JP2007305826A - Silicon-based thin film solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a highly efficient and inexpensive silicon-based thin film solar cell capable of exerting a predominant light confinement effect and keeping a carrier recombination in the solar cell few even though the layer having low refraction factor is arranged, by arranging a layer having a refraction factor lower than that of a photoelectric layer at the rear of the photoelectric conversion layer from a view of light entering side without using a facility different from that forming the phtotelectric conversion layer. <P>SOLUTION: This invention can form the silicon-based low reafraction factor layer having the refraction factor lower than that of the photoelectric conversion layer at the rear of the photoelectric conversion layer from the view of light enterng side, without using the facility different from that forming the photoelectric conversion layer to be able to exert the predominant light confinement effect. Further, the arrangement of the thin conductive silicon-based interface layer at the rear of the silicon-based low refcation factor layer can keep the series resistance of the solar cell small. This can provide the highly efficient and inexpensive silicon-based thin film solar cell. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a silicon-based thin film solar cell, and in particular, by arranging a thin layer having a refractive index smaller than that of the photoelectric conversion layer behind the photoelectric conversion layer viewed from the light incident side, The present invention relates to a thin film solar cell that simultaneously exhibits an effect of reducing carrier recombination.

  In recent years, thin film solar cells that require less raw materials to achieve both cost reduction and high efficiency of photoelectric conversion devices have attracted attention and are being developed vigorously. At present, in addition to conventional amorphous thin-film solar cells, crystalline thin-film solar cells have also been developed, and stacked thin-film solar cells called hybrid solar cells in which these are stacked have been put into practical use.

  A thin film solar cell generally includes a first electrode, one or more semiconductor thin film photoelectric conversion units, and a second electrode that are sequentially stacked on a substrate. One photoelectric conversion unit includes an i-type layer sandwiched between a p-type layer and an n-type layer.

  The i-type layer is a substantially intrinsic semiconductor layer and occupies most of the thickness of the photoelectric conversion unit, and the photoelectric conversion action mainly occurs in the i-type layer. For this reason, this i-type layer is usually called an i-type photoelectric conversion layer or simply a photoelectric conversion layer. The photoelectric conversion layer is not limited to an intrinsic semiconductor layer, and may be a layer doped in a small amount of p-type or n-type within a range where loss of light absorbed by a doped impurity does not cause a problem. The photoelectric conversion layer is preferably thicker for light absorption, but if it is thicker than necessary, the cost and time for film formation will increase.

  On the other hand, the p-type and n-type conductive layers serve to generate a diffusion potential in the photoelectric conversion unit, and the open circuit voltage (Voc), which is one of the important characteristics of the thin-film solar cell, depending on the magnitude of this diffusion potential. The value of depends on. However, these conductive layers are inactive layers that do not directly contribute to photoelectric conversion, and light absorbed by impurities doped in the conductive layer results in a loss that does not contribute to power generation. Therefore, it is preferable to keep the p-type and n-type conductive layers as small as possible as long as they are within a range in which a sufficient diffusion potential can be generated. For the p-type and n-type conductive layers, the same material as the photoelectric conversion layer may be used. For example, by using a material having a wider band gap than the photoelectric conversion layer, such as silicon carbide for silicon. A technique for forming a new electric field at the interface between the conductive type layer and the photoelectric conversion layer and suppressing recombination of carriers generated along with light absorption at the interface is also widely used.

  Here, the pin (nip) type photoelectric conversion unit or thin film solar cell as described above has its main part regardless of whether the p-type and n-type conductive layers contained therein are amorphous or crystalline. An amorphous photoelectric conversion layer is referred to as an amorphous unit or an amorphous thin film solar cell, and a crystalline photoelectric conversion layer is referred to as a crystalline unit or a crystalline thin film solar cell.

  As a method for improving the conversion efficiency of the thin-film solar cell, there is a method of stacking two or more photoelectric conversion units into a tandem type. In this method, a front unit including a photoelectric conversion layer having a wide band gap is disposed on the light incident side of the thin film solar cell, and a rear unit including a photoelectric conversion layer having a narrow band gap is sequentially disposed behind the front unit. The photoelectric conversion is enabled over a wide wavelength range of the incident light, thereby improving the conversion efficiency of the entire solar cell. Among such tandem solar cells, a laminate in which an amorphous photoelectric conversion unit and a crystalline photoelectric conversion unit are stacked is called a hybrid thin film solar cell.

  For example, the wavelength of light that can be photoelectrically converted by i-type amorphous silicon is up to about 800 nm on the long wavelength side, but i-type crystalline silicon photoelectrically converts light up to a wavelength of about 1100 nm longer than that. Can do. However, a thickness of about 0.3 μm or less is sufficient for an amorphous silicon photoelectric conversion layer having a large light absorption coefficient to absorb light sufficiently, but a crystalline silicon photoelectric conversion layer having a small light absorption coefficient is long. In order to sufficiently absorb light having a wavelength, it is preferable to have a thickness of about 1.5 to 3 μm. That is, the crystalline photoelectric conversion layer is usually desired to have a thickness of about 5 to 10 times that of the amorphous photoelectric conversion layer.

  In both the amorphous silicon single-layer thin-film solar cell and the hybrid thin-film solar cell described above, it is desirable to keep the thickness of the photoelectric conversion layer as small as possible from the viewpoint of productivity improvement, that is, cost reduction. For this reason, a structure using a so-called light confinement effect that effectively reflects light of a specific wavelength by arranging a layer having a smaller refractive index than the photoelectric conversion layer behind the photoelectric conversion layer when viewed from the light incident side is common. It is used for. Arranged behind the photoelectric conversion layer when viewed from the light incident side is that it is in contact with the photoelectric conversion layer on the back side, or another layer is arranged on the back side of the photoelectric conversion layer, It is arranged on the back side with the layer in between.

  As a conventional technique, Patent Document 1 discloses, from the light incident side, a translucent first electrode, an amorphous silicon semiconductor thin film (hereinafter simply referred to as a semiconductor thin film), a zinc oxide film having a thickness of less than 1200 mm, and an opaqueness. A structure of a solar cell in which a second electrode (metal electrode) is sequentially laminated is disclosed.

As a conventional technique, Patent Document 2 discloses a structure of a photoelectric conversion layer / a conductive silicon-based low refractive index layer / a conductive silicon-based interface layer, which is the same structure as the present invention.
Japanese Patent Laid-Open No. 2-73672 International Patent Publication No. WO2005 / 011002

  In Patent Document 1, from the light incident side, a translucent first electrode, an amorphous silicon semiconductor thin film (hereinafter simply referred to as a semiconductor thin film), a zinc oxide film having a thickness of less than 1200 mm, and a translucent second electrode ( A structure of a solar cell in which metal electrodes) are sequentially stacked is disclosed. The zinc oxide film has a function of preventing an increase in absorption loss due to silicide generated at the interface between the semiconductor thin film and the metal electrode. Further, since there is a difference in refractive index between the zinc oxide film and the semiconductor thin film, if the thickness of the zinc oxide film is limited to less than 1200 mm, preferably 300 to 900 mm, the reflectance at the semiconductor thin film / zinc oxide film interface can be reduced. Has the effect of improving. For this reason, the short circuit current density of a solar cell improves, and conversion efficiency improves. However, since the zinc oxide film is formed by a technique such as sputtering or spraying, it is necessary to use equipment different from the semiconductor thin film generally formed by plasma CVD or the like, which requires equipment costs and increases production tact time. Problem arises. Furthermore, in particular, when a sputtering method is used for forming a zinc oxide film, there is a problem that the performance may be deteriorated due to sputtering damage to the underlying semiconductor thin film.

  Patent Document 2 discloses the structure of a photoelectric conversion layer / a conductive silicon-based low refractive index layer / a conductive silicon-based interface layer, which is the same structure as the present invention. However, in Patent Document 2, the thickness of the silicon-based low refractive index layer is designed to be substantially 30 nm or more from an optical viewpoint. In addition to this thickness, adding the thickness of the conductive silicon-based interface layer, which plays a role in reducing the contact resistance generated between the silicon-based low refractive index layer and the back electrode, gives the total of the conductive layer. The thickness is substantially about 40 nm or more. For this reason, there is a case where the absorption loss of light when passing through the conductive layer cannot be ignored.

  In view of the situation as described above, the present invention provides a photoelectric conversion layer in which a silicon-based layer having a refractive index lower than that of the photoelectric conversion layer is viewed from the light incident side without using a photoelectric conversion layer formation and other types of equipment. By arranging a very thin layer behind the conversion layer, the light absorption loss within the layer is kept small while exhibiting a sufficient light confinement effect, and a layer having such a low refractive index is disposed. Another object of the present invention is to provide a silicon-based thin film solar cell with high efficiency and low cost that can keep the recombination loss of carriers generated in the solar cell small.

  Based on the knowledge of Patent Document 2, the present inventors have further studied the back surface structure of the solar cell. As a result, the inventors have completed the following invention having remarkable effects that cannot be easily conceived by those skilled in the art.

The first of the present invention is
“A conductive silicon-based low refractive index layer and a conductive silicon-based interface layer are sequentially arranged behind the photoelectric conversion layer as viewed from the light incident side, and the conductive silicon-based low refractive index layer and the conductive silicon-based interface layer” The thickness of each is a silicon-based thin-film solar cell characterized in that it is greater than 0 nm and less than or equal to 10 nm.

  The present invention is also “a silicon-based thin film solar cell characterized in that a refractive index of the silicon-based low refractive index layer at a wavelength of 600 nm is 2.5 or less”.

  The present invention is also “a silicon-based thin film solar cell characterized in that the most constituent element excluding silicon in the silicon-based low refractive index layer is oxygen”.

  The present invention is also “a silicon-based thin film solar cell characterized in that the silicon-based low refractive index layer contains a crystalline silicon component in the layer”.

  The present invention is also “a silicon-based thin film solar cell characterized in that the conductive silicon-based interface layer has a thickness of 5 nm or more”.

  The present invention is also “the above-described silicon-based thin film solar cell, which is a hybrid thin film solar cell including at least one amorphous photoelectric conversion unit and at least one crystalline photoelectric conversion unit”. .

  As a result of further intensive studies on the back surface structure of the solar cell based on the knowledge of Patent Document 2, the present inventors have completed the following invention having remarkable effects that cannot be easily conceived by those skilled in the art.

  In the first aspect of the present invention, a conductive silicon-based low refractive index layer and a conductive silicon-based interface layer are sequentially disposed behind the photoelectric conversion layer as viewed from the light incident side, the conductive silicon-based low refractive index layer, The silicon-based thin film solar cell is characterized in that each of the conductive silicon-based interface layers has a thickness of greater than 0 nm and 10 nm or less.

  According to the first configuration of the present invention, Voc and the fill factor are within the range where the thickness of the silicon-based low-refractive index layer is greater than 0 nm and less than or equal to 10 nm, which was not particularly expected as common sense of those skilled in the art at the time of Patent Document 2. (FF) was improved. Surprisingly, it was thought at that time that the short circuit current density (Jsc) could not be improved by optical confinement when the thickness of the silicon-based low refractive index layer was 10 nm or less. It was found that improvement was also seen.

  The silicon-based low refractive index layer plays a role of generating a diffusion potential in the photoelectric conversion layer, and is a layer doped n-type with impurities. Since the thickness of the silicon-based low refractive index layer of the present invention is set to 10 nm or less, light is effectively reflected on the photoelectric conversion layer side on the surface, and absorption loss of light in the layer is as much as possible. Can be kept small.

  The present invention is also the silicon-based thin film solar cell, wherein the silicon-based low refractive index layer has a refractive index of 2.5 or less at a wavelength of 600 nm.

  In the present invention, the thickness of the silicon-based low-refractive index layer is reduced to 10 nm or less, so that it becomes apparent when the refractive index of the conductive silicon-based interface layer is lowered. Recombination can be reduced. Therefore, even if the “refractive index of the silicon-based low refractive index layer at a wavelength of 600 nm is 2.5 or less” as in the second configuration of the present invention, the problem of the present invention can be solved.

  The present invention is also the silicon-based thin-film solar cell, wherein the most constituent element excluding silicon occupied in the silicon-based low refractive index layer is oxygen.

  The present invention is also the silicon-based thin-film solar cell, wherein the silicon-based low refractive index layer includes a crystalline silicon component in the layer.

  The silicon-based thin film solar cell of the present invention has a contact resistance with a transparent oxide layer disposed in contact with the conductive silicon-based interface layer by including a crystalline silicon component in the layer of the conductive silicon-based interface layer. Can be kept small. Moreover, the refractive index of the conductive silicon-based interface layer can be lowered while maintaining the crystallization ratio of the conductive silicon-based interface layer by using oxygen as the most abundant constituent element excluding silicon in the conductive silicon-based interface layer. Thus, the conductivity of the interface layer can be maintained even if the refractive index of the conductive silicon-based interface layer is lowered.

  The present invention is also the silicon-based thin film solar cell, wherein the conductive silicon-based interface layer has a thickness of 5 nm or more.

  The conductivity type silicon-based interface layer is a conductivity type layer mainly composed of silicon. The conductive silicon-based interface layer is set to a thickness of 10 nm or less in order to keep the light absorption loss in the layer as small as possible. Moreover, in order to compensate for the generation of a diffusion potential in the photoelectric conversion layer, which may be insufficient only with a thin silicon-based low refractive index layer, the thickness is preferably 5 nm or more.

  Due to the fifth configuration of the present invention, that is, when the thickness of the conductive silicon-based interface layer is 5 nm or more, a thin silicon-based low-refractive index layer alone may be insufficient. Generation of a diffusion potential in the layer can be compensated.

  In particular, when silicon oxide is used as a silicon-based low refractive index layer as shown in FIG. 1 and the amount of oxygen in the layer is increased to lower the refractive index to 2.5 or less, the silicon-based low refractive index layer and the back electrode Although it is difficult to lower the contact resistance of the layer, such a problem can be solved by inserting a conductive silicon-based interface layer.

  According to the present invention, a silicon-based low refractive index layer having a refractive index lower than that of the photoelectric conversion layer is formed behind the photoelectric conversion layer when viewed from the light incident side without using a separate type of equipment from the formation of the photoelectric conversion layer. Since it can be formed, a sufficient light confinement effect can be exhibited at low cost. Furthermore, the series resistance of the solar cell can be kept small by disposing a thin conductive silicon-based interface layer behind the silicon-based low refractive index layer. As a result, a silicon-based thin film solar cell can be provided with high efficiency and low cost.

  The present invention is also “the above-described silicon-based thin film solar cell, which is a hybrid thin film solar cell including at least one amorphous photoelectric conversion unit and at least one crystalline photoelectric conversion unit”. .

  Below, the silicon-type thin film solar cell as embodiment of this invention is demonstrated, referring FIG.

A transparent electrode layer 2 is formed on the translucent substrate 1. As the translucent substrate 1, a plate-like member or a sheet-like member made of glass, transparent resin or the like is used. The transparent electrode layer 2 is preferably made of a conductive metal oxide such as SnO ₂ or ZnO, and is preferably formed using a method such as CVD, sputtering, or vapor deposition. The transparent electrode layer 2 desirably has the effect of increasing the scattering of incident light by having fine irregularities on its surface. An amorphous photoelectric conversion unit 3 is formed on the transparent electrode layer 2. The amorphous photoelectric conversion unit 3 includes an amorphous p-type silicon carbide layer 3p, a non-doped amorphous i-type silicon photoelectric conversion layer 3i, and an n-type silicon-based interface layer 3n. A crystalline photoelectric conversion unit 4 is formed on the amorphous photoelectric conversion unit 3. A high-frequency plasma CVD method is suitable for forming the amorphous photoelectric conversion unit 3 and the crystalline photoelectric conversion unit 4 (hereinafter, both units are collectively referred to as a photoelectric conversion unit). As conditions for forming the photoelectric conversion unit, a substrate temperature of 100 to 300 ° C., a pressure of 30 to 1500 Pa, and a high frequency power density of 0.01 to 0.5 W / cm ² are preferably used. As a source gas used for forming the photoelectric conversion unit, a silicon-containing gas such as SiH ₄ or Si ₂ H ₆ or a mixture of these gases and H ₂ is used. B ₂ H ₆ or PH ₃ is preferably used as the dopant gas for forming the p-type or n-type layer in the photoelectric conversion unit.

The crystalline photoelectric conversion unit 4 includes a crystalline p-type silicon layer 4p, a crystalline i-type silicon photoelectric conversion layer 4i, an n-type silicon-based low refractive index layer 4on, and an n-type silicon-based interface layer 4n. As the n-type silicon-based low refractive index layer 4on, silicon oxide is typically used. In this case, a mixed gas of SiH ₄ , H ₂ , CO ₂ , and PH ₃ is suitable as the source gas used. The thickness of the silicon-based low refractive index layer 4on is set to 10 nm or less. The silicon-based low refractive index layer 4on may not contain a crystalline silicon component, but it is more preferred that it be contained. The refractive index at a wavelength of 600 nm of the silicon-based low refractive index layer 4on is preferably 2.5 or less, and more preferably the refractive index at 600 nm is 2.2 or less. When silicon oxide is used as the silicon-based low refractive index layer 4on, a film whose layer conductivity is in the range of 1 × 10 ⁻³ S / cm to 1 × 10 ⁻⁶ S / cm is used. The silicon-based low refractive index layer 4on may have a constant refractive index in the film thickness direction, or the refractive index may change midway. Furthermore, the refractive index may be increased or decreased periodically. In FIG. 2, a structure in which an n-type silicon-based low refractive index layer 4on is disposed in contact with the crystalline i-type silicon photoelectric conversion layer 4i behind the crystalline i-type silicon photoelectric conversion layer 4i when viewed from the light incident side. However, another layer such as an n-type silicon layer may be interposed between the crystalline i-type silicon photoelectric conversion layer 4i and the n-type silicon-based low refractive index layer 4on. Further, as the silicon-based low refractive index layer 4on, instead of silicon oxide, silicon nitride, silicon carbide, silicon oxynitride, silicon oxycarbide, etc., silicon, nitrogen, carbon, oxygen, or any one or more elements May be included in the layer.

An n-type silicon-based interface layer 4n is formed on the n-type silicon-based low refractive index layer 4on. Crystalline silicon is mainly used for the n-type silicon-based interface layer 4n. The n-type silicon-based interface layer 4n is used for the purpose of improving the contact resistance between the n-type silicon-based low refractive index layer 4on and the transparent oxide layer 5, and is as small as possible in order to minimize the light absorption loss in this layer. It is desirable to have a thickness. Specifically, a thickness of 10 nm or less is used. On the other hand, the n-type silicon-based interface layer 4n also plays a role of supplementing the generation of a diffusion potential in the photoelectric conversion layer, so that the thickness is preferably 5 nm or more. Furthermore, the n-type silicon-based interface layer 4n may be one having a conductivity of about 1 to 10 ² S / cm. The n-type silicon-based interface layer 4n may contain one or more elements of oxygen, carbon, and nitrogen to the extent that the contact resistance with the transparent oxide layer 5 is not increased.
A transparent oxide layer 5 and a back reflective electrode layer 6 are formed on the n-type silicon-based interface layer 4n. A metal oxide such as ZnO or ITO is used for the transparent oxide 5, and Ag, Al, or an alloy thereof is preferably used for the back reflective electrode layer 6. In forming the transparent oxide layer 5 and the back reflective electrode layer 6, methods such as sputtering and vapor deposition are preferably used. Although FIG. 2 shows the structure of a hybrid thin film solar cell, the number of photoelectric conversion units 4 is not necessarily two, and an amorphous or crystalline single structure, a stacked solar cell having three or more layers. It may be a battery structure. Further, FIG. 2 shows a structure in which a photoelectric conversion layer, a silicon-based low refractive index layer, and an n-type silicon-based interface layer are sequentially disposed on a light-transmitting substrate. A so-called reverse type structure in which an n-type silicon-based interface layer, a silicon-based low refractive index layer, and a photoelectric conversion layer are sequentially deposited may be used.

  As a result of further intensive studies on the back surface structure of the solar cell based on the knowledge of Patent Document 2, the present inventors have completed the following invention having remarkable effects that cannot be easily conceived by those skilled in the art.

  According to a first aspect of the present invention, “a conductive silicon-based low refractive index layer and a conductive silicon-based interface layer are sequentially arranged behind the photoelectric conversion layer as viewed from the light incident side, and the conductive silicon-based low refractive index layer and A silicon-based thin-film solar cell, wherein the thickness of the conductive silicon-based interface layer is greater than 0 nm and 10 nm or less, respectively.

  With the configuration of the present invention, Voc and fill factor (FF) were improved in the range where the thickness of the silicon-based low refractive index layer was 10 nm or less, which was not particularly expected as the common sense of those skilled in the art at the time of Patent Document 2. It was. Surprisingly, it was thought at that time that the short circuit current density (Jsc) could not be improved by optical confinement when the thickness of the silicon-based low refractive index layer was 10 nm or less. It was found that improvement was also seen.

  This is presumed to be due to the following reasons, but further study is awaited. The silicon-based low refractive index layer has a wider band gap than the photoelectric conversion layer and has few defects due to lattice mismatch at the interface with the photoelectric conversion layer, so that a so-called passive layer that reduces carrier recombination at the interface is used. To act as a social layer. However, the silicon-based low refractive index layer itself contains a large amount of elements other than silicon. For this reason, if the silicon-based low-refractive index layer is a little too thick, carrier recombination within the layer is greatly promoted compared with, for example, a silicon-only conductive type layer, and the carrier at the interface obtained with great care is obtained. The effect of reducing recombination is canceled, and Voc and FF are not improved. Furthermore, if the silicon-based low refractive index layer is too thick, absorption loss increases when light passes through the layer, and Jsc decreases.

  The silicon-based thin film solar cell according to the present invention is characterized in that a silicon-based low refractive index layer having a thickness of 10 nm or less and a conductive silicon-based interface layer are sequentially arranged behind the photoelectric conversion layer as viewed from the light incident side. This is a silicon-based thin film solar cell.

  The silicon-based low refractive index layer plays a role of generating a diffusion potential in the photoelectric conversion layer, and is a layer doped n-type with impurities. The thickness of the silicon-based low refractive index layer is set to 10 nm or less in order to effectively reflect light on the surface thereof to the photoelectric conversion layer side and keep light absorption loss in the layer as small as possible.

  The present invention is also “a silicon-based thin film solar cell characterized in that a refractive index of the silicon-based low refractive index layer at a wavelength of 600 nm is 2.5 or less.

Moreover, it is preferable that the refractive index in wavelength 600nm is 2.5 or less. Also, by reducing the thickness of the silicon-based low refractive index layer to 10 nm or less, carrier recombination caused by defects in the layer, which becomes apparent when the refractive index of the conductive silicon-based interface layer is lowered Can be reduced. The silicon-based low refractive index layer is an alloy layer composed of elements such as silicon and oxygen, represented by silicon oxide, and generates a sufficient diffusion potential in the photoelectric conversion layer even if it is thin. Is preferably 1 × 10 ⁻³ S / cm or less and in the range of 1 × 10 ⁻⁶ S / cm or more, and is formed by the same manufacturing method as the photoelectric conversion layer, that is, a method such as high-frequency plasma CVD. It is preferred that The silicon-based low refractive index layer preferably contains a crystalline silicon component in the layer in order to obtain the above-described conductivity and reduce the interface resistance with the conductive silicon-based interface layer formed in contact therewith. .

  The conductivity type silicon-based interface layer is a conductivity type layer mainly composed of silicon. The conductive silicon-based interface layer is set to a thickness of 10 nm or less in order to keep the light absorption loss in the layer as small as possible. Moreover, in order to compensate for the generation of a diffusion potential in the photoelectric conversion layer, which may be insufficient only with a thin silicon-based low refractive index layer, the thickness is preferably 5 nm or more. Furthermore, in order to keep the contact resistance with the transparent oxide layer disposed in contact with the conductive silicon-based interface layer small, the conductive silicon-based interface layer preferably contains a crystalline silicon component in the layer.

  In particular, when silicon oxide is used as a silicon-based low refractive index layer as shown in FIG. 1 and the amount of oxygen in the layer is increased to lower the refractive index to 2.5 or less, the silicon-based low refractive index layer and the back electrode Although it is difficult to lower the contact resistance of the layer, such a problem can be solved by inserting a conductive silicon-based interface layer.

  The present invention is also the silicon-based thin-film solar cell, wherein the most constituent element excluding silicon occupied in the silicon-based low refractive index layer is oxygen.

  The present invention is also the silicon-based thin-film solar cell, wherein the silicon-based low refractive index layer includes a crystalline silicon component in the layer.

  The present invention is also “a silicon-based thin film solar cell characterized in that the conductive silicon-based interface layer has a thickness of 5 nm or more”.

  The present invention is also “the above-described silicon-based thin film solar cell, which is a hybrid thin film solar cell including at least one amorphous photoelectric conversion unit and at least one crystalline photoelectric conversion unit”. .

  Hereinafter, Examples 1, 2 and 3 as silicon-based thin film solar cells according to the present invention will be described in comparison with Comparative Examples 1 and 2 with reference to FIG.

Example 1
FIG. 3 is a cross-sectional view schematically showing a hybrid thin film solar cell produced in each example and each comparative example.

First, the transparent electrode layer 2 having a fine concavo-convex structure on the surface made of SnO ₂ was formed on one main surface of the translucent substrate 1 made of 0.7 mm thick blue plate glass by a thermal CVD method.

  Next, in order to form the amorphous photoelectric conversion unit 3, the transparent substrate 1 on which the transparent electrode layer 2 is formed is introduced into a high-frequency plasma CVD apparatus, heated to a predetermined temperature, and then has a thickness of 15 nm. The amorphous p-type silicon carbide layer 3p, the non-doped amorphous i-type silicon photoelectric conversion layer 3i having a thickness of 330 nm, and the n-type silicon layer 3n having a thickness of 30 nm were sequentially stacked.

Further, in order to form the crystalline photoelectric conversion unit 4, a plasma CVD apparatus is used to form a p-type crystalline silicon layer 4p having a thickness of 15 nm and a crystalline i-type silicon photoelectric conversion layer 4i having a thickness of 1.4 μm, n. An n-type crystalline silicon-based interface layer 4n having a thickness of 5 nm and a thickness of 7.5 nm was sequentially stacked. The film forming conditions of the n-type silicon-based low refractive index layer 4on at that time are as follows: the distance between the substrate film forming surface and the electrode is 8 to 15 mm, the pressure is 5 to 10 Torr, the high frequency power density is 0.1 W / cm ² , and SiH ₄ / CO ₂ The / PH ₃ / H ₂ flow rate ratio was 1/5 / 0.042 / 380, respectively. The refractive index measured by spectroscopic ellipsometry of the n-type silicon low refractive index layer deposited on glass under the same film forming conditions as the n type silicon low refractive index layer used in this structure is 600 nm. The conductivity was 1.3 × 10 ⁻⁴ S / cm. Further calculating the ratio of the peak intensity of the amorphous silicon in the vicinity of the peak intensity and 480 cm ^-1 in crystalline Si of 520cm around ^-1 when measuring n-type silicon based low refractive index layer on the glass by Raman scattering spectroscopy 4 .8. On the other hand, the film forming conditions of the n-type silicon-based interface layer 4n are as follows: the distance between the substrate forming surface and the electrode is 10 to 15 mm, the pressure is 350 to 1300 Pa, the high frequency power density is 0.11 W / cm ² , and SiH ₄ / PH ₃ / H _2. The flow rate was 20 / 0.5 / 2500 sccm, respectively. Further, the conductivity of the n-type silicon-based interface layer deposited on the glass at 360 nm under the same film forming conditions was 100 S / cm.

  Thereafter, a transparent oxide layer 5 made of ZnO having a thickness of 30 nm and a back surface reflective electrode layer 6 made of Ag having a thickness of 200 nm were formed by DC sputtering.

Furthermore, in order to leave the transparent electrode layer 2 and separate the amorphous photoelectric conversion unit 3, the crystalline photoelectric conversion unit 4, the transparent oxide layer 5, and the back reflective electrode layer 6 into island shapes, the YAG second harmonic By irradiating the translucent substrate 1 with a pulsed laser, a plurality of back electrode layer separation grooves 5a were formed. Although not shown, island-shaped separation regions were formed by forming a plurality of back surface electrode separation grooves that intersect perpendicularly with the back surface electrode layer separation grooves 5a. Further, a back surface electrode layer separation groove is further formed outside the island-shaped separation region adjacent to the single back surface electrode layer separation groove 5a, and solder is infiltrated into the inside thereof to contact the transparent electrode layer 2. A hybrid thin film solar cell was fabricated by forming This hybrid thin-film solar cell has an effective area of 1 cm ² , and in Example 1, a total of 25 solar cells were produced on one substrate.

The hybrid thin film solar cell produced in Example 1 was irradiated with pseudo-sunlight having a spectral distribution of AM1.5 and an energy density of 100 mW / cm ² under a measurement atmosphere and a temperature of the solar cell of 25 ± 1 ° C. The output characteristics of the thin film solar cell were measured by measuring the voltage and current between the positive electrode probe 7 brought into contact with the layer 2 through the contact region 6 and the negative electrode probe 8 brought into contact with the back electrode 5. Table 1 shows the average performance of the 25 hybrid thin film solar cells produced in Example 1.

(Example 2)
In Example 2, almost the same process as in Example 1 was performed, but the point that only the film thickness of the n-type silicon-based low refractive index layer 4on was changed to 10 nm was different from Example 1. Table 1 shows the average performance of the 25 hybrid thin film solar cells produced in Example 2.

(Comparative Example 1)
Comparative Example 1 was different from Example 1 only in the following points. Instead of sequentially laminating the n-type silicon-based low refractive index layer 4on and the n-type crystalline silicon-based interface layer 4n, a 20-nm thick n-type crystalline silicon layer and a 90-nm thick ZnO transparent oxide layer 5 are sequentially stacked. did. The ZnO layer was formed by DC sputtering. Moreover, the refractive index measured by spectroscopic ellipsometry of a ZnO layer deposited on glass at 250 nm under the same film forming conditions was 1.9 at a wavelength of 600 nm. Table 1 shows the average performance of the 25 hybrid thin film solar cells produced in Comparative Example 1.

(Comparative Example 2)
In Comparative Example 2, almost the same process as in Example 1 was performed, but the point that only the film thickness of the n-type silicon-based low refractive index layer 4on was changed to 60 nm was different from Example 1. Table 1 shows the average performance of 25 integrated hybrid thin film solar cells fabricated in Comparative Example 2.

  From comparison between Example 1 and Comparative Example 1, it can be seen that in Example 1, the short-circuit current is improved by 2.5% or more compared to Comparative Example 1. This is because in Example 1, most of the light that reaches the back of the crystalline i-type silicon photoelectric conversion layer 4i is crystallized at the interface between the crystalline i-type silicon photoelectric conversion layer 4i and the n-type silicon-based low refractive index layer 4on. The ratio of light passing through the n-type crystalline silicon-based interface layer 4n that is reflected toward the crystalline i-type silicon photoelectric conversion layer 4i and has a large light absorption loss is small, whereas in Comparative Example 1, crystalline i-type silicon This is because an n-type crystalline silicon layer and a ZnO layer are sequentially stacked behind the photoelectric conversion layer 4i, and the ratio of light passing through the n-type crystalline silicon layer having a large light absorption loss is large. Furthermore, in Example 1, it turns out that the open circuit voltage is improved about 1% compared with the comparative example 1. This is because in Example 1, damage to the underlying crystalline silicon layer during sputtering of the ZnO layer that occurs in the process of Comparative Example 1 can be prevented.

Next, it can be seen from the comparison between Example 1 and Example 2 that the short-circuit current is increased in Example 2. This is because the effect of light confinement by the n-type silicon-based low refractive index layer is increased because the thickness of the n-type silicon-based low refractive index layer is increased. However, paying attention to the open circuit voltage here, the open circuit voltage equivalent to that of the first embodiment is obtained in the second embodiment, and the effect of improving the open circuit voltage by the thin n-type silicon-based low refractive index layer is maintained. I understand.
Further, from comparison between Examples 1 and 2 and Comparative Example 2, in Comparative Example 2, the short-circuit current is inferior to about 1% in the open circuit voltage although the same value as in Examples 1 and 2 is obtained. I understand that. This is because in Comparative Example 2, the n-type silicon-based low-refractive index layer 4on is as thick as 60 nm, which increases the probability of carrier recombination in the n-type silicon-based low-refractive index layer, thereby reducing the open-circuit voltage. Conceivable.

  From the above, according to the present invention, it is possible to exert a sufficient light confinement effect by forming a silicon-based low refractive index layer having a refractive index lower than that of the photoelectric conversion layer thinly behind the photoelectric conversion layer. In addition, carrier recombination generated in the silicon-based low refractive index layer can be reduced. As a result, a silicon-based thin film solar cell can be provided with high efficiency and low cost.

It is a figure showing the relationship between the amount of oxygen in a layer of a silicon system low refractive index layer, and a refractive index. It is a typical sectional view of a thin film solar cell containing a silicon system low refractive index layer by the present invention. It is typical sectional drawing of the hybrid thin film solar cell produced in each Example and the comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Translucent substrate 2 Transparent conductive film 3 Amorphous silicon photoelectric conversion unit 3p Amorphous p-type silicon carbide layer 3i Amorphous i-type silicon photoelectric conversion layer 3n n-type crystalline silicon layer 4 Crystalline silicon photoelectric conversion unit 4p p-type crystalline silicon layer 4i crystalline i-type silicon photoelectric conversion layer 4on n-type silicon-based low refractive index layer 4n n-type crystalline silicon-based interface layer 5a back electrode layer separation groove 5 transparent oxide layer 6 back reflective electrode layer 7 Positive probe 8 Negative probe

Claims (6)

  A conductive silicon-based low refractive index layer and a conductive silicon-based interface layer are sequentially arranged behind the photoelectric conversion layer as viewed from the light incident side, and the conductive silicon-based low refractive index layer and the conductive silicon-based interface layer A silicon-based thin-film solar cell having a thickness greater than 0 nm and less than or equal to 10 nm.   2. The silicon-based thin film solar cell according to claim 1, wherein a refractive index of the silicon-based low refractive index layer at a wavelength of 600 nm is 2.5 or less.   3. The silicon-based thin film solar cell according to claim 1, wherein the most abundant constituent element excluding silicon occupied in the silicon-based low refractive index layer is oxygen.   The silicon-based thin-film solar cell according to claim 1, wherein the silicon-based low refractive index layer includes a crystalline silicon component in the layer.   5. The silicon-based thin-film solar cell according to claim 1, wherein a thickness of the conductive silicon-based interface layer is 5 nm or more.   The hybrid thin film solar cell according to any one of claims 1 to 5, comprising at least one amorphous photoelectric conversion unit and at least one crystalline photoelectric conversion unit. .