JP2015162524A - Photoelectric conversion element, solar battery, and method for manufacturing photoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a photoelectric conversion element which enables the further simplification of its manufacturing process without the need for performing an etching process by use of potassium cyanide or the like after CIGS layer formation; a method for manufacturing such a photoelectric conversion element; and a solar battery arranged by use of such a photoelectric conversion element.SOLUTION: A photoelectric conversion element 10 comprises: a substrate 1; a backside electrode 2; a photoelectric conversion layer 3 having a thickness of D, the photoelectric conversion layer 3 comprising a compound semiconductor layer including Cu, In, Ga and Se; a buffer layer 4; and a transparent electrode 5, which are stacked in this order. In the photoelectric conversion element, the average value of Ga/(In+Ga) is less than 0.010 in the buffer layer from the surface of the photoelectric conversion layer 3 on the side of the buffer layer 4 to a distance of D/20 toward the backside electrode 5. The maximum of Ga/(In+Ga) in a thickness direction of the photoelectric conversion layer 3 is located in a range from the surface 2a of the photoelectric conversion layer 3 on the side of the backside electrode 2 to a thickness of D/4 toward the buffer layer 4.

Description

  The present invention relates to a photoelectric conversion element, a solar cell, and a method for manufacturing a photoelectric conversion element.

A photoelectric conversion element is an element that has a stacked structure in which a photoelectric conversion layer including a semiconductor that generates current by light absorption is sandwiched between two electrodes, and forms the basis of a power generation device that uses light energy such as a solar cell.
A photoelectric conversion element using a compound semiconductor having a chalcopyrite structure for a photoelectric conversion layer is attracting attention as a next-generation solar cell element because of its excellent photoelectric conversion efficiency. Among them, shown in a photoelectric conversion layer, Cu (In, Ga) as the compound semiconductor Se ₂ (hereinafter, CIGS referred.) Element using a (hereinafter, referred to as CIGS photoelectric conversion element or CIGS device.) Is a high photoelectric conversion efficiency It has been reported that the device can be made thinner due to its high light absorption rate.
The CIGS element typically has a structure in which a substrate, a back electrode, a CIGS layer (p-type semiconductor layer), a buffer layer (n-type semiconductor layer), and a transparent electrode are laminated in this order from the opposite side of the light receiving surface. In this structure, a pn junction is formed by the CIGS layer and the buffer layer.

It is known that the band gap profile in the thickness direction of the CIGS layer affects the photoelectric conversion efficiency of the CIGS element. The band gap of the CIGS layer changes according to the ratio of In and Ga, which are constituent elements of the CIGS layer. As Ga / (In + Ga), which is an index of the composition ratio of In and Ga, increases, the band gap of the CIGS layer increases. Therefore, by changing the composition ratio of In and Ga in the depth (thickness) direction of the CIGS layer, the band gap can be changed in the depth direction of the CIGS layer, and the photoelectric conversion efficiency is controlled. Can do.
For example, Patent Documents 1 and 2 describe that the photoelectric conversion efficiency is improved by gradually increasing the ratio of Ga / (In + Ga) from the buffer layer side to the back electrode side of the CIGS layer. Yes. By gradually increasing the ratio of Ga / (In + Ga) as described above, the band gap gradually increases from the buffer layer side to the back electrode side of the CIGS layer. As a result, an electric field is generated inside the CIGS layer, and carriers excited by light irradiation are easily transported to the junction (pn junction) between the CIGS layer and the buffer layer, thereby improving the photoelectric conversion efficiency. It is considered to be.

Japanese Patent No. 3249407 International Publication No. 2004/090995

  As a CIGS layer forming method, for example, multi-source vapor deposition, selenization, sputtering, hybrid sputtering, mechanochemical process, screen printing, proximity sublimation, MOCVD, and spray are known. By using these methods, a CIGS layer that is a compound semiconductor layer containing Cu, In, Ga, and Se can be formed on a substrate having a back electrode. In the formation of the CIGS layer, a different phase from the CIGS layer such as CuxSe (x is a number greater than 0) is usually generated. If this heterogeneous phase is present on the CIGS layer surface, a leakage path is formed and the photoelectric conversion efficiency is lowered. Further, formation of an appropriate pn junction with the buffer layer formed thereon is also prevented. In order to prevent a decrease in photoelectric conversion efficiency, after forming a CIGS layer, an etching process is performed using potassium cyanide or the like to remove foreign phases such as CuxSe.

The present invention is a photoelectric conversion element having a CIGS layer as a photoelectric conversion layer, and in the manufacture thereof, it is not necessary to perform an etching treatment using potassium cyanide or the like after the CIGS layer is formed, and the manufacturing process can be further simplified. An object is to provide a possible photoelectric conversion element and a solar cell using the photoelectric conversion element.
The present invention also provides a method for producing a photoelectric conversion element having a CIGS layer as a photoelectric conversion layer, which does not require an etching treatment using potassium cyanide after the CIGS layer is formed, and provides a production method having excellent production efficiency. The task is to do.

In the case where the CIGS layer is formed by a multi-source vapor deposition method, Cu, In, Ga, and Se are vapor-deposited to finally form a compound semiconductor layer (that is, a CIGS layer) made of “CuIn _1-x Ga _x Se ₂ ”. In the formation of the CIGS layer by the so-called three-stage vapor deposition method, as a first stage, In, Ga, and Se are vapor-deposited at a temperature of about 350 ° C. to form a first compound semiconductor layer. Next, as a second step, Cu and Se are vapor-deposited at a high temperature of about 500 ° C. so that the atomic composition of the entire layer is Cu / (In + Ga)> 1, and the first compound semiconductor layer is formed into the second compound semiconductor layer. And convert. Next, as a third step, In, Ga, and Se are deposited so that the atomic composition of the entire layer is Cu / (In + Ga) <1, and the second compound semiconductor layer is converted into a third compound semiconductor layer. Thereby, a double graded band gap is formed, and the photoelectric conversion efficiency is improved.

In manufacturing a photoelectric conversion element having a CIGS layer, the present inventors formed a CIGS layer by suppressing Ga / (In + Ga) at the buffer layer side surface and in the vicinity thereof to a specific level, thereby providing a buffer for the CIGS layer. Generation of heterogeneous phase such as CuxSe on the surface of the layer side can be suppressed, and a photoelectric conversion element exhibiting high photoelectric conversion efficiency as in the case where etching treatment is performed without removing the heterogeneous phase by etching treatment after forming the CIGS layer I found out that
The present invention has been completed based on these findings.

The above problems have been achieved by the following means.
[1]
A photoelectric conversion element having a structure in which a substrate, a back electrode, a photoelectric conversion layer having a layer thickness D composed of a compound semiconductor layer containing Cu, In, Ga, and Se, a buffer layer, and a transparent electrode are stacked in this order. There,
The average value of Ga / (In + Ga) from the buffer layer side surface of the photoelectric conversion layer to the distance of D / 20 from the buffer layer side surface to the back electrode side is less than 0.010, Photoelectric conversion layer in which the maximum value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer exists within a thickness range of D / 4 from the back electrode side surface of the photoelectric conversion layer toward the buffer layer side. Conversion element.
[2]
The maximum value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.15 or more,
The average value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.050 or more, and
The photoelectric conversion element according to [1], wherein an average value of Cu / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.70 or more and 0.95 or less.
[3]
The maximum value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.20 or more and 0.70 or less,
The photoelectric conversion element according to [2], wherein an average value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.10 or more.
[4]
The photoelectric conversion element according to any one of [1] to [3], wherein the substrate is a flexible substrate.
[5]
The photoelectric conversion element according to any one of [1] to [4], wherein the buffer layer does not contain a hydroxyl group.
[6]
The solar cell which has a photoelectric conversion element in any one of [1]-[5].
[7]
A photoelectric conversion element having a structure in which a substrate, a back electrode, a photoelectric conversion layer having a layer thickness D composed of a compound semiconductor layer containing Cu, In, Ga, and Se, a buffer layer, and a transparent electrode are stacked in this order. A manufacturing method comprising:
In the formation of the photoelectric conversion layer, an average value of Ga / (In + Ga) between the buffer layer side surface of the photoelectric conversion layer and a distance of D / 20 from the buffer layer side surface toward the back electrode side is determined. The manufacturing method including making it less than 0.010.
[8]
The manufacturing method according to [7], wherein a buffer layer is formed on the photoelectric conversion layer without performing an etching treatment after the photoelectric conversion layer is formed.

  In the present specification, the atomic composition (for example, “Ga / (In + Ga)” and “Cu / (In + Ga)”) is on a molar basis unless otherwise specified. “In + Ga” indicates the total mole of In and Ga.

The photoelectric conversion element of this invention can simplify a manufacturing process, and shows the outstanding photoelectric conversion efficiency.
The solar cell of the present invention includes the photoelectric conversion element of the present invention, and is excellent in production efficiency and photoelectric conversion efficiency.
The method for producing a photoelectric conversion element of the present invention is excellent in production efficiency because an etching process using potassium cyanide or the like is not required after forming a CIGS layer which is a photoelectric conversion layer.

It is typical sectional drawing which shows an example of the photoelectric conversion element of this invention. It is typical sectional drawing which shows an example of the solar cell of this invention.

  Hereinafter, the present invention will be described in detail.

[Structure of photoelectric conversion element]
The structure of the photoelectric conversion element of the present invention will be described with reference to FIG. The photoelectric conversion element shown in FIG. 1 is a schematic diagram for facilitating the understanding of the present invention, and the size or relative size relationship of each member may be changed for convenience of explanation. It does not show the actual relationship as it is. Moreover, it is not limited to the external shape and shape shown by these drawings except the matter prescribed | regulated by this invention.
1 includes a substrate 1, a back electrode 2 formed on the substrate 1, a photoelectric conversion layer 3 formed on the back electrode 2, and a buffer formed on the photoelectric conversion layer 3. Layer 4, transparent electrode 5 formed on buffer layer 4, and upper electrode terminal 6 formed in a partial region of surface 5 a of transparent electrode 5. Light enters the photoelectric conversion element 10 from the surface 5 a side of the transparent electrode 5.

<Board>
If the board | substrate 1 of the photoelectric conversion element 10 is a board | substrate normally used as a substrate of a photoelectric conversion element thru | or a solar cell, it can be especially used without a restriction | limiting. The substrate 1 is usually an insulating substrate. As the substrate 1, for example, a soda lime glass substrate, a ceramic substrate, a resin substrate, a metal substrate with an insulating layer, or the like can be used. Especially, it is preferable that the board | substrate 1 is a flexible substrate. Preferred examples of the flexible substrate include a resin substrate and a metal substrate with an insulating layer.
The metal substrate with an insulating layer is a substrate in which an insulating layer is formed on a metal substrate. Examples of the metal substrate include metal substrates such as an Al substrate and a SUS substrate. Moreover, the board | substrate (composite metal board | substrate) which consists of a composite material containing Al and metals other than Al may be sufficient. The insulating layer on the metal substrate is also preferably an anodized film formed by anodizing the surface of the metal substrate. For example, as the substrate 1, an anodized aluminum substrate obtained by anodizing the surface of an Al substrate can be used.

  There is no restriction | limiting in particular in the thickness of the board | substrate 1, It adjusts suitably according to the magnitude | size of the photoelectric conversion element 10, the formation material of the board | substrate 1, the flexibility of the board | substrate 1 and the mechanical strength of the target element. The thickness of the substrate 1 is usually 0.02 to 10 mm. The substrate 1 is usually flat.

<Back electrode>
For example, the back electrode 2 is preferably formed of Mo, Cr, W, or a combination of one or more of these, and more preferably formed of Mo. The back electrode 2 may have a single layer structure or a multilayer structure of two or more layers. Although there is no restriction | limiting in particular in the thickness of the back surface electrode 2, It is preferable to set it as 200 nm-1000 nm from a viewpoint of resistance value and productivity.

<Photoelectric conversion layer>
The photoelectric conversion layer 3 is a layer that generates current by absorbing light that has passed through the transparent electrode 5 and the buffer layer 4.
The film thickness of the photoelectric conversion layer 3 is preferably 1.0 to 3.0 μm, and particularly preferably 1.5 to 2.0 μm. The photoelectric conversion layer 3 is composed of a compound semiconductor layer containing Cu, In, Ga, and Se. More specifically, this compound semiconductor layer is a CIGS layer (p-type compound semiconductor layer) having a chalcopyrite crystal structure. The configuration of the photoelectric conversion layer 3 will be described in further detail below.

When the thickness of the photoelectric conversion layer 3 is D, the distance from the buffer layer side surface to the distance D / 20 from the buffer layer side surface to the back electrode side (hereinafter simply referred to as “D / 20”). The average value of Ga / (In + Ga) is less than 0.010. By forming the photoelectric conversion layer so that the average value of Ga / (In + Ga) in the region of D / 20 is less than 0.010, a different phase such as unreacted CuxSe is hardly generated on the surface of the photoelectric conversion layer, High photoelectric conversion efficiency can be realized without performing an etching process after the formation of the photoelectric conversion layer. By setting the average value of Ga / (In + Ga) in the D / 20 region to be less than 0.010, the mechanism that suppresses the formation of heterogeneous phases such as CuxSe is not clear, but is estimated as follows. In general, In is easier to diffuse in CIGS than Ga and is more reactive. For this reason, it is presumed that the crystal phase having the chalcopyrite structure, which is the target product, can be easily formed by reducing the abundance of low-reactivity Ga, and as a result, the formation of heterogeneous phases can be suppressed. .
Since the photoelectric conversion element of the present invention does not require an etching process in the process of forming the photoelectric conversion layer, it can be manufactured by a dry process, and roll-to-roll formation in an all-dry process can be performed by using a flexible substrate. It is also possible to employ a membrane. Therefore, productivity is remarkably improved as compared with conventional CIGS photoelectric conversion elements.
The lower limit of the average value of Ga / (In + Ga) in the D / 20 region is not particularly limited, but is more preferably larger than 0.001.
In the photoelectric conversion layer 3, it is preferable that the average value of Ga / (In + Ga) on the buffer layer side surface is the same as or smaller than the average value of Ga / (In + Ga) in the D / 20 region.
The change in Ga / (In + Ga) in the depth direction of the photoelectric conversion layer 3 correlates with the change in the band gap in the depth direction of the photoelectric conversion layer 3.

The photoelectric conversion layer 3 has a maximum value of Ga / (In + Ga) in the thickness direction within a thickness range of D / 4 from the back electrode side surface of the photoelectric conversion layer 3 toward the buffer layer side. Is preferred. Thereby, since the carrier transport effect becomes large, the photoelectric conversion efficiency can be further improved.
Moreover, from the viewpoint of further improving the photoelectric conversion efficiency, the photoelectric conversion layer 3 preferably has a maximum value of Ga / (In + Ga) in the thickness direction of 0.15 or more, and is 0.20 or more and 0.70 or less. It is more preferable that it is 0.25 or more and 0.70 or less, and it is more preferable that it is 0.3 or more and 0.65 or less.
From the same viewpoint, the photoelectric conversion layer 3 preferably has an average value of Ga / (In + Ga) in the thickness direction of 0.050 or more, more preferably 0.050 or more and 0.50 or less. Preferably, it is 0.10 or more and 0.40 or less, more preferably 0.15 or more and 0.30 or less.
From the same viewpoint, the photoelectric conversion layer 3 preferably has an average value of Cu / (In + Ga) in the thickness direction of 0.70 or more and 0.95 or less, and 0.72 or more and 0.92 or less. More preferably.
The average value of Ga / (In + Ga) in the region of D / 20, the maximum value and average value of Ga / (In + Ga) in the thickness direction, and the average value of Cu / (In + Ga) in the thickness direction are described in the examples described later. It can be calculated by the method described.

There is no restriction | limiting in particular in the formation method of the photoelectric converting layer 3, A various method is employable. Especially, it is preferable to form by a vapor deposition method. As the vapor deposition method, it is preferable to employ a method including the following steps (i) to (iii).
(I) depositing In, Ga, and Se on a substrate having a back electrode to form a first compound semiconductor layer containing In, Ga, and Se;
(Ii) Cu and Se are vapor-deposited on the first compound semiconductor layer, and the first compound semiconductor layer is a second compound semiconductor layer in which the atomic composition of the entire layer satisfies Cu / (In + Ga)> 1. And (iii) a third compound semiconductor in which In and Se are vapor-deposited on the second compound semiconductor layer, and the second compound semiconductor layer has a total atomic composition of Cu / (In + Ga) <1. The process of making a layer.
The step (i) is preferably performed at a substrate temperature of 200 to 400 ° C., more preferably at a substrate temperature of 300 to 400 ° C. The step (ii) is preferably performed at a substrate temperature of 450 to 620 ° C, more preferably at a substrate temperature of 480 to 580 ° C. The step (iii) is preferably performed at a substrate temperature of 450 to 620 ° C., more preferably 480 to 580 ° C.

  It is known that when an alkali metal, particularly Na, diffuses into the CIGS compound semiconductor layer, the photoelectric conversion efficiency increases. Therefore, an alkali supply layer may be provided between the insulating substrate 1 and the back electrode 2 in order to efficiently supply the alkali metal to the photoelectric conversion layer 3. By having the alkali supply layer, the alkali metal diffuses into the photoelectric conversion layer 3 through the back electrode 2 when the photoelectric conversion layer 3 is formed, and the photoelectric conversion efficiency of the resulting element can be further improved.

The alkali supply layer is composed of a compound containing an alkali metal. Examples of the compound constituting the alkali supply layer may be, for _{example, NaO 2, Na 2 S, Na} 2 Se, NaCl, one or two or more of NaF and sodium molybdate salts. The alkali supply layer can be formed by, for example, a sputtering method or a coating method. In addition, the supply source of the alkali metal to the photoelectric conversion layer 3 is not limited to the alkali supply layer.

<Buffer layer>
The buffer layer 4 includes an n-type semiconductor for forming a pn junction with the photoelectric conversion layer 3. The buffer layer 4 also plays a role of protecting the photoelectric conversion layer 3 from damage when forming the transparent electrode 5.
The buffer layer 4 preferably contains a metal sulfide containing at least one metal element selected from the group consisting of Cd, Zn, Sn, and In. As specific examples, CdS; ZnS, Zn (S, O) and / or Zn (S, O, OH); SnS, Sn (S, O) and / or Sn (S, O, OH); InS, In ( S, O) and / or In (S, O, OH). The layer thickness of the buffer layer 4 is preferably 10 nm to 2 μm, and more preferably 15 to 200 nm. The buffer layer 4 can be formed by, for example, a chemical bath deposition method (hereinafter referred to as a CBD method) or a sputtering method, but in order to realize device fabrication in a highly productive all-dry process, a sputtering method is used. It is preferable to adopt (a method that requires solution processing requires the construction of a dedicated supply facility and a waste liquid facility, and is inferior in productivity compared to a dry process). Furthermore, when a dry process is employed, a hydroxyl group is not contained in the buffer layer because an aqueous solvent is not used for forming the buffer layer. Since the hydroxyl group works as a charge trap, variation in conversion efficiency can be further suppressed by suppressing the hydroxyl group content in the buffer layer.

<Transparent electrode>
The transparent electrode 5 has translucency and functions as an electrode through which a current generated in the photoelectric conversion layer 3 flows in a pair with the back electrode 2. The transparent electrode 5 is made of, for example, ZnO doped with Al, B, Ga, In or the like, or ITO (indium tin oxide). The transparent electrode 5 may have a single layer structure or a multilayer structure of two or more layers. The thickness of the transparent electrode 5 is usually 50 nm to 2 μm.
The method for forming the transparent electrode 5 is not particularly limited, and methods such as an electron beam vapor deposition method, a vapor deposition method such as a sputtering method or a CVD method, and a coating method can be widely adopted. Further, an antireflection film made of MgF ₂ or the like may be formed on the surface 5 a of the transparent electrode 5.

<Upper electrode>
The upper electrode 6 is an electrode for taking out the current generated in the photoelectric conversion layer 3 from the transparent electrode 5 when the photoelectric conversion element 10 is a cell. For this reason, the photoelectric conversion element of the present invention may or may not have the upper electrode 6.
There is no restriction | limiting in particular in the shape of the upper electrode 6, For example, it can be set as a rectangular shape, a cylinder shape, etc. The upper electrode 6 is preferably provided on the end of the surface 5 a of the transparent electrode 5 and the surface 2 a of the back electrode 2.
The upper electrode 6 can be made of aluminum, for example. The upper electrode 6 can be formed by, for example, a sputtering method, a vapor deposition method, or a CVD method.

  In the photoelectric conversion element 10, a window layer may be formed between the buffer layer 4 and the transparent electrode 5. The window layer is formed on the buffer layer 4 to suppress a parallel resistance component generated at the pn junction, and is a high-resistance insulating film made of undoped ZnO (i-ZnO) or the like. Consists of. This window layer is formed by, for example, sputtering. In particular, a window layer made of a high resistance film such as ZnO may be formed between the buffer layer 4 made of CdS or the like formed by CBD and the transparent electrode 5 made of ZnO or the like doped with Al. preferable.

There is no restriction | limiting in particular in the magnitude | size of the photoelectric conversion element of this invention, According to the objective, it can adjust suitably. For example, the area of one surface of the photoelectric conversion ^layer, it is possible to 0.5cm ² ~10cm 2.

Next, the solar cell of the present invention will be described.
FIG. 2 is a schematic cross-sectional view showing a solar cell according to an embodiment of the present invention. In addition, the solar cell shown in FIG. 2 is a schematic diagram for facilitating understanding of the present invention, and the size or relative size relationship of each member may be changed for convenience of explanation, It does not show the actual relationship as it is. Moreover, it is not limited to the external shape and shape shown by these drawings except the matter prescribed | regulated by this invention.

  A solar cell 20 shown in FIG. 2 is obtained by integrating the photoelectric conversion elements 10 shown in FIG. In the solar cell 20, the same components as those of the photoelectric conversion element 10 shown in FIG.

  The solar cell 20 has a structure in which the back electrode 2, the photoelectric conversion layer 3, the buffer layer 4, and the transparent electrode 5 are stacked, and the first groove P <b> 1 that penetrates only the back electrode 2, the photoelectric conversion layer 3. And a second groove portion P2 penetrating the buffer layer 4 and a third groove portion P3 penetrating the photoelectric conversion layer 3, the buffer layer 4, and the transparent electrode 5.

In the solar cell 20, it is isolate | separated into the some photoelectric conversion element 21 by the 1st groove part P1-the 3rd groove part P3. By filling the transparent electrode 5 in the second groove portion P2, a structure in which the transparent electrode 5 of a certain photoelectric conversion element 21 is connected in series to the back electrode 2 of the adjacent photoelectric conversion element 21 is obtained. In this case, the voltage generated in each photoelectric conversion element 21 is electrically connected in series so that the voltage is added. At this time, the effective portion of the photoelectric conversion function is the region 22.
The solar cell 20 is configured such that electrons flow in the direction D shown in FIG. 2, the back electrode 2 is a positive electrode, and the transparent electrode 5 is a negative electrode.
Note that FIG. 2 illustrates the repeated series connection structure of the photoelectric conversion elements 21 in an easy-to-understand manner, and the connection of the minus lead electrode may be the transparent electrode 5 as shown, or the second groove portion P2 may be connected. The back electrode 2 located below may be sufficient.

  In addition, the solar cell 20 is the structure similar to a well-known solar cell or a solar cell module except the structure of the CIGS layer which is a photoelectric converting layer. For this reason, also about the manufacturing method, the manufacturing method of a well-known solar cell or a solar cell module can be employ | adopted suitably.

  The present invention will be described below in more detail based on examples, but the present invention is not limited to these examples.

Example 1 Production of Photoelectric Conversion Element-1
Using a 3 cm square size anodized Al substrate (thickness: 150 μm) as an insulating substrate, a photoelectric conversion element was produced as follows.
First, a sodium supply layer was formed on an anodized Al substrate by sputtering, and a molybdenum film having a thickness of 600 nm was formed on the sodium supply layer as a back electrode by sputtering.
A CIGS layer having a thickness of 2 μm was formed on the molybdenum film by vapor deposition. The CIGS layer forming process was performed in three stages. As a first step, the substrate was heated to 350 ° C., and In, Ga, and Se were deposited while heating the substrate, and a compound semiconductor layer containing In, Ga, and Se was formed on the Mo layer. Next, as the second stage, the heating temperature of the substrate was raised to 500 ° C., and Cu and Se were deposited on the compound semiconductor layer containing In, Ga, and Se. The second stage of Cu and Se deposition was carried out until Cu / (In + Ga) of the entire layer was 1.3. Finally, as a third stage, In and Se were deposited on a compound semiconductor layer containing Cu, In, Ga, and Se without changing the heating temperature of the substrate, and a CIGS layer as a photoelectric conversion layer was formed. The third stage of In and Se deposition was such that the Cu / (In + Ga) ratio of the entire layer was less than 1. The composition adjustment in each film formation stage was performed after measuring the film formation rate of each deposition source of Cu, In, Ga, and Se in advance and determining the film formation conditions for obtaining a desired composition. The CIGS layer forming step was carried out over 10 minutes.
Next, the substrate on which the CIGS layer was formed (CIGS layer forming substrate) was placed in a reaction vessel containing a KCN 3% aqueous solution and immersed in the KCN 3% aqueous solution for 3 minutes to remove impurities on the CIGS layer surface (etching treatment). ). The CIGS layer forming substrate taken out from the solution was sufficiently washed with water, and then a CdS layer having a thickness of 50 nm was formed on the CIGS layer as a buffer layer by the CBD method. Thereafter, a ZnO: Al film (Al-doped ZnO film) having a thickness of 300 nm was continuously formed as a transparent electrode on the buffer layer (CdS layer) by sputtering.
Next, a collector electrode (upper electrode) made of aluminum is formed on the surface of the transparent electrode by sputtering, and eight photoelectric conversion elements (light-receiving area of a single cell of 0.493 cm ² ) are formed on a 3 cm square substrate. ) Was produced.
In order to investigate the influence of etching on the photoelectric conversion efficiency, photoelectric conversion elements prepared in the same manner as described above were prepared except that the etching process (KCN process) was not performed (Examples 2 to 3 and Comparative Examples 1 to 4 below). The same).

Example 2 Production of Photoelectric Conversion Element-2
In Example 1, a photoelectric conversion element was produced in the same manner as in Example 1 except that a 3 cm square size heat-resistant polyimide substrate (thickness: 25 μm) was used as the insulating substrate.

[Example 3] Production of photoelectric conversion element-3
In Example 1, a 3 cm square size soda lime glass substrate (thickness: 1100 μm) was used as the insulating substrate, and a photoelectric conversion element was produced in the same manner as in Example 1 except that the sodium supply layer was not formed.

Comparative Example 1 Production of Photoelectric Conversion Element-4
In Example 1, photoelectric conversion was performed in the same manner as in Example 1 except that Ga and Se were deposited without changing the heating temperature of the substrate from 500 ° C. following the third stage deposition in the CIGS layer forming process. An element was produced.

Comparative Example 2 Production of Photoelectric Conversion Element-5
In Example 1, a photoelectric conversion element was produced in the same manner as in Example 1 except that In, Ga, and Se were vapor deposited instead of vapor deposition of In and Se as the third stage of the CIGS layer forming process.

[Comparative Example 3] Production of photoelectric conversion element-6
In Example 1, the photoelectric conversion element was produced like Example 1 except having vapor-deposited In and Se and then vapor-deposited Ga and Se as the 1st step of the formation process of a CIGS layer.

Comparative Example 4 Production of Photoelectric Conversion Element-7
In Example 1, a photoelectric conversion element was produced in the same manner as in Example 1 except that In and Se were vapor-deposited and Ga was not vapor-deposited as the first stage of the CIGS layer forming process.

[Measurement of Ga / (In + Ga) Average Value in D / 20 Region]
About the CIGS layer of each photoelectric conversion element, the composition analysis of Ga and In was performed using SIMS (secondary ion mass spectrometer). Specifically, for one photoelectric conversion element, Ga concentration and In between the center of the surface of the CIGS layer on the buffer layer side and the distance of D / 20 from the center of the surface on the buffer layer side toward the back electrode side. The concentration was determined. Composition analysis by SIMS (secondary ion mass spectrometer) was performed using Cs ⁺ as the primary ion species, accelerating voltage 5.0 kV, and detection region 60 μm × 60 μm.
The measurement of Ga concentration and In concentration from the center of the surface on the buffer layer side to the distance of D / 20 from the center of the surface on the buffer layer side to the back electrode side is first performed on the surface center on the buffer layer side. Ga concentration and In concentration are measured, and then the Ga concentration so that the distance of D / 20 from the center of the surface on the buffer layer side toward the back electrode side becomes the final measurement point every about 10 nm toward the back electrode side. And In concentration were measured sequentially. Ga / (In + Ga) is calculated based on the Ga concentration and In concentration at each measurement point, and an average value of the obtained values of each Ga / (In + Ga) is calculated. This is calculated as Ga / (D / 20 region). The average value of In + Ga).
The results are shown in Table 1 below.

[Measurement of average value and maximum value of Ga / (In + Ga) in the thickness direction of the CIGS layer, and average value of Cu / (In + Ga) in the thickness direction of the CIGS layer]
The CIGS layer of each photoelectric conversion element was subjected to composition analysis of Cu, Ga, and In using SIMS (secondary ion mass spectrometer). Specifically, the Cu concentration, the Ga concentration, and the In concentration were determined over the entire thickness direction from the center of the CIGS layer on the buffer layer side toward the center of the back electrode side surface. The composition analysis by SIMS (secondary ion mass spectrometer) was performed using Cs ⁺ as the primary ion species, the acceleration voltage was 5.0 kV, and the detection region was 60 μm × 60 μm. The measurement interval along the thickness direction was about 10 nm as described above.
Ga / (In + Ga) at each measurement point was calculated using data on Ga concentration and In concentration measured at intervals of about 10 nm over the entire thickness direction. The average value of each Ga / (In + Ga) value calculated in this way was used as the average value of Ga / (In + Ga) in the thickness direction of the CIGS layer. Further, the maximum value among the calculated values of Ga / (In + Ga) was set as the maximum value of Ga / (In + Ga) in the thickness direction of the CIGS layer.
Similarly, Cu / (In + Ga) at each measurement point was calculated using Cu concentration, Ga concentration, and In concentration measured at intervals of about 10 nm over the entire thickness direction. The average value of each Cu / (In + Ga) value calculated in this way was taken as the average value of Cu / (In + Ga) in the thickness direction of the CIGS layer.
The results are shown in Table 1 below.
Table 1 below also shows the position of the maximum value of Ga / (In + Ga) in the thickness direction of the CIGS layer. That is, the case where the maximum value of Ga / (In + Ga) in the thickness direction of the CIGS layer is within the range of the thickness of D / 4 from the back electrode side surface of the CIGS layer toward the buffer layer side is not (A). The case is shown in Table 1 as (B).

[Measurement of photoelectric conversion efficiency]
About each photoelectric conversion element, the current-voltage characteristic (IV characteristic) under the said pseudo sun was measured using AM (Air mass) 1.5, 100 mW / cm < ² > pseudo sun, and the measurement result was shown. Using, photoelectric conversion efficiency (%) was measured. In each Example and Comparative Example, the highest photoelectric conversion efficiency in 8 cells and the average of the photoelectric conversion efficiency of 8 cells were compared. The results are shown in Table 1 below.

Comparative Example 1 is an example in which the average value of Ga / (In + Ga) in the D / 20 region of the photoelectric conversion layer is considerably higher than that defined in the present invention. In the photoelectric conversion element of Comparative Example 1, when the etching process (KCN process) was performed in the formation of the photoelectric conversion layer, both the maximum value and the average value of the photoelectric conversion efficiency of 8 cells were high. However, the photoelectric conversion efficiency was remarkably lowered without etching. This indicates that a heterogeneous phase such as CuxSe is generated on the surface of the CIGS layer during the formation of the CIGS layer, and this causes a leak path to reduce the photoelectric conversion efficiency.
Comparative Example 2 is a photoelectric conversion element in which the average value of Ga / (In + Ga) is slightly higher than that defined in the present invention, although the average value of Ga / (In + Ga) in the D / 20 region of the photoelectric conversion layer is low. is there. In the photoelectric conversion element of Comparative Example 2, when the etching treatment was not performed, the average value of the photoelectric conversion efficiency of 8 cells was significantly reduced. That is, a photoelectric conversion element with large variation in characteristics was obtained.
Comparative Example 3 is an example in which the position of the maximum value of Ga / (In + Ga) does not exist within the thickness range of D / 4 from the back electrode side surface of the photoelectric conversion layer. The photoelectric conversion element of Comparative Example 3 had no significant difference in the maximum value and average value of the photoelectric conversion efficiency of 8 cells with or without etching, but the photoelectric conversion efficiency was a low value of less than 10%.
Comparative Example 4 is an example in which the photoelectric conversion layer does not contain Ga. The photoelectric conversion element of Comparative Example 3 had no significant difference in the maximum value and average value of the photoelectric conversion efficiency of 8 cells with or without etching, but the photoelectric conversion efficiency was a low value of less than 10%.

On the other hand, the photoelectric conversion elements of Examples 1 to 3 showed high photoelectric conversion efficiencies in which the maximum value and average value of the photoelectric conversion efficiencies of 8 cells exceeded 10% regardless of the presence or absence of the etching treatment.
A region where Ga / (In + Ga) is less than 0.001 is a region that exceeds the detection limit of atomic composition analysis. Therefore, in Table 1 above, the case where Ga / (In + Ga) is less than 0.001 is uniformly indicated as “less than 0.001”.

10 Photoelectric conversion elements 1 Substrate (insulating substrate)
2 Back electrode 3 Photoelectric conversion layer 4 Buffer layer 5 Transparent electrode 6 Upper electrode terminal 20 Solar cell 21 Photoelectric conversion element

Claims (8)

A photoelectric conversion element having a structure in which a substrate, a back electrode, a photoelectric conversion layer having a layer thickness D made of a compound semiconductor layer containing Cu, In, Ga, and Se, a buffer layer, and a transparent electrode are stacked in this order. There,
The average value of Ga / (In + Ga) from the buffer layer side surface of the photoelectric conversion layer to a distance of D / 20 from the buffer layer side surface to the back electrode side is less than 0.010, Photoelectric conversion layer in which the maximum value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer exists within a thickness range of D / 4 from the back electrode side surface of the photoelectric conversion layer toward the buffer layer side. Conversion element. The maximum value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.15 or more,
The average value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.050 or more, and
The photoelectric conversion element of Claim 1 whose average value of Cu / (In + Ga) in the thickness direction of the said photoelectric converting layer is 0.70 or more and 0.95 or less. The maximum value of Ga / (In + Ga) in the thickness direction of the photoelectric conversion layer is 0.20 or more and 0.70 or less,
The photoelectric conversion element of Claim 2 whose average value of Ga / (In + Ga) in the thickness direction of the said photoelectric converting layer is 0.10 or more.   The photoelectric conversion element according to claim 1, wherein the substrate is a flexible substrate.   The photoelectric conversion element according to claim 1, wherein the buffer layer does not contain a hydroxyl group.   The solar cell which has a photoelectric conversion element of any one of Claims 1-5. A photoelectric conversion element having a structure in which a substrate, a back electrode, a photoelectric conversion layer having a layer thickness D composed of a compound semiconductor layer containing Cu, In, Ga, and Se, a buffer layer, and a transparent electrode are stacked in this order. A manufacturing method comprising:
In the formation of the photoelectric conversion layer, an average value of Ga / (In + Ga) between the buffer layer side surface of the photoelectric conversion layer and a distance of D / 20 from the buffer layer side surface toward the back electrode side is determined. The manufacturing method including making it less than 0.010.   The manufacturing method of Claim 7 which forms a buffer layer on the said photoelectric converting layer, without performing an etching process after forming the said photoelectric converting layer.

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