JP2016154172A - Cigs solar cell and manufacturing method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a CIGS solar cell including a CIGS layer with the excellent characteristics and a manufacturing method for the same.SOLUTION: The CIGS solar cell includes a substrate 1, a back electrode layer 3, a CIGS light absorption layer 4, a buffer layer 5, and a transparent electrode 6 in this order. The CIGS light absorption layer 4 includes a first light absorption layer 4a formed on the substrate 1 side, and a second light absorption layer 4b formed on the first light absorption layer 4a. The average grain size Nof the crystal grains included in the second light absorption layer 4b is smaller than the average grain size Nof the crystal grains included in the first light absorption layer 4a. The coverage of the second light absorption layer 4b relative to the first light absorption layer 4a is in the range of 40% or more and 90% or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a CIGS solar cell including a CIGS light absorption layer having good characteristics and a method for manufacturing the same.

  Thin solar cells typified by amorphous silicon solar cells and compound solar cells can significantly reduce material costs and manufacturing costs as compared to conventional crystalline silicon solar cells. For this reason, in recent years, these research and development have been advanced rapidly. Especially, it is a compound solar cell using Group I, III, and VI elements as constituents, and the light absorption layer is made of a copper (Cu), indium (In), gallium (Ga), or selenium (Se) alloy. Since the CIGS solar cell which does not use silicon at all and has excellent photoelectric conversion efficiency (hereinafter referred to as “conversion efficiency”), it is particularly attracting attention among thin solar cells.

  Such a CIGS solar cell usually has a structure in which a substrate, a back electrode layer, a CIGS light absorption layer, a buffer layer, and a transparent electrode layer are laminated in this order, and the CIGS light absorption layer, the buffer layer, A pn junction is formed at the interface. In order to achieve high conversion efficiency, it is necessary to increase the grain size of the crystal in the CIGS light absorption layer to obtain a high-quality crystal (for example, see Patent Document 1).

  Patent Document 2 discloses that chalcopyrite is obtained by heat-treating a metal precursor layer in a Se flux atmosphere with a Cu excess composition (Cu / (Ga + In)> 1) in a crystal growth process of a CIGS light absorption layer. It is disclosed to grow into a crystal of structure. At this time, the Cu—Se-based liquid phase serves as a flux that promotes crystallization. After the crystal has grown sufficiently, the In, Ga, Se flux is added to make the final composition of the layer (In, Ga) excess composition [Cu / (Ga + In) <1]. In the process of adding In, Ga, Se flux, if the Cu—Se system starts to be insufficient, the crystal growth becomes insufficient, and fine crystal grains having an average grain size of 50 nm to 300 nm are formed (bulk average The particle size is 1000 nm or more). If such fine crystal grains are present in the CIGS light absorption layer, it is said that the conversion efficiency of the solar cell is lowered.

  On the other hand, although the conversion efficiency is improved to some extent by increasing the crystal grain size in the CIGS light absorption layer or preventing the formation of fine crystal grains, in order to spread it to the general market However, the conversion efficiency obtained by these improvement measures is inadequate, and further development of a CIGS solar cell having a higher conversion efficiency is required.

Japanese National Patent Publication No. 10-513606 JP 2013-58540 A

  This invention is made | formed in view of such a situation, The objective is provision of the CIGS solar cell which can implement | achieve much higher conversion efficiency, and its manufacturing method.

In order to achieve the above object, the present invention is a CIGS solar cell having a substrate, a back electrode layer, a CIGS light absorption layer, a buffer layer, and a transparent electrode layer in this order, and the CIGS light absorption layer Comprises a first light absorption layer formed on the substrate side and a second light absorption layer formed on the first light absorption layer, and constitutes the second light absorption layer the average particle size N ₂ crystal grains, with grain of smaller average particle size N ₁ constituting the first light-absorbing layer, the coverage for the first light-absorbing layer of the second light absorption layer 40 The first summary is a CIGS solar cell set in a range of not less than 90% and not more than 90%.

And it is a CIGS solar cell which has a board | substrate, a back surface electrode layer, a CIGS light absorption layer, a buffer layer, and a transparent electrode layer in this order, Comprising: The said CIGS light absorption layer is formed in the said substrate side. a first light-absorbing layer, and a second light absorption layer formed on the first light-absorbing layer, the average particle size of N ₂ crystal grains constituting the second light-absorbing layer , set with the first crystal grains of a mean particle diameter smaller than N ₁ constituting the light-absorbing layer, the first range coverage less 90% 40% with respect to the light absorbing layer of the second light absorbing layer A method of manufacturing a CIGS solar cell, wherein a layer (α) containing indium, gallium and selenium and a layer (β) containing copper and selenium are formed on the substrate in a solid state. In this order, and heating the laminated body in which the layer (α) and the layer (β) are laminated. Perform crystal growth, forming a first light-absorbing layer consisting of crystal grains having an average grain size of N _1, on the first light-absorbing layer, indium, gallium and selenium in the gas phase supplied performs growth and supply crystal of indium and gallium and selenium at the same time, consists of crystal grains having an average grain size of N _2, the average crystal grain N ₂ is an average crystal grain N ₁ is smaller than the second light-absorbing layer And a step of forming the CIGS light absorption layer via the step of forming the CIGS light absorption layer through the step of forming a coverage ratio in the range of 40% to 90% with respect to the first light absorption layer. The gist of

  That is, the present inventors have repeatedly studied the crystals constituting the CIGS light absorption layer in order to obtain a CIGS solar cell that achieves high conversion efficiency. As a result, contrary to the trend of increasing the grain size of crystals constituting the CIGS light absorption layer and eliminating fine crystal grains that have been studied for achieving high conversion efficiency in the past, the CIGS light absorption layer is composed of fine crystal grains. It was found that a high conversion efficiency of the CIGS solar cell can be achieved unexpectedly by providing a layer and arranging it on a crystal layer having a larger grain size to achieve a specific coverage, and the present invention has been conceived. did.

  In the present invention, a solid phase refers to a phase that is in a solid state at that temperature, and a liquid phase refers to a phase that is in a liquid state at that temperature. The gas phase means a phase that is in a gaseous state at that temperature.

In the present invention, the average grain size (N ₁ , N ₂ ) of the crystal grains constituting the light absorption layer is determined by measuring each of the cross sections of the first and second light absorption layers with a scanning electron microscope (SEM). It can be obtained by observing the central part at a magnification of 20,000 using a microscope, measuring the particle size of all crystals that can be confirmed on the screen, and calculating the average. The diameter of the crystal is measured when the crystal is spherical, and when the crystal is in other shapes, the maximum length is measured.

  Moreover, in this invention, the coverage (%) with respect to the 1st light absorption layer of a 2nd light absorption layer can be calculated | required as follows. That is, first, the measurement target portion is observed from the normal direction at a magnification of 10,000 using an SEM, and an observation photograph (secondary electron image or reflected electron image) is taken. Then, the electronic image is binarized by image processing software, and the area where the crystal grains constituting the second light absorption layer are recognized, and the area where the crystal grains constituting the first light absorption layer are recognized Is obtained, and the coverage ratio (%) of the second light absorption layer to the first light absorption layer can be calculated.

The CIGS solar cell of the present invention is a CIGS solar cell having a substrate, a back electrode layer, a CIGS light absorption layer, a buffer layer, and a transparent electrode layer in this order. A first light absorption layer formed on the substrate side, and a second light absorption layer formed on the first light absorption layer, the crystal grains constituting the second light absorption layer average particle diameter of N _2, is smaller than the average particle size N ₁ crystal grains constituting the first light-absorbing layer. For this reason, not only can the electrons generated by the first light absorption layer disposed on the substrate side be extracted without loss, but the second light absorption layer made of relatively small crystal grains is also provided on the buffer layer side (CIGS light). By being arranged on the surface side of the absorption layer, a large amount of Na existing in the CIGS grain boundary can be supplied in the vicinity of the surface (light incident side) when viewed from the whole CIGS light absorption layer. The acceptor concentration can be improved. Further, the second light absorption layer made of relatively small crystal grains is arranged on the surface side (light incident side) when viewed from the whole CIGS light absorption layer, so that the surface (light incident) side is fine. The unevenness is formed, and the light confinement effect of the unevenness can be expected.

  And the coverage with respect to the 1st light absorption layer of the said 2nd light absorption layer is set to the range of 40% or more and 90% or less. For this reason, higher conversion efficiency can be realized. That is, if the coverage of the second light absorption layer is too high, the recombination of electrons and holes may increase excessively, leading to a decrease in conversion efficiency. Conversely, if the coverage is too low, This is because the above-described effect of providing the light absorption layer 4b may not be sufficiently exhibited.

Further, the first average particle size _{N 1} of the crystal grains constituting the light-absorbing layer is at 1000nm or more 2000nm or less, an average particle size of _{N 2} crystal grains constituting the second light-absorbing layer is 50nm or more 300nm When it is below, the current value is further improved, and a CIGS solar cell with high conversion efficiency is obtained.

Furthermore, when the CIGS solar cell of the present invention is obtained, a layer (α) containing indium, gallium and selenium and a layer (β) containing copper and selenium are formed on the substrate in a solid state. stacked in this order, perform the growth of the layer (alpha) and the layer (beta) and crystal by heating the laminate are stacked to form a first light-absorbing layer consisting of crystal grains having an average grain size of N ₁ a step, on the first light-absorbing layer, indium, gallium and selenium supplied in gas phase, performed crystal growth with the supply of indium, gallium and selenium at the same time, the average particle size of N ₂ crystals The second light absorption layer is made of grains, and the average crystal grain N ₂ is smaller than the average crystal grain N _1. The second light absorption layer is formed so as to have a coverage of 40% to 90% with respect to the first light absorption layer. And forming the CIGS light absorption layer through the step of The size of the crystal grains in the CIGS light absorption layer can be controlled only by changing the conditions such as temperature, and the first light absorption layer and the second light absorption layer having different crystal grain sizes can be used. Since it can form continuously with the same apparatus, manufacturing efficiency and cost reduction can be achieved.

It is sectional drawing which shows typically the CIGS solar cell which is one embodiment of this invention. (A) is the secondary electron image obtained by observing the CIGS light absorption layer (coverage ratio of the second light absorption layer to the first light absorption layer of the second light absorption layer) of the CIGS solar cell of the present invention using SEM. (B) is an explanatory diagram schematically showing the backscattered electron image, and (c) is a schematic diagram showing the binarized image of the backscattered electron image. It is explanatory drawing. (A) is explanatory drawing which showed typically the secondary electron image obtained by observing the CIGS light absorption layer of the conventional CIGS solar cell using SEM, (b) is this light absorption layer, It is explanatory drawing which showed typically the reflected electron image obtained by observing using SEM.

  Next, an embodiment for carrying out the present invention will be described.

  FIG. 1 is a cross-sectional view schematically showing a CIGS solar cell obtained by an embodiment of the present invention. This CIGS solar cell includes a substrate 1, an impurity diffusion prevention layer 2, a back electrode layer 3, a CIGS light absorption layer 4 including a first light absorption layer 4a and a second light absorption layer 4b, and a first buffer layer 5a. And the transparent electrode layer 6 are laminated in this order, and the coverage ratio of the second light absorption layer 4b to the first light absorption layer 4a is specified. It is set to be a range. When light is irradiated from the transparent electrode layer 6 side, a current can be generated at the interface between the CIGS light absorption layer 4 and the buffer layer 5. Below, this CIGS solar cell is demonstrated in detail. In addition, in FIG. 1, each part is shown typically and is different from an actual thickness, size, etc. (the same applies to FIG. 2).

That is, in this CIGS solar cell, a stainless steel foil (SUS) having a width of 10 mm, a length of 100 mm, and a thickness of 50 μm is used as the substrate 1, and an impurity diffusion prevention layer made of chromium (Cr) is formed on the substrate 1. 2, and a 500 nm-thick back electrode layer 3 made of molybdenum (Mo) is provided on the impurity diffusion preventing layer 2. Then, as shown in FIG. 2, a first light absorption layer 4 a having a thickness of 2000 nm made of crystal grains having an average grain diameter N ₁ = 1000 nm and crystal grains having an average grain diameter N ₂ = 200 nm on the back electrode layer 3. The second light absorption layer 4b having a thickness of 100 nm is provided in this order, and the coverage of the second light absorption layer 4b with respect to the first light absorption layer 4a is set to 70%. A first buffer layer 5a made of CdS having a thickness of 50 nm and a second buffer layer 5b made of ZnO having a thickness of 70 nm are provided in this order on the second light absorption layer 4b (FIG. 1). Back to). Furthermore, a transparent electrode layer 6 made of ITO and having a thickness of 200 nm is provided on the second buffer layer 5b.

According to the CIGS solar cell having the above-described configuration, the CIGS light absorption layer 4 has a special configuration including the first light absorption layer 4a and the second light absorption layer 4b, and has relatively large crystal grains (average A first light absorption layer 4a having a particle size N ₁ ) is disposed on the substrate 1 side. Therefore, the electrons generated in the first light absorption layer 4a can be taken out without loss. Further, the second light absorption layer 4b made of relatively small crystal grains (average particle diameter N ₂ ) is arranged on the buffer layer 5 side (the surface side of the CIGS light absorption layer 4), and there are many in the crystal grain boundary. A large amount of Na to be supplied can be supplied to the vicinity of the surface of the CIGS light absorption layer 4. For this reason, the acceptor density | concentration in the surface vicinity of the CIGS light absorption layer 4 can be improved, and the conversion efficiency of a CIGS solar cell can be raised further.

  Further, since the second light absorption layer 4b made of relatively small crystal grains is thinly formed on the surface side of the CIGS light absorption layer 4, the surface of the CIGS light absorption layer 4 (light incident surface) is fine. Concavities and convexities are formed, and the current value of the CIGS solar cell can be expected to be improved by the light confinement effect of the fine irregularities. Furthermore, the coverage of the second light absorption layer 4b with respect to the first light absorption layer 4a is set to 70%, and the crystal grains of the second light absorption layer 4b are completely free of the first light absorption. Since it is not arranged on the layer 4a but with some gaps, the recombination of electrons and holes does not increase excessively, and higher conversion efficiency can be realized. ing.

  In addition to the stainless steel foil (SUS) shown in the above embodiment, the substrate 1 may be a glass substrate, a metal substrate, a resin substrate, or the like, depending on the purpose and design needs. it can. Examples of the glass substrate include blue plate glass, low alkali glass (high strain point glass) having a very low content of alkali metal elements, and alkali-free glass containing no alkali metal elements.

  Moreover, when manufacturing this CIGS solar cell by a roll-to-roll system or a stepping roll system, it is suitable that the said board | substrate 1 is elongate and has flexibility. The “long shape” means that the length in the length direction is 10 times or more of the length in the width direction, and more preferably 30 times or more.

  And it is preferable that the thickness of the said board | substrate 1 exists in the range of 30 micrometers or more and 200 micrometers or less, More preferably, it is the range of 50 micrometers or more and 100 micrometers or less. That is, if the thickness is too thick, the flexibility of the CIGS solar cell is lost, and the stress applied when it is bent may increase and damage the internal structure of the CIGS solar cell. This is because, when the battery is manufactured, the substrate 1 is buckled, and the product defect rate of the CIGS solar battery tends to increase.

The material for forming the impurity diffusion preventing layer 2 formed on the substrate 1 may be SiO ₂ , Al ₂ O ₃ , TiN, TiO ₂ , Ni, NiCr, Co other than Cr shown in the above embodiment. Etc. can be used. Moreover, it is preferable that the thickness is 50 nm or more and 1000 nm or less from a viewpoint of the balance of an effect and cost. The impurity diffusion preventing layer 2 can be formed not on the substrate 1 but on the back electrode layer 3. Further, when there is little possibility that impurities derived from the substrate 1 adversely affect the CIGS solar cell, the impurity diffusion prevention layer 2 may not be provided.

  As a material for forming the back electrode layer 3 formed on the impurity diffusion preventing layer 2, W, Cr, Ti, or the like can be used in addition to molybdenum (Mo) shown in the above embodiment. And the back electrode layer 3 may be not a single layer but a multilayer. Further, the thickness (in the case of multiple layers, the total thickness of each layer) is preferably 50 nm or more and 1000 nm or less.

As the CIGS light absorption layer 4 formed on the back electrode layer 3, not only the above embodiment but also a material satisfying the above requirements can be used. Of these, the first light-absorbing layer 4a, from the standpoint of can be extracted without loss of generated electrons, that the average particle size N ₁ is formed by the following grain 2000nm or 1000nm preferable. The thickness is preferably 1000 nm or more and 3000 nm or less, and more preferably 1500 nm or more and 2500 nm or less. If the thickness is too thin, there is a possibility that sunlight cannot be absorbed sufficiently. Conversely, if the thickness is too thick, the cost due to the increase in material costs tends to increase.

Further, the second light absorbing layer 4b is preferably an average particle size _{N 2} is constituted by the following grain 300nm or 50nm. If it is too small, the recombination of electrons and holes may increase, and if it is too large, there is a tendency that Na is not sufficiently supplied to the crystal grain boundaries.

  And in the said embodiment, although the coverage with respect to the 1st light absorption layer 4a of the 2nd light absorption layer 4b is set to 70%, a coverage is not restricted to this, The range of 40 to 90% As long as it is set to. By setting the coverage in this range, it is possible to effectively prevent the recombination of electrons and holes from increasing excessively, and higher conversion efficiency can be realized.

  Each of the first light absorption layer 4a and the second light absorption layer 4b is formed of a compound semiconductor having a chalcopyrite crystal structure composed of four elements of Cu, In, Ga, and Se. The total thickness of the first light absorption layer 4a and the second light absorption layer 4b (the thickness of the CIGS light absorption layer 4) is preferably in the range of 1000 nm to 3000 nm, preferably 1500 nm to 2500 nm. It is more preferable that it is in the range. If the thickness is too thin, the amount of light absorption decreases, and the performance of the CIGS solar cell tends to be reduced. Conversely, if it is too thick, the time required for forming the CIGS light absorption layer 4 increases, resulting in increased productivity. This is because an inferior tendency is seen.

Moreover, it is preferable that the composition ratio of Cu, In, and Ga in the CIGS light absorption layer 4 satisfies the equation 0.7 <Cu / (Ga + In) <0.95 (molar ratio). If this formula is satisfied, it is possible to further prevent the Cu ( _2-x ) Se from being excessively taken into the CIGS light absorption layer 4 and to make the entire layer slightly deficient in Cu. Because. Moreover, it is preferable that the ratio of Ga and In which are the same element is in the range of 0.10 <Ga / (Ga + In) <0.60 (molar ratio).

The buffer layer 5 formed on the CIGS light absorption layer 4 includes a first buffer layer 5a formed on the substrate 1 side and a second buffer layer formed on the first buffer layer 5a. 5b, and the formation materials thereof include ZnMgO, Zn (OH) ₂ , In ₂ O ₃ , In ₂ S ₃ , and mixed crystals thereof other than CdS and ZnO shown in the above embodiment. Zn (O, S, OH) or the like can be used. In addition, when a high-resistance n-type semiconductor capable of pn coupling with the CIGS light absorption layer 4 is used, the buffer layer 5 can be a single layer. Moreover, it is preferable that the thickness of the 1st buffer layer 5a and the 2nd buffer layer 5b is 30 nm or more and 200 nm or less, respectively. Even when the buffer layer 5 is a single layer, the thickness is preferably 30 nm or more and 200 nm or less.

Since the transparent electrode layer 6 formed on the buffer layer 5 is located on the light incident side, it is preferable to use a material having as high a light transmittance as possible so as not to disturb the incident light. Examples of such a material include ZnO, In ₂ O ₃ , SnO _{2 and the} like other than ITO shown in the above embodiment. In addition, for the purpose of increasing electrical conductivity or adjusting the band alignment, those materials containing a small amount of a doping material can be suitably used. As such a doping material, for example, Al: ZnO (AZO), B: ZnO (BZO), Ga: ZnO (GZO), Sn: In ₂ O ₃ (ITO), F: SnO ₂ (FTO), Zn : In ₂ O ₃ , Ti: In ₂ Oe, Zr: In ₂ O ₃ , W: In ₂ O _{3 and the} like. The thickness is preferably 100 nm or more and 2000 nm or less from the viewpoints of light transmittance and electrical conductivity.

  Such a CIGS solar cell can be manufactured, for example, by the following method.

[Up to the formation of the back electrode layer 3]
A substrate 1 made of long SUS is prepared, and while the substrate 1 is run by a roll-to-roll method, an impurity diffusion prevention layer 2 made of Cr having a thickness of 300 nm by sputtering and a thickness of 500 nm by sputtering. The back electrode layer 3 made of Mo is formed in this order.

[Formation of CIGS light absorption layer 4]
On the back electrode layer 3, gallium selenide and indium selenide are laminated in this order in a solid state with the substrate 1 held at 420 ° C., and a layer (α) having indium, gallium and selenium. Form. Then, with the temperature of the substrate 1 kept at 420 ° C., copper selenide is laminated on the layer (α) to form a layer (β) having copper and selenium. The substrate 1 is heated to 650 ° C. while supplying a small amount of Se vapor to the laminate in which the impurity diffusion preventing layer 2, the back electrode layer 3, the layer (α) and the layer (β) are laminated on the substrate 1. And hold that state for 15 minutes. Thereby, the said layer ((beta)) will be in a liquid phase state, the copper in this layer ((beta)) will diffuse in a layer ((alpha)), and a crystal will grow. Next, In, Ga, and Se are vapor-deposited while gradually increasing the vapor deposition amount while supplying a small amount of Se vapor while keeping the temperature of the substrate 1 at 650 ° C. Thus, the first light absorption layer 4a having crystal grains with an average grain size N ₁ = 1000 nm and having a thickness of 2000 nm is formed on the back electrode layer 3.

Next, the temperature of the substrate 1 is cooled to 500 ° C., and indium selenide and gallium selenide are vapor-deposited on the first light absorption layer 4a at the same time, thereby obtaining crystal grains having an average grain size N ₂ = 200 nm. A second light absorption layer 4b having a thickness of 70% is formed with respect to the first light absorption layer 4a. That is, if In, Ga, and Se are supplied in a gas phase state, and the supply and the growth of the crystal are performed simultaneously, new layer deposition species will fly continuously before the crystal grows sufficiently. This is preferable because the crystal growth is hindered and the crystal grains are sized. In addition, the said coverage can be controlled by increasing / decreasing the supply amount of In, Ga, and Se at the time of forming the 2nd light absorption layer 4b. Then, NaF is vapor-deposited on the second light absorption layer 4b, and then the temperature of the substrate 1 is set to 420 ° C., and this state is maintained for 10 minutes, whereby the first light absorption layer 4a and the second light absorption layer 4b. The CIGS light absorption layer 4 is formed by diffusing Na into the grain interface of the absorption layer 4b.

[Formation of Buffer Layer 5]
The impurity diffusion prevention layer 2, the back electrode layer 3, and the CIGS light absorption layer 4 are formed by the roll-to-roll method, and the substrate 1 wound up in a roll shape is unwound again using a cutting device. A predetermined length of the laminated body (substrate 1 + impurity diffusion preventing layer 2 + back electrode layer 3 + CIGS light absorption layer 4) is obtained by cutting every predetermined length. Then, a first buffer layer 5a (thickness 50 nm) made of CdS is formed on the CIGS light absorption layer 4 (second light absorption layer 4b) of the laminate by a solution growth method (CBD method). A buffer composed of the first buffer layer 5a and the second buffer layer 5b is formed on the first buffer layer 5a by forming a second buffer layer 5b (thickness 70 nm) made of ZnO by sputtering. Layer 5 is formed.

[Process for forming transparent electrode layer 6]
A transparent electrode layer 6 (thickness: 200 nm) made of ITO is formed on the buffer layer 5 (second buffer layer 5b) by sputtering. Note that an extraction electrode (not shown) having a grid shape or the like may be formed on the transparent electrode layer 6 by using the same technique as that for the back electrode layer 3. In this way, the CIGS solar cell of the present invention can be obtained.

  According to this method, the excellent CIGS light absorption layer 4 capable of achieving high conversion efficiency can be continuously used by using a general-purpose vapor deposition apparatus (roll-to-roll method) only by changing conditions such as temperature. Since it can be formed, manufacturing efficiency and cost can be reduced.

  In the above embodiment, the CIGS light absorption layer 4 up to the CIGS light absorption layer 4 is formed by roll-to-roll. However, it may be formed for each sheet by adjusting the size of the substrate 1 or the like. Moreover, although both the impurity diffusion prevention layer 2 and the back electrode layer 3 are formed by the sputtering method, these layers may be formed by using a vapor deposition method, an inkjet method, an electrolytic plating method, or the like.

Furthermore, in the above embodiment, Na is added to the CIGS light absorption layer 4 using NaF, but other Na compounds such as Na ₂ Se and Na ₂ S are used. May be. In the above embodiment, Na is added to the CIGS light absorption layer 4 until the second light absorption layer 4b is formed, and then NaF is deposited and post-annealed. Prior to the formation of the light absorption layer 4, NaF may be deposited on the back electrode layer 3.

  In the above embodiment, the first buffer layer 5a is formed by a solution growth method (CBD method), and the second buffer layer 5b is formed by a sputtering method. It can also be formed by any of these methods, and can be formed in any of vacuum, air and aqueous solution. For example, methods performed in vacuum include molecular beam epitaxy, electron beam vapor deposition, resistance heating vapor deposition, plasma CVD, and organic metal vapor deposition in addition to sputtering. Examples of the method performed in an aqueous solution include a CBD method and an electrolytic plating method.

  Further, in the above embodiment, the transparent electrode layer 6 is formed by the sputtering method, but other than that, it can be formed by a vacuum deposition method, a metal organic chemical vapor deposition method or the like.

  Next, examples will be described together with comparative examples. However, the present invention is not limited to this.

[Example 1]
First, a substrate 1 of stainless steel foil SUS430 (size 10 × 100 mm, thickness 50 μm) is prepared, and an impurity diffusion prevention layer 2 made of Cr having a thickness of 300 nm and a back electrode layer 3 made of Mo having a thickness of 500 nm are formed thereon. Were stacked in this order. Next, the substrate 1 formed up to the back electrode layer 3 is carried into a vacuum deposition apparatus, and gallium selenide (thickness 400 nm) is formed on the back electrode layer 3 in a state where the substrate temperature is maintained at 420 ° C. Indium selenide (thickness 1000 nm) was laminated in this order to form a layer (α) containing selenium, gallium and indium. Subsequently, with the substrate temperature kept at 420 ° C., copper selenide (thickness: 600 nm) was laminated on the layer (α) to form a layer (β) having copper and selenium. The laminate in which these layers (α) and (β) are laminated is heated while supplying a small amount of Se vapor, and the substrate temperature is maintained at 650 ° C. for 15 minutes to grow crystals. In addition, while supplying a small amount of Se vapor and keeping the substrate temperature at 650 ° C., In, Ga, and Se were vapor-deposited while gradually increasing the amount of vapor-deposited Ga, and crystals with an average grain size N ₁ = 1000 nm The 1st light absorption layer 4a (thickness 2000nm) which has a grain was formed.

Next, the stacked body on which the first light absorption layer 4a is formed is cooled, and indium selenide and gallium selenide are placed on the first light absorption layer 4a in a state where the substrate temperature is 500 ° C. The average particle size N ₂ is deposited on the first light absorption layer 4a by vapor deposition at the same time, that is, by supplying In, Ga and Se in a gas phase state and crystal growth simultaneously. = The second light absorption layer 4b having crystal grains of 200 nm was formed so as to have a coverage of 50%. Then, the substrate temperature is maintained at 400 ° C., and NaF is vacuum-deposited on the second light absorption layer 4b by using a NaF vapor deposition source set at 750 ° C., and then the substrate temperature becomes 420 ° C. By heating and holding the substrate temperature for 10 minutes, the CIGS light absorption layer 4 was formed by diffusing Na into the grain boundaries of the first light absorption layer 4a and the second light absorption layer 4b. In addition, what represented typically the SEM secondary electron image of this CIGS light absorption layer 4 (coverage 50% with respect to the 1st light absorption layer of a 2nd light absorption layer) is shown to Fig.2 (a), FIG. 2B shows a schematic representation of the reflected electron image. Further, FIG. 2C shows a schematic representation of the binarized image of the reflected electron image.

  Then, on the CIGS light absorption layer 4 (second light absorption layer 4b), a first buffer layer 5a (thickness 50 nm) made of CdS, a second buffer layer 5b (thickness 70 nm) made of ZnO, ITO The objective CIGS solar cell was manufactured by laminating | stacking the transparent electrode layer 6 (thickness 200nm) which consists of in this order.

[Examples 2 to 7]
The average particle diameter of the first light absorption layer 4a and the second light absorption layer 4b and the coverage of the second light absorption layer 4b with respect to the first light absorption layer 4a were changed as shown in Table 1 below. Except for this, a CIGS solar cell was produced in the same manner as in Example 1. In addition, the grain size of the crystal grains constituting each of the light absorption layers 4a and 4b can be set to an arbitrary size, for example, by controlling the substrate temperature.

[Comparative Example 1]
A CIGS solar cell was manufactured in the same manner as in Example 1 except that the second light absorption layer 4b was not formed. That is, the CIGS solar cell of Comparative Example 1 is a conventional product itself in which the CIGS light absorption layer 4 is composed only of large crystal grains. A schematic representation of the SEM secondary electron image of the CIGS light absorption layer 4 (without the second light absorption layer) of Comparative Example 1 is shown in FIG. 3A, and the reflected electron image is schematically represented. This is shown in FIG.

[Comparative Examples 2 to 4]
A CIGS solar cell was manufactured in the same manner as in Example 1 except that the coverage of the second light absorption layer 4b with respect to the first light absorption layer 4a was changed as shown in Table 1 below.

  10 CIGS solar cells of Examples 1 to 7 and Comparative Examples 1 to 4 are manufactured, respectively, and their conversion efficiencies and crystal grains constituting the first light absorption layer 4a and the second light absorption layer 4b And the coverage of the second light absorption layer 4b with respect to the first light absorption layer 4a were measured according to the following procedure. These results are also shown in Table 1 below.

〔Conversion efficiency〕
Pseudo sunlight (AM1.5) was irradiated to each Example product and Comparative product, and the conversion efficiency was measured with a solar simulator (Yamashita Denso Co., Ltd., Cell Tester YSS150). And the average of each value was computed and it was set as the conversion efficiency (%) of each Example and the comparative example.

[Average particle size]
First and second light-absorbing layers (first in Comparative Example 1) of the CIGS solar cells of Examples 1 to 7 and Comparative Examples 1 to 4 using SEM (manufactured by Hitachi High-Technologies Corporation, S-3400N) The light-absorbing layer only) was observed at a magnification of 20,000 times, and the crystal grain size in the vicinity of the center of each light-absorbing layer was measured. Then, the average of the measured particle size was calculated, and the average particle size N _1, N ₂ of each light absorbing layer. Note that the diameter is measured when the crystal is spherical, and the maximum length is measured when the crystal has other shapes.

[Coverage]
Using SEM (manufactured by JEOL Ltd., JSM-7001F), the buffer layer is scheduled to be formed in the normal direction at the central portion of the light absorption layer of the CIGS solar cells of Examples 1-7 and Comparative Examples 1-4. A secondary electron image or a backscattered electron image is photographed at a magnification of 10,000 from the side. The electronic image is visually divided into crystal grains constituting the first light absorption layer and crystal grains constituting the second light absorption layer, and binarization processing is performed by image processing software. In the binarized image, the area where the crystal grains constituting the second light absorption layer are recognized and the area where the crystal grains constituting the first light absorption layer are found, respectively, By calculating the area ratio, the coverage ratio (%) of the second light absorption layer to the first light absorption layer was calculated. Note that it is preferable to adjust the contour enhancement, brightness, and contrast in the secondary electron image or the reflected electron image as necessary, because the consistency before and after the binarization process can be improved.

  From the above results, it can be seen that Examples 1 to 7 show a high conversion efficiency of 13% or more. On the other hand, Comparative Example 1 in which the second light absorption layer 4b was not formed had a conversion efficiency of 9.0%. In addition, Comparative Examples 2 to 4 in which the coverage of the second light absorption layer 4b with respect to the first light absorption layer 4a is outside the range defined in the present invention show lower conversion efficiency than the product of the example. It was.

  Since the CIGS solar cell of the present invention is thin and has very high conversion efficiency, it is suitable not only for installation with a large area but also for installation in a space with a low load resistance or a curved surface. .

DESCRIPTION OF SYMBOLS 1 Substrate 3 Back surface electrode layer 4 CIGS light absorption layer 4a 1st light absorption layer 4b 2nd light absorption layer 5 Buffer layer 6 Transparent electrode layer

Claims (3)

A CIGS solar cell having a substrate, a back electrode layer, a CIGS light absorption layer, a buffer layer, and a transparent electrode layer in this order, wherein the CIGS light absorption layer is formed on the substrate side. of the light absorbing layer, and a second light absorption layer formed on the first light-absorbing layer, the average particle size of N ₂ crystal grains constituting the second light-absorbing layer is, the 1 together with the crystal grains of average particle size of less than N ₁ constituting the light-absorbing layer, is set to the first range coverage less 90% 40% with respect to the light absorbing layer of the second light absorbing layer A CIGS solar cell characterized by comprising: Said first average particle size _{N 1} of the crystal grains constituting the light-absorbing layer is at 1000nm or more 2000nm or less, below the second 300nm mean particle size _{N 2} crystal grains 50nm or more constituting the light-absorbing layer The CIGS solar cell according to claim 1. A CIGS solar cell having a substrate, a back electrode layer, a CIGS light absorption layer, a buffer layer, and a transparent electrode layer in this order, wherein the CIGS light absorption layer is formed on the substrate side. of the light absorbing layer, and a second light absorption layer formed on the first light-absorbing layer, the average particle size of N ₂ crystal grains constituting the second light-absorbing layer is, the 1 together with the crystal grains of average particle size of less than N ₁ constituting the light-absorbing layer, is set to the first range coverage less 90% 40% with respect to the light absorbing layer of the second light absorbing layer A CIGS solar cell manufacturing method comprising: a layer (α) containing indium, gallium, and selenium; and a layer (β) containing copper and selenium on the substrate in a solid state. The layers are sequentially laminated, and the crystal structure is formed by heating the laminated body in which the layers (α) and (β) are laminated. Perform long, forming a first light-absorbing layer consisting of crystal grains having an average grain size of N _1, on the first light-absorbing layer, and supplying the indium, gallium and selenium in the gas phase Indium, gallium, and selenium are simultaneously supplied and crystals are grown. The second light-absorbing layer is made of crystal grains having an average grain size N ₂ , and the average crystal grains N ₂ are smaller than the average crystal grains N ₁ . Forming the CIGS light absorbing layer through a step of forming the first light absorbing layer so as to have a coverage of 40% or more and 90% or less. The method of manufacturing a CIGS solar cell .

Images