HK1225512B - Buffer layer film-forming method and buffer layer - Google Patents

Description

Method for forming buffer layer and buffer layer

Technical Field

The present invention relates to a buffer layer for a solar cell and a method for forming the buffer layer.

Background

In the CIS solar cell, in order to improve conversion efficiency, a buffer layer is provided between the light absorbing layer and the transparent conductive film, and a band offset is formed between the light absorbing layer and the buffer layer. The band offset is defined as the energy difference between the lower ends of the conduction bands of the buffer layer and the light absorbing layer. When the energy at the lower end of the conduction band of the buffer layer is larger than that of the light absorption layer, the band offset is represented as "+". On the other hand, when the energy at the lower end of the conduction band of the buffer layer is smaller than the energy at the lower end of the conduction band of the light absorbing layer, the band offset is represented by "-". The optimum band offset is considered to be 0 to +0.4 eV. In the future, when the band gap of the light absorbing layer is widened to further improve the conversion efficiency, the band gap control of the buffer layer becomes important.

The buffer layer is a thin film layer, and the formation of the conduction band offset greatly affects the performance of the solar cell. As the buffer layer, a metal compound containing cadmium and indium is mainly used. In addition, a solution growth method has been conventionally used as a method for forming a buffer layer.

As a conventional technique relating to the production of a solar cell including such a buffer layer, for example, patent documents 1 and 2 exist.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 8-330614

Patent document 2: japanese laid-open patent publication No. 2006-332440

Disclosure of Invention

Problems to be solved by the invention

Conventionally, a material such as cadmium or sulfur that is difficult to process (e.g., dispose of) and an expensive material such as indium have been used as buffer layers. Therefore, it is desired to develop a novel buffer layer which is easy to manufacture and can reduce manufacturing cost.

In addition, a buffer layer capable of controlling a band gap in a wide range is also desired. For the buffer layer, the following relationship is given: as the band gap becomes larger, the conduction band offset also becomes larger. Therefore, the use of the buffer layer capable of controlling the band gap in a wide range can generate conduction band offsets having various sizes, which is advantageous from the viewpoint of the expansion of applications.

In addition, when the buffer layer is formed by the solution growth method, a large amount of waste liquid treatment is required, and the production cost is increased. In addition, it is extremely important to generate a buffer layer having a desired band gap with high accuracy (referred to as band gap controllability). However, this solution growth method has a problem that the controllability of the band gap is poor. That is, it is difficult to produce a buffer layer having a desired band gap value with high accuracy by the solution growth method (band gap varies between the generated buffer layers).

Accordingly, an object of the present invention is to provide a method for forming a buffer layer, which has simple steps, low cost, and excellent band gap controllability. Another object of the present invention is to provide a novel buffer layer which can be easily manufactured, can be manufactured at a reduced cost, and can control a band gap in a wide range.

Means for solving the problems

In order to achieve the above object, a method for forming a buffer layer according to the present invention is a method for forming a buffer layer for a solar cell, which is disposed between a light absorbing layer and a transparent conductive film, the method including: (A) atomizing a solution containing zinc and aluminum as metal materials of the buffer layer; (B) heating a substrate disposed under atmospheric pressure; and (C) spraying the solution atomized in the step (a) onto the substrate in the step (B).

In order to achieve the above object, the buffer layer according to the present invention is a buffer layer for a solar cell disposed between a light absorbing layer and a transparent conductive film, and comprises Zn containing zinc and aluminum_1-xAl_xAnd (3) an O film. Wherein x is more than 0 and less than 1.

Effects of the invention

The present invention relates to a method for forming a buffer layer for a solar cell, which is disposed between a light-absorbing layer and a transparent conductive film, the method including: (A) atomizing a solution containing zinc and aluminum as metal materials of the buffer layer; (B) heating a substrate disposed under atmospheric pressure; and (C) spraying the solution atomized in the step (a) onto the substrate in the step (B).

In this method for forming a buffer layer, since it is not necessary to reduce the pressure in the reaction vessel and a liquid atomized solution is used without using a gas in the film forming process, the buffer layer can be formed by a low-cost and simple process.

In addition, the method for forming the buffer layer employs an atomization method. Therefore, the device is not easily affected by external interference. Therefore, a buffer layer having a desired band gap (i.e., excellent in band gap controllability and resistivity controllability) can be formed on the substrate by merely adjusting the content ratio of (Al/Zn) in the solution.

The buffer layer according to the present invention is a buffer layer for a solar cell, which is disposed between the light absorbing layer and the transparent conductive film, and contains Zn and aluminum_1-xAl_xAnd (3) an O film. Wherein x is more than 0 and less than 1.

The buffer layer according to the present invention is formed using zinc and aluminum which are easily handled and discarded and are inexpensive. Therefore, the buffer layer can be easily produced, and the production cost can be reduced. In addition, in the buffer layer, by changing the amount of aluminum relative to the amount of zinc, the band gap and the resistivity can be controlled (adjusted) in a wide range.

The objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description and the accompanying drawings.

Drawings

Fig. 1 is a diagram showing a configuration of a film formation apparatus for carrying out a method for forming a buffer layer according to the present invention.

FIG. 2 is a view showing another configuration of a film formation apparatus for carrying out the method for forming a buffer layer according to the present invention.

Fig. 3 is a graph showing the results of band gap measurement performed on the buffer layer formed.

Fig. 4 is a graph showing the results of resistivity measurement performed on the buffer layer formed.

Fig. 5 is a graph showing the ratio of aluminum contained as mixed crystals in the buffer layer formed using the solution containing zinc and aluminum.

Fig. 6 is a graph showing the band gap and the resistivity of a film produced using a solution containing zinc and magnesium.

Fig. 7 is a graph showing the results of transmittance measurement performed on the buffer layer formed.

Detailed Description

The present invention relates to a buffer layer for a solar cell, which is disposed between a light absorbing layer and a transparent conductive film, and a method for forming the buffer layer. Further, the inventors have made extensive studies and developments and found that Zn containing Zn and Al as a novel buffer layer_1-xAl_xAn O film is beneficial. Here, 0 < x < 1. The inventors have also found that, in the method for forming a buffer layer, an atomization method performed under atmospheric pressure, which will be described later, is advantageous.

As described above, the object of the present invention is a "buffer layer" rather than a "conductive film" having low resistance. Here, a small amount of aluminum may be introduced as a dopant into the zinc oxide film. The film (Al-doped ZnO) is an electrical conductor (resistivity of 10)^-3On the order of Ω · cm or less), the amount of aluminum introduced is only about 1% with respect to the amount of zinc. When aluminum is introduced as a dopant into the zinc oxide film, the transmittance of the formed film tends to decrease (particularly, the transmittance of light having a wavelength of 1500nm or more tends to decrease significantly).

In contrast, the buffer layer to which the present invention is directed is Zn_1-xAl_xAn O film (0 < x < 1) which can maintain high transparency (for example, a Zn film having a transmittance for light having a wavelength of 1500nm or more) even when the amount of Al to be mixed is changed_1-xAl_xHigher values are maintained in the O film).

In the present invention, the mixed aluminum is used as a metal material of the buffer layer to be formed (that is, aluminum is contained as mixed crystals in the film to be formed). Thus, the present invention relates to Zn_1-xAl_xThe O film contains more aluminum than when aluminum is introduced as a dopant. In the present invention, the amount of aluminum in the raw material solution is, for example, 10% or more with respect to the amount of zinc. In addition, Zn_1-xAl_xThe O film has a resistivity of 10 Ω · cm or more and cannot be said to be a conductor.

Zn as buffer layer_1-xAl_xThe O film is produced using zinc and aluminum which are easy to handle and discard and are inexpensive. Therefore, the buffer layer can be easily produced, and the production cost can be reduced. In addition, the inventors found that by changing the amount of aluminum relative to the amount of zinc in the buffer layer, the band gap and the resistivity can be controlled (adjusted) over a wide range.

Hereinafter, in the embodiments, a method for forming the buffer layer will be specifically described with reference to the drawings.

< embodiment >

Fig. 1 is a diagram showing a schematic configuration of a buffer layer deposition apparatus according to the present embodiment.

As shown in fig. 1, a film formation apparatus 100 for a buffer layer according to embodiment 1 includes a reaction vessel 1, a heater 3, a solution container 5, and an atomizer 6.

The film formation apparatus 100 performs an atomization method. That is, the buffer layer is formed on the substrate 2 by spraying the atomized raw material solution 4 onto the substrate 2 disposed under atmospheric pressure.

Here, a heater 3 is disposed in the reaction container 1. The substrate 2 is heated by the heater 3 during the film formation process. Further, a light absorbing layer is formed on the surface of the substrate 2.

The heater 3 is a heating device or the like, and the substrate 2 is heated to a desired temperature by adjusting the heating temperature of the heater 3 by an external control unit during the film formation process. In the present invention, the substrate 2 is heated at a temperature of 120 ℃ to 300 ℃ by the heater 3, for example, in order to lower the temperature.

Zn is filled in the solution container 5 as a buffer layer for film formation_1-xAl_xA raw material solution (hereinafter referred to as a solution) 4 of an O film, the solution 4 containing a metal source (mixed crystal in the film to be formed), zinc (Zn), and aluminum (Al).

The solvent of the solution 4 may be water, an alcohol such as methanol, or another organic solvent, or a mixture of these liquids.

As the atomizer 6, for example, an ultrasonic atomizing device can be used. The atomizer 6 as the ultrasonic atomizing device atomizes the solution 4 in the solution container 5 by applying ultrasonic waves to the solution 4 in the solution container 5. The atomized solution 4 was supplied into the reaction vessel 1 through a path L1.

Then, in the reaction vessel 1, the atomized solution 4 is sprayed on the substrate 2 being heated, and Zn as a buffer layer is formed on the substrate 2_1-xAl_xAnd (3) an O film. Here, the unreacted solution 4 in the reaction vessel 1 is continuously (continuously) discharged to the outside of the reaction vessel 1 through the path L3.

Next, a method for forming a buffer layer according to the present embodiment will be described.

First, a solution 4 containing zinc and aluminum is prepared. Here, when the amount of aluminum in the solution 4 is changed with respect to the amount of zinc in the solution 4, the resistivity and the band gap of the buffer layer to be formed change.

Specifically, in the solution 4, when the amount of aluminum is increased relative to the amount of zinc, the resistivity of the buffer layer formed becomes large, and the band gap of the buffer layer formed becomes large.

Therefore, the content of aluminum relative to the content of zinc is adjusted in the solution 4 so as to form a buffer layer having a desired resistance value and a desired band gap.

The prepared solution 4 is atomized in a solution container 5 by means of an atomizer 6. The atomized solution 4 was supplied to the reaction vessel 1 through a path L1.

On the other hand, the substrate 2 is heated to a predetermined temperature by the heater 3, and the heating temperature of the substrate 2 is maintained at a desired temperature. For example, the heating temperature of the substrate 2 is maintained at a certain temperature, for example, about 120 to 300 ℃.

The substrate 2 in the heated state is sprayed with a mist of the solution 4. Thus, Zn as a buffer layer is formed on the substrate 2 existing in the reaction vessel 1_1-xAl_xAnd (3) an O film. Here, the value of x is 0 < x < 1 and varies depending on the (Al/Zn) content ratio in the prepared solution 4.

As described above, the atomized solution 4 is sprayed on the substrate 2 disposed at atmospheric pressure.

As described above, in the method for forming a buffer layer according to the present embodiment, an atomization method of spraying the solution 4 onto the substrate 2 disposed at atmospheric pressure is employed.

Thus, in this film formation method, since the inside of the reaction container 1 does not need to be depressurized, and the liquid atomized solution 4 is used without using a gas in the film formation process, the buffer layer can be formed by a low-cost and simple process.

In addition, in the case of forming a buffer layer by a solution growth method, a large-scale apparatus for immersing a substrate is required when forming a buffer layer having a large area, and the apparatus is considered to be large-scale. However, the film formation apparatus 100 using the atomization method can be more miniaturized than an apparatus for performing a solution growth method.

In addition, when the buffer layer is formed by a solution growth method, it is difficult to balance the reaction at 1 point due to various external disturbances during the reaction. Therefore, the buffer layer produced by the solution growth method has poor controllability of the band gap (that is, a buffer layer having a band gap having a value different from the band gap of the target value is formed).

However, the method for forming the buffer layer according to the present embodiment employs an atomization method. Therefore, it is difficult to be affected by external disturbance. Therefore, by merely adjusting the content ratio of (Al/Zn) in the solution 4, a buffer layer having a desired band gap can be formed on the substrate 2 (that is, the controllability of the band gap is excellent).

In addition, for example, when the buffer layer of the present invention is formed by a CVD method, it is necessary to vaporize an organic metal solution such as diethyl zinc. This leads to complication of the constitution of the manufacturing apparatus and increase in the manufacturing cost.

In contrast, in the present invention, the solution 4 can be prepared using, for example, a β -diketone metal compound which is inexpensive and stable, and only the solution 4 can be atomized. Therefore, the film formation method according to the present invention can form the buffer layer at a lower cost and in a simpler step than the film formation method using the CVD method.

In addition to the atomized spray of the solution 4, ozone may be supplied into the reaction container 1 in which the heated substrate 2 is disposed. FIG. 2 is a diagram showing a schematic configuration of a buffer layer film forming apparatus 200 for supplying ozone.

As is clear from a comparison between fig. 1 and fig. 2, the film formation apparatus 200 has an ozone generator 7 added to the structure of the film formation apparatus 100. In the film forming apparatus 200, a path L2, which is a path different from the path L1, is provided to supply ozone from the ozone generator 7 to the reaction container 1.

The film forming apparatuses 100 and 200 have the same configuration except that the ozone generator 7 and the path L2 are added.

The ozone generator 7 is capable of generating ozone. The ozone generated in the ozone generator 7 is supplied into the reaction container 1 through a path L2 different from the path L1. In the ozone generator 7, for example, a high voltage is applied between parallel electrodes arranged in parallel, oxygen is passed between the electrodes to decompose oxygen molecules, and the oxygen molecules are bonded to other oxygen molecules, thereby generating ozone.

In the film forming apparatus 200, ozone and a mist-like solution 4 are supplied into a reaction container 1 in which a substrate 2 being heated is disposed. Here, the unreacted ozone and the solution 4 in the reaction vessel 1 are continuously (continuously) discharged to the outside of the reaction vessel 1 through the path L3. The substrate 2 is disposed under atmospheric pressure.

As described above, the inventors have found that when ozone is supplied during the process of forming a buffer layer, a desired buffer layer can be formed even when the heating temperature of the substrate 2 is lowered (for example, 120 to 200 ℃).

In the film forming apparatuses 100 and 200, ammonia may be contained in the solution 4. The inventors have found that by containing ammonia in the solution 4 in this manner, a desired buffer layer can be formed even when the heating temperature of the substrate 2 is lowered (for example, 120 to 200 ℃). Here, the solution 4 may contain ammonia without supplying ozone, or the solution 4 may contain ammonia and the ozone may be supplied.

The inventors used the film forming apparatus 200 shown in fig. 2 (i.e., supplied ozone to the reaction vessel 1) to make ammonia (NH in terms of molar ratio) contained in the solution 4₃Ca. 22.2/Zn), a buffer layer is formed. The film forming temperature is in the range of 120 to 300 ℃. Here, the buffer layer can be formed even at a heating temperature higher than 300 ℃, but from the viewpoint of lowering the temperature, experiments are omitted. Further, the buffer layer may not be formed at a film formation temperature lower than 120 ℃.

Fig. 3 shows the results of measuring the band gap value (eV) of each buffer layer, in which a plurality of buffer layers were produced by changing the Al content with respect to the Zn content and the film formation temperature.

As shown in fig. 3, by changing the amount of Al, Al/(Al + Zn) in the solution 4 was changed to 0, 0.1, 0.2, 0.35, 0.5.

Here, the buffer layer was formed by changing the film formation temperature under the condition of Al/(Al + Zn) ═ 0.75. The buffer layers were formed at the respective film formation temperatures, but the band gap value of each buffer layer was not less than the upper limit of the measuring instrument. Therefore, in fig. 3, the band gap value of the buffer layer formed under the condition of Al/(Al + Zn) ═ 0.75 is not recorded.

Further, as shown in FIG. 3, the buffer layers were formed at film formation temperatures of 120 ℃, 150 ℃, 200 ℃, 250 ℃ and 300 ℃. The buffer layer was not formed at a film formation temperature of 150 ℃, 200 ℃, 250 ℃, and 300 ℃ under the condition that Al/(Al + Zn) ═ 0.35.

As is clear from fig. 3, increasing the amount of aluminum added increases the value of the band gap of the buffer layer formed. In addition, within the range of the results shown in FIG. 3, the buffer layer (Zn) can be formed by changing the amount of aluminum added and the film formation temperature_1-xAl_xO film) varies at least in a range of 3.52eV to 4.25 eV. That is, the buffer layer according to the present invention can control the band gap in a wide range.

Fig. 4 shows the results of measuring the resistivity (Ω · cm) of each buffer layer, which was produced by changing the Al content with respect to the Zn content and also changing the film formation temperature.

As shown in fig. 4, by changing the amount of Al, Al/(Al + Zn) in the solution 4 was changed to 0, 0.1, 0.2, 0.35, 0.5, 0.75.

Further, as shown in FIG. 4, the buffer layers were formed at film formation temperatures of 120 ℃, 150 ℃, 200 ℃, 250 ℃ and 300 ℃. The buffer layer was not formed at the film formation temperature of 150 ℃, 200 ℃, 250 ℃, and 300 ℃ under the condition that Al/(Al + Zn) ═ 0.35. Similarly, under the condition of Al/(Al + Zn) ═ 0.75, the buffer layer was not formed at a film formation temperature of 120 ℃.

As is clear from fig. 4, increasing the amount of aluminum added increases the resistivity of the buffer layer formed. In addition, inIn the range of the results shown in FIG. 4, the buffer layer (Zn) can be formed by changing the amount of aluminum added and the film formation temperature_1-xAl_xO film) has a resistivity of at least 8.0 × 10^-1Ω·cm～2.2×10⁷In the range of Ω · cm. That is, the buffer layer according to the present invention can control resistivity in a wide range.

As is clear from the values of the resistivities shown in fig. 4, the buffer layer according to the present invention is neither a conductor nor an insulator, and has an intermediate conductivity between a conductor and an insulator.

FIG. 5 shows the detection of the buffer layer (Zn) subjected to the above-described measurements by an energy-dispersive X-ray analyzer (EDX apparatus)_1-xAl_xO film) of the composition ratio.

That is, the buffer layers (Zn) were formed at the film forming temperatures (120 ℃, 150 ℃, 200 ℃, 250 ℃, 300 ℃) using the solutions 4 having Al/(Al + Zn) of 0.1, 0.2, 0.35, 0.5, 0.75 (that is, the aluminum content in the solution 4 was changed with respect to the zinc content)_1-xAl_xO film). Then, for each buffer layer (Zn)_1-xAl_xO film), and the composition ratio (Al/(Al + Zn) EDX) was measured using an EDX apparatus. The measurement results are shown in the table of fig. 5.

As is clear from fig. 5, when zinc and aluminum (which become mixed crystals) are contained as metal materials in the solution 4 and a buffer layer is formed using the solution 4, a predetermined amount of aluminum is contained as mixed crystals in the buffer layer formed. Namely, Zn is formed by using this solution 4_1-xAl_xAnd (3) an O film.

The inventors also performed the following experiments.

That is, zinc and magnesium are contained in the solution 4 as metal materials, and a film is formed using the solution 4. Here, the film formation temperature was 150 ℃, and ammonia and ozone were also supplied to the substrate during film formation. Then, the band gap and the resistivity of the film formed were measured, and the composition ratio was also measured by using an EDX apparatus.

Here, the content of magnesium relative to the content of zinc in the solution 4 was changed as follows. Namely, Mg/(Mg + Zn) was set to 0, 0.17, 0.29, 0.375, 0.44, 0.5.

The results of measuring the band gap and the resistivity of each film formed using each solution 4 are shown in fig. 6.

As can be seen from fig. 6, in the solution 4, the band gap was substantially constant even when the content of magnesium relative to the content of zinc was changed. Likewise, in the solution 4, even if the content of magnesium is changed with respect to the content of zinc, the amount of change in resistivity is small.

As a result of measuring the composition ratio of each film using the EDX apparatus, magnesium was not detected in the film. From this result, it was found that magnesium was not substantially contained as mixed crystals in the film formed. Further, the fact that magnesium is not substantially contained in the film is considered to be the reason for the measurement result in fig. 6 (that is, even if the amount of magnesium in the solution 4 is changed, the band gap and the resistivity do not change greatly in the film formed).

In the buffer layer of the present invention, as shown in fig. 5, aluminum is contained as mixed crystals in the buffer layer formed. Therefore, as shown in fig. 3 and 4, the formed buffer layer can control the band gap and the resistivity in a wide range.

FIG. 7 shows the buffer layer (Zn) for each of the above measurements (FIGS. 3 and 4)_1-xAl_xO film) is shown.

That is, the buffer layers (Zn) were formed using solutions 4 having Al/(Al + Zn) of 0.1, 0.2, 0.35, 0.5, and 0.75 (i.e., the content of aluminum relative to the content of zinc in the solution 4 was varied) at film formation temperatures (120 ℃, 150 ℃, 200 ℃, 250 ℃, and 300 ℃)_1-xAl_xO film). Then, by applying each buffer layer (Zn)_1-xAl_xO film) was irradiated with light of a predetermined wavelength (light of a plurality of wavelengths), and the respective transmittances were measured.

In fig. 7, each buffer film is irradiated with a wavelength of 400nm to 1500nm (substantially in the visible light region) to measure each transmittance, and the average transmittance over the above wavelength range is shown for each buffer film. The thickness of each buffer layer is about 500nm to 700 nm.

As described above, the buffer layer was not formed at the film formation temperature of 150 ℃, 200 ℃, 250 ℃, and 300 ℃ under the condition that Al/(Al + Zn) ═ 0.35. Similarly, under the condition of Al/(Al + Zn) ═ 0.75, the buffer layer was not formed at a film formation temperature of 120 ℃.

As is clear from the results of fig. 7, the average transmittance of each buffer layer was about 90% or more. This indicates that the buffer layer according to the present invention does not function as a dopant, but functions as a mixed crystal of aluminum. That is, in the case where aluminum is contained as a dopant in the film of zinc oxide, the transmittance is lowered, but the buffer layer of the present invention does not show a reduction in transmittance.

In the experiment of fig. 7, the buffer layer was irradiated with a wavelength of more than 1500nm, and the transmittance was also measured. As a result, even when the film is irradiated with a wavelength of about 1500nm to 1700nm, the transmittance of more than 80% is maintained.

The buffer layer of the invention is stated to be Zn_1-xAl_xAnd (3) an O film. Here, the ZnO film containing no Al has a small offset barrier and cannot sufficiently function as a buffer layer. On the other hand, the Zn-free AlO film does not function as a buffer layer because it is a complete insulator. Therefore, as the buffer layer, a structure in which Al and Zn are contained as mixed crystals in the film is important.

The present invention has been described in detail, but the above description is illustrative in all aspects, and the present invention is not limited thereto. Innumerable modifications that are not illustrated are to be construed as conceivable without departing from the scope of the present invention.

Description of the symbols

1 reaction vessel

2 base plate

3 heating device

4 stock solution (solution)

5 solution container

6 atomizer

7 ozone generator

100. 200 film forming apparatus

L1, L2 Path

Claims (4)

1. A method for forming a buffer layer for a solar cell, the buffer layer being disposed between a light-absorbing layer and a transparent conductive film, the method comprising:

(A) atomizing a solution (4) containing zinc and aluminum as metal materials of the buffer layer;

(B) a step of heating a substrate (2) disposed at atmospheric pressure; and

(C) spraying the solution atomized in the step (a) onto the substrate in the step (B).

2. The method of forming a buffer layer according to claim 1, comprising: (D) and (c) supplying ozone to the substrate in the step (B).

3. The method for forming a buffer layer according to claim 1, wherein the solution contains ammonia.

4. The method for forming a buffer layer according to claim 1, wherein the solution contains a metal compound containing zinc and aluminum,

the metal compound is a beta-diketone compound.