JP7273537B2 - Solar cells, multi-junction solar cells, solar cell modules and photovoltaic power generation systems - Google Patents

Description

Embodiments relate to solar cells, multi-junction solar cells, solar cell modules, and photovoltaic systems.

There is a multi-junction (tandem) solar cell as a highly efficient solar cell. A tandem solar cell can use a cell with high spectral sensitivity for each wavelength band, and therefore can be more efficient than a single junction. Cuprous oxide compounds, which are inexpensive materials and have a wide bandgap, are expected as top cells of tandem solar cells. However, cuprous oxide solar cells fabricated by oxidizing copper foil have been reported to have an efficiency of about 8%, which is lower than the theoretical limit efficiency. This is because the different phases such as copper oxide on the outermost surface are removed by etching after the copper foil is oxidized, but it cannot be completely removed and the constituent elements of the etching solution remain. Therefore, a good pn junction cannot be formed. It is considered to be for Moreover, in this method, it is necessary to oxidize a foil having a thickness of about 0.1 mm and then polish it to a thickness of about 20 μm, which makes it difficult to increase the area.

On the other hand, there is an example of a thin film produced by a method using a reaction in a liquid phase, but the efficiency is about 4% at maximum. The main reason for this is thought to be that not only different phases but also impurities contained in the solution are taken into the film, and these become recombination centers of photoexcited carriers. Since such a thin film also absorbs light with a wavelength of 600 nm or more, which it does not normally absorb, it cannot be used for the top cell of a tandem solar cell. Sputtering is generally well known as a method for producing a thin film with little contamination of impurities, and although there have been reports of production by this method, the conversion efficiency was 1% or less. The reason for this is thought to be that, even without contamination with impurities, heterogeneous phases of copper and copper oxide tend to occur, making it difficult to obtain pure cuprous oxide.

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 photovoltaic power generation system with improved conversion efficiency.

A solar cell according to an embodiment includes a first electrode, a second electrode, a photoelectric conversion layer provided on the first electrode and between the first electrode and the second electrode, the second electrode and the photoelectric conversion layer. and an n-type layer between conversion layers, wherein the average transmittance of the solar cell is 60% or more when measured in a wavelength range of 700 to 1000 nm. The photoelectric conversion layer has a thickness of 500 nm or more and 0.3 mm or less. 2/3 of the peak top value of cuprous oxide in the peak of cuprous oxide seen in the range of binding energy values of 930 eV or more and 934 eV or less when the photoelectric conversion layer is analyzed by X-ray photoelectron spectroscopy. There are a first intersection point and a second intersection point where the horizontal straight line passing through and the cuprous oxide peak intersect, and a third intersection point where the vertical line drawn from the peak top to the horizontal straight line and the horizontal straight line intersect and the average 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 first electrode and the second electrode are transparent conductive films.

FIG. 2 is a conceptual cross-sectional view of the solar cell according to the first embodiment; Schematic diagram of a method for measuring a photoelectric conversion layer using X-ray photoelectron spectroscopy. 4 is a flow chart of a method for manufacturing the solar cell of the first embodiment; FIG. 2 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 which concerns on 3rd Embodiment. The cross-sectional conceptual diagram of the solar cell module which concerns on 3rd Embodiment. The conceptual diagram of the photovoltaic power generation system which concerns on 4th Embodiment.

(First embodiment)
A first embodiment relates to a solar cell. FIG. 1 shows a conceptual diagram of a solar cell 100 of the first embodiment. As shown in FIG. 1, a solar cell 100 according to this embodiment includes a substrate 1, a first electrode 2 on the substrate 1, a photoelectric conversion layer 3 on the first electrode 2, and an n electrode on the photoelectric conversion layer 3. A type layer 4 and a second electrode 5 on the n-type layer 4 are provided. An intermediate layer (not shown) may be included between the first electrode 2 and the photoelectric conversion layer 3 and between the n-type layer 4 and the second electrode 5 .

(substrate)
As the substrate 1 of the embodiment, it is desirable to use a white plate glass, and general glass such as quartz, soda lime glass, chemically strengthened glass, stainless steel (SUS), metal plates 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. As a transparent conductive film, Indium Tin Oxide (ITO), Al-doped Zinc Oxide (AZO), Boron-doped Zinc Oxide (BZO), 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 (IZO), Indium Gallium Zinc Oxide (IGZO), Ta-doped Tin Oxide (SnO2:Ta), Nb-doped Tin Oxide : SnO ₂ : Nb), tungsten-doped tin oxide (W-doped Tin Oxide: SnO ₂ : W), molybdenum-doped tin oxide (Mo-doped Tin Oxide: SnO ₂ : Mo), fluorine-doped tin oxide (F-doped Tin Oxide: SnO 2 : Mo). Oxide: SnO ₂ :F), hydrogen-doped indium oxide (IOH), etc. are not particularly limited.

The transparent conductive film may be a laminated film having a plurality of films, and the laminated film may contain a film of tin oxide or the like in addition to the above oxides. As a dopant for a film such as tin oxide, In, Si, Ge, Ti, Cu, Sb, Nb, F, Ta, W, Mo, F, Cl and the like are not particularly limited. The metal film is not particularly limited and may be Mo, Au, Cu, Ag, Al, Ta or W films.

Also, the first electrode 2 may be an electrode in which a dot-like, line-like or mesh-like metal is provided on a transparent conductive film. At this time, the dot-shaped, line-shaped or mesh-shaped metal is arranged between the transparent conductive film and the photoelectric conversion layer 3 or on the opposite side of the transparent conductive film from the photoelectric conversion layer 3 . The dot-shaped, line-shaped or mesh-shaped 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 and may be Mo, Au, Cu, Ag, Al, Ta or W.

A metal substrate may be used for the first electrode 2 instead of the transparent conductive film. For example, W, Cr, Ti, Ta, Al, SUS (for example, SUS430), etc. can be used for the metal substrate. A later-described photoelectric conversion layer 3 may be formed directly on this metal substrate.

The photoelectric conversion layer 3 of the embodiment is a p-type compound semiconductor layer. The photoelectric conversion layer 3 is a layer existing between the first electrode 2 and the n-type compound semiconductor layer 4 . Hereinafter, an n-type compound semiconductor layer will be referred to as an n-type layer. In FIG. 1, photoelectric conversion layer 3 is in direct contact with first electrode 1 and n-type layer 4 . The photoelectric conversion layer 3 is a layer containing cuprous oxide. Cuprous oxide is non-doped or doped cuprous oxide. The thickness of cuprous oxide is typically 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 the cost of the solar cell 100 can be reduced. Cuprous oxide has a bandgap of about 2.1 eV, which is a wide bandgap. When the photoelectric conversion layer 3 of the solar cell 100 according to the present embodiment has a wide bandgap, when a solar cell having a narrow bandgap photoelectric conversion layer 3 such as Si is used as a bottom cell to form a multi-junction, the first embodiment The solar cell 100 according to the first embodiment is preferable in that the solar cell 100 according to the embodiment has a high transmittance of wavelengths that contribute to power generation on the bottom cell side, and thus the amount of power generation on the bottom cell side is high. 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 this embodiment will be described.

The photoelectric conversion layer 3 is produced using sputtering. First, a cuprous oxide thin film 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 higher in a mixed gas of Ar and oxygen. Other power such as a DC power supply may be supplied instead of the RF power supply. At this time, the presence of oxide on the electrode, for example, at least a part of the substrate during sputtering appropriately suppresses the reaction between oxygen and the electrode (corresponding to the substrate during film formation) during the formation of cuprous oxide. can be obtained, and good crystals are easily obtained. For example, it is desirable to use a transparent conductive oxide because it can be used as an electrode and the structure is simple. Therefore, it is preferable that an oxide exists in at least part of the electrode. The power used for sputtering may be direct current. At this time, the substrate is heated to 400° C. or higher and 700° C. or lower, preferably 450° C. or higher and 550° C. or lower. Thus, a cuprous oxide thin film can be produced. Depending on the flow rate ratio of Ar and oxygen, different phases such as copper and copper oxide may be formed. In order to suppress the formation of copper and copper oxide, it is necessary to adjust the ratio of Ar flow rate and oxygen flow rate, for example. Note that copper oxide is CuO.

Since sputtering is performed under reduced pressure, there is almost no oxygen in the surroundings, and if the film formation temperature of the photoelectric conversion layer 3 is high, the metal is easily reduced. Therefore, in order to produce cuprous oxide, it is necessary to oxidize moderately, so it is necessary to increase the amount of oxidizing agent, that is, oxygen, in an environment where reduction is easy, that is, at a high temperature. In other words, when the temperature is high, the oxygen flow rate needs to be increased because the atmosphere is more likely to be reduced. Therefore, when forming the photoelectric conversion layer 3 at a high temperature, the photoelectric conversion layer 3 with high quality can be obtained by increasing the oxygen flow rate.

It is preferable to use a substrate having transparency as the substrate on which the cuprous oxide film is formed. This is because, by using a transparent material, it is possible to effectively use the wavelength region in which cuprous oxide cannot absorb on the side opposite to the light incident direction, taking advantage of the wide gap.

Copper and copper oxide form a different phase in the photoelectric conversion layer 3 provided 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 contained 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 a heterogeneous phase causes recombination centers of photoexcited carriers to lower the conversion efficiency and lower the quality of the photoelectric conversion layer 3 itself. This is because copper-derived leakage or the like occurs, and the fill factor (FF: form factor) deteriorates. This is because when copper oxide is contained, it adversely affects the overall cell characteristics by promoting recombination. In addition, absorption of light in an unintended wavelength range leads to a reduction in the conversion efficiency of the bottom cell in the case of a multi-junction solar cell, which will be described later.

In order to measure heterogeneous phases present in the photoelectric conversion layer 3 of the solar cell, analysis is performed by the X-ray diffraction method, and high conversion efficiency is not necessarily obtained even if no heterophase is detected in the photoelectric conversion layer 3 . This is because there is a heterogeneous phase that cannot be measured by the X-ray diffraction method. Therefore, by measuring the transmittance of the solar cell, the existence of the heterogeneous phase in the photoelectric conversion layer 3 can be measured with high accuracy. Transmittance is an analytical method that exposes a solar cell to light of various wavelengths and measures the percentage of light that is transmitted. The measuring method will be described later in detail. The transmittance of the solar cell increases as the number of different phases decreases, and the efficiency of the solar cell can be improved. Therefore, it is preferable that the solar cell has an average transmittance of 60% or more in the wavelength region of 700 to 1000 nm. This is because a high conversion efficiency can be achieved when the average transmittance is within this range. In the case of a multi-junction solar cell, which will be described later, the wavelength region of 700 to 1000 nm is well transmitted by the top cell, so that light can be efficiently absorbed by the bottom cell. Moreover, it is more preferably 70% or more. This is because a higher conversion efficiency can be achieved by having the average transmittance in this range. If the average transmittance in the wavelength region of 700 to 1000 nm is less than 60%, the amount of power generated by the bottom cell will decrease, and the total amount of power generated will decrease, which is not preferable.

Here, a method for measuring transmittance will be described. Ultraviolet-visible-near-infrared spectroscopy is used to measure transmittance. An ultraviolet-visible-near-infrared spectrophotometer, for example, Shimadzu Model: UV-3101PC is used as an apparatus, and a sample is placed just before the integrating sphere to measure the transmittance in the target wavelength range of 700 to 1000 nm. Transmittance measurement is performed by dividing the solar cell vertically and horizontally into four equal parts. Next, excluding the scribed portion, five locations in the portion where the photoelectric conversion layer 3 exists are selected as measurement points, and the transmittance is measured at each point. For the transmittance at the measurement point, obtain the transmittance in increments of 1 nm in the range of 700 to 1000 nm, and average the transmittance. After that, by averaging the transmittance at these five measurement points, the average transmittance of the solar cell to be measured can be obtained. This measurement point is selected from a range within 5% of the vertical and horizontal lengths of the solar cell to be measured from the intersection of the lines that vertically and horizontally divide the solar cell into four equal parts. The measurement range is narrower than the scribing interval.

Moreover, when the photoelectric conversion layer 3 is applied not on the transparent electrode but on the metal electrode, it is possible to evaluate the heterogeneous phase present in the photoelectric conversion layer 3 by measuring the reflectance. This is because a heterogeneous phase exists inside the photoelectric conversion layer 3, and when the light in the corresponding region is absorbed, the reflected light is reduced. If the surface roughness is large, it is necessary to consider the reflected light spreading around.

The photoelectric conversion layer 3 of the solar cell according to this embodiment preferably has an average reflectance of 49% or more in the wavelength range of 700 to 1000 nm. This is because when the average reflectance is within this range, the absorption of the corresponding wavelength range inside the photoelectric conversion layer 3 is small and the film quality is good. More preferably, it is 58% or more. This is because when the average reflectance is 58% or more, the film quality is further improved.

The ultraviolet-visible-near-infrared spectrophotometer used for measuring the transmittance is used for measuring the reflectance. A sample is set so that the incident angle is 5 degrees and the reflection angle is 5 degrees, and the reflectance in the target wavelength region is evaluated. As for the method of measuring the reflectance, the average reflectance can be obtained by setting the measurement points in the same manner as the transmittance.

When the constituent elements of the photoelectric conversion layer 3 provided in the solar cell according to the first embodiment are measured by X-ray Photoelectron Spectroscopy (XPS), the binding energy value falls within the range of 930 eV or more and 934 eV or less. A copper oxide peak is present. The cuprous oxide peak observed in the range of 930 eV to 934 eV in binding energy obtained by measuring the photoelectric conversion layer 3 with XPS is Cu (0 ) peak, a Cu (I) peak that is considered to be derived from monovalent copper that constitutes cuprous oxide (cuprous oxide), and a divalent copper that constitutes copper oxide (cupric oxide). It is believed to consist of Cu(II) peaks. Therefore, when the photoelectric conversion layer 3 with few heterogeneous phases is measured by XPS, no other peaks appear, and the (I) peak derived from cuprous oxide (cuprous oxide) becomes a single peak. The shape is almost symmetrical with the top as the center.

FIG. 2 is a schematic diagram of a method for measuring the photoelectric conversion layer 3 using X-ray photoelectron spectroscopy. Referring to FIG. 2, first, when the photoelectric conversion layer 3 provided in the solar cell according to the first embodiment is analyzed by XPS, the binding energy value of cuprous oxide found in the range of 930 eV or more and 934 eV or less In the peak, the point with the largest value, that is, the peak top 6 value of cuprous oxide is examined. A first crossing point 8 and a second crossing point 9 where the cuprous oxide peak intersects with a horizontal straight line 7 passing through 2/3 of the value of this peak top 6 is examined. Further, a third intersection point 11 where a vertical line 10 drawn from the peak top 6 to the horizontal straight line 7 intersects with the horizontal straight line 7 is examined. After that, the difference between the first length 12 formed by the first intersection point 8 and the third intersection point 11 and the second length 13 formed by the second intersection point 9 and the third intersection point 11 is examined. The first length 12 is conveniently labeled L1 and the second length 13 is conveniently labeled L2. This measurement is performed at the measurement points set in the same way as for the transmittance measurement. Specifically, the solar cell is divided vertically and horizontally into four equal parts. Next, except for the scribed portion, five locations in the portion where the photoelectric conversion layer 3 exists are selected as measurement points, and each of them is measured by XPS. The ratio of the difference between the first length (L1) and the second length (L2) at each of these five measurement points (the absolute value of the difference between L1 and L2/L1 or L2, whichever is longer) demand. Whichever of L1 and L2 is longer means, for example, comparing L1 and L2 and using L1 as the denominator if L1 is longer. By averaging the calculated difference ratios, the average difference ratio can be calculated. This measurement point is selected from a range within 5% of the vertical and horizontal lengths of the solar cell to be measured from the intersection of the lines that vertically and horizontally divide the solar cell into four equal parts. The measurement range is narrower than the scribing interval. The smaller the number of different phases, the smaller the difference between the first length and the second length.

Therefore, the average ratio of the difference between the first length 12 formed by the first intersection point 8 and the third intersection point 11 and the second length 13 formed by the second intersection point 9 and the third intersection point 11 is 15%. If it is below, it can mean that the heterogeneous phase contained in the photoelectric conversion layer 3 is sufficiently small. Therefore, it is possible to prevent the conversion efficiency from deteriorating as a center of recombination of photoexcited carriers and the deterioration of the quality of the photoelectric conversion layer 3 itself, thereby improving the efficiency of the solar cell. Furthermore, the average ratio of the difference between the first length 12 formed by the first intersection point 8 and the third intersection point 11 and the second length 13 formed by the second intersection point 9 and the third intersection point 11 is 10%. The following cases indicate that the heterogeneous phase contained in the photoelectric conversion layer 3 is further reduced, so that the efficiency of the solar cell can be further improved.

In addition, when measuring the photoelectric conversion layer 3 by XPS as described above, the n-type layer on the photoelectric conversion layer 3 and the second layer thereon, which will be described later, are placed so that the state inside the photoelectric conversion layer 3 can be analyzed. 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 .

It is desirable that the n-type layer 4 formed on the photoelectric conversion layer 3 does not contain excess oxygen during its production. This is because the presence of oxygen in the n-type layer 4 causes the oxygen contained in the n-type layer 4 to react with the cuprous oxide at the interface between the n-type layer 4 and the n-type layer 4, thereby generating a heterogeneous phase such as copper oxide. be. This different phase is more likely to occur as the n-type layer 4 contains more oxygen. Therefore, it is desirable that excessive oxygen does not exist in the n-type layer 4 during fabrication. The n-layer may be composed of multiple 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 sputtering targets of ZnO and GeO ₂ in an Ar stream.

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 leakage current may occur when the coverage of the n-type layer 4 is poor, which may deteriorate the characteristics. If the thickness of the n-type layer 4 exceeds 100 nm, the resistance of the n-type layer 4 may be excessively increased, resulting in a decrease in characteristics or a decrease in short-circuit current due to a decrease in transmittance. Therefore, the thickness of the n-type layer 4 is more preferably 10 nm or more and 50 nm or less. Further, the surface roughness of the n-type layer 4 is preferably 5 nm or less in order to realize a film with good coverage. If the quality of the n-type layer is high, a solar cell that operates even with a thickness of about 200 nm can be constructed.

Conduction _band offset ₍ 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 greater than 0, the conduction band at the pn junction interface becomes discontinuous and spikes occur. If the conduction band offset is less than 0, the conduction band at the pn junction interface becomes discontinuous and a cliff occurs. Since both spikes and cliffs act as barriers to photogenerated electrons, smaller ones are preferable. Therefore, the conduction band offset is more preferably 0.0 eV or more and 0.4 eV or less (0.0 eV≦ΔE≦+0.4 eV). However, this is not the case when the in-gap level is used for conduction. The location of the CBM can be estimated using the following technique. The valence band maximum (VBM) is actually measured by photoelectron spectroscopy, which is an evaluation method for electron occupied levels, and then the CBM is calculated assuming a known bandgap. However, since an actual pn junction interface does not maintain an ideal interface due to interdiffusion and generation of cation vacancies, there is a high possibility that the bandgap will change. Therefore, it is preferable to evaluate CBM by reverse photoelectron spectroscopy that directly utilizes the reverse process of photoelectron emission. 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 can be determined by X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS). Further, the thickness and grain size of each layer can be determined by observing the cross section of the solar cell 100 at a magnification of 100,000 with a transmission electron microscope (TEM). 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. A conductive metal film such as Ni or Al can be used as the third electrode. The film thickness of the third electrode is, for example, 200 nm or more and 2000 nm or less. Further, when the resistance value of the second electrode 5 is so low that the series resistance component can be ignored, the third electrode may be omitted.

(Anti-reflection film)
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 5 or the third electrode on the side opposite to the light absorption layer 3 side. there is As the antireflection film, it is desirable to use MgF2 or SiO2, for example. Note that the antireflection film can be omitted in the embodiment. Although it is necessary to adjust the film thickness according to 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 a solar cell according to this embodiment will be described.

(Manufacturing method)
FIG. 3 shows a flowchart of the method for manufacturing a solar cell according to this embodiment.
A material for the first electrode 22 is deposited on the substrate 1 by sputtering or the like. (S1). Next, it is introduced into a vacuum device and vacuumed (S2). A material for the photoelectric conversion layer 3 is formed into a film by sputtering or the like under vacuum conditions (S3). After forming the photoelectric conversion layer 3, the n-type layer 4 is formed (S4). After that, a material for the second electrode 5 is formed into a film by sputtering or the like (S5). When it is manufactured, it may be a superstrate type or a substrate type.

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

A solar cell according to the present embodiment is a solar cell including a first electrode, a second electrode, and a photoelectric conversion layer provided between the first electrode and the second electrode, and the transmittance of the solar cell is The average transmittance is 60% or more when measured in the wavelength range of 700 to 1000 nm. Therefore, it can be prevented from becoming a recombination center of photoexcited carriers to lower the conversion efficiency and from lowering the quality of the photoelectric conversion layer itself, and a highly efficient solar cell can be obtained.

(Second embodiment)
The second embodiment relates to a multijunction solar cell. FIG. 4 shows a conceptual cross-sectional view of a multi-junction solar cell 200 of the second embodiment. A multi-junction solar cell 200 in FIG. 4 has the solar cell (first solar cell) 100 of the first embodiment and a second solar cell 201 on the light incident side. The photoelectric conversion layer 3 of the second solar cell 201 has a smaller bandgap than the photoelectric conversion layer 3 of the solar cell 100 of the first embodiment. Note that the multi-junction solar cell of the embodiment also includes a solar cell in which three or more solar cells are joined.

Since the bandgap of the photoelectric conversion layer 3 of the solar cell 100 of the first embodiment is about 2.0 eV, the bandgap of the photoelectric conversion layer 3 of the second solar cell 201 is 1.0 eV or more and 1.4 eV or less. is preferred. The photoelectric conversion layer 3 of the second solar cell 201 is preferably one or more of CIGS, CIT, CdTe, and CuO compound semiconductor layers or crystalline silicon having a high In content. .

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, it is possible to obtain an efficient multi-junction solar cell.

(Third embodiment)
The third embodiment relates to a solar cell module. FIG. 5 shows a conceptual perspective view of a solar cell module 300 of the third embodiment. A solar cell module 300 in FIG. 5 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. Second solar cell 201 is preferably used for second solar cell module 302 .

FIG. 6 shows a conceptual cross-sectional view of the solar cell module 300. As shown in FIG. FIG. 6 shows the structure of the first solar cell module 301 in detail and does not show the structure of the second solar cell module 302 . In the second solar cell module 302, the structure of the solar cell module is appropriately selected according to the photoelectric conversion layer 3 of the solar cell used. The solar cell module of FIG. 6 includes a plurality of sub-modules 303 surrounded by dashed lines in which a plurality of solar cells 100 (solar cells) are horizontally arranged and electrically connected in series. They are electrically connected in parallel or in series.

The solar cells 100 are scribed, and adjacent solar cells 100 are connected by the upper and second electrodes 55 and the lower first electrodes 22 . The solar cell 100 of the third embodiment also has a substrate 1, a first electrode 22, a photoelectric conversion layer 33, an n-type layer 44 and a second electrode 55, like the solar cell 100 of the first embodiment.

If the output voltage is different for each module, the current may flow backward to the part with the lower voltage, or excessive heat may be generated, leading to a decrease in the output of the module.

In addition, by using the solar cell of the present application, it is possible to use a solar cell suitable for each wavelength band, so that it is possible to generate power more efficiently than when it is used alone, and the overall output of the module increases. desirable.

If the conversion efficiency of the entire module is high, it is possible to reduce the proportion of energy that becomes heat in the irradiated light energy. Therefore, it is possible to suppress a decrease in efficiency due to an increase in the temperature of the entire module.

(Fourth embodiment)
4th Embodiment is related with a photovoltaic 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. A solar power generation system according to an embodiment generates power using a solar cell module. It has a storage means for storing electricity or a load for consuming the generated electricity. FIG. 7 shows a configuration conceptual diagram of a photovoltaic power generation system 400 of the embodiment. The photovoltaic power generation system of FIG. 7 has a photovoltaic module 401 (300), a converter 402, a storage battery 403, and a load 404. Either one of the storage battery 403 and the load 404 may be omitted. The load 404 may be configured to use the electrical energy stored in the storage battery 403 . The converter 402 is a device that includes a circuit or element that performs power conversion such as DC-DC converter, DC-AC converter, AC-AC converter, or the like. A suitable configuration may be adopted for the configuration of the converter 402 according to the generated voltage, the configuration of the storage battery 403 and the load 404 .

Photovoltaic cells included in the sub-module 301 included in the photovoltaic module 300 that received the light generate power, and the electrical 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 is provided with a sunlight tracking drive device for always directing the solar cell module 401 toward the sun, a light collector for collecting sunlight, a device for improving power generation efficiency, and the like. Adding is preferred.

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

EXAMPLES The present invention will be more specifically described below based on examples, but the present invention is not limited to the following examples.

[Example]
A top cell is fabricated and the light transmittance, XPS value and conversion efficiency are measured.

(Example 1)
On a white plate glass substrate, an ITO transparent conductive film is deposited as a first electrode on the back side, and an Sb-doped SnO ₂ transparent conductive film is deposited thereon. A cuprous oxide compound was formed on the transparent first electrode by heating at 450° C. by a sputtering method in an atmosphere in which the ratio of nitrogen and oxygen to argon gas (O ₂ /(Ar+O ₂ ) ratio) was 0.078. film. Thereafter, n-type Zn _0.8 Ge _0.2 O _x is deposited on the p-cuprous oxide layer by sputtering at room temperature, and MgF ₂ is deposited thereon as an antireflection film. After that, an AZO transparent conductive film is deposited as a second electrode on the surface side. Further, when depositing the second electrode on the surface side, it is necessary to form the film at room temperature in order to suppress the oxidation of the cuprous oxide. The AZO target preferably has a ratio of Al ₂ O ₃ to ZnO of about 2 wt % to 3 wt %.

Measurement of average transmittance is as follows.

An ultraviolet-visible-near-infrared spectrophotometer (Shimadzu Corporation model: UV-3101PC) was used as an apparatus. Divide the solar cell vertically and horizontally into 4 equal parts, and select 5 points of intersection of the lines that divide the solar cell into 4 equal parts vertically and horizontally. From each intersection point, measurement points were selected from within 5% of the vertical and horizontal lengths of the solar cell to be measured, and each transmittance was obtained in 1 nm steps over the range of 700 to 1000 nm and averaged. . The average transmittance of the solar cell was determined from the transmittances measured at the five measurement points in this manner, and is shown in Table 1.

The XPS value was measured as follows. Elemental analysis of the photoelectric conversion layer by XPS first removed the second electrode and the n-type layer from the solar cell using Ar ion etching to expose the p-type semiconductor layer. Next, the exposed photoelectric conversion layer was measured using the following equipment and measurement conditions. The apparatus used was AXIS Ultra DLD manufactured by Shimadzu Corporation, and monochro (Al-Kα) (15 kV×15 mA) was used as an excitation source. The measurement mode was Spectrum for Analyzer mode and Hybrid for Lens mode. In addition, the photoelectron extraction angle was 45°. It was performed in the range of 278 to 294 eV. At this time, Pass Energy was set at 160 eV for Wide scan and 10 eV for Narrow scan. The measurement was made so that the binding energy value was incremented by 0.1 eV. Charge correction was performed with the C1s peak of surface-contaminated hydrocarbons set at 284.8 eV.

This measurement was performed at 5 locations on the photoelectric conversion layer, and the average of the respective differences was obtained. In Table 1, ⊚ indicates that the average difference ratio is within 10%, ◯ indicates that the average difference ratio is within 15%, and x indicates more than 15%.

A method for measuring the conversion efficiency is as follows.

A solar simulator simulating an AM1.5G light source is used, and the amount of light is adjusted to 1 sun using a reference Si cell under the light source. The temperature is 25°C. When the horizontal axis is voltage and the vertical axis is current density, the point where the horizontal axis intersects is Voc, and the range where Jsc can be measured from a value (for example, 1.6 V) that covers Voc with a voltmeter (negative area , for example, −0.4 V), and the current value at that time is measured. The value obtained by dividing by 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).
Efficiency η is η=Voc×Jsc×FF/P×100
P is the incident power density, AM1.5 simulated sunlight is calibrated 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.

These results are listed in Table 1. Further, Tables 2 to 4 show the results of Examples 2 to 24 and Comparative Examples 6 to 9. In Tables 1 to 3, solar cell efficiencies (FF) of other examples and comparative examples were calculated using Example 1 as a reference.

(Examples 2 to 6)
It was prepared and measured in the same manner as in Example 1, except that the ratio of oxygen and argon gas was as shown in Table 1.

(Comparative example 2)
Manufacture and measurement were performed in the same manner as in Example 1, except that an oxygen stream was present during the deposition of the n-type layer and Zn+Ge was used as the target.

(Comparative Example 3)
Manufacture and measurement were performed in the same manner as in Comparative Example 2, except that ZnO+Ge was used as the target when depositing the n-type layer.

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

(Examples 7 to 10, Comparative Example 8, Comparative Example 9)
It was fabricated and measured in the same manner as in Example 1, except that tungsten (W) was used as the first electrode and the proportions of oxygen and argon gas were set as shown in Table 2.

(Example 13)
It was prepared and measured in the same manner as in Example 8, except that the cuprous oxide film was formed at 400°C.

(Example 14)
It was prepared and measured in the same manner as in Example 9, except that the cuprous oxide compound was formed into a film at 600°C.

(Example 15)
It was prepared and measured in the same manner as in Example 1, except that the cuprous oxide compound was formed into a film at 400°C.

(Example 16)
It was prepared and measured in the same manner as in Example 1, except that the cuprous oxide compound was formed into a film at 600°C.

(Comparative Example 6)
It was prepared and measured in the same manner as in Example 1, except that the cuprous oxide compound was formed into a film at 350°C.

(Comparative Example 7)
It was prepared and measured in the same manner as in Example 1, except that the cuprous oxide compound was formed into a film at 750°C.

From Tables 1 to 3, when Examples 1 to 6 and Comparative Examples 2 to 4 are compared, the average transmittance of the solar cell when measured in the wavelength range of 700 to 1000 nm is 60% or more. It can be seen that excellent efficiency can be achieved by Furthermore, a better efficiency can be achieved with an average transmission of 70%.

In addition, when comparing Examples 7 to 14 with Comparative Examples 8 and 9, the average reflectance when the transmittance of the solar cell was measured in the wavelength range of 700 to 1000 nm was 49% or more. It can be seen that efficiency can be achieved.

In the case of solar cells fabricated at the same temperature, the efficiency ratios of Examples 1 to 6 using transparent electrodes were higher than those of Examples 7 to 10 using metal electrodes. This is because the presence of the oxide on at least a portion of the first electrode can appropriately suppress the reaction between oxygen and the electrode during the formation of cuprous oxide.

In addition, XPS analysis was performed on the photoelectric conversion layer, and in the cuprous oxide peak seen in the range of 930 eV or more and 934 eV or less of the binding energy value when the photoelectric conversion layer was analyzed by XPS, the peak top of the cuprous oxide was There are a first intersection point and a second intersection point where a horizontal line passing through 2/3 of the value and the cuprous oxide peak intersect, and a third point where a vertical line drawn down from the peak top and the horizontal line intersect. and the average 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 can also improve the conversion efficiency of solar cells. This is because the number of heterogeneous phases present inside the photoelectric conversion layer is small, so that it becomes the center of recombination of photoexcited carriers to prevent a decrease in conversion efficiency, improve the quality of the photoelectric conversion layer, and prevent light in an unintended wavelength range. 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 good interface, the defect level of the interface can be reduced and the open-circuit voltage can be increased. Furthermore, the suppression of leakage components and the formation of good-quality contacts improve the form factor and improve the efficiency.

Although the embodiments of the present invention have been described above, the present invention is not to be construed as being limited to the above-described embodiments, and can be embodied by modifying constituent elements without departing from the gist of the present invention. Also, various inventions can be formed by appropriate combinations of the plurality of constituent elements disclosed in the above embodiments. For example, like modifications, constituent elements of different embodiments may be appropriately combined.

DESCRIPTION OF SYMBOLS 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, DESCRIPTION OF SYMBOLS 200... Multi-junction-type solar cell, 201... Second solar cell (bottom cell), 300... Solar cell module, 301... First solar cell module, 302... Second solar cell module, 303... Sub-module, 400... Photovoltaic 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 on the first electrode;
an n-type layer between the second electrode and the photoelectric conversion layer;
A solar cell comprising
When the transmittance of the solar cell is measured in a wavelength range of 700 to 1000 nm, the average transmittance is 60% or more,
The photoelectric conversion layer has a thickness of 500 nm or more and 0.3 mm or less,
2/3 of the peak top value of cuprous oxide in the peak of cuprous oxide seen in the range of binding energy values of 930 eV or more and 934 eV or less when the photoelectric conversion layer is analyzed by X-ray photoelectron spectroscopy. There are a first intersection point and a second intersection point where the horizontal straight line passing through and the cuprous oxide peak intersect, and a third intersection point where the vertical line drawn from the peak top to the horizontal straight line and the horizontal straight line intersect and the average 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 law of nature,
The solar cell , wherein the first electrode and the second electrode are transparent conductive films . An antireflection film is provided on the second electrode,
2. The solar cell according to claim 1, wherein the average transmittance is 70% or more. 3. The solar cell of any one of claims 1 or 2, wherein the photoelectric conversion layer comprises cuprous oxide. 2. The solar cell according to claim 1, wherein the average ratio of the length difference is 10% or less. The solar cell according to any one of claims 1 to 4 , wherein the n-type layer has a thickness of 5 nm or more and 100 nm or less. A multi-junction solar cell using the solar cell according to any one of claims 1 to 5 . 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 photovoltaic power generation system for photovoltaic power generation using the photovoltaic module according to claim 7 .

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