Heat treatments in chemically deposited SnS thin films and theirinfluence in CdS/SnS photovoltaic structures
D. Avellaneda • B. Krishnan • A. C. Rodriguez •
T. K. Das Roy • S. Shaji
Received: 30 June 2013 / Accepted: 28 August 2014
� Springer Science+Business Media New York 2014
Abstract Chemical deposition technique is used to produce
good quality tin sulphide (SnS) thin films of 450 nm in
thickness, from a chemical bath containing tin chloride
(SnCl2�2H2O) and thioacetamide. The SnS thin films were
annealed at 300, 350 and 400 �C for 1 h in vacuum, nitrogen
and argon atmospheres, resulting in the modification of their
optical and electrical properties. The optical band gap showed
a decrease from 1.2 to 1 eV whereas the electrical conduc-
tivity increased by more than two orders in the heat treated
films at 400 �C in comparison with those of pristine samples.
The morphology analysis of the as prepared and heat treated
SnS thin films was done using scanning electron microscopy;
X-ray diffraction patterns showed orthorhombic SnS phase
and the chemical states of the elements were confirmed by
X-ray photoelectron spectroscopy. The heat treated SnS thin
films (1.8 lm thickness) were incorporated to photovoltaic
structures of the type: Glass/TCO/CdS/SnS/C/Ag, showing
open circuit voltages (Voc) from 270 to 330 mV and a short
circuit current density (Jsc) up to 6 mA/cm2.
1 Introduction
One of the most important challenges in solar cell tech-
nology is the development of thin film solar cells based on
inexpensive processes using materials of high abundance.
Many attempts have been made for searching these types of
abundant, non-toxic and cost-effective new materials
which are suitable for that purpose. One of the emerging
materials is CZTS (copper-zinc-tin-sulphide) that has
already been incorporated as absorber material in solar
cells, with conversion efficiencies close to 10 % [1].
Another material that has been received more attention
recently is the IV–VI compound semiconductor tin sul-
phide (SnS), due to the abundance of Sn in the earth’s crust
([2 ppm), and the facility to produce it in the thin film
form by various techniques such as electrochemical depo-
sition [2, 3], spray pyrolysis [4], thermal evaporation [5],
chemical bath deposition [6], etc. Thin films of SnS exhibit
p-type conductivity, optical band gap from 1–1.7 eV, and
orthorhombic or zinc blende crystalline structure, depend-
ing on the preparation method [6–9].
Thin films of tin sulphide have been incorporated to
photovoltaic structures giving open circuit voltages (Voc) in
the range of 120–370 mV and short circuit current densities
(Jsc) between 1 and up to 9 mA/cm2 [10–13]. Wang et al.
reported the fabrication of photovoltaic structures using
nanocrystals of SnS giving Voc = 471 mV, Jsc = 0.3 mA/
cm2, FF = 0.71 and conversion efficiency of 0.1 % [14].
Recently, Bashkirov et al. reported the formation of hetero-
junctions of the type: Mo/SnS/CdS/ZnO:Al with SnS
obtained by hot wall deposition method, CdS by chemical
bath and ZnO by magnetron sputtering. The parameters
reported were: Voc = 132 mV, Jsc = 3.6 mA/cm2, FF =
0.29 and conversion efficiency up to 0.5 % [15]. Among all
photovoltaic parameters reported, the best results were
Voc = 260 mV, Jsc = 9.6 mA/cm2 and FF = 0.53 to
achieve conversion efficiencies of 1.3 % [16], using SnS thin
films of 600 nm in thickness with a band gap close to
1.32 eV, prepared by spray pyrolysis.
D. Avellaneda (&) � B. Krishnan � A. C. Rodriguez �T. K. Das Roy � S. Shaji
Facultad de Ingenierıa Mecanica y Electrica, Universidad
Autonoma de Nuevo Leon, C.P. 66450 San Nicolas de los Garza,
Nuevo Leon, Mexico
e-mail: [email protected]
B. Krishnan � S. Shaji
CIIDIT- Universidad Autonoma de Nuevo Leon, Apodaca,
Nuevo Leon, Mexico
123
J Mater Sci: Mater Electron
DOI 10.1007/s10854-014-2295-2
In the present work, we report modifications in the
structural, morphological, optical and electrical character-
istics of chemical bath deposited SnS thin films due to post
deposition treatments (annealing) in vacuum (V), nitrogen
(N2) and argon (Ar) atmospheres at temperatures of 300, 350
and 400 �C. Such films were incorporated as absorber layer
in photovoltaic structures using chemical bath deposited CdS
as a window material. Photovoltaic characterizations of the
cells in dark and under illumination are also presented.
2 Experimental
2.1 Chemical deposition
(a) SnS thin films: SnS thin films were deposited on
microscope (Corning) glass substrates (25 9 75 9 1 mm)
cleaned using distilled water and neutral soap, and finally
dried with air. The SnS bath was prepared following the
procedure reported previously [6, 10, 18]: in a 100 ml
beaker, 1 g of tin chloride (SnCl2�2H2O) was dissolved in
5 ml of acetone, followed by the sequential addition of
12 ml of 3.7 M triethanolamine (TEA), 65 ml of distilled
water, 8 ml of 1 M thioacetamide (TA) and 10 ml of 4 M
ammonia, NH3(aq). The substrates were placed vertically
into the beaker with the bath composition, and placed in a
controlled temperature of 40 �C and 17 h to obtain thick-
ness of SnS thin films close to 450 nm, for the character-
ization and annealing of the films.
(b) CdS thin films: The bath composition used to obtain
CdS thin films was reported previously [17]: to prepare
100 ml of the deposition bath, 30 ml of 0.1 M cadmium
acetate, 10 ml of 1 M sodium citrate, 10 ml of 1.5 M
ammonia (aq), 8 ml of 1 M thiourea, and finally distilled
water. The bath temperature was kept at 70 �C for 3 h,
which resulted in good quality CdS thin films of thickness
close to 200 nm. The thickness of the samples was mea-
sured using an Alpha Step 100 thickness measurement unit
(Tencore, CA).
2.2 Preparation of photovoltaic structures
To develop the PV structures, commercial (TEC-15 from
Pilkington, Toledo) transparent conductive oxide, TCO,
(SnO2:F) coated glass substrates were used to deposit CdS
thin films. These substrates of TCO coated with a layer of
CdS, were placed in four consecutive fresh baths of SnS:
the first one at 40 �C for 17 h, and the next three at 60 �C
for 6 h each, to obtain final thickness close to 1.8 lm. The
photovoltaic structures were completed with a 50 nm of
evaporated Ag (99.999 %) onto colloidal carbon suspen-
sion as electrodes. Thermal evaporation was done in a
high vacuum system (INTERCOVAMEX-TE12P), and
thickness of Ag layer was measured using a quartz crystal
thickness monitor incorporated in the evaporation system.
2.3 Annealing the thin films
The samples of tin sulphide (SnS) deposited on glass
substrates and the photovoltaic structures deposited on
TCO were heated at 300, 350 and 400 �C for 1 h in vac-
uum, nitrogen (300 mTorr) and argon (300 mTorr). The
annealing was done in a vacuum oven (T-M High Vacuum
Products). To heat in nitrogen atmosphere, the samples
were introduced into the oven and the chamber was purged
twice with N2 at a pressure of 100 mTorr, and after that the
nitrogen was introduced at the work pressure (300 mTorr)
the heating of the samples began. Similar procedure was
done in the case of annealing in argon.
2.4 Characterization
X-ray diffraction patterns (XRD) were recorded using an
Empyrean diffractometer with CuKa radiation (k = 1.5406
A), operated at 45 kV and 40 mA; the scans were per-
formed in the 2h range from 20� to 60� in a step of 0.016�and 80 s per step in a continuous mode. Optical transmit-
tance (T) and near-normal specular reflectance (R) were
measured using a UV–Vis spectrophotometer (Shimadzu
UV-1800) in the 300–1,100 nm wavelength range; the
films on one side of the substrates were removed with
cotton swabs moistened in dilute HCl, cleaned with water,
and dried. The spectrometer was calibrated (base line set-
ting process) using air as sample as well as reference for
the transmittance measurements. Similarly, for the reflec-
tance measurements, a front aluminized mirror supplied by
the manufacturer was used. The morphological analysis of
the films was done using a scanning electron microscope
(FEI Nova Nano, SEM), and an energy dispersive X-ray
analyzer (EDX) associated with the SEM was used for the
elemental detection. The X-ray photoelectron spectra were
recorded on K-Alpha (Thermo Scientific) X-ray photo-
electron spectrometer, using Al Ka radiation (1,486.6 eV)
as the exciting source. The electrical characterization was
done using a Keithley Picoammeter/Voltage Source 6487
connected with a computer through RS-232 interface. Pairs
of silver paint electrodes of 5 mm in length separated by
5 mm were printed on the surface of the films for the
photocurrent measurements. The samples were allowed to
stabilize in the dark inside the measurement chamber prior
to applying the voltage. Photocurrent response of the films
was measured under an illumination intensity of 85 mW/
cm2 from a tungsten halogen lamp.
For the characterization of the photovoltaic structures,
carbon electrodes were painted in the top absorber layer
(SnS), and Ag was evaporated on the carbon electrodes after
J Mater Sci: Mater Electron
123
the annealing of the structure. TCO was taken directly as the
negative contact to measure the I–V characteristics in the
dark and under illumination, using an Oriel solar simulator
(model 96000) under an AM1.5 radiation of 100 mW/cm2.
3 Results and discussion
3.1 XRD
Figure 1a–c shows the XRD patterns for SnS thin films of
0.45 lm in thickness, annealed in vacuum, nitrogen and
argon atmospheres, at 300, 350 and 400 �C, respectively. At
different conditions, the recorded patterns are similar: well
defined peaks are present at 2h = 22�, 26�, 30.4�, 31.5�, 39�and 49�. All these peaks match the standard pattern for
mineral herzenbergite (PDF# 39-0354) of chemical formula
SnS. The observed peaks correspond to the crystallographic
planes: (110), (120), (101), (111), (131) and (211), respec-
tively. The shape of the peaks in the range of 2h = 42–46�,
shows overlap of the weak planes: (210), (141) and (002)
located at 2h = 42.5�, 44.7� and 45.5�, as marked in the
figure. Thus, the thin films annealed at 300–400 �C show
orthorhombic SnS as reported previously [10]. There were
no peaks corresponding to other phases such as SnS2, Sn2S3
or SnO2 due to the heat treatment.
3.2 SEM with EDAX
Figure 2a–d shows the scanning electron micrographs for
the as prepared SnS thin films and heat treated films at
350 �C. In each case, the micrograph captured under
magnification 20,000 is given. The micrographs show
folded sheet like structure of an estimated thickness of the
individual sheet less than 100 nm. Also, flower like
assemblies were formed on the surface of all the samples.
This type of morphology was observed in SnS thin films
prepared by chemical bath deposition and other techniques
[19–22]. Similar growth was shown for SnS samples
annealed at 300 and 400 �C irrespective of the annealing
atmosphere. Semi-quantitative analysis for all the samples
using EDAX gave atomic ratios (at%) of Sn/S close to 1.1
for the as prepared, vacuum and nitrogen annealed sam-
ples, and 0.95 for the sample annealed in argon.
3.3 XPS analysis
Figure 3a shows a wide scan X-ray photoelectron spectra
(XPS) of SnS thin films annealed at 350 �C in argon
recorded after etching by 3 keV Ar? ions for 180 s.
Binding energies (BE) of all the peaks were corrected using
C 1 s energy at 284.6 eV corresponding to adventitious
carbon in addition to the charge compensation by the flood
gun associated with the spectrometer. The pattern of the
survey spectrum shows the presence of Sn, S, C and O. The
peaks corresponding to C 1s and O 1s were detected only
on the surface probably due to contamination in the air; no
other impurities were present in the film. The high reso-
lution spectra of S 2p and Sn 3d peaks are given in Fig. 3b
and c, respectively. The peak at 160.9 eV corresponds to
the energies of S 2p3/2 for S in SnS (reduction state -2) and
the peaks at 485.7 and 494.1 eV coincide with Sn 3d5/2 and
Sn 3d3/2 binding energies of Sn in the oxidation state ?2
[23]. The gap between Sn 3d5/2 and Sn 3d3/2 level is
8.4 eV, separated by spin orbit coupling. From the spectra,
the Sn/S ration was evaluated using the relation between
the integral area for S 2p and Sn 3d peaks and the value
was 1.02:1. This result is consistent with that obtained from
EDAX (Sect. 3.2). The BE values and the spectral char-
acteristics are in agreement with previous reports of similar
studies on SnS thin films [24, 25]. Comparable results were
obtained in the case of as prepared, vacuum and nitrogen
annealed samples.
3.4 Optical properties
Figure 4 shows the reflectance (R) and transmittance (T)
spectra of the as prepared and annealed SnS thin films.
From these data we can observe clearly that the transmit-
tance spectra of the annealed samples are shifted to longer
wavelengths, and this is more evident for the samples
annealed at 400 �C. The reflectance spectra are also shifted
in the same way. From this figure, we can observe that for
the visible region, almost all the radiation is absorbed in all
Fig. 1 X-ray diffraction patterns of SnS thin film samples annealed
in vacuum, nitrogen and argon at different temperatures: a 300 �C,
b 350 �C and c 400 �C. Principal planes that match the standard
pattern of the mineral herzenbergite (PDF#39-0354) are indicated
J Mater Sci: Mater Electron
123
the annealed films. From these transmittance and reflec-
tance data, the optical absorption coefficients (a) of the
films at different wavelengths were calculated using the
equation [26]:
a ¼ 1
dln
1� Rð Þ2þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1� Rð Þ4þ 2RTð Þ2q
2T
2
4
3
5;
where d is the thickness of the film. In semiconductors,avaries with hm according to the empirical relation [27]:
a ¼ Aðhm� EgÞm
hm
� �
;
where A is a constant, m = 1/2 or 3/2, for direct allowed or
forbidden transitions and m = 2, for indirect allowed
transitions respectively, across a band gap (Eg). To calcu-
late the value of Eg, we considered the Tauc plot of (ahm)1/m
versus (hm-Eg) from which the straight line region was
extrapolated to the hm axis. In our experiments the best
fitting was obtained for m = 2 corresponding to indirect
transitions, i.e. plots of (ahm)1/2 versus hm given in Fig. 5.
The plots show the values of Eg close to 1.25 eV for the ‘as
prepared’ SnS thin film, whereas for the samples annealed
at 300 �C the values slowly decreases to a minimum of
1.1 eV for the vacuum annealed film (V). For the 350 �C
samples the values of Eg remain *1.1 eV irrespective of
the annealing atmosphere. In the case of the annealed
samples at 400 �C, a similar behavior of slight reduction in
Eg is observed, with values close to 1.1 eV (N2, Ar) and
1 eV for the V annealed samples. The reduction in Eg value
from 1.25 to 1.1 eV in all the samples annealed at different
conditions can be due to an improvement in the crystal-
linity after annealing. Transmittance and reflectance mea-
surements were repeated for various times on the same
sample as well as on different sample prepared at same
conditions. The virtually identical results in spectra of T%
and R% indicated that the analyzed samples were homo-
geneous. The spectrophotometer used in our experiments
can detect 1 nm increase in wavelength. Considering the
propagation of errors for the calculation of absorption
spectra and the optical band gap, the estimated energy gap
is presented with an experimental error of ±0.01 eV.
3.5 Electrical properties
The photocurrent response curves of as prepared and
annealed SnS thin films are shown in Fig. 6. An applied
bias of 100 V was used in all cases, and the measurements
were carried out by collecting the current for 10 s in the
dark, 20 s under illumination and finally 10 s in the dark
again. The dark conductivity (rD) of the as prepared SnS
sample shown in the figure is 2.5 9 10-5 X-1 cm-1, and it
increases to 6.5 9 10-5 X-1 cm-1 under illumination (rL).
The values of rD and rL for the SnS thin films annealed in
vacuum (V), nitrogen (N2) and argon (Ar) atmospheres, at
300, 350 and 400 �C, are given in Table 1. From the table
Fig. 2 SEM images of thin films of SnS as prepared (a) and after annealing at 350 �C in different atmospheres: b vacuum, c nitrogen and
d argon
J Mater Sci: Mater Electron
123
for all annealed samples, conductivity is increased com-
pared to that of as prepared films. However for 300 and
400 �C samples, the vacuum annealed films show the
maximum value, while for 350 �C samples nitrogen
annealed films show the maximum value. All the samples
(as prepared and annealed) show photoconductivity: when
light was turned on (during 20 s) number of charge carriers
increased instantly and hence the photocurrent, as seen
from Fig. 6. However during illumination, there was a
slight increase in the current due to thermal effect of the
light source (tungsten halogen lamp). When light was
turned off, the thermal effect delayed the decay process of
the photocurrent. In general, the samples annealed above
300 �C showed better optical absorption and crystallinity;
hence the increase in conductivity during illumination.
The equipment used for the measurement of electrical
properties (Keithley) was calibrated and certified to operate
in the established ranges. The experimental error in the
electrical properties was mainly originated from the mea-
surement error of the printed electrodes (±1 mm) and the
film thickness (±5 nm) used which was measured with a
profiler. Taking these data into account, the conductivity
value was estimated with an error ±10 %.
3.6 Photovoltaic structures
For the development of photovoltaic structures, we selected
vacuum and argon annealed SnS thin films at 350 �C on the
basis of their electrical conductivity. Even though the
Fig. 3 XPS analysis of the SnS thin film annealed at 350 �C in argon
atmosphere, recorded after etching by 3 keV Ar? ions for 180 s:
a survey scan, b S region and c Sn region
Fig. 4 Transmittance (T %)
and reflectance (R %) spectra of
SnS thin films as-prepared and
annealed at 300, 350 and
400 �C, 1 h in vacuum, nitrogen
and argon (300 mTorr). SnS
thin film of four sequential
deposits (4d-1.8 lm) annealed
at 350 �C in argon atmosphere
is also included
J Mater Sci: Mater Electron
123
400 �C annealed samples were more conductive, the pho-
tovoltaic structures were short circuited probably due to the
diffusion of Sn into CdS layer at that temperature. The
photovoltaic structures were of the type: glass/FTO/CdS/
SnS/C/Ag. The J–V characterization of the structures was
made in the dark and under illumination (AM1.5, 100 mW/
cm2), and the cell parameters (short circuit current density,
Jsc, open circuit voltage, Voc, fill-factor FF and energy
conversion efficiency) were determined using the standard
procedures.
Figure 7a depicts the J–V curve under illumination of
the photovoltaic structure using as prepared SnS. The curve
under dark condition showing good rectification properties
is given in the inset. The principal parameters of the
structure were: Voc = 310 mV, Jsc = 0.43 mA/cm2 and
FF = 0.34, having low conversion efficiencies (0.04 %).
Figure 7b shows the J–V characteristics under dark and
illumination for photovoltaic structures annealed at 350 �C
in vacuum, which presents the parameters: Voc = 323 mV,
Jsc = 3.9 mA/cm2 and FF = 0.44, for conversion effi-
ciencies close to 0.55 %. The curves under dark and light
for the photovoltaic structures annealed at 350 �C in argon,
are presented in Fig. 7c giving the best results obtained
in this work: Voc = 270 mV, Jsc = 6 mA/cm2 and FF =
0.44, conversion efficiencies up to 0.7 %. It is worth to
note that these results are the best results reported so far for
SnS thin films as absorber layer, prepared by chemical bath
deposition technique. A comparative analysis of the present
results revealed that for the PV structures formed by
annealing the SnS thin films Jsc values were higher than
that of using as prepared SnS films. This may be due to
Fig. 5 Band gap energy (Eg) of
SnS samples at the different
annealed temperatures. As
prepared SnS is given for
comparison. Also Eg of SnS
(1.8 lm) annealed at 350 �C in
argon is shown
Fig. 6 Photocurrent response of SnS thin films as prepared and
annealed at 300, 350 and 400 �C in vacuum, nitrogen and argon. Bias
voltage of 100 V was applied in all cases. Light ON and OFF periods
were of 20 and 10 s respectively
Table 1 Electrical conductivity of annealed SnS thin films
Temperature Condition rD (X cm)-1
(9 10-3)
rL (X cm)-1
(9 10-3)
As prepared 0.025 0.065
300 �C Vacuum (V) 0.48 1.4
Nitrogen (N2) 0.04 0.15
Argon (Ar) 0.2 0.8
350 �C Vacuum (V) 2.6 4.9
Nitrogen (N2) 3.0 6.0
Argon (Ar) 0.95 3.0
400 �C Vacuum (V) 4.2 6.6
Nitrogen (N2) 2.7 5.6
Argon (Ar) 3.0 5.6
J Mater Sci: Mater Electron
123
increase in conductivity by the annealing process. The best
results for argon annealed structures, Jsc was increased by
more than ten times.
4 Conclusions
Heat treatments in SnS thin films prepared by chemical
bath deposition were done at 300, 350 and 400 �C for 1 h
in vacuum, nitrogen and argon atmospheres. The XRD
analysis showed that SnS thin films were of orthorhombic
crystal structure and XPS results confirmed the presence of
sulphur and tin with chemical states corresponding to that
in SnS. Heat treatments resulted in the modification of
optical and electrical properties of the thin films. The
optical characterization of these films showed a decrease in
the value of the band gap energy from 1.2 eV for the as
prepared film to a value close to 1 eV for the sample
annealed in vacuum at 400 �C. The maximum increase in
the values of electrical conductivities were observed in the
vacuum annealed thin films at 400 �C corresponding to 4.2
and 6.6 9 10-3 X-1 cm-1, for rD and rL, respectively.
The best photovoltaic results obtained in this work, were an
open circuit voltage, Voc = 270 mV, short circuit current
density, Jsc = 6 mA/cm2, fill factor of 0.44 and efficiency
of 0.7 %. Values of Jsc are smaller than that expected
theoretically from the band gap of absorber layer; this
could be mainly due to insufficient diffusion length in tin
sulphide. Further research on the control and character-
ization of the interface properties of photovoltaic structures
are needed to improve the efficiency of the photovoltaic
structures.
Acknowledgments The authors are thankful to PROMEP, Mexico,
CEMIE-SOL (CONACyT-SENER-UNAM) and PAICYT-UANL for
the financial assistance, CIIDIT-UANL for SEM and Dr. Josue
Amilcar Aguilar from CIMAV-Monterrey for XRD.
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