JP2006196771A - Chalcopyrite thin film solar cell and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a chalcopyrite thin film solar cell capable of improving the photoelectric conversion efficiency and a method for producing the same.
A precursor forming step of forming a precursor layer 4 containing Cu, In, and Ga on a Mo electrode layer 2 formed on a soda lime glass substrate 1 by a sputtering method, and a substrate 1 on which the precursor is formed. In contrast, a selenization process for forming the light absorption layer 4 by performing heat treatment in an H ₂ Se gas atmosphere at a temperature range higher than 500 ° C. and lower than the melting point of the substrate, and InAlS on the light absorption layer 4 A chalcopyrite thin film solar cell is obtained by performing a buffer layer forming step for forming the n-type buffer layer 5 and a transparent electrode layer forming step for forming the transparent electrode layer 6 on the buffer layer 5.
[Selection] Figure 1

Description

  The present invention relates to a chalcopyrite thin film solar cell and a method for manufacturing the same, and more particularly to a chalcopyrite thin film solar cell in which a buffer layer is formed between a light absorption layer and a transparent electrode layer and a method for manufacturing the same.

Of the various types of solar cells that are broadly classified into silicon solar cells, thin film solar cells, compound solar cells, etc., the thin film type has the advantage that the manufacturing process is simple and requires low energy as an optical device applying thin film technology. Commercialization development is progressing. A chalcopyrite thin film solar cell belongs to a thin film type, and a CIGS layer made of a chalcopyrite compound (Cu (In + Ga) Se ₂ ) containing elements of Group I, Group III and Group VI as a p-type light absorption layer. Prepare as. And it is known that the light absorption layer formed with such a compound composition can provide high photoelectric efficiency particularly when used in combination with an alkali metal-containing glass substrate such as soda lime glass. In addition, high reliability based on drastic reduction of light degradation (aging) phenomenon due to impurity contamination and defect grating, photosensitivity characteristics obtained in a wide light absorption wavelength region including long wavelength band, high level light absorption coefficient, etc. In addition to these advantages, it has excellent radiation resistance, and research and development for the purpose of commercial production is progressing.

  A general structure of a thin-film solar cell provided with this CIGS layer is shown in FIG. Referring to FIG. 1, this solar cell prevents a non-uniformity of Na originating from a back electrode layer as a positive electrode made of Mo metal layer 2 and SLG substrate 1 on soda lime glass (SLG) substrate 1. The multilayer structure 7 including the Na dip layer 3, the CIGS light absorption layer 4, the n-type buffer layer 5, and the outermost surface layer formed of the transparent electrode layer 6 serving as the negative electrode.

  And when irradiation light, such as sunlight, enters from the surface light-receiving part of this multilayer laminated structure 7, in the vicinity of the pn junction of the multilayer laminated structure, it is excited by the irradiation light having energy greater than or equal to the band gap, and a pair of electrons and Holes are generated. The excited electrons and holes reach the pn junction by diffusion, and the electrons are collected in the n region and the holes are separated in the p region due to the internal electric field of the junction. As a result, the n region is negatively charged, the p region is positively charged, and a potential difference is generated between the electrodes 8 and 9 provided in each region. Then, using this potential difference as an electromotive force, a photocurrent is obtained when the electrodes are connected by a conductive wire, which is the principle of the solar cell.

  FIG. 2 is a process diagram showing a manufacturing process of the multilayer laminated structure 7 constituting the chalcopyrite thin film solar cell shown in FIG.

  In order to describe in order according to each step, when the multilayer laminated structure 7 is manufactured, first, a Mo electrode layer is formed on a glass substrate such as SLG by a sputtering method using a metal Mo target ( Mo electrode layer film forming step: FIG. 2 (a)).

  Next, the 1st scribing process which divides | segments the board | substrate with which Mo electrode layer was formed into desired size by laser cutting is performed (FIG.2 (b)).

  Thereafter, to remove shavings and the like, the substrate is cleaned by water washing or the like, and this is immersed in a sodium sulfide diluted solution or the like to form an Na dip layer. Then, an In metal target and a Cu—Ga alloy target are formed. By using the sputter film forming method used for each, a laminated film having a two-layer structure of an In layer and a Cu—Ga layer is formed, and this is used as a precursor of the light absorption layer (FIG. 2C).

In order to obtain a desired CIGS light absorption layer, the conventional method using this precursor is as shown in FIG. 2D, in which an annealing process chamber is provided for each substrate including an In layer and a Cu—Ga layer as a precursor in a laminated state. In this state, preheating is performed for 10 minutes under a temperature condition of 100 ° C. Then, H ₂ Se gas is introduced from the gas introduction pipe inserted into the annealing chamber, and the chamber is heated to a temperature range of 500 to 520 ° C. while flowing through the chamber. Further, the H ₂ Se gas, which is a reaction gas, is exchanged with a purge gas such as Ar gas as a flow gas. Thereby, the process of converting the precursor composed of the laminated structure of the In layer and the Cu—Ga layer into the CIGS single layer composed of the chalcopyrite compound is completed.

  Then, the substrate with the CIGS layer taken out from the annealing chamber is subjected to CdS, ZnO which is an n-type semiconductor material by immersion bath deposition (CBD) or sputtering as shown in FIG. , InS or the like is formed (see, for example, Patent Document 1).

  Further, a second scribing step is performed on the obtained laminated structure by laser irradiation or cutting using a metal needle (FIG. 2 (f)).

  Then, a transparent conductive film (TCO: Transparent Conductive Oxide) made of a ZnOAl layer is laminated as the outermost surface layer by a spack film forming method using a ZnO—Al alloy target (FIG. 2G).

  Finally, a third scribe process is performed again by laser irradiation or cutting using a metal needle (FIG. 2 (h)).

  A thin film solar cell having such a laminated structure is obtained as a single cell having a uniform size by cutting, and the final product is a planar integrated structure in which these single cells are connected in series.

  For this type of CIGS-based thin film solar cell, improvement in photoelectric conversion efficiency is desired, and as one of the techniques for this purpose, a wide band gap of the light absorption layer is effective. It has been proved that the band gap of the light absorption layer for obtaining the maximum conversion efficiency is theoretically 1.4 to 1.15 eV. Further, in order to increase the wide band gap of the light absorption layer, it is necessary to increase the solid solution ratio of metal Ga in the layer. On the other hand, the actual band gap remains at about 1.1 eV.

By the way, the light absorption layer having a band gap of about 1.1 eV is as described above.
(1) A substrate provided with an In layer and a Cu—Ga layer as a precursor in a laminated state is accommodated in an annealing chamber and pre-heated at a temperature of 100 ° C. for 10 minutes.
(2) The H ₂ Se gas is introduced from the gas introduction pipe inserted into the annealing chamber, and the chamber is heated to a temperature range of 500 to 520 ° C. while flowing through the chamber.
(3) Furthermore, as a flow gas, the H ₂ Se gas as a reaction gas is replaced with a purge gas such as Ar gas.
It is obtained through

At this time, in order to increase the solid solution rate of the metal Ga, the temperature condition in the above (2) needs to be set to a high temperature of about 650 ° C., for example.
JP 2003-282909 A (second page)

  However, by setting the temperature condition higher than the conventional one, even if the band gap of the light absorption layer is to be increased (increased) from about 1.1 eV at present, the obtained solar cell product has an open circuit voltage. Tends to saturate, resulting in a decrease in photoelectric conversion efficiency.

  As a cause of this, it is assumed that a band gap mismatch (band mismatch) occurs between the light absorption layer having a wide band gap and the buffer layer adjacent thereto.

  On the other hand, an InS-based material that is attracting attention as a buffer layer that does not use harmful cadmium has a relatively large band gap of about 2.5 eV, and it is further possible to form a light absorption layer having a band gap matching this. is important.

  This invention makes it a subject to provide the chalcopyrite type thin film solar cell which can implement | achieve the improvement of a photoelectric conversion efficiency, and its manufacturing method in view of the said problem.

  In order to solve the above problems, a chalcopyrite thin film solar cell of the present invention includes a back electrode layer formed on a substrate, and a p formed on the electrode layer and containing at least a group I, group III and group VI element. Type light absorption layer, an n-type buffer layer formed on the light absorption layer and containing InAIS, and a transparent electrode layer formed on the buffer layer.

  According to the present invention, even when a wide band gap is formed in a p-type light absorption layer containing a group I, group III, and group VI element, band mismatching hardly occurs due to the interposition of the InAIS buffer layer. For this reason, the photoelectric conversion efficiency can be improved without the open circuit voltage of the solar cell product being moderated.

Further, the present invention provides a precursor forming step of forming a precursor containing Cu, In and Ga on a back electrode layer formed on a substrate by a sputtering method, and H _{2 with} respect to the substrate on which the precursor is formed. A selenization step of forming a light absorption layer by performing a heat treatment in a Se gas atmosphere, a buffer layer formation step of forming an n-type buffer layer containing InAIS on the light absorption layer, and a transparent electrode layer on the buffer layer By the manufacturing method comprising the transparent electrode layer forming step for forming the film, the photoelectric conversion efficiency of the obtained CIGS thin film solar cell is improved.

  In this case, it is practically desirable to perform the heat treatment in the selenization step in a temperature range higher than 500 ° C. and lower than the melting point of the substrate.

  Since the chalcopyrite thin film solar cell according to the present invention includes an InAIS component as a buffer layer, matching of the band gap between the p-type light absorption layer and the n-type buffer layer can be obtained. For this reason, the thin film solar cell product obtained is not accompanied by the fall of an open circuit voltage, Therefore, the improvement of a photoelectric conversion efficiency is realizable.

  In addition, since the InAIS buffer layer can reduce the amount of expensive In rare metal used compared to the InS buffer layer, a cost reduction effect can also be obtained.

  FIG. 3 shows a film forming apparatus for manufacturing the precursor of the light absorption layer according to the present invention corresponding to the film forming process of the precursor light absorption layer shown in FIG.

  FIG. 3 shows an in-line type sputtering apparatus in which a purchase chamber 31, a first sputter film forming chamber 32, a second sputter film forming chamber 33, and a take-out chamber 34 are connected to each other through gate valves 35, 36, and 37, respectively. FIG. Each component chamber 31, 32, 33, 34 of the apparatus 38 is provided with a vacuum exhaust mechanism (not shown), and a film forming process is performed under a desired pressure condition.

  A substrate support table (not shown) capable of storing a plurality of batches of substrates 1a is mounted inside the purchase chamber 31. The substrate 1a to be accommodated has already been formed with a Mo electrode layer (see (a)). Then, among the plurality of substrates 1a with Mo electrode layers, the substrate 1b proceeding to the film forming step is held by a substrate holder (not shown) such as a substrate transfer tray, and the next through the gate valve 35. The film is transferred to the first film formation chamber 32.

  In the first film formation chamber 32, the gate valves 35 and 36 on both sides are closed, and a metal In layer is formed as a surface layer on the substrate 1b by a spack film formation method using an In target under a predetermined pressure condition. A film is formed (see (b)). Similarly, in the next second film forming chamber 33, a Cu—Ga alloy layer is formed on the substrate 1b by a sputtering film forming method using a Cu—Ga target (see (c)), and a precursor is formed. The film forming step is completed. The substrate 1 c after film formation is transferred to the take-out chamber 34 via the gate valve 37. A substrate support table similar to that in the purchase chamber 31 is mounted in the take-out chamber 34, and the number of substrates 1c corresponding to the number of batch units that have been formed in the same film formation process cycle as described above are supported by the support table. When all the film formation processes are completed, the film formation process is completed.

FIG. 4 is a schematic view of a heat treatment chamber 40 for performing selenization of the light absorption layer according to the present invention, corresponding to the selenization process of the precursor light absorption layer shown in FIG. The heat treatment chamber 40 is heated by a heater 41 disposed on both sides of the heat treatment chamber 40. In addition, a quartz boat 42 capable of accommodating a predetermined batch number of substrates is installed therein, and a plurality of substrates 1c are accommodated in a vertical state on the bottom surface of the boat 42. Further, quartz susceptors 43a and 43b are provided for keeping the substrate 1c on the boat 42 in an upright state. The quartz boat 42 provided with the susceptors 43a and 43b is connected to a rotation drive shaft 44 connected to an external drive mechanism via a connecting portion 45, and the rotation of the rotation shaft 44 keeps the entire substrate 1c in a vertical state. It can be rotated as it is. Further, the quartz boat 42 on which all the substrates 1c are mounted is surrounded by a quartz process tube 46. The sealed space formed by being surrounded by the process tube 46 has variable pressure conditions by a vacuum exhaust mechanism (not shown), and a gas introduction pipe 47 is inserted into the sealed space. The gas introduction pipe 47 is for introducing selenization gas, and H ₂ Se gas flows in from a large number of nozzle holes 48 provided on the peripheral wall of the introduction pipe 47. Further, the nozzle hole 48 is drilled with an outer diameter of 1 to 2 mm so that a uniform flow of H ₂ Se gas is obtained in the process tube 46.

  When producing the light absorption layer 4 (see FIG. 1), the glass substrate 1c on which a laminated structure composed of a metal In layer and a Cu—Ga alloy layer is formed using an inline-type sputter deposition apparatus 38 shown in FIG. Aligned to the number of sheets, and accommodated in the heat treatment chamber 40 as shown in FIG. Then, selenization is performed according to the temperature profile shown in FIG.

That is, the internal temperature of the process tube 46 in the heat treatment chamber 40 of FIG. 4 is raised to about 250 ° C. by the heater 41 while maintaining a reduced pressure of 50 to 95 kPa by the operation of an exhaust mechanism not shown. While maintaining these temperature conditions and pressure conditions, a predetermined flow rate of H ₂ Se gas is allowed to flow from the nozzle hole 48 of the gas introduction pipe 47 over a predetermined time (1), which is defined as a first selenization process. This step is provided for preheating in the heat treatment chamber 40 and stabilizing the H ₂ Se gas atmosphere. The time (1) at this time is preferably about 10 minutes, for example.

Further, by rotating the rotational drive shaft 44 of the heat treatment chamber 40 in FIG. 4 at a constant speed of 1 to 2 rpm, the ambient environment of the simultaneously rotating substrate 1c, that is, the H ₂ Se gas atmosphere under the preheating temperature condition can be obtained. Furthermore, it is ensured stably. The rotation of the substrate 1c becomes more effective by performing not only the first selenization step but also the second and third selenization steps and the cooling step described later.

Next, after the introduction of the H ₂ Se gas at time (1), the internal temperature A is raised to about 250 to 450 ° C. by the heater 41 while the process tube 46 is kept in a reduced pressure state of 50 to 95 kPa. Then, with these temperature conditions and pressure conditions maintained, a predetermined flow rate of H ₂ Se gas is allowed to flow from the nozzle hole 48 of the gas introduction pipe 47 over a period of time (2), which is used as the second selenization step. . This step is provided to capture the Se component while diffusing each component of In, Cu, and Ga in a light absorption layer precursor having a laminated structure of an In layer and a Cu—Ga layer formed on the substrate 1c. It is done. The time (2) at this time is preferably about 10 to 120 minutes, for example.

Further, after the temperature is raised to about 250 to 450 ° C., the process tube 46 is evacuated and held in a high vacuum state at a time (2) immediately before the time (3). As a result, the Se component raw material H ₂ Se gas incorporated in the second selenization step is obtained as a highly active material. In addition, since it is not necessary to consider the influence of the residual gas derived from the first selenization process, the introduction of the Se component in the second selenization process can be accurately controlled by introducing a required set amount of H ₂ Se gas. Alternatively, when the required amount of H ₂ Se gas reaches a relatively large flow rate in the _second selenization step, a method may be adopted in which the flow is divided into a plurality of times in order to accurately control the flow rate. However, in that case, it is necessary to perform the above-described vacuum process immediately before the divided flow. As a result, the flow rate control of the H ₂ Se gas is performed more accurately.

Next, after the introduction of the H ₂ Se gas at time (3), the internal temperature B is raised to about 500 to 650 ° C. by the heater 41 while keeping the inside of the process tube 46 in a reduced pressure state of 50 to 95 kPa.

  Note that the upper limit value 650 ° C. of the internal temperature B is the upper limit of the softening temperature of the glass substrate used in this embodiment. In substrate type solar cells, there are no major restrictions on the material used for the substrate. For example, when a substrate such as stainless steel, carbon, mica (mica) or the like is used, selenization can be performed at a higher temperature. It becomes possible.

  And this state is hold | maintained for about 10 to 120 minutes, and let this be a 3rd selenization process. This step crystallizes the light absorption layer precursor that has been homogenized by the diffusion of each component of In, Cu, and Ga and the incorporation of the Se component performed so far, and stably rearranges the internal film structure. Provided for.

Thereafter, the heating temperature by the heater 41 is gradually lowered and cooled to room temperature, and then the substrate 1c on which the light absorption layer has been formed by the steps up to the third selenization step is taken out to complete the preparation of the light absorption layer. Residual H ₂ Se gas may act on the substrate 1c being cooled to cause unnecessary Se precipitation in the surface layer. In order to prevent this, a vacuum process for evacuating the process tube 46 and keeping it in a high vacuum state during the cooling time (4) may be interposed. In addition, the rotation of the substrate 1c by the constant speed rotation of the rotation drive shaft 44 is desirably performed until the substrate 1c is taken out, which is the time when the light absorption layer fabrication is completed.

  Next, in order to obtain a hetero bond with the p-type light absorption layer, a high-resistance n-type buffer layer is formed. In forming the buffer layer, a wet CBD method is employed, and an InAlS buffer layer is formed using a mixed aqueous solution of thioacetamide, indium chloride, and aluminum chloride. In actual practice of the CBD method, the surface of the light absorption layer is immersed in a temperature-controlled aqueous solution, fine particles are deposited by colloidal growth in the aqueous solution, and thin film growth proceeds.

  Thereafter, a transparent electrode layer made of a ZnOAl layer or the like is formed as an outermost surface layer by a normal sputtering film forming method using a ZnO—Al alloy target or the like, thereby completing a thin film solar cell.

A thin film solar cell was produced using an InAlS-based buffer layer as described above, and the performance was measured for a plurality of thin film solar cell products manufactured under the same conditions. The photoelectric conversion efficiency shown in FIG. Got the value.
[Comparative Example 1]
A thin film solar cell having a conventional InS buffer layer was produced instead of the InAlS system, and the performance was measured for a plurality of thin film solar cell products manufactured under the same conditions. The photoelectric conversion efficiency shown in FIG. Got the value.

  When [Example 1] and [Comparative Example 1] are compared, in the thin-film solar cell product according to the present invention, the photoelectric conversion efficiency as the product characteristic index is remarkably improved from 8.0% to 8.5%. . This is because the band gap matching between the light absorption layer and the buffer layer is improved. As a result, in the thin-film solar cell product provided with the InAlS buffer layer, the open circuit voltage tends to decrease, but the short circuit current and the fill factor increase, so that the result of improved photoelectric conversion efficiency is obtained.

  FIG. 7 is a schematic diagram conceptually showing band gap matching. FIG. 7A shows a stacked structure in which an InS buffer layer is interposed between a ZnOAl transparent electrode layer and a CIGS light absorption layer, and FIG. 7B shows an InAlS buffer layer with a ZnOAl transparent layer. A laminated structure interposed between the electrode layer and the CIGS light absorption layer is shown. Compared to the bandgap matching shown in FIG. 7A, in FIG. 7B, the discontinuous portions between the transparent electrode layer, the buffer layer, and the light absorption layer are reduced on both the upper and lower sides of the interface, and the light absorption is reduced. Coupled with the wide gap of the layer, the band gap mismatch is improved.

  Further, when the InAlS buffer layer was formed by the CBD method, the mixing ratio of indium chloride and aluminum chloride in the aqueous solution was changed, and the photoelectric conversion efficiency of the thin film solar cell product in each case was measured, as shown in FIG. Results were obtained. Thus, it can be seen that the maximum conversion efficiency is obtained when the mixing ratio of indium chloride and aluminum chloride is 1: 1 to 1: 2 (corresponding to columns 3 and 4).

  In the chalcopyrite thin film solar cell of the present invention, a part of the indium metal used in the conventional buffer layer is replaced with aluminum. That is, since the expensive rare metal indium is replaced with aluminum which is an inexpensive general-purpose metal, an effect of reducing the manufacturing cost of the solar cell can be obtained.

  INDUSTRIAL APPLICATION This invention can be utilized for manufacture of the buffer layer in the chalcopyrite thin film solar cell provided with the light absorption layer by which wide gap formation was requested | required.

Schematic showing the general structure of thin film solar cells General manufacturing process diagram of thin film solar cell Schematic diagram of an in-line type sputter deposition apparatus for producing a laminated precursor composed of an In metal layer and a Cu-Ga alloy layer Schematic of heat treatment chamber for producing CIGS light absorption layer Temperature profile diagram of selenization process The graph which shows the photoelectric conversion efficiency characteristic of the thin film solar cell produced by [Example 1] and [Comparative Example 1] (A) Conceptual schematic diagram of band gap matching of a laminated structure in which an InS buffer layer is interposed between a ZnOAl transparent electrode layer and a CIGS light absorption layer, (b) an InAlS buffer layer as a ZnOAl transparent electrode layer and a CIGS light absorption layer Schematic diagram of band gap matching of laminated structure interposed between Graph showing the photoelectric conversion efficiency characteristics of thin-film solar cell products produced by changing the mixing ratio of indium chloride and aluminum chloride

Explanation of symbols

1 1a 1b 1c Glass substrate 2 Mo electrode layer 4 CIGS light absorption layer 7 Multilayer laminated structure (thin film solar cell)
32 In metal layer first sputter deposition chamber 33 Cu—Ga alloy layer second sputter deposition chamber 38 In-line sputter deposition apparatus 40 Heat treatment chamber 41 Heater 42 Quartz boat 43 Susceptor 44 Rotation drive shaft 46 Process tube 47 Gas inlet pipe 48 Nozzle hole

Claims (3)

A back electrode layer formed on the substrate;
A p-type light absorption layer formed on the electrode layer and containing at least a group I, group III and group VI element;
An n-type buffer layer formed on the light absorption layer and containing InAIS;
A transparent electrode layer formed on the buffer layer;
A chalcopyrite thin film solar cell comprising: A precursor forming step of forming a precursor containing Cu, In, and Ga on the back electrode layer formed on the substrate by a sputtering method;
A selenization step of forming a light absorption layer by performing a heat treatment on the substrate on which the precursor has been formed in an H ₂ Se gas atmosphere;
A buffer layer forming step of forming an n-type buffer layer containing InAIS on the light absorption layer;
A transparent electrode layer forming step of forming a transparent electrode layer on the buffer layer;
A method for producing a chalcopyrite thin film solar cell comprising:   The method for manufacturing a chalcopyrite thin film solar cell according to claim 2, wherein the heat treatment in the selenization step is performed in a temperature range higher than 500 ° C and lower than the melting point of the substrate.

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