Method of manufacturing perovskite solar cell and perovskite solar cell manufactured by the method

Abstract

The present application relates to a method of manufacturing a perovskite solar cell and a perovskite solar cell manufactured by the method, and more particularly, to a method of manufacturing a perovskite solar cell and a perovskite solar cell manufactured by the method, the method including the steps of: (S1) performing a) oxidant treatment, or b) ultraviolet and ozone treatment, or c) oxygen plasma treatment, or d) nitrogen dioxide gas treatment on a hole transport layer (HTL) of a laminate sequentially layering a base layer, a first electrode layer, and the HTL containing a metal oxide, to oxidize the metal oxide; and (S2) sequentially layering a perovskite layer, an electron transport layer, and a second electrode layer on the HTL of the laminate.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a perovskite solar cell and a perovskite solar cell manufactured by the method, and more specifically, to a method for manufacturing a perovskite solar cell and a perovskite solar cell manufactured by the method, which can improve the hole mobility of the hole transport layer while minimizing damage to other components. Background Technology

[0002] Solar cells are the core component of solar power generation, directly converting sunlight into electrical energy. They are currently widely used for power supply not only in homes but also in space. Recently, they have been applied in fields such as aviation, meteorology, and communications, and products such as solar-powered cars and solar-powered air conditioners have also attracted considerable attention.

[0003] These types of solar cells primarily use silicon semiconductors, but due to the high price of high-purity silicon semiconductor raw materials and the complexity of manufacturing processes for solar cells using these materials, the unit cost of power generation is very high. That is, it is 3 to 10 times higher than the unit cost of traditional fossil fuel power generation. Therefore, with government subsidies around the world, the market has growth limits. For this reason, the development and research of silicon-free solar cells have been actively underway. Since the 1990s, research on dye-sensitized solar cells (DSSCs) utilizing organic semiconductor dyes and polymer solar cells utilizing conductive polymers has been formally promoted. Despite many efforts from academia and industry, organic semiconductor-based solar cells, such as DSSCs and polymer solar cells, have not yet been commercialized. However, with the recent emergence of perovskite solar cells (PSCs), which combine the advantages of DSSCs and polymer solar cells, expectations for the next generation of solar cells are rising.

[0004] Perovskite solar cells are a type of solar cell that combines traditional DSSC and polymer solar cells. Unlike DSSC, which uses a liquid electrolyte, perovskite solar cells have improved reliability. At the same time, due to the excellent optical properties of perovskite, perovskite solar cells can be used as a high-efficiency solar cell, and their efficiency has been continuously improved recently through process improvements, material improvements, and structural improvements.

[0005] Figure 1 This is a view showing a perovskite solar cell. (See reference...) Figure 1The perovskite solar cell 100 includes a substrate layer 10, a first electrode layer 20, a hole transport layer 30, a perovskite layer 40, an electron transport layer 50, and a second electrode layer 60.

[0006] In the perovskite solar cell 100, the mobility of electrons or holes in each of the aforementioned layers is important, but charge extraction at the interfaces between the layers is also crucial. If charge cannot be rapidly extracted at the interfaces, electrons and holes may recombine.

[0007] For example, compared to organic hole transporters, the NiO contained in the hole transport layer 30 x While the material exhibits high hole mobility, hole extraction is not effectively performed at the interface of the perovskite layer 40, potentially negatively impacting the characteristics of the solar cell 100. Ni vacancies can be adjusted using additives or high heat to improve hole extraction efficiency. However, the indium tin oxide (ITO) primarily used in the substrate layer 10 or the first electrode layer 20, which is stacked on the lower surface of the hole transport layer 30, is damaged by high-temperature heat treatment; for example, its resistance increases significantly at temperatures of 200°C or higher. Therefore, a method is needed to improve hole extraction efficiency without damaging the substrate layer 10 or the first electrode layer 20. Summary of the Invention

[0008] Technical issues

[0009] Therefore, the problem to be solved by the present invention is to provide a method for manufacturing a perovskite solar cell that can improve the hole mobility and hole extraction efficiency of the hole transport layer while minimizing damage to the substrate or electrode layer, and a perovskite solar cell manufactured by the method.

[0010] Problem-solving methods

[0011] To achieve the above objectives, according to one aspect of the present invention, a method for manufacturing a perovskite solar cell includes the following steps: (S1) performing an a) oxidant treatment, or b) ultraviolet and ozone treatment, or c) oxygen plasma treatment, or d) nitrogen dioxide gas treatment on the hole transport layer of a stack of a substrate layer, a first electrode layer, and a hole transport layer (HTL) containing a metal oxide, in sequence, to oxidize the metal oxide; and (S2) sequentially stacking a perovskite layer, an electron transport layer, and a second electrode layer on the hole transport layer of the stack.

[0012] The step (S1) may further include: treating the hole transport layer with a solution containing the oxidant to oxidize the metal oxide, and then removing the solvent contained in the solution.

[0013] Additionally, the metal oxide in step (S1) may be NiO. x .

[0014] At this point, in step (S1), the NiO can be oxidized. x To improve the Ni vacancy in the hole transport layer.

[0015] Additionally, in step (S1), the NiO can be oxidized. x To include a portion of Ni contained in the hole transport layer 2+ Oxidation to Ni 3+ .

[0016] At this time, the Ni 3+ The content of Ni 2+ and Ni 3+ The ratio of the total content can be below 0.6.

[0017] Meanwhile, the first electrode layer and the second electrode layer may independently include at least one of the following combinations: indium tin oxide (ITO), indium cerium oxide (ICO), indium tungsten oxide (IWO), zinc indium tin oxide (ZITO), zinc indium oxide (ZIO), zinc tin oxide (ZTO), gallium indium tin oxide (GITO), gallium indium oxide (GIO), gallium zinc oxide (GZO), aluminum doped zinc oxide (AZO), fluorine tin oxide (FTO), and zinc oxide (ZnO).

[0018] Furthermore, the electron transport layer may include at least one selected from a combination of Ti oxide, Zn oxide, In oxide, Sn oxide, W oxide, Nb oxide, Mo oxide, Mg oxide, Zr oxide, Sr oxide, Yr oxide, La oxide, V oxide, Al oxide, Y oxide, Sc oxide, Sm oxide, Ga oxide, and SrTi oxide.

[0019] According to another aspect of the present invention, a perovskite solar cell is formed by sequentially stacking a substrate layer, a first electrode layer, a hole transport layer (HTL) comprising a metal oxide, a perovskite layer, an electron transport layer, and a second electrode layer, wherein the metal oxide is NiO. x The hole transport layer includes Ni 2+ and Ni 3+ .

[0020] The effects of the invention

[0021] According to the present invention, without subjecting the hole transport layer to high-temperature heat treatment at 200°C or above, the metal oxide contained in the hole transport layer is oxidized by a) oxidant treatment, or b) ultraviolet and ozone treatment, or c) oxygen plasma treatment, or d) nitrogen dioxide gas treatment, thereby improving the hole mobility or hole extraction efficiency of the hole transport layer without damaging the substrate layer or electrode layer.

[0022] Therefore, when the hole extraction efficiency of the hole transport layer is improved, recombination caused by inefficient hole extraction at the interface with the perovskite layer can be prevented, ultimately improving the photoelectric conversion efficiency. Attached Figure Description

[0023] The following drawings, which are included with this specification, illustrate preferred embodiments of the invention and, together with the detailed description of the invention described below, help to further understand the technical concept of the invention. Therefore, the invention should not be construed as being limited to the contents shown in the drawings.

[0024] Figure 1 This is a side view showing a perovskite solar cell.

[0025] Figure 2 This is a side view showing a laminate of a substrate layer, a first electrode layer and a hole transport layer according to the present invention.

[0026] Figure 3 This is a conceptual diagram illustrating how, according to an embodiment of the present invention, Ni vacancy in a hole transport layer is improved by treatment with an oxidant.

[0027] Figure 4 This indicates the presence of NiO. x The UPS analysis results are shown in the figure when the hole transport layer is not oxidized.

[0028] Figure 5 This indicates the presence of NiO. x The UPS analysis results are shown in the figure when the hole transport layer is oxidized. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings. The terms or words used in this specification and claims should not be construed as limited to their general or dictionary meanings. Inventors should interpret them in accordance with the principle of appropriately defining terms and concepts to best interpret their invention, ensuring that they conform to the technical concept of the present invention.

[0030] Therefore, the configuration described in this specification is only one preferred embodiment of the present invention and does not represent all the technical ideas of the present invention. It should be understood that in this application, they can have various equivalents and variations that can be substituted.

[0031] Figure 2 This is a view showing a laminate comprising a substrate layer, a first electrode layer, and a hole transport layer according to the present invention. (Refer to...) Figure 2 According to the present invention, a method for manufacturing a perovskite solar cell is as follows.

[0032] First, the hole transport layer 30 of the stack consisting of a substrate layer 10, a first electrode layer 20, and a hole transport layer (HTL) 30 containing a metal oxide is subjected to a) oxidant treatment, or b) ultraviolet and ozone treatment, or c) oxygen plasma treatment, or d) nitrogen dioxide gas treatment to oxidize the metal oxide (step S1).

[0033] Therefore, by oxidizing the metal oxides included in the hole transport layer 30 through the above-mentioned methods such as oxidant treatment, without applying high-temperature heat treatment above 200°C to the hole transport layer 30, the hole mobility or hole extraction efficiency of the hole transport layer 30 can be improved without damaging the substrate layer 10 or the first electrode layer 20.

[0034] At this time, the oxidant can be any substance that can oxidize metal oxides to improve metal vacancy in the hole transport layer or increase the oxidation number of metal ions, but specifically, H2O2, HNO3, H2SO4, KNO3, etc. can be used.

[0035] Step S1 may include treating the hole transport layer 30 with a solution containing the oxidant to oxidize the metal oxide, and then removing the solvent contained in the solution.

[0036] At this point, solution processing is performed, for example, by spin-coating a solution containing the oxidant onto the upper surface of the hole transport layer 30, or by immersing the laminate in the solution for dip coating, and then oxidizing the metal oxide, and then removing the solvent by evaporation.

[0037] The solvent can be a volatile solvent to facilitate subsequent evaporation. More specifically, it can be an alcoholic substance including, but not limited to, deionized water, diethyl ether, acetone, ethanol, methanol, isopropanol, etc. During the evaporation of the solvent, heating can be carried out at a temperature below 150°C to prevent damage to the perovskite.

[0038] Furthermore, the ultraviolet and ozone treatment according to the present invention can be carried out for at least 5 minutes to oxidize metal oxides.

[0039] In addition, the oxygen plasma treatment of the present invention can be a low-temperature oxygen plasma treatment maintained at a temperature below 200°C.

[0040] Furthermore, the nitrogen dioxide gas treatment of the present invention can be performed by flowing dry air containing nitrogen dioxide to the upper surface of the hole transport layer 30 to oxidize the metal oxide. In this case, the nitrogen dioxide concentration in the dry air can be 5 to 1000 ppm, and the temperature can be maintained at 25 to 35°C.

[0041] The step of oxidizing the metal oxide can be used to oxidize only the surface of the hole transport layer 30 or to oxidize the entire hole transport layer 30.

[0042] Additionally, the substrate layer 10 may include a transparent material that allows light to pass through. Furthermore, the substrate layer 10 may include a material that selectively transmits light of a desired wavelength. For example, the substrate layer 10 may include a transparent conductive oxide (TCO), such as silicon oxide, aluminum oxide, indium tin oxide (ITO), fluorine tin oxide (FTO), glass, quartz, or a polymer, for example, the polymer may include at least one of polyimide, polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), or polydimethylsiloxane (PDMS).

[0043] The thickness of the substrate layer 10 can range from, for example, 100 μm to 150 μm, and can be, for example, 125 μm. However, the material and thickness of the substrate layer 10 are not limited to those described above, and can be appropriately selected according to the technical concept of the present invention.

[0044] Furthermore, the first electrode layer 20 can be formed of a transparent conductive material. For example, transparent conductive materials can include transparent conductive oxides, carbonaceous conductive materials, and metallic materials. Transparent conductive oxides can be, for example, indium tin oxide (ITO), indium cerium oxide (ICO), indium tungsten oxide (IWO), zinc indium tin oxide (ZITO), zinc indium oxide (ZIO), zinc tin oxide (ZTO), gallium indium tin oxide (GITO), gallium indium oxide (GIO), gallium zinc oxide (GZO), aluminum doped zinc oxide (AZO), fluorine tin oxide (FTO), and zinc oxide (ZnO). For example, carbon-based conductive materials can use graphene or carbon nanotubes, and metallic materials can use metal (Ag) nanowires or multilayered metal films, such as Au / Ag / Cu / Mg / Mo / Ti. In this specification, the term "transparent" refers to the ability to transmit light to a certain extent, and is not necessarily interpreted as completely transparent. The substances described above are not limited to the embodiments described above, but can be made of various materials and can undergo various modifications in their structure, including single-layer and multi-layer constructions.

[0045] At this time, the first electrode layer 20 can be formed by stacking on the base layer 10 or by being integrally formed with the base layer 10.

[0046] Additionally, a hole transport layer 30 can be stacked on the first electrode layer 20, and its function is to transport holes generated in the perovskite layer 40 to the first electrode layer 20. The hole transport layer 30 may include materials selected from tungsten oxide (WO3). x ), molybdenum oxide (MoO) x ), vanadium oxide (V₂O₅), nickel oxide (NiO) xAt least one metal oxide thereof and mixtures thereof. Furthermore, at least one may be included from combinations of single-molecule hole transport materials and polymeric hole transport materials, but is not limited thereto; the substances used in the art are not limited thereto. For example, spiro-MeOTAD [2,2',7,7'-tetrakis(N,Np-dimethoxy-phenylamino)-9,9'-spirobifluore ne] may be used as the single-molecule hole transport material, and P3HT [poly(3-hexylthiophene)], PTAA (polytriarylamine), poly(3,4-ethylenedioxythiophene), or polystyrene sulfonate (PEDOT:PSS) may be used as the polymeric hole transport material, but the invention is not limited thereto.

[0047] Furthermore, the hole transport layer 30 may further include a doped material, which may include, but is not limited to, dopants selected from Li-based dopants, Co-based dopants, Cu-based dopants, Cs-based dopants, and combinations thereof.

[0048] The hole transport layer 30 can be formed by coating a precursor solution for the hole transport layer onto the first electrode layer and drying the precursor solution. Before coating the precursor solution, the first electrode layer can be treated with ultraviolet light and ozone to reduce the work function of the first electrode layer 20, remove surface impurities, and undergo hydrophilic treatment. The precursor solution can be coated using methods such as spin coating, but is not limited to these methods. The thickness of the formed hole transport layer 30 can be from 10 to 500 nm.

[0049] At this time, the metal oxide of the hole transport layer 30 is preferably NiO. x Compared with other organic hole transporters or other metal oxides, it has the advantage of high hole mobility in matter.

[0050] in addition, Figure 3 This is a conceptual diagram illustrating how, according to an embodiment of the present invention, treatment with an oxidant improves Ni vacancy in hole transport layer 30. (Refer to...) Figure 3 In step (S1), the NiO can be oxidized. x This improves the Ni vacancy in the hole transport layer 30. Furthermore, the NiO can be oxidized. x To include a portion of Ni contained in the hole transport layer 30 2+ Oxidation to Ni 3+ Therefore, when Ni vacancies increase or some Ni... 2+ Oxidized to Ni3+ At this time, the hole mobility increases and the resistance decreases, thereby improving the hole extraction efficiency of the hole transport layer 30.

[0051] Wherein, Ni 3+ The content of Ni 2+ and Ni 3+ The total content ratio is 0.6 or less, specifically 0.3 or less. At this point, if the content ratio exceeds 0.6, that is, if Ni... 3+ Excessive Ni content may lead to a decrease in optical transmittance. Furthermore, with the increasing content of Ni... 3+ As the proportion increases, the valence band maximum (VBM) of the hole transport layer 30 shifts downward. When the Ni 3+ The content of Ni 2+ and Ni 3+ When the total content ratio is approximately 0.6, more preferably approximately 0.3, good energy matching with the perovskite layer occurs, leading to effective charge extraction. Furthermore, if Ni... 3+ If the content of the ore is too high, the work function (VBM) will decrease excessively, which will cause an energy level alignment mismatch at the interface with the perovskite, resulting in interference with hole extraction at the interface.

[0052] Figure 4 This indicates the presence of NiO. x The UPS analysis results are shown in the figure when the hole transport layer is not oxidized. Figure 5 This indicates the presence of NiO. x The UPS analysis results are shown in the figure below when the hole transport layer is oxidized. Table 1 below shows the work function and valence band edge values ​​for each of the above cases.

[0053] [Table 1]

[0054] <![CDATA[NiO x (No processing) <![CDATA[NiO x (Processed) Work function 4.66 5.16 Price band edge 5.61 5.69

[0055] It can be confirmed that with NiO x Ni after oxidation treatment 3+As the proportion increases, the work function value increases and approaches the valence band edge value, indicating an effect similar to p-type doping. 1) In NiO x (Unprocessed)

[0056] Work function: 21.22 eV (He|UPS spectrum) - 16.56 eV = 4.66 eV

[0057] Valence band edge: 4.66 eV (work function) + 0.95 eV = 5.61 eV

[0058] 2) In NiO x (In cases where processing is required)

[0059] Work function: 21.22 eV (He|UPS spectrum) - 16.56 eV = 5.16 eV

[0060] Valence band edge: 5.16 eV (work function) + 0.53 eV = 5.69 eV

[0061] Subsequently, a perovskite layer 40, an electron transport layer 50, and a second electrode layer 60 are sequentially stacked on the hole transport layer 30 of the laminate (step S2).

[0062] In the perovskite solar cell 100 according to the present invention, a perovskite compound is used as a photoactive material to absorb sunlight and generate photoelectron-hole pairs. Perovskite has a direct band gap and a light absorption coefficient as high as 1.5 × 10⁻⁶ at 550 nm. 4 cm -1 It possesses excellent charge transfer characteristics and outstanding resistance to defects.

[0063] Furthermore, the advantage of perovskite compounds is that the light absorbers that make up the photoactive layer can be formed through a simple, easy and low-cost solution coating and drying process. Due to the spontaneous crystallization caused by the drying of the coating solution, coarse-grained light absorbers can be formed, especially with good conductivity of electrons and holes.

[0064] This perovskite compound can be represented by the structure of the following formula 1.

[0065] [Formula 1]

[0066] ABX3

[0067] (Where A represents a monovalent organic ammonium cation or a metal cation, B represents a divalent metal cation, and X represents a halide anion.)

[0068] Perovskite compounds can be used, such as CH3NH3PbI3 and CH3NH3PbI. x Cl 3-x MAPbI3, CH3NH3PbI x Br 3-x CH3NH3PbCl x Br 3-x HC(NH2)2PbI3, HC(NH2)2PbI x Cl 3-x HC(NH2)2PbI x Br 3-x HC(NH2)2PbCl x Br 3-x (CH3NH3)(HC(NH2)2) 1-y PbI3, (CH3NH3)(HC(NH2)2) 1-y PbI x Cl 3-x (CH3NH3)(HC(NH2)2) 1-y PbI x Br 3-x (CH3NH3)(HC(NH2)2) 1-y PbCl x Br 3-x (0≤x,y≤1). Alternatively, compounds in which Cs are partially doped into the A portion of ABX3 can also be used.

[0069] Furthermore, the electron transport layer 50, located on the perovskite layer 40, can facilitate the transfer of electrons generated in the perovskite layer 40 to the second electrode layer 60. The electron transport layer 50 may include metal oxides, such as Ti oxide, Zn oxide, In oxide, Sn oxide, W oxide, Nb oxide, Mo oxide, Mg oxide, Zr oxide, Sr oxide, Yr oxide, La oxide, V oxide, Al oxide, Y oxide, Sc oxide, Sm oxide, Ga oxide, SrTi oxide, etc. The electron transport layer 50 according to the present invention may include a compact structure of TiO2, SnO2, WO3, or TiSrO3, etc. Such an electron transport layer 50 may further include n-type or p-type dopants as needed.

[0070] In addition to the interlayer structure and / or materials of the hole transport layer 30 / perovskite layer 40 / electron transport layer 50 described above, various layer structures and materials constituting the perovskite solar cell 100 can also be applied, and the hole transport layer 30 and the electron transport layer 50 can be formed by exchanging their positions.

[0071] Furthermore, the second electrode layer 60 can be formed of a transparent conductive material. For example, transparent conductive materials can include transparent conductive oxides, carbonaceous conductive materials, and metallic materials. Transparent conductive oxides can be, for example, indium tin oxide (ITO), indium cerium oxide (ICO), indium tungsten oxide (IWO), zinc indium tin oxide (ZITO), zinc indium oxide (ZIO), zinc tin oxide (ZTO), gallium indium tin oxide (GITO), gallium indium oxide (GIO), gallium zinc oxide (GZO), aluminum doped zinc oxide (AZO), fluorine tin oxide (FTO), and zinc oxide (ZnO). For example, carbon-based conductive materials can use graphene or carbon nanotubes, and metallic materials can use metal (Ag) nanowires or multilayered metal films, such as Au / Ag / Cu / Mg / Mo / Ti. In this specification, the term "transparent" refers to the ability to transmit light to a certain extent, and is not necessarily interpreted as completely transparent. The substances described above are not limited to the embodiments described above, but can be made of various materials and can undergo various modifications in their structure, including single-layer and multi-layer constructions.

[0072] Additionally, although not illustrated, a bus electrode (not shown) can be further disposed on the second electrode layer 60 to reduce the resistance of the second electrode layer 60 and further promote charge transfer. The bus electrode may be formed of Ag, Mg, Al, Pt, Pd, Au, Ni, Nd, Ir, Cr and / or compounds thereof.

[0073] Furthermore, the embodiments of the present invention described in this specification and accompanying drawings are specific examples shown to facilitate the description of the technical content of the present invention and to aid in understanding the present invention, and are not intended to limit the scope of the present invention. In addition to the embodiments described herein, it will be apparent to those skilled in the art that other modifications based on the technical concept of the present invention can be implemented.

Claims

1. A method for manufacturing perovskite solar cells, characterized in that, Includes the following steps: (S1) Without high-temperature heat treatment at temperatures above 200°C, the hole transport layer of a stack consisting of a substrate layer, a first electrode layer, and a hole transport layer (HTL) containing a metal oxide is subjected to a) oxidant treatment, or c) oxygen plasma treatment, or d) nitrogen dioxide gas treatment to oxidize the metal oxide; and (S2) A perovskite layer, an electron transport layer, and a second electrode layer are sequentially stacked on the hole transport layer of the laminate. In step (S1), the metal oxide is oxidized to make Ni 3+ The content of Ni 2+ and Ni 3+ The total content ratio is below 0.6 to prevent a decrease in optical transmittance and energy level mismatch with the perovskite layer interface.

2. The method for manufacturing perovskite solar cells as described in claim 1, characterized in that, The step (S1) further includes: treating the hole transport layer with a solution containing the oxidant to oxidize the metal oxide, and then removing the solvent contained in the solution.

3. The method for manufacturing perovskite solar cells as described in claim 1, characterized in that, The metal oxide in step (S1) is NiO. x .

4. The method for manufacturing a perovskite solar cell as described in claim 3, characterized in that, In step (S1), the NiO is oxidized. x To improve the Ni vacancy in the hole transport layer.

5. The method for manufacturing a perovskite solar cell as described in claim 3, characterized in that, In step (S1), the NiO is oxidized. x To include a portion of Ni contained in the hole transport layer 2+ Oxidation to Ni 3 + .

6. The method for manufacturing a perovskite solar cell as described in claim 5, characterized in that, The Ni 3+ The content of Ni 2+ and Ni 3+ The ratio of the total content is below 0.

6.

7. The method for manufacturing a perovskite solar cell as described in claim 1, characterized in that, The first electrode layer and the second electrode layer independently comprise at least one of the following: indium tin oxide (ITO), indium cerium oxide (ICO), indium tungsten oxide (IWO), zinc indium tin oxide (ZITO), zinc indium oxide (ZIO), zinc tin oxide (ZTO), gallium indium tin oxide (GITO), gallium indium oxide (GIO), gallium zinc oxide (GZO), aluminum doped zinc oxide (AZO), fluorine tin oxide (FTO), and zinc oxide (ZnO).

8. The method for manufacturing a perovskite solar cell as described in claim 1, characterized in that, The electron transport layer includes at least one selected from a combination of Ti oxide, Zn oxide, In oxide, Sn oxide, W oxide, Nb oxide, Mo oxide, Mg oxide, Zr oxide, Sr oxide, Yr oxide, La oxide, V oxide, Al oxide, Y oxide, Sc oxide, Sm oxide, Ga oxide, and SrTi oxide.

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