffusion by annealing, so that the manufacturing equipment tends to be complicated, and production There is a problem in sex and cost. In addition, the transparent conductive oxide layer has problems that productivity is difficult due to a dopant concentration gradient in ZnO and that wet heat durability is poor.

  The transparent conductive oxide has an absorption loss in the material, and a great improvement in power generation characteristics cannot be expected. On the other hand, an inorganic dielectric compound has a problem in electrical conductivity, and electrical bonding may be deteriorated and power generation characteristics may be deteriorated. In particular, when silicon oxide is used as the inorganic dielectric compound, since silicon oxide is a dielectric, there is a concern that the series resistance of the solar cell increases and the curve factor of the solar cell characteristics deteriorates.

  Patent Document 2 describes a technology in which ZnO used as a transparent electrode on the light incident side is simultaneously doped with a trivalent atom such as aluminum and silicon. In that example, aluminum and silicon are doped in approximately the same amount. However, when this composite oxide is used as a transparent conductive oxide layer on the back electrode side, it is considered that transparency deteriorates due to the coexistence of a plurality of dopants, and it is difficult to improve conversion efficiency. .

  Patent Document 3 describes a thin film solar cell having a two-layer translucent conductive film made of a metal oxide material or the like on the back electrode side. In this thin film solar cell, the short circuit current is reduced and the light confinement effect is improved.

  Patent Document 4 discloses that plasma treatment is performed on the surface of a transparent conductive oxide layer (zinc oxide) having a first impurity in an atmosphere containing a dopant gas having a second impurity and a hydrocarbon gas. It is described that a transparent conductive oxide layer of substantially two layers is formed, thereby improving transparency and conductivity.

  Patent Document 5 describes that the ZnO dopant between the back metal electrode and the photoelectric conversion semiconductor layer exhibits a concentration profile in the film thickness direction. In this method, sputtering is performed using a gas containing dopant atoms as a carrier gas for sputtering.

JP 2007-266095 A JP 2002-217429 A JP 2003-179241 A International Publication No. 2009/116467 Japanese Patent No. 2962897

  However, although Patent Document 3 describes that ZnO is used as the two-layer light-transmitting conductive film and a doping element is used as necessary, the specific doping element and the amount thereof are not specified.

  In Patent Document 4, although transparency and conductivity are improved, carbon is present on the outermost surface (that is, the back surface side of the two transparent conductive oxide layers), and the series resistance is optimized. From this point of view, there is still room for improvement.

  In view of the above situation, an object of the present invention is to provide a thin-film solar cell with further improved power generation characteristics.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies. As a result, by providing a transparent conductive oxide layer mainly composed of zinc oxide and doped with silicon between the photoelectric conversion layer and the back electrode of the thin-film solar cell, the parasitic caused by the back electrode It was found that absorption can be suppressed, and as a result, the amount of light taken into the photoelectric conversion layer can be increased, and the amount of generated current can be improved.

  In addition to the transparent conductive oxide layer doped with silicon as described above, a transparent conductive oxide layer mainly composed of zinc oxide, which is doped with aluminum, is used on the back electrode side, thereby achieving high transparency. It has been found that the power and conductivity can be balanced at a high level, and as a result, the power generation characteristics can be improved. Furthermore, by adopting this configuration, the electrical connection between the transparent conductive oxide layer and the semiconductor layer and the electrical connection between the transparent conductive oxide layer and the back electrode are improved, improving the performance of the solar cell. I found out that I can do it.

  That is, the present invention relates to the following.

One aspect of the present invention is a thin film solar cell in which a transparent electrode layer made of a transparent conductive oxide, at least one photoelectric conversion unit, and a back electrode are arranged in this order from the light incident side,
Between the photoelectric conversion unit and the back electrode, a transparent conductive oxide layer (A) mainly composed of zinc oxide is formed,
The transparent conductive oxide layer (A) includes a transparent conductive oxide layer (B) containing conductive impurities mainly composed of silicon and a transparent containing conductive impurities mainly composed of elements other than silicon. A conductive oxide layer (C),
The thin film solar cell is characterized in that the film thickness of the transparent conductive oxide layer (B) occupies 30% to 80% of the film thickness of the transparent conductive oxide layer (A).

  Here, “having zinc oxide as a main component” in the transparent conductive oxide layer (A) means that the component that forms the transparent conductive oxide layer contains more than 50% of zinc oxide. .

  Here, “having silicon as a main component” in the conductive impurities of the transparent conductive oxide layer (B) means that 85% or more of the conductive impurities are silicon atoms. Similarly, “having an element other than silicon as the main component” in the conductive impurity of the transparent conductive oxide layer (C) means that 85% or more of the conductive impurity is an atom other than silicon.

Preferably, the content of conductive impurities mainly composed of silicon in the transparent conductive oxide layer (B) is 1 to 8% by weight, and the X-ray photoelectron spectroscopy of the transparent conductive oxide layer (B) Silicon is substantially tetravalent as seen from the Si2P1 _{/ 2} peak.

  Preferably, the transparent conductive oxide layer (A) has a thickness of 10 to 120 nm.

  Preferably, the conductive impurity in the transparent conductive oxide layer (C) is mainly composed of aluminum.

  Preferably, the transparent conductive oxide layer (B) is disposed in contact with the n-type semiconductor layer of the layers constituting the photoelectric conversion unit, and the transparent conductive oxide layer (C) is disposed in contact with the back electrode. ing.

  With this configuration, the conductive carrier generated by the photoelectric conversion unit can be taken out without loss.

  Preferably, the transparent conductive oxide layer (B) contains 1 to 5% by weight of silicon oxide as a conductive impurity, and the transparent conductive oxide layer (C) contains 1 to 7 aluminum oxide as a conductive impurity. Contains by weight.

  Preferably, the film thickness of the transparent conductive oxide layer (B) accounts for 60% to 80% of the film thickness of the transparent conductive oxide layer (A).

  Preferably, the conductive impurities in the transparent conductive oxide layer (C) are mainly composed of one or more selected from the group consisting of aluminum, gallium and boron.

  Preferably, the photoelectric conversion unit is made of non-single crystal silicon made of a pin type junction.

  Preferably, the power generation efficiency retention rate is 90% or more when treated in an environment of 85 ° C. and 85% relative humidity for 1000 hours.

  According to the present invention, the electrical junction between the photoelectric conversion unit and the transparent conductive oxide layer and the electrical junction between the transparent conductive oxide layer and the back electrode are good, and as a result, the photovoltaic Conversion characteristics can be improved.

It is sectional drawing which represents typically the laminated structure of the thin film solar cell which concerns on one Embodiment of this invention. It is an X-ray photoelectron spectroscopy spectrum in Example 1-1 and Comparative Example 1-4.

  One aspect of the present invention is “a thin film solar cell in which a transparent electrode layer made of a transparent conductive oxide, at least one photoelectric conversion unit, and a back electrode are arranged in this order from the light incident side, A transparent conductive oxide layer (A) mainly composed of zinc oxide is formed between the photoelectric conversion unit and the back electrode, and the transparent conductive oxide layer (A) is composed mainly of silicon. A transparent conductive oxide layer (B) containing a conductive impurity and a transparent conductive oxide layer (C) containing a conductive impurity containing an element other than silicon as a main component. The thin film solar cell is characterized in that the thickness of the conductive oxide layer (B) occupies 30% to 80% of the thickness of the transparent conductive oxide layer (A).

  The solar cell functions as a power generator by taking out the conductive carrier excited by the light entering the photoelectric conversion unit through the electrode. However, conductive carriers may cause recombination at the interface of the laminate, which is one factor that reduces power generation efficiency. Further, there is a loss due to consumption of heat as the conductive carriers transition to the interface state. In this invention, the transparent conductive oxide layer of an above-described structure is arrange | positioned between a photoelectric conversion unit and a back surface electrode. As a result, the bonding at the interface of the photoelectric conversion unit / transparent conductive oxide layer and the bonding at the interface of the transparent conductive oxide layer / back surface electrode are improved, and recombination of conductive carriers at the interface is suppressed, It is thought that the loss due to can be suppressed.

  Hereinafter, typical embodiments of the substrate with a transparent electrode according to the present invention will be described. FIG. 1 schematically shows a cross-sectional structure of a thin film solar cell according to an embodiment of the present invention. In the thin film solar cell 10 of the present embodiment, the transparent electrode layer 2, the photoelectric conversion unit 3, the transparent conductive oxide layer (transparent conductive oxide layer (A)) 4 and the back electrode 5 are arranged in this order on the substrate 1. It has an arranged structure. Furthermore, the photoelectric conversion unit 3 is formed by the first photoelectric conversion unit 3-1, the transparent conductive intermediate layer 3-2, and the second photoelectric conversion unit 3-3.

  For the substrate 1, a known transparent material can be used. For example, it is preferable to use a transparent insulating material, and among these, it is more preferable to use glass, sapphire, or the like. Specific examples of the glass include alkali glass, borosilicate glass, and non-alkali glass.

  The thickness of the substrate 1 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. In the case of 0.5 mm or more, cracks due to insufficient strength can be prevented. Moreover, when it is 10.0 mm or less, since it is lightweight, it can be used for portable devices and the like, which is preferable in terms of transparency and cost.

  A known transparent electrode material can be used for the transparent electrode layer 2. Examples of the transparent electrode material include transparent conductive oxides such as indium oxide, tin oxide and composite oxides thereof, and zinc oxide. When the transparent electrode layer 2 is used as a transparent electrode for a thin film silicon solar cell, fluorinated tin oxide, zinc oxide, or the like is preferably used among the above because of resistance to hydrogen plasma.

  It is particularly important that the transparent electrode layer 2 is transparent and has high conductivity. In order to achieve both of these, the transparent electrode material preferably has high crystallinity. By increasing the crystallinity, the transparent electrode layer 2 having high transparency can be obtained. Further, the higher the electrical conductivity, that is, the lower the sheet resistance, the better. However, 5 to 30Ω / □ is preferable from the viewpoint of balance with transparency and as a result that a solar cell with good performance can be manufactured.

  The manufacturing method of the transparent electrode layer 2 may be any method as long as it can achieve transparency and conductivity. Preferably, a technique such as a wet process or a dry process can be employed. Among these, metal organic chemical vapor deposition (MOCVD) using a reaction between an organometallic compound and water is more preferable because a transparent electrode layer with good crystallinity can be formed.

  Furthermore, when using a transparent conductive oxide as the transparent electrode layer 2, it is preferable to form a texture on the surface of the transparent electrode layer by controlling the crystal orientation of the transparent conductive oxide. As a result, the light confinement efficiency in the photoelectric conversion layer can be increased, and as a result, the power generation characteristics can be improved.

  For example, the photoelectric conversion unit 3 can be configured by arranging at least one silicon semiconductor laminated structure in which one unit is 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 produced by, for example, a plasma CVD method. That is, a silicon semiconductor layer can be formed using plasma energy by using silane gas as a silicon material. When a p-type layer or an n-type layer is formed, an appropriate amount of impurity gas such as diborane or phosphine may be added to the silane gas.

  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. The photoelectric conversion unit 3 in the present embodiment has two photoelectric conversion units, that is, a first photoelectric conversion unit 3-1 and a second photoelectric conversion unit 3-3.

  For example, when the thin film solar cell 10 is a thin film silicon solar cell, the first photoelectric conversion unit 3-1 with a wide band gap is arranged on the light incident side, and the second photoelectric conversion unit 3 with a narrow band gap is formed thereon. -3 may be arranged. In this case, a photoelectric conversion unit made of amorphous silicon can be arranged as the first photoelectric conversion unit, and a photoelectric conversion unit made of microcrystalline silicon can be arranged as the second photoelectric conversion unit. Further, three or more photoelectric conversion units may be arranged.

  A transparent conductive intermediate layer 3-2 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 3-1, and the transmitted light can contribute to the power generation of the second photoelectric conversion unit 3-3. . As the transparent conductive intermediate layer 3-2, for example, silicon oxide containing non-oxidized silicon can be used.

  Between the transparent electrode layer 2 and the photoelectric conversion unit 3, a layer for the purpose of improving electrical contact can be provided. As this layer, a semiconductor layer having a wider band gap than the photoelectric conversion unit is preferably used. Because this suppresses electron-hole recombination in the vicinity of the interface between the transparent electrode layer and the photoelectric conversion layer, the electron-hole generated in the photoelectric conversion layer can be efficiently extracted to the electrode. As a result, the conversion efficiency can be improved. An example of such a semiconductor is p-type silicon carbide.

  As the back electrode 5, it is preferable to form at least one metal layer made of at least one material selected from Al, Ag, Au, Cu, Pt and Cr by sputtering or vapor deposition.

  The film thickness of the back electrode 5 is not particularly limited, but is preferably 200 to 1000 nm (2000 to 10,000 mm). A thickness of 200 nm (2000 mm) or more is preferable because sufficient conductivity can be secured and a function as a light reflecting material important as a back electrode can be sufficiently achieved. On the other hand, the case of 1000 nm (10000 mm) or less is preferable from the viewpoint of cost and the like.

  The thin film solar cell of the present invention is characterized by the structure of the transparent conductive oxide layer (A). Details will be described below.

  The transparent conductive oxide layer (transparent conductive oxide layer (A)) 4 shown in FIG. 1 is a transparent and conductive layer mainly composed of zinc oxide. The transparent conductive oxide layer 4 is formed of two or more layers having a discontinuous composition. Specifically, a transparent conductive oxide layer (B) containing conductive impurities mainly composed of silicon, and a transparent conductive oxide layer containing conductive impurities mainly composed of elements other than silicon ( C) having at least two layers. That is, the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C) have different main components of the conductive impurities contained therein. The conductive impurities can be imparted with conductivity by being ionized in the transparent conductive oxide.

  These conductive impurities are added separately from “minimum impurities”. Here, “minimum impurities” refers to atoms that are unintentionally contained in the target manufacturing process, impurity atoms derived from equipment in the sputtering film forming process, and photoelectric conversion units and back electrodes of thin film solar cells. This represents a minute amount of atomic diffusion of 0.1% or less. The minimum amount of the above-mentioned minimum impurities is preferable, and the concentration is more preferably 0%, that is, not included.

  As described above, “having zinc oxide as a main component” means containing more than 50% of zinc oxide among the components forming the transparent conductive oxide layer 4. It is particularly preferable to contain 70% or more of zinc oxide. Here, as long as the function of the thin film solar cell of the present invention is not impaired, a composite oxide containing a transparent conductive oxide such as indium oxide, tin oxide and titanium oxide in addition to zinc oxide as a main component should be used. However, it is more preferable to use a transparent conductive oxide layer which is all zinc oxide except for conductive impurities. In this case, formation of the transparent conductive oxide layer 4 having a high transmittance in a wide wavelength region can be expected.

  The transparent conductive oxide layer (B) contains a conductive impurity mainly composed of silicon. As described above, “having silicon as a main component” means that 85% or more of the conductive impurities are silicon atoms. Preferably, 90% or more, more preferably 95% or more of the conductive impurities are silicon atoms, more preferably 100% are silicon atoms, that is, the transparent conductive oxide layer (B) is substantially only silicon atoms. Is contained as a conductive impurity. By using conductive impurities containing silicon in these ranges, the transparent conductive oxide layer 4 excellent in conductivity and transparency and the thin film solar cell 10 excellent in wet heat durability can be produced. In addition, aluminum and gallium are mentioned as components (components other than a main component) other than silicon in a conductive impurity.

  The content of conductive impurities mainly composed of silicon in the transparent conductive oxide layer (B) is not particularly limited, but usually silicon oxide is 1.0 to 8.0% by weight, preferably Is contained in an amount of 1.5 to 6.0% by weight, more preferably 1.5 to 5.0% by weight, and still more preferably 2.0 to 4.5% by weight. By introducing silicon oxide, it is possible to improve wet heat durability during practical use. That is, it is considered that by introducing silicon oxide, it is possible to improve the wet heat durability of the thin film solar cell by applying an appropriate strain to the crystal structure and balancing the residual stress. When silicon oxide is added to zinc oxide, it exhibits substantially the same function as that in which silicon is doped as a conductive impurity. When the doping amount of silicon atoms is less than the above range, the effect of improving wet heat durability is reduced. On the other hand, when the doping amount of silicon atoms is large, the conductivity is remarkably lowered and becomes resistance, so the power generation efficiency is lowered. This is not preferable.

  The doping amount of silicon can be detected by various elemental analysis methods. For example, the number of atoms can be accurately counted by secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), or the like.

  In the film formation of the transparent conductive oxide layer (B), the atomic composition of the sputter target is equal to the atomic composition of the transparent conductive oxide layer (B) to be formed. It is known from elemental analysis such as (EDS). Therefore, the doping amount of silicon atoms can be controlled by the atomic composition ratio of the sputter target. The atomic composition ratio of the sputter target can be controlled by kneading and sintering zinc oxide and silicon or silicon oxide according to the composition ratio at the time of producing the sintered body.

  Silicon in the transparent conductive oxide layer (B) is preferably substantially tetravalent. Details about the effect of tetravalent silicon on durability in particular are unclear, but supply of a stable conductive carrier when silicon becomes tetravalent, the tetravalence of silicon being a stable oxidation number, Is estimated to be effective. In addition, in the case of tetravalent silicon, all valence electrons become conductive carriers, and therefore, electron transition in the visible light region is reduced, and as a result, transparency is excellent.

Such oxidation number of silicon can be detected by X-ray photoelectron spectroscopy (XPS). Although a general X-ray source can be arbitrarily used, it is preferable to use AlKα or MgKα because a wide energy range can be measured with high resolution. In XPS, the kinetic energy of photoelectrons generated by incident X-rays is detected in normal measurement, but the difference from incident X-ray energy, for example, about 1254 eV in MgKα, is evaluated by the binding energy. Focusing on the Si2P1 _{/ 2} peak, it is expected that the Si0-valent peak is detected at a binding energy of about 100 eV. In this case, the Si 3 valence is detected at about 103 eV, and the Si 4 valence peak is detected at about 104 eV (FIG. 2).

Here, “substantially tetravalent” means that the ratio of Si2P _1/2 tetravalent to trivalent (Si ⁴⁺ / Si ³⁺ ) measured by XPS is 0.5 or more. By taking this value, silicon has a stable oxidation number, and a transparent conductive oxide excellent in transparency and durability, particularly wet heat durability, can be obtained. As shown in FIG. 2, such a ratio draws a linear baseline when the spectrum binding energy measured by XPS is around 100 to 108 eV, and is 103 eV for Si ^{3+ and} 104 eV for Si ⁴⁺ . The ratio (Si ⁴⁺ / Si ³⁺ ) is calculated by taking the difference value obtained by subtracting the baseline value from each measured value as the peak values, ie, Si ⁴⁺ and Si ^3+. You can ask for it.

  The transparent conductive oxide layer (transparent conductive oxide layer (A)) 4 is a transparent conductive material containing conductive impurities mainly composed of elements other than silicon in addition to the transparent conductive oxide layer (B). The oxide layer (C) is included. Examples of the “element other than silicon” that is the main component of the conductive impurity include aluminum, gallium, and boron. Specifically, the transparent conductive oxide layer (C) can be formed from zinc oxide to which aluminum oxide, gallium oxide, boron oxide or the like is added as a conductive impurity. Only one element other than silicon may be used, or two or more elements may be used. Due to the presence of the transparent conductive oxide layer (C), the conductivity of the transparent conductive oxide layer 4 is increased, and the electrical contact between the transparent conductive oxide layer 4 and the photoelectric conversion unit 3 or the back electrode 5 is good. It is possible to become.

  As described above, the transparent conductive oxide (B) and the transparent conductive oxide layer (C) are laminated with a discontinuous composition. By setting it as such a structure, the electrical conductivity and transparency are compatible and a thin film solar cell with a high characteristic can be produced. Here, the “discontinuous composition” means that the interface between the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C) is a boundary, excluding mutual conductive impurities and atomic diffusion of impurities. , Which means that the composition is different. The transparent conductive oxide layer (A) having such a structure is produced by sputtering the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C) using different targets. it can.

Thickness d _B of the transparent conductive oxide layer (B) accounts for 30% to 80% of the transparent conductive oxide layer (transparent conductive oxide layer (A)) 4 having a thickness d _A. More preferably, it is 45 to 80%. By setting it as these ranges, the electroconductivity and transparency are compatible and a thin film solar cell with a high characteristic can be produced.

The film thickness d _A of the transparent conductive oxide layer 4 can be freely designed according to the required optical effect and conductivity, but is preferably 10 to 120 nm (100 to 1200 mm), more preferably 25 to 110 nm ( 250 to 1100 Å), more preferably 30 to 110 nm (300 to 1100 Å), and particularly preferably 40 to 110 nm (400 to 1100 Å).

When the film thickness d _A is 10 nm (100 mm) or more, the optical characteristics are good, and the metal material forming the back electrode is less likely to diffuse into the photoelectric conversion unit, resulting in power generation characteristics. Becomes better.

Here, the transparent conductive oxide layer containing zinc oxide as a main component is characterized by having higher transparency than the transparent conductive oxide layer containing other components as a main component. Light is absorbed in the region of 3 eV or more) and 1 eV or less which is a free electron reflection / absorption region, and the amount of absorption increases as the film thickness increases. Therefore, it is preferable that the film thickness d _A is 120 nm (1200 mm) or less because absorption loss or the like of the transparent conductive oxide layer 4 hardly occurs and the optical characteristics and performance are considered to be good. On the other hand, when the film thickness d _A is greater than 120 nm, not only the optical characteristics but also the effect of light absorption by the transparent conductive oxide layer 4 itself is increased, which is not preferable.

  “Optical characteristics” as used herein refers to interference of light reflected at the interface of the photoelectric conversion unit / transparent conductive oxide layer and the interface of the transparent conductive oxide layer / back electrode, and the required wavelength. This is a characteristic necessary for incorporating the light into the photoelectric conversion unit. Such light interference is a characteristic necessary for incorporating light of a necessary wavelength into the photoelectric conversion unit.

  Each transparent conductive oxide layer can be formed by chemical vapor deposition using plasma or heat in addition to physical deposition by a magnetron sputtering method or various vapor deposition methods. Among these, the magnetron sputtering method is preferable because it can be easily formed into a film. The target used for magnetron sputtering is preferably a material having the same composition as the transparent conductive oxide layer to be deposited. At this time, the film can be formed by ionizing a carrier gas such as argon gas and sputtering the target. In addition to argon gas, a reactive gas such as oxygen or hydrogen, an inert gas such as nitrogen, or a mixed gas thereof can be used as the carrier gas.

  When the transparent conductive oxide layer is formed by sputtering as described above, the power source used for sputtering can be a high frequency power source in addition to a direct current (DC) power source or a low frequency (AC) power source. The high frequency power source has a frequency such as RF or VHF, and the transparent conductive oxide layer 4 can be suitably formed at any frequency.

In the case of the transparent conductive oxide layer (B), the power density at the time of film formation is preferably 1.5 to 15.0 W / cm ² , more preferably 3.5 to 12.0 W / cm ² , especially among them. 7.0-12.0 W / cm < ² > is preferable. The transparent conductive oxide layer (B) may affect the solar cell characteristics of the present invention depending on the power density at the time of film formation. When the power density is low, the film formation speed is not improved and the crystallinity is increased. The wet heat durability may be poor. On the other hand, when the power density is too high, the transparent conductive oxide layer is re-sputtered by oxygen ions generated in the plasma, which may result in a substrate with a transparent electrode having poor transparency and conductivity. Absent.
The power application method may be a continuous wave or a pulse wave, and can be arbitrarily determined according to the optimum conditions of the film forming apparatus.

  On the other hand, layers other than the transparent conductive oxide layer (B) such as the transparent conductive oxide layer (C) can be formed at an arbitrary power density. In this case, unlike the transparent conductive oxide layer (B), the possibility of affecting the solar cell characteristics is low.

  The pressure in the film forming chamber during sputtering is preferably 0.7 Pa or less, and more preferably 0.1 to 0.4 Pa. The pressure in the film forming chamber represents the number of carrier gas atoms (including atomic groups and ion states) existing in the film forming chamber, and these are constituent atoms (atomic groups) of the transparent conductive oxide layer flying from the target. (Including ion state). In the present invention, by setting the pressure in the film forming chamber within the above range, the substrate can be adhered with sufficient kinetic energy, and a transparent conductive oxide layer effective in the present invention can be formed.

  The temperature of the substrate during sputtering can be any temperature as long as it is lower than the softening temperature of the substrate. In particular, it is preferable to form a film at 25 ° C. or lower. By lowering the substrate temperature, the kinetic energy of the sputtered particles can be drastically reduced, the crystal grains can be made dense, and depending on the composition, it can be brought into an amorphous state. This makes it possible to improve wet heat durability.

  The order of arrangement of the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C) in the transparent conductive oxide layer (A) is not particularly limited, and from the photoelectric conversion unit 3 side (B) -The order of (C) or the order of (C)-(B) may be used. In other words, the order of “n-type semiconductor layer / transparent conductive oxide layer (B) / transparent conductive oxide layer (C) / back electrode 5” may be used. The order of “layer (C) / transparent conductive oxide layer (B) / back electrode 5” may be used. Furthermore, when the transparent conductive oxide layer (A) comprises three or more layers, the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C) may be alternately laminated. As one example, when the highly conductive transparent conductive oxide layer (C) is disposed on the side in contact with the photoelectric conversion unit 3 (n-type semiconductor layer), the electrical contact becomes high and the solar cell structure is connected in series. It is expected that the resistance will be reduced.

  On the other hand, in the embodiment in which the transparent conductive oxide layer (B) is disposed on the side in contact with the photoelectric conversion unit 3 and the transparent conductive oxide layer (C) is disposed on the side in contact with the back electrode 5, excellent Performance can be obtained. That is, in a preferred embodiment, the conductive impurities in the transparent conductive oxide layer (C) are mainly composed of aluminum, and the transparent conductive oxide layer (B) constitutes the photoelectric conversion unit 3. The transparent conductive oxide layer (C) is disposed in contact with the back electrode 5. Hereinafter, this embodiment (second embodiment) will be described. That is, the second embodiment relates to a thin film solar cell arranged in the order of “n-type semiconductor layer / transparent conductive oxide layer (B) / transparent conductive oxide layer (C) / back electrode 5”. is there.

  In 2nd embodiment, it is preferable that a transparent conductive oxide layer (B) contains 1 to 5 weight% of silicon oxide as a conductive impurity, and it is more preferable to contain 1.5 to 3 weight%. The transparent conductive oxide layer (B) may contain conductive impurities other than silicon oxide. For example, the transparent conductive oxide layer (B) may contain about 0 to 0.5% by weight of aluminum oxide. The aluminum content is preferably close to 0% by weight, more preferably 0% by weight (not included). In the transparent conductive oxide layer (B), by making the content of silicon oxide, which is a conductive impurity, 1 to 5% by weight, both the characteristics of conductivity and electrical bondability should be improved. Can do. Moreover, electrical junction of a photoelectric conversion unit and a transparent conductive oxide layer (B) can be made favorable by reducing content, such as aluminum oxide, as much as possible.

  Further, when the transparent conductive oxide layer (B) is in contact with the n-type semiconductor layer among the semiconductor layers forming the photoelectric conversion unit 3, further improvement in electrical junction can be expected.

  In 2nd embodiment, it is preferable that the transparent conductive oxide layer (C) contains 1 to 7 weight% of aluminum oxide as a conductive impurity, and it is more preferable to contain 1 to 4 weight%. The transparent conductive oxide layer (C) may contain conductive impurities other than aluminum oxide. For example, the transparent conductive oxide layer (C) may contain about 0 to 0.5% by weight of silicon oxide. The silicon content is preferably close to 0% by weight, more preferably 0% by weight (not included). In the transparent conductive oxide layer (C), by making 1 to 7% by weight of aluminum oxide, which is a conductive impurity, electrical connection with the back electrode can be improved. Further, by reducing the content of silicon oxide or the like as much as possible, the electrical connection between the back electrode and the transparent conductive oxide layer (C) can be improved.

  Further, when the transparent conductive oxide layer (C) is in contact with the back electrode 5, further improvement in electrical bonding can be expected.

  Here, content of a conductive impurity is content with respect to a transparent conductive oxide layer. For example, “containing 1 to 5% by weight of silicon oxide” for the transparent conductive oxide layer (B) means that silicon oxide is included in the weight of all the components forming the transparent conductive oxide layer (B). It means containing 1 to 5%. The same applies to the content of aluminum oxide in the transparent conductive oxide layer (C).

In the second embodiment, the thickness d _B of the transparent conductive oxide layer (B) is, it is preferable to account for 60% to 80% of the thickness d _A of the transparent conductive oxide layer (A), 65% More preferably, it accounts for ~ 80%.

  Although details regarding the improvement of electrical junction by the transparent conductive oxide layers (B) and (C) in the second embodiment are not clear, the configuration of the present embodiment, that is, “n-type semiconductor layer / transparent conductive”. By arranging in the order of “conductive oxide layer (B) / transparent conductive oxide layer (C) / back surface electrode”, it is presumed that the bonding at each interface becomes good. In addition, it is presumed that the above configuration reduces band bending due to the Fermi level relationship of each layer and compensates for defect levels.

  In the present invention, the arrangement order of the transparent conductive oxide layers (B) and (C) is arbitrary, and the transparent conductive oxide layer (B) is disposed on the back electrode 5 side. A thin film solar cell in which (C) is arranged on the photoelectric conversion unit 3 side is also included in the present invention. As shown in the examples described later, according to the thin-film solar cell having this configuration, the power generation efficiency retention rate is improved. However, as shown in the examples described later, according to the second embodiment, the fill factor is further improved. This is considered to be because the electrical connection between the photoelectric conversion unit 3 and the transparent conductive oxide layer (B), and between the transparent conductive oxide layer (C) and the back electrode 5 is improved.

  In the above-described embodiment, the transparent conductive oxide layer 4 has the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C), but further includes layers other than these. Also good. For example, another transparent conductive oxide layer can be formed between the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C). As such another transparent conductive oxide layer, any conductive oxide can be used that contains indium oxide, tin oxide, zinc oxide, or the like as its main component. It is preferable from the viewpoint of transparency to use a transparent conductive oxide layer. In this case, various conductive impurities can be added to the other transparent conductive oxide layers. As a conductive impurity, well-known things, such as aluminum, gallium, boron, silicon, carbon, niobium, selenium, are employable, for example. In addition, also in another transparent conductive oxide layer, like a transparent conductive oxide layer (B) and (C), when adding a conductive impurity, it is preferable to add only 1 type. This is because the carrier density is smaller than when a plurality of types of conductive impurities are added at the same time, so that the transmittance can be expected to increase.

  As described above, the addition amount of silicon, aluminum, and other conductive impurities can be detected by various elemental analysis methods. For example, the number of atoms can be accurately counted by an energy dispersive measurement device (EDS), secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), or the like.

  As described above, in the formation of the transparent conductive oxide layer in the present invention, the atomic composition of the sputter target and the atomic composition of the transparent conductive oxide layer are equal to each other, such as an energy dispersive measuring device (EDS). It is known from elemental analysis. Therefore, the amount of conductive impurities contained in the transparent conductive oxide layer can be controlled by controlling the atomic composition of the sputter target. The control of the atomic composition ratio of the sputter target is achieved by preparing a transparent conductive oxide layer containing silicon oxide as a conductive impurity, and kneading zinc oxide and silicon or silicon oxide in accordance with the composition ratio when preparing the sintered body. It can be carried out by sintering. In addition, when producing a transparent conductive oxide layer containing aluminum oxide as a conductive impurity, it can be carried out by kneading and sintering zinc oxide and aluminum or aluminum oxide according to the composition ratio during the production of the sintered body. is there.

  The transparent conductive oxide layer in the present invention has high wet heat durability. Here, “high wet heat durability” indicates that the change in conductivity before and after leaving the wet heat durability test before and after leaving for 1000 hours in an environment of 85 ° C. and 85% relative humidity is small. As a typical measurement, the change (ratio) before and after the wet heat durability test of the sheet resistance of the transparent conductive oxide layer is small. This is implemented as an accelerated test for harsh conditions in the assumed usage environment of the solar cell. Taking a thin film silicon solar cell as an example, the biggest cause of the performance degradation in the wet heat durability test is the transparent conductive oxide layer provided between the photoelectric conversion unit and the back electrode.

  In the transparent conductive oxide layer 4 containing zinc oxide as a main component, the sheet resistance ratio before and after the wet heat durability test, that is, “(sheet resistance after the wet heat durability test) / (sheet resistance before the wet heat durability test)”. "Is preferably 1.0 to 3.0, more preferably 1.01 to 2.0, still more preferably 1.02 to 1.8, and particularly preferably 1.03 to 1.40.

  In addition, the influence of the transparent conductive oxide layer on the solar cell characteristics has “series resistance”, and an extreme increase in sheet resistance means an increase in series resistance. Thereby, a curve factor falls among solar cell characteristics. Therefore, by setting the sheet resistance ratio within the above range, the characteristics of the solar cell can be kept good over a long period of time.

  Note that when the change in sheet resistance increases, the transparent conductive oxide layer becomes resistance, and carriers generated in the photoelectric conversion unit are consumed as heat in the transparent conductive oxide layer, and do not contribute to power generation. The transparent conductive oxide layer employed in the present invention suppresses this and enables stable power generation over a long period of time.

  The wet heat durability test can be carried out by removing only the back electrode after the solar cell is made up to the back electrode, or can be carried out in the state before the back electrode production, but transparent conductive oxidation on a substrate such as glass. The present invention can also be implemented in a so-called single film state in which the film is formed under the same conditions as the physical layer 4.

  In the above-described embodiment, the transparent conductive oxide layer 4 has the transparent conductive oxide layer (B) and the transparent conductive oxide layer (C), but further includes layers other than these. Also good. Moreover, you may have a some transparent conductive oxide layer (B) and a some transparent conductive oxide layer (C).

In the present invention, the “generation efficiency retention ratio” of a thin-film solar cell is defined by a change in power generation efficiency before and after the wet heat durability test. Specifically, it is as shown in the following formula (1).
(Retention rate) = [(Power generation efficiency after wet heat durability test) / (Power generation efficiency before wet heat durability test)] × 100 (1)

  The retention rate of power generation efficiency in the thin-film solar cell of the present invention is preferably 90% or more, and more preferably 94% or more. A thin-film solar cell having a high power generation efficiency retention rate is preferable because it is less likely to deteriorate in performance for a long period of use in an environment where the solar cell is actually used, such as outdoor use, and is highly practical.

  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 following examples, a scanning electron microscope JSM-6390-LA (manufactured by JEOL Ltd.) was used for the doping amount measurement. The film thickness of the transparent conductive oxide layer was calculated by fitting a measurement result using a spectroscopic ellipsometer VASE (manufactured by JA Woollam Co., Ltd.) with a Chaucy model. X-ray photoelectron spectroscopy (XPS) was calculated using MgKα (1254 eV) as the X-ray source and the binding energy of the Si ₂ P _1/2 peak as 0 valence of about 100 eV. Photoelectric conversion characteristics were measured using solar simulator with AM1.5 spectral distribution and irradiated with simulated sunlight at an energy density of 100 mW / cm ² at 25 ° C., and the output characteristics were measured, and the open circuit voltage (Voc) , Short-circuit current density (Jsc), fill factor (FF), power generation efficiency (Eff), and voltage-current characteristics.

  The wet heat durability test was carried out by leaving the produced thin-film solar cell in an environment of 85 ° C. and a relative humidity of 85% for 1000 hours, and comparing the power generation efficiency before and after that. The retention rate of power generation efficiency was calculated according to the above formula (1).

As the substrate 1, a translucent non-alkali glass substrate (thickness 1.1 mm, manufactured by Nippon Electric Glass Co., Ltd.) was used. The transparent electrode layer 2 was made of fluorinated tin oxide (F: SnO ₂ ) produced by a thermal CVD method. The film thickness of the transparent electrode layer 2 at this time was 800 nm, the sheet resistance was 10Ω / □, and the haze value was 15 to 20%.

  On this, 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 20 nm thick using a high-frequency plasma CVD apparatus. To form a film. As a result, a first photoelectric conversion unit 3-1 (top cell) made of amorphous silicon having a pin junction, which is a front photoelectric conversion unit, was formed.

The conductive intermediate layer 3-2 made of a conductive oxygenated silicon layer was formed by a plasma CVD apparatus without taking the substrate on which the first photoelectric conversion unit 3-1 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 pressure in the reaction chamber was 666 Pa. SiH ₄ , PH ₃ , CO ₂ , and H ₂ were used as source gases introduced into the plasma CVD reaction chamber. A conductive oxygenated silicon layer 3-2 having a thickness of 60 nm (600 cm) was formed under the above conditions.

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

  The work-in-process product in which the crystalline silicon photoelectric conversion unit has been formed is taken out from the high-frequency plasma CVD apparatus into the atmosphere, and then introduced into the film-forming chamber of the high-frequency magnetron sputtering apparatus, on the second photoelectric conversion unit 3-3. A transparent conductive oxide layer 4 was formed.

  The transparent conductive oxide layer 4 is formed by using the sputtering target material and the deposition power density shown in Table 1, the deposition pressure is 0.2 Pa, the substrate temperature is 150 ° C., and the substrate / target distance is 60 mm. Set and implemented. The parentheses (%) in the film thickness column of Table 1 indicate the ratio of the film thickness of the transparent conductive oxide layer (B) to the film thickness of the transparent conductive oxide layer 4.

  In Examples 1-1 to 1-11, the transparent conductive oxide layer 4 was composed of two or more layers having different types of conductive impurities. As an operation, a first layer was formed on the second photoelectric conversion unit 3-3, and a second layer was formed thereon. In Example 1-5, a third layer was further formed on the second layer. In addition, the first layer “transparent conductive oxide layer (C) containing conductive impurities mainly containing elements other than silicon” and the second layer “contains conductive impurities mainly containing silicon”. A transparent conductive oxide layer (B) ”was employed. Furthermore, in Example 1-5, the “transparent conductive oxide layer (C) containing conductive impurities whose main component is an element other than silicon” was adopted as the third layer. The first, second, and third layers in Table 1 correspond to the first, second, and third layers of the transparent conductive oxide layer 4, respectively.

Specifically, first, in Examples 1-1 to 1-11, silicon oxide (SiO ₂ ) was used as a conductive impurity in the transparent conductive oxide layer (B).
In Examples 1-6, 1-9, 1-10, and 1-11, aluminum oxide (Al ₂ O ₃ ) was used as a conductive impurity in the transparent conductive oxide layer (C). In Example 1-5, two transparent conductive oxide layers (C) were provided and arranged so as to sandwich the transparent conductive oxide layer (B). In Example 1-7, gallium oxide (Ga ₂ O ₃ ) was used as a conductive impurity in the transparent conductive oxide layer (C). In Example 1-8, boron oxide (B ₂ O ₃ ) was used as a conductive impurity in the transparent conductive oxide layer (C).

  In Examples 1-1, 1-3, 1-4, 1-7, 1-8, 1-11, the transparent conductive oxide layer (B) is disposed in contact with the back electrode 5, and transparent conductive oxidation is performed. The physical layer (C) was placed in contact with the photoelectric conversion unit 3. In Examples 1-2, 1-6, 1-9, and 1-10, the transparent conductive oxide layer (B) is disposed in contact with the photoelectric conversion unit 3, and the transparent conductive oxide layer (C) is disposed on the back surface. Arranged in contact with the electrode 5. In Example 1-5, one transparent conductive oxide layer (C) was disposed in contact with the back electrode 5, and the other transparent conductive oxide layer (C) was disposed in contact with the photoelectric conversion unit 3. In Example 1-5, the transparent conductive oxide layer (B) is not in contact with either the photoelectric conversion unit 3 or the back electrode 5.

On the other hand, in the comparative examples, the transparent conductive oxide layer 4 consisting of only one layer was used. Specifically, in Comparative Example 1-1, aluminum oxide (Al ₂ O ₃ ) was used as the conductive impurity. In Comparative Examples 1-2 and 1-3, silicon oxide (SiO ₂ ) was used as the conductive impurity. In Comparative Example 1-4, silicon oxide (SiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) were used in combination as conductive impurities (simultaneous doping).

Subsequently, an Ag film having a film thickness of 250 nm was formed as the back electrode 5 using a vacuum vapor 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 nm / second. As described above, a total of 15 types (11 types of Examples and 4 types of Comparative Examples) of thin film solar cells were produced.

Table 2 shows the photoelectric conversion characteristics of the produced thin-film solar cells. That is, the thin film solar cells of the examples all showed high retention rates. This is considered because the wet heat durability of the zinc oxide transparent conductive oxide doped with silicon oxide is excellent.
Moreover, in the thin film solar cell of the Example, it turned out that the short circuit current density (Jsc) and the open circuit voltage (Voc) are also improving, respectively. This is because zinc oxide doped with silicon oxide tends to have higher transmittance than zinc oxide doped with aluminum oxide, which increases the amount of light taken into the photoelectric conversion unit, As a result, it is considered that the short circuit current density is improved. Regarding the improvement of the open circuit voltage, the influence on the internal stress of the transparent conductive oxide layer caused by the difference in ionic radius and electronegativity of silicon and aluminum doped as impurities, and the defect level generated by the internal stress. Can be explained by thinking that there will be less.
In addition, Comparative Example 1-3 did not show the function as a thin film solar cell. This is considered because the current did not flow because the resistance of the transparent conductive oxide layer was very large.

  In FIG. 2, the X-ray photoelectron spectroscopy spectrum in Example 1-1 and Comparative Example 1-4 is shown. In FIG. 2, the solid line indicates Example 1-1 and the broken line indicates Comparative Example 1-4. As a result of XPS measurement, as shown in Table 1 and FIG. 2, in the transparent conductive oxide layer formed under the conditions of the example, the peak of the binding energy of about 104 eV indicating tetravalent silicon was preferential. On the other hand, in the transparent conductive oxide layer formed under the conditions of the comparative example, the peak of the binding energy of about 103 eV indicating trivalent silicon was preferential. From this, it was found that by using a transparent conductive oxide layer containing tetravalent silicon, a thin film solar cell having good characteristics and excellent durability can be produced.

  As a result of the above, between the photoelectric conversion unit and the back electrode, a transparent conductive oxide layer (B) containing a conductive impurity mainly composed of silicon and a conductive impurity mainly composed of an element other than silicon. It turned out that the thin film solar cell which is excellent in electric power generation efficiency can be produced by arrange | positioning the transparent conductive oxide layer (C) containing this. This is considered to be because the amount of light taken into the photoelectric conversion unit is increased by improving the transmittance of the transparent conductive oxide layer. The reason why the transmittance is improved is considered to be that the carrier concentration in the transparent conductive oxide is lowered.

  Furthermore, it turned out that a high retention can be implement | achieved by comprising the transparent conductive oxide layer 4 with an above-described transparent conductive oxide layer (B) and a transparent conductive oxide layer (C).

  In addition, as shown in FIG. 2, it turned out that silicon is not fully oxidized in Comparative Example 1-4. This is probably because aluminum was preferentially oxidized.

[Example 2-1]
In the same manner as in Example 1 above, the transparent electrode layer 2 was provided on the substrate 1.

  On this, a boron-doped p-type silicon carbide (SiC) layer is 10 nm, a non-doped amorphous silicon photoelectric conversion layer is 250 nm, and a phosphorus-doped n-type μc-Si layer is 20 nm thick using a high-frequency plasma CVD apparatus. To form a film. As a result, a first photoelectric conversion unit 3-1 (top cell) made of amorphous silicon having a pin junction, which is a front photoelectric conversion unit, was formed.

  Subsequently, a conductive oxygenated silicon layer 3-2 was formed in the same manner as in Example 1 above.

  Further, a boron-doped p-type microcrystalline silicon layer was formed by plasma CVD at a thickness of 15 nm, a non-doped crystalline silicon photoelectric conversion layer was formed at 1750 nm, and a phosphorus-doped n-type microcrystalline silicon layer was formed at a thickness of 20 nm. Thereby, the 2nd photoelectric conversion unit 3-3 (bottom cell) which consists of crystalline silicon of a pin junction which is a back photoelectric conversion unit was formed.

  Subsequently, in the same manner as in Example 1, the transparent conductive oxide layer 4 was formed on the second photoelectric conversion unit 3-3.

As the transparent conductive oxide layer 4, two layers of transparent conductive oxide layers (B) and (C) were formed. Film formation was performed by sputtering at a film forming pressure of 0.2 Pa, a substrate temperature of 150 ° C., and a substrate / target distance of 60 mm. As a transparent conductive oxide layer (B), a film of 60 nm (600 mm) was formed at a power density of 10 W / cm ^{2 using} zinc oxide containing 2.0% of silicon oxide as a target (first layer), and transparent on it As the conductive oxide layer (C), 30 nm (300 mm) was formed at a power density of 1.5 W / cm ^{2 using} zinc oxide containing 2.0% of aluminum oxide as a target (second layer).

Subsequently, an Ag film having a film thickness of 250 nanometers was formed as the back electrode 5 using a vacuum evaporation 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 nm / second. A thin film solar cell was produced as described above.

[Examples 2-2 to 2-6]
As the transparent conductive oxide layers (B) and (C), a thin film solar cell under the same conditions as in Example 2-1, except that the content of silicon oxide or aluminum oxide contained in the target or the film thickness was changed. A battery was produced.

[Reference Example 2-1]
As a first layer, a film of 20 nm (200 mm) is formed with a power density of 1.5 W / cm ^{2 using} zinc oxide containing 2.0% of aluminum oxide as a target, and 2.0 nm of silicon oxide is formed thereon as a second layer. 60 nm (600 mm) was formed at a power density of 7.0 W / cm ² using zinc oxide containing 1% as a target. Otherwise, a thin-film solar cell was produced under the same conditions as in Example 2-1.

[Reference Example 2-2]
As a first layer, a film having a power density of 1.5 W / cm ^{2 and} a thickness of 60 nm (600 mm) was formed using zinc oxide containing 2.0% of aluminum oxide as a target, and silicon oxide was added as a second layer to a thickness of 2.0 nm. 30 nm (300 mm) was formed at a power density of 7.0 W / cm ^{2 using} as a target zinc oxide containing 1%. Otherwise, a thin-film solar cell was produced under the same conditions as in Example 2-1.

  That is, in Examples 2-1 to 2-8, the transparent conductive oxide layer (B) is disposed in contact with the photoelectric conversion unit 3, and the transparent conductive oxide layer (C) is disposed in contact with the back electrode 5. did. On the other hand, in Reference Examples 2-1 and 2-2, the transparent conductive oxide layer (B) is disposed in contact with the back electrode 5, and the transparent conductive oxide layer (C) is disposed in contact with the photoelectric conversion unit 3. did.

[Comparative Examples 2-1 to 2-3]
As the transparent conductive oxide layer 4, 2.0% of aluminum oxide (Comparative Example 2-1), 2.0% of silicon oxide (Comparative Example 2-2), or 7.0% of silicon oxide (Comparative Example) 2-3) Only one layer was formed using the contained zinc oxide target. Other conditions were formed under the same conditions as in Example 2-1.
As described above, a total of 13 types of thin film solar cells (Example 8 type, Reference Example 2 type, Comparative Example 3 type) were produced. Table 3 shows the film forming conditions of the transparent conductive oxide layer 4 for each thin film solar cell. The first layer and the second layer in Table 3 correspond to the first layer and the second layer of the transparent conductive oxide layer 4, respectively. The parentheses (%) in the film thickness column of Table 3 indicate the ratio of the film thickness of the transparent conductive oxide layer (B) to the film thickness of the transparent conductive oxide layer 4.

  Table 4 shows the photoelectric conversion characteristics of the produced thin-film solar cells. In Comparative Example 2-3, the solar cell could not be formed because the conductivity of the transparent conductive oxide layer 4 was extremely poor.

  From Table 4, it can be seen that the retention rate of power generation efficiency is improved in Examples and Reference Examples in which two layers are used as compared with Comparative Examples in which only one layer is used as the transparent conductive oxide layer. This is considered to be due to the effect of maintaining the conductivity by maintaining the durability of the transparent conductive oxide layer and maintaining the electrical connection.

  Specifically, when Comparative Example 2-1 and Examples 2-1 to 2-8 are compared, the transparent conductive oxide layer (B) containing a predetermined amount of silicon oxide and the n-type semiconductor of the photoelectric conversion unit By making it contact | connect a layer, especially the improvement of a fill factor became clear compared with the comparative example 2-1 which has only the transparent conductive oxide layer containing an aluminum oxide. As described above, this is considered because the electrical junction between the n-type semiconductor layer forming the photoelectric conversion unit and the transparent conductive oxide layer was improved.

  Further, comparing Comparative Examples 2-2 and 2-3 with Examples 2-1 to 2-8, the transparent conductive oxide layer (C) containing a predetermined amount of aluminum oxide and the back electrode 5 were compared. By making it contact, especially the improvement of a fill factor became clear compared with Comparative Examples 2-2 and 2-3 which have only the transparent conductive oxide layer containing a silicon oxide. This is probably because the electrical connection between the transparent conductive oxide layer 4 and the back metal 5 was improved in Examples 2-1 to 2-8 as compared to Comparative Example 2-2. In contrast to Comparative Example 2-3 in which the resistance of the transparent conductive oxide layer 4 was increased due to the large amount of silicon oxide doped, in Examples 2-1 to 2-8, the resistance was low and the conductivity was reduced. It is thought to be superior.

  Moreover, when Examples 2-1 to 2-8 and Reference Examples 2-1 and 2-2 were compared, Examples 2-1 to 2-8 had higher values of the curve factor. That is, the transparent conductive oxide layer (B) containing silicon oxide is placed in contact with the photoelectric conversion unit 3, and the transparent conductive oxide layer (C) containing aluminum oxide is placed in contact with the back electrode 5. As a result, a higher value of the fill factor was obtained.

  From the above results, a zinc oxide transparent conductive oxide layer containing silicon is formed between the photoelectric conversion unit 3 and the back electrode 5, and zinc oxide containing silicon is formed on the side in contact with the photoelectric conversion unit 3. It was found that by forming zinc oxide containing aluminum on the side in contact with the back electrode 5, a thin film solar cell having excellent power generation efficiency can be produced.

This is because the electrical junction between the transparent conductive oxide layer 4 and the n-type semiconductor layer forming the photoelectric conversion unit 3 and the electrical junction between the transparent conductive oxide layer 4 and the back electrode 5 are both This is considered to be due to the fact that carriers induced by light can be efficiently extracted.
Further, it was found that a high retention rate was exhibited even after the wet heat durability test.

  In Reference Example 2-1, the “retention ratio” was improved as compared with Comparative Examples 2-1 and 2-2. In Examples 2-1 to 2-8, it was possible to improve the “curve factor” in particular while maintaining this high retention rate. This is considered to be because the conditions such as the order of lamination and film thickness of the predetermined transparent conductive oxide layer were optimized. Similarly to Reference Example 2-2, improvement of “curve factor” was confirmed in Examples 2-1 to 2-8.

  From the above, it is considered that the photoelectric conversion characteristics of the solar cell could be improved by adopting the configuration of the present invention.

DESCRIPTION OF SYMBOLS 1 Substrate 2 Transparent electrode layer 3 Photoelectric conversion unit 3-1 1st photoelectric conversion unit 3-2 Transparent conductive intermediate layer 3-3 2nd photoelectric conversion unit 4 Transparent conductive oxide layer 5 Back electrode 10 Thin film solar cell

Claims (10)

From the light incident side, a transparent electrode layer made of a transparent conductive oxide, at least one photoelectric conversion unit, and a back surface electrode are thin film solar cells arranged in this order,
Between the photoelectric conversion unit and the back electrode, a transparent conductive oxide layer (A) mainly composed of zinc oxide is formed,
The transparent conductive oxide layer (A) includes a transparent conductive oxide layer (B) containing conductive impurities mainly composed of silicon and a transparent containing conductive impurities mainly composed of elements other than silicon. A conductive oxide layer (C),
The thin film solar cell, wherein the film thickness of the transparent conductive oxide layer (B) occupies 30% to 80% of the film thickness of the transparent conductive oxide layer (A). In the transparent conductive oxide layer (B), the content of conductive impurities mainly composed of silicon is 1 to 8% by weight, and the X-ray photoelectron spectroscopy Si2P _{1 / of the} transparent conductive oxide layer (B) The thin film solar cell according to claim 1, wherein silicon is substantially tetravalent as viewed from the _two peaks.   The thin film solar cell according to claim 1 or 2, wherein the thickness of the transparent conductive oxide layer (A) is 10 to 120 nm.   The thin film solar cell according to claim 1, wherein the conductive impurity in the transparent conductive oxide layer (C) is mainly composed of aluminum.   The transparent conductive oxide layer (B) is disposed in contact with the n-type semiconductor layer of the layers constituting the photoelectric conversion unit, and the transparent conductive oxide layer (C) is disposed in contact with the back electrode. Item 5. The thin-film solar cell according to Item 4.   The transparent conductive oxide layer (B) contains 1 to 5% by weight of silicon oxide as a conductive impurity, and the transparent conductive oxide layer (C) contains 1 to 7% by weight of aluminum oxide as a conductive impurity. The thin film solar cell according to claim 4 or 5.   The thin film solar cell according to any one of claims 4 to 6, wherein the film thickness of the transparent conductive oxide layer (B) occupies 60% to 80% of the film thickness of the transparent conductive oxide layer (A).   The conductive impurities in the transparent conductive oxide layer (C) are mainly composed of one or more selected from the group consisting of aluminum, gallium, and boron. Thin film solar cell.   The thin film solar cell according to any one of claims 1 to 8, wherein the photoelectric conversion unit is made of non-single crystal silicon composed of a pin type junction.   The thin film solar cell according to any one of claims 1 to 9, wherein a retention rate of power generation efficiency is 90% or more when treated in an environment of 85 ° C and a relative humidity of 85% for 1000 hours.

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