Effect of pre-annealing on the phase formation and efficiency of CZTS solar cell prepared by sulfurization of Zn:(Cu,Sn) precursor with H2S gas

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Solar Energy 136 (2016) 499–504
Contents lists available at ScienceDirect
Solar Energy
journal homepage: www.elsevier.com/locate/solener
Effect of pre-annealing on the phase formation and efficiency of CZTS
solar cell prepared by sulfurization of Zn/(Cu,Sn) precursor with H2S gas
Jung Hun Lee a, Heon Jin Choi b, Won Mok Kim c, Jeung Hyun Jeong d, Jong Keuk Park c,⇑
a
Agency for Defense Development, Yuseong, Daejeon 305-600, Republic of Korea
Department of Materials Science & Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
c
Center for Electronic Materials Research, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
d
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
b
a r t i c l e
i n f o
Article history:
Received 15 June 2016
Received in revised form 15 July 2016
Accepted 18 July 2016
Available online 25 July 2016
Keywords:
Cu2ZnSnS4(CZTS)
Pre-annealing
Metal precursor
H2S gas sulfurization
Thin film solar cells
a b s t r a c t
The effect of pre-annealing on the phase formation behavior and efficiency of CZTS thin film solar cell prepared by sulfurization of sputtered Zn/(Cu,Sn) metal precursor with H2S gas was investigated. Precursor
with stacking structure of Zn/(Cu,Sn) was deposited by sputtering of Cu, Zn, and Sn metal targets. The
depth profile of metal elements and cell efficiency of the sulfurized CZTS films with H2S were observed
to be critically dependent on the pre-annealing conditions. For the CZTS film prepared by sulfurization in
N2-5 vol.% H2S at 550 °C after pre-annealing at 350 °C in Ar, segregation of SnS phase at the surface region
was observed to be pronounced. When the pre-annealing was performed at 350 °C in N2-5 vol.% H2S,
however, uniform depth profile of metal elements with a small amount of CuS phase was observed.
The CuS phase was disappeared with increase in the pre-annealing temperature in N2-5 vol.% H2S. The
phase formation behavior influenced by pre-annealing condition was observed to affect solar cell performance of the CZTS thin film synthesized at 550 °C in N2-5 vol.% H2S. In contrast to the CZTS thin film prepared with pre-annealing at 350 °C in Ar showing bad efficiency (0.93%), the CZTS solar cells fabricated
with pre-annealing at 450 °C in H2S shows higher efficiency of 3.04%. By the optimization of Zn layer
thickness, solar cell efficiency of 4.40% was obtained in the CZTS thin film prepared with pre-annealing
at 450 °C in N2-5 vol.% H2S. This phenomenon was due to the change in the secondary phase formation
behavior during sulfurization of the Zn/(Cu,Sn) metal precursor with various pre-annealing conditions.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Cu2ZnSnS4 (CZTS) using earth abundant and non-toxic elements
such as Cu, Zn and Sn is known to be a candidate to replace Cu(In,
Ga)Se2 (CIGS) which shows cell efficiency beyond 20% but has rare
elements such as In and Ga as well as toxic material of Se (Jackson
et al., 2011; Chirilă et al., 2013). Since the photovoltaic effect of
CZTS was first reported by Ito and Nakazawa in 1988 (Ito and
Nakazawa, 1988), the phase formation behavior and cell efficiency
of CZTS thin film synthesized with various techniques have been
extensively investigated (Mitzi et al., 2011; Katagiri et al., 2008;
Shin et al., 2013; Woo et al., 2012; Moholkar et al., 2012).
In general, CZTS phase can be obtained by two sequential processes, the sulfurization of prepared precursor containing Cu-ZnSn-(S) elements. As reported previously, there are many techniques
to prepare the precursor such as sputtering (Chalapathy et al.,
⇑ Corresponding author.
E-mail address: [email protected] (J.K. Park).
http://dx.doi.org/10.1016/j.solener.2016.07.031
0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
2011; Shin et al., 2011; Lee et al., 2015), co-evaporation (Shin
et al., 2013), and PLD (Moholkar et al., 2012) as well as spray coating (Kamoun et al., 2007), electro-deposition (Ahmed et al., 2012).
Among them, sputtering process with metal target is favorable to
obtain thin film with uniform and controllable composition as well
as for commercialization due to its low cost. As for the sulfurization, contrary to the sulfur supply with sulfur powder or pellet,
the annealing with H2S gas has advantage for preparing CZTS solar
cell under the controlled sulfur partial pressure in mass production. However, the report on the efficiency of CZTS thin film solar
cells prepared by sulfurization of metallic precursor with H2S is
very few, except for the CZTS solar cell prepared with very slow
heating rate and long annealing time (Maeda et al., 2011; Emrani
et al., 2013).
In previous paper (Lee et al., 2015), we have reported that the
metal precursor with a stacking structure of Zn/(Cu,Sn) was
observed to be favorable to synthesize CZTS phase by annealing
the precursor under N2-5 vol.% H2S atmosphere. However, the CZTS
thin film obtained from Zn/(Cu,Sn) has a second phase of CuS and
500
J.H. Lee et al. / Solar Energy 136 (2016) 499–504
shows a low solar cell efficiency of 2.64%. In the present study, the
effect of pre-annealing on the phase formation and efficiency of
CZTS thin film solar cell prepared by sulfurization of Zn/(Cu,Sn)
metal precursor with H2S gas was investigated. The preannealing conditions such as annealing temperature and atmosphere were checked for the CZTS phase synthesized at 550 °C in
N2-5 vol.% H2S.
2. Experimental
2.1. Precursor deposition and sulfurization
Soda-lime glass was used as a substrate after Molybdenum
(Mo) back contact with 500 nm in thickness was deposited on
the soda-lime glass by DC magnetron sputtering. The detailed procedure for the deposition of Mo back contact was described in previous report (Lee et al., 2015; Yoon et al., 2011). The Zn/(Cu,Sn)
metal precursor was deposited by sputtering metal targets of Cu
(99.995%), Zn(99.999%) and Sn(99.999%) with 7.5 cm in diameter.
After the base pressure reached approximately 4 10 4 Pa, precursor was deposited at a working pressure of 0.67 Pa in an Ar
atmosphere. The Zn/(Cu,Sn) precursor in thickness of
170 nm/300 nm was prepared by the deposition of Zn layer followed by co-sputtered (Cu,Sn) layer.
Sulfurization was performed by rapid thermal annealing (RTA)
apparatus equipped with quartz tube and halogen lamp at atmospheric pressure in N2-5 vol.% H2S gas. The Zn/(Cu,Sn) metal precursor was put into the quartz tube. After the base pressure of
the quartz tube was less than 1 Pa, the pressure in the tube was
controlled to be 105 Pa by the flow of Ar or N2-5 vol.% H2S gases.
The precursor sample was then heated to pre-annealing temperature of 350 °C and 450 °C at a ramping rate of 10 °C/min and maintained for 60 min under the gas flow. CZTS film was obtained by
sulfurization of the pre-annealed sample at 550 °C in N2-5 vol.%
H2S gas for 15 min. The H2S gas flow stopped when the samples
cooled to 200 °C. Hereafter, N2-5 vol.% H2S gas atmosphere was
referred to as H2S.
2.2. Fabrication of CZTS solar cells
The sulfurized CZTS thin films were etched in solutions of KCN
and HCl for the removal of Cu- and Zn-based secondary phases at
surface before the fabrication of CZTS thin film solar cells. After
the etching process, CdS buffer layer (50 nm) was deposited by
chemical bath deposition on the CZTS thin films. Intrinsic zinc
oxide (i-ZnO, 50 nm) and aluminum doped zinc oxide (AZO,
250 nm) were deposited by RF-magnetron sputtering on the CdS
buffer layer, followed by the deposition of Ni(50 nm)/Al(800 nm)
metal grid with electron-beam evaporation.
2.3. Characterization of CZTS thin film solar cell
The surface and cross sectional morphology of CZTS thin films
were characterized by field emission scanning electron microscopy
(FE-SEM; Hitachi S-4200). The compositions of CZTS thin films
were analyzed by electron probe X-ray microanalysis with 5 different points in each sample. The area at each point for the chemical
composition measurement was a circle with a diameter of 30 lm.
The relative atomic concentration profiles across the thicknesses of
the sulfurized thin films were analyzed by Auger electron spectroscopy (AES; Scanning auger nanoprobe PHI-700 & LC-TOFMS
LECO). The crystal structures of CZTS thin films were investigated
with X-ray diffraction (XRD; Bruker D8 ADVANCE) in h–2h scan
mode. The secondary phases formed in CZTS were analyzed by
Raman spectroscopy with Nd:YAG laser of 532 nm wavelength.
The photovoltaic performances of CZTS thin film solar cells were
evaluated by measuring the illuminated current-voltage (I-V) using
solar simulator (Newport’s Oriel Sol3A) under AM 1.5 one sun
condition.
3. Results and discussion
Fig. 1 show the surface and cross-sectional images of CZTS thin
films obtained by the sulfurization of Zn/(Cu,Sn) precursor at
550 °C in H2S after pre-annealing at 350 °C in Ar and H2S. In contrast to the CZTS thin film prepared with pre-annealing in H2S
which shows clear grain boundary between larger grains (Fig. 1
(d)), a large number of smaller grains were observed at the surface
region of CZTS thin film prepared with pre-annealing in Ar (Fig. 1
(c)). In the cross-sectional images of CZTS thin films shown in
Fig. 1(a) and (b), voids are observed to form at the CZTS/Mo interface for the CZTS thin film, which was believed to be due to the
high diffusivity of Cu in Sn and Zn during the sulfurization of metal
precursor (Lee et al., 2015; Fairbrother et al., 2013).
The chemical compositions of the CZTS thin films synthesized
with Zn/(Cu,Sn) precursor were observed to be critically dependent
on the pre-annealing atmosphere, as represented in Table 1. The
CZTS film synthesized with pre-annealing in H2S shows Cu-poor
and Zn-rich composition, whereas Cu-poor and Zn-poor composition was detected for the CZTS film prepared with pre-annealing
in Ar. The atomic compositional depth profiles of CZTS thin films
obtained with different pre-annealing condition, which were
determined by AES, were shown in Fig. 2. The CZTS films prepared
with pre-annealing in Ar, elemental segregation of Sn at surface
was pronounced, which is believed to be due to the SnS formed.
The Sn segregation at the surface region makes Sn content (lower
ratio of Zn to Sn) higher in the composition analysis as shown in
Table 1. However, the segregation of Sn at the surface region was
greatly suppressed and a more uniform distribution of Cu, Zn, Sn
and S were observed through the thickness for the CZTS thin film
obtained with pre-annealing in H2S. For the specimen, a little bit
higher Zn concentration at the surface region was also observed
to make Sn concentration higher at the back contact region.
The change in compositional depth profile was checked by XRD
and Raman spectra. Fig. 3 show the XRD patterns for the CZTS thin
films obtained with pre-annealing in Ar and H2S. From the XRD
patterns, (112) oriented kesterite CZTS phase is clearly confirmed
in the films (JCPDS no. 26-0575). In addition to the CZTS phase, secondary phases of Cu2S (JCPDS no. 53-0522) and SnS (JCPDS no. 390354) was observed to be pronounced for the CZTS film synthesized with pre-annealing in Ar. For the CZTS film prepared with
pre-annealing in H2S, second phase of CuS (JCPDS no. 06-0464)
was observed. In previous paper (Lee et al., 2015), we have
reported that only CuS was detected in the XRD patterns in the
CZTS film prepared by the sulfurization of Zn/(Cu,Sn) precursor in
H2S. Although pre-annealing step at 350 °C in H2S was adopted, the
CuS second phase still exist in the CZTS film. Other secondary
phases such as Cu2SnS3 and ZnS cannot be distinguished from CZTS
in the XRD patterns, because the diffraction peaks were apparent at
the same diffraction angle (Araki et al., 2008). Fig. 4 show Raman
spectra of CZTS thin films using the excitation laser wavelength
of 532 nm. The two strongest peaks of 286 cm 1, and 337 cm 1,
and the peak of 373 cm 1 correspond to kesterite CZTS, which is
dominant phase in all the sulfurized precursors. Besides these kesterite CZTS peaks, SnS peaks of 163 cm 1, 189 cm 1 and 218 cm 1
(Parkin et al., 2001) were observed to be pronounced in the CZTS
film synthesized after pre-annealing step at 350 °C in Ar. For the
CZTS film prepared with pre-annealing in Ar, although Cu2S phase
was detected in the XRD patterns, the Cu2S phase cannot be
observed in the Raman spectra, which is believed to be due to a
J.H. Lee et al. / Solar Energy 136 (2016) 499–504
501
Fig. 1. Surface morphologies and cross sectional images of CZTS thin films prepared by sulfurization of Zn/(Cu,Sn) precursor with pre-annealing at 350 °C in Ar ((a) and (c))
and N2-5 vol.% H2S ((b) and (d)).
Table 1
Chemical compositions of CZTS thin films prepared with pre-annealing.
Pre-annealing
atmosphere
Cu
Zn
Sn
S
Zn/Sn
Cu/(Zn + Sn)
Ar
H2S
22.05
26.01
12.29
14.27
17.01
11.90
48.65
47.83
0.72
1.20
0.75
0.99
small detection area (circle in diameter of 2 lm) of Raman spectroscopy used in this study.
In our previous report (Lee et al., 2015), the CZTS and second
phase formation behaviors in the CZTS film obtained by the sulfurization of metal precursors was shown to be critically dependent
on the stacking structure of metal layer. Considering the synthesis
path of CZTS phase, which is represented by the reaction of ZnS and
Cu2SnS3 over 500 °C (Fairbrother et al., 2013; Han et al., 2013; Yoo
et al., 2012), ZnS and Cu2SnS3 phases should be formed to synthesize CZTS phase by the sulfurization of metal precursor. The stable
metal alloys and sulfide phases in Cu-Sn-Zn-S material system
were represented in Table 2. Although there is a stable Cu2SnS3
phase as well as Cu-Sn metal alloys such as Cu6Sn5, Cu5Sn4 and
Cu3Sn in Cu-Sn-S system, there is no stable ternary sulfide phase
(Cu-Zn-S) in contrast to the stable metal alloys such as Cu5Zn8,
CuZn and CuZn5 (Scragg, 2011) in Cu-Zn system. Therefore, ZnS,
which is necessary phase to synthesize CZTS, should be formed
by the sulfurization of Zn or Cu-Zn alloy through the outdiffusion of Cu. In this respect, Zn/(Cu,Sn) metal precursor was
reported to be the favorable to synthesize CZTS phase by sulfurization with smaller second phases and pores (Lee et al., 2015),
because Cu and Sn in the (Cu,Sn) layer is possible to form Cu2SnS3
with less diffusion. In the co-sputtered (Cu,Zn,Sn) metal precursor,
on the contrary, pores and second phase (Cu2 xS) remained during
sulfurization to form Cu2SnS3 and ZnS due to the fast out-diffusion
of Cu from Cu-Zn alloy (Scragg, 2011).
In fact, Ahn et al. reported by the XRD analysis (Chalapathy
et al., 2011) that Cu6Sn5, Sn, Cu3Sn and CuZn phases were observed
in the Cu/ZnSn/Cu metal precursor layers prepared by sputtering
Cu and ZnSn alloy targets. Although Cu-Sn and Cu-Zn alloy targets
were not used for preparing Cu/ZnSn/Cu metal precursor layer, CuSn and Cu-Zn phases were synthesized by the sputtering process at
room temperature, which clearly indicates that Cu easily reacts
with Sn and Zn in ZnSn to make Cu-Sn and Cu-Zn phases at room
temperature. Therefore Cu-Zn phase in the Zn/(Cu,Sn) precursor
pre-annealed in Ar at 350 °C as well as co-sputtered (Cu,Zn,Sn) precursor is believed to leave Cu2 xS second phase throughoutdiffusion of Cu in the synthesis of ZnS during sulfurization. Furthermore low melting temperature of Sn (231.9 °C) promoted the
Sn segregation at the surface region during pre-annealing at
350 °C in Ar through the formation of Sn-based liquid, which promoted SnS phase formation. However, pre-annealing Zn/(Cu,Sn)
precursor in H2S under the CZTS formation temperature of 500 °C
has a possibility to promote formation of sulfide phase (Cu2SnS3
and ZnS), because there is no other stable ternary sulfide phases
of (Cu-Zn-S) and (Zn-Sn-S). Lee et al. reported (Shin et al., 2011)
that a single kesterite crystal structure without second phases
was obtained from Cu/SnS2/ZnS/glass precursor by sulfurization
in a N2(95%)+H2S(5%) atmosphere. The stable sulfide phases represented in Table 2 indicate that the stacking structure Cn/SnS2/ZnS
is favorable to synthesize CZTS with less second phases. The sulfide
layer SnS2 cannot react with ZnS but only with Cu to form Cu2SnS3
because there is no stable Sn-Zn-S ternary sulfide.
In this respect, the phases formed from the Zn/(Cu,Sn) precursor
during the pre-annealing step was analyzed by XRD. Fig. 5(a)
shows the XRD pattern for the Zn/(Cu,Sn) precursor obtained by
the sputtering of Cu, Zn and Sn metal targets. The peak at
2h 43.2° with the maximum intensity corresponds to the Cu
(JCPDS no. 04-0836), Zn (JCPDS no. 04-0831) and Cu6Sn5 (JCPDS
no. 45-1488) phases. Besides those phases, Sn (JCPDS no. 040673) was also observed in the XRD patterns. The Cu6Sn5 phase
is believed to be synthesized during the co-deposition of Cu and
Sn. After the pre-annealing in Ar at 350 °C for 1 h, alloy phases such
as Cu6Sn5 and CuZn (JCPDS no. 08-0439) in addition to Zn were
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J.H. Lee et al. / Solar Energy 136 (2016) 499–504
100
(a)
SnS
Mo
218cm
80
70
60
50
S
40
Cu
30
20
Sn
Zn
CZTS
-1
337cm
-1
(a) Ar, 350oC
(b) H2S, 350oC
(c) H2S, 450oC
SnS
Intensity (arbit.u.)
Atomic concentration (%)
90
189cm
-1
CZTS
-1
373cm
CZTS
-1
286cm
SnS
163cm-1
(c)
(b)
10
(a)
0
0
5
10
15
20
25
100
Sputter time (min)
100
Atomic concentration (%)
200
250
300
350
400
450
500
550
Fig. 4. Raman spectra of CZTS thin films prepared with pre-annealing (a) at 350 °C
in Ar, (b) at 350 °C in N2-5 vol.% H2S and (c) 450 °C in N2-5 vol.% H2S.
Mo
80
Table 2
Stable metal alloys and sulfides in Cu-Zn-Sn-S system.
70
60
Elements
50
30
S
Cu
20
Zn
10
Sn
40
Phase
Metal
Cu-Sn
Cu-Zn
Zn-Sn
Cu-Zn-Sn
Cu6Sn5, Cu5Sn4, Cu3Sn
CuZn, Cu5Zn8, CuZn5
–
–
Sulfide
Cu-S
Sn-S
Zn-S
Cu-Zn-S
Cu-Sn-S
Zn-Sn-S
CuS, Cu2S, Cu2 xS
SnS, SnS2, Sn2S3
ZnS
–
Cu2SnS3, Cu4SnS4, Cu4SnS6
–
0
0
5
10
15
20
25
Sputter time (min)
Fig. 2. Atomic compositional depth profiles of CZTS thin films prepared by
sulfurization of Zn/(Cu,Sn) precursor with pre-annealing at 350 °C in (a) Ar and
(b) N2-5 vol.% H2S. The profiles were obtained by AES.
ZnS, %
Mo
# Cu6Sn 5
%
* Sn
%
% Cu2SnS3
%
(a) Ar, 350 C
o
(b) H2S, 350 C
# Cu2S
% CuS
Mo
o
(c) H2S, 450 C
& SnS
Mo
(c)
%
Intensity (arbit. unit)
o
* CZTS
(112)*
CuS CuS
(c)
(b)
Mo
& SnS
(d)
&
%
%
%
%
%
%
%
%
&&
&
&
%
#
Cu, Zn, #
CuZn
Zn
#
(b)
#
#
#
CuZn
#
#
(220/204)*
20
25
30
35
40
45
(a)
50
55
60
65
70
(332)*
(008)*
(200)*
(a) &&
(103)*
#
(312)*
Intensity (arbit. u.)
600
Raman shift (cm-1)
(b)
90
150
75
20
**
30
#
*
*
#
40
#
50
#
60
#
70
80
2θ(o)
80
2θ (°)
Fig. 3. XRD diffraction patterns of CZTS thin films with prepared with preannealing (a) at 350 °C in Ar, (b) at 350 °C in N2-5 vol.% H2S and (c) 450 °C in N25 vol.% H2S.
observed, as shown in Fig. 1(b). The Sn in Zn/(Cu,Sn) precursor
almost disappeared through the synthesis of Cu6Sn5 phase with
Cu. The alloy phases such as Cu6Sn5 and CuZn should be sulfurized
to ZnS and Cu2SnS3 to synthesize the CZTS phase at 550 °C, which
Fig. 5. XRD diffraction patterns obtained from (a) Zn/(Cu,Sn) precursor, (b)
annealed Zn/(Cu,Sn) precursor at 350 °C in Ar, (c) annealed Zn/(Cu,Sn) precursor
at 350 °C in N2-5 vol.% H2S and (d) annealed Zn/(Cu,Sn) precursor at 450°Cin N25 vol.% H2S. The annealing time was 1 h.
leaves SnS and Cu2S during the sulfurization at 550 °C as previously mentioned. On the contrary, the pre-annealing step under
the H2S atmosphere promoted the synthesis of Cu2SnS3 phase
(JCPDS no. 27-0198) as shown in Fig. 5(c) and (d). When the preannealing temperature was 350 °C, second phases such as CuS
and SnS in addition to Cu2SnS3 phase was clearly observed. The
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J.H. Lee et al. / Solar Energy 136 (2016) 499–504
Fig. 6. (a) Surface and (b) cross-sectional morphologies of CZTS thin films prepared with pre-annealing at 450 °C in N2-5 vol.% H2S.
ZnS cannot be distinguished from Cu2SnS3 in the XRD pattern due
to the same diffraction angle (Araki et al., 2008). If the preannealing temperature was increased to 450 °C, the peaks from
the CuS and SnS phases was dramatically decreased, which corresponds to the XRD data obtained from the CZTS film prepared with
pre-annealing at 450 °C as shown in Fig. 3.
The formation of Cu2SnS3 and ZnS phases, which promotes the
synthesis of CZTS phase, can be expedited in sulfurization of Zn/
(Cu,Sn) precursor by the increase in the pre-annealing temperature. Fig. 6 show the surface and cross-sectional images of CZTS
thin films prepared by the sulfurization of Zn/(Cu,Sn) precursor
at 550 °C in H2S after pre-annealing at 450 °C in H2S. As shown
in Figs. 1(b) and 6, the lager grained film was obtained with
increase in the pre-annealing temperature from 350 °C to 450 °C.
The ratio of sulfur to metal, S/(Cu + Zn + Sn), measured after the
pre-annealing step at 350 °C and 450 °C for 10 min was 0.54 and
0.96, respectively. The higher S/(Cu + Zn + Sn) value for the
pre-annealed film at 450 °C in H2S indicates that it takes less
time to obtain Cu2SnS3 and ZnS from Zn/(Cu,Sn) precursor with
the increase in pre-annealing temperature. In the pre-annealed
Zn/(Cu,Sn) film in 450 °C, therefore, Cu2SnS3 and ZnS phases can
be formed with a less amount of metal alloys, which reduces the
amount of second phase during the synthesis of CZTS phase over
500 °C, as described previously. From the XRD patterns (Fig. 3(c))
a second phase of CuS observed in the CZTS film prepared with
pre-annealing at 350 °C in H2S disappeared by the increase in the
pre-annealing temperature to 450 °C. A more uniform distribution
of Cu, Zn, Sn and S through the thickness of CZTS film was observed
for the CZTS thin film obtained with pre-annealing at 450 °C in H2S,
as shown in Fig. 7.
The pre-annealing conditions, which influenced second phase
and microstructure of CZTS film synthesized by sulfurization of
Zn/(Cu,Sn) precursor in H2S, affect solar cell performances of the
film as shown in Table 3. The CZTS film prepared without preannealing showed solar cell efficiency of 2.13%. In contrast, the
CZTS film prepared with pre-annealing at 350 °C in Ar, which has
second phases of SnS and Cu2 xS, showed low cell efficiency of
0.93%. The value is comparable with the efficiency of the CZTS film
synthesized with (Cu,Zn,Sn) precursor (Lee et al., 2015). The second
phase Cu2 xS observed in XRD patterns, as shown in Fig. 4, results
18
100
Mo
Current density (mA/cm2)
Atomic concentration (%)
90
80
70
60
50
S
40
30
Cu
Zn
20
Jsc :15.9 mA/cm2
14
FF :47.5%
12
η:4.4%
10
8
6
4
2
Sn
10
Voc: 576 mV
16
0
0
0.0
0
5
10
15
20
25
0.1
0.2
30
Sputter time (min)
Fig. 7. Atomic compositional depth profiles of CZTS thin films prepared with preannealing at 450 °C in N2-5 vol.% H2S.
0.3
0.4
0.5
0.6
Voltage (V)
Fig. 8. J-V characteristic of the CZTS solar cell prepared by sulfurization of Zn/(Cu,
Sn) metal precursor at 550 °C in N2-5 vol.% H2S via pre-annealing at 450 °C in N25 vol.% H2S. The cell performances were optimized by controlling Zn layer thickness.
Table 3
Atomic ratios and photovoltaic performances of CZTS thin film solar cells prepared with various pre-annealing conditions.
Pre-annealing conditions
Zn/Sn
Cu/(Zn + Sn)
Cell area (cm2)
Voc (V)
Jsc (mA/cm2)
FF
Efficiency (%)
350 °C,
350 °C,
450 °C,
450 °C,
0.72
1.20
1.19
1.07
0.75
0.99
0.94
0.93
0.433
0.415
0.422
0.414
0.296
0.498
0.539
0.576
9.03
13.18
12.54
15.90
0.347
0.421
0.450
0.475
0.93
2.76
3.04
4.40
Ar
H2S
H2S
H2S (best cell)
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J.H. Lee et al. / Solar Energy 136 (2016) 499–504
in lower Voc. due to the low band gap of Cu2 xS (Gang et al., 2015).
The SnS observed at the surface region of CZTS film would act as
shunt pass or high recombination source, which also lowers Voc
(Gang et al., 2015). However, the cell efficiency was improved to
2.76% by the change in pre-annealing atmosphere (H2S). Under
the H2S atmosphere, the cell efficiency was observed to improve
to 3.04% by the increase in pre-annealing temperature, which
was due to the increased Voc and fill factor. By the optimization
of Zn layer thickness to be 165 nm which induced lower Zn to Sn
ratio (1.07) in the CZTS film, 4.40% solar cell efficiency with Voc
of 576 mV, Jsc of 15.9 mA/cm2 and fill factor of 0.475 was obtained
for the CZTS film prepared by the sulfurization of Zn/(Cu,Sn) precursor via pre-annealing at 450 °C in H2S, as shown in Table 3
and Fig. 8.
4. Conclusion
The effect of pre-annealing on the phase formation behavior
and efficiency of CZTS thin film solar cell prepared by sulfurization
of sputtered Zn/(Cu,Sn) metal precursor with H2S gas was investigated. For the CZTS film prepared by sulfurization in H2S at 550 °C,
segregation of SnS phase at the surface region was observed to be
pronounced by the adoption of pre-annealing at 350 °C in Ar.
When the pre-annealing was performed in H2S at the same temperature, however, more uniform depth profile of metal elements
was observed with a small amount of CuS phase, which was disappeared by the increase in the pre-annealing temperature to 450 °C.
In contrast to the CZTS thin film prepared with pre-annealing at
350 °C in Ar showing bad efficiency (0.93%), the CZTS solar cells
fabricated with pre-annealing at 450 °C in H2S shows higher efficiency of 3.04%. The cell efficiency was further improved to 4.40%
by the optimization of Zn layer thickness in the Zn/(Cu,Sn)
precursor.
Acknowledgement
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of
Trade, Industry & Energy (MOTIE) of the Republic of Korea
(20158520000060).
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