Solution Processed p-Type Cu2ZnSnS4 Thin

Films for Absorber Layer

G. Genifer Silvena, Bincy John, R. Anne

Sarah Christinal, M. C. Santhosh Kumar,
Sujay Chakravarty & A. Leo Rajesh

Journal of Inorganic and

Organometallic Polymers and
Materials

ISSN 1574-1443
Volume 27
Number 5

J Inorg Organomet Polym (2017)

27:1556-1562
DOI 10.1007/s10904-017-0616-7

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 Author's personal copy
J Inorg Organomet Polym (2017) 27:1556–1562
DOI 10.1007/s10904-017-0616-7

Solution Processed p-Type ­Cu2ZnSnS4 Thin Films for Absorber

Layer
G. Genifer Silvena1 · Bincy John1 · R. Anne Sarah Christinal1 ·
M. C. Santhosh Kumar2 · Sujay Chakravarty3 · A. Leo Rajesh1

Received: 1 March 2017 / Accepted: 26 June 2017 / Published online: 11 July 2017
© Springer Science+Business Media, LLC 2017

Abstract Thin films of quaternary chalcogenide Keywords Cu2ZnSnS4 · Thin films · 2-Methoxyethanol
­Cu2ZnSnS4 (CZTS) were prepared due to its abundance solvent · Temperature · Spray pyrolysis
and non-toxic nature by chemical spray pyrolysis technique
with 2-methoxyethanol as solvent for the precursor solution
that avoids the insoluble base by-products of zinc and tin. 1 Introduction
The solution was clear without precipitates as the solvent
has higher viscosity compared to DI water which resulted The modern cognizance of clean energy usage leads to the
in the successful deposition of smoother films. The struc- renovation of solar energy research which progresses to
tural, morphological, optical and electrical properties of the the state-of-the-art thin film solar cell technology. The thin
films at different temperatures 280, 300, 350 and 400 °C film solar cell technology has been developing with differ-
were investigated. The structural formation of kesterite ent emergent materials. CdTe is a binary material with a
­Cu2ZnSnS4 was confirmed with a preferential (112) phase latest efficiency of 16.5% and ­CuInGaSe2 is a quaternary
and with an intensed 334 cm−1 Raman frequency mode. material with a newest maximum efficiency of >21.5% [1,
Uniform film growth and crack-free surface was obtained 2]. ­Cu2ZnSn(SSe)4 and C ­ u2ZnSnS4 are also quaternary
at a temperature of 350 °C which was confirmed from mor- chalcogenide materials with the most recent efficiencies
phological studies. The optical absorption showed a contin- of 12.6 and 8.4% respectively [3, 4]. Among these materi-
uous absorption in the UV–Visible region and the obtained als ­Cu2ZnSnS4 (CZTS) attained a distinctive consideration
band gap for the films was in the range of 1.45–1.62 eV. because of its innoxious nature, less expensive and suffi-
The electrical study using Hall measurement proved the cient source availability aspects [5]. Moreover, CZTS is a
p-type conductivity of CZTS thin films which is pertinent p-type absorber layer in the photovoltaic cell with a direct
for an absorber layer in thin film photovoltaic cell. A p–n optimum band gap of 1.45 eV and has an absorption coef-
junction was formed with ­TiO2 and CZTS materials and the ficient of >104 cm−1 [6]. The UV-Vis region reaches maxi-
light and dark current response was studied. mum from the solar spectrum to Earth’s surface and this
material is suitable for absorbance in this region [7]. This
material has a wide range of application in all the fields
including antibacterial activity and biomedical application
with a specific interest in photovoltaics [8, 9].
* A. Leo Rajesh A number of deposition techniques were employed to
aleorajesh@gmail.com
prepare CZTS thin films for the fabrication of solar cell.
1
Department of Physics, St. Joseph’s College (Autonomous), Vacuum based techniques like pulsed laser deposition,[10]
Tiruchirappalli, Tamilnadu 620002, India RF sputtering, [11] thermal evaporation, [12] and chemical
2
Department of Physics, National Institute of Technology, methods such as chemical bath deposition, [13] spin coat-
Tiruchirappalli, Tamilnadu 620015, India ing, [14, 15] dip coating [16] and spray pyrolysis [17, 18]
3
UGC-DAE Consortium for Scientific Research, Kalpakkam, were used. Among these techniques spray pyrolysis is an
Tamilnadu 603104, India inexpensive and wide surface area applicable technique.

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The CZTS thin films deposited using chemical spray pyrol- spray pyrolysis technique. The solution was prepared
ysis technique are uniform and thick which is suitable for by dissolving the above mentioned respective precursor
absorber layer in photovoltaic cell. Since both the method materials in 80 ml of solvent. 2-methoxyethanol was used
and the material are low-cost and high yield, it is of sig- as the solvent in order to dissolve the precursor solution
nificant interest to the researchers. Various levels of optimi- without any formation of precipitates. The solution was
zations like concentration variation, [19] precursor effect, stirred for half an hour in the magnetic stirrer at room
[20] doping, [21] pH [22] etc. are already being done on temperature and a colorless solution was obtained. The
this material using spray pyrolysis technique. The main prepared solution was sprayed onto the preheated SLG
drawback experienced in the spray technique for CZTS thin substrates. The substrates were previously cleaned by
films is the precursor solution preparation. The instability sonicating in soap solution followed by HCl and finally
in the precursor solution preparation leads to the precipi- with double distilled water and it was used immediately
tate formation which complicates the spraying of solution. after washing. The pressure was fixed as 1.2 Kg cm−2
In order to overcome this, the precursor solution is being using compressed air as the carrier gas and the spray
prepared in different approaches like the use of different rate was maintained at 6 ml ­min−1. The substrate to noz-
solvents methanol, hydrazine, ethanol/water, dimethyl sul- zle distance was approximately kept at 25 cm. The films
foxide, etc.[23–25] and the arrangement of mixing order of were deposited continuously with 30 s spraying and 30 s
source materials [26] are done. decomposing process for eight times. The films were
The novelty of this work stipulates the use of 2-meth- prepared at different substrate temperatures such as 280,
oxyethanol as solvent in precursor solution preparation for 300, 350 and 400 °C. The film deposited at 250 °C did not
chemical spray pyrolysis deposition. The precursor sources spread uniformly due to less evaporation and pyrolysis
chosen for this work is C ­ uCl2, ­ZnCl2, ­SnCl2 and thiourea. effect at lower temperature and the films appeared as a
Among these elements ­ZnCl2 and ­SnCl2 are not easily solu- solution sprinkled over the substrate. The samples were
ble in water and forms insoluble base materials in the solu- denoted as 280C, 300C, 350C and 400C for the films
tion. In order to avoid this problem the solvent 2-methox- deposited at 280, 300, 350 and 400 °C respectively in the
yethanol is used which has a boiling point of 124–125 °C following discussions.
and is mostly used in sol–gel preparation due to its opti-
mum viscosity. The effect of solvent is studied at different
temperatures 280, 300, 350 and 400 °C and the precursor 2.3 Characterization
solutions were free from precipitates which lead to the
easy and repeatable preparation of solution on necessary The prepared films were characterized to confirm the
situations. structure and phase identification by using (Bruker D8
Discover) Glancing incidence X-Ray Diffractometer
and (Renishaw inVia) Raman microscope at room tem-
2 Experimental Preparation perature. The Bruker D8 Discover High resolution X-ray
Diffractometer was used to study the GIXRD pattern of
2.1 Materials ­Cu2ZnSnS4 thin film at a scanning range of 15° to 70°
and a step size of 0.04° with the C ­ uKα1 source radia-
The precursor materials for Copper, Zinc, Tin and Sulfur tion (= 1.54051 Å). The Raman spectrum was recorded
elements were Copper(II) chloride dihydrate (­ CuCl2·2H2O) at room temperature using 785 nm near IR laser source
10 mM, Zinc chloride ­(ZnCl2·2H2O) 5 mM, Tin(II) chlo- wavelength.
ride dihydrate ­(SnCl2·2H2O) 5 mM and Thiourea ­(CH4N2S) The surface morphology and cross-section was studied
40 mM respectively. Titanium(IV) isopropoxide was used using (Carl Zeiss) field emission scanning electron micro-
as precursor for Titanium. 2-methoxyethanol was used as scope with 10 kV extra-high tension (EHT) voltage. The
solvent for CZTS preparation and isopropyl alcohol was optical properties of the CZTS thin films were studied to
used as solvent for T­ iO2 preparation. The precursors were observe the absorption region and to acquire the band gap
purchased in Merck grade and used as purchased without using (Jasco V670) UV–Visible-NIR spectrophotometer.
any further purification. The electrical properties of the prepared films were stud-
ied using (ECOPIA AMP 55) Hall Effect measurement
2.2 Preparation of Precursor Solution and Deposition system. The type of conductivity, carrier concentration,
of Thin Films resistivity, mobility and Hall coefficient were all studied
using this system. The photocurrent response was stud-
A precursor solution has to be prepared without precipi- ied using keithley (source meter 2450) and Xenon lamp
tates in order to deposit CZTS thin films using chemical source.

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3 Results and Discussion The crystallite size was calculated using the
Debye–Scherrer’s formula [28]
3.1 Structural Analysis
0.9𝜆
D= (1)
𝛽 cos 𝜃
Figure 1a gives the glancing incidence X-ray diffrac-
tion (GIXRD) pattern of CZTS thin films with prominent where D is the crystallite size, λ is the wavelength of the
peaks at Bragg angle 2θ = 28.5°, 47.5° and 56.2° which target CuK𝛼1(= 1.54051), β is the full-width half maximum
agrees with the standard value for Kesterite tetragonal value and θ is the position of respective Bragg peaks.
­Cu2ZnSnS4 phase (JCPDS: 26–0575) with lattice plane The crystallite size is in the range of 3–5 nm which
orientation along (112), (220) and (312) direction respec- shows broad diffraction peaks in Fig. 1a. The condensed
tively [27]. The CZTS thin films deposited at 280, 300 and crystallite size is resulted from the super saturation of pre-
350 °C using 2-methoxyethanol solvent shows polycrys- cursors in the solvent. The 350 °C was reported as opti-
talline nature with a strong orientation along (112) phase. mized temperature for the aqueous CZTS thin films as per
From the GIXRD pattern it is evident that the crystalline the literature [29].
nature of the material deposited with solvent increases with The phase confirmation of ­ Cu2ZnSnS4 thin films is
increase in temperature and there is no significant change insufficient with XRD patterns alone since other struc-
in the crystallite size with respect to the temperature. The tures like ZnS and ­Cu2SnS3 also show similar XRD pat-
350 °C film contributes maximum intensed peaks showing tern [30]. In order to confirm the crystal structure and the
good crystallinity. phase purity, Raman spectrum details are necessary for
­Cu2ZnSnS4 thin films. Figure 1b gives the room tempera-
ture Raman spectrum of ­Cu2ZnSnS4 thin films. The utili-
zation of higher excitation wavelength like 785 nm would
(a) be helpful in fluorescence suppression and the detection
of various secondary phases and oxides [31]. The Raman
spectra were covered in the range from 100 to 600 cm−1.
The broad and intensed mode located at 334 cm−1 indi-
cates the formation of ­Cu2ZnSnS4 phase and a shoulder
at 289 cm−1 also shows the presence of ­Cu2ZnSnS4 phase
[32–34]. The prominent peaks 334 and 289 cm−1 arises
from A1 vibration mode in which only Sulphur ­S2− anions
are involved whereas the weak mode at 367 cm−1 corre-
sponds to B1 vibrational mode with cation moving in the
z-direction [35–37]. The presence of 472 cm−1 [38] mode
in 280 °C shows the ­Cu2S impurity phase formation which
was later missing in the films prepared at higher tempera-
tures. This infers that the impurity phase formation reduces
(b) with increase in temperature.

3.2 Morphological Studies

Figure 2 shows the surface morphology and the cross-sec-

tional FESEM images. There is a significant variation in the
surface morphology of the films with respect to temperature.
The surface morphology details explain that the film 350 °C
acquire nano flakes structure and are uniform without cracks
[39]. From Fig. 2c it is observed that at 350 °C temperature
the film growth is uniform and for all other temperatures
micro cracks are seen which show the discontinuity in the
upper surface of the coating. Since CZTS is a polycrystalline
material the grain boundaries are seen in the image. The inset
shows cross-sectional image of the samples which is used to
Fig. 1  a GIXRD pattern of CZTS thin films at different temperatures measure the thickness of thin film. The thickness of the sam-
and b Raman spectrum of ­Cu2ZnSnS4 thin films ples is 548.8, 628.4, 767.7 and 891.5 nm for 280, 300, 350

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Fig. 2  FESEM morphology of a 280 °C b 300 °C c 350 °C d 400 °C and inset is the cross-section of each sample respectively

and 400 °C respectively. There is no agglomeration in the where absorption coefficient, A is a constant, Eg is the
surface since there is no precipitate formation. band gap energy and n = ½ for direct allowed transitions.
The Tauc plot gives the direct band gap of the sam-
3.3 Optical Studies ples which is suitable for absorber layer. The band gap
is obtained as 1.62, 1.60, 1.45 and 1.54 eV for 280C,
Figure 3a shows the absorption spectra of ­Cu2ZnSnS4 thin 300C, 350C and 400C respectively. The optimum band
films taken by UV–Visible spectrophotometer. The spectra gap applicable for absorber layer in photovoltaic cell
show maximum absorption at 300–350 nm followed by a was obtained for 350C film. The band gap was reducing
continuous absorption in the visible region and reduces down from 280C to 350C, reached the optimum value at 350C
to the near IR region about 900 nm. The cut-off wavelength in and again increases to higher value at 400C [15]. As the
the near IR region shows that the material is suitable for solar thickness increases the color of the film improves from
cell application. The optical band gap of the films was cal- dark brown to black color [41] and it is seen that the
culated using Tauc plot as shown in Fig. 3b. The direct band absorption edge is in the near IR region. The absorbance
gap is obtained from the extrapolation of the linear part of the also improves at higher temperature due to the thick-
curve in the plot of hν in eV versus (αhν)2 in ­(cm−1 eV)2. The ness of the film. The absorption coefficient of the films
optical band gap is calculated using the formula [40]. is found to be ­104 cm−1 which is suitable for photovoltaic
application.
𝛼h𝜈 = A[h𝜈 − Eg ]n (2)

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(a) Table 1  Carrier concentration, mobility, resistivity and Hall co-effi-

cient of CZTS thin films
Sample (ºC) Carrier Mobility Resis- Hall coef-
concentration ­(cm2 V−1 s−1) tivity ficient
­(cm−3) (Ω cm) ­(cm3 C−1)

280 3.24 × 1013 180 1070 193,000

300 6.92 × 1014 4.60 1960 9020
350 1.25 × 1015 17.1 292 4980
400 4.53 × 1017 1.95 7.08 13.8

of CZTS thin films. The variation in mobility, carrier con-

centration and Hall coefficient with temperature is shown
in Fig. 4. It shows that mobility and Hall coefficient are
inversely proportional to carrier concentration.
The photocurrent response of the device was studied and
is shown in Fig. 5. The heterojunction FTO/TiO2/CZTS/Ag
was formed and studied. The response shows increase in
(b)
current value for increase in temperature. It showed a maxi-
mum current response of 10 mA for 350 °C film which is
shown in Fig. 5c. The Fig. 5a–d shows the light (l) and dark
(d) current response.

4 Conclusion

The spray pyrolysed CZTS thin films were deposited with-

out precipitates using 2-methoxyethanol solvent. The effect
of solvent at different substrate temperatures revealed sig-
nificant difference in the crystalline nature, surface mor-
phology, optical absorption, band gap energy and electri-
cal parameters. The carrier concentration and mobility had
increased with temperature and all the films showed p-type
conductivity. The photocurrent response also showed maxi-
Fig. 3  a Absorption spectrum of C
­ u2ZnSnS4 thin films b Tauc plot mum current value for 350 °C. At 350 °C the film obtained

3.4 Electrical Studies

Hall measurement for the CZTS thin films was performed

by applying 0.59 T magnetic field at room temperature
and the contact was made with silver paste. All the films
showed p-type conductivity which is pertinent for photovol-
taic application. The samples show low resistivity at higher
deposition temperature which indicates that the resistance
decreases with increase in deposition temperature [42]. The
carrier concentration, mobility, resistivity and Hall coef-
ficient values are listed in Table 1. The carrier concentra-
tion increases with increase in temperature from ­1013 to
­1017 cm−3, whereas the resistivity and mobility decreases
with increase in temperature from 1070 to 7.08 Ω cm and
180 to 1.95 cm2 V−1 s−1 respectively [15, 43]. The positive Fig. 4  Carrier concentration, mobility and Hall coefficient variation
value of Hall coefficient denotes the p-type conductivity of CZTS films at different substrate temperatures

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(a) (b)

(c) (d)

Fig. 5  Photocurrent response of a 280 °C b 300 °C c 350 °C and d 400 °C

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