U.S. patent application number 16/023859 was filed with the patent office on 2019-10-17 for method for manufacturing cigs thin film for solar cell.
The applicant listed for this patent is KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY. Invention is credited to Yun Jeong HWANG, Byoung Koun MIN, Hyung Suk OH, Gi Soon PARK.
Application Number | 20190319141 16/023859 |
Document ID | / |
Family ID | 67775813 |
Filed Date | 2019-10-17 |
United States Patent
Application |
20190319141 |
Kind Code |
A1 |
MIN; Byoung Koun ; et
al. |
October 17, 2019 |
METHOD FOR MANUFACTURING CIGS THIN FILM FOR SOLAR CELL
Abstract
Methods of manufacturing a CIGS thin film for a solar cell are
provided. According to the method, a CIGS thin film having an ideal
double band gap grade structure with a large particle size may be
obtained by heat-treating a solution-treated CIG oxide thin film by
a three-step chalcogenization process. Accordingly, performance of
the solar cell may be improved.
Inventors: |
MIN; Byoung Koun; (Seoul,
KR) ; PARK; Gi Soon; (Seoul, KR) ; OH; Hyung
Suk; (Seoul, KR) ; HWANG; Yun Jeong; (Seoul,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY |
Seoul |
|
KR |
|
|
Family ID: |
67775813 |
Appl. No.: |
16/023859 |
Filed: |
June 29, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 18/1241 20130101;
C23C 16/0218 20130101; C23C 16/305 20130101; H01L 31/0272 20130101;
H01L 31/0322 20130101; C23C 18/1295 20130101; C23C 18/1216
20130101; H01L 31/03923 20130101; H01L 31/0749 20130101 |
International
Class: |
H01L 31/032 20060101
H01L031/032; H01L 31/0392 20060101 H01L031/0392; C23C 16/02
20060101 C23C016/02; C23C 16/30 20060101 C23C016/30 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2018 |
KR |
10-2018-0044533 |
Claims
1. A method of manufacturing a CIGS thin film for a solar cell, the
method comprising: first heat-treating a CIG oxide thin film coated
on a substrate by a solution process, the heat-treating being
performed under an inert gas atmosphere; second heat-treating the
CIG oxide thin film while supplying a gaseous phase selenium
precursor to the CIG oxide thin film, thereby forming a
Cu.sub.2-xSe (0.ltoreq.x<1) phase; and third heat-treating the
thin film in which the Cu.sub.2-xSe phase is formed under an
atmosphere comprising a gaseous phase sulfur precursor, thereby
forming a CIGS thin film.
2. The method of claim 1, wherein the first heat-treating is
performed at a temperature of about 200.degree. C. to about
400.degree. C. for about 5 minutes to about 90 minutes.
3. The method of claim 1, wherein the second heat-treating
comprises raising a temperature to a higher temperature than that
of the first heat-treating.
4. The method of claim 1, wherein the second-treating is performed
by heat-treating the CIG oxide thin film at a temperature of about
200.degree. C. to about 600.degree. C. for about 5 minutes to about
120 minutes under an inert gas atmosphere while supplying the
gaseous phase selenium precursor to the CIG oxide thin film.
5. The method of claim 1, wherein the gaseous phase selenium
precursor is selenium vapor.
6. The method of claim 1, wherein the third heat-treating comprises
raising a temperature to a higher temperature than that of the
second heat-treating.
7. The method of claim 1, wherein the third heat-treating is
performed by increasing the temperature stepwise from the
temperature of the second heat-treating.
8. The method of claim 1, wherein the third heat-treating is
performed at a temperature about 10.degree. C. to about 100.degree.
C. higher than that of the second heat-treating.
9. The method of claim 1, wherein the gaseous phase sulfur
precursor is H.sub.2S.
10. The method of claim 1, wherein the CIG oxide thin film is
obtained by coating a solution comprising Cu, In, and Ga precursors
in an alcohol solvent on the substrate and heat-treating the coated
solution under an air atmosphere.
11. The method of claim 10, wherein the coating is performed by at
least one solution process selected from spin coating, doctor
blading, and screen printing.
12. The method of claim 1, wherein the substrate comprises at least
one selected from molybdenum, fluorine tin oxide, and indium tin
oxide.
13. The method of claim 1, wherein the CIGS thin film has a double
band gap grade structure.
14. A method of manufacturing a junction structure of a buffer
layer and a CIGS thin film for a solar cell, the method comprising:
manufacturing a CIGS thin film for a solar cell according to the
method of claim 1; and forming a buffer layer comprising cadmium
zinc sulfide on the CIGS thin film.
15. A method of manufacturing a solar cell, the method comprising:
forming a CIGS thin film on a first electrode by using the method
of claim 1; and forming a second electrode on the CIGS thin
film.
16. The method of claim 15, wherein the first electrode comprises
at least one selected from molybdenum, fluorine tin oxide, and
indium tin oxide.
17. The method of claim 15, wherein the second electrode comprises
at least one selected from molybdenum, fluorine tin oxide, indium
tin oxide, nickel, and aluminum.
18. The method of claim 15, wherein the solar cell further
comprises a buffer layer comprising cadmium zinc sulfide.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of Korean Patent
Application No. 10-2018-0044533, filed on Apr. 17, 2018, in the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein in its entirety by reference.
BACKGROUND
1. Field
[0002] One or more embodiments relate to a method of manufacturing
a CIGS thin film for a solar cell, and more particularly, to a
method of manufacturing a CIGS thin film for a solar cell capable
of improving solar cell performance, a method of manufacturing a
junction structure of a buffer layer and a CIGS thin film for a
solar cell, a method of manufacturing a solar cell using the method
of manufacturing a CIGS thin film, and a solar cell including a
CIGS thin film manufactured according to the method of
manufacturing a CIGS thin film.
2. Description of the Related Art
[0003] Photovoltaic cells, i.e., solar cells, refer to devices
capable of converting solar energy into electrical energy.
Particularly, when light is incident on a photosensitive material
included in the photovoltaic cell, electrons and holes created via
photovoltaic effects generate current-voltage. Since such a
photovoltaic cell may obtain electrical energy from pollution-free
solar energy, which is the source of all energy, extensive research
and development have been carried out in terms of development of
alternative energy sources.
[0004] Solar cells are classified into various types according to a
material used to form a light absorbing layer, and silicon solar
cells using silicon wafers have been the most widely used solar
cells in recent years. However, price competitiveness of silicon
solar cells has reached a limit, and thus, in order to further
enhance price competitiveness, thin-film solar cells have drawn
considerable attention. Since thin-film solar cells are
manufactured with a small thickness, the consumption of materials
and total weight may be reduced. Thus, thin film solar cells may be
used in a wide variety of applications.
[0005] A thin-film solar cell including a chalcopyrite thin film as
a light absorbing layer, the chalcopyrite thin film including
elements of Groups IB, IIIA, and VIA known as CIS or CIGS, is one
of the well-known types of thin-film solar cells. In general, a
light absorbing layer having a Cu(In,Ga)(S,Se).sub.2 (CIGS)
composition is one of the most important factors determining
performance of solar cells.
[0006] A CIS or CIGS light absorbing layer is generally
manufactured by coevaporating or sputtering metal elements.
Particularly, a CIS or CIGS thin film may generally be deposited by
coevaporating three elements using several operations.
Alternatively, a CIS or CIGS thin film may be manufactured by
sputtering metal targets such as Cu, In, and Ga and performing a
selenization process. However, since these processes are performed
under vacuum conditions, high-priced vacuum equipment is required.
In addition, the use of such vacuum equipment may not only cause
considerable losses of high-priced raw materials such as indium or
gallium but may also make large-area production and a high
processing speed more difficult to achieve.
[0007] Solution processes, as low-priced chemical methods that do
not use vacuum equipment, have been used to replace vacuum
deposition processes. Solution processes are cost effective and
suitable for mass production. For example, a method of
manufacturing a CIGS thin film by using a highly reactive hydrazine
solution capable of directly dissolving a precursor metal compound
has been known in the art. However, since the hydrazine solution is
highly toxic and reactive, an additional device is required to
maintain an inert atmosphere during manufacturing processes. Thus,
there is a need for a non-toxic and easy-to-perform method to
improve the benefits of solution processes.
[0008] Although much progress has been made in solar cell
performance, there is a great difference in efficiency between
vacuum processes and solution processes. Thus, there is a need for
a solution process for manufacturing a CIGS thin film capable of
improving solar cell performance.
SUMMARY
[0009] One or more embodiments include a method of manufacturing a
CIGS thin film for a solar cell, the method capable of obtaining a
solution-processed CIGS thin film suitable for interface
engineering, in order to improve performance of the solar cell.
[0010] One or more embodiments include a method of manufacturing a
junction structure of a buffer layer and a CIGS thin film for a
solar cell by using the method of manufacturing a CIGS thin
film.
[0011] One or more embodiments include a method of manufacturing a
solar cell including the method of manufacturing a CIGS thin
film.
[0012] One or more embodiments include a solar cell including the
CIGS thin film manufactured according to the method of
manufacturing a CIGS thin film.
[0013] Additional aspects will be set forth in part in the
description which follows and, in part, will be apparent from the
description, or may be learned by practice of the presented
embodiments.
[0014] According to one or more embodiments, a method of
manufacturing a CIGS thin film for a solar cell includes first
heat-treating a CIG oxide thin film coated on a substrate by a
solution process, the heat-treating being performed under an inert
gas atmosphere, second heat-treating the CIG oxide thin film while
supplying a gaseous phase selenium precursor to the CIG oxide thin
film, thereby forming a Cu.sub.2-xSe (0.ltoreq.x<1) phase, and
third heat-treating the thin film in which the Cu.sub.2-xSe phase
is formed under an atmosphere including a gaseous phase sulfur
precursor, thereby forming a CIGS thin film.
[0015] According to one or more embodiments, a method of
manufacturing a junction structure of a buffer layer and a CIGS
thin film for a solar cell includes manufacturing a CIGS thin film
for a solar cell according to the method of manufacturing a CIGS
thin film, and forming a buffer layer including cadmium zinc
sulfide on the CIGS thin film.
[0016] According to one or more embodiments, a method of
manufacturing a solar cell includes forming a CIGS thin film on a
first electrode by using the method of manufacturing a CIGS thin
film, and forming a second electrode on the CIGS thin film.
[0017] According to one or more embodiments, a solar cell includes
a CIGS thin film manufactured according to the method of
manufacturing a CIGS thin film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] These and/or other aspects will become apparent and more
readily appreciated from the following description of the
embodiments, taken in conjunction with the accompanying drawings in
which:
[0019] FIG. 1 is a schematic diagram for describing a method of
manufacturing a CIGS thin film for solar cells, according to an
embodiment;
[0020] FIG. 2 is a graph exemplarily illustrating a temperature
profile of respective operations of a chalcogenization process
according to an embodiment;
[0021] FIG. 3 is a graph illustrating X-ray diffraction (XRD)
patterns of a CIGS thin film manufactured according to Example 1,
in respective operations of a three-step chalcogenization
process;
[0022] FIG. 4 shows scanning electron microscope (SEM) images of
surfaces of the CIGS thin film manufactured according to Example 1,
in respective operations of the three-step chalcogenization
process;
[0023] FIG. 5 shows SEM cross-sectional images of the CIGS thin
film manufactured according to Example 1, in respective operations
of the three-step chalcogenization process;
[0024] FIG. 6 is a graph illustrating an atomic depth profile of
the CIGS thin film manufactured according to Example 1, analyzed by
using a dynamic secondary ion mass spectrometer (D-SIMS);
[0025] FIG. 7 shows schematic band diagrams of a CdS/CIGS p-n
structure and a (Cd,Zn)S/CIGS p-n junction structure;
[0026] FIG. 8 is a graph illustrating photocurrent-voltage curves
of unit cells of solar cells manufactured using CIGS thin films
prepared according to Example 1 and Comparative Example 1, for
confirming the quality of the CIGS thin film manufactured according
to Example 1;
[0027] FIG. 9 is a graph illustrating photocurrent-voltage curves
of CIGS thin film solar cells manufactured according to Examples 2
and 3;
[0028] FIG. 10 is a graph illustrating external quantum
efficiencies (EQE) of the CIGS thin film solar cells manufactured
according to Examples 2 and 3, wherein the EQE was analyzed by
photon-to-current conversion efficiency; and
[0029] FIG. 11 is a graph illustrating Aln(J.sub.0) vs 1/kT plots
of a CdS/CIGS sample of Example 2 and a (Cd,Zn)S/CIGS sample of
Example 3 obtained by temperature-dependent current density voltage
(J-V-T) analysis performed in dark conditions with no light.
DETAILED DESCRIPTION
[0030] Reference will now be made in detail to embodiments,
examples of which are illustrated in the accompanying drawings,
wherein like reference numerals refer to like elements throughout.
In this regard, the present embodiments may have different forms
and should not be construed as being limited to the descriptions
set forth herein. Accordingly, the embodiments are merely described
below, by referring to the figures, to explain aspects of the
present description. As used herein, the term "and/or" includes any
and all combinations of one or more of the associated listed items.
Expressions such as "at least one of," when preceding a list of
elements, modify the entire list of elements and do not modify the
individual elements of the list.
[0031] Hereinafter, a method of manufacturing a CIGS thin film for
solar cells, a method of manufacturing a junction structure of a
buffer layer and a CIGS thin film for a solar cell and a method of
manufacturing a solar cell using the method of manufacturing a CIGS
thin film, and a solar cell including a CIGS thin film manufactured
according to the method of manufacturing a CIGS thin film.
[0032] Although much progress has been made in performance of solar
cells, insufficient growth of particles by solution processes is
one of the many reasons for a big difference in efficiency between
vacuum processes and solution processes. This is because boundaries
of particles may act as recombination centers leading to losses of
photovoltaic carriers. Thus, present inventors have applied a
three-step chalcogenization process to a method of manufacturing a
CIGS thin film having an ideal double band gap grade with a large
particle size to improve performance of the CIGS thin film by a
solution process.
[0033] A method of manufacturing a CIGS thin film for solar cells
according to an embodiment includes a three-step chalcogenization
process. The method of manufacturing a CIGS thin film for solar
cells according to an embodiment includes first heat-treating a CIG
oxide thin film coated on a substrate by a solution process under
an inert gas atmosphere, second heat-treating the CIG oxide thin
film while supplying a gaseous phase selenium precursor, thereby
forming a Cu.sub.2-xSe (0.ltoreq.x<1) phase, and third
heat-treating the thin film in which the Cu.sub.2-xSe phase is
formed under an atmosphere including a gaseous phase sulfur
precursor, thereby forming a CIGS thin film.
[0034] According to the method of manufacturing a CIGS thin film
for solar cells suitable for interface engineering may be obtained
by growing particles derived from Cu.sub.2-xSe and forming a CIGS
thin film having a double band gap grade structure via a three-step
chalcogenization process as described above on the CIG oxide thin
film coated on the substrate. Since the Cu.sub.2-xSe phase formed
during the three-step chalcogenization process acts as a fluxing
agent in a crystallization process, the growth of CIGS particles
may be enhanced and performance of the CIGS thin film may be
improved by a solution process.
[0035] The CIG oxide thin film coated on the substrate may be
formed by a solution process. For example, the CIG oxide thin film
may be obtained by coating a solution including Cu, In, and Ca
precursors in an alcohol solvent on the substrate and heat-treating
the coated solution under an air atmosphere. The coating may be
performed by spin coating or doctor blading. For example, the CIG
oxide thin film may be obtained by coating a paste including Cu,
In, and Ga precursors on a substrate by spin coating or doctor
blading and heat-treating the coated paste under an air atmosphere
at a temperature of about 250.degree. C. to about 350.degree. C.
for about 1 minute to about 60 minutes. By the heat treatment,
carbon impurities are removed from the thin film and Cu, In, and Ga
react with oxygen to form a noncrystalline CIG oxide thin film.
[0036] Although the CIG oxide includes Cu, In, and Ga as main
components but may further include other doping components.
[0037] In this regard, the substrate may include at least one
substrate coated with a conductive layer selected from, for
example, molybdenum (Mo), fluorine tin oxide (FTO), and indium tin
oxide (ITO).
[0038] The CIG oxide thin film coated on the substrate by the
solution process as described above may be subjected to the
three-step chalcogenization process to form a CIGS thin film.
[0039] FIG. 1 is a schematic diagram for describing a method of
manufacturing a CIGS thin film for solar cells according to an
embodiment. FIG. 2 is a graph exemplarily illustrating a
temperature profile of respective operations of a chalcogenization
process according to an embodiment.
[0040] As illustrated in FIG. 1, the CIG oxide thin film coated on
the substrate may be subjected to the three-step chalcogenization
process to obtain a CIGS thin film for solar cells. By the
three-step chalcogenization process, the growth of CIGS particles
may be promoted by forming the Cu.sub.2-xSe phase by using a
2-stage tube furnace in which a sample and a Se source (e.g., Se
pellets) are separated from each other and a CIGS thin film having
a desired double band gap grade structure may be manufactured.
[0041] In the first heat-treating of the chalcogenization process,
the first heat-treating may be performed under an inert N.sub.2 gas
atmosphere. By the first heat-treating, an amount of oxygen atoms
present in the substrate is reduced, and thus formation of an oxide
layer by a substrate-derived component may be inhibited at a
CIGS/substrate interface. For example, the first heat-treating of
the CIG oxide thin film coated on the Mo substrate under an inert
N.sub.2 gas atmosphere may prevent formation of a MoO.sub.3 layer.
Since the MoO.sub.3 layer inhibits formation of a MoSe.sub.2 layer,
formation of the MoO.sub.3 layer is undesirable. The MoSe.sub.2
layer is well known to improve adhesion at a CIGS/Mo interface.
[0042] The first heat-treating may be performed at a temperature
of, for example, about 200.degree. C. to about 400.degree. C. for
about 5 minutes to about 90 minutes. Within these ranges, the
amount of oxygen atoms present in the substrate may be reduced.
[0043] The method may further include cooling the CIG oxide thin
film before the second heat-treating after the first
heat-treating.
[0044] In the second heat-treating step of the chalcogenization
process, the CIG oxide thin film is heat-treated while a gaseous
phase selenium (Se) precursor is supplied thereto to form a
Cu.sub.2-xSe (0.ltoreq.x<1) phase. The second heat-treating may
also be performed under an inert N.sub.2 gas atmosphere. For
example, in the second heat-treating, Se vapor may be provided to
the CIG oxide thin film by heating Se pellets under an inert
N.sub.2 gas atmosphere.
[0045] The Cu.sub.2-xSe phase may be formed by the second
heat-treating. Since the Cu.sub.2-xSe phase acts as a fluxing agent
during a crystallization process, the growth of CIGS particles may
be promoted.
[0046] The second heat-treating may include raising a temperature
to a higher temperature than that of the first heat-treating.
[0047] According to an embodiment, the CIG oxide thin film may be
heat-treated at a temperature of about 200.degree. C. to about
600.degree. C. under an inert gas atmosphere while supplying the
gaseous phase selenium precursor thereto. For example, the CIG
oxide thin film may be heat-treated at a temperature of 300.degree.
C. or higher and lower than 450.degree. C. under an inert gas
atmosphere while supplying the gaseous phase selenium precursor
thereto. The second heat-treating may be performed at a temperature
of, for example, about 200.degree. C. to about 600.degree. C. for
about 5 minutes to about 120 minutes. Within theses ranges,
formatin of the Cu.sub.2-xSe phase may be maximized.
[0048] The gaseous phase selenium precursor may be selenium
vapor.
[0049] For example, the gaseous phase selenium precursor may be
supplied by supplying selenium vapor by heating selenium pellets in
the second heating-treating. By using the 2-stage tube furnace in
which a sample and a Se source (Se pellets) are separated from each
other, a CIGS thin film having a desired double band gap grade
structure may be manufactured.
[0050] In the third heat-treating of the chalcogenization process,
the thin film having the Cu.sub.2-xSe phase is heat-treated under
an atmosphere including a gaseous phase sulfur precursor to form
the CISG thin film. the gaseous phase sulfur precursor may be
H.sub.2S.
[0051] The third heat-treating may include raising a temperature to
a higher temperature than that of the second heat-treating. For
example, the third heat-treating may be performed by raising a
temperature to a temperature 10.degree. C. to 100.degree. C. higher
than that of the second heat-treating. Since the third
heat-treating is performed at a higher temperature than that of the
second heat-treating, a band gap may be designed as a double
grading model by adjusting amounts of sulfur and selenium in the
thin film in a non-vacuum solution process. This may alleviate
difficulty in adjusting amounts of elements in a solution process
in the related art to increase effects on realizing a high
efficiency solar cell.
[0052] The third heat-treating may be performed by raising the
temperature stepwise from the temperature of the second
heat-treating. The temperature may be increased in several stages
in an atmosphere in which a gaseous phase sulfur precursor flows to
provide a sufficient time for growing particles derived from the
Cu.sub.2-xSe phase.
[0053] For example, the third heat-treating may be performed at a
temperature of about 400.degree. C. to about 600.degree. C. for
about 5 minutes to about 120 minutes. Within these ranges, a CIGS
thin film having an increased particle size may be obtained.
[0054] The CIGS thin film for solar cells manufactured as described
above may have a large particle size and an ideal double band gap
grade structure. For example, the particle size of the CIGS
particles constituting the CIGS thin film may be in the range of
about 600 nm to about 1000 nm. Since the CIGS particles having a
particle size of about 300 nm to about 400 nm are obtained
according to heat treatment of blowing selenium particles under a
hydrogen sulfide atmosphere from the beginning of the heat
treatment commonly used in the art, the particle size of the CIGS
particles may be doubled by using the method according to the
present embodiment.
[0055] A solar cell according to another embodiment includes the
CIGS thin film for solar cells manufactured as described above.
[0056] According to an embodiment, the solar cell may further
include a buffer layer including cadmium zinc sulfide on the CIGS
thin film. By a p-n junction between the CIGS thin film for solar
cells and the buffer layer including cadmium zinc sulfide and
formed on the thin film, interface recombination may be
prevented.
[0057] Since the surface of the CIGS thin film is rich in S, a
ternary cadmium zinc sulfide (Cd,Zn)S buffer layer may be formed to
create a desirable "spike" conduction band alignment instead of a
"cliff" alignment, thereby inhibiting interface recombination.
[0058] In the p-n junction structure between the CIGS thin film and
the (Cd,Zn)S buffer layer, inhibition of interface recombination is
explained by comparing recombination activation energies of the
buffer layer and the CIGS thin film.
[0059] FIG. 7 shows schematic band diagrams of a CdS/CIGS p-n
junction structure and a (Cd,Zn)S/CIGS p-n junction structure. In
this regard, interface recombination barriers are also shown. This
will be described with reference to FIG. 7.
[0060] A band alignment structure of a p-n junction is one of the
important factors determining performance of a CIGS thin film solar
cell. This is because the band alignment structure is closely
related to the interface recombination barrier Eb that is an energy
difference between a lowest point of a conduction band (CBM) of an
n-type buffer layer and a highest point of a valence band (VBM) of
a p-type CIGS absorbing layer at the p-n junction interface. In
addition, it has been widely reported that a serious interface
recombination is caused in a "cliff" type conduction band alignment
since the "cliff" type conduction band alignment in which a CBM of
a CIGS absorber is higher than a CMB of a butter layer has a
relatively low Eb. On the contrary, interface recommunication may
be inhibited in a "spike" type conduction band alignment in which a
CBM of a CIGS absorber is lower than a CBM of a buffer layer at a
p-n junction interface since the "spike" type conduction band
alignment has a relatively high Eb value. Unless a height of a
"spike" structure exceeds 0.4 eV, disturbance of electron movement
toward the buffer layer is negligible.
[0061] In the case of a CIGS absorber having a wide band gap
favorable for higher photovoltage, it is difficult for a CBM of a
CIGS absorber to form a "spike" conduction band alignment at a p-n
junction interface. Thus, a CIGS absorber which is rich in Ga
and/or S has poor solar cell performance.
[0062] On the contrary, since the surface of the CIGS thin film for
solar cells is rich in S, an excellent "spike" conduction band
alignment may be formed at a p-n junction interface via the p-n
junction with the (Cd,Zn)S buffer layer instead of the CdS buffer
layer. A relatively large recombination activation energy may be
identified in the (Cd,Zn)S buffer layer. This indicates that
interface recombination is considerably inhibited via optimization
of bandgap alignment.
[0063] A method of manufacturing a junction structure of a buffer
layer and a CIGS thin film for a solar cell according to another
embodiment includes manufacturing the CIGS thin film by the
above-described method of manufacturing a CIGS thin film, and
forming a buffer layer including cadmium zinc sulfide on the CIGS
thin film.
[0064] A method of manufacturing a solar cell according to another
embodiment includes forming a CIGS thin film on a first electrode
layer, and forming a second electrode layer on the CIGS thin
film.
[0065] Since the method of manufacturing the CIGS thin film is
described in detail above, descriptions thereof will not be
repeated.
[0066] According to an embodiment, the first electrode may include
at least one selected from molybdenum, fluorine tin oxide, and
indium tin oxide. However, any other conductive and transparent
material may also be used without limitation.
[0067] The second electrode is formed on the CIGS thin film and may
include at least one selected from molybdenum, fluorine tin oxide,
indium tin oxide, nickel, and aluminum.
[0068] The solar cell may further include a buffer layer including
cadmium sulfide or cadmium zinc sulfide. The solar cell may further
include at least one metal oxide selected from titanium oxide, zinc
oxide, and tin oxide.
[0069] The CIGS thin film solar cell obtained as described above
may exhibit remarkably improved power conversion efficiency (PCE)
as compared with CIGS thin film solar cells efficiently
solution-treated except for a toxic and reactive hydrazine
solution.
[0070] One or more embodiments will be described in more detail,
according to the following examples and comparative examples.
However, the following examples are merely presented to exemplify
the present invention, and the scope of the present invention is
not limited thereto.
Example 1: Preparation of CIGS Thin Film
[0071] Copper nitrate hydrate (Cu(NO.sub.3).sub.2.xH.sub.2O,
99.999%, Sigma-Aldrich, 0.94 g), indium nitrate hydrate
(In(NO.sub.3).sub.3.xH.sub.2O, 99.99%, Sigma-Aldrich, 1.15 g), and
gallium nitrate hydrate (Ga(NO.sub.3).sub.3.xH.sub.2O, 99.999%,
Alfa Aesar, 0.49 g) were dissolved in a methanol solvent (8 mL) to
prepare a metal precursor solution. Meanwhile, polyvinyl acetate
(average molecular weight: 100,000 g/mol, Sigma-Aldrich, 1.0 g) was
dissolved in a vinyl acetate solvent (8 mL) and the solution was
vigorously stirred to prepare a binder solution. The two solutions
were mixed at 25.degree. C. for 30 minutes and filtered to obtain a
CIG solution.
[0072] The CIG solution was spin-coated on a molybdenum (Mo) layer,
which was sputtered on a soda-lime glass (SLG) to a thickness of
500 nm, at 2000 rpm for 40 seconds and air-annealed in a box
furnace at 300.degree. C. for 30 minutes. This deposition process
was repeated six times to prepare a CIG oxide thin film having a
thickness of 1 .mu.m.
[0073] The CIG oxide thin film was subjected to a three-step
chalcogenization process to be described below by using a 2-stage
tube furnace in which a sample and Se pellets (99.99%,
Sigma-Aldrich, 0.5 g) are separated from each other to prepare a
CIGS thin film. In the first heat-treating, the prepared CIG oxide
thin film was annealed at 300.degree. C. for 5 minutes under an
inert N.sub.2 gas atmosphere without heating Se pellets. In the
second heat-treating, the sample was heat-treated at 25.degree. C.
for 35 minutes and heated to 400.degree. C. under an inert N.sub.2
gas atmosphere. Meanwhile, the Se pellets were heat-treated at
550.degree. C. for 15 minutes to supply Se vapor to the CIG oxide
thin film. In the third heat-treating, a flowing gas was changed to
H.sub.2S (H.sub.2S(1%)/N.sub.2) and the sample was heated to
475.degree. C. stepwise and strongly sulfurated for 15 minutes to
obtain a CIGS thin film.
Example 2: Preparation of CIGS Thin Film Solar Cell
[0074] A CIGS thin film solar cell was manufactured by forming a
CdS buffer layer having a thickness of about 50 nm on the CIGS thin
film by a chemical wet process using cadmium sulfate (CdSO.sub.4,
99.99%, Sigma-Aldrich, 0.16 g), depositing an i-ZnO (50 nm)/Al:ZnO
(500 nm) window layer by RF sputtering, and depositing an Ni/Al
upper electrode by using an electron beam.
Example 3: Preparation of CIGS Thin Film Solar Cell
[0075] A CIGS thin film solar cell was manufactured in the same
manner as in Example 2, except that a (Cd,Zn)S buffer layer having
a thickness of about 50 nm was formed instead of the CdS buffer
layer by replacing the CdSO.sub.4 precursor (0.08 g) with zinc
sulfate heptahydrate (Zn(SO.sub.4).7H.sub.2O, 99.999%,
Sigma-Aldrich, 0.11 g).
Comparative Example 1
[0076] Copper nitrate hydrate (Cu(NO.sub.3).sub.2.xH.sub.2O,
99.999%, Sigma-Aldrich, 0.82 g), indium nitrate hydrate
(In(NO.sub.3).sub.3.xH.sub.2O, 99.99%, Sigma-Aldrich, 1.12 g), and
gallium nitrate hydrate (Ga(NO.sub.3).sub.3.xH.sub.2O, 99.999%,
Alfa Aesar, 0.41 g) were dissolved in a methanol solvent (8.5 mL)
to prepare a metal precursor solution. Meanwhile, polyvinyl acetate
(average molecular weight: 100,000 g/mol, Sigma-Aldrich, 1.0 g) was
dissolved in a vinyl acetate solvent (8.5 mL) and the solution was
vigorously stirred to prepare a binder solution. The two solutions
were mixed at 25.degree. C. for 30 minutes and filtered to obtain a
CIG solution.
[0077] The CIG solution was spin-coated on a Mo layer, which was
sputtered on a SLG to a thickness of 500 nm, at 2000 rpm for 40
seconds and air-annealed in a box furnace at 300.degree. C. for 30
minutes. This deposition process was repeated six times to prepare
a CIG oxide thin film having a thickness of 1 .mu.m.
[0078] The CIG oxide thin film was subjected to a three-step
chalcogenization process to be described below by using a 2-stage
tube furnace in which a sample and Se pellets (99.99%,
Sigma-Aldrich, 0.5 g) are separated from each other to prepare a
CIGS thin film. First, the CIG oxide thin film and Se pellets were
located on respective stages of the 2-stage tube furnace, and the
Se pelles were maintained at 550.degree. C. for 50 minutes under a
H.sub.2S (H.sub.2S(1%)/N.sub.2) gas atmosphere to supply gaseous
phase Se to the CIGS oxide thin film. In the beginning of the
supplying of the gaseous phase Se to the CIG oxide thin film, the
temperature of the thin film was increased to 500.degree. C. for 25
minutes and maintained for 15 minutes to simultaneously supply S
and Se to the CIG oxide thin film for reactions.
[0079] A CIGS thin film solar cell was manufactured by using the
manufactured CIGS thin film in the same process as in Example
2.
Evaluation Example 1: XRD Analysis
[0080] Phase transformation of the CIGS thin film manufactured
according to Example 1 was identified by X-ray diffraction (XRD)
analysis during the three-step chalcogenization process. FIG. 3
illustrates XRD patterns of the CIGS thin film in respective
operations of the chalcogenization process.
[0081] As shown in FIG. 3, only an Mo peak (110) was observed at
40.5.degree. (JCPDS 42-1120) after the first heat-treating
indicating that there was no significant crystalline phases of Cu,
In, Ga, and O. On the other hand, a strong Cu.sub.2+xSe peak (111)
was observed at 26.98.degree. (JCPDS 06-0680) as a result of
optimized heat treatment in the second heat-treating indicating
that Cu, which is the most reactive among Cu, In and Ga,
selectively reacted with the Se vapor. Finally, chalcopyrite CIGS
peaks were observed at 27.32.degree., 45.82.degree. and
54.53.degree. (JCPDS 35-1101) respectively corresponding to (112),
(220), and (312) planes.
Evaluation Example 2: SEM Analysis
[0082] Scanning electron microscope (SEM) images of surfaces of the
CIGS thin film manufactured according to Example 1 were obtained in
respective operations of the three-step chalcogenization process
and the results are shown in FIG. 4.
[0083] As shown in FIG. 4, a CIGO.sub.x thin film has a porous
surface with nanoparticles after the first heat-treating. Since Cu
atoms may easily diffuse to the surface of the thin film in this
for, the Cu.sub.2-xSe phase may efficiently be formed.
[0084] After the second heat-treating, numerous micron-sized
Cu.sub.2-xSe particles were identified on the surface of an
(In,Ga)O.sub.x thin film indicating selective reactions between Cu
atoms and Se vapor. According to previous reports, formation of a
binary phase of Cu.sub.2-xSe and InSe is started at 270.degree. C.
and a ternary CuInSe.sub.2 is formed at 340.degree. C. However,
although a maximum temperature of the second heat-treating of the
chalcogenization process was 400.degree. C. in Example 1, no phase
other than the Cu.sub.2-xSe phase was formed. It is considered that
this phenomenon is caused by an amorphouse oxide structure of the
thin film which may interfere with reactions between In or Ga and
Se.
[0085] After the third heat-treating, Cu.sub.2-xSe particles
completely disappeared and a CIGS thin film was obtained by heat
treatment under a H.sub.2S gas atmosphere. The surface of the CIGS
thin film was highly dense without having large cracks or
pinholes.
[0086] SEM cross-sectional images of the CIGS thin film
manufactured according to Example 1 in respective operations of the
three-step chalcogenization process are shown in FIG. 5. As shown
in FIG. 5, a cross-section of the thin film after the third
heat-treating shows a large particle size of 700 nm and particles
well connected in the transverse direction. This result indicates
that Cu.sub.2-xSe successfully promotes the growth of CIGS
particles during the third heat-treating.
Evaluation Example 3: D-SIMS Depth Profile Analysis
[0087] An atomic depth profile was analyzed by using a dynamic
secondary ion mass spectrometer (D-SIMS) to investigate a band gap
grade structure of the CIGS thin film manufactured according to
Example 1 and the results are shown in FIG. 6. FIG. 6 illustrates
atomic ratios of Ga/(In+Ga) and S/(S+Se) and a band gap
profile.
[0088] As shown in FIG. 6, an atomic ratio of Ga/(In+Ga)
continuously increases from 0.05 at a surface of the thin film to
0.42 at an CIGS/Mo interface. However, distribution of S/(S+Se) is
divided into two regions. Although a profile of S/(S+Se) is uniform
in a bulk CIGS region (300 nm to 1000 nm), an atomic ratio thereof
rapidly increases on a surface and in a space charge region (SCR, 0
nm to 300 nm) to a maximum value of 0.65. Thus, the bulk region of
CIGS is estimated as back band gap graded mainly due to a monotone
increase of Ga/(In+Ga). In addition, a strong bond of S on the
surface and in the SCR has resulted in front band gap graded. As a
result, it may be confirmed that the double band gap grade
structure is successfully implemented by the three-step
chalcogenization process.
Evaluation Example 4: Evaluation of Performance of CIGS Absorbing
Layer
[0089] The CIGS thin film of Example 1 was compared with a CIGS
thin film of the related art manufactured by the process of
Comparative Example 1 (simultaneously supplying selenium vapor and
H.sub.2S gas) to identify a quality of the CIGS thin film of
Example 1 manufactured according to the three-step chalcogenization
process. Unit cells of CIGS thin film solar cells were manufactured
by using each of the CIGS thin film and a CdS buffer layer and
photocurrent-voltate curves thereof were compared. The analysis was
performed using a Sun2000 solar simulator available from ABET
Technologies (USA) under conditions of 1 SUN (100 mW/cm.sup.2).
[0090] Photocurrent-voltage curve comparison results are shown in
FIG. 8. As shown in FIG. 8, the solar cell manufactured using the
CIGS absorbing layer according to Example 1 by the three-step
chalcogenization process exhibits improved solar cell parameters
such as open voltage, short-circuit current, and filling rate when
compared with the solar cell according to Comparative Example 1.
Particularly, the solar cell of Example 1 had an open voltage about
50 mV higher and a short-circuit current about 2.5 mA/cm.sup.2
higher than those of Comparative Example 1, indicating that the
growth of CIGS particles by Cu.sub.2-xSe reduced recombination
losses. As a result, according to the three-step chalcogenization
process, an efficiency of 12.7%, which is about 1.5% higher than
that of the solar cell of the related art, may be obtained.
Evaluation Example 5: Evaluation of Performance of Solar Cell
According to Buffer Layer
[0091] Current-voltage curves of the CIGS thin film solar cells
manufactured according to Examples 2 and 3 were analyzed by a
Sun2000 solar simulator available from ABET Technologies (USA) was
used for AM 1.5.
[0092] The photocurrent-voltage curves of the CIGS thin film solar
cells are shown in FIG. 9. Voltage (V.sub.oc), current density
(J.sub.sc), filling factor (FF), and conversion efficiency (Eff,
.eta.) calculated from the photocurrent-voltage curves are shown in
Table 1 below. In Table 1, J.sub.sc, V.sub.oc, and FF and n are
obtained from photo J-V curves. Series resistance (R.sub.s), shunt
resistance (R.sub.sh), ideality factor (A), and J.sub.0 were
calculated by using dark J-V curve data.
TABLE-US-00001 TABLE 1 J.sub.sc V.sub.oc FF R.sub.s R.sub.sh
J.sub.0 .eta. [mA cm.sup.-2] [V] [%] [.OMEGA. cm.sup.2] [.OMEGA.
cm.sup.2] A [mA cm.sup.-2] [%] Example 2 32.61 0.549 70.91 1.10
3.57 .times. 10.sup.5 1.64 3.59 .times. 10.sup.-5 12.7 CdS/CIGS
Example 3 34.73 0.584 71.00 0.79 1.32 .times. 10.sup.5 1.58 1.38
.times. 10.sup.-6 14.4 (Cd, Zn)S/CIGS
[0093] As shown in FIG. 9 and Table 1, the CIGS thin film solar
cell of Example 2 had a short circuit current density (J.sub.sc) of
32.61 mAcm.sup.-2, an open circuit voltage (V.sub.oc) of 0.549 V, a
fill factor (FF) of 70.91%, and a PCE(n) of 12.7% indicating
acceptable performance.
[0094] The CIGS thin film solar cell of Example 3 had a short
circuit current density (J.sub.sc) of 34.73 mAcm.sup.-2, an open
circuit voltage (V.sub.oc) of 0.584 V, a fill factor (FF) of
71.00%, and a PCE(n) of 14.4% indicating better performance than
the CdS/CIGS sample of Example 2. The PCE of 14.4% of the
(Cd,Zn)S/CIGS sample according to Example 3 is similar to that of
the CIGS thin film solar cell manufactured by a solution process,
which is the most efficient except for the highly toxic and
reactive hydrazine solution.
[0095] For better understanding of enhancement of J.sub.sc in the
(Cd,Zn)S/CIGS sample, external quantum efficiencies (EQE) of the
CIGS thin film solar cells manufactured according to Examples 2 and
3 were analyzed by photon to current conversion efficiency, and the
results are shown in FIG. 10.
[0096] As shown in FIG. 10, an EQE of the (Cd,Zn)S/CIGS sample is
higher than that of the CdS/CIGS sample in a wavelength range of
300 nm to 550 nm, leading to an additional J.sub.sc gain of 0.79
mAcm.sup.-2. This may be explained in terms of the fact that the
(Cd,Zn)S buffer layer derived by addition of Zn had a transmittance
16.57% higher and a band gap 0.47 eV larger than those of the
CdS/CIGS buffer layer. In addition, a considerable increase in EQE
was observed in a wavelength range of 820 nm to 1170 nm together
with a J.sub.sc gain of 1.60 mAcm.sup.-2, indicating that
inhibition of interface recombination improves collection of
minority carriers generated in a deeper portion of the CIGS
absorber.
[0097] In order to identify inhibition of interface recombination
in a (Cd,Zn)S/CIGS junction, temperature-dependent current density
voltage (J-V-T) analysis was performed on the CdS/CIGS sample of
Example 2 and the (Cd,Zn)S/CIGS of Example 3 at a temperature of
160 K to 300 K in a dark condition with no light. Aln(J.sub.0) vs
1/kT plots of the two samples obtained from dark J-V-T data are
shown in FIG. 11.
[0098] As shown in FIG. 11, the CdS/CIGS sample had an E.sub.a
value of 0.96 eV which is far lower than a band gap of the CIGS
absorber, indicating that a dominant recombination path is formed
in the CdS/CIGS p-n junction interface. Meanwhile, the
(Cd,Zn)S/CIGS sample had an E.sub.a value of 1.30 eV which is
similar to the band gap of the CIGS absorber, indicating that bulk
recombination of the CIGS absorber is more dominant than the
recombination in the (Cd,Zn)S/CIGS interface. Since the dominant
recombination path migrates from the p-n junction interface to the
bulk region of the CIGS absorber, it may be confirmed that
interface recombination is significantly reduced by using the
(Cd,Zn)S buffer layer.
[0099] According to the method of manufacturing a CIGS thin film
for solar cells according to an embodiment, a CIGS thin film having
an ideal double band gap grade structure with a large particle size
may be obtained by performing heat treatment on the
solution-treated CIG oxide thin film by the three-step
chalcogenization process. Accordingly, performance of the solar
cell may be improved.
[0100] It should be understood that embodiments described herein
should be considered in a descriptive sense only and not for
purposes of limitation. Descriptions of features or aspects within
each embodiment should typically be considered as available for
other similar features or aspects in other embodiments.
[0101] While one or more embodiments have been described with
reference to the figures, it will be understood by those of
ordinary skill in the art that various changes in form and details
may be made therein without departing from the spirit and scope of
the disclosure as defined by the following claims.
* * * * *