JP2010034525A - Thin-film solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin-film solar cell forming a series connection structure, suppressing degradation of light absorption of an n-layer in the lowest layer, and high in conversion efficiency. <P>SOLUTION: A first electrode 2, a photoelectric conversion layer 4 and a second electrode 3 are sequentially formed on an insulating substrate 1, the photoelectric conversion layer 4 is formed with a pin structure cell comprising a plurality of nonmonocrystalline silicon-based thin films, the photoelectric conversion layer 4 formed on the first electrode 2 has an i-layer 6 formed of a microcrystalline silicon thin film, and an under layer 5 of the i-layer 6 in the photoelectric conversion layer 4 is formed of an amorphous silicon-based material having a film thickness of 200-600 nm, whereby this thin-film solar cell of an SCAF structure suppressing degradation of light absorption of the under layer 5 in the photoelectric conversion layer 4, and high in conversion efficiency can be provided. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thin film solar cell using a microcrystalline silicon-based thin film as a photoelectric conversion layer.

  A thin film solar cell using a thin film made of microcrystalline silicon or polycrystalline silicon as a photoelectric conversion layer has a light deterioration phenomenon as seen in a thin film solar cell using amorphous silicon (a-Si) as a photoelectric conversion layer. Hard to occur. In addition, since a sensitivity to long wavelength light is high, a thin film solar cell using a thin film made of microcrystalline silicon or polycrystalline silicon as a photoelectric conversion layer has been actively developed in recent years.

At present, conversion efficiency exceeding 10% is obtained in a solar cell module having a tandem structure or a triple structure in which a microcrystalline silicon layer and an a-Si layer are stacked.
On the other hand, a thin film solar cell using polycrystalline silicon often employs a pin structure as its structure, similar to a thin film solar cell using a-Si. Specifically, a p layer, an i layer, and an n layer made of a polycrystalline silicon thin film are sequentially laminated on a glass substrate on which a transparent electrode is formed, or n on an opaque substrate such as stainless steel. A layer, an i layer, and a p layer are sequentially stacked.

  As a material of the substrate, a plastic film is preferable in terms of realizing flexibility and weight reduction for increasing the degree of freedom of mounting on a roof or an outer wall. Among them, aramid, polyimide, etc. excellent in heat resistance are preferable. Particularly preferred. However, since these materials are opaque, a so-called substrate structure in which a metal electrode, a photoelectric conversion layer, and a transparent electrode are sequentially laminated on a substrate is employed. Examples of thin film solar cells that employ a substrate structure are described in Patent Document 1 and Patent Document 2.

  FIG. 10 is a cross-sectional view showing the structure of a conventional solar cell disclosed in Patent Document 1. In FIG. The solar cell illustrated in FIG. 10 is manufactured by sequentially forming strip-shaped first electrodes 102 a to 102 c, photoelectric conversion layers 103 a to 103 c, and second electrodes 104 a to 104 c on a substrate 101. For the separation of each layer, a mask film forming method or a patterning method using a laser or an ultrasonic vibrator is used.

FIG. 11 is a cross-sectional view showing the structure of a conventional solar cell disclosed in Patent Document 2. In the solar cell shown in FIG. 11, a back base electrode 210, a silicon-based photoelectric conversion unit 220, a transparent conductive film ITO 202 as an upper electrode, and a comb-shaped Ag electrode 203 for extracting current are stacked in this order on a glass substrate 201. Being done. The back base electrode 210 is formed by laminating a Ti film 211, an Ag film 212, and a ZnO film 213 in this order from the glass substrate 201 side. The silicon-based photoelectric conversion unit 220 is formed by laminating an n-type silicon layer 221, a non-doped photoelectric conversion layer (i layer) 222, and a p-type silicon layer 223 in this order from the back base electrode 210 side. Here, the average concentration of oxygen atoms in the photoelectric conversion layer (i layer) 222 is in the range of 1 × 10 ¹⁹ cm ^{−3 to} 1 × 10 ²⁰ cm ⁻³ and the photoelectric conversion layer (i layer). The average concentration of carbon atoms in 222 is 1 × 10 ¹⁹ cm ⁻³ or less.

  Here, in a solar cell having a substrate structure in which strip-shaped electrodes as disclosed in Patent Document 1 are connected in series, laser patterning (particularly, patterning of a transparent electrode) is very difficult. Not reached. In addition, when a large-area solar cell is manufactured using an insulating substrate, a substrate is used to achieve the two purposes of “simplification of external electrical wiring” and “reduction of current collection loss”. In many cases, a series connection structure of the mold is adopted. An example of a thin film solar cell employing a substrate type series connection structure is described in Patent Document 3.

  12 is a cross-sectional view showing the structure of a conventional solar cell disclosed in Patent Document 3, and FIG. 13A is a cross-sectional view taken along line 11a-11a in FIG. 12, and FIG. ) Is a cross-sectional view taken along line 11b-11b of FIG. The solar cell shown in FIG. 12 has a structure in which a hole is formed in a film substrate 301 and series connection is formed by through-hole contact, and is called SCAF (Series-Connection through Apertures on Film). In this structure, a metal electrode 302, an a-Si film 303 as a photoelectric conversion layer, and a transparent electrode 304 are laminated in this order on one surface of the film substrate 301. A back electrode 305 is formed on the other surface of the film substrate 301.

Here, a current collection hole 306 and a series connection hole 307 are formed in order to serially connect the metal electrode 302 and the back electrode 305 formed on both surfaces of the film substrate 301 (see FIG. 13A). ) And FIG. 13B).
As shown in FIG. 13A, an a-Si-based film 303, a transparent electrode 304, and a back electrode 305 are sequentially formed on the end surface of the film substrate 301 exposed by forming the current collecting hole 306. The transparent electrode 304 and the back electrode 305 are connected. On the other hand, as shown in FIG. 13B, an a-Si-based film 303, a transparent electrode 304, and a back electrode 305 are sequentially formed on the end surface of the film substrate 301 exposed by forming the series connection holes 307. The metal electrode 302 and the back electrode 305 are connected.

JP-A-55-123177 JP 2000-58889 A Y. Ichikawa, K .; Tabuchi, T .; Yoshida, S .; Kato, A .; Takano, S .; Saito, H .; Sato, S .; Fujikake, T .; Yoshida and H.K. Sakai: Conference Record 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion (1994) 441. Sakai: Conference Record 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion (1994) 441.

However, when a solar cell having a series connection structure is manufactured using a film substrate, there is a problem that cell characteristics are deteriorated due to degassing and the influence of impurities.
First, as an example in which cell characteristics deteriorate due to the effect of degassing, a thin film laminated on a substrate having a series connection hole (Reference Example 1) and a substrate having no series connection hole (Reference Example 2). Table 1 shows the result of forming the cell and evaluating the characteristics. In Table 1, “V _OC ” indicates an open circuit voltage value (V), “J _SC ” indicates a short circuit photocurrent density (mA / cm ² ), “FF” indicates a fill factor, and “Eff” indicates a conversion. Indicates efficiency (%).

  From the results shown in Table 1, Reference Example 1 in which series connection holes are formed has higher oxygen concentration in the thin film formed on the film substrate and lower conversion efficiency than Reference Example 2. I understand that. This is due to the following reason. That is, when a thin film such as a-Si or microcrystalline Si is formed on a film substrate, moisture absorbed in the film substrate is vaporized and released (degassing) in the atmosphere. Concentration increases. Degassing can be prevented if the entire surface of the film substrate is covered with metal electrodes, but in the production of a thin film solar cell having a series connection structure, a process of forming a series connection hole exposing a part of the film substrate is required. The problem of degassing is inevitable. For example, when forming a laser patterning line, a current collecting hole, and a series connection hole for the back electrode, each exposed end face becomes a degassing source.

As a method for suppressing degassing, there is a method of performing a heat treatment for about 30 minutes after carrying a film substrate into a CVD apparatus. However, it is difficult to carry out such a heat treatment with a tact time of about 3 to 5 minutes so as not to reduce the yield in the manufacturing process of the thin film solar cell.
Therefore, in a thin film solar cell in which a thin film such as a-Si or microcrystalline Si is formed on a film substrate to produce a cell having a series connection structure, oxygen of about 1 × 10 ²⁰ atoms / cm ³ is usually used. Will be mixed in the thin film.

Next, the reason why the cell characteristics deteriorate due to the influence of impurities is that the defect density of the microcrystalline Si film is lower by an order of magnitude than that of the a-Si film (actual use state: after photodegradation). The structure sensitivity is high and the Fermi level changes greatly with a small amount of impurities.
Further, since the absorption coefficient of polycrystalline silicon is about an order of magnitude smaller than that of a-Si, it is necessary to make the thickness of the thin film relatively thick in order to obtain a sufficient photocurrent. However, when the film thickness is large in this way, the electric field profile is easily distorted, and the region of the internal electric field 0 is easily formed. In particular, in the i layer whose film thickness is increased to about 1 to 3 μm, if the amount of oxygen in the film is large, a dead layer that cannot extract carriers can be formed on the n layer side of the i layer due to weak n-type ( In FIG. 11B, the short-circuit photocurrent density (J _sc ) decreases.

Thus, in a solar cell having a series connection structure using a film substrate, degassing and the presence of impurities are the conversion efficiency (Eff) and the short-circuit photocurrent density (J _sc ), which are indicators of cell characteristics of the thin-film solar cell. Cause a decline.
Therefore, the present invention has been made paying attention to the above problems, the purpose of which is a thin-film solar cell having a series connection structure, suppressing the decrease in light absorption of the n layer in the lowest layer cell, An object of the present invention is to provide a thin film solar cell having an SCAF structure with high conversion efficiency.

In order to solve the above-mentioned problems, the present inventor has made extensive studies and as a result, it functions as a protective layer for suppressing degassing, and it is effective to define the conditions of the n layer in order to suppress the decrease in light absorption. I found out.
This invention is based on the said knowledge by this inventor, The thin film solar cell which concerns on Claim 1 of this invention for solving the said subject is a 1st electrode, a photoelectric converting layer on an insulating board | substrate, In the thin film solar cell formed by sequentially forming the second electrode and the second electrode, the photoelectric conversion layer is formed by laminating one or more pin structure cells made of a plurality of non-single-crystal silicon thin films on the first electrode. The formed lowermost cell has an i layer made of a microcrystalline silicon thin film, and the lower layer of the i layer in the lowermost cell is an amorphous silicon-based material having a thickness of 200 to 600 nm. .

  In the thin film solar cell according to claim 1, in order to make the lower layer in the lowermost layer cell (photoelectric conversion layer) function as a protective layer for suppressing degassing, the lower layer is made of an amorphous silicon-based material, The film thickness was defined. Here, the lower layer is one or more layers formed between the i layer and the first electrode in the lowermost layer cell. An example of the lower layer is an n layer.

  The first reason that the lower layer material is an amorphous silicon material is that it has been confirmed that an amorphous silicon material has a higher degassing suppression function than microcrystalline silicon. This is because the amorphous silicon-based material has a uniform structure without a columnar crystal structure like the microcrystalline silicon-based material, and thus is considered to have a high water vapor barrier effect.

  The second reason why the lower layer material is an amorphous silicon material is that the absorption coefficient for light having a wavelength of 700 nm or more is lower than that of microcrystalline silicon. Since an amorphous silicon-based material behaves as a direct transition type, the absorption coefficient for short-wavelength light with a wavelength of 600 nm or less is higher than that of a crystalline silicon-based material. However, due to the wide band gap, the absorption coefficient for light is lower for amorphous silicon-based materials at wavelengths of 700 nm or more. Due to this effect, the decrease in light absorption when applied to the lower layer is smaller for amorphous silicon-based materials.

Further, when the thickness of the lower layer of the lowermost layer cell is around 200 nm, the presence of a peak of conversion efficiency can be confirmed. Further, when the film thickness of the lower layer of the lowermost cell is around 600 nm, the oxygen concentration in the i layer of the lowermost cell is not significantly reduced.
Therefore, by setting the film thickness of the lower layer made of an amorphous silicon-based material to 200 to 600 nm, it is possible to provide a thin-film solar cell that suppresses a decrease in light absorption and has high conversion efficiency.

  The thin-film solar battery according to claim 2 of the present invention is the thin-film solar battery according to claim 1, wherein the lower layer is a laminate of an n layer and an n / i interface layer substantially intrinsic to the n layer. It is a structure. That is, it is effective that the lower layer has a stacked structure of an n layer and an n / i interface layer substantially intrinsic to the n layer. When the lower layer is formed of a single thick film n layer, an absorption loss of several percent occurs. However, if the n layer is thinned by stacking with an active n / i interface layer, the absorption loss can be further suppressed. Become.

  The thin-film solar battery according to claim 3 of the present invention is the thin-film solar battery according to claim 1 or 2, wherein the amorphous silicon-based material is amorphous silicon, amorphous silicon oxide, amorphous silicon carbide, amorphous silicon. It is one of nitride, amorphous silicon oxycarbide, and amorphous silicon oxynitride.

Moreover, the thin film solar cell according to claim 4 of the present invention is the thin film solar cell according to any one of claims 1 to 3, wherein the oxygen concentration in the i layer of the lowermost cell is 4 × 10 ¹⁹ atoms / It is characterized by being cm ³ or less.

  As described above, according to the thin film solar cell according to the present invention, it is possible to provide a SCAF structure thin film solar cell that suppresses a decrease in light absorption of the n layer in the lowermost layer cell and has high conversion efficiency.

It is sectional drawing which shows the structure in 1st Embodiment of the thin film solar cell concerning this invention. It is a figure which shows the manufacturing method of the thin film solar cell which concerns on this invention. It is a graph which shows the relationship between the film thickness of n layer, and the oxygen concentration in i layer. It is a graph which shows the fall of the light absorption in n layer. It is a graph which shows the relationship between the film thickness of n layer, a short circuit photocurrent, and conversion efficiency. It is a graph which shows the relationship between the oxygen amount in i layer which consists of microcrystal silicon, and the fall rate of _Jsc resulting from oxygen. It is the schematic which shows the _Jsc fall model resulting from oxygen. It is sectional drawing which shows the structure in 2nd Embodiment of the thin film solar cell which concerns on this invention. It is a graph which shows the relationship between the film thickness of the n / i interface layer in 2nd Embodiment of the thin film solar cell which concerns on this invention, a short circuit photoelectric current, and conversion efficiency. It is the schematic which shows the structure of the conventional thin film solar cell. It is the schematic which shows the structure of the conventional thin film solar cell. It is the schematic which shows the structure of the conventional thin film solar cell. It is the schematic which shows the structure of the conventional thin film solar cell.

Hereinafter, embodiments of a thin film solar cell according to the present invention will be described with reference to the drawings.
(First embodiment)
FIG. 1 is a schematic cross-sectional view showing a cell configuration of a thin-film solar battery according to the present invention. The thin film solar cell of this embodiment is formed by sequentially forming at least a first electrode, a photoelectric conversion layer, and a second electrode on an insulating film substrate 1. Specifically, as shown in FIG. 1, a photoelectric conversion layer 4 formed on a film substrate 1, metal electrodes 2 and 3 formed on both surfaces of the film substrate 1, and a metal electrode 2 as a first electrode. And a transparent electrode 8 and a current collecting electrode 9 as second electrodes sequentially formed on the photoelectric conversion layer 4. The photoelectric conversion layer 4 includes a lower layer 5, an i layer 6, and a p layer 7 in order from the metal electrode 2 side. The photoelectric conversion layer 4 is formed of one layer or a plurality of layers. In the present embodiment, the n layer 5 a is employed as the lower layer 5. Although not shown, the film substrate 1 has a current collecting hole and a series connection hole. Since the film substrate 1 is exposed at the end faces of these current collecting holes and series connection holes, the film substrate 1 absorbs moisture in the atmosphere, and water vapor is released as degassing during film formation.

(Method for manufacturing thin film solar cell)
As an apparatus for manufacturing the thin film solar cell of the present invention, a manufacturing apparatus including a CVD 3 chamber and a sputtering 2 chamber is used. The electrode size of the CVD chamber is 625 cm ² , and a 20 cm □ thin film solar cell can be produced on the film substrate 1 having a width of about 25 cm. Each chamber can be completely sealed with the film substrate 1 interposed therebetween, and a cell can be manufactured by a stepping roll film forming method in which film formation and conveyance are alternately repeated.

Hereinafter, a method for manufacturing a thin-film solar cell having a cell area of 1 cm ² using this manufacturing apparatus will be described with reference to FIG.
<Formation of the first electrode>
First, as shown in FIGS. 2A to 2B, series connection holes 11 were formed in the film substrate 1.

Next, as shown in FIG. 2 (c), after forming the first electrode layer 2 and the third electrode layer (a part of the back electrode) 3 on both surfaces of the film substrate 1, as shown in FIG. 2 (c). In addition, a current collecting hole 10 was formed at a position away from the series connection hole 11 by a predetermined distance.
Next, as shown in FIG. 2 (d), the first electrode layer 2 is laser-processed into a predetermined shape between the formation of the third electrode layer 3 and the formation of the current collecting holes 10, so that the first electrode 2 was patterned.

<Formation of n layer>
Thereafter, as shown in FIG. 1 and FIG. 2 (e), the photoelectric conversion layer 4 is formed on the first electrode layer 2. The photoelectric conversion layer 4 formed on the first electrode layer 2 first sends the film substrate 1 to the first CVD chamber, and after degassing for 3 minutes, the n layer 5a is formed on the first electrode layer 2 by the CVD apparatus. Is formed. As the material of the n layer 5a, "microcrystalline silicon (Si)", "amorphous silicon (a-Si)", and "amorphous silicon oxide (a-SiO)" are respectively employed, and different n layers 5a are formed. Three types of film substrates 1 were produced.

Here, in the cell employing a-SiO for the n layer 5a, the oxygen concentration in the film was set to 5 to 10 at% so as not to increase the series resistance of the cell. The optical band gaps of the cell employing a-Si for the n layer 5a and the cell employing a-SiO for the n layer 5a were 1.75ev and 1.82ev, respectively.
Both the cell employing microcrystalline Si for the n layer 5a and the cell employing a-Si for the n layer 5a used monosilane as the main gas, hydrogen as the diluent gas, and phosphine as the doping gas. It was formed by adding CO ₂ as adopted cell further main gas a-SiO the n layer 5a. The three types of n layers 5a were formed at a film formation rate of 60 nm / min.

<Formation of i layer>
Next, after evacuation, each film substrate 1 on which the n layer 5a was formed was sent to the second CVD chamber, and an i layer 6 having a thickness of 1 μm was formed on the n layer 5a. The film formation temperature at this time was 200 to 250 ° C., monosilane was used as the main gas, and hydrogen was used as the dilution gas.

<Formation of p layer>
Then, after vacuuming, each film substrate 1 on which the n layer 5a and the i layer 6 were formed was sent to the third CVD chamber, and a p-layer of microcrystalline Si having a thickness of 20 nm was formed on the i layer. . The film formation temperature at this time was 150 to 200 ° C., monosilane was used as the main gas, hydrogen was used as the dilution gas, and diborane was used as the doping gas.

<Formation of second electrode>
Thereafter, as shown in FIGS. 1 and 2 (f), after evacuation and release to the atmosphere, the film substrate 1 on which the photoelectric conversion layer 4 composed of the n layer 5a, the i layer 6 and the p layer 7 is formed is carried out. Then, a mask film is formed on the p-layer 7 using the transparent electrode 8 of about 70 nm as the second electrode by a small sputtering apparatus.

Further, the collector electrode 9 is formed on the transparent electrode 8 by a small vapor deposition apparatus, and the back electrode layer 3 is formed on the third electrode layer 3 as shown in FIG. Thereafter, the thin film on both sides of the film substrate 1 is separated using a laser beam to make the cell area 1 cm ² , and the lowermost cell (the photoelectric conversion layer 4 of one unit cell formed on the first electrode 2). A thin film solar cell having a series connection structure in which the material of the n layer 5a was made of three types was prepared.

(Discussion)
Regarding the three types of thin film solar cells produced as described above, the oxygen concentration in the film, the decrease in light absorption, the short-circuit photocurrent density, the conversion efficiency, and the decrease in J _SC were considered as follows.
<Relationship between film thickness and oxygen concentration in film>
FIG. 3 is a graph showing the relationship between the film thickness of the n layer of the lowermost layer cell and the oxygen concentration in the film of the i layer of the lowermost layer cell in three types of thin film solar cells. Since the oxygen concentration in the film has a profile in the depth direction, the average oxygen concentration in the i layer of the lowermost cell was measured using a SIMS (Secondary Ion Mass Spectrometry) apparatus.

  From the results shown in FIG. 3, it can be seen that the oxygen concentration in the film decreases as the film thickness increases for the n layer of the lowermost cell in the three types of thin film solar cells. In particular, the thin film solar cell employing a-Si and a-SiO for the n-layer of the lowermost cell has a lower degree of oxygen concentration in the film than the thin-film solar cell employing polycrystalline silicon for the n-layer of the lowermost cell. It turns out that is remarkable. This is presumably because the amorphous silicon-based material does not have a columnar structure and thus has a higher gas barrier property against water vapor than microcrystalline silicon.

From the results shown in FIG. 3, it was also found that when an amorphous silicon-based material is used for the n layer, the oxygen concentration in the film becomes 4 × 10 ¹⁹ atoms / cm ³ or less when the film thickness is 200 nm or more. In addition, when the film thickness of the n layer exceeds 600 nm, no significant reduction is observed in the decrease in the oxygen concentration in the film, so it is considered that the film thickness of the n layer is preferably 600 nm or less.

<Relationship between film thickness and reduction rate of light absorption>
Next, a decrease in the light absorption of the n layer was derived using the absorption coefficient data of the n layer of the lowest layer cell in the three types of thin film solar cells and a simulation program. As a simulation program, a simulation program considering light confinement due to scattering of incident light was used. It has been separately confirmed that the simulation results and the spectral sensitivity characteristics of the actual solar cell agree well when various n layers are applied.

FIG. 4 is a graph showing the relationship between the film thickness of the n layer and the reduction rate of light absorption.
As shown in FIG. 4, the thin-film solar cell employing an amorphous silicon-based material for the n-layer of the lowermost cell is lower than the thin-film solar cell employing microcrystalline Si for the n-layer of the lowermost cell. It can be seen that the rate of decrease in light absorption decreases as the thickness of the n layer of the lower layer cell increases. This is because the light absorption of the amorphous silicon-based material is smaller with respect to the long wavelength light incident on the n layer. In particular, the decrease in light absorption of a-SiO is about half that of microcrystalline Si. This suggests that application of an a-Si wide gap alloy material is effective.

<Relationship between film thickness, short-circuit photocurrent and conversion efficiency>
FIG. 5 is a graph showing the relationship between the film thickness of the n-layer of the lowest layer cell in three types of thin film solar cells, the short-circuit photocurrent density, and the conversion efficiency, and FIG. It is a graph which shows the relationship between the film thickness of n layer of the lowest layer cell in a battery, and a short circuit photocurrent density, FIG.5 (b) is the film thickness of n layer of the lowest layer cell in three types of thin film solar cells. It is a graph which shows the relationship with conversion efficiency.

As shown in FIGS. 5A and 5B, it can be seen that the tendency of the short-circuit photocurrent density with respect to the thickness of the n layer is similar to the tendency of the conversion efficiency with respect to the thickness of the n layer. In particular, in FIG. 5B, the presence of a peak of conversion efficiency can be confirmed when the thickness of the n layer of the lowermost cell in the three types of thin film solar cells is around 200 nm. It can also be seen that the thin film solar cell using amorphous silicon-based material has higher short-circuit photocurrent density and conversion efficiency than the thin film solar cell using microcrystalline Si for the n layer. This is considered to be due to the high gas barrier property and low light absorption property of the n layer described above. In particular, the conversion efficiency of the thin film solar cell employing a-SiO, which has low light absorption, is higher than that of the thin film solar cell employing a-Si.
As described above, from the results shown in FIGS. 3 to 5, it is concluded that it is effective to use an amorphous silicon-based material so that the thickness of the n layer is 200 nm to 600 nm.

<Relationship between oxygen concentration in film and _JSC reduction rate>
FIG. 6 is a graph showing the relationship between the oxygen concentration in the i-layer of the lowermost cell and the J _sc decrease rate due to oxygen based on the results of FIGS. is there. Specifically, for the n layer of the lowermost layer cell in the three types of thin film solar cells, lower components other than optical absorption were derived and plotted against the amount of oxygen in the film of the i layer of the lowermost layer cell.

From the results shown in FIG. 6, it can be seen that in the three types of thin-film solar cells, the J _sc decrease rate due to this increases as the oxygen concentration in the i-layer increases. This can be explained by the model shown in FIG. That is, when there is no space charge in the i layer, the electric field profile is linear as shown in FIG. 7A. However, when oxygen increases, this acts as a donor ion and the band profile is distorted. It is thought that there will be a dead layer where carriers cannot be collected.

From the results shown in FIG. 6, it is considered that the amount of oxygen in the film should be low. However, if the degassing time is shortened for mass production, the amount of oxygen in the film increases. That is, it is considered that the amount of oxygen in the film and the throughput when using a film substrate are in a trade-off relationship.
Therefore, it is considered appropriate to set the rate of decrease in J _sc due to oxygen in FIG. 6 to about 10% or less, and the oxygen concentration in the i layer is set to 4 × 10 ¹⁹ atoms / cm ³ or less. It is considered reasonable.

(Second Embodiment)
FIG. 8 is a cross-sectional view showing the structure in the second embodiment of the thin-film solar cell according to the present invention.
As shown in FIG. 8, in the present embodiment, the lower layer 5 formed between the i layer 6 and the first electrode 2 has a two-layer structure of an n layer 5a and an n / i interface layer 5b. This is different from the first embodiment.
Regarding the thin-film solar cell of the present embodiment having such a configuration, the dependency on the film thickness of the n / i interface layer 5b was examined as in the first embodiment. In the present embodiment, the material of the n layer 5a is a-SiO, and the material of the n / i interface layer 5b is a-Si. The n / i interface layer 5b was formed using monosilane as a source gas and hydrogen as a diluent gas, and without adding a doping gas. Furthermore, in order to make the conditions as a protective layer uniform, the total thickness of the n layer 5a and the n / i interface layer 5b was set to a constant value (400 nm).

FIG. 9 shows the relationship between the film thickness of the n / i interface layer thus obtained, J _sc, and conversion efficiency. FIG. 9A is a diagram showing the relationship between the film thickness of the n / i interface layer and J _sc, and FIG. 9B shows the relationship between the film thickness of the n / i interface layer and the conversion efficiency. FIG. As shown in FIG. 9A, it can be seen that J _sc increases as the thickness of the n / i interface layer increases (the thickness of the n layer decreases). Further, as shown in FIG. 9B, it can be seen that the conversion efficiency increases as the thickness of the n / i interface layer increases (the thickness of the n layer decreases). This is because the n layer is a dead layer, whereas the n / i interface layer is an active layer, so that the light absorption loss decreases as the film thickness of the n layer decreases, and J _sc increases. From these results, it has been found that according to the present embodiment, higher conversion efficiency can be obtained than in the first embodiment.

  As mentioned above, although embodiment of this invention has been described, this invention is not limited to this, A various change and improvement can be performed. For example, the photoelectric conversion layer may be configured by a plurality of pin structures. Further, it is apparent that the present invention is effective for improving the conversion efficiency of a tandem or triple structure thin film solar cell. The present invention is also effective for a substrate type solar cell in which strip-shaped electrodes are shifted and stacked. Furthermore, the amorphous silicon-based material may be a material other than a-Si and a-SiO. As an amorphous silicon-based material whose absorption coefficient for light with a wavelength of 700 nm or more is lower than that of microcrystalline silicon, the simplest material is amorphous silicon. However, when the band gap is widened by alloying, It is valid. As typical materials, amorphous silicon oxide, amorphous silicon carbide, amorphous silicon nitride, amorphous silicon oxycarbide, and amorphous silicon oxynitride are effective.

  Here, the characteristics of the microcrystalline silicon solar cell may deteriorate when left in the atmosphere for about one month depending on the film formation conditions of the i layer. The cause of this is not well understood, but it is thought that oxygen and water vapor enter the columnar structure from the surface of microcrystalline silicon and oxidize. For example, when a tandem cell or a triple cell is produced using a microcrystalline silicon cell on a plastic film substrate, the front side is protected by a homogeneous a-Si top cell, but the back side is protected. Absent. Therefore, if water vapor or oxygen enters from the back surface in a high temperature and high humidity test or the like, the microcrystalline cell may be deteriorated.

  On the other hand, in the present invention, not only the front side but also the back side is protected by an amorphous silicon thin film having a thickness of 200 nm to 600 nm, so that the influence of water vapor and the like can be completely blocked. Also play.

DESCRIPTION OF SYMBOLS 1 Film substrate 2,3 Metal electrode 4 Photoelectric conversion layer 5 Lower layer 5a n layer 5b n / i interface layer 6 i layer 7 p layer 8 Transparent electrode 9 Current collecting electrode 10 Current collecting hole 11 Series connection hole

Claims (4)

In a thin film solar cell in which a first electrode, a photoelectric conversion layer, and a second electrode are sequentially formed on an insulating substrate,
The photoelectric conversion layer is formed by laminating one or more pin structure cells made of a plurality of non-single-crystal silicon thin films, and the lowermost cell formed on the first electrode is made of a microcrystalline silicon thin film. A thin-film solar cell comprising: an amorphous silicon-based material having a thickness of 200 to 600 nm as a lower layer of the i layer in the lowermost cell.   The thin film solar cell according to claim 1, wherein the lower layer has a stacked structure of an n layer and an n / i interface layer substantially intrinsic to the n layer.   The amorphous silicon-based material is any one of amorphous silicon, amorphous silicon oxide, amorphous silicon carbide, amorphous silicon nitride, amorphous silicon oxycarbide, and amorphous silicon oxynitride. The thin film solar cell according to 1. 4. The thin film solar cell according to claim 1, wherein an oxygen concentration in the i layer of the lowermost cell is 4 × 10 ¹⁹ atoms / cm ³ or less.

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