JP2014130858A - Photoelectric conversion element and process of manufacturing buffer layer of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve element characteristics of a photoelectric conversion element provided with a buffer layer composed of a zinc compound.SOLUTION: A photoelectric conversion element 1 includes a lower electrode layer 20, a photoelectric conversion layer 30, a buffer layer 40, and an upper electrode layer 60, laminated on a substrate 10, where the buffer layer 40 is composed of a compound including zinc and sulfur, and further including at least one kind of element whose valence is larger than that of zinc among group 3 elements, group 7 elements, and group 13 elements.

Description

  The present invention relates to a photoelectric conversion element used for solar cells, CCD sensors, and the like, a method for manufacturing a buffer layer of the photoelectric conversion element, and a method for manufacturing the photoelectric conversion element.

A photoelectric conversion element including a photoelectric conversion layer and an electrode connected to the photoelectric conversion layer is used for applications such as solar cells. Conventionally, in the case of solar cells, Si-based solar cells using bulk single crystal Si or polycrystalline Si, or thin-film amorphous Si have been mainstream, but research and development of Si-independent compound semiconductor solar cells has been made. ing. Known compound semiconductor solar cells include bulk systems such as GaAs systems and thin film systems such as CIS or CIGS systems composed of group Ib elements, group IIIb elements, and group VIb elements. CI (G) S is represented by the general formula _{Cu 1-z In 1-x} Ga x Se 2-y S y ( where, 0 ≦ x ≦ 1,0 ≦ y ≦ 2,0 ≦ z ≦ 1) When x = 0, it is a CIS system, and when x> 0, it is a CIGS system. In this specification, CIS and CIGS are collectively described as “CI (G) S”.

  In conventional thin film photoelectric conversion elements such as CI (G) S, a CdS buffer layer is generally provided between a photoelectric conversion layer and a translucent conductive layer formed thereon. In such a system, the buffer layer is usually formed by a chemical bath deposition method (CBD method).

  The role of the buffer layer includes (1) prevention of recombination of photogenerated carriers, (2) band discontinuous matching, (3) lattice matching, (4) coverage of surface irregularities of the photoelectric conversion layer, and (5) It is conceivable to protect from damage when forming a transparent conductive film on the buffer layer. In the CI (G) S system or the like, the surface irregularity of the photoelectric conversion layer is relatively large. In particular, in order to satisfy the conditions (4) and (5) well, the CBD method which is a liquid phase method is considered preferable.

  In recent years, the Cd-free buffer layer has been studied in consideration of environmental load. As the main component of the Cd-free buffer layer, zinc-based materials such as Zn (S, O) have been studied.

Patent Documents 1 and 2 propose a method for manufacturing a photoelectric conversion element including a Zn (S, O) -based buffer layer.
On the other hand, Patent Document 3 discloses a method for improving the fill factor by making the buffer layer into a three-layer structure and suppressing series resistance. Photoelectric conversion layer A ZnS film of 1 to 3 nm is formed by the CBD method, a layer that continuously changes from ZnS to ZnO is formed by the CBD method, and doping impurities such as Al, Ga, B, etc. are formed in the third layer by the MOCVD method It is disclosed that a ZnO film containing an element is formed to a thickness of 100 nm or more to obtain a buffer layer having a three-layer structure.

  On the other hand, Non-Patent Document 1 shows that the resistance of CdS changes by doping impurities in the CdS buffer layer.

JP 2011-119964 A JP 2012-18953 A JP 2012-004287 A

M. Altosaar et al. "Comparison of CdS films deposited from chemical baths containing different doping impurities" Thin Solid Films 480-481 (2005) 147-150

  A photoelectric conversion element using a zinc-based buffer layer such as Zn (S, O) has a problem that the photoelectric conversion efficiency is lower than that of a photoelectric conversion element using a CdS buffer layer. According to the study by the inventors, in a photoelectric conversion element using a zinc-based buffer layer such as Zn (S, O), among others, the photoelectric conversion efficiency is low because the fill factor (curve factor) is low. It became clear that it did not improve.

The fill factor is a numerical value derived from the IV characteristic curve as follows. This will be described with reference to FIG. First, the open circuit voltage Voc is obtained from the intersection between the IV characteristic curve and the voltage axis, and the short circuit current Isc is obtained from the intersection with the current axis. Further, the maximum output Pm is obtained from the product (output P) of the voltage V and the current I along the IV characteristic curve, and the voltage Vm and the current Im when the maximum output Pm is obtained. The fill factor F can be obtained from the following equation.
F = (Im × Vm) / (Isc × Voc)

  On the other hand, the slope in the vicinity of Voc of the IV characteristic curve corresponds to the series resistance. Specifically, the series resistance can be obtained from the reciprocal of the slope near Voc.

  Factors that decrease the fill factor include an increase in series resistance, a decrease in shunt resistance, and an increase in diode factor. In Patent Document 3, the resistance of the buffer layer is reduced in order to suppress the increase in series resistance among these factors of lowering the fill factor. If the method described in Patent Literature 3 is used, the series resistance can be suppressed as a result by suppressing the resistance of the buffer layer. However, it is necessary to form a plurality of buffer layers, and it is necessary to use both the CBD method and the MCVD method, leading to an increase in the number of steps and an increase in cost.

Further, as a method of reducing the resistance of the buffer layer, it is conceivable to make the buffer layer a thin film.
However, the series resistance is the sum of multiple resistance components that can be considered as an upright connection, and each layer and interface that make up the photoelectric conversion element contribute. Therefore, there is a limit to reducing the series resistance only by reducing the resistance of the buffer layer. is there.

  This invention is made | formed in view of the said situation, Comprising: It is a photoelectric conversion element provided with the buffer layer which consists of Zn-type compounds, Comprising: It aims at providing the photoelectric conversion element which improved the photoelectric conversion characteristic. To do.

  In addition, the present invention can improve the photoelectric conversion characteristics applicable to the production of a photoelectric conversion element provided with a buffer layer made of a Zn-based compound, and the method for producing the buffer layer with reduced number of steps and cost, and the production of the buffer layer It aims at providing the manufacturing method of the photoelectric conversion element to which the method is applied.

The photoelectric conversion element of the present invention is a photoelectric conversion element in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are laminated on a substrate,
The buffer layer is composed of a compound containing zinc and sulfur and further containing at least one element having a valence higher than that of zinc among group 3 elements, group 7 elements and group 13 elements. .

  Here, the Group 3 elements include lanthanoids and actinoids.

  The element having a higher valence than zinc is preferably boron, aluminum, gallium, indium, manganese, yttrium, lanthanum, or cerium.

It is preferable that the content of the element having a valence larger than that of zinc in the compound constituting the buffer layer is 0.01 at% or more and 5 at% or less.
In the present specification, the content of elements having a higher valence than zinc is defined as the content at the center in the thickness direction of the buffer layer.

It is preferable that carbonyl ions are provided on the surface of the buffer layer on the upper electrode layer side.
The carbonyl ion is particularly preferably a citrate ion.

  Furthermore, it is preferable that the buffer layer contains sodium.

  The thickness of the buffer layer is preferably 5 nm or more and 200 nm or less.

If the buffer layer is thinner than 5 nm, the effects of (4) and (5) among the roles of the buffer layer described in the [Background Art] section may not be exhibited, and the photoelectric conversion characteristics may be deteriorated. Is not preferable. On the other hand, if the buffer layer is thicker than 200 nm, it is difficult to obtain the effect of reducing the series resistance, which may lead to deterioration in photoelectric conversion characteristics, which is not preferable.
The thickness of the buffer layer is more preferably 100 nm or less.
Furthermore, when the thickness of the buffer layer is 30 nm or more, the effect of the present invention is more remarkable and preferable.

  The photoelectric conversion layer is a photoelectric conversion layer mainly composed of a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element, and the upper electrode layer is a translucent conductive layer It is preferable that

The main component of the photoelectric conversion layer is
At least one group Ib element selected from the group consisting of Cu and Ag;
At least one compound semiconductor comprising at least one IIIb group element selected from the group consisting of Al, Ga and In and at least one VIb group element selected from the group consisting of S, Se and Te It is particularly preferred that

  In the present specification, unless otherwise specified, the “main component” means a component having a content of 80% by mass or more.

  The solar cell of the present invention comprises the photoelectric conversion element of the present invention.

The method for producing a buffer layer of the present invention is the production of a buffer layer in a photoelectric conversion element having a laminated structure in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are laminated in this order on a substrate. A method,
Chemistry containing zinc ions and sulfur ions, and ions of elements having a higher valence than zinc among group 3, 7 or 13 elements (excluding boron) or oxo acid ions containing boron Prepare a reaction solution for bath deposition,
By bringing at least the surface of the photoelectric conversion layer into contact with the reaction solution of the buffer film forming substrate in which the lower electrode layer and the photoelectric conversion layer are laminated in this order on the substrate, the reaction liquid is formed on the photoelectric conversion layer. A buffer layer is deposited.

  It is preferable that the reaction solution further contains thiourea.

  The reaction solution preferably contains one or more of boric acid, tetraboric acid, aluminum ions, gallium ions, manganese ions, yttrium ions, lanthanum ions, and cerium ions.

  Moreover, it is preferable that a carbonyl ion is contained in a reaction liquid, and as a carbonyl ion, a citrate ion is especially preferable.

  Furthermore, it is preferable that the reaction solution contains sodium ions.

The method for producing a photoelectric conversion element of the present invention is a method for producing a photoelectric conversion element having a laminated structure in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are laminated on a substrate.
The buffer layer is formed by the buffer layer manufacturing method of the present invention.

  The photoelectric conversion element of the present invention is a photoelectric conversion element in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are laminated on a substrate, and the buffer layer contains zinc and sulfur, Further, it is composed of a compound containing at least one element having a valence higher than that of zinc among the group 3 element, the group 7 element and the group 13 element, and an element having a higher valence than zinc is added to the compound. As a result, the fill factor can be improved and the photoelectric conversion efficiency can be improved.

  Although the mechanism of action for improving the fill factor is not necessarily clear, one of the factors is considered to be an increase in the diode factor. As the diode factor increases, the fill factor decreases. When the series resistance and the shunt resistance do not change and only the diode factor increases, the IV curve indicated by the solid line in FIG. 4 changes as the IV curve indicated by the dotted line in FIG. As the diode factor increases, the convex shape of the IV curve changes and Pmax moves in the direction of the origin, resulting in a decrease in fill factor. In the band gap of the Zn (S, O) buffer layer formed by the chemical bath deposition method, it is considered that inevitably formed impurity levels exist, and carrier recombination through these levels is a diode factor. It is estimated that this is a factor that increases the fill factor. It is expected that a large number of donor levels are formed near the conduction band in the band gap by adding an element having a higher valence than zinc among the group 3 element, group 7 element and group 13 element. . This level of electrons compensates for the impurity level, thereby suppressing carrier recombination via the impurity level and improving the fill factor.

Even when the thickness of the buffer layer is increased to some extent, the addition of an element having a higher valence than zinc to the compound can suppress an increase in electrical resistance associated with increasing the thickness of the buffer layer. As a result, an increase in the series resistance of the element can be suppressed, so that deterioration in characteristics can be suppressed. That is, when the buffer layer is a thick film, the effect of the present invention is remarkable.
It is known that when the upper electrode is formed by sputtering, there is a decrease in performance due to sputtering damage (see paragraph [0008] of Japanese Patent Laid-Open No. 2012-204477). In order to suppress this sputter damage, it is necessary to make the buffer layer relatively thick. However, increasing the thickness of the buffer layer has a trade-off relationship with the increase in series resistance in the conventional technology, and the photoelectric conversion efficiency is improved. I couldn't plan enough. According to the present invention, an increase in series resistance can be suppressed even when the buffer layer is thick enough to prevent damage to the CIGS layer during sputter deposition of the upper electrode. That is, when the present invention is applied to an element having a buffer layer that is thick enough to suppress sputter damage, the effect of reducing the electrical resistance of the buffer layer is synergistically added to the effect of improving the diode characteristics described above. Further, the fill factor is improved and high conversion efficiency can be obtained.

According to the method for manufacturing a buffer layer of the present invention, a buffer layer having a low electrical resistance can be obtained, so that the thickness can be increased to some extent.
By using the buffer layer manufacturing method of the present invention, a photoelectric conversion element having a large fill factor can be obtained, and the buffer layer can be formed to a certain degree of thickness, thereby preventing damage during formation of the upper electrode and element characteristics. It is possible to obtain a photoelectric conversion element that can achieve both prevention of deterioration.
Note that since the buffer layer may have a single-layer structure formed by the CBD method, a photoelectric conversion element having a high photoelectric conversion rate can be obtained with the same number of steps as in the normal buffer layer formation and without increasing the cost. .

Schematic sectional view showing a layer structure of a photoelectric conversion element according to an embodiment of the present invention A partially enlarged view of FIG. 1A showing a configuration having carbonyl ions on the surface of the buffer layer Schematic sectional view showing an example of an anodized substrate applicable to the photoelectric conversion element of the present invention The figure which shows the compositional analysis result of the depth direction about the sample of Example 11 The figure which shows the IV curve for demonstrating a fill factor The figure which shows the quantum conversion efficiency curve for demonstrating a quantum efficiency fall rate

  Hereinafter, embodiments of the present invention will be described in detail.

  With reference to drawings, the structure of the photoelectric conversion element of one Embodiment which concerns on this invention, and its manufacturing method are demonstrated. FIG. 1A is a cross-sectional view illustrating a schematic configuration of a photoelectric conversion element. In order to facilitate visual recognition, the scale of each component in the figure is appropriately different from the actual one.

The photoelectric conversion element 1 includes a lower electrode layer (back electrode) 20, a photoelectric conversion layer 30 containing a compound semiconductor as a main component, a buffer layer 40, and a translucent conductive layer (upper electrode layer) on a substrate 10. Transparent electrode) 60 and grid electrode 70 are sequentially stacked, and buffer layer 40 contains zinc and sulfur, and is more valent than zinc among group 3 elements, group 7 elements and group 13 elements. It contains at least one element having a large number.
A so-called window layer may be provided between the buffer layer 40 and the translucent conductive layer 60.

  The buffer layer 40 is provided for the purpose of preventing recombination of photogenerated carriers, band discontinuous matching, lattice matching, surface irregularity coverage of the photoelectric conversion layer, and mitigating damage during the formation of the light-transmitting conductive layer. Layer.

In the present embodiment, the buffer layer 40 contains at least zinc and sulfur, and further has an element having a valence higher than that of zinc among group 3 elements, group 7 elements and group 13 elements (hereinafter referred to as “D”). And a compound containing. At this time, the element D having a higher valence than zinc is a dopant, and has an effect of improving the fill factor. Zinc is divalent (Zn ²⁺ ), and D may be trivalent or more. Note that two or more elements having a higher valence than zinc may be contained in the buffer layer.

  In particular, it is preferable that any of boron, aluminum, gallium, indium, manganese, yttrium, lanthanum, or cerium is contained in the buffer layer 40 as D.

  The main component constituting the buffer layer 40 is a compound containing at least ZnS, and Zn (S, O) and Zn (S, O, OH) are more preferable. The description of Zn (S, O) and Zn (S, O, OH) means a mixed crystal of zinc sulfide and zinc oxide and a mixed crystal of zinc sulfide, zinc oxide and zinc hydroxide. In addition, zinc oxide and zinc hydroxide may be present without forming a mixed crystal because they are amorphous. Here, “the element D is contained in the buffer layer” means that the buffer layer is composed of a compound in which the element D is added to ZnS, Zn (S, O), and Zn (S, O, OH) as main components. Means that

  The D content in the buffer layer 40 is preferably 0.01 at% or more and 5 at% or less. In addition, when 2 or more types of D are contained in the buffer layer, it is preferable that the total content is 0.01 at% or more and 5 at% or less. If it is less than 0.01 at%, the effect as a dopant is low, and the resistance of the buffer layer cannot be lowered. On the other hand, when the buffer layer is formed by a chemical bath deposition method (CBD method) described later, it is difficult to dope the dopant content to exceed 5 at%.

The composition of the buffer layer may be, for example, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), glow discharge spectroscopy (GD-OES), Rutherford backscattering (RBS), or Auger It can identify by the measurement by electron spectroscopy (AES), and can determine the content rate of a dopant from the measurement result. Since the content of elements that can be detected varies depending on the measurement method (the detection limit varies), these methods are appropriately used depending on the content of the dopant.
Here, the content rate of a dopant shall be defined by the content rate in the center position in the thickness direction of a buffer layer.

  The buffer layer 40 preferably further contains alkali metal ions. By including alkali metal ions, the in-plane uniformity of the buffer layer 40 can be increased, and variations in characteristics between elements can be suppressed. As the alkali metal ion to be added, lithium, sodium or potassium is preferable.

  The thickness of the buffer layer 40 is 10 nm or more and 1 μm or less, preferably 10 nm or more and 100 nm or less, more preferably 40 nm or more, and particularly preferably 50 nm or more.

  The buffer layer 40 preferably has a crystalline part and an amorphous part. The molar ratio of sulfur atoms to the total number of moles of sulfur atoms and oxygen atoms in the buffer layer 40 is preferably 0.2 to 0.8.

  Further, the conductivity type of the buffer layer 40 is not particularly limited, but is preferably n-type.

  Further, as schematically shown in a partially enlarged view of the photoelectric conversion element of FIG. 1A in FIG. 1B, it is preferable that the surface 40s of the buffer layer 40 on the light-transmitting conductive layer 60 side is provided with carbonyl ions. According to such a configuration, the denseness of the buffer layer is increased due to the presence of carbonyl ions. Therefore, it is possible to provide a photoelectric conversion element having a high in-plane uniformity of photoelectric conversion efficiency, including a buffer layer that covers the base layer substantially uniformly.

  The carbonyl ion is preferably a carbonyl ion having a plurality of carbonyl groups, and particularly preferably a citrate ion.

  The carbonyl ions are preferably present on the surface 40s of the buffer layer 40, and more preferably adsorbed on the surface 40s. When the carbonyl ions are adsorbed on the surface 40s, the adsorption mode is not particularly limited. However, when the charge state of the surface 40s is positive, the carbonyl ions are bound to the surface 40s by the interaction between the surface 40s and oxygen of the carbonyl ions. It becomes an aspect adsorbed on. In such an embodiment, the carbonyl ions are likely to be densely arranged on the surface 40s, and thus the surface of the photoelectric conversion layer 30 that is the base layer is likely to be covered substantially uniformly, and thus the denseness of the buffer layer 40 becomes higher, which is preferable. The carbonyl ion is not particularly limited, but is preferably a carbonyl ion having a plurality of carbonyl groups. This is considered to result from the fact that the growth of the buffer layer is suppressed in the thickness direction and the growth in the in-plane direction is easier to proceed. Examples of such carbonyl ions include at least one selected from the group consisting of citrate ions, tartrate ions, and maleate ions. Since citric acid is a trianion, there are three sites that can be adsorbed to the surface 40s, so that adsorption to the surface 40s is denser and stronger at the same time. As a result, a dense film may be obtained, which is preferable.

  Below, the manufacturing method of the buffer layer 40 which consists of Zn compound layer which has ZnS, Zn (S, O) and / or Zn (S, O, OH) as a main component is demonstrated. The compound may contain inevitable impurities.

<Film formation process>
A CBD method is used to form a Zn compound layer mainly composed of ZnS, Zn (S, O) and / or Zn (S, O, OH).
The “CBD method” is a general formula [M (L) _i ] ^{m +} Ｍ M ^{n +} + iL (wherein, M: metal element, L: ligand, m, n, i: each represents a positive number). This is a method for precipitating crystals on a substrate at a moderate rate in a stable environment by setting the supersaturation condition by an equilibrium as expressed.

Hereinafter, the preferable composition of the reaction solution for chemical bath deposition will be described.
The component (Z) which is a Zn source preferably contains at least one selected from the group consisting of zinc sulfate, zinc acetate, zinc nitrate, zinc citrate, and hydrates thereof. When using zinc citrate, the zinc citrate also serves as component (C) described later. The concentration of the component (Z) is not particularly limited and is preferably 0.001 to 0.5M.
It is preferable that thiourea is included as the component (S) as the S source. The concentration of the component (S) is not particularly limited and is preferably 0.01 to 1.0M.

As the component (N) that functions as a pH adjuster or the like, an ammonium salt such as NH ₄ OH is preferable. This component (N) is also a component that functions as a complexing agent or the like. The concentration of component (N) is preferably 0.001 to 0.40M. The solubility and supersaturation degree of metal ions can be adjusted by adjusting the pH with the component (N). If the concentration of component (N) is in the range of 0.001 to 0.40M, the reaction rate is fast, and film formation is carried out at a practical production rate without providing a fine particle layer formation step before the film formation step. can do. When the concentration of the component (N) exceeds 0.40M, the reaction rate becomes slow. The concentration of component (N) is preferably 0.01 to 0.30M.

The pH of the reaction solution before starting the reaction is 9.0 to 12.0.
When the pH of the reaction solution before the start of reaction is less than 9.0, the decomposition reaction of the component (S) such as thiourea does not proceed, or even if it proceeds, the precipitation reaction does not proceed. The decomposition reaction of thiourea is as follows. The decomposition reaction of thiourea is described in Journal of the Electrochemical Society, 141, 205-210 (1994) and Journal of Crystal Growth 299, 136-141 (2007).
SC (NH ₂ ) ₂ + OH ⁻ ⇔ SH ⁻ + CH ₂ N ₂ + H ₂ O
SH ⁻ + OH ⁻ ⇔ S ² + + H ₂ O
When the pH of the reaction solution before the start of the reaction exceeds 12.5, the effect that the component (N) that also functions as a complexing agent or the like forms a stable solution is increased, and the precipitation reaction does not proceed or proceeds. It will be very slow.
In addition, among the group 3 elements, group 7 elements, or group 13 elements (except boron) used in the present invention, ions of elements having a higher valence than zinc are precipitated as hydroxides in a high pH region. The reaction solution is preferably weakly basic. The pH of the reaction solution before starting the reaction is preferably 9.5 to 11.5.

In order to make the reaction solution weakly basic, a carboxylic acid such as citric acid, tartaric acid or maleic acid is preferably included. Since these carboxylic acids act as a complexing agent for zinc, the concentration of ammonia that also acts as a complexing agent can be lowered. That is, a reaction solution containing a carboxylic acid such as citric acid can cause a chemical bath precipitation reaction stably even at a lower pH.
In addition, as the carboxylic acid content in the reaction solution increases, the pH can be lowered, and the doping element ions other than boron are stabilized by forming a complex with the carboxylic acid ion in the solution. The content limit of the doping element in the liquid can be increased, and as a result, the doping amount into the buffer layer can be increased.

In particular, it is preferable to contain trisodium citrate and / or a hydrate thereof as the component (C) of the citric acid source which is a carboxylic acid. The concentration of component (C) is preferably 0.001 to 1M. If the concentration of the component (C) is within this range, the complex is formed satisfactorily, and a film that satisfactorily covers the base can be stably formed. When the concentration of component (C) exceeds 1M, a stable aqueous solution in which the complex is well formed is obtained. On the other hand, the progress of the precipitation reaction on the substrate is slow, or the reaction does not proceed at all. There is. The concentration of component (C) is preferably 0.001 to 0.1M.
When a reaction solution containing trisodium citrate is used, sodium that is an alkali metal can be added to the buffer layer at the same time, and the effect of improving the in-plane uniformity of the buffer layer can be obtained.

  In the reaction solution used in the present invention, the concentration of the component (N) is 0.001 to 10.0 M, and if it is such a concentration, special pH adjustment such as using a pH adjusting agent other than the component (N) is performed. Even if not, the pH of the reaction solution before starting the reaction is usually in the range of 9.0 to 12.5.

  The pH after completion of the reaction of the reaction solution is not particularly limited. The pH of the reaction solution after completion of the reaction is preferably 7.5 to 12.0. When the pH of the reaction solution after completion of the reaction is less than 7.5, it means that the reaction does not proceed, and it is meaningless when considering efficient production. In addition, if there is such a pH drop in a system containing ammonia that has a buffering effect, it is highly possible that ammonia has volatilized excessively in the heating process, and it is considered that manufacturing improvements are necessary. It is done. When the pH of the reaction solution after the reaction is over 12.0, the decomposition of thiourea is promoted, but since most of the zinc ions are stabilized as ammonium complexes, the progress of the precipitation reaction may be remarkably slowed. The pH of the reaction solution after completion of the reaction is more preferably 9.5 to 11.5.

  The reaction temperature is 60 to 95 ° C. If the reaction temperature is less than 60 ° C., the reaction rate becomes slow, and the thin film does not grow, or even if the thin film is grown, it becomes difficult to obtain a desired thickness (for example, 50 nm or more) at a practical reaction rate. When the reaction temperature exceeds 95 ° C., generation of bubbles and the like increases in the reaction solution, which adheres to the film surface and makes it difficult to grow a flat and uniform film. Furthermore, when the reaction is carried out in an open system, a concentration change due to evaporation of the solvent or the like occurs, making it difficult to maintain stable thin film deposition conditions. The reaction temperature is preferably 70 to 90 ° C.

  The reaction time is not particularly limited. In the present invention, the reaction can be carried out at a practical reaction rate without providing a fine particle layer. Although the reaction time depends on the reaction temperature, for example, the base can be satisfactorily covered in 10 to 90 minutes, and a layer having a sufficient thickness as a buffer layer can be formed.

The reaction solution used in the present invention is aqueous, and the pH of the reaction solution is not a strong acid condition. Although the pH of the reaction solution may be 11.0 to 12.5, the reaction can be carried out even under a mild pH condition of less than 11.0. Therefore, alkali such as a metal capable of forming a complex ion with hydroxide ions. Anodization mainly composed of Al ₂ O ₃ on at least one surface side of an Al base material mainly composed of Al, which can be applied as a substrate including a metal that is easily dissolved in a solvent, for example, a flexible substrate An anodized substrate having a film formed thereon, Al ₂ O ₃ on at least one surface side of a composite base material in which an Al material containing Al as a main component is composited on at least one surface side of an Fe material containing Fe as a main component An anodized substrate having an anodic oxide film mainly composed of Fe, and at least one of a base material having an Al film mainly composed of Al formed on at least one surface side of an Fe material mainly composed of Fe to mainly composed of _Al 2 _{O 3} on the side Even when using the electrode oxide film anodized substrate of the anodized substrate and the like are formed such, there is no possibility of damaging the substrate.

  In addition, after the film formation process, at least the buffer layer is annealed at a temperature lower than the heat resistant temperature of the substrate. An annealing treatment temperature of 150 ° C. or higher is effective for improving photoelectric conversion efficiency. The method for the annealing treatment is not particularly limited, and may be heated in a heater or a dryer, or may be light annealing such as laser annealing or flash lamp annealing.

  The photoelectric conversion element of the present invention is not particularly limited with respect to the configuration other than the buffer layer. Below, the preferable example about each component other than a buffer layer is demonstrated in order.

(substrate)
The substrate 10 is not particularly limited, and a glass substrate, a metal substrate such as stainless steel having an insulating film formed on the surface, and Al ₂ O ₃ is mainly used on at least one surface side of an Al base material mainly composed of Al. An anodized substrate on which an anodized film as a component is formed, at least one surface side of a composite base material in which an Al material containing Al as a main component is combined with at least one surface side of an Fe material containing Fe as a main component An anodized substrate on which an anodized film mainly composed of Al ₂ O ₃ is formed, and an Al film mainly composed of Al formed on at least one surface side of an Fe material mainly composed of Fe An anodized substrate in which an anodized film mainly composed of Al ₂ O ₃ is formed on at least one surface side of the material, a resin substrate such as polyimide, and the like can be used.

  Since production by a roll-to-roll process is possible, a flexible substrate such as a metal substrate having an insulating film formed on the surface, an anodized substrate, and a resin substrate is preferable.

In consideration of thermal expansion coefficient, heat resistance, substrate insulation, etc., an anodized film mainly composed of Al ₂ O ₃ was formed on at least one surface side of an Al base composed mainly of Al. The main component is Al ₂ O ₃ on at least one surface side of a composite base material in which an Al material containing Al as a main component is combined with at least one surface side of an Fe material containing Fe as a main component. An anodized substrate on which an anodic oxide film is formed, and Al on at least one surface side of a base material on which an Al film mainly composed of Al is formed on at least one surface side of an Fe material containing Fe as a main component An anodized substrate selected from the group consisting of an anodized substrate on which an anodized film composed mainly of ₂ O ₃ is formed is particularly preferred.

The left figure of FIG. 2 is a board | substrate with which the anodic oxide film 12 was formed in both surfaces of the Al base material 11, and the right figure of FIG. It is shown in. The anodic oxide film 12 is a film containing Al ₂ O ₃ as a main component.
In order to suppress the warpage of the substrate due to the difference in thermal expansion coefficient between Al and Al ₂ O ₃ and the film peeling due to this in the device manufacturing process, as shown in the left diagram of FIG. It is preferable that the anodic oxide film 12 is formed on both sides of the film.

  Further, a soda lime glass (SLG) layer may be provided on the anodized film of the anodized substrate. By providing the soda lime glass layer, Na can be diffused in the photoelectric conversion layer. When the photoelectric conversion layer contains Na, the photoelectric conversion efficiency can be further improved.

(Lower electrode)
The main component of the lower electrode (back electrode) 20 is not particularly limited, and Mo, Cr, W, and combinations thereof are preferable, and Mo is particularly preferable. The film thickness of the lower electrode (back electrode) 20 is not limited and is preferably about 200 to 1000 nm. The lower electrode 20 can be formed on the substrate by sputtering, for example.

(Photoelectric conversion layer)
The main component of the photoelectric conversion layer 30 is not particularly limited and is preferably a compound semiconductor having at least one chalcopyrite structure because high photoelectric conversion efficiency is obtained. The Ib group element, the IIIb group element, and the VIb group More preferably, it is at least one compound semiconductor composed of an element.

  As a main component of the photoelectric conversion layer 30, at least one type Ib group element selected from the group consisting of Cu and Ag, and at least one type IIIb group element selected from the group consisting of Al, Ga, and In, It is preferably at least one compound semiconductor comprising at least one VIb group element selected from the group consisting of S, Se, and Te.

Examples of the compound _{semiconductor, CuAlS 2, CuGaS 2, CuInS} 2, CuAlSe 2, CuGaSe 2, AgAlS 2, AgGaS 2, AgInS 2, AgAlSe 2, AgGaSe 2, AgInSe 2, AgAlTe 2, AgGaTe 2, AgInTe 2, Cu ( _{in, Al) Se 2, Cu} (in, Ga) (S, Se) 2, Cu in _{1-z in 1-x Ga} x Se 2-y S y ( wherein, 0 ≦ x ≦ 1,0 ≦ y ≦ 2, 0 ≦ z ≦ 1) (CI (G) S), Ag (In, Ga) Se ₂ , Ag (In, Ga) (S, Se) _{2 and the} like.

_{Further, Cu 2 ZnSnS 4, Cu 2} ZnSnSe 4, Cu 2 ZnSn (S, Se) may be ₄ or the like.

  The film thickness of the photoelectric conversion layer 30 is not particularly limited, and is preferably 1.0 to 4.0 μm, particularly preferably 1.5 to 3.5 μm.

  The method for forming the photoelectric conversion layer 30 is not particularly limited, and can be formed by a vacuum deposition method, a sputtering method, an MOCVD method, or the like.

(Window layer)
As described above, an insulating layer may be provided as a window layer between the buffer layer 40 and the translucent conductive layer 60. This insulating layer is an intermediate layer that captures light and inhibits recombination of photoexcited electrons and holes, thereby contributing to improvement in power generation efficiency. The composition of the insulating layer is not particularly limited, but i-ZnO or the like is preferable. The film thickness is not particularly limited, preferably 10 nm to 2 μm, and more preferably 15 to 200 nm. The film forming method is not particularly limited, but a sputtering method or an MOCVD method is suitable. On the other hand, when the buffer layer 40 is manufactured by the liquid phase method, it is also preferable to use the liquid phase method in order to simplify the manufacturing process.

(Translucent conductive layer)
The translucent conductive layer (transparent electrode) 60 is a layer that captures light and functions as an upper electrode that is paired with the lower electrode 20 and through which a current generated in the photoelectric conversion layer 30 flows. The composition of the translucent conductive layer 60 is not particularly limited, and n-ZnO such as ZnO: Al is preferable. The film thickness of the translucent conductive layer 60 is not particularly limited, and is preferably 50 nm to 2 μm. The film forming method of the translucent conductive layer 60 is not particularly limited, but the sputtering method and the MOCVD method are suitable as with the window layer. In order to form the buffer layer with a thinner film, a reactive plasma deposition (RPD) method, an MOCVD method, or the like is preferable because the film-forming damage is small. On the other hand, in order to simplify the manufacturing process, it is also preferable to use a liquid phase method.

(Grid electrode)
The grid electrode 70 is an electrode used to extract electric power from the element, and the main component thereof is not particularly limited, and examples thereof include Al. The film thickness of the grid electrode 70 is not particularly limited, and is preferably 0.1 to 3 μm.

  The photoelectric conversion element 1 of this embodiment is configured as described above.

  If the manufacturing method of the photoelectric conversion element 1 forms a buffer layer using the manufacturing method of the said buffer layer, the formation method about another layer will not have a restriction | limiting in particular, The various demonstrated in the description part of each said layer. A film formation method can be applied.

  The photoelectric conversion element 1 can be preferably used for a solar cell or the like. While integrating the photoelectric conversion element 1, a cover glass, a protective film, etc. can be attached as needed and it can be set as a solar cell.

  As described above, the photoelectric conversion element 1 is made of a compound in which the buffer layer 40 includes zinc and sulfur and includes at least one element having a higher valence than zinc. According to this configuration, the photoelectric conversion element exhibits a high fill factor and can improve the photoelectric conversion characteristics. Furthermore, the resistance of the buffer layer can be lowered to such an extent that the upper electrode and the photoelectric conversion layer are not short-circuited and the series resistance between the upper and lower electrodes can be suppressed. Photoelectric conversion efficiency can also be improved when the thickness is such that it can be protected from spatter damage during formation.

"Design changes"
The present invention is not limited to the above-described embodiment, and the design can be changed as appropriate without departing from the spirit of the present invention.

  Examples and comparative examples according to the present invention will be described.

<Substrate to photoelectric conversion layer>
The following substrates 1 and 2 were prepared as substrates.
Substrate 1: Soda lime glass (SLG) substrate.
Substrate 2: A substrate having a soda lime glass (SLG) layer on the AAO surface of an anodized substrate in which an aluminum anodic oxide film (AAO) is formed on an Al surface on a 100 μm thick stainless steel (SUS) -30 μm thick Al composite base material . Here, SUS (100 μm), Al (30 μm), AAO (20 μm), and SLG (0.2 μm).

A Mo lower electrode having a thickness of 0.8 μm is formed on the substrate 1 or the substrate 2 by sputtering, and a Cu (In) film having a thickness of 1.8 μm is formed on the Mo lower electrode as a photoelectric conversion layer using a three-step method. _{A 0.7} Ga _0.3 ) Se ₂ layer was deposited. This is referred to as a buffer film forming substrate.

<Surface treatment>
A reaction vessel containing a 10% aqueous solution of KCN was prepared, and the surface of the CIGS layer, which is the outermost surface of the buffer film formation substrate, was immersed at room temperature for 3 minutes to remove impurities from the CIGS layer surface. After taking out, it fully washed with water.

  After the formation of the Mo electrode and CIGS layer and the surface treatment using the substrate 1 or the substrate 2, the buffer layer is formed on the surface of the CIGS layer under the conditions of the following Examples 1 to 7, 11 to 17, and Comparative Examples 1 and 11. A film was formed.

Example 1
Substrate 1 was used.
Zinc sulfate aqueous solution (0.18 [M]) as aqueous solution (I), thiourea aqueous solution (thiourea 0.30 [M]) as aqueous solution (II), and trisodium citrate aqueous solution (0.18 as aqueous solution (III)) [M]) and aqueous ammonia (0.3 [M]) were prepared as an aqueous solution (IV). Next, among these aqueous solutions, I, II, and III are mixed in the same volume, zinc sulfate 0.06 [M], thiourea 0.10 [M], trisodium citrate 0.06 [M]. A mixed solution to be obtained was completed, and this mixed solution and 0.3 [M] aqueous ammonia were mixed in equal volumes to obtain a reaction solution 1. When mixing the aqueous solutions (I) to (IV), the aqueous solution (IV) was added last. In order to obtain a transparent reaction solution, it is important to add the aqueous solution (IV) last. Reaction liquid 1 is zinc sulfate 0.03 [M], thiourea 0.05 [M], trisodium citrate 0.03 [M], and ammonia 0.15 [M]. The pH of the obtained reaction liquid 1 was 10.3. Boric acid was added to the reaction solution 1 so as to be 0.03M. At this time, the doping element in the buffer layer to be formed is boron.

Next, Zn (S, O) and / or Zn (S, S, O) is applied to the surface of the outermost photoelectric conversion layer (CIGS) of the buffer film-forming substrate by the CBD method using the reaction solution 1 to which boric acid is added. A buffer layer mainly composed of O, OH) was formed. Specifically, the substrate for buffer film formation was immersed in 500 mL of the reaction solution adjusted to 90 ° C. for 30 minutes, and then the substrate was taken out. The surface was sufficiently washed with pure water, and then dried at room temperature. In the step of immersing the substrate in the reaction solution, the substrate was placed so that the substrate surface was perpendicular to the bottom surface of the reaction solution container.
After the above buffer layer was formed, the buffer layer was formed by annealing at 200 ° C. for 1 hour.

(Example 2)
Instead of boric acid, gallium nitrate was added to the reaction solution 1 so as to be 0.003M. At this time, the doping element is gallium. Except for these points, the buffer layer was formed in the same manner as in Example 1.

(Example 3)
Instead of boric acid, yttrium sulfate was added to the reaction solution 1 so as to be 0.003M. At this time, the doping element is yttrium. Except for these points, the buffer layer was formed in the same manner as in Example 1.

Example 4
In place of boric acid, lanthanum sulfate was added to the reaction solution 1 so as to be 0.003M. At this time, the doping element is lanthanum. Except for these points, the buffer layer was formed in the same manner as in Example 1.

(Example 5)
Instead of boric acid, cerium sulfate was added to the reaction solution 1 so as to be 0.003M. At this time, the doping element is cerium. Except for these points, the buffer layer was formed in the same manner as in Example 1.

(Example 6)
As reaction solution 2, an aqueous solution containing 0.16 [M] zinc sulfate, 0.60 [M] thiourea, and 7.5 [M] ammonia was prepared. The pH was 12.2.
Boric acid was added to the reaction solution 2 so as to be 0.003M. At this time, the doping element in the buffer layer to be formed is boron.
Next, Zn (S, O) and / or Zn (S, S, O) is applied to the surface of the outermost photoelectric conversion layer (CIGS) of the buffer film-forming substrate by the CBD method using the reaction liquid 2 to which boric acid is added. A buffer layer mainly composed of O, OH) was formed. Specifically, the substrate for buffer film formation was immersed for 15 minutes in 500 mL of the reaction solution adjusted to 80 ° C., then the substrate was taken out, and the surface was sufficiently washed with pure water, and then dried at room temperature. In the step of immersing the substrate in the reaction solution, the substrate was placed so that the substrate surface was perpendicular to the bottom surface of the reaction solution container.

(Example 7)
In place of boric acid, gallium nitrate was added to the reaction solution 2 so as to be 0.0003M. At this time, the doping element is gallium. Except for these points, a buffer layer was formed in the same manner as in Example 6.

(Comparative Example 1)
Using the reaction solution 1, a buffer layer was formed in the same manner as in Example 1 except that the doping element source was not added.

  In Examples 11 to 15 and Comparative Example 11, the substrate 2 was used in place of the substrate 1 in Examples 1 to 5 and Comparative Example 1, respectively, and the film formation time was changed to 60 minutes.

  In Examples 16 and 17, the substrate 2 was used in place of the substrate 1 in Examples 6 and 7, respectively, and the film formation time was changed to 40 minutes.

<Evaluation of buffer layer surface>
The buffer layer surface was evaluated for each example and comparative example. In the evaluation, the surface material was identified by infrared spectroscopic analysis (IR). For the measurement, an infrared spectroscopic analyzer manufactured by JASCO Corporation, MFT-2000 was used.

The presence or absence of carbonyl ions on the surface of the buffer layer was specified by the technique described in Japanese Patent No. 4615067. That is, the IR spectrum of the buffer layer surface for each sample of each example and comparative example was obtained, and measured by the IR spectrum of CIGS before the buffer layer deposition measured by the ATR (attenuated total reflection) method, and the KBr disk method Trisodium citrate dihydrate raw material powder (manufactured by Wako Pure Chemical Industries, Ltd.), thiourea raw material powder (manufactured by Wako Pure Chemical Industries, Ltd.), zinc sulfate heptahydrate (manufactured by Kanto Chemical Co., Ltd.), Zn Comparison of IR spectra and peaks of (OH) ₂ particles (synthetic product), ZnO particles (manufactured by Koyo Chemical Co., Ltd.), and ZnS particles (manufactured by Kanto Chemical Co., Ltd.) was performed to identify surface materials.

  The presence of citrate ions can be confirmed from samples prepared using a reaction solution containing trisodium citrate, and citrate ions are not detected from samples prepared using a reaction solution not containing trisodium citrate. It was. The presence or absence of a carbonyl ion (here, citrate ion) for each example and comparative example is shown in Tables 1 and 2.

<Evaluation of composition of buffer layer>
A CIGS substrate cut in advance to a 1 cm square (with a CIGS layer formed on the substrate) was formed up to a buffer layer under the same conditions as in Examples 1 to 6 and Comparative Examples 1 and 2. Sample composition analysis was performed. For the measurement, a buffer layer was formed, and composition analysis in the depth direction was performed by X-ray photoelectron spectroscopy (XPS). As an example, the results of Example 11 are shown in FIG. In addition, since the composition ratio (content rate) of dope element is very small, it does not publish in FIG. The identification and quantification of the dope element will be described later.

In FIG. 3, the vertical axis represents the composition ratio (content ratio) of each element, and the horizontal axis represents the etching depth (nm) obtained from the sputtering time (minutes). The conversion from the sputtering time to the etching depth can be calculated based on the etching rate obtained using a SiO ₂ film (substrate is Si wafer) whose thickness is known as a standard sample. Note that the thickness of the buffer layer was calculated based on the position at which the concentration becomes half of the maximum concentration of S.

The XPS measurement with etching was performed under the following conditions.
XPS analysis apparatus name: X-ray photoelectron spectroscopy analyzer PHI Quantum-2000
X-ray source: Monochromated-Al-Kα ray (1486.6 eV) 20 W
Photoelectron extraction angle: 45 ° (measurement depth at one measurement point: about 4 nm)
Measurement area: φ100μm (oval)
Ar ion sputtering conditions:
Acceleration voltage: 4 kV
Sputtering rate: 19.0 nm / min (SiO ₂ equivalent value)

It was confirmed by XPS measurement as shown in FIG. 3 that both S and O exist in the buffer layer. The thickness for each example and comparative example is shown in Tables 1 and 2, respectively.
It was also confirmed that Zn was present even at a position where S was no longer detected. This is probably because Zn ²⁺ diffused in the CIGS layer in the CBD process.

  Further, the ratio of the S component in the buffer layer is shown in Table 1 as S / (S + O) value. S / (S + O) values ranged from 0.42 to 0.45 in all examples and comparative examples. Therefore, it can be said that the composition of the buffer layer produced under the conditions of the examples and comparative examples includes S, O or S, O, OH.

  The quantification of the dope element and the quantification of the dope amount were carried out by analyzing the X-ray photoelectron spectrum at the position etched to half the thickness of the buffer layer (the center position in the thickness direction of the buffer layer thickness). The analysis results for each example and comparative example are shown in Tables 1 and 2.

<Preparation of photoelectric conversion element-continued>
A translucent conductive film made of a 300 nm-thick Ga-doped conductive zinc oxide thin film was formed on the buffer layers prepared in Examples 1 to 7 and Comparative Example 1 by a reactive plasma deposition (RPD) method. .
On the buffer layers prepared in Examples 11 to 17 and Comparative Example 11, a translucent conductive film made of an Al-doped conductive zinc oxide thin film having a thickness of 300 nm was formed by sputtering.
Next, an Al electrode was formed as a grid electrode on the translucent conductive film of each Example and Comparative Example by vapor deposition to produce a photoelectric conversion element (single cell solar cell).
In addition, 16 cells were produced for each example and comparative example.

<Measurement of photoelectric conversion efficiency>
About the photoelectric conversion element (each 16 cells) of the obtained Example and Comparative Example, using a solar simulator, under the conditions using the artificial sunlight of Air Mass (AM) = 1.5, 100 mW / cm ² , The photoelectric conversion efficiency was measured. The photoelectric conversion efficiency was measured after 30 minutes of light irradiation.

<Measurement of fill factor and series resistance>
The IV characteristics were measured to determine the fill factor and series resistance.
For each of the examples and comparative examples, the IV characteristics as shown in FIG. 4 were measured, and the open circuit voltage Voc, the short circuit current Isc, the voltage Vm at the maximum output Pm, and the current Im were obtained, and the fill factor was calculated.
Further, the series resistance was calculated from the slope of the IV characteristic curve near Voc.

<Measurement of spectral sensitivity characteristics>
Spectral sensitivity measurement was performed. In the spectral sensitivity spectrum, since the decrease in quantum efficiency on the long wavelength side is due to carrier recombination, the quantum efficiency decrease rate defined below was introduced to evaluate the degree of carrier recombination. FIG. 5 shows a general quantum conversion efficiency curve (QE curve) for reference. QE curves were obtained for each of the examples and comparative examples, and the quantum efficiency reduction rate was determined.
here,
Quantum efficiency reduction rate = 1− (average quantum efficiency at 800 to 900 nm) / (average quantum efficiency at 500 to 600 nm)
It is. It shows that carrier recombination is suppressed, so that a quantum efficiency fall rate is small.

Table 1 summarizes the main manufacturing conditions and evaluation results of Examples 1 to 7 and Comparative Example 1. In Table 1, for each example and comparative example, the photoelectric conversion efficiency, fill factor, series resistance, and quantum efficiency reduction rate are the maximum values among the measured values obtained from the 16 cells. For each example and comparative example, the variation coefficient of photoelectric conversion efficiency is a value obtained by dividing the standard deviation of photoelectric conversion efficiency of 16 cells by the average value of photoelectric conversion efficiency of 16 cells (here, It is shown as a percentage.)

From Table 1, it is clear that the photoelectric conversion efficiency of the photoelectric conversion elements of Examples 1 to 7 and the fill factor are large compared to Comparative Example 1. Moreover, the quantum efficiency fall rate of Examples 1-7 is smaller than the quantum efficiency fall rate of the comparative example 1. The reason why the rate of decrease in quantum efficiency is small is thought to be because carrier recombination is suppressed. This means that Examples 1 to 7 have improved diode characteristics (reduced diode factor) compared to Comparative Example 1. To do. In other words, the addition of trivalent or higher elements to divalent zinc changed the electronic state of the buffer layer, and it was considered that the diode characteristics were improved by suppressing carrier recombination originating from defect levels. It is done.
Here, since the series resistances of Comparative Example 1 and Examples 1 to 7 are equivalent, it is considered that the improvement of the diode characteristics contributes to the increase of the fill factor.

Table 2 summarizes the main manufacturing conditions and evaluation results of Examples 11 to 17 and Comparative Example 11. In Table 1, each measured value for each example and comparative example is an average value of 16 cells.

From Table 2, it is clear that the photoelectric conversion efficiency and the fill factor of the photoelectric conversion elements of Examples 11 to 17 are large as compared with Comparative Example 11. Moreover, compared with the comparative example 11, the series resistance of the photoelectric conversion element of Examples 11-17 is small, and a quantum efficiency fall rate is small.
As described in Table 1, a small quantum efficiency reduction rate means that the diode characteristics are improved (the diode factor is reduced). On the other hand, a small series resistance indicates that the electron conductivity of the buffer layer is increased by doping, and in Examples 11 to 17 having a relatively thick thickness of 50 nm, the diode factor decreases and the series resistance increases. This shows that the effect of the decrease in the power is synergistic and the fill factor is improved.

Examples 1 to 7 and Comparative Example 1 described in Table 1 and Examples 11 to 17 and Comparative Example 11 described in Table 2 are the configurations of the photoelectric conversion elements (substrate type, buffer layer thickness, upper electrode Since the film forming method and the compound) are different, the characteristics cannot be simply compared between the two.
However, as described above, in both Table 1 and Table 2, the addition of a trivalent or higher element to divalent zinc changes the electronic state of the buffer layer, resulting in carrier relocation from the defect level. It was shown that the diode characteristics were improved and the fill factor was increased by suppressing the coupling.

  In addition, the series resistance reduction effect obtained only in the case of the thick film shown in Table 2 can reduce the electric resistance of the buffer layer by adding a trivalent or higher element to divalent zinc. This is considered to be an effect. The reason why the series resistance did not change greatly between the example and the comparative example in Table 1 is considered that the contribution of the resistance of the buffer layer to the series resistance is small because the buffer film thickness is thin.

In the above-described embodiment, the case where boron, gallium, yttrium, lanthanum, and cerium are used as the dopant was verified, but among these group 3 element, group 7 element, and group 13 element, the valence of zinc is larger. It is considered that the same effect can be obtained if the element is a valence element.
In particular, the devices of Examples 2, 7, 12, 17 and Examples 4 and 14 doped with gallium or lanthanum were very large with conversion efficiency and fill factor, and the effect of doping was high.

  Further, from comparison between Examples 1 and 6, 11 and 16, and 2 and 7, and 12 and 17, it can be seen that the dope amount can be controlled by the concentration of the dope source in the reaction solution.

  As described above, from the surface of the buffer layer of Examples 1 to 5, Comparative Example 1, Examples 11 to 15, and Comparative Example 11 in which citric acid is contained in the reaction solution, carbonyl ions Was detected. In Examples 1 to 5, a variation coefficient of photoelectric conversion efficiency was smaller than that of Examples 6 and 7 formed using a reaction solution not containing citric acid, and a photoelectric conversion element having high in-plane uniformity could be obtained. . Similarly, in Examples 11 to 15, a photoelectric conversion element having a smaller coefficient of variation in photoelectric conversion efficiency and higher in-plane uniformity than those in Examples 16 and 17 formed using a reaction solution not containing citric acid is obtained. I was able to.

DESCRIPTION OF SYMBOLS 1 Photoelectric conversion element 10 Board | substrate 11 Base material 12 Anodized film 20 Lower electrode (back surface electrode)
30 Photoelectric conversion layer 40 Buffer layer (Zn-based compound layer)
60 Upper electrode (translucent conductive layer)
70 Grid electrode

Claims (20)

A photoelectric conversion element in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are stacked on a substrate,
The buffer layer is composed of a compound containing zinc and sulfur, and further comprising at least one element having a valence higher than that of zinc among group 3, element 7 and group 13 elements. A photoelectric conversion element.   2. The photoelectric conversion element according to claim 1, wherein the element having a valence higher than that of zinc is at least one of boron, aluminum, gallium, indium, manganese, yttrium, lanthanum, and cerium.   The photoelectric conversion element according to claim 1, wherein the element having a higher valence than zinc is gallium or lanthanum.   4. The content of an element having a valence larger than that of zinc in the compound constituting the buffer layer is 0.01 at% and 5 at% or less. 5. Photoelectric conversion element.   5. The photoelectric conversion element according to claim 1, wherein carbonyl ions are provided on a surface of the buffer layer on the upper electrode layer side.   6. The photoelectric conversion element according to claim 5, wherein the carbonyl ion is a citrate ion.   The photoelectric conversion element according to claim 1, wherein the buffer layer contains sodium.   The thickness of the said buffer layer is 10 nm or more and 200 nm or less, The photoelectric conversion element of any one of Claim 1 to 7 characterized by the above-mentioned.   The photoelectric conversion element according to claim 8, wherein the buffer layer has a thickness of 30 nm or more.   The photoelectric conversion layer is a photoelectric conversion layer mainly composed of a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element, and the upper electrode layer has a light-transmitting property. The photoelectric conversion element according to claim 1, wherein the photoelectric conversion element is a conductive layer. The main component of the photoelectric conversion layer is
At least one group Ib element selected from the group consisting of Cu and Ag;
At least one compound semiconductor comprising at least one IIIb group element selected from the group consisting of Al, Ga and In and at least one VIb group element selected from the group consisting of S, Se and Te The photoelectric conversion element according to claim 10, wherein   A solar cell comprising the photoelectric conversion element according to any one of claims 1 to 11. A method for producing a buffer layer in a photoelectric conversion element having a stacked structure in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are stacked in this order on a substrate,
Chemistry containing zinc ions and sulfur ions, and ions of elements having a higher valence than zinc among group 3, 7 or 13 elements (excluding boron) or oxo acid ions containing boron Prepare a reaction solution for bath deposition,
By bringing at least the surface of the photoelectric conversion layer into contact with the reaction solution of the buffer film forming substrate in which the lower electrode layer and the photoelectric conversion layer are laminated in this order on the substrate, the reaction liquid is formed on the photoelectric conversion layer. A method for producing a buffer layer, comprising depositing the buffer layer.   14. The method for producing a buffer layer according to claim 13, wherein the reaction solution further contains thiourea.   The buffer layer according to claim 13 or 14, wherein the reaction solution contains at least one of boric acid, tetraboric acid, aluminum ions, gallium ions, manganese ions, yttrium ions, lanthanum ions, and cerium ions. Manufacturing method.   15. The method for producing a buffer layer according to claim 13, wherein the reaction solution contains gallium ions or lanthanum ions.   The method for producing a buffer layer according to any one of claims 13 to 16, wherein the reaction liquid contains a carbonyl ion.   The method for producing a buffer layer according to claim 17, wherein the carbonyl ion is a citrate ion.   The method for producing a buffer layer according to claim 13, wherein the reaction solution contains sodium ions. In the method for producing a photoelectric conversion element having a laminated structure in which a lower electrode layer, a photoelectric conversion layer, a buffer layer, and an upper electrode layer are laminated on a substrate,
The method for manufacturing a photoelectric conversion element, wherein the buffer layer is formed by the method for manufacturing a buffer layer according to any one of claims 13 to 19.

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