pssc.201700170

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The Impact of Na and K on Properties of Cu2ZnSnS4
Thin Films Prepared by Ultrasonic Spray Technique
Mouaad Sekkati,* Mhamed Taibi, Guy Schmerber, El Bachir Benamar,
Mohammed Regragui, Fouzia Cherkaoui El Moursli, Cedric Leuvrey, Zineb Edfouf,
Zouheir Sekkat, Aziz Dinia, Abdelilah Slaoui, and Mohammed Abd-Lefdil
The later can be controlled by the addition
of Se in the CZTS that it becomes
Cu2ZnSn(S,Se)4. In addition, CZTS has
similar properties as CIGS which is already
marketed. The world record for CZTS and
CZTSSe is 9.7 and 12.6% respectively,[1,2]
even though it has a theoretical yield of
32%.[3] The performance of CZTS is
limited by defects like secondary phases
and interfaces states. So it is very important
to understand the role of these defects,
then control them or reduce their amounts.
CZTS has been elaborated by different
techniques such as physical way like
PLD,[4] sputtering,[5] evaporation[6] or by
chemical way like electrodeposition,[7]
spray,[8] and sol-gel.[9] Among these methods, the ultrasonic spray technique allows to deposit large area
thin-films in an easy way. The effects of Na is still far from being
explained in commonly accepted terms, in spite of its benefit
effect on CIGS PV efficiency.[10]
In this work, we study the effect of the alkali addition (Na, K)
in the CZTS, on the structural, optical, and electrical properties
of CZTS thin films. For this investigation, we have deposited by
ultrasonic spray CZTS thin films on glass and Mo-coated glass
substrates using different amounts of the alkali in the sprayed
solution.
CZTS thin films are deposited onto glass and Mo-coated glass substrates by
ultrasonic spray technique, followed by a sulfurization under argon atmosphere at 500 C during 1 h. We have investigated how the addition of the
alkali can influence Cu2ZnSnS4 thin films properties. X-ray diffraction and
Raman spectroscopy proved that CZTS has the kesterite structure with a
preferential orientation along 112. The values of Cu/(ZnþSn) and (Zn/Sn)
ratios, determined by energy dispersive spectroscopy, for undoped CZTS and
alkali-doped CZTS are observed to be affected by the alkali doping. The
electrical resistivity, for CZTS films deposited on glass substrates, decreased
after alkali doping from 9.2 101 Ω cm for undoped CZTS to reach 1.8 101
and 4.2 101 Ω cm for 15% content of Na and K, respectively.
1. Introduction
Cu2ZnSnS4 (CZTS) kesterite is a quaternary material which has
attracted the attention of researches in the thin films
photovoltaic field due to its optical and electrical properties
characterized by a very high absorption coefficient, more than
104 cm1, and a direct optical gap with an optimal value of 1.5 eV.
Dr. M. Sekkati, Dr. E. B. Benamar, Prof. M. Regragui,
Prof. F. Cherkaoui El Moursli, Prof. Z. Edfouf, Prof. Z. Sekkat,
Prof. M. Abd-Lefdil
MANAPSE
Faculty of Sciences
University of Mohammed V-Rabat, Rabat 10000, Morocco
E-mail: mouaad.sekkati@gmail.com
2. Experimental Section
Prof. M. Taibi
Ecole Normale Superieure, University of Mohammed V, LPCMIN,
Rabat, Morocco
Dr. G. Schmerber, Dr. C. Leuvrey, Prof. A. Dinia
IPCMS, Universite de Strasbourg CNRS UMR 7504, 23 rue du Loess,
B. P. 43, 67034 Strasbourg Cedex 2, France
Dr. Z. Sekkat
Optics and Photonics Center
Moroccan Foundation for Advanced Science Innovation and Research
(MAScIR),
Rabat, Morocco
Dr. A. Slaoui
ICube, Universite de Strasbourg, CNRS UMR 7357, 23 rue du Loess,
B.P. 20 CR, 67037 Strasbourg Cedex 2, France
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssc.201700170.
DOI: 10.1002/pssc.201700170
Phys. Status Solidi C 2017, 1700170
The spray solution of the CZTS contained CuCl2, SnCl2, ZnCl2
SC(NH2)2, and NaCl or KCl with the ratio Cu/(ZnþSn) ¼ 1, Zn/
Sn ¼ 1, S/(CuþZnþSn) ¼ 2, and (Na, K)/Zn ¼ (0.05; 0.1; 0.15).
All the precursors were dissolved in deionized water. The
solution was sprayed with ultrasonic spray on glass and Mocoated glass substrates, cleaned as we reported in a previous
paper,[7] and heated at 200 C. We choose this temperature to
avoid oxidation and stabilization of the binary secondary oxide
phases and to minimize the sulfur losses by evaporation during
the spray process. The as-deposited samples were submitted to a
sulfurization in a tubular furnace under argon and amount of
sulfur S8 at 500 C during 1 h.
The structural properties of the films were analyzed in the
10–80 range 2θ range by means of a Rigaku Smartlab (9 kW)
diffractometer equipped with CuKa1 source (λ ¼ 1.54056 Å ) and
a Ge (220) x2-bounce front monochromator. Raman
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Table 1. Chemical composition of CZTS thin films.
Sample
Cu/(ZnþSn)
Zn/Sn
S/metal
CZTS
0.88
1.06
1.01
CZTS:Na5%
1.04
0.95
0.89
CZTS:Na10%
0.96
1.13
0.93
CZTS:Na15%
1.03
1.02
0.94
CZTS:K5%
0.96
1.20
0.94
CZTS:K10%
1.12
1.00
0.93
CZTS:K15%
0.87
1.42
0.99
spectroscopy was carried out at room temperature using a
Horiba LabRam ARAMIS spectrometer equipped with a
multichannel CCD detection system in the backscattering
configuration. The incident laser wavelength was 532 nm.
Absorbance and reflectance of the films were recorded in the
wavelength ranging from 300 to 1200 nm using a Perkin-Elmer
Lambda 950 spectrophotometer with an integrating sphere of
Figure 1. Surface and cross-section images of CZTS thin films with
different Na nominal doping concentrations (0, 5, 10, and 15%) and K
nominal doping concentrations (5 and 15%).
Figure 2. XRD diffractogram of CZTS thin films: (a) CZTS thin films with
Na nominal concentration (5, 10, and 15%), (b) CZTS thin films with K
nominal concentration (5, 10, and 15%).
Phys. Status Solidi C 2017, 1700170
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Table 2. Structural parameters of CZTS thin films.
Sample
2θ ( )
FWHM ( )
TC (112)
D (nm)
a (Å ) 0.001 Å
c (Å ) 0.003 Å
CZTS
28.53
0.178
1.57
45
5.415
10.82
CZTS:Na5%
28.54
0.150
1.90
55
5.412
10.802
CZTS:Na10%
28.51
0.211
1.72
39
5.418
10.811
CZTS:Na15%
28.53
0.193
1.76
43
5.415
10.825
CZTS:K5%
28.54
0.111
2.06
74
5.412
10.783
CZTS:K10%
28.54
0.178
1.99
47
5.413
10.815
CZTS:K15%
28.56
0.146
1.74
56
5.412
10.789
150 mm diameter. The surface morphology and the cross section
were observed using a JEOL JSM-6700F scanning electron
microscope (SEM) coupled with energy dispersive X-ray
spectroscopy (EDS) analyzer to determine the chemical
composition of the films. The electrical resistivity, the carrier
concentration, and the mobility were measured at room
temperature by an ECOPIA Hall effect system (HMS 5500).
Figure 3. Raman spectra of CZTS thin films: (a) CZTS films with Na
nominal concentration (0, 5, 10, and 15%). b) CZTS thin films with K
nominal concentration (0, 5, 10, and 15%).
Phys. Status Solidi C 2017, 1700170
3. Results and Discussion
Surface and cross-section images of CZTS thin films with
different doping concentrations deposited onto Mo glass
substrates are shown in Figure 1.
The surface morphology of the samples presents the same
aspect with agglomerated particles, as it is usually noted for
samples prepared by spray process. The density of this
agglomeration depends on the alkali doping content. The
Figure 4. Optical properties of CZTS films: (a) absorbance of CZTS films
with Na nominal concentration (5, 10, and 15%). b) Absorbance of CZTS
films with K nominal concentration (5, 10, and 15%).
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4. Conclusion
Table 3. Electrical parameters of CZTS thin films at room
temperature.
ρ (101 Ω cm)
μ (101 cm2 V1 s1)
N (1019 cm3)
CZTS
9.2
120.3
0.2
CZTS:Na5%
3.0
6.5
3.2
CZTS:Na10%
1.8
10.1
3.3
CZTS:Na15%
1.5
21.5
4.3
CZTS:K5%
8.3
7.9
0.3
CZTS:K10%
5.3
4.9
2.1
CZTS:K15%
4.2
4.2
3.4
Sample
thickness of the CZTS film, estimated from cross-section view,
was found to be between 1 and 3 μm, while the Mo thickness was
around 600 nm.
The chemical composition of the CZTS films obtained by EDS
is illustrated in Table 1. The composition value was calculated, as
an average of the compositions, obtained at several points
distributed arbitrarily on the surface of the sample. The best
compositions were obtained for CZTS: Na (10%) and CZTS: K
(5%) where the films are copper-poor and zinc-rich, as it is
desired for solar cells.
Figure 2 exhibits the X-ray diffraction diffractograms of CZTS
thin films. All the films have a polycrystalline structure of
kesterite Cu2ZnSnS4 (JCPDS card No. 00-026-0575 or No. 04015-7542) with a preferential orientation according to (112)
crystallographic plane. No binary or ternary secondary phases
were observed in the detection limit of X-ray diffraction
measurement.
The Scherrer formula was used to determine the average
crystallite size of our samples. We used the most important
crystallographic orientation (112). For undoped CZTS, D was
found around 45 nm (Table 2). After alkali doping, D reached 55
and 74 nm for CZTS:Na (5%) and CZTS:K (5%), respectively. For
larger content, the crystallite size decreased, in agreement with
Gershon et al.[11] results. However, the crystalline structures of
ZnS and Cu2SnS3 compounds can not be distinguished from the
kesterite CZTS by only the X-ray diffraction. Therefore, we use
the Raman spectroscopy to better identify the crystal structure.
The Raman spectra exhibited in Figure 3 show three peaks at
144, 287, and 338 cm1 corresponding to the vibration modes
B/E and A, respectively, of kesterite CZTS structure for all the
samples. The red shift of Raman peak obtained using Na and K
doping is attributed to the tensile stress of the crystal lattice.[12]
Absorbance spectra of CZTS films are shown in Figure 4. We
note an increase in absorbance after increasing Na concentration. The same observation is also valid for K doping.
Table 3 shows the electrical parameter values of undoped and
alkali-doped CZTS deposited on glass substrates at room
temperature. The best electrical resistivity values were 1.5
101 and 4.2 101 Ω cm for Na (15%) and K (15%) doping,
respectively.
Phys. Status Solidi C 2017, 1700170
XRD and Raman spectroscopy, for CZTS thin films, undoped
and doped with alkali metals (Na, K), prepared by ultrasonic
spray technique on Mo-coated glass substrate, showed that all
the samples had the kesterite structure with a best improvement
of the preferential orientation along 112 axis for 5% content for
both metals. Alkali doping was benefit for CZTS crystallite size
and also for its electrical resistivity.
Acknowledgments
The research leading to these results has received funding from the
European Union Seventh Framework Programme (FP7/2007-2013) under
grant agreement no. 608593 (EUROSUNMED Project).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
alkali, CZTS, sulfurization, ultrasonic spray
Received: June 10, 2017
Published online:
[1] W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov,
Y. Zhu, D. B. Mitzi, Adv. Energy Mater. 2014, 4, 1301465-1-1301465-5.
[2] H. S. Takuya Kato, H. Hiroi, N. Sakai, S. Muraoka, 27th European
Photovoltaic Solar Energy Conference and Exhibition, 2009, pp.
2236–2239.
[3] W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510.
[4] S. M. Pawar, A. V. Moholkar, I. K. Kim, S. W. Shin, J. H. Moon,
J. I. Rhee, J. H. Kim, Curr. Appl. Phys. 2010, 10, 565.
[5] J. Seol, S. Lee, J. Lee, H. Nam, K. Kim, Sol. Energy Mater. Sol. Cells
2003, 75, 155.
[6] K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey,
N. A. Bojarczuk, D. Mitzi, S. Guha, Appl. Phys. Lett. 2010, 97,
143508.
[7] T. SlimaniTlemSc ani, E. B. Benamar, F. Cherkaoui El Moursli, F. Hajji,
Z. Edfouf, M. Taibi, H. Labrim, B. Belhorma, S. Aazou, G. Schmerber,
K. Bouras, Z. Sekkat, A. Dinia, A. Ulyashin, A. Slaoui, M. Abd-Lefdil,
Energy Procedia 2015, 84, 127.
[8] N. Kamoun, H. Bouzouita, B. Rezig, Thin Solid Films 2007, 515,
5949.
[9] K. Tanaka, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 2007,
91, 1199.
[10] Y. M. Shin, D. H. Shin, J. H. Kim, B. T. Ahn, Curr. Appl. Phys. 2011, 11,
S59.
[11] T. Gershon, B. Shin, N. Bojarczuk, M. Hopstaken, D. B. Mitzi,
S. Guha, Adv. Energy Mater. 2015, 5, 1400849.
[12] Z. Tong, C. Yan, Z. Su, F. Zeng, J. Yang, Y. Li, L. Jiang, Y. Lai, F. Liu,
Appl. Phys. Lett. 2014, 105, 223903.
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