Title of Invention

SILICON BASED THIN FILM SOLAR CELL

Abstract A silicon based thin film solar cell exhibiting a sufficient light confinement effect while sustaining a low series resistance and can be produced with high efficiency at a low cost by forming a silicon based low refractive index layer (4on) and a thin silicon based interface layer (4n) sequentially rearwardly of a photoelectric conversion layer (4i) when viewed from the light incident side. The silicon based low refractive index layer (4on) preferably has a refractive index not higher than 2.5 at a wavelength of 600 nm and a thickness not smaller than 300 angstrom and it is preferably an alloy layer of silicon and oxygen represented by silicon oxide. Furthermore, the thin silicon based interface layer (4n) is preferably a conductivity type layer principally comprising silicon having a thickness not larger than 150 angstrom and containing a crystalline silicon component.
Full Text DESCRIPTION
SILICON BASED THIN FILM SOLAR CELL
Technical Field
The present invention relates to a silicon based thin film solar
cell, and in particular to a thin film solar cell enabling demonstration
of light trapping effect by disposing a layer having a smaller refractive
index than a refractive index of the photoelectric conversion layer
on a backside of a photoelectric conversion layer observed from a light
incident side.
Background Art
In recent years, various thin film solar cells have come into
use, in addition to conventional amorphous thin film solar cells,
crystalline thin film solar cells are also being developed, and moreover
hybrid thin film solar cells obtained by laminating these solar cells
together are also put in practical use.
Thin film solar cells in general comprise a first electrode, one
or more semiconductor thin film photoelectric conversion units, and
a second electrode laminated in an order on a substrate. And one
photoelectric conversion unit comprises an i type layer sandwiched by
a p type layer and an n type layer.
The i type layer is substantially an intrinsic semiconductor layer,
occupies a large percentage of a thickness of the photoelectric
conversion unit, and then photoelectric conversion effect is generated
mainly within this i type layer. For this reason, this i type layer
is usually referred to as an i type photoelectric conversion layer,
or simply as a photoelectric conversion layer. The photoelectric
conversion layer is not limited to an intrinsic semiconductor layer,
but may be a layer obtained by being doped, within a range in which
loss of light absorbed with doped impurity does not cause problems,
into a p type or an n type in a very small quantity range. Although
a thicker photoelectric conversion layer is more preferable for light
absorption, a layer thicker than necessary may cause results of
increasing cost for film-forming and time for production.
On the other hand, conductivity type layers of a p type or an
n type exhibit function to generate a diffusion potential in a
photoelectric conversion unit, a magnitude of this diffusion potential
influences a value of an open circuit voltage as one of important
characteristics of a thin film solar cell. However, these conductivity
type layers are inert layers not directly contributing to photoelectric
conversion, and thus light absorbed with impurity doped in the
conductivity type layer gives loss not contributing to generation of
electric power. Therefore, the conductivity type layers of the p type
and the n type are preferably maintained for a smallest possible
thickness within a range for generation of a sufficient diffusion
potential.
Here, in the above-mentioned a pin (nip) type photoelectric
conversion unit or a thin film solar cell, when a photoelectric
conversion layer occupying a principal portion is amorphous, it is
called an amorphous unit or an amorphous thin film solar cell, and
when a photoelectric conversion layer is crystalline, it is called a
crystalline unit or a crystalline thin film solar cell, regardless of
whether conductivity type layers of p type and n type included therein
are amorphous or crystalline.
As a method of improving conversion efficiency of a thin film
solar cell, a method of laminating two or more photoelectric conversion
units to obtain a tandem unit may be mentioned. In this method, a front
unit comprising a photoelectric conversion layer having a larger band
gap is disposed on a light incident side of a thin film solar cell,
and a back unit comprising a photoelectric conversion layer having a
smaller band gap is disposed in an order on a back side of the front
unit, and this configuration thereby enables photoelectric conversion
over a large wave range of an incident light, and realizes improvement
in conversion efficiency as a whole solar cell. Among such tandem solar
cells, especially a solar cell having an amorphous photoelectric
conversion unit and a crystalline photoelectric conversion unit
laminated together is referred to as a hybrid thin film solar cell.
For example, in a longer wavelength side, an i type amorphous
silicone exhibits photoelectric conversion function in wavelength of
a light up to about 800 nm, while an i type crystalline silicone can
exhibit photoelectric conversion function with a light of longer
wavelength of about 1100 nm. However, although a light absorption even
with a sufficient thickness of about not more than 0.3 micrometers can
be realized in an amorphous silicone photoelectric conversion layer
having a larger light absorption coefficient, in a crystalline silicone
photoelectric conversion layer having a smaller light absorption
coefficient, in order to fully absorb light of a longer wavelength,
the layer preferably has a thickness of about 1.5 to 3 micrometers.
That is, usually a crystalline photoelectric conversion layer
preferably has a thickness of about 5 to 10 times as large as a thickness
of an amorphous photoelectric conversion layer.
In a monolayer amorphous silicon thin film solar cell, and also
in the above-mentioned hybrid thin film solar cell, a thickness of a
photoelectric conversion layer is desirably maintained as small as
possible, from a viewpoint of improvement in productivity, that is,
cost reduction. For this reason, generally used is a structure using
what is called light trapping effect in which a disposition of a layer
having a refractive index smaller than a refractive index of a
photoelectric conversion layer, on a backside of the photoelectric
conversion layer observed from a light incident side enables effective
reflection of light of a particular wavelength. A disposition on a
backside of a photoelectric conversion layer observed from a light
incident side means a disposition contacting to the photoelectric
conversion layer on a side of a back face, or a disposition on a side
of a back face in a state of sandwiching an other layer disposed on
a back face of the photoelectric conversion layer.
Japanese Patent Laid-Open No. 02-73672 official report discloses
a structure of a solar cell in which a translucent first electrode,
an amorphous silicon semiconductor thin film (hereinafter referred to
as simply semiconductor thin film), a zinc oxide film having a thickness
of less than 1200 angstroms, a non-translucent second electrode (metal
electrode) are laminated in this order from a light incident side. The
zinc oxide film has a function for preventing a silicide formed in an
interface between the semiconductor thin film and the metal electrode
increase absorption loss. Since refractive index difference exists
between the zinc oxide film and the semiconductor thin film, a thickness
of the zinc oxide film limited to a range of less than 1200 angstroms
and preferably to a range of 300 angstroms to 900 angstroms has an effect
of improving reflectance in an interface of the semiconductor thin film
/the zinc oxide film. For this reason, a short-circuit current density
of the solar cell and consequently a conversion efficiency improves.
However, since the zinc oxide film is formed by a technique of sputtering,
spraying, etc., different facilities from that for semiconductor thin
film formed in general by plasma CVD methods etc. are required, leading
to occurrence of problems of facility cost rise and longer production
tact. Furthermore, there may occur problems that especially use of
sputtering method in formation of the zinc oxide film may cause
performance reduction by sputter damage to a ground semiconductor thin
film. According to examples, the above-mentioned semiconductor thin
film consists of a P type a-SiC:H film, a non doped a-Si:H film, and
an n type a-Si:H film. In this case in order to generate sufficient
diffusion potential in a non doped a-Si:H film, a thickness of an n
type a-Si:H film requires 150 angstroms to 300 angstroms in general,
which will not permit ignoring absorption loss of light passing through
the n type a-Si:H film.
Japanese Patent Laid-Open No. 4-167473 official report discloses
a structure, in a sequential order from light incident side, of a
transparent electrode/ one electric conductive type amorphous
semiconductor layer / an intrinsic amorphous semiconductor layer / an
amorphous silicon oxynitride or amorphous silicon oxide (hereinafter
referred as a-SiON or a-SiO) / a metal oxide layer / a high reflective
metal layer / a substrate. However, this a-SiON (a-SiO) layer is formed
for prevention of increase in absorption loss by reduction of the metal
oxide layer that may be obtained when.forming-the amorphous
semiconductor layer on the metal oxide layer, and no description is
disclosed that light trapping may be performed using refractive index
difference between the a-SiON (a-SiO) layer and the intrinsic amorphous
semiconductor layer. Specifically, in Examples, a thickness of a-SiON
(a-SiO) layer set thin as 200 angstroms does not permit expectation
of sufficient light trapping effect.
Japanese Patent Laid-Open No. 6-267868 official report discloses
a method for forming a film of a-SiO including microcrystalline phase
of silicon characterized by being based on decomposition of a source
gas having not more than 0.6 of a value of CO2 / (S1H4+ CO2). The official
report describes that this film represents a high photoconductivity
not less than 10"6 S/cm, and a low absorption coefficient, and is suitable
for a window layer of amorphous silicon based solar cells. However,
this official report fails to describe about a refractive index of the
obtained film, and fails to describe that the film can be disposed on
a backside of a photoelectric conversion layer of the solar cell observed
from a light incident side. The present inventors carried out
investigation for application of a silicon oxide layer by a high
frequency plasma CVD method for an n type layer of pin type silicon
based thin film solar cell using SiH4, CO2, H2, and PH3 as reactive gas,
based on teachings obtained by the documentary materials. As a result,
it was found our that using a technique of disposing a silicon oxide
layer on a backside of a photoelectric conversion layer, and of setting
a ratio of CO2 / S1H4 larger, light trapping effect was exhibited and
a short-circuit current of the solar cell was increased when increasing
an amount of oxygen in the layer and a difference of refractive index
with the photoelectric conversion "layer. However, only simple use of
the silicon oxide as an n type layer increased a series resistance of
the solar cell, leading to a problem of reduction of conversion
efficiency. This is considered to originate in a contact resistance
between silicon oxide and a metal oxide layers, such as ZnO as a part
of a back electrode.
Thus, conventional technique cannot solve a problem of series
resistance of solar cells that is believed to be caused by a contact
resistance generated between a silicon based low-refractive index layer
represented by silicon oxides, and a back electrode.
Disclosure of Invention
Taking the above situations into consideration, by disposing a
layer having a lower refractive index compared with that of a
photoelectric conversion layer, on a backside of the photoelectric
conversion layer observed from a light incident side, without using
different facilities from those for formation of the photoelectric
conversion layer, the present invention aims at providing a silicon
based thin film solar cell, efficiently and at a low cost, that can
exhibit sufficient light trapping effect and can keep a series
resistance of the solar cell smaller even if a layer having a low
refractive index is disposed.
A silicon based thin film solar cell by the present invention
is characterized in that a silicon based low refractive index layer
and a silicon based interface layer are disposed in this order on a
backside of a photoelectric conversion layer observed from a light
incident side.
The silicon based low refractive index layer, has a function to
generate a diffusion potential in the photoelectric conversion layer,
which is a layer doped with impurity to give a p type or an n type.
In order to effectively reflect light on a surface thereof to the
photoelectric conversion layer side, and to keep absorption loss of
light in the layer as small as possible, the silicon based low refractive
index layer preferably has a refractive index of not more than 2.5 at
a wavelength of 600 nm, and has a thickness of not less than 300
angstroms.
A silicon based low refractive index layer is an alloy layer
comprising silicon and elements, such as oxygen, typically a silicon
oxides, preferably a ratio of a most abundantly existing constituent
element, excluding silicon, in the layer is not less than 25 atom %,
and the layer is preferably formed by methods, such as a high freguency
plasma CVD that are same kind as methods for a photoelectric conversion
layer. The silicon based low refractive index layer preferably
includes crystalline silicon components in the layer, in order to reduce
a resistance in a thickness direction of the layer itself.
A silicon based interface layer is a conductivity type layer
having silicon as a principal component. Since the silicon based
interface layer does not need to contribute to generation of a diffusion
potential in the photoelectric conversion layer, it preferably has a
thickness of not more than 150 angstroms, and more preferably has a
thickness of not more than 100 angstroms in order to keep light
absorption loss in the layer as small as possible. Furthermore, in
order to keep a contact resistance with back electrode small, it
preferably comprises a crystalline silicon component in the layer.
In order to solve problems of increase in a series resistance.
of a solar cell caused by disposition of a silicon based low refractive
index layer in a backside of a photoelectric conversion layer, the
present inventors wholeheartedly investigated structures for optimal
solar cells. As a result, it.was found out that by disposing a thin
silicon based interface layer in a backside of a silicon based low
refractive index layer, a contact resistance to a back electrode layer
comprising metal oxide layer disposed in a backside thereof was improved,
and thereby a series resistance of the solar cell became smaller and
conversion efficiency was improved.
In the present invention, a silicon based interface layer disposed
between a silicon based low refractive index layer and a back electrode
layer has small contact resistances with either of the silicon based
low refractive index layer and the back electrode layer, and as a result
a small series resistance of the solar cell is believed to be realized.
Especially, as shown in Figure 1, when silicon oxide is used as a silicon
based low refractive index layer, and an amount of oxygen in the layer
is increased to lower a refractive index to not more than 2.5, it is
difficult to lower a contact resistance between the silicon based low
refractive index layer and the back electrode layer. However, such
a problem is solved by inserting a silicon based interface layer.
Therefore, this technique enables design of the silicon based low
refractive index layer to an optimal thickness and an optimal refractive
index for light trapping. Furthermore, since simple change of
film-forming conditions permits adjustment of a refractive index of
the silicon based low refractive index layer, increase in light trapping
effect by more delicate optical designs can also be expected, such as
periodic variation of a refractive index in a thickness direction.
Herein a description will be given about a silicon based thin
film solar cell as an embodiment of the invention referring to Figure
2.
A transparent electrode layer 2 is formed on a translucent board
1. As the translucent board 1, a tabular member and a sheet shaped
member comprising a glass, a transparent resin, etc. are used. The
transparent electrode layer 2 preferably comprises conductive metal
oxides, such as SnO2 and ZnO, and preferably formed using methods, such
as CVD, sputtering, and vapor deposition. The transparent electrode
layer 2 preferably has minute unevenness formed on a surface thereof,
and preferably has an effect of increasing dispersion of incident light.
An amorphous photoelectric conversion unit 3 is formed on the
transparent electrode layer 2. The amorphous photoelectric conversion
unit 3 comprises an amorphous p type silicon carbide layer 3p, non doped
amorphous i type silicon photoelectric conversion layer 3i, and an n
type silicon based interface layers 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 formation
of the amorphous photoelectric conversion unit 3 and the crystalline
photoelectric conversion unit 4 (both of the units are simply,
hereinafter, collectively referred to as photoelectric conversion
unit) . As formation conditions for the photoelectric conversion unit,
preferably used are conditions of: substrate temperature 100 to 300
degrees C; pressure 30 to 1500 Pa; and high frequency power density
0.01 to 0.5 W/cm2. As a source gas used for photoelectric conversion
unit formation, silicon including gases, such as SiH4 and Si2H6 etc.
. or a mixed gas thereof with H2 are used. As dopant gas for forming a
p type or an n type layer in the photoelectric conversion unit, B2H6
or PH3 etc. is preferably used.
The crystalline photoelectric conversion unit 4 comprises
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 a n type silicon based low refractive index layer 4on, silicon
oxide is typically used, and in the case a mixed gas of SiH4, H2, CO2,
and PH3 is suitable for a source gas to be used. The silicon based low
refractive index layer 4on may or may not comprise crystalline silicon
components. A refractive index at a wavelength of 600 ran of the silicon
based low refractive index layer 4on is preferably not more than 2.5.
A percentage of a most abundantly existing constituent element except
silicon in a layer in the silicon based low refractive index layer 4on
is preferably not less than 25 atomic %. A thickness of the silicon
based low refractive index layer 4on is preferably not less than 300
angstroms, and more preferably 500 angstroms to 900 angstroms. When
silicon oxide is used as the silicon based low refractive index layer
4on, in order to realize a percentage of oxygen occupied in the layer,
or a refractive index thereof, a gas ratio of C02/SiH4 is approximately
2 to 10. The silicon based low refractive index layer 4on may have
a fixed refractive index in a thickness direction, or may have refractive
indexes varying in a thickness direction. Furthermore, it may have
refractive indexes periodically variable. Figure 2 shows a structure
that the n type silicon based low refractive index layer 4on is disposed,
contacting the crystalline i type silicon photoelectric conversion
layer 4i, on a backside of the crystalline i type silicon photoelectric
conversion layer 4i observed from a light incident side. Other layers,
such as an n type silicon layer, may be disposed in a state sandwiched
between the crystalline i type silicon photoelectric conversion layer
4i and the n type silicon based low refractive index layer 4on. And
the silicon based low refractive index layer 4on may be a layer, instead
of silicon oxide, including any one or more elements of nitrogen, carbon,
and oxygen in addition to silicon, such as silicon nitride, silicon
carbide, silicon oxy-nitride, silicon oxy-carbide, etc.
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 in order to improve
a contact resistance between the n type silicon based low refractive
index layer 4on and a back electrode 5, and thus preferably has a
thickness as small as possible in order to minimize light absorption
loss in this layer. Specifically, the thickness is not more than 150
angstroms, and more preferably not more than 100 angstroms.
Furthermore, as the n type silicon based interface layer 4n, a layer
having an electric conductivity of about 1 to 102 S/cm is used. The
n type silicon based interface layer 4n may include any one or more
elements of oxygen, carbon, and nitrogen in a range not increasing a
contact resistance with the back electrode 5.
The back electrode 5 is formed on the n type silicon based
interface layer 4n. The back electrode 5 consists of a transparent
reflecting layer 5t, and a back reflecting layer 5m. Metal oxides,
such as ZnO and ITO, are preferably used for the transparent reflecting
layer 5t, and Ag, Al, or alloys thereof are preferably used for the
back reflecting layer 5m. In formation of the back electrode 5, methods
such as sputtering and vapor deposition, are preferably used. Although
Figure 2 indicates a structure of a hybrid thin film solar cell, a number
of the photoelectric conversion units 4 needs not necessarily to be
two, and it may be amorphous or crystalline and may have a solar cell
structure with monolayer or three or more layer-type. Furthermore
Figure 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 disposed on a translucent board in this order, or
it may have what is called a 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 deposited in this order
on a conductive boards of such as metal, or an insulated substrate.
The present invention corresponds to a patent application concerning
achievement of sponsored research by the government. (Japanese, New
Energy and Industrial Technology Development Organization in Heisei
15 fiscal year "Photovoltaic power generation technical research
development commission enterprise", Article 30 of the Law concerning
Temporary Measures for Industrial Revitalization is applied to the
present application.)
Brief Description of the Drawings
Figure 1 is a figure showing a relationship of an amount of oxygen
in a layer and a refractive index of silicon based low refractive index
layer;
Figure 2 is a schematic sectional view of a thin film solar cell
comprising a silicon.based low refractive index layer.by the present
invention;
Figure 3 is a schematic sectional view of hybrid thin film solar
cells produced in each Example and Comparative Example;
Figure 4 is a figure showing a reflection spectrum in which light
was entered and was measured from a surface exposed by etching removal
of a back electrode of solar cells produced by Example 1 and Comparative
Example 1;
Figure 5 is a figure showing a relationship between a refractive
index of a silicon based low refractive index layer, and conversion
efficiency of a hybrid thin film solar cell;
Figure 6 is a figure showing a relationship between a thickness
of silicon based low refractive index layer, and conversion efficiency
of a hybrid thin film solar cell; and
Figure 7 is an expanded sectional view by transmission electron
microscope (TEM) photograph of a silicon based thin film solar cell
of the present invention obtained in Example 1.
Best Mode for Carrying Out the Invention
Descriptions will, hereinafter, be given for Examples 1, 2, and
3 as a silicon based thin film solar cell by the present invention,
comparing with Comparative Examples 1 and 2 with reference to Figure
3.
Example 1
Figure 3 is a sectional view showing schematically a hybrid thin
film solar cell produced in each Example and each Comparative Example.
First, a transparent electrode layer 2 consisting of Sn02 and
having a minute uneven structure on a surface thereof was formed by
a heat CVD method on a principal surface of a translucent board 1
consisting a blue plate glass of a thickness of 0.7 mm.
Next, in order to form an amorphous photoelectric conversion unit
3, the translucent board 1 having a transparent electrode layer 2 formed
thereon was introduced in a high frequency plasma CVD device. After
the equipment was heated up to a predetermined temperature, an amorphous
p type silicon carbide layer 3p with a thickness of 160 angstroms, a
non doped amorphous i type silicon photoelectric conversion layer 3i
with a thickness of 3000 angstroms, and an n type silicon layer 3n with
a thickness of 300 angstroms were sequentially laminated.
Furthermore, in order to form a crystalline photoelectric
conversion unit 4, using a plasma CVD device, a p type crystalline
silicon layer 4p with a thickness of 150 angstroms, a crystalline i
type silicon photoelectric conversion layer 4i with a thickness of 1.4
micrometers, an n type silicon based low refractive index layer 4on
with a thickness of 600 angstroms, and an n type crystalline silicon
based interface layer 4nwith a thickness of 50 angstroms to 70 angstroms
were sequentially laminated. Film-forming conditions of the n type
silicon based low refractive index layer 4on in the case are shown below:
a distance between substrate film-forming, side-electrode of 10 to 15
mm; a pressure of 350 to 1300 Pa; a high frequency power density of
0.1 to 0.13 W/cm2; and flow rates of SiH4 / CO2 / PH3 / H2 of 15 / 120
/ 0.5 / 9000 seem, respectively. A refractive index of an n type silicon
based low refractive index layer deposited by a thickness of 2500
angstroms on a glass using identical film-forming conditions was
measured for by spectrum ellipsometry to give 1.9 at a wavelength of
600 nm. On the other hand, film-forming conditions of the n type silicon
based interface layer 4n are shown below: a distance between substrate
film-forming side-electrode of 10 to 15 mm; a pressure of 350 to 1300
Pa; a high frequency power density of 0.11 W/cm2; and flow rates of SiFU
/ PH3 / H2 of 20 / 0. 5 / 2500 seem respectively. An electric conductivity
of an n type silicon based interface layer deposited with a thickness
of 2500 angstroms on a glass on identical film-forming conditions gave
12 S/cm.
Then, as back electrodes 5, a transparent reflecting layer
consisting of ZnO with a thickness of 300 angstroms (not shown) and
a back reflecting layer consisting of Ag with a thickness of 2000
angstroms (not shown) were formed using a DC sputtering method.
Furthermore, in order to isolate the amorphous photoelectric
conversion unit 3, the crystalline photoelectric conversion unit 4,
and the back electrode 5 in a shape of an island, while leaving the
transparent electrode layer 2, two or more of back electrode layer
isolation grooves 5a were formed by irradiating a YAG second harmonics
pulsed laser to the translucent board 1. Although not shown, two or
more back electrode isolation grooves perpendicularly intersecting to
the back electrode layer isolation groove 5a were also formed to give
island-like isolated areas. Furthermore, on outside of the island-like
isolated area adjacent to the back electrode layer isolation groove
5a a back electrode layer isolation groove was further formed, and then
solder was permeated to an inside thereof to form a contact area 6 with
respect to the transparent electrode layer 2. Thus, a hybrid thin film
solar cell was produced. This hybrid thin film solar cell has an
effective area of 1 cm2. In Example 1, totally 25 of the solar cells
were produced on one substrate.
Pseudo-solar light having a spectrum distribution AM 1.5 and an
energy density 100 mW/cm2 was irradiated to the hybrid thin film solar
cell produced in Example 1 under a condition of temperature of
measurement atmosphere and solar cell as 25±1 degrees C. A voltage
and an electric current between a positive electrode probe 7 in contact
with the transparent electrode layer 2 through the contact area 6, and
a negative electrode probe 8 in contact with the back electrode 5 were
measured to obtain an output characteristic of the thin film solar cell.
Table 1 shows an average performance of 25 hybrid thin film solar cells
produced in Example 1.
A part of the solar cell was dipped in a nitric acid aqueous
solution, and etching removal of the back electrode 5 was carried out
to expose the n type silicon based interface layer 4n. In this state,
light was irradiated from a side of the n type silicon based interface
layer 4n, and a reflection spectrum was measured for. Figure 4 shows
the reflection spectrum. Next, the n type silicon based interface layer
4n was removed by a reactive ion etching (RIE) method to expose the
n type silicon based low refractive index layer 4on. A refractive index
of this silicon based low refractive index layer measured by a spectrum
ellipsometry gave 1.93 at a wavelength of 600 nm. Moreover, an amount
of oxygen in the silicon based low refractive index layer measured by
an X-ray photoelectron spectroscopy (XPS) gave 4 8 atomic %.
Example 2
In Example 2, an almost similar process as in Example 1 was carried
out except for having varied a refractive index at a wavelength of 600
ran in a range of 1.65 to 2.65 by varying only film-forming conditions
of an n type silicon based low refractive index layer 4on. Figure 5
shows a relationship between refractive indexes of the silicon based
low refractive index layer, and conversion efficiency of the hybrid
thin film solar cell.
Example 3
In Example 3, an almost similar process as in Example 1 was carried
out except for having varied a thickness of an n type silicon based
low refractive index layer 4on in a range of 100 angstroms to 1000
angstroms. Figure 6 shows a relationship between a thickness of the
silicon based low refractive index layer and obtained conversion
efficiency of the hybrid thin film solar cell.
Comparative Example 1
In Comparative Example 1, only following points differed from
in Example 1. Instead of laminating sequentially an n type silicon
based low refractive index layer 4on and an n type crystalline silicon
based interface layer 4n, an n type crystalline silicon layer with a
thickness of 150 angstroms and a ZnO layer with a thickness of 600
angstroms were laminated sequentially. Film-forming of ZnO layer was
performed by a DC sputtering method. A ZnO layer deposited with a
thickness of 2500 angstroms on a glass under an identical film-forming
condition was measured for a refractive index by spectrum ellipsometry
to give 1.9 at a wavelength of 600 nm. Table 1 shows an average
performance of 25 hybrid thin film solar cells produced in Comparative
Example.1." A part of the solar cells produced in Comparative Example
1 was dipped in a nitric acid aqueous solution, and etching removal
of the back electrode 5 was carried out to expose the n type silicon
based interface layer 4n. Light was irradiated from the n type
crystalline silicon layer side in this state to obtain a reflection
spectrum. Figure 4 shows the reflection spectrum.
Comparative Example 2
In Comparative Example 2, a similar process as in Example 1 was
carried out except for a point of having omitted formation of an n type
silicon based interface layer 4n on an n type silicon based low
refractive index layer 4on. Table 1 shows an average performance of
25 integrated hybrid thin film solar cells produced in Comparative
Example 2.
Comparison between Example 1 and Comparative Example 1 shows that
in Example 1 a short-circuit current is improved not less than 4% as
compared to that in Comparative Example 1. The reason is shown below.
In Example 1, a great portion of a light reaching on a backside of the
crystalline i type silicon photoelectric conversion layer 4i was
reflected into a side of the crystalline i type silicon photoelectric
conversion layer 4i,'at an interface between the crystalline i type
silicon photoelectric conversion layer 4i and the n type silicon based
low refractive index layer 4on, and consequently a percentage of light
passing through the n type crystalline silicon based interface layer
4n having a large light absorption loss decreased. On the other hand,
in Comparative Example 1, the n type crystalline silicon layer and the
ZnO layer are sequentially laminated on a backside of the crystalline
i type silicon photoelectric conversion layer 4i, and therefore, a
percentage of light passing through the n type crystalline silicon layer
having a large light absorption loss increased. And furthermore, in
Example 1, damage given to a ground crystalline silicon layer at the
time of sputtering of ZnO layer possibly formed in the process of
Comparative Example 1 was prevented.
Next, comparison between Example 1 and Comparative Example 2 shows
that a fill factor in Example 1 is improved about 5% as compared to
that in Comparative Example 2. This is based on a reason that in Example
1, disposition by insertion of the n type crystalline silicon based
interface layer 4n between the n type silicon based low refractive index
layer 4on and the transparent reflecting layer 5t improves a series
resistance of the solar cell.
A test result of a reflection spectrum obtained by measuring with
irradiated light from a light incident side and an opposite direction
at the time of solar cell characteristics measurement shown in Figure
4 shows that etching removal of the back electrode 5 enables detection
of whether a silicon based low refractive index layer 4on having a
smaller refractive index is disposed on a backside of a crystalline
i type silicon photoelectric conversion layer 4i. Results of Example
2 in Figure 5 shows that a refractive index of the silicon based low
refractive index layer has an optimal value, which is preferably not
more than 2.5.
Figure 1 shows that this condition corresponds to a value of not
less than 25 atomic % of an amount of oxygen in the layer. This is
based on a reason that a refractive index exceeding 2.5 makes a
refractive index difference with an adjoining crystalline i type
silicon photoelectric conversion layer smaller, which reduces light
trapping effect. Results of Example 3 shown in Figure 6 shows that
a thickness of the silicon based low refractive index layer has an
optimal value, which.is preferably not less than 300 angstroms.
According to the present invention from the above description,
sufficient light trapping effect at low cost can be exhibited by
disposing a layer having a lower refractive index compared to that of
a photoelectric conversion layer, on a backside of the photoelectric
conversion layer observed from a light incident side, without using
different facilities from those for formation of the photoelectric
conversion layer. Furthermore, by disposing a thin silicon based
interface layer on a backside of a silicon based low refractive index
layer, a series resistance of a solar cell can be kept small. As a
result, a silicon based thin film solar cell can be provided efficiently
and at low cost.
Industrial Applicability
According to the present invention, sufficient light trapping
effect at low cost can be exhibited by disposing a layer having a lower
refractive index compared with that of a photoelectric conversion layer,
on a backside of the photoelectric conversion layer observed from a
light incident side, without using different facilities from those for
formation of the photoelectric conversion layer. Furthermore, by
disposing a thin silicon based interface layer on a backside of a silicon
based low refractive index layer, a series resistance of a solar cell
can be kept small. As a result, a silicon based thin film solar cell
can be provided efficiently and at low cost.
We Claim :
1. A silicon based thin film solar cell, wherein a conducted type silicon based low
refractive index layer, a silicon based interface layer, and a back electrode are disposed
and contact one another in this order on a backside of a photoelectric conversion layer
observed from a light incident side.
2. The silicon based thin film solar cell as claimed in claim 1, wherein the silicon
based low refractive index layer has a refractive index not more than 2.5 at a wavelength
of 600 nm.
3. The silicon based thin film solar cell as claimed in claim 1, wherein a most
abundantly existing constituent element, excluding silicon, in the silicon based low
refractive index layer is not less than 25 atomic %.
4. The silicon based thin film solar cell as claimed in claim 3, wherein the most
abundantly existing constituent element is oxygen.
5. The silicon based thin film solar cell as claimed in claim 1, wherein the silicon
based low refractive index layer has a thickness of not less than 300 angstroms.
6. The silicon based thin film solar cell as claimed in claim 1, wherein the silicon
based low refractive index layer comprises a crystalline silicon component in the layer.
7. The silicon based thin film solar cell as claimed in claim 1, wherein the silicon
based interface layer has a thickness not more than 150 angstroms.
8. The silicon based thin film solar cell as claimed in claim 7, wherein the silicon
based interface layer comprises a crystalline silicon component in the layery" ^


A silicon based thin film solar cell exhibiting a sufficient light confinement
effect while sustaining a low series resistance and can be produced with high
efficiency at a low cost by forming a silicon based low refractive index layer (4on)
and a thin silicon based interface layer (4n) sequentially rearwardly of a
photoelectric conversion layer (4i) when viewed from the light incident side. The
silicon based low refractive index layer (4on) preferably has a refractive index not
higher than 2.5 at a wavelength of 600 nm and a thickness not smaller than 300
angstrom and it is preferably an alloy layer of silicon and oxygen represented by
silicon oxide. Furthermore, the thin silicon based interface layer (4n) is preferably
a conductivity type layer principally comprising silicon having a thickness not
larger than 150 angstrom and containing a crystalline silicon component.

Documents:

02742-kolnp-2005-abstract.pdf

02742-kolnp-2005-claims.pdf

02742-kolnp-2005-description complete.pdf

02742-kolnp-2005-drawings.pdf

02742-kolnp-2005-form 1.pdf

02742-kolnp-2005-form 3.pdf

02742-kolnp-2005-form 5.pdf

02742-kolnp-2005-international publication.pdf

02742-kolnp-2005-priority document.pdf

2742-kolnp-2005-assignment.pdf

2742-KOLNP-2005-CORRESPONDENCE 1.1.pdf

2742-KOLNP-2005-CORRESPONDENCE-1.1.pdf

2742-KOLNP-2005-CORRESPONDENCE-1.2.pdf

2742-KOLNP-2005-CORRESPONDENCE.pdf

2742-kolnp-2005-correspondence1.3.pdf

2742-kolnp-2005-examination report.pdf

2742-kolnp-2005-form 18.pdf

2742-kolnp-2005-form 3.pdf

2742-kolnp-2005-form 5.pdf

2742-KOLNP-2005-FORM-27.pdf

2742-kolnp-2005-gpa.pdf

2742-kolnp-2005-granted-abstract.pdf

2742-kolnp-2005-granted-claims.pdf

2742-kolnp-2005-granted-description (complete).pdf

2742-kolnp-2005-granted-drawings.pdf

2742-kolnp-2005-granted-form 1.pdf

2742-kolnp-2005-granted-specification.pdf

2742-KOLNP-2005-OTHERS 1.1.pdf

2742-KOLNP-2005-OTHERS.pdf

2742-kolnp-2005-reply to examination report.pdf

2742-kolnp-2005-translated copy of priority document.pdf

abstract-02742-kolnp-2005.jpg


Patent Number 242866
Indian Patent Application Number 2742/KOLNP/2005
PG Journal Number 38/2010
Publication Date 17-Sep-2010
Grant Date 16-Sep-2010
Date of Filing 28-Dec-2005
Name of Patentee KANEKA CORPORATION
Applicant Address 2-4, NAKANOSHIMA 3-CHOME, KITA-KU, OSAKA-SHI, OSAKA
Inventors:
# Inventor's Name Inventor's Address
1 KOI, YOHEI 24-8-304, KONOOKACHO, OHTSU-SHI SHIGA 520-0103
2 SAWADA, TORU 64-32, SHINMEI MIYAKITA, UJI-SHI, KYOTO 611-0025
3 SASAKI, TOSHIAKI 2-1-2-131, HIEITSUJI, OHTSU-SHI, SHIGA 520-0104
4 GOTO, MASAHIRO 1-25-1, HIEITSUJI, OTSU-SHIGA 520-0104
5 YOSHIMI, MASASHI 6-6-4, IBUKIDAI-NISHIMACHI, NISHI-KU, KOBE-SHI, HYOGO 651-2243
6 YAMAMOTO, KENJI 2-W1406, MIKATA-DAI 1-CHOME, NISHI-KU KOBE-SHI, HYOGO 651-2277
PCT International Classification Number H01L 31/075
PCT International Application Number PCT/JP2004/010248
PCT International Filing date 2004-07-12
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 2003-279491 2003-07-24 Japan
2 2003-358362 2003-10-17 Japan