JP2020053668A - Solar cell, multi-junction type solar cell, solar cell module and photovoltaic power generation system - Google Patents

Abstract

To provide solar cell, multi-junction type solar cell, solar cell module and photovoltaic power generation system, capable of achieving high conversion efficiency.SOLUTION: The solar cell includes: a first electrode; a second electrode and a photoelectric conversion layer provided between the first electrode and the second electrode. When a photoluminescence spectrum of the photoelectric conversion layer is measured at a temperature of 100 K or less, a first maximum value (A) which is a maximum value of light emission intensity in a wavelength region of a range of more than 650 nm and 1000 nm or less is 100 times or less (A≤100B) of a second maximum value (B) which is a maximum value of light emission intensity of 600 nm or more and 650 nm or less.SELECTED DRAWING: Figure 1

Description

  Embodiments relate to a solar cell, a multi-junction solar cell, a solar cell module, and a solar power generation system.

  As a high-efficiency solar cell, there is a multi-junction (tandem) solar cell. Since a tandem solar cell can use a cell having high spectral sensitivity for each wavelength band, higher efficiency can be achieved than a single junction. In addition, as a top cell of a tandem solar cell, a cuprous oxide compound which is an inexpensive material and has a wide band gap is expected. However, an efficiency of about 8% has been reported so far in a cuprous oxide solar cell manufactured by oxidizing a copper foil, but it is lower than the theoretical limit efficiency. This is because a foreign phase such as copper oxide on the outermost surface is removed by etching after oxidizing the copper foil, but a good pn junction has not been formed because it cannot be completely removed and a constituent element of the etching solution remains. It is thought to be. Further, in this method, it is necessary to oxidize a foil having a thickness of about 0.1 mm and then polish the foil to about 20 μm, and it is difficult to increase the area.

  On the other hand, there is an example in which a thin film is manufactured by a method using a reaction in a liquid phase, but the efficiency is at most about 4%. It is considered that the main reason is that not only the heterophase but also impurities contained in the solution are taken into the film, and these become the recombination centers of the photoexcited carriers. Such a thin film also absorbs light having a wavelength of 600 nm or more, which is not originally absorbed, and therefore cannot be used for a tandem solar cell top cell. In general, a sputtering method is well known as a method for producing a thin film with a small amount of impurities mixed therein, and there is a report example produced by this method, but the conversion efficiency was 1% or less. It is considered that the reason is that even if there is no contamination of impurities, a different phase of copper or copper oxide is likely to be generated, and pure cuprous oxide cannot be easily obtained.

Tadatsugu Minami et al. Applied Physics Express 9, 052301 (2016)

  Embodiments provide a solar cell, a multi-junction solar cell, a solar cell module, and a solar power generation system with improved conversion efficiency.

  The solar cell according to the embodiment includes a first electrode, a second electrode, and a photoelectric conversion layer provided between the first electrode and the second electrode. The photoluminescence layer has a photoluminescence spectrum at a temperature of 100 K or less. Is measured, the first maximum value (A) which is the highest value of the light emission intensity in the wavelength range of 650 nm to 1000 nm is the second highest value (the highest value of the light emission intensity at 600 nm to 650 nm). B) 100 times or less (A ≦ 100B).

FIG. 2 is a conceptual cross-sectional view of the solar cell according to the embodiment. FIG. 2 is an example diagram when a photoluminescence spectrum of the solar cell according to the first embodiment is measured. The schematic diagram of the measuring method of the photoelectric conversion layer using X-ray photoelectron spectroscopy. 3 is a flowchart of a manufacturing method according to the solar cell of the first embodiment. FIG. 5 is a conceptual cross-sectional view of a multi-junction solar cell according to a second embodiment. The conceptual diagram of the solar cell module concerning 3rd Embodiment. FIG. 10 is a conceptual sectional view of a solar cell module according to a third embodiment. The conceptual diagram of the solar power generation system concerning 4th Embodiment.

(1st Embodiment)
The first embodiment relates to a solar cell. FIG. 1 shows a conceptual diagram of the solar cell 100 of the first embodiment. As shown in FIG. 1, the solar cell 100 according to the present embodiment includes a substrate 1, a first electrode 2 on the substrate 1, a photoelectric conversion layer 3 on the first electrode 2, and n on the photoelectric conversion layer 3. A mold layer; and a second electrode on the n-type layer. An intermediate layer (not shown) may be included between the first electrode 2 and the photoelectric conversion layer 3 or between the n-type layer 4 and the second electrode 5.

(substrate)
As the substrate 1 of the embodiment, it is desirable to use white plate glass, and it is preferable to use glass such as quartz, soda lime glass, and chemically strengthened glass in general, stainless steel (SUS), a metal plate such as W, Ta, Al, Ti, or Cr, or polyimide; A resin such as acrylic can also be used.

(First electrode)
The first electrode 2 of the embodiment is a layer existing between the substrate 1 and the photoelectric conversion layer 3. In FIG. 1, the first electrode 2 is in direct contact with the substrate 1 and the photoelectric conversion layer 3. The first electrode 2 is preferably a transparent conductive film, or a laminate of a metal film, a transparent conductive film and a metal film. The transparent conductive film includes indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), boron-doped zinc oxide (BZO), and gallium-doped zinc oxide (Gallium). -doped Zinc Oxide: GZO), Fluorine-doped Tin Oxide (FTO), Antimony-doped Tin Oxide (ATO), Titanium-doped Indium Oxide (ITiO), Indium zinc oxide (Indium Zinc Oxide: IZO), indium gallium zinc oxide (IGO), tantalum-doped tin oxide (Ta-doped Tin Oxide: SnO ₂ : Ta), niobium-doped tin oxide (Nb-doped Tin) Oxide: SnO ₂ : Nb), tungsten-doped tin oxide (SnO ₂ : W), molybdenum-doped tin oxide (Mo-doped Tin Oxide: SnO ₂ : Mo), fluorine-doped There is no particular limitation on tin oxide (F-doped Tin Oxide: SnO ₂ : F) and hydrogen-doped indium oxide (Hydrogen-doped Indium Oxide: IOH). The transparent conductive film may be a stacked film having a plurality of films, and a film such as tin oxide may be included in the stacked film in addition to the oxide. The dopant for the film such as tin oxide is not particularly limited, such as In, Si, Ge, Ti, copper, Sb, Nb, F, Ta, W, Mo, F, and Cl. The metal film is not particularly limited, such as a film of Mo, Au, Cu, Ag, Al, Ta, or W. Further, the first electrode 2 may be an electrode in which a dot-shaped, line-shaped, or mesh-shaped metal is provided on a transparent conductive film. At this time, the dot-shaped, line-shaped, or mesh-shaped metal is disposed between the transparent conductive film and the photoelectric conversion layer 3 or on the side of the transparent conductive film opposite to the photoelectric conversion layer 3. The dot, line or mesh metal preferably has an aperture ratio of 50% or more with respect to the transparent conductive film. The dot-shaped, line-shaped or mesh-shaped metal is not particularly limited, such as Mo, Au, Cu, Ag, Al, Ta or W.

  Further, a metal substrate may be used for the first electrode instead of the transparent conductive film. For the metal substrate, for example, W, Cr, Ti, Ta, Al, SUS (for example, SUS430) can be used. A photoelectric conversion layer described later may be directly formed on the metal substrate.

(Photoelectric conversion layer)
The photoelectric conversion layer 3 of the embodiment is a p-type compound semiconductor layer. The photoelectric conversion layer is a layer existing between the first electrode 2 and the n-type compound semiconductor layer 4. Hereinafter, the n-type compound semiconductor layer is referred to as an n-type layer. In FIG. 1, the photoelectric conversion layer 3 is in direct contact with the first electrode 1 and the n-type layer 4. The photoelectric conversion layer 3 is a layer containing cuprous oxide. The cuprous oxide in the photoelectric conversion layer 3 is non-doped or doped cuprous oxide. The thickness of the cuprous oxide is generally 500 nm or more and 0.3 mm or less, but is not limited thereto. Cuprous oxide is less expensive than a compound having a chalcopyrite structure, so that the cost of the solar cell 100 can be reduced. Cuprous oxide has a band gap of about 2.1 eV, which is a wide band gap. When the band gap of the photoelectric conversion layer 3 of the solar cell 100 according to the present embodiment is wide, the first embodiment is performed when the solar cell having the narrow band gap photoelectric conversion layer 3 of Si or the like is used as a bottom cell to form a multi-junction. The solar cell 100 according to the first embodiment is preferable in that the solar cell 100 according to the embodiment has high transmittance of wavelengths that contribute to power generation on the bottom cell side, and thus has a high power generation amount on the bottom cell side. When the solar cell 100 according to the first embodiment is used as a multi-junction solar cell, the solar cell 100 according to the first embodiment is preferably provided on the light incident side.

  Here, a method for manufacturing the photoelectric conversion layer 3 included in the solar cell according to the present embodiment will be described.

  The photoelectric conversion layer 3 is formed using sputtering. First, a thin film of cuprous oxide can be obtained by supplying, for example, RF power from a high-frequency power source to a target of oxygen-free copper having a purity of 4N or more in a mixed gas of Ar and oxygen. Other electric power such as a DC power supply may be supplied instead of the RF power supply. At this time, the reaction between oxygen and the electrode (corresponding to the substrate during film formation) during formation of cuprous oxide is appropriately suppressed due to the presence of an oxide on at least a part of the electrode, for example, the substrate during sputtering. And good crystals are easily obtained. For example, when a transparent conductive oxide is used, it can be used as an electrode, which is preferable in that the structure is simplified. Therefore, an oxide is preferably present in at least a part of the electrode. The power used for sputtering may be DC. At this time, the substrate is heated to 400 ° C to 700 ° C, preferably 450 ° C to 550 ° C. Thus, a cuprous oxide thin film can be manufactured. A different phase such as copper or copper oxide may be formed depending on the flow ratio of Ar and oxygen. For example, it is necessary to adjust the ratio of the Ar flow rate to the oxygen flow rate in order to suppress the formation of copper and copper oxide. Note that copper oxide is CuO.

  Note that since sputtering is performed under reduced pressure, there is almost no oxygen around, and when the film formation temperature of the photoelectric conversion layer 3 is high, the metal is easily reduced. Therefore, in order to prepare cuprous oxide, it is necessary to appropriately oxidize, and therefore, it is necessary to increase the amount of the oxidizing agent, that is, oxygen, in an environment where reduction is easy, that is, in a higher temperature. In other words, when the temperature is high, the atmosphere is more easily reduced, so that it is necessary to increase the oxygen flow rate. Therefore, when the photoelectric conversion layer 3 is formed at a high temperature, a high-quality photoelectric conversion layer 3 can be obtained by increasing the oxygen flow rate.

  It is preferable to use a substrate having transparency for a substrate on which cuprous oxide is formed. This is because, by using a material having transparency, a wavelength region in which cuprous oxide cannot be absorbed can be effectively used on the side opposite to the light incident direction by utilizing the wide gap.

  Copper and copper oxide have different phases in the photoelectric conversion layer 3 included in the solar cell according to the present embodiment. Therefore, by manufacturing a solar cell in which these different phases are reduced, the efficiency of the solar cell can be improved.

  The hetero-phase included in the photoelectric conversion layer 3 greatly affects the photoelectric conversion efficiency (also referred to as conversion efficiency) of the solar cell. As described above, the presence of the hetero-phase serves as a recombination center for photoexcited carriers to lower the conversion efficiency and to lower the quality of the photoelectric conversion layer 3 itself. This is because copper-derived leaks and the like occur, and the fill factor (FF: shape factor) deteriorates. This is because, when copper oxide is contained, the overall cell characteristics are adversely affected by promoting recombination and the like. In addition, by absorbing light in an unintended wavelength range, in the case of a multi-junction solar cell to be described later, the conversion efficiency of the bottom cell is reduced.

  In order to measure a different phase existing in the photoelectric conversion layer 3 of the solar cell, analysis is performed by an X-ray diffraction method, and even when a different phase is not detected in the photoelectric conversion layer 3, high conversion efficiency is not always obtained. This is because there is a different phase that cannot be measured by the X-ray diffraction method. Therefore, a photoluminescence spectrum of a solar cell manufactured using a photoluminescence method (Photo Luminescence; PL method) is measured.

  The PL method is an analysis method in which light having energy equal to or greater than the band gap is applied to a thin film and secondary light emission is examined. Therefore, by using the PL method, the film quality of the photoelectric conversion layer 3 of the solar cell can be examined. The better the film quality of the photoelectric conversion layer 3 is, the higher the peak intensity is measured. By measuring the photoluminescence spectrum, the quality of the photoelectric conversion layer 3 of the solar cell, for example, a different phase, a defect, or the like can be measured.

  In measuring the PL intensity ratio, first, the solar cell is divided into four equal parts vertically and horizontally. Next, excluding the scribe part, five points are selected as measurement points from among the parts where the photoelectric conversion layer 3 exists, and the respective PL intensity ratios are measured. By averaging the PL intensity ratios at these five measurement points, an average PL intensity ratio can be obtained. The measurement point is selected from a crossing point of a line obtained by dividing the solar cell vertically and horizontally into four equal portions within a range of 5% or less with respect to the length and width of the solar cell to be measured. The measurement is performed in a range narrower than the scribe interval.

  With respect to the PL intensity ratio, for cuprous oxide, for example, a laser beam having a wavelength of 532 nm can be used. If a laser beam having an excessively short wavelength is used, a signal from a portion other than the photoelectric conversion layer 3 may be detected. Therefore, it is preferable to use a light source having an energy slightly larger than the band gap of the photoelectric conversion layer 3.

  In a solar cell, the maximum value (A) of the light emission intensity in the wavelength range of 650 nm to 1000 nm in the photoluminescence spectrum at a very low temperature is 100 times or less the maximum value (B) of the light emission intensity in the wavelength range of 600 nm to 650 nm ( A ≦ 100B) is preferable. The wavelengths of 650 nm and below are caused by oxygen deficiency and copper deficiency in the cuprous oxide, and the range of 600 nm to 650 nm is caused by band edge emission of the cuprous oxide. Therefore, high conversion efficiency can be achieved when the maximum value of the light emission intensity in the wavelength range from 650 nm to 1000 nm is 100 times or less the maximum value of the light emission intensity in the wavelength range of 600 nm to 650 nm. Further, it is more preferably 30 times or less (A ≦ 30B). Since the maximum value of the emission intensity in the wavelength range of greater than 650 nm and 1000 nm or less is 30 times or less the maximum value of the emission intensity at 600 nm or more and 650 nm or less, recombination of photoexcited carriers can be further prevented. This is because higher conversion efficiency can be achieved. It is even more preferably 1.5 times or less (A ≦ 1.5B). This is because, since there is less heterogeneous phase, higher conversion efficiency can be achieved, and recombination of photoexcited carriers can be prevented. If the maximum value of the light emission intensity in the wavelength range from 650 nm to 1000 nm is more than 100 times the maximum value of the light emission intensity in the range from 600 nm to 650 nm, the conversion efficiency decreases, which is not preferable (A> 100B).

The method for measuring the photoluminescence spectrum of the solar cell is as follows.
In measuring the PL intensity ratio, first, the solar cell is divided into four equal parts vertically and horizontally. Next, excluding the scribe part, five points are selected as measurement points from among the parts where the photoelectric conversion layer 3 exists, and the respective PL intensity ratios are measured. By averaging the PL intensity ratios at these five measurement points, an average PL intensity ratio can be obtained. The measurement point is selected from a crossing point of a line obtained by dividing the solar cell vertically and horizontally into four equal portions within a range of 5% or less with respect to the length and width of the solar cell to be measured. The measurement is performed in a range narrower than the scribe interval.

As for the PL intensity ratio, laser light having a wavelength of 532 nm can be used for cuprous oxide. If a laser beam having an excessively short wavelength is used, a signal from a portion other than the photoelectric conversion layer 3 may be detected. Therefore, it is preferable to use a light source having an energy slightly larger than the band gap of the photoelectric conversion layer 3. For example, the solar cell is cut into a square of about 1 cm ² around the measurement point, and cooled to about 10 K using a cryostat. The photoelectric conversion layer 3 is irradiated with a 532 nm YAG laser (excitation light intensity: 0.2 mW) using a microphotoluminescence measuring device, for example, a model: LabRAM-HR PL device manufactured by Horiba, Ltd. The measurement is performed with the integration time set to 5 seconds and the integration count set to 3 times. The hole diameter is 100 μm, and the objective lens has a magnification of 15 times. A CCD is used as a detector. Thus, a photoluminescence spectrum as shown in FIG. 2 can be obtained. By averaging the PL intensity ratios at these five measurement points, an average PL intensity ratio can be obtained.

  When the constituent elements of the photoelectric conversion layer 3 included in the solar cell according to the first embodiment are measured by X-ray Photoelectron Spectroscopy (XPS), the binding energy value is in the range of 930 eV to 934 eV. There is a peak for copper oxide. The peak of cuprous oxide whose binding energy value obtained by measuring the photoelectric conversion layer 3 by XPS is in the range of 930 eV to 934 eV is considered to be derived from zero-valent copper constituting metallic copper. ) Peak, Cu (I) peak considered to be derived from monovalent copper forming cuprous oxide (cuprous oxide), and divalent copper forming copper oxide (cupric oxide) It is thought to consist of a Cu (II) peak. Therefore, when the photoelectric conversion layer 3 having a small number of different phases is measured by XPS, no other peak appears and the peak (I) derived from cuprous oxide (cuprous oxide) becomes a single peak. It is almost symmetrical about the top.

  FIG. 3 is a schematic diagram illustrating a method for measuring the photoelectric conversion layer 3 using X-ray photoelectron spectroscopy. Explaining with reference to FIG. 3, first, when the photoelectric conversion layer 3 included in the solar cell according to the first embodiment is analyzed by XPS, a cuprous oxide having a binding energy value in a range of 930 eV or more and 934 eV or less is used. The first point 8 and the second point of intersection of the point where the peak takes the largest value, that is, the horizontal straight line 7 passing through 2/3 of the value of the peak top 6 of cuprous oxide and the peak of cuprous oxide 9, there is a third intersection 11 at which a perpendicular 10 extending from the peak top 6 to the horizontal straight line 7 intersects with the horizontal straight line 7, and a first length formed by the first intersection 8 and the third intersection 11. The difference between the length 12 and the second length 13 formed by the second intersection 9 and the third intersection 11 is examined. The first length 12 is L1 for convenience and the second length 13 is L2 for convenience. This measurement is performed at measurement points set in the same manner as the measurement of the transmittance. More specifically, the length and width of the solar cell are divided into four equal parts. Next, excluding the scribe part, five points are selected as measurement points from among the parts where the photoelectric conversion layer 3 exists, and each is measured by XPS. At each of these five measurement points, the ratio of the difference between the first length (L1) and the second length (L2) (the absolute value of the difference L1-L2 / L1 or L2, whichever is longer) is determined. . The longer one of L1 and L2 means that, for example, L1 and L2 are compared, and if L1 is longer, L1 is used as a denominator. By averaging the obtained difference ratios, an average of the difference ratios can be obtained. The measurement point is selected from a crossing point of a line obtained by dividing the solar cell vertically and horizontally into four equal portions within a range of 5% or less with respect to the length and width of the solar cell to be measured. The measurement is performed in a range narrower than the scribe interval. The smaller the number of different phases, the smaller the difference between the first length and the second length.

  Therefore, the average of the ratio of the difference between the first length 12 formed by the first intersection 8 and the third intersection 11 and the second length 13 formed by the second intersection 9 and the third intersection 11 is 15%. If it is below, it can mean that the hetero phase included in the photoelectric conversion layer 3 is sufficiently small. Therefore, it becomes the center of the recombination of the photoexcited carriers, and the conversion efficiency can be reduced. Further, the quality of the photoelectric conversion layer 3 itself can be prevented from being lowered, and the efficiency of the solar cell can be improved. Furthermore, the average of the ratio of the difference between the first length 12 formed by the first intersection 8 and the third intersection 11 and the second length 13 formed by the second intersection 9 and the third intersection 11 is 10%. The following cases indicate that the number of different phases contained in the photoelectric conversion layer 3 is further reduced, so that the efficiency of the solar cell can be further improved.

  When measuring the photoelectric conversion layer 3 by the XPS as described above, the n-type layer on the photoelectric conversion layer 3 and the n-type layer on the n It is preferable to measure after removing the two electrodes 5 by Ar ion etching or the like.

(N-type layer)
The n-type layer 4 is a layer existing between the photoelectric conversion layer 3 and the second electrode 5. In FIG. 1, the n-type layer 4 is in direct contact with the photoelectric conversion layer 3 and the second electrode 5.

The n-type layer 4 formed on the photoelectric conversion layer 3 desirably does not contain excessive oxygen during the preparation. This is because the presence of oxygen in the n-type layer 4 may cause the oxygen contained in the n-type layer 4 to react with the cuprous oxide at the interface between the cuprous oxide and the n-type layer 4 to generate a different phase such as copper oxide. is there. This different phase is more likely to occur as the n-type layer 4 contains more oxygen. Therefore, it is desirable that excess oxygen does not exist in the n-type layer 4 during fabrication. The n-layer may be composed of a plurality of layers including a buffer layer and the like. In the production of the n-type layer 4, for example, the n-type layer 4 can be produced by co-sputtering a sputtering target of ZnO and GeO _{2 in} an Ar gas flow.

  The thickness of the n-type layer 4 is preferably 5 nm or more and 100 nm or less. If the thickness of the n-type layer 4 is 5 nm or less, a leak current may be generated when the coverage of the n-type layer 4 is poor, and the characteristics may be degraded. If the thickness of the n-type layer 4 exceeds 100 nm, the characteristics may be deteriorated due to the excessive increase in the resistance of the n-type layer 4, or the short-circuit current may be reduced due to the decrease in the transmittance. Therefore, the thickness of the n-type layer 4 is more preferably 10 nm or more and 50 nm or less. Further, in order to realize a film having good coverage, the surface roughness of the n-type layer 4 is preferably 5 nm or less. When the quality of the n-type layer is high, a solar cell that operates even with a thickness of about 200 nm can be configured.

The conduction band offset (E _cn (eV)), which is the difference between the position (E _cp (eV)) of the conduction band lower end (Conduction Band Minimum: CBM) of the photoelectric conversion layer 3 and the position (E _cn (eV)) below the conduction band of the n-type layer 4. ΔE = E _cp −E _cn ) is preferably −0.2 eV or more and 0.6 eV or less (−0.2 eV ≦ ΔE ≦ + 0.6 eV). If the conduction band offset is larger than 0, the conduction band at the pn junction interface becomes discontinuous and spikes occur. If the conduction band offset is smaller than 0, the conduction band at the pn junction interface becomes discontinuous, and a cliff occurs. Since both the spike and the cliff are barriers for photogenerated electrons, it is preferable that the spike and the cliff are small. Therefore, the conduction band offset is more preferably not less than 0.0 eV and not more than 0.4 eV (0.0 eV ≦ ΔE ≦ + 0.4 eV). However, this is not the case when conduction is performed using the level in the gap. The position of the CBM can be estimated using the following method. The valence band maximum (Valence Band Maximum: VBM) is actually measured by photoelectron spectroscopy, which is a method for evaluating the electron occupied level, and then the CBM is calculated assuming the band gap of the material to be measured. However, at an actual pn junction interface, an ideal interface such as interdiffusion and generation of cation vacancies is not maintained, so that the band gap is likely to change. For this reason, it is preferable that the CBM is also evaluated by inverse photoelectron spectroscopy using the reverse process of photoelectron emission directly. Specifically, the electronic state of the pn junction interface can be evaluated by repeating low energy ion etching and forward / reverse photoelectron spectroscopy on the surface of the solar cell.

(Second electrode)
In FIG. 1, the second electrode 5 is in direct contact with the n-type layer 4. As the second electrode 5, a transparent conductive film is preferable. It is preferable to use the same material as the first electrode 2 for the transparent conductive film.
The composition of the solar cell 100 and the like are determined by X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS). The thickness and the particle size of each layer may be obtained by observing the cross section of the solar cell 100 with a transmission electron microscope (TEM) at a magnification of 100,000. The surface roughness may be observed with an atomic force microscope (AFM).

(Third electrode)
The third electrode of the embodiment is an electrode of the photoelectric conversion element 100, and is a metal film formed on the second electrode 5 on the side opposite to the light absorbing layer 3 side. As the third electrode, a conductive metal film such as Ni or Al can be used. The thickness of the third electrode is, for example, not less than 200 nm and not more than 2000 nm. When the resistance value of the second electrode 5 is low and the series resistance component is negligible, the third electrode may be omitted.

(Anti-reflective coating)
The antireflection film of the embodiment is a film for facilitating the introduction of light into the light absorption layer 3 and is formed on the second electrode 56 or on the third electrode on the side opposite to the light absorption layer 3 side. I have. It is desirable to use, for example, MgF ₂ or SiO ₂ as the antireflection film. In the embodiment, the anti-reflection film can be omitted. Although it is necessary to adjust the film thickness in accordance with the refractive index of each layer, it is preferable to deposit about 70 to 130 nm (preferably 80 to 120 nm).

  Here, a method for manufacturing the solar cell according to the present embodiment will be described.

(Production method)
FIG. 4 shows a flowchart of the method for manufacturing a solar cell according to the present embodiment.
A material for the first electrode 222 is formed on the substrate 1 by sputtering or the like. (S1). Next, it is introduced into a vacuum device and evacuated (S2). A material for the photoelectric conversion layer 33 is formed under vacuum conditions by sputtering or the like (S3). After forming the photoelectric conversion layer 3, an n-type layer is formed (S4). Thereafter, a material for forming the second electrode 5 is formed by sputtering or the like (S5). When manufacturing, a superstrate type or a substrate type may be used.

  The method for manufacturing the n-type layer 4 is not limited to the above. For example, there are CBD (Chemical Bath Deposition), CVD (Chemical Vapor Deposition), ALD (Atomic Layer Deposition), a coating method, an electrodeposition method, and the like.

  The solar cell according to the present embodiment includes a first electrode, a second electrode, and a photoelectric conversion layer provided between the first electrode and the second electrode, and a photoluminescence spectrum of the photoelectric conversion layer at a temperature of 100 K or less. Is measured, the first maximum value (A) which is the highest value of the light emission intensity in the wavelength range of 650 nm to 1000 nm is the second highest value (the highest value of the light emission intensity at 600 nm to 650 nm). B) 100 times or less (A ≦ 100B). Therefore, it is possible to prevent a reduction in conversion efficiency as a center of recombination of photoexcited carriers and a reduction in quality of the photoelectric conversion layer itself, and to provide a highly efficient solar cell.

(2nd Embodiment)
The second embodiment relates to a multi-junction solar cell. FIG. 5 is a conceptual cross-sectional view of the multi-junction solar cell 200 according to the second embodiment. The multi-junction solar cell 200 of FIG. 5 has the solar cell (first solar cell) 100 of the first embodiment and the second solar cell 201 on the light incident side. The band gap of the photoelectric conversion layer of the second solar cell 201 has a smaller band gap than that of the photoelectric conversion layer 3 of the solar cell 100 of the first embodiment. The multi-junction solar cell of the embodiment includes a solar cell in which three or more solar cells are joined.

  Since the band gap of the photoelectric conversion layer 3 of the solar cell 100 of the first embodiment is about 2.0 eV, the band gap of the photoelectric conversion layer of the second solar cell 201 is not less than 1.0 eV and not more than 1.4 eV. Is preferred. The photoelectric conversion layer of the second solar cell 201 is preferably a compound semiconductor layer of at least one of CIGS-based, CIT-based, CdTe-based, and copper oxide-based compounds having a high In content ratio or crystalline silicon. .

  By using the solar cell according to the first embodiment as the first solar cell, the conversion efficiency of the bottom cell (second solar cell) is reduced by absorbing light in an unintended wavelength range in the first solar cell. Therefore, an efficient multi-junction solar cell can be obtained.

(Third embodiment)
The third embodiment relates to a solar cell module. FIG. 6 is a perspective conceptual view of a solar cell module 300 according to the third embodiment. The solar cell module 300 in FIG. 6 is a solar cell module in which a first solar cell module 301 and a second solar cell module 302 are stacked. The first solar cell module 301 is on the light incident side, and uses the solar cell 100 of the first embodiment. It is preferable to use the second solar cell 201 for the second solar cell module 302.

  FIG. 7 shows a conceptual sectional view of the solar cell module 300. In FIG. 7, the structure of the first solar cell module 301 is shown in detail, and the structure of the second solar cell module 302 is not shown. In the second solar cell module 302, the structure of the solar cell module is appropriately selected according to the photoelectric conversion layer of the solar cell to be used. The solar cell module in FIG. 7 includes a plurality of submodules 303 surrounded by a broken line in which a plurality of solar cells 100 (solar cells) are arranged in a horizontal direction and electrically connected in series. They are electrically connected in parallel or in series.

  The solar cell 100 is scribed, and the adjacent second solar cell 100 is connected to the upper second electrode 5 and the lower first electrode 2. The solar cell 100 according to the third embodiment also includes a substrate 1, a first electrode 2, a photoelectric conversion layer 3, an n-type layer 4, and a second electrode 5, similarly to the solar cell 100 according to the first embodiment.

  If the output voltage is different for each module, the current may flow backward to a low voltage portion or generate extra heat, which leads to a decrease in the output of the module.

In addition, since the use of the solar cell of the present application makes it possible to use a solar cell suitable for each wavelength band, power can be generated more efficiently than when used alone, and the overall output of the module increases. desirable.

  If the conversion efficiency of the entire module is high, the proportion of the emitted light energy that becomes heat can be reduced. Therefore, a decrease in efficiency due to an increase in the temperature of the entire module can be suppressed.

(Fourth embodiment)
The fourth embodiment relates to a solar power generation system. The solar cell module 300 of the third embodiment can be used as a generator for generating power in the solar power generation system of the fourth embodiment. The photovoltaic power generation system of the embodiment generates power using a solar cell module, and specifically, a solar cell module that generates power, a unit that converts the generated power into power, and And a load that consumes generated electricity. FIG. 8 shows a conceptual configuration diagram of a photovoltaic power generation system 400 according to the embodiment. The solar power generation system in FIG. 8 includes a solar cell module 401 (300), a converter 402, a storage battery 403, and a load 404. Either the storage battery 403 or the load 404 may be omitted. The load 404 may be configured to be able to use the electric energy stored in the storage battery 403. The converter 402 is a device including a circuit or an element such as a DC-DC converter, a DC-AC converter, or an AC-AC converter that performs power conversion such as voltage transformation or DC-AC conversion. The configuration of the converter 402 may be a suitable configuration according to the configuration of the generated voltage, the storage battery 403, and the load 404.

  A photovoltaic cell included in the sub-module 301 that receives light included in the solar cell module 300 generates power, and its electric energy is converted by the converter 402 and stored in the storage battery 403 or consumed by the load 404. The solar cell module 401 includes a solar tracking driving device for constantly directing the solar cell module 401 toward the sun, a light collector for condensing sunlight, a device for improving power generation efficiency, and the like. It is preferable to add.

  The photovoltaic power generation system 400 is preferably used for real estate such as dwellings, commercial facilities, factories, and the like, or used for movable products such as vehicles, aircraft, and electronic devices. By using the solar cell having excellent conversion efficiency of the embodiment for the solar cell module 401, an increase in power generation is expected.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to the following Examples.

[Example]
Prepare a top cell and measure the PL intensity ratio, XPS value, and conversion efficiency.

(Example 1)
On a white glass substrate, an ITO transparent conductive film is deposited as a backside first electrode, and an Sb-doped SnO ₂ transparent conductive film is deposited thereon. The cuprous oxide compound was formed on the transparent first electrode by heating at 450 ° C. by sputtering in an atmosphere in which the ratio of nitrogen, oxygen and argon gas (O ₂ / (Ar + O ₂ ) ratio) was 0.078. Film. Then depositing an n-type _Zn 0.8 _Ge 0.2 _{O x} in p- cuprous oxide layer by sputtering at room temperature, depositing MgF ₂ as an anti-reflection film thereon. Thereafter, an AZO transparent conductive film is deposited as a second electrode on the surface side. When depositing the second electrode on the surface side, it is necessary to form a film at room temperature in order to suppress oxidation of cuprous oxide. For example, by using AZO, a film having low resistance can be obtained even at room temperature. The target of AZO is preferably such that the ratio of Al ₂ O _{3 to} ZnO is about 2 wt% to 3 wt%, but this is not limited as long as it has a sufficiently low resistance value and high transmittance for the device.

  The method for measuring the PL intensity ratio is as follows.

  Divide the solar cell vertically and horizontally into four equal parts, and select five intersections of the lines obtained by dividing the solar cell into four equal parts vertically and horizontally. From each intersection, a measurement point was selected from a range of 5% or less with respect to the vertical and horizontal lengths of the solar cell to be measured, and each PL intensity ratio was measured. The measurement point was cooled to about 10K using a cryostat, and a 532 nm YAG laser (excitation light intensity: 0.2 mW) was used using a microphotoluminescence measurement device (Model: LabRAM-HR PL manufactured by Horiba, Ltd.). To irradiate the photoelectric conversion layer. The irradiation was measured with an integration time of 5 seconds and an integration count of 3 times. The hole diameter was 100 μm, and the objective lens used was 15 times. The average PL intensity ratio was determined from the PL intensity ratio at each of the five measurement points, and is shown in Table 1.

  The measurement of the XPS value was performed as follows.

  In the elemental analysis of the photoelectric conversion layer by XPS, first, the second electrode and the n-type layer were removed from the solar cell by using Ar ion etching to expose the photoelectric conversion layer. Next, the exposed photoelectric conversion layer was measured using the following apparatus and measurement conditions. The instrument used was AXIS Ultra DLD, Shimadzu Corporation, and monochro (Al-Kα) (15 kV × 15 mA) was used as the excitation source. The mode at the time of measurement was set to Spectrum for Analyzer mode and Hybrid for Lens Mode. The photoelectron take-out angle is 45 °, and the take-in region has a binding energy value of 0 to 1200 eV in Wide scan, a 926 to 942 eV range of Cu2p in which a main peak of cuprous oxide can be seen in Narrow scan, and C1s. Performed in the range of 278-294 eV. In this case, Pass Energy was performed at 160 eV in the Wide scan and 10 eV in the Narrow scan. The measurement was performed so that the binding energy value was in steps of 0.1 eV. The charge correction was performed with the C1s peak of the surface-contaminated hydrocarbon set at 284.8 eV. In this way, the photoelectric conversion layer was measured at five points by XPS, and the average of error ranges of the respective differences was obtained. Table 1 also summarizes the results. In Table 1, ◎ indicates that the average range of the difference ratio is within 10%, ○ indicates that the average range of the difference ratio is within 15%, and X indicates that the average range is greater than 15%.

  The method for measuring the conversion efficiency is as follows.

Using a solar simulator simulating an AM1.5G light source, the light amount is adjusted to 1 sun using a reference Si cell under the light source. The temperature is 25 ° C. When the horizontal axis represents voltage and the vertical axis represents current density, the point at which the horizontal axis intersects is Voc, and a range in which Jsc can be measured from a value (eg, 1.6 V) that covers Voc with a voltmeter (minus region). , For example, -0.4 V), and the current value at that time is measured. The value obtained by dividing the area of the solar cell is the current density (mA / cm ² ), and the value of the current density at an applied voltage of 0 V is Jsc (short-circuit current density).
The efficiency η is η = Voc × Jsc × FF / P × 100
P calibrates simulated sunlight having an incident power density of AM1.5 with a reference solar cell.
FF is obtained by FF = Vmpp × Jmpp / (Voc × Jsc). Vmpp and Jmpp are the values of V and J at the point where the product of V × J is the largest.

  At this time, in Table 1, the solar cell efficiencies (FF) of the other Examples and Comparative Examples were calculated based on Example 1.

  Table 1 also summarizes the measurement results of Examples 2 to 6 and Comparative Examples 1 to 5 described later.

(Examples 2 to 6)
Preparation and measurement were performed in the same manner as in Example 1 except that the ratios of oxygen and argon gas were as shown in Table 1.

(Comparative Example 1)
Production and measurement were performed in the same manner as in Example 1 except that an oxygen gas flow was present when n-type was deposited.

(Comparative Example 2)
Fabrication and measurement were performed in the same manner as in Comparative Example 1 except that Zn + Ge was used as the target when depositing the n-type layer.

(Comparative Example 3)
Fabrication and measurement were performed in the same manner as in Comparative Example 1 except that ZnO + Ge was used as a target when depositing the n-type layer.

(Comparative Example 4)
Production and measurement were performed in the same manner as in Comparative Example 1, except that Zn + GeO ₂ was used as a target when depositing the n-type layer.

(Examples 7 to 12)
Production and measurement were performed in the same manner as in Example 1 except that tungsten (W) was used as the first electrode and the ratio of oxygen and argon gas was as shown in Table 2.

(Example 13)
Production and measurement were performed in the same manner as in Example 8 except that the formation of cuprous oxide was performed at 400 ° C.

(Example 14)
It was prepared and measured in the same manner as in Example 9 except that the formation of the cuprous oxide compound was performed at 600 ° C.

(Comparative Example 5)
Production and measurement were performed in the same manner as in Example 7 except that an oxygen gas flow was present when depositing the n-type.

(Example 15)
Production and measurement were performed in the same manner as in Example 1 except that the formation of the cuprous oxide compound was performed at 400 ° C.

(Example 16)
Preparation and measurement were performed in the same manner as in Example 1 except that the formation of the cuprous oxide compound was performed at 600 ° C.

(Comparative Example 6)
Production and measurement were performed in the same manner as in Example 1 except that the formation of the cuprous oxide compound was performed at 350 ° C.

(Comparative Example 7)
Production and measurement were performed in the same manner as in Example 1 except that the formation of the cuprous oxide compound was performed at 750 ° C.

  From Tables 1 to 3, when Examples 1 to 16 and Comparative Examples 1 to 7 are compared, in the photoluminescence spectrum, the first highest value (the highest value of the emission intensity in the wavelength range of more than 650 nm and not more than 1000 nm) ( When A) is 100 times or less (A ≦ 100B) of the second highest value (B), which is the highest value of the emission intensity at 600 nm or more and 650 nm or less, it can be seen that excellent efficiency can be achieved. Furthermore, when the photoluminescence spectrum is measured at a temperature of 100 K or less in the photoluminescence spectrum, the first maximum value (A), which is the highest value of the emission intensity in the wavelength range from 650 nm to 1000 nm, is 600 nm to 650 nm. It can be seen that better efficiency can be achieved by being 30 times or less (A ≦ 30B) the second highest value (B), which is the highest value of the emission intensity below.

  In the case of solar cells manufactured at the same temperature, the efficiency ratios of Examples 1 to 6 using the transparent electrode were higher than those of Examples 7 to 12 using the metal electrode. This is because the reaction between oxygen and the electrode during the formation of cuprous oxide can be appropriately suppressed when the oxide is present on at least a part of the first electrode.

  In addition, the XPS analysis was performed on the photoelectric conversion layer, and when the photoelectric conversion layer was analyzed by XPS, the binding energy value was found to be in the range of 930 eV or more and 934 eV or less. There are a first intersection and a second intersection where a horizontal line passing through a value of 2/3 of the value intersects the peak of cuprous oxide, and a third line where a vertical line lowered from the peak top to the horizontal line intersects the horizontal line And the average of the ratio of the difference between the first length formed by the first and third intersections and the second length formed by the second and third intersections is 15% or less. Also, the conversion efficiency of the solar cell can be improved. This is because there is little heterogeneous phase present inside the photoelectric conversion layer, which prevents recombination of photoexcited carriers and prevents reduction in conversion efficiency, improves the quality of the photoelectric conversion layer, and prevents light in unintended wavelength ranges. This is because absorption can be prevented. Therefore, the short-circuit current density inside the photoelectric conversion layer (p layer, cuprous oxide) can be improved. In addition, by forming a favorable interface, defect levels at the interface can be reduced and the open-circuit voltage can be increased. Further, the suppression of the leak component and the formation of a good-quality contact improve the shape factor and improve the efficiency.

  Although the embodiment of the present invention has been described above, the present invention is not construed as being limited to the above-described embodiment, and can be embodied by modifying components in an implementation stage without departing from the scope of the invention. Further, various inventions can be formed by appropriately combining a plurality of components disclosed in the above embodiments. For example, components of different embodiments may be appropriately combined as in a modification.

100 solar cell (first solar cell, top cell), 1 substrate, 2 first electrode, 3 photoelectric conversion layer, 4 n-type compound semiconductor layer (n-type layer), 5 second electrode, 200: Multi-junction solar cell, 201: Second solar cell (bottom cell), 300: Solar cell module, 301: First solar cell module, 302: Second solar cell module, 303: Submodule, 400: Solar power generation System, 401: solar cell module, 402: converter, 403: storage battery, 404: load

Claims (8)

A first electrode;
A second electrode;
A photoelectric conversion layer provided between the first electrode and the second electrode,
When the photoluminescence spectrum of the photoelectric conversion layer is measured at a temperature of 100 K or less, the first highest value (A) of the emission intensity in a wavelength range of more than 650 nm and 1000 nm or less is 600 nm or more and 650 nm or less. A solar cell having 100 times or less (A ≦ 100B) of the second highest value (B), which is the highest value of the light emission intensity in the above.   2. The solar cell according to claim 1, wherein the first maximum value (A) is 30 times or less (A ≦ 30B) of the second maximum value (B).   The solar cell according to claim 1, wherein the photoelectric conversion layer includes cuprous oxide.   When the photoelectric conversion layer was analyzed by X-ray photoelectron spectroscopy, the value of 亜 of the peak of cuprous oxide whose binding energy value was in the range of 930 eV to 934 eV was 2/3 of the peak top value of cuprous oxide. There is a first intersection and a second intersection where the horizontal straight line passing through the peak of the cuprous oxide intersects, and a third intersection where the perpendicular drawn from the peak top to the horizontal straight line and the horizontal straight line intersects The average of the ratio of the difference between the first length formed by the first intersection and the third intersection and the second length formed by the second intersection and the third intersection is 15% or less. The solar cell according to claim 1.   The solar cell according to claim 4, wherein the average of the ratio of the length difference is 10% or less.   A multi-junction solar cell using the solar cell according to claim 1.   A solar cell module using the solar cell according to any one of claims 1 to 5 or the multi-junction solar cell according to claim 6.   A solar power generation system that performs solar power generation using the solar cell module according to claim 7.