Synthesis of CZTS nanoparticles for low-cost solar cells

Article abstract of DOI:10.1166/jnn.2016.12175

In this work, uniformly sized Cu₂ZnSnS₄ (CZTS) nanoparticles with easy control of chemical composition were synthesized and printable ink containing CZTS nanoparticles was prepared for low-cost solar cell applications. In addition, we studied the effects of synthesis conditions, such as reaction temperature and time, on properties of the CZTS nanoparticles. For CZTS nanoparticles synthesis process, the reactants were mixed as the 2:1:1:4 molar ratios. The reaction temperature and time was varied from 220°C to 320°C and from 3 hours to 5 hours, respectively. The crystal structure and morphology of CZTS nanoparticles prepared under the various conditions were investigated by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray spectroscopy (EDS) was used for compositional analysis of the CZTS nanoparticles.

Full text of DOI:10.1166/jnn.2016.12175

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 5082–5086, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Synthesis of CZTS Nanoparticles for
Low-Cost Solar Cells
1
1
2
3
4
Donguk Kim , Minha Kim , Joongpyo Shim , Doyoung Kim , Wonseok Choi ,
5
6
1ꢀ∗
Yong Seob Park , Youngkwan Choi , and Jaehyeong Lee
1School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon, 440-746, Republic of Korea
Department of Nano and Chemical Engineering, Kunsan National University, Kunsan, 573-701, Republic of Korea
2
3
School of Electrical and Electronic Engineering, Ulsan College, Ulsan, 680-749, Republic of Korea
4
Department of Electrical Engineering, Hanbat National University, Daejeon, 305-719, Republic of Korea
5
Department of Photoelectronics, Chosun College of Science and Technology, Gwangju 501-744, Republic of Korea
6
Water Facility Research Center, K-Water, Daejeon, 305-730, Republic of Korea
In this work, uniformly sized Cu ZnSnS (CZTS) nanoparticles with easy control of chemical compo-
2
4
sition were synthesized and printable ink containing CZTS nanoparticles was prepared for low-cost
solar cell applications. In addition, we studied the effects of synthesis conditions, such as reaction
temperature and time, on properties of the CZTS nanoparticles. For CZTS nanoparticles synthesis
process, the reactants were mixed as the 2:1:1:4 molar ratios. The reaction temperature and time
ꢀ
ꢀ
was varied from 220 C Dt oe 3l i 2v 0e r eC d ab n yd I fnr og me n 3t ah too u:r sM tco M5a hs ot eu rr s U, rn ei vs pe er sc it ti vy ely. The crystal structure
and morphology of CI ZP T: S1 8n 8a .n 7o 2p .a1 r 2t i 6c l.e1 s0 p3 r eOp na :r eMd ou nn ,d 0e r9 t hM ea vya r2i o0 u1 s6 c 1o 2n :d 0i t6i o: 0n s7 were investigated by
X-ray diffraction (XRD) andCfi oe pl dy - re i gm hi st :s i Ao nm se cr ai cn an ni n gS ce il ee cn t tr iof inc mP iuc br ol issc ho ep ry s(FE-SEM), and energy dis-
persive X-ray spectroscopy (EDS) was used for compositional analysis of the CZTS nanoparticles.
Keywords: Cu ZnSnS (CZTS), Nanoparticles, Synthesis, Solar Cells.
2
4
1
. INTRODUCTION
replacing In(III) of the chalcopyrite CIS structure by Zn(II)
5
ꢁ6
In the PV industry, a lot of solar cell companies have
reduced solar power cost toward the current conven-
tional cost of electricity. For low-cost solar cell, among
the various compound semiconductors, the I–III–VI fam-
ily of chalcogenide as CuInSe (CIS), CuGaSe (CGS),
Cu(In ,Ga )(S ,Se ꢀ2 (CIGS) have been developed as
thin film solar cell. Today, the laboratory CIGS solar cell
with small size has led to the highest efficiency of over
and Sn(IV) in a 50:50 ratio. The bandgap of CZTS was
7
reported as the direct bandgap of 1.45 eV, which is close
to the optimum direct band gap value for single junc-
8
tion solar cell. The absorption coefficient of CZTS was
2
4
−1
investigated in the order of 10 cm near a fundamental
2
2
9
ꢁ10
5
−1
absorption edge,
which is similar to about 10 cm of
x
1−x
y
1−y
1
1
CIGS. Thanks to these good properties, CZTS has been
expected as a promising candidate for low cost absorber
layers.
1
2
0% in the chalcogenide thin film solar cell, and CIGS
modules have been successfully produced on an industrial
Today, the highest performance of CZTS based solar
cells are 8.4% for pure sulfur CZTS absorber deposited
2
–4
large scale.
However, due to relatively expensive and
1
2
scarce materials as In and Ga, the CIGS solar cell has been
still issues of long term sustainability in terms of cost and
by vacuum thermal evaporation process, and 12.6% for
CZTS absorber with S and Se coated by an ink made by
5
13
availability.
hydrazine solution. There have been various high vac-
For these days, the p-type quaternary compound
uum such as sputtering, evaporation and non-vacuum such
as solution(ink), electrode deposition, for the deposition of
CZTS thin films.
Cu ZnSnS (CZTS) have been investigated as new mate-
2
4
1
4
rial, which is nontoxic, earth-abundant and cheap. The
CZTS structure is tetragonal and could be obtained by
Here, we report the CZTS synthesis of nanopowder
that will be dispersed in an ink by the method referred
∗
15
Author to whom correspondence should be addressed.
to as Steinhagen et al. In this work, uniformly sized
5082
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 5
1533-4880/2016/16/5082/005
doi:10.1166/jnn.2016.12175

Kim et al.
Synthesis of CZTS Nanoparticles for Low-Cost Solar Cells
Table I. Synthesis conditions for CZTS nanoparticles.
Sigma-Aldrich) in 1 liter five-neck flask, and heated to the
different temperature with magnetic stirrer under an inert
atmosphere made by nitrogen purging for enough time,
instead of Schlenk line. To prevent mass-loss during reac-
tion, Liebig condenser to circulate water was set up on the
flask. At the different temperature, the mixture was reacted
for the various time. After the CZTS synthesis, the reacted
mixture was cooled to room temperature. In addition, 100
mL Hexane (95%, from DAEJUNG CHEMICALS AND
METALS Co.) was put in the flask, and the reacted mix-
ture was stirred for 1 hour to remove solid reaction byprod-
Weight Atomic
Chemicals
MW
Mmol
(g)
ratio
Cu(II) acetylacetonate
Zinc acetylacetonate hydrate
Tin(IV) bis(acetylacetonate)
dibromide
261.76
281.62
476.73
2.275
1.138
1.138
0.5956
0.3204
0.5424
2.00
1.00
1.00
Sulfur
32.07
4.549
0.1459
4.00
Cu ZnSnS (CZTS) nanoparticles with easy control of
2
4
chemical composition were synthesized and printable ink
containing CZTS nanoparticles was prepared for low-cost
solar cell applications. In addition, we studied the effects
of synthesis conditions, such as reaction temperature and
time, on properties of the CZTS nanoparticles.
ucts and poorly capped nanocrystals by redispersion of
15ꢁ16
hexane.
The reacted mixture was equally divided by
six 50 mL conical tubes, then some amount of ethanol
99.9%, from SAMCHUN PURE CHEMICALS Co.) was
(
added in considering weight balance for centrifugation and
to clean the CZTS nanoparticles from OLA. CZTS nano-
crystals were obtained, using centrifugation at 8000 rpm
for 10 minutes for isolation of CZTS nanocrystals from the
mixtures. The obtained CZTS nanocrystals were washed
five more times by solvent/antisolvent precipitation with
2
. EXPERIMENTAL DETAILS
CZTS nanoparticles were synthesized at different tem-
ꢀ
peratures (220–320 C) for 3 hours and for var-
ꢀ
ious reaction times (2–5 hours) at 240 C, using
high-temperature arrested precipitation in the coordi-
1
5
(a)
nating solvent, oleylamine (OLA). Under the reac-
tion time of 3 hours, the reactants for synthesis of
CZTS nanoparticles didn’t dissolve enough in OLA. Cop-
per(II) acetylacetonate (>99.99%, from Sigma-Aldrich),
Cu
Zn
Sn
S
Yield
5
4
zinc acetylacetonate (from DONGSAN Chemical Co.)
Delivered by Ingenta to: McMaster University
tin(IV) bis(acetylacetonate) dibr oI Pm :i d1 e8 8( 9. 77 2% . 1, f2 r 6o .m1 0 S3 ig Om na :- Mon, 09 May 2016 12:06:07
3
Copyright: American Scientific Publishers
Aldrich) and elemental sulfur (80%, from Sigma-Aldrich)
(
as shown in Table I) were put in 100 mL OLA (70%, from
2
1
(a) 220ºC
(b) 240ºC
(d) 280ºC
(f) 320ºC
0
2
20
240
260
280
300
320
Temperature (ºC)
(
b)
2
1
1
0
0
.0
.5
.0
.5
.0
Cu
Zn
Sn
S
Yield
(c) 260ºC
(e) 300ºC
220
240
260
280
300
320
340
Temperature (ºC)
Figure 1. SEM images of the CZTS nanoparticles synthesis at different
Figure 2. EDS analysis of CZTS nanoparticles prepared at different
temperatures (a) when Sn ratio is fixed to a reference (Sn = 1) and
(b) when when Sn ratio is fixed to a reference (Cu = 1).
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
temperatures: (a) 220 C, (b) 240 C, (c) 260 C, (d) 280 C, (e) 300 C,
ꢀ
(
f) 320 C. The reaction time was 3 hours.
J. Nanosci. Nanotechnol. 16, 5082–5086, 2016
5083

Synthesis of CZTS Nanoparticles for Low-Cost Solar Cells
Kim et al.
ꢀ
be under 260 C for synthesis of uniformly sized CZTS
nanoparticles.
o
3
20ºC
C
■
䃨
Figure 2 shows EDS analysis of CZTS nanoparticles
prepared at different temperatures. As the synthesis tem-
perature increased, a stoichiometry of CZTS nanoparticles
became Cu rich and S poor due to Sn and chalcogen loss
■
■
■
■ ■
ඔ
▲○ ■ ■ ■
●
■
■
■
■
00ºC
3
○
○ ○○
1
7ꢁ18
during reaction.
When Cu ratio is fixed to a refer-
2
2
80ºC
60ºC
ence (Fig. 2(b)), S amount decreased sharply and Sn ratio
was slightly reduced with the reaction temperature. All the
reactants was put into the flask at once for synthesis, so
the compositional variation of CZTS nanoparticles may be
2
2
40ºC
20ºC
1
9
19
○
due to different dissolving rates in OLA. Byeon et al.
reported that a significant compositional change of CZTS
♠
30
films was observed when the film was annealed above
1
0
20
40
50
60
70
80
90
ꢀ
3
00 C, due to some losses of S and Sn. In this work, the
2
θ(degrees)
ꢀ
reaction temperature of 240 C was selected because of
more uniform nanoparticles’ size.
Figure 3. XRD patterns of the CZTS nanoparticles synthesized at dif-
ferent temperatures.
Figure 3 shows the XRD patterns of the CZTS nanopar-
ticles synthesized at different temperatures. The XRD
hexane/ethanol.15ꢁ16 The slurry of the CZTS nanocrystals
ꢀ
was dried in vacuum dry oven at 100 C to get CZTS
(a)
5
4
Cu
Zn
Sn
S
Yield
nanopowder. The CZTS ink was made by sonification of
mL hexanethiol (95%, from Sigma-Aldrich) with the
CZTS nanoparticles for dispersion. The CZTS ink was
printed directly onto soda lime glass by blade coating.
After the blade coating, the film was dried in the vacuum
1
ꢀ
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oven at 150 C for 15 min.
3
. RESULTS AND DISCUSSION
2
Figure 1 shows SEM images of the CZTS nanoparti-
cles synthesized at different temperatures. When the reac-
tion temperature was elevated, the particle size increased.
In addition, some large particles appeared as the synthesis
temperature increased. In particular, non-uniform and large
CZTS nanoparticles were exhibited at higher temperature
1
180
200
220
240
260
280
300
Reaction Time (ºC)
ꢀ
than 260 C. Therefore, the synthesis temperature should
(
b)
5
4
3
2
1
0
Cu
Zn
Sn
S
Yield
(a) 180 min
(b) 195 min
(c) 210 min
(d) 300 min
180
200
220
240
260
280
300
Reaction Time (min)
Figure 4. SEM images of the CZTS nanoparticles synthesized for var-
ious times: (a) 180 min, (b) 195 min, (c) 210 min, (d) 300 min. The
reaction temperature was 3 hours.
Figure 5. EDS analysis of CZTS nanoparticles synthesized for various
times (a) when Sn ratio is fixed to a reference (Sn = 1) and (b) when
when Sn ratio is fixed to a reference (Cu = 1).
5084
J. Nanosci. Nanotechnol. 16, 5082–5086, 2016

Kim et al.
Synthesis of CZTS Nanoparticles for Low-Cost Solar Cells
peaks were analyzed and compared with the data of
JCPDS card No 26-0575 for tetragonal CZTS (referred
Figure 6 shows the XRD patterns of the CZTS nanopar-
ticles synthesized for various reaction times. The diffrac-
tion peaks related to the secondary CuS and ternary
Cu SnS were observed because of Cu too rich and
Sn poor system. However, the additional secondary or
ternary phases were not formed, although the reaction time
increases.
2
0
as red box ꢀ). Other secondary or ternary XRD peaks
were referred CuS as empty orange circle (ꢀ), Cu S as
2
3
4
2
2
orange dot (•), Cu Sn S as purple upper triangle (ꢁ),
2
3 7
Cu SnS as black clover (♣) and Cu SnS as left trian-
3
4
4
4
2
1
gle ( ). All the samples exhibited the strongest peak near
ꢀ
2
ꢂ = 28ꢃ5 , which correspond to (1 1 2) plane of tetrago-
nal CZTS, and several small peaks associated with CZTS
phase. In addition, some secondary and ternary phases
were observed from the XRD patterns. This may be due
to non-stoichiometric composition of Cu rich and S poor,
as shown in Figure 3. As the reaction temerature was
higher, the intensity of the (1 1 2) plane peak increased,
indicating improvement of crystallinity. When the reac-
4
. CONCLUSION
The effects of synthesis conditions including reaction tem-
perature and time on the properties of the CZTS nanopar-
ticles. As the synthesis temperature and time increased,
the nanoparticles became larger. The CZTS nanoparticles
with uniform size were obtained at the reaction tempera-
ꢀ
ture of 240 C. In addition, the significant deviation from
ꢀ
tion temperature further increased over 300 C, a strong
the stoichiometry of CZTS was observed for longer reac-
tion times. From the XRD analysis, the strongest peak cor-
responded to (1 1 2) plane of tetragonal CZTS and some
peaks associated with secondary and ternary phases were
observed, regardless of the synthesis conditions. In parti-
clular, a strong diffraction peak appeared corresponded the
ꢀ
diffraction peak appeared at 2ꢂ = 32ꢃ1 , which is related
to the secondary SnS phase. The formation of SnS phase
is attributed to thermal decomposition of CZTS, as the fol-
1
7ꢁ19
lowing reaction,
or abnormal stoichiometry states of
Cu rich, Sn and S poor.
ꢀ
secondary SnS phase over 300 C.
Cu ZnSnS ꢄsꢀ → Cu S ꢄsꢀ+ZnS ꢄsꢀ+1/2 S ꢄgꢀ (1)
2
4
2
2
Acknowledgment: This work was supported by a grant
from the Korea Research Foundation (KRF), funded by the
Korean government (MEST) (No. 2012R1A1A2008591).
Figure 4 shows the SEM images of the CZTS nanopar-
ticles synthesized for various times. Frome the figures, the
nanoparticles became larger when the reaction time was
longer.
Figure 5 shows the EDS analysis of CZTS nanoparti-
Delivered by Ingenta to: IMn c aMd da i st it oe nr , Ut hn ii sv ew r so ir tky was also supported by the Human
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Resources Development program (No. 20144030200580)
Copyright: American Scientific Publishers
of the Korea Institute of Energy Technology Evaluation
and Planning (KETEP) grant funded by the Korean gov-
ernment Ministry of Trade, Industry and Energy.
cles synthesized for various times. When the reaction time
increased, the ratio of Cu and S increased. In particular, the
significant deviation from the stoichiometry of CZTS due
to higher Cu ratio than 4 was observed for 300 min, com-
pared to the results of the reaction temperature, as shown
in Figure 2. This is attributed to smaller chalcogen loss
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Delivered by Ingenta to: McMaster University
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Copyright: American Scientific Publishers
5086
J. Nanosci. Nanotechnol. 16, 5082–5086, 2016