U.S. patent application number 15/093155 was filed with the patent office on 2017-03-30 for cztse-based thin film and method for preparing the same, and solar cell using the same.
The applicant listed for this patent is EWHA UNIVERSITY - INDUSTRY COLLABORATION FOUNDATION. Invention is credited to William Jo, Gee-Yeong Kim.
Application Number | 20170092792 15/093155 |
Document ID | / |
Family ID | 55630967 |
Filed Date | 2017-03-30 |
United States Patent
Application |
20170092792 |
Kind Code |
A1 |
Jo; William ; et
al. |
March 30, 2017 |
CZTSe-Based Thin Film and Method for Preparing the Same, and Solar
Cell Using the Same
Abstract
The present invention relates to a CZTSe-based composite thin
film, a method for preparing the CZTSe-based composite thin film, a
solar cell using the CZTSe-based composite thin film, and a method
for preparing the solar cell using the CZTSe-based composite thin
film.
Inventors: |
Jo; William; (Seoul, KR)
; Kim; Gee-Yeong; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
EWHA UNIVERSITY - INDUSTRY COLLABORATION FOUNDATION |
Seoul |
|
KR |
|
|
Family ID: |
55630967 |
Appl. No.: |
15/093155 |
Filed: |
April 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/KR2015/010351 |
Sep 30, 2015 |
|
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15093155 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/072 20130101;
H01L 31/0326 20130101; Y02E 10/50 20130101; H01L 31/18 20130101;
Y02P 70/50 20151101 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/18 20060101 H01L031/18 |
Claims
1. A method for preparing a CZTSe-based composite thin film,
comprising: forming a Cu.sub.2ZnSnSe.sub.4 thin film on a
substrate; and forming a metal component M-containing layer on the
Cu.sub.2ZnSnSe.sub.4 thin film, followed by annealing to form a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film, wherein x is from
0.2 to 0.5 and the component M includes Ge and/or Si.
2. The method for preparing the CZTSe-based composite thin film of
claim 1, wherein the Cu.sub.2ZnSnSe.sub.4 thin film is formed by
co-evaporating a precursor including Cu, Zn, Sn, and Se; or the
Cu.sub.2ZnSnSe.sub.4 thin film is formed by forming a precursor
thin film by depositing a Cu-containing precursor, a Zn-containing
precursor, and a Sn-containing precursor, followed by performing
selenization of the precursor thin film.
3. The method for preparing the CZTSe-based composite thin film of
claim 1, further comprising: performing selenization after forming
the Cu.sub.2ZnSnSe.sub.4 thin film and
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film.
4. The method for preparing the CZTSe-based composite thin film of
claim 2, wherein the precursor thin film is deposited one or more
times in order of Sn/Zn/Cu, Sn/Cu/Zn, Zn/Cu/Sn, Zn/Sn/Cu, Cu/Sn/Zn,
or Cu/Zn/Sn.
5. The method for preparing the CZTSe-based composite thin film of
claim 1, further comprising: forming a
Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film below the
Cu.sub.2ZnSnSe.sub.4 thin film, by using SnS as a Sn-containing
precursor and/or ZnS as a Zn-containing precursor; or by further
injecting a S-containing gas phase source, prior to forming the
Cu.sub.2ZnSnSe.sub.4 thin film, wherein 0.ltoreq.y.ltoreq.1.
6. The method for preparing the CZTSe-based composite thin film of
claim 1, wherein the metal component M-containing layer is prepared
by depositing the metal M on a surface of the Cu.sub.2ZnSnSe.sub.4
thin film by a vacuum evaporation method; by decomposing and
diffusing a metal component M-containing gas phase precursor;
and/or by thermally treating the surface of the
Cu.sub.2ZnSnSe.sub.4 thin film in the metal component M-containing
atmosphere.
7. The method for preparing the CZTSe-based composite thin film of
claim 1, wherein the Cu.sub.2ZnSnSe.sub.4 thin film is formed to a
thickness of from 0.1 .mu.m to 3 .mu.m.
8. The method for preparing the CZTSe-based composite thin film of
claim 6, wherein the metal component M-containing layer is formed
to a thickness of from 10 nm to 1,500 nm on the surface of the
Cu.sub.2ZnSnSe.sub.4 thin film.
9. A CZTSe-based composite thin film, comprising: a
Cu.sub.2ZnSnSe.sub.4 thin film layer; and a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film, wherein x is from 0.2 to 0.5 and
the component M includes Ge and/or Si.
10. The CZTSe-based composite thin film of claim 9, further
comprising: a Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film
below the Cu.sub.2ZnSnSe.sub.4 thin film, wherein
0.ltoreq.y.ltoreq.1.
11. The CZTSe-based composite thin film of claim 9, wherein a band
gap of the Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4thin film is
higher than that of the Cu.sub.2ZnSnSe.sub.4 thin film.
12. A method for preparing a solar cell using a CZTSe-based
composite thin film, comprising: forming a back electrode on a
substrate; and forming a light absorbing layer including a
CZTSe-based composite thin film prepared by a method according to
any one of claim 1 on the back electrode.
13. A solar cell using a CZTSe-based composite thin film,
comprising: a substrate; a back electrode on the substrate; and a
light absorbing layer including a CZTSe-based composite thin film
according to any one of claim 9 formed on the back electrode.
Description
TECHNICAL FIELD
[0001] The present invention relates to a CZTSe-based composite
thin film, a method for preparing the CZTSe-based composite thin
film, a solar cell using the CZTSe-based composite thin film, and a
method for preparing the solar cell using the CZTSe-based composite
thin film.
BACKGROUND
[0002] Recently, the development of next-generation clean energy
has become more important due to severe environmental pollution and
depletion of fossil fuels. Particularly, it is expected that solar
cells, which are devices capable of converting solar energy
directly into electrical energy, will become an energy source
capable of solving future energy problems since they generate
little air pollution, their resources are inexhaustible and they
have semi-permanent life time.
[0003] Such solar cells are classified into various types depending
on a material used as a light absorbing layer. Silicon solar cells
using silicon are most commonly used presently. However, the price
of silicon has soared due to a recent deficient supply of silicon.
Accordingly, there is a growing interest in thin-film solar cells.
Since the thin-film solar cells are manufactured to have a thin
thickness, the thin-film solar cells require less consumption of
materials and are light, and, thus, have a wide range of
applications. Studies on amorphous silicon, CdTe, and CIS
(CuInSe.sub.2, CuIn.sub.1-xGa.sub.xSe.sub.2,
CuIn.sub.1-xGa.sub.xS.sub.2, etc.) have been actively conducted as
materials of such thin-film solar cells. A CIS-based thin film is
one of I-III-IV compound semiconductors, and particularly, a CIGS
solar cell has the highest conversion efficiency (about 22.3%) out
of thin-film solar cells prepared experimentally. However, in used
therein is a rare element found in a relatively small amount of its
resource, and the price thereof is on an upward trend due to the
demand for ITO used in the display industry, which may constitute
an obstacle to mass production. In order to solve this problem,
compound semiconductors such as Cu.sub.2ZnSnSe.sub.4 (CZTSe) and
Cu.sub.2ZnSnS.sub.4 (CZTS) have been actively researched to be used
for the development of a low-price solar cell as alternatives to
CIGS-based thin film materials by substituting abundant elements Zn
and Sn for rare elements of In and Ga. The compound semiconductors
are advantageous in preparing a high-efficiency solar cell suitable
for the solar spectrum and expected simultaneously to solve other
problems of the CIGS-based solar cells.
[0004] Meanwhile, Korean Patent No. 10-1339874 discloses a method
for preparing a double-grading CZTS-based thin film.
DETAILED DESCRIPTION OF THE INVENTION
Problems to be Solved by the Invention
[0005] In view of the foregoing problems, one purpose of the
present disclosure is to provide a CZTSe-based composite thin film,
a method for preparing the CZTSe-based composite thin film, a solar
cell using the CZTSe-based composite thin film, and a method for
preparing the solar cell using the CZTSe-based composite thin
film.
[0006] However, problems to be solved by the present disclosure are
not limited to the above-described problems. Although not described
herein, other problems to be solved by the present disclosure can
be clearly understood by those skilled in the art from the
following descriptions.
Means for Solving the Problems
[0007] In a first aspect of the present disclosure, there is
provided a method for preparing a CZTSe-based composite thin film,
including: forming a Cu.sub.2ZnSnSe.sub.4 thin film on a substrate;
and forming a metal component M-containing layer on the
Cu.sub.2ZnSnSe.sub.4 thin film, followed by annealing to form a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film; wherein, x is
from 0.2 to 0.5 and the component M includes Ge and/or Si.
[0008] In a second aspect of the present disclosure, there is
provided a CZTSe-based composite thin film including: a
Cu.sub.2ZnSnSe.sub.4 thin film; and a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film; wherein, x is from 0.2 to 0.5 and
the component M includes Ge and/or Si.
[0009] In a third aspect of the present disclosure, there is
provided a method for preparing a solar cell using a CZTSe-based
composite thin film, including: forming a back electrode on a
substrate; and forming a light absorbing layer including a
CZTSe-based composite thin film prepared by the method in
accordance with the first aspect on the back electrode.
[0010] In a fourth aspect of the present disclosure, there is
provided a solar cell using a CZTSe-based composite thin film,
including: a substrate; a back electrode on the substrate; and a
light absorbing layer including the CZTSe-based composite thin film
in accordance with the second aspect formed on the back
electrode.
Effect of the Invention
[0011] A CZTSe-based composite thin film in accordance with an
embodiment of the present disclosure includes a
Cu.sub.2ZnSnSe.sub.4 thin film and a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film (herein, x=0.2 to
0.5) on the Cu.sub.2ZnSnSe.sub.4 thin film. Thus, since a band gap
on a front surface layer is higher, an open-circuit voltage is
increased and factors of electron-hole recombination that impedes
efficiency are reduced. Further, since a band gap on a back surface
layer is high, an electron mobility is increased. Accordingly, the
efficiency of a solar cell using the CZTSe-based composite thin
film as a light absorbing layer can be improved.
[0012] In an embodiment of the present disclosure, when the
CZTSe-based composite thin film further includes a
Cu.sub.2ZnSn(Se,S).sub.4 thin film below the Cu.sub.2ZnSnSe.sub.4
thin film, the CZTSe-based composite thin film may include thin
films having three different band gaps. In this case, the
CZTSe-based composite thin film is manufactured to have band gaps
which are not simply increased or decreased but have a dual or
triple slope and thus include a higher band gap at an upper part
including a component M (Ge or Si), the lowest band gap at an
intermediate part including Se only, and a high band gap at a lower
part including S. The band gap slopes at the upper part and the
intermediate part lead to absorb light in a broad wavelength
region, and, thus, can improve the efficiency. Further, the band
gap slopes at the intermediate part and the lower part lead to
inhibit electron-hole recombination, and, thus, can contribute to
improvement of the efficiency of a solar cell using the same.
[0013] In an embodiment of the present disclosure, the CZTSe-based
composite thin film having various band gaps can lead to absorb the
solar spectrum in various manners, and, thus, can improve the solar
absorbance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic diagram illustrating a solar cell
using a CZTSe-based composite thin film in accordance with an
example of the present disclosure.
[0015] FIG. 2 is a graph showing temperature conditions according
to a deposition time of a CZTSe-based composite thin film in
accordance with an example of the present disclosure.
[0016] FIG. 3 and FIG. 4A to FIG. 4D are scanning electron
microscopic images of a Cu.sub.2Zn(Sn,Ge)Se.sub.4 thin film in
accordance with an example of the present disclosure.
[0017] FIG. 5 is a graph showing a measurement result of a band gap
of a Cu.sub.2Zn(Sn,Ge)Se.sub.4 thin film in accordance with an
example of the present disclosure.
[0018] FIG. 6 is a graph showing a change in a band gap depending
on an amount of Ge which substitutes for Sn in a
Cu.sub.2Zn(Sn,Ge)Se.sub.4 thin film in accordance with an example
of the present disclosure.
[0019] FIG. 7 is a graph showing a measurement result of an element
composition while a CZTSe-based composite thin film in accordance
with an example of the present disclosure is etched by
sputtering.
[0020] FIG. 8A to FIG. 8D show measurement results of an element
composition according to a depth of a CZTSe-based composite thin
film in accordance with an example of the present disclosure.
[0021] FIG. 9 is a spectrum showing an X-ray diffraction analysis
result of a CZTSe-based composite thin film in accordance with an
example of the present disclosure.
[0022] FIG. 10A to FIG. 10D are spectrums showing Raman
spectroscopy analysis results of a CZTSe-based composite thin film
in accordance with an example of the present disclosure.
[0023] FIG. 11 is a graph showing band gap grading of a light
absorbing layer of a CZTSe-based composite thin film in accordance
with an example of the present disclosure.
[0024] FIG. 12 is a graph showing a change in a band gap depending
on a composition of a CZTSe-based composite thin film in accordance
with an example of the present disclosure.
[0025] FIG. 13 is a graph showing a current-voltage relationship of
a CZTSe-based composite thin film in accordance with an example of
the present disclosure
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] Hereinafter, embodiments of the present disclosure will be
described in detail with reference to the accompanying drawings so
that the present disclosure may be readily implemented by those
skilled in the art. However, it is to be noted that the present
disclosure is not limited to the embodiments but can be embodied in
various other ways. In drawings, parts irrelevant to the
description are omitted for the simplicity of explanation, and like
reference numerals denote like parts through the whole document of
the present disclosure.
[0027] Through the whole document of the present disclosure, the
term "connected to" or "coupled to" that is used to designate a
connection or coupling of one element to another element includes
both a case that an element is "directly connected or coupled to"
another element and a case that an element is "electronically
connected or coupled to" another element via still another
element.
[0028] Through the whole document of the present disclosure, the
term "on" that is used to designate a position of one element with
respect to another element includes both a case that the one
element is adjacent to the another element and a case that any
other element exists between these two elements.
[0029] Through the whole document of the present disclosure, the
term "comprises or includes" and/or "comprising or including" used
in the document means that one or more other components, steps,
operation and/or existence or addition of elements are not excluded
in addition to the described components, steps, operation and/or
elements unless context dictates otherwise.
[0030] Through the whole document of the present disclosure, the
term "about or approximately" or "substantially" is intended to
have meanings close to numerical values or ranges specified with an
allowable error and intended to prevent accurate or absolute
numerical values disclosed for understanding of the present
disclosure from being illegally or unfairly used by any
unconscionable third party.
[0031] Through the whole document of the present disclosure, the
term "step of" does not mean "step for".
[0032] Through the whole document of the present disclosure, the
term "combinations of" included in Markush type description means
mixture or combination of one or more components, steps, operations
and/or elements selected from a group consisting of components,
steps, operation and/or elements described in Markush type and
thereby means that the disclosure includes one or more components,
steps, operations and/or elements selected from the Markush
group.
[0033] Through the whole document of the present disclosure, a
phrase in the form "A and/or B" means "A or B, or A and B".
[0034] Hereinafter, embodiments of the present disclosure will be
described, but the present disclosure may not be limited
thereto.
[0035] In a first aspect of the present disclosure, there is
provided a method for preparing a CZTSe-based composite thin film,
including: forming a Cu.sub.2ZnSnSe.sub.4 thin film on a substrate;
and forming a metal component M-containing layer on the
Cu.sub.2ZnSnSe.sub.4 thin film, followed by annealing to form a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film; wherein, x is
from 0.2 to 0.5 and the component M includes Ge and/or Si.
[0036] In the Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film
(herein, x=0.2 to 0.5), it is found that when x is in the range of
from 0.2 to 0.5, a band gap is increased from about 1.15 eV to
about 1.4 eV and optimized to improve a voltage and reduce a
current loss. It is determined that when x is higher than 0.5, a
voltage may be increased but a current loss occurs due to a high
band gap, and when the content of M is higher than that of Sn, the
probability of semiconductor defects may be increased, which may
have a negative effect on the efficiency. By way of example, CZTS
(including S only without Se) has a band gap of 1.5 eV in the case
of CZT(S.sub.1-x,Se.sub.x).sub.4 (x=0). A high band gap on a front
surface may cause a current loss. Therefore, adjustment of a band
gap using Ge may have a positive effect to the efficiency. If S is
introduced to CZTSe, a valence band level is decreased, and, thus,
a band gap is increased. If Ge is introduced, a conduction band
level is increased, and, thus, a band gap is increased. Such
adjustment of a band gap can reduce electron-hole recombination and
thus contribute to improvement of the efficiency.
[0037] In an embodiment of the present disclosure, the substrate
may include a metal or metal sulfide, but may not be limited
thereto.
[0038] In an embodiment of the present disclosure, the
Cu.sub.2ZnSnSe.sub.4 thin film may be formed by co-evaporating a
precursor including Cu, Zn, Sn, and Se, or by forming a precursor
thin film by depositing a Cu-containing precursor, a Zn-containing
precursor, and a Sn-containing precursor, followed by performing
selenization of the precursor thin film, but may not be limited
thereto.
[0039] In an embodiment of the present disclosure, the
Cu.sub.2ZnSnSe.sub.4 thin film may be prepared using a solution
process, but may not be limited thereto.
[0040] In an embodiment of the present disclosure, the
co-evaporation may be performed at a temperature of the substrate
in the range of from about 300.degree. C. to about 600.degree. C.
under a deposition pressure in the range of from about
5.times.10.sup.-6 torr to about 5.times.10.sup.-5 torr for from
about 40 minutes to about 90 minutes, but may not be limited
thereto. By way of example, a temperature of the substrate may be
in the range of from about 300.degree. C. to about 600.degree. C.,
from about 300.degree. C. to about 500.degree. C., from about
300.degree. C. to about 400.degree. C., from about 400.degree. C.
to about 600.degree. C., from about 400.degree. C. to about
500.degree. C., or from about 500.degree. C. to about 600.degree.
C., but may not be limited thereto.
[0041] In an embodiment of the present disclosure, when the
Cu.sub.2ZnSnSe.sub.4 thin film is formed, the method may include
forming the Cu/Zn/Sn thin film, followed by performing a
selenization process within a thin film forming chamber or a
separate furnace, but may not be limited thereto. The selenization
process may improve the density and crystallinity of the thin film,
but may not be limited thereto.
[0042] In an embodiment of the present disclosure, the selenization
process is a process for introducing Se onto the thin film using a
Se-containing precursor. To be specific, there are, but not limited
to, a method of introducing Se by evaporating and diffusing a Se
metal source or a method of introducing Se by using H.sub.2Se gas
phase source. The selenization process may be performed at a
temperature of the substrate in the range of from about 300.degree.
C. to about 600.degree. C. under a deposition pressure of from
about 5.times.10.sup.-6 torr to about 5.times.10.sup.-5 torr for
from about 10 minutes to about 30 minutes, but may not be limited
thereto. By way of example, during the selenization process, a
temperature of the substrate may be in the range of from about
300.degree. C. to about 600.degree. C., from about 300.degree. C.
to about 500.degree. C., from about 300.degree. C. to about
400.degree. C., from about 400.degree. C. to about 600.degree. C.,
from about 400.degree. C. to about 500.degree. C., or from about
500.degree. C. to about 600.degree. C., but may not be limited
thereto.
[0043] In an embodiment of the present disclosure, the selenization
process may be performed within a separate quartz furnace in a
range of from about 300.degree. C. to about 600.degree. C., but may
not be limited thereto. By way of example, a temperature of the
quartz furnace may be in the range of from about 300.degree. C. to
about 600.degree. C., from about 300.degree. C. to about
500.degree. C., from about 300.degree. C. to about 400.degree. C.,
from about 400.degree. C. to about 600.degree. C., from about
400.degree. C. to about 500.degree. C., or from about 500.degree.
C. to about 600.degree. C., but may not be limited thereto.
[0044] In an embodiment of the present disclosure, if the
Cu.sub.2ZnSnSe.sub.4 thin film is formed by co-evaporating a
precursor including Cu, Zn, Sn, and Se, a CZTSe thin film may be
prepared using a Cu--Se, ZnSe, Sn--Se, Cu--Sn--Se, Cu--Sn--Se,
Cu--Sn, Cu--Zn, Zn--Sn, and/or Cu.sub.2ZnSnSe.sub.4 (CZTSe)
precursor such that all of Cu, Zn, Sn, and Se are included on a
metal substrate. By way of example, a CZTSe thin film may be
prepared using one to three combinations of the above-described
precursors by the co-evaporation method such that all of Cu, Zn,
Sn, and Se are included, but may not be limited thereto.
[0045] In an embodiment of the present disclosure, a metal
precursor may be prepared by co-evaporating (Cu, Zn, and Sn at the
same time, and/or Cu, Zn, and SnSe at the same time). By way of
example, a CZTSe may be prepared by co-evaporating Cu--Zn--Sn
followed by performing selenization, co-evaporating Cu--Zn--Sn--Se,
co-evaporating Cu--Zn--SnSe followed by performing selenization, or
co-evaporating Cu--Zn--SnSe--Se.
[0046] In an embodiment of the present disclosure, if the
Cu.sub.2ZnSnSe.sub.4 thin film is formed by forming a precursor
thin film by depositing a Cu-containing precursor, a Zn-containing
precursor, and a Sn-containing precursor, followed by performing
selenization, the precursor thin film may be deposited one or more
times in order of Sn/Zn/Cu, Sn/Cu/Zn, Zn/Cu/Sn, Zn/Sn/Cu, Cu/Sn/Zn,
or Cu/Zn/Sn, but may not be limited thereto.
[0047] In an embodiment of the present disclosure, each of the Sn,
Zn, and Cu layers included in the precursor thin film may be formed
to a thickness of from about 100 nm to about 1,800 nm, but may not
be limited thereto. By adjusting the thickness of each layer, it
may be possible to adjust a composition of the CZTSe-based thin
film according to the present disclosure, but may not be limited
thereto. By adjusting the composition, it may be possible to adjust
a size of a grain growing when the thin film is formed, but may not
be limited thereto. By way of example, each of the Sn, Zn, and Cu
layers included in the precursor thin film may be formed to a
thickness of from about 100 nm to about 1,800 nm, from about 300 nm
to about 1,800 nm, from about 600 nm to about 1,800 nm, from about
900 nm to about 1,800 nm, from about 1,200 nm to about 1,800 nm,
from about 1,500 nm to about 1,800 nm, from about 300 nm to about
1,500 nm, from about 600 nm to about 1,500 nm, from about 600 nm to
about 1,500 nm, from about 900 nm to about 1,500 nm, from about 300
nm to about 1,200 nm, from about 600 nm to about 1,200 nm, from
about 900 nm to about 1,200 nm, from about 300 nm to about 900 nm,
from about 600 nm to about 900 nm, or from about 300 nm to about
600 nm, but may not be limited thereto.
[0048] In an embodiment of the present disclosure, the Sn, Zn, and
Cu layers included in the precursor thin film may be formed at a
temperature of the substrate in the range of from about room
temperature to about 600.degree. C. under a deposition pressure of
from about 5.times.10.sup.-6 torr to about 5.times.10.sup.-5 torr,
but may not be limited thereto. By way of example, when the Sn, Zn,
and Cu layers are formed, a temperature of the substrate may be in
the range of from about room temperature to about 600.degree. C.,
from about room temperature to about 500.degree. C., from about
room temperature to about 400.degree. C., from about room
temperature to about 300.degree. C., from about room temperature to
about 200.degree. C., from about room temperature to about
100.degree. C., from about 100.degree. C. to about 500.degree. C.,
from about 100.degree. C. to about 400.degree. C., from about
100.degree. C. to about 300.degree. C., from about 100.degree. C.
to about 200.degree. C., from about 200.degree. C. to about
500.degree. C., from about 200.degree. C. to about 400.degree. C.,
from about 200.degree. C. to about 300.degree. C., from about
300.degree. C. to about 500.degree. C., from about 300.degree. C.
to about 400.degree. C., or from about 400.degree. C. to about
500.degree. C., but may not be limited thereto.
[0049] In an embodiment of the present disclosure, before the
Cu.sub.2ZnSnSe.sub.4 thin film is formed, a
Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film
(0.ltoreq.y.ltoreq.0 may be formed below the Cu.sub.2ZnSnSe.sub.4
thin film, by using SnS as a Sn-containing precursor and/or ZnS as
a Zn-containing precursor; or by introducing a S-containing gas
phase source, instead of using a Se precursor, by the same
deposition method as performed to form the Cu.sub.2ZnSnSe.sub.4
thin film, but may not be limited thereto. By way of example, the
S-containing gas phase source may include H.sub.2S, but may not be
limited thereto. By way of example, a Cu.sub.2ZnSnS.sub.4 thin film
may be formed below the Cu.sub.2ZnSnSe.sub.4 thin film and a
Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film (0<y<1) may
be formed at an interface between these thin films.
[0050] In an embodiment of the present disclosure, a band gap of
the Cu.sub.2ZnSnS.sub.4 thin film or
Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film (0<y<1) may
be higher than that of the Cu.sub.2ZnSnSe.sub.4 thin film.
[0051] In an embodiment of the present disclosure, as described
above, a sulfurization process of reacting S included in a
precursor; or additionally injecting a S-containing gas phase
source may be performed within a furnace in a S atmosphere, but may
not be limited thereto.
[0052] By way of example, the precursor thin film is formed on the
substrate in order of Cu/Sn/Zn, Sn/Zn/Cu, or Sn/Cu/Zn, and when the
precursor thin film is formed, SnS as a Sn-containing precursor and
ZnS as a Zn-containing precursor are deposited. Therefore, the
Cu.sub.2ZnSnS.sub.4 thin film and/or the
Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film (0<y<1) may
be formed below the Cu.sub.2ZnSnSe.sub.4 thin film, but may not be
limited thereto.
[0053] In an embodiment of the present disclosure, the metal
component M-containing layer may be prepared by depositing the
metal M on a surface of the Cu.sub.2ZnSnSe.sub.4 thin film by a
vacuum evaporation method; by decomposing and diffusing a metal
component M-containing gas phase precursor; and/or by thermally
treating the surface of the Cu.sub.2ZnSnSe.sub.4 thin film in the
metal component M-containing atmosphere, but may not be limited
thereto. By way of example, the metal component M may be diffused
to about 100 nm from the surface of the Cu.sub.2ZnSnSe.sub.4 thin
film to create band gap grading, but may not be limited
thereto.
[0054] In an embodiment of the present disclosure, the
Cu.sub.2ZnSnSe.sub.4 thin film may be formed to a thickness of from
about 0.1 .mu.m to about 3 .mu.m, but may not be limited thereto.
By way of example, the Cu.sub.2ZnSnSe.sub.4 thin film may be formed
to a thickness of from about 0.1 .mu.m to about 3 .mu.m, from about
0.1 .mu.m to about 2 .mu.m, from about 0.1 .mu.m to about 1 .mu.m,
from about 0.1 .mu.m to about 0.5 .mu.m, from about 0.5 .mu.m to
about 3 .mu.m, from about 0.5 .mu.m to about 2 .mu.m, from about
0.5 .mu.m to about 1 .mu.m, from about 1 .mu.m to about 3 .mu.m,
from about 1 .mu.m to about 2 .mu.m, or from about 2 .mu.m to about
3 .mu.m in order to absorb light, but may not be limited
thereto.
[0055] In an embodiment of the present disclosure, the metal
component M-containing layer may be formed to a thickness of from
about 10 nm to about 1,500 nm on the surface of the
Cu.sub.2ZnSnSe.sub.4 thin film, but may not be limited thereto. By
way of example, the metal component M-containing layer may be
formed to a thickness of from about 10 nm to about 1,500 nm, from
about 10 nm to about 1,000 nm, from about 10 nm to about 500 nm,
from about 10 nm to about 250 nm, from about 10 nm to about 100 nm,
from about 250 nm to about 1,500 nm, from about 250 nm to about
1,000 nm, from about 250 nm to about 500 nm, from about 500 nm to
about 1,500 nm, from about 500 nm to about 1,000 nm, or from about
1,000 nm to about 1,500 nm on the surface of the
Cu.sub.2ZnSnSe.sub.4 thin film, but may not be limited thereto.
[0056] In an embodiment of the present disclosure, the metal
component M-containing layer may be formed by depositing the
component M on the surface of the Cu.sub.2ZnSnSe.sub.4 thin film at
a temperature in the range of from about 250.degree. C. to about
600.degree. C. under a deposition pressure in the range of from
about 5.times.10.sup.-6 torr to about 5.times.10.sup.-5 torr and
then, the Cu.sub.2Zn(Sn,M)Se.sub.4 thin film may be formed by
additionally performing annealing for from about 10 minutes to
about 30 minutes at a temperature in the range of from about
200.degree. C. to about 400.degree. C., but may not be limited
thereto.
[0057] By way of example, when the metal component M-containing
layer is formed, a temperature may be in the range of from about
250.degree. C. to about 600.degree. C., from about 250.degree. C.
to about 500.degree. C., from about 250.degree. C. to about
400.degree. C., from about 400.degree. C. to about 600.degree. C.,
from about 400.degree. C. to about 500.degree. C., or from about
500.degree. C. to about 600.degree. C. If the temperature is higher
than 600.degree. C., a glass substrate is melted, and if the
temperature is lower than 250.degree. C., the reaction does not
occur.
[0058] By way of example, when the annealing is additionally
performed, a temperature may be in the range of from about
200.degree. C. to about 400.degree. C., from about 200.degree. C.
to about 350.degree. C., from about 200.degree. C. to about
300.degree. C., from about 200.degree. C. to about 250.degree. C.,
from about 250.degree. C. to about 400.degree. C., from about
250.degree. C. to about 350.degree. C., from about 250.degree. C.
to about 300.degree. C., from about 300.degree. C. to about
400.degree. C., from about 300.degree. C. to about 350.degree. C.,
or from about 350.degree. C. to about 400.degree. C., but may not
be limited thereto.
[0059] In a second aspect of the present disclosure, there is
provided a CZTSe-based composite thin film including: a
Cu.sub.2ZnSnSe.sub.4 thin film layer; and a
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film; wherein, x is from 0.2 to 0.5 and
the component M includes Ge and/or Si. All the descriptions
relating to the first aspect of the present disclosure may be
applied to a method for preparing the CZTSe-based composite thin
film in accordance with the present aspect.
[0060] In an embodiment of the present disclosure, a
Cu.sub.2ZnSn(Se.sub.1-y, S.sub.y).sub.4 thin film
(0.ltoreq.y.ltoreq.1) may be further formed below the
Cu.sub.2ZnSnSe.sub.4 thin film, but may not be limited thereto. By
way of example, a Cu.sub.2ZnSnS.sub.4 thin film may be further
formed below the Cu.sub.2ZnSnSe.sub.4 thin film and a
Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film (0<y<1) may
be formed at an interface between these thin films.
[0061] In an embodiment of the present disclosure, the
Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film may include
Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 or
Cu.sub.2Zn(Sn.sub.1-x,Si.sub.x)Se.sub.4, and the
Cu.sub.2ZnSn(Se.sub.1-x,S.sub.x).sub.4 thin film
(0.ltoreq.y.ltoreq.1) may include Cu.sub.2ZnSnS.sub.4, but may not
be limited thereto.
[0062] In an embodiment of the present disclosure, band gap energy
of the Cu.sub.2ZnSnSe.sub.4 is about 1.0 eV, band gap energy of the
Cu.sub.2ZnGeSe.sub.4 is about 1.4 eV, band gap energy of the
Cu.sub.2ZnSiSe.sub.4 is about 2.4 eV, and band gap energy of the
Cu.sub.2ZnSnS.sub.4 is about 1.5 eV. A band gap value of a CZTGSe
thin film including Se may be adjusted depending on a composition
ratio of Ge/(Ge+Sn) and Si/(Si+Sn). By way of example,
Cu.sub.2ZnSnSe.sub.4, Cu.sub.2ZnSn.sub.0.8Ge.sub.0.2Se.sub.4, and
Cu.sub.2ZnSn.sub.0.5Ge.sub.0.5Se.sub.4 may have band gaps of 1.0
eV, 1.15 eV, and 1.25 eV close to ideal band gaps, respectively,
and Cu.sub.2Zn(Sn.sub.0.75,Si.sub.0.25)Se.sub.4,
Cu.sub.2Zn(Sn.sub.0.5Si.sub.0.5)Se.sub.4, and Cu.sub.2ZnSiSe.sub.4
can be adjusted to 1.3 eV, 1.5 eV, and 2.3 eV, respectively.
[0063] A band gap value of a Cu.sub.2ZnSnGeS.sub.4 thin film
including S may be adjusted in the range of from 1.4 eV to 1.8 eV
depending on a content of Ge. However, since the
Cu.sub.2ZnSnGeS.sub.4 thin film has a large band gap, the CZTSe
composite thin film in accordance with an embodiment of the present
disclosure has an advantage of improving the efficiency when used
as a light absorbing layer of a solar cell.
[0064] In an embodiment of the present disclosure, a band gap of
the Cu.sub.2Zn(Sn.sub.1-xGe.sub.x)Se.sub.4 thin film or the
Cu.sub.2ZnSn(S,Se).sub.4 thin film is higher than that of the
Cu.sub.2ZnSnSe.sub.4 thin film. A band gap of Cu.sub.2ZnSnSe.sub.4
may be adjusted in the range of from 0.9 eV to 1.0 eV, a band gap
of Cu.sub.2ZnSn(S.sub.0.5Se.sub.0.5).sub.4 may be adjusted to 1.2
eV, and a band gap of Cu.sub.2ZnSnS.sub.4 may be adjusted to 1.5
eV. Further, for example, a band gap of the
Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 thin film may be higher
than that of the Cu.sub.2ZnSnSe.sub.4 thin film, and may be in the
range of higher than about 1.0 eV to lower than about 1.5 eV, and a
band gap of Cu.sub.2Zn(Sn.sub.0.5Ge.sub.0.5)Se.sub.4 may be
adjusted to 1.15 eV and a band gap of
Cu.sub.2Zn(Sn.sub.0.5Si.sub.0.5)Se.sub.4 may be adjusted to 1.5 eV.
Since Cu.sub.2ZnSiSe.sub.4 has a very high band gap of 2.3 eV or
more, it is not suitable for a light absorbing layer. If it is used
to create band gap grading, it may be desirable to set x in
Cu.sub.2ZnSn.sub.1-xSi.sub.xSe.sub.4 to 0.5 or less. By way of
example, a band gap of the Cu.sub.2Zn(Sn.sub.1-x,Si.sub.x)Se.sub.4
thin film may be higher than that of the Cu.sub.2ZnSnSe.sub.4 thin
film, and may be in the range of higher than about 1.0 eV to lower
than about 2.4 eV, but may not be limited thereto.
[0065] In an embodiment of the present disclosure, in the
CZTSe-based composite thin film in accordance with the present
disclosure, since a band gap on a front surface layer is higher, an
open-circuit voltage is increased and electron-hole recombination
is reduced, and since a band gap on a back surface layer is high,
an electron mobility is increased, which results in that and the
efficiency of a solar cell is improved. If the CZTSe-based
composite thin film further includes a Cu.sub.2ZnSn(Se,S).sub.4
thin film below the Cu.sub.2ZnSnSe.sub.4 thin film, the CZTSe-based
composite thin film may include thin films having three different
band gaps. In this case, the CZTSe-based composite thin film is
manufactured to have band gaps which are not simply increased or
decreased but have a dual or triple slope and include the highest
band gap at an upper part by introducing Ge or Si, the lowest band
gap at an intermediate part including Se only, and a high band gap
at a lower part including S. The band gap slopes at the upper part
and the intermediate part lead to absorb light in a broad
wavelength region, and, thus, can improve the efficiency. Further,
the band gap slopes at the intermediate part and the lower part
lead to inhibit electron-hole recombination, and, thus, can
contribute to improvement of the efficiency of a solar cell using
the same.
[0066] In a third aspect of the present disclosure, there is
provided a method for preparing a solar cell using a CZTSe-based
composite thin film, including: forming a back electrode on a
substrate; and forming a light absorbing layer including a
CZTSe-based composite thin film prepared by the method in
accordance with the first aspect on the back electrode. All the
descriptions relating to the first aspect and the second aspect of
the present disclosure may be applied to the CZTSe-based composite
thin film in accordance with the present aspect.
[0067] In an embodiment of the present disclosure, the CZTSe-based
composite thin film includes a Cu.sub.2ZnSnSe.sub.4 thin film and
the Cu.sub.2Zn(Sn.sub.1-x, M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film, and the component M includes Ge
and/or Si.
[0068] In an embodiment of the present disclosure, the CZTSe-based
composite thin film includes a Cu.sub.2ZnSnSe.sub.4 thin film and
the Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film, and a Cu.sub.2ZnSn(Se.sub.1-y,
S.sub.y).sub.4 thin film (0.ltoreq.y.ltoreq.1) may be further
formed below the Cu.sub.2ZnSnSe.sub.4 thin film, but may not be
limited thereto. By way of example, a Cu.sub.2ZnSnS.sub.4 thin film
may be further formed below the Cu.sub.2ZnSnSe.sub.4 thin film and
a Cu.sub.2ZnSn(Se.sub.1-y,S.sub.y).sub.4 thin film (0<y<1)
may be formed at an interface between these thin films.
[0069] In an embodiment of the present disclosure, the substrate
may employ glass, metal foil, polyimide, and the like, but may not
be limited thereto.
[0070] In an embodiment of the present disclosure, the back
electrode may include Mo, but may not be limited thereto.
[0071] In a fourth aspect of the present disclosure, there is
provided a solar cell using a CZTSe-based composite thin film,
including: a substrate; a back electrode on the substrate; and a
light absorbing layer including the CZTSe-based composite thin film
in accordance with the second aspect formed on the back
electrode.
[0072] The solar cell using a CZTSe-based composite thin film in
accordance with the present aspect may be prepared by the method in
accordance with the third aspect of the present disclosure. All the
descriptions relating to the first aspect and the second aspect of
the present disclosure may be applied to the CZTSe-based composite
thin film in accordance with the present aspect.
[0073] In an embodiment of the present disclosure, the CZTSe-based
composite thin film includes a Cu.sub.2ZnSnSe.sub.4 thin film and
the Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film, and the component M includes Ge
and/or Si.
[0074] In an embodiment of the present disclosure, the CZTSe-based
composite thin film includes a Cu.sub.2ZnSnSe.sub.4 thin film and
the Cu.sub.2Zn(Sn.sub.1-x,M.sub.x)Se.sub.4 thin film formed on the
Cu.sub.2ZnSnSe.sub.4 thin film, and a Cu.sub.2ZnSn(Se.sub.1-y,
S.sub.y).sub.4 thin film (0.ltoreq.y.ltoreq.1) may be further
formed below the Cu.sub.2ZnSnSe.sub.4 thin film, but may not be
limited thereto. By way of example, a Cu.sub.2ZnSnS.sub.4 thin film
may be further formed below the Cu.sub.2ZnSnSe.sub.4 thin film and
a Cu.sub.2ZnSn(Se.sub.1-y, S.sub.y).sub.4 thin film (0<y<1)
may be formed at an interface between these thin films.
[0075] In an embodiment of the present disclosure, the solar cell
using a CZTSe-based composite thin film according to the present
disclosure includes the CZTSe-based composite thin film in which a
band gap on a front surface layer is high, and, thus, an
open-circuit voltage is increased and electron-hole recombination
is reduced and a band gap on a back surface layer is high, and,
thus, an electron mobility is increased. Thus, the efficiency of
the solar cell is improved, but the present disclosure may not be
limited thereto. Assuming that a solar cell using a conventional
CZTSe thin film has the efficiency of about 3%, when grading is
created using CZTGS, a band gap on a front surface is increased too
much. Thus, it can be seen that an open-circuit voltage is improved
but the efficiency is decreased due to a decrease in a
short-circuit current and a fill factor. Meanwhile, it can be seen
that CZTGSe achieves a voltage improvement and reduces a current
loss, and, thus, achieves the efficiency of about 4% or more which
is improved by about 33% as compared with the conventional CZTSe
thin film. In particular, unlike S grading in CIGSSe, it has not
yet been reported that S grading in CZTGSSe is achieved. If a
surface treatment is performed using Ge and/or Si as described in
the present example, it is possible to further improve device
characteristics than the conventionally known efficiency. In this
regard, S changes a valence band, whereas Ge changes a conduction
band. Therefore, as described in an embodiment of the present
disclosure, if a solar cell is optimized by creating band gap
grading using Ge and/or Si, it is possible to achieve the
efficiency of about 15% or more.
[0076] If a solar cell is optimized by creating band gap grading,
it is possible to suppress reduction of electron-hole interface
recombination and an efficiency loss caused by an electric field on
a back surface through back grading and front grading of a
conventional device. Thus, it is possible to achieve the efficiency
of about 4% or more. In particular, unlike S grading in CIGSSE, it
has not yet been reported that S grading in CZTGSSe is achieved. If
a surface treatment is performed using Ge and/or Si as described in
the present example, it is possible to further improve device
characteristics than the conventionally known efficiency. In this
regard, S changes a valence band, whereas Ge changes a conduction
band. Therefore, as described in an embodiment of the present
disclosure, if a solar cell is optimized by creating band gap
grading using Ge and/or Si, it is possible to achieve the
efficiency of about 15% or more.
[0077] Accordingly, as described in an embodiment of the present
disclosure, if a solar cell is optimized by creating band gap
grading using Ge and/or Si, it is possible to achieve the
efficiency of about 15% or more.
[0078] Hereinafter, the present disclosure will be explained in
detail with reference to examples. However, the following examples
are provided only for understanding of the present disclosure, but
the present disclosure is not limited thereto.
MODE FOR CARRYING OUT THE INVENTION
Example
Example 1
Preparation of Cu.sub.2ZnSnSe.sub.4 (CZTSe) Thin Film Using
Co-Evaporation
[0079] (1) Preparation of Cu.sub.2ZnSnSe.sub.4 Thin Film
[0080] A CZTSe thin film was prepared on a metal substrate using
Cu.sub.2ZnSnSe.sub.4 (Cu--Zn--Sn--Se precursor)(layer) including
all of Cu, Zn, Sn, and Se.
[0081] When the thin film was deposited, a temperature was
500.degree. C. and a deposition pressure was 1.times.10.sup.-5
torr. The thin film was deposited for about 60 minutes. Finally, a
Cu.sub.2ZnSnSe.sub.4 thin film was formed to a thickness of from
1,200 nm to 1,300 nm.
[0082] (2) Selenization of Cu.sub.2ZnSnSe.sub.4 Thin Film
[0083] After the Cu.sub.2ZnSnSe.sub.4 thin film was formed,
selenization was additionally performed using a Se metal source for
about 20 minutes in order to improve crystallinity of the thin
film. In this case, the temperature was 500.degree. C. and the
pressure was 1.times.10.sup.-5 torr as did in the above
deposition.
Example 2
Preparation of Cu.sub.2ZnSnSe.sub.4 Thin Film Using a
Layer-by-Layer Deposition
[0084] (1) Preparation of Cu.sub.2ZnSnSe.sub.4 Thin Film
[0085] A thin film was deposited in a layer-by-layer manner on a
metal substrate using Cu metal, Zn metal and Sn metal sources by a
vacuum evaporation method. In this case, a Cu/Zn/Sn layered thin
film was formed in order of the above-described precursors. In the
layered thin film, Sn was deposited as the lowermost layer, which
is due to that if Sn is present at an upper part, a CZTSe thin film
cannot be formed well due to Sn-loss. Further, the reason why Cu
was deposited at the uppermost part is that it is possible to
improve a grain size and it is easy to adjust a composition. Thus,
the thin film was formed using the Cu/Zn/Sn layered structure. Each
component layer was formed to a thickness of about 1,200 nm. A
composition of the layered thin film was adjusted by adjusting a
thickness of each layer. When the layered thin film was formed, a
temperature of the substrate was about 500.degree. C. and a
deposition pressure was 1.times.10.sup.-5 torr.
[0086] As described above, after the layered thin film including
components Cu, Zn, and Sn was formed, selenization was performed to
the layered thin film at a temperature of 500.degree. C. under a
deposition pressure of 1.times.10.sup.-5 torr for about 20 minutes
in order to improve crystallinity of the thin film. Finally, a
Cu.sub.2ZnSnSe.sub.4 thin film was prepared.
[0087] (2) Preparation of Cu.sub.2ZnSnSe.sub.4 Thin Film
(Selenization in a Separate Furnace)
[0088] Alternatively, in order to perform selenization to the
prepared layered thin film including components Cu, Zn, and Sn, the
Cu.sub.2ZnSnSe.sub.4 thin film was put into a quartz furnace and
selenization was performed thereto at a temperature of 580.degree.
C. under a deposition pressure of 1.times.10.sup.-3 torr for about
20 minutes. Through this process, grains of CZTSe were formed to a
size of several hundreds nm to several .mu.m or more. Solar
absorption was improved by adjusting a grain size.
Example 3
Preparation of Cu.sub.2ZnSnSe.sub.4 Thin Film Formed on
Cu.sub.2ZnSn(Se,S).sub.4 Thin Film
[0089] A Cu.sub.2ZnSnSe.sub.4 thin film was prepared in the same
manner as Example 1 or 2, but before the Cu.sub.2ZnSnSe.sub.4 thin
film was prepared, a Cu.sub.2ZnSnS.sub.4 (CZTS) thin film was
formed below the light absorbing layer using a SnS metal source,
ZnS, and a Cu metal source, a CZTSe thin film was formed thereon
(using the method of Examples 1 and 2) and then a CZTGSe thin film
was formed by introducing Ge into a surface of the CZTSe thin film
(see Example 4 to be described below), so that front band gap
grading (CZTGSe thin film) and back band gap grading (CZTS thin
film) were created.
Example 4
Preparation of Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 (CZTGSe)
Thin Film
[0090] A Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 thin film was
formed on a surface of the Cu.sub.2ZnSnSe.sub.4 thin film prepared
in Example 1 to Example 3 by depositing Ge on the surface of the
Cu.sub.2ZnSnSe.sub.4 thin film (see FIG. 1). To be specific, the Ge
was deposited on the surface of the Cu.sub.2ZnSnSe.sub.4 thin film
to a thickness of from 30 nm to 1,000 nm. In this case, the Ge was
deposited at a temperature in the range of from 200.degree. C. to
500.degree. C. under a pressure in the range of from
5.times.10.sup.-6 torr to 5.times.10.sup.-5 torr for from about 10
minutes to about 30 minutes (5-minute intervals). Finally, a
CZTSe-based thin film including the
Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 thin film on the
Cu.sub.2ZnSnSe.sub.4 thin film was formed. If a thickness of Ge was
greater than 100 nm, a Sn-loss is worsened and an amount of Ge
becomes greater than that of the CZTSe thin film, and, thus, it is
difficult to form a proper thin film.
[0091] It is possible to adjust a composition of the CZTGSe thin
film and a diffusion rate of Ge by adjusting a thickness of Ge and
a heat treatment time. If a heat treatment condition for Ge is
higher than 400.degree. C., all of Sn forming CZTSe is lost and a
proper thin film cannot be formed. Further, at 400.degree. C. or
higher, Ge is not deposited but lost immediately. Thus, a heat
treatment temperature of 400.degree. C. or less was applied. The
CZTSe-based thin film had a structure of a
Cu.sub.2ZnSnSe.sub.4/Cu.sub.2Zn(Sn.sub.1-xGe.sub.x)Se.sub.4 thin
film (see FIG. 1(a)) or Cu.sub.2ZnSn(Se.sub.1-y,
S.sub.y).sub.4/Cu.sub.2ZnSnSe.sub.4/Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub-
.4 thin film (see FIG. 1(b)). FIG. 2 is a graph showing temperature
conditions according to a deposition time.
[0092] The following Table 1 shows a result of a heat treatment of
the CZTGSe thin film performed at 300.degree. C. to minimize loss
of Sn and Ge and adjust a ratio of Ge/(Ge+Sn). In particular, a
grain shape on the surface was adjusted by varying a heat treatment
time, and a diffusion rate of Ge and a ratio of Ge/(Ge+Sn) were
adjusted.
TABLE-US-00001 TABLE 1 Cu/(Zn + Zn/(Ge + Ge/(Ge + Substance Ge
Condition Cu Zn Ge Sn Se Sn + Ge) Sn) Sn) (a) 150 CZTGSe 100 nm
300.degree. C., 10 mm 20.54 21.41 6.01 7.86 44.19 0.58 1.54 0.43
(b) 162 CZTGSe 100 nm 300.degree. C., 15 min 16.95 18.81 3.85 14.63
45.77 0.45 1.02 0.21 (c) 160 CZTGSe 100 nm 300.degree. C., 20 min
21.31 17.87 7.43 10.07 43.31 0.60 1.02 0.42 (d) 157 CZTGSe 100 nm
300.degree. C., 30 min 18.50 12.44 15.72 15.44 37.91 0.42 0.40
0.50
Experimental Example 1
Field Emission Scanning Electron Microscopic (FE-SEM) Analysis
[0093] A FE-SEM analysis (JSM-6700F, acceleration voltage of 10 kV)
was conducted to the Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 thin
film formed on the surface of the Cu.sub.2ZnSnSe.sub.4 thin film
prepared in Example 4. A result thereof was as shown in FIG. 3 and
FIG. 4A to FIG. 4D. To be specific, as shown in FIG. 3, it was
observed that grains of the Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4
thin film were sufficiently grown and prepared to make it easy to
improve light absorbance and transport electrons and holes.
Experimental Example 2
Measurement and Analysis of Band Gap
[0094] A band gap of the Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4
thin film formed on the surface of the Cu.sub.2ZnSnSe.sub.4 thin
film prepared in Example 4 was measured [Perkin-Elmer UV/VIS
spectrometer (Lambda 750), wavelength range of from 200 nm to 1,200
nm]. A result thereof was as shown in FIG. 5. To be specific, FIG.
5 illustrates measured light transmission according to a
wavelength, and a wavelength (i.e., corresponding to light energy)
meeting an extrapolation line indicates a band gap of a
substance.
[0095] Further, a change in a band gap depending on an amount of Ge
substituting for Sn in the Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4
thin film was measured and shown in FIG. 6. It can be seen that as
an amount of Ge substituting for Sn increases, a band gap also
increases.
[0096] In case of the thin film prepared in Example 3, the CZTS
thin film was formed below the CZTSe thin film in order to create
back grading of the CZTSe thin film. Thus, when an electric field
was formed within a light absorbing layer and electrons and holes
were formed by solar light, the CZTS thin film acts to assist the
electrons in moving well toward a back electrode due to a potential
difference.
[0097] In the thin film prepared in Example 4, Ge was introduced
into the CZTSe thin film to create front grading and improve a band
gap on a surface of a light absorbing layer, and, thus, an
open-circuit voltage was improved. Therefore, a band offset was
reduced at the time of interfacial bonding with a buffer layer
formed on the light absorbing layer when a solar cell was prepared,
thereby reducing electron-hole recombination. Therefore, it is
possible to obtain the advantage in efficiency when using in the
solar cell. In this regard, if the band bap is increased to 1.4 eV
or more, a short-circuit current and a fill factor decrease, which
may cause a loss of efficiency. Therefore, Ge grading needs to be
created such that a band gap is formed in the range of 1.2 eV to
1.4 eV (see FIG. 11).
[0098] A thickness of the Ge grading is very important in
determining the efficiency when using in the solar cell. As
described above, if a band gap is higher than 1.4 eV in the front
grading, a great loss of efficiency may occur due to a loss of a
short-circuit current. Therefore, it is important to create grading
with a band gap of 1.4 eV or less. Accordingly, x in
Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4 needs to be set in the
range of from 0.2 to 0.5 and a graded depth needs not to exceed 100
nm (see FIG. 12).
Experimental Example 3
Measurement of Element Composition According to Depth
[0099] While the CZTSe-based thin film prepared in Example 4 was
etched by sputtering, an element composition was measured according
to a depth (Secondary ion mass spectroscopy, IMS-4FE7). A result
thereof was as shown in FIG. 7. The measurement was carried out
using a Cs.sup.+ ion gun under conditions including impact energy
of 5.5 keV, a current of 20 mA, and an analysis area of 33 .mu.m.
On the surface, Ge was present in substitution for Sn, and a large
amount of Se was included in the center. At the interface, a large
amount of S was present and an amount of Se was decreased. Cu and
Zn were uniformly distributed overall. Distribution of Se and Cu in
a small amount on the surface was a measure to improve the
performance of a p-n junction.
[0100] According to the element composition according to a depth in
the CZTGSe-based thin film prepared in Example, it can be seen from
FIG. 8A to FIG. 8D that as a heat treatment time increased, a
diffusion rate of Ge distributed on the surface toward the CZTSe
light absorbing layer increased (Ge diffusion depth=about 100 nm).
Therefore, it is possible to control a depth of band gap grading by
adjusting the heat treatment time. Accordingly, it is possible to
reduce a ratio of recombination and also possible to improve an
open-circuit voltage by controlling the depth of band gap
grading.
Experimental Example 4
X-Ray Diffraction and Raman Spectroscopy Analysis
[0101] FIG. 9 is an XRD result showing crystallinity of a thin film
[X-ray diffraction--New D8 Advance, Bruker]. It can be seen that
since a CZTSe phase appeared in (112), (200)/(204), (312), and
(316) directions, the thin film was properly formed of CZTSe. From
FIG. 10A to FIG. 10D showing a Raman spectrum measurement result
[Raman Spectroscopy--McPherson 207 spectrometer equipped with a
nitrogen-cooled charged-coupled device (CCD)], it was confirmed
that a CZTSe phase was uniformly distributed when a laser
wavelength was varied (observation was conducted at various depths
of penetration), and it was observed that a main peak of CZTSe had
a high intensity at 300.degree. C. between 10 minutes and 20
minutes. Further, it was confirmed that since a peak of CZTSe was
not much changed according to a laser wavelength, a phase was
properly formed, and it was observed that secondary phases such as
MoSe.sub.2 and ZnSe were present.
Example 5
Preparation of Solar Cell Using CZTSe-Based Composite Thin Film
Including CZTGSe Thin Film
[0102] A molybdenum (Mo) layer was coated to a thickness of 600 nm
as a back electrode on a glass substrate. The Cu.sub.2ZnSnSe.sub.4
thin film and/or the Cu.sub.2Zn(Sn.sub.1-x,Ge.sub.x)Se.sub.4
(CZTGSe) thin film prepared in the same manner as Example 4 was
formed on the molybdenum layer, and a CdS layer as a buffer layer,
an i-ZnO/Al--ZnO as a transparent electrode layer, and an Al layer
as an electrode were laminated in sequence, so that a thin film
solar cell was prepared.
[0103] Assuming that a solar cell using a conventional CZTSe thin
film has the efficiency of about 3%, when grading is created using
CZTGS, a band gap on a front surface is increased too much. Thus,
it can be seen that an open-circuit voltage is improved but the
efficiency is decreased due to a decrease in a short-circuit
current and a curve factor. Meanwhile, it can be seen that a
CZTSe-based composite thin film including CZTGSe achieves a voltage
improvement and reduces a current loss, and, thus, achieves the
efficiency of about 4% or more which is improved by 25% as compared
with the conventional CZTSe thin film (see FIG. 13).
[0104] If a solar cell is optimized by creating band gap grading,
it is possible to decrease reduction of electron-hole interface
recombination and suppress an efficiency loss caused by an electric
field on a back surface through back grading and front grading of a
conventional device. Thus, it is possible to achieve the efficiency
of about 4% or more. In particular, unlike S grading in CIGSSe, it
has not yet been reported that S grading in CZTGSSe is achieved. If
a surface treatment is performed using Ge and/or Si as described in
the present example, it is possible to further improve device
characteristics than the conventionally known efficiency. In this
regard, S changes a conduction band, whereas Ge changes a valence
band. Therefore, it has the advantage of further reducing a
conduction band offset than S grading, which is very advantageous
in improving the efficiency. For this reason, even if S grading is
successfully accomplished, theoretically, it is difficult to
achieve efficiency improvement of 0.4% or more.
[0105] Accordingly, as described in the present embodiment, if a
solar cell is optimized by creating band gap grading using Ge
and/or Si, it is possible to achieve the efficiency of about 15% or
more.
[0106] The above description of the present disclosure is provided
for the purpose of illustration, and it would be understood by
those skilled in the art that various changes and modifications may
be made without changing technical conception and essential
features of the present disclosure. Thus, it is clear that the
above-described embodiments are illustrative in all aspects and do
not limit the present disclosure. For example, each component
described to be of a single type can be implemented in a
distributed manner. Likewise, components described to be
distributed can be implemented in a combined manner.
[0107] The scope of the present disclosure is defined by the
following claims rather than by the detailed description of the
embodiment. It shall be understood that all modifications and
embodiments conceived from the meaning and scope of the claims and
their equivalents are included in the scope of the present
disclosure.
This invention was made with Korean government support under
Project No. 1415132277 sponsored by Ministry of Trade, Industry and
Energy and managed by Korea Institute Energy Technology Evaluation
and Planning (under Research Project Title: Technology Innovation
Program for Knowledge Economy; Subject Title: Development of
non-vacuum-based technology for the deposition of Cu--Zn--Sn--S--Se
thin films for low-cost and high-efficiency thin film solar cells;
Research period: Dec. 1, 2011 through Nov. 30, 2014) with the
beneficiary of sponsorship being Korea Institute of Science and
Technology.
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