JP2002222969A - Laminated solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated solar battery having a plurality of photoelectric conversion layers laminated so that the layers are electrically connected in series with each other to enable characteristics such as a conversion efficiency or the like to be improved and to enable a yield in the case of producing in a large area and a large quantity to be improved. SOLUTION: At least one or more of photoelectric conversion layers 503 to 505 has a transparent conductive film layer 504 having a ruggedness on a light incident side surface 508. The layer 505 of the light incident side is formed in a state in which the ruggedness is reflected. A sheet resistance of the layer 504 is 30 Ω/(square) or more, and a resistance in a thickness direction is 3 Ω.cm2 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stacked solar cell, and more particularly, to a solar cell in which a plurality of photoelectric conversion layers are stacked and the photoelectric conversion layers are electrically connected in series.

[0002]

2. Description of the Related Art As a laminated solar cell of this type, photoelectric conversion layers made of materials having different band gaps are laminated,
It is known that they are electrically connected in series (for example, JP-A-1-289173). In this stacked solar cell, a photoelectric conversion layer made of amorphous silicon having a large band gap is arranged on the light incident side (upper), and a photoelectric conversion layer made of crystalline silicon having a small band gap is arranged below. Light energy in a wider wavelength range is efficiently extracted. That is, in semiconductor materials, the forbidden band width (Eg)
Only photons having the above energies are absorbed, and photons having an energy lower than Eg are transmitted and lost. However, the energy (hν) of the absorbed photon is Eg
If it is larger than the above, the energy corresponding to (hv-Eg) is absorbed by the semiconductor lattice and becomes thermal energy,
Since it cannot be electrically extracted outside, the utilization efficiency of the absorbed energy deteriorates. Therefore, by arranging a plurality of semiconductor materials in order of Eg from the light incident side, energy loss is reduced, and light is more efficiently used.

In such a stacked solar cell, Japanese Patent No. 2738557 discloses that the output current of each photoelectric conversion layer is matched and the output current of the entire solar cell is maximized so that the output current is prohibited in order from the light incident side. A device provided with a selective reflection film between two photoelectric conversion layers having a small band width has been proposed. In this stacked solar cell, the reflectance of the surface of the selective reflection film is maximized in the wavelength region where the photoelectric conversion layer on the light incident side (upper) has the maximum photosensitivity by optimally selecting the thickness of the selective reflection film. At the same time, the reflectance of the surface of the selective reflection film is lowered in a wavelength range where the lower photoelectric conversion layer can absorb the light. Thereby, the output current of each photoelectric conversion layer can be adjusted to be equal, and the output current of the entire solar cell can be optimized.

[0004] Separately, Japanese Patent Application Laid-Open No. H11-214728 discloses that the surface of a substrate or underlayer (or the surface of a back electrode)
Has been proposed in which a light incident side surface of a photoelectric conversion layer on the light incident side is made uneven by forming a texture structure including fine irregularities. The unevenness formed suppresses multiple reflection in the photoelectric conversion layer on the light incident side, reduces surface reflectance in a long wavelength region (reflectance on the surface on the light incident side), and reduces the photoelectric conversion layer on the light incident side. By increasing the optical path length inside, the amount of light absorption in the photoelectric conversion layer can be increased (light confinement effect). In particular, in a photoelectric conversion layer made of an amorphous silicon-based material, the distance over which carriers generated by light irradiation move can be shortened, and the internal electric field in the film can be increased. Characteristic deterioration due to irradiation can be suppressed.

[0005]

However, the above two proposals for the stacked solar cell have the following problems, respectively.

[0006] In the proposal described in Japanese Patent No. 2738557, a selective reflection film utilizing interference due to multiple reflection inside the film is used between photoelectric conversion layers, and control of the film thickness becomes important. When an amorphous silicon-based thin film is used as the photoelectric conversion layer on the light incident side, its thickness is about several hundred nm, which is on the same order as the wavelength at which the amorphous silicon-based thin-film solar cell has the highest sensitivity. It is difficult to control the reflection spectrum of the selective reflection film when interference due to multiple reflection in the layer is taken into account. Further, if irregularities are formed as described in JP-A-11-214728, the effect of interference due to multiple reflection in the selective reflection film is lost, and the function of selective reflection cannot be performed. Furthermore, when a transparent conductive film having a small sheet resistance is inserted as a selective reflection film, a shunt path leak occurs due to a short circuit through a defect in the photoelectric conversion layer between the surface electrode for taking out electric power and the transparent conductive film layer. In addition, there is a problem that the characteristics of the power generation region other than the leak portion are adversely affected.

In the proposal described in Japanese Patent Application Laid-Open No. H11-214728, the optimum unevenness on the surface of the stacked solar cell depends on crystalline silicon and the unevenness on the surface of the back electrode. Therefore, the conditions for forming the crystalline silicon and the back electrode are limited,
It is extremely difficult to simultaneously optimize these film properties and the structure of the stacked solar cell. In particular, the film quality of crystalline silicon has a great influence on the conversion efficiency, and therefore needs to be optimized separately from other elements. When an amorphous silicon-based material is used as the photoelectric conversion layer on the light incident side, there is almost no difference in the refractive index between the amorphous silicon material and the lower crystalline silicon (the refractive index is 3 to 4 for both amorphous and crystalline silicon). Therefore, the reflection at the interface between the photoelectric conversion layers is reduced, and the output current of the photoelectric conversion layer on the light incident side is reduced. Therefore, there is a problem that the output current of the entire solar cell is limited by the output current of the photoelectric conversion layer on the light incident side, and the output current of the entire solar cell is reduced.

SUMMARY OF THE INVENTION An object of the present invention is to provide a stacked solar cell which can improve characteristics such as conversion efficiency while solving the above problems, and can also improve the yield in large-area and mass production. is there.

[0009]

Means for Solving the Problems To achieve the above object, a laminated solar cell according to the present invention comprises a laminated solar cell in which a plurality of photoelectric conversion layers are laminated and the photoelectric conversion layers are electrically connected in series. In the battery, a transparent conductive film layer having irregularities on the light incident side surface is provided on at least one or more of the photoelectric conversion layers, and the photoelectric conversion layer on the light incident side is formed in a state reflecting the irregularities, The sheet resistance of the transparent conductive film layer is 30Ω / □ or more, and the resistance in the film thickness direction is 3Ω · cm.
² or less.

Here, the “photoelectric conversion layer on the light incident side” is
When the stacked solar cell is used, it refers to a photoelectric conversion layer disposed on the light incident side of the transparent conductive film layer.

The term "irregularities" does not mean a fixed shape but a predetermined surface roughness that scatters sunlight.

In the stacked solar cell of the present invention, since at least one or more transparent conductive layers are provided between the photoelectric conversion layers, the material of the photoelectric conversion layer and the material of the transparent conductive layer on the light incident side are different. The difference in the refractive index can be easily increased during. Amorphous or crystalline silicon, which is generally used as a material for a photoelectric conversion layer, has a refractive index of about 3 to 4, whereas ZnO (zinc oxide) or I, which is generally used as a material for a transparent conductive film layer, is used.
This is because TO (tin-added indium oxide) has a refractive index of about 2. As a result, the reflectance at the interface between the photoelectric conversion layer on the light incident side and the transparent conductive film layer can be increased, and the output current of the photoelectric conversion layer on the light incident side can be increased. Therefore, the state where the output current of the entire solar cell is limited by the output current of the photoelectric conversion layer on the light incident side (usually in this tendency) can be eliminated, and the output current of the entire solar cell can be increased. Further, since the transparent conductive film layer has irregularities on the light incident side surface, and the photoelectric conversion layer on the light incident side is formed in a state reflecting the irregularities, multiple reflection in the light incident side photoelectric conversion layer is prevented. In addition to reducing the surface reflectivity in the long wavelength region, the optical path length inside the photoelectric conversion layer on the light incident side can be increased to increase the amount of light absorption in the photoelectric conversion layer (light confinement effect) ). In addition, since the sheet resistance of the transparent conductive film layer is as large as 30 Ω / □ or more, shunt path leak due to a short circuit between the surface electrode for taking out power to the outside and the transparent conductive film layer through a defective portion in the photoelectric conversion layer is reduced. Even if it occurs, the resistance component between the leaked portion and the other power generation region other than the leaked portion becomes large, so that the photoelectric conversion characteristics of the other power generation regions are not adversely affected. In addition, since the resistance in the thickness direction of the transparent conductive film layer is as small as 3 Ω · cm ² or less, a decrease in characteristics due to an increase in series resistance between the photoelectric conversion layers does not occur.

In general, in forming a transparent conductive film of a metal oxide by a sputtering method, in order to increase the specific resistance, as shown in FIG. 1, the film is formed by increasing the oxygen partial pressure / argon partial pressure ratio. This is realized by reducing the oxygen vacancies in the inside and reducing the carrier density. However, as the carrier density decreases, as shown in FIG. 2, the amount of light absorbed decreases, which contributes to an increase in output current. Further, in order to reduce the series resistance in the film thickness direction, it is possible to reduce the thickness of the transparent conductive film layer. FIG. 3 shows the results of experimentally examining the FF (fill factor) reduction with respect to the series resistance. If the series resistance is 3 Ω · cm ² or less, the decrease in FF can be suppressed to within 5%. In the present invention, a series resistance of 3Ω · c
It is defined as m ² or less.

FIG. 4 shows a simulation result of the surface reflectance of the stacked solar cell when the surface shape is flat. The thickness of the transparent conductive film layer between the photoelectric conversion layers is 50 n
When it is increased to m, the surface reflectivity in the long wavelength region increases due to the effect of multiple reflection on the light incident side photoelectric conversion layer, and the amount of incident light in the long wavelength region where the lower photoelectric conversion thin film layer has sensitivity decreases. Would. Conventional technology (Japanese Unexamined Patent Application Publication
As described in relation to JP-A-214728, if the surface of the substrate and the underlying layer is made to have a texture structure including fine irregularities, and if the light incident side surface of the light incident side photoelectric conversion layer is made uneven, the light incident side Multiple reflection in the photoelectric conversion layer can be suppressed, and the surface reflectance in a long wavelength region can be reduced. However, due to the formed irregularities, a short circuit occurs between the surface electrode and the transparent conductive layer through a defective portion of the photoelectric conversion layer, so that a shunt path leak is likely to occur, resulting in deterioration of characteristics. In the present invention, since the sheet resistance of the underlayer is increased to 30 Ω / □ or more, even if a shunt path leak occurs due to irregularities aiming at a light confinement effect,
It is possible to suppress the deterioration of the solar cell characteristics. Therefore, the yield at the time of large area and mass production can be improved.

In one embodiment, the stacked solar cell is characterized in that the plurality of photoelectric conversion layers are stacked on a supporting substrate.

In the stacked solar cell of this embodiment, the plurality of photoelectric conversion layers are supported by a support substrate. Thereby, mechanical strength is obtained.

In one embodiment, at least one of the plurality of photoelectric conversion layers includes a supporting material for supporting the remaining photoelectric conversion layers.

In the stacked solar cell of this embodiment, at least one photoelectric conversion layer supports the remaining photoelectric conversion layers. Thereby, mechanical strength is obtained.

In one embodiment, the unevenness on the light incident side surface of the transparent conductive film layer is formed by etching the transparent conductive film.

In the stacked solar cell of this embodiment, the irregularities on the light incident side surface of the transparent conductive film layer are easily formed.

In one embodiment, the surface of the support substrate has irregularities.

In the stacked solar cell of this embodiment, if a photoelectric conversion layer is deposited on the surface of the support substrate and the transparent conductive film layer is deposited thereon, the unevenness on the surface of the support substrate is reflected. Thus, irregularities of the transparent conductive film layer are formed.

In one embodiment, the surface of the photoelectric conversion layer containing the support material has irregularities.

In the stacked solar cell of this embodiment, if the transparent conductive film layer is deposited on the surface of the photoelectric conversion layer containing the support material, the unevenness on the surface of the photoelectric conversion layer containing the support material is reflected. Thus, irregularities of the transparent conductive film layer are formed.

In one embodiment, the stacked solar cell is characterized in that the electrode surface on the support substrate has irregularities.

In the stacked solar cell of this embodiment, if the photoelectric conversion layer and the transparent conductive film layer are deposited on the electrode surface, the unevenness of the transparent conductive film layer reflects the unevenness of the electrode surface. It is formed.

The stacked solar cell according to one embodiment is characterized in that the surface of the photoelectric conversion layer below the transparent conductive film layer has irregularities.

In the stacked solar cell of this embodiment, if the transparent conductive film layer is deposited on the surface of the photoelectric conversion layer, the transparent conductive film layer reflects the unevenness of the surface of the photoelectric conversion layer. Are formed.

In one embodiment, the stacked solar cell is characterized in that the transparent conductive film layer contains zinc oxide as a main component.

In the stacked solar cell of this embodiment, the transparent conductive film layer is easily formed by a known sputtering method, vacuum evaporation method or the like. In addition, irregularities on the surface of the transparent conductive film layer can be easily formed by known wet etching and dry etching techniques. Furthermore, since zinc oxide has plasma resistance, the unevenness formed on the surface of the transparent conductive film layer is favorably maintained in a later step, that is, a photoelectric conversion layer deposition step.

In one embodiment of the stacked solar cell, two photoelectric conversion layers are provided, the photoelectric conversion layer on the light incident side is made of amorphous silicon or an amorphous silicon alloy, and the lower photoelectric conversion layer Is composed of a material containing crystalline silicon.

In the stacked solar cell of this embodiment, the photoelectric conversion layer on the light incident side and the lower photoelectric conversion layer are easily formed by a known semiconductor process.

In one embodiment of the stacked solar cell, three photoelectric conversion layers are provided, the photoelectric conversion layer on the light incident side is made of amorphous silicon or an amorphous silicon alloy, and an intermediate photoelectric conversion layer is provided. And the lower photoelectric conversion layers are each made of a material containing crystalline silicon.

In the stacked solar cell of this embodiment, the photoelectric conversion layer on the light incident side, the intermediate photoelectric conversion layer, and the lower photoelectric conversion layer are easily formed by a known semiconductor process.

In one embodiment of the stacked solar cell, three photoelectric conversion layers are provided, and the photoelectric conversion layer on the light incident side and the intermediate photoelectric conversion layer are each made of amorphous silicon or an amorphous silicon alloy. Wherein the lower photoelectric conversion layer is made of a material containing crystalline silicon.

In the stacked solar cell of this embodiment, the photoelectric conversion layer on the light incident side, the intermediate photoelectric conversion layer, and the lower photoelectric conversion layer are easily formed by a known semiconductor process.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a laminated solar cell according to the present invention will be described in detail with reference to the illustrated embodiments.

(First Embodiment) FIG. 5 schematically shows a sectional structure of a laminated solar cell according to a first embodiment. This stacked solar cell has a structure in which amorphous silicon / transparent conductive film / polycrystalline silicon substrate are stacked. Specifically, in the stacked solar cell, the n-type semiconductor substrate 501
And the p-type semiconductor layer 503a formed on the surface thereof constitutes the first photoelectric conversion layer 503. On the first photoelectric conversion layer 503, a transparent conductive film layer 504 having irregularities on the surface 508, a second photoelectric conversion layer 505, and an upper transparent electrode 5
06 and the upper comb-shaped electrode 507 are stacked in this order. Back side of n-type semiconductor substrate 501 (transparent conductive layer 504)
The back electrode 502 is formed on the surface opposite to the surface on which is formed). The irradiation light 509 indicates light to be irradiated at the time of measuring solar cell characteristics.

This laminated solar cell is manufactured as follows.

First, as the n-type semiconductor substrate 501, an n-type polycrystalline silicon substrate containing 10 ¹⁷ cm ^{-3 of} phosphorus is prepared. As such an n-type semiconductor substrate, a single crystal silicon substrate can be used.

Next, the surface of the substrate 501
RCA cleaning (RC
H developed by Company A₂O₂, NH₄Using OH and Hcl
Cleaning method). Subsequently, the substrate 5 is formed by a thermal diffusion method.
01, a p-type semiconductor layer 503a is formed. In detail
Heats the n-type semiconductor substrate 501 in a furnace, ₃
Is used as a diffusion source, and the surface region of the n-type semiconductor substrate 501 is
Forming a p-type semiconductor layer 503a by diffusing silicon
You. The other one for forming the p-type semiconductor layer 503a
The methods include public methods such as plasma CVD and ion implantation.
Known techniques can be used.

Thereafter, a back electrode 502 made of Ti / Pd / Ag is formed on the back surface of the n-type semiconductor substrate 501 by electron beam evaporation. In addition, as the material of the back surface electrode 502, a simple metal such as Al or an alloy thereof can be used. The method for forming the back electrode 502 is not limited to the electron beam evaporation method, and a known technique such as a sputtering method can be used.

After the formation of the back electrode 502, a transparent conductive film layer 504 containing zinc oxide (ZnO) as a main component is formed on the first photoelectric conversion layer 503. As a material of the transparent conductive film layer 504, in addition to zinc oxide (ZnO), ITO (a material containing several percent by weight of tin in indium oxide), tin dioxide (SnO ₂ ), and the like may be used. Excellent ZnO is desirable. Further, the ZnO may be a mixture of dopants such as gallium (Ga), aluminum (Al), and boron (B). As a method for forming the transparent conductive film layer 504, a sputtering method, a vacuum evaporation method, or the like can be employed. In this example, ZnO containing gallium was deposited as the transparent conductive film layer 504 by a sputtering method. In this film formation, in a sputtering apparatus, with the substrate 501 heated to 200 ° C., oxygen is flowed at a flow rate of 1.5 sccm, argon is flowed at a flow rate of 100 sccm, the pressure is set to 0.7 Pa, and the substrate 501 and the cathode ( (Not shown target)
And a DC bias of 500 V is applied. This film formation condition is that the specific resistance of the transparent conductive film layer 504 is 1 × 10 ⁻² Ω.
・ This is a condition that is as large as cm. The thickness of the transparent conductive film layer 504 after this film formation is about 600 nm, and the surface roughness is Rmax.
= 42 nm. Subsequently, the transparent conductive film layer 504
In order to form irregularities on the surface of the transparent conductive layer 504, a 0.5% by weight aqueous solution of acetic acid is used as an etchant.
Is wet-etched for 50 seconds (this is called "irregularity etching"). As a result, the roughness of the surface 508 of the transparent conductive film layer 504 is reduced to Rmax
= 198 nm. The thickness of the transparent conductive film 504 after the uneven etching is 400 nm ± 100 nm.
The sheet resistance was 200 Ω / □ and the resistance in the film thickness direction was 5 × 10 ⁻⁷ Ω · cm ² . Note that a method for forming unevenness on the surface 508 of the transparent conductive film layer 504 is not limited to wet etching, and other methods such as dry etching can be adopted.

Next, a second photoelectric conversion layer 505 as a light incident side photoelectric conversion layer is formed on the transparent conductive film layer 504 having irregularities (texture) on the surface 508 by a sputtering method. As a material of the second photoelectric conversion layer 505, amorphous silicon having a wider band gap than crystalline silicon was employed. This is because amorphous silicon has a wider band gap than that of crystalline silicon and is therefore suitable as a material for the light incident side photoelectric conversion layer of the stacked solar cell. The surface roughness of the second photoelectric conversion layer 505 after this film formation is:
This reflects the roughness of the surface 508 of the transparent conductive film layer 504. In this example, the conditions for forming the second photoelectric conversion layer (including the n, i, and p layers) 505 were as shown in Table 1 below.

[Table 1]

Next, an upper transparent electrode 506 is formed on the second photoelectric conversion layer 505. The upper transparent electrode 506 is formed by depositing ITO to a thickness of 60 nm by a sputtering method. In this film formation, in a sputtering apparatus,
With the substrate 501 heated to 220 ° C., oxygen was flowed at a flow rate of 1.4 sccm, argon was flowed at a flow rate of 250 sccm, the gas pressure was set at 3.8 Pa, and 450 V was applied between the substrate 501 and a cathode (not shown). Apply DC bias. The surface roughness of the upper transparent electrode 506 after the film formation reflects the surface roughness of the second photoelectric conversion layer 505. The surface roughness of the upper transparent electrode 506 after film formation is R
max = 186 nm. This value can reduce the reflection on the surface of the stacked solar cell and obtain a light confinement effect.

Next, an upper comb-shaped electrode 507 is formed on the upper transparent electrode 506. The upper comb-shaped electrode 507 is formed of silver to a thickness of 300 nm by an electron beam evaporation method. In forming the upper comb-shaped electrode 507, silver is selectively deposited on the substrate surface using a metal mask while the substrate 501 is heated to 180 ° C. in an evaporation apparatus. By using a low-resistance material such as silver as the material of the upper comb-shaped electrode 507, the current collecting capability of the upper transparent electrode 506 can be complemented, and current collection can be performed more efficiently. As a method for forming the upper comb-shaped electrode 507, in addition to the vacuum evaporation method,
A sputtering method is exemplified.

The following Table 2 shows that the laminated solar cell fabricated in this manner was subjected to AM-1.
The results of measuring solar cell characteristics (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) by giving irradiation light 509 of ⁵ , 100 mW / cm ² are shown together with the results of the comparative example. In addition, as a comparative example, a transparent conductive film is not inserted between the photoelectric conversion layers (this is referred to as a “comparative example”),
A transparent conductive film having no irregularities on its surface (this is referred to as “Comparative Example”) was produced. Other manufacturing conditions of the comparative example are the same as those of the above-described stacked solar cell.

[Table 2]

As can be seen from Table 2, in the stacked solar cell of this embodiment, the output current (short-circuit current density) of the entire solar cell is larger than that of the comparative example.
The first reason is that, by inserting the transparent conductive film layer 504 between the photoelectric conversion layer 505 on the light incident side and the lower photoelectric conversion layer 503, the reflectance at the interface can be increased,
This is because the output current of the light incident side photoelectric conversion layer 505 can be increased. On the other hand, in the comparative example, the output current of the entire solar cell is controlled by the output current of the light incident side photoelectric conversion layer 505, and the output current of the entire solar cell is low. Second, the transparent conductive film layer 50
4 has irregularities on the light incident side surface 508, so that multiple reflection in the light incident side photoelectric conversion layer 505 can be suppressed to reduce the surface reflectance in a long wavelength region, and that the light incident side photoelectric conversion layer 505 has The optical path length of the photoelectric conversion layer 505
This is because the amount of light absorbed in the inside can be increased (light confinement effect). On the other hand, in the comparative example, the output current of the entire solar cell is reduced by the extent that such an effect is not obtained. Note that in order to form a light confinement structure, irregularities may be formed on the surface of the crystalline silicon substrate by chemical etching or mechanical processing.

FIG. 6 shows that in the step of forming the transparent conductive film 504 containing ZnO as a main component, the flow rate of oxygen is 0 to 1.5 sc.
cm, the resulting transparent conductive film 50 is obtained.
4 shows the correlation between the sheet resistance and the characteristic yield of the stacked solar cell when the sheet resistance value of FIG. 4 is changed. Here, the threshold value of the characteristic yield was set to 95% of the maximum value. From the results of FIG. 6, it can be seen that a yield of 100% is obtained at a sheet resistance of 30 Ω / □ or more. Therefore, if the sheet resistance of the transparent conductive film layer 504 is set to 30Ω / □ or more, the surface electrodes 506 and 507 and the transparent conductive film layer 50
Even when a shunt path leak occurs due to a short circuit through a defective portion in the photoelectric conversion layer 505 between the leaked portion and the power generation region other than the leaked portion, the resistance component between the leaked portion and another power generation region other than the leaked portion becomes large. It can be said that there is no adverse effect on the photoelectric conversion characteristics of the power generation region. Here, the resistance value in the thickness direction of the transparent conductive film layer 504 after the above-mentioned uneven etching is 3 Ω · cm, which has an influence on the characteristics according to the calculation result.
² (described above), which is a very small value of 5 × 10 ⁻⁷ Ω · cm ² , so that there is almost no effect on the solar cell characteristics.

As described above, in the stacked solar cell of the first embodiment, the transparent conductive film layer 504 having the unevenness on the light incident side surface 508 is inserted between the two photoelectric conversion layers 503 and 505. Characteristics can be improved. In addition, since the sheet resistance of the transparent conductive film 504 is increased to 30Ω / □, the yield can be improved. That is, both characteristics and yield can be achieved. As a result, manufacturing costs can be reduced.

(Second Embodiment) FIG. 7 schematically shows a sectional structure of a stacked solar cell according to a second embodiment. This stacked solar cell has an amorphous silicon / transparent conductive film / crystalline silicon thin film / support substrate structure. Specifically, in the stacked solar cell, a back electrode 702, a first photoelectric conversion layer 703, a transparent conductive film layer 704 having a texture (unevenness) on a surface 708, and a second photoelectric conversion layer 705, an upper transparent electrode 706, and an upper comb-shaped electrode 707 are stacked in this order. The irradiation light 7
Reference numeral 09 denotes light to be applied when measuring solar cell characteristics.

This laminated solar cell is manufactured as follows.

First, a stainless steel substrate is prepared as the substrate 701. As the substrate 701, in addition to materials such as glass, metal, ceramic, and silicon, a material in which a metal film or an insulating material is deposited over these materials can be used.

Next, the stainless steel substrate 701 is washed with pure water and acetone. Then, by electron beam evaporation,
On one surface of the substrate 701, silver and zinc oxide (ZnO) are deposited as a material of the back electrode 702. When depositing silver, the substrate is deposited at a film thickness of 100 nm with a substrate temperature of 180 ° C. and a flat surface without irregularities. Further, when depositing ZnO, the substrate temperature was set to 220 ° C., oxygen was supplied at a flow rate of 42 sccm, and ZnO as a target was irradiated with an electron beam to have a thickness of 50 nm.
m. As the material of the back electrode 702, a metal or a conductive metal oxide film may be used.
A composite layer may be used. The method for forming the back electrode 702 may be a sputtering method or the like.

Next, using a parallel plate type plasma CVD apparatus, as a material of the first photoelectric conversion layer 703, a crystalline silicon thin film having an electrical conductivity type of n, i, and p, respectively, is formed on the back electrode 702. Are continuously formed. In forming the first photoelectric conversion layer 703, the substrate 701 and a cathode (a target (not shown)) are heated while the substrate 701 is heated.
And a high frequency power is applied between them to generate plasma.
Thereby, a target crystalline silicon thin film can be obtained. In this example, the conditions for forming the first photoelectric conversion layer (including the n, i, and p layers) 703 were as shown in Table 3 below.

[Table 3]

Next, a transparent conductive film layer 704 is formed on the first photoelectric conversion layer 703. As a material of the transparent conductive film layer 704, in addition to zinc oxide (ZnO), ITO (a material containing several weight% of tin in indium oxide), tin dioxide (SnO ₂ ), and the like may be used. Excellent ZnO is desirable. Further, the ZnO may be a mixture of dopants such as gallium (Ga), aluminum (Al), and boron (B). As a method for forming the transparent conductive film layer 704, a sputtering method, a vacuum evaporation method, or the like can be used. In this example, ZnO containing gallium was deposited as the transparent conductive film layer 704 by a sputtering method. In this film formation, while the substrate 701 is heated to 200 ° C. in a sputtering apparatus, oxygen is flowed at a flow rate of 1.5 sccm, argon is flowed at a flow rate of 100 sccm, the pressure is set to 0.7 Pa, and the substrate 701 and the cathode ( (Not shown target)
And a DC bias of 500 V is applied. After this film formation, the thickness of ZnO is about 600 nm, and the surface roughness is Rmax.
= 42 nm. Subsequently, in order to form irregularities on the surface of the transparent conductive film layer 704, an etching solution of 0.5
The surface of the transparent conductive film layer 704 is wet-etched for 50 seconds (this is referred to as “irregularity etching”) using an aqueous solution of acetic acid of weight%. As a result, the roughness of the surface 708 of the transparent conductive film layer 704 was reduced to Rmax = 189.
nm. The thickness of the transparent conductive film layer 704 after the uneven etching is about 400 nm ± 100 nm,
The sheet resistance is about 300Ω / □, and the resistance in the film thickness direction is 3 ×
It was 10 ⁻⁷ Ω · cm ² . Note that the transparent conductive film layer 70
The method for forming the irregularities on the surface of 4 is not limited to wet etching, and other methods such as dry etching can also be adopted.

Next, a second photoelectric conversion layer 705 as a light incident side photoelectric conversion layer is formed on the transparent conductive film layer 704 having unevenness (texture) on the surface 708 by a sputtering method. As the material of the second photoelectric conversion layer 705, as in the first embodiment, amorphous silicon having a wider band gap than crystalline silicon was employed. This is because amorphous silicon has a wider band gap than that of crystalline silicon and is therefore suitable as a material for the light incident side photoelectric conversion layer of the stacked solar cell. The surface roughness of the second photoelectric conversion layer 705 after this film formation depends on the surface roughness of the transparent conductive film layer 704.
08 is reflected. In this example, the conditions for forming the second photoelectric conversion layer (including the n, i, and p layers) 705 were as shown in Table 4 below.

[Table 4]

As a material of the second photoelectric conversion layer 705, n-type and p-type microcrystalline silicon may be used instead of n-type or p-type amorphous silicon.

Next, an upper transparent electrode 706 is formed on the second photoelectric conversion layer 705. The upper transparent electrode 706 is formed by depositing ITO to a thickness of 60 nm by a sputtering method. In this film formation, in a sputtering apparatus,
With the substrate 701 heated to 220 ° C., oxygen was flowed at a flow rate of 1.5 sccm and argon was flowed at a flow rate of 250 sccm, the gas pressure was set at 3.8 Pa, and 450 V was applied between the substrate 701 and a cathode (not shown target). Apply DC bias. The surface roughness of the upper transparent electrode 706 after film formation reflects the surface roughness of the second photoelectric conversion layer 705. The surface roughness of the upper transparent electrode 706 after film formation is R
max = 170 nm. This value can reduce the reflection on the surface of the stacked solar cell and obtain a light confinement effect.

Next, an upper comb-shaped electrode 707 is formed on the upper transparent electrode 706. The upper comb-shaped electrode 707 is formed of silver to a thickness of 300 nm by an electron beam evaporation method. In forming the upper comb-shaped electrode 707, silver is selectively deposited on the substrate surface using a metal mask while the substrate 701 is heated to 180 ° C. in an evaporation apparatus. By using a low-resistance material such as silver as the material of the upper comb-shaped electrode 707, the current collecting capability of the upper transparent electrode 706 can be complemented, and current collection can be performed more efficiently. As a method for forming the upper comb-shaped electrode 707, in addition to the vacuum evaporation method,
A sputtering method is exemplified.

The following Table 5 shows that the laminated solar cell fabricated in this manner was subjected to AM-1.
The results of measuring solar cell characteristics (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) by giving irradiation light 709 of ⁵ , 100 mW / cm ² are shown together with the results of the comparative example. As a comparative example, a transparent conductive film having no irregularities on its surface was manufactured. Other manufacturing conditions of the comparative example are the same as those of the above-described stacked solar cell. The surface roughness of the second photoelectric conversion layer of the comparative example was Rmax = 37 nm,
The sheet resistance of the transparent conductive film which was the base of the photoelectric conversion layer on the light incident side was about 150 Ω / □, and the yield of solar cell characteristics was 100%.

[Table 5]

As can be seen from Table 5, in the stacked solar cell of this embodiment, the output current (short-circuit current density) of the entire solar cell is larger than that of the comparative example. The reason is that the transparent conductive film layer 704 has irregularities on the light incident side surface 708, so that multiple reflection in the light incident side photoelectric conversion layer 705 can be suppressed to reduce the surface reflectance in a long wavelength region, This is because the optical path length inside the incident-side photoelectric conversion layer 705 can be lengthened to increase the amount of light absorbed in the photoelectric conversion layer 705 (light confinement effect). On the other hand, in the comparative example, the output current of the entire solar cell is reduced by the extent that such an effect is not obtained.

As described above with reference to FIG. 6, if the sheet resistance of the transparent conductive layer 704 is set to 30 Ω / □ or more, the surface electrodes 706 and 707 and the transparent conductive layer 7
Even if a shunt path leak occurs due to a short circuit through the defective portion in the photoelectric conversion layer 705 between the leaked portion and the other portion, the resistance component between the leaked portion and another power generation region other than the leaked portion becomes large. It can be said that there is no adverse effect on the photoelectric conversion characteristics of the power generation region. Here, the resistance value in the thickness direction of the transparent conductive film layer 704 after the above-mentioned uneven etching is 3 Ω · cm ² , which has an effect on the characteristics according to the calculation result.
Compared with (described above), it is a very small value of 3 × 10 ⁻⁷ Ω · cm ^2, and thus has little effect on the solar cell characteristics.

As described above, in the stacked solar cell of the second embodiment, since the transparent conductive film layer 704 having irregularities on the light incident side surface 708 is inserted between the two photoelectric conversion layers 703 and 705, Characteristics can be improved. Moreover, since the sheet resistance of the transparent conductive film 704 is increased to 30 Ω / □ or more, the yield can be improved. actually,
The characteristic yield of the stacked solar cell of the second embodiment is 10
It was 0%. That is, it was possible to achieve both the characteristics and the yield. As a result, the manufacturing cost is reduced.

(Third Embodiment) FIG. 8 schematically shows a sectional structure of a stacked solar cell according to a third embodiment. This stacked solar cell has a so-called super straight structure. Specifically, in this laminated solar cell, the first transparent electrode 80 having a texture (unevenness) on the surface 803 is provided on one surface of a glass substrate 801 which becomes a front panel when used.
2, the first photoelectric conversion layer 804, and the transparent conductive film layer 805
, A second photoelectric conversion layer 806 and a back electrode 807 are stacked in this order. Note that irradiation light 808 indicates light to be irradiated when measuring solar cell characteristics.

This laminated solar cell is manufactured as follows.

First, a glass substrate 801 having an insulating property and a light transmitting property is prepared. One side of the glass substrate 801 is coated with SiO ₂ (thickness 10 nm) by a normal pressure CVD method.
And SnO ₂ (film thickness 80) as the first transparent electrode layer 802.
0 nm). The first transparent electrode layer 802 after this film formation has irregularities on the surface 803,
The surface roughness was Rmax = 200 nm. This film formation method is not limited to only the normal pressure CVD method,
For example, an evaporation method or a sputtering method may be used. Further, as a material of the first transparent electrode layer 802, ZnO, IT
A material such as O may be used.

Next, the first surface 803 having irregularities
A first photoelectric conversion layer (including p, i, and n layers) 804 is formed over the transparent electrode layer 802. As the material of the first photoelectric conversion layer, amorphous silicon was used as in the first and second embodiments. The conditions for forming the first photoelectric conversion layer 804 were set to be the same as those in the first embodiment.

Next, a transparent conductive layer is formed on the first photoelectric conversion layer 804.
A film layer 805 is formed. Material of this transparent conductive film layer 805
In addition to zinc oxide (ZnO),
Material containing several weight percent tin in indium), tin dioxide
(SnO₂) May be used, but it is particularly excellent in plasma resistance
ZnO is desirable. Furthermore, this ZnO is gallium
(Ga), aluminum (Al), boron (B), etc.
-Punting may be used. Transparent
As a method for forming the conductive film layer 805, sputtering
Method, a vacuum evaporation method, or the like can also be adopted. This example
Then, the transparent conductive film layer 805 is the same as that of the first embodiment.
Gallium-containing ZnO by sputtering.
Was deposited. The resistivity of the transparent conductive film layer 805 after this film formation
Anti 1 × 10 ^-2Ω · cm, thickness about 400nm, surface
The roughness is Rmax = 100 nm, and the sheet resistance is 300Ω /
□.

Next, a second photoelectric conversion layer 806 is formed on the transparent conductive film layer 805. As the second photoelectric conversion layer 806, a crystalline silicon thin film is formed using a parallel plate type plasma CVD apparatus under the same conditions as in the second embodiment. The surface roughness of the second photoelectric conversion layer 806 after the film formation reflects the surface roughness of the transparent conductive film layer 805.

After that, a back electrode 807 is formed on the second photoelectric conversion layer 806. As the back electrode 807, D
The substrate 801 is formed by C magnetron sputtering.
Is heated to 180 ° C., and ITO (film thickness: 50 nm)
And aluminum (film thickness 500 nm) are deposited. As the back electrode 807, a metal or a conductive metal oxide film may be used as a material, or a single layer or a composite layer thereof may be used. The back electrode 807 may be formed by a vapor deposition method or the like.

The following Table 6 shows that the laminated solar cell manufactured in this manner was subjected to AM-1.
The results of measuring the solar cell characteristics (short-circuit current density, open-circuit voltage, fill factor, and conversion efficiency) by giving irradiation light 808 of ⁵ , 100 mW / cm ² are shown together with the results of the comparative example. As a comparative example, a device in which a transparent conductive film was not inserted between the photoelectric conversion layers was manufactured. Other manufacturing conditions of the comparative example are the same as those of the above-described stacked solar cell. In addition, the solar cell characteristic yield of the comparative example was 100%.

[Table 6]

As can be seen from Table 6, in the stacked solar cell of this embodiment, the output current (short-circuit current density) of the entire solar cell is larger than that of the comparative example. The first reason is that the reflectance at the interface can be increased by inserting the transparent conductive film layer 805 between the photoelectric conversion layer 804 on the light incident side and the photoelectric conversion layer 806 on the lower side. This is because the output current of the photoelectric conversion layer 804 can be increased. On the other hand, in the comparative example, the output current of the entire solar cell is controlled by the output current of the light incident side photoelectric conversion layer, and the output current of the entire solar cell is low.

As described above, in the stacked solar cell according to the third embodiment, since the transparent conductive film layer 805 having irregularities on the light incident side surface is inserted between the two photoelectric conversion layers 804 and 806, the characteristics are improved. Can be improved. Moreover, since the sheet resistance of the transparent conductive film layer 805 is increased to 30 Ω / □ or more, the yield can be improved. Actually, the characteristic yield of the stacked solar cell of the third embodiment is 100%.
Met. That is, it was possible to achieve both the characteristics and the yield. As a result, manufacturing costs can be reduced.

In each of the above embodiments, the number of photoelectric conversion layers included in the stack is two. However, the number is not limited to this, and three or more photoelectric conversion layers may be included in the stack. In that case, two or more transparent conductive layers may be provided between the photoelectric conversion layers.

[0076]

As is clear from the above, according to the stacked solar cell of the present invention, characteristics such as conversion efficiency can be improved.
In addition, the yield in large area and mass production can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the specific resistance of a transparent conductive film and the oxygen partial pressure during film formation.

FIG. 2 is a diagram showing the relationship between the transmittance of a transparent conductive film at an incident wavelength of 600 nm and the oxygen partial pressure during film formation.

FIG. 3 is a diagram showing a correlation between a series resistance and a fill factor (FF).

FIG. 4 is a view showing simulation results of surface reflectivity of a flat structure laminated solar cell.

FIG. 5 is a view schematically showing a structure of the stacked solar cell of the first embodiment.

FIG. 6 is a diagram showing the relationship between the sheet resistance of a transparent conductive film of the stacked solar cell and the yield of solar cell characteristics.

FIG. 7 is a diagram schematically illustrating a structure of a stacked solar cell according to a second embodiment.

FIG. 8 is a diagram schematically illustrating a structure of a stacked solar cell according to a third embodiment.

[Explanation of symbols]

 501 n-type semiconductor substrate 701 stainless steel substrate 502, 702, 807 back electrode 503a p-type semiconductor layer 503, 703, 804 first photoelectric conversion layer 504, 704, 805 transparent conductive film layer 505, 705, 806 second photoelectric conversion layer 506 , 706 Upper transparent electrode 507, 707 Upper comb-shaped electrode 508, 708 Surface of transparent conductive film layer 509, 709, 808 Irradiation light 801 Glass substrate 802 First transparent electrode layer 803 Surface of first transparent electrode layer

Claims (12)

[Claims] 1. A stacked solar cell in which a plurality of photoelectric conversion layers are stacked and the photoelectric conversion layers are electrically connected in series, wherein at least one or more between the photoelectric conversion layers is provided on a light incident side surface. A transparent conductive film layer having irregularities is provided, and the photoelectric conversion layer on the light incident side is formed so as to reflect the irregularities. The sheet resistance of the transparent conductive film layer is 30Ω / □ or more, and the resistance in the film thickness direction. Is 3Ω · cm ² or less. 2. The stacked solar cell according to claim 1, wherein the plurality of photoelectric conversion layers are stacked on a support substrate. 3. The stacked solar cell according to claim 1, wherein at least one of the plurality of photoelectric conversion layers includes a supporting material for supporting the remaining photoelectric conversion layers. Stacked solar cells. 4. The stacked solar cell according to claim 1, wherein the irregularities on the light incident side surface of the transparent conductive film layer are formed by etching the transparent conductive film. Characteristic stacked solar cell. 5. The stacked solar cell according to claim 2, wherein the surface of the support substrate has irregularities. 6. The stacked solar cell according to claim 3, wherein the surface of the photoelectric conversion layer including the support material has irregularities. 7. The stacked solar cell according to claim 2, wherein the electrode surface on the support substrate has irregularities. 8. The stacked solar cell according to claim 1, wherein the surface of the photoelectric conversion layer below the transparent conductive film layer has irregularities. 9. The stacked solar cell according to claim 1, wherein the transparent conductive film layer contains zinc oxide as a main component. 10. The stacked solar cell according to claim 1, wherein two photoelectric conversion layers are provided, and the photoelectric conversion layer on the light incident side is made of amorphous silicon or amorphous silicon. A stacked solar cell comprising a silicon alloy and a lower photoelectric conversion layer comprising a material containing crystalline silicon. 11. The stacked solar cell according to claim 1, wherein three photoelectric conversion layers are provided, and the photoelectric conversion layer on the light incident side is made of amorphous silicon or amorphous silicon. A stacked solar cell comprising a silicon alloy, wherein an intermediate photoelectric conversion layer and a lower photoelectric conversion layer are each made of a material containing crystalline silicon. 12. The stacked solar cell according to claim 1, wherein three photoelectric conversion layers are provided, and the photoelectric conversion layer on the light incident side and the intermediate photoelectric conversion layer are non-conductive. A stacked solar cell comprising a crystalline silicon or an amorphous silicon alloy, wherein a lower photoelectric conversion layer comprises a material containing crystalline silicon.