JP2896247B2 - Photoelectric conversion element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improving the performance of amorphous solar cells, and more particularly, to improving the open-circuit voltage and the photoelectric conversion efficiency by applying a semiconductor thin film having an extremely thin crystalline state to a conductive thin film. Related to technology.

[0002]

2. Description of the Related Art Amorphous solar cells have already been put into practical use as energy sources with a small output for driving calculators and watches. However, as an energy source with a large output of 0.1 W or more, such as for photovoltaic power generation, its performance and stability are not sufficient, and various studies have been conducted with the aim of improving performance. I have. Therefore, the photoelectric conversion efficiency of the solar cell is the open-circuit voltage,
It is expressed by the product of the short-circuit photocurrent and the fill factor. As a result of various studies, with regard to the short-circuit photocurrent and fill factor, the current achieved value has approached the theoretically expected value.
That is, the open-circuit voltage has not been sufficiently improved.

In order to improve the reliability of the solar cell, a pin type amorphous solar cell having a p-layer on the light incident side has recently been studied. In this amorphous solar cell, in order to improve the open-circuit voltage, the photoelectric characteristics of the p-type semiconductor thin film must be improved. In particular, it is necessary to simultaneously increase the optical band gap and improve the electrical conductivity. Where it wasn't, there was technical difficulty.

A microcrystalline thin film has been proposed as a material that satisfies these conditions. However, a conventional technique such as a plasma CVD method is used to form a thin film of a p-type under a condition for forming a p-type microcrystalline thin film on a transparent electrode. Even if the formation is attempted, the open-circuit voltage of the amorphous solar cell is not improved as a result. For this reason,
50-30 necessary and sufficient for the p-layer of the pin type amorphous solar cell
It has been reported that it is difficult to form a p-type microcrystalline thin film on a transparent electrode at a film thickness of 0 ° (for example, B. Goldstein et al. Characteristics of p-type SiC: H microcrystalline thin film ", Applied Physics Letter, 53, 2672-
2674, 1988 (B. Goldstein et al., "Properties
of P ⁺ microcrystalline films of SiC: H deposited by
conventional rf glow discharge ", Applied Physics
letters, 53, p2672-2674 (1988)). That is, 50-300
At the film thickness of Å, crystallization does not occur, the resistance is increased, the fill factor is low, and the improvement of the open-circuit voltage cannot be obtained. on the other hand,
ECR (Electron Cyclotron Resonane) Plasma CVD
When the p-layer is formed by using the method, it is reported that the open-circuit voltage is improved and the photoelectric conversion efficiency is improved. However, in this method, damage to the underlying material due to ion bombardment is severe. At present, the characteristics of p-type microcrystalline thin films have not been fully exploited (for example, Yutaka Hattori et al. "High-efficiency amorphous heterojunction solar cells using p-type microcrystalline SiC films formed by ECR-CVD")
Digest of International PVSEC-3,
171-174, published in 1987 (Y. Hattori et al., "High Ef
ficiency Amorphous Heterojunction Solar cell Emplo
ying ECR-CVD Produced p-Type Microcrystalline SiC
Film ", Technical Digest of the International PVSEC
-3, p.171-174 (1987)).

In recent years, it has been reported that a microcrystalline thin film can be obtained by repeating the treatment with atomic hydrogen generated by microwave plasma, high-frequency plasma, or the like after film formation. It is not mentioned whether microcrystallization can be achieved even when the total film thickness is 300 mm or less (for example, A. Asano, "Effect of hydrogen atoms on network structure of hydrogenated amorphous silicon and microcrystalline silicon thin film", Applied Physics Letter, Volume 56, 533
535 pages, 1990 (A. Asano, "Effects of hydrogen
atoms on the network structure of hydrogenated am
orphous andmicrocrystalline silicon thin films ", A
pplied Physics letters, 56, 533-535 (1990)), Isamu. Shimizu et al., "Chemical Reaction Control for Crystalline Silicon Growth at Low Substrate Temperature," Material Research Society Symposium Proceeding, Volume 164,
195-204, 1990 (I. Shimizu et al., "CONTROL
OF CHEMICAL REACTIONS FOR GROWTH OF CRYSTALLINE Si
AT LOW SUBSTRATE TEMPERATURE ", Materials Research
Society Symposium Proceeding Vol.164, pp.195-204
(1990))).

It has also been reported that a microcrystalline thin film can be obtained by forming a silicon thin film having a thickness of 900 to 1000 ° and then treating it with an ion beam.
It must be irradiated with high-energy ions in the order of ~ MeV (for example, J. S. Im et al., "Crystal nucleation by ion irradiation in amorphous silicon thin film", Applied Physics Letter, vol. 57, pp. 1766-1768)
Page, 1990 (JSImet al., "Ion irradiation enh
anced crystal nucleationin amorphous Si thin film
s ", Applied Physics letters, 57, p. 1766-1768 (19
90)), Sea. Spinella et al., "Grain Growth Mechanism of Amorphous Silicon by Ion Beam Irradiation by Chemical Vapor Deposition" Applied Physics Letters, Vol. 57, 554-
556 pages, 1990 (C. Spinella et al., "Grain growt
h kinetics during ion beam irradiation of chemical
vapor deposited amorphous silicon ", Applied Physi
cs letters, 57, p.554-556 (1990)), damage to the underlying material occurs, and the equipment becomes extremely large and expensive, making it impractical.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a photoelectric conversion element in which a crystalline semiconductor thin film is formed on a conductive thin film to improve the open-circuit voltage.

[0008]

Means for Solving the Problems The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and as a result, after irradiating the growing surface with low-energy ions of 1 keV or less, forming an ultrathin film of 100 ° or less, Further, they have found that by continuing ion irradiation, the crystallization of the film can be promoted and a semiconductor thin film having crystallinity can be formed even with a thickness of 300 mm or less, and the present invention has been completed.

By using the crystalline thin film for the p-layer or the n-layer, a photoelectric conversion element having extremely high open-circuit voltage and photoelectric conversion efficiency can be obtained.

[0010] The present invention relates to a method for forming a first electrode, a first conductive thin film, a thinner first substantially intrinsic thin film, a thicker second substantially intrinsic thin film, a second thin film on a substrate. In a photoelectric conversion element in which a conductive thin film and a second electrode are formed in this order, at least a step of forming a thin film in an atmosphere containing charged particles (hereinafter, abbreviated as a film forming step) and a step of forming a thin film A step of stopping the supply of the raw material and leading only charged particles to the surface on which the thin film is formed (hereinafter, abbreviated as a reforming step), and the thickness of the thin film formed in each of the steps. Is a crystalline semiconductor thin film having a total thickness of 1 to 100 ° and a total thickness of 300 ° or less.

In the present invention, at least it is essential to apply a crystalline semiconductor thin film to the first conductive thin film, but even if a similar crystalline thin film is applied to the second conductive thin film, It is not a hindrance to the effects of the present invention, but rather a preferred form. Hereinafter, the formation of the conductive thin film will be described in detail.

In the formation of the conductive thin film, the first conductive thin film and the second conductive thin film have different conductivity types. For example, if the first conductivity type is p-type, the second conductivity type is n-type. It goes without saying that the reverse is also possible.

The film forming step is characterized in that the thin film is grown in an atmosphere containing charged particles, and the means for supplying the material for forming the thin film, that is, the film forming means itself is not particularly limited. Specifically, it is performed by a physical film forming method such as vacuum evaporation, sputtering, or ion plating, or a chemical vapor deposition (CVD) method such as optical CVD or plasma CVD. The ion irradiation is to generate an atmosphere containing cations or anions and to guide (collide) with a thin film surface of the substrate by means such as applying a bias voltage to the substrate.

On the other hand, in the reforming step, the supply of the thin film forming raw material in the film forming step is stopped, only the atmosphere containing the charged particles is continued, and the atmosphere is guided (collides) with the thin film forming surface of the substrate, and the properties of the semiconductor thin film are changed. This is the step of improving

An effective physical film forming method will be described below. As described above, the atmosphere containing the charged particles in the film forming step is performed in conjunction with the physical film forming method or the chemical vapor film forming method described below, but can be controlled independently. Since the generation method is the same as the atmosphere containing charged particles in the reforming step, an effective generation method and the like will be described in a specific section of the reforming step described later.

As starting materials for film formation, silicon,
Elements, compounds and alloys such as silicon carbide, silicon nitride, silicon-germanium alloy or composite powder, silicon-tin alloy or composite powder can be used effectively. In addition, elements, compounds, and alloys such as carbon, germanium, and tin can also be used.

In order to impart p-type conductivity, a compound containing an element belonging to Group III of the periodic table of the element (for example, boron) is used as a starting material, or diborane, boron halide, trimethylboron gas or the like is used. This can be performed by forming a film in an atmosphere. In order to impart n-type conductivity, a compound containing a Group V element (for example, phosphorus) is used as a starting material,
It can be formed by forming a film in an atmosphere of phosphine, arsine, phosphorus halide, arsenic halide, alkyl phosphorus, alkyl arsenic gas or the like.

The film forming conditions are not particularly limited except for irradiating ions during the growth of the thin film. Rare gases such as argon, xenon, helium, neon and krypton, hydrogen, hydrocarbons, fluorine, nitrogen The film can be formed in an atmosphere such as oxygen gas. As specific conditions, the gas flow rate is 0.1 to 100 sccm, and the reaction pressure is 0.0001 mtorr to
It is in the range of 100mtorr. In addition, film forming conditions such as a flow rate, a pressure, and an electric power are appropriately selected according to a film forming speed.

Regarding the film forming temperature, the film is formed by controlling the substrate temperature. Although the temperature range is not basically limited, it is preferable to set the temperature in accordance with the reforming step. Specifically, it is selected in a temperature range of 500 ° C. or less.

Next, a specific example of an effective chemical vapor deposition method will be described.

As a source gas for film formation, a general formula Si _n
Monosilane, disilane represented by H _{2n + 2} (n is a natural number),
A silane compound such as trisilane and tetrasilane, a fluorinated silane, an organic silane, a hydrocarbon, a germane compound, and the like are used. Further, a gas such as hydrogen, deuterium, fluorine, chlorine, helium, argon, neon, xenon, krypton, or nitrogen may be introduced together with the source gas. When these gases are used, 0.01 to 1
It is effective to use it in the range of 00% (volume ratio), and it is appropriately selected in consideration of the film forming speed and film characteristics.

In order to impart p-type conductivity, a film can be formed by adding diborane, boron halide, trimethylboron gas or the like to a source gas. In order to impart n-type conductivity, a film can be formed by adding phosphine, arsine, phosphorus halide, arsenic halide, alkyl phosphorus, alkyl arsenic gas, or the like to a source gas.

Like the physical film forming method, the film forming conditions are not particularly limited, except that the film is formed in an atmosphere containing charged particles during the growth of the thin film. Specific conditions are disclosed below.

In the photo CVD, a source gas is decomposed by using an ultraviolet light source having a wavelength of 350 nm or less, such as a low-pressure mercury lamp, a deuterium lamp, or a rare gas lamp, to form a film. Conditions for film formation include gas flow rate of 1 to 100 sccm, reaction pressure of 15 mtorr to atmospheric pressure, substrate temperature of 200 to 600 ° C, heat resistance of substrate, film formation time considered from film formation speed, temperature of reforming process, etc. Then, more preferably, it is appropriately selected in the range of 300 to 500 ° C.

The plasma CVD will be specifically described below. As a discharge method, a method such as a high-frequency discharge, a DC discharge, a microwave discharge, and an ECR discharge can be effectively used. A flow rate of the raw material gas of 1 to 900 sccm, a reaction pressure of 0.001 mtorr to atmospheric pressure, and a power of 1 mW / cm ² to 10 W / cm ² are sufficient. These film forming conditions are appropriately changed according to the film forming speed and the discharge method. Substrate temperature is 200 to 6
The temperature is 00 ° C, more preferably 300 to 500 ° C.

In the present invention, the atmosphere containing charged particles in the reforming step and the film forming step is an atmosphere in which charged particles are generated by discharge using a non-deposition gas and exposed to the growth surface. The presence of non-ionized substances in the atmosphere does not hinder the effects of the present invention. Exposure to the discharge atmosphere and application of a bias voltage to the substrate to effectively guide ionized components onto the substrate are more preferable means as a modification method. As a method for generating a discharge, a high-frequency discharge, a DC discharge, a microwave discharge, an ECR discharge, or the like can be effectively used. It is also a useful method in the present invention to effectively generate ions by an ion generator and guide the ions to the substrate surface. Specifically, various ion generators such as a Kauffman ion gun and an ECR ion gun are used. Non-depositable gases include reactive gases such as hydrogen, fluorine, and fluorine compounds and rare gases such as argon, neon, helium, xenon, and krypton, and include hydrogen gas, deuterium gas, hydrogen fluoride gas, and fluorine gas. Gas, nitrogen trifluoride, carbon tetrafluoride, argon gas, neon gas, helium gas, xenon gas, krypton gas, and the like can be effectively used. Also, a mixture of these gases can be used effectively.

Next, specific conditions of the reforming step will be disclosed. In the case of using discharge, the discharge is maintained at a discharge power of 1 to 500 W, a flow rate of a non-deposition gas of 5 to 500 sccm, and a pressure of 0.001 mtorr to atmospheric pressure. When using an ion gun, the flow rate of the non-deposition gas is 0.1 to 50 sccm, and the pressure is 0.000.
A pressure range of 1 mtorr to 100 mtorr with ion generation and sufficient lifetime is used. Further, the ion energy in the range of 10 to 1000 eV is sufficient, and preferably 100 to 600 eV. If the energy of the ions is increased beyond this range, the damage due to the ions and the sputtering phenomenon become more intense than the effect of the modification, which is not effective.

The temperature condition in the reforming step is controlled by the temperature of the substrate. This substrate temperature is the same as or lower than the substrate temperature in the film formation process, and is between room temperature and 600 ° C.
° C, preferably 200-500 ° C.

In one film forming step, 1 to 100 mm,
It is preferably formed to a thickness of 3 to 50 °. 100Å film thickness
If the ratio exceeds, the effect of the present invention is reduced. Also, 1
A film thickness of less than Å is not preferable because the number of times of film formation and modification is increased from the viewpoint of practicality. The time required for one cycle is not particularly limited.
Within 00 seconds.

In the present invention, the substantially intrinsic thin film is
Such as a silicon hydride thin film, a silicon germanium hydride thin film, and a silicon hydride silicon thin film, which form a photoactive region of an amorphous solar cell. These substantially intrinsic thin films are compounds having silicon in the molecule,
A plasma CVD method or an optical CVD method is applied to a source gas appropriately selected from a compound having germanium in a molecule such as germane and silylgermane, an organic silane compound such as methylsilane, and a hydrocarbon gas depending on a target semiconductor thin film.
It is easily formed by applying a method. Use conventional techniques for forming substantially intrinsic thin films, such as diluting the source gas with hydrogen, deuterium, helium, argon, xenon, neon, krypton, etc., or adding a very small amount of diborane to the source gas This does not hinder the effects of the present invention.

The forming conditions are a temperature of 150 to 500 ° C., preferably 175 to 350 ° C., and a forming pressure of 0.01 to 5 torr, preferably 0.03 to 1.5 torr. The thickness of the substantially intrinsic thin film is appropriately determined according to the use of the solar cell, and is not a limiting condition of the present invention. In order to achieve the effect of the present invention, 1000 to 10,000 ° is sufficient.

The materials of the substrate and the electrodes used in the present invention are not particularly limited, and conventionally used substances are effectively used. For example, the substrate may have any of insulating or conductive, transparent, and opaque properties. Basically, a film or plate-like material formed of a substance such as glass, alumina, silicon, stainless steel, aluminum, molybdenum, chromium, or a heat-resistant polymer can be effectively used. Of course, a transparent or transparent material must be used on the light incident side as the electrode material, but there is no other practical limitation. It can be appropriately selected from metals such as aluminum, silver, chromium, titanium, nickel-chromium, gold and platinum and metal oxides such as tin oxide, zinc oxide and indium oxide.

[0033]

【Example】

Example 1 An apparatus for carrying out the present invention includes a substrate insertion chamber (1), a first conductive thin film forming chamber (2) as shown in FIG.
The chamber comprises a substantially intrinsic thin film forming chamber (3), a second conductive thin film forming chamber (4), and a second electrode forming chamber (5), and silicon is deposited in the first conductive thin film forming chamber. An apparatus having an electron beam evaporation apparatus (14) for performing ionization and an ion generator (15) for generating ions was used. The glass substrate (7) on which the tin oxide thin film has been formed as the first electrode is heated in a vacuum in the substrate insertion chamber and transferred to the first conductive thin film forming chamber. The first conductive thin film is formed by setting high-purity silicon as a starting material in a crucible, injecting an electron beam, and evaporating the electron beam. Hydrogen / diborane at a ratio of 20 / 0.1 and a pressure of 1.5x10 ^-3
A torr was introduced, an ion beam voltage of 300 V and an acceleration voltage of 50 V were applied to generate an ion beam, the shutter of the ion beam was opened, and the substrate was irradiated with the ion beam. at the same time,
The shutter of the electron beam evaporation apparatus was opened for 20 seconds, and a 20 ° silicon thin film was evaporated on the substrate heated to 250 ° C. Next, the shutter of the electron beam evaporation apparatus was closed, and only ion beam irradiation was performed for 10 seconds. Thus, opening and closing of the shutter of the electron beam evaporation apparatus were repeated at each time interval. By repeating 10 times, a p-type crystalline silicon film having a thickness of about 200 ° was formed. Then, the substrate is transferred to a substantially intrinsic thin film forming chamber, monosilane is introduced at a flow rate of 10 sccm, the amorphous silicon thin film is formed by plasma CVD at a pressure of 0.05 torr and a forming temperature of 250 ° C. for about 6000 mm.
Was formed. The plasma CVD method uses a 13.56 MHz R
An F discharge was used. At this time, the RF power was 5 W. After forming the substantially intrinsic thin film, the substrate was transferred to a second conductive thin film forming chamber. To impart n-type conductivity to the second conductive thin film, disilane / phosphine / hydrogen was introduced at a ratio of 10 / 0.01 / 100 as a source gas at a pressure of 0.2 torr, a forming temperature of 250 ° C. Approx. 500Å at 80W RF power
An n-type microcrystalline silicon film having a thickness of? Then, it was transferred to a second electrode forming chamber, and an aluminum film was formed as a second electrode by vacuum evaporation.

The photoelectric conversion characteristics of the thus obtained amorphous solar cell were measured by irradiating light of AM-1.5, 100 mW / cm ² with a solar simulator. As a result, a very high value of 0.970V was obtained for the open-circuit voltage. Short-circuit photocurrent 17.70mA /
cm ² and fill factor 0.745, which are large values.
The photoelectric conversion efficiency was 12.8%, an extremely excellent performance.

When only the thin film formed under the conditions for forming the first conductive thin film was measured by Raman scattering spectrum, a Raman scattering spectrum of 520 cm ^-1 characteristic of crystalline silicon was obtained.

Example 2 In Example 1, the p-type crystalline thin film was changed to about 4 ° and 6 seconds, respectively, by changing only the deposition thickness per one time and the modification time in the case of forming the first conductive thin film. Was formed. The deposition thickness was changed by changing the time for opening the shutter of the electron beam deposition apparatus. Example 1
In this example, the deposition rate by electron beam deposition was found to be about 1Å / sec, so in this example, the time for one film formation was set to 4 seconds. A p-type crystalline thin film of about 200 ° was obtained by repeating the deposition step and the modification step 50 times. The photoelectric conversion characteristics were measured in the same manner as in Example 1. As a result, the open-end voltage was 0.950 V, the short-circuit photocurrent was 17.40 mA / cm ² , and the fill factor was 0.755, which was a large value. As a result, the photoelectric conversion efficiency was extremely excellent at 12.5%. Met.

Comparative Example 1 In Example 1, when forming the first conductive thin film,
The film was formed to a thickness of 200 mm only by ion irradiation without going through the modification process. That is, the shutters of the ion beam evaporation apparatus and the electron beam evaporation apparatus are simultaneously opened.
Seconds, the shutters were closed after 200 seconds to complete the formation of the first conductive thin film. This sample was also measured for the same photoelectric conversion characteristics as in the example.
0.830 V, short-circuit photocurrent 15.67 mA / cm ² , fill factor 0.702, and photoelectric conversion efficiency 9.13%, which were lower than those of the examples. In particular, the open-circuit voltage and the short-circuit photocurrent were low. When only the thin film formed under the conditions for forming the first conductive thin film was measured by Raman scattering spectrum, it was not crystallized, and the decrease in open-circuit voltage and short-circuit photocurrent corresponded to the presence or absence of this crystallization. Turned out to be.

Comparative Example 2 In Example 1, when forming the first conductive thin film,
After forming a Si thin film to a thickness of 200 mm, only ion irradiation was continued. The irradiation-only time was 1000 seconds. The photoelectric conversion characteristics of the device obtained under these conditions are such that the open-circuit voltage is 0.
825 V, short-circuit photocurrent 15.75 mA / cm ² , fill factor 0.698, photoelectric conversion efficiency 9.07%, almost the same characteristics as Comparative Example 1. Even under the conditions for forming the first conductive thin film, since the thin film is not crystallized, the open-circuit voltage and the short-circuit photocurrent have not been improved.

Comparative Example 3 In Example 1, when forming the first conductive thin film,
Film formation was performed without performing ion irradiation in the film forming process. That is, open the shutter of the electron beam evaporation apparatus for 20 seconds,
After depositing a 20 ° silicon thin film on the substrate, the shutter of the electron beam evaporation apparatus was closed, the shutter of the ion beam was opened, and ion beam irradiation was performed for 10 seconds. next,
While the shutter of the ion beam is closed, the shutter of the electron beam evaporation apparatus is opened again to form a film. By repeating this film formation and modification 10 times, about 2
A p-type crystalline thin film of 00 ° was obtained. The photoelectric conversion characteristics of the device obtained under these conditions are: open-circuit voltage 0.832 V, short-circuit photocurrent
The characteristics were almost the same as Comparative Example 1 with 15.57 mA / cm ² , fill factor of 0.701, and photoelectric conversion efficiency of 9.08%. Even under the conditions for forming the first conductive thin film, since the thin film is not crystallized, the open-circuit voltage and the short-circuit photocurrent have not been improved.

[0040]

As is clear from the above Examples and Comparative Examples, the semiconductor thin film produced by using this method has a thickness of 300
ÅEven at the following film thickness, it has crystallinity, which enables the microcrystalline semiconductor thin film to be applied to the p-layer of the amorphous solar cell, which has been difficult, This leads to significant improvements in open-circuit voltage and short-circuit photocurrent.
Therefore, the present invention can provide a technology that enables high conversion efficiency and high reliability required for power solar cells, and is a very useful invention for the energy industry.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a preferred apparatus for forming an element of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate insertion room 2 First conductive thin film formation room 3 Substantially intrinsic thin film formation room 4 Second conductive thin film formation room 5 Second electrode formation room 6 Substrate removal room 7 Substrate 8 Discharge power application electrode 9 Substrate Heating electrode 10 Metal mask 11 Second electrode material evaporation source 12 Vacuum exhaust line 13 Source gas introduction line 14 Silicon evaporation source 15 Ion generator 16 Gate valve

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-177371 (JP, A) JP-A-2-166727 (JP, A) JP-A-61-136220 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) H01L 31/04

Claims (1)

(57) [Claims] 1. A photoelectric conversion element formed on a substrate in the order of a first electrode, a first conductive thin film, a substantially intrinsic thin film, a second conductive thin film, and a second electrode. One conductive thin film, the step of forming a thin film in an atmosphere containing charged particles and the supply of thin film forming material is stopped, and the step of further modifying the thin film forming surface in an atmosphere containing charged particles is repeated, In addition, the total thickness of the thin film formed in one repetition is 1 to 100 mm and the total thickness is 300 mm.
A photoelectric conversion element comprising the following crystalline semiconductor thin film.