Patent Application
20120118384
The present invention relates to a CIS-based thin film solar cell and, in particular, relates to a CIS-based thin film solar cell having a novel structure that can achieve high photoelectric conversion efficiency.
In recent years, a CIS-based thin film solar cell that uses a chalcopyrite structure I-III-VI2 group compound semiconductor containing Cu, In, Ga, Se and S as a p-type light absorbing layer has been attracting attention. A solar cell of this type can be manufactured at relatively low cost. Moreover, because a solar cell of this type has a large absorption coefficient in a wavelength range from visible light to near-infrared light, it is anticipated that high photoelectric conversion efficiency can be attained. It is, therefore, widely considered as a leading candidate for the next-generation solar cell. Typical materials include Cu(In, Ga) See, Cu(In, Ga) (Se, S)2, CuInS2 and so on.
In the CIS-based thin film solar cell, a metal back electrode layer is formed on a glass substrate. Then, a p-type light absorbing layer comprised of the I-III-VI2 group compound semiconductor is formed on this layer and, further, an n-type buffer layer and an n-type transparent conductive window layer are formed. In this CIS-based thin film solar cell, it is reported that high photoelectric conversion efficiency can be achieved when a soda lime glass is used as the glass substrate.
It is thought that this is because Na, or a Ia group element contained in the soda lime glass is thermal-diffused into the p-type light absorbing layer in the formation process of this layer and affects carrier concentration. On the other hand, there is a problem that, when the amount of Na introduced into the p-type light absorbing layer is too high, the p-type light absorbing layer may be separated from the electrode layer. When the CIS-based thin film solar cell is manufactured, it is, therefore, very important to introduce an optimal amount of Na into the p-type light absorbing layer so as to improve its photoelectric conversion efficiency.
In order to introduce an optimal amount of the Ia group element such as, for example, Na contained in the soda lime glass into the p-type light absorbing layer during the film forming process of this layer, a technique in which an alkali control layer made of silica and the like between the soda lime glass substrate and the back electrode layer is provided so as to control the amount of Na diffused into the p-type light absorbing layer has been proposed. (For example, see Japanese Unexamined Patent Publication No. 2006-165386.) According to this technique, the present inventors have succeeded in manufacturing a CIS-based thin film solar cell having a photoelectric conversion efficiency of 14.3% by using an alkali control layer having a thickness of 30 nm.
On the other hand, in order to improve the photoelectric conversion efficiency of the CIS-based thin film solar cell, it is necessary to increase the film forming temperature of the p-type light absorbing layer or the temperature of sulfurization/selenization. However, the soda lime glass has a relatively low distortion point. If the p-type light absorbing layer is formed at a higher film forming temperature such as, for example, at 550° C. or higher in order to further improve the photoelectric conversion efficiency, the soda lime glass substrate is therefore deformed. Thus, on the soda lime glass substrate, the film forming temperature of the p-type light absorbing layer cannot be sufficiently increased. In order to form the p-type light absorbing layer at a sufficiently high temperature, the glass substrate has to be made of high distortion point glass that is low alkali or non-alkali glass. However, because these glass materials contain too low a concentration of the alkali components or do not contain them at all, the p-type light absorbing layer cannot be supplied with sufficient alkali components.
Also in the prior art, Japanese Unexamined Patent Publication No. H11-135819 discloses a technique in which high distortion point glass as the substrate of the CIS-based thin film solar cell is used. However, in this patent publication, it is considered important to inhibit deformation of the glass substrate due to heat history and distortion due to a difference in thermal expansion coefficient between the substrate and the CIS-based semiconductor layers by using the high distortion glass as the substrate, so that the CIS-based thin film solar cell can be manufactured inexpensively. This prior art does not, therefore, take in account improving photoelectric conversion efficiency due to the optimal diffusion of Na from the glass substrate and the solar cell according to this prior art is not provided with the alkali control layer.
The present invention has been made to solve the above problems of a CIS-based thin film solar cell and to obtain a CIS-based thin film solar cell that has higher photoelectric conversion efficiency. Thus, it is an object of the present invention to provide a CIS-based thin film solar cell having a novel structure that can introduce an optimal amount of a Ia group element, such as Na into a p-type light absorbing layer, even when a glass substrate is made of high distortion point glass in place of soda lime glass having a low distortion point.
In order to solve the above problems, according to an embodiment of the present invention, a CIS-based thin film solar cell laminated in order to produce a high distortion point glass substrate, an alkali control layer, a back electrode layer, a p-type CIS-based light absorbing layer, and an n-type transparent conductive film, wherein said alkali control layer is a silica film whose film thickness is within a range of 2.00-10.00 nm and whose refractive index is within a range of 1.450-1.500 is provided.
In the CIS-based thin film solar cell, the film thickness of the alkali control layer may be within a range of 2.00-7.00 nm. Further, the refractive index of the alkali control layer may be within a range of 1.470-1.490.
Further, in the CIS-based thin film solar cell, a distortion point of the high distortion point glass substrate may be 560° C. or higher. Further, the annealing point may be 610° C. or higher. Further, the thermal expansion coefficient may be within a range of 8×10−6/° C.-9×10−6/° C. Moreover, the density may be within a range of 2.7-2.9 g/cm3.
In the CIS-based thin film solar cell, the high distortion point glass may contain 1-7 weight % of Na2O. In particular, the high distortion point glass may contain 3-5 weight % of Na2O. Further, the high distortion point glass may contain 1-15 weight % or, in particular, 5-10 weight % of K2O. Moreover, the high distortion point glass may contain 1-15 weight % or, in particular, 4-10 weight % of CaO.
Further, the p-type CIS-based light absorbing layer may be formed of a five component compound that is mainly comprised of Cu, In, Ga, Se and S. Further, the p-type CIS-based light absorbing layer may be formed by selenizing and sulfurizing a laminated structure containing Cu, In and Ga or a mixed crystal.
According to the present invention, the alkali control layer that is a silica film having a film thickness within a range of 2.00-10.00 nm and a refractive index within a range of 1.450-1.500 is provided between the high distortion point glass substrate and the back electrode. The alkali control layer of this structure can efficiently diffuse an alkali element of a low concentration contained in the high distortion point glass into the p-type light absorbing layer. As a result, by forming the p-type light absorbing layer at a high temperature such as, for example, 600° C. or more, the CIS-based thin film solar cell having high photoelectric conversion efficiency can be implemented.
In comparison with the present invention,
Table 1 indicates physical properties of high distortion point glass substrate 1 of this embodiment.
The contents of Na2O, K2O and CaO in the high distortion point glass used in this embodiment are as indicated in Table 1. Typical high distortion point glass contains 1-7 weight % of Na2O, 1-15 weight % of K2O and 1-15 weight % of CaO and the high distortion point glass like this can also be used to constitute the CIS-based thin film solar cell according to the present invention. Further, there is a high distortion point glass that does not satisfy the above conditions and even such glass may be used to manufacture the solar cell of the present invention.
Next, a film forming method for alkali control layer 2 will be described. Alkali control layer 2 can be formed, for example, by using SiOx or Si as a target with the help of 1) RF sputtering method, 2) AC sputtering method or 3) DC sputtering method. In the film forming method using these sputtering methods, the alkali control layer having various film thickness and refractive index can be formed by changing parameters such as applied electric power, O2 concentration and film forming pressure. The parameters may include gas flow rate, substrate carrying speed and the like.
An example of the parameters is as follows:
RF sputtering: SiO2 target
Applied electric power: 0.1-3 W/cm2
O2 concentration (O2/O2+Ar): 0-20%
Film formation pressure: 0.3-2.0 Pa
In place of the sputtering methods described above, alkali control layer 2 can also be formed by plasma CVD method, electron beam evaporation method and the like.
Table 2 indicates a construction of back electrode 3.
Next, p-type light absorbing layer 4 will be described in detail.
In order to form p-type light absorbing layer 4, a metal precursor film of a laminated structure or a mixed crystal containing Cu, In and Ga is formed on metal back electrode 3 by the sputtering or evaporation method and, then, selenized and sulfurized. In this example, when a ratio of atomicity of Cu to III group element or In and Ga (Cu/III group ratio) is fixed to 0.85-0.95 and a ratio of atomicity of Ga in III group element (Ga/III group ratio) is fixed to 0.15-0.4, the selenization is performed at 350° C.-500° C. and the sulfurization is performed at 550° C.-650° C., so that a light absorbing layer of a film thickness of 1-3 μm having p-type electrical conduction can be formed.
In the embodiment of
cooper indium di-selenide (CuInSe2);
cooper indium di-sulfide (CuInS2);
cooper indium di-selenide/sulfide (CuIn(SeS)2);
cooper gallium di-selenide (CuGaSe2);
cooper gallium di-sulfide (CuGaS2);
copper indium gallium di-selenide (Cu(InGa)Se2); and
copper indium gallium di-sulfide (Cu(InGa)S2).
Next, buffer layer 5 will be described in detail.
In the embodiment of
Next, window layer (transparent conductive film) 6 will be described in detail.
In the embodiment of
In the CIS-based thin film solar cell according to the embodiment of the present invention illustrated in
The values of the film thickness and refractive index of the alkali control layer indicated in Table 3 are merely fixed according to an embodiment of the present invention and the present invention is not limited to this embodiment. According to the present invention, the alkali control layer desirably has a film thickness of 2-10 nm and a refractive index of 1.45-1.50 (at a wavelength of 633 nm) and, more desirably, it has a film thickness of 2-7 nm and a refractive index of 1.47-1.49.
Hereinafter, results of an experiment carried out to determine the above structure of the CIS-based thin film solar cell according to the present invention will be described.
The high distortion point glass typically contains 1-7 weight % of Na2O, 1-15 weight % of K2O and 1-10 weight % of CaO. The Na content in the high distortion point glass is about half or less than that in the soda lime glass. However, the present inventors believe that, if these elements could be efficiently diffused into the p-type light absorbing layer by optimizing the structural and physical properties of the alkali control layer, with the help of high temperature treatment making use of the properties of the high distortion point glass, the CIS-based thin film solar cell having high photoelectric conversion efficiency could be obtained. In CIS-based thin film solar cells whose alkali control layer was composed of silica (SiOx), the film thickness was selected as a structural factor and the refractive index was selected as a physical factor. Then, changing these factors variously, the present inventors measured the photoelectric conversion efficiency. In this experiment, a plurality of CIS-based thin film solar cells whose film thickness was within a range of 0-30 nm and whose refractive index was within a range of 1.407-1.507 could be obtained. This refractive index was a value measured at a wavelength of 633 nm.
As indicated in
From the graphs in
Next, from the graph of
As a result, the present inventors have reached the conclusion that, when the high distortion point glass is used as the glass substrate, in the case of the film thickness T of the alkali control layer within a range of 2.00-10.00 nm and the refractive index n within a range of 1.450-1.500, a good CIS-based thin film solar cell having high photoelectric efficiency can be obtained and, more preferably, in the case of the film thickness T of the alkali control layer within a range of 2.00-7.00 nm and the refractive index n within a range of 1.470-1.490, a CIS-based thin film solar cell having higher photoelectric efficiency can be obtained.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-148768 | Jun 2009 | JP | national |
This application is the national stage under 35 USC 371 of International Application No. PCT/JP2010/060793, filed Jun. 18, 2010, which claims the priority of Japanese Patent Application No. 2009-148768, filed Jun. 23, 2009, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/060793 | 6/18/2010 | WO | 00 | 12/21/2011 |
