Effect of deposition temperature on the properties of

Superlattices and Microstructures 103 (2017) 335e342
Contents lists available at ScienceDirect
Superlattices and Microstructures
journal homepage: www.elsevier.com/locate/superlattices
Effect of deposition temperature on the properties of
Cu2ZnSnS4 (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
Chonnam National University, Gawanju, Seoul, South Korea
c
Dongguk University, Seoul, South Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 January 2017
Accepted 3 February 2017
Available online 4 February 2017
Cu2ZnSnS4 (CZTS) thin films were deposited viva spray pyrolysis technique at different
substrate temperature. The effect of substrate temperature on the structural, morphological, 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 (104 cm1) 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.
Keywords:
Cu2ZnSnS4
Thin film
Spray
Raman
Optical properties
1. Introduction
Kesterite Cu2ZnSnS4 (CZTS) semiconductor has attracted world-wide attention due to its excellent optical and electronic
properties comparable to traditional Cu(In,Ga)Se2 (CIGS) and CdTe materials for thin film solar cells while consisting of earthabundant and low-toxic constituent elements. Literature survey shows that, due to most favorable band gap energy (1.5 eV),
the large absorption coefficient over 104 cm1 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], electrospinning 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: [email protected] (S.M. Pawar), [email protected], [email protected] (R.J. Deokate).
http://dx.doi.org/10.1016/j.spmi.2017.02.003
0749-6036/© 2017 Elsevier Ltd. All rights reserved.
336
S.A. Khalate et al. / 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 M and
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 CuKa radiation (l ¼ 1.54 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 þ Sn) < 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 temperature. 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].
atomic percentage (%)
48
44
40
Cu
Zn
Sn
S
36
32
28
24
20
16
12
8
290
300
310
320
330
340
350
360
370
380
o
Substrate temperature ( C)
Fig. 1. Variation of chemical composition of CZTS thin films for different substrate temperature.
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342
337
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 2q ¼ 28.49, 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 C to 375 C are
similar as the other reports [17,21,22]. The lattice parameters have been found to be a ¼ 5.42 Å and c ¼ 10.84 Å, which give a
value of c/2a ¼ 0.99. 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 C to 375 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].
D¼
0:9l
bcosq
(1)
where D is 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
bcosq
(2)
4
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].
d¼
a¼
1
(3)
D2
2p2
pffiffiffi
Dð2qÞ
45 3tanq
(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 1016 lines/m2 to value 0.56 1016 lines/m2 and again reached to the value 1.7 1016 lines/m2. 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 þ Sn)
Zn/Sn
Metal/S
300
325
350
375
0.75
0.76
0.75
0.88
1.45
1.34
1
1.19
1.40
1.36
1.31
1.42
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342
o
375 C
Intensity (A.U.)
312
220
112
338
o
350 C
o
325 C
o
300 C
20
30
40
50
60
70
80
2θ (Degree)
0.028
2800
0.026
2600
0.024
2400
0.022
2200
0.020
2000
0.018
1800
0.016
1600
0.014
0.012
1400
0.010
1200
0.008
1000
300
320
340
360
Intensity (arb. unit)
FWHM (deg.) for (112)
Fig. 2. X-ray diffraction patterns of CZTS films deposited for different substrate temperature.
380
o
SubstrateTemperature ( C)
Fig. 3. The FWHM values and relatively peak intensity of (112) diffraction peak of CZTS for different substrate temperature.
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 cm1 which is good indication of the kesterite phase of prepared CZTS thin films. Further, there is no any evidence of
binary phases like SnS, SnS2, CuS, Cu2S 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
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342
339
Table 2
The Micro structural properties of CZTS thin films deposited for different substrate temperature.
Substrate
temp. C
hkl
Diffraction
angle 2q (deg.)
Intensity
d(obs.)
(nm)
FWHM (rad.)
Crystallite
size D (nm)
Micro-strain (3 )
(103 lines2 m4)
Dislocation density
(d) (1016 lines/m2)
Stacking
fault
300
112
220
312
112
220
312
112
220
312
112
220
312
28.458
47.699
56.103
28.502
47.402
56.302
28.502
47.404
56.3
28.492
47.302
56.202
1086
448
330
1919
823
558
2797
1146
741
1777
560
414
3.1339
1.9051
1.638
3.1291
1.9163
1.6327
3.1292
1.9161
1.6327
3.1302
1.9201
1.6354
0.0223463
0.0271958
0.017951
0.0145137
0.01631
0.017061
0.00942
0.01043
0.010885
0.0131007
0.016572
0.014078
6.398664
4.661183
4.31457
9.255724
7.780842
7.162945
14.260595
14.510397
11.226644
2.60347
9.130281
11.161062
5.415112
6.218438
3.9605527
3.516838
3.73358
3.6707
2.282528
2.387554
2.399368
3.17453
3.795013
3.104616
2.44243
4.60265
5.37186
1.16729
1.65176
1.94902
4.91728
4.74943
7.93414
1.47535
1.19959
8.02766
0.0729069
0.2102407
0.035645
0.0289164
0.040383
0.0586877
0.0289164
0.0415349
0.0577436
0.0389018
0.0173482
0.0113828
325
350
375
6
5
3.1275
o
d-spacing (A )
-3
Micro-Strain (×10 )
3.1300
3.1250
4
3.1225
3
3.1200
3.1175
2
3.1150
300
320
340
360
380
o
SubstrateTemperature ( C)
Fig. 4. The variation of interplanar spacing and strain of CZTS thin film for different substrate temperature.
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 (Eg). The band
gap values are determined by extrapolating the straight line to (ahn)2 versus photo energy (hn) curve with the intercept on
horizontal photon energy axis, is shown in Fig. 8 [40,41]. The determined Eg 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
340
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342
6
5
2
12
δ , X1016 (lines/cm )
Crystallite Size (nm)
14
4
10
3
8
2
6
1
4
0
300
320
340
360
380
o
Substrate Temperature ( C)
Fig. 5. Crystallite size and dislocation density of CZTS thin film for different substrate temperature.
Raman spectra results confirm the internal compressive stress relaxation. The electronic transition from valence band to
conduction band becomes easy due to the decrease in the possibility of the scattering of electron at grain boundaries, results
the decrease of Eg. In addition, the CZTS thin film deposited at 350 C gives the relatively optimum band gap value (1.49 eV).
4. Conclusions
In summary, polycrystalline Cu2ZnSnS4 (CZTS) absorber thin films have been deposited using spray pyrolysis technique
with kesterite phase. The XRD and Raman results reveal that the formation of single phase CZTS with kesterite structure and
the crystalline sizes depends upon substrate temperature. The interplanar spacing and internal microstrain relaxation
denoted changes in CZTS thin film with temperature. The surface morphological study indicated that with increasing substrate temperature, the film crystallinity is improved. The optical absorption data reveal that the CZTS film has an optical
bandgap of 1.59 eV. The thin film deposited at 350 C substrate temperature shows the suitable structural, morphological and
optical properties for solar cell application.
300 0C
325 0C
350 0C
375 0C
500 nm
Fig. 6. Surface FE-SEM images of CZTS thin film for different substrate temperature.
341
328
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342
Intensity (A.U.)
328
o
375 C
328
o
350 C
328
o
325 C
o
300 C
100
200
300
400
500
600
700
800
-1
Raman Shift (cm
)
Fig. 7. Raman spectra of CZTS thin films for different substrate temperature.
2
0
1.0
2
(α hν ) , (ev/cm)
2
1.5
300 C
0
325 C
0
350 C
0
375 C
0.5
0
1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
hν ( eV)
Fig. 8. Plot of (ahn)2 versus the photon energy (hn) of CZTS thin films for different.
Acknowledgements
The author Dr. R.J. Deokate wish to acknowledge to Science and Engineering Research Board, Department of Science and
Technology, New Delhi, India for financial assistance through project under SERB FAST Track Scheme for Young Scientist (SB/
FTP/PS-079/2014).
342
S.A. Khalate et al. / Superlattices and Microstructures 103 (2017) 335e342
References
[1] K. Kim, I. Kim, Y. Oh, D. Lee, K. Woo, S. Jeong, J. Moon, Influence of precursor type on non-toxic hybrid inks for high-efficiency Cu2ZnSnS4 thin-film
solar cells, Green Chem. 16 (2014) 4323e4332.
[2] K. Woo, K. Kim, Z. Zhong, I. Kim, Y. Oh, S. Jeong, J. Moon, Non-toxic ethanol based particulate inks for low temperature processed Cu2ZnSn(S,Se)4 solar
cells without S/Se treatment, Sol. Energy Mater. Sol. Cells 128 (2014) 362e368.
[3] K.V. Gurav, S.W. Shin, U.M. Patil, M.P. Suryawanshi, S.M. Pawar, M.G. Gang, S.A. Vanalakar, J.H. Yun, J.H. Kim, Improvement in the properties of CZTSSe
thin films by selenizing single-step electrodeposited CZTS thin films, J. Alloys Compd. 631 (2015) 178e182.
[4] T.K. Todorov, K.B. Reuter, D.B. Mitzi, High-efficiency solar cell with earth-abundant liquid-processed absorber, Adv. Mater 22 (2010) E156eE159.
[5] D.B. Mitzi, O. Gunawan, T.K. Todorov, K. Wang, S. Guha, The path towards a high-performance solution-processed kesterite solar cell, Sol. Energy Mater.
Sol. Cells 95 (6) (2011) 1421e1436.
[6] S. Siebentritt, S. Schorr, Kesterites-a challenging material for solar cells, Prog. Photovoltaics Res. Appl. 20 (5) (2012) 512e519.
[7] J.S. Seol, S.Y. Lee, J.C. Lee, H.D. Nam, K.H. Kim, Electrical and optical properties of Cu2ZnSnS4 thin films prepared by RF magnetron sputtering process,
Sol. Energy Mater. Sol. Cells 75 (2003) 55e162.
[8] H. Yoo, J. Kim, Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films, Thin Solid Films 518 (2010) 6567e6572.
[9] K. Ito, T. Nakazawa, Electrical and optical properties of stannite-type quaternary semiconductor thin films, Jpn. J. Appl. Phys. 27 (1988) 2094e2097.
[10] K.C. Hsu, J.J. Liao, J.R. Yang, Y.S. Fu, Cellulose acetate assisted synthesis and characterization of kesterite quaternary semiconductor Cu2ZnSnS4 mesoporous fibers by an electrospinning process, Cryst. Eng. Commun. 15 (2013) 4303e4308.
[11] S.M. Pawar, A.V. Moholkar, I.K. Kim, S.W. Shin, J.H. Moon, J.I. Rhee, J.H. Kim, Effect of laser incident energy on the structural, morphological and optical
properties of Cu2ZnSnS4 (CZTS) thin films, Curr. Appl. Phys. 10 (2010) 565e569.
[12] H. Katagiri, N. Ishigaki, T. Ishida, K. Saito, Characterization of Cu2ZnSnS4 thin films prepared by vapor phase sulfurization, Jpn. J. Appl. Phys. 40 (2001)
500e504.
[13] J. Scragg, P. Dale, L. Peter, Towards sustainable materials for solar energy conversion: preparation and photoelectrochemical characterization of
Cu2ZnSnS4, Electrochem. Commun. 10 (2008) 639e642.
[14] F. Liu, Y. Li, K. Zhang, B. Wang, C. Yan, Y. Lai, Z. Zhang, J. Li, Y. Liu, In situ growth of Cu2ZnSnS4 thin films by reactive magnetron co-sputtering, Sol.
Energy Mater. Sol. Cells 94 (2010) 2431e2434.
[15] K. Oishi, G. Saito, K. Ebina, M. Nagahashi, K. Jimbo, W.S. Maw, H. Katagiri, M. Yamazaki, H. Araki, A. Takeuchi, Growth of Cu2ZnSnS4 thin films on Si
(100) substrates by multisource evaporation, Thin Solid Films 517 (2008) 1449e1452.
[16] N. Nakayama, K. Ito, Sprayed films of stannite Cu2ZnSnS4, Appl. Surf. Sci. 92 (1996) 171e175.
[17] R.J. Deokate, N.S. Shinde, A.A. Adsool, S.M. Pawar, C.D. Lokhande, Structural and optical properties of spray-deposited Cu2ZnSnS4 thin films, Energy
Proced. 54 (2014) 627e633.
[18] K. Moriya, J. Watabe, K. Tanaka, H. Uchiki, Characterization of Cu2ZnSnS4 thin films prepared by photo-chemical deposition, Phys. Status Solidi. C 3 (8)
(2006) 2848e2852.
[19] K. Moriya, K. Tanaka, H. Uchiki, Characterization of Cu2ZnSnS4 thin films prepared by photo-chemical deposition, Jpn. J. Appl. Phys. 44 (2005) 715e717.
[20] K. Tanaka, N. Moritake, H. Uchiki, Preparation of Cu2ZnSnS4 thin films by sulfurizing solegel deposited precursors, Sol. Energy Mater. Sol. Cells 91
(2007) 1199e1201.
[21] Y.B. Kishore Kumar, G. Suresh Babu, P. Uday Bhaskar, V. Sundara Raja, Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films, Sol.
Energy Mater. Sol. Cells 93 (2009) 1230e1237.
[22] J. Madara, P. Bombicz, M. Okuya, S. Kaneko, G. Pokol, Thermal decomposition of thiourea complexes of Cu(I), Zn(II), and Sn(II) chlorides as precursors
for the spray pyrolysis deposition of sulfide thin films, Solid State Ionics 141e142 (2001) 439e446.
[23] N.M. Shinde, R.J. Deokate, C.D. Lokhande, Properties of spray deposited Cu2ZnSnS4 (CZTS) thin films, J. Anal. Appl. Pyrolysis 100 (2013) 12e16.
[24] Y.B. Kumar, P.U. Bhaskar, G.S. Babu, V.S. Raja, Effect of copper salt and thiourea concentrations on the formation of Cu2ZnSnS4 thin films by spray
pyrolysis, Phys. Status Solidi A 207 (2010) 149e156.
[25] T. Tanaka, A. Yoshida, D. Saiki, K. Saito, Q. Guo, M. Nishio, T. Yamaguchi, Influence of composition ratio on properties of Cu2ZnSnS4 thin films fabricated
by co-evaporation, Thin Solid Films 518 (2010) S29eS33.
[26] W. Daranfed, M.S. Aida, N. Attaf, J. Bougdira, H. Rinnert, Cu2ZnSnS4 thin films deposition by ultrasonic spray pyrolysis, J. Alloys Compd. 542 (2012)
22e27.
s, G. Santoro, M. Va
zquez, Spray deposition of Cu2ZnSnS4 thin films, J. Alloys Compd. 585 (2014) 776e782.
[27] M. Valde
[28] L. Sun, J. He, H. Kong, F. Yue, P. Yang, J. Chu, Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by pulsed laser deposition
method, Sol. Energy Mater. Sol. Cells 95 (2011) 2907e2913.
[29] JCPDS # 26e0575.
[30] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Palo Alto, CA, 1956.
[31] B.G. Jeyaprakash, K. Kesavan, R.A. Kumar, S. Mohan, A. Amalarani, Temperature dependent grain-size and microstrain of CdO thin films prepared by
spray pyrolysis method, Bull. Mater. Sci. 34 (2011) 601e605.
[32] S.R. Vishwakarma, R.N. Tripathi, A.K. Verma, Afr. Phys. Rev. 6 (2011) 103.
[33] A. Moses Ezhil Raj, K.C. Lalithambika, V.S. Vidhya, G. Rajagopal, A. Thayumanavan, M. Jayachandran, C. Sanjeeviraja, Optical properties of Er3þ/Yb3þcodoped transparent PLZT ceramic, Phys. B 403 (2008) 44e49.
[34] J. He, L. Sun, K. Zhang, W. Wang, J. Jiang, Y. Chen, P. Yang, J. Chu, Effect of post-sulfurization on the composition, structure and optical properties of
Cu2ZnSnS4 thin films deposited by sputtering from a single quaternary target, Appl. Surf. Sci. 264 (2013) 133e138.
[35] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969, p. 270.
[36] R. Touati, M.B. Rabeh, M. Kanzari, Structural and optical properties of the new absorber Cu2ZnSnS4 thin films grown by vacuum evaporation method,
Energy Proced. 44 (2014) 44e51.
[37] G. Gouadec, P. Colomban, Raman Spectroscopy of nanomaterials: how spectra relate to disorder, particle size and mechanical properties, Prog. Cryst.
Growth Charact. Mater. 53 (2007) 1e58.
[38] X. Fontane, L. Calvo-Barrio, V. Izquierdo-Roca, E. Saucedo, A. Perez-Rodriguez, R. Morante, D.M. Berg, P.J. Dale, S. Siebentritt, In-depth resolved Raman
scattering analysis for the identification of secondary phases: characterization of Cu2ZnSnS4 layers for solar cell applications, Appl. Phys. Lett. 98
(2011), 181905e1-181905-3.
[39] M. Venkatachalam, M.D. Kannan, S. Jayakumar, R. Balasundaraprabhu, N. Muthukumarasamy, Effect of annealing on the structural properties of
electron beam deposited CIGS thin films, Thin Solid Films 516 (2008) 6848e6852.
[40] J.H. Tao, J.F. Liu, L.L. Chen, H.Y. Cao, X.K. Meng, Y.B. Zhang, C.J. Zhang, L. Sun, P.X. Yang, J.H. Chu, 7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar
cells with sputtered CdS buffer layers, Green Chem. 18 (2016) 550e557.
[41] A. Fischereder, T. Rath, W. Haas, H. Amenitsch, J. Albering, D. Meischler, S. Larissegger, M. Edler, R. Saf, F. Hofer, G. Trimmel, Investigation of Cu2ZnSnS4
formation from metal salts and thioacetamide, Chem. Mater. 22 (2010) 3399e3406.
[42] B. Welber, M. Cardona, C.K. Kim, S. Rodriguez, Dependence of the direct energy gap of GaAs on hydrostatic pressure, Phys. Rev. B 12 (1975) 5729e5731.
[43] E. Ghahramani, J.E. Sipe, Pressure dependence of the band gaps of semiconductors, Phys. Rev. B 40 (1989) 12516e12519.