JP5307280B2 - Thin film photoelectric conversion element - Google Patents

Description

  The present invention relates to a thin film photoelectric conversion element. In particular, the present invention relates to improvement of a transparent electrode layer of a thin film photoelectric conversion element.

  Solar cells, which are photoelectric conversion elements that convert sunlight into electrical energy, are expected as clean energy to replace fossil energy such as petroleum and nuclear energy. Problems to be solved in order to put solar cells into practical use as an energy source for electric power are high efficiency and low cost. In particular, a thin film photoelectric conversion element that can be easily increased in area can be greatly reduced in cost, and therefore, it is strongly desired to improve its energy conversion efficiency.

An example of a thin film solar cell is described in JP2012-114296A (Patent Document 1). In Patent Document 1, it is preferable to provide a transparent electrode made of ZnO, ITO, or SnO ₂ between the back electrode and the photoelectric conversion layer.

  A report on the results of the New Energy and Industrial Technology Development Organization (NEDO) (Management Number: 20100000000638) “Results of FY2006-21 Research and Development of New Energy Technology Future Technology Research and Development of Solar Power Generation Systems High Current From the description of "Research and development of type high-efficiency thin-film silicon solar cell (bottom cell)" (Non-patent Document 1), the unit cell size of crystal in microcrystalline silicon germanium is proportional to the increase in germanium concentration. It can be seen that it varies in the range from the lattice size to the unit cell size of crystalline germanium.

JP 2012-114296 A NEDO Results Report (Management Number: 20100000000638) “Fiscal Year 2006-21 Results Report New Energy Technology Research and Development Future Technology Research and Development of Solar Power Generation System Research and Development of High Current Type High Efficiency Thin Film Silicon Solar Cells (Bottom Cell)”

As disclosed in Patent Document 1 described above, the inventors have caused the conversion efficiency of a thin-film solar cell to be reduced when ZnO, ITO, or SnO ₂ is formed as a transparent electrode on the back side. I found out. This is because the work function relationship between the n-type semiconductor layer and the transparent electrode on the back side is not a suitable design.

  Therefore, an object of the present invention is to provide a thin film photoelectric conversion element that solves these problems and has improved photoelectric conversion efficiency.

In order to achieve the above object, in one aspect of the thin-film photoelectric conversion element according to the present invention, a light-transmitting substrate having a main surface, and the main surface are sequentially stacked from the side closer to the light-transmitting substrate. A transparent electrode layer, a p-type semiconductor layer, an i-type semiconductor layer, an n-type semiconductor layer, a back surface transparent electrode layer, and a back surface reflective layer, and a vacuum is formed at the junction interface between the n-type semiconductor layer and the back surface transparent electrode layer. When the level is set to zero and the work function of the material is defined as a negative value, the relationship between the work function Φ _smi of the n-type semiconductor layer and the work function Φ _ox of the back transparent electrode layer is Φ _smi <Φ It becomes _ox .

  The back transparent electrode layer preferably contains ZnO as a main component. Furthermore, the back transparent electrode layer preferably contains Mg. Further, the back transparent electrode layer preferably contains a Group 13 element. Furthermore, the back transparent electrode layer preferably contains Al.

  In order to achieve the above object, in another aspect of the thin film photoelectric conversion element according to the present invention, a transparent electrode layer having a main surface, and a transparent electrode layer on the main surface in order from the side close to the light transparent substrate. A multi-junction pin semiconductor junction layer including an intermediate layer, a back surface transparent electrode layer, and a back surface reflective film, wherein the back surface transparent electrode layer includes Mg, Zn, and at least one group 13 element Includes metal oxide film.

Here, Φ _smi <Φ _ox means that the work function Φ _ox of the back transparent electrode layer is closer to the vacuum level than the work function of the n-type semiconductor layer. By satisfying the above-described work function relationship at the junction interface, a desired built-in potential, that is, a built-in potential at which an electron carrier accumulation layer is formed in the n-side semiconductor layer can be applied to the junction interface. The characteristics of the thin film photoelectric conversion element can be improved by increasing the open-circuit voltage and increasing the fill factor (FF).

  A metal oxide composed of a metal whose work function is relatively shallow with respect to the vacuum level, such as Al, Mg, etc., can reduce the work function of the metal oxide itself and provide a stable oxide. Therefore, a desired built-in potential can be applied to the bonding interface by satisfying the above-described work function magnitude relationship at the bonding interface, and a thin film can be formed by increasing the open circuit voltage and the high fill factor. The characteristics of the photoelectric conversion element can be improved.

  In the thin film photoelectric conversion element, the back transparent electrode layer preferably contains a metal oxide, and the band gap of the metal oxide is preferably larger than 2.8 eV. When the composition ratio of Mg to the contained metal in the metal oxide is in the range of 5 wt% to 30 wt%, the band gap of the metal oxide becomes larger than 2.8 eV and the refractive index is lowered. Therefore, the reflectance of light at the bonding interface increases, and the characteristics of the thin film photoelectric conversion element can be improved by increasing the short-circuit current. The work function of the metal oxide is preferably larger than −4.5 eV when the vacuum level is zero.

  In the thin film photoelectric conversion element, the back transparent electrode layer preferably contains a metal oxide, and the work function of the metal oxide is preferably larger than −4.5 eV when the vacuum level is zero. When the work function of the metal oxide is greater than −4.5 eV when the vacuum level is zero, the recombination probability of photoexcited carriers at the junction interface can be reduced, and the high fill factor enables thin film photoelectric conversion. The characteristics of the element can be improved.

  In the thin film photoelectric conversion element, it is preferable that the back transparent electrode has a multilayer structure of a plurality of types of metal oxides. High conductivity metal oxide such as ITO (InSnO metal oxide) and IGZO (InGaZnO metal oxide) with good electrical conductivity as well as metal oxide film composed of Al, Mg, Zn as the back transparent electrode layer By further stacking the films, the series resistance can be reduced, and the characteristics of the thin film photoelectric conversion element can be improved by increasing the fill factor.

The back transparent electrode layer preferably contains 3 wt% or more and 10 wt% or less of Mg.
The back transparent electrode layer preferably contains 1 wt% or more and 5 wt% or less of Al.

  The back surface reflective layer contains Ag, the back surface transparent electrode layer is composed of a plurality of layers, and the back surface transparent electrode layer is in contact with the pin semiconductor junction layer and includes Mg, and the back surface reflective layer is in contact with and does not contain Mg. It is preferable to have a layer.

It is sectional drawing of the thin film photoelectric conversion element in Embodiment 1 based on this invention. It is sectional drawing of the thin film photoelectric conversion element in Embodiment 2 based on this invention. It is sectional drawing of the thin film photoelectric conversion element in Embodiment 3 based on this invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conceptual diagram of a substrate moving magnetron sputtering apparatus that can be used to manufacture a thin film photoelectric conversion element in Embodiment 1 according to the present invention.

  In the present specification, “amorphous” is synonymous with “amorphous” generally used in the art. In addition, as generally used in the field, “microcrystal” includes not only a state consisting essentially of a crystalline phase but also a state where a crystalline phase and an amorphous phase are mixed.

For example, in the Raman scattering spectrum, it is considered that “microcrystalline silicon” is one in which even a sharp peak near 520 cm ⁻¹ attributed to a silicon-silicon bond in crystalline silicon is detected. “Microcrystalline silicon” is also used in the same sense in the book.

  Note that in this specification, “microcrystalline silicon germanium” refers to silicon germanium in the above-described microcrystalline state. That is, microcrystalline silicon germanium includes not only a state substantially consisting of a crystalline phase but also an amorphous silicon germanium phase and a crystalline silicon germanium phase.

  In microcrystalline silicon germanium, as described in Non-Patent Document 1, the unit cell size of crystal is changed from the unit cell size of crystal silicon to the unit cell size of crystal germanium in proportion to the increase in germanium concentration. It is known to change in the range up to.

  This means that the proportion of unit cell silicon-germanium bonds present in the microcrystalline silicon germanium layer increases with increasing germanium concentration. Therefore, the presence of the crystalline silicon germanium phase can be known by measuring the size of the unit cell using, for example, X-ray diffraction. In addition, in the analysis of the Raman scattering spectrum described later, the microcrystalline silicon is also observed when the peak attributed to crystalline silicon germanium is observed or the peak position attributed to silicon-silicon of crystalline silicon is changed. The presence of the germanium phase can be known.

Therefore, the determination is made as follows. First, the 1st-5th conditions relevant to judgment are shown.
First condition: The presence of silicon and germanium can be confirmed by secondary ion mass spectrometry.

  Second condition: the size of the unit cell obtained from the (220) diffraction peak angle in the X-ray diffraction method is larger than the unit cell size of crystalline silicon, which is 5.43 mm, and the unit cell size of crystalline germanium, which is 5.67 mm. Small.

  Third condition: A peak attributed to a silicon-silicon bond of crystalline silicon is observed in a Raman scattering spectrum.

  Fourth condition: The Raman shift value of the peak is shifted to a lower wave number side than the Raman shift value of crystalline silicon not containing germanium.

Fifth condition: A peak around 400 cm ⁻¹ attributed to crystalline silicon germanium is observed.

  Among these five conditions, when the first, second and third conditions are satisfied simultaneously, when the first, third and fourth conditions are satisfied simultaneously, or when the first, third and fifth conditions are satisfied simultaneously Therefore, it is determined that a crystalline silicon germanium phase is present, that is, it is determined to be microcrystalline silicon germanium.

  When the germanium concentration is relatively low, it is difficult to observe a peak attributed to crystalline silicon germanium related to the fifth condition. Therefore, the above first to fourth conditions are mainly used as conditions for determining the presence or absence of crystalline silicon germanium.

  Moreover, a super straight type is mentioned as a structure of a general thin film silicon solar cell. In the super straight type structure, a transparent electrode layer, a semiconductor photoelectric conversion layer, a back surface transparent electrode layer, and a back surface reflection layer are laminated in this order on a light transmissive substrate so that light enters from the light transmissive substrate side. Placed in.

  The photoelectric conversion layer includes a semiconductor layer exhibiting p conductivity type (hereinafter referred to as “p-type semiconductor layer”), a light absorption layer (hereinafter referred to as “i-type semiconductor layer”), and a semiconductor layer exhibiting n conductivity type ( Hereinafter, it is often provided with a pin junction composed of “n-type semiconductor layer”. In the first and second embodiments described below, a thin film solar cell having a super straight pin structure will be described as an example.

(Embodiment 1)
Hereinafter, the thin film solar cell and the manufacturing method thereof according to the first embodiment of the present invention will be described. In the description of the following embodiments, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated.

  With reference to FIG. 1, the thin film solar cell 101 as a thin film photoelectric conversion element in Embodiment 1 based on this invention is demonstrated. As shown in FIG. 1, in the thin film solar cell 101, a transparent electrode layer 2, a semiconductor photoelectric conversion layer 3, and a back surface transparent electrode layer that is an AZMO (Al, Zn, Mg, O) layer on a light transmissive substrate 1. 4 and the back reflective layer 5 are laminated in this order.

<Base material>
A transparent electrode layer 2 is formed on the light transmissive substrate 1. As the light-transmitting substrate 1, a glass plate, a heat-transmitting resin plate having heat resistance such as polyimide or polyvinyl, and a laminate thereof are preferably used. However, the material of the light-transmitting substrate 1 is not particularly limited as long as it has a high light-transmitting property and can structurally support the entire thin-film solar cell 101. The surface of the light transmissive substrate 1 may be covered with a metal film, a light transmissive conductive film, an insulating film, or the like.

  The transparent electrode layer 2 is made of a transparent conductive material. As the transparent electrode layer 2, for example, a single layer or a plurality of layers of transparent conductive films such as ITO (Indium Tin Oxide), tin oxide, or zinc oxide can be used. Since the transparent electrode layer 2 functions as an electrode, it is preferable to have high electrical conductivity. Therefore, the transparent electrode layer 2 can be used which has improved electrical conductivity by adding a small amount of impurities.

  As a method for forming the transparent electrode layer 2, known methods such as sputtering, CVD (Chemical Vapor Deposition), electron beam evaporation, sol-gel, spray, and electrodeposition can be used.

  Moreover, it is preferable that an uneven shape is formed on the surface of the transparent electrode layer 2. This uneven shape can scatter and refract incident light incident from the light-transmitting substrate 1 side to extend the optical path length, so that the light confinement effect in the semiconductor photoelectric conversion layer 3 is enhanced and the short-circuit current density is increased. Can be achieved.

  As a method of forming a concavo-convex shape on the surface of the transparent electrode layer 2, after forming the transparent electrode layer 2 on the light-transmitting substrate 1, a method of forming the concavo-convex shape by mechanical processing such as etching or sand blasting, transparent electrode A method using the uneven surface shape formed by crystal growth of the film material when forming the layer 2, or a regular uneven surface shape is formed because the crystal growth surface is oriented. You may use the method of using.

  In addition, it is more preferable to form the zinc oxide layer 2a on the transparent electrode layer 2 by a sputtering method or the like because the transparent electrode layer 2 can be prevented from being damaged by plasma when the semiconductor photoelectric conversion layer 3 is formed later.

  In the present embodiment, a transparent electrode layer 2 made of tin oxide is deposited on a translucent substrate 1 made of soda-lime glass, utilizing the uneven shape formed when a film material crystal is grown by CVD. A product obtained by depositing a zinc oxide layer 2a by a sputtering method is used on the product (manufactured by Asahi Glass Co., Ltd., trade name: Asahi-U).

<Semiconductor photoelectric conversion layer>
In the semiconductor photoelectric conversion layer 3, a p-type semiconductor layer 3p, an i-type semiconductor layer 3i, and an n-type semiconductor layer 3n are stacked in this order from the light-transmitting substrate 1 side to form a pin junction.

  The thickness of each semiconductor layer is not particularly limited, but the thickness of the p-type semiconductor layer 3p is 5 nm to 50 nm, the thickness of the i-type semiconductor layer 3i is 100 nm to 5000 nm, and the n-type semiconductor layer 3n The film thickness is preferably 5 nm or more and 100 nm or less. More preferably, the p-type semiconductor layer 3p has a thickness of 10 nm to 30 nm, the i-type semiconductor layer 3i has a thickness of 200 nm to 4000 nm, and the n-type semiconductor layer 3n has a thickness of 10 nm to 30 nm.

  The main material of the constituent material of the semiconductor photoelectric conversion layer 3 is silicon, and amorphous silicon or microcrystalline silicon is particularly preferable. Here, “amorphous silicon” and “microcrystalline silicon” include “hydrogenated amorphous silicon” and “hydrogenated microcrystalline silicon”, which are generally used in the art.

  The p-type semiconductor layer 3p is a silicon layer doped with a p-type conductivity determining element. As the p-type conductivity determining element, impurity atoms such as boron, aluminum and gallium can be used. The main material of the p-type semiconductor layer 3p may be amorphous silicon or microcrystalline silicon.

  When the p-type semiconductor layer 3p includes a crystalline silicon phase, high conductivity can be obtained, the series resistance of the semiconductor photoelectric conversion layer 3 can be reduced, and the fill factor of the thin film solar cell 101 can be increased to obtain high conversion efficiency. preferable.

  The p-type semiconductor layer 3p containing a crystalline silicon phase is excellent as a base layer for crystallization of the i-type semiconductor layer 3i. Specifically, the p-type semiconductor layer 3p containing a crystalline silicon phase makes it easy to grow a crystal component at the initial stage when the i-type semiconductor layer 3i is formed, and the high-quality i-type semiconductor layer 3i having a high crystallization rate. Enables film formation. Thus, the p-type semiconductor layer 3p including the crystalline silicon phase increases the short-circuit current density of the thin-film solar cell 101 and contributes to the improvement of conversion efficiency.

  The i-type semiconductor layer 3i is a microcrystalline silicon layer to which no impurity is added. However, the i-type semiconductor layer 3i may contain a small amount of an impurity element as long as it is a substantially intrinsic semiconductor. In this case, amorphous silicon may be used as the material of the i-type semiconductor layer 3i instead of microcrystalline silicon.

  However, microcrystalline silicon is preferable to amorphous silicon as a material for the i-type semiconductor layer 3i in that high photoelectric conversion efficiency can be obtained because it does not cause photodegradation. Further, as will be described later, in a thin film solar cell having an n-type semiconductor layer 3n made of microcrystalline silicon, in order to reduce recombination at the interface between the n-type semiconductor layer 3n and the i-type semiconductor layer 3i, i The type semiconductor layer 3i is preferably microcrystalline silicon.

  In order to increase the sensitivity of the i-type semiconductor layer 3i to long wavelengths, the i-type semiconductor layer 3i may contain at least one of amorphous silicon germanium and microcrystalline silicon germanium. In this case, the atomic composition percentage of germanium in the i-type semiconductor layer 3i is preferably 5 atomic percent or more and 30 atomic percent or less.

  If the atomic composition percentage of germanium is less than 5 atomic%, the band gap is hardly reduced, and therefore the short circuit current density of the thin film solar cell cannot be increased, which is not preferable. When the atomic composition percentage of germanium exceeds 30 atomic%, the band gap decreases and the open-circuit voltage of the thin-film solar cell decreases, or the crystal grain size of the i-type semiconductor layer 3i decreases and the conversion efficiency of the thin-film solar cell Is unfavorable because of lowering.

  The n-type semiconductor layer 3n is a silicon layer doped with an n-type conductivity determining element. As the n-type conductivity determining element, impurity atoms such as phosphorus, nitrogen and oxygen can be used. The main material of the n-type semiconductor layer 3n may be amorphous silicon or microcrystalline silicon. It is preferable that the n-type semiconductor layer 3n contains crystalline silicon because high conductivity can be obtained, the series resistance of the photoelectric conversion layer 11 can be reduced, and the conversion factor of the thin film solar cell 101 can be increased to obtain high conversion efficiency.

  As a method for forming the semiconductor photoelectric conversion layer 3, a CVD method can be used. Examples of the CVD method include an atmospheric pressure CVD method, a low pressure CVD method, a plasma CVD method, a thermal CVD method, a hot wire CVD method, and a MOCVD (Metal Organic Chemical Vapor Deposition) method. In this embodiment, the plasma CVD method is used. Is used.

The silicon-containing gas used when forming the semiconductor photoelectric conversion layer 3 by the plasma CVD method is not particularly limited as long as it contains silicon atoms such as SiH ₄ and Si ₂ H _6, but generally SiH ₄ is used. Use.

As a dilution gas used together with the silicon-containing gas, H ₂ gas, Ar gas, and He gas can be used, but H ₂ gas is generally used when forming amorphous silicon and microcrystalline silicon.

Further, when forming the p-type semiconductor layer 3p and the n-type semiconductor layer 3n, a doping gas is used together with the silicon-containing gas and the dilution gas. The doping gas is not particularly limited as long as it contains a target type conductivity determining element. Generally, when the p-type conductivity determining element is boron, B ₂ H ₆ is used as the n-type conductivity. PH ₃ is used when the sex determining element is phosphorus.

  When the semiconductor photoelectric conversion layer 3 is formed by the plasma CVD method, the abundance ratio of the amorphous phase and the crystalline phase is controlled by appropriately controlling the film forming parameters such as the substrate temperature, pressure, gas flow rate, and power input to the plasma. Can be controlled.

<Back reflective film>
The back surface reflection layer 5 should just be comprised by the 1 or more conductive layer, and the light reflectance and electrical conductivity of a conductive layer are so preferable that it is high. As a material of the back surface reflection layer 5, a metal material such as silver, aluminum, titanium and palladium having high visible light reflectance, or an alloy thereof can be used. The back surface reflection film 5 is formed on the semiconductor photoelectric conversion layer 3 by a CVD method, a sputtering method, a vacuum evaporation method, an electron beam evaporation method, a spray method, a screen printing method, or the like.

  The back surface reflection layer 5 reflects light that has not been absorbed in the semiconductor photoelectric conversion layer 3 and returns it to the semiconductor photoelectric conversion layer 3, thereby contributing to an improvement in the conversion efficiency of the thin film solar cell 101. The thickness of the back reflective film 5 is preferably 100 nm or more and 400 nm or less.

<Backside transparent electrode layer>
By forming the back surface transparent electrode layer 4 between the semiconductor photoelectric conversion layer 3 and the back surface reflection layer 5, it is possible to improve the light confinement effect and the light reflectivity with respect to incident light. Furthermore, by forming the back surface transparent electrode layer 4, it is possible to prevent the elements contained in the back surface reflection film 5 from diffusing into the semiconductor photoelectric conversion layer 3.

  Typically, the back transparent electrode layer 4 is formed by the same material and manufacturing method as the transparent electrode layer 2. In addition, it is preferable that the thickness of the back surface transparent electrode layer 4 is 20 nm or more. However, if the back surface transparent electrode layer 4 is too thick, the amount of light re-entering the semiconductor photoelectric conversion layer 3 decreases due to light absorption generated in the back surface transparent electrode layer 4. Therefore, it is preferable that the thickness of the back surface transparent electrode layer 4 is 20 nm or more and 3000 nm or less.

  Here, in the present embodiment, the back transparent electrode layer 4 is a layer mainly composed of elements of Al, Zn, Mg, and O (oxygen). The main components are Zn and O, and as will be described later, they contain several weight% of Al and Mg. The reason is described below.

  The back transparent electrode layer 4 receives mainly light having a long wavelength that could not be absorbed by the semiconductor photoelectric conversion layer 3. Since this long wavelength light is reflected by the back surface reflection layer 5 and is preferably reabsorbed as much as possible in the semiconductor photoelectric conversion layer 3, in the back surface transparent electrode layer 4, a material having a long wavelength light absorption is selected. However, it is essential for improving the cell characteristics. Moreover, it is preferable that the back surface transparent electrode layer 4 has electrical conductivity. This is because the carriers generated in the semiconductor photoelectric conversion layer 3 are taken out. Further, as a material for the back surface transparent electrode layer 4, it is possible to select a material that can be formed by an inexpensive method without causing thermal or mechanical damage to the semiconductor photoelectric conversion layer 3 as much as possible. In addition, the manufacturing cost is reduced. Therefore, it can be said that it is preferable to select ZnO as the material of the back transparent electrode layer 4 from such a viewpoint. This is because ZnO absorbs light of a long wavelength and has appropriate electrical conductivity depending on the degree of oxygen defects. Moreover, it is because a film | membrane can be obtained using sputtering method which is a typical thin film formation method.

  Here, the back surface transparent electrode layer 4 of the present invention contains Mg. By including Mg, Mg is preferentially bonded to O (oxygen) of ZnO, so that a ZnO film having oxygen defects that cannot be obtained usually only by controlling the deposition conditions of ZnO is obtained. Is possible. As a result, the work function changes as the bond orbitals of ZnO change greatly. In typical ZnO, the work function is −4.7 eV, but in ZnO containing Mg, the absolute value of the work function decreases. ZnO containing Mg has a work function of about −4.4 eV, depending on the Mg content. That is, the work function of ZnO containing Mg is larger than the work function of ZnO not containing Mg. Although clear natural scientific considerations have not been completed, this work function change suggests that the Fermi level has shifted to the vacuum level. Here, considering the band diagram of the contact state of the semiconductor photoelectric conversion layer 3 and the back surface transparent electrode layer 4 in a thermal equilibrium state, the valence band of the back surface transparent electrode layer 4 is reduced by the absolute value of the work function, The shape protrudes to the far side from the vacuum level. Since the protruding portion can play a role of blocking recombination of holes generated in the semiconductor photoelectric conversion layer 3, it is considered that the open circuit voltage and the fill factor are improved. For this reason, the back transparent electrode layer 4 of the present invention contains Mg.

  Further, the back transparent electrode layer 4 further preferably contains a Group 13 element. This is because since Zn is a Group 12 element, the Group 13 element is replaced with Zn in ZnO to become a carrier source. Therefore, since the electrical conductivity of the back surface transparent electrode layer 4 which consists of ZnO containing Mg increases, it is preferable. Of the Group 13 elements, Al is most preferably used. This is because the optical absorption edge of the ZnO film containing the element shifts to the longer wavelength side as the atomic group radius is smaller, among the various Group 13 elements, that is, the atomic radius is smaller. This is because light absorption decreases and light absorption on the short wavelength side increases. This is described as the Burstein-Moss shift effect. As described above, it is preferable that the back transparent electrode layer 4 has less light absorption of long wavelength light. However, when B (boron) having the smallest atomic radius among Group 13 elements is used, the electrical conductivity cannot be increased so much as compared with other Group 13 elements. This is presumably because B is too different in size from Zn and causes the crystal structure of ZnO to be greatly distorted, resulting in a decrease in film mobility. For the above reasons, it is most preferable to use Al as the Group 13 element.

  From the above, the most desirable backside transparent electrode layer 4 is composed of ZnO as a main component and added with Mg and Al.

  In addition, it is preferable that the density | concentration of Mg contained in a back surface transparent electrode layer is 3 to 10 weight%. This is because if the Mg content is 3% by weight or more, the work function can be sufficiently changed, so that the effect of improving the open circuit voltage, the fill factor, and the like can be sufficiently obtained. Moreover, it is because the electrical conductivity of the back surface transparent electrode layer 4 does not become low too much that Mg is 10 weight% or less, a series resistance does not generate | occur | produce, and a cell characteristic is not deteriorated.

  The concentration of Al contained in the back transparent electrode layer 4 is preferably 1% by weight or more and 5% by weight or less. This is because Al functions as a carrier source for imparting electrical conductivity in a film containing ZnO as a main component, but sufficient electrical conductivity can be secured when Al is 1% by weight or more. Further, when Al is 5% by weight or less, Al acts as a dopant without causing Al to be a carrier scattering factor and reducing the electrical conductivity. Therefore, series resistance is not generated and cell characteristics are not deteriorated.

As described above, the back transparent electrode layer 4 is more preferably in contact with the semiconductor photoelectric conversion layer 3 and includes a layer containing Mg and a layer in contact with the back reflection layer 5 and not containing Mg.
This is to prevent Mg and Ag contained in the back reflective layer from being alloyed when the thin film solar cell 101 is heat-treated. The layer that is in contact with the back surface reflection layer 5 and does not contain Mg is formed by the same material and manufacturing method as the transparent electrode layer 2. Among these, it is preferable to use ITO and ZnO as materials.

  ITO has a relatively high electrical conductivity among various transparent conductive films, and has a low light absorption at a long wavelength as much as ZnO, although it depends on the tin concentration. The major difference from ZnO is that it has the advantage of higher resistance to humidity and chemicals than ZnO, and the disadvantage of high cost. Therefore, after forming the back surface transparent electrode layer 4 and before forming the back surface reflection layer 5, it is preferable to use ITO when performing a manufacturing process that exposes to the atmosphere by taking it out of a vacuum device or the like. The advantages of using ZnO are as described above. To this ZnO, a group 13 element may be added in order to increase electrical conductivity.

(Manufacturing method of AZMO layer)
When the AZMO layer is formed, the same manufacturing method as that for the transparent electrode layer 2 and the back surface transparent electrode layer 4 can be employed. However, it is preferable to prepare an AZMO target and prepare it by sputtering.

Prior to film formation, the substrate is suitably heated. The heating temperature is not particularly limited, but about 100 ° C. is appropriate. The film formation chamber is preferably evacuated in advance. For example, the degree of vacuum is preferably about 5 × 10 ⁻³ Pa or higher.

When sputtering is performed, it is preferable that both the AZMO target and the ITO target are formed by DC sputtering. The sputtering conditions are not particularly limited. For example, the power level may be a discharge density of about 0.5 W / cm ² . Regarding the setting of the magnetic field strength, specifically, the magnetic field strength of both the AZMO target and the ITO target may be about 200 to 250 G or more.

  By the manufacturing method described above, it is possible to obtain a super straight type thin film solar cell with high short-circuit current density and high conversion efficiency.

(Embodiment 2)
A thin film solar cell as a thin film photoelectric conversion element in Embodiment 2 based on this invention is demonstrated. The thin-film solar cell in the present embodiment is different from the thin-film solar cell 101 in the first embodiment only in that it is a tandem type, so the description of the other components will not be repeated.

  FIG. 2 is a cross-sectional view of tandem thin-film solar cell 200 according to Embodiment 2 of the present invention. The tandem thin film solar cell 200 has a second semiconductor photoelectric conversion layer 20 in addition to the first semiconductor photoelectric conversion layer 3. In the second semiconductor photoelectric conversion layer 20, a p-type semiconductor layer 22, an i-type semiconductor layer 23, and an n-type semiconductor layer 24 are stacked in order from the light-transmitting substrate 1 side to form a pin junction.

  In a tandem thin film silicon solar cell, the pin junction closest to the light incident side is often referred to as a top cell, and the pin junction farthest from the light incident side is referred to as a bottom cell. In a tandem thin film solar cell having three or more pin junctions, the pin junction located between the top cell and the bottom cell is often called a middle cell.

  The tandem thin film solar cell 200 in the present embodiment is a thin film solar cell having a super straight type structure. In the tandem thin film solar cell 200, the transparent electrode layer 2, the second semiconductor photoelectric conversion layer 20, the first semiconductor photoelectric conversion layer 3, and the back surface transparent electrode layer which is an AZMO layer are formed on the light transmissive substrate 1. 4 and the back reflective layer 5 are laminated in this order.

  Since the second semiconductor photoelectric conversion layer 20 is located on the light incident side, only the light transmitted through the second semiconductor photoelectric conversion layer 20 is incident on the first semiconductor photoelectric conversion layer 3. In the tandem thin film solar cell 200, the first and second semiconductor photoelectric conversion layers 3 and 20 can receive different spectral regions of incident light, so that the light use efficiency can be improved. A high open-circuit voltage almost equal to the sum of the open-circuit voltages of the semiconductor photoelectric conversion layers 3 and 20 can be obtained.

  Further, by making the band gap of the second semiconductor photoelectric conversion layer 20 located on the light incident side larger than the band gap of the first semiconductor photoelectric conversion layer 3, the short wavelength light of the incident light is mainly the second. In the semiconductor photoelectric conversion layer 20, long-wavelength light is mainly absorbed by the first semiconductor photoelectric conversion layer 3, so that the conversion efficiency of the tandem-type thin film solar cell 200 can be further improved.

  Examples of silicon-based materials having different band gaps include amorphous silicon carbide, amorphous silicon, amorphous silicon germanium, amorphous germanium, microcrystalline silicon, microcrystalline silicon germanium, and microcrystalline germanium. These can be appropriately selected and used as the light absorption layer.

  For example, in a typical two-junction thin-film silicon solar cell using amorphous silicon for the top cell and microcrystalline silicon for the bottom cell, amorphous silicon having a large band gap absorbs light in the short wavelength region of the incident light. However, microcrystalline silicon having a small band gap is designed to absorb the remaining light in the long wavelength region.

  Microcrystalline silicon and microcrystalline silicon germanium are known not to cause photodegradation as described above, and are generally preferred for use as materials that absorb light of long wavelengths in multi-junction thin-film silicon solar cells. It has been.

  If the first and second semiconductor photoelectric conversion layers 3 and 20 are arranged such that the stacking directions of the pin junctions are the same and the p-type semiconductor layers 3p and 22 are located on the light incident side. The same applies when there are three or more photoelectric conversion layers. That is, when the first semiconductor photoelectric conversion layer 3 has a pin junction, the second semiconductor photoelectric conversion layer 20 also has a pin junction, and when the first semiconductor photoelectric conversion layer 3 has a nip junction, the second semiconductor photoelectric conversion layer 3 has a pin junction. The semiconductor photoelectric conversion layer 20 also has a nip junction.

  The film thickness of each semiconductor layer in the first and second semiconductor photoelectric conversion layers 3 and 20 is not particularly limited, but in the second semiconductor photoelectric conversion layer 20, the film thickness of the p-type semiconductor layer 22 is It is preferable that the thickness of the i-type semiconductor layer 23 is 100 nm to 500 nm and the thickness of the n-type semiconductor layer 24 is 5 nm to 50 nm. More preferably, the p-type semiconductor layer 22 has a thickness of 10 nm to 30 nm, the i-type semiconductor layer 23 has a thickness of 200 nm to 400 nm, and the n-type semiconductor layer 24 has a thickness of 10 nm to 30 nm.

  In the first semiconductor photoelectric conversion layer 3, the thickness of the p-type semiconductor layer 3p is 5 nm or more and 50 nm or less, the thickness of the i-type semiconductor layer 3i is 1000 nm or more and 5000 nm or less, and the thickness of the n-type semiconductor layer 3n is 5 nm or more. It is preferable that it is 100 nm or less. More preferably, the p-type semiconductor layer 3p has a thickness of 10 nm to 30 nm, the i-type semiconductor layer 3i has a thickness of 2000 nm to 4000 nm, and the n-type semiconductor layer 3n has a thickness of 10 nm to 30 nm.

  An intermediate layer may be formed between at least one pair of mutually adjacent pin junctions or nip junctions among the plurality of pin junctions or the plurality of nip junctions. That is, an intermediate layer may be formed between the first semiconductor photoelectric conversion layer 3 and the second semiconductor photoelectric conversion layer 20. When there are three or more photoelectric conversion layers, an intermediate layer may be formed in at least one place between the photoelectric conversion layers. The intermediate layer is preferably composed of a light-transmitting conductive film.

  By providing the intermediate layer, part of the light that has passed through the second semiconductor photoelectric conversion layer 20 and entered the intermediate layer is reflected by the intermediate layer, and the remaining part is transmitted through the intermediate layer and transmitted through the first semiconductor photoelectric conversion layer. Therefore, the amount of incident light on each photoelectric conversion layer can be controlled.

  Thereby, the value of the photocurrent in the first and second semiconductor photoelectric conversion layers 3 and 20 can be equalized. By efficiently using photogenerated carriers generated in the first and second semiconductor photoelectric conversion layers 3 and 20, the conversion efficiency can be improved by increasing the short-circuit current value of the tandem-type thin film solar cell 200.

(Embodiment 3)
The present invention can also be applied to a substrate-type thin film solar cell. Next, Embodiment 3 in which the present invention is applied to a substrate type thin film solar cell will be described. Thin-film solar cell 300 in the present embodiment differs from thin-film solar cell 101 in the first embodiment only in that it is a substrate type, and therefore the description of the other components will not be repeated.

  FIG. 3 is a cross-sectional view of a thin-film solar cell according to Embodiment 3 based on the present invention. As shown in FIG. 3, in the substrate type thin film solar cell 300 according to the third embodiment of the present invention, the back surface reflection film 5 and the back surface transparent electrode layer 4 which is an AZMO layer are formed on the support substrate 31c. The semiconductor photoelectric conversion layer 3, the transparent electrode layer 2, and the surface extraction electrode 32 are formed in this order.

  In the present embodiment, since light enters from the transparent electrode layer 2 side, the support substrate 31c may be made of a non-translucent material. Alternatively, like the light transmissive substrate 1, the support substrate 31c may be made of a light transmissive material. As the material of the support substrate 31c, the same material as that of the light transmissive substrate 1 can be used. In addition, the support substrate 31c may be made of a metal plate such as a stainless plate or a copper plate. The thickness of the support substrate 31c is not particularly limited as long as it is strong enough to structurally support the entire thin film solar cell 300.

  In the present embodiment, irregularities (not shown) for improving the light confinement effect are formed on the support substrate 31c. However, the support substrate 31c does not have unevenness, and a transparent conductive film having unevenness may be formed on the support substrate 31c, or the back surface reflective film 5 may be processed to form unevenness. As a result, it is desirable that irregularities having a surface roughness RMS (root mean square roughness) of 20 nm or more and 200 nm or less are formed on the back surface reflecting film 5 as a result.

  The unevenness described above exhibits a light confinement effect in the back surface reflection film 5 itself, and unevenness due to the uneven shape on the back surface reflection film 5 is formed on the surface after the transparent electrode layer 2 is deposited. The light confinement effect can also be obtained.

  In the present embodiment, since a light confinement effect can be obtained, the photoelectric conversion efficiency of the substrate type thin film solar cell 300 can be improved.

  In addition, the substrate type thin film solar cell 300 in the present embodiment has a pin structure from the light incident side, similar to the structure of the super straight type thin film solar cell 101 in the first embodiment. It may have a nip structure from the light incident side.

  A surface extraction electrode 32 may be formed on the transparent electrode layer 2. As the material of the front surface extraction electrode 32, a material having high conductivity such as silver is preferably used similarly to the back surface reflection layer 5. When the surface extraction electrode 32 is formed of a non-translucent material, the surface extraction electrode 32 may block light incident on the photoelectric conversion layer 10 and reduce the photoelectric conversion efficiency of the thin-film solar cell. It is preferable that the surface extraction electrode 32 has a small area.

Hereinafter, experimental examples in which the power generation characteristics of the thin film solar cell according to the present invention are confirmed will be described.
(Experimental example)
Example 1
In Example 1, the super straight tandem thin film solar cell 200 shown in FIG. 2 was produced as follows.

  Blue plate glass (manufactured by Asahi Glass Co., Ltd., trade name: Asahi-) having a transparent electrode layer 2 made of a tin oxide-based transparent conductive film formed on the surface of the light-transmitting substrate 1 and having a length of 115 mm × width of 115 mm × thickness of 3.9 mm VU) was used.

  Thereafter, the p-type semiconductor layer 3p, the i-type semiconductor layer 3i, and the n-type semiconductor layer 3n were sequentially stacked by plasma CVD under the conditions described later to form the semiconductor photoelectric conversion layer 3. Subsequently, a second semiconductor photoelectric conversion layer 20 was formed on the semiconductor photoelectric conversion layer 3. Then, the back surface transparent electrode layer 4 and the back surface reflection layer 5 were formed on the 2nd semiconductor photoelectric converting layer 20, and the super straight type tandem-type thin film solar cell 200 was produced.

Each layer of the semiconductor photoelectric conversion layer 3 was formed under the following conditions. For the formation of the p-type semiconductor layer 3p, a mixed gas containing SiH ₄ , H ₂ and B ₂ H ₆ was used as a source gas. The gas flow ratio of H _{2 to} SiH ₄ was 20 times, and the gas flow ratio of B ₂ H ₆ to SiH ₄ was 0.3 times.

  The p-type semiconductor layer 3p is desirably thin so as not to impair the function as the p-type semiconductor layer in order to increase the amount of light incident on the i-type semiconductor layer 3i, which is a photoactive layer. The film thickness was taken.

For forming the i-type semiconductor layer 3i, a mixed gas containing SiH ₄ and H ₂ was used as a source gas. The gas flow ratio of H _{2 to} SiH ₄ was 15 times. In this example, the film thickness of the i-type semiconductor layer 3i was 250 nm.

For forming the n-type semiconductor layer 3n, a mixed gas containing SiH ₄ , H ₂ , PH ₃ and N ₂ was used as a source gas. The gas flow ratio of H _{2 to} SiH ₄ was 20 times, and the gas flow ratio of PH _{3 to} SiH ₄ was 0.3 times.

  The film thickness of the n-type semiconductor layer 3n is desirably thin enough not to impair the function as the n-type semiconductor layer in order to suppress light absorption in this layer. In this embodiment, the film thickness is set to 20 nm. The substrate temperature at the time of forming the p-type, i-type and n-type semiconductor layers 3p, 3i, 3n by plasma CVD was set to 180 ° C.

Subsequently, each layer of the second semiconductor photoelectric conversion layer 20 was formed under the following conditions. For the formation of the p-type semiconductor layer 22, a mixed gas containing SiH ₄ , H ₂ and B ₂ H ₆ was used as a source gas. The gas flow ratio of H _{2 to} SiH ₄ was 150 times, and the gas flow ratio of B ₂ H ₆ to SiH ₄ was 0.003 times.

  The p-type semiconductor layer 22 is desirably thin so as not to impair the function as the p-type semiconductor layer in order to increase the amount of light incident on the i-type semiconductor layer 23 that is a photoactive layer. In this embodiment, the p-type semiconductor layer 22 has a thickness of 20 nm. The film thickness was taken.

In forming the i-type semiconductor layer 23, a mixed gas containing SiH ₄ and H ₂ was used as a source gas. The gas flow ratio of H _{2 to} SiH ₄ was 80 times. In this example, the film thickness of the i-type semiconductor layer 23 was 1900 nm.

For forming the n-type semiconductor layer 24, a mixed gas containing SiH ₄ , H ₂ , PH ₃ and N ₂ was used as a source gas. The gas flow ratio of H _{2 to} SiH ₄ was 100 times, and the gas flow ratio of PH 3 to SiH ₄ was 0.03 times.

  The film thickness of the n-type semiconductor layer 24 is desirably thin enough not to impair the function as the n-type semiconductor layer in order to suppress absorption of reflected light from the back surface reflection layer 5. In this embodiment, the film thickness is 20 nm. It was. The substrate temperature at the time of film formation by plasma CVD of the p-type, i-type, and n-type semiconductor layers 22, 23, and 24 was set to 180 ° C.

  Subsequently, AZMO was deposited as the back transparent electrode layer 4 to a thickness of 80 nm by sputtering. Thereafter, silver was deposited as the back reflective layer 5 so as to have a thickness of 200 nm by sputtering.

At the time of forming the AZMO film by the sputtering method, after evacuating until the pressure in the film forming chamber became 1 × 10 ⁻⁴ Pa or less, Ar gas was supplied at 20 sccm, and the pressure was maintained at 1 mTorr in this state. Thereafter, high frequency power of 13.56 MHz was applied so that the power density was 0.43 W / cm ² , and plasma discharge was generated on the AZMO target.

  Similarly, as a result of analyzing the composition of the AZMO film formed on the glass by the SIMS (secondary ion mass spectrometry) method, it was confirmed that Zn, Al, Mg, and O were contained. As for the composition, Mg was 4% by weight, Al was 2.6% by weight, and the others were mainly ZnO.

  Further, the band gap was obtained by plotting the transmission / reflection spectrum of this AZMO film at a wavelength of 300 to 1200 nm to 3.3 eV. Moreover, as a result of obtaining the work functions of the AZMO film and the n-type semiconductor layer 24 by the Kelvin probe method using FAC-2 manufactured by Riken Keiki, the work function of the AZMO film is −4.3 eV, and n-type. The work function of the semiconductor layer 24 was −4.5 eV.

For such a photoelectric conversion device of Example 1 obtained in the, AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. Further, after heat treatment (annealing) for 45 minutes at the cell 170 ° C., AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. These results are shown in Table 1 together with other examples and comparative examples. In addition, as each characteristic in Table 1, the normalized value which is a value divided by the characteristic obtained in Comparative Example 1 is listed.

(Example 2)
In Example 2, a superstrate was formed in the same manner as Example 1 except that an ITO layer was formed as a layer that did not contain Mg and was in contact with the back surface reflective layer 5 following the formation of the back surface transparent electrode layer 4. Type tandem-type thin film solar cell 200 was produced. For the photoelectric conversion device obtained in Examples 2, AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. Further, after heat treatment (annealing) for 45 minutes at the cell 170 ° C., AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. These results are shown in Table 1 together with other examples and comparative examples. Each characteristic includes a normalized value that is a value divided by the characteristic obtained in Comparative Example 1.

(Example 3)
In Example 3, following the formation of the back surface transparent electrode layer 4, a superstrate was carried out in the same manner as in Example 1 except that a ZnO layer was formed as a layer not containing Mg and in contact with the back surface reflection layer 5. Type tandem-type thin film solar cell 200 was produced. For the photoelectric conversion device obtained in Examples 2, AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. Further, after heat treatment (annealing) for 45 minutes at the cell 170 ° C., AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. These results are shown in Table 1 together with other examples and comparative examples. Each characteristic includes a normalized value that is a value divided by the characteristic obtained in Comparative Example 1.

(Comparative Example 1)
In Comparative Example 1, a super straight type tandem thin film solar cell 200 was fabricated in the same manner as Example 1 except that a ZnO layer was formed as the back transparent electrode layer 4. For the photoelectric conversion device obtained in Examples 2, AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. Further, after heat treatment (annealing) for 45 minutes at the cell 170 ° C., AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. These results are shown in Table 1 together with other examples and comparative examples. Each characteristic includes a normalized value that is a value divided by the characteristic obtained in Comparative Example 1.

(Comparative Examples 2 to 5)
In Comparative Examples 2 to 5, the composition of the AZMO target was changed, and the backside transparent electrode layer 4 having a different Mg concentration or Al concentration in the film was formed. A tandem thin film solar cell 200 was produced. For the photoelectric conversion device obtained in Examples 2, AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. Further, after heat treatment (annealing) for 45 minutes at the cell 170 ° C., AM 1.5, under the conditions of irradiance 100 mW / cm ^2, current cell area 1 cm ² - voltage characteristics were measured. These results and the Mg concentration and Al concentration in the film are shown in Table 1 together with other examples and comparative examples. Each characteristic shows a normalized value which is a value divided by the characteristic obtained in Comparative Example 1.

(Consideration from comparison between Example 1 and Comparative Example 1)
In Example 1 in which the back transparent electrode layer 4 contains Al, Mg, Zn, and O, all of the short-circuit current density, the open-circuit voltage, and the fill factor are improved as compared with Comparative Example 1 in which the back transparent electrode layer 4 is ZnO. As a result, the conversion efficiency was improved.

  Since the back surface transparent electrode layer 4 of Example 1 contains Mg, the work function is shallower than the n-type semiconductor layer 24 (close to the vacuum level side). 4 is expected to be deeper (farther than the vacuum level) than the valence band of the n-type semiconductor layer 24. As a result of this energy barrier suppressing the recombination of holes, the conversion efficiency is improved. Expected to improve.

  Moreover, since favorable electrical conductivity can be ensured by including Al, an increase in series resistance is not observed, and there is no factor for lowering the fill factor.

  From the above, it was shown that the present Example in which the back surface transparent electrode layer 4 contains Al, Mg, Zn, and O has an advantage in increasing the efficiency of the thin film solar cell.

(Consideration from Examples 2 and 3 and Comparative Example 1)
Following the formation of the back transparent electrode layer 4 containing Al, Mg, Zn, and O, Example 2 in which ITO was formed as a layer that did not contain Mg and was in contact with the back reflective layer 5 and Example 3 in which ZnO was formed Compared with the comparative example 1 whose back surface transparent electrode layer 4 is ZnO, as a result of improving all of a short circuit current density, an open circuit voltage, and a fill factor, conversion efficiency improved. In particular, regarding the improvement rate of the cell characteristics after the heat treatment, Example 2 and Example 3 were higher than Comparative Example 1 and Example 1.

  In general, when a thin film material is heat-treated, the electrical characteristics and optical characteristics of the film are often improved by improving the crystallinity and optimizing the atomic arrangement. However, care must be taken at the contact interface with dissimilar materials because thermal energy causes alloying due to atom migration and bond formation, which may result in unexpected compositional changes.

  In Example 1, the characteristic of the thin film solar cell after heat processing became a value which is not largely different from the comparative example 1. This is because the film quality of the back surface transparent electrode layer 4 is improved by heat treatment, while Mg contained in the back surface transparent electrode layer 4 and Ag contained in the back surface reflective layer 5 are alloyed from the back surface transparent electrode layer 4. It is presumed that Mg removal also occurred at the same time, and as a result, the Mg concentration was reduced in the back surface transparent electrode layer 4 after the heat treatment, so that the effect of improving the properties by Mg was lost.

  On the other hand, in Example 2 and Example 3, ITO or ZnO, which is in contact with the back reflective layer 5 and does not contain Mg, hinders alloying between Mg and Ag. It can be obtained appropriately, and as a result, it seems that higher conversion efficiency than Comparative Example 1 was realized.

(Consideration from comparison between Example 1 and Comparative Examples 2 to 5)
In Comparative Examples 2 to 5 in which the Mg concentration and the Al concentration were increased or decreased with respect to Example 1, the cell characteristics were not improved or decreased compared to Example 1.

  In Comparative Example 2 where the Mg concentration was lower than that of Example 1, the cell characteristics were not improved. This is presumed that the effect of improving the characteristics was not obtained because the Mg concentration was too low. The work function of the single film of Comparative Example 2 is −4.5 eV, which is not much different from that of the backside transparent electrode layer 4 not containing Mg. In Comparative Example 2, the work concentration was too low. The function does not change, and therefore the energy barrier that blocks the recombination of holes does not occur, so it seems that the characteristics were not improved.

  Further, in Comparative Example 3 in which the Mg concentration was higher than that in Example 1, the cell characteristics were lowered due to a decrease in the fill factor. The decrease in fill factor was due to an increase in series resistance. Since the sheet resistance of the single film of Comparative Example 3 is several MΩ / □, which is 10 times or more larger than that of Example 1, the electrical conductivity of the back surface transparent electrode layer 4 deteriorated due to an increase in the amount of Mg. It seems that the fill factor decreased and the cell characteristics decreased.

  Similar results were obtained in Comparative Example 4 in which the Al concentration was lower than that in Example 1. The sheet resistance of the single film of Comparative Example 4 was also several MΩ / □. This is presumably due to the low carrier density of the backside transparent electrode layer 4 and low electrical conductivity due to the small amount of Al. As a result, since the electrical conductivity of the back surface transparent electrode layer 4 was deteriorated, the fill factor was decreased, and the cell characteristics were decreased.

  Further, in Comparative Example 5 in which the Al concentration is higher than that in Example 1, both the open circuit voltage and the fill factor are the same as those in Example 1, but the cell characteristics are lower than in Example 5 due to the decrease in the short circuit current density. It was. This is probably because the light absorption amount of the back surface transparent electrode layer 4 increased due to excessive Al doping, and as a result, the amount of reflected light from the back surface reflection layer 5 decreased, resulting in a decrease in short circuit current density in the photoelectric conversion layer. It is.

  In addition, the said embodiment disclosed this time is an illustration in all the points, Comprising: It is not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and includes all modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 1 Light transmissive substrate, 2 Transparent electrode layer, 2a Zinc oxide layer, 3 (1st) semiconductor photoelectric conversion layer, 3i i type semiconductor layer, 3n n type semiconductor layer, 3p p type semiconductor layer, 4 back surface transparent electrode layer 5 Back surface reflection layer, 6 preparation chamber, 7 film formation chamber, 8 AZMO target, 9 ITO target, 10 magnet, 11 power source, 20 second semiconductor photoelectric conversion layer, 22 p-type semiconductor layer, 23 i-type semiconductor layer, 24 n-type semiconductor layer, 31c support substrate, 32 surface extraction electrode, 40 (conventional) back surface transparent electrode layer, 100 (based on conventional technology) thin film solar cell, 101,300 thin film solar cell, 200 tandem thin film solar cell, 201 Sputtering device.

Claims (3)

A light transmissive substrate having a main surface;
In order from the side closer to the light transmissive substrate on the main surface, a transparent electrode layer, a pin semiconductor bonding layer, a back surface transparent electrode layer, and a back surface reflection layer,
The back transparent electrode layer, Mg and Zn, seen containing a metal oxide containing at least one Group 13 element, the back reflecting layer comprises Ag, the back transparent electrode layer comprises a plurality of layers, wherein The back transparent electrode layer is a thin film photoelectric conversion element having a layer containing Mg in contact with the pin semiconductor bonding layer and a layer not in contact with the back reflective layer . The thin film photoelectric conversion element according to claim 1 , wherein the back transparent electrode layer contains 3 wt% or more and 10 wt% or less of Mg. The back transparent electrode layer includes Al 1 wt% to 5 wt% or less, thin film photoelectric conversion device according to claim 1 or 2.

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