JPH04115579A - Amorphous solar cell - Google Patents

Abstract

PURPOSE:To obtain an amorphous solar cell which has excellent performance and bar a method wherein a thicker second practically intrinsic thin film is formed by the repetition of a forming process in which a thin semiconductor film having a specific content of bound hydrogen is formed and modifying process in which the formed thin film is exposed to a discharge atmosphere containing reactive gas. CONSTITUTION:A first electrode 2, a first thin conductive film 3, a thinner first practically intrinsic thin film 4, a thicker second practically intrinsic thin film 5, a second thin conductive film 6 and a second electrode 7 are formed successively on a glass substrate 1 to constitute an amorphous solar cell. The thicker second practically intrinsic thin film 5 is formed by the repetition of a forming process in which a thin semiconductor film having a bound hydrogen content of 20 atomic % is formed and a modifying process in which the formed thin film is exposed to a discharge atmosphere containing reactive gas. Further, the thickness of the thin semiconductor film which is formed by one repetition of both the processes is controlled to be 5-100Angstrom . With this constitution, an amorphous solar cell which has high performance and stability against light application can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an amorphous solar cell, and particularly to an amorphous solar cell that stably has excellent photoelectric properties.

[Background Art] Amorphous solar cells have already been put into practical use as a low-output energy supply source for driving calculators and watches. However, as an energy supply source with a large output of o, iw or more, such as for photovoltaic power generation, its performance and stability are not sufficient, and various studies have been conducted to improve performance. ing. In fact, although some improvements have been seen with device structures such as tandem solar cells, as a practical solar cell, the photoelectric characteristics deteriorate by about 20% in the -afternoon. That is the reality. The factors contributing to these performance improvements and deterioration of photoelectric properties are plasma CVD method, photo CVD method,
Thinking that this problem lies in the characteristics of the hydrogenated amorphous silicon film itself, which is formed by a film-forming method such as thermal CVD, the inventors proposed a solution to this problem in Japanese Patent Applications No. 63-308909 and No. 01-1999.
163710.165402.165403, patent application Hei 0
2-011803. That is, in the film formation process, amorphous silicon with a small amount of hydrogen is formed, and then this film is exposed to hydrogen gas, hydrogen fluoride gas, fluorine gas,
As a result of alternating treatments in a discharge atmosphere containing non-depositing reactive gases such as nitrogen trifluoride and carbon tetrafluoride to form a film, the optical band is narrower than before. With the gap, it became possible to obtain a film with good photoelectric properties and fairly stable against light irradiation.

Therefore, in this application, we aim to apply this new film formation method to the formation of a thicker, second, substantially intrinsic thin film that is mainly used for power generation in amorphous solar cells, thereby improving performance and stability. This has made it possible to provide an excellent amorphous solar cell.

[Basic idea of the invention]

The performance of current amorphous solar cells and their stability against light irradiation are largely limited by the properties of the substantially intrinsic thin film used for power generation, especially the amount of hydrogen in the film and the state of hydrogen bonding. . Therefore, we are recommending at least the formation of a second substantially intrinsic thin film, which is thicker, by controlling the amount of hydrogen, and has a carrier with a line-optical bandgap and better photoelectric properties than before. A formation method that allows for the development of transport properties was used. In other words, a semiconductor FWJ film is formed by only 5 to 100 people using the plasma CVD method (hereinafter referred to as
By repeating the process of exposing to a discharge atmosphere containing a reactive gas, a thicker second substantially intrinsic thin film is formed and an amorphous solar cell is fabricated. We were able to provide a product with high performance and extremely stability against light irradiation.

[Disclosure of the invention]

The present invention provides a first electrode, a first conductive thin film,
a thinner first substantially intrinsic thin film, a thicker second substantially intrinsic thin film, a second electrically conductive thin film, and a second electrode.
In the amorphous solar cell formed in @, at least the formation of a thicker second substantially intrinsic thin film is performed in a step of forming a semiconductor thin film with a bound hydrogen content of 20 atomic % or less (hereinafter abbreviated as film forming step). ) and the step of exposing the formed thin film to a discharge atmosphere containing a reactive gas (hereinafter abbreviated as the modification step), and in repeating the film forming step and the modification step, - The semiconductor thin film is formed by repeating the steps 5 to 100 times thick.

The present invention will be explained in detail below.

In the present invention, it is essential to form a thicker second substantially intrinsic thin film by repeating the film forming step and the modification step, but of course, a second substantially intrinsic thin film thinner than the other first conductive Ti1H1 is essential. Even if both the first substantially intrinsic thin film and the second conductive thin film are formed by repeating the film forming step and the modification step, the effects of the present invention will not be hindered in any way.
This is a rather preferable method. Details of the repeated formation of the film forming process and the modifying process will be described below.

In the present invention, the film-forming process is not particularly limited, and specifically includes physical film-forming methods such as vacuum evaporation, sputtering, and ion blasting, and chemical vapor deposition methods such as photo-CVD and plasma CVD. This is a process of forming a film by (CVD) method, and the modification process is a process of forming a semiconductor thin film by generating a discharge of a non-depositional gas containing a reactive gas and exposing the thin film to the atmosphere of this discharge. It is a process that improves the properties of

In the present invention, the thickness of the semiconductor thin film formed by repeating the film forming process and the modification process to form a thin film is increased from 5 to 100, preferably from 10 to 80. More preferably, it is an essential requirement that the number of people be 30 to 50, and other film forming conditions do not particularly impede the effects of the present invention.

An effective physical film formation method will be explained below.

First, in the case of forming a substantially intrinsic thin film, silicon, silicon carbide, silicon nitride, silicon-germanium alloy (or composite powder), silicon-tin alloy (or composite powder), etc. are used as starting materials for film formation. elements and compounds,
Alloys can be used effectively. In addition, carbon,
Elements, compounds, and alloys such as germanium and tin can also be used.

The film-forming conditions are not particularly limited other than reducing the amount of bound hydrogen. The amount of bonded hydrogen in the semiconductor thin film is 20
Film forming conditions are selected such that the concentration is at most 10 at%, preferably at most 10 at%. If there is too much bound hydrogen, exceeding 20%, exposure to a discharge atmosphere containing reactive gases will be ineffective. Of course, as long as these conditions are satisfied, the film can be formed in an atmosphere of inert gas, hydrogen, hydrocarbon, fluorine, oxygen gas, or the like. As specific conditions, the gas flow rate is 1 to 100 sec,
The reaction pressure is 0.001 mtorr to 10 II torr
is within the range of In addition, the flow rate, pressure,
Film forming conditions such as electric power are selected as appropriate. Regarding the film formation temperature, film formation is performed by controlling the substrate temperature. Although the temperature range is basically not subject to any restrictions, it is preferable to set the temperature in accordance with the reforming process. Specifically, it is selected within a temperature range of soo'c or less.

On the other hand, in the case of forming a conductive thin film, the first conductive thin film and the second conductive thin film have conductivity types different from each other. For example, if the first conductivity type is p-type, the second conductivity type is n-type. Needless to say, the opposite case is also possible. P-type conductivity can be imparted by using a compound containing a group (I) element (for example, boron) as a starting material, or by forming a film in an atmosphere of diborane, boron halide, trimethylboron gas, or the like. To impart n-type conductivity, a group ■ element (for example, phosphorus) is added to the starting material.
The film can be formed in an atmosphere of phosphine, arsine, phosphorus halide, arsenic halide, alkyl phosphorus, alkyl arsenic gas, or the like.

Specific examples of effective CVD methods are shown below.

First, in the case of forming a substantially intrinsic thin film, the general formula Si, Hz, , -z, (n
is a natural number), silane compounds such as monosilane, disilane, trisilane, and tetrasilane, fluorinated silane,
Organic silanes, hydrocarbons, etc. are used. Further, a gas such as hydrogen, fluorine, chlorine, helium, argon, neon, nitrogen, etc. may be introduced together with the raw material gas. When using these gases, 0.01 to 100
It is effective when used within a range of % (volume ratio), and is appropriately selected in consideration of the film formation rate and film characteristics (hydrogen content, etc.).

As with the physical film-forming method, the film-forming conditions are not particularly limited other than reducing the amount of bonded hydrogen. That is, the film forming conditions are selected so that the amount of bound hydrogen in the semiconductor thin film is 20 at % or less, preferably 10 at % or less. Specific conditions will be disclosed below. In photo-CVD, film formation is performed by decomposing a source gas using an ultraviolet light source with a wavelength of 350 nm or less, such as a low-pressure mercury lamp, deuterium lamp, or rare gas lamp. The conditions during film formation were a gas flow rate of 1 to 100 seam and a reaction pressure of 15
mtorr ~ atmospheric pressure, substrate temperature 200~600°C
Considering the heat resistance of one substrate, the film formation time considered from the film formation rate, the plasma treatment temperature of post-treatment, etc., it is more preferably selected appropriately in the range of 300 to 500°C.

Further, plasma CVD will be specifically shown below.

As the discharge method, methods such as high frequency discharge, direct current discharge, microwave discharge, and ECR discharge can be effectively used. Flow rate of raw material gas is 1 to 9005 ecta, reaction pressure is 0.
OO1mtorr~atmospheric pressure, power 1mW/c+i~IO
A range of W/c+J is sufficient. These film forming conditions are changed as appropriate depending on the film forming rate and the discharge method.

The substrate temperature is 200 to 600°C, more preferably 300 to 500°C.

On the other hand, in the case of forming a conductive thin film, the first conductive thin film and the second conductive thin film have different conductivity types, similar to the physical film formation method. For example, if the first conductivity type is p-type, the second conductivity type is n-type. Needless to say, the opposite case is also possible. P-type conductivity can be imparted by adding diborane, boron halide, trimethylboron gas, etc. to the raw material gas and forming a film. To impart n-type conductivity, phosphine, arsine, phosphorus halide, arsenic halide,
This can be done by adding alkyl phosphorus, alkyl arsenic gas, etc. to form a film.

In the present invention, the modification step is a step in which a non-depositional reactive gas is introduced into the modification chamber, a discharge is generated, and the thin film formed in the film formation step is exposed to this iff atmosphere.

As a method for generating discharge, high frequency discharge, direct current discharge, microwave discharge, ECR discharge, etc. can be effectively used. Further, the non-depositing reactive gas includes hydrogen, fluorine, fluorine compounds, etc., and hydrogen gas, fluorine, hydrogen fluoride gas, nitrogen trifluoride, carbon tetrafluoride, etc. can be effectively used. Further, a mixed gas of these gases may be used.

Specific conditions for the modification step will be disclosed next. The discharge is
' with a power of 1 to 500 W and a flow rate of non-depositing reactive gas of 5 to 500 W.
It is generated and maintained within a range of 500 sec+a, pressure, O,OOlmtorr to atmospheric pressure. The temperature conditions in the modification process are controlled by the temperature of the substrate. This substrate temperature is the same as or lower than the substrate temperature in the film forming process, and is preferably 600°C to 200°C to 50°C from room temperature.
0″C.

In one film-forming process, 5 to 100 people, preferably 10 to 808 people, more preferably 30 to 50 people
Formed to the thickness of a person. Note that if the film thickness is made too thin, there is a risk of crystallization, so it cannot be made too thin as an amorphous thin film.

The time required for the I cycle is not particularly limited, but is within 1000 seconds and several seconds or more. It is preferable that the time from the film formation process to the modification process and the time from the modification process to the film formation process be as short as possible. This time depends on the shape and dimensions of the device, the vacuum evacuation system, etc. Specifically, the time can be shortened to less than 1 second.

There are no particular limitations on the forming apparatus for carrying out the present invention, but for example, a substrate insertion chamber l, as shown in FIG.
Consisting of a first conductive thin film forming chamber 2, a thicker second substantially intrinsic thin film forming chamber 3, a second conductive thin film forming chamber 4, and a second t-electrode forming chamber 5, a substrate insertion chamber, The plasma CVD method can be applied to everything except the electrode forming chamber. The materials of the substrate and electrodes used in the present invention are not particularly limited;
Conventionally used substances can be used effectively. for example,
The substrate may be insulating or conductive, transparent, or opaque.

Basically, film or plate-shaped materials formed from substances such as glass, alumina, silicon, stainless steel, aluminum, molybdenum, chromium, and heat-resistant polymers can be effectively used. As for the electrode material, a transparent or transparent material must of course be used on the light incident side, but there are no other practical limitations. Aluminum, silver, chromium, titanium, nickel-chromium,
It can be appropriately selected and used from metals such as gold and platinum, and metal oxides such as tin oxide, zinc oxide, and indium oxide. The structure of the amorphous solar cell obtained by the present invention is shown in FIG. 1, but the thickness of each constituent layer is not particularly defined, and the thickness of the second substantially intrinsic thin film, which is thicker, is From the thickness of the thin film formed by repeating , - degrees,
Except for the obvious requirement that the solar cell be thick, the thickness of conventionally known amorphous solar cells can be used as is. For example, the thickness of the thicker second substantially intrinsic film can vary by as much as 500 to 50,000.

[Example 1] As a forming apparatus for implementing the present invention, an apparatus as shown in FIG. 2 was used. It consists of a substrate insertion chamber, a first conductive thin film formation chamber, a thicker second substantially intrinsic thin film formation chamber, a second conductive thin film formation chamber, and a second electrode formation chamber. All rooms except the electrode formation room are plasma C.
A material that can be used with the VD method was used. The structure of the amorphous solar cell obtained here is shown in FIG. A glass substrate 1 on which a tin oxide thin film was formed as a first electrode 2 was heated in a vacuum in a substrate insertion chamber, and then transferred to a first conductive thin film forming chamber. First; 1g I P! In order to impart p-type conductivity to the thin film 3, disilane/dimethylsilane/diborane/hydrogen is used as a source gas at 571°510.01/2.
A p-type amorphous silicon carbide film with a thickness of about 10 OA was formed at a pressure Q of 2 Torr, a formation temperature of 250° C., and a high frequency power IW. Then, the diborane feed was stopped, the dimethylsilane/disilane ratio was decreased from 175 to 0, and the pressure was reduced to 0. ITorr,
A first substantially intrinsic thin film 4 having a thickness of approximately 150 mm was formed at a formation temperature of 275° C. and a high frequency power IW.

The substrate was then transferred to a second, thicker, substantially intrinsic film formation chamber. thicker second substantially intrinsic thin film 5
First, the film forming conditions were set as disilane 2 sec, hydrogen 50 sec, reaction pressure 0.25 Torr, high frequency power 15 W, and substrate temperature 300°C.
After the film was formed, the supply of disilane was immediately stopped without stopping the discharge, and the supply of hydrogen as a reactive gas was continued for 50 seconds.
m, pressure 0.25T.

rr, high frequency power 15W, substrate temperature 400°C 10
The Si thin film was exposed during the discharge for seconds. After the modification, disilane was introduced again for 2 seconds, and the film formation process and modification process were repeated under the same conditions. Approximately 6,000 people were formed by repeating 150 times. Next, it was transferred to a second conductive thin film forming chamber. In order to impart n-type conductivity to the second conductive thin film 6, disilane/phosphine/hydrogen was introduced as raw material gases in a ratio of 1010.01/100, at a pressure of 0.2 Torr and a formation temperature of 250. °C, RF power 80
An n-type microcrystalline silicon film with a thickness of about 500 wafers was formed using W. Then, it was transferred to a second electrode forming chamber, and an aluminum film was formed as the second electrode 7 by vacuum coating.

The photoelectric conversion characteristics of the amorphous solar cell thus obtained are A
M 1.5. Light of 100mW/cm2
, / Umilator, irradiated and measured. As a result, the photoelectric conversion efficiency was 12.8%, and the short-circuit photocurrent was 18.70 mA.
/cm”, open circuit voltage 0.900 V, fill factor 0.7
60, showing excellent performance. Moreover, in order to control the photostability of this amorphous solar cell, AM-1,5,100m
Light of W/cffl was continuously irradiated for 20 hours, and changes in photoelectric conversion efficiency were observed. The change in photoelectric conversion efficiency after 20 hours with respect to the initial photoelectric conversion efficiency was 5% or less, indicating that it was an extremely stable amorphous solar cell.

[Comparative Example 1] In Example 1, in the formation of a thicker second substantially intrinsic thin film, after forming the Si thin film, it was formed to a thickness of 6000 nm without going through a modification process, and was made into an amorphous film. A high-quality solar cell was fabricated. The performance of the amorphous solar cell obtained by this method is that the photoelectric conversion efficiency is 11.7%, and the short-circuit photocurrent is 17.
15a+A/cm", open-circuit voltage 0.911■, and fill factor 0.752, which were lower than the performance shown in Example 1. 2. When the photostability was measured, it was found that the photoelectric conversion efficiency It was found that the rate of change was about 20%, which indicates that the property still lacks credibility.This comparative example shows that the effect of the present invention cannot be exhibited unless the reforming process is performed. It is.

〔Effect of the invention〕

As is clear from the above Examples and Comparative Examples, a second (and thicker) substantially intrinsic thin film is repeatedly formed at a thickness of 5 to 100 times, and a non-depositing reactive gas is used in the modification process. An amorphous solar cell manufactured by processing the thin film in a discharge atmosphere has better optical 1i characteristics than an amorphous solar cell manufactured without performing a modification process on the thin film, and is essentially free from problems. The stability against light irradiation, which is said to be important, has also been significantly improved.

Therefore, the present invention can provide a technology that enables high conversion efficiency and high multiplier performance required for power solar cells, and is an extremely useful invention for the energy industry.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing an example of the structure of an amorphous solar cell obtained by the present invention. FIG. 2 is a schematic diagram showing an example of a preferred apparatus for forming the element of the present invention. In FIG. 2, 1----substrate insertion chamber, 2----
---First conductive thin film forming chamber, 3--------
1--a second substantially intrinsic film formation chamber that is thicker; 4.
--------Second conductive thin film forming chamber, 5--Second electrode forming chamber, 6----Substrate removal chamber, 7---
-11 base, 8-------1 release force applying electrode,
9.-Substrate heating electrode, 1l----Metal mask,
11 evaporation source, 12--...- vacuum exhaust line, 13
---- Raw material gas introduction line, 1 t --- 1.- Three-way switching valve, 15 --- Gate valve is shown. Patent applicant: Mitsui Toatsu Chemical Co., Ltd. - Second electrode - Second conductive thin film - Thicker second substantially intrinsic thin film formed by repeating the deposition process and the modification process - Thinner first film Substantially Intrinsic Thin Film - First Conductive Thin Film - First Electrode - Glass Substrate FIG.

Claims (1)

[Claims] (1) on a substrate, a first electrode, a first conductive thin film, a thinner first substantially intrinsic thin film, a thicker second substantially intrinsic thin film, a second conductive thin film; In an amorphous solar cell formed in the order of the second electrode, [1] Formation of at least a thicker second substantially intrinsic thin film is a step of forming a semiconductor thin film with a bound hydrogen content of 20 at % or less. [2] The thickness of the semiconductor thin film formed in one repetition is from 5 to 100 Å. Production method.