JP6529326B2 - Compound semiconductor thin film solar cell and method of manufacturing the same - Google Patents

Description

  The present invention relates to a compound semiconductor thin film solar cell and a method of manufacturing the same, and more particularly, to a compound semiconductor thin film solar cell which improves photoelectric conversion efficiency by providing a window layer having a novel structure, and a method of manufacturing the same.

A CIS-based thin film solar cell or a CIGS-based thin film solar cell using a chalcopyrite I-III-VI group ₂ compound semiconductor containing Cu, In, Ga, Se, or S as a p-type light absorbing layer is generally a glass A substrate, a metal back electrode made of Mo or the like formed on a glass substrate, a p-type light absorbing layer formed on the metal back electrode layer, and a window layer formed on the p-type light absorbing layer It is comprised by the n-type transparent conductive film formed on the window layer. A very thin n-type high resistance buffer layer (tens of nm) may be formed between the p-type light absorbing layer and the window layer.

  In this structure, the p-type light absorbing layer formed of a CIS-based or CIGS-based (hereinafter referred to as CIS-based) material is an n-type compound material having a narrow energy band gap and a wide energy band gap. A pn hetero junction is formed between the formed window layers, whereby a solar cell is configured. In such a solar cell, in order to obtain high photoelectric conversion efficiency, 1) optimize the optical design of the window layer, p-type light absorption layer, etc. to generate more photoexcitation carriers, and 2) further In order to suppress recombination of generated carriers, it is necessary to optimize the electrical structure of the window layer and the p-type light absorbing layer.

  Here, by using a ZnS and CdS gradient composition layer (ZnCdS thin film) as the window layer, an internal electric field is generated in the window layer, and the photocarrier generated in the window layer is the interface between the window layer and the p-type light absorption layer It is known that the photoelectric conversion efficiency of a solar cell can be improved by reaching the depletion layer of (see Patent Document 1). However, in this case, it is necessary to provide a region where the energy band gap of the window layer narrows in order to generate an internal electric field in the window layer, and the short wavelength sensitivity of the solar cell is determined by the energy band gap of this region. Therefore, there is a problem that a sufficiently large photocurrent can not be extracted.

  Furthermore, a solar cell is proposed in which an internal electric field is generated in the window layer by laminating the three ZnO layers having different impurity concentrations to form the window layer, and the photoelectric conversion efficiency is improved (Patent Document 1) 2). However, in this solar cell, the energy band gaps of the three ZnO layers are different from one another, and hence the short wavelength sensitivity is determined by the ZnO layer having the narrowest energy band gap among the three ZnO layers. As a result, carriers generated in the vicinity of the light receiving surface side surface of the window layer can not be effectively extracted as a photocurrent. Moreover, in order to form a multilayer window layer, it also has the fault which a manufacturing process becomes complicated.

Unexamined-Japanese-Patent No. 5-218480 JP-A-8-330609

  The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to improve the photoelectric conversion efficiency of a compound semiconductor thin film solar cell by providing a window layer of a novel structure and a method for producing the same. I assume.

  In a first aspect of the present invention, in order to solve the problems described above, a substrate, a first electrode layer formed on the substrate, and a p of a compound semiconductor formed on the first electrode layer Window layer formed on the p-type light absorption layer and having an energy band gap wider than the p-type light absorption layer and forming a pn hetero junction at the interface with the p-type light absorption layer And a transparent electrode layer formed on the window layer, wherein the energy band gap of the window layer is constant in the thickness direction of the window layer, and the window layer has a Fermi level. In the compound semiconductor thin film solar cell, the semiconductor device is inclined in a direction approaching the lower end of the conduction band from the interface toward the light receiving surface side of the window layer.

  In the first aspect, the slope of the Fermi level in the window layer may be larger in the vicinity of the light receiving side surface of the window layer than in the vicinity of the interface of the p-type light absorption layer.

  Further, the inclination of the Fermi level in the window layer may be formed from a surface on the light receiving surface side of the window layer to a half of the film thickness direction in the film thickness direction of the window layer. .

  Further, the p-type light absorption layer may be formed of a CIS-based semiconductor or a CIGS-based semiconductor, and the window layer may be formed of non-doped ZnO.

  In addition, an n-type high resistance buffer layer may be provided between the window layer and the p-type light absorbing layer.

  Further, the buffer layer may be made of Zn (S, O, OH).

  In a second aspect of the present invention, in order to solve the above-mentioned problems, a metal back electrode layer is formed on a substrate, and a metal of a laminated structure or mixed crystal containing Cu, In, Ga on the metal back electrode layer. A precursor film is formed, the metal precursor film is selenized and / or sulfurized to form a p-type light absorption layer, and a buffer layer of Zn (S, O, OH) is formed on the p-type light absorption layer Forming a window layer of ZnO on the buffer layer by MOCVD using DEZ and water, and forming a transparent electrode on the window layer, wherein the window layer is formed The present invention provides a method for producing a compound semiconductor thin film solar cell, characterized in that the method is performed by reducing the ratio of water to DEZ with the passage of film forming time.

  In the third aspect of the present invention, in order to solve the above-mentioned problems, a metal back electrode layer is formed on a substrate, and a metal of a laminated structure or mixed crystal containing Cu, In, Ga on the metal back electrode layer. A precursor film is formed, the metal precursor film is selenized and / or sulfurized to form a p-type light absorption layer, and a buffer layer of Zn (S, O, OH) is formed on the p-type light absorption layer Forming a window layer of ZnO on the buffer layer by MOCVD method or ALD method, and forming a transparent electrode on the window layer, the window layer is formed by forming the window layer Provided is a method for producing a compound semiconductor thin film solar cell, characterized in that the substrate temperature is raised with the passage of time.

  In the fourth aspect of the present invention, in order to solve the above-mentioned problems, a metal back electrode layer is formed on a substrate, and a metal of a laminated structure or mixed crystal containing Cu, In, Ga on the metal back electrode layer. A precursor film is formed, the metal precursor film is selenized and / or sulfurized to form a p-type light absorption layer, and a buffer layer of Zn (S, O, OH) is formed on the p-type light absorption layer Forming a window layer of ZnO on the buffer layer by MOCVD or ALD, and forming a transparent electrode on the window layer, wherein the window layer is formed at a wavelength of 300 nm. The method for producing a compound semiconductor thin film solar cell is characterized in that the irradiation is performed while irradiating the following short wavelength light to the substrate, and the irradiation intensity of the short wavelength light is increased with the passage of film forming time. Do.

  In the compound semiconductor thin film solar cell of the present invention, the energy band gap of the window layer is constant in the film thickness direction of the window layer, and the Fermi level approaches the lower end of the conduction band toward the light receiving surface side surface of the window layer. The internal electric field is formed by the inclination. Therefore, the photocarriers generated near the surface on the light receiving surface side of the window layer can be efficiently extracted as photocurrent, and a solar cell having high photoelectric conversion efficiency can be obtained.

  Further, in the film thickness direction of the window layer, a portion having a slope and a portion having a constant Fermi level are provided, and the portion having the slope is the side opposite to the p-type light absorbing layer side of the window layer, ie, the window layer By arranging on the light receiving surface side, the photocarrier can be extracted more efficiently as photocurrent.

The window layer is formed by MOCVD method (metal organic chemical vapor deposition method) using DEZ (diethyl zinc (Zn (C ₂ H ₅ ) ₂ )) and water, and at this time, water is formed along with the film forming time. The concentration of oxygen defects is increased on the surface side of the window layer, and the carrier concentration is increased. As a result, the Fermi level inclines toward the lower end of the conduction band toward the surface side of the window layer. As a result, an internal electric field is generated in the window layer, and carriers generated by light absorption near the surface of the window layer by the internal electric field are smooth to the depletion region formed at the interface between the window layer and the p-type light absorption layer. To improve the photoelectric conversion efficiency of the solar cell.

  Further, by raising the substrate temperature in the process of film formation by the MOCVD method or ALD method of the window layer, it is possible to provide the window layer with a slope of the Fermi level while maintaining the energy band gap constant. The ZnO film formed at high temperature has a high carrier concentration, and the Fermi level is closer to the lower end of the conduction band.

  Furthermore, in the process of film formation by the MOCVD method or ALD method of the window layer, the window layer can be provided with a Fermi level inclination by irradiating the substrate with short wavelength light (wavelength 300 nm or less) while increasing the intensity. it can. The ZnO film formed under irradiation of short wavelength light has a high carrier concentration and the Fermi level is closer to the lower end of the conduction band.

BRIEF DESCRIPTION OF THE DRAWINGS The figure which shows schematic structure of the compound semiconductor thin film solar cell which concerns on one Embodiment of this invention. The figure which shows the energy band structure of a window layer and a p-type light absorption layer in the compound semiconductor thin film solar cell which concerns on one Embodiment of this invention. The figure which shows the energy band structure of a window layer and a p-type light absorption layer in the compound semiconductor thin film solar cell of conventional structure. In ZnO film forming process, a graph showing the relationship between the H ₂ O / DEZ ratio and the carrier concentration in the film. The graph which shows the relationship between the film forming temperature and the carrier concentration in a film | membrane in the ZnO film forming process.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, in order to facilitate understanding, the thickness of each layer is shown in a relation different from the actual one.

  FIG. 1 is a schematic cross-sectional view showing the structure of a compound semiconductor thin film solar cell according to an embodiment of the present invention. In FIG. 1, 1 is a glass substrate, 2 is a metal back electrode layer made of metal such as Mo, 3 is a p-type CIS light absorbing layer, 4 is a buffer layer, 5 is a window layer, 6 is a transparent electrode And an n-type transparent conductive film. The p-type light absorption layer 3 is formed, for example, by forming a metal precursor film of a laminated structure or mixed crystal containing Cu, In, and Ga on the metal back electrode layer 2 by a sputtering method, a vacuum evaporation method, etc. And / or sulfided. The buffer layer 4 and the window layer 5 have an n-type conductivity as a whole, and a pn hetero junction is formed with the p-type light absorption layer 3.

  The buffer layer 4 is a transparent n-type high-resistance layer having a composition of, for example, Zn (S, O, OH) (mixed crystal), and has a very thin film thickness (2 nm to 50 nm). The buffer layer 4 can be deposited by a solution growth method, an MOCVD method, or the like.

  The window layer 5 is entirely formed of, for example, non-doped ZnO. Therefore, the energy band gap of the window layer 5 is constant in the layer thickness direction. On the other hand, in the window layer 5 of the present invention, the Fermi level is inclined in the film thickness direction. Specifically, the Fermi level has a slope which approaches the lower end of the conduction band in the film thickness direction of the window layer 5 as it goes away from the p-type CIS-based light absorption layer 3 side. The inclination of the Fermi level does not have to be constant in the film thickness direction of the window layer, and has a large inclination on the light receiving surface side, while the Fermi level is constant on the p-type CIS light absorption layer 3 side. You may keep the position.

  FIG. 2 shows the energy band structure of the window layer 5 of the solar cell and the p-type CIS light absorbing layer 3 according to one embodiment of the present invention, and FIG. 3 shows the window of the solar cell provided with the window layer of the conventional structure. The energy band structure of a layer and a p-type CIS type | system | group light absorption layer part is shown.

2 and 3 show UPS analysis results of the valence band upper end (VBM) using VersaProbe manufactured by PHI, and the sputtering time on the horizontal axis represents the integration time of sputtering, and the vertical axis is the vertical axis , VBM and CBM (the lower end of the conduction band) are represented by the difference from the Fermi level. The sputtering conditions are Ar ⁺ ion, 3.0 kV. Since the film thickness of the window layer is about 150 nm, the sputtering rate is calculated to be less than 1 nm / min. In FIGS. 2 and 3, plots A and a are plots of UPS analysis results of the top of the valence band (VBM) using a VersaProbe manufactured by PHI.

  The ultraviolet light source at the time of UPS analysis is I (21.22 eV) of He, the detection area of UPS analysis is 8 mm square or less, and the detection depth is about 1 nm. By alternately performing sputter etching for 1 minute and UPS measurement, a depth profile of VBM is obtained. In FIGS. 2 and 3, plots B and b represent the lower end of the conduction band (CBM), and the calculated values of VBM plus the value of the energy band gap are plotted.

  As shown in FIG. 2, in the structure of the present invention, the window layer 5 extends from the light receiving surface side (the transparent electrode 6 side, the left end of the window layer 5 in FIG. 2) to a thickness of about 60 nm (sputtering time about 50 min) There is a slope to Since the material of the entire window layer 5 is non-doped ZnO, the energy band gap is constant in the film thickness direction, and as a result, the CBM has a similar slope. Due to the presence of this inclination, that is, the internal electric field, carriers generated by light absorption near the surface of the window layer 5 are smooth to the depletion layer region formed near the interface between the window layer 5 and the light absorption layer 3. And as a result, the photocurrent extracted from the solar cell is increased. Further, the presence of the internal electric field in the window layer 5 suppresses recombination in the window layer 5. Therefore, higher voltage can be obtained.

  The VBM or CBM may have a slope in the entire region of the window layer 5, but as shown in FIG. 2, the surface side (the opposite side to the light absorbing layer, the portion surrounded by the circle in FIG. Further, the following effects can be obtained by limiting the inclination to a region of approximately 1/2 or less of. That is, when the Fermi level is located at an energy level far from the conduction band, the carrier concentration in the window layer 5 is low and the electrical resistance is high. Therefore, when the VBM or CBM is inclined in the entire region of the window layer 5, the conversion efficiency of the solar cell may be reduced due to the increase in the electrical resistance of the window layer 5 in the vicinity of the light absorption layer 3. is there. On the other hand, in the case of the energy band structure of FIG. 2 in which the inclined region is at most about half the film thickness or less from the surface side of the window layer, the electrical resistance of the window layer 5 on the light absorption layer 3 side is increased. The level can be suppressed to a certain level, and a decrease in conversion efficiency can be avoided.

  The size of the slope of the Fermi level in the window layer varies depending on the material of the window layer, but in the case of a non-doped ZnO window layer, it is about 0.5 eV at a thickness of 60 nm.

  The solar cell used in the experiment of FIG. 2 has a structure of p-type CIS light absorbing layer / Zn (S, O, OH) layer / non-doped ZnO layer, where Zn (S, O, OH) The layer functions as an n-type high resistance buffer layer, and the non-doped ZnO layer functions as a window layer. In general, the n-type high-resistance buffer layer 4 has a very thin film thickness, and therefore, UPS analysis does not give accurate results. However, it is apparent that the entire n-type high resistance buffer layer 4 and the window layer 5 have n-type conductivity and have the energy band structure shown in FIG.

  FIG. 3 shows the energy band structure of the window layer and the p-type CIS based light absorbing layer according to the prior art for comparison with the solar cell according to the present invention. The measurement of the upper end (VBM) of the valence band and the calculation of the lower end of the conduction band (CBM) are performed in the same manner as the measurement and calculation of FIG. The solar cell used for the measurement of FIG. 3 has a structure of p-type CIS-based light absorption layer / non-doped ZnO window layer.

  As shown in FIG. 3, in the solar cell of the conventional structure, the energy band gap of the window layer is constant in the film thickness direction, and unlike the present invention, the plot a of the valence band upper end (VBM) is a film An almost constant energy value is maintained in the thickness direction. Therefore, there is no slope in the Fermi level, and as a result there is no internal electric field in the window layer. Therefore, carriers generated by light absorption in the vicinity of the light receiving surface side surface of the window layer can not smoothly move to the depletion layer region formed in the vicinity of the interface between the window layer and the light absorption layer. The photoelectric conversion efficiency of the solar cell can not be improved.

  Below, the manufacturing method of the window layer especially the compound semiconductor thin film solar cell which concerns on one Embodiment of this invention shown in FIG.1 and FIG.2 is demonstrated in comparison with the compound semiconductor thin film solar cell which concerns on a prior art.

In the prior art, for example, a ZnO film which is a window layer is formed by MOCVD using water and diethylzinc (Zn (C ₂ H ₅ ) ₂ , hereinafter, DEZ) as materials. That is, diethyl zinc (Zn (C ₂ H ₅ ) ₂ , hereinafter DEZ) and water are reacted in the gas phase to form a non-doped ZnO layer on the p-type CIS light absorbing layer or the high resistance buffer layer. In this gas phase reaction, in the prior art, the window layer is formed at a constant DEZ: water ratio.

On the other hand, in the present invention, the window layer 5 is manufactured by the following method.
a) When the ZnO window layer 5 is formed by MOCVD using water and diethylzinc (Zn (C ₂ H ₅ ) ₂ , hereinafter, DEZ) as a material, the film formation time elapses in the film thickness direction. Change the ratio of DEZ: water according to. More specifically, the concentration of oxygen defects is increased on the surface side of the film by decreasing the introduction amount of water with the passage of time, and the Fermi level is inclined toward the conduction band.
b) Another production method is a method of raising the substrate temperature with the passage of film forming time. The ZnO film formed at a high temperature has a high carrier concentration, and the Fermi level is closer to the conduction band, so that the gradient of the Fermi level can be formed in the film thickness direction by the temperature change during the film formation. This method is applicable not only to the formation of a ZnO window layer by MOCVD but also to the formation of a ZnO window layer by ALD.

  c) Another production method is a method of increasing the irradiation intensity of short wavelength light (about 300 nm or less) to the substrate with the passage of film forming time. The ZnO film formed under irradiation of short wavelength light has a high carrier concentration, and the Fermi level is closer to the conduction band. Therefore, the Fermi level in the film thickness direction is adjusted by adjusting the irradiation intensity of short wavelength light in the film formation. Can form a gradient of This method is also applicable not only to the formation of a ZnO window layer by MOCVD but also to the formation of a ZnO window layer by ALD.

FIG. 4 is a graph showing the effects of the manufacturing method a) described above, in which the molar ratio of water to DEZ is changed in forming a ZnO film by MOCVD, and the carrier concentration in the ZnO film is measured at that time. It is. The vertical axis of the figure shows the carrier concentration (/ cm ³ ) in the ZnO film, and the horizontal axis of the figure shows H ₂ O / DEZ in molar ratio. As is clear from this graph, it can be seen that the carrier concentration of the ZnO film increases as the amount of water introduced is reduced (the molar ratio is reduced).

Table 1 shows details of the H ₂ O / DEZ molar ratio and the carrier concentration at each measurement point shown in FIG.

  An increase in carrier concentration means that the Fermi level approaches the lower end of the conduction band. Therefore, when depositing the window layer 5 of ZnO using the MOCVD method, the carrier concentration can be increased in the surface direction of the window layer 5 by gradually reducing the ratio of water to DEZ. it can. As a result, the Fermi level can be inclined toward the surface of the window layer 5 toward the lower end of the conduction band while keeping the energy band gap of the window layer 5 constant.

  FIG. 5 is a graph showing the effect of the above-mentioned production method b), particularly showing the effect of forming a ZnO film by ALD (Atomic layer Deposition, atomic layer deposition). The vertical axis of the figure indicates the carrier concentration of the ZnO film formed by the ALD method, and the horizontal axis indicates the film forming temperature. As apparent from this graph, when the film forming temperature of ZnO is increased, the carrier concentration of the ZnO film is increased. The high carrier concentration means that the Fermi level is close to the conduction band. Therefore, when forming the ZnO window layer 5, the film formation temperature is gradually increased to keep the energy band gap of the ZnO window layer 5 constant, while the Fermi level is directed toward the surface of the ZnO window layer 5. Can be inclined toward the lower end of the conduction band.

Table 2 shows details of the film forming temperature and the carrier concentration at each measurement point shown in FIG.

  In the above embodiment, the window layer 5 is formed of ZnO because the energy band gap of ZnO is as wide as 3.2 eV, and a relatively large photocurrent can be extracted. According to the optical design of the solar cell, it is clear that various materials can be considered as the window layer, and therefore the present invention does not limit the material of the window layer 5 to non-doped ZnO.

Although the substrate 1 is a glass substrate in the embodiment shown in FIG. 1, the present invention is not limited to the glass substrate, and it goes without saying that a metal substrate or the like can also be used. Furthermore, although the layer 3 is shown as a p-type CIS-based light absorption layer, specific examples of this material include Cu (In, Ga) Se ₂ , Cu (In, Ga) (Se, S) ₂ , CuInS ₂ Etc. The transparent electrode 6 may be formed of ITO, B, Al or ZnO doped with Ga or the like.

1 substrate 2 first electrode layer (metal back electrode layer)
3 p-type CIS light absorbing layer 4 n-type high-resistance buffer layer 5 non-doped ZnO layer 6 transparent electrode

Claims (8)

A substrate,
A first electrode layer formed on the substrate;
A p-type light absorption layer of a compound semiconductor formed on the first electrode layer;
A window layer formed on the p-type light absorption layer, having an energy band gap wider than the p-type light absorption layer, and forming a pn hetero junction at the interface with the p-type light absorption layer;
A transparent electrode layer formed on the window layer, in a compound semiconductor thin film solar cell,
The window layer, the energy band gap is constant in the thickness direction of the window layer is inclined in a direction in which the Fermi level is closer to the conduction band toward the light-receiving surface side of the window layer from the interface ,
The compound semiconductor thin film solar cell , wherein the slope of the Fermi level in the window layer is larger on the light receiving surface side of the window layer than on the p-type light absorption layer side . A substrate,
A first electrode layer formed on the substrate;
A p-type light absorption layer of a compound semiconductor formed on the first electrode layer;
A window layer formed on the p-type light absorption layer, having an energy band gap wider than the p-type light absorption layer, and forming a pn hetero junction at the interface with the p-type light absorption layer;
A transparent electrode layer formed on the window layer, in a compound semiconductor thin film solar cell,
The window layer, the a constant energy band gap in the film thickness direction of the window layer, the half of the portion from the light-receiving surface side surface of the thickness of the window layer in the thickness direction before Kimadoso The compound semiconductor thin film solar cell whose Fermi level inclines in the direction which approaches a conduction band lower end toward the light-receiving surface side of the said window layer from the said interface . The compound semiconductor thin film solar cell according to claim 1 or 2 , wherein the p-type light absorption layer is formed of a CIS-based semiconductor or a CIGS-based semiconductor, and the window layer is formed of non-doped ZnO. Semiconductor thin film solar cells. The compound semiconductor thin film solar cell according to any one of claims 1 to 3 , further comprising a buffer layer formed between the window layer and the p-type light absorbing layer. The compound semiconductor thin film solar cell according to claim 4 , wherein the buffer layer is made of Zn (S, O, OH). Forming a metal back electrode layer on the substrate,
Forming a metal precursor film of a laminated structure or mixed crystal containing Cu, In, Ga on the metal back electrode layer,
The metal precursor film is selenized and / or sulfurized to form a p-type light absorbing layer,
Forming a buffer layer of Zn (S, O, OH) on the p-type light absorbing layer;
A window layer of ZnO is formed on the buffer layer by MOCVD using DEZ and water,
In the manufacturing method of the compound semiconductor thin film solar cell which forms a transparent electrode on the said window layer,
The method for manufacturing a compound semiconductor thin film solar cell, wherein the formation of the window layer is performed by decreasing the ratio of water to DEZ with the passage of film forming time. Forming a metal back electrode layer on the substrate,
Forming a metal precursor film of a laminated structure or mixed crystal containing Cu, In, Ga on the metal back electrode layer,
The metal precursor film is selenized and / or sulfurized to form a p-type light absorbing layer,
Forming a buffer layer of Zn (S, O, OH) on the p-type light absorbing layer;
Forming a window layer of ZnO on the buffer layer by MOCVD or ALD;
In the manufacturing method of the compound semiconductor thin film solar cell which forms a transparent electrode on the said window layer,
The method for manufacturing a compound semiconductor thin film solar cell, wherein the formation of the window layer is performed by raising the film formation temperature with the passage of a film formation time. Forming a metal back electrode layer on the substrate,
Forming a metal precursor film of a laminated structure or mixed crystal containing Cu, In, Ga on the metal back electrode layer,
The metal precursor film is selenized and / or sulfurized to form a p-type light absorbing layer,
Forming a buffer layer of Zn (S, O, OH) on the p-type light absorbing layer;
Forming a window layer of ZnO on the buffer layer by MOCVD or ALD;
In the manufacturing method of the compound semiconductor thin film solar cell which forms a transparent electrode on the said window layer,
The compound is characterized in that the formation of the window layer is performed while irradiating the substrate with short wavelength light having a wavelength of 300 nm or less, and the irradiation intensity of the short wavelength light is increased with the lapse of film forming time. Method of manufacturing a semiconductor thin film solar cell