JP2688219B2 - Photoelectric conversion element - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to improving the performance of a photoelectric conversion element, and more particularly to a technology for increasing the efficiency of a photoelectric conversion element by increasing the open circuit voltage.

(Background technology)

Photoelectric conversion elements have already been put into practical use as low-output energy supply sources for driving calculators and watches. However, performance is insufficient as an energy supply source for a device with a large output, and various studies have been carried out to improve the performance. Therefore, the photoelectric conversion efficiency of the solar cell is represented by the product of the open circuit voltage, the short circuit current and the fill factor. As a result of various studies, the current reached values of the short-circuit current and the fill factor have come close to the theoretically expected values, but the open-circuit voltage has not been sufficiently improved. In order to improve the open-circuit voltage, the photoelectric characteristics of the p-type semiconductor thin film must be improved, and in particular, the technology has difficulty in that the optical bandgap and the electrical conductivity must be increased at the same time. there were. This is because, if the optical band gap is widened, the electrical conductivity generally decreases and the operation ends.

A microcrystalline thin film has been proposed as a material that satisfies these requirements, and we are using a conventional technique such as a plasma CVD method or an optical CVD method to form a p-type microcrystalline thin film on a transparent electrode. An attempt was made to form a thin film under the conditions, but as a result, it was found that the open circuit voltage of the photoelectric conversion element did not substantially improve, which did not lead to improvement in photoelectric conversion efficiency. In this way, a p-type microcrystalline thin film is needed for the photoelectric conversion element at 50 Å
Is it extremely difficult to form on a transparent electrode with a film thickness of ~ 500Å, or is it not possible to improve performance because the transparent electrode was damaged due to the p-type microcrystalline thin film forming conditions? Although it is not clear in the state of the art, anyway, the performance improvement using the p-type microcrystalline thin film has not been sufficiently achieved.

[Basic idea of invention]

The inventors of the present invention have made extensive studies in order to solve such a problem, and as a result, a highly conductive n-type semiconductor thin film is easily formed, is easily formed on a transparent electrode, and is p-type. Since the pn junction with the microcrystalline thin film does not show rectification,
A new idea is obtained that a p-type microcrystalline thin film may be formed on a silicon material more easily than on a different material, and based on this idea, a highly conductive n-type semiconductor thin film is formed on a transparent electrode. It was found that it is possible to form a photoelectric conversion element having a high open-circuit voltage by forming a p-type microcrystalline thin film to form a photoelectric conversion element after the formation.
The present invention has been completed.

[Disclosure of the Invention]

That is, according to the present invention, a transparent substrate, a transparent electrode,
In a photoelectric conversion element formed by laminating a p-type semiconductor thin film, an i-type semiconductor thin film, an n-type semiconductor thin film, and a back electrode in this order, a highly conductive n-type film is provided between the transparent electrode and the p-type semiconductor thin film.
Type photoelectric conversion element having a semiconductor thin film interposed therein, preferably an electric conductivity of 0.0001 S / cm or more, more preferably an electric conductivity of 0.01 S / cm or more, and even more preferably 0.1 S / cm or more. A photoelectric conversion element having a semiconductor thin film interposed therein is provided, which provides an element having a sufficiently high open circuit voltage and high photoelectric conversion efficiency.

 Hereinafter, the present invention will be described in detail.

FIG. 1 schematically shows the configuration of the photoelectric conversion element of the present invention.

That is, in the photoelectric conversion element formed by laminating the transparent substrate 1, the transparent electrode 2, the p-type semiconductor thin film 4, the i-type semiconductor thin film 5, the n-type semiconductor thin film 6, and the back electrode 7 in this order, the transparent electrode 2 And a p-type semiconductor thin film 4 between which a highly conductive n-type semiconductor thin film 3 is interposed.

Incidentally, FIG. 2 shows the structure of a conventional photoelectric conversion element,
A transparent substrate 1, a transparent electrode 2, a p-type semiconductor thin film 4, an i-type semiconductor thin film 5, an n-type semiconductor thin film 6, and a back electrode 7 are laminated in this order without the interposition of a highly conductive n-type semiconductor thin film 3. The photoelectric conversion element thus formed is shown.

The most characteristic feature of the present invention is that the highly conductive n-type semiconductor thin film 3 is interposed between the transparent electrode 2 and the p-type semiconductor thin film 4. An n-type microcrystalline thin film or an amorphous thin film can be used as the thin film. As the n-type microcrystalline thin film, an n-type microcrystalline silicon thin film, a carbon-containing microcrystalline silicon thin film, a microcrystalline silicon carbide thin film, or the like can be effectively used. An amorphous silicon thin film, an amorphous silicon carbon thin film, or the like can be effectively used as the n-type amorphous thin film. These n-type semiconductor thin films are formed by mixing a compound having silicon in the molecule, a hydrocarbon gas, etc., with a group V compound of the periodic table such as phosphine, arsine, etc., and hydrogen, and plasma CVD (chemical vapor deposition). ) Method or optical CVD (Chemical Vapor Deposition) method is applied. Further, diluting the source gas with an inert gas such as helium or argon does not hinder the effect of the present invention. As the forming conditions, the forming temperature is 100 to 400 ° C, preferably 175 to 300 ° C,
Particularly preferably, it is 200 to 250 ° C., and the forming pressure is 0.01 to
5 Torr, preferably 0.03 to 1.5 Torr, particularly preferably 0.03
It is done at 5 to 1.0 Torr. By appropriately selecting these forming conditions within this range, the value of the conductivity S can be controlled. For example, when the forming pressure is kept constant and the forming temperature is increased, the S value is increased, and when it is further increased, the S value is decreased. In addition, under the conditions other than the above range, it is not possible to obtain the predetermined conductivity.

In the present invention, the required thickness of the n-type semiconductor thin film is 5 Å or more and 100 Å at most. It is preferably 10Å to 50Å. If it is less than 5Å, the effect of forming a thin film cannot be achieved, and conversely, if it is more than 100Å, the effect of the invention cannot be achieved because the reduction effect of the current cancels the improvement effect of the open end voltage. In the current technology, the conductivity of the n-type semiconductor thin film itself formed on the transparent electrode at 100 Å or less is directly measured and evaluated, and it is sufficiently high, preferably a thin film of 0.0001 S / cm is formed. Basically, it is difficult to make sure of this. However, in the present invention, a thin film that is effective as an n-type semiconductor thin film has a sufficient thickness that allows direct measurement, for example, 1000
Any n-type microcrystalline thin film having a sufficiently high conductivity when formed to have a film thickness of about Å or more, for example, 0.0001 S / cm or more is sufficient. That is, since the conductivity can be directly measured by depositing on an insulating substrate, for example, a glass substrate in a film thickness of 1000 Å or more, which is sufficiently thick enough to be measured, the present invention can be used. Under the condition that the n-type semiconductor thin film having the necessary conductivity is formed (n-type semiconductor thin film forming condition), the film forming time corresponding to the required film thickness is calculated from the film forming speed at 100Å or less By doing so, an n-type semiconductor thin film having a predetermined thickness, which is the object of the present invention, can be obtained.

In the present invention, it is preferable to use a p-type microcrystalline thin film as the p-type semiconductor thin film. As the p-type microcrystalline thin film, a p-type microcrystalline silicon thin film, a carbon-containing microcrystalline silicon thin film, a microcrystalline silicon carbide thin film, or the like can be effectively used. The required thickness of the p-type microcrystalline thin film is 50 Å or more and at most 400 Å, and it is sufficient, preferably 100 Å to 250 Å. If it is less than 50 Å, it cannot fully function as a p-type semiconductor thin film, and if it is formed over 400 Å, the effect of the invention is achieved by canceling the improvement effect of the open end voltage by the decrease of the current. Can not. In the current technology, it is difficult to directly and sufficiently prove the formation of the p-type semiconductor thin film itself of 400 Å or less. However, in the present invention, a thin film that is effective as a p-type microcrystalline thin film may be one that exhibits the properties of a p-type microcrystalline thin film when formed to a thickness that can be directly measured, for example, 1000 Å or more. . That is, by depositing to a film thickness of 1000 Å or more, the diffraction peak due to the silicon crystal can be observed by using X-ray diffraction. Under the formation conditions (formation conditions for p-type microcrystalline thin film) in which this diffraction peak appears, the film formation time corresponding to the required film thickness is calculated from the film formation speed at 400 Å or less, and the film having a predetermined thickness is formed. It is possible to obtain the p-type microcrystalline thin film of the invention. It should be noted that such a p-type microcrystalline thin film usually has a high conductivity of 0.01 S / cm or more even when the optical bandgap is as wide as 1.9 eV or more. The upper limit is not particularly specified, but a practical upper limit is usually about 10,000 S / cm.

The p-type microcrystalline thin film is a compound gas having silicon in the molecule, a compound of Group III of the periodic table typified by diborane, and a mixed gas consisting of hydrogen as a source gas,
It is easily formed by performing a plasma CVD (chemical vapor deposition) method or an optical CVD (chemical vapor deposition) method. The present invention does not prevent addition of a hydrocarbon gas or an inert gas such as helium or argon to the mixed gas as necessary. The forming conditions are such that the forming temperature is 150 to 400 ° C., preferably 175 to 300 ° C., particularly preferably 200 to 250 ° C., and the forming pressure is 0.01 to 5 Torr, preferably 0.03 to 1.5 Torr, and particularly preferably 0.035 to 1.0. Done in Torr.

In the present invention, the i-type semiconductor thin film is a hydrogenated silicon thin film, a hydrogenated silicon germane thin film, a hydrogenated silicon carbon thin film, or the like, and forms the photoactive region of the photoelectric conversion element. These i-type semiconductor thin films are compounds having silicon in the molecule; compounds having germanium in the molecule such as germane and silylgermane; or hydrocarbon gases and the like as source gases appropriately selected according to the intended semiconductor thin film. , Plasma CVD (chemical vapor deposition) method and optical CVD
It is easily formed by applying the (chemical vapor deposition) method. Diluting the source gas with hydrogen, helium, etc., adding a very small amount of diborane to the source gas, etc.
The combined use of the conventional techniques for forming the i-type semiconductor thin film does not hinder the effect of the present invention. The forming condition is that the forming temperature is 150 to 400 ° C., preferably 175 to 350 ° C., and the forming pressure is 0.01 to 5 Torr, preferably 0.03 to 1.5 Torr. The film thickness of the i-type semiconductor thin film is appropriately determined according to the application of the photoelectric conversion element, and is not a limitation condition of the present invention. In order to achieve the effect of the present invention, about 1000 to 10000Å is sufficient.

In the present invention, as the n-type semiconductor thin film provided in contact with the back surface electrode, an n-type microcrystalline thin film or an n-type amorphous thin film is effectively used. For these, an n-type microcrystalline silicon thin film, a carbon-containing microcrystalline silicon thin film, a microcrystalline silicon carbide thin film, an amorphous silicon thin film, an amorphous silicon carbon thin film, an amorphous silicon germane thin film, etc. can be effectively used. These n-type semiconductor thin films include compounds having silicon in the molecule; compounds having germanium in the molecule such as germane and silylgermane; hydrocarbon gases and the like, which are appropriately selected as raw materials according to the intended semiconductor thin film. It is easily formed by mixing a compound of Group V of the periodic table such as phosphine and arsine, and hydrogen, and applying a plasma CVD (chemical vapor deposition) method or an optical CVD (chemical vapor deposition) method. It Further, diluting the source gas with an inert gas such as helium or argon does not hinder the effect of the present invention. As the forming conditions, the forming temperature is 150 to 400.
℃, preferably 175 ~ 350 ℃, forming pressure 0.01 ~ 5T
It is performed at orr, preferably 0.03 to 1.5 Torr. It is sufficient that the thickness of the n-type semiconductor thin film is 200 to 500Å.

In the present invention, the raw material gas preferable for use will be described more specifically with reference to examples. For compounds having silicon in the molecule, monosilane, disilane,
Hydrogenated silicon such as trisilane; alkyl group-substituted hydrogenated silicon such as monomethylsilane, dimethylsilane, trimethylsilane, tetramethylsilane, ethylsilane, diethylsilane; vinylsilane, divinylsilane, trivinylsilane, vinyldisilane, divinyldisilane,
Silicon hydrides having radically polymerizable unsaturated hydrocarbon groups such as propenylsilane and ethenylsilane in the molecule; Silicon fluorides in which some or all of the hydrogen atoms of these hydrogenated silicons are replaced with fluorine are effective. Can be used.

Further, as specific examples of hydrocarbon gas, hydrocarbon gas such as methane, ethane, propane, ethylene, propylene, acetylene, etc. is useful. These hydrocarbon gases are
It is convenient to use when changing the optical band gap in the formation of a carbon-containing microcrystalline silicon thin film, a microcrystalline silicon carbide thin film, or the like. Further, for this purpose, a silicon hydride substituted with an alkyl group, a silicon hydride having an unsaturated hydrocarbon group capable of radical polymerization in the molecule, and a fluorine in which a part or all of the hydrogen of the silicon hydride is substituted with fluorine. Materials such as silicon oxide are also useful.

In the present invention, the translucent substrate, the transparent electrode, the back electrode, etc. are not particularly limited. Conventionally used glass substrate materials such as soda lime glass, borosilicate glass, and quartz glass are useful as the translucent substrate.
Furthermore, metals and plastics can also be used as the substrate material. As a plastic material, a material that can withstand a temperature of 100 ° C. or higher can be used more effectively. As the transparent electrode, a metal oxide such as tin oxide, indium oxide, or zinc oxide, a translucent metal, or the like can be effectively used. Since the back electrode does not necessarily need to be translucent, aluminum, chromium, nickel-
It can be appropriately selected and used from metals such as chromium, silver, gold and platinum and metal oxides such as tin oxide, indium oxide and zinc oxide.

〔Example〕

Hereinafter, the present invention will be described more specifically with reference to examples.

[Example 1] As a photoelectric conversion element forming apparatus, a film forming apparatus to which a plasma CVD method and a photo CVD method can be applied was used. A glass substrate is installed in a forming apparatus to which the photo CVD method can be applied.
The substrate is heated by introducing vacuum exhaust and raw material gas,
At a substrate temperature of 200 ° C. and a reaction pressure of 0.4 Torr, ultraviolet rays were radiated by a low-pressure mercury lamp to perform photo-CVD, and first, the film forming conditions of the highly conductive n-type semiconductor thin film were determined. That is, 5 / 0.05 / 100 of disilane / phosphine / hydrogen as source gas
Introduced at the rate of. A mercury lamp was irradiated for 2 hours to form a film.
The film thickness was measured and it was confirmed that a thin film of about 5000 Å was formed. In X-ray diffraction of this thin film, diffraction lines due to silicon crystals clearly appeared, and it was confirmed that the thin film was an n-type semiconductor thin film. The conductivity was as high as 5 S / cm. The average film formation rate obtained by dividing the film thickness by the film formation time is 0.7
It was Å / s.

Using these film forming conditions, a highly conductive n-type semiconductor thin film applied to the photoelectric conversion element was formed. A glass substrate with a transparent electrode made of tin oxide was also placed in the forming apparatus, and the film formation time was set to 20 seconds to form an n-type semiconductor thin film having a high conductivity of 15Å. Then, the composition of the raw material gas was changed from phosphine to diborane. As a raw material gas, disilane / diborane / hydrogen was introduced at a ratio of 5 / 0.01 / 200. The p-type microcrystalline thin film was formed at a rate of 0.2 Å / s and a film formation time of 850 seconds to a film thickness of 170 Å. Next, the substrate is transferred to the i-type semiconductor thin film forming chamber, monosilane is introduced, and the pressure is adjusted to 0.05 To.
An i-type amorphous silicon thin film was formed to a thickness of about 7,000 Å by the plasma CVD method under the conditions of rr and the formation temperature of 240 ° C.
The plasma CVD method used 13.56 MHz RF discharge. At this time, the RF power was 10W. After forming the i-type semiconductor thin film, the substrate was transferred to the n-type semiconductor thin film forming chamber. Monosilane /
The flow rate of each source gas consisting of phosphine / hydrogen is
Introduced at a ratio of 10 / 0.01 / 100, pressure 0.2Torr,
An n-type semiconductor thin film was formed to a thickness of 500Å by the plasma CVD method under the condition of the formation temperature of 240 ° C. Plasma CVD method is 13.56M
RF discharge of Hz was used. At this time, the RF power was 50W. Then, it was taken out from the thin film forming apparatus to form a metal electrode. AM 1 and 100 mW / cm ² of light were irradiated by a solar simulator to measure the photoelectric characteristics of the photoelectric conversion element, specifically, an amorphous silicon solar cell. As a result, the open circuit voltage is obtained a very high value of 0.928V, the after confirming the effects of the present invention, short-circuit photoelectric current is also large value of 17.12mA / cm ^2, as a result, the photoelectric conversion efficiency It was extremely excellent at 11.12%. The fill factor was 0.700.

[Example 2] In Example 1, the conditions for forming the n-type semiconductor thin film were changed. That is, the substrate was heated by evacuation and introduction of a source gas, and the plasma CVD method was carried out at a substrate temperature of 250 ° C. and a reaction pressure of 0.05 Torr to determine the conditions for forming a highly conductive n-type semiconductor thin film. As a raw material gas, disilane / phosphine / hydrogen was introduced at a ratio of 10 / 0.01 / 10. frequency
Glow discharge was generated at 13.56 MHz and power of 5 W to form an n-type semiconductor thin film. It was confirmed that a thin film of about 3600Å was formed in 1 hour of forming time. From the X-ray diffraction of this thin film, no diffraction line due to silicon crystals was observed. It was confirmed to be an n-type amorphous thin film. Conductivity is 0.01
It was as high as S / cm. The average film formation rate obtained by dividing the film thickness by the film formation time was 1.0Å / s.

An n-type semiconductor thin film applied to the photoelectric conversion element was formed under these film forming conditions. A glass substrate with a transparent electrode made of tin oxide was also placed in the forming apparatus, and the film formation time was set to 20 seconds to form an n-type semiconductor thin film of 20Å. Less than,
An amorphous silicon solar cell was formed in exactly the same manner as in Example 1. As a result of measuring the photoelectric characteristics of the amorphous silicon solar cell, an open-ended voltage of 0.926V was obtained, which was a very high value, and after confirming the effect of the present invention, the short-circuit photocurrent was also 16.5.
The value was as large as 5 mA / cm ^2, and as a result, the photoelectric conversion efficiency was excellent at 10.56%. The fill factor is 0.689
Met.

[Comparative Example 1] In Example 1, except that the p-type microcrystalline thin film was directly formed on the glass substrate with the transparent electrode without using the n-type semiconductor thin film having high conductivity. An amorphous silicon solar cell was formed in exactly the same process as 1. When the performance of the obtained solar cell was measured, the open-ended voltage was 0.762 V, and the short-circuit photocurrent was also reduced to 15.20 mA / cm ² ,
The photoelectric conversion efficiency dropped to 7.66%, and it ended. The fill factor was 0.661.

Comparative Example 2 In Example 1, the conditions for forming the highly conductive n-type semiconductor thin film were changed. That is, substrate temperature 80 ° C, reaction pressure
5 Torr, source gas disilane / phosphine / hydrogen 10/0.
Change to the ratio of 00001/1, irradiate the mercury lamp for 1 hour,
When an n-type semiconductor thin film was formed with a thickness of about 3600Å, the conductivity was
It was 0.00005 S / cm.

Using these film forming conditions, a highly conductive n-type semiconductor thin film applied to the photoelectric conversion element was formed. The film formation time was set to 15 seconds, and 15 Å of high conductivity n-type semiconductor thin film was formed. Thereafter, an amorphous silicon solar cell was formed in exactly the same manner as in Example 1. When the photoelectric characteristics of the amorphous silicon solar cell were measured, the open end voltage was 0.758V and the short-circuit photocurrent was 16.67m.
A / cm ² , fill factor of 0.596, and photoelectric conversion efficiency of 7.53% were observed, and the reductions in open-circuit voltage and fill factor were particularly remarkable.

[Effects of the Invention and Industrial Applicability] As is clear from the above examples and comparative examples, by providing a highly conductive n-type semiconductor thin film between a transparent electrode and a p-type semiconductor thin film, the conventional technique Practical optical CV
By using the D method and the plasma CVD method, the photoelectric conversion element of the present invention having a high open circuit voltage is formed. That is,
The present invention greatly contributes to the improvement of the photoelectric conversion efficiency of the photoelectric conversion element at a practical level. in this way,
The present invention can provide a technique that enables high conversion efficiency required for a power photoelectric conversion element, and it must be said that the present invention is an extremely useful invention for the energy industry.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration example of a photoelectric conversion element of the present invention, and FIG. 2 is a schematic diagram showing a configuration example of a photoelectric conversion element according to a conventional technique. In the figure, 1 ... Translucent substrate, 2 ... Transparent electrode, 3 ... Highly conductive n-type semiconductor thin film, 4 ... P-type semiconductor thin film, 5 ...
i-type semiconductor thin film, 6 ... N-type semiconductor thin film, 7 ... back electrode.

Claims (3)

(57) [Claims] 1. A transparent substrate, a transparent electrode, a p-type semiconductor thin film,
In a photoelectric conversion element formed by laminating an i-type semiconductor thin film, an n-type semiconductor thin film, and a back electrode in this order, the transparent electrode and p
A photoelectric conversion element, characterized in that a highly conductive n-type semiconductor thin film is interposed between the type semiconductor thin films. 2. The device according to claim 1, wherein the conductivity of the n-type semiconductor thin film is 0.0001 S / cm or more. 3. The device according to claim 1, wherein the open circuit voltage is increased.