JPH07115216A - Solar cell manufacturing method - Google Patents

Abstract

(57) [Summary] [Objective] To provide a solar cell in which crystallinity of a chalcopyrite structure semiconductor thin film which becomes a light absorption layer deposited on a semiconductor thin film containing cadmium which becomes a window layer is improved and high energy conversion efficiency is obtained. To do. A semiconductor thin film containing cadmium formed on a translucent substrate is heat-treated in an atmosphere containing cadmium chloride, a chalcopyrite structure semiconductor thin film is deposited on the semiconductor film, and an electrode film is further formed. The crystallinity of the light absorption layer is improved by increasing the crystal grain size of the semiconductor thin film containing cadmium.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a solar cell having high energy conversion efficiency.

[0002]

2. Description of the Related Art Various solar cells having a structure in which a transparent conductive film, a window layer, a light absorbing layer and an electrode film are sequentially formed on a transparent substrate have been reported. Above all, solar cells using amorphous Si or CdTe as the light absorption layer have been commercialized. In the solar cell with this configuration, light is incident from the surface of the transparent substrate, and therefore, when packaged as a solar cell, the protective layer necessary for the configuration in which light is directly incident on the transparent conductive film, the window layer or the light absorption layer. Can be simplified. Therefore, it is advantageous in manufacturing a solar cell. As a light absorption layer of a solar cell having a structure in which light is incident from such a transparent substrate surface, a group I,
Several thin film solar cells using CuInSe ₂ which is a chalcopyrite structure semiconductor thin film composed of Group III and Group VI elements have been reported. In this structure, a CdS film having a lattice constant close to that of CuInSe ₂ is used as the window layer. In this case, since the CuInSe2 film is grown by utilizing the orientation or crystallinity of the underlying CdS film, the quality of the CdS film has a great influence on the performance of the solar cell.

[0003]

In order to obtain a solar cell having high conversion efficiency, it is necessary to prepare a CuInSe ₂ film having few defects and good crystallinity. In general, a high film formation temperature of 500 ° C. or higher is required to form a CuInSe ₂ film having a large grain size and good crystallinity. However, when the CuInSe ₂ film is deposited on the CdS film at a high temperature, excessive interdiffusion of elements between the two films occurs, and as a result, the leakage current of the diode composed of CdS and CuInSe ₂ increases and the conversion efficiency increases. descend. Therefore, it is important to form a CuInSe ₂ film having excellent crystallinity at low temperature. Therefore, a method of growing a CuInSe ₂ film by utilizing the crystallinity of the underlying CdS film is effective. In particular, CuI with good crystallinity and large grain size
It is considered necessary to increase the grain size of the CdS film in order to obtain the nSe ₂ film.

Conventionally, a vapor deposition film has been mainly used as the window layer CdS film. In the case of a vapor-deposited film, the grain size of the crystal grain is small, and its size is proportional to the film thickness. For example, about 1.0μ
When the film thickness is m, the particle diameter is about several tens nm, and when the film thickness is 10 μm, it is about 1 μm. Therefore, in order to obtain a film having a large grain size, that is, a CdS film having good crystallinity, it is necessary to increase the film thickness. However, as the film thickness increases, the resistance component in the CdS film increases and current loss occurs. Also, the use of a large amount of Cd poses a problem of pollution when the solar cell is burnt out or discarded. Therefore, it is important to manufacture a solar cell using a thin CdS film having large crystal grains.

[0005]

In order to solve the above-mentioned problems, a method of manufacturing a solar cell according to the present invention uses a cadmium chloride in order to increase the grain size of a semiconductor thin film containing cadmium to be a window layer and improve its crystallinity. On the window layer semiconductor film having a large grain size obtained by the heat treatment step using
The method is characterized by including a step of depositing a chalcopyrite structure semiconductor thin film made of a group I element and a group VI element.
That is, the first method for manufacturing a solar cell according to the present invention includes a step of depositing a semiconductor thin film containing cadmium on a light-transmitting substrate and then heat-treating it in an atmosphere containing cadmium chloride, and cadmium heat-treated in the step. The method is characterized by including a step of depositing a chalcopyrite structure semiconductor thin film made of a group I element, a group III element and a group VI element on the semiconductor thin film. A second method for manufacturing a solar cell according to the present invention comprises a step of forming a semiconductor thin film containing cadmium and cadmium chloride on a transparent substrate, followed by heat treatment, and a group I or III group on the semiconductor thin film heat-treated in the step. And a step of depositing a chalcopyrite structure semiconductor thin film made of a group VI element.

The above-mentioned manufacturing method includes a step of depositing at least one thin film of a metal film, a carbon film and a transparent conductive film on a chalcopyrite structure semiconductor thin film made of a group I, group III and group VI element. Is desirable. Further, in the manufacturing method, any one of a transparent insulator as a transparent substrate, a transparent insulator covered with a transparent conductor thin film, and a transparent insulator covered with a transparent conductor thin film on which the transparent insulator thin film is deposited. Is preferably used. Further, in the above manufacturing method, a step of ultrasonically cleaning the heat-treated semiconductor thin film containing cadmium in pure water, or a step of washing with hot water, and a step of heat-treating the heat-treated semiconductor thin film containing cadmium in a vacuum or a gas atmosphere It is preferable to provide at least one of the above.

[0007]

According to the above-described structure of the present invention, the semiconductor thin film containing cadmium is heat-treated in the atmosphere containing cadmium chloride, or the semiconductor thin film containing cadmium and cadmium chloride is heat-treated. The crystal grains of the thin film increase, and a semiconductor thin film having a large grain size and forming a window layer excellent in crystallinity can be obtained. By depositing a chalcopyrite structure semiconductor composed of group I, group III and group VI elements on the window layer semiconductor thin film having a large grain size obtained in the above step, a film excellent in crystallinity can be obtained at low temperature film formation. To be Therefore, since defects and the like are reduced, the open-circuit voltage and the short-circuit photocurrent at the time of sunlight irradiation are increased, and the conversion efficiency of the solar cell is improved. Further, as an electrode formed on the chalcopyrite thin film, Au, P, which is excellent in ohmic contact with the chalcopyrite thin film and is easy to form a thin film,
It is advantageous to use at least one thin film of a metal film such as t or Ni, a carbon film, and a transparent conductive film such as SnO ₂ . Further, when the window layer semiconductor thin film has low resistance and can be used as an electrode, it is advantageous to use only a transparent insulating substrate such as glass as a light-transmissive substrate on which light is incident. . If the window layer semiconductor thin film has a high resistance, a transparent electrode is required,
It is necessary to use a transparent insulator substrate coated with a transparent conductor film such as ITO (indium tin oxide) or SnO ₂ . Further, in order to reduce mutual diffusion of elements of the transparent conductor film and the window layer semiconductor thin film by heat treatment, when a substrate having a transparent insulator film such as ZnO deposited on the transparent conductor film is used as a buffer layer, The fill factor, which is one of the characteristics of solar cells, is improved.

Further, after the heat treatment of the atmosphere containing cadmium chloride or after the heat treatment of the semiconductor film containing cadmium chloride, cadmium chloride may adhere or accumulate on the film surface. When a chalcopyrite structure semiconductor thin film is deposited on a film to which cadmium chloride has adhered, cadmium and chlorine diffuse into the film, which lowers the crystallinity. Further, when the entire chalcopyrite film is an n-type semiconductor and the window layer semiconductor is an n-type semiconductor, the pn junction is not formed. The cadmium chloride that has adhered to or accumulated on the surface of the semiconductor thin film to be the window layer is removed by ultrasonic cleaning in pure water, washing with hot water, or heat treatment in a vacuum or gas atmosphere. From the above, a large window layer semiconductor thin film having excellent crystallinity is obtained, and by depositing a chalcopyrite structure semiconductor thin film on the semiconductor thin film, it is possible to form a light absorption layer with excellent crystallinity and less defects, A solar cell with high energy conversion efficiency can be manufactured.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. [Embodiment 1] FIG. 1 shows a heat treatment process which is a part of a manufacturing process of a solar cell in one embodiment of the present invention. 1
Is a translucent substrate in which a transparent conductor film ITO is coated on a glass plate and a transparent insulator film ZnO is further deposited thereon. A CdS film as a semiconductor thin film 2 containing cadmium is vapor-deposited on the substrate 1 in an amount of 1.0 μm. The grain size of this CdS film is about 20 nm. This laminated film is placed on the upper part of an alumina boat 5 having a double structure inside, and cadmium chloride (CdCl ₂ ) 6 is arranged on the lower part. Reference numeral 7 is a lid of the boat 5. This semi-enclosed alumina boat is used as an electric furnace 8
It is put in the inside and heated by energizing the heater 9 in a nitrogen stream 45
Heat treatment is performed at a temperature of 0 ° C to 650 ° C. The optimum heat treatment temperature depends on the thickness and area of the CdS film. In this embodiment, heat treatment is performed at 570 ° C. After heat treatment, the film surface was observed with an optical microscope to find that the average grain size of the crystal grains was about 3 μm.
Which is about 100 times larger than that before the heat treatment. This CdS film is ultrasonically cleaned with pure water for 5 minutes, and then rinsed with hot water (about 80 ° C.) for 15 minutes. After drying the membrane,
Place in a vacuum container and in a vacuum of 10 ^-7 Torr, 4
Heat treatment at 50 ° C. for 5 minutes. After that, Cu, In, Se
Are independently evaporated at the same time to form a CuInSe ₂ film 3, which is a chalcopyrite structure semiconductor, at a substrate temperature of 300 ° C. for about 2.0.
μm vapor deposition. This film was heat-treated in air at 300 ° C. for 1 hour to form a back electrode 4 on the CuInSe ₂ film.
evaporate u.

FIG. 2 is a schematic vertical sectional view of the device manufactured.
For comparison, a chalcopyrite structure semiconductor thin film was similarly deposited on the vapor-deposited CdS film without heat treatment in a cadmium chloride atmosphere, and a device having a conventional structure was also manufactured. AM1 is added to the solar cell elements of this example and comparative example 1.
Table 5 shows the results of comparing the characteristics of the solar cells obtained by irradiating light with a light intensity of 100 mW / cm ² .

[0011]

[Table 1]

As can be seen from Table 1, all characteristics of the solar cell of the present invention are improved as compared with the conventional one. In particular,
Short-circuit photocurrent has improved dramatically. In order to investigate this in detail, the result of measuring the quantum efficiency at each wavelength is shown in FIG. The conventional solar cell shown as Comparative Example 1 has a reduced quantum efficiency in the long wavelength region, whereas the solar cell of the present invention can sufficiently extract carriers generated by light in the long wavelength region. Recognize. In the solar cell of the present invention, this is due to CuI on the CdS film having a large grain size.
It is considered that since the nSe ₂ film was deposited, the crystal grains of the CuInSe ₂ film were increased and the crystallinity was improved, and the carrier life and diffusion length of the light absorption layer were extended. Therefore, CuInSe _{2 which} light in the long wavelength region penetrates
The recombination probability of the carriers generated near the Au electrode in the deep portion of the film is reduced, and it is effectively extracted as a photocurrent.

[Embodiment 2] FIG. 4 shows a part of the manufacturing process of the solar cell of this embodiment. Reference numeral 11 is a transparent substrate made of glass, on which 0.1 atom% of In and 2 wt% of C are formed.
1 of CdS: In: CdCl ₂ film 12 containing dCl ₂
It is vapor-deposited to a thickness of 0 μm. This film is placed on an alumina boat 15, heated by a heater 9 in an electric furnace 8 and heat-treated at 600 ° C. in a nitrogen atmosphere. Thereafter, as in Example 1, ultrasonic cleaning and hot water cleaning are performed with pure water, and then heat treatment is performed at 450 ° C. for 15 minutes in a nitrogen atmosphere. CuG is deposited on this film at a substrate temperature of 350 ° C. by a sputtering method.
An aSe ₂ film is deposited to a thickness of about 2.5 μm. After that, after heat treatment in air at 250 ° C. for 1 hour, SnO ₂ is deposited as a back electrode by a sputtering method. Similarly, a device of a comparative example in which a CuGaSe ₂ film is deposited on a CdS: In film containing no cadmium chloride is also manufactured. The solar cell elements of this example and comparative example 2 had an AM of 1.5 and a light intensity of 10
Table 2 shows the solar cell characteristics obtained by irradiating light of 0 mW / cm ² . Similar to Example 1, the device obtained by heat-treating the CdS: In film containing cadmium chloride has higher conversion efficiency, and in particular, the short-circuit photocurrent is larger. The reason for this is that the CuGaSe ₂ film serving as the light absorption layer has good crystallinity as described above, and in this structure, C is used as the electrode on the light incident side.
Because the dS: In film is used, the CdS: In film whose grain size is increased by the heat treatment of cadmium chloride has a lower resistance and a smaller current loss.

[0014]

[Table 2]

Example 3 In the same manner as in Example 1, the window layer Cd _0.98 Zn _0.02 S thin film (film thickness 1.0 μm) is heat-treated in a cadmium chloride atmosphere. Here, glass coated with ITO is used as the translucent substrate. After performing ultrasonic cleaning with pure water and hot water washing in the same manner as in Example 1, heat treatment is performed in vacuum at 450 ° C. for 5 minutes. Then, Cu, In, Se, and S are independently and simultaneously evaporated in the same vacuum to form a CuInSe _0.55 S _0.45 film at a substrate temperature of 380 ° C.
After heat treatment at 300 ° C for 1 hour in air, CuI
the nSe _{0. 5} 5 S _0.45 film dried after coating the carbon paste, to form a carbon film. The solar cell element of the present example manufactured in this way and the comparative example not including the heat treatment step in the cadmium chloride atmosphere
Table 3 shows the results of measuring the solar cell characteristics under the light irradiation under the same conditions as described above. From Table 3, it can be seen that the fill factor is increased in the element that has been heat-treated with cadmium chloride. This is because when the window layer is an evaporated film as in the conventional structure, a large amount of mutual diffusion of elements occurs at the deposition temperature of 380 ° C. in the window layer and the absorption layer through the grain boundaries that exist at high density. is there. It is considered that this mutual diffusion increases the leakage current of the diode and reduces the fill factor. On the other hand, since the Cd _0.98 Zn _0.02 S thin film heat-treated in a cadmium chloride atmosphere has a large grain size,
It is considered that the grain boundary density is low and mutual diffusion is suppressed.

[0016]

[Table 3]

[0017]

According to the present invention, since the grain size and the crystallinity of the semiconductor thin film which becomes the window layer of the solar cell can be increased, the chalcopyrite structure semiconductor thin film which becomes the light absorption layer deposited thereon can be formed. Improvement of crystallinity and reduction of defects can be realized. Therefore, by increasing the carrier life and the diffusion length, it is possible to improve the efficiency of the solar cell using the chalcopyrite structure semiconductor thin film.

[Brief description of drawings]

FIG. 1 is a schematic vertical sectional view showing a heat treatment process in one embodiment of the present invention.

FIG. 2 is a schematic vertical sectional view showing a structure of a solar cell obtained according to an embodiment of the present invention.

FIG. 3 is a diagram showing changes in quantum efficiency with respect to light wavelengths of solar cells of Examples and Comparative Examples.

FIG. 4 is a diagram showing a part of a manufacturing process of the solar cell of the present invention.

[Explanation of symbols]

 1 Translucent Substrate 2 Semiconductor Thin Film Containing Cadmium 3 Chalcopyrite Structure Semiconductor Thin Film 4 Electrode Film 5 Alumina Boat 6 Cadmium Chloride 7 Boat Lid 8 Electric Furnace 9 Heater

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Takahiro Wada 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A step of depositing a semiconductor thin film containing cadmium on a transparent substrate, a step of heat-treating the semiconductor thin film in an atmosphere containing cadmium chloride, and a group I on the heat-treated semiconductor thin film containing cadmium, A method for manufacturing a solar cell, comprising a step of depositing a chalcopyrite structure semiconductor thin film made of a group III element and a group VI element. 2. A step of forming a semiconductor thin film containing cadmium and cadmium chloride on a transparent substrate, a step of heat-treating the semiconductor thin film, and a group I, III or VI element on the heat-treated semiconductor thin film. A method for manufacturing a solar cell, comprising the step of depositing a chalcopyrite structure semiconductor thin film.