Properties of In2S3 thin films deposited onto ITO/glass substrates by chemical bath deposition

Article abstract of DOI:10.1016/j.jpcs.2010.09.011

In₂S₃ films have been chemically deposited on ITO coated glass substrates by chemical bath deposition, using different deposition times and precursor concentrations. The bilayers are intended for photovoltaic applications. Different characterization methods have been employed: optical properties of the films were investigated from transmittance measurements, structural properties by XRD and micro-Raman, and surface morphology by SEM microscopy analysis. Also, the direct and indirect band-gaps and the surface gap states were studied with surface photovoltage spectroscopy (SPS). We proposed that electronic properties of the In₂S₃ samples are controlled by two features: shallow tail states and a broad band centred at 1.5 eV approximately. Their relation with the structure is discussed, suggesting that their origin is related to defects created on the S sub-lattice, and then both defects are intrinsic to the material.

Full text of DOI:10.1016/j.jpcs.2010.09.011

Journal of Physics and Chemistry of Solids 71 (2010) 1629–1633
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Properties of In2S3 thin films deposited onto ITO/glass substrates
by chemical bath deposition
n
a
a
b
b
b
B. Asenjo a, , C. Guillen , A.M. Chaparro , E. Saucedo , V. Bermudez , D. Lincot ,
´
a
a
´
J. Herrero , M.T. Gutierrez
a Department of Energy, CIEMAT, Avda. Complutense, 22, 28040 Madrid, Spain
b Institute of Research and Development of Photovoltaic Energy (IRDEP-UMR 7174-CNRS-EDF-ENSCP), 6 quai Watier, 78401 Paris, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In2S3 films have been chemically deposited on ITO coated glass substrates by chemical bath deposition,
using different deposition times and precursor concentrations. The bilayers are intended for
photovoltaic applications. Different characterization methods have been employed: optical properties
of the films were investigated from transmittance measurements, structural properties by XRD and
micro-Raman, and surface morphology by SEM microscopy analysis. Also, the direct and indirect band-
gaps and the surface gap states were studied with surface photovoltage spectroscopy (SPS). We
proposed that electronic properties of the In2S3 samples are controlled by two features: shallow tail
states and a broad band centred at 1.5 eV approximately. Their relation with the structure is discussed,
suggesting that their origin is related to defects created on the S sub-lattice, and then both defects are
intrinsic to the material.
Received 24 March 2009
Received in revised form
7 September 2010
Accepted 14 September 2010
Keywords:
A. Thin films
B. Chemical synthesis
C. Raman spectroscopy
& 2010 Elsevier Ltd. All rights reserved.
1. Introduction
together with surface photovoltage spectroscopy (SPS), in order to
characterize the surface defect structure. Attention is mostly
concentrated to study the effect of the different growth conditions
on the formation, origin (intrinsic, extrinsic) and properties of
the surface states, and their possible impact on the electronic
properties of the buffer layer. Samples were prepared using
different precursor concentrations and growth times with the aim
of identifying the effect of both parameters.
Indium sulphide buffer layers, prepared by chemical bath
deposition (CBD), have demonstrated their potential to obtain
high efficiency solar cells [1,2]; their band gap energy values of
between 2 and 3.7 eV [3], their transparency, etc., are suitable
characteristics for application in solar energy conversion. For
obtaining In2S3 with the adequate properties for such application,
we must take into account several considerations about the
material.
These properties of In2S3 obtained by the CBD method depend
on different factors such as precursor concentration, deposition
time and reaction temperature. However, the effect of the
different conditions on the final layer properties is still not well
understood and is one of the key points for the further
development of this material. In particular for example, the
presence of surface states is very important because of their effect
on junction formation in solar cells; hence, it is mandatory to
know their origin and properties [4]. Commonly, this type of
surface defects is possibly due to recombination centres, which
strongly deteriorate the interface between the absorber and the
buffer layers, and as a consequence the final device performance.
In this work, a combination of optical, morphological and
structural characterization is used to explore the In2S3 layers,
2. Experimental
ITO thin films have been prepared by rf magnetron sputtering
from an oxide ceramic target 90 wt% In2O3 and 10 wt% SnO2. The
substrates were soda lime glasses (2 mm thick) that were placed
in equivalent positions in an adjustable vertical frame and moved
in front of the target to obtain simultaneous ITO deposition,
without intentional heating. High purity Ar and O2 were
introduced through independent mass flow controllers after the
vacuum chamber was evacuated below 1 ꢀ 10ꢁ4 Pa. The oxygen
content in the sputter gas, as determined by the oxygen to argon
partial pressure ratio, was adjusted in order to optimize the ITO
properties according to previous studies [5,6]. The process total
pressure was kept at 4 ꢀ 10ꢁ1 Pa and the sputtering power
density employed was below 1.5 W/cm2. The samples were later
annealed at 350 1C in vacuum in order to improve their optical
and electrical characteristics [7], achieving visible transmittance
n Corresponding author. Tel.: +34 913466364; fax: +34 913466037.
E-mail address: mbegona.asenjo@ciemat.es (B. Asenjo).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2010.09.011

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B. Asenjo et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1629–1633
Table 1
In2S3 films prepared on glass/ITO substrate.
90
F1
F2
F3
F4
ITO
Sample
[TA] (M)
Deposition
Thickness
(nm)
80
70
60
50
40
30
20
10
0
time (min)
F1
F2
F3
F4
0.3
0.3
0.5
0.5
25
35
25
35
55
70
80
85
Other growth conditions: 70 1C bath temperature, InCl3, 0.01 M HCl, 0.3 M
CH3COOH.
near 90% and sheet resistance of 7
with about 350 nm thickness.
O/sq for the present ITO layers
The deposition of In2S3 was carried from an acidic solution of
thioacetamide concentration (TA) (CH3CSNH3, Fluka), and indium
(III) chloride (InCl3, Fluka) 0.01 M at 70 1C, for different deposition
times (Table 1). Other additives necessary to obtain films with
photovoltaic quality are hydrochloric acid and acetic acid. Some of
the samples were annealed at 200 1C in a tubular furnace under
flowing nitrogen.
0
500
1000
λ (nm)
1500
2000
2500
90
80
70
60
50
40
30
20
10
0
The thickness of the In2S3 films obtained was found using a
profilometer (Dektak 3030).
Raman microprobe measurements were made on In2S3 layers
before and after thermal treatment using a HR800-UV Horiba-Jobin
Yvon spectrometer coupled with an Olympus metallographic
microscope. The spectra were obtained using as excitation the green
F1
F2
F3
F4
line of an Ar+ laser (
software defining the peaks as Gaussian–Lorentzian.
l¼514 nm) and were fitted using the LabSpec
Surface photovoltage spectroscopy (SPS) was carried out using
a home-made equipment consisting in an isolated camera with a
gold grid (Kelvin probe S, Besocke Delta Phi GmbH), controlled by
a Kelvin Control 07. Ar pressure of 1 atm was imposed and the
measurements were carried out at room temperature. Excitation
was made with the light of a monochromator Spectra Pro 2300i
0.3 m (350–1400 nm) with an ARC Model TS-428 250 W
Tungsten-Halogen lamp. The surface photovoltage signal was
monitored with an oscilloscope Iso-Tech ISR635 35 MHz and
measured with a Keithley 2010 Multimeter. The collected signal
was corrected using the dark contact potential difference (CPDD)
and the wavelength dependence of the lamp emission spectra, on
the studied energy region. The indirect and direct band-gaps and
the energy of the exponential defect tails were obtained using
analysis of the SPS spectra.
Optical transmission measurements of the samples referred to
the air were carried out with unpolarised light at normal
incidence in the wavelength range from 300 to 2500 nm, with a
double beam spectrophotometer Perkin Elmer lambda 9.
X-ray diffraction (Philips X’pert-MPD diffractometer) was used
to determine the crystallographic structure of the films;
phase identification was carried out by comparing the observed
d-spacing with the powder diffraction standards card files and
SEM microscopic analysis (Hitachi S-2500) to study the surface
morphology.
0
500
1000
1500
λ (nm)
2000
2500
Fig. 1. Optical transmittance spectra for indium sulfide films on ITO/glass
substrates (a) and (b) after the thermal treatment at 200 1C under nitrogen
atmosphere.
suggesting the presence of high shallow state density as will be
confirmed later by SPS.
The X-ray diffractograms corresponding to the samples are
shown in Fig. 2. The In2S3/ITO films were annealed at 200 1C in
nitrogen atmosphere due to the low crystallinity exhibited by the
In2S3 films. The diffraction peaks of 2y associated with ITO are at
21.51, 30.51, 35.51, 41.71, 45.71, 50.71, 60.61, 62.01 and 63.61 (JCPDS
Card no. 06-0416) corresponding to a cubic structure with (2 2 2)
preferential orientation. The diffraction peaks of 2
43.61 and 47.61 corresponding to In2S3 (JCPDS Card no. 32-0456)
appear with small intensity in the F1, F2 and F4 samples. This is
not surprising because of the very low sample thickness, their
nanocrystalline structure and their poor quality.
y at 33.101,
The optical transmittance spectra corresponding to these samples
(Fig. 1b) show an increase in transmittance between 5% and 10% in
the range 400–800 nm, the transmittance being lower for films with
high time deposition, as the samples without heat treatment.
The Raman spectra of as-grown In2S3 are shown in Fig. 3a for
samples F1–F4. Raman modes are observed in the energy region
between 200 and 500 cmꢁ1, but all of them are broad and have
very low intensity, confirming the nanocrystalline or amorphous
nature of these samples. Bands observed between 200 and
3. Results and discussion
The optical transmittance spectra corresponding to the
different indium sulfide films are shown in Fig. 1. The transmit-
tance of samples (Fig. 1a) in the visible spectral range
(400–800 nm) is above 70–80%, with the transmittance being
lower for films with high time deposition (and then thicker),
whereas transmittance in the near-infrared region decreases
owing to interaction with free-carriers in the ITO coating [5–7],
400 cmꢁ1 can be assigned to
b-In2S3 [8,9], but the band at

B. Asenjo et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1629–1633
1631
*
*
*
F4
F3
F2
F1
ITO
0
10
20
30
40
50
60
70
80
90
2θ/deg
Fig. 2. XRD of samples F1–F4 after the thermal treatment at 200 1C under nitrogen
atmosphere (n¼corresponding In2S3 CJCPDS Card no. 32-0456).
460 cmꢁ1 could be related to the S–S mode of elemental S [10], an
indication being that free S is present in the samples. From Fig. 3a
it is evident that, the lower the growth time (F1 and F3), the higher
the intensity of the band at 460 cmꢁ1, suggesting that for higher
growth times there is a lack of S in the bath. After the thermal
treatment at 200 1C, spectra change as is clearly observed in
Fig. 3b. The band at 460 cmꢁ1 assigned to the S–S vibrational
mode of elemental S almost disappears, presumably due to S
evaporation and/or reaction with the as-grown layers, suggesting
that many free S atoms cannot form the In–S bonds before the
thermal treatment. Also, several peaks and bands are enhanced in
the whole frequency region. The active modes of
b-In2S3 are
present at 105, 117, 250, 305 and 363 cmꢁ1 [7,8], confirming the
composition and structure of the layers. Their nanocrystalline or
quasi-amorphous nature is reflected in the broadening of the
bands (mostly in the high frequency bands), indicating that
although some crystallization occurs after thermal treatment, the
layers still remain polycrystallines and/or nanocrystallines, mak-
ing an in-deepth study of the spectra difficult. In any case, the
Raman bands do not show any tendence, either with deposition
time or with bath concentration. This means that both conditions
seem not to affect considerably the sample’s structure after the
thermal treatment (almost at large scale), the active Raman modes
Fig. 3. Raman spectra of samples F1–F4 before (a) and after (b) the thermal
treatment at 200 1C under nitrogen atmosphere.
lower the concentration and growth time, the higher the broad
onset. In any case, the bath concentration seems to affect more
than the growth time. Similar to the as-grown samples, this band
could be related to the first growth steps where surface
instabilities are largely expected, due to unsatisfied bonds, for
example. Further growth steps lead to a lower density of tail
states on the surface, probably because the last growth steps are
under a more stable regimen.
In addition, when the growth time exceeds 25 min and/or the
precursor concentration 0.3 M, a broad band centred at 1.5 eV
starts to dominate the spectra. There are few reports on defect
levels on In2S3 in general and about this energy region in
particular [13], but with a simple reasoning we can try to obtain
more information on it. The material being n-type (holes are
minority carriers), the increase in photovoltage implies the
generation of minority carriers, i.e. holes. Then, in the spectra of
Fig. 4a at 1.5 eV we can observe the excitation of holes from a
hole-trap centre (probably positively charged and with donor
character), in agreement with the observation in Ref. [13].
Although it is not possible to establish the actual origin of this
centre, their hole-trap properties could be related for example
corresponding to the b-In2S3 phase being present in all the cases.
Concerning the SPS measurements, the first important point is
that all the samples have n-type conductivity as was expected. In
Fig. 4a the squared (W-CPD) difference is plotted for the as-grown
samples, where W is the maximum contact potential that is
obtained at high energies (the value was taken nominally at
3.5 eV in all the cases) and CPD is the contact potential difference
[11,12]. Note that before the thermal treatment, the surface
photovoltage signal is very low. Hence, it is not possible to
accurately determine the direct and indirect band-gaps and then,
these spectra will be only qualitatively discussed taking into
account two important features: the broad onset at high energies
and the broad band centred at 1.5 eV approximately. The broad
onset can be attributed to the presence of shallow states,
extending from their corresponding band up to the band-gap
(tail states) [11]. In such a case, there is a high density of shallow
states in the samples (especially in the samples grown from low
concentration solutions), in agreement with their poor crystal-
linity confirmed by XRD and Raman spectroscopy. Observing
Fig. 4a, it is possible to establish two important correlations: the

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B. Asenjo et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1629–1633
Table 2
Summary of the values extracted from SPS measurements (room temperature) of
samples with the thermal treatment at 200 1C under nitrogen atmosphere.
Sample
Eg dir. (eV)
Eg ind. (eV)
Et (meV)
F1
F2
F3
F4
2.5770.01
2.6170.01
2.5170.02
2.4870.03
1.6970.06
1.7570.10
1.7770.05
—
6172
6971
14172
4271
the bath concentration or with the growth time. It is evident that
samples F1 and F2 present similar spectra and although the signal
is considerably improved after heating, the presence of the band
onset is also clear. Both samples being grown at low bath
concentration, this is an indication that the deposition regime in
the two cases was the same. Conversely, sample F3 strongly
increases its signal after crystallization and is very different from
that of sample F4. It is worthnoticing that shallow states for
sample F3 are the major contribution factor to the photovoltage at
high energies, indicating a disordered system in agreement with
the XRD spectrum. To explain this phenomenon, we can take into
account the growth conditions. At higher bath concentration, the
first steps on the growth occur at high deposition rates (as is the
case of sample F3), and the layer gets a low crystalline quality.
After a certain point of time, the bath concentration decreases and
the growth rate starts to decrease, explaining the better crystalline
quality of the sample F4. In fact, the sample F4 behaves differently
from others as is clearly observed in Fig. 4b. The broad onset
diminishes and almost disappears, implying the minimization of
the tail states density [11], and is accompaigned by a low signal.
These tendencies are confirmed by the value of the Urbach-
energy (Et) presented in Table 2. While samples F1 and F2 are
comparable, the value of Et for sample F3 is considerably higher
and for F4 it is considerably lower. In such a case, the disorder on
the systems is related to the shallow surface states (tail states)
and as a consequence the lower the Urbach-energy, the lower the
band onset. Finally, the broad band centred at 1.5 eV for the
precursors strongly decreases and shifts to higher energies after
thermal treatment. The rearrangement of the atoms after the
thermal process can explain this behaviour and it seems to be
effective in eliminating this undesirable band absorption, prob-
ably for the diffusion of S in order to compensate the VS.
Fig. 4. SPS spectra of samples F1–F4 before (a) and after (b) the thermal treatment
at 200 1C under nitrogen atmosphere.
with positive charged centres, formed by anion vacancy in the
In2S3 lattice: VS (the charge of the centre could be +1 or +2,
in agreement with their hole-trap properties). This is also in
agreement with the Raman measurements, where free S was
observed, indicating that some S atoms were not included on the
In2S3 structure, the origin being the VS and also with the
conductivity type of samples, because this intrinsic defect (VS)
can explain n-type conductivity. This band seems to be more
dependent on growth time.
SEM images (Fig. 5) show the surface morphology of the films.
For the F1 layer the surface is very compact and homogeneous
microstructure compared with the other samples, but all of the
samples show nanocrystalline structure as is expected, with grain
sizes lower than 100 nm.
4. Conclusions
After the thermal treatment the surface photovoltage spectra
dramatically change as is clearly observed in Fig. 4b. The
photovoltage markedly increases and the SPS spectra exhibit a
well defined band edge. In order to analyse the spectra, numerical
fitting using sigmoidal logistic functions was employed. Means
these fittings we obtain both band gaps which are summarized in
Table 2. Also, with the slope of the plot of the difference (W-CPD)
as a function of the photon excitation energy in semi-log scale, we
obtain the exponential defect tails below the band-gap (Et), or
Urbach-energy, which is directly related to the disorder on the
surface [12], and is also summarized in Table 2.
b-In2S3 layers were prepared by the CBD method with
nanocrystalline and/or amorphous structure, as confirmed by XRD
and Raman spectroscopy. As-grown samples exhibit poor crystalline
quality with free S as the main secondary phase. The crystalline
quality of the layers is enhanced after thermal treatment at 200 1C,
as observed from the elimination of the free S by evaporation and/or
diffusion onto the layer. The surface photovoltage spectra of as-
grown CBD In2S3 shows two distinct features: a broad onset at high
energies related to the tail states and a broad band at 1.5 eV with
donor character and presumably related to anion vacancies. The
contribution of both bands to the total surface photovoltage depends
on the growth conditions, which seems to be responsible for the
control of the optical and electrical properties of the layers. With a
bath concentration of 0.5 M and a growth time of 35 min and after
the crystallization process at 200 1C (sample F4), the surface tail
The results are in agreement with the reported values for In2S3
layers [14–17] and, as can be observed in Table 2, the direct band-
gap tends to decrease with bath concentration. No special
correlations of the indirect band-gap are observed, either with

B. Asenjo et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1629–1633
1633
Fig. 5. SEM images of In2S3 films: F1 (a), F2 (b), F3 (c) and F4 (c) deposited at 70 1C on glass and annealing at 200 1C under nitrogen atmosphere.
states are minimized and the Urbach-energy is also considerably
reduced, obtaining In2S3 layers with electronic and structural
properties suitable for their application as buffer layers in photo-
voltaic devices. Unfortunately, the transparency of the film is
reduced due to their thicker nature at undesirable values. Future
works will be focused on the optimization of the optical parameters
without deterioration of the electronic and structural ones.
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Acknowledgements
This work has been financed by the FOTOFLEX project (Comu-
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