KR20230167842A - Manufacturing method of solar cell to prevent short circuit - Google Patents

Abstract

The present invention relates to a method of manufacturing a solar cell to prevent short circuit, and a solar cell manufactured accordingly. The solar cell manufacturing method uses a thermal release tape to prepare solar cells, compared to the existing photolithography manufacturing method. The upper electrode is manufactured by attaching it to the outer part of the battery, depositing the upper electrode, and then removing the heat release tape, which not only simplifies the manufacturing process and shortens the process time, but also has the advantage of efficiently preventing short circuits. .

Description

Manufacturing method of solar cell to prevent short circuit}

The present invention relates to a method of manufacturing a solar cell to prevent short circuit, and a solar cell manufactured thereby.

Currently, fossil fuels, the most widely used energy material, are gradually being depleted, and the use of fossil fuels is also causing environmental pollution and climate change. Therefore, many scientists and engineers are devoting themselves to research to develop eco-friendly energy sources that can replace fossil fuels.

A solar cell or photovoltaic (PV) cell is an energy device that can directly convert sunlight into electricity, and is attracting attention as a next-generation eco-friendly energy technology because it can produce pollution-free electrical energy from sunlight. there is.

However, in the case of existing solar cell manufacturing, when manufacturing using methods such as photolithography, there was a disadvantage in that the manufacturing process was very complicated and the process took a long time, and a solar cell manufacturing method was needed to efficiently prevent short circuits.

Republic of Korea Patent Publication No. 10-2021-0032649

The purpose of the present invention is to provide a method of manufacturing a solar cell using a thermal release tape and a solar cell manufactured thereby.

A solar cell manufacturing method according to an aspect of the present invention includes preparing a preliminary solar cell including a first substrate, a lower electrode layer, a hole transport layer, and an electron transport layer; Attaching a thermal release tape to the outer portion of the preliminary solar cell; Depositing an upper electrode layer inside the preliminary solar cell; and removing the heat release tape.

The thickness of the heat release tape may be 0.3 mm to 2.5 mm.

In the step of removing the heat release tape, the heat release tape may be removed by performing heat treatment at a temperature of 120°C to 180°C for 1 second to 2 seconds.

Preparing the preliminary transparent solar cell includes preparing a first laminate in which a first substrate and a lower electrode layer are stacked; manufacturing a second laminate by disposing a hole transport layer on the lower electrode layer of the first laminate; and disposing an electron transport layer on the hole transport layer of the second laminate.

The step of manufacturing a second laminate by disposing a hole transport layer on the lower electrode layer of the first laminate includes preparing a 2-1 laminate in which a second substrate and a hole transport layer are stacked; manufacturing a 2-2 laminate by disposing a sacrificial layer on the hole transport layer of the third laminate; removing the second substrate of the 2-2 laminate; disposing the hole transport layer of the 2-2 stack from which the second substrate has been removed on the lower electrode layer of the first stack; and removing the sacrificial layer.

The step of disposing the electron transport layer on the hole transport layer of the second laminate includes preparing a 3-1 stack in which a third substrate and an electron transport layer are stacked; manufacturing a 3-2 laminate by disposing a sacrificial layer on the electron transport layer of the 3-1 laminate; removing the third substrate of the 3-2 laminate; disposing the electron transport layer of the 3-2 stack from which the third substrate has been removed, on the hole transport layer of the second stack; and removing the sacrificial layer.

In the step of depositing the upper electrode inside the preliminary solar cell stack, the upper electrode may be deposited through a sputtering method.

The first substrate is glass, polyethyleneterephthalate (PET), polyethylene naphthelate (PEN), polyperopylene (PP), polyimide (PI), polycarbornate (PC), polystylene (PS), polyoxyethlene (POM), and AS resin (acrylonitrile). It may include one or more selected from the group consisting of styrene copolymer), ABS resin (acrylonitrile butadiene styrene copolymer), and TAC (Triacetyl cellulose).

The lower electrode layer may be a transparent electrode or a metal electrode.

The hole transport layer may be one or more selected from the group consisting of molybdenum sulfide (MoSx), tungsten sulfide (WSx), molybdenum oxide (MoOx), nickel oxide (NiOx), tungsten oxide (WOx), and vanadium oxide (VOx). .

The electron transport layer includes WSe2, CVD graphene, 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, and Al. It may be one or more types selected from the group consisting of oxide, Y oxide, Sc oxide, Sm oxide, Ga oxide, and SrTi oxide.

The upper electrode may be one or more types selected from the group consisting of transparent conductive oxide, carbonaceous conductive material, and metallic material.

The sacrificial layer may include one or more selected from the group consisting of PMMA and PDMS.

The solar cell according to another aspect of the present invention is manufactured using the above manufacturing method, so that short circuit between the upper and lower electrodes does not occur.

Compared to the existing photolithography manufacturing method, the solar cell manufacturing method according to the present invention uses a thermal release tape to attach it to the outer part of a preliminary solar cell, deposits an upper electrode, and then removes the thermal release tape. Since the upper electrode is manufactured, the manufacturing process is simple and the process time is shortened, and it has the advantage of efficiently preventing short circuits.

1 is a schematic cross-sectional view of a preliminary solar cell according to an embodiment.
Figure 2 is a graph confirming the operation of a solar cell manufactured using a solar cell manufacturing method according to an embodiment.

The above objects, other objects, features and advantages will be easily understood through the following preferred embodiments in conjunction with the accompanying drawings. However, it is not limited to the embodiments described here and may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosed content will be thorough and complete and the technical ideas can be sufficiently conveyed to those skilled in the art.

While describing each drawing, similar reference numerals are used for similar components. In the attached drawings, the dimensions of the structures are enlarged from the actual size for clarity of the present invention. Terms such as first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, a first component may be named a second component, and similarly, the second component may also be named a first component without departing from the scope of the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

In this specification, terms such as “comprise” or “have” are intended to designate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, but are not intended to indicate the presence of one or more other features. It should be understood that this does not exclude in advance the possibility of the existence or addition of elements, numbers, steps, operations, components, parts, or combinations thereof. Additionally, when a part of a layer, membrane, region, plate, etc. is said to be “on” another part, this includes not only being “directly above” the other part, but also cases where there is another part in between. Conversely, when a part of a layer, membrane, region, plate, etc. is said to be "underneath" another part, this includes not only being "immediately below" the other part, but also cases where there is another part in between.

Unless otherwise specified, all numbers, values, and/or expressions used herein expressing quantities of components, reaction conditions, polymer compositions, and formulations are intended to represent, among other things, how such numbers inherently occur in obtaining such values. Since they are approximations reflecting the various uncertainties of measurement, they should be understood in all cases as being qualified by the term "approximately". Additionally, where a numerical range is disclosed herein, such range is continuous and, unless otherwise indicated, includes all values from the minimum to the maximum of such range inclusively. Furthermore, when such range refers to an integer, all integers from the minimum value up to and including the maximum value are included, unless otherwise indicated.

In this specification, when a range is stated for a variable, the variable will be understood to include all values within the stated range, including the stated endpoints of the range. For example, the range "5 to 10" includes the values 5, 6, 7, 8, 9, and 10, as well as any subranges such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, etc. It will be understood that it also includes any values between integers that fall within the scope of the stated range, such as 5.5, 6.5, 7.5, 5.5 to 8.5, and 6.5 to 9, etc. Also, for example, the range "10% to 30%" includes values such as 10%, 11%, 12%, 13%, etc. and all integers up to and including 30%, as well as 10% to 15%, 12% to 12%, etc. It will be understood that it includes any subranges, such as 18%, 20% to 30%, etc., and any value between reasonable integers within the range of the stated range, such as 10.5%, 15.5%, 25.5%, etc.

Conventionally, when manufacturing solar cells, electrodes are deposited through processes such as photolithography, which has the disadvantage of being complicated and taking a long time.

Accordingly, as a result of intensive research to solve the above problem, the present inventors prepared a preliminary solar cell including a first substrate, a lower electrode layer, a hole transport layer, and an electron transport layer, and applied a thermal peeling tape (thermal peeling tape) to the outer portion of the preliminary solar cell. realese tape) is attached, then the upper electrode is deposited inside the spare solar cell, and the heat release tape is removed to manufacture the solar cell. Not only is the manufacturing process simple and the process time is shortened, but short circuits can also be effectively prevented. discovered and completed the present invention.

According to one aspect of the present invention, preparing a preliminary solar cell including a first substrate, a lower electrode layer, a hole transport layer, and an electron transport layer (S100); Attaching a thermal release tape to the outer portion of the preliminary solar cell (S200); Depositing an upper electrode layer inside the preliminary solar cell (S300); and removing the thermal release tape (S400).

According to the present invention, the step of preparing a preliminary solar cell (S100) is a step of preparing a preliminary solar cell for attaching a heat release tape, and preparing a first laminate in which a first substrate and a lower electrode layer are stacked ( S110); Manufacturing a second laminate by disposing a hole transport layer on the lower electrode layer of the first laminate (S120); and disposing an electron transport layer on the hole transport layer of the second laminate (S130).

In one embodiment, the first substrate used in step S110 may be a general substrate used in solar cell manufacturing. The first substrate may be a transparent substrate such as glass or various types of flexible plastic substrates. Preferably, it may be a transparent substrate, and in the case of a transparent substrate, it is desirable to have good mechanical strength, thermal stability, and transparency. Examples of transparent substrates include transparent plastic films. Glass and quartz are mainly used as substrates, but other materials include PET (polyethyleneterephthalate), PEN (polyethylene naphthelate), PP (polyperopylene), PI (polyimide), PC (polycarbornate), PS (polystylene), POM (polyoxyethlene), and AS resin. It can be manufactured from flexible and transparent film materials such as plastics containing (acrylonitrile styrene copolymer), ABS resin (acrylonitrile butadiene styrene copolymer), and TAC (Triacetyl cellulose), etc., and can be appropriately selected according to the purpose of use.

In step S110, the lower electrode layer laminated on the first substrate may serve as an anode (cathode) that allows electrons excited by solar light to flow. Preferably, the lower electrode may be a transparent electrode or a metal electrode made of a transparent material.

In one embodiment, the lower electrode layer is a transparent electrode and may be one or more types selected from the group consisting of indium tin oxide (ITO), graphene, zinc oxide (ZnO), and fluorinated tin oxide (FTO). Additionally, the lower electrode layer is a metal electrode and may be one type selected from the group consisting of Cr/Au, Pd/Au, and Ti/Au.

In step S120, manufacturing a second laminate by disposing a hole transport layer on the lower electrode layer of the first laminate includes preparing a 2-1 laminate in which a second substrate and a hole transport layer are stacked (S121). ; Manufacturing a 2-2 laminate by disposing a sacrificial layer on the hole transport layer of the 2-1 laminate (S122); Removing the second substrate of the 2-2 stack (S123); Disposing the hole transport layer of the 2-2 stack from which the second substrate has been removed on the lower electrode layer of the first stack (S124); and removing the sacrificial layer (S125).

In step S121, the second substrate is not particularly limited as long as it is a substrate capable of supporting the hole transport layer and can be removed through an etching process in step S123 (removal of the second substrate). According to one embodiment, the second substrate may be one or more types selected from the group consisting of silicon dioxide (SiO ₂ ), Cu Foil, and Si.

In step S121, the hole transport layer serves to transfer holes generated in the solar cell to the lower electrode layer. According to one embodiment, _the hole transport layer is molybdenum sulfide (MoS _x ), tungsten sulfide (WS _x ), molybdenum oxide (MoO _x ), nickel oxide (NiO _x ), tungsten oxide ( _WO ) may be one or more types selected from the group consisting of, and may be deposited and disposed on a second substrate by CVD (Chemical vapor deposition).

In step S122, the sacrificial layer may be disposed to transfer the hole transport layer. According to one embodiment, it can be placed on the hole transport layer of the 2-1 laminate by spin coating using a spin coater. At this time, the sacrificial layer may be one or more types selected from the group consisting of PMMA, PDMS, etc.

In step S123, when the second substrate is removed from the 2-2 stack, the 2-2 stack includes only the sacrificial layer and the hole transport layer. According to one embodiment, the second substrate of the 2-2 stack may be removed by etching using a Buffered Ocide Etch (BOE) method.

In step S124, the hole transport layer of the 2-2 laminate including only the sacrificial layer and the hole transport layer is placed on the lower electrode layer of the first laminate, and the hole transport layer and the sacrificial layer are sequentially stacked on the first laminate. According to one embodiment, the 2-2 laminate including only the sacrificial layer and the hole transport layer may be laminated on the lower electrode layer of the first laminate through a WET transfer method.

In step S125, the hole transport layer may be separated and the sacrificial layer may be removed to prevent performance degradation. According to one embodiment, the result of step S124 is immersed in acetone for 2 hours to perform IPA washing, and then CVD Vacuum Annealing (250 ℃ 3 H, Ar 50 sccm) can be performed to remove the sacrificial layer.

The step of disposing the electron transport layer on the hole transport layer of the second laminate (S130) includes preparing the 3-1 stack in which the third substrate and the electron transport layer are stacked (S131); Manufacturing a 3-2 laminated body by disposing a sacrificial layer on the electron transport layer of the 3-1 laminated body (S132); Removing the third substrate of the 3-2 laminate (S133); Disposing the electron transport layer of the 3-2 stack from which the third substrate has been removed on the hole transport layer of the second stack (S134); and removing the sacrificial layer (S135).

In step S131, the electron transport layer serves to transfer electrons generated in the solar cell to the upper electrode layer. According to one embodiment, the electron transport layer is WSe2, CVD graphene, 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 may be selected from the group consisting of V oxide, Al oxide, Y oxide, and SrTi oxide, and may be deposited and placed on a third substrate by CVD (Chemical vapor deposition). there is.

In addition, the remaining steps S131, S132, S133, S134, and S135 may use the same materials and be performed in the same manner as the corresponding steps of step S120 described above.

According to the present invention, the step of attaching a thermal release tape (thermal realese tape) to the outer part of the preliminary solar cell (S200) is to attach the thermal release tape to the outer part of the preliminary solar cell prepared in step S100 to attach the upper electrode in step S300. This is a preparatory step for efficient and simple deposition without short circuits.

1 is a schematic cross-sectional view of a preliminary solar cell according to an embodiment.

Referring to FIG. 1, the outer portion of the preliminary solar cell may include a first region that does not overlap with the width region of the hole transport layer or charge transport layer among the width regions of the first substrate and the lower electrode layer based on the cross-sectional view of the preliminary solar cell, preferably, It may extend from an area that does not overlap the width area of the hole transport layer or charge transport layer to include a second area that is a partial area that overlaps the width area of the hole transport layer or charge transport layer.

According to one embodiment, the length of the second region, which is a portion of the region that extends from an area that does not overlap the width and length of the hole transport layer or the charge transport layer and overlaps the width and length of the hole transport layer or the charge transport layer, is long enough to cover the area of the hole transport layer (horizontally and vertically). It may be 1.5 cm to 2.0 cm. Outside of the above range, if the length of the second region is too short, there is a disadvantage that the hole transport layer may be damaged during separation, and if the length of the second region is too long, there is a disadvantage that separation may not be possible due to insufficient heat transfer.

In step S200, the heat release tape is not particularly limited as long as it is a tape that is removed under certain heat treatment conditions. According to one embodiment, the heat release tape may be one or more types selected from the group of products composed of heat conductive silicone elastomer.

According to one embodiment, the thickness of the heat release tape may be 0.3 mm to 2.5 mm, preferably 0.5 mm to 2.0 mm. Outside of the above range, if the thickness of the heat release tape is too thin, it is denatured at too low a temperature, making it difficult to control. If the thickness of the heat release tape is too thick, heat transfer is not possible and the device may be damaged by heat during the process of raising the temperature. There is a downside to this.

According to the present invention, the step of depositing the upper electrode inside the preliminary solar cell (S300) involves attaching a thermal release tape to the outer part of the preliminary solar cell prepared in step S200, and then attaching the thermal release tape to the inside of the preliminary solar cell to which the thermal release tape is not attached. This is the step of depositing the upper electrode layer in the third region.

In step S300, the inside of the preliminary solar cell is a third area, which may be the remaining area excluding the first area and the second area, referring to FIG. 1 .

In step S300, the upper electrode layer may be a conductive material with light transparency. Preferably, it may be a transparent conductive oxide, a carbonaceous conductive material, a metallic material, etc. According to one embodiment, the upper electrode layer is made of Indium Tin Oxide (ITO), Indium Cerium Oxide (ICO), Indium Tungsten Oxide (IWO), Zinc Indium Tin Oxide (ZITO), Zinc Indium Oxide (ZIO), and Zinc Tin Oxide (ZTO). ), GITO (Gallium Indium Tin Oxide), GIO (Gallium Indium Oxide), GZO (Gallium Zinc Oxide), AZO (Aluminum doped Zinc Oxide), FTO (Fluorine Tin Oxide), ZnO, etc. can be used. As a carbonaceous conductive material, for example, graphene or carbon nanotubes can be used, and as a metallic material, for example, a metal (Ag) nanowire, a multi-layered metal such as Au/Ag/Cu/Mg/Mo/Ti. Thin films may be used.

In this specification, the term transparent refers to being able to transmit light to a certain extent, and is not necessarily interpreted to mean complete transparency. The materials described above are not necessarily limited to the embodiments described above and can be formed of various materials, and their structures can also be modified in various ways, such as single-layer or multi-layer.

In step S300, the upper electrode layer may be deposited on the electron transport layer of the preliminary solar cell, and preferably, the upper electrode layer may be deposited on a third region inside the preliminary solar cell where the heat release tape is not attached. . According to one embodiment, the upper electrode layer can be deposited efficiently and stably without short circuiting with the lower electrode layer through a metal mask method.

According to the present invention, the step of removing the heat release tape (S400) is a step of depositing the upper electrode layer inside the preliminary solar cell according to step S300 and then removing the heat release tape through heat treatment to finally manufacture the solar cell. .

내지 180

의 온도에서 1초 내지 2초의 시간 동안 열처리를 수행하여 열박리 테이프를 제거할 수 있다. 상기 범위를 벗어나, 열처리 온도가 너무 낮으면 열전달이 제대로 되지 않아 테이프의 박리가 일어나지 않아 정공수송층이 분리되지 않을 수 있다는 단점이 있고, 열처리 온도가 너무 높으면 열전달 과정 중에 소자가 훼손될 수도 있다는 단점이 있다. 또한, 열처리 시간이 너무 짧으면 온도 제어가 어렵다는 단점이 있고, 열처리 시간이 너무 길면 소자에 가해지는 열에너지 총량이 커져 소자 성능 저하 또는 훼손이 발생할 수 있다는 단점이 있다.According to one embodiment, the step of removing the heat release tape is 120

to 180

The heat release tape can be removed by performing heat treatment at a temperature of 1 second to 2 seconds. If the heat treatment temperature is outside the above range, if the heat treatment temperature is too low, heat transfer may not be performed properly and the tape may not peel off, resulting in the hole transport layer not being separated. If the heat treatment temperature is too high, the device may be damaged during the heat transfer process. there is. In addition, if the heat treatment time is too short, there is a disadvantage that temperature control is difficult, and if the heat treatment time is too long, the total amount of heat energy applied to the device increases, which may lead to deterioration or damage to device performance.

According to another aspect of the present invention, a solar cell is provided that is manufactured by the solar cell manufacturing method according to an aspect of the present invention, and does not cause a short circuit between the upper electrode layer and the lower electrode layer.

Figure 2 is a graph confirming the operation of a solar cell manufactured using a solar cell manufacturing method according to an embodiment.

Referring to Figure 2, it was confirmed that energy conversion occurred and operated as a solar cell even when the entire manufacturing process was reduced by using heat release tape.

That is, compared to the existing photolithography manufacturing method, the solar cell manufacturing method according to the present invention uses a thermal peel tape to attach it to the outer part of a preliminary solar cell, deposits an upper electrode, and then removes the thermal peel tape. Since the upper electrode is manufactured using this method, not only is the manufacturing process simple and the process time is shortened, but the solar cell manufactured using this method has the advantage of preventing short circuits.

Claims (14)

Preparing a preliminary solar cell including a first substrate, a lower electrode layer, a hole transport layer, and an electron transport layer;
Attaching a thermal release tape to the outer portion of the preliminary solar cell;
Depositing an upper electrode layer inside the preliminary solar cell; and
A solar cell manufacturing method comprising: removing the thermal release tape. According to paragraph 1,
A solar cell manufacturing method, characterized in that the thickness of the heat release tape is 0.3 mm to 2.5 mm. According to paragraph 1,
The step of removing the heat release tape is,
A solar cell manufacturing method, characterized in that heat treatment is performed at a temperature of 120°C to 180°C for 1 to 2 seconds to remove the heat release tape. According to paragraph 1,
The step of preparing the preliminary transparent solar cell is
Preparing a first laminate including a first substrate and a lower electrode layer;
manufacturing a second laminate by disposing a hole transport layer on the lower electrode layer of the first laminate; and
A solar cell manufacturing method comprising: disposing an electron transport layer on the hole transport layer of the second laminate. According to clause 4,
The step of manufacturing a second laminate by disposing a hole transport layer on the lower electrode layer of the first laminate
Preparing a 2-1 laminate in which a second substrate and a hole transport layer are stacked;
manufacturing a 2-2 laminate by disposing a sacrificial layer on the hole transport layer of the third laminate;
removing the second substrate of the 2-2 laminate;
disposing the hole transport layer of the 2-2 stack from which the second substrate has been removed on the lower electrode layer of the first stack; and
A solar cell manufacturing method comprising: removing the sacrificial layer. According to clause 4,
The step of disposing the electron transport layer on the hole transport layer of the second laminate is
Preparing a 3-1 laminate in which a third substrate and an electron transport layer are stacked;
manufacturing a 3-2 laminate by disposing a sacrificial layer on the electron transport layer of the 3-1 laminate;
removing the third substrate of the 3-2 laminate;
disposing the electron transport layer of the 3-2 stack from which the third substrate has been removed, on the hole transport layer of the second stack; and
A solar cell manufacturing method comprising: removing the sacrificial layer. According to paragraph 1,
In the step of depositing the upper electrode inside the preliminary solar cell stack,
A solar cell manufacturing method, characterized in that the upper electrode is deposited through a sputtering method. According to paragraph 1,
The first substrate is glass, polyethyleneterephthalate (PET), polyethylene naphthelate (PEN), polyperopylene (PP), polyimide (PI), polycarbornate (PC), polystylene (PS), polyoxyethlene (POM), and AS resin (acrylonitrile). A solar cell manufacturing method comprising at least one selected from the group consisting of styrene copolymer), ABS resin (acrylonitrile butadiene styrene copolymer), and TAC (Triacetyl cellulose). According to paragraph 1,
A solar cell manufacturing method, wherein the lower electrode layer is a transparent electrode or a metal electrode. According to paragraph 1,
The hole transport layer is characterized in that it is one or more selected from the group consisting of molybdenum sulfide (MoSx), tungsten sulfide (WSx), molybdenum oxide (MoOx), nickel oxide (NiOx), tungsten oxide (WOx), and vanadium oxide (VOx). A method of manufacturing a solar cell. According to paragraph 1,
The electron transport layer includes WSe2, CVD graphene, 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, and Al. A solar cell manufacturing method, characterized in that at least one selected from the group consisting of oxide, Y oxide, Sc oxide, Sm oxide, Ga oxide, and SrTi oxide. According to paragraph 1,
A solar cell manufacturing method, wherein the upper electrode is one or more selected from the group consisting of transparent conductive oxide, carbonaceous conductive material, and metallic material. According to claim 5 or 6,
A method of manufacturing a solar cell, wherein the sacrificial layer includes at least one selected from the group consisting of PMMA and PDMS. Manufactured by the manufacturing method of paragraph 1,
A solar cell that does not cause short circuits between the upper and lower electrodes.