J. Nig. Soc. Phys. Sci. 3 (2021) 455–458

Journal of the
Nigerian Society

of Physical
Sciences

Deposition Time induced Structural and Optical Properties of
Lead Tin Sulphide Thin Films

J. Damisaa,∗, J.O. Emeghaa, I.L. Ikhioyab

aDepartment of Physics, University of Benin, Benin City, Edo State, Nigeria
bDepartment of Physics and Astronomy, University of Nigeria, Nsukka, Enugu State, Nigeria

Abstract

Lead tin sulphide (Pb-Sn-S) thin films (TFs) were deposited on fluorine-doped tin oxide (FTO) substrates via the electrochemical deposition
process using lead (II) nitrate [Pb(NO3)2], tin (II) chloride dehydrate [SnCl2.2H2O] and thiacetamide [C2H5NS] precursors as sources of lead
(Pb), tin (Sn) and sulphur (S). The solution of all the compounds was harmonized with a stirrer (magnetic) at 300k. In this study, we reported on
the improvements in the properties (structural and optical) of Pb-Sn-S TFs by varying the deposition time. We observed from X-ray diffractometer
(XRD) that the prepared material is polycrystalline in nature. UV-Vis measurements were done for the optical characterizations and the band gap
values were seen to be increasing from 1.52 to 1.54 eV with deposition time. In addition to this, the absorption coefficient and refractive index
were also estimated and discussed.

DOI:10.46481/jnsps.2021.157

Keywords: Thin films, XRD, band gap, Pb-Sn-S, refractive index, absorption coefficient

Article History :
Received: 19 January 2021
Received in revised form: 17 September 2021
Accepted for publication: 18 September 2021
Published: 29 November 2021

c©2021 Journal of the Nigerian Society of Physical Sciences. All rights reserved.
Communicated by: B. J. Falaye

1. Introduction

Tin sulphide (SnS) is a potential material for the production
of thin solar cells because of its high absorption coefficient (
α ≈ 104 cm−1 near the fundamental edge), suitable band gap (
Eg = 1.1–2.1 eV) and high hole mobility of 90 cm3 V−1 S −1

[1-3]. SnS semiconductors could present p or n-type conduc-
tivity owing to the preparation conditions and doping materi-
als, which allow the films to be used as an absorption layers in
hetero-junction solar cells fabrication [1, 4, 5]. Theoretically,
SnS thin films solar cells can be optimized such that a conver-
sion efficiency of above 25 % can be reached [6]. SnS thin films

∗Corresponding author tel. no: +234 (0) 7060515540
Email address: john.damisa@uniben.edu (I.L. Ikhioya)

has been prepared by several methods like thermal evaporation,
spray pyrolysis, electron beam evaporation, SILAR, hot injec-
tion, aqueous solution, colloidal route, single solid approach,
precipitation and electrochemical deposition (ECD) [5-7]. ECD
technique was used to fabricate Pb-Sn-S thin films in this study.
The method presents a simple route of depositing TFs due to its
low cost of experimental system, uniformity of films thickness
as well as its large area deposition at low temperature [8-10].

Recently, investigation into new photovoltaic materials with
improved efficiency have assumed a considerable interest and
researchers are investigating for understanding and engineering
the properties of SnS TFs for photovoltaic application [5]. Dop-
ing with elements like lead has shown to enhance the structural
as well as the optical properties of SnS TFs. The study presents
the preparation of lead tin sulphide (Pb-Sn-S) thin films using

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Damisa et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 455–458 456

Figure 1. Schematic diagram of Pb-Sn-S deposition process

the electrochemical deposition technique. Our aim is to develop
a better growth approach of new and non-toxic material for the
production of low cost solar cells. Improvement in the proper-
ties of SnS TFs due to lead incorporation in SnS-system will
enhance the device efficiency of the material. Particularly, this
communication is concerned with the influenced of deposition
duration on the properties (structural and optical) of Pb-Sn-S
TFs via the electrochemical deposition technique which hith-
erto has not been studied using this route.

2. Materials and Method

Analytically graded chemical (Sigma-Aldrich) was used to
deposit the Pb-Sn-S TFs. These chemicals include; lead (II) ni-
trate [Pb(NO3)2], tin (II) chloride dehydrate [SnCl2.2H2O] and
thiacetamide [C2H5NS]. The electrochemical deposition bath
system consist of a cationic precursor of 0.05 mol of Pb (NO3)2,
0.01 mol of SnCl2.2H2O and an anionic (precursor) of 0.15 mol
of C2H5NS mixed in distilled water. A stirrer (magnetic) was
used to harmonize the reaction bath. Fluorine doped Tin Oxide
(FTO) and carbons were employed as the cathode and anode
electrodes respectively. The deposition using the electrochemi-
cal deposition method was achieved according to the scheme in
Figure 1. The deposition were repeated for 20, 25, 30, 35 and
40 seconds and the samples were later coded as J0, J1, J2, J3,
and J4 respectively.

2.1. Characterization of thin films

The Pb-Sn-S films used for this study have good adherent
with the FTO substrates. The XRD patterns of Pb-Sn-S TFs
were observed using Advanced X-ray diffractometer (Bruker
D8) operating with a wavelength of 1.5406Ȧ and, at a scan-
ning range of 15 to 80o. The UV-Visible optical measurements
of Pb-Sn-S thin films were done using a UV-1800 Spectropho-
tometer in the range of 300 to 1000 nm at room temperature.
The optical band gaps (Eg), absorption coefficient (α) as well
as refractive index (n) were estimated from the optical data.

3. Results and Discussion

3.1. X-ray diffraction (XRD) studies
The XRD patterns of Pb-Sn-S TFs are shown in Figure

2. Although the deposition time is increased, the XRD spec-
tra showed four similar main peaks at 21.70◦, 23.50◦, 24.94◦

and 33.62◦, which correspond to the diffraction peaks of (200),
(201), (211) and (221). The presence of higher intensity peaks
in the Pb-Sn-S films with narrower spectral widths indicated
that the films are polycrystalline in nature [11, 12]. From the
XRD patterns also, it is obvious that the Pb-Sn-S structures con-
sist of mixtures of several phases including the orthorhombic
Sn2S3 (JCPDS no 014-0619), hexagonal SnS2 (JCPDS no 023-
0677) and cubic structure PbS thin films (JCPDS 01-0880). As
known, the presence of secondary phases within the Pb-Sn-S
system may have deteriorated the structural crystallization as
well as the peaks patterns.

Figure 2. XRD patterns of the prepared PbSnS thin film

3.2. Optical studies
To ascertain the potentials of the electrochemical prepared

Pb-Sn-S TFs for device fabrications, the absorbance was in-
vestigated in the spectra wavelength of 300 to 1000 nm. The
absorbance A, was measured using the relation in equation(1)
[13];

A = Log
(

1
T

)
(1)

Figure 3 shows the relationship between the optical absorbance
and wavelength of Pb-Sn-S FTs. In the plot, it could be ob-
served that the absorption was decreasing along the wavelength
regions. As well from the figure, the absorbance decreases with
deposition time. Such decrease in absorbance may be due to
structural defects such as surface irregularity and defect density
in the Pb-Sn-S system as a result of increase in deposition time
[14], as indicated from the XRD measurement. The low absorb-
ing nature of the films consequently indicates an improvement
in the transmission [13]. The absorption coefficient (α) of the
material was evaluated by means of the equation (2) [15];

α =
1
t

ln
(

1
T

)
(2)

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Damisa et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 455–458 457

Table 1. Some optical properties of Pb-Sn-S TFs
Samples Band gap Absorption coefficient Refractive index

(eV) (cm−1) × 104 (n)
JO 1.51 6.10 2.944
J1 1.52 15.0 2.939
J2 1.53 17.5 2.934
J3 1.54 13.2 2.929
J4 1.51 6.80 2.944

Equation (2) assumed a negligible reflectance while T and t
are the respective transmittance and thickness of Pb-Sn-S films.
The observed variation of the absorption coefficient with depo-
sition time is shown in Table 1. It was observed that α increased
from 6.1 × 104 to 17.5 × 104 cm−1 with deposition time of 30
seconds (Sample J2) and was decreased on further increase of
deposition time. Also from the table, the magnitude of the ab-
sorption coefficient was found to be greater than the 104 cm−1

which makes Pb-Sn-S thin film a better alternative than GaAs
and CdTe as absorber layers in photovoltaic applications [5,16].
The band gap (direct) was determined using the following rela-

Figure 3. Absorbance against wavelength of the prepared PbSnS thin films

tion in equation(3) [17];

(αhv)2 = k (hv − Eg) (3)

where hv is the photon energy, α is the absorption coefficient, k
is a proportionality constant, and Eg is the optical band gap. A
plot of the square of absorption coefficient against photon en-
ergy gives a curve illustrated in Figure 4. Since the prepared
Pb-Sn-S films is a direct semiconducting material [2], extrapo-
lating the linear portion of Figure 4 to the x-axis at x = 0 gives
the band gap energy (Eg). The band gap obtained was seen to
be between 1.52 – 1.54 eV as indicated in Table 1. Also from
the table, the band gap energy values were observed to be in-
creasing with increase in deposition time. Generally, band gap

energy in semiconducting materials are mostly influenced by
their structural defects, crystallinity, impurities, grain sizes as
well as grain boundary disorders [18].

Consequently, the observed increase in band gap energy in
Pb-Sn-S samples can be explained in terms of the effect of im-
purities in their lattice system as indicated from the XRD stud-
ies. Sebastian et al. [2] have reported a band gap range of
1.60 to 1.90 eV for lead doped tin sulphide (SnS:Pb) TFs grown
by varying lead concentration using Nebulized spray pyrolysis
(NSP) technique. Orimi et al. [19] estimated an optical band
gap of 1.63 to 1.80 eV for Pb1-xSnxS nano-powder using chem-
ical precipitate technique. Our obtained values are relatively
lower than these values which could be the direct effect of the
electrochemical deposition method employed in the preparation
of this film.

The refractive index (n) is an essential property of optical
materials. It is closely related to the electronic polarization of
ions as well as the local field within the optical materials [20].
Many optoelectronic devices such as switches, modulators, fil-
ters, waveguides, solar cells and detectors are based on refrac-
tive index [21]. Generally, the n of Pb-Sn-S TFs is related to
the optical band gaps. Moreover, the refractive index of the pre-
pared Pb-Sn-S films was estimated using the proposed relation
by Herve and Vandamme, given in equation (3) [21, 22]

n =

1 +
(

A
Eg + B

)2
1
2

(4)

where Eg is the band gap, A (13.6 eV), and B (3.4 eV) are
constants. The estimated values of the refractive index are in-
dicated in Table 1. The results indicated that the n values of
Pb-Sn-S TFs decreased from 2.944 to 2.929 with enhance de-
position time. The range of the refractive indices of the material
indicate that Pb-Sn-S film is a ternary material whose properties
falls between the binary constituents of PbS (1.8 – 6.0) [23] and
SnS (3.5 – 5.5) [4], and compares favorable well with values in
literature.

4. Conclusion

Thin films of Pb-Sn-S have been prepared by the electro-
chemical deposition method and the structural and optical prop-
erties were investigated as function of the deposition time. The
XRD measurements indicated a polycrystalline film with no
much difference in their crystallinity as deposition time increases.
The increased deposition time resulted in the variation of the

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Damisa et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 455–458 458

Figure 4. Square of absorption coefficient and photon energy for Pb-Sn-S TFs

absorption coefficient and band gap from 6.1 × 104 to 17.5 ×
104 cm−1 and 1.51 to 1.54 eV respectively. The refractive in-
dex was observed to decrease with deposition time except for
sample J4, and obeyed the Harve and Vandamme model. The
high absorption coefficient obtained for the material suggests
that the prepared Pb-Sn-S would create good absorbing proper-
ties in solar cell fabrications.

Acknowledgments

We thank the reviewers as well as the editorial team for
their positive contributions and ideas, which have markedly en-
hanced the value of this paper.

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