materials
Article
Copper-Zinc-Tin-Sulfur Thin Film Using
Spin-Coating Technology
Min-Yen Yeh 1 , Po-Hsun Lei 2, *, Shao-Hsein Lin 2 and Chyi-Da Yang 1
1
2
*
Department of Microelectronic Engineering, National Kaohsiung Marine University, Kaohsiung 811, Taiwan;
[email protected] (M.-Y.Y.); [email protected] (C.-D.Y.)
Institute of Electro-Optical and Materials Science, National Formosa University, 64 Wen-Hwa Rd, Hu-Wei,
Yun-Lin 632, Taiwan; [email protected]
Correspondence: [email protected]; Tel.:+886-5-6315668; Fax: +886-5-6329257
Academic Editor: Dirk Poelman
Received: 22 May 2016; Accepted: 27 June 2016; Published: 29 June 2016
Abstract: Cu2 ZnSnS4 (CZTS) thin films were deposited on glass substrates by using spin-coating
and an annealing process, which can improve the crystallinity and morphology of the thin films.
The grain size, optical gap, and atomic contents of copper (Cu), zinc (Zn), tin (Sn), and sulfur (S) in a
CZTS thin film absorber relate to the concentrations of aqueous precursor solutions containing copper
chloride (CuCl2 ), zinc chloride (ZnCl2 ), tin chloride (SnCl2 ), and thiourea (SC(NH2 )2 ), whereas
the electrical properties of CZTS thin films depend on the annealing temperature and the atomic
content ratios of Cu/(Zn + Sn) and Zn/Sn. All of the CZTS films were characterized using X-ray
diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS),
Raman spectroscopy, and Hall measurements. Furthermore, CZTS thin film was deposited on an
n-type silicon substrate by using spin-coating to form an Mo/p-CZTS/n-Si/Al heterostructured
solar cell. The p-CZTS/n-Si heterostructured solar cell showed a conversion efficiency of 1.13% with
V oc = 520 mV, Jsc = 3.28 mA/cm2 , and fill-factor (FF) = 66%.
Keywords: CZTS thin film; solar cell; optical band gap; conversion efficiency
1. Introduction
Semiconductor solar cells incorporating thin-film absorbers are an attractive option for replacing
classical silicon-based solar cells. In recent years, several thin-film absorbers such as III–V compound
material (GaInP/GaAs) [1–3], I–III–VI compound material (Cu(In, Ga)Se2 , CIGS) [4–6], and I–II–VI
compound material (Cu2 ZnSnS4 , CZTS) [7–9] have been investigated for semiconductor thin-film
solar cells. Among these studied materials, direct band gap chalcopyrite-structured CIGS and
kesterite-structured CZTS are potential absorber materials for the next generation of thin-film solar cells
because of their large optical absorption coefficients over the visual light range. A record conversion
efficiency of 20.3% was reported for a CIGS thin-film solar cell [4]. However, use of toxic Se and scarce
or expensive elements, such as gallium (Ga) and indium (In), may limit the production development
of CIGS thin-film solar cells. Researchers strongly desire to identify a nontoxic inexpensive thin-film
absorber with high conversion efficiency to replace CIGS. CZTS has been considered as an alternative
to CIGS because of its majority carrier type (p-type), direct band gap of 1.4–1.5 eV, and large optical
absorption coefficient (>104 cm´1 ) over the visible light range [10]. In addition, CZTS thin films
are composed of elements that are nontoxic, abundant, and inexpensive, namely copper, zinc, tin,
and sulfur.
CZTS thin films are generally fabricated using vacuum-based techniques such as thermal
evaporation [11], sputtering [12], and pulsed laser deposition [13]. These methods can deposit
high-quality CZTS thin films on molybdenum (Mo)-coated glass substrates. However, these techniques
Materials 2016, 9, 526; doi:10.3390/ma9070526
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Materials 2016, 9, 526
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require complicated equipment to maintain a vacuum and high process temperatures. In addition
to high conversion efficiency, film content and composition as well as large-scale rather than
small-area cell structural uniformity are challenges in developing CZTS thin-film solar cells for
industrial use. Many nonvacuum methods of reducing production costs and satisfying the large-area
requirements have been studied, including electrodeposition [14], spray pyrolysis techniques [15],
sol-gel processing [16], spin-coating methods [17], and chemical bath methods [18]. Among these,
the spin-coating method has the advantages of simple construction, low cost, ecological safety, and
the large-scale deposition of different semiconductor thin films. However, few studies have reported
the preparation of spin-coated CZTS thin films without toxic precursors such as hydrazine [19] or
ethanolamine (MEA) [17]. Researchers do not completely and clearly understand the relationships
of the deposition parameters to the atomic contents of Cu, Zn, Sn, and S, surface morphology,
and electrical properties of spin-coated CZTS thin films.
In this study, we developed spin-coating Cu-poor and Zn-rich CZTS thin films with optimal
atomic contents of Cu, Zn, Sn, and S onto glass and silicon (Si) substrates. The atomic contents of
Cu, Zn, Sn, and S, crystalline morphology, and surface morphology of these CZTS thin films were
characterized using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) patterns,
field-emission scanning electron microscopy (FE-SEM) images, and Raman measurements. The optical
band gap and electrical properties were measured using absorption spectra and Hall measurements.
Finally, an optimal CZTS thin was spun on an n-type Si substrate to form a Mo/p-CZTS/n-Si/Al
solar cell.
2. Results and Discussion
The crystallinity, morphology, electrical properties, and band gap energy of CZTS thin films show
a strong dependence on the stoichiometry of Cu, Zn, Sn, and S in those CZTS thin films [9,16,20–22].
Well-controlled atomic contents of Cu, Zn, Sn, and S in CZTS thin films has become a crucial issue for
fabricating high-performance kesterite CZTS solar cells. To determine the optimal atomic contents
of Cu, Zn, Sn, and S in CZTS thin films, the chemical concentrations of CuCl2 , ZnCl2 , SnCl2 , and
thiourea aqueous precursor solution should be analyzed. Figure 1a–d show the atomic content ratios
of Cu/(Zn + Sn), Cu/Zn, Zn/Sn, and S/(Cu + Zn + Sn) for CZTS thin films as functions of the
concentrations of CuCl2 , ZnCl2 , SnCl2 , and thiourea aqueous precursor solution, respectively, for
an annealing temperature of 350 ˝ C. As shown in Figure 1a–c, the atomic contents of Cu, Zn, and
Sn increase with raising the concentration of CuCl2 , ZnCl2 , and SnCl2 aqueous precursor solution,
respectively, implying that the atomic contents of Cu, Zn, Sn, and S in CZTS thin films can be modulated
by changing the concentrations of aqueous precursor solution. Cu-rich CZTS thin films can facilitate
the formation of passivated defect clusters such as CuZn + SnZn and 2CuZn + SnZn . The former
produces a deep donor level in the band gap of CZTS, whereas the latter can significantly reduce
the band gap of CZTS [20]. However, a high Zn/Sn ratio or high Zn atomic content is necessary to
form the ZnSn acceptors and reduce the SnZn donors to increase the conductivity of the CZTS thin
film [23]. However, high Zn atomic content, which results in a low Cu/Zn ratio, can bring about a p/n
conduction type [22]. To avoid defect clusters and increase the conductivity of p-type CZTS thin films,
the Cu/(Zn + Sn) atomic content ratio should be less than 1, the Zn/Sn atomic content ratio should
be more than 1, and the Cu/Zn atomic content ratio should not be too low. In Figure 1a, the atomic
content ratio of Cu/(Zn + Sn) is lower than 1 for the CuCl2 concentration below 0.14 M and approaches
1 at the CuCl2 concentration of 0.16 M. When the ZnCl2 , SnCl2 , and thiourea concentrations increase
with a CuCl2 concentration of 0.14 M, the Cu/(Zn + Sn) atomic content ratio remains below 1, as seen
in Figure 1b–d. These results indicate that Cu-poor and Zn-rich CZTS thin films can be produced
from properly controlled concentrations of aqueous precursor solutions. The atomic content ratio
of S/Cu + Zn + Sn in CZTS thin films can approach 1 for the concentrations of CuCl2 , ZnCl2 , and
SnCl2 , as shown in Figure 1a–c, respectively. In addition, in Figure 1d, the atomic content of S increases
Materials 2016, 9, 526
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slightly as the thiourea concentration rises from 0.6 to 0.8 M, and saturates at a thiourea concentration
above 0.8
M,9,possibly
because of the solubility of S in the CZT thin film.
Materials
2016,
526
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3.0
3.0
Conten Ratio
Conten Ratio
3.5
Cu/Zn+Sn
Cu/Zn
Zn/Sn
S/Cu+Zn+Sn
2.5
2.0
1.5
1.0
2.0
1.5
1.0
0.5
0.0
2.5
Cu/Zn+Sn
Cu/Zn
Zn/Sn
S/Cu+Zn+Sn
0.5
0.10
0.12
0.14
0.0
0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11
0.16
ZnCl2 Concentration (M)
CuCl2 Concentration (M)
(a)
Cu/Zn+Sn
Cu/Zn
Zn/Sn
S/Cu+Zn+Sn
Cu/Zn+Sn
Cu/Zn
Zn/Sn
S/Cu+Zn+Sn
2.5
2.0
Content Ratio
Content Ratio
2.5
1.5
1.0
0.5
0.0
0.02
(b)
3.0
3.0
2.0
1.5
1.0
0.5
0.03
0.04
0.05
0.06
0.07
0.08
SnCl2 Concentration (M)
(c)
0.09
0.10
0.0
0.6
0.7
0.8
Thiourea Concentration (M)
0.9
(d)
Figure 1. Atomic content ratios of Cu/(Zn + Sn), Cu/Zn, Zn/Sn, and S/(Cu + Zn + Sn) for copper-zincFigure 1. Atomic content ratios of Cu/(Zn + Sn), Cu/Zn, Zn/Sn, and S/(Cu + Zn + Sn) for
tin-sulfur (CZTS) thin films with (a) CuCl2; (b) ZnCl2; (c) SnCl2; and (d) thiourea concentrations,
copper-zinc-tin-sulfur (CZTS) thin films with (a) CuCl2 ; (b) ZnCl2 ; (c) SnCl2 ; and (d) thiourea
respectively.
concentrations, respectively.
Figure 2a–c represent the XRD patterns for films with various concentrations of CuCl2, ZnCl2,
Figure
2a–c represent the XRD patterns for films with various concentrations of CuCl2 , ZnCl2 ,
and SnCl
2 aqueous precursor solutions that were annealed at a temperature of 350 °C. The XRD peaks
˝ C. The XRD
and
SnCl
aqueous
solutions
that
annealed at
a temperature
350CZTS
2
were indexed using precursor
JADE software
(Jade
5.0,were
Christchurch,
New
Zealand) asofthe
kesterite
peaks
were
indexed
using
JADE
software
(Jade
5.0,
Christchurch,
New
Zealand)
as
the
CZTS
kesterite
structure; and they closely matched the standard data (PDF#26-5075). All of the spin-coated
CZTS
structure;
they closely
matched the
standard
data (PDF#26-5075).
All broad
of the spin-coated
thin
thin
films and
exhibited
a polycrystalline
kesterite
crystal
structure with six
peaks alongCZTS
the (101),
films
exhibited
a
polycrystalline
kesterite
crystal
structure
with
six
broad
peaks
along
the
(101),
(112),
(112), (200), (220), (312) and (224) planes as shown in Figure 2a–c. The secondary phases including
(200),
(220), (312) and (224) planes as shown in Figure 2a–c. The secondary phases including Cu2 SnS
Cu
2SnS3, CuS, SnS and ZnS would form during growth of CZTS thin films. They will degrade
the3 ,
CuS, SnS andof
ZnS
would
form
during
growth of to
CZTS
thin
They will
degrade the
performance
performance
CZTS
solar
cells.
It is important
avoid
thefilms.
formation
of secondary
phases
in CZTS
of
CZTS
solar
cells.
It
is
important
to
avoid
the
formation
of
secondary
phases
in
CZTS
thincubicfilms.
thin films. Among these secondary phases, tetragonal-structured Cu2SnS3 (PDF#33-0501) and
Among these
phases,
tetragonal-structured
Cu2 SnSto
and cubic-structured
3 (PDF#33-0501)
structured
ZnSsecondary
(PDF#05-0566)
show
the similar XRD patterns
kesterite-structured
CZTS between
ZnS
(PDF#05-0566)
show
the
similar
XRD
patterns
to
kesterite-structured
CZTS
between
diffraction angles of 20 and 80 degrees because of the same lattice constants [15].
As a diffraction
result, the
angles
of
20
and
80
degrees
because
of
the
same
lattice
constants
[15].
As
a
result,
the
measured
measured XRD patterns of CZTS thin film might be related to either CZTS or Cu2SnS3 or ZnS
pahse.
XRDweak
patterns
of CZTS
thin
film might
related
to either
CZTS
or Cu2 SnS3 or ZnS pahse.
The
peak
at 18.2°
attributed
to be
(101)
plane
indicates
a kesterite-structured
CZTS The
thinweak
film
˝ attributed to (101) plane indicates a kesterite-structured CZTS thin film because it is not
peak at 18.2
because
it is not
found in the XRD patterns of tetragonal-structured Cu2SnS3 and cubic-structured
foundConsequently,
in the XRD patterns
of tetragonal-structured
cubic-structured
ZnS.
2 SnS3 and
ZnS.
a CZTS
thin film without Cu
secondary
phase,
which leads
toConsequently,
a degraded
a CZTS thin film
without
which
leads
to a degraded
performance
of CZTS
solar
performance
of CZTS
solarsecondary
cells, can bephase,
obtained
using
properly
controlled
concentrations
of aqueous
cells,
can
be
obtained
using
properly
controlled
concentrations
of
aqueous
precursor
solutions
and
precursor solutions and a proper annealing process. To further confirm the XRD characteristics ofa
proper
annealingCZTS
process.
further
confirm
XRD
characteristics
of this
spin-coated
this
spin-coated
thinTo
film,
which
differ the
from
similar
XRD patterns
such
as those ofCZTS
ZnS thin
and
film,
which
differ
from
similar
XRD
patterns
such
as
those
of
ZnS
and
Cu
SnS
,
Raman
spectroscopy
2
3
Cu2SnS3, Raman spectroscopy was performed. Figure 2e shows the Raman spectrum for a spin-coated
was performed.
Figure 2e
shows
Raman
spectrum for a spin-coated CZTS thin film prepared
CZTS
thin film prepared
from
0.14 the
M CuCl
2, 0.07 M ZnCl2, 0.07 M SnCl2, and 0.8 M thiourea aqueous
from 0.14 M
CuCl2 , 0.07
ZnCl2 , at
0.07
M°C.
SnCl
0.8 M thiourea
precursor
and
2 , and
−1,
precursor
solutions
and M
annealed
350
The
characteristic
CZTSaqueous
peak in Figure
2e issolutions
at 336.2 cm
˝ C. The characteristic CZTS peak in Figure 2e is at 336.2 cm´1 , which is different from
annealed
at
350
which is different from the Raman peaks of Cu2–xS at 475 cm−1, ZnS at 273 and 351 cm−1, Cu2SnS3 at 290
and 352 cm−1, and of Sn2S3 at 234 and 307 cm−1, indicating a spin-coated CZTS thin film deposited on
glass substrate [24,25].
Materials 2016, 9, 526
4 of 12
the Raman peaks of Cu2–x S at 475 cm´1 , ZnS at 273 and 351 cm´1 , Cu2 SnS3 at 290 and 352 cm´1 , and of
´1
Sn2 S3 at2016,
234 9,and
Materials
526 307 cm , indicating a spin-coated CZTS thin film deposited on glass substrate [24,25].
4 of 12
CZTS (112)
CZTS (112)
CZTS (220) CZTS (312)
CZTS (101)
CZTS (220) CZTS (312)
CZTS (200)
CZTS (224)
Intensity (A. U.)
Intensity (A. U.)
CZTS (224)
CZTS (101) CZTS (200)
0.16 M
0.14 M
0.09 M
0.07 M
0.05 M
0.12 M
0.03 M
0.1 M
20
30
40
50
60
70
20
80
30
Two Theta (Deg.)
40
60
70
80
(b)
(a)
CZTS (112)
50
Two Theta (Deg.)
CZTS (220) CZTS (312)
CZTS (224)
CZTS (101) CZTS (200)
[CuCl2] = 0.14 M
[ZnCl2] = 0.07 M
CZTS
0.09 M
0.07 M
0.05 M
[SnCl2] = 0.07M
[CH4N2S] = 0.8 M
Intensity (A. U.)
Intensity (A. U.)
336.2 cm-1
Annealing Temp. = 350 oC
0.03 M
20
30
40
50
60
Two Theta (Deg.)
(c)
70
80
200 250 300 350 400 450 500 550 600 650 700
Raman Shift (cm-1)
(d)
Figure 2. X-ray diffraction (XRD) patterns for CZTS thin film with varied concentration of (a) CuCl2;
Figure 2. X-ray diffraction (XRD) patterns for CZTS thin film with varied concentration of (a) CuCl2 ;
(b) ZnCl2; and (c) SnCl2 aqueous precursor solution under an annealing temperature of 350 °C. The
(b) ZnCl2 ; and (c) SnCl2 aqueous precursor solution under an annealing temperature of 350 ˝ C.
2, ZnCl2, SnCl2, and thiourea aqueous precursor
Raman
spectrum
for for
a CZTS
thin
film
with
The Raman
spectrum
a CZTS
thin
film
withCuCl
CuCl
2 , ZnCl2 , SnCl2 , and thiourea aqueous precursor
solution
concentration
of
0.14,
0.07,
0.07,
and
0.8
M,
after after
annealing
at 350 at
°C350
is shown
˝ C is
solution concentration of 0.14, 0.07, 0.07, and 0.8 respectively,
M, respectively,
annealing
in
(d).
shown in (d).
The mean crystallite grain size (D) of CZTS thin film can be estimated by the Scherrer formula [26]:
The mean crystallite grain size (D) of CZTS thin film can be estimated by the Scherrer formula [26]:
0.9λ
=
(1)
0.9λ
β cos θ
D“c
(1)
βcosθ
where β, λ, and θB are the line width at full width at half the maximum of the film diffraction peak at
2è,
X-ray
wavelength
nm), and
Bragg
diffraction
angle,
respectively.
Figure
3a–c show
where
β, λ,
and θB are(0.15406
the line width
at full
width
at half the
maximum
of the film
diffraction
peakthe
at
full
widthwavelength
at half maximum
(FWHM)
of (112)
peak
and calculated
mean crystallite
grain
sizeshow
of CZTS
2θ, X-ray
(0.15406
nm), and
Bragg
diffraction
angle, respectively.
Figure
3a–c
the
thin
films at
as half
a function
of the
variedofconcentration
of calculated
CuCl2, ZnCl
2, and
SnCl2 aqueous
precursor
full width
maximum
(FWHM)
(112) peak and
mean
crystallite
grain size
of CZTS
solutions
annealed
at varied
a temperature
of 350 °C.
The grain
size
enlarges
with
increasing
CuCl2
thin filmsthat
as awere
function
of the
concentration
of CuCl
,
ZnCl
,
and
SnCl
aqueous
precursor
2
2
2
concentration,
as shown
in Figure
3a becauseof
of350
the˝increase
of Cu
atomic
content
the CZTS
thin2
solutions that were
annealed
at a temperature
C. The grain
size
enlarges
with in
increasing
CuCl
films
[27,28]. This
increaseininFigure
the grain
size might
be due
to agglomeration
grains.inInthe
addition,
the
concentration,
as shown
3a because
of the
increase
of Cu atomicof
content
CZTS thin
grain
size of CZTS
thin films
represents
a slight
2, and SnClof
2 concentrations
increase
films [27,28].
This increase
in the
grain size
mightchange
be dueas
toZnCl
agglomeration
grains. In addition,
the
or
decrease
while
CuCl
2
concentration
is
maintained
at
0.14
M,
as
shown
in
Figure
3b,c.
This
is
grain size of CZTS thin films represents a slight change as ZnCl2 , and SnCl2 concentrations increase or
attributed
to theCuCl
fairly
constant Cu atomic
contentatin0.14
these
thinin
films.
Consequently,
the grain
decrease while
is maintained
M, CZTS
as shown
Figure
3b,c. This is attributed
2 concentration
size
offairly
CZTSconstant
thin films
the content
same annealing
temperature
is determined
by the
atomic
to the
Cuwith
atomic
in these CZTS
thin films.
Consequently,
theCu
grain
size content.
of CZTS
thin films with the same annealing temperature is determined by the Cu atomic content.
Materials 2016, 9, 526
Materials 2016, 9, 526
14
20
18
12
16
14
10
12
0.10 0.11 0.12 0.13 0.14 0.15 0.16
24
20
22
18
20
16
18
14
16
12
14
10
12
8
8
10
0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
CuCl2 Concentration (M)
ZnCl2 Concentration (M)
Cu Content (at %)
(a)
(b)
24
20
22
18
20
16
18
14
16
12
14
10
12
Calculated Grain Size (nm)
10
Cu Content (at %)
Cu Content (at %)
22
Calculated Grain Size (nm)
16
24
Calculated Grain Size (nm)
5 of 12
5 of 12
8
10
0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10
SnCl2 Concentration (M)
(c)
Figure
Figure 3.
3. Full
Full width
width at
at half
half maximum
maximum (FWHM)
(FWHM) of
of (112)
(112) peak
peak and
and calculated
calculated grain
grain size
size for
for CZTS
CZTS thin
thin
films
with
varied
concentration
of
(a)
CuCl
2; (b) ZnCl2; and (c) SnCl2 aqueous precursor solution
films with varied concentration of (a) CuCl2 ; (b) ZnCl2 ; and (c) SnCl2 aqueous precursor solution under
under
an annealing
temperature
an annealing
temperature
of 350 ˝ofC.350 °C.
The conversion efficiency of a CZTS thin-film solar cell is strongly associated with the grain size
The conversion efficiency of a CZTS thin-film solar cell is strongly associated with the grain size
because an absorber layer with a large grain size maximizes the minority carrier diffusion length and
because an absorber layer with a large grain size maximizes the minority carrier diffusion length and
the inherent potential of a polycrystalline solar cell [16,24]. Figure 4 shows the FE–SEM images of the
the inherent potential of a polycrystalline solar cell [16,24]. Figure 4 shows the FE–SEM images of the
CZTS thin film. Figure 4a–c show plane views of CZTS thin films with CuCl2 concentrations of 0.1,
CZTS thin film. Figure 4a–c show plane views of CZTS thin films with CuCl concentrations of 0.1, 0.14,
0.14, and 0.16 M with concentrations of ZnCl2, SnCl2, and thiourea 2at 0.07, 0.07, and 0.8 M,
and 0.16 M with concentrations of ZnCl , SnCl , and thiourea at 0.07, 0.07, and 0.8 M, respectively. The
respectively. The thickness of all the 2CZTS 2thin films was approximately 2.4 ìm, and Figure 4d
thickness of all the CZTS thin films was approximately 2.4 µm, and Figure 4d represents the FE-SEM
represents the FE-SEM cross section image of CZTS thin film with concentrations of CuCl2, ZnCl2,
cross section image of CZTS thin film with concentrations of CuCl2 , ZnCl2 , SnCl2 , and thiourea at
SnCl2, and thiourea at 0.14, 0.07, 0.07, and 0.8 M. As shown in Figure
4a–c, large numbers of voids
0.14, 0.07, 0.07, and 0.8 M. As shown in Figure 4a–c, large numbers of voids were observed in the
were observed in the Cu-rich CZTS thin films. Voids of Cu-rich CZTS absorbers in thin-film solar
Cu-rich CZTS thin films. Voids of Cu-rich CZTS absorbers in thin-film solar cells cause low conversion
cells cause low conversion efficiencies for photovoltaic applications [16,29]. Well-controlled Cu
efficiencies for photovoltaic applications [16,29]. Well-controlled Cu atomic content is important to
atomic content is important to obtain a large grain size and less number of voids in CZTS thin films.
obtain a large grain size and less number of voids in CZTS thin films. Figure 4e shows the plane
Figure 4e shows the plane view of a CZTS thin film with a high ZnCl2 concentration of 0.1 M and
view of a CZTS thin film with a high ZnCl2 concentration of 0.1 M and CuCl2 , SnCl2 , and thiourea
CuCl2, SnCl2, and thiourea concentrations of
0.14, 0.07, and 0.8 M. The surface of Zn-rich CZTS thin
concentrations of 0.14, 0.07, and 0.8 M. The surface of Zn-rich CZTS thin film is smoother than that of
film is smoother than that of Zn-poor CZTS thin films because of the large columnar grains at the
Zn-poor CZTS thin films because of the large columnar grains at the bottom [30].
bottom [30].
Materials 2016, 9, 526
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(a)
(b)
(c)
(d)
(e)
Figure 4. Plane-view field-emission scanning electron microscopy (FE–SEM) images of CZTS thin
Figure 4. Plane-view field-emission scanning electron microscopy (FE–SEM) images of CZTS thin films
films deposited
with ZnCl
2, SnCl2, and thiourea concentrations of 0.07, 0.07, and 0.8 M, respectively,
deposited
with ZnCl
2 , SnCl2 , and thiourea concentrations of 0.07, 0.07, and 0.8 M, respectively, and
2 concentrations of (a) 0.1 M; (b) 0.14 M; and (c) 0.16 M, respectively. The cross section image
and CuCl
CuCl
2 concentrations of (a) 0.1 M; (b) 0.14 M; and (c) 0.16 M, respectively. The cross section image of a
2, ZnCl2, SnCl2, and thiourea concentrations of 0.14, 0.07, 0.07, and 0.8
of
a
CZTS
film with
CZTS thin thin
film with
CuClCuCl
2 , ZnCl2 , SnCl2 , and thiourea concentrations of 0.14, 0.07, 0.07, and 0.8 M is
M
is
represented
in
(d).
A
CZTS
thin film with
fabricated
2, SnCl2, CuCl2, and thiourea
represented in (d). A CZTS thin film fabricated
ZnCl2 , with
SnCl2 ZnCl
, CuCl
2 , and thiourea concentrations
concentrations
of
0.1,
0.07,
0.14,
and
0.8
M,
respectively,
is
shown
in
(e).
of 0.1, 0.07, 0.14, and 0.8 M, respectively, is shown in (e).
Figure 5 shows the graph of optical band gap energy as a function of the Cu/(Zn + Sn) atomic
content ratio. The optical band gaps of these CZTS thin films range from 1.39 to 1.69, whereas the
Cu/(Zn + Sn) atomic content ratios range from 0.58 to 0.76. This shift is attributed to the change in the
p-d hybridization between Cu d-levels and S p-levels [16]. In addition, the relation of the extended
Materials 2016, 9, 526
7 of 12
1 0 1 0 e V 2 c m -2 )
1.75
1.70
1.65
1.60
1.55 10
1.50
8
1.45
6
1.40
1.35
4
1.30
2
1.25
1.20
0
-3
1.15
1.10
0.56
(a h v )2 (
Optical Band gap (eV)
Figure 5 shows the graph of optical band gap energy as a function of the Cu/(Zn + Sn) atomic
content ratio. The optical band gaps of these CZTS thin films range from 1.39 to 1.69, whereas the
Cu/(Zn
+ Sn)
content ratios range from 0.58 to 0.76. This shift is attributed to the change in
the
Materials 2016,
9, atomic
526
7 of
12
p-d hybridization between Cu d-levels and S p-levels [16]. In addition, the relation of the extended
optical
optical band
band gap to
to the
the decrease
decrease in
in grain
grain size
size is
is attributed
attributed to the fact
fact that
that the
the band
band gap
gap energy
energy of
of aa
material
material is dependent
dependent on the particle size of that material according to the quantum size effect [15].
As
shown
in
As shown in Figure
Figure 3a–c,
3a–c, the
the grain
grain size
size of
of aa Cu-poor
Cu-poor CZTS
CZTS thin
thin film
film is
is smaller
smaller than
than that
that of
of aa Cu-rich
Cu-rich
CZTS
CZTS thin
thin film,
film, resulting
resulting in
in aa wide
wide optical
optical band
band gap.
gap. The
The inset
inset of
of Figure
Figure 55 shows
shows aa plot
plot of
of(αhν)
(αhν)22
against
1.12) CZTS thin film where α
against the
the photon
photon energy
energy of
of aa Cu-poor
Cu-poor and
and Zn-rich
Zn-rich (Cu/Zn
(Cu/Zn == 1.12)
α is the
the
absorption
coefficient
and
hν
is
the
photon
energy.
The
optical
band
gap
energy
is
estimated
absorption
hν is the photon energy.
estimated by
2
2
extrapolating
in the
(αhν)
plotplot
and and
determining
the intercept
with the
photon
energy
extrapolatingthe
thelinear
linearregion
region
in the
(αhν)
determining
the intercept
with
the photon
axis.
The
optical
gap
for gap
an optimal
CZTS absorber
is approximately
1.5 eV. 1.5 eV.
energy
axis.
The band
optical
band
for an optimal
CZTS absorber
is approximately
o
Annealing Temp. = 350 C
[CuCl2] = 0.14 M
[ZnCl2] = 0.07 M
[SnCl2] = 0.07 M
[Thiourea] = 0.8 M
-2 -1 0 1 2 3 4 5 6 7 8 9 10
hv (eV)
0.60
0.64
0.68
0.72
0.76
Cu/(Zn+Sn) Ratio
Figure 5.
5. Optical
Optical band
band gap
gap as
as aa function
function of
of the
theCu/(Zn
Cu/(Zn +
+ Sn)
ratio, and
and the
the inset
inset shows
shows the
the plot
plot of
of
Figure
Sn) ratio,
(αhν)22 against
energy of
of aa Cu-poor
Cu-poor and
and Zn-rich
Zn-rich (Cu/Zn
(Cu/Zn ==1.12)
(αhν)
against the
the photon
photon energy
1.12)CZTS
CZTSthin
thinfilm.
film.
Figure 6a,b show the carrier concentration and resistivity of CZTS thin films as a function of
Figure 6a,b show the carrier concentration and resistivity of CZTS thin films as a function
CuCl2 and ZnCl2 concentration, respectively. The carrier concentration shown in Figure 6a increases
of CuCl2 and ZnCl2 concentration, respectively. The carrier concentration shown in Figure 6a
with CuCl2 concentration. The formation of a CuZn + SnZn deep level depends on the Cu/(Zn + Sn) and
increases with CuCl2 concentration. The formation of a CuZn + SnZn deep level depends on the
Zn/Sn atomic content ratio, and an optimal condition occurs at Cu/(Zn + Sn) ≈ 0.8 and Zn/Sn ≈ 1.2
Cu/(Zn + Sn) and Zn/Sn atomic content ratio, and an optimal condition occurs at Cu/(Zn + Sn) « 0.8
[20]. As the CuCl2 concentration increases from 0.1 to 0.14 M, the Cu/(Zn + Sn) atomic content ratio
and Zn/Sn « 1.2 [20]. As the CuCl2 concentration increases from 0.1 to 0.14 M, the Cu/(Zn + Sn)
approaches the optimal value because of the suppression of the formation of a CuZn + SnZn deep donor
atomic content ratio approaches the optimal value because of the suppression of the formation of
level in the CZTS band gap, leading to a significant increase in hole concentration. Nevertheless, the
a CuZn + SnZn deep donor level in the CZTS band gap, leading to a significant increase in hole
Cu/(Zn + Sn) atomic content ratio increases with CuCl2 concentration to 1.6 M, causing the generation
concentration. Nevertheless, the Cu/(Zn + Sn) atomic content ratio increases with CuCl2 concentration
of a CuZn + SnZn deep donor defect and a reduction of the CuZn antisite acceptor defects, resulting in
to 1.6 M, causing the generation of a CuZn + SnZn deep donor defect and a reduction of the CuZn antisite
a decrease of carrier concentration. Under the optimal Cu/(Zn + Sn) atomic content ratio of 0.714, the
acceptor defects, resulting in a decrease of carrier concentration.
Under the optimal Cu/(Zn + Sn)
CZTS thin film has a high carrier concentration of 1.07 × 1019 cm−3 and a resistivity of 1.19 Ωcm.
The
atomic content ratio of 0.714, the CZTS thin film has a high carrier concentration of 1.07 ˆ 1019 cm´3
atomic content ratio of Zn/Sn is also a crucial factor determining the electrical properties of a CZTS
and a resistivity of 1.19 Ωcm. The atomic content ratio of Zn/Sn is also a crucial factor determining the
thin film. The ZnSn antisite defect with an inherent p-type conduction type in a CZTS thin film can
electrical properties of a CZTS thin film. The ZnSn antisite defect with an inherent p-type conduction
easily form under a high Zn/Sn atomic content ratio. However, further increases in Zn/Sn atomic
type in a CZTS thin film can easily form under a high Zn/Sn atomic content ratio. However, further
content ratio would generate a large population of intrinsic defects, causing a detrimental influence
increases in Zn/Sn atomic content ratio would generate a large population of intrinsic defects, causing
relative to a CZTS thin film with a atomic content ratio of Cu/(Zn + Sn) = 1 and Zn/Sn = 1. In Figure
a detrimental influence relative to a CZTS thin film with a atomic content ratio of Cu/(Zn + Sn) = 1 and
6b, the hole concentration decreases as the ZnCl2 concentration rises from 0.7 to 0.9 M because of the
Zn/Sn = 1. In Figure 6b, the hole concentration decreases as the ZnCl2 concentration rises from 0.7 to
population of donor defect clusters [20]. Further increases in ZnCl2 concentration would move the
0.9 M because of the population of donor defect clusters [20]. Further increases in ZnCl2 concentration
Cu/(Zn + Sn) atomic content ratio away from 0.8 (see Figure 1b), leading to a translation of conduction
types.
Materials 2016, 9, 526
8 of 12
12000
10000
1500
8000
1000
6000
4000
500
2000
0
0
0.06
0.08
0.10
0.12
0.14
0.16
2000
2500
Annealing Temp. = 350 oC
2000
1500
1500
1000
1000
500
Resistivity ( cm )
Annealing Temp. = 350 oC
Carrier Concentration ( x10 1 6 cm -3 )
2000
Resistivity ( cm )
Carrier Concentration ( x10 16 cm -3 )
would move the Cu/(Zn + Sn) atomic content ratio away from 0.8 (see Figure 1b), leading to a
translation
conduction types.
Materials
2016,of
9, 526
8 of 12
500
0
0.06
0.07
0.08
0.09
CuCl2 Concentration (M)
ZnCl2 Concentration (M)
(a)
(b)
0.10
0
0.11
Figure 6. Carrier concentration and resistivity of the CZTS thin film as a function of the (a) CuCl2; and
Figure 6. Carrier concentration and resistivity of the CZTS thin film as a function of the (a) CuCl2 ;
(b) ZnCl2 concentrations.
and (b) ZnCl2 concentrations.
The sulfer vacancy (VS) is also a dominant donor-like defect in a CZTS thin film because of its
The sulfer vacancy
also a dominant
defectfrom
in a CZTS
because
low
S ) isconduction
low formation
energy. (V
The
type donor-like
would change
p-typethin
to film
n-type
and of
theitshole
formation
energy.
The
conduction
type
would
change
from
p-type
to
n-type
and
the
hole
concentration
concentration might decrease in an S-poor CZTS thin film due to a high concentration of VS. High S
might decrease
S-poor
CZTS
film due
to a high
S content
or S-rich
S .S.High
content
or S-richininanCZTS
thin
filmthin
is crucial
to reduce
theconcentration
concentrationofofVV
Kosyak
et al. [31]
have
in
CZTS
thin
film
is
crucial
to
reduce
the
concentration
of
V
.
Kosyak
et
al.
[31]
have
calculated
S
calculated the concentration of ZnSn antisite and VS in CZTS thin
films with different Cu, Zn, andthe
Sn
concentration
of
Zn
antisite
and
V
in
CZTS
thin
films
with
different
Cu,
Zn,
and
Sn
content,
and
Sn
S
content, and represented that the concentration of acceptor-like ZnSn antisite is higher than that of
representedVthat
the concentration of acceptor-like Zn Based
antisite
is higher
that of donor-like
VS
donor-like
S in a Zn-rich and Sn-poor CZTS thin film. Sn
on this
result,than
we measure
the electrical
in
a
Zn-rich
and
Sn-poor
CZTS
thin
film.
Based
on
this
result,
we
measure
the
electrical
properties
properties of CZTS thin films with various thiourea concentration under optimal Cu/(Zn + Sn) atomic
of CZTSratio
thin of
films
with
various
thiourea
concentration
optimal
+ Sn)
atomicratio
content
content
0.714.
Table
1 lists
the measured
results under
of S/(Cu
+ Zn +Cu/(Zn
Sn) atomic
content
and
ratio
of
0.714.
Table
1
lists
the
measured
results
of
S/(Cu
+
Zn
+
Sn)
atomic
content
ratio
and
electrical
electrical characteristics of CZTS thin films depending on various thiourea concentration. The S/(Cu
of CZTS
thin
films
depending
various
thiourea
concentration.
The S/(Cu
Zna +high
Sn)
+characteristics
Zn + Sn) atomic
content
ratio
increases
withonrising
thiourea
concentration,
implying
that +
the
atomic
content
ratioCZTS
increases
withcan
rising
thiourea concentration,
implying
that thethiourea
a high Saqueous
content
S
content
or S-rich
thin film
be obtained
by changing the
concentration
or
S-rich
CZTS
thin
film
can
be
obtained
by
changing
the
concentration
thiourea
aqueous
solution.
solution. As thiourea concentration rises from 0.6 to 0.8 M, the hole concentration and conductivity
As
thiourea
rises
from 0.6
to 0.8 of
M,reduction
the hole concentration
and conductivity
of the
of the
p-typeconcentration
CZTS thin film
increase
because
of VS concentration.
However, further
p-type
thin film
increase because
of reduction
VS concentration.
further increase
increaseCZTS
in thiourea
concentration
would
reduce theofhole
concentration However,
and conductivity
possible
in
thiourea
concentration
would
reduce
the
hole
concentration
and
conductivity
possible
due to the
due to the formation of ZnS [32].
formation of ZnS [32].
Table 1. The S/(Cu + Zn + Sn) atomic content ratio and electrical characteristics of copper-zinc-tinTable 1. The S/(Cu + Zn + Sn) atomic content ratio and electrical characteristics of copper-zinc-tin-sulfur
sulfur (CZTS) thin films with different thiourea concentration.
(CZTS) thin films with different thiourea concentration.
Mobility
Resistivity
Thiourea
S/(Cu +
Conduction
Carrier Concentration
−3)
2/Vs)
(cm
(Ùcm)
Concentration
Zn + Sn)
Type
(cmConcentration
Thiourea (M)
Conduction Carrier
Mobility
Resistivity
S/(Cu + Zn + Sn)
´3 )
Concentration
(M)
(cm
(Ùcm)
0.6
1.04
p/n Type
9.13 ×(cm
1017
0.342 /Vs)
20.7
18
17
0.7
1.07
p/n
1.32
×
10
0.22
19.2
0.6
1.04
p/n
0.34
20.7
9.13 ˆ 10
0.49
1.19
0.8
1.091.07
p p/n
1.071.32
× 10ˆ191018
0.7
0.22
19.2
0.9
1.191.09
p
8.871.07
× 10ˆ181019
0.55
1.23
0.8
p
0.49
1.19
0.9
1.19
p
0.55
1.23
8.87 ˆ 1018
Table 2 shows the relation between annealing temperature and conduction type, carrier
concentration,
mobility,
resistivity
of CZTS
thin films
with concentrations
of CuCl2, type,
ZnCl2,carrier
SnCl2,
Table 2 shows
theand
relation
between
annealing
temperature
and conduction
and
thiourea
aqueous
precursor
solution
at
0.14,
0.07,
0.07,
and
0.8
M,
respectively.
The
hole,
concentration, mobility, and resistivity of CZTS thin films with concentrations of CuCl2 , ZnCl
2
14
concentration
for annealing
of 300 °C,
350 0.07,
°C, 400
°C,and
and0.8
450M,°Crespectively.
are 2.02 × 10The
, 1.07
×
SnCl2 , and thiourea
aqueoustemperatures
precursor solution
at 0.14,
0.07,
hole
19, 2.80 × 1015, and 1.24 × 1016 cm−3. The varied ˝carrier ˝
10
concentrations
of
CZTS
thin
films
with
˝
˝
14
concentration for annealing temperatures of 300 C, 350 C, 400 C, and 450 C are 2.02 ˆ 10 ,
increasing
temperatures
is 16
attributed
to varied
the changes
Cu/(Zn + Sn) and
Zn/Sn
content
1.07
ˆ 1019 ,annealing
2.80 ˆ 1015
, and 1.24 ˆ 10
cm´3 . The
carrierofconcentrations
of CZTS
thin
films
ratios, which cause the formation of CuZn + SnZn deep donor defects by reducing the CuZn antisite
acceptor defects [20,25]. The mobility of CZTS thin films shows a counterclockwise dependence on
the carrier concentration because of the impurity scattering. In conclusion, with an annealing
temperature of 350 °C, CZTS thin films have a stable p-type conduction type with a high hole
concentration of 1.07 × 1019 cm−3 and a low resistivity of 1.19 Ωcm. To realize the optimal CZTS
Materials 2016, 9, 526
9 of 12
with increasing annealing temperatures is attributed to the changes of Cu/(Zn + Sn) and Zn/Sn content
ratios, which cause the formation of CuZn + SnZn deep donor defects by reducing the CuZn antisite
acceptor defects [20,25]. The mobility of CZTS thin films shows a counterclockwise dependence on the
carrier concentration because of the impurity scattering. In conclusion, with an annealing temperature
of 350 ˝ C, CZTS thin films have a stable p-type conduction type with a high hole concentration
Materials 2016, 9, 526
9 of 12
of 1.07 ˆ 1019 cm´3 and a low resistivity of 1.19 Ωcm. To realize the optimal CZTS absorber for
photovoltaic
solar cell applications,
a CZTS thin film
wasthin
spin-coated
onto an Si substrate
to substrate
fabricate
absorber for photovoltaic
solar cell applications,
a CZTS
film was spin-coated
onto an Si
atoMo/p-CZTS/n-Si/Al
heterostructured
solar cell. solar cell.
fabricate a Mo/p-CZTS/n-Si/Al
heterostructured
Table 2. Temperature dependence of conduction type, carrier concentration, mobility, and resistivity of
Table 2. Temperature dependence of conduction type, carrier concentration, mobility, and resistivity
the CZTS thin film measured using Hall measurements.
of the CZTS thin film measured using Hall measurements.
Annealing
Conduction
Annealing
Conduction
˝ C)
Temperature
(
Type
Temperature (°C)
Type
p/n
300 300
p/n
350
350
pp
400
p/n
400
p/n
450
p/n
450
p/n
Carrier Carrier
Concentration
´3 )
(cm
Concentration
(cm−3)
2.02
2.02ˆ× 10
101414
19
1.07
1.07ˆ× 10
101519
2.80 ˆ 10 15
2.80 × 10
1.24 ˆ 1016
Mobility
Mobility Resistivity
Resistivity
2 /Vs)
(cm
(Ùcm)
2
(cm /Vs)
(Ùcm)
126126
0.49
0.49
7.95
7.95
0.094
1.24 × 1016
0.094
245
1.19
280
5310
245
1.19
280
5310
Figure
solar cell
cell under
Figure 77 shows
showsthe
the(current-voltage)
(current-voltage)I–V
I–Vcharacteristics
characteristicsofofthis
thisMo/CZTS/Si/Al
Mo/CZTS/Si/Al solar
under
(air
mass)
AM
1.5
G.
The
open
circuit
voltage,
short
current
density,
fill
factor,
and
efficiency
(air mass) AM 1.5 G. The open circuit voltage, short current density, fill factor, and efficiency were
were
2 , 66%, and 1.13%, respectively. The Mo/CZTS/Si/Al heterostructured solar
520
mV,
3.28
mA/cm
2
520 mV, 3.28 mA/cm , 66%, and 1.13%, respectively. The Mo/CZTS/Si/Al heterostructured solar cell
cell
prepared
in work
this work
was simple
and
the characteristics
of thethin
CZTS
film
used
as the
prepared
in this
was simple
and the
characteristics
of the CZTS
filmthin
used
as the
absorber
absorber
in this
solar
cell should
be investigated
as soon
as possible.
Compared
withthe
thetechniques
techniques of
in this solar
cell
should
be investigated
as soon
as possible.
Compared
with
of
preparing
CZTS
thin
films
reported
by
other
authors,
our
CZTS
thin
film
preparation
method
of
using
preparing CZTS thin films reported by other authors, our CZTS thin film preparation method of
spin-coating
exhibits
superior
performance
for photovoltaic
solar cell
applications.
using spin-coating
exhibits
superior
performance
for photovoltaic
solar
cell applications.
0.3
Current (mA)
0.2
0.1
0.0
-0.1
-0.2
Voc = 520 mV,
2
Jsc = 3.28 mA/cm ,
FF = 66 %
Efficiency = 1.13 %
-0.3
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Voltage (V)
7. Current-voltage
Current-voltage(I–V)
(I–V) characteristics
characteristicsof
ofaaMo/CZTS/Si/Al
Mo/CZTS/Si/Al heterostructured
Figure 7.
heterostructured solar cell under
(air
(air mass)
mass) AM
AM 1.5
1.5 G.
G.
3. Experimental
ExperimentalDetails
Details
3.1. Preparation of CZTS Thin Film
Because
and thiourea
thiourea powder in deionized (DI) water
Because the
the solubility
solubility levels
levelsof
of CuCl
CuCl22,, ZnCl
ZnCl22, SnCl22,, and
differ,
these
powders
were
dissolved
in
DI
water
individually
differ, these powders were dissolved in DI water individually to
to form
form aqueous
aqueous precursor
precursor solutions
solutions
with
with varied
varied molar
molar concentrations.
concentrations. If the aqueous precursor solutions do not contain toxic organic
organic
materials, such as MEA, spin-coating them uniformly onto glass substrates can be difficult. To spread
precursor solutions uniformly without using toxic materials, the surfaces of the glass substrates were
modified by using plasma cleaning to generate hydrophilic surfaces. To increase the S content of the
CZTS thin film and prevent the mixed aqueous solution from becoming turbid, two separate spincoating steps were applied to deposit CZTS thin films. For the first step, CuCl2, ZnCl2, and SnCl2
Materials 2016, 9, 526
10 of 12
materials, such as MEA, spin-coating them uniformly onto glass substrates can be difficult. To spread
precursor solutions uniformly without using toxic materials, the surfaces of the glass substrates were
modified by using plasma cleaning to generate hydrophilic surfaces. To increase the S content of
the CZTS thin film and prevent the mixed aqueous solution from becoming turbid, two separate
spin-coating steps were applied to deposit CZTS thin films. For the first step, CuCl2 , ZnCl2 , and SnCl2
aqueous precursor solutions were mixed and stirred uniformly to form a metallic aqueous solution
that was spin-coated onto soda lime glass substrates. The glass substrates, coated with uniformly
distributed metallic aqueous solution, were treated using thermal processes, namely baking on a
hot plate at 100 ˝ C for 3 min to dewater and annealing in a furnace at 350 ˝ C for 5 min to tailor the
crystallinity and morphology of the CZT layer. In the second step, aqueous thiourea solution was spun
onto the CZT layers, and then the samples were treated by being baked on a hot plate for 3 min at
100 ˝ C. Then, thermal annealed in a furnace for 180 min at 350 ˝ C to form CZTS thin films. The surface
morphologies of the CZTS thin films were characterized using FE-SEM and XRD patterns by using a
Bruker D8 advanced diffractometer (XRD: Bruker AXS-DS Discov, Taichung Taiwan) equipped with a
CuKá (λ = 0.154 nm). The atomic contents of Cu, Zn, Sn, and S in these CZTS thin films were measured
using EDX with the same diffractometer. The absorption of CZTS thin films in the visual range was
measured using a UV–Vis–NIR spectrophotometer (UVD-350, Yunlin Taiwan). The electrical properties
of CZTS thin films, including carrier concentration, carrier mobility, and resistivity, were measured
using Hall measurements (HMS-5000) at room temperature.
3.2. Fabrication of the Mo/CZTS/n-Si/Al Heterostructured Solar Cell
The preparation of the Mo/p-CZTS/n-Si/Al heterostructured solar cell can be described as
follows. First, aluminum (Al) metal was deposited on the back of an n–Si substrate using a thermal
evaporation process and then a thermal annealing treatment was applied to the sample to form an
ohmic contact between Si and Al. Second, the optimal CZTS absorber layer was deposited on the
surface of the n-Si substrate through spin-coating, followed by a thermal annealing procedure. Third,
the Mo metal was sputtered onto the CZTS absorber contact region to serve as ohmic contact metal.
The sample was then annealed in N2 atmosphere for 5 min at 400 ˝ C.
4. Conclusions
In this study, we spin-coated CZTS thin films onto glass substrates and annealed them to improve
the crystallinity and morphology of the thin films. The amounts of Cu, Zn, Sn, and S in these CZTS
thin films depend on the CuCl2 , ZnCl2 , SnCl2 , and SC(NH2 )2 concentrations of the precursor solutions.
The surface morphology and optical band gap properties of these CZTS thin films relate to the Cu
content and the electrical properties of these CZTS thin films depend on the annealing temperature
and the Cu/(Zn + Sn) and Zn/Sn ratios. All of the CZTS thin films exhibited a polycrystalline
kesterite crystal structure with three major broad peaks along the (112), (220), and (312) planes in XRD
patterns. The optimal CZTS thin film can be deposited on a glass and Si substrate with proportions
of [CuCl2 ] = 0.14 M, [ZnCl2 ] = 0.07 M, [SnCl2 ] = 0.07 M, and [SC(NH2 )2 ] = 0.8 M and an annealing
temperature of 350 ˝ C. Finally, an optimal CZTS thin film was deposited on an n-type silicon substrate
to produce a Mo/p-CZTS/n-Si/Al heterostructured solar cell with a conversion efficiency of 1.13%
and with V oc = 520 mV, Jsc = 3.28 mA/cm2 , and fill-factor (FF) = 66%.
Acknowledgments: The authors gratefully acknowledge the financial support of the Ministry of Science and
Technology, R.O.C. (101-2622-E-150-013-CC3).
Author Contributions: Min-Yen Yeh and Po-Hsu Lei organized and designed the experiment procedure,
and wrote the paper. Shao-Hsein Lin execute the deposition of CZTS thin films. Chyi-Da Yang performed
the measurement of CZTS thin films. All of the authors read and approved the final version of the manuscript to
be submitted.
Conflicts of Interest: The authors declare that there is no conflict of interests.
Materials 2016, 9, 526
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References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Nelson, J. The Physics of Solar Cells; Imperial College Press: London, UK, 2003; p. 211.
Shahrjerdi, D.; Bedell, S.W.; Ebert, C.; Bayram, C.; Hekmatshoar, B.; Fogel, K.; Lauro, P.; Gaynes, M.;
Gokmen, T.; Ott, J.A.; et al. High-efficiency thin-film InGaP/InGaAs/Ge tandem solar cells enabled by
controlled spalling technology. Appl. Phys. Lett. 2012. [CrossRef]
Lei, P.H.; Lin, C.T.; Ye, S.J. Improved efficiency of GaInP/(In)GaAs/Ge solar cells using textured
liquid-phase-deposited (LPD) ZnO. J. Phys. D Appl. Phys. 2012. [CrossRef]
Jackson, P.; Hariskos, D.; Lotter, E.; Paetel, S.; Wuerz, R.; Menner, R.; Wischmann, W.; Powalla, M. New world
record efficiency for Cu (InGa)Se2 . Prog. Photovol. Res. Appl. 2012, 19, 894–897. [CrossRef]
Wada, T.; Nakamura, S.; Maeda, T. Ternary and multinary Cu-chalcogenide photovoltaic materials from
CuInSe2 to Cu2 ZnSeS4 and other compounds. Prog. Photovol. Res. Appl. 2012, 20, 520–525. [CrossRef]
Zhou, D.; Zhu, H.; Liang, X.; Zhang, C.; Li, Z.; Xu, Y.; Chen, J.; Zhang, L.; Mai, Y. Sputtered molybdenum
thin films and the application in CIGS solar cell. Appl. Surf. Sci. 2016, 362, 202–209. [CrossRef]
Yoo, H.S.; Kim, J.H. Comparative study of Cu2 ZnSnS4 film growth. Sol. Energy Mater. Sol. Cells 2011,
95, 239–244. [CrossRef]
Jiang, F.; Shen, H.L. Fabrication and photovoltaic properties of Cu2 ZnSnS4 /i-a-Si/n-a-Si thin film solar cells.
Appl. Surf. Sci. 2013, 280, 138–143. [CrossRef]
Peng, C.Y.; Dhakal, T.P.; Garner, S.; Cimo, P.; Lu, S.; Westgate, C.R. Fabrication of Cu2 ZnSnS4 solar cell on a
flexible glass substrate. Thin Solid Films 2014, 562, 574–577. [CrossRef]
Katagiri, H.; Jimbo, K.; Maw, W.S.; Oishi, K.; Yamazaki, M.; Araki, H.; Takeuchi, A. Development of
CZTS-based thin film solar cells. Thin Solid Films 2009, 517, 2455–2460. [CrossRef]
Wang, K.; Gunawan, O.; Todorov, T.; Shin, B.; Chey, S.J.; Bojarczuk, N.A.; Mitzi, D.; Guha, S. Thermally
evaporated Cu2 ZnSnS4 solar cells. Appl. Phys. Lett. 2010. [CrossRef]
Pawar, S.M.; Inamdar, A.I.; Pawar, B.S.; Gurav, K.V.; Shin, S.W.; Xiao, Y.J.; Kolekar, S.S.; Lee, J.H.; Kim, J.H.;
Im, H.S. Synthesis of Cu2 ZnSnS4 (CZTS) absorber by rapid thermal processing (RTP) sulfurization of stacked
metallic precursor films for solar cell applications. Mater. Lett. 2014, 118, 76–79. [CrossRef]
Vanalakar, S.A.; Agawane, G.L.; Shin, S.W.; Suryawanshi, M.P.; Gurav, K.V.; Jeon, K.S.; Patil, P.S.; Jeong, C.W.;
Kim, J.Y.; Kim, J.H. A review on pulsed laser deposited CZTS thin films for solar cell applications.
J. Alloys Compd. 2015, 619, 109–121. [CrossRef]
Lee, S.G.; Kim, J.M.; Woo, H.S.; Jo, Y.C.; Inamdar, A.I.; Pawar, S.M.; Kim, H.S.; Jung, W.; Im, H.S. Structural,
morphological, compositional, and optical properties of single step electrodeposited Cu2 ZnSnS4 (CZTS) thin
films for solar cell application. Cur. Appl. Phys. 2014, 14, 254–258. [CrossRef]
Bhosale, S.M.; Suryawanshi, M.P.; Gaikwad, M.A.; Bhosale, P.N.; Kim, J.H. Influence of growth temperatures
on the properties of photoactive CZTS thin films using a spray pyrolysis technique. Mater. Lett. 2014,
129, 153–155. [CrossRef]
Tanaka, K.; Fukui, Y.; Moritake, N.; Uchiki, H. Chemical composition dependence of morphological and
optical properties of Cu2 ZnSnS4 thin films deposited by sol-gel sulfurization and Cu2 ZnSnS4 thin film solar
cell efficiency. Sol. Energy Mater. Sol. Cells 2011, 95, 838–842. [CrossRef]
Swami, S.K.; Kumar, A.; Dutta, V. Deposition of kesterite Cu2 ZnSnS4 (CZTS) thin films by spin coating
technique for solar cell application. Energy Proc. 2013, 33, 198–202. [CrossRef]
Gao, C.; Schnabel, T.; Abzieher, T.; Krammer, C.; Powalla, M.; Kalt, H.; Hetterich, M. Cu2 ZnSn(S,Se)4 solar
cells based on chemical bath deposited precursors. Thin Solid Films 2014, 562, 621–624. [CrossRef]
Todorov, T.K.; Tang, J.; Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Mitzi, D.B. Beyond 11% efficiency:
Characteristics of state-of-the-art Cu2 ZnSn(S,Se)4 solar cells. Adv. Mater. 2013, 3, 34–38. [CrossRef]
Chen, S.Y.; Wang, L.W.; Walsh, A.; Gong, X.G.; Wei, S.H. Abundance of CuZn + SnZn and 2CuZn + SnZn
defect clusters in kesterite solar cell. Appl. Phys. Lett. 2012. [CrossRef]
Malerba, C.; Biccary, F.; Ricardo, C.L.A.; Valentini, M.; Chierchia, R.; Muller, M.; Santoni, A.; Esposito, E.;
Mangiapane, P.; Scardi, P.; et al. CZTS stoichiometry effects on the band gap energy. J. Alloys Compd. 2014,
582, 528–534. [CrossRef]
Ruan, C.H.; Huang, C.C.; Lin, Y.J.; He, G.R.; Chang, H.C.; Chen, Y.H. Electrical properties of CuxZnySnS4
films with different Cu/Zn ratios. Thin Solid Films 2014, 550, 525–529. [CrossRef]
Materials 2016, 9, 526
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
12 of 12
Kosyak, V.; Karmarkar, M.A.; Scarpulla, M.A. Temperature dependent conductivity of polycrystalline
Cu2 ZnSnS4 thin films. Appl. Phys. Lett. 2012. [CrossRef]
Shin, S.W.; Pawar, S.M.; Park, C.Y.; Yun, J.H.; Moon, J.H.; Kim, J.H.; Lee, J.Y. Studies on Cu2 ZnSnS4 (CZTS)
absorber layer using different stacking orders in precursor thin films. Sol. Energy Mater. Sol. Cells 2011,
95, 3202–3206. [CrossRef]
Fairbrother, A.; Fontane, X.; Roca, V.I.; Rodriguez, M.E.; Marino, S.L.; Placidi, M.; Barrio, L.C.; Rodriguez, A.P.;
Saucedo, E. On the formation mechanisms of Zn-rich Cu2 ZnSnS4 films prepared by sulfurization of metallic
stacks. Sol. Energy Mater. Sol. Cells 2013, 112, 97–105. [CrossRef]
Birkholz, M. Thin Film Analysis by X-ray Scattering; Addison-Wesley: New York, NY, USA, 2006; p. 110.
Bhosale, S.M.; Suryawanshi, M.P.; Kim, J.H.; Moholkar, A.V. Influence of copper concentration on sprayed
CZTS thin films deposited at high temperature. Cream. Int. 2015, 41, 8299–8304. [CrossRef]
Shinde, N.M.; Deokate, R.J.; Lokhande, C.D. Properties of spray deposited Cu2 ZnSnS4 (CZTS) thin films.
J. Anal. Appl. Pyrol. 2013, 100, 12–16. [CrossRef]
Todorov, T.; Kita, M.; Garda, J.; Escribano, P. Cu2 ZnSnS4 films deposited by a soft-chemistry method.
Thin Solid Films 2009, 517, 2541–2544. [CrossRef]
Inamdar, A.I.; Lee, S.G.; Jeon, K.Y.; Lee, C.H.; Pawar, S.M.; Kalubarme, R.S.; Park, C.J.; Im, H.S.; Jung, W.;
Kim, H.S. Optimized fabrication of sputter deposited Cu2 ZnSnS4 (CZTS) thin films. Sol. Energy 2013,
91, 196–203. [CrossRef]
Kosyok, V.; Mortazavi Amiri, N.B.; Postnikov, A.V.; Scarpulla, M.A. Model of native point defect equilibrium
in Cu2 ZnSnS4 application to one-zone annealing. J. Appl. Phys. 2013. [CrossRef]
Long, B.; Cheng, S.Y.; Zheng, Q.; Yu, J.L.; Jai, H.J. Effects of sulfurization time and H2 S concentration on
electrical properties of Cu2 ZnSnS4 films prepared by sol-gel method. Mater. Res. Bull. 2016, 73, 140–144.
[CrossRef]
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