JP2001345463A - Photovoltaic device and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve junction characteristics by enhancing crystalline and amorphous interface characteristics. SOLUTION: In a photovoltaic device where an i-type amorphous silicon thin film 2 is provided between an n-type single crystal silicon substrate 1 and a p-type amorphous silicon based thin film 3, boron atoms are introduced to an interface region of the silicon substrate 1 and the i-type amorphous silicon thin film 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a photovoltaic device using a hetero semiconductor junction and a method for manufacturing the same.

[0002]

2. Description of the Related Art When forming a pn heterojunction by forming an amorphous silicon film of another conductivity type on a single crystal silicon substrate showing one conductivity type, the single crystal silicon substrate and the amorphous silicon film are formed. A so-called HIT type solar cell structure has been reported in which the junction characteristics are improved by not performing valence electron control during this period, or by inserting a very small amount of boron into a substantially intrinsic amorphous silicon film. Since the pn junction of this structure can be formed at a low temperature of 200 ° C. or less, good junction characteristics can be obtained even when the purity of the substrate is low and the influence of impurities or oxygen-induced defects is concerned in a high-temperature process.

Since the above-mentioned method is a low-temperature process, it is difficult to completely remove moisture and organic substances adhering to the substrate surface, and impurities such as oxygen, carbon, and nitrogen exist on the substrate surface. Among them, the oxygen with the highest content has a concentration of about 1 × 10 ²⁰ cm ⁻³ , and there is a concern that the impurities may lower the interface characteristics.

On the other hand, a method of obtaining a substantially intrinsic amorphous silicon film by doping a trace amount of boron is described in, for example, A
applied Physics Letters vo
l. 68, 1996 As disclosed in P1201 to P1203, a concentration of about 1/1000 (〜１０10 ¹⁷ cm ⁻³ ) for an amorphous silicon film containing a certain concentration of oxygen.
It can be compensated by introducing boron (B).

[0005]

However, even if the above amount of boron is introduced and compensated, the solar cell characteristics are hardly affected. The improvement of the interface characteristics mainly affects the open-circuit voltage of the solar cell characteristics, but the open-circuit voltage did not change regardless of whether or not there was compensation.

In view of the above problems, an object of the present invention is to improve the interface characteristics between a crystal and an amorphous phase and to improve the junction characteristics, and particularly to increase the open-circuit voltage in a solar cell.

[0007]

The photovoltaic device of the present invention does not control valence electrons between a crystalline silicon substrate of one conductivity type and an amorphous silicon-based thin film of valence control of another conductivity type. Or, in a photovoltaic device in which a substantially intrinsic amorphous silicon thin film is interposed, an interface formed between the crystalline silicon substrate and the valence-free or substantially intrinsic amorphous silicon thin film It is characterized in that boron atoms are present in the region.

The concentration of the boron atom is 1 × 10 ¹⁸ cm ^-3.
It is preferable that the ^{thickness be} 5 × 10 ¹⁹ cm ⁻³ or less.

A method for manufacturing a photovoltaic device according to the present invention includes the steps of introducing a gas containing low-concentration boron along with hydrogen gas to clean the surface of a one-conductivity-type crystalline silicon substrate by plasma discharge; A step of forming a non-charge-controlled or substantially intrinsic amorphous silicon thin film on the surface of the silicon substrate, and a step of forming an amorphous silicon-based thin film which is charge-controlled to another conductivity type thereon; , Is included.

In the method of manufacturing a photovoltaic device according to the present invention, a gas containing silicon and a gas containing boron at a low concentration are introduced into the surface of a crystalline silicon substrate, and the first gas is introduced by a gas phase reaction.
Forming an amorphous silicon thin film, and introducing a gas containing silicon onto the first amorphous silicon thin film to form a substantially intrinsic second amorphous silicon thin film by a gas phase reaction. A step of forming a thin film and a step of forming an amorphous silicon-based thin film with valence control of another conductivity type on the second amorphous silicon thin film.

The method of manufacturing a photovoltaic device according to the present invention includes a step of exposing a crystalline silicon substrate of one conductivity type to a gas containing low-concentration boron together with hydrogen gas while heating the crystalline silicon substrate. A step of forming a non-charged or substantially intrinsic amorphous silicon-based thin film on the surface, and a step of forming an amorphous silicon-based thin film controlled by another conductivity type on the surface thereof. It is characterized by including.

As described above, according to the present invention, the amorphous silicon-based thin film is formed at the interface between the crystalline silicon substrate and the amorphous silicon-based thin film, which is not charged or substantially controlled. By introducing more boron than necessary to compensate, carrier recombination at the interface can be suppressed, and junction characteristics can be improved. In particular, in a solar cell, the open-circuit voltage can be increased.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of the present invention will be described.

The cleaned n-type single-crystal silicon substrate is introduced into a vacuum chamber and heated to an appropriate temperature (200 ° C. or lower) to remove water adhering to the substrate surface as much as possible.

Next, a hydrogen gas is introduced, and the surface of the substrate is cleaned by plasma discharge. This process has been found to be effective in reducing the amount of carbon on the substrate surface.

In this embodiment, during this hydrogen plasma treatment, diborane (B ₂ H ₆ ) gas is introduced to decompose boron (B) and adsorb boron to the surface, thereby introducing boron to the substrate interface. went.

Thereafter, a silane gas (SiH ₄ ) and a hydrogen gas (H ₂ ) as a diluting gas are introduced, and the plasma CV is introduced.
A non-doped amorphous silicon layer is formed by the D method.
Subsequently, a silane gas (SiH ₄ ), a hydrogen gas (H ₂ ) as a diluting gas, and a diborane gas (B ₂ H ₆ ) as a dopant gas are introduced to form a p-type amorphous silicon layer by a plasma CVD method. Complete the pn junction.

Further, an indium tin oxide (ITO) layer is formed as a surface electrode on the p-type amorphous silicon layer by sputtering, and a silver electrode is formed thereon as a collecting electrode by screen printing. The photovoltaic device is completed.

On the other side of the substrate, a non-doped amorphous silicon layer, an n-type amorphous silicon layer, and a back electrode layer are formed to form a so-called Back Surface Fie.
An ld structure may be formed. At this time, the order of forming the amorphous silicon layer may be from the back side (n-type side) or from the front side (p side).

The substrate is p-type, and a non-doped amorphous silicon layer, an n-type amorphous silicon layer and an indium tin oxide layer and a silver collector electrode are provided on the front side, and a non-doped amorphous silicon layer is provided on the back side. Needless to say, the same can be applied to the case of producing a p-type amorphous silicon layer and a back electrode layer. A similar effect can be obtained when a photovoltaic device is manufactured using n-type or p-type polycrystalline silicon substrates as the crystalline substrate.

FIG. 1 shows a band structure of a semiconductor junction according to the present invention. In FIG. 1, a solid line indicates a band structure according to the present invention, and a broken line indicates a conventional band structure in which B ₂ H ₆ is not introduced in hydrogen plasma treatment.

As shown in FIG. 1, the photovoltaic device formed by the above-described method has a substantially i-type amorphous silicon layer 2 on an n-type single-crystal silicon substrate 1 and an n-type amorphous silicon layer 2 thereon. An amorphous silicon layer 3 is formed, and an ITO layer 4 is further formed thereon. At the interface between the substrate 1 and the i-type amorphous silicon layer 2, more boron is introduced than necessary to compensate for the i-type amorphous layer.

As shown in FIG. 1, by introducing more boron than necessary to compensate for the amorphous layer at the interface between the substrate 1 and the i-type amorphous silicon layer 2, amorphous The silicon layer 2 becomes extremely weak p-type, and at the same time, a localized electric field generated in the single crystal silicon substrate 1 near the interface becomes strong. Therefore, the separation of electrons and holes existing near the interface is more effectively performed, and the probability of recombination at the interface is reduced. Further, the barrier at the discontinuity of the band at the interface in the valence band becomes relatively small, and carrier movement becomes easy. As a result, the open-circuit voltage in the solar cell characteristics is improved.

At this time, although the electric field intensity existing in the non-doped amorphous silicon layer is weakened, an electric field is applied still enough to easily move carriers in the amorphous layer.

When boron is excessively supplied, the electric field intensity of the non-doped amorphous silicon layer 2 decreases, and the recombination of carriers in this portion increases. Is directly equivalent to that directly deposited on an n-type single-crystal silicon substrate, recombination via excessive boron is performed, and the solar cell characteristics deteriorate.

Table 1 shows a photovoltaic device according to the present invention.
A photovoltaic device was prepared under the following forming conditions. Table 2 shows the output characteristics of the photovoltaic device thus manufactured. Here, as a comparative example, in the hydrogen plasma treatment described above, B
Those without the introduction of ₂ H ₆ were used. At this time, the BSF layer on the back side is formed at the same time.

[0027]

[Table 1]

[0028]

[Table 2]

As is apparent from Table 2, when the B ₂ H ₆ was introduced in the hydrogen plasma treatment, the open-circuit voltage was improved, and the effectiveness of the present invention was confirmed.

The thickness of the amorphous layer in the embodiment shown in Table 1 is about 7 nm for the non-doped amorphous silicon layer and about 5 nm for the p-type layer, and the depth of the existing secondary ion mass spectrometer is Since it cannot be seen with a resolution in the direction (10 nm to 20 nm), the thickness of the non-doped layer is 160 nm,
FIG. 2 shows the result of observing the impurity profile in the depth direction with the p-type layer being about 8 nm.

In the drawing, broken lines indicate the interface positions formed by the respective layers, which are expected from the profile. About 3 × 1 at the substrate interface
A boron (B) concentration of 0 ¹⁸ cm ^-3 is present. The monotonous decrease in the amount of boron from the surface of the p-type amorphous silicon into the intrinsic (i-type) amorphous silicon layer and the monotonous decrease in the amount of boron from the substrate interface into the substrate are both gentler than the Gaussian distribution. SIMS (secnda)
ry ion mass spectroscopy)
It can be seen that this is due to the driving effect during the sputtering during the analysis. Therefore, it is considered that the boron concentration actually decreases sharply at each interface, and hardly diffuses into the substrate from the i-type amorphous silicon layer or the substrate interface. Even when an actual solar cell is observed, a profile similar to the profile of boron (B) in FIG. It is thought that it becomes the measurement result which I had.

Next, a second embodiment of the present invention will be described.

The cleaned n-type single-crystal silicon substrate is introduced into a vacuum chamber and heated to an appropriate temperature (200 ° C. or lower) to remove moisture adhering to the substrate surface as much as possible.

Next, a hydrogen gas is introduced, and the surface of the substrate is cleaned by plasma discharge. Next, a diborane gas, a hydrogen gas, and a silane gas are introduced, and a very thin and low-concentration first amorphous silicon layer of about 1 nm is deposited on the substrate surface by plasma discharge. Introduced. After that, as in the first embodiment,
A non-doped amorphous silicon layer and a p-type amorphous silicon layer are sequentially formed to complete a pn junction. Further, an indium tin oxide (ITO) layer and a silver collecting electrode are formed as surface electrodes to complete the process.

A photovoltaic device according to the present invention was prepared under the forming conditions shown in Table 3.

[0036]

[Table 3]

Changes in the amount of boron introduced into a very thin p-type amorphous silicon layer into which a small amount of B ₂ H ₆ was introduced near the interface between the substrate and the amorphous layer formed under the conditions shown in Table 3 above. FIG. 3 shows the change in the open-circuit voltage characteristics with respect to the above.

[0038] From FIG. 3, compared with B ₂ H ₆ when not doped, the open-circuit voltage is stepwise increased, As you further doped with B ₂ H ₆ content, the open-circuit voltage starts to decrease. The amount of boron introduced is 1 × 10 ¹⁸ cm ⁻³ as compared with the case where no boron is introduced.
The open-circuit voltage could be improved in the range of 5 × 10 ¹⁹ cm ⁻³ or less. From FIG. 3, the amount of boron introduced at the interface between the substrate and the amorphous layer is 1 × 10 ¹⁸ cm ⁻³ or more and 5 ×
The doping amount of B ₂ H ₆ is preferably controlled so as to be in the range of 10 ¹⁹ cm ⁻³ or less.

Hereinafter, a third embodiment of the present invention will be described.

The cleaned n-type single-crystal silicon substrate is introduced into a vacuum chamber and heated to an appropriate temperature (200 ° C. or lower) to remove as much water as possible from the substrate surface.

Next, diborane gas and hydrogen gas are introduced to expose the substrate to the gas. The diborane (B ₂ H ₆ ) gas is decomposed on the heated substrate surface, adsorbs on the substrate surface, and boron can be introduced into the interface. Exposure conditions include
At a substrate temperature of 170 ° C., a 2% diborane (B ₂ H ₆ ) gas was used at 100 sccm and a pressure of 40 Pa for 1 to 600 seconds.

Thereafter, similarly to the first embodiment, a non-doped amorphous silicon layer and a p-type amorphous silicon layer are sequentially formed to complete a pn junction. Further, an indium tin oxide (ITO) layer and a silver collecting electrode are formed as a surface electrode to complete the process.

Also in this case, similar to the first embodiment, an improvement in the open circuit voltage was confirmed. Further, there was no significant difference in the characteristics due to the change in the exposure condition, and the change due to the presence or absence of the exposure was large.

[0044]

As described above, in a pn junction composed of crystalline silicon and an amorphous silicon semiconductor, an appropriate amount of boron is present near the interface between the amorphous silicon semiconductor substrate and the amorphous silicon semiconductor thin film. By doing so, the recombination of carriers at the interface can be suppressed, and the bonding characteristics can be improved. In a solar cell using this junction, the open-circuit voltage can be improved.

[Brief description of the drawings]

FIG. 1 is a band structure diagram of a semiconductor junction in a photovoltaic device of the present invention.

FIG. 2 is a diagram showing a result of observing an impurity profile in a depth direction when a thickness of a non-doped layer is 160 nm and a thickness of a p-type layer is about 8 nm.

FIG. 3 shows B ₂ H _{6 formed} near the interface between the substrate and the amorphous layer.
B ₂ H _{6 of} ultra-thin p-type amorphous silicon layer
It is an open-circuit voltage characteristic diagram with respect to the change of the introduction amount.

[Explanation of symbols]

 Reference Signs List 1 n-type single crystal substrate 2 i-type amorphous silicon layer 3 p-type amorphous silicon layer 4 ITO layer

Claims (5)

[Claims] An amorphous silicon thin film having no or no valence control between a crystalline silicon substrate of one conductivity type and an amorphous silicon thin film of valence control of another conductivity type. A photovoltaic device interposed by a light source, characterized in that boron atoms are present in the interface region between the crystalline silicon substrate and the non-charge-controlled or substantially intrinsic amorphous silicon thin film. Electromotive device. 2. The concentration of boron atoms is 1 × 10 ¹⁸ cm.
A photovoltaic device having a size of not less than ^{−3 and not} more than 5 × 10 ¹⁹ cm ⁻³ . A step of introducing a gas containing a low concentration of boron together with hydrogen gas to clean the surface of the one-conductivity-type crystalline silicon substrate by plasma discharge; Forming a substantially intrinsic amorphous silicon thin film, and forming an amorphous silicon thin film with valence control of another conductivity type thereon. A method for manufacturing a power device. 4. A step of introducing a gas containing silicon and a gas containing low concentration of boron to the surface of the crystalline silicon substrate to form a first amorphous silicon thin film by a gas phase reaction;
Introducing a gas containing silicon onto the first amorphous silicon thin film to form a substantially intrinsic second amorphous silicon thin film by a gas phase reaction; Forming an amorphous silicon-based thin film of which charge conductivity is controlled to another conductivity type on the porous silicon thin film. 5. A step of exposing a crystalline silicon substrate of one conductivity type to a gas containing a low concentration of boron together with hydrogen gas while heating the crystalline silicon substrate, and controlling or substantially not controlling valence electrons on the surface of the crystalline silicon substrate. Manufacturing a photovoltaic device, comprising the steps of: forming a thin amorphous silicon thin film; and forming an amorphous silicon thin film of which charge conductivity is controlled to another conductivity type thereon. Method.