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zinc sulfide composites deposited by spray pyrolysis
Accepted Manuscript Thin film Bismuth (III) sulfide /Zinc sulfide composites deposited by spray pyrolysis H. Benattou, N. Benramdane, M. Berouaken PII: DOI: Reference:
S2211-3797(17)31225-1 https://doi.org/10.1016/j.rinp.2017.09.038 RINP 956
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
2 August 2017 18 September 2017 18 September 2017
Please cite this article as: Benattou, H., Benramdane, N., Berouaken, M., Thin film Bismuth (III) sulfide /Zinc sulfide composites deposited by spray pyrolysis, Results in Physics (2017), doi: https://doi.org/10.1016/j.rinp.2017.09.038
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Thin film Bismuth (III) sulfide /Zinc sulfide composites deposited by spray pyrolysis
1
H. BENATTOU1*, N. BENRAMDANE1, M. BEROUAKEN2 Laboratory of elaboration and characterizations of the materials, Djillali Liabes University of Sidi Bel-Abbes, Algeria. 2 Center for Research in Semiconductor Technology for Energy, Division of Thin Films Surfaces and Interfaces, Algeria.
ABSTRACT (Bi2S3)(x)(ZnS)(1-x) composites in thin films were successfully grown on glass substrates by
the spray pyrolysis technique. The films growth were prepared by the reaction of aqueous solutions of bismuth (III) chloride (BiCl3) and zinc chloride (ZnCl) with Thiourea on substrates heated to a temperature of 280°C. The structural properties have been identified using X-ray diffraction spectra. The deposited films are of polycrystalline natures. The both of the two phases mixed (Bi2S3 and ZnS) were well observed in the X-ray diffraction plots. The optical properties were also studied using transmittance and reflectance measurements in the wavelength range (200nm - 2500nm). Optical gaps were evaluated; we are found that (Bi2S3)(x)(ZnS)(1-x) (x= 0 to 1) composites in thin films are characterized by two optical gaps limited between the gap of Bi2S3 and that of ZnS films in the pure phase. Keywords: Thin films; (Bi2S3)(x)(ZnS)(1-x); Composite materials; Structural and optical properties. 1. INTRODUCTION ZnS and Bi2S3 binary Chalcogenides are two material semiconductors, the first compound forms part of the II-VI family of compounds [1], it has a larger gap of 3.3eV to 3.8eV [2], [3], [4]. This allows it to have direct transitions between the valence band and the conduction band, and also to have radiative transitions. This band gap value may vary, depending on the preparation method and doping rate. ZnS in thin films has the particularity of being relatively transparent. For this purpose, and in addition to their use in various applications, it is often used as window layers in solar cells [5], [6], [7]. Whereas for the second compound Bi2S3 forms part of the V-VI family of semiconductor compounds. In thin films forms, it presents a rather narrow optical gap of 1.6eV to 1.71eV [8][9][10], and this gives it a privileged applications like photo detector and especially as infrared detector.
Despite the solubility difference distinct existing between ZnS (2,93.10-25) and Bi2S3 (1,82.10 99
) [11], we were able successfully synthesized a series of composites in thin films between
these two components by the pyrolysis spray technique. ZnS is transparent in the visible regions and close to the infrared of the solar spectrum. On the other hand, Bi2S3 is absorbent in the visible and UV regions of the solar spectrum [12, 13, 14, 15, and 16]. Bi2S3 is useful for photovoltaic conversions [17, 18]. So there are no approaches in the optical properties nor in the structural properties between these two materials. Although many compounds have been extensively studied using several techniques, studies of the composition between Bi2S3 and ZnS in thin films were not available in literature, so this investigation in this paper is the first. We were look on the possibility of the formation of Bi2S3–ZnS composites using a low cost process such as spray pyrolysis and hence to present the effect of the compositional variation on the films properties. The main aim is to exploit the different properties of Bi2S3 and those of ZnS in thin films to increase the yield in the photovoltaic cells and reduce the cost of manufacture thanks to the reduction of the raw material. 2. EXPERIMENTAL DETAILS 2.1 Preparation of Thin Films 0,1M solution of bismuth chloride (BiCl4) and 0.1M solution of zinc chloride were prepared separately by dissolving them in distilled water. Thiourea (CS(NH2)2), using it source of sulfide ions, was dissolved in distilled water too. For the Bi2S3 solutions, we mixed 0.1M solution of bismuth chloride and those of Thiourea in appropriate volumes to obtain a Bi:S ratio of 2:3. For the two solutions respectively (Bi2S3 and ZnS), a transparent yellow and transparent white solutions were obtained, they were mixed for preparation of the sprayed solution. The HCl droplets were added to dissolve the precipitates in the solution. The final solution was sprayed onto a hot glass substrate at temperature of 280°C. The nozzle to substrate distance was 30cm. The pressure of air was of 7 bars. These deposition conditions were kept constant during the spray process. Before the deposition of the films, the glass substrates were firstly cleaned by acetone acid during 5min at least and washed in the ionized water then in the methanol alcohol and washed in the ionized water still a second liver. 2.2 Characterization techniques X-ray diffraction (XRD) analysis was carried out by a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ = 0.15406 nm) over the 2θ collection range of 5 to 88°. UV–visible– NIR transmittance and reflectance spectra were recorded at normal incidence. The wavelength
range of 200–2500nm operated, was offer by UV-Visible JASCO type V-570 double beam spectrophotometer. 3. RESULTS AND DISCUSSION 3.1 Structural study Our composites obtained were of colors varied between metallic gray as color of Bi2S3 and yellowish white as color of the pure phase of ZnS, they perfectly adhered to the substrate surfaces. X-ray diffraction profiles of (Bi2S3)(x)(ZnS)(1-x) composites with different stœchiometries (x) are shown in Fig.1. The various diffractions peak existing in Fig.(1.a) corresponding to the deposited Bi2S3 films are narrower and well intense indicates the polycrystalline nature of the material with orthorhombic structure, they could be easily indexed to the standard PDF card of orthorhombic Bi2S3 (JCPDS N°: 43-1471). For deposited ZnS film, the XRD pattern of ZnS in Fig. (1.g) was indexed to the standard PDF card of pure center-face cubic phase of ZnS ((JCPDS N°: 77-2100) with a lattice parameter of a = 5.419 Å. As for fig.(1.b,c,d,e and f), the different diffraction peaks DRX of films correspond to the orthorhombic Bi2S3 and the ZnS with centered cubic face structures. No distinct impurity is detected indicating that the (Bi2S3)(x)(ZnS)(1-x) are composed of only two phases: ZnS and Bi2S3. The peak (110) of the orthorhombic phase at 2θ= 11.3° appeared the most intense thanks to the addition of HCl used to dissolve the precipitates in the mixture. Moreover, the absence of any other peaks corresponding to impurities reveals the good quality of asdeposited films. Also we can see the reduction of Bi2S3 peaks in intensity and in number with addition of ZnS in solution. Some peaks of two phases are convoluted and some others are shifted at 2θ angle in the x-axis. Crystallinity of Bi2S3 seems to be affected; the intensity of the peaks is decreased as the fraction of ZnS is increases. The reason can be due to the differences in the thicknesses of the thin films and their densities on the substrates and thus the smallness of the crystallites size. Bi2S3 undergoes a compression and then a bursting into small crystallites by the addition of ZnS. The change in lattice parameters and crystallite size is associated with the incorporation of ZnS. In Table 1, we listed these parameters. The lattice parameters (a, b and c) of Bi2S3 were estimated from the inter-reticular data (d hkl) using the formula giving the relationship between dhkl and these parameters: =
1
ℎ + +
Where h, k and l are the Miller indices of the diffraction planes. However for ZnS, we used the following quadratic relation of center-face cubic system:
= ℎ + +
The crystallite size (G) was estimated from full width half maximum (FWHM) of slow scan of XRD peaks of the two phases: Bi2S3 and the ZnS. We used the following Sherrer formula: =
.
Where k=1 is the shape factor, λ is the wavelength in Å, B is the half width of the peak, and θ is the diffraction angle in radian, λ = X-ray wave length. From table 1, the crystallite size of Bi2S3 weakened with the increase of ZnS contents in (Bi2S3)(x)(ZnS)(1-x) composites, while those of ZnS strengthened as its content was increased. Bi2S3 decomposes into small crystallites; its size is reduced because Bi2S3 has undergone external forces exerted by ZnS which lead to its breakup into small crystallites. Despite the low crystalline of these samples, it should be noted; that XRD models of deposited films reveal the presence of orthorhombic and cubic phases. These observations confirm the formation of composites films and the coexistence of two phases.
(310)
(a)
35
40
2θ (deg)
65
70
75
80
(391)
(901)
60
(721)
55
(810) (612) (811) (362) (313)
50
(222)
45
(312)
(501)
(440)
(420) (041) (430) (421)
(221) (410) 30
(640) (720) (171)
25
(311)
(021)
(101) 20
(161) (611)
15
(060)
10
(520)
5
(200) (120)
(110)
(220)
Intensity (a.u)
(211)
Bi2S3
85
(c)
(Bi2S3) (0.6) (ZnS)(0.4) *: Bi2S3 #: ZnS
*
(Bi2S3) (0.8) (ZnS)(0.2)
*
*
(b)
5
** * # *
*
#
*
#
*#
#
# 5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2 θ (deg)
(e)
(Bi2S3) (0.5) (ZnS)(0.5)
*
(Bi2S3) (0.4) (ZnS)(0.6) *: Bi2S3 #: ZnS
** * *# * **
# #
*
*
#
*
#
#
*
*#
*#
*
Intensity (a.u)
*
*: Bi2S3 #: ZnS
#*
(d)
*
2 θ (deg)
Intensity (a.u)
#
* *
*
**
* * *#
*
*#
#
**
Intensity (a.u)
*
*
Intensity (a.u)
*
*: Bi2S3 #: ZnS
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
5
2 θ (deg)
(Bi2S3) (0.2)(Zns)(0.8)
(g)
(11 1)
*
(f)
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 2 θ (deg)
ZnS
(2 2 0 )
(40 0 )
(3 11 ) (4 2 0)
(2 0 0)
(3 11 ) (2 22 )
70
75
# #
#
*
*
#
** *
**
#*
Intensity (a.u)
*
In ten sity (a .u )
* *
*: Bi2S3 #: ZnS
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
10
15
20
25
30
35
40
45
50
55
60
65
80
85
2θ (deg)
2 θ (deg)
Fig.1. (a) - (g): X-Ray diffraction patterns for different compositions x of (Bi2S3)(x)(ZnS)(1-x) (x= 0 to 1) in thin films.
Table 1: Lattice parameters and crystallite size values of thin films of (Bi2S3)(x)(ZnS)(1-x) composites with (x= 0 to 1) Lattice parameters of
crystallite
Lattice
crystallite
Bi2S3
size
parameters of
size
of Bi2S3
ZnS (a=b=c)
of ZnS
a (Å)
b (Å)
C (Å)
(nm) ZnS
/
(Bi2S3)(0.2)(ZnS)(0.8)
(nm)
/
/
/
5,419
2.0098
11.205
10.836
3.944
20.6
5.39
160.2
(Bi2S3)(0.4)(ZnS)(0.6)
11.204
10.838
3.953
15.14
5.4388
94.06
(Bi2S3)(0.5)(ZnS)(0.5)
11.219
10.916
3.943
15.47
5.420
74,25
(Bi2S3)(0.6)(ZnS)(0.4)
11.234
10.799
3.9426
10.218
5.4201
14.637
(Bi2S3)(0.8)(ZnS)(0.2)
11.261
10.592
3.9598
21.193
5.4201
32.322
Bi2S3
11,0696
11,12
3,968
22.276
/
/
3.2 Optical properties Fig. 2 shows optical transmittance spectrum of (Bi2S3)(x)(ZnS)(1-x) composites (x= 0 to 1), It can be seen that ZnS film in pure phase has relatively high transmittance especially in visible and near infrared light ranges. The transmittance of ZnS film in pure phase reached 80% in visible and near infrared regions, while those of Bi2S3 reduced fewer than 45%. ZnS has a high transparency in visible and the near infrared regions of solar spectrum; this characteristic is a consequence of its wide gap (3.7eV) with a fundamental absorption in ultraviolet range. The color of the films changes with the incorporation of ZnS. This indicates the variation of their average transmittance. It is about 20% for (Bi2S3)(0.8)(ZnS)(0.2) composite to 50% for those of (Bi2S3)(0.2)(ZnS)(0.8) in visible region. For (Bi2S3)(0.5)(ZnS)(0.5) and (Bi2S3)(0.6)(ZnS)(0.4) composites, the difference in their transmittance values is attributed to the difference in their thicknesses and roughness of the two films. The transmittance represents a plate level in infrared region which attains 80%. The transmittance of composites increased when the quantity of ZnS increases in the sprayed solution. They all clearly show a good optical response in visible and the near infrared regions and UV region too from 312 to 2500 nm. The absorption coefficient (α) values were calculated from transmittance and the reflectance values. We used the following formula:
=
1 − 1 log " !
Where d is the film thickness, T and R (in %) are the transmittance and the reflectance respectively. From Fig.2, Bi2S3 orthorhombic show intense absorption coefficient over the visible range, it extend to near infrared region, however for the white ZnS, almost no absorption coefficient has been recorded after photon energy of 3.7eV (a negligible absorption). The absorption coefficient of (Bi2S3)(x)(ZnS)(1-x) composites was higher than that individual ZnS or even individual Bi2S3. It increased with the incorporation of proportion of ZnS in the solution. This indicates that composition of two Bi2S3 and ZnS materials can enhance the absorbance under visible light and especially in UV light, implying that these composites can used in photovoltaic applications.
90 80
Transmittance (%)
70 60 50 40 30 20
Bi2S3(x)ZnS(1-x) x=1 (Bi2S3) x=0.8 x=0.6
10 0
x=0.5 x=0.4 x=0.2 x=0 (ZnS)
-10 0
250
500
750
1000
1250
1500
1750
2000
2250
2500
λ (nm)
Fig.2 UV-Vis-NIR transmittance spectrum of (Bi2S3)(x)(ZnS)(1-x) composites (x= 0 to1)
140 130
Bi2S3(x)ZnS(1-x) x=1 (Bi2S3) x=0.8 x=0.6 x=0.5 x=0.4 x=0.2 x=0 (ZnS)
110 100
4
-1
Absorption coefficient: α x10 (cm )
120
90 80 70 60 50 40 30 20 10 0 0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
Photon energy (eV)
Fig.3 Absorption coefficient spectrum of (Bi2S3)(x)(ZnS)(1-x) composites (x= 0 to1) In Fig.3, we note two quadratic growths. The coefficient value increases around 1.6 eV and 3.7 eV; it shift towards high energies when the concentration of Bi2S3 decreases and that of ZnS increases in the solution. We suggest two electronic transitions inherent in this samples and existence of two optical gaps. The absorption coefficient of ZnS, Bi2S3 in pure phases and of the (Bi2S3)(x)(ZnS)(1-x) composites was plotted versus the photon energy on Fig (4. a-g). To determine the optical gap of our composites, we used the following formula: ℏ$% = &'ℏ$ − () * Where ℏ$ is the photon energy, () is the gap energy, α is the absorption coefficient, and A is a transition probability parameter.
Direct optical gap values were obtained from (αhν)2 curves extrapolated to the energy axis. It was found that electronic transitions are direct allowed transitions. Optical gaps of (Bi2S3)(x)(ZnS)(1-x) composites were limited between the gap of Bi2S3 (1.6ev) and that of ZnS (3.7eV) (see fig.4.a-g). Indeed, we can see a modulation in the optical gap of Bi2S3 phases from 1.6eV to 1.94eV in (Bi2S3)(x)(ZnS)(1-x) composites, caused by the incorporation of ZnS. Also by this result, we show the formation of (Bi2S3)(x)(ZnS)(1-x) composites and the coexistence of the two phases: orthorhombic Bi2S3 and ZnS face-centered cubic.
0,8
1,8
(a)
1,6
(b)
ZnS, Tsub =280°c
1,4 2 -1
(α hν) [x10 . cm eV]
1,0
(α hν )
5
6
2
2
0,6
-1
(x10 cm .eV)
1,2
Bi 2S3 (0.2)ZnS(0.8)
0,6
0,4
2
0,8
0,2
0,4
Eg1 =1.94eV
0,0
0,0 1
2
3
4
5
6
0
hν (eV)
2,0
1
2
2 -1 5
1
2
3
4
hν [eV]
5
Eg1 =1.9eV
0
6
Eg2 =3.64eV
1
2
3
4
5
6
7
hν [eV]
8
(e)
14
(f)
7
Bi2S3 (0.8)ZnS(0.2)
6
12 2
5
-1
(α hν) (x10 cm . eV)
2
Bi2S3 (0.6)ZnS(0.4)
-1
20
10
16
5
30
0
0
2
40
Eg2 =3.67eV
0,0
5
4 3
2
4
2
Eg1 =1.907eV
Eg1 =1.8eV
Eg2 =3.65eV
2
Eg2 =3.69eV
1
0
0 0
1
2
3
4
5
6
7
1
2
3
4
5
6
hν (eV)
hν [eV]
2,0 1,8
(g)
Bi2S3, Tsub =280°c
1,6
2
1,4 1,2
-1
(α hυ ) (10 .cm .eV)
5
1,0 0,8
2
(αhν ) [x10 . cm eV]
7
2
2 -1 6 2
(α hν) [x10 . cm eV]
(αhν ) [x10 . cm eV]
Eg1 =1.99eV
0,2
6
6
Bi2S3 (0.5)ZnS(0.5)
0,4
8
5
(d)
0,6
10
4
50
1,4
0,8
hν [eV]
60
1,6
1,0
3
(c)
Bi2S3 (0.4)ZnS(0.6)
1,8
1,2
Eg2 =3.68eV
Eg= 3.7eV
0,2
0,6 0,4
Eg=1,61eV
0,2 0,0 0,0
0,5
1,0
1,5
2,0
2,5
3,0
hυ (eV)
Fig.4.(a-g) Optical gap of (Bi2S3)(x)(ZnS)(1-x) composites in thin films (x= 0 to 1)
4. CONCLUSION The ZnS, Bi2S3 and (Bi2S3)(x)(ZnS)(1-x) (x= 0 to 1) composites in thin films were grown on glasses substrates by spray pyrolysis method. The samples were characterized by XRD and UV-Vis-NIR spectroscopy. The diffraction spectra reveal the presence of both Bi2S3 orthorhombic and ZnS cubic phases, and so (Bi2S3)(x)(ZnS)(1-x) composites in thin films were established. In optical study, it was found out that the absorption of Bi2S3 is intense over the visible range; it extends to UV range. Also, ZnS film has a high transparency in visible and the near infrared region of solar spectrum. Therefore, the absorption and transmittance of the (Bi2S3)(x)(ZnS)(1-x) composites can enhanced and improved by the mixing of the two materials. We conclude that the (Bi2S3)(x)(ZnS)(1-x) composites have a good visible light response. Ultimately, we summarize that it is useful to synthesis of composites with zinc sulfide and bismuth (III) sulfide in aqueous solution despite the difference in their solubility. 5. ACKNOWLEDGEMENTS I would like to thank Professor GABOUZE NOUREDDINE: the Director of the Center for Research in Semiconductor Technology for Energy (DRTSE). 6. REFERENCES [1] X.D. Gao, X.M. Li, W.D. Yu, ‘Morphology and optical properties of amorphous ZnS films deposited by ultrasonic-assisted successive ionic layer adsorption and reaction method’, Thin Solid Films 468, (2004), 43-47, doi: https://doi.org/10.1016/j.tsf.2004.04.005 [2] F. Gode, C. Gumus, M. Zora, ‘Investigations on the physical properties of the polycrystalline ZnS thin films deposited by the chemical bath deposition method’, Journal of Crystal Growth 299, (2007), 136–141. [3] R. G. Kaufman and P. Dowbor, ‘Mechanism of formation of ohmic contacts to ZnSe, ZnS, and crystals mixed ZnS(x) Se (1-x)’, J. Appl. Phys. 10, (1974), 4487-4490, doi: http://dx.doi.org/10.1063/1.1663075 [4] S. Yamaga, A. Yoshokawa, H. Kasain, ‘Electrical and optical properties of donor doped ZnS films grown by low-pressure MOCVD’, J. Crystal Growth 86, (1998), 252, doi: https://doi.org/10.1016/0022-0248(90)90725-Z
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S2211-3797(17)31225-1 https://doi.org/10.1016/j.rinp.2017.09.038 RINP 956
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
2 August 2017 18 September 2017 18 September 2017
Please cite this article as: Benattou, H., Benramdane, N., Berouaken, M., Thin film Bismuth (III) sulfide /Zinc sulfide composites deposited by spray pyrolysis, Results in Physics (2017), doi: https://doi.org/10.1016/j.rinp.2017.09.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thin film Bismuth (III) sulfide /Zinc sulfide composites deposited by spray pyrolysis
1
H. BENATTOU1*, N. BENRAMDANE1, M. BEROUAKEN2 Laboratory of elaboration and characterizations of the materials, Djillali Liabes University of Sidi Bel-Abbes, Algeria. 2 Center for Research in Semiconductor Technology for Energy, Division of Thin Films Surfaces and Interfaces, Algeria.
ABSTRACT (Bi2S3)(x)(ZnS)(1-x) composites in thin films were successfully grown on glass substrates by
the spray pyrolysis technique. The films growth were prepared by the reaction of aqueous solutions of bismuth (III) chloride (BiCl3) and zinc chloride (ZnCl) with Thiourea on substrates heated to a temperature of 280°C. The structural properties have been identified using X-ray diffraction spectra. The deposited films are of polycrystalline natures. The both of the two phases mixed (Bi2S3 and ZnS) were well observed in the X-ray diffraction plots. The optical properties were also studied using transmittance and reflectance measurements in the wavelength range (200nm - 2500nm). Optical gaps were evaluated; we are found that (Bi2S3)(x)(ZnS)(1-x) (x= 0 to 1) composites in thin films are characterized by two optical gaps limited between the gap of Bi2S3 and that of ZnS films in the pure phase. Keywords: Thin films; (Bi2S3)(x)(ZnS)(1-x); Composite materials; Structural and optical properties. 1. INTRODUCTION ZnS and Bi2S3 binary Chalcogenides are two material semiconductors, the first compound forms part of the II-VI family of compounds [1], it has a larger gap of 3.3eV to 3.8eV [2], [3], [4]. This allows it to have direct transitions between the valence band and the conduction band, and also to have radiative transitions. This band gap value may vary, depending on the preparation method and doping rate. ZnS in thin films has the particularity of being relatively transparent. For this purpose, and in addition to their use in various applications, it is often used as window layers in solar cells [5], [6], [7]. Whereas for the second compound Bi2S3 forms part of the V-VI family of semiconductor compounds. In thin films forms, it presents a rather narrow optical gap of 1.6eV to 1.71eV [8][9][10], and this gives it a privileged applications like photo detector and especially as infrared detector.
Despite the solubility difference distinct existing between ZnS (2,93.10-25) and Bi2S3 (1,82.10 99
) [11], we were able successfully synthesized a series of composites in thin films between
these two components by the pyrolysis spray technique. ZnS is transparent in the visible regions and close to the infrared of the solar spectrum. On the other hand, Bi2S3 is absorbent in the visible and UV regions of the solar spectrum [12, 13, 14, 15, and 16]. Bi2S3 is useful for photovoltaic conversions [17, 18]. So there are no approaches in the optical properties nor in the structural properties between these two materials. Although many compounds have been extensively studied using several techniques, studies of the composition between Bi2S3 and ZnS in thin films were not available in literature, so this investigation in this paper is the first. We were look on the possibility of the formation of Bi2S3–ZnS composites using a low cost process such as spray pyrolysis and hence to present the effect of the compositional variation on the films properties. The main aim is to exploit the different properties of Bi2S3 and those of ZnS in thin films to increase the yield in the photovoltaic cells and reduce the cost of manufacture thanks to the reduction of the raw material. 2. EXPERIMENTAL DETAILS 2.1 Preparation of Thin Films 0,1M solution of bismuth chloride (BiCl4) and 0.1M solution of zinc chloride were prepared separately by dissolving them in distilled water. Thiourea (CS(NH2)2), using it source of sulfide ions, was dissolved in distilled water too. For the Bi2S3 solutions, we mixed 0.1M solution of bismuth chloride and those of Thiourea in appropriate volumes to obtain a Bi:S ratio of 2:3. For the two solutions respectively (Bi2S3 and ZnS), a transparent yellow and transparent white solutions were obtained, they were mixed for preparation of the sprayed solution. The HCl droplets were added to dissolve the precipitates in the solution. The final solution was sprayed onto a hot glass substrate at temperature of 280°C. The nozzle to substrate distance was 30cm. The pressure of air was of 7 bars. These deposition conditions were kept constant during the spray process. Before the deposition of the films, the glass substrates were firstly cleaned by acetone acid during 5min at least and washed in the ionized water then in the methanol alcohol and washed in the ionized water still a second liver. 2.2 Characterization techniques X-ray diffraction (XRD) analysis was carried out by a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ = 0.15406 nm) over the 2θ collection range of 5 to 88°. UV–visible– NIR transmittance and reflectance spectra were recorded at normal incidence. The wavelength
range of 200–2500nm operated, was offer by UV-Visible JASCO type V-570 double beam spectrophotometer. 3. RESULTS AND DISCUSSION 3.1 Structural study Our composites obtained were of colors varied between metallic gray as color of Bi2S3 and yellowish white as color of the pure phase of ZnS, they perfectly adhered to the substrate surfaces. X-ray diffraction profiles of (Bi2S3)(x)(ZnS)(1-x) composites with different stœchiometries (x) are shown in Fig.1. The various diffractions peak existing in Fig.(1.a) corresponding to the deposited Bi2S3 films are narrower and well intense indicates the polycrystalline nature of the material with orthorhombic structure, they could be easily indexed to the standard PDF card of orthorhombic Bi2S3 (JCPDS N°: 43-1471). For deposited ZnS film, the XRD pattern of ZnS in Fig. (1.g) was indexed to the standard PDF card of pure center-face cubic phase of ZnS ((JCPDS N°: 77-2100) with a lattice parameter of a = 5.419 Å. As for fig.(1.b,c,d,e and f), the different diffraction peaks DRX of films correspond to the orthorhombic Bi2S3 and the ZnS with centered cubic face structures. No distinct impurity is detected indicating that the (Bi2S3)(x)(ZnS)(1-x) are composed of only two phases: ZnS and Bi2S3. The peak (110) of the orthorhombic phase at 2θ= 11.3° appeared the most intense thanks to the addition of HCl used to dissolve the precipitates in the mixture. Moreover, the absence of any other peaks corresponding to impurities reveals the good quality of asdeposited films. Also we can see the reduction of Bi2S3 peaks in intensity and in number with addition of ZnS in solution. Some peaks of two phases are convoluted and some others are shifted at 2θ angle in the x-axis. Crystallinity of Bi2S3 seems to be affected; the intensity of the peaks is decreased as the fraction of ZnS is increases. The reason can be due to the differences in the thicknesses of the thin films and their densities on the substrates and thus the smallness of the crystallites size. Bi2S3 undergoes a compression and then a bursting into small crystallites by the addition of ZnS. The change in lattice parameters and crystallite size is associated with the incorporation of ZnS. In Table 1, we listed these parameters. The lattice parameters (a, b and c) of Bi2S3 were estimated from the inter-reticular data (d hkl) using the formula giving the relationship between dhkl and these parameters: =
1
ℎ + +
Where h, k and l are the Miller indices of the diffraction planes. However for ZnS, we used the following quadratic relation of center-face cubic system:
= ℎ + +
The crystallite size (G) was estimated from full width half maximum (FWHM) of slow scan of XRD peaks of the two phases: Bi2S3 and the ZnS. We used the following Sherrer formula: =
.
Where k=1 is the shape factor, λ is the wavelength in Å, B is the half width of the peak, and θ is the diffraction angle in radian, λ = X-ray wave length. From table 1, the crystallite size of Bi2S3 weakened with the increase of ZnS contents in (Bi2S3)(x)(ZnS)(1-x) composites, while those of ZnS strengthened as its content was increased. Bi2S3 decomposes into small crystallites; its size is reduced because Bi2S3 has undergone external forces exerted by ZnS which lead to its breakup into small crystallites. Despite the low crystalline of these samples, it should be noted; that XRD models of deposited films reveal the presence of orthorhombic and cubic phases. These observations confirm the formation of composites films and the coexistence of two phases.
(310)
(a)
35
40
2θ (deg)
65
70
75
80
(391)
(901)
60
(721)
55
(810) (612) (811) (362) (313)
50
(222)
45
(312)
(501)
(440)
(420) (041) (430) (421)
(221) (410) 30
(640) (720) (171)
25
(311)
(021)
(101) 20
(161) (611)
15
(060)
10
(520)
5
(200) (120)
(110)
(220)
Intensity (a.u)
(211)
Bi2S3
85
(c)
(Bi2S3) (0.6) (ZnS)(0.4) *: Bi2S3 #: ZnS
*
(Bi2S3) (0.8) (ZnS)(0.2)
*
*
(b)
5
** * # *
*
#
*
#
*#
#
# 5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
2 θ (deg)
(e)
(Bi2S3) (0.5) (ZnS)(0.5)
*
(Bi2S3) (0.4) (ZnS)(0.6) *: Bi2S3 #: ZnS
** * *# * **
# #
*
*
#
*
#
#
*
*#
*#
*
Intensity (a.u)
*
*: Bi2S3 #: ZnS
#*
(d)
*
2 θ (deg)
Intensity (a.u)
#
* *
*
**
* * *#
*
*#
#
**
Intensity (a.u)
*
*
Intensity (a.u)
*
*: Bi2S3 #: ZnS
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
5
2 θ (deg)
(Bi2S3) (0.2)(Zns)(0.8)
(g)
(11 1)
*
(f)
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 2 θ (deg)
ZnS
(2 2 0 )
(40 0 )
(3 11 ) (4 2 0)
(2 0 0)
(3 11 ) (2 22 )
70
75
# #
#
*
*
#
** *
**
#*
Intensity (a.u)
*
In ten sity (a .u )
* *
*: Bi2S3 #: ZnS
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
10
15
20
25
30
35
40
45
50
55
60
65
80
85
2θ (deg)
2 θ (deg)
Fig.1. (a) - (g): X-Ray diffraction patterns for different compositions x of (Bi2S3)(x)(ZnS)(1-x) (x= 0 to 1) in thin films.
Table 1: Lattice parameters and crystallite size values of thin films of (Bi2S3)(x)(ZnS)(1-x) composites with (x= 0 to 1) Lattice parameters of
crystallite
Lattice
crystallite
Bi2S3
size
parameters of
size
of Bi2S3
ZnS (a=b=c)
of ZnS
a (Å)
b (Å)
C (Å)
(nm) ZnS
/
(Bi2S3)(0.2)(ZnS)(0.8)
(nm)
/
/
/
5,419
2.0098
11.205
10.836
3.944
20.6
5.39
160.2
(Bi2S3)(0.4)(ZnS)(0.6)
11.204
10.838
3.953
15.14
5.4388
94.06
(Bi2S3)(0.5)(ZnS)(0.5)
11.219
10.916
3.943
15.47
5.420
74,25
(Bi2S3)(0.6)(ZnS)(0.4)
11.234
10.799
3.9426
10.218
5.4201
14.637
(Bi2S3)(0.8)(ZnS)(0.2)
11.261
10.592
3.9598
21.193
5.4201
32.322
Bi2S3
11,0696
11,12
3,968
22.276
/
/
3.2 Optical properties Fig. 2 shows optical transmittance spectrum of (Bi2S3)(x)(ZnS)(1-x) composites (x= 0 to 1), It can be seen that ZnS film in pure phase has relatively high transmittance especially in visible and near infrared light ranges. The transmittance of ZnS film in pure phase reached 80% in visible and near infrared regions, while those of Bi2S3 reduced fewer than 45%. ZnS has a high transparency in visible and the near infrared regions of solar spectrum; this characteristic is a consequence of its wide gap (3.7eV) with a fundamental absorption in ultraviolet range. The color of the films changes with the incorporation of ZnS. This indicates the variation of their average transmittance. It is about 20% for (Bi2S3)(0.8)(ZnS)(0.2) composite to 50% for those of (Bi2S3)(0.2)(ZnS)(0.8) in visible region. For (Bi2S3)(0.5)(ZnS)(0.5) and (Bi2S3)(0.6)(ZnS)(0.4) composites, the difference in their transmittance values is attributed to the difference in their thicknesses and roughness of the two films. The transmittance represents a plate level in infrared region which attains 80%. The transmittance of composites increased when the quantity of ZnS increases in the sprayed solution. They all clearly show a good optical response in visible and the near infrared regions and UV region too from 312 to 2500 nm. The absorption coefficient (α) values were calculated from transmittance and the reflectance values. We used the following formula:
=
1 − 1 log " !
Where d is the film thickness, T and R (in %) are the transmittance and the reflectance respectively. From Fig.2, Bi2S3 orthorhombic show intense absorption coefficient over the visible range, it extend to near infrared region, however for the white ZnS, almost no absorption coefficient has been recorded after photon energy of 3.7eV (a negligible absorption). The absorption coefficient of (Bi2S3)(x)(ZnS)(1-x) composites was higher than that individual ZnS or even individual Bi2S3. It increased with the incorporation of proportion of ZnS in the solution. This indicates that composition of two Bi2S3 and ZnS materials can enhance the absorbance under visible light and especially in UV light, implying that these composites can used in photovoltaic applications.
90 80
Transmittance (%)
70 60 50 40 30 20
Bi2S3(x)ZnS(1-x) x=1 (Bi2S3) x=0.8 x=0.6
10 0
x=0.5 x=0.4 x=0.2 x=0 (ZnS)
-10 0
250
500
750
1000
1250
1500
1750
2000
2250
2500
λ (nm)
Fig.2 UV-Vis-NIR transmittance spectrum of (Bi2S3)(x)(ZnS)(1-x) composites (x= 0 to1)
140 130
Bi2S3(x)ZnS(1-x) x=1 (Bi2S3) x=0.8 x=0.6 x=0.5 x=0.4 x=0.2 x=0 (ZnS)
110 100
4
-1
Absorption coefficient: α x10 (cm )
120
90 80 70 60 50 40 30 20 10 0 0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
Photon energy (eV)
Fig.3 Absorption coefficient spectrum of (Bi2S3)(x)(ZnS)(1-x) composites (x= 0 to1) In Fig.3, we note two quadratic growths. The coefficient value increases around 1.6 eV and 3.7 eV; it shift towards high energies when the concentration of Bi2S3 decreases and that of ZnS increases in the solution. We suggest two electronic transitions inherent in this samples and existence of two optical gaps. The absorption coefficient of ZnS, Bi2S3 in pure phases and of the (Bi2S3)(x)(ZnS)(1-x) composites was plotted versus the photon energy on Fig (4. a-g). To determine the optical gap of our composites, we used the following formula: ℏ$% = &'ℏ$ − () * Where ℏ$ is the photon energy, () is the gap energy, α is the absorption coefficient, and A is a transition probability parameter.
Direct optical gap values were obtained from (αhν)2 curves extrapolated to the energy axis. It was found that electronic transitions are direct allowed transitions. Optical gaps of (Bi2S3)(x)(ZnS)(1-x) composites were limited between the gap of Bi2S3 (1.6ev) and that of ZnS (3.7eV) (see fig.4.a-g). Indeed, we can see a modulation in the optical gap of Bi2S3 phases from 1.6eV to 1.94eV in (Bi2S3)(x)(ZnS)(1-x) composites, caused by the incorporation of ZnS. Also by this result, we show the formation of (Bi2S3)(x)(ZnS)(1-x) composites and the coexistence of the two phases: orthorhombic Bi2S3 and ZnS face-centered cubic.
0,8
1,8
(a)
1,6
(b)
ZnS, Tsub =280°c
1,4 2 -1
(α hν) [x10 . cm eV]
1,0
(α hν )
5
6
2
2
0,6
-1
(x10 cm .eV)
1,2
Bi 2S3 (0.2)ZnS(0.8)
0,6
0,4
2
0,8
0,2
0,4
Eg1 =1.94eV
0,0
0,0 1
2
3
4
5
6
0
hν (eV)
2,0
1
2
2 -1 5
1
2
3
4
hν [eV]
5
Eg1 =1.9eV
0
6
Eg2 =3.64eV
1
2
3
4
5
6
7
hν [eV]
8
(e)
14
(f)
7
Bi2S3 (0.8)ZnS(0.2)
6
12 2
5
-1
(α hν) (x10 cm . eV)
2
Bi2S3 (0.6)ZnS(0.4)
-1
20
10
16
5
30
0
0
2
40
Eg2 =3.67eV
0,0
5
4 3
2
4
2
Eg1 =1.907eV
Eg1 =1.8eV
Eg2 =3.65eV
2
Eg2 =3.69eV
1
0
0 0
1
2
3
4
5
6
7
1
2
3
4
5
6
hν (eV)
hν [eV]
2,0 1,8
(g)
Bi2S3, Tsub =280°c
1,6
2
1,4 1,2
-1
(α hυ ) (10 .cm .eV)
5
1,0 0,8
2
(αhν ) [x10 . cm eV]
7
2
2 -1 6 2
(α hν) [x10 . cm eV]
(αhν ) [x10 . cm eV]
Eg1 =1.99eV
0,2
6
6
Bi2S3 (0.5)ZnS(0.5)
0,4
8
5
(d)
0,6
10
4
50
1,4
0,8
hν [eV]
60
1,6
1,0
3
(c)
Bi2S3 (0.4)ZnS(0.6)
1,8
1,2
Eg2 =3.68eV
Eg= 3.7eV
0,2
0,6 0,4
Eg=1,61eV
0,2 0,0 0,0
0,5
1,0
1,5
2,0
2,5
3,0
hυ (eV)
Fig.4.(a-g) Optical gap of (Bi2S3)(x)(ZnS)(1-x) composites in thin films (x= 0 to 1)
4. CONCLUSION The ZnS, Bi2S3 and (Bi2S3)(x)(ZnS)(1-x) (x= 0 to 1) composites in thin films were grown on glasses substrates by spray pyrolysis method. The samples were characterized by XRD and UV-Vis-NIR spectroscopy. The diffraction spectra reveal the presence of both Bi2S3 orthorhombic and ZnS cubic phases, and so (Bi2S3)(x)(ZnS)(1-x) composites in thin films were established. In optical study, it was found out that the absorption of Bi2S3 is intense over the visible range; it extends to UV range. Also, ZnS film has a high transparency in visible and the near infrared region of solar spectrum. Therefore, the absorption and transmittance of the (Bi2S3)(x)(ZnS)(1-x) composites can enhanced and improved by the mixing of the two materials. We conclude that the (Bi2S3)(x)(ZnS)(1-x) composites have a good visible light response. Ultimately, we summarize that it is useful to synthesis of composites with zinc sulfide and bismuth (III) sulfide in aqueous solution despite the difference in their solubility. 5. ACKNOWLEDGEMENTS I would like to thank Professor GABOUZE NOUREDDINE: the Director of the Center for Research in Semiconductor Technology for Energy (DRTSE). 6. REFERENCES [1] X.D. Gao, X.M. Li, W.D. Yu, ‘Morphology and optical properties of amorphous ZnS films deposited by ultrasonic-assisted successive ionic layer adsorption and reaction method’, Thin Solid Films 468, (2004), 43-47, doi: https://doi.org/10.1016/j.tsf.2004.04.005 [2] F. Gode, C. Gumus, M. Zora, ‘Investigations on the physical properties of the polycrystalline ZnS thin films deposited by the chemical bath deposition method’, Journal of Crystal Growth 299, (2007), 136–141. [3] R. G. Kaufman and P. Dowbor, ‘Mechanism of formation of ohmic contacts to ZnSe, ZnS, and crystals mixed ZnS(x) Se (1-x)’, J. Appl. Phys. 10, (1974), 4487-4490, doi: http://dx.doi.org/10.1063/1.1663075 [4] S. Yamaga, A. Yoshokawa, H. Kasain, ‘Electrical and optical properties of donor doped ZnS films grown by low-pressure MOCVD’, J. Crystal Growth 86, (1998), 252, doi: https://doi.org/10.1016/0022-0248(90)90725-Z
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