Patent Application
20150059845
The present invention relates to a CZTS-based thin film solar cell and a method of production of the same, more particularly relates to a CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency and a method of production of the same.
In recent years, thin film solar cells which use chalcogenide-based compounds generally called CZTS as the p-type light absorption layer have come under the spotlight. This type of solar cell is made of a relatively inexpensive material and has a band gap energy which is suitable for solar light, so holds forth the promise of inexpensive production of highly efficient solar cells. CZTS is a Group I2-(II-IV)-VI4 compound semiconductor which contains Cu, Zn, Sn, S, or Se. As typical ones, there are Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnSn(S,Se)4, etc.
A CZTS-based thin film solar cell comprises a glass substrate on which a metal back electrode layer is formed, on which a p-type CZTS-based light absorption layer is then formed, and further on which an n-type high resistance buffer layer and n-type transparent conductive film are successively stacked. As the material of the metal back electrode layer, molybdenum (Mo) or titanium (Ti), chromium (Cr), or another high corrosion resistant and high melting point metal is used. The p-type CZTS-based light absorption layer is, for example, formed by forming a Cu—Zn—Sn or Cu—Zn—Sn—S precursor film on a substrate formed with a molybdenum (Mo) back electrode layer by sputtering, vapor deposition, etc. and sulfurizing and/or selenizing this in an atmosphere which contains hydrogen sulfide or hydrogen selenide (for example, see PLT 1).
PLT 1. Japanese Patent Publication No. 2009-135316A
NPLT 1. “Fabrication of p-type ZnO Thin Films via DC Reactive Magnetron Sputtering by Using Na as the Dopant Source”, L. L. Yang et al., Feb. 6, 2007, Journal of ELECTRONIC MATERIALS, Vol. 36, No. 4
As explained above, a CZTS-based thin film solar cell is high in latent possibility, but at the present time, a product with a high photovoltaic conversion efficiency which can withstand practical use cannot be obtained. Much more progress in production technology is being sought. The present invention was made relating to this point. In particular, the object is the provision of a novel structure of a CZTS-based thin film solar cell and a method of production of the same which effectively suppress recombination of carriers at the back electrode side of the p-type CZTS-based light absorption layer to realize a high photovoltaic conversion efficiency.
To solve the problem, in a first aspect of the present invention, there is provided a CZTS-based thin film solar cell which is provided with a substrate, a metal back electrode layer which is formed on the substrate, a p-type CZTS-based light absorption layer which is formed on the metal back electrode layer, and an n-type transparent conductive film which is formed on the p-type CZTS-based light absorption layer and which has a dispersed layer of ZnS-based fine particles at the interface between the p-type CZTS-based light absorption layer and the metal back electrode layer.
In the CZTS-based thin film solar cell, the dispersed layer of ZnS-based fine particles need not completely cover the surface of the metal back electrode layer. Further, the ZnS-based fine particles may have a size of 10 nm to 200 nmφ or so.
Further, the substrate may be alkali-containing glass which contains Na. Furthermore, an alkali barrier layer may be formed between the substrate and the metal back electrode layer.
Further, the ZnS-based fine particles may be any of ZnS, ZnSe, or ZnSSe. Furthermore, an n-type high resistance buffer layer may be provided between the p-type CZTS-based light absorption layer and the n-type transparent conductive film.
To solve the above problem, in a second aspect of the present invention, there is provided a method of production of a CZTS-based thin film solar cell which is comprised of forming a metal back electrode layer on a substrate, forming a metal precursor film which contains Cu, Sn, and Zn on the metal back electrode layer, sulfurizing and/or selenizing the metal precursor film to form a p-type CZTS-based light absorption layer, and forming an n-type transparent conductive film on the p-type CZTS-based light absorption layer, where the metal precursor film has a bottommost layer at the metal back electrode layer side, the bottommost layer being formed by Zn, a mixture of Zn and Cu, a mixture of Zn and a Group VI element, or a mixture of Zn, Cu and a Group VI element, and the sulfurization and/or selenization is performed at an ambient temperature of 540° C. to 600° C. for 5 minutes to 25 minutes.
In the method of production of a CZTS-based thin film solar cell, the substrate can also be formed by Na-containing alkali glass. In this case, it is also possible to form an alkali barrier layer between the substrate and the metal back electrode layer.
Further, the metal precursor film may be formed by first depositing a ZnS layer on the metal back electrode layer, then depositing Sn, Cu, or a mixture which contains these on the same. Furthermore, it is also possible to form an n-type high resistance buffer layer after forming the p-type CZTS-based light absorption layer and before forming the n-type transparent conductive film.
The CZTS-based thin film solar cell of the present invention is provided with a dispersed layer of ZnS-based fine particles at the interface of the p-type CZTS-based light absorption layer and metal back electrode layer. Among ZnS-based compounds, for example, ZnS has a bandgap of 3.7 eV or so, ZnSe 2.7 eV, and ZnSSe one between the same. These are all larger than the bandgaps of CZTS-based compounds. If such a dispersed layer of ZnS-based fine particles is present at the back surface of the p-type CZTS-based light absorption layer, a barrier to electrons is formed and the rate of back surface recombination of electrons falls. Due to this, the efficiency of collection of electrons is improved. On the other hand, so long as the dispersed layer is formed by ZnS-based fine particles, there are clearances between the fine particles. Holes can pass through these clearances to reach the metal back electrode layer, so the efficiency of collection of holes does not fall. As a result, the solar cell increases in photovoltaic conversion efficiency.
ZnS-based fine particles better exhibit the effect of an electron barrier the more the conductivity type is the p-type. It is known that a ZnS or other Zn (Group VI)-based semiconductor becomes the p-type if Na is added. For this reason, if using alkali glass which contains Na for the substrate, Na is supplied from the back surface side of the p-type CZTS-based light absorption layer and the ZnS-based fine particles are converted to p-types more. Due to this, a higher photovoltaic conversion efficiency can be realized.
The dispersed layer of ZnS-based fine particles is formed by forming the bottommost layer of the metal precursor film (metal back electrode layer side) by Zn, a mixture of Zn and Cu, a mixture of Zn and a Group VI element, or a mixture of Zn, Cu, and a Group VI element and sulfurizing and/or selenizing the metal precursor film at an ambient temperature of 540° C. to 600° C. for 5 minutes to 25 minutes. By 5 minute to 25 minute short sulfurization and/or selenization, the Zn or the mixture of Zn and Cu, the mixture of the Zn and a Group VI element, or the mixture of the Zn, Cu, and a Group VI element which is formed at the bottommost layer does not completely react to form a CZTS-based compound. Part remains as ZnS-based fine particles and forms the fine particles dispersed layer to thereby increase the photovoltaic conversion efficiency. Therefore, the present invention does not provide a passivation layer of an insulator different from the material of the solar cell such as in a crystalline solar cell. It can increase the photovoltaic conversion efficiency by just changing part of the steps in the conventional production process, so does not invite an increase in the production costs.
In general, the contact interface between the light absorption layer (semiconductor layer) and metal back electrode layer in a solar cell is a part which has a high density of dangling bonds and other crystal defects and where the speed of recombination of carriers becomes the fastest. Therefore, in a crystalline solar cell which is made using a conventional silicon wafer as a material, a point contact structure which uses a high quality passivation film is employed between the light absorption layer and the back electrode layer and reduction of the speed of recombination of carriers is sought.
However, in current CZTS-based thin film solar cells, realization of the point contact which is employed in crystalline solar cells is difficult. When applying the above art relating to crystalline silicon solar cells to a CZTS-based thin film solar cell, it is necessary to form a reduced defect, high quality insulating film (passivation film) between the semiconductor layer and electrode layer, but art for forming such an insulating film by a thin film has not yet been established. While there is a possibility of realization by future technical innovation, in that case as well, the production process would be complicated and an increase in manufacturing costs would be invited.
Therefore, in the present invention, a novel structure of a CZTS-based thin film solar cell and a method of production of the same which can realize an effect of inhibiting carrier recombination equal to or better than that of conventional point contact without using an insulating film are proposed.
In
“ZnS-based” indicates a compound of Zn (zinc) and a Group VI element, for example, ZnS, ZnSe, and ZnSSe. When forming a p-type CZTS-based light absorption layer 3 by only sulfurization, the ZnS-based fine particles become ZnS fine particles. When forming the p-type CZTS-based light absorption layer 3 by just selenization, the compound becomes ZnSe, while when performing both selenization and sulfurization, it becomes ZnSSe. Further, in the dispersed layer of ZnS-based fine particles, the ZnS-based compound does not cover the entire surface of the metal back electrode layer 2 as a layer. It is present as a group of fine particles. The clearances between the fine particles or in the group form paths of passage of holes. In this case, part of the metal back electrode layer 2 is not covered by the ZnS-based fine particle dispersed layer 30. Part of the layer may be exposed in state. The dispersed layer 30 of ZnS-based fine particles is, for example, formed to a thickness of up to 300 nm or so. The particles have sizes of 10 nm to 200 nmφ or so.
Usually, the p-type CZTS-based light absorption layer 3 is formed by forming the metal precursor film which contains Cu, Zn, and Sn on the metal back electrode layer 2, then sulfurizing and/or selenizing this a 500° C. to 650° C. atmosphere which contains hydrogen sulfide and/or hydrogen selenide. When sulfurizing a metal precursor film which contains Cu, Zn, and Sn in a hydrogen-sulfide containing atmosphere, a p-type CZTS-based light absorption layer 3 which is comprised of Cu2ZnSnS4 is formed. On the other hand, if selenizing the metal precursor film in a hydrogen selenide-containing atmosphere, a p-type CZTS-based light absorption layer 3 which is comprised of Cu2ZnSnSe4 is formed. Alternatively, by selenizing and sulfurizing the same metal precursor film, a p-type CZTS-based light absorption layer 3 which is comprised of Cu2ZnSn(S,Se)4 is formed.
The n-type high resistance buffer layer 4, for example, is a thin film of a compound which contains Cd, Zn, and In (film thickness 3 nm to 50 nm or so). Typically, it is formed by CdS, ZnO, ZnS, Zn(OH)2, In2O3, In2S3, or a mixed crystal of the same comprised of Zn(O,S,OH) or In(O,S,OH). This layer is generally formed by the chemical bath deposition (CBD), but as a dry process, metalorganic chemical vapor deposition (MOCVD) or atomic layer deposition (ALD) may also be applied. Note that, the “CBD” dips a base material in a solution which contains a chemical species forming a precursor and causes an uneven reaction to proceed between the solution and the surface of the base material to deposit a thin film on the base material. The n-type high resistance buffer layer 4 is not a layer which is necessarily required as a solar cell. An n-type transparent conductive film 5 may also be directly formed on the p-type CZTS-based light absorption layer 3.
The n-type transparent conductive film 5 is formed by a material which has an n-type of conductivity, has a broad bandgap, is transparent, and is low in resistance to a film thickness of 0.05 to 2.5 μm or so. Typically, there is a zinc oxide-based thin film (ZnO) or ITO thin film. In the case of a ZnO film, Group III elements (for example, Al, Ga, and B) can be added as a dopant to obtain a low resistance film. The n-type transparent conductive film 5 can be formed by sputtering (DC, RF) etc. other than MOCVD.
The inventors thought that by forming a dispersed layer 30 of ZnS-based fine particles at the interface of the p-type CZTS-based light absorption layer 3 and the metal back electrode layer 2 as shown in
Note that, it is important that the ZnS-based fine particles which are present at the interface between the metal back electrode layer 2 and p-type CZTS-based light absorption layer 3 be dispersed rather than layered. The clearance between the fine particles has to form paths for passage for holes 10 to reach the metal back electrode layer 2. Further, the more the ZnS-based fine particles are p-type in conductivity type, the more the effect as a barrier of electrons is exhibited. It is known that ZnS or another Zn(Group VI)-based semiconductor becomes the p-type if Na is added (see NPLT 1). Therefore, if using glass which contains Na as the substrate 1, Na is supplied by diffusion from the metal back electrode layer 2 side to the p-type CZTS-based light absorption layer 3 and therefore the ZnS-based fine particles contain a large amount of Na. Due to this, the ZnS-based fine particles are further converted to the p-type (higher concentration) and the effect as an electron barrier is increased.
Below, the method of effectively forming the dispersed layer 30 of ZnS-based fine particles will be explained. The inventors ran experiments on forming the above p-type CZTS-based light absorption layer 3 by changing the production conditions in various ways and as a result discovered that by adjusting the order of stacking of the metal precursor film and the temperature and time of sulfurization/selenization, it is possible to form this layer 30 and obtain a CZTS-based thin film solar cell with superior photovoltaic conversion efficiency without providing any special step for forming a dispersed layer 30 of ZnS-based fine particles. In particular, by forming as the bottommost layer of the metal precursor film (layer which contacts metal back electrode layer 2) Zn or a mixture of Zn and Cu, a mixture of Zn and a Group VI element, or a mixture of Zn, Cu, and a Group VI element (below, mixture which contains Zn) and further by shortening the sulfurization/selenization time of the metal precursor film, the photovoltaic conversion efficiency is greatly improved.
In the past, the sulfurization/selenization when producing a CZTS-based thin film solar cell was considered to require a considerably long time of 2 or 3 hours (for example, see PLT 1, paragraph 0031). In this regard, in the experiments of the inventors etc., sulfurization of a considerably shorter time than the conventional method of production, for example, 15 minutes or so, enabled improvement of the photovoltaic conversion efficiency in the CZTS-based thin film solar cell. Therefore, to confirm this effect, Experiments 1 to 3 which are shown in the following Table 1 were performed while changing the sulfurization conditions of the metal precursor film. In these experiments, the bottommost layer of the metal precursor film (on interface with metal back electrode layer) was formed by ZnS.
Note that, the VOC of the solar cell indicates the open voltage, Jsc indicates the short-circuit current density, and FF indicates the fill factor.
In Experiments 1 to 3, as the substrate 1, Na-containing alkali glass is used, but in Experiments 1 and 2, a thick alkali barrier layer is formed between the substrate 1 and the metal back electrode layer 2 to suppress the diffusion of Na from the substrate 1. On the other hand, in Experiment 3, a thin alkali barrier layer is formed between the substrate 1 and the metal back electrode layer 2. As a result, the p-type CZTS-based light absorption layer 3 of the sample which was formed in Experiment 3 contains a considerable amount of Na supplied from the substrate 1 through the metal back electrode layer 2.
As clear from Table 1, compared with the Experiment 1 sample with a sulfurization time of 45 minutes, in the Experiment 2 sample and the Experiment 3 sample with a sulfurization time of 15 minutes, large improvements are seen in the photovoltaic conversion efficiency. Furthermore, even with the same sulfurization time of 15 minutes, in the Experiment 3 sample with a high Na concentration, the photovoltaic conversion efficiency is further improved. In particular, in the Experiment 2 sample and the Experiment 3 sample, the short-circuit current density Jsc greatly increases compared with the Experiment 1 sample. The increase of the short-circuit current density Jsc greatly contributes to improvement of the photovoltaic conversion efficiency.
As clear from
The surface of
As a result, in the samples of Experiments 2 and 3 in which large improvements in photovoltaic conversion efficiency were seen, high temperature, short sulfurization treatment enabled the formation of a dispersed layer 30 of ZnS-based fine particles at the interface between the p-type CZTS-based light absorption layer 3 and the metal back electrode layer 2. On the other hand, in the Experiment 1 sample which has a low photovoltaic conversion efficiency, since the high temperature sulfurization treatment extended over a long period of time, it is learned that the Zn at the interface completely reacts to be uniformly contained in the layer of the p-type CZTS-based light absorption layer 3 as a whole and a dispersed layer 30 of ZnS-based fine particles is not formed. In this way, improvement of the photovoltaic conversion efficiency is governed by whether there is a dispersed layer 30 of ZnS-based fine particles present at the interface between the p-type CZTS-based light absorption layer 3 and the metal back electrode layer 2. Note that, in Experiment 3, the concentration of Na at the interface is higher than in the sample of Experiment 2. It is guessed that the ZnS fine particles of the Experiment 3 sample contain a larger amount of Na than the Experiment 2 sample. The photovoltaic conversion efficiency of the Experiment 3 sample is higher than the photovoltaic conversion efficiency of the Experiment 2 sample, so to obtain a high photovoltaic conversion efficiency, it is believed that a dispersed layer of ZnS-based fine particles which contains a larger amount of Na is desirable.
In the examples which are shown in Table 1, ZnS was used for the bottommost layer of the metal precursor film, but Table 2 shows the results of experiments for finding the optimum stacking order of the metal precursor film.
In Sample 4 and Sample 5, as shown in Table 2, the vapor deposited layer of ZnS is formed at the bottommost layer of the metal precursor film (interface with Mo metal back electrode layer 2), while in Sample 6 to Sample 9, it is formed at the middle layer or topmost layer of the metal precursor film. In each sample, the temperature of sulfurization of the metal precursor film was 560° C., while the sulfurization time was 15 minutes. Further, the rest of the film structure is based on the one which is shown in
In Samples 6 to 9, as shown in Table 2, a layer other than ZnS (Cu layer or Sn layer) is formed at the first layer of the metal precursor film 15 (bottommost layer), and a ZnS layer is formed as the second layer 12 or the third layer 13.
If a metal precursor film 15 is formed in this way, high temperature, short sulfurization is performed in an atmosphere which contains hydrogen sulfide to sulfurize the metal precursor film 15 and form a p-type CZTS-based light absorption layer 3. At this time, a dispersed layer 30 of ZnS-based fine particles is formed at the interface between the metal back electrode layer 2 and p-type CZTS-based light absorption layer 3.
As clear from Table 2, in Sample 4 or 5 where ZnS is formed at the first layer of the precursor film 15, the photovoltaic conversion efficiency is in the high 6% range in each case. On the other hand, in Samples 6 to 9 where ZnS is formed as the top layer or middle layer of the metal precursor film 15, the photovoltaic conversion efficiency is in the 2% range or 3% range. This is much lower than Samples 4 and 5. The sulfurization temperature is a high temperature of 560° C. and the sulfurization time is a short time of 15 minutes, so in Samples 4 and 5, a dispersed layer of ZnS fine particles is formed at the interface and, as a result, it is believed the photovoltaic conversion efficiency is improved.
On the other hand, in Sample 6 to Sample 9, the photovoltaic conversion efficiency is considerably low. Therefore, it is believed that a dispersed layer of ZnS fine particles is not sufficiently formed at the interface. This means that even if treating the metal precursor film by high temperature short sulfurization, if a vapor deposition layer of ZnS is not formed at the interface of the precursor film 15, a dispersed layer 30 of ZnS-based fine particles will be hard to form at the interface.
Note that, the layer which is formed as the first layer of the precursor film 15 does not necessarily have to be ZnS. For example, it may be Zn, ZnSe, ZnSSe, CuZn, CuZnS, CuZnSe, CuZnSSe, or other ingredient of Zn, a mixture of Zn and Cu, a mixture of Zn and a Group VI element, or a mixture of Zn, Cu, and a Group VI element (below, these called mixtures which contain Zn). Here, as the first layer of the precursor film 15, the example of Zn or a Zn mixture not containing a Group VI element was given, but the reason is that the energy of formation of a Zn (Group VI) compound is much lower compared with other compounds and formation is easy. This is because even if the first layer of the precursor film 15 does not contain a Group VI element, a dispersed layer 30 of ZnS-based fine particles can be formed in the later process of sulfurization and/or selenization. Further, when using a mixture which contains Zn to form the bottommost layer of the precursor film, the second layer or the third layer which is formed on that may also be Sn, SnS, SnSe, or SnSSe or Cu, CuS, CuSe, or CuSSe. Further, the second layer may be formed by any of CuSn, CuSnS, CuSnSe, and CuSnSSe. In this case, the third layer need not be formed. Furthermore, similar effects are obtained not only with high temperature short sulfurization, but also with high temperature short selenization and high temperature short sulfurization/selenization.
Table 3 shows the results of experiments which were run to clarify the effect which the high temperature short sulfurization of the precursor film and/or temperature and treatment time at the time of selenization have on the photovoltaic conversion efficiency.
The samples which are used in the experiments have a first layer of the metal precursor film 15 formed by a mixture which contains Zn, have glass substrates 1 using alkali-containing glass containing Na, and are provided with alkali barrier layers 6. The time in Table 3 shows the sulfurization time of the metal precursor film 15, while the temperature shows the sulfurization temperature. The structure of the film other than these are based on the one which is shown in
As clear from Table 3, when making the sulfurization temperature 580° C., it was possible to obtain the highest photovoltaic conversion efficiency with a sulfurization time of 5 minutes of 7.12%. When the sulfurization temperature is lowered to 560° C., it was possible to obtain the highest photovoltaic conversion efficiency with a sulfurization time of 15 minutes of 6.94%. This means that if simply shortening the treatment time, it not possible to obtain a CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency and that formation of a dispersed layer of ZnS-based fine particles requires a certain extent of heat. Further, overall, if the treatment time is within 25 minutes, a relatively high photovoltaic conversion efficiency can be obtained. If the treatment time exceeds 45 minutes, the dispersed layer of the ZnS-based fine particles is not effectively formed and the photovoltaic conversion efficiency is not improved.
Summarizing the above, by 1) forming the bottommost layer of the metal precursor film by a mixture which contains Zn and by 2) sulfurizing/selenizing the metal precursor film at a high temperature for a short time, a dispersed layer of ZnS-based fine particles is effectively formed at the interface and a CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency can be obtained.
As explained above, if comparing
The results are shown in the following Table 4.
The samples which were used for the experiments are provided with layers of mixtures which contain Zn (deposited layers) at the bottommost layers of the metal precursor films. Further, for the glass substrate 1, either alkali glass which contains Na: 3 to 13 wt % or alkali-free glass which does not contain Na is used. Between the glass substrate 1 and the Mo metal back electrode layer 2, an alkali barrier layer 6 which is formed by silica etc. is provided. This layer 6 is provided to keep the various types of metals or impurities which are contained in the glass substrate 1 from diffusing in the p-type CZTS-based light absorption layer 3 in the process of high temperature selenization of the precursor film 15.
The alkali barrier layer 6 is, for example, formed by RF sputtering using SiO2 as the target. In this case, by adjusting the power which is input to the RF sputter system, the thickness can be controlled. The alkali barrier layer 6 of the samples which are shown in Table 4 is one which is formed using a power input to the RF sputter system of 0.6 kW.
Table 4 classifies eight solar cell samples which have the above structure by the temperature of the sulfurization treatment, whether the glass substrate 1 is alkali glass or alkali-free glass, and the high temperature sulfurization treatment time, and shows the photovoltaic conversion efficiency of the same. From Table 4, it will be understood that in Samples 10 and 12 to 14 which use alkali glass for the glass substrate 1, a relatively high photovoltaic conversion efficiency is obtained. In particular, it was possible to obtain a high photovoltaic conversion efficiency from Sample 13 which was treated for sulfurization at 560° C. for 15 minutes. In Samples 11 and 15 to 17 which use alkali-free glass for the substrate 1, the photovoltaic conversion efficiency is considerably low. However, even in samples of alkali-free glass substrates, a relatively high photovoltaic conversion efficiency is exhibited with treatment at 560° C. for 15 minutes. The effect of the dispersed layer 30 of ZnS fine particles which is formed by high temperature short sulfurization can be confirmed in the same way as Na-containing alkali glass substrate.
Table 5 shows the results of experiments for detecting the effect which the film thickness of the alkali barrier layer 6 has on the photovoltaic conversion efficiency when using alkali-containing glass as the substrate 1.
In Table 5, the film thickness of the alkali barrier layer 6 is shown by the deposition power. The layer with a deposition power shown as 0.6 kW has the thinnest film thickness, while that shown as 2.0 kW has the thickest film thickness. “Na diffusion amount large” indicates that due to the film thickness of the alkali barrier layer being thin, a large amount of Na diffuses from the substrate 1 into the p-type CZTS-based light absorption layer 3. “Na diffusion amount small” and “Na diffusion amount very small” have similar meanings. The sulfurization treatment temperature, the film thickness of the alkali barrier layer, and the sulfurization treatment temperature were changed to form six solar cell samples. These were measured for photovoltaic conversion efficiency. Note that, in the case of these experiments as well, a Zn-containing layer was formed at the bottommost layer of the metal precursor film and a glass substrate 1 constituted by alkali-containing glass which contains Na: 3 to 13 wt % was used.
From Table 5, it will be understood that the thinner the film thickness of the alkali barrier layer, the higher the photovoltaic conversion efficiency. This is believed to be because by the alkali barrier layer being thin, the content of Na of the p-type CZTS-based light absorption layer 3, in particular near the interface with metal back electrode layer, becomes greater and the electron barrier effect which is shown in
Summarizing the above, to effectively form a dispersed layer of ZnS-based fine particles to obtain a CZTS-based thin film solar cell with a high photovoltaic conversion efficiency, it is understood that it is important to 1) form a layer which contains Zn as a bottommost layer of the metal precursor film, 2) perform treatment in the sulfurization/selenization at a high temperature (540° C. to 600° C.) for a short time (5 minutes to 25 minutes), and 3) raise the Na concentration near the interface of the p-type CZTS-based light absorption layer and the metal back electrode layer.
Table 6 shows an example of the method of production, film thickness, etc. other than that explained above for the CZTS-based thin film solar cell of
Note that, as the substrate 1, in addition to blue sheet glass, low alkali glass, or other glass substrate, a stainless steel sheet or other metal substrate, polyimide resin substrate, etc. may be used. As the method of formation of a metal back electrode layer 2, in addition to the DC sputtering which is described in Table 6, there are electron beam deposition, atomic layer deposition (ALD), etc. As the material of the metal back electrode layer 2, a high corrosion resistance and high melting point metal, for example, chromium (Cr), titanium (Ti), etc. may be used.
The n-type transparent conductive film 5 is formed using a material which has an n-type of conductivity, has a broad band gap, is transparent, and has a low resistance to a film thickness of 0.05 to 2.5 μm or so. Typically, there is a zinc oxide-based thin film (ZnO) or ITO thin film. In the case of a ZnO film, a Group III element (for example, Al, Ga, and B) is added as a dopant to obtain a low resistance film. An n-type transparent conductive film 5 can be formed by sputtering (DC, RF) etc. in addition to MOCVD.
1 glass substrate
2 metal back electrode layer
3 p-type CZTS-based light absorption layer
4 n-type high resistance buffer layer
5 n-type transparent conductive film
6 alkali barrier layer
10 hole
12 first layer of metal precursor film
13 second layer of metal precursor film
14 third layer of metal precursor film
15 metal precursor film
20 electrons
30 dispersed layer of ZnS-based fine particles
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-257893 | Nov 2011 | JP | national |
This application is a U.S. National Phase patent application of PCT/JP2012/080347, filed on Nov. 22, 2012, which claims priority to Japanese Patent Application No. 2011-257893, filed on Nov. 25, 2011, each of which is hereby incorporated by reference in the present disclosure in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2012/080347 | 11/22/2012 | WO | 00 |
