Patent Application
20130074933
1. Field of the Invention
This invention relates to a photovoltaic device and a method for making the same, more particularly to a photovoltaic device including an n-type semiconductor layer of a chalcopyrite-type compound formed on a p-type semiconductor layer.
2. Description of the Related Art
Conventional photovoltaic devices, such as CIGS-based solar cells, normally include a soda lime glass substrate, a back electrode formed on the soda lime glass substrate, an absorption layer of a p-type CIGS (Cu—In—Ga—Se) material formed on the back electrode, a buffer layer of a weak n-type CdS material formed on the absorption layer and forming a p-n junction with the absorption layer, a window layer of ZnO formed on the buffer layer, a TCO (transparent conductive oxide) layer of a highly doped ZnO material formed on the window layer and serving as a front electrode, and a top electrode contact formed on the TCO layer.
The conversion efficiency of the CIGS-based solar cells highly depends on the quality and the composition of the p-type CIGS material in the absorption layer. Conventionally, the p-type CIGS material can be made by co-evaporating techniques, two-stage sputtering techniques, or electrodepositing techniques.
Co-evaporating techniques have the advantage of freely regulating the concentration of the components in the absorption layer so as to obtain a desired concentration gradient in the absorption layer, thereby achieving a higher conversion efficiency. However, it has the problems that production of a large area absorption layer is difficult to achieve due to a non-uniform problem over different areas of the absorption layer, which is caused by the evaporation equipment, and that mass production is infeasible.
U.S. Pat. No. 6,048,442 discloses a two-stage sputtering method for making a CIGS film. The method includes: sequentially forming a stack of precursor films on a back electrode on a substrate by sputtering techniques, the precursor films including a first copper-gallium film, a second copper-gallium film, and a pure indium film; and heating the precursor films in an atmosphere of selenium and/or sulfur so as to obtain a CIGS absorption layer with a gallium concentration gradient that increases from a side closest to an interfacial layer (or buffer layer) to a side closest to the back electrode. Although the aforementioned two-stage sputtering method can improve the non-uniform problem of the co-evaporating techniques and the solar cell thus formed can achieve a good efficiency, it is relatively complicated because the heating step in the atmosphere of selenium is required.
An object of the present invention is to provide a photovoltaic device and a method for making the same that is relatively easy and cost effective while still possessing a satisfactory efficiency.
According to one aspect of the present invention, a photovoltaic device comprises: a back electrode; a transparent front electrode; a p-type semiconductor layer disposed between the transparent front electrode and the back electrode and made from a first semiconductor compound comprising M1, M2, and A1, where M1 is selected from Cu, Au, Ag, Na, Li, K and combinations thereof, M2 is selected from In, Ga, Al, Ti, Zn, Cd, Sn, Mg and combinations thereof, and A1 is selected from S, Se, Te and combinations thereof, the p-type semiconductor layer having a substantially uniform M1/M2 atomic ratio throughout an entire layer thickness thereof; and an n-type layered structure disposed between the p-type semiconductor layer and, the transparent front electrode and cooperating with the p-type semiconductor layer to form a p-n junction therebetween. Then-type layered structure includes an n-type semiconductor layer made from a second semiconductor compound comprising M3, M4, and A2, where M3 is selected from Cu, Au, Ag, Na, Li, K and combinations thereof, M4 is selected from In, Ga, Al, Ti, Zn, Cd, Sn, Mg and combinations thereof, and A2 is selected from S, Se, Te and combinations thereof. The n-type semiconductor layer has a substantially uniform M3/M4 atomic ratio throughout an entire layer thickness thereof. The M3/M4 atomic ratio is less than the M1/M2 atomic ratio and is greater than 0.1 and less than 0.9.
According to another aspect of the present invention, a method for making a photovoltaic device comprises: providing a first sputtering target of a first chalcopyrite-type compound comprising M1, M2, and A1, in which M1 is selected from Cu, Au, Ag, Na, Li, K and combinations thereof, M2 is selected from In, Ga, Al, Ti, Zn, Cd, Sn, Mg and combinations thereof, and A1 is selected from S, Se, Te and combinations thereof; providing a second sputtering target of a second chalcopyrite-type compound comprising M3, M4, and A2, in which M3 is selected from Cu, Au, Ag, Na, Li, K and combinations thereof, M4 is selected from In, Ga, Al, Ti, Zn, Cd, Sn, Mg and combinations thereof, and A2 is selected from S, Se, Te and combinations thereof; forming a p-type semiconductor layer on a back electrode by sputtering the first sputtering target such that the p-type semiconductor layer thus formed is made of a first semiconductor compound comprising M1, M2, and A1 and has a substantially uniform M1/M2 atomic ratio throughout an entire layer thickness thereof; forming an n-type semiconductor layer on the p-type semiconductor layer by sputtering the second sputtering target such that the n-type semiconductor layer thus formed is made of a second semiconductor compound comprising M3, M4, and A2 and has a substantially uniform M3/M4 atomic ratio throughout an entire layer thickness thereof; forming a′ buffer layer on the n-type semiconductor layer; and forming a window layer on the buffer layer.
Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiment of this invention, with reference to the accompanying drawings, in which:
The n-type semiconductor layer 61 is made from a second semiconductor compound comprising M3, M4, and A2, where M3 is selected from Cu, Au, Ag, Na, Li, K and combinations thereof, M4 is selected from In, Ga, Al, Ti, Zn, Cd, Sn, Mg and combinations thereof, and A2 is selected from S, Se, Te and combinations thereof. The n-type semiconductor layer 61 has a substantially uniform M3/M4 atomic ratio throughout an entire layer thickness thereof. The M3/M4 atomic ratio is less than the M1/M2 atomic ratio, and is greater than 0.1 and less than 0.9.
Preferably, the M1/M2 atomic ratio ranges from 0.91 to 1.3, and the M3/M4 atomic ratio ranges from 0.4 to 0.7.
Preferably, the layer thickness of the p-type semiconductor layer 5 ranges from 0.2 to 2 μm, and the layer thickness of the n-type semiconductor layer 61 ranges from 0.02 to 0.7 μm.
Preferably, the p-type semiconductor layer 5 is formed by sputtering a first sputtering target of a first chalcopyrite-type compound, and the n-type semiconductor layer 61 is formed by sputtering a second sputtering target of a second chalcopyrite-type compound. Sputtering techniques have the advantages of mass production of a large area thin film with a uniform composition throughout the entire area of the thin film thus formed.
Preferably, the first chalcopyrite-type compound is selected from the group consisting of p-type CuInSe2, p-type CuInS2, p-type CuIn1-xGaxSe2, and p-type CuIn1-xGaxSe2-ySy, where and 0≦x≦1 and 0≦y≦2, and the second chalcopyrite-type compound is selected from the group consisting of n-type CuInSe2, n-type CuInS2, n-type CuIn1-xGaxSe2, and n-type CuIn1-xGaxSe2-ySy, where 0≦x≦1 and 0≦y≦2. Other examples of the first chalcopyrite-type compound may be selected from p-type CuIn3Se5 and p-type CuIn5Se9, and other examples of the second chalcopyrite-type compound may be selected from n-type CuIn3Se5 and n-type CuIn5SeB.
The substrate 2 can be made from soda lime glass, stainless steel, or polymers, and is preferably made from soda lime glass.
The back electrode 3 is preferably made from a metallic material, such as Mo and stainless steel.
The buffer layer 62 can be made from a material selected from one of CdS, ZnS, Zn2Se3, CdZnS, and combinations thereof, and is preferably made from CdS.
The window layer 63 can be made from a material selected from ZnO, ZnS, AZO, and combinations thereof, and is preferably made from ZnO.
The top electrode contact 7 is made from a metallic material, such as Ni/Al, Au and Ag, and is preferably made from Ni/Al.
The photovoltaic device of this preferred embodiment includes only one p-type semiconductor layer 5 and one n-type semiconductor layer 61. Alternatively, the photovoltaic device of the present invention can include a stack of p-type semiconductor layers having different M1/M2 atomic ratios and a stack of n-type semiconductor layers having different M3/M4 atomic ratios and formed on the stack of the p-type semiconductor layers.
Preparation of the first chalcopyrite-type compound and the second chalcopyrite-type compound can be conducted using a conventional technique, such as solvothermal synthesis techniques, and a coordinating solvent-based reaction scheme as disclosed in U.S. Pat. No. 7,591,990.
The merits of the method for making the photovoltaic device of this invention will become apparent with reference to the following Examples and Comparative Examples. This invention is not restricted to the following Examples.
Preparation of the Photovoltaic Device
A first sputtering target made of a first CIGS compound (i.e., first chalcopyrite-type compound) comprising 34.12 atomic % Cu, 8.68 atomic % Ga, 16.85 atomic % In, and 40.35 atomic % Se (the atomic ratio of Cu/(Ga+In) being 1.34) and a second sputtering target made of a second CIGS compound (i.e., second chalcopyrite-type compound) comprising 23.25 atomic % Cu, 8.5 atomic % Ga, 19.1 atomic % In, and 49.15 atomic % Se (the atomic ratio of Cu/(Ga+In) being 0.84) were prepared using solvothermal synthesis techniques, followed by sintering. A Mo-coated soda lime glass was cleaned and was deposited with a p-type CIGS semiconductor layer of a first semiconductor compound thereon by sputtering (DC sputtering) the first sputtering target. The sputtering output power was 0.55 KW. The p-type CIGS semiconductor layer thus formed had a layer thickness of 0.3 μm and the first semiconductor compound thus formed comprised 30.27 atomic % Cu, 6.33 atomic % Ga, 16.99 atomic % In, and 46.41 atomic % Se (the atomic ratio of Cu/(Ga+In) being 1.30 throughout the entire layer thickness of the p-type CIGS semiconductor layer). An n-type CIGS semiconductor layer of a second semiconductor compound was formed on the p-type CIGS semiconductor layer by sputtering the second sputtering target. The sputtering output power was 0.4 KW. The n-type CIGS semiconductor layer thus formed had a layer thickness of 0.38 μm and the second semiconductor compound thus formed comprised 15.43 atomic % Cu, 5.24 atomic % Ga, 18.17 atomic % In, and 61.17 atomic % Se (the atomic ratio of Cu/(Ga+/n) being 0.66 throughout the entire layer thickness of the n-type CIGS semiconductor layer). A buffer layer of CdS was formed on the n-type CIGS semiconductor layer by sputtering. The sputtering output power was 0.5 KW. The buffer layer thus formed had a layer thickness of 0.16 μm. A window layer of ZnO was formed on the buffer layer by sputtering. The sputtering output power was 3 KW. The window layer thus formed had a layer thickness of 0.12 μm. A transparent front electrode of AZO was formed on the window layer by sputtering. The sputtering output power was 4 KW. The transparent front electrode thus formed had a layer thickness of 0.44 μm. A top electrode contact of Ni/Al was formed on the transparent front electrode by sputtering.
Performance Test
The current and voltage (I-V) characteristic curve of the photovoltaic device of Example 1 was measured using a solar simulator according to IEC 60904-9 standard test. The measured I-V curve is shown in
Preparation of the Photovoltaic Device
The preparation procedures of the photovoltaic device of Example 2 are similar to those of Example 1. In Example 2, the first CIGS compound of the first sputtering target comprises 28.13 atomic % Cu, 8.5 atomic % Ga, 18.22 atomic % In, and 45.15 atomic Se (the atomic ratio of Cu/(Ga+/n) is 1.05), and the second CIGS compound of the second sputtering target comprises 23.25 atomic % Cu, 8.5 atomic % Ga, 19.1 atomic % In, and 49.15 atomic % Se (the atomic ratio of Cu/(Ga+In) being 0.84). The first semiconductor compound of Example 2 thus formed comprises 25.4 atomic 15% Cu, 9.72 atomic % Ga, 18.13 atomic % In, and 46.75 atomic % Se (the atomic ratio of Cu/(Ga+In) is 0.912 throughout the entire layer thickness of the p-type CIGS semiconductor layer), and the second semiconductor compound of Example 2 thus formed comprises 11.28 atomic % Cu, 6.34 atomic Ga, 19.2 atomic % In, and 63.18 atomic % Se (the atomic ratio of Cu/(Ga+In) is 0.44 throughout the entire layer thickness of the n-type CIGS semiconductor layer).
Performance Test
The current and voltage (I-V) characteristic curve of the photovoltaic device of Example 2 was measured using the solar simulator according to IEC 60904-9 standard test. The measured I-V curve is shown in
Preparation of the Photovoltaic Device
The preparation procedures of the photovoltaic device of Comparative Example 1 are similar to those of Example 1, except that the photovoltaic device of Comparative Example 1 is formed without formation of the n-type CIGS semiconductor layer and that the buffer layer is formed on the p-type CIGS semiconductor layer. In Comparative Example 1, the first CIGS compound of the first sputtering target comprises 34.12 atomic % Cu, 8.68 atomic % Ga, 16.85 atomic % In, and 40.35 atomic % Se (the atomic ratio of Cu/(Ga+In) being 1.34). The first semiconductor compound of Comparative Example 1 thus formed comprises 30.27 atomic % Cu, 6.33 atomic % Ga, 16.99 atomic % In, and 46.41 atomic (the atomic ratio of Cu/(Ga+In) is 1.30 throughout the entire layer thickness of the p-type CIGS semiconductor layer).
Performance Test
The current and voltage (I-V) characteristic curve of the photovoltaic device of Comparative Example 1 was measured using the solar simulator according to IEC 60904-9 standard test. The measured I-V curve is shown in
Preparation of the Photovoltaic Device
The preparation procedures of the photovoltaic device of Comparative Example 2 are similar to those of Example 1. In Comparative Example 2, the first CIGS compound of the first sputtering target comprises 28.13 atomic % Cu, 8.5 atomic % Ga, 18.22 atomic % In, and 45.15 atomic % Se (the atomic ratio of Cu/(Ga+In) is 1.05), and the second CIGS compound of the second sputtering target comprises 23.25 atomic % Cu, 8.5 atomic % Ga, 19.1 atomic % In, and 49.15 atomic % Se (the atomic ratio of Cu/(Ga+In) being 0.84). The first semiconductor compound of Comparative Example 2 thus formed comprises 24.24 atomic % Cu, 7.7 atomic % Ga, 20.65 atomic % In, and 47.41 atomic % Se (the atomic ratio of Cu/(Ga+In) is 0.855 throughout the entire layer thickness of the p-type CIGS semiconductor layer), and the second semiconductor compound of Comparative Example 2 thus formed comprises 11.28 atomic % Cu, 6.34 atomic % Ga, 19.2 atomic % In, and 63.18 atomic % Se (the atomic ratio of Cu/(Ga+In) is 0.44 throughout the entire layer thickness of the n-type CIGS semiconductor layer).
Performance Test
The current and voltage (I-V) characteristic curve of the photovoltaic device of Comparative Example 2 was measured using the solar simulator according to IEC 60904-9 standard test. The measured I-V curve is shown in
Preparation of the Photovoltaic Device
The preparation procedures of the photovoltaic device of Comparative Example 3 are similar to those of Example 1. In Comparative Example 3, the first CIGS compound of the first sputtering target comprises 34.12 atomic % Cu, 8.68 atomic % Ga, 16.85 atomic % In, and 40.35 atomic % Se (the atomic ratio of Cu/(Ga+In) being 1.39), and the second CIGS compound of the second sputtering target comprises 7.25 atomic % Cu, 9.92 atomic % Ga, 27.28 atomic % In, and 55.55 atomic % Se (the atomic ratio of Cu/(Ga+In) is 0.195). The first semiconductor compound of Comparative Example 3 thus formed comprises 30.27 atomic % Cu, 6.33 atomic % Ga, 16.99 atomic % In, and 46.41 atomic % Se (the atomic ratio of Cu/(Ga+In) is 1.30 throughout the entire layer thickness of the p-type CIGS semiconductor layer), and the second semiconductor compound of Comparative Example 3 thus formed comprises 3.54 atomic % Cu, 10.42 atomic % Ga, 28.63 atomic % In, and 57.41 atomic % Se (the atomic ratio of Cu/(Ga+In) is 0.09 throughout the entire layer thickness of the n-type CIGS semiconductor layer).
Performance Test.
The current and voltage (I-V) characteristic curve of the photovoltaic device of Comparative Example 3 was measured using the solar simulator according to IEC 60904-9 standard test. The measured I-V curve is shown in
With the inclusion of the n-type semiconductor layer 61 in the photovoltaic device of the present invention, the aforesaid drawbacks associated with the prior art can be eliminated. Moreover, by virtue of precise control of the atomic ratios in the first and second semiconductor compounds, the solar cell efficiency could be improved. In the method of this invention, no heating step is required, thereby simplifying the process for making the photovoltaic device.
With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention. It is therefore intended that the invention be limited only as recited in the appended claims.
