SOLAR CELL AND METHOD OF PRODUCING SOLAR CELL

A solar cell according to the present disclosure comprises a support, a photoelectric conversion element, and a sealing member. The photoelectric conversion element is in a sealed space defined by the support and the sealing member. The photoelectric conversion element comprises, in this order, a first electrode, a photoelectric conversion layer, and a second electrode, and an oxygen concentration in the sealed space is greater than or equal to 10 ppm and less than or equal to 3000 ppm in terms of volume fraction.

BACKGROUND

1. Technical Field

The present disclosure relates to a solar cell and a method of producing a solar cell.

2. Description of the Related Art

In recent years, a perovskite solar cell, which uses a perovskite crystal structure represented by a composition formula ABX3(in which A is a monovalent cation, B is a divalent cation, and X is a halogen anion) or a similar crystal structure (hereinafter referred to as a “perovskite compound”) as a photoelectric conversion material, has been under research and development. Various efforts are being made to improve the photoelectric conversion efficiency and the durability of the perovskite solar cell.

Q. Sun and eight others, “Advanced Energy Materials”, July 2017, Vol. 7, p. 1700977 (hereinafter, referred to as NPL 1) report that oxygen reacts with cation in a perovskite compound under illumination of light, and the reaction generates metal oxide or hydroxide on a surface or a grain boundary of the perovskite compound.

SUMMARY

One non-limiting and exemplary embodiment provides a solar cell having improved durability.

In one general aspect, the techniques disclosed here feature a solar cell comprising a support; a photoelectric conversion element, and a sealing member, wherein the photoelectric conversion element is in a sealed space defined by the support and the sealing member, the photoelectric conversion element comprises, in this order, a first electrode, a photoelectric conversion layer, and a second electrode, and the sealed space has an oxygen concentration of greater than or equal to 10 ppm and less than or equal to 3000 ppm in terms of volume fraction.

According to the present disclosure, a solar cell can have improved durability.

DETAILED DESCRIPTIONS

As reported in NPL 1, a perovskite compound containing lead reacts with oxygen under illumination of light and generates lead oxide (PbO) or lead hydroxide (Pb(OH)2) on a surface or a grain boundary of the perovskite compound. If the reaction products, which have high-insulating properties, are produced in large amounts, movement of photogenerated carriers is inhibited. This deteriorates the characteristics of the perovskite solar cell. To avoid this problem, perovskite solar cells are generally used in a nitrogen atmosphere that has very little oxygen (e.g., an oxygen concentration of less than or equal to 0.1 ppm in terms of volume fraction). In this specification, the oxygen concentrations described below are all concentrations in terms of volume fraction.

NPL 1 discloses that normalized photoelectric conversion efficiency of perovskite solar cells after light exposure tests decreases monotonically, based on when an oxygen concentration around the solar cell is 0%, as the oxygen concentration increases from 1% to 20%.

Japanese Unexamined Patent Application Publication No. 2017-103450 (hereinafter, referred to as PTL 1) discloses that a perovskite solar cell having a sealed structure has oxygen and moisture in the sealed space. It is disclosed that the heat resistance of the solar cell is improved by controlling the oxygen concentration in the space to greater than or equal to 5% and the moisture concentration (volume fraction) to less than or equal to 300 ppm. It is disclosed here that the moisture concentration needs to be controlled in the above range because the oxidant of the hole transport material is reduced by moisture.

However, NPL 1 reports that the light resistance decreases at the oxygen concentration of 5%. Thus, when the oxygen concentration is greater than or equal to 5%, the durability of the solar cell may be insufficient.

PTL 1 does not disclose data for oxygen concentrations of less than 2%.

Considering these prior technologies, the inventors conducted detailed research to find the oxygen concentration that significantly affects the durability. The research found that the optimal oxygen concentration for the durability of solar cells exists within a certain range where the oxygen concentration is less than 2% in terms of volume fraction. Specifically, in a solar cell having a sealing structure, the photoelectric conversion efficiency after a light exposure test is improved when the oxygen concentration in the sealed space is in the range of greater than or equal to 10 ppm and less than or equal to 3000 ppm. Furthermore, when the oxygen concentration in the sealed space is greater than or equal to 100 ppm and less than or equal to 3000 ppm, the photoelectric conversion efficiency after a heat resistance test is also improved.

Description of Embodiments

A solar cell according to an embodiment of the present disclosure comprises a support, a photoelectric conversion element, and a sealing member, wherein the photoelectric conversion element is in a sealed space defined by the support and the sealing member. The photoelectric conversion element comprises, in this order, a first electrode, a photoelectric conversion layer, and a second electrode. The oxygen concentration in the sealed space is greater than or equal to 10 ppm and less than or equal to 3000 ppm.

The above-described configuration can reduce the photodegradation induced by defects in the photoelectric conversion material. Thus, the solar cell can have improved durability.

In the solar cell according to this embodiment, the oxygen concentration in the sealed space may be greater than or equal to 100 ppm and less than or equal to 3000 ppm.

The above-described configuration can sufficiently passivate defects in the photoelectric conversion material, and thus heat-induced structural changes in the photoelectric conversion material are less likely to occur, resulting in improvement in the thermal stability of the solar cell. Thus, the above-described configuration enables the solar cell to have both the photostability and the thermal stability. In other words, the above-described configuration can provide a solar cell having further improved durability.

The oxygen concentration in the sealed space can be measured by an atmospheric pressure ionization mass spectrometer, a gas chromatograph, and an electrochemical oxygen sensor. The gas whose oxygen concentration to be measured is the gas that is taken out from the sealed space with a syringe at room temperature (25° C.) or the gas that is allowed to escape from the sealed space by breaking the solar cell module in a closed space. The components of the gas are measured, and the percentage of oxygen is calculated to determine the oxygen concentration. Here, an example of a method of measuring the oxygen concentration is described. In the method, the gas in the sealed space of a solar cell is allowed to escape to a closed space, and the oxygen concentration in the outflowed gas is measured by using an atmospheric pressure ionization mass spectrometer. For example, the solar cell module is placed in a chamber filled with an inert gas, such as argon or krypton. The module is broken in the chamber to let the gas out the sealed space of the solar cell. Next, quantitative analysis is performed on the gas in the chamber by using an atmospheric pressure ionization mass spectrometer. All the components of the gas in the chamber are quantified, and the percentage of oxygen in the sum of the quantities is calculated to determine the oxygen concentration. Examples of gases in the sealed space, other than oxygen, include an inert gas, such as nitrogen and noble gases, carbon dioxide, and water vapor. If the sealed space contains the inert gas that is the same in type as the inert gas filling the chamber used for the gas analysis, accurate gas analysis may be difficult. To solve the problem, when the type of gas contained in the sealed space is unknown, two identical solar cell modules are provided, and the analysis of the gas is performed for each of the two solar cell modules by the above-described procedure using different types of inert gases as the inert gas filling the chamber. The composition of the gas contained in the sealed space can be determined when the results of the two analyses are compared.

The partial pressure of oxygen in the sealed space may be greater than or equal to 1×10−5atm and less than or equal to 3×10−3atm.

The above-described configuration can more readily reduce the photodegradation induced by defects in the photoelectric conversion material. Thus, the solar cell can have improved durability. When the solar cells according to the present disclosure are produced under atmospheric pressure, the pressure in the sealed space is about atmospheric pressure. In this case, the partial pressure of oxygen in the sealed space is greater than or equal to 1×10−5atm and less than or equal to 3×10−3atm.

To calculate the partial pressure of oxygen in the sealed space, the mass or molar concentration of the gas containing oxygen in the sealed space is determined by using the above-described method of measuring the oxygen concentration, and the volume of the sealed space is measured. Then, the partial pressure is calculated by using an equation of state of a gas. The volume of the sealed space can be determined by a method other than size measurement. For example, after a vacuum is drawn in the sealed space, an inert gas of known pressure and volume is injected into the sealed space, and the volume may be determined from the pressure change, or a liquid of known density is injected into the sealed space, and the volume may be determined from the weight change. The above-described measurements are usually performed at room temperature, but the temperature is not limited to this. In other words, the measurements may be performed at an actual operating temperature such that the effects of adsorption and desorption of gases in the sealed space can be taken into consideration.

Modifications of the solar cell according to the embodiment of the present disclosure will be described. The same explanation can be appropriately omitted.

As described above, the solar cell according to the present disclosure comprises a support, a photoelectric conversion element, and a sealing member, wherein the photoelectric conversion element is in a sealed space defined by the support and the sealing member. The photoelectric conversion element comprises, in this order, a first electrode, a photoelectric conversion layer, and a second electrode. The amount of oxygen in the sealed space per surface area of the surface facing the sealed space of the photoelectric conversion element may be greater than or equal to 2.3×10−7mol/m2and less than or equal to 7.0×10−5mol/m2. In other words, the value obtained by dividing the amount of oxygen in the sealed space by the area of the surface of the photoelectric conversion element facing the sealed space may be greater than or equal to 2.3×10−7mol/m2and less than or equal to 7.0×10−5mol/m2. This can reduce degradation due to long-term operation of the solar cell.

In the photoelectric conversion element, the main surface of the first electrode may face the support, or the main surface of the second electrode may face the support.

The solar cell according to the present disclosure can be produced, for example, by the following method. First, a photoelectric conversion element is produced by the method described below.

The produced photoelectric conversion element is sealed with a sealing member in a glove box having the controlled oxygen concentration.

The oxygen concentration in the glove box is greater than or equal to 10 ppm and less than or equal to 3000 ppm. Alternatively, the oxygen concentration in the glove box is adjusted such that the amount of oxygen in the sealed space per surface area of the surface facing the sealed space of the photoelectric conversion element is greater than or equal to 2.3×10−7mol/m2and less than or equal to 7.0×10−5mol/m2.

The pressure and an oxygen concentration in the glove box may be controlled such that the partial pressure of oxygen in the sealed space becomes greater than or equal to 1×10−5atm and less than or equal to 3×10−3atm.

With the above-described configuration, a solar cell having a predetermined oxygen concentration, a predetermined oxygen content, or a predetermined oxygen partial pressure can be produced.

As described above, a photoelectric conversion element comprising, in this order, a first electrode, a photoelectric conversion layer, and a second electrode may be sealed in an atmosphere having an oxygen concentration of greater than or equal to 10 ppm and less than or equal to 3000 ppm in terms of volume fraction to produce a solar cell.

FIG.1illustrates a schematic configuration of the solar cell1000according to the embodiment of the present disclosure.

The solar cell1000according to this embodiment includes a photoelectric conversion element1, a support2, and a sealing member3. The photoelectric conversion element1is in the sealed space defined by the support2and the sealing member3.

The photoelectric conversion element1may be in contact with the support2.

The support2and the sealing member3may be formed of the same material. The material may, for example, have a gas barrier function. The material may be glass.

Hereinafter, the photoelectric conversion element1is described in more detail with reference to first to third configuration examples. The photoelectric conversion element1is not limited to the photoelectric conversion elements in the following first to third configuration examples.

FIG.2is a cross-sectional view illustrating a schematic configuration of the first configuration example of the photoelectric conversion element1in the solar cell1000according to an embodiment of the present disclosure.

The first configuration example of the photoelectric conversion element100comprises, in this order, a substrate4, a first electrode5, an electron transport layer6, a photoelectric conversion layer7, a hole transport layer8, and a second electrode9. As the first configuration example of the photoelectric conversion element100, the photoelectric conversion element1in the solar cell1000according to this embodiment may further comprise an electron transport layer between the first electrode and the photoelectric conversion layer and may further comprise a hole transport layer between the photoelectric conversion layer and the second electrode.

When the photoelectric conversion element100is irradiated with light, the photoelectric conversion layer7absorbs the light and generates excited electrons and holes. The excited electrons move through the electron transport layer6to the first electrode5. The holes generated at the photoelectric conversion layer7move through the hole transport layer8to the second electrode9. This allows current to be drawn from the photoelectric conversion element100through the first electrode5as the negative electrode and the second electrode9as the positive electrode.

The photoelectric conversion element100may or may not include the substrate4.

The photoelectric conversion element100may or may not include the electron transport layer6. When the photoelectric conversion element100includes the electron transport layer6, electrons can be efficiently moved to the first electrode5. Thus, current can be efficiently drawn from the photoelectric conversion element100.

The photoelectric conversion element100may or may not include the hole transport layer8. When the photoelectric conversion element100includes the hole transport layer8, holes can be efficiently moved to the second electrode9. Thus, current can be efficiently drawn from the photoelectric conversion element100.

The photoelectric conversion element100can be produced, for example, by the following method.

First, the first electrode5is formed on the surface of the substrate4by, for example, chemical vapor deposition or sputtering. Then, the electron transport layer6is formed by, for example, chemical vapor deposition, sputtering, or solution coating. Then, the photoelectric conversion layer7is formed on the electron transport layer6. The photoelectric conversion layer7may be formed, for example, by solution coating, printing, or vapor deposition. Alternatively, for example, a perovskite compound cut to a predetermined thickness may be used as the photoelectric conversion layer7and placed on the electron transport layer6. Then, the hole transport layer8is formed on the photoelectric conversion layer7by, for example, chemical vapor deposition, sputtering, or solution coating. Then, the second electrode9is formed on the hole transport layer8by, for example, chemical vapor deposition, sputtering, or solution coating. The photoelectric conversion element100is produced as above.

FIG.3is a cross-sectional view illustrating a schematic configuration of a second configuration example of the photoelectric conversion element1in the solar cell1000according to the embodiment of the present disclosure.

A photoelectric conversion element200comprises, in this order, the substrate4, the first electrode5, the electron transport layer6, a porous layer10, the photoelectric conversion layer7, the hole transport layer8, and the second electrode9. As the photoelectric conversion element200of the second configuration example, the photoelectric conversion element1in the solar cell1000according to this embodiment may further include the porous layer. The porous layer is located, for example, between the electron transport layer and the photoelectric conversion layer.

The porous layer10contains a porous material. The porous material contains voids.

The photoelectric conversion element200may or may not include the substrate4.

The photoelectric conversion element200may or may not include the electron transport layer6. If the photoelectric conversion element200does not include the electron transport layer6, the porous layer10is located between the first electrode5and the photoelectric conversion layer7. When the photoelectric conversion element200includes the electron transport layer6, electrons can be efficiently moved to the first electrode5. Thus, current can be efficiently drawn from the photoelectric conversion element200.

The photoelectric conversion element200may or may not include the hole transport layer8. When the photoelectric conversion element200includes the hole transport layer8, holes can be efficiently moved to the second electrode9. Thus, current can be efficiently drawn from the photoelectric conversion element200.

FIG.4is a cross-sectional view illustrating a schematic configuration of a third configuration example of the photoelectric conversion element1in the solar cell1000according to the embodiment of the present disclosure.

A photoelectric conversion element300of the third configuration example comprises, in this order, the substrate4, the first electrode5, the electron transport layer6, the porous layer10, an intermediate layer11, the photoelectric conversion layer7, the hole transport layer8, and the second electrode9. As the photoelectric conversion element300of the third configuration example, the photoelectric conversion element1in the solar cell1000according to this embodiment may further include the intermediate layer. The intermediate layer is located, for example, between the porous layer and the photoelectric conversion layer.

The photoelectric conversion element300may or may not include the substrate4.

The photoelectric conversion element300may or may not include the electron transport layer6. When the photoelectric conversion element300includes the electron transport layer6, electrons can be efficiently moved to the first electrode5. Thus, current can be efficiently drawn from the photoelectric conversion element300.

The photoelectric conversion element300may or may not include the hole transport layer8. When the photoelectric conversion element300includes the hole transport layer8, holes can be efficiently moved to the second electrode9. Thus, current can be efficiently drawn from the photoelectric conversion element300.

The photoelectric conversion element300may or may not include the porous layer10. When the photoelectric conversion element300does not include the porous layer10, the intermediate layer11is located between the electron transport layer6and the photoelectric conversion layer7.

Hereinafter, the components of the photoelectric conversion element will be described specifically.

The substrate4is an optional component. The substrate4holds the layers of the photoelectric conversion element. The substrate4can be formed of a transparent material. For example, the substrate4may be a glass substrate or a plastic substrate. The plastic substrate may be, for example, a plastic film.

If the second electrode9has light-transmitting properties, the substrate4may be formed of a non-light-transmitting material. Examples of the material include metals, ceramics, and resin materials having low light-transmitting properties.

If the first electrode5has sufficient strength, the first electrode5can hold the layers, eliminating the need for the substrate4.

The first electrode5is conductive.

The first electrode5has light-transmitting properties. For example, the first electrode5transmits light in the visible range to the near-infrared range.

The first electrode5is composed of, for example, a transparent and conductive material. Examples of the material include metal oxides and metal nitrides. Examples of them include: (i) titanium dioxide doped with at least one selected from the group consisting of lithium, magnesium, niobium, and fluorine; (ii) gallium oxide doped with at least one selected from the group consisting of tin and silicon; (iii) gallium nitride doped with at least one selected from the group consisting of silicon and oxygen; (iv) tin oxide doped with at least one selected from the group consisting of antimony and fluorine; (v) zinc oxide doped with at least one selected from the group consisting of boron, aluminum, gallium, and indium; (vi) indium-tin composite oxide; and (vii) a compound of the above materials.

The first electrode5may have a light-transmitting pattern. Examples of the light-transmitting pattern include a line pattern, a wave pattern, a lattice pattern, a perforated metal-like pattern having many fine through holes regularly or irregularly arranged. When the first electrode5has any of these patterns, light can pass through portions having no electrode material. Thus, the light-transmitting pattern enables non-transparent materials to be used. Examples of the non-transparent electronic material include platinum, gold, silver, copper, aluminum, rhodium, indium, titanium, iron, nickel, tin, zinc, and alloys containing at least one of these. A conductive carbon material may be used as a non-transparent electrode material.

When the photoelectric conversion element does not include the electron transport layer6, the first electrode5has blocking properties against holes from the photoelectric conversion layer7. In such a case, the first electrode5is not in ohmic contact with the photoelectric conversion layer7. Furthermore, the blocking properties against holes from the photoelectric conversion layer7are properties that only allow passage of electrons generated in the photoelectric conversion layer7and does not allow passage of holes. The Fermi energy of a material having such properties is higher than the energy of the photoelectric conversion layer7at the upper end of the valence band. The Fermi energy of a material having such properties may be higher than the Fermi energy of the photoelectric conversion layer7. A specific example of the material is aluminum.

When the photoelectric conversion element includes the electron transport layer6, the first electrode5may have no blocking properties against holes from the photoelectric conversion layer7. In this case, the first electrode5may be formed of a material that can form an ohmic contact with the photoelectric conversion layer7. In this case, the first electrode5may or may not be in ohmic contact with the photoelectric conversion layer7.

The light transmittance of the first electrode5may be, for example, greater than or equal to 50%, or greater than or equal to 80%. The wavelength of light to be transmitted through the first electrode5depends on the absorption wavelength of the photoelectric conversion layer7.

The thickness of the first electrode5may be, for example, greater than or equal to 1 nm and less than or equal to 1000 nm.

The electron transport layer6contains a semiconductor. The electron transport layer6may be formed of a semiconductor having a band gap of greater than or equal to 3.0 eV. This allows transmission of visible and infrared light to the photoelectric conversion layer7. An example of the semiconductor is an inorganic n-type semiconductor.

The electron transport layer6may contain a material having a band gap of greater than 6.0 eV. Examples of such a material include: (i) halides of alkali metals such as lithium fluoride and halides of alkaline earth metals such as calcium fluoride; (ii) alkali metal oxides, such as magnesium oxide; and (iii) silicon dioxide. In this case, the electron transport layer6may have a thickness of less than or equal to 10 nm, for example, to have electron transport properties.

The electron transport layer6may include multiple layers formed of different materials.

The photoelectric conversion layer7contains a photoelectric conversion material.

The photoelectric conversion material may be, for example, a perovskite compound. In other words, the photoelectric conversion layer7may comprise a perovskite compound. The perovskite compound has a high optical absorption coefficient in the wavelength range of the solar spectrum and high carrier mobility. Thus, the photoelectric conversion element containing a perovskite compound has high photoelectric conversion efficiency.

The perovskite compound is represented, for example, by the composition ABX3. A is a monovalent cation. Examples of the monovalent cation include alkali metal cations and organic cations. Examples of the alkali metal cations include a potassium cation (K+), a cesium cation (Cs+), and a rubidium cation (Rb+). Examples of organic cations include a methylammonium cation (MA+or CH3NH3+), a formamidinium cation (FA+or HC(NH2)2+), an ethylammonium cation (CH3CH2NH3+), and a guanidinium cation (CH6N3+). B is a divalent cation. Examples of the divalent cation include a lead cation (Pb2+) and a tin cation (Sn2+). X is a monovalent anion. Examples of the monovalent anion include a halogen anion. Each of the A, B, and X sites may be occupied by two or more types of ions.

The photoelectric conversion material may be, for example, a lead-containing perovskite compound.

The thickness of the photoelectric conversion layer7is, for example, greater than or equal to 50 nm and less than or equal to 10 μm.

The photoelectric conversion layer7is formed, for example, by solution coating, printing, or vapor deposition. The photoelectric conversion layer7may be formed by placing a perovskite compound that has been cut out.

The photoelectric conversion layer7may contain a perovskite compound represented by the composition formula ABX3as a main component. Here, “the photoelectric conversion layer7contains a perovskite compound represented by the composition formula ABX3as a main component” means that the photoelectric conversion layer7contains greater than or equal to 90% by mass of a perovskite compound represented by the composition formula ABX3. The photoelectric conversion layer7may contain greater than or equal to 95% by mass of a perovskite compound represented by the composition formula ABX3. The photoelectric conversion layer7may be composed of a perovskite compound represented by the composition formula ABX3. The photoelectric conversion layer7only needs to contain a perovskite compound represented by the composition formula ABX3and may have defects or impurities.

The photoelectric conversion layer7may further contain a different compound from the perovskite compound represented by the composition formula ABX3. Examples of the different other compounds include a compound having a Ruddlesden-Popper type layered perovskite structure.

The hole transport layer8contains a hole transport material. The hole transport material transport holes. The hole transport material is, for example, an organic or inorganic semiconductor.

The hole transport layer8may comprise an organic semiconductor. The organic semiconductor forms a food interface with the photoelectric conversion layer7and can reduce the formation of interface defects during bonding. This enables the photoelectric conversion element to have high photoelectric conversion efficiency and durability.

Typical examples of the organic semiconductor used as the hole transport material include 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (hereinafter, may be referred to as “PTAA”), poly(3-hexylthiophene-2,5-diyl), poly(3,4-ethylenedioxythiophene), and copper phthalocyanine. These organic semiconductors have high hole transport properties. Thus, the photoelectric conversion efficiency of the photoelectric conversion element can be improved.

The organic semiconductor may comprise at least one selected from the group consisting of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene and PTAA.

The inorganic semiconductor used as the hole transport material is a p-type semiconductor. Examples of the inorganic semiconductor include carbon materials such as Cu2O, CuGaO2, CuSCN, CuI, NiOx, MoOx, V2O5, and a graphene oxide. Here, x>0 is satisfied.

The hole transport layer8may include multiple layers formed of different materials. For example, the layers may be laminated such that the ionization potential of the hole transport layer8gradually decreases relative to the ionization potential of the photoelectric conversion layer7. This improves the hole transport properties.

The thickness of the hole transport layer8may be greater than or equal to 1 nm and less than or equal to 1000 nm, or greater than or equal to 10 nm and less than or equal to 50 nm. This allows exhibition of sufficient hole transport properties. Thus, the solar cell can keep the low resistance, and thus high photoelectric conversion efficiency can be achieved.

The hole transport layer8is formed, for example, by coating, printing, or vapor deposition. This is the same as the photoelectric conversion layer7. Examples of the coating include a doctor blade method, a bar coating method, a spray method, a dip coating method, and a spin coating method. An example of the printing is a screen-printing method. The hole transport layer8may be formed of a mixture of multiple materials and may be pressurized or heat-treated as necessary. If the hole transport layer8is formed of an organic low-molecular material or an inorganic semiconductor, the hole transport layer8can also be produced by vacuum deposition.

The hole transport layer8may comprise an additive in addition to the hole transport material to improve the conductivity. Examples of the additive include a supporting electrolyte, a solvent, and a dopant. The supporting electrolyte and the solvent stabilize the holes in the hole transport layer8. The dopant increases the number of holes in the hole transport layer8.

Examples of the supporting electrolyte include an ammonium salt, an alkaline earth metal salt, and a transition metal salt. Examples of the ammonium salt include tetrabutylammonium perchlorate, tetraethylammonium hexafluorophosphate, an imidazolium salt, and a pyridinium salt. Examples of the alkali metal salt include lithium perchlorate and potassium boron tetrafluoride. An example of the alkaline earth metal salt is bis(trifluoromethanesulfonyl)imide calcium(II). Examples of the transition metal salt include bis(trifluoromethanesulfonyl)imide zinc(II) and tris[4-tert-butyl-2-(1H-pyrazol-1-yl)pyridine] cobalt(III) tris(trifluoromethanesulfonyl)imide.

An example of the dopant is a fluorinated aromatic boron compound. An example of the fluorinated aromatic boron compound is tris(pentafluorophenyl)borane.

The solvent in the hole transport layer8may have high ionic conductivity. The solvent may be an aqueous solvent or an organic solvent. To make the solute more stable, the solvent in the hole transport layer8may be an organic solvent. Examples of the organic solvent include a heterocyclic compound solvent such as tert-butylpyridine, pyridine, and n-methylpyrrolidone.

The solvent may be an ionic liquid. The ionic liquid may be used alone or in combination with another solvent. The ionic liquid is preferable because of its low volatility and high flame resistance.

The hole transport layer8may comprise, as an additive, at least one selected from the group consisting of tert-butylpyridine, bis(trifluoromethanesulfonyl)imide calcium(II), bis(trifluoromethanesulfonyl)imide zinc(II), tris[4-tert-butyl-2-(1H-pyrazol-1-yl)pyridine] cobalt(III) tris(trifluoromethanesulfonyl)imide, and tris(pentafluorophenyl)borane. This improves the hole transport properties of the hole transport layer8. This can improve the photoelectric conversion efficiency of the photoelectric conversion element.

The second electrode9is conductive.

When the photoelectric conversion element does not include the hole transport layer8, the second electrode9has blocking properties against electrons from the photoelectric conversion layer7. In this case, the second electrode9is not in ohmic contact with the photoelectric conversion layer7. The blocking properties against electrons from the photoelectric conversion layer7are properties that only allow passage of holes generated at the photoelectric conversion layer7and does not allow passage of electrons. The Fermi energy of a material having such properties may be lower than the energy of the photoelectric conversion layer7at the lower end of the valence band. The Fermi energy of a material having such properties may be lower than the Fermi energy of the photoelectric conversion layer7. Specific examples of the material include platinum, gold, and a carbon material such as graphene.

When the photoelectric conversion element includes the hole transport layer8, the second electrode9may have no blocking properties against holes from the photoelectric conversion layer7. In this case, the second electrode9may be formed of a material that can form an ohmic contact with the photoelectric conversion layer7. This allows the second electrode9to have light-transmitting properties.

Of the first electrode5and the second electrode9, only at least one electrode that is located on a light-receiving side needs to have light-transmitting properties. Thus, one of the first electrode5and the second electrode9may have no light-transmitting properties. In other words, one of the first electrode5and the second electrode9does not need to contain a light-transmitting material and does not need to have a pattern having openings through which light passes.

The porous layer10is formed on the electron transport layer6by, for example, coating. If the photoelectric conversion element does not include the electron transport layer6, the porous layer10is formed on the first electrode5.

The pore structure given by the porous layer10serves as the foundation of the photoelectric conversion layer7. The porous layer10does not inhibit light absorption of the photoelectric conversion layer7and electron transport from the photoelectric conversion layer7to the electron transport layer6.

The porous layer10contains a porous material.

The porous material is formed, for example, of a series of insulating or semiconducting particles. Examples of the insulating particle include an aluminum oxide particle and a silicon oxide particle. An example of the semiconductor particle is an inorganic semiconductor particle. Examples of the inorganic semiconductor include metal oxide, perovskite oxide of a metal element, a sulfide of a metal element, and a metal chalcogenide. Examples of the metal oxide include oxides of Cd, Zn, In, Pb, Mo, W, Sb, Bi, Cu, Hg, Ti, Ag, Mn, Fe, V, Sn, Zr, Sr, Ga, Si, and Cr. An example of the metal oxide is TiO2. Examples of the perovskite oxide of a metal element include SrTiO3and CaTiO3. Examples of the sulfide of a metal element include CdS, ZnS, In2S3, PbS, Mo2S, WS2, Sb2S3, Bi2S3, ZnCdS2, and Cu2S. Examples of the metal chalcogenide include CsSe, In2Se3, WSe2, HgS, PbSe, and CdTe.

The thickness of the porous layer10may be greater than or equal to 0.01 μm and less than or equal to 10 or greater than or equal to 0.05 μm and less than or equal to 1 μm.

For the surface roughness of the porous layer10, the surface roughness coefficient given by the effective area/the projected area may be greater than or equal to 10 or greater than or equal to 100. The projected area is the area of shadow created behind an object that is illuminated by light directly from the front. The effective area is the actual surface area of the object. The effective area can be calculated from the volume, which is determined from the projected area and the thickness of the object, and the specific surface area and the bulk density of the material constituting the object. The specific surface area is measured, for example, by nitrogen adsorption.

The voids in the porous layer10extend continuously through the porous layer10from one main surface to the other main surface. In other words, the voids in the porous layer10extend continuously from the main surface of the porous layer10in contact with the photoelectric conversion layer7to the main surface of the porous layer10in contact with the electron transport layer6. This enables the material of the photoelectric conversion layer7to fill the voids in the porous layer10to the electron transport layer6. Thus, the presence of the porous layer10does not prevent transport of electrons, because the photoelectric conversion layer7is in direct contact with the electron transport layer6.

The presence of the porous layer10can make the formation of the photoelectric conversion layer7easy. Since the porous layer10is present, the material of the photoelectric conversion layer7enters the voids of the porous layer10, allowing the porous layer10to function as a foothold for the photoelectric conversion layer7. This reduces the possibility that the material of the photoelectric conversion layer7will be repelled by or agglomerated on the surface of the porous layer10. Thus, the photoelectric conversion layer7can be readily formed as a uniform film. The photoelectric conversion layer7can be formed by the above coating, printing, or vapor deposition.

Light scattering caused by the porous layer10is also expected to elongate the optical path length of light passing through the photoelectric conversion layer7. The increase in the optical path length will increase the number of electrons and holes generated in the photoelectric conversion layer7.

The intermediate layer11includes a self-assembled monolayer having fullerene (C60), a C60derivative, or C60(also referred to as “C60SAM”). The intermediate layer11efficiently collects electrons, reducing the resistance loss in transporting electrons to the electron transport layer6.

EXAMPLES

Hereafter, the present disclosure will be described in more detail with reference to examples and comparative examples.

In Examples and Comparative Examples, perovskite solar cells were produced, and the solar cells were evaluated in terms of the initial characteristics, the characteristics after the light resistance test, and the characteristics after the heat resistance test.

Components of the photoelectric conversion elements of the solar cells of Examples 1 to 3 and Comparative Examples 1 to 4 are as follows: Substrate, a glass substrate (thickness: 0.7 mm); First electrode, a transparent electrode (indium-tin composite oxide layer) (thickness: 200 nm); Electron transport layer, titanium dioxide (TiO2) (thickness: 10 nm); Porous layer, a mesoporous titanium dioxide (TiO2); Intermediate layer, 4-(1′,5′-Dihydro-1′-methyl-2′H-[5,6]fullereno-C60-Ih-[1,9-c]pyrrol-2′-yl)benzoic acid (i.e., C60SAM) (manufactured by Sigma-Aldrich Co. LLC); Photoelectric conversion layer, a layer containing HC(NH2)2PbI3as a main component (thickness: 500 nm); Hole transport layer, a layer containing n-butylammonium bromide (manufactured by GreatCell Solar Limited) and a layer containing PTAA as a main component (and tris(pentafluorophenyl)borane (TPFPB) (manufactured by Tokyo Chemical Industry Co., Ltd.) as an additive (thickness: 50 nm)); and Second electrode, Au (thickness: 200 nm).

First, a glass substrate having a thickness of 0.7 mm was provided. The substrate serves as the support of the solar cell of the present disclosure.

An indium-tin composite oxide layer was formed on the substrate by sputtering. In this way, the first electrode was formed.

Next, a titanium oxide layer was formed on the first electrode by sputtering. In this way, the electron transport layer was formed.

The electron transport layer was coated with 30NR-D (manufactured by GreatCell Solar Limited) by spin coating and then heat-treated at 500° C. for 30 minutes to form a titanium oxide layer having a mesoporous structure. In this way, a porous layer was formed.

Next, the substrate having the porous layer and the layers formed prior to the porous layer was immersed in the C60SAM solution for 30 minutes and then taken out. Here, the C60SAM solution was obtained by adding C60SAM into a mixture containing tetrahydrofuran (manufactured by FUJIFILM Wako Pure Chemical Corporation) and ethanol (manufactured by FUJIFILM Wako Pure Chemical Corporation) in a 1:1 volume ratio so that its concentration becomes 1×10−5mol/L. The substrate that was taken out was rinsed thoroughly with an ethanol solution and then annealed on a hot plate at 100° C. for 30 minutes. After annealing, the substrate was allowed to cool naturally to room temperature, and a C60SAM-modified substrate was produced. In this way, the intermediate layer was formed.

Next, a raw material solution of a photoelectric conversion material was applied by spin coating to form a photoelectric conversion layer containing a perovskite compound. The raw material solution contains 0.92 mol/L of lead(II) iodide (manufactured by Tokyo Chemical Industry Co., Ltd.), 0.17 mol/L of lead(II) bromide (manufactured by Tokyo Chemical Industry Co., Ltd.), 0.83 mol/L of formamidinium iodide (manufactured by GreatCell Solar Limited), 0.17 mol/L of methylammonium bromide (manufactured by GreatCell Solar Limited), 0.05 mol/L of cesium iodide (manufactured by Iwatani Corporation), and 0.05 mol/L of rubidium iodide (manufactured by Iwatani Corporation). The solvent of the solution was a mixture of dimethyl sulfoxide (Acros) and N,N-dimethylformamide (Acros). The mixing ratio (DMSO:DMF) of dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) in this raw solution was 1:4 by volume.

Next, a solution containing 6.49×10−3mol/L of n-butylammonium bromide was applied on the photoelectric conversion layer by spin coating. The solvent was 2-propanol. Furthermore, a raw material solution of a hole transport material was applied by spin coating to form the hole transport layer. The raw material solution contains 10 g/L of PTAA and TPFPB, and the solvent is toluene (manufactured by Acros).

Next, an Au film was deposited on the hole transport layer by vacuum evaporation to form the second electrode9. As above, the photoelectric conversion element was produced on the glass substrate, which is the support.

Next, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 100 ppm. Here, the moisture concentration in the glove box was less than or equal to 0.1 ppm in terms of volume fraction. The photoelectric conversion element was sealed with a UV-curing resin, a cover glass, and the glass substrate. In other words, the photoelectric conversion element including the first electrode, the electron transport layer, the porous layer, the photoelectric conversion layer, the hole transport layer, and the second electrode was sealed with the UV-cured resin, the cover glass, and the glass substrate. As above, the solar cell according to Example 1 was produced. In the solar cell according to Example 1, the oxygen concentration in the sealed space is 100 ppm.

In Example 2, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 1000 ppm. The solar cell according to Example 2 was produced in the same way as in Example 1 except for this.

In Example 3, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 3000 ppm. The solar cell according to Example 3 was produced in the same way as in Example 1 except for this.

In Example 4, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 10 ppm. The solar cell according to Example 4 was produced in the same way as in Example 1 except for the above.

Comparative Example 1

In Comparative Example 1, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 1 ppm. The solar cell according to Comparative Example 1 was produced in the same way as in Example 1 except for this.

Comparative Example 2

In Comparative Example 2, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 10000 ppm. The solar cell according to Comparative Example 2 was produced in the same way as in Example 1 except for this.

Comparative Example 3

In Comparative Example 3, sealing of the photoelectric conversion element was performed in a glove box in which the oxygen concentration was adjusted to 100000 ppm. The solar cell according to Comparative Example 3 was produced in the same way as in Example 1 except for this.

The photoelectric conversion efficiency of solar cells according to Examples 1 to 4 and Comparative Examples 1 to 3 were measured.

The photoelectric conversion efficiency of the solar cells was measured using an electrochemical analyzer (ALS440B, manufactured by BAS) and a xenon light source (BPS X300BA, manufactured by Bunkoukeiki Co., Ltd.) for the initial state, after the light resistance test, and after the heat resistance test. Before the measurement, the light intensity was calibrated to 1 Sun (100 mW/cm2) by using a silicon photodiode. The voltage sweep rate was 100 mV/s. No preconditioning, such as light irradiation and prolonged forward bias application, was performed prior to the start of the measurement. To fix the effective area and reduce the influence of scattered light, the solar cell was masked with a black mask having an aperture of 0.1 cm2 and irradiated with light from the masked glass substrate side. Photoelectric conversion efficiency was measured at room temperature and under dry air (<2% RH). Table 1 shows the open circuit voltage (VOC), the short circuit current density (Jsc), the fill factor (FF), and the photoelectric conversion efficiency (Eff) measured as above.

Light resistance tests were conducted on solar cells according to Examples 1 to 4 and Comparative Examples 1 to 3. In the light resistance test, the voltage and the current of the solar cells were kept near the optimal operating point, and the solar cells were exposed to light equivalent to 1 Sun from the substrate side for 71 hours with the temperature of the substrate being kept at 50° C. After the light resistance test, the photoelectric conversion efficiency of the solar cells was measured using the method described above. Table 2 shows the measurement results.

A heat resistance test was conducted on the solar cells according to Examples 1 to 4 and Comparative Examples 1 to 3. The solar cells were kept in a thermostatic chamber at 85° C. for 331 hours. After the heat resistance test, the photoelectric conversion efficiency of the solar cells was measured using the method described above. Table 3 shows the measurement results.

FIG.5is a graph showing the oxygen concentration dependence of the normalized photoelectric conversion efficiency of the solar cells of Examples 1 to 4 and Comparative Examples 1 to 3.FIG.6is a graph showing the oxygen amount dependence of the normalized photoelectric conversion efficiency of the solar cells according to Examples 1 to 4 and Comparative Examples 1 to 3. The normalized photoelectric conversion efficiency inFIGS.5and6is normalized for the initial state, the state after light resistance test, and the state after heat resistance test in which the highest photoelectric conversion efficiency among Examples 1 to 4 and Comparative Examples 1 to 3 is set to 0. InFIG.6, the oxygen concentrations in the sealed space indicated by the horizontal axis ofFIG.5are converted to the values obtained by dividing the amount of oxygen by the surface area of the surface of the photoelectric conversion element facing the sealed space. The ideal gas law was used for the conversion. For the solar cells of Examples 1 to 4 and Comparative Examples 1 to 3, the volume of the sealed space was about 1.4×10−7m3, and the surface area of the surface of the photoelectric conversion element facing the sealed space was 2.6×10−4m2.

<Oxygen Concentration Effect on Initial Solar Cell Characteristics>

As shown in Table 1 andFIG.5, there is no significant difference in the initial photoelectric conversion efficiency when the oxygen concentrations in the sealed space were less than or equal to 3000 ppm. The initial photoelectric conversion efficiency was stable without light irradiation and heating. However, when the oxygen concentrations were greater than or equal to 10000 ppm, the photoelectric conversion efficiency was lower. As the oxygen concentration increases, the drop rate of the photoelectric conversion efficiency increases. This may result from oxidation of the perovskite compound by excess oxygen.

<Effect of Oxygen Concentration on Solar Cell Characteristics after Light Resistance Test>

As shown in Table 2 andFIG.5, the photoelectric conversion efficiency after the light resistance test was high when the oxygen concentration in the sealed space is in the range of greater than or equal to 10 ppm and less than or equal to 3000 ppm. In contrast, the photoelectric conversion efficiency decreased in the low oxygen concentration region having the oxygen concentration of 1 ppm and in the high oxygen concentration region having the oxygen concentration of greater than or equal to 10000 ppm. The decrease in light resistance was particularly noticeable in Comparative Examples 2 and 3. When the oxygen concentration is greater than or equal to 10000 ppm, the degradation phenomenon induced by the excess oxygen may generate a large amount of reaction products with oxygen (metal oxides or metal hydroxides) or decomposition products of perovskite (metal iodides) on the surface and/or grain boundaries of the perovskite compound. This seems to have inhibited the movement of carriers and caused the decrease. In contrast, when the amount of oxygen is small, the products contribute to the passivation of defects in the perovskite compound, but when the oxygen concentration is less than or equal to 1 ppm, defect passivation under light may be insufficient, and carrier recombination through the defect level seems to have degraded the solar cell characteristics.

<Effect of Oxygen Concentration on Solar Cell Characteristics after Heat Resistance Test>

As shown in Table 3 andFIG.5, the photoelectric conversion efficiency after the heat resistance test was also high when the oxygen concentration in the sealed space was greater than or equal to 100 ppm and less than or equal to 3000 ppm. In contrast, the photoelectric conversion efficiency was lower than 15% in the low oxygen concentration region having an oxygen concentration of 10 ppm and in the high oxygen concentration region having an oxygen concentration of greater than or equal to 10000 ppm. The optimal oxygen concentration ranges exist after the light resistance test and the heat resistance test for the same reason. However, unlike the light resistance test, the lower limit of the optimal oxygen concentration range is high for the heat resistance test. This may be caused by differences in types and densities of defects caused in the perovskite compound under light and under heat. An increase in the energy of light applied in the light resistance test and an increase in the heating temperature in the heat resistance test may generate new defects depending on the activation energy for the generation of various defects. However, the conditions for the light resistance test and the heat resistance test in this example were set for actual outdoor operation. Thus, the optimal oxygen concentration range found by Examples for improving the durability of solar cells will remain the same.

From the above results, the durability of solar cells can be improved if the oxygen concentration in the sealed space is greater than or equal to 10 ppm and less than or equal to 3000 ppm. When the oxygen concentration in the sealed space is in the range of greater than or equal to 100 ppm and less than or equal to 3000 ppm, both the photostability and the thermal stability can be achieved.

This disclosure can significantly improve the durability of solar cells and has remarkably high industrial applicability.