U.S. patent application number 13/331787 was filed with the patent office on 2012-06-21 for solar cell and method for manufacturing the same.
Invention is credited to Junyong Ahn, Jungmin Ha, Jinho Kim.
Application Number | 20120152338 13/331787 |
Document ID | / |
Family ID | 46232750 |
Filed Date | 2012-06-21 |
United States Patent
Application |
20120152338 |
Kind Code |
A1 |
Ha; Jungmin ; et
al. |
June 21, 2012 |
SOLAR CELL AND METHOD FOR MANUFACTURING THE SAME
Abstract
A method for manufacturing a solar cell is discussed. The method
may include injecting first impurity ions at a first surface of a
substrate by using a first ion implantation method to form a first
impurity region, the substrate having a first conductivity type and
the first impurity region having a second conductivity type,
heating the substrate with the first impurity region to activate
the first impurity region to form an emitter region, etching the
emitter region from a surface of the emitter region to a
predetermined depth to form an emitter part, and forming a first
electrode on the emitter part to connect to the emitter part and a
second electrode on a second surface of the substrate, which is
opposite the first surface of the substrate to connect to the
second surface of the substrate.
Inventors: |
Ha; Jungmin; (Seoul, KR)
; Ahn; Junyong; (Seoul, KR) ; Kim; Jinho;
(Seoul, KR) |
Family ID: |
46232750 |
Appl. No.: |
13/331787 |
Filed: |
December 20, 2011 |
Current U.S.
Class: |
136/255 ;
257/E31.124; 438/87 |
Current CPC
Class: |
Y02E 10/52 20130101;
H01L 31/186 20130101; H01L 31/068 20130101; H01L 31/02168 20130101;
Y02E 10/547 20130101; H01L 31/022425 20130101; H01L 31/18 20130101;
H01L 31/1864 20130101; H01L 31/03529 20130101 |
Class at
Publication: |
136/255 ; 438/87;
257/E31.124 |
International
Class: |
H01L 31/0352 20060101
H01L031/0352; H01L 31/18 20060101 H01L031/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2010 |
KR |
10-2010-0131823 |
Claims
1. A method for manufacturing a solar cell, the method comprising:
injecting first impurity ions at a first surface of a substrate by
using a first ion implantation method to form a first impurity
region, the substrate having a first conductivity type and the
first impurity ions having a second conductivity type opposite the
first conductivity type, and the first impurity region having the
second conductivity type; heating the substrate with the first
impurity region to activate the first impurity region to form an
emitter region from the first impurity region; etching the emitter
region from a surface of the emitter region to a predetermined
depth to form an emitter part from the emitter region; and forming
a first electrode on the emitter part to connect to the emitter
part and a second electrode on a second surface of the substrate,
which is opposite the first surface of the substrate to connect to
the second surface of the substrate.
2. The method of claim 1, wherein the heating of the substrate
heats the first impurity portion at 800.degree. C. to 1100.degree.
C. in a nitrogen atmosphere.
3. The method of claim 1, wherein the etching of the emitter region
removes a portion of the emitter region from the surface of the
emitter region to a depth of 5 nm to 20 nm.
4. The method of claim 3, wherein the emitter region is etched by
an etchant composed of nitric acid HNO.sub.3, hydrofluoric acid HF
and pure wafer.
5. The method of claim 1, wherein the heating of the substrate
heats the first impurity portion at 800.degree. C. to 1100.degree.
C. in an oxygen atmosphere.
6. The method of claim 5, wherein the etching of the emitter region
removes a portion of the emitter region from the surface of the
emitter region to a depth of 20 nm to 35 nm.
7. The method of claim 6, wherein the emitter region is etched by
an etchant composed of hydrofluoric acid HF and pure wafer.
8. The method of claim 1, wherein the emitter part comprises a
first emitter portion having a first impurity doped thickness and a
second emitter portion having a second impurity doped thickness
greater than the first impurity doped thickness, and wherein the
etching of the emitter region comprises: selectively forming an
etch prevention layer on the emitter region to expose a portion of
the emitter region and to cover a remaining portion of the emitter
region; and etching the exposed portion of the emitter region from
the surface of the emitter region to the predetermined depth using
the etch prevention layer as a mask; and removing the etch
prevention layer, wherein the etched exposed portion of the emitter
region is formed as the first emitter portion and the remaining
portion of the emitter region is formed as the second emitter
portion.
9. The method of claim 1, wherein the first impurity region
comprises a first impurity portion having a first impurity doped
thickness and a second impurity portion having a second impurity
doped thickness greater than the first impurity doped thickness,
and wherein the injecting of the first impurity ions forms the
first and second impurity portions by use of a mask positioned at
the first surface of the substrate and use of the first ion
implantation method.
10. The method of claim 9, wherein the mask comprises a first
portion having a first exposing area for forming the first impurity
portion and a second portion having a second exposing area for
forming the second impurity portion, the first and second exposing
areas being areas exposing the substrate in a unit area
thereof.
11. The method of claim 9, further comprising forming the first
impurity region not having the first and second impurity portions
at an entire first surface of the substrate by injecting the first
impurity ions of the second conductivity type at the entire first
surface of the substrate without a mask, before forming the first
and second impurity portions of the first impurity region, wherein
the forming of the first and second impurity portions of the first
impurity region forms the first and second impurity portions by use
of the mask positioned at the first impurity region not having the
first and second impurity portions and use of the first ion
implantation method.
12. The method of claim 1, further comprising; injecting second
impurity ions at a second surface of the substrate by using a
second ion implantation method to form a second impurity region of
the first conductivity type, the second surface being opposite the
first surface of the substrate; heating the substrate with the
second impurity region to activate the second impurity region to
form an surface field region from the second impurity region; and
etching the surface field region from a surface of the surface
field region to a predetermined depth to form a surface field part
from the surface field region, wherein the second electrode is
connected to the second surface of the substrate through the
surface field part.
13. The method of claim 12, wherein the surface field part
comprises a first surface field portion having a first impurity
doped thickness and a second surface field portion having a second
impurity doped thickness greater than the first impurity doped
thickness, wherein the etching of the surface field region
comprises: selectively forming an etch prevention layer on the
surface field region to expose a portion of the surface field
region and to cover a remaining portion of the surface field
region; etching the exposed portion of the surface field region
from the surface of the surface field region to the predetermined
depth using the etch prevention layer as a mask; and removing the
etch prevention layer, wherein the etched exposed portion of the
surface field region is formed as the first surface field portion
and the remaining portion is formed as the second surface field
portion.
14. The method of claim 13, wherein the second electrode is in
contact with the second surface field portion and is connected to
the second surface of the substrate through the second surface
field portion.
15. The method of claim 14, wherein the first and second surfaces
of the substrate are light incident surfaces on which light is
incident.
16. The method of claim 12, wherein the second impurity region
comprises a first impurity portion having a first impurity doped
thickness and a second impurity portion having a second impurity
doped thickness greater than the first impurity doped thickness,
wherein the injecting of the second impurity ions forms the first
and second impurity portions by use of a mask positioned at first
surface of the substrate and use of the second ion implantation
method.
17. The method of claim 16, wherein the mask comprises a first
portion having a first exposing area for forming the first impurity
portion and a second portion having a second exposing area for
forming the second impurity portion, the first and second exposing
areas being areas exposing the substrate in a unit area
thereof.
18. The method of claim 16, further comprising forming the second
impurity region not having the first and second impurity portions
at an entire second surface of the substrate by injecting the
second impurity ions of the first conductivity type at the entire
second surface of the substrate without a mask, before forming the
first and second impurity portions of the second impurity region,
wherein the forming of the first and second impurity portions of
the second impurity region forms the first and second impurity
portions by use of the mask positioned at the second impurity part
not having the first and second impurity portions and use of the
second ion implantation method.
19. The method of claim 16, wherein the second electrode is in
contact with the second surface field portion and is connected to
the second surface of the substrate through the second surface
field portion.
20. The method of claim 19, wherein the first and second surfaces
of the substrate are light incident surfaces on which light is
incident.
21. A solar cell, comprising: a substrate into which an impurity of
a first conductivity type is doped; an emitter part positioned at a
first surface of the substrate, into which an impurity of a second
conductivity type opposite the first conductivity type is doped,
and comprising a first emitter portion having a first impurity
doped thickness and a second emitter portion having a second
impurity doped thickness greater than the first impurity doped
thickness; a first electrode positioned at the second emitter
portion and connected to the second emitter portion; and a second
electrode positioned at a second surface of the substrate and
connected to the substrate, the second surface being opposite the
first surface of the substrate, wherein a junction surface between
the first emitter portion and the substrate is positioned at a same
height as a junction surface between the second emitter portion and
the substrate, and a damage amount existing at the second emitter
portion is more than a damage amount existing at the first emitter
portion.
22. The solar cell of claim 21, further comprising an
anti-reflection layer positioned on the first emitter portion of
the emitter part.
23. The solar cell of claim 22, further comprising a surface field
part positioned between the second surface of the substrate and the
second electrode and doped with an impurity of the first
conductivity type.
24. The solar cell of claim 22, wherein the surface field part
comprises a first surface field portion having a third impurity
doped thickness and a second surface field portion having a fourth
impurity doped thickness greater than the third impurity doped
thickness, a first electrode positioned at the second emitter
portion and connected to the second emitter portion, and a damage
amount existing at the second surface field portion is more than a
damage amount existing at the first surface field portion.
25. The solar cell of claim 24, wherein the second electrode is
positioned on the second surface field portion and connected to the
second surface field portion.
26. The solar cell of claim 25, wherein the first and second
surfaces of the substrate are light incident surfaces on which
light is incident.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2010-0131823, filed in the Korean
Intellectual Property Office on Dec. 21, 2010, the entire contents
of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] Embodiments of the invention relate to a solar cell and a
method for manufacturing the same.
[0004] (b) Description of the Related Art
[0005] Recently, as existing energy sources such as petroleum and
coal are expected to be depleted, interests in alternative energy
sources for replacing the existing energy sources are increasing.
Among the alternative energy sources, solar cells for generating
electric energy from solar energy have been particularly
spotlighted.
[0006] A solar cell generally includes semiconductor parts that
have different conductivity types, such as a p-type and an n-type,
and form a p-n junction, and electrodes respectively connected to
the semiconductor parts of the different conductivity types.
[0007] When light is incident on the solar cell, electron-hole
pairs are generated in the semiconductor parts. The electrons move
to the n-type semiconductor part and the holes move to the p-type
semiconductor part, and then the electrons and holes are collected
by the electrodes connected to the n-type semiconductor part and
the p-type semiconductor part, respectively. The electrodes are
connected to each other using electric wires to thereby obtain
electric power.
SUMMARY OF THE INVENTION
[0008] According to an aspect of the invention, a method for
manufacturing a solar cell includes injecting first impurity ions
at a first surface of a substrate by using a first ion implantation
method to form a first impurity region, the substrate having a
first conductivity type and the first impurity ions having a second
conductivity type opposite the first conductivity type, and the
first impurity region having the second conductivity type, heating
the substrate with the first impurity region to activate the first
impurity region to form an emitter region from the first impurity
region, etching the emitter region from a surface of the emitter
region to a predetermined depth to form an emitter part from the
emitter region, and forming a first electrode on the emitter part
to connect to the emitter part and a second electrode on a second
surface of the substrate, which is opposite the first surface of
the substrate to connect to the second surface of the
substrate.
[0009] The heating of the substrate may heat the first impurity
portion at 800.degree. C. to 1100.degree. C. in a nitrogen
atmosphere.
[0010] The etching of the emitter region may remove a portion of
the emitter region from the surface of the emitter region to a
depth of 5 nm to 20 nm.
[0011] The emitter region may be etched by an etchant composed of
nitric acid HNO.sub.3, hydrofluoric acid HF and pure wafer.
[0012] The heating of the substrate may heat the first impurity
portion at 800.degree. C. to 1100.degree. C. in an oxygen
atmosphere.
[0013] The etching of the emitter region may remove a portion of
the emitter region from the surface of the emitter region to a
depth of 20 nm to 35 nm.
[0014] The emitter region may be etched by an etchant composed of
hydrofluoric acid HF and pure wafer.
[0015] The emitter part may include a first emitter portion having
a first impurity doped thickness and a second emitter portion
having a second impurity doped thickness greater than the first
impurity doped thickness, wherein the etching of the emitter region
includes: selectively forming an etch prevention layer on the
emitter region to expose a portion of the emitter region and to
cover a remaining portion of the emitter region, and etching the
exposed portion of the emitter region from the surface of the
emitter region to the predetermined depth using the etch prevention
layer as a mask, and removing the etch prevention layer, wherein
the etched exposed portion of the emitter region is formed as the
first emitter portion and the remaining portion of the emitter
region is formed as the second emitter portion.
[0016] The first impurity region may include a first impurity
portion having a first impurity doped thickness and a second
impurity portion having a second impurity doped thickness greater
than the first impurity doped thickness, and wherein the injecting
of the first impurity ions forms the first and second impurity
portions by use of a mask positioned at the first surface of the
substrate and use of the first ion implantation method.
[0017] The mask may include a first portion having a first exposing
area for forming the first impurity portion and a second portion
having a second exposing area for forming the second impurity
portion, the first and second exposing areas being areas exposing
the substrate in a unit area thereof.
[0018] The method may further include forming the first impurity
region not having the first and second impurity portions at an
entire first surface of the substrate by injecting the first
impurity ions of the second conductivity type at the entire first
surface of the substrate without a mask, before forming the first
and second impurity portions of the first impurity region, wherein
the forming of the first and second impurity portions of the first
impurity region forms the first and second impurity portions by use
of the mask positioned at the first impurity region not having the
first and second impurity portions and use of the first ion
implantation method.
[0019] The method may further include injecting second impurity
ions at a second surface of the substrate by using an ion
implantation method to form a second impurity region of the first
conductivity type, the second surface being opposite the first
surface of the substrate, heating the substrate with the second
impurity region to activate the second impurity region to form an
surface field region from the second impurity region, and etching
the surface field region from a surface of the surface field region
to a predetermined depth to form a surface field part from the
surface field region, wherein the second electrode is connected to
the second surface of the substrate through the surface field
part.
[0020] The surface field part may include a first surface field
portion having a first impurity doped thickness and a second
surface field portion having a second impurity doped thickness
greater than the first impurity doped thickness, wherein the
etching of the surface field region includes: selectively forming
an etch prevention layer on the surface field region to expose a
portion of the surface field region and to cover a remaining
portion of the surface field region, etching the exposed portion of
the surface field region from the surface of the surface field
region to the predetermined depth using the etch prevention layer
as a mask, and removing the etch prevention layer, wherein the
etched exposed portion of the surface field region is formed as the
first surface field portion and the remaining portion is formed is
formed as the second surface field portion.
[0021] The second electrode may be in contact with the second
surface field portion and is connected to the second surface of the
substrate through the second surface field portion.
[0022] The first and second surfaces of the substrate may be light
incident surfaces on which light is incident.
[0023] The second impurity region may include a first impurity
portion having a first impurity doped thickness and a second
impurity portion having a second impurity doped thickness greater
than the first impurity doped thickness, wherein the injecting of
the second impurity ions forms the first and second impurity
portions by use of a mask positioned at first surface of the
substrate and use of the second ion implantation method.
[0024] The mask may include a first portion having a first exposing
area for forming the first impurity portion and a second portion
having a second exposing area for forming the second impurity
portion, the first and second exposing areas being areas exposing
the substrate in a unit area thereof.
[0025] The method may further include forming the second impurity
region not having the first and second impurity portions at an
entire second surface of the substrate by injecting the second
impurity ions of the first conductivity type at the entire second
surface of the substrate without a mask, before forming the first
and second impurity portions of the second impurity region, wherein
the forming of the first and second impurity portions of the second
impurity region forms the first and second impurity portions by use
of the mask positioned at the second impurity part not having the
first and second impurity portions and use of the second ion
implantation method.
[0026] The second electrode may be in contact with the second
surface field portion and is connected to the second surface of the
substrate through the second surface field portion.
[0027] The first and second surfaces of the substrate may be light
incident surfaces on which light is incident.
[0028] According to another aspect of the invention, a solar cell
may include a substrate into which an impurity of a first
conductivity type is doped, an emitter part positioned at a first
surface of the substrate, into which an impurity of a second
conductivity type opposite the first conductivity type is doped,
and comprising a first emitter portion having a first impurity
doped thickness and a second emitter portion having a second
impurity doped thickness greater than the first impurity doped
thickness, a first electrode positioned at the second emitter
portion and connected to the second emitter portion, and a second
electrode positioned at a second surface of the substrate and
connected to the substrate, the second surface being opposite the
first surface of the substrate, wherein a junction surface between
the first emitter portion and the substrate is positioned at a same
height as a junction surface between the second emitter portion and
the substrate, and a damage amount existing at the second emitter
portion is more than a damage amount existing at the first emitter
portion.
[0029] The solar cell may further include an anti-reflection layer
positioned on the first emitter portion of the emitter part.
[0030] The solar cell may further include a surface field part
positioned between the second surface of the substrate and the
second electrode and doped with an impurity of the first
conductivity type.
[0031] The surface field part may include a first surface field
portion having a third impurity doped thickness and a second
surface field portion having a fourth impurity doped thickness
greater than the third impurity doped thickness, a first electrode
positioned at the second emitter portion and connected to the
second emitter portion, and a damage amount existing at the second
surface field portion is more than a damage amount existing at the
first surface field portion.
[0032] The second electrode may be positioned on the second surface
field portion and connected to the second surface field
portion.
[0033] The first and second surfaces of the substrate may be light
incident surfaces on which light is incident.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The accompanying drawings, which are included to provide a
further understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate example
embodiments of the invention and together with the description
serve to explain the principles of the invention. In the
drawings:
[0035] FIG. 1 is a partial perspective view of a solar cell
according to an example embodiment of the invention;
[0036] FIG. 2 is a cross-sectional view taken along line II-II of
FIG. 1;
[0037] FIGS. 3A to 3G are cross-sectional views sequentially
illustrating a method for manufacturing a solar cell according to
an example embodiment of the invention;
[0038] FIG. 4 shows graphs illustrating impurity doped
concentrations depending on changes in depths of an emitter region
and an emitter part according to an example embodiment of the
invention;
[0039] FIG. 5 depicts a damage portion existing at an emitter part
in a solar cell according to an comparative example;
[0040] FIG. 6 is a partial perspective view of a solar cell
according to another example embodiment of the invention;
[0041] FIG. 7 is a cross-sectional view taken along line VII-VII of
FIG. 6;
[0042] FIGS. 8A to 8D are cross-sectional views illustrating
portions of processes for manufacturing the solar cell shown in
FIGS. 6 and 7;
[0043] FIG. 9 is a partial cross-sectional view of a solar cell
according to yet another example embodiment of the invention;
[0044] FIGS. 10A to 10C are cross-sectional views illustrating
portions of processes for manufacturing the solar cell shown in
FIG. 9;
[0045] FIG. 11 is a partial cross-sectional view of a solar cell
according to yet another example embodiment of the invention;
and
[0046] FIG. 12 shows graphs illustrating an external quantum
efficiency and an internal quantum efficiency of the solar cell
shown in FIG. 11 depending on a wavelength variation of light.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0047] The invention will be described more fully hereinafter with
reference to the accompanying drawings, in which example
embodiments of the inventions are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the example embodiments set forth
herein.
[0048] In the drawings, the thickness of layers, films, panels,
regions, etc., are exaggerated for clarity. Like reference numerals
designate like elements throughout the specification. It will be
understood that when an element such as a layer, film, region, or
substrate is referred to as being "on" another element, it can be
directly on the other element or intervening elements may also be
present. In contrast, when an element is referred to as being
"directly on" another element, there are no intervening elements
present. Further, it will be understood that when an element such
as a layer, film, region, or substrate is referred to as being
"entirely" on another element, it may be on the entire surface of
the other element and may not be on a portion of an edge of the
other element.
[0049] Reference will now be made in detail to example embodiments
of the invention, examples of which are illustrated in the
accompanying drawings.
[0050] An example of a solar cell according to an example
embodiment of the invention is described in detail with reference
to FIGS. 1 and 2.
[0051] As shown in FIGS. 1 and 2, a solar cell 1 according to an
example embodiment of the invention includes a substrate 110, an
emitter part 121 positioned at an incident surface (hereinafter,
referred to as "a front surface" or "a first surface") of the
substrate 110 on which light is incident, an anti-reflection layer
130 positioned on the emitter part 121, a front electrode part 140
positioned on the emitter part 121, a back surface field part 172
positioned at a surface (hereinafter, referred to as "a back
surface" or "a second surface") opposite the front surface of the
substrate 110, and a back electrode part 150 positioned on the back
surface of the substrate 110 and the back surface field part
172.
[0052] The substrate 110 is a semiconductor substrate such as
silicon of a first conductivity type, for example, an n-type,
though not required. The semiconductor is a crystalline
semiconductor of single crystalline silicon or poly crystal
silicon.
[0053] When the substrate 110 is of an n-type, the substrate 110
may be doped with impurities of a group V element such as
phosphorus (P), arsenic (As), and antimony (Sb). Alternatively, the
substrate 110 may be of a p-type. When the substrate 110 is of the
p-type, the substrate 110 may be doped with impurities of a group
III element such as boron (B), gallium (Ga), and indium (In).
[0054] The front surface of the substrate 110 is textured to form a
textured surface corresponding to an uneven surface or having
uneven characteristics and including a plurality of projections and
a plurality of depressions. FIG. 1 shows that only an edge of the
substrate 110 and only an edge of the anti-reflection layer 130 on
the substrate 110 have a plurality of uneven portions for the sake
of brevity. However, the entire front surface of the substrate 110
is the textured surface having the plurality of uneven portions,
and thus the anti-reflection layer 130 on the front surface of the
substrate 110 has a textured surface having a plurality of uneven
portions. In an alternative example, the back surface as well as
the front surface of the substrate 110 may have a textured
surface.
[0055] By the textured surface of the substrate 110 having the
plurality of projections, the surface area of the substrate 110 and
the surface area of the anti-reflection layer 130 increase and an
amount of light reflected by the substrate 110 decreases and
thereby, an amount of light incident on the substrate 110
increases.
[0056] The emitter part 121 is an impurity region obtained by
doping the substrate 110 with impurities of a second conductivity
type (for example, a p-type) opposite the first conductivity type
of the substrate 110. Thus, the emitter part 121 forms a p-n
junction with the substrate 110 (that is, the first conductivity
type portion of the substrate 110).
[0057] In this example, the emitter part 121 is formed by an ion
implantation method. Thereby, when compared with an emitter part
formed by a thermal diffusion method, the maximum impurity doped
concentration (that is, the maximum surface impurity doped
concentration) of a surface (that is, the front electrode part 140
is positioned thereat and a surface opposite a p-n junction surface
between the first conductivity portion of the substrate 110 and the
emitter part 121) of the emitter part 121 increases and an impurity
doped thickness of the emitter part 121 decreases.
[0058] In the thermal diffusion method, for increasing an impurity
doped concentration of a surface of the emitter part, it is
required to increase a process time, but since the impurity doped
thickness increases in proportion to the process time, the impurity
doped thickness also increases by the increased impurity doped
concentration.
[0059] However, when the emitter part 121 is formed by the ion
implantation method, the maximum surface impurity doped
concentration is greater than that of the thermal diffusion method,
but the impurity doped thickness of the emitter part 121
decreases.
[0060] Thereby, when compared with the emitter part formed by the
thermal diffusion method, the impurity doped thickness (depth)
decreases and the maximum surface impurity doped concentration of
the emitter part 121 increases.
[0061] By a built-in potential difference due to the p-n junction
of the substrate 110 and the emitter part 121, a plurality of
electrons and a plurality of holes produced by light incident on
the substrate 110 move to the n-type semiconductor and the p-type
semiconductor, respectively. Thus, when the substrate 110 is of the
n-type and the emitter part 121 is of the p-type, the holes move to
the emitter part 121 and the electrons move to the back surface of
the substrate 110.
[0062] Because the substrate 110 and the emitter part 121 form the
p-n junction, the emitter part 121 may be of the n-type when the
substrate 110 is of the p-type unlike the example embodiment
described above. In this instance, the electrons move to the
emitter part 121, and the holes move to the back surface of the
substrate 110.
[0063] When the emitter part 121 is of the p-type, the emitter part
121 may be formed by doping impurities of a group III element into
the substrate 110. On the contrary, when the emitter part 121 is of
the n-type, the emitter part 121 may be formed by doping impurities
of a group V element into the substrate 110.
[0064] The anti-reflection layer 130 positioned on the emitter part
121 is made of hydrogenated silicon nitride (SiNx:H), hydrogenated
silicon oxide (SiOx:H), or hydrogenated silicon oxy nitride
(SiOxNy:H), etc.
[0065] The anti-reflection layer 130 reduces reflectance of light
incident onto the substrate 110 and increases selectivity of a
specific wavelength band, thereby increasing the efficiency of the
solar cell 1. Further, by the hydrogen (H) supplied when the
anti-reflection layer 130 is formed, the anti-reflection layer 130
performs a passivation function that converts a defect, for
example, dangling bonds existing on the surface of the substrate
110 and around the surface of the substrate 110, into stable bonds
to thereby prevent or reduce a recombination and/or a disappearance
of charges moving to the front surface of the substrate 110
resulting from the defect. Hence, the anti-reflection layer 130
reduces loss of charges caused by disappearance of the charges due
to the defect on or around the surface of the substrate 110, to
further improve the efficiency of the solar cell 1.
[0066] In this example embodiment, the anti-reflection layer 130
has a single-layered structure, but the anti-reflection layer 130
may have a multi-layered structure such as a double-layered
structure in other example embodiments. The anti-reflection layer
130 may be omitted, if desired.
[0067] The front electrode part 140 includes a plurality of front
electrodes (a plurality of first electrodes) 141 and a plurality of
front bus bars (a plurality of first bus bars) 142 connected to the
plurality of front electrodes 141.
[0068] The plurality of front electrodes 141 are electrically and
physically connected to portions of the emitter part 121 and extend
substantially parallel to one another in a predetermined direction
at a distance therebetween. The plurality of front electrodes 141
collect carriers (e.g., electrons) moving to the emitter part
121.
[0069] The plurality of front bus bars 142 are electrically and
physically connected to portions of the first emitter part 121 and
extend substantially parallel to one another in a direction
crossing an extending direction of the front electrodes 141.
[0070] The front electrodes 141 and the front bus bars 142 are
placed on the same level layer (or are coplanar). The front
electrodes 141 and the front bus bars 142 are electrically and
physically connected to one another at crossings of the front
electrodes 141 and the front bus bars 142.
[0071] As shown in FIG. 1, the plurality of front electrodes 141
have a stripe shape extending in a transverse (or longitudinal)
direction, and the plurality of front bus bars 142 have a stripe
shape extending in a longitudinal (or transverse) direction. Thus,
the front electrode part 140 has a lattice shape on the front
surface of the substrate 110.
[0072] By the front electrode part 140, the anti-reflection layer
130 is not positioned on the portions of the emitter part 121 on
which the front electrodes 141 and the front bus bars 142 are
positioned, but is positioned on portion of the emitter part 121
between portions the front electrode part 140 (for example,
portions between the front electrodes 141 and between the front bus
bars 142).
[0073] The plurality of front bus bars 142 collect not only
carriers transferred from portions of the emitter part 121
contacting the plurality of front bus bars 142 but also the
carriers collected by the plurality of front electrodes 141.
[0074] Because the plurality of front bus bars 142 collect the
carriers collected by the plurality of front electrodes 141 and
move the carriers to a desired location, a width of each of the
plurality of front bus bars 142 is greater than a width of each of
the plurality of front electrodes 141.
[0075] When the plurality of front bus bars 142 are connected to an
external device, the carriers (for example, electrons) collected by
the front bus bars 142 are output to the external device.
[0076] The front electrode part 140 including the plurality of
front electrodes 141 and the plurality of front bus bars 142 is
formed of at least one conductive material, for example, silver
(Ag).
[0077] Although FIG. 1 shows a predetermined number of front
electrodes 141 and a predetermined number of front bus bars 142 on
the substrate 110, the number of front electrodes 141 and the
number of front bus bars 142 may vary.
[0078] The back surface field part 172 is a region (for example, a
p.sup.+-type region) that is more heavily doped than the substrate
110 with impurities of the same conductivity type as the substrate
110. Thus, the back surface field part 172 has a sheet resistance
less than that of the substrate 110.
[0079] A potential barrier is formed by a difference between
impurity concentrations of a first conductivity region of the
substrate 110 and the back surface field part 172. Hence, the
potential barrier prevents or reduces electrons from moving to the
back surface field part 172 used as a moving path of holes and
makes it easier for holes to move to the back surface field part
172. Thus, an amount of carriers lost by a recombination and/or a
disappearance of the electrons and the holes at and near the back
surface of the substrate 110 is reduced, and a movement of carriers
to the back electrode part 150 increases by accelerating a movement
of desired carriers (for example, holes).
[0080] The back electrode part 150 includes a back electrode (a
second electrode) 151 and a plurality of back bus bars 152
connected to the back electrode 151.
[0081] The back electrode 151 contacts the back surface field part
172 positioned at the back surface of the substrate 110 and is
positioned on the entire back surface of the substrate 110 except a
formation area of the back bus bars 152. Thereby, the back
electrode 151 is positioned at a portion of the back surface of the
substrate 110, which is positioned between the back bus bars 152.
However, if necessary or desired, the back electrode 151 may be not
positioned at an edge of the back surface of the substrate 110 as
well as the formation area of the back bus bars 152.
[0082] The back electrode 151 contains a conductive material such
as aluminum (Al).
[0083] The back electrode 151 collects carriers (for example,
holes) moving to the back surface field part 172.
[0084] Because the back electrode 151 contacts the back surface
field part 172 having the impurity concentration higher than the
substrate 110, a contact resistance between the substrate 110
(i.e., the back surface field part 172) and the back electrode 151
decreases. Hence, the carrier transfer efficiency from the
substrate 110 to the back electrode 151 is improved.
[0085] The plurality of back bus bars 152 are positioned on the
back surface of the substrate 110, on which the back electrode 151
is not positioned, and are connected to the back electrode 151.
[0086] Further, the plurality of back bus bars 152 are positioned
opposite the plurality of front bus bars 142 with the substrate 110
therebetween. That is, the back bus bars 152 and the front bus bars
142 may be aligned, but such is not required.
[0087] The plurality of back bus bars 152 collect carriers from the
back electrode 151 in the same manner as the plurality of front bus
bars 142.
[0088] The plurality of back bus bars 152 are connected to the
external device and output the carriers (for example, holes)
collected by the back bus bars 152 to the external device.
[0089] The plurality of back bus bars 152 may be formed of a
material having better conductivity than the back electrode 151.
Further, the plurality of back bus bars 152 may contain at least
one conductive material, for example, silver (Ag).
[0090] Alternatively, the back electrode 151 may be positioned on
the entire back surface of the substrate 110. In this instance, the
back bus bars 152 may be positioned opposite the front bus bars 142
with the substrate 110 therebetween on the back electrode 151.
Further, the back electrode 151 may be positioned on substantially
the entire back surface of the substrate 110 except the edge of the
back surface of the substrate 110, if necessary or desired.
[0091] An operation of the solar cell 1 having the above-described
structure is described below.
[0092] When light irradiated to the solar cell 1 is incident on the
emitter part 121 and the substrate 110 through the anti-reflection
layer 130, a plurality of electron-hole pairs are generated in the
emitter part 121 and the substrate 110 by light energy based on the
incident light. In this instance, because a reflection loss of the
light incident on the substrate 110 is reduced by the textured
surface of the substrate 110 and the anti-reflection layer 130, an
amount of light incident on the substrate 110 further
increases.
[0093] By the p-n junction of the substrate 110 and the emitter
layer 121, the electrons and the holes move to the n-type
semiconductor part (for example, the emitter part 121) and the
p-type semiconductor part (for example, the substrate 110),
respectively. The electrons moving to the n-type emitter part 121
are collected by the front electrodes 141 and the front bus bars
142 and then move to the front bus bars 142. The holes moving to
the p-type substrate 110 are collected by the back electrodes 151
and the back bus bars 152 and then move to the back bus bars 152.
When the front bus bars 142 are connected to the back bus bars 152
using electric wires, current flows therein to thereby enable use
of the current for electric power.
[0094] Referring to FIGS. 3A to 3G, a method for the solar cell 1
according to an example embodiment of the invention is
described.
[0095] As shown in FIG. 3A, by using a dry etching method such as a
reaction ion etching (ME) method, etc., or a wet etching method, an
exposed surface, for example, a front surface of a substrate 110 of
a first conductivity type is etched to form a textured surface (an
uneven surface) having a plurality of projections 11 and a
plurality of depressions 12. A projected height H1 of each
projection 11 and a width H2 of each projection 11 are of various
magnitudes, respectively.
[0096] Next, as shown in FIG. 3B, by an ion implantation method,
impurities are injected into only the front surface of the
substrate 110 having the uneven surface, to form an impurity region
120. Since the impurities are of a second conductivity, for
example, a n-type, opposite the first conductivity type, the
impurity region 120 is of the second conductivity type. The
impurity region 120 is a region obtained by injecting ions (that
is, impurity ions) of the n-type impurities into the substrate 110
using the ion implantation method, and thereby, the impurity region
120 has a thickness measured from the front surface of the
substrate 110. An energy for injecting the impurity ions may be
about 5 KeW to 30 KeW.
[0097] When the impurity ions are injected into the substrate 110,
the impurity ions come into collision with the front surface of the
substrate 110 and thereby a damage portion 21 in which normal
silicon bonds in the substrate 110 are broken is formed at and/or
near the front surface of the substrate 110, that is, a surface of
the impurity region 120. The damage portion 21 may be totally or
partially formed at and/or near the entire front surface of the
substrate 110 exposed to the impurity ions for the ion implantation
method. The damage portion 21 is mainly formed at and near the
surface of the substrate 110 but may be formed into the impurity
region 120.
[0098] Since the injection of the impurity ions is performed only
at the front surface of the substrate 110 which is exposed to the
impurity ions, unlike a thermal diffusion method, the impurity
region 120 is not formed at a back surface of the substrate
110.
[0099] As shown in FIG. 3C, the substrate 110 is heated in an
atmosphere of a nitrogen gas (N.sub.2) to activate the impurity
region 120 formed in the substrate 110, and to thereby, an emitter
region 1201. That is, by the activation process using the heat
treatment, the impurity ions existing in the impurity region 120 in
an interstitial state are reconfigured with silicon (Si) such that
a state of the impurity ions is changed from the interstitial state
into a substitutional state. Thereby, the impurity region 120
functions as an emitter part of a p-type or an n-type and thereby,
forms form a p-n junction with the first conductivity portion of
the substrate 110.
[0100] Further, by the heat treatment, the impurity ions existing
in the impurity region 120 is more deeply diffused into the
substrate 110, and thereby a thickness (a depth) (that is, an
impurity doped thickness) of the emitter region 1201 that is the
activated impurity region is greater than a thickness of the
impurity region 120. The heat treatment may be performed at about
800.degree. C. to 1100.degree. C. For example, when the impurity
region 120 is of an n-type, for example, containing phosphorous
(P), the heat treatment may be performed at about 800.degree. C. to
1100.degree. C. and when the impurity region 120 is of a p-type,
for example, containing boron (B), the heat treatment may be
performed at about 900.degree. C. to 1100.degree. C.
[0101] An example of a variation of an impurity doped concentration
of the emitter region 1201 is shown in a graph G1 of FIG. 4. The
graph G1 of the FIG. 4 shows the impurity doped concentration in
accordance with a thickness change of the emitter region 1201 from
a surface of the emitter region 1201 to the back surface of the
substrate 110.
[0102] As shown in the graph G1, the impurity doped concentration
from the surface of the emitter region 1201 to a thickness of about
0.07 .mu.m adjacent to the surface of the emitter region 1201
increases from about 4.0E+19 cm.sup.-3 to about 6.5E+19 cm.sup.-3
and the impurity doped concentration of the emitter region 1201
from a thickness of about 0.07 .mu.m to a thickness about 0.48
.mu.m gradually decreases from about 6.5E+19 cm.sup.-3 to about
4.0E+19 cm.sup.-3.
[0103] During the heat treatment for activating the impurity region
120, a recrystallization of silicon Si is performed at a
recrystallization temperature of silicon, whereby damaged silicon
lattices of the damaged portion 21 may be reconfigured. When the
silicon recrystallization has occurred, the damaged silicon
lattices in the damage portion 21 may be reconfigured into stable
silicon lattices to anneal the damaged silicon lattices.
[0104] The activation process of the impurity region 120 may be
performed in an atmosphere of an oxygen (O.sub.2) gas. In this
instance, a silicon oxide layer may be formed on the impurity
region 120 by coupling oxygen of the atmosphere and silicon of the
impurity region 120. The silicon oxide layer may have a thickness
of about 15 nm to 30 nm.
[0105] When forming the emitter region by using the ion
implantation method, the damage portion existing on the emitter
region is shown in FIG. 5.
[0106] FIG. 5 depicts a portion of a solar cell formed with an
anti-reflection layer after forming an emitter region by the ion
implantation and activation processes by using a TEM (transmission
electron microscopy) equipment. The emitter region having the
damage portion functions as an emitter part.
[0107] In FIG. 5, a reference numeral {circle around (4)} denotes a
silicon substrate, a reference numeral {circle around (3)} denotes
the damage portion formed at a surface of the silicon substrate, a
reference numeral {circle around (2)} denotes a native silicon
oxide portion generated by the exposure to an air, and a reference
numeral {circle around (1)} denotes the anti-reflection layer.
[0108] Next, referring to FIG. 3D, a surface (a front surface which
is a surface opposite a p-n junction surface with the substrate 110
and the emitter region 1201) of the emitter region 1201, is removed
to form an emitter part 121. By the removal, the entire surface of
the emitter region 1201 is removed by a predetermined thickness and
thereby the damage portion 21 formed at and/or near the surface of
the emitter region 1201 is also removed. An impurity doped
concentration of the emitter part 121 may be about
1.times.10.sup.19 cm.sup.-3 to 1.times.10.sup.20 cm.sup.-3.
[0109] A removed thickness of the impurity region 120 may be about
5 nm to 35 nm. For example, when the activation process is
performed in the nitrogen atmosphere, a separate layer such as the
silicon oxide layer is not formed on the emitter region 1201
activated by the nitrogen N.sub.2. Thus, the emitter region 1201 is
removed by a thickness of about 5 nm to 20 nm to further remove an
amount greater than a thickness of the damage portion 21 existing
at and/or near the surface of the emitter region 1201. However,
when the activation process is performed in the oxygen atmosphere,
since the silicon oxide layer formed on the emitter region 1201 as
well as the damage portion 21 should be removed, the removal
thickness increases. Thus, for example, the removed thickness of
the emitter region 1201 may be about 20 nm to 35 nm.
[0110] As described above, since the emitter region 1201 is etched
by an etchant and removed, and the entire front surface of the
emitter region 1201 is exposed to the etchant, the thickness of the
emitter portion 121 is reduced by the thickness removed from the
emitter region 1201. In this instance, the removed thickness of the
emitter region 1201 may be adjusted using a concentration of the
etchant and/or an etching time, etc.
[0111] By the removal of a portion of the emitter region 1201
adjacent to the surface thereof, into which the impurity ions are
injected, the impurity doped concentration of the emitter part 121
in accordance with a depth change of emitter portion 121 is varied
as a graph G2 of FIG. 4.
[0112] For example, as shown in the graph G2 of FIG. 4, when a
thickness of emitter part 121 from the surface of emitter part 121
is about 0.01 .mu.m, the impurity doped concentration of the
emitter part 121 has the maximum value, about 6.0E+19 cm.sup.-3,
and then gradually decreases from about 6.0E+19 cm.sup.-3 to about
1.0E+19 cm.sup.-3. Thereby, the minimum impurity doped
concentration of the emitter part 121 is about 1.0E+19 cm.sup.-3
and the thickness of the emitter part 121 having the minimum
impurity doped concentration is about 0.42 .mu.m.
[0113] Since the front surface of the emitter region 1201, at
and/or near which the damage portion 21 exists is removed by a
predetermined thickness, the thickness of the emitter part 121 is
less than that of the emitter region 1201.
[0114] Thus, referring to FIG. 4, when the front surface of the
emitter part 1201 is not removed, the thickness of the emitter
region 1201 is about 0.47 .mu.m, and the thickness of the emitter
part 121 is about 0.42 .mu.m after the front surface of the emitter
region 1201 is removed.
[0115] For removing the entire front surface of the emitter region
1201 including the damage portion 21 by the predetermined
thickness, an etch prevention layer is formed on a desired portion
(for example, a back surface of the substrate 110 on which the
etching is not desired) of the substrate 110 and then the substrate
110 is immersed into the etchant. Thus, the entire front surface of
the emitter region 1201 on which the etch prevention layer is not
formed is removed by the predetermined thickness, to remove the
damage portion 21 existing at and/or near the surface of the
emitter region 1201.
[0116] Alternatively, without the formation process of the etch
prevention layer, the emitter region 1201 of the predetermined
thickness may be removed by immersing a desired thickness of only
the front surface of the substrate 110 (that is, the front surface
of the emitter region 1201) into the etchant.
[0117] The etchant for removing the emitter region 1201 may be
composed of nitric acid HNO.sub.3, hydrofluoric acid HF and pure
wafer. The nitric acid HNO.sub.3 is used to oxygenate silicon
composing the emitter region 1201 and the hydrofluoric acid HF is
used to remove the oxygenated silicon.
[0118] When the heat treatment for activating the impurity region
120 is performed in the oxygen (O.sub.2) atmosphere, the nitric
acid HNO.sub.3 may be omitted from the etchant. That is, since the
impurity region 120 is oxygenated by oxygen (O.sub.2) supplied
during the heat treatment, the nitric acid HNO.sub.3 oxygenating
silicon Si is possible to omit. Further, as already described, the
silicon oxide layer generated by the oxygenation of silicon on the
impurity region 120 should be removed along with the emitter region
120, at least one of a concentration of the hydrofluoric acid HF
and an etching time may be increased as compared with a case in
which the impurity region 120 is activated in the nitrogen
(N.sub.2) atmosphere.
[0119] By the etching process of the emitter region 1201, the
impurity doped concentration of the emitter portion 121 has the
maximum value at and/or near the surface of the emitter portion 121
contacting the front electrode part 140. Thus, conductivity of
regions of the emitter portion 121 contacting the front electrode
part 140 increase. In addition, since the thickness of the emitter
portion 121 decreases, charge transfer distances of charges moving
to the surface of the emitter portion 121 are reduced. Thus, the
charges further easily move from the emitter portion 121 to the
front electrode part 140 adjacent to the emitter portion 121.
[0120] After forming the emitter portion 121 using the ion
implantation method including the ion implantation process, the
activation process and the etching process, as shown in FIG. 3E, an
anti-reflection layer 130 is formed on the emitter part 121 on the
front surface of the substrate 110 using a plasma enhanced chemical
vapor deposition (PECVD), etc. In this example, the anti-reflection
layer 130 may be made of hydrogenated silicon nitride (SiNx) or
hydrogenated silicon oxide (SiOx) etc.
[0121] Next, referring to FIG. 3F, a paste containing metals such
as silver (Ag) is printed on corresponding portions of the
anti-reflection layer 130 using a screen printing method and then
is dried to form a front electrode part pattern 40.
[0122] The front electrode pattern 40 includes a front electrode
pattern 41 and a front bus bar pattern 42.
[0123] Next, referring to FIG. 3G, a paste containing metals such
as aluminum (Al) is selectively or partially printed on the back
surface of the substrate 110 using a screen printing method and
then is dried to form a back electrode pattern 51 and a paste
containing metals such as silver (Ag) is printed on portions of the
back surface of the substrate 110 on which the back electrode
pattern 51 is not formed using a screen printing method and then is
dried to form a back bus bar pattern 52. Thereby, a back electrode
part pattern 50 having the back electrode pattern 51 and the back
bus bar pattern 52 is completed. The back bus bar pattern 52 is
opposite the front bus bar pattern 42 with the substrate 110
therebetween.
[0124] The patterns 40 and 50 may be dried at about 120.degree. C.
to 200.degree. C., and a formation order of the patterns 40 and 50
may be changed.
[0125] Next, the substrate 110 having the patterns 40 and 50 is
heated at about 750.degree. C. to 800.degree. C.
[0126] By the heat treatment, a front electrode part 140 having a
plurality of front electrodes 141 and a plurality of front bus bars
142 electrically and physically connected to the emitter part 121,
a back surface field part 172 at the back surface of the substrate
110 on which the back electrode pattern 51 is formed, and a back
electrode part 150 including a back electrode 151 electrically
connected to the substrate 110 through the back surface field part
172 and a plurality of back bus bars 152 electrically and
physically connected to the substrate 110 and the back electrode
151 are formed, to complete a solar cell 1 (refer to FIGS. 1 and
2).
[0127] By the heat process, by an etching material such as lead (or
PbO) contained in the front electrode pattern 41, the front
electrode pattern 41 penetrates through portions of the
anti-reflection layer 130 underlying the front electrode pattern 41
and is connected to the emitter part 121, thereby forming the
plurality of front electrodes 141 and the plurality of front bus
bars 142, to complete the front electrode part 140.
[0128] In this instance, the front electrode pattern 41 of the
front electrode part pattern 40 is formed as the plurality of front
electrodes 141 of the front electrode part 140, and the front bus
bar pattern 42 of the front electrode part pattern 40 is formed as
the plurality of front bus bars 142 of the front electrode part
140.
[0129] In addition, during the heat process, the back electrode
pattern 51 and the back bus bar pattern 52 of the back electrode
part pattern 50 are formed as the back electrode 151 and the
plurality of back bus bars 152, respectively, and aluminum (Al)
contained in the back electrode pattern 51 of the back electrode
part pattern 50 is diffused (or doped) into the substrate 110 to
form an impurity region, that is, the back surface field part 172
that is highly doped with impurities of the same conductivity type
as the substrate 110. In this instance, an impurity doped
concentration of the back surface field region 172 is higher than
that of the substrate 110. The back electrode pattern 51 and the
back bus bar pattern 52 do not contain an etching material (e.g.,
Pb). Even though the back electrode pattern 51 and the back bus bar
pattern 52 contain the etching material, the back electrode pattern
51 and the back bus bar pattern 52 contains the etching material
equal to or less than a predetermined amount (e.g., 1000 ppm) not
influencing the etching of the underlying layer (that is, the
substrate of the back electrode pattern 51 and the back bus bar
pattern 52. Thus, unlike the front electrode part pattern 40,
portions of the substrate 110 contacting the back electrode pattern
51 and the back bus bar pattern 52 are not etched during the
thermal treatment process. Thereby, the back electrode 151 is in
contact with the back surface field part 172 to be electrically
connected to the substrate 110.
[0130] In this example, since by the ion implantation method, the
emitter part 121 is formed only at (in) the front surface of the
substrate 110 and is not formed at (in) the back surface of the
substrate 110, characteristics of the back surface field part 172
are improved.
[0131] That is, when the emitter part 121 having the opposite
conductivity type to the conductivity type of the substrate 110 is
positioned at the back surface of the substrate 110, impurities
having the opposite conductivity type and contained in the emitter
part 121 are mixed to the back surface field part 172 of the same
conductivity type as the substrate 110, and thereby a field effect
by the back surface field part 172 is weakened.
[0132] However, in this instance, since the emitter part 121 is not
formed at the back surface of the substrate, the reduction of the
field effect of the back surface field part 172 due to the emitter
part 121 does not occur and the field effect of the back surface
field part 172 is further improved. Thus, an amount of charges
moving to the back surface of the substrate 110 increases to
improve an efficiency of the solar cell 1.
[0133] Moreover, in performing the heat process, metal components
contained in the patterns 40 and 50 are chemically coupled to the
contacted emitter part 121 and the substrate 110, respectively,
such that a contact resistance is reduced and thereby a charge
transfer efficiency is improved to improve a current flow.
[0134] Further, since the emitter part 121 is formed at only the
front surface of the substrate 110, an edge isolation process
separating the emitter part 121 formed in the front surface of the
substrate 110 and the emitter part formed in the back surface of
the substrate 110 or a separate process for removing the emitter
part formed in the back surface of the substrate 110 are not
necessary. Thus, a manufacturing time of the solar cell 1 is
reduced to increase productivity of the solar cell 1 and a
manufacturing cost of the solar cell 1 is also reduced.
[0135] Next, referring to FIGS. 6 and 7, a solar cell according to
another example embodiment of the invention is described.
[0136] A solar cell 2 shown in FIGS. 6 and 7 has the same structure
as the solar cell of FIGS. 1 and 2 except an emitter part 121a and
a front electrode part 140 connected to the emitter part 121a.
[0137] That is, the emitter part 121a includes first and second
emitter portions 1211 and 1212 each having different impurity doped
concentrations and different impurity doped thicknesses from each
other. Thus, the solar cell 2 of the example includes a selective
emitter structure.
[0138] In this example, the impurity doped thicknesses of the first
emitter portion 1211 is less than the impurity doped thicknesses of
the second emitter portion 1212, and thereby, the impurity doped
concentration of the first emitter portion 1211 is less than the
impurity doped concentration of the second emitter portion 1212.
Thus, conductivity of the second emitter portion 1212 is greater
than that of the first emitter portion 1211 and a sheet resistance
of the second emitter portion 1212 is less than that of the first
emitter portion 1211.
[0139] An amount (that is, a damage amount) of damage portion
generated at and/or near a surface of the second emitter portion
1212 in injecting impurity ions is more than an amount (a damage
amount) of damage portion generated at and/or near a surface of the
first emitter portion 1211. Thus, the damage amount existing at the
second emitter portion 1212 is more than that of the first emitter
portion 1211. In particular, the damage amount existing at and/or
near the surface of the second emitter portion 1212 is more than
that at and near the surface of the first emitter portion 1211.
[0140] When comparing a TEM photograph of the second emitter
portion 1212 and a TEM photograph of the first emitter portion
1211, the damage amount observed in the TEM photograph of the
second emitter portion 1212 is more than that observed in the TEM
photograph of the first emitter portion 1211. Thus, the damage
amounts of the first and second emitter portions 1211 and 1212 may
be measured by using a TEM equipment. As another method for
measuring the damage amount of the first and second emitter
portions 1211 and 1212, a CV (capacitance voltage) measurement
equipment may be used. For example, charge mobility or leakage
current in the first and second emitter portions 1211 and 1212 is
measured by the CV measurement equipment and the damage amounts of
the first and second emitter portions 1211 are calculated based on
the charge mobility or leakage current. In general, as the damage
amount increases, the leakage current increases and the charge
mobility decreases.
[0141] In this example, the front electrode part 140 is connected
to the second emitter portion 1212 of the emitter part 121a having
the conductivity more than that of the first emitter portion
1211.
[0142] Since the front electrode part 140 is connected to the
second emitter portion 1212, charges transferring to the emitter
part 121a move to the surface of the first emitter portion 1211 and
then move to the front electrode part 140 along the surface of
first emitter portion 1211. In this instance, since the impurity
doped thickness of the first emitter portion 1211 is less than that
of the second emitter portion 1212, the charge transfer distances
of charges moving to the surface of the first emitter portion 1211
are reduced. Thus, an amount of charges collected by the front
electrode part 140 increases and an efficiency of the solar cell 2
is improved.
[0143] Since the first emitter portion 1211 through which charges
mainly move to adjacent portions of the front electrode part 140
has the impurity doped concentration less than that of the second
emitter portion 1212, when the charges move from the first emitter
portion 1211 to the second emitter portion 1212, a loss amount of
the charges due to the impurities of the first emitter portion 1211
decreases and mobility of the charges increases. Thus, an amount of
charges moving from the first emitter portion 1211 to the second
emitter portion 1212 increases.
[0144] Further, since the front electrode part 140 is connected to
the second emitter portion 1212 having larger conductivity and less
resistance than the first emitter portion 1211, a charge transfer
efficiency from the second emitter portion 1212 and the front
electrode part 140 is increased. Thus, the efficiency of the solar
cell 1 is improved.
[0145] As described above, the impurity doped concentration of the
second emitter portion 1212 formed by the ion implantation method
is greater than that of a second emitter portion formed by the
thermal diffusion method. Thus, the conductivity of the second
emitter portion 1212 contacting the front electrode part 140 is
greater than that of the second emitter portion formed by the
thermal diffusion method, and thereby, contact resistance between
the second emitter portion 1212 and the front electrode part 140
further decreases. Thereby, an amount of charges transferring from
the second emitter portion 1212 to the front electrode part 140
increases and an amount of charges collected by the front bus bars
142 also increases.
[0146] As described above, since the first and second emitter
portions 1211 and 1212 are different impurity doped thicknesses
from each other, a distance (hereinafter, referred to as `a first
shortest distance`) d1 from the front surface of the substrate 110
to a p-n junction surface (hereinafter, referred to as `a first
junction surface`) between the first emitter portion 1211 and the
substrate 110 is different from a distance (hereinafter, referred
to as `a second shortest distance`) d2 from the front surface of
the substrate 110 to a p-n junction surface (hereinafter, referred
to as `a second junction surface`) d2 between the second emitter
portion 1212 and the substrate 110. That is, as shown in FIGS. 6
and 7, the first shortest distance d1 is shorter than the second
shortest distance d2.
[0147] In the substrate 110, the first and second junction surfaces
are positioned at the same level (i.e., the same height) as each
other. Thus, a third shortest distance from the back surface of the
substrate 110 to the first junction surface is substantially equal
to a fourth shortest distance from the back surface of the
substrate 110 to the second junction surface. In this instance, the
first to fourth shortest distances are substantially equal to each
other within the margin of error obtained by a difference between
the heights of each projection of the textured surface of the
substrate 110.
[0148] Since the emitter part 121 is formed at the front surface of
the substrate which is the textured surface, the junction surface
between the substrate 110 and the emitter part 121a may be not a
flat surface but an uneven surface under the influence of the
textured surface of the substrate 110.
[0149] As described above, since the front electrode part 140 is
connected to only the second emitter portion 1212 of the emitter
part 121a, the anti-reflection layer 130 is mainly positioned on
the first emitter portion 1211 of the emitter part 121a positioned
between portions of the front electrode part 140.
[0150] The emitter part 121a may be formed as discussed below.
[0151] As described referring to FIGS. 3A to 3C, after forming a
textured surface at a front surface of the substrate 110, an
emitter region 1201 is formed by an ion implantation method and an
activation process. The emitter region 1201 has a damage portion 21
existing at least one of a surface of the emitter region 1201.
[0152] Next, as shown in FIG. 8A, an etch protection layer 81 is
selectively or partially formed on the emitter region 1201 to
expose portions of the emitter region 1201 on which the etch
protection layer 81 is not formed.
[0153] As shown in FIG. 8B, the front surface of the substrate 110
is exposed to an etchant, to remove the exposed portions of the
emitter region 1201 by a predetermined thickness (a desired
thickness).
[0154] The etchant may be composed of nitric acid HNO.sub.3,
hydrofluoric acid HF and pure wafer, etc., or may be composed of
the hydrofluoric acid HF and pure wafer, excluding the nitric acid
HNO.sub.3.
[0155] Thus, a portion etched of the emitter region 1201 is formed
as the first emitter portion 1211 and a portion not etched of the
emitter region 1201 is formed as the second emitter portion 1212,
to form an emitter part 121a (FIG. 8B). Then, the etch protection
layer 81 is removed by a cleansing liquid such as water, etc. Since
a reason for removing the portions of the emitter region 1201 is to
form the first emitter portion 1211 having a thickness less than
that of the second emitter portion 1212, a removed thickness of the
emitter region 1201 may be about 30 nm to 100 nm.
[0156] The second emitter portion 1212 includes a portion 1212a (a
front electrode second emitter portion) for a plurality of front
electrodes extending in a predetermined direction and a portion (a
front bus bar second emitter portion) 1212b for a plurality of
front bus bars extending in a direction crossing the front
electrode second emitter portion 1212a. A width of the front bus
bar second emitter portion 1212b is greater than that of the front
electrode second emitter portion 1212a.
[0157] In an alternative example, the etch protection layer 81 may
be formed on the back surface as well as the front surface of the
substrate 110, and then the entire surface of the substrate 110 may
be exposed to the etchant to form a selective emitter structure
having the first and second emitter portions 1211 and 1212.
[0158] Thereby, since the damage portion 21 existing at and/or near
the front surface of the emitter region 1201 for the first emitter
portion 1211 is removed, the first emitter portion 1211 may have an
impurity doping graph such as a shape of the graph G2 of FIG. 4, an
impurity doped thickness of first emitter portion 1211 is reduced
by the removed thickness of the emitter region 1201. Thereby, an
impurity doped thickness of the first emitter portion 1211 is less
than that of the second emitter portion 1212 at which the etching
is not performed. In this instance, the second emitter portion 1212
may have an impurity doping graph such as a shape of the graph G1
of FIG. 4 since portions of the emitter region 1201 corresponding
to the second emitter portion 1212 are not removed. Thereby, since
the surface of the emitter region 1201 having a higher impurity
doped concentration than other portion of the emitter region 1201
is removed, a surface impurity doped concentration at the surface
of the first emitter portion 1211 is less than that of the second
emitter portion 1212. For example, the surface impurity doped
concentration of the first emitter portion 1211 may be about
2.times.10.sup.19 cm.sup.-3 and the surface impurity doped
concentration of the second emitter portion 1212 may be about
4.times.10.sup.19 cm.sup.-3.
[0159] Thus, the impurity doped thickness of the first emitter
portion 1211 is reduced, and the charge transfer distance of
charges moving to the surface of the first emitter portion 1211
decreases.
[0160] Next, as shown in FIG. 8C, an anti-reflection layer 130 is
formed on the entire front surface of the substrate 110, that is,
on the first emitter portion 1211 and the second emitter portion
1212 of the emitter part 121a.
[0161] Since the anti-reflection layer 130 is positioned on the
first emitter portion 1211 at which the damage portion 21 of the
emitter region 1201 is removed, both a reduction effect of a charge
loss due to the damage portion 21 and a passivation effect by the
anti-reflection layer 130 are obtained at the first emitter portion
1211. Thus, a loss amount of charges moving from first emitter
portion 1211 to the second emitter portion 1212 is further
reduced.
[0162] As described referring to FIG. 8D, a paste containing silver
(Ag) and an etching material is printed on the second emitter
portion 1212 and then is dried to form a front electrode part
pattern 40 having a front electrode pattern 41 and a front bus bar
pattern 42, and a paste containing aluminum (Al) and a paste
containing silver (Ag) are printed on the back surface of the
substrate 110 and then are dried to form a back electrode part
pattern 50 having a back electrode pattern 51 and a back bus bar
pattern 52 (referring to FIG. 8D.) The front electrode pattern 41
is formed on and along the front electrode second emitter portion
1212a and the front bus bar pattern 42 is formed on and along the
front bus bar second emitter portion 1212b.
[0163] Then, in the same manner as described above, by heating the
substrate 110 with the pattern 40 and 50, a front electrode part
140 having a plurality of front electrodes 141 and a plurality of
first bus bars 142 penetrating the anti-reflection layer 130 and
contacting the second emitter portion 1212, a back surface field
part 172, that is highly doped with impurities of the same
conductivity type as the substrate 110, a back electrode part 150
including a back electrode 151 contacting the back surface field
part 172 and electrically connected to the substrate 110 through
the back surface field part 172 and a plurality of back bus bars
152 electrically and physically connected to the substrate 110 and
the back electrode 151 are formed. Thereby, the solar cell 2 is
completed.
[0164] When an activation process for forming the emitter region
1201 is performed in an oxygen atmosphere, a silicon oxide layer
may be formed on the second emitter portion 1212. However, when the
front electrode pattern 40 penetrates the anti-reflection layer 130
for forming the front electrode part 140, the silicon oxide layer
underlying the anti-reflection layer 130 is also penetrated by the
front electrode part pattern 40. Thereby, the silicon oxide layer
generated during the activation process does not negatively
influence the contact between the front electrode part 140 and the
emitter part 121a.
[0165] Another example of the selective emitter structure of the
solar cell is shown in FIG. 9.
[0166] As shown in FIG. 9, an emitter part 121a having first and
second emitter portions 1211 and 1212 has a different structure
from the emitter part 121a of FIGS. 6 and 7.
[0167] That is, in FIGS. 6 and 7, a p-n junction surface of the
first emitter portion 1211 is the same level as a p-n junction
surface of the second emitter portion 1212, and the second emitter
portion 1212 having the impunity doped thickness greater than that
of the first emitter portion 1211 is projected toward the substrate
110. Thus, a surface (i.e., a front surface, that is, an opposite
surface of the p-n junction surface) of the first emitter portion
1211 and a surface (i.e., a front surface, that is, an opposite
surface of the p-n junction surface) of the second emitter portion
1212 are not positioned at the same level (i.e., the same height)
as each other and thereby, the surface (the front surface) of the
emitter part 121a is not a flat surface but an uneven surface
having projections at which the second emitter portion 1212 is
projected to the incident surface of the substrate 110. Thereby, a
front surface position of the second emitter portion 1212 is higher
than that of the first emitter portion 1211.
[0168] However, in FIG. 9, a surface (i.e., a front surface) of the
first emitter portion 1211 and a surface (i.e., a front surface) of
the second emitter portion 1212 are positioned at the same level
(i.e., the same height) and thereby, the front surface of the
emitter part 121a is substantially level. However, a p-n junction
of the first emitter portion 1211 and a p-n junction of the second
emitter portion 1212 are not of the same level, but are positioned
at different positions (i.e., different heights), respectively.
Thus, a front position of the first emitter portion 1211 is equal
to the front position of the second emitter portion 1212 and the
p-n junction surface of the second emitter portion 1212 is
projected from the p-n junction of the first emitter portion 1211
to the back surface of the substrate 110. The p-n junction of the
emitter part 121a is an uneven surface having projections at which
the second emitter portion 1212 is projected to the back surface of
the substrate 110.
[0169] Functions of the first and second emitter portions 1211 and
1212 of the solar cell shown in FIG. 9 are equal to that of the
first and second emitter portions 1211 and 1212, and thereby the
functions of the first and second emitter portion 1211 and 1212 are
not described in detail.
[0170] A method for manufacturing the emitter part 121a is
described below.
[0171] As described referring to FIG. 10A, first and second
impurity portions 120a and 120b which are of a second conductivity
type are formed at a front surface of the substrate 110 by
injecting impurity ions of the second conductivity type using an
ion implantation method. The first impurity portion 120a has
different impurity doped concentration and impurity doped thickness
from the second impurity portion 120b. The first and second
impurity portions 120a and 120b are an impurity region.
[0172] The first and second impurity portions 120a and 120b having
different impurity doped thicknesses from each other are formed by
using a mask 85. The mask 85 may be equipped to an ion implantation
equipment for the ion implantation method. An amount of ions
injected into portions of the substrate 110 over which the mask 85
is positioned is less than an amount of ion injected into the
remaining portion of the substrate 110 over which the mask 85 is
not positioned. Thus, the portions of the substrate 110
corresponding the mask 85 is formed as the first impurity portion
120a, and the remaining portion of the substrate 110 (that is,
portions over which the mask 85 is not positioned) are formed as
the second impurity portion 120b.
[0173] As another example for manufacturing the first and second
impurity portions 120a and 120b, the mask 85 may be formed on the
entire front surface of the substrate 110. In this instance, an
exposed area of the substrate 110 exposed through the mask 85 in a
unit area thereof is changed depending on positions of the
substrate 110. For example, the mask 85 may include a first portion
having a first exposed area of the substrate 110 in the unit area
and a second portion having a second exposed area of the substrate
110 in the unit area, and the second exposed area is greater than
the first exposed area. Thereby, when, by using an ion implantation
method, impurity ions are applied on the mask 85 having the first
and second portions and positioned over the entire front surface of
the substrate 110, a portion of the substrate 110 facing the first
portion of the mask 85 may be formed as the first impurity portion
120a having a first impurity doped thickness (concentration) and a
portion of the substrate 110 facing the second portion of the mask
85 may be formed as the second impurity portion 120b having a
second impurity doped thickness (concentration) greater than the
first impurity doped thickness (concentration).
[0174] The mask 85 having the first and second portions may be
positioned directly on the substrate 110, and the first and second
impurity portions 120a and 120b may be formed at the substrate 110
in the same manner as that described above.
[0175] Alternatively, after forming a first impurity portion 120a
having a desired impurity doped thickness at the entire front
surface of the substrate 110, impurity ions being further
selectively or partially injected at the first impurity portion
120a by using a mask such that portions of the first impurity
portion 120a are formed as the second impurity portion 120b. In
this instance, the second impurity portion 120b is the portion of
the substrate 110 at which the impurity ions are further injected,
and the first impurity portion 120a is the remaining portion of the
substrate 110 at which the impurity ions are not further
injected.
[0176] Further, the first and second impurity portions 120a and
120b may be formed by using various known methods using the ion
implantation method.
[0177] Since the first and second impurity portions 120a and 120b
are formed by using the ion implantation method, a damage portion
21 due to the impurity ions is generated at and/or near surfaces
(front surfaces) of the first and second impurity portions 120a and
120b.
[0178] Next, as shown in FIG. 10B, the front surface of the
substrate 110 having the first and second impurity portions 120a
and 120b is heated in a nitrogen (N.sub.2) or oxygen (O.sub.2)
atmosphere and thereby the first and second impurity portions 120a
and 120b are formed as first and second emitter regions 12011 and
12012 of the emitter region 1201a, respectively. In this instance,
the damage portion 21 still exists at and/or near a front surface
of the emitter region 1201a.
[0179] Next, the entire front surface of the emitter region 1201a
is etched by an etchant and then removed to a desired thickness (a
predetermined thickness). In this instance, the damage portion 21
existing at and/or near the front surface of the emitter region
1201a is removed by the etchant. Thereby, an emitter part 121a
having the first and second emitter portions 1211 and 1212 is
completed (FIG. 10C). Since the removal of the emitter region 1201a
is enough to remove the portion at and/or near the entire front
surface of the emitter region 1201a, at which the damage portion 21
largely exists is removed, a removed thickness of the emitter
region 1201a may be 5 nm to 35 nm.
[0180] The thicknesses of the first and second emitter portions
1211 and 1212 of the emitter part 121a are less than thicknesses of
the first and second emitter regions 12011 and 12012 of the emitter
region 1201a, respectively, and the damage portion 21 is removed
and does not exist at and/or near not only the first emitter
portion 12011 but also at the second emitter portion 12012.
[0181] Since all the first and second emitter regions 12011 and
12012 are removed by the etchant, variations of the impurity doped
concentrations of the first and second emitter portions 1211 and
1212 in accordance thickness variations of the first and second
emitter portions 1211 and 1212 have shapes as the graph G2 of FIG.
4, respectively. Further, a surface impurity doped concentration of
the first emitter portion 1211 is less than a surface impurity
doped concentration of the second emitter portion 1212.
[0182] Next, formation processes of an anti-reflection layer 130, a
front electrode part 140, a back electrode part 150, and a back
surface field part 172 are equal to the processes described
referring to FIGS. 8C and 8D.
[0183] The process for forming the emitter part 121 by using the
ion implantation process, the activation process and the etching
process may be adopted for manufacturing the back surface field
part 172.
[0184] That is, after forming an impurity portion of a first
conductivity type at a back surface of the substrate 110 by
injecting impurity ions of the first conductivity type into the
back surface of the substrate 110 in the same manner as described
above, the impurity portion of the first conductivity type is
activated by heating at a predetermined temperature (for example,
about 800.degree. C. to 1100.degree. C.) to form the impurity
portion of the first conductivity type into a back surface field
region. The back surface field region includes a damage portion due
to the impurity ions at and/or near a surface of the back surface
field region. Then, the back surface field region is removed by a
predetermined (desired) thickness from the surface of back surface
field region to form the back surface field part 172 at the back
surface of the substrate 110. The removal of the back surface field
region is performed by the etchant described above, and the damage
portion of the back surface field region is removed during removing
of the back surface field region.
[0185] For reducing or preventing deterioration of the substrate
110 by the heat treatment process for the activation process, after
forming the impurity portion of the second conductivity type for
the emitter part 121 and the impurity portion of the first
conductivity type for the back surface field part 172 at the front
and back surfaces of the substrate 110, respectively, the impurity
portions formed at the front and back surfaces of the substrate 110
may be simultaneously activated by one heat treatment process to
form the emitter part 121 and the back surface field part 172.
[0186] Since the back surface field part 172 is not formed during
forming of the back electrode part 150 but formed by a separate
process such as the ion implantation process, the back surface
field part 172 is further stably formed and the impurity doped
concentration of the back surface field part 172 is further
accurately controlled. As described above, when the back surface
field part 172 is formed by the ion implantation method, a surface
impurity doped concentration of the back surface field part 172 is
greater than that of a back surface field part formed by heat
applied during forming the back electrode part 150, and thereby,
contact resistance between the back surface field part 172 and a
back electrode 151 is reduced to increase an amount of charges
moving from the substrate 110 to the back electrode 151.
[0187] After at least one of the emitter part 121 and the back
surface field part 172 is formed by the ion implantation process,
the activation process and the etching process, an anti-reflection
layer 130, a front electrode part 140 and a back electrode part 150
are formed as described referring to FIGS. 3E to 3G. Since the back
surface field part 172 is already formed before the formation of
the front and back electrode parts 140 and 150, the further
generation of the back surface field part 172 should be prevented
during forming the back electrode 151. For example, a material of a
back electrode pattern for forming the back electrode 151 may be
changed, or after the formation of the front electrode part 140,
the back electrode part 150 should be formed at a low temperature
not to further generate the back surface field part 172 by the back
electrode pattern.
[0188] Further, as shown in FIG. 11, at least one of the emitter
part 121a and the back surface field part 172a formed by the ion
implantation process, the activation process, and the etching
process is applied to a bifacial solar cell of which light is
incident on front and back surfaces.
[0189] As shown in FIG. 11, in the same method as shown in FIGS. 6
and 7 and referring to FIGS. 8A to 8C and 10A to 10C, the emitter
part 121a of the selective emitter structure includes the first and
second emitter portions 1211 and 1212, and a back surface field
part 172a of a selective back surface field structure includes
first and second back surface field portions 1721 and 1722.
[0190] All the first and second back surface field portions 1721
and 1722 have impurity doped concentrations greater than an
impurity doped concentration of the substrate 110. The second back
surface field portion 1722 has the impurity doped concentration
greater than that of the first back surface field portions 1721,
and thus the second back surface field portion 1722 has
conductivity greater than that of the first back surface field
portions 1721.
[0191] Like a plurality of front electrodes 141, the back electrode
includes a plurality of electrodes 151 extending substantially
parallel to one another in a predetermined direction (that is, in
the same direction as the front electrodes 141) at a distance
therebetween, and a back bus bar also includes a plurality of back
bus bars 152 extending in a direction (that is, in the same
direction as the front bus bars 142) crossing the plurality of back
electrodes 151. In this instance, the plurality of back electrodes
151 may face the plurality of front electrodes 141 with the
substrate 110 therebetwen and the plurality of back bus bars 152
may face the plurality of front bus bars 142 with the substrate 110
therebetwen.
[0192] When the solar cell have the selective back surface field
structure, contact resistance between the second back surface field
portion 1722 and the back electrodes 151 is reduced and an amount
of charges moving from the second back surface field portion 1722
to the back electrodes 151 increases, and when the charges move
along a surface of the first back surface field portion 1721 to the
second back surface field portion 1722, a loss amount of the
charges due to the impurities is decreased, to increase an amount
of the charges moving from the first back surface field portion
1721 to the second back surface field portion 1722.
[0193] In this instance, even though the conductivity types of the
emitter part 121a and the back surface field part 172a are
different from each other, methods for forming the selective
emitter structure and the selective back surface field structure
are equal to the processes of FIGS. 8A to 8C or FIGS. 10A to 10C.
That is, as described, impurity ions for forming the emitter part
121a injected into the substrate 110 has a second conductivity type
different from that of the substrate 110, but impurity ions for
forming the back surface field part 172a injected into the
substrate 110 has a first conductivity type equal to that of the
substrate 110.
[0194] Thus, when the selective back surface field structure is
formed in the manner shown in FIGS. 8A to 8C, a back surface field
region of the first conductivity type at the back surface of the
substrate 110 is formed, and then an etch prevention layer is
selective or partially formed on the back surface field region to
expose portions of the back surface field region. Then the exposed
portions of the back surface field region are removed from a
surface of the back surface field region by a predetermined
thickness and the etch prevention layer is removed. Thus, etched
portions of the back surface field region is formed as the first
back surface field portion and the remaining portion (that is,
portions of back surface field region on which the etch prevention
layer is positioned) of the back surface field region is formed as
the second back surface field portion. When the portions of the
back surface field region are removed, a damage portion existing at
the etched portion of the back surface field region is also
removed.
[0195] When the selective back surface field structure is formed in
the manner shown in FIGS. 10A to 10C, a first impurity portion
having a first impurity doped thickness (concentration) and a
second impurity portion having a second impurity doped thickness
(concentration) greater than the first impurity doped thickness
(concentration) are formed by using a mask 85 equipped an ion
implantation equipment or positioned directly on the substrate 110,
or a first impurity portion of the first conductivity type may be
formed at the entire front surface of the substrate 110 by the ion
implantation method, and then impurity ions of the first
conductivity type may be further selectively or partially injected
into the first impurity portion, to form portions of the first
impurity portion as a second impurity portion.
[0196] Next, a heat treatment is performed on the first and second
impurity portions to form a back surface field region of first and
second back surface field regions, and the back surface field
region is removed by a predetermined thickness to form the back
surface field part 172a including the first and second back surface
field portions. Since the back surface field region includes a
damage portion at and/or near the back surface field region, the
damage portion is removed during removing of the back surface field
region.
[0197] In an alternative example, the back surface field part of
the bifacial solar cell is in contact with only the back electrode
151 and the back bus bars 152, and in this instance, when the back
bus bars 152 are omitted, the back surface field part contacts only
the back electrodes 151. That is, the back surface field part may
be not positioned at portions of the substrate 110 between adjacent
back electrodes 152. In this instance, a mask is selectively or
partially positioned on the back surface of the substrate 110 and
impurity ions of the first conductivity type are injected into the
back surface of the substrate 110 to selectively or partially form
an impurity portion of the first conductivity type on the back
surface of the substrate 110. Then, a portion of the impurity
portion is formed as the back surface field region by heating the
impurity and a portion of the back surface field region is removed
from a surface of the back surface field region by a predetermined
thickness, to form the back surface field part.
[0198] Unlike the solar cell shown in FIG. 11, at least one of the
emitter part 121a and the back surface field part 172a may have the
selective emitter structure or the selective back surface field
structure.
[0199] When the back surface field part has the selective back
surface field structure formed by the ion implantation process, the
activation process and the etching process, the solar cell may
obtain a quantum efficiency depending on a wavelength of light, as
shown in (A) and (B) of FIG. 12.
[0200] (A) of FIG. 12 shows a simulated graph of an external
quantum efficiency (E.Q.E) depending on a variation of the
wavelength of light, and in the graph, the first back surface field
portion which is a low impurity doped portion has an impurity doped
concentration of about 3.times.10.sup.19 cm.sup.-3 and the second
back surface field portion which is a high impurity doped portion
has an impurity doped concentration of about 2.times.10.sup.20
cm.sup.-3.
[0201] As shown in (A) of FIG. 12, the maximum wavelength of light
absorbed into the substrate 110 not passing through the substrate
110 is about 1200 nm, and at the back surface of the substrate 110
absorbed light of a wavelength of 900 nm and more, an external
quantum efficiency LG1 measured at the first back surface field
portion, at which a damage portion is removed is larger than an
external quantum efficiency HG1 measured at the second back surface
field portion at the back surface of the substrate 110, at which a
damage portion exists.
[0202] (B) of FIG. 12 shows a graph of an internal quantum
efficiency (I.Q.E) depending on a variation of a wavelength of
light obtained based on the external quantum efficiency shown in
(A) of FIG. 12. As shown in (B) of FIG. 12, an internal quantum
efficiency LG2 measured at the first back surface field portion, at
which a damage portion is removed is larger than an internal
quantum efficiency HG2 measured at the second back surface field
portion at the back surface of the substrate 110, at which a damage
portion exists.
[0203] Although embodiments have been described with reference to a
number of illustrative embodiments thereof, it should be understood
that numerous other modifications and embodiments can be devised by
those skilled in the art that will fall within the scope of the
principles of this disclosure. More particularly, various
variations and modifications are possible in the component parts
and/or arrangements of the subject combination arrangement within
the scope of the disclosure, the drawings and the appended claims.
In addition to variations and modifications in the component parts
and/or arrangements, alternative uses will also be apparent to
those skilled in the art.
* * * * *