CZTS Thin Films: Deposition Temperature & Properties

Effect of deposition temperature on the properties of
Cu
2
ZnSnS
4
(CZTS) thin films
S.A. Khalate
a
, R.S. Kate
a
, J.H. Kim
b
, S.M. Pawar
c , *
, R.J. Deokate
a , **
a
Vidya Pratishthan’s, Arts Science and Commerce College, Baramati, 413 133, MS, India
b
Chonnam National University, Gawanju, Seoul, South Korea
c
Dongguk University, Seoul, South Korea
article info
Article history:
Received 31 January 2017
Accepted 3 February 2017
Available online 4 February 2017
Keywords:
Cu
2
ZnSnS
4
Thin film
Spray
Raman
Optical properties
abstract
Cu
2
ZnSnS
4
(CZTS) thin films were deposited viva spray pyrolysis technique at different
substrate temperature. The effect of substrate temperature on the structural, morpho-
logical, compositional and optical properties was reported. The X-ray diffraction and
Raman analysis revealed that prepared CZTS thin film show kesterite phase without any
secondary phases. Moreover, these analyses indicated the internal compressive stress
relaxed with substrate temperature for all CZTS thin films. The homogeneous nature of
CZTS thin films were observed from surface morphology and chemical composition study.
The optical study provided good optical absorption (10
4
cm
1
) in the visible region and
band gap energy was decreased and found quite close to the optimum value of about
1.57 eVe1.49 eV for solar cell application.
©2017 Elsevier Ltd. All rights reserved.
1. Introduction
Kesterite Cu
2
ZnSnS
4
(CZTS) semiconductor has attracted world-wide attention due to its excellent optical and electronic
properties comparable to traditional Cu(In,Ga)Se
2
(CIGS) and CdTe materials for thin film solar cells while consisting of earth-
abundant and low-toxic constituent elements. Literature survey shows that, due to most favorable band gap energy (1.5 eV),
the large absorption coefficient over 10
4
cm
1
in the visible solar spectrum and a p-type conductivity of copper-based
quaternary semiconductors have attracted much attention in photovoltaic application [1e3]. These properties stimulate
CZTS as a potential candidate for absorber in thin film solar cells. Thus, great efforts have been made for the fabrication of CZTS
thin films in order to develop eco-friendly solar cells with high efficiency and low-cost technology [4e6].
CZTS thin films can be prepared by numerous techniques which include sputtering [7,8], thermal evaporation [9], elec-
trospinning process [10], pulsed laser deposition [11], electron-beam-evaporated precursors [12,13], electrodeposition [14],
co-evaporation [15] or by vacuum free chemical methods such as spray-pyrolysis [16,17], photochemical deposition [18,19]
and solegel [20]. However, few studies have been devoted to CZTS deposition by spray pyrolysis [21,22] and it is observed
that the properties of prepared films are intensely dependent on the deposition method and conditions of the films.
*Corresponding author.
** Corresponding author.
E-mail addresses: [e-mail] (S.M. Pawar), [email protected],deokate200[email protected] (R.J. Deokate).
Contents lists available at ScienceDirect
Superlattices and Microstructures
journal homepage: www.elsevier.com/locate/superlattices
http://dx.doi.org/10.1016/j.spmi.2017.02.003
0749-6036/©2017 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 103 (2017) 335e342

In the present investigation, we have made an attempt to synthesize CZTS thin films by spray with various substrate
temperatures. The structural, morphological and optical properties of CZTS thin films are studied and relative study supported
to the substrate temperature.
2. Experimental
2.1. Preparation of CZTS thin films
The spray pyrolysis method was used to deposit the CZTS thin films using the mixture of an aqueous solutions of cupric
chloride (AR Grade 99.99%) 0.025 M, zinc chloride (AR Grade 99.99%) 0.025 M, stannic chloride (AR Grade 99.99%) 0.025 Mand
thiourea (AR Grade 99.99%) 0.2 M. The solutions were mixed in different composition ratios then sprayed on to glass substrate
at various temperature (300e375
C) using SPD technique with flow rate 3 ml/min to achieve uniform and well adhesive CZTS
thin films. Air was used as a carrier gas. Additional sintering of this thin film was not carried out [23]. It was observed that for
higher temperature (>350
C) degradation of films occurred, which was probably due to with excess temperature.
2.2. Characterization techniques
The structural characterization of these thin film samples was carried out using Bruker AXS D8 X-Ray Diffractometer
(XRD), with CuK
a radiation ( l ¼1.54 Å) and Renishaw InVia Raman Microscope in the range of 10e80 and micro-Raman
spectrometer JobineYvon LabRAM HR 800UV with the excitation by 514.5 nm photons of 20 mW Argon ion laser from
spectra physics. The surface morphology and chemical composition were studied using field emission scanning electron
microscope (FE-SEM) SU8000, Hitachi. Perkin Elmer Lambda 1050 UVeViseNIR spectrophotometer used to study optical
properties.
3. Results and discussion
3.1. EDAX study
Fig.1 shows the composition variation of spray deposited CZTS thin films prepared at different substrate temperatures. The
CZTS thin films deposited at substrate temperatures from 300 to 375
C were close to the stoichiometric ratio. The relative
compositions of CZTS thin films were Cu-poor and Zn-rich (i.e. Cu/(Zn þSn) <1 and Zn/Sn >1) to all substrate temperature. To
get stoichiometric results by spray method are very difficult to the quaternary compound films and similar results were found
to the other studies [24e26].Table 1 also shows the atomic percent of the CZTS thin deposited at different substrate tem-
perature. With increasing substrate temperature the compositional ratios of sulfur content is slightly increased means sulfur
vacancies are reduces and hence crystalline quality of CZTS thin films improved. However, at higher substrate temperature Sn
and Zn elements are volatile; and the effect leads to the Cu-rich state. Due to volatile property more losses of Zn is found in all
CZTS thin films. Thus, the atomic percentage of copper, zinc, tin and sulfur are well dependent on substrate temperature [27].
290 300 310 320 330 340 350 360 370 380
8
12
16
20
24
28
32
36
40
44
48
atomic percentage (%)
Substrate temperature (oC)
Cu
Zn
Sn
S
Fig. 1. Variation of chemical composition of CZTS thin films for different substrate temperature.
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342336

3.2. Xeray diffraction studies
The XRD patterns of spray deposited CZTS thin films deposited at different substrate temperatures (300, 325, 350 and
375
C) are shown in Fig. 2 and prepared CZTS samples are polycrystalline nature with kesterite phase with substrate
temperature [28]. The diffraction peaks at angles 2
q ¼28.49, 47.40 and 56.25 correspond to diffraction planes (112), (220)
and (312), respectively [29] for all samples. Besides, as-prepared thin films might have a complete synthesization without
post-annealing. The intensity and full width at half maximum (FWHM) of (112) peak showed strong and narrow behavior
with good crystallinity.
Fig. 3 gives the relative intensity and full width at half maximum (FWHM) of the (112) peak with respect to substrate
temperature for CZTS films. The relative intensity of the (112) peak continuously increased and FWHM is decreased with
substrate temperature. However, at high substrate temperature (>350
C), the (112) peak intensity dramatically decreased,
shows the weak in crystallinity. Additionally, the lattice parameters of CZTS thin film prepared from 300
Cto375 Care
similar as the other reports [17,21,22]. The lattice parameters have been found to be a ¼5.42 Å and c ¼10.84 Å, which give a
value of c/2a ¼0.99. As the value of c/2a is close to 1 showed the unit cell is tetragonal and enlarged to different substrate
temperature. With the increase of temperature from 300
Cto375 C, the crystallite size increased from 5 to 14 nm, and
decreased the values 7 nm at 375
C substrate temperature. Table 2 shows the microstructural properties of the CZTS thin
films deposited to various substrate temperature. The (hkl) and d-values of CZTS thin films are agree well with the values
found in the JCPDS card [29]. The crystallite size of the films, calculated using Scherrer’s formula [30].
D¼0:9
l
b
cos q
(1)
where Dis the crystallite size, b is the broadening of the diffraction line measured at half of its maximum intensity (rad)
(FWHM) and
l the X-ray wavelength (1.5418 Å). The prepared CZTS films are polycrystalline in nature. The stresses are one of
the most important critical factors affecting on the structural properties which changes geometric mismatch at boundaries
between crystalline lattices of thin films and substrate [31]. The microstrains (
3 ) are developed in the thin films, and
calculated from the following relation [32].
3 ¼
b cos q
4(2)
The variation of micro structural parameters like microstrain and stacking fault probability to all CZTS samples are shown
in Fig. 4. The value of the interplanar spacing has been increased and internal microstrain relaxation in CZTS thin films is
decreased. The microstrain relaxation is consistent with the expansion of the interplanar spacing of (112) plane and internal
microstrain induces the formation of defect center [33,34]. However, this internal microstrain could be relaxed at the higher
substrate temperature and deposited at low temperature CZTS thin films may be present smaller amounts of defects. The
dislocation density (
d ) and stacking fault probability ( a ) of the CZTS thin films are estimated by the following equations
[35,36].
d ¼1
D2(3)
a ¼2
p
2
45 ffiffiffi 3
ptan
q
D ð2 q Þ(4)
Fig. 5 shows the variation of crystallite size (D) and dislocation density ( d ) of CZTS thin films. The dislocation density
decreased from value 3.8 10
16
lines/m
2
to value 0.56 10
16
lines/m
2
and again reached to the value 1.7 10
16
lines/m
2
. The
minimum values are obtained to the CZTS thin film prepared at 350
C substrate temperature. The value of dislocation density
gives the amount of defects in the CZTS thin film, the maximum value of (D) obtained for the CZTS thin film prepared at 350
C
substrate temperature confirms the good crystallinity.
Table 1
The atomic percent of CZTS thin films deposited for different substrate temperature.
Substrate temperature (
C) Cu/(Zn þSn) Zn/Sn Metal/S
300 0.75 1.45 1.40
325 0.76 1.34 1.36
350 0.75 1 1.31
375 0.88 1.19 1.42
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342 337

3.3. Raman spectroscopy of CZTS thin film
Fig. 6 shows the recorded Raman spectra to all the prepared CZTS thin films. All spectra exhibited single Raman peak at
328 cm
1
which is good indication of the kesterite phase of prepared CZTS thin films. Further, there is no any evidence of
binary phases like SnS, SnS
2
, CuS, Cu
2
S and b -ZnS from Raman spectra [37].
3.4. Surface morphology of CZTS thin film
Fig. 7 shows the FE-SEM images of CZTS thin films to all substrate temperatures. The surface morphology is strongly
dependent on the substrate temperature. The morphological studies shows that films deposited at 300
C have absence of
20 30 40 50 60 70 8
0
375 oC
350 oC
325 oC
312
220
Intensity (A.U.)
2
θ
(Degree)
112
300 oC
Fig. 2. X-ray diffraction patterns of CZTS films deposited for different substrate temperature.
300 320 340 360 380
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0.022
0.024
0.026
0.028
SubstrateTemperature (oC)
FWHM (deg.) for (112)
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
Intensit
y
(
arb. unit
)
Fig. 3. The FWHM values and relatively peak intensity of (112) diffraction peak of CZTS for different substrate temperature.
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342338

well-defined grains. The CZTS thin film deposited at higher temperature (>300 C) has observed the dense structure with
large grains. The average size determined from FE-SEM studies approximately found 50e100 nm to the CZTS thin film
deposited at 350
C substrate temperature. The enhancement in crystalline quality in the CZTS thin film due to well
decomposition and agglomeration of the neighboring crystallites at the higher substrate temperature [38]. The improvement
of the grain size of CZTS thin films is consistent with the X-ray diffraction and Raman spectrum analyses. The large grains with
less grain boundaries are beneficial for the devices performance due to less chance for the recombination of photogenerated
carriers at the grain boundaries [39].
3.5. Optical properties of CZTS thin film
The optical transmittance spectra of the CZTS thin films were measured to resolve the values of band gap (E
g
). The band
gap values are determined by extrapolating the straight line to (
a h n )
2
versus photo energy (h n ) curve with the intercept on
horizontal photon energy axis, is shown in Fig. 8 [40,41]. The determined E
g
values of CZTS thin films at 300, 325, 350, and
375
C are 1.57, 1.55, 1.49, and 1.52 eV, respectively. The obtained values are greater at lower substrate temperature due to the
effect of internal compressive stress [34]. The internal compressive stress releases at higher temperature and the relaxations
of internal compressive stress, crystal lattice expand subsequently diminish the band gaps of the thin films [42,43]. XRD and
Table 2
The Micro structural properties of CZTS thin films deposited for different substrate temperature.
Substrate
temp.
C
hkl Diffraction
angle 2 q (deg.)
Intensity d
(obs.)
(nm)
FWHM (rad.) Crystallite
size D(nm)
Micro-strain (
3 )
(10
3
lines
2
m
4
)
Dislocation density
( d )(10
16
lines/m
2
)
Stacking
fault
300 112 28.458 1086 3.1339 0.0223463 6.398664 5.415112 2.44243 0.0729069
220 47.699 448 1.9051 0.0271958 4.661183 6.218438 4.60265 0.2102407
312 56.103 330 1.638 0.017951 4.31457 3.9605527 5.37186 0.035645
325 112 28.502 1919 3.1291 0.0145137 9.255724 3.516838 1.16729 0.0289164
220 47.402 823 1.9163 0.01631 7.780842 3.73358 1.65176 0.040383
312 56.302 558 1.6327 0.017061 7.162945 3.6707 1.94902 0.0586877
350 112 28.502 2797 3.1292 0.00942 14.260595 2.282528 4.91728 0.0289164
220 47.404 1146 1.9161 0.01043 14.510397 2.387554 4.74943 0.0415349
312 56.3 741 1.6327 0.010885 11.226644 2.399368 7.93414 0.0577436
375 112 28.492 1777 3.1302 0.0131007 2.60347 3.17453 1.47535 0.0389018
220 47.302 560 1.9201 0.016572 9.130281 3.795013 1.19959 0.0173482
312 56.202 414 1.6354 0.014078 11.161062 3.104616 8.02766 0.0113828
300 320 340 360 380
2
3
4
5
6
SubstrateTemperature (oC)
Micro-Strain (×10-3)
3.1150
3.1175
3.1200
3.1225
3.1250
3.1275
3.1300
d-spacing (Ao)
Fig. 4. The variation of interplanar spacing and strain of CZTS thin film for different substrate temperature.
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342 339