Effect of indium doping on physical properties of
nanocrystallized SnS Zinc blend thin films grown by chemical bath
deposition.
Meriem Reghima(a) , Anis Akkari (a,b)*, Michel Castagn (b) and
Najoua Kamoun-Turki(a)
(a) Laboratoire de Physique de la Matire Condense, Facult des
Sciences de Tunis El Manar, Tunisie (2092), Tunisia. (b) Institut
dElectronique du Sud, Unit Mixte de Recherche 5214 UM2-CNRS (ST2i)
- Universit Montpellier 2 Place Eugne Bataillon Bat 21 cc083
F-34095 Montpellier Cedex 05 France. Corresponding author. Tel:
+21698347470; fax: +21671885073
E-mail adress [email protected]
SnS:In thin films have been successfully prepared on Pyrex
substrates using low cost chemical bath deposition (CBD) technique
with different Indium-doped concentration (y = [In]/[Sn]= 4%, 6%,
8% and 10%). The structure, the surface morphology and the optical
properties of the SnS:In films were studied by X-ray diffraction
(XRD), scanning electron microscope (SEM), atomic force microscopy
(AFM) and spectrophotometry measurements, respectively. To obtain a
thickness of the order of 308 nm for absorber material in solar
cell devices, a system of multilayer have been prepared. It is
found that the physical properties of tin sulphide are affected by
Indium-doped concentration. In fact, X ray diffraction study showed
that the better cristallinity in zinc blend structure with
preferential orientations (111)ZB and (200)ZB , was obtained for y
equal to 6%. According to the AFM analysis we can remark that low
average surface roughness (RMS) value of SnS(ZB) thin film is
obtained with In-doped concentrations equal to y = 6%, is about of
26 nm. Energy dispersive spectroscopy (EDS) showed the existence of
In, S and Sn in the films. Optical analyses by means of
transmission T() and reflection R() measurements show 1.57 eV as a
band gap value of SnS:In(6%). On the other hand, In doped tin
sulphide exhibits a high absorption coefficient up to 2.5 106 cm-1
, indicating that SnS:In can be used as absorber thin layer in
photovoltaic structure such, SnS:In/In2S3:Al/SnO2 (F) and SnS/ZnS:/
SnO2 (F) where ZnS and In2S3 are chemically deposited in a previous
works. In this study the hetero-junctions SnS/In2S3 (Al) and
SnS/ZnS(In) are analyzed.
Figure 1. pattern of doped thin layers system obtained by for
different y=[In]/[Sn]=4,6,8 and 10%
concentrations . Each system is formed by three successive
depositions runs which each one having the same indium doping
concentration and grown
for 4h .
Table 1 Variation of indium dopped SnS spaincing d(111)ZB ,
grain size d,
dislocation density dis , number of crystallites per unit
surface area nc for(111)ZB reticular plan and average surface
roughness as a function of the indium doping concentration
y=[In]/[Sn] .
Multilayer of doped SnS thin films have been prepared on Pyrex
substrates by CBD technique. It is found, after three deposition
runs, that the thickness of the film is relatively uniform with an
average thickness close to 308 nm. Indium doping of multideposited
layer of SnS thin film affect the structural and optical properties
of these layers. Indeed, for y =[In]/[Sn] equal to 6%, we obtained
the better cristallinity with zinc blend structure and grain size
equal to 45 nm, the lower value of average surface roughness is
obtained which is equal to 26 nm. Moreover, it is noted that in
case of y = 6% tin sulphide exhibit a high absorption coefficient
in the fort absorption region (> 2.5 10 cm-1) and the better
optical band gap Eg= 1.57 eV which is nearly equal to the optimum
theoritical value of 1.50 eV for efficient light absorption.
Therefore, the SnS:In thin films are suitable for absorber layers
in solar cells. The fabrication of p n heterojunction including
this binary absorber such as SnS:In/ In2S3 (Al)/ SnO2 (F) and
SnS:In/ZnS(In)/ SnO2 (F) where ZnS and In2S3 are chemically
deposited in a previous works , could be tested as an alternative
cell in comparison with CuInS2/ CdS ones. Other physical
characterizations especially electrical ones are in progress to
reach better thin films with no higher resistivity by means of an
appropriate heat treatment under controlled atmosphere.
Table 2. EDS analysis of indium doped tin sulphide multilayer
thin films formed by three successive thin layers, prepared by CBD
for different y=[In]/[Sn] concentrations ratios.
Figure 4. Spectral shapes of transmission T() and reflection
R()
of In doped SnS(ZB) multilayer formed by three successive thin
layers prepared for y=[In]/[Sn]=4,6,8 and 10% .
Figure 5. Plot of versus photon energy (h)2/3 of indium doped
tin sulphide multilayer formed by three
successive layers grown for different indium concentration
y=[In]/[Sn]=4,6,8 and 10% .
Table 3. Optical band gap energy (Eg) of indium doped SnS (ZB)
multilayer system grown by for different y=[In]/[Sn]
concentrations. Each system is formed by three successive
depositions runs which each one has
the same indium doping concentration and grown for 4h .
Facult des Sciences de Tunis
y=[In]/[Sn] (%)
FWHM [2th.]
d(111)
d(nm) dis (*1010 cm-2) nc (*10
18 cm- 3)
RMS(nm)
4 0.227 3.345 45 4.9 2.5 31
6 0.227 3.356 45 4.9 3.2 26
8 0.252 3.359 38 6.9 4.4 48
10 0.252 3.355 38 6.9 4.7 33 Y=[In]/[Sn] (%) 4 6 8 10
Sn (%) 77.77 73.59 75.56 76.41
S (%) 20.88 20.56 19.92 20.38
In (%) 1.36 5.85 4.52 3.21
In/Sn(%) 1.7 7.9 5.9 4.2
Y=[In]/[Sn] (%)
0 4 6 8 10
Eg (V) 1.76 1.62 1.57 1.59 1.59
Figure 3. MEB (a) and SEM cross-section (b) images of SnS:In
multilayer system grown by CBD after three
successive depositions and for y=[In]/[Sn]=6%.
Figure 2. 2D and 3D AFM topography of indium doped tin sulphide
multilayer formed by three successive thin films prepared by CBD
for y=[In]/[Sn] equal to 6%.
In the same line, SnS:In thin films was also Chemically analyzed
by energy dispersive spectroscopy (EDS), we selected several points
on the surface and the atomic percentage of Sn, S, In and In/Sn,
values are gathered in table 2. It is obvious shown that In element
exists.
Optical band gap energy of the SnS(ZB) thin film prepared for
different indium doping concentration .y =[In]/[Sn] = 4%, 6%, 8%
and 10%. are calculated and summarized in table 3. It is clearly
shown that the optical properties of tin sulphide are affected by
In-doped concentration. In fact, the optical band gap E. is
decreased from 1.76 eV for undoped SnS thin film [1] to 1.57 eV for
SnS:In thin film doped with indium concentration ratio equal to y
=[In]/[Sn] = 6%, this value is nearly equal to the theoretical
optimum value of 1.50 eV for efficient light absorption.
Photovoltaic parameter SnS(ZB)/In2S3:Al(40%)
SnS(ZB)/ZnS/In(10%)
Imax (A) 5x10-5 3x10-6
Vmax (V) 0,13 4.8
Isc (A)/cm2
9,98x10-5 8.48x10-6
Voc (mV) 211 10
Rs (K) 89x103 44x104
Rsh (K) 20x106 39x104
FF 0,30 0,17
Is (A) 2,9x10-5 9x10-12
Iph (A) 1,2x10-4 8.9x10-11
Table 4. parameters of the hetero-junctions
Figure 7. The characteristics in the dark and under illumination
of the elaborated hetero-junction
Figure 6. Schematic representation of fabricated Hetero-junction
structure: (1):SnO2:F, (2): In2S3:Al or ZnS:In, SnS and (4):Ag.
The hetero-structure is obtained after four distinct steps which
are:
1)Deposition by spray of the front contact layer SnO2:F thin
film (of low
electrical resistivity and of high optical transmittance) on
substrate.
2)Deposition by CBD of the n-type In2S3:Al or ZnS:In window
layers [2,3].
3)Deposition by CBD of the SnS absorber material [1].
4)Painting of the electrical back contact for I(V)
characteristics (silver dag).
At first glance, this crystallographic investigation can be
resumed to be the best cristallinity of tin sulphide thin film
system with grain size of about using indium doping concentration
equal to 6%.
Photovoltaic technical conference THIN FILMS & ADVANCED
SOLUTIONS 2011
[1] A. Akkari, C. Guasch, N. Kamoun-Turki, Journal of Alloys and
Compounds 490 (2010) 180183. [2] Anis Akkari, Cathy Guasch, Najoua
Kamoun-Turki and Michel Castagne, journal of material science,
uncorrected proof . (May 2011) . [3] T. Ben Nasr, N. Kamoun-Turki,
C. Guasch, Materials Chemistry and Physics 96 (2006) 8489.
