JPH11145494A - Thin-film silicon photoelectric converting device and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin-film crystalline silicon photoelectric converting device, wherein an excellent semiconductor junction can be formed and photoelectric converting characteristics can be improved. SOLUTION: A thin-film silicon photoelectric converting device contains a thin-film crystalline silicon photoelectric converting layer 3 and a microscopic crystal silicon conductive thin film layer 4 containing a conductivity determinant impurity atom of 0.01 atomic % or more, formed by coming into contact with the photoelectric converting layer through a semiconductor junction interface. The rate of interfacial region having an angle of deviation of 15 deg. or less of the equivalent crystal orientational axis corresponding to both sides of junction interface is in the range of 5 to 50%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin-film photoelectric conversion device, and more particularly to improvement in performance of a thin-film crystalline silicon-based photoelectric conversion device which can be manufactured at low cost.

[0002] In this specification, the terms "crystalline silicon" and "microcrystalline silicon" also mean those partially containing amorphous silicon.

[0003]

2. Description of the Related Art One of the most important technologies relating to semiconductor devices such as a photoelectric conversion device such as a solar cell is a semiconductor junction forming process. In the photoelectric conversion device, particularly, the characteristics of a semiconductor junction formed on the light incident side of the photoelectric conversion layer have a large direct influence on the performance of the device. At present, methods widely used in industry for forming a semiconductor junction based on a bulk crystalline silicon-based material include thermal diffusion of dopant atoms, ion implantation, and an epitaxial growth method by a film forming process such as a CVD method. There is.

On the other hand, an amorphous silicon photoelectric conversion device, which is one of the thin film photoelectric conversion devices which can be manufactured at low temperature at low cost, has a layer of one conductivity type, a photoelectric conversion layer and a layer of opposite conductivity type sequentially deposited by a plasma CVD method. It includes a pin junction. In this case, a material having a larger energy band gap than that of the photoelectric conversion layer is deposited as a conductive type layer on the light incident side to form a junction.
The photoelectric conversion efficiency can be improved by a so-called heterojunction window effect such as an increase in diffusion potential and a reduction in light absorption loss. Representative materials for such a window layer include amorphous silicon carbide and microcrystalline silicon.

In Japanese Patent Application Laid-Open No. Hei 8-116080, if microcrystalline silicon used for the light-incident side conductive type layer is deposited at a low temperature of 150 ° C. or less, the conventional condition of 150 ° C. or more or amorphous silicon carbide is used. It has also been reported that the diffusion potential increases and the photoelectric conversion efficiency also increases. However, in this case, the photoelectric conversion layer to be bonded is an amorphous film, and more specifically, a conductive microcrystalline silicon layer is deposited on the amorphous silicon film photoelectric conversion layer, There is no intention to actively reduce defects at the bonding interface. Also, in combination with an amorphous photoelectric conversion layer having an originally high defect density, it is substantially difficult to greatly improve the junction interface characteristics.

[0006] In recent years, as represented by a thin-film polycrystalline silicon solar cell, research and development aiming at practical use of a low-cost and high-performance thin-film photoelectric conversion device have been conducted. In order to realize cost reduction, it is desirable to use a simple low-temperature film forming process that can use inexpensive substrates such as glass, metal, and organic films, can easily increase the area of thin films, and is mature. . At this time, not only the low temperature formation of the photoelectric conversion layer, which is the center of the device constituent material, but also the low temperature of the semiconductor junction formation process is desired. High temperature process cannot be used.

Accordingly, many attempts to form a semiconductor junction easily by forming a conductive layer at a low temperature by a plasma CVD method in a thin-film crystalline silicon-based photoelectric conversion device, similarly to an amorphous silicon-based photoelectric conversion device. This is performed, for example, in JP-A-3-218683. Also in this case, amorphous silicon, microcrystalline silicon, or an alloy thereof is used as the material of the conductive type layer to be deposited, and all have a large energy band gap as compared with crystalline silicon. Again, improvement in photoelectric conversion efficiency due to the heterojunction window effect in the photoelectric conversion device is expected.

[0008]

On the other hand, at the bonding interface between the conductive type material formed at such a low temperature and the underlying crystalline silicon-based material, an established high-temperature process such as the above-mentioned thermal diffusion is used. Compared to the case of forming a junction, there are generally more defect levels in the energy level based on crystal grain boundaries, lattice mismatch, etc., which generally exist near the junction interface. Such a problem limits the photoelectric conversion characteristics 1
Despite these factors, no approach has been attempted to solve them.

From such a viewpoint, a main object of the present invention is to obtain good semiconductor junction characteristics and photoelectric conversion characteristics in a thin-film crystalline silicon-based photoelectric conversion device formed at a low temperature. It is an object of the present invention to provide a method capable of forming a high-quality semiconductor junction with a reduced number of crystal grain boundaries at an interface between a conductive layer and a photoelectric conversion layer and having few defects by a simple process at a low temperature.

[0010]

A thin-film silicon-based photoelectric conversion device according to one embodiment of the present invention is formed in contact with a thin-film crystalline silicon-based photoelectric conversion layer and a photoelectric conversion layer via a semiconductor junction interface. A microcrystalline silicon-based conductive type thin film layer containing 0.01 atomic% or more of a conductive type determining impurity atom; at least one misalignment angle of a corresponding equivalent crystal orientation axis on both sides of a semiconductor junction interface is 15 degrees or less. The ratio of the interface region is within a range of 5% or more and 50% or less.

According to another aspect of the present invention, there is provided a method of manufacturing a thin film silicon-based photoelectric conversion device, wherein a conductive type is formed on a photoelectric conversion layer by a plasma CVD method at a base temperature in a range of 20 ° C. or more and less than 200 ° C. The method is characterized in that a thin film layer is deposited.

[0012]

FIG. 1 is a schematic exploded perspective view showing a basic structure of a thin-film crystalline silicon-based photoelectric conversion device applicable as an example of an embodiment of the present invention. In this photoelectric conversion device, as the substrate 1, a metal, an organic film, a low-melting-point inexpensive glass, or the like can be used. The base electrode 2 is formed, for example, by subjecting a metal electrode such as Ag formed by vapor deposition or sputtering, a transparent conductive oxide film such as ZnO, or a highly doped amorphous silicon layer formed by plasma CVD to pulse laser annealing. Any of a low-resistance silicon film obtained by polycrystallization, a microcrystalline silicon-based conductive thin film formed by a plasma CVD method, or the like, or a combination of a plurality of these layers can be used.

The thin-film crystalline silicon-based photoelectric conversion layer 3 is formed by a plasma CVD method at a relatively low temperature of 550 ° C. or less so that an inexpensive substrate as described above can be used. As a main source gas for film formation, not only a hydrogenated silane-based gas such as monosilane, but also a halogen-based gas such as silicon tetrafluoride or dichlorosilane can be used. In addition to these main source gases, a hydrogen gas or an argon gas is mixed and used as a diluent gas or a carrier gas.
The film deposited as the photoelectric conversion layer 3 may be a non-doped intrinsic semiconductor thin film polycrystalline silicon or a volume crystallization fraction of 80%.
The above microcrystalline silicon, or a silicon alloy material similar to these, or a thin p-type or n-type thin film silicon material which contains a small amount of impurities and has a sufficient photoelectric conversion function. Since such a photoelectric conversion layer 3 is formed under a low temperature condition, it contains a relatively large amount of hydrogen atoms.
It is in the range of 0 atomic% or less. The film thickness is preferably set in the range of 0.5 to 20 μm, which is a necessary and sufficient film thickness for the thin-film silicon-based photoelectric conversion layer 3. Note that many of the crystal grains included in the photoelectric conversion layer 3 extend upwardly from the base electrode 2 in a columnar shape. Many of these crystal grains have a preferential crystal orientation plane of (110) parallel to the film plane, and the intensity ratio of the (111) diffraction peak to the (220) diffraction peak determined by X-ray diffraction is 0.1%.
2 or less.

Subsequent to the deposition of the photoelectric conversion layer 3, a microcrystalline silicon-based conductive thin film layer having a volume crystallization fraction of 20% or more is deposited as a window layer 4 for receiving the light 7 by a plasma CVD method. As the microcrystalline silicon-based conductive type thin film layer 4, for example, boron atoms that are conductive type determining impurity atoms are 0.01%.
P-type microcrystalline silicon doped with at least atomic% or n-type microcrystalline silicon doped with at least 0.01 atomic% of phosphorus atoms can be used. However, these conditions are not limited, and for example, aluminum may be used as a conductivity type determining impurity atom to obtain p-type microcrystalline silicon. Further, as the window layer 4, an alloy material such as microcrystalline silicon carbide having an optical band gap larger than that of the microcrystalline silicon film may be used.

The film forming process of the microcrystalline silicon layer 4 includes:
An RF plasma CVD method generally used widely, an ECR plasma CVD method, a combination thereof, or the like may be used. As a source gas for film formation, a halogenated gas such as silicon tetrafluoride or dichlorosilane may be used in addition to a hydrogenated silane gas such as monosilane. Further, for example, diborane or phosphine may be used as the dopant gas. Further, in addition to these gases, a hydrogen gas is mixed and used.

More specifically, as an example of the window layer 4 of p-type microcrystalline silicon formed by RF plasma CVD, the substrate temperature is set in a range from room temperature to less than 200 ° C. The forming temperature is set at a relatively lower temperature than in the conventional case. The pressure in the reaction chamber is in the range of 0.1 to 2.0 Torr, and the RF power is in the range of 10 to 500 mV / cm ² . Silane is used as a source gas, and diborane is used as a doping gas. At this time, the mixing ratio of the diborane gas to the silane is preferably in the range of 0.02 to 10%, more preferably 0.1 to 5%. Within the range. In addition, a hydrogen gas 10 times or more the silane gas is mixed. The thickness of the microcrystalline silicon layer 4 thus formed is preferably in the range of 3 to 100 nm, more preferably 5 to 5 nm.
It is in the range of 0 nm.

Under these conditions, the p-type microcrystalline silicon film directly deposited on the glass substrate has a dark conductivity of less than 30 S / cm and an optical band gap of 2.0 e.
V or more, and can have both sufficient conductivity and a wide band gap for forming a heterojunction. The dark conductivity in this case was determined from the resistance between the coplanar metal electrodes deposited on the microcrystalline silicon film and the thickness of the silicon film. The optical band gap E _OPT is obtained by using the equation (αhν) ^1/2 αhν-E _OPT of Tauc based on the light energy hν dependency of the light absorption coefficient alpha. On the other hand, the hydrogen atom content quantitatively determined from the infrared absorption spectrum
It can be within the range of 5 atomic% or less, and contains a large amount of hydrogen atoms in the film because it is formed at a low temperature.
Due to the presence of hydrogen, the effects of passivation at the crystal grain boundaries in the microcrystalline film and widening the band gap can be obtained. Since the various physical property values and film compositions shown here relate to a microcrystalline silicon film directly deposited on a glass substrate, a thin film crystalline silicon photoelectric converter in an actual photoelectric conversion device as shown in FIG. Although it is considered that the absolute values of the physical properties are slightly different from those in the case of being deposited on the conversion layer 3, it can be considered that they show almost similar tendencies.

FIG. 2 is an example of a transmission electron microscope (TEM) photograph of a cross section of the thin-film crystalline silicon-based photoelectric conversion device obtained under the above-described manufacturing conditions, and shows the photoelectric conversion layer 3 and the window-side microcrystalline silicon. The crystal lattice image in which the vicinity of the interface with the layer 4 is enlarged is shown, and white dots correspond to Si atoms. As is apparent from this TEM photograph, there is a portion where there is no clearly structurally discontinuous interface between these two layers 3 and 4. That is, it is understood that there is a portion where the microcrystalline silicon conductive type film 4 is epitaxially grown under the influence of the crystal orientation of the underlying crystalline silicon photoelectric conversion layer 3. Such epitaxial growth reduces defect levels due to lattice mismatch or the like at the junction interface. In addition, since the window-side microcrystalline silicon layer 4 is deposited under low-temperature conditions, the effect of reducing damage to the vicinity of the surface of the deposited film due to energetic particles such as ions present in the plasma is also superimposed, and a more favorable effect is obtained. A semiconductor junction having interface properties is formed. Such an improvement in the junction interface characteristics directly contributes to reduction of carrier recombination near the interface and the like, so that a high photoelectric conversion efficiency can be obtained in the thin-film crystalline silicon-based photoelectric conversion device.

If the deposition temperature is relatively high and the region where epitaxial growth occurs at the junction interface increases, the formed microcrystalline silicon conductive type layer 4 approaches a crystalline film of the same quality as the photoelectric conversion layer 3. . In this case, the junction between the two is substantially close to a homojunction and interface defects and the like are reduced. However, at a relatively high deposition temperature, the hydrogen content in the microcrystalline conductivity type layer 4 is reduced. Since the band gap is reduced and the heterojunction window effect is reduced, the photoelectric conversion efficiency of the photoelectric conversion device is reduced. For these reasons, there is a preferable range of the ratio of the region where the epitaxial growth occurs at the junction interface. In the present invention, the deposition of the microcrystalline silicon conductive type layer 4 is performed at 200 ° C.
Since it is performed at a low temperature of less than, it does not grow epitaxially in the entire region of the junction interface,
It is characterized in that the ratio of the interface region in which at least one corresponding crystal orientation axis has a shift angle of 15 degrees or less on both sides of the bonding interface is within a range of 5% or more and 50% or less.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a photoelectric conversion device according to an embodiment of the present invention and a photoelectric conversion device of a comparative example according to the prior art will be described with reference to FIG.

(Example 1) An Ag thin film layer, a ZnO thin film layer and a phosphorus-doped n-type polycrystalline silicon layer having a thickness of 30 nm are sequentially deposited on a glass substrate 1 to form a base electrode layer 2, and then, After depositing a non-doped thin-film polycrystalline silicon layer as the photoelectric conversion layer 3 to a thickness of 3 μm,
By forming the p-type microcrystalline silicon film 4 to a thickness of 15 nm by RF plasma CVD, a nip junction was formed. Further, an 80 nm thick I
The TO transparent conductive film and the Ag comb-shaped metal electrode 6 for current extraction were formed.

The thin polycrystalline silicon layer 3 was deposited at a deposition temperature of 400 ° C. and contained 4.5 atomic% of hydrogen. The p-type microcrystalline silicon film 4 has a lower temperature of 100.degree.
At a film formation temperature of. Under the same film forming conditions, p deposited directly on a glass substrate to a thickness of 200 nm
In the type microcrystalline silicon film, the dark conductivity is 0.68 S /
cm, the optical band gap was 2.1 eV, and the hydrogen content was 15 atomic%. Also, according to the cross-sectional TEM image observation, at the junction interface between the thin-film polycrystalline silicon layer 3 and the p-type microcrystalline silicon film 4, the interface region where epitaxial growth appears to be occurring, that is, both sides of the junction interface The ratio of the interface region in which at least one shift angle of the equivalent crystal orientation axis is 15 degrees or less was 40%.

The output characteristics of the solar cell of Example 1 under the light intensity of 100 mA / cm ^{2 with} the light of AM 1.5 are such that the open-circuit voltage is 0.510 volt and the short-circuit current is 2
6.8 mA / cm ² , fill factor 73.5%, and conversion efficiency 10.0%.

Comparative Example 1 Comparative Example 1 was performed under the same conditions as in Example 1 except that the film forming temperature of the p-type microcrystalline silicon film 4 was 250 ° C., which was higher than 200 ° C. as in the conventional case. Was manufactured. Under the same film forming conditions, p deposited directly on a glass substrate to a thickness of 200 nm
Type microcrystalline silicon film has a dark conductivity of 1.2 S /
cm, the optical band gap was 1.9 eV, and the hydrogen content was 8.5 atomic%. In the cross-sectional TEM image observation in Comparative Example 1, the ratio of the interface region where epitaxial growth had occurred was 70%.

The output characteristics of the solar cell of Comparative Example 1 under the light intensity of 100 mA / cm ^{2 with} the light of AM 1.5 were as follows: the open-circuit voltage was 0.460 volt and the short-circuit current was 2
4.3 mA / cm ² , fill factor 72.8%, and conversion efficiency 8.1%. In other words, in Comparative Example 1, the conversion efficiency is lower because the optical band gap of the p-type microcrystalline silicon film 4 is narrower and closer to the homojunction characteristics than in Example 1. .

(Comparative Example 2) A photoelectric conversion device as Comparative Example 2 under the same conditions as in Example 1 except that the film forming temperature of the p-type microcrystalline silicon film 4 was considerably low and was 65 ° C. Was produced. A p-type microcrystalline silicon film deposited directly on a glass substrate to a thickness of 200 nm under the same film forming conditions has a low dark conductivity of 0.0021 s / c.
m, the optical band gap was 1.95 eV, and the hydrogen content was 27.4 atomic%. According to the cross-sectional TEM image observation, at the junction interface between the thin-film polycrystalline silicon layer 3 and the p-type microcrystalline silicon layer 4, the ratio of the region where epitaxial growth has occurred is very small, and one of the interface regions
% Or less.

The output characteristics of the solar cell of Comparative Example 2 under the light intensity of 100 mA / cm ^{2 with} the light of AM 1.5 are as follows:
The open-circuit voltage is 0.490 volt and the short-circuit current density is 2
The charge factor was 4.5 mA / cm ² , the fill factor was 64.9%, and the conversion efficiency was 7.8%. That is, in Comparative Example 2, the p-type microcrystalline silicon film 4 is almost similar to an amorphous film, and the defect density at the junction interface between the photoelectric conversion layer 3 and the p-type layer 4 is high. , The conversion efficiency is lower than that of the first embodiment.

(Embodiment 2) A thin-film crystalline silicon-based solar cell having improved photoelectric conversion efficiency by forming a junction at a low temperature as shown in the above-described embodiment 1 is an amorphous silicon solar cell having a large sensitivity in the visible light region. It can be preferably used also for producing a stack type cell in which a silicon-based solar cell is additionally laminated. In a tandem cell in which a thin-film crystalline silicon solar cell according to the present invention and an amorphous silicon solar cell including a non-doped layer having a thickness of 0.4 μm were stacked, 13.0 mA / cm was obtained under the same light irradiation conditions as in Example 1. ^Two
Short circuit current, open circuit voltage of 1.38 volt, and 1
A conversion efficiency of 3.0% was obtained.

[0029]

As described above, according to the present invention, a semiconductor junction having excellent interface characteristics can be formed in a thin-film crystalline silicon-based material. System photoelectric conversion device,
High photoelectric conversion efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view illustrating an example of a basic structure of a photoelectric conversion device.

FIG. 2 is a photomicrograph by a transmission electron microscope showing an example of a crystal structure included in the photoelectric conversion device according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate made of glass etc. 2 Base electrode layer 3 Photoelectric conversion layer 4 Window layer 5 Transparent electrode layer 6 Comb-shaped metal electrode 7 Incident light

Claims (4)

[Claims] 1. A microcrystalline silicon-based photoelectric conversion layer comprising: a thin-film crystalline silicon-based photoelectric conversion layer; and a microcrystalline silicon-based photoelectric conversion layer formed by being in contact with the photoelectric conversion layer via a semiconductor junction interface and containing not less than 0.01 atomic% of conductivity-type determining impurity atoms. A conductive type thin film layer, and a ratio of an interface region in which at least one equivalent crystal orientation axis has a shift angle of 15 degrees or less on both sides of the bonding interface is in a range of 5% or more and 50% or less. A thin-film silicon-based photoelectric conversion device characterized by the above-mentioned. 2. The method according to claim 1, wherein the photoelectric conversion layer is formed at a base temperature of 550 ° C. or less by a plasma CVD method, and has a volume crystallization fraction of 80% or more and a volume crystallization fraction of 30% or more. Hydrogen content in the range of at most atomic%,
m and a thickness in the range of 20 μm or less, and has a (110) preferred crystal orientation plane parallel to the film plane, and the (111) diffraction peak of the (220) diffraction peak in the X-ray diffraction thereof. 2. The thin-film silicon-based photoelectric conversion device according to claim 1, wherein the intensity ratio is 0.2 or less. 3. The method according to claim 1, wherein the conductive type thin film layer is not less than
Contains hydrogen in the range of 5 atomic% or less and 2.0 eV
3. The thin-film silicon-based photoelectric conversion device according to claim 1, having the above optical band gap. 4. The method for manufacturing a thin-film silicon-based photoelectric conversion device according to claim 1, wherein the conductive thin-film layer is formed at a temperature of 80 ° C. or higher and 200 ° C. by a plasma CVD method. A method for manufacturing a thin-film silicon-based photoelectric conversion device, wherein the thin-film silicon-based photoelectric conversion device is deposited on the photoelectric conversion layer at a base temperature within the range of