Solar cell and manufacturing method therefor

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a method for manufacturing a solar cell, specifically, a method for manufacturing a solar cell having excellent photoelectric conversion efficiency.

【0002】

[Conventional technology]

In recent years, due to concerns about the depletion of energy resources in the future, great interest has been focused on improving the efficiency and cost of solar cells that use the sun as an energy source. Among them, compound semiconductor solar cells, which can easily increase the area, can be significantly reduced in cost, and therefore, improvement in energy conversion efficiency is strongly desired. As a compound semiconductor solar cell, a solar cell using an n-type CdS film with a wide bandgap as a window layer and a p-type CdTe film with a narrow bandgap as an absorption layer (hereinafter referred to as CdS / CdTe solar cell). Etc. are known.

【0003】

Here, FIG. 1 shows a schematic cross-sectional view of the CdS / CdTe solar cell, and the conventional method for manufacturing the CdS / CdTe solar cell will be described with reference to the schematic cross-sectional view. First, a transparent conductive film 2 made of tin dioxide-doped indium oxide (ITO) or fluorine-doped tin dioxide (FTO) is sequentially formed on a translucent substrate 1 such as a glass substrate, and then a CdS layer 3 is formed. .. Then, a CdTe layer 4 having a thickness of 1 to 6 μm is formed on the CdS layer 3 by a proximity sublimation method or the like. Next, a carbon electrode 5 is formed on the CdTe layer 4, and an AgIn electrode is formed on the CdS layer 3 and the carbon electrode 5 as a current collector to form a solar cell element.

【0004】

In such a CdS / CdTe solar cell, the carbon electrode plays a role of collecting the photocurrent generated near the CdS layer / CdTe layer interface. Therefore, the configuration of the carbon electrode / CdTe layer interface is a very important point for improving the performance of the solar cell. In other words, among the generated photocurrents, it is important to efficiently collect holes, which are p-type carriers, at the carbon electrode / CdTe layer interface. Therefore, the surface of the CdTe layer on the side where the carbon electrode is formed has a higher p-type carrier. It is desirable to have a concentration.

【0005】

From this point of view, in order to improve the conversion efficiency of the CdS / CdTe solar cell, a technique of mainly modifying the surface of the CdTe layer to increase the Te concentration has been proposed. For example, Effect of nitric-phosphoric acid etches on material properties and back-contact formation of CdTe-based solar cells (Xiaonan Li, David W. Niles, Falah S. Hasoon, Richard J. Matson, and Peter Sheldon: J. Vac. Sci. Technol. A 17 (3), p.805-809) discloses a technique for selectively etching elements on the surface of a CdTe film before forming a current collector electrode. Specifically, the open-end voltage of the obtained solar cell is increased by modifying the surface of the CdTe film by using an acidic solution of phosphorus nitride as the etching solution. However, the selective etching of the semiconductor surface as described above is a process utilizing a chemical reaction and involves a special and complicated process such as using a solution.

【0006】

[Problems to be Solved by the Invention]

An object of the present invention is to provide a method for manufacturing a solar cell, which can inexpensively increase the p-type carrier concentration on the surface of a p-type semiconductor layer by a simple method. In particular, in CdS / CdTe solar cells, the Te concentration on the surface of the CdTe layer is increased cheaply by a simpler method than before, the p-type carrier concentration on the surface of the CdTe layer is improved, and the current is generated at the CdS layer / CdTe layer interface. The purpose is to efficiently collect the generated photocurrent and improve the conversion efficiency of the solar cell. Furthermore, it is an object of increasing the adhesion strength at the carbon electrode / CdTe layer interface to obtain a highly reliable CdS / CdTe solar cell.

【0007】

[Means for solving problems]

The present invention relates to (1) a step of forming an n-type semiconductor layer on a conductive film formed on a translucent substrate, (2) a step of forming a p-type semiconductor layer on the n-type semiconductor layer, and ( 3) The present invention relates to a method for manufacturing a solar cell having a step of applying a solution containing a melting point lowering agent and a Group 6 element to the surface of the p-type semiconductor layer and heat-treating it. Here, it is preferable to use cadmium telluride as the p-type semiconductor layer, cadmium halide as the melting point lowering agent, and Te element as the Group 6 element. Further, as the halogenated cadmium, cadmium chloride is preferable. The concentration of the Group 6 element in the solution containing the melting point lowering agent and the Group 6 element is preferably 0.01 to 3 mol / liter. In the present invention, the molar ratio of tellurium atoms to cadmium atoms on the surface of the p-type semiconductor layer is 1.5 to 2.5, and the p-type carrier concentration is 10.¹⁸cm^-3The above is related to solar cells.

【0008】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the method for manufacturing the solar cell of the present invention will be described with reference to FIG. First, an n-type semiconductor layer is formed on the transparent conductive film 2 provided on the translucent substrate 1. As the translucent substrate, a substrate having a property of transmitting light used for photoelectric conversion at the pn junction can be used without particular limitation, and for example, a glass substrate is preferable. The thickness of the substrate is preferably 1 to 5 mm.

【0009】

A conductive film having the same translucency is formed on the surface of the translucent substrate. As the conductive film, in addition to having high light transmittance, it is preferable that the conductive film has a low resistance, and a conductive film having a resistance of 20 Ω / or less is particularly preferable. The thickness of the conductive film is preferably 200 to 600 nm.

【0010】

As the conductive film, for example, In₂O₃, In₂O₃ SnO₂(ITO), Fluorine-doped SnO₂(FTO), Antimony-doped SnO₂, In, Al, B, F, Ga or Si-doped ZnO, Cd₂In₂O₄, Cd₂SnO₄, CdO, Cd₂SnO₄, Zn₂SnO₄, In₂O₃-ZnO and so on. These may be used alone or in combination of two or more. Among them, In because it has higher light transmission and lower resistance.₂O₃, ITO, FTO, Antimony-doped SnO₂, In, Al or Si doped ZnO, Cd₂In₂O₄, Cd₂SnO₄Is preferable. These conductive films may be formed by conventionally known methods such as a spray method, a normal pressure CVD (chemical vapor deposition) method, a reduced pressure CVD method, a vacuum vapor deposition method, and a sputtering method so as not to impair the original effects of the solar cell. ..

【0011】

Examples of the n-type semiconductor include cadmium sulfide (CdS), zinc sulfide (ZnS), cadmium oxide (CdO), zinc oxide (ZnO) and the like. Among them, it is preferable to use CdS because it has a wide bandgap. If the n-type semiconductor layer is formed by, for example, a coating sintering method, an electrolysis method, a sputtering method, a solution growth method, a chemical vapor deposition method, an organic metal pyrolysis method, a vapor deposition method, a proximity sublimation method, a spray method, or the like. Good. The thickness of the n-type semiconductor layer is not particularly limited as long as it does not impair the original effect of the solar cell, but is preferably 50 to 130 nm.

【0012】

Next, a p-type semiconductor layer is formed on the n-type semiconductor layer. The p-type semiconductor is not particularly limited as long as it forms a pn junction with the n-type semiconductor, but for example, cadmium telluride (CdTe) and CuInSe.₂, CuInGaSeS and the like. Of these, it is preferable to use CdTe because it is suitable for combination with CdS. Here, CdTe has lattice defects when nothing is mixed in, and when elements such as copper, silver, phosphorus, lithium, and sodium are mixed in the cadmium lattice position, those elements are p-type carriers. It causes generation. The p-type semiconductor layer can be formed by a conventionally known method such as a coating sintering method, an electrolysis method, a sputtering method, a solution growth method, a chemical vapor deposition method, an organic metal pyrolysis method, a vapor deposition method, a proximity sublimation method, or a spray method. It may be formed. The thickness of the p-type semiconductor layer is not particularly limited as long as it does not impair the original effect of the solar cell, but is preferably 1 to 8 μm.

【0013】

The most preferable combination of the n-type semiconductor layer and the p-type semiconductor layer is a combination of CdS and CdTe, but CdS and Cu (InGa) Se and ZnO are also preferable._XS_Y(OH)_ZIt may be a combination of (X + Y + Z = 1) and Cu (InGa) Se.

【0014】

Here, as described above, an electrode is formed on the p-type semiconductor layer, which plays a role of collecting the photocurrent generated near the pn junction surface. As this electrode, a carbon electrode is preferably used. Since the p-type carrier is responsible for the conductivity of the p-type semiconductor, the higher the p-type carrier concentration near the interface between the p-type semiconductor layer and the electrode, the more the p-type carrier generated near the pn junction surface is on the electrode side. It is efficiently collected and the characteristics of solar cells such as open end voltage are improved.

【0015】

Therefore, in the present invention, the surface of the p-type semiconductor layer is coated with a solution containing a melting point lowering agent and a Group 6 element, and then heat treatment is performed. The greatest feature of the present invention is that the solution contains Group 6 elements. The melting point lowering agent is for lowering the melting point of the p-type semiconductor, and has the effect of improving the crystallinity of the p-type semiconductor layer and purifying and activating the surface of the p-type semiconductor layer by subsequent heat treatment. As a result, the adhesion between the p-type semiconductor layer and the carbon electrode is also improved. In addition, Group 6 elements have the effect of increasing the p-type carrier concentration on the surface of the p-type semiconductor layer.

【0016】

The melting point lowering agent may be any as long as it can exert the above-mentioned effect, for example, cadmium chloride (CdCl).₂), Zinc chloride (ZnCl)₂) And other inorganic chlorides. Among them, in the CdS / CdTe solar cell, it is preferable to use cadmium chloride because it contains Cd of the same type as the constituent elements.

【0017】

Group 6 elements contained in the solution refer to tellurium, oxygen (O), sulfur (S), selenium (Se) and the like. These may be used alone or in combination of two or more. Of these elements, tellurium, which has characteristics such as exhibiting the behavior of a p-type semiconductor by itself when crystallized, is most preferable. Further, when the CdTe layer is formed as the p-type semiconductor layer, tellurium is particularly preferable because it is itself a constituent element of the p-type semiconductor layer. That is, by spraying tellurium on the surface of the CdTe layer and then forming the electrode, the p-type carrier concentration near the CdTe layer / electrode interface can be increased, and an excellent CdS / CdTe solar cell can be obtained. ..

【0018】

When the Group 6 element is contained in the solution, for example, tellurium chloride (TeCl)₄), Tellurium oxide (TeO)₂) May be used.

【0019】

The semiconductor properties depend on the group properties defined in the periodic table of the constituent elements. Therefore, the combination of the n-type semiconductor, the p-type semiconductor, and the Group 6 element is not particularly limited, and the same effect can be expected with a combination other than the above.

【0020】

As a method of spraying a Group 6 element such as tellurium on the surface of a p-type semiconductor such as CdTe, a vacuum vapor deposition method or the like can be mentioned. However, these methods have the disadvantages of requiring a vacuum device, complicating the process, and increasing costs. On the other hand, in the step of improving the crystallinity of the p-type semiconductor layer by using a solution of a melting point lowering agent and purifying and activating the surface thereof, when a Group 6 element is mixed in the solution, Group 6 elements can be easily sprayed on the surface of p-type semiconductors, costs can be reduced, and variations between products can be reduced.

【0021】

Examples of the solvent used in the solution include highly volatile organic solvents such as methanol and ethanol, water and the like.

【0022】

As the solution, an aqueous solution containing cadmium chloride and a Group 6 element, particularly cadmium chloride, tellurium chloride, and tellurium oxide, is used because the insoluble matter completely volatilizes during the heat treatment and therefore does not have an adverse effect as a residual residue. It is preferable to use an aqueous solution of a tellurium compound such as.

【0023】

The concentration of the melting point lowering agent in the solution is preferably 0.01 to 3 mol / liter. If the concentration is less than 0.01 mol / liter, the effect is not sufficiently obtained, and if it exceeds 3 mol / liter, the p-type semiconductor layer may be eroded.

【0024】

The concentration of the Group 6 element in the solution is preferably 0.01 to 3 mol / liter. If the concentration is less than 0.01 mol / liter, the effect is not sufficiently obtained, and if it exceeds 3 mol / liter, the Group 6 element may segregate on the p-type semiconductor layer.

【0025】

In the solution, the amount of the Group 6 element per 1 mol of the melting point lowering agent is preferably 0.01 to 1 mol. If the ratio of Group 6 elements is too large, for example, Te atoms will aggregate, and if it is too small, the effects of Group 6 elements will not be sufficient.

【0026】

The method for applying the solution to the p-type semiconductor layer is not particularly limited as long as it is a conventionally known method. For example, the solution may be applied by a spray method in which the solution is placed in a mist blower and atomized into fine particles. The amount of the solution applied to the p-type semiconductor layer of a unit area is 1 × 10 for the melting point lowering agent.^-5~3×10^-3Mol / cm²The amount that becomes, or the group 6 element is 1 × 10^-⁵~1×10^-3Mol / cm²It is preferable that the amount is such that. If the coating amount is too small, the effects of the melting point lowering agent and the Group 6 element cannot be sufficiently obtained, and if the coating amount is too large, the same inconvenience as when the solution concentration is too high occurs.

【0027】

The heat treatment temperature in the heat treatment step is preferably 300 to 500 ° C. The treatment time is preferably 5 to 60 minutes. If the heat treatment temperature is too low or the treatment time is too short, the p-type semiconductor layer is not sufficiently modified, and if the heat treatment temperature is too high or the treatment time is too long, characteristics such as conversion efficiency of the solar cell are obtained. This is because it causes a decrease in. The heat treatment step is preferably performed in an atmosphere containing oxygen. In addition, other conditions may be in accordance with the conventional ones.

【0028】

The p-type carrier concentration near the surface of the p-type semiconductor layer after the heat treatment process is 1 × 10 from the viewpoint that a solar cell with good performance can be obtained.¹⁸cm^-3The above is preferable. The higher the p-type carrier concentration, the more preferable, but the currently possible upper limit is 1 × 10.²⁰cm^-3Degree. Among them, when the p-type semiconductor layer is a CdTe layer, cadmium halide is used as a melting point lowering agent, and tellurium is used as a Group 6 element, the molar ratio of tellurium atoms to cadmium atoms on the surface of the p-type semiconductor layer is 1.5 to The one with 2.5 is the most preferable.

【0029】

[Example]

Next, the present invention will be described in more detail based on Examples, but the present invention is not limited thereto.

【0030】

<< Example 1 >> Group 6 element concentration dependence A solar cell element having the structure shown in FIG. 1 was manufactured. A transparent conductive film (fluorine-doped SnO) with a thickness of 400 nm is previously placed on a glass substrate 1 having a thickness of 3 mm and a size of 10 cm × 10 cm.₂The film-forming substrate on which (2) was formed and the source substrate on which the paste obtained by kneading cadmium diethyldithiocarbamate and an organic solvent (propylene glycol) were screen-printed were opposed to each other with a gap in between, and the source substrate was placed at 450 ° C. The paste was decomposed by heating to C to form a CdS layer 3 having a thickness of 100 nm.

【0031】

Next, the CdTe layer 4 was formed as follows. First, the substrate was placed in a container containing a source substrate screen-printed with a paste obtained by kneading CdTe powder and an organic solvent (propylene glycol) so as to face each other with a gap in between, and the inside of the container was placed. Was reduced to 1 Torr. Then, the temperature of the CdTe source was set to 640 ° C, the temperature of the substrate was set to 600 ° C, and the mixture was held for 2 minutes to form a CdTe layer 4 having a thickness of 5 μm on the CdS layer 3.

【0032】

Next, on CdTe layer 4, CdCl₂And an aqueous solution containing the tellurium element was applied by a spray method. Specifically, the aqueous solution was placed in a mist blower, atomized into fine particles, and applied onto the CdTe layer 4. However, CdCl₂The concentration was 0.3 mol / liter, and the tellurium element concentration was varied in the range of 0.01 to 3 mol / liter. In addition, the amount of the solution applied to the CdTe layer of a unit area is 1 × 10 for the melting point lowering agent.^-3Mol / cm²The amount was set to Here, tellurium chloride was used as a raw material for the tellurium element. After that, heat treatment was performed at 430 ° C. for 30 minutes in a belt-type electric heating furnace in which the oxygen volume concentration in the furnace atmosphere was 1%. Finally, the substrate was washed with water.

【0033】

Then, the atomic concentration ratio was examined on the surface of the CdTe layer by using X-ray photoelectron spectroscopy. According to X-ray photoelectron spectroscopy, the surface atomic composition can be investigated by irradiating the sample surface with X-rays, detecting the emitted photoelectrons, and analyzing the energy thereof.

【0034】

FIG. 2 shows the relationship between the concentration of tellurium element mixed in the aqueous solution and the molar ratio of tellurium atom to cadmium atom on the surface of the CdTe layer after the heat treatment step (hereinafter referred to as Te / Cd ratio). From FIG. 2, it can be seen that when the tellurium element concentration is less than 0.01 mol / liter, the Te / Cd ratio is 1.5 or less, and when it exceeds 3 mol / liter, it exceeds 2.5. On the other hand, it was found that when the tellurium element concentration was 2.5 or more, Te segregated on the CdTe layer.

【0035】

<< Example 2 >> Heat treatment temperature dependence In Example 1, the aqueous solution of CdCl₂The same operation was performed except that the concentration was 0.3 mol / liter, the tellurium element concentration was 0.3 mol, and the heat treatment temperature in the belt-type electric heating furnace was changed in the range of 200 to 600 ° C. The treatment time was constant at 30 minutes. Then, the atomic concentration ratio was examined on the surface of the CdTe layer by using X-ray photoelectron spectroscopy. Figure 3 shows the relationship between the heat treatment temperature and the Te / Cd ratio. From FIG. 3, it can be seen that when the heat treatment temperature is less than 300 ° C, the Te / Cd ratio is less than 1.5, and when it exceeds 500 ° C, it exceeds 2.5.

【0036】

<< Example 3 >> Evaluation of solar cell characteristics A CdS / CdTe solar cell was prepared using the substrate having the CdTe layer 4 of various Te / Cd ratios obtained in Example 1. Specifically, the carbon electrode 5 was formed on the CdTe layer, and the AgIn electrode 6 was formed on the carbon electrode 5 and on the CdS layer to complete the solar cell. The conversion efficiency and open-edge voltage of the obtained solar cell were measured using a solar simulator. Fig. 4 shows the relationship between the Te / Cd ratio and the conversion efficiency, and Fig. 5 shows the relationship between the Te / Cd ratio and the open end voltage. From these figures, it can be seen that the improvement in open-end voltage and conversion efficiency is remarkable in the range of Te / Cd ratio of 1.5 to 2.5.

【0037】

<< Example 4 >> p-type carrier concentration on the surface of the CdTe layer The p-type carrier concentration of the CdTe layer with various Te / Cd ratios obtained in Example 1 was measured. Here, PN4300 (device name) manufactured by Bio-Rad was used, and the measurement was performed by the Electrochemical CV method. Figure 6 shows the relationship between the Te / Cd ratio and the p-type carrier concentration. From FIG. 6, it can be seen that the p-type carrier concentration changes as the Te / Cd ratio changes. When the Te / Cd ratio is 1.5 or more, the p-type carrier concentration is 1 × 10.¹⁸cm^-3That is all. On the other hand, when the Te / Cd ratio is 2.5 or more, the p-type carrier concentration is 1 × 10.²⁰cm^-3Is saturated with.

【0038】

It is considered that such an improvement in the p-type carrier concentration on the surface of the CdTe layer facilitates the collection of photocurrents formed at the CdS layer / CdTe layer interface, and improves the open-end voltage and conversion efficiency of the solar cell. ..

【0039】

[Effect of the invention]

According to the present invention, it is possible to obtain a solar cell having high conversion efficiency extremely easily without requiring a complicated process and at low cost without requiring an expensive device. Further, according to the method of the present invention, not only the p-type carrier concentration of the p-type semiconductor layer is improved, but also the adhesion between the p-type semiconductor layer and the carbon electrode is improved. As a result, a solar cell with high long-term reliability can be obtained.

[Simple explanation of drawings]

[Figure 1]

It is the schematic sectional drawing of the CdS / CdTe solar cell.

[Figure 2]

It is a figure which shows the relationship between the tellurium element concentration mixed in the melting point lowering agent aqueous solution, and the Te / Cd ratio of the obtained CdTe layer surface.

[Fig. 3]

It is a figure which shows the relationship between the heat treatment temperature in a heat treatment process, and the Te / Cd ratio of the surface of a CdTe layer.

[Fig. 4]

It is a figure which shows the relationship between the Te / Cd ratio of the surface of a CdTe layer, and the conversion efficiency of a solar cell.

[Fig. 5]

It is a figure which shows the relationship between the Te / Cd ratio of the surface of a CdTe layer, and the open end voltage of a solar cell.

[Fig. 6]

It is a figure which shows the relationship between the Te / Cd ratio of the CdTe layer surface, and the p-type carrier concentration.

[Explanation of symbols]

1 Glass substrate 2 Transparent conductive film 3 CdS layer 4 CdTe layer 5 carbon electrode 6 AgIn electrode