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SnS trilayer films for solar cell application
Journal Pre-proof Influence of Ag thickness on the structural, optical, and electrical properties of the SnS/Ag/SnS trilayer films for solar cell application Vinaya Kumar Arepalli, Tien Dai Nguyen, Jeha Kim PII:
S1567-1739(20)30002-X
DOI:
https://doi.org/10.1016/j.cap.2020.01.002
Reference:
CAP 5128
To appear in:
Current Applied Physics
Received Date: 12 September 2019 Revised Date:
13 November 2019
Accepted Date: 2 January 2020
Please cite this article as: V.K. Arepalli, T.D. Nguyen, J. Kim, Influence of Ag thickness on the structural, optical, and electrical properties of the SnS/Ag/SnS trilayer films for solar cell application, Current Applied Physics (2020), doi: https://doi.org/10.1016/j.cap.2020.01.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V. on behalf of Korean Physical Society.
1
Influence of Ag Thickness on the Structural, Optical, and Electrical
2
Properties of the SnS/Ag/SnS Trilayer Films for Solar Cell Application
3
Vinaya Kumar Arepalli1, Tien Dai Nguyen2, and Jeha Kim1*
4
1
Department of Energy Convergence Engineering, Cheongju University, Cheongju, Korea.
5
2
Institute of Theoretical and Applied Research, Duy Tan University, Hanoi 100000, Vietnam.
6 7 8 9 10 11 12 13
* corresponding author ([email protected]) Keywords: RF-sputtering; SnS/Ag/SnS trilayer films; Ag interlayer; sandwich structure
Abstract: We fabricated the SnS/Ag/SnS (SAS) trilayer thin films by a sputtering method at 200 ℃.
14
The structural, optical, and electrical properties of the films were studied by varying the Ag
15
interlayer thickness from 9 to 27 nm. The EDS analysis revealed that all SAS trilayer films
16
showed an increase in the atomic percentage of Ag from 1.87 to 6.18. The X-ray diffraction
17
studies confirmed that SAS films with Ag-18 nm thickness showed a preferred (111) peak of the
18
SnS with improved crystallinity. The optical absorption coefficient of the SAS films increased by
19
a factor of 18 when compared to the SnS films without Ag. Also, the optical band gap decreased
20
from 1.53 to 1.28 eV with Ag thickness. All SAS films exhibited the p-type conductivity with
21
increased hole- concentration from 1.94 ×
22
1.31 to 81.6 cm2.V-1s-1.
to 4.15 ×
cm-3 and also the mobility from
23 24
1. Introduction
25
Tin monosulphide (SnS) is IV–VI group semiconductor compound and has attracted
26
more attention recently due to its potential application as a p-type absorber material in the
27
heterojunction solar cells [1]. It has a direct optical energy band gap (1.2–1.4 eV) with the high 1
28
absorption coefficient (>104 cm-1) [2]. The constituent elements of the SnS are earth-abundant,
29
non-toxic, and inexpensive [2]. According to Alber’s group investigations, the carrier
30
concentration of the SnS was nearly 1018 cm-3 [3]. Moreover, the theoretical energy conversion
31
efficiency of the SnS is >25% [4]. The highest efficiency of the SnS based solar cell is 4.36%
32
with the hole density of 1015 cm-3 and the resistivity of 280 Ω.cm which are far from the
33
theoretical limit [3, 5]. Moreover, the SnS suffers from the loss of light absorption at near
34
bandgap region due to its indirect bandgap [6]. The important two typical issues to improve the
35
SnS thin film based solar cell performance are the higher light absorption in the active SnS layer
36
and the lower electrical resistivity. In general, the optical absorption increases with the SnS film
37
thickness. But the thickness of the SnS is limited below its carrier diffusion length (<900 nm)
38
due to the charge carrier recombination at a higher thickness (>900 nm) [7]. Moreover, there is a
39
need to reduce the film thickness (<1 µm) of any solar absorber to use at a large scale and more
40
economical way. Despite the optical properties, the electrical properties such as the mobility,
41
carrier concentration and electrical resistivities are also necessary to get improved. The lower
42
electrical resistivity (5 Ω.cm) [8] with the high carrier concentration (>1018) can be expected to
43
make an efficient solar absorber material. To meet these requirements, we adopted the
44
fundamental idea of a metal-semiconductor system in which the metal nanoparticles embedded
45
in a semiconductor. When the sun light is striking on a metal-semiconductor interface, the metal
46
nanoparticles exhibit a collective oscillation of electrons known as the localized surface plasmon
47
resonance (LSPR) through the excitation of the conduction electrons at the interface between the
48
metal and the semiconductor [9-11]. Thus, the light can be trapped into a thin semiconductor
49
layer, thereby the absorption is increased.
2
50
Noble metal nanoparticles such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al)
51
embedded in a semiconductor or dielectric material show the enhanced optical properties [12].
52
Thus, they gain much attention in the applications of plasmonic solar cells [13], surface plasmon
53
enhanced sensors [14], and photonic devices [15]. Among those, silver (Ag) is a potential metal
54
that can be used to improve the opto-electronic properties in the active solar absorber due to its
55
lowest optical absorption coefficient in the visible region and its superior conductivity when
56
compared with other metals [16]. The amount of light either absorbed or scattered depends on
57
the size and shape of the Ag nanoparticles. As per the previous literature reports, Ag can exhibit
58
plasmonic effect with different shapes such as cylindrical, hemispherical, spherical, cubic,
59
triangular nanoprism [6, 29]. The Ag nanoparticles with a diameter between 100 – 150 nm in a
60
hemispherical shape are suited well for the light trapping [17]. Till date, we have not seen the
61
thickness effect of the Ag interlayer on the structural, optical, and the electrical properties of the
62
SnS thin films. Thus, we prepared a trilayer films with a sandwich structure of Ag interlayer
63
between the top and bottom SnS layers by using a sputtering method. The trilayer film structure
64
was denoted as SnS/Ag/SnS (SAS). The structural, optical, and electrical properties of the as-
65
grown SAS films were investigated.
66
2. Experimental section
67
The SnS/Ag/SnS (SAS) trilayer films were deposited on both the soda lime glass (SLG)
68
and Mo/SLG substrates (25 × 25 mm2) by sputtering method. In this, SnS (purity; 99.99%) and
69
Ag (purity; 99.999%) targets were used. Prior to deposition, the substrates were ultrasonically
70
cleaned with acetone, methanol, and deionized water for 30 min and dried with N2 gas. Then the
71
substrates were placed inside the sputter chamber. The substrate temperature was maintained at
72
200 ℃. The distances between the substrate to both the SnS and Ag targets were 65 and 75 mm,
3
73
respectively. By using a turbomolecular pump (TMP), the base pressure of the sputter chamber
74
around 6.0 ×
75
with 30 sccm flow rate through a mass flow controller. Each top and bottom SnS layers were
76
deposited at 200 ℃ by using 100 W radio frequency (RF) power at 30 mTorr working pressure
77
for 5 min. Whereas, the center Ag layer was deposited at 200 ℃ by using 100 W direct current
78
(DC) power at 8 mTorr working pressure for various deposition times from 20 s to 60 s. The
79
above experimental details are shown in Table 2.
Torr was achieved. The argon (purity; 99.999%) was used as a sputter gas
80
The surface and cross-sectional morphologies were characterized by using scanning
81
electron microscopy (JEOL, JSM-7610F). The elemental composition of the SAS trilayer films
82
was measured using an energy dispersive spectrometer. The X-ray diffraction (SmartLab, Rigaku)
83
was used to study the structural properties with a CuKα radiation of λ = 1.54 Å. The elemental
84
concentration depth profiles were measured by a secondary ion mass spectroscopy (CAMECA,
85
IMS 6F) with O2 primary ion at 1 keV DUO (+3 kV) / Secondary (+2 kV) without oxygen
86
flooding/Cs. The optical analysis was performed by using UV-Vis-NIR spectrophotometry
87
(Perkin Elmer, Lambda 1050) in the wavelength range of 300 – 1400 nm. The structural defects
88
were characterized by using micro-Raman spectroscopy (Uninanotech, UR1207J) with a diode-
89
pumped solid-state (DPSS) laser at 532 nm excitation wavelength. The electrical properties such
90
as carrier concentration, mobility, and resistivity of the SnSAg/SnS trilayer films were evaluated
91
from the Hall measurement system (ECOPIA, HMS-3000) using Van-der Pauw method.
92 93
3. Results and Discussion
94
Based on the growth rate of Ag (0.45 nm/s), the SnS/Ag/SnS (SAS) trilayer films were
95
prpepared by varying the Ag interlayer thickness from 9 – 27 nm while maintaining the SnS film
4
96
thickness of 300 nm constant on both sides. Also, these films were compared with SnS film (600
97
nm) without Ag.
98
3.1. Microstructure
99
Fig. 1 shows the surface FESEM image of a bilayer films that consist of the Ag film on
100
top of the SnS (Ag/SnS) with various Ag layer thicknesses. Also, the morphology of the SnS
101
film without Ag is compared with the Ag layer on top of the SnS. As shown in Fig. 1a the
102
surface feature of the SnS film without Ag seems to have a densely packed plate-like grain
103
structure. The grain size is around 100 – 200 nm with slightly roughed surface. Whereas, the
104
morphology of the Ag layer on the SnS film is critically dependent on its growth nature at
105
particular temperature (200 ℃). To observe the growth behavior based on the growth rate (0.45
106
nm/s), the Ag was deposited on top of the SnS film at various deposition times: 20, 40, and 60 s.
107
The corresponding thicknesses of the Ag layer are 9, 18, and 27 nm, respectively. It is clearly
108
shown that an increase of Ag layer thickness results in transformation of its shape from an island
109
to a continuous film. The morphology of the Ag at 9 nm thickness (Fig. 1b), the nucleation and
110
the growth have been proceeded already. Thus, the Ag islands are observed on the surface of the
111
SnS as shown in Fig. 1b. Moreover, the average size of the Ag island is ≤100 nm and are
112
disconnected to each other (arrow marks). After 40 s, the thickness of the Ag becomes 18 nm
113
and the islands are randomly connected to each other by a coalescence phenomenon. As a result,
114
an incomplete surface coverage is observed (Fig. 1c) with an ellipsoid shape of slightly larger Ag
115
particles (≥100 nm). Finally, as shown in Fig. 1d, a fully covered continuous Ag film is
116
observed due to the agglomeration of the particles for 60 s growth time with 27 nm thickness.
117
Fig. 1e shows the cross-sectional image of SAS trilayer film on Mo-coated SLG in which two
118
SnS layers seperated by a small interfacial line that could be the Ag layer.
5
119
The elemental composition of the SAS trilayer films with varying Ag thickness was
120
analyzed by the EDS measurement. The atomic percentage of Sn, S, and Ag are listed in Table 2.
121
It is observed that, with increasing Ag thickness, the atomic percentage of Ag increases from
122
1.87 to 6.18. Also, the [S/Sn] ratio of the SAS films slightly increases with Ag thickness. It
123
indicates the presence of Ag layer affects slightly on the composition of the SnS. Moreover, the
124
SAS trilayer film at 18 nm of Ag shows the same [S/Sn] ratio as the SnS film (1.08) without Ag.
125
3.2. Structural phase identification
126
The structural phase of the SAS trilayer films was investigated by X-ray diffraction
127
pattern shown in Fig. 2a. The observed peaks at 2θ of 22.1°, 30.6°, 31.6°, 39.1°, 45.4°, and 66.0°
128
are assigned to (110), (120), (101), (111), (131), (002), and (251), respectively. These are well
129
matched with the standard JCPDS #39-0354 of the orthorhombic crystal phase of the SnS. All
130
samples show the polycrystalline nature with a preferred orientation along the (111) plane.
131
Importantly, from Fig. 2b, the SAS trilayer films grown at Ag of 22.5 nm and 27 nm the peaks at
132
38.1° and 45.3° are well matched with the (111) and (200) planes of the cubic Ag and are
133
consistent with the standard JCPDS #01-1164. It is clearly shown that the (111) peak intensity of
134
the SnS increases with an increase of Ag layer thickness from 0 nm to 18 nm and followed by a
135
dcrease of Ag thickness to 27 nm. It indicates an improvement in the crystallinity of the SnS
136
films due to the formation of new nucleation centers of the SnS on the Ag interlayer [18]. The
137
crystallite size that corresponds to the (111) peak of the SAS trilayer films can be evaluated from
138
Scherrer relation as follows [19] ,
139 140 141
D=
.
----------------(1)
where β is the full width at the half maxima (FWHM) of the SnS and Ag preferred (111) peaks, is the X-ray wavelength (1.54 Å) and
is Bragg’s angle. The evaluated crystallite sizes of the 6
142
both SnS and Ag are shown in Table 2. The crystallite size increases from 34.5 nm to 43.1 nm
143
for the SAS trilayer films with an increase of Ag thickness from 0 to 18 nm and followed by
144
gradually decreases to 37.1 nm at the Ag thickness of 27 nm. Other impurity phases such as SnS2
145
and Sn2S3 are not detected in the films. Based on the XRD results, the SAS trilayer structure with
146
18 nm-thick Ag interlayer exhibited the improved crystallinity than the other structures.
147
The micro-Raman analysis was used to know the secondary phases in the SAS trilayer
148
films. Fig. 3 shows the micro-Raman spectra of the SAS trilayer films. Usually, the
149
orthorhombic SnS shows 12 Raman active modes of 4Ag, 2B1g, 4B2g, 2B3g [27]. All SAS trilayer
150
films exhibit the major Raman intense peaks at 92 and 218.9 cm−1 which are assigned to Ag
151
Raman mode of the SnS [20]. Also, the minor peaks appeared at 139 cm–1 belong to the B2u
152
mode of the SnS [21]. Besides, the peak intensities are amplified with an increase of Ag
153
thickness from 9 nm to 18 nm due to the plasmon-induced electric field enhancement of Ag
154
interlayer film [22]. However, secondary impurity phases such as SnS2, Sn2S3, and Ag8SnS6
155
were not detected in the SAS films.
156
Fig. 4 shows SIMS elemental depth distribution profiles of Sn, S, Ag, and Mo in the
157
SAS trilayer film at 9 nm thick Ag. The SIMS depth profile shows a well-defined top and bottom
158
SnS layers. The green area in the elemental profiles indicates the Ag interlayer region between
159
the top and SnS layers without interfacial reactions. The constant atomic concentrations of Sn
160
and S atoms in the top and the bottom SnS films clearly denote the identical composition and
161
thickness of the top and bottom SnS layers. In addition, there is no evidence of either the Ag
162
diffusion into top and bottom SnS layers or the tin or sulfur diffusion into the Ag layer at 200 ℃.
163
3.3. Optical Properties
7
164
Fig. 5a represents the optical transmittance spectra of the SAS trilayer films deposited by
165
varying the Ag film thickness. In the transmittance spectra, all films show an apparent shift of
166
fundamental absorption edge in the wavelength range of 800 – 900 nm indicating the more
167
absorption of photons in the near IR region. When compared to the SnS film without Ag, the
168
SAS trilayer film deposited at the Ag thickness of 9 nm shows the higher reduction in the
169
transmittance. However, as the Ag thickness increases from 9 nm to 27 nm, the transmittance of
170
the SAS trilayer films futher decreases gradually. This can be attributed to the light scattering
171
effect of Ag film due to the increased surface roughness of Ag interlayer in the sandwich
172
structure [23]. Fig. 5b denotes the optical refelctance spectra of the SAS trilayer films without
173
and with Ag interlayer. The SnS film without Ag (Ag-0 nm) shows the higher reflectance due to
174
more surface smoothness compared to the SAS trilayer films with Ag thicknesses from 9 to 27
175
nm. Also, there is no proper fall of the fundamental absorption edge within the wavelength range
176
of 800 – 1300 nm for the SnS films in the reflection spectra. It can be because of the presence of
177
sputter induced ionic defects. While, as inserting the Ag layer between two SnS layers reduces
178
the optical reflection which is associated with the scattering effect of the Ag particles. Moreover,
179
with increasing Ag thickness from 9 to 27 nm, the reflectance gradually decreases, while, the
180
fundamental absorption edge shifts towards the longer wavelength side due to the transformation
181
of the shape and the increase of size of the Ag particles (Fig. 1). The larger Ag particles show the
182
higher scattering effect that increases the optical path of incident light which is beneficial for the
183
solar absorber.
184 185 186
Fig. 5c shows the optical absorption coefficient (α) of the SAS trilayer films evaluated from the above transmittance and reflectance spectra by using the following relation [24].
α = - ln (
)
8
--------------(2)
= thickness, R= reflectance and ! = transmittance of the trilayer film. As shown in Fig.
187
Where
188
5c the absorption coefficient is enhanced by a factor of 18 in the SAS trilayer films (3.24 × 10
189
cm-1) from that of 0.18 × 10 for the SnS films without Ag interlayer (Ag-0 nm). An abrupt
190
increase of absorption coefficient with the thickness of Ag from 0 nm (α = 0.18 × 10 cm-1) to
191
Ag of 9 nm (α = 1.94 × 10 cm-1) is due to the light scattering effect by the Ag islands.
192
However, the absorption coefficient increases slightly on further increase of the Ag thickness
193
from 9 to 27 nm. This can be understood by the change of Ag particle size and shape due to the
194
agglomeration of Ag islands. As a result, the surface of the bottom SnS layer is gradually
195
covered by Ag interlayer which shows a reduction in the light scattering effect of the Ag
196
particles.
197 198
The direct optical band gap of the SAS trilayer films can be estimated from the following relation [24] αhν = A (hν-Eg) k
199
-------------(3)
200
Where Eg is the optical band gap, k = ½, hν is the incident photon, and A is constant of the direct
201
allowed transitions. According to Tauc plot, as shown in Fig. 6, an intercept of tangent has
202
drawn towards X-axis in the (αhν)2 vs. hν spectra gives the optical energy band gap (Eg) of the
203
SAS trilayer films. The observed band gaps of 1.53, 1.36, 1.32, and 1.28 eV are assigned to the
204
SAS trilayer films with Ag-0 nm, Ag-9 nm, Ag-18 nm, and Ag-27 nm thicknesses, respectively.
205
These band gaps are consistant with the previously reported direct optical energy band gaps [25,
206
26]. The decrease in the band gap is attributed to the increased crystallite size of the SnS (Table
207
2) and the density of the Ag particles and their shapes in the sandwich structure (Fig. 1).
208
3.3. Electrical properties
9
209
The electrical properties such as resistivity, mobility, and hole concentration of the SAS trilayer
210
films are strongly affected by the thickness of the Ag in the sandwich structure. The change in
211
the electrical properties of the SAS trilyaer films was determined by using the Hall effect
212
measurement as a function of increasing Ag interlayer thickness and are shown in Fig. 7. As Ag
213
thickness increases, all the SAS films exhibit a p-type conductivity with an increase of hole-
214
carrier concentration (Fig. 7a) from 1.94 × 10
215
mobility (Fig. 7a) increases from 1.31 to 81.6 cm2.V-1s-1. This is attributed to the transformation
216
of Ag grain shape from island to a continuous film by the aggregation of particles with increase
217
Ag thickness and also the increase of inplane Ag grain size (Fig. 1). The hole-carriers travel
218
intitally through randomily distributed Ag islands (Ag-9 nm) which are not connected together.
219
The hole-carriers can travel much easier as Ag thickness increases to 18 and 22.5 nm. Because,
220
some of the Ag particles are connected together due to the agglomeration of these densily
221
distributed Ag islands. However, the hole-carrier mobility and concentration show a better
222
improvement at 27 nm thickess of Ag due to the formation of a continuous Ag film.
to 4.15 × 10
$
cm-3. Similarly, the hole-
223
As shown in Fig. 7b the electrical resistivity drastically decreases from 6060 to 0.018
224
Ω.cm with Ag thickness. Moreover, the resistivities of the SAS trilayer films are very high when
225
compared to the resistivity of the pure Ag metal on SLG as increasing Ag thickness. The only
226
SnS (Ag-0 nm) film shows the high electrical resistivity of 6060 Ω.cm is due to the surface
227
defects, poor crystallinity, and the higher surface roughness which can strongly reflect the
228
electrical properties. In the case of the SAS trilayer film with the Ag thicknesses of 9 and 13.5
229
nm, a fairly high resistivities 4460 and 2080 Ω.cm, respectively are observed, due to the
230
disconnection of the Ag islands (Fig. 1). However, it is noteworthy that the insertion of an 18-
231
nm-thick Ag layer between the SnS layers reduces the resistivity to 18.3 Ω.cm which is suitable
10
232
to act as an active SnS solar absorber [8]. The reason could be the improved crystallinity [28] of
233
the SAS trilayer films (Fig. 2) due to the increased crystallite size (Table 2). On further increase
234
of Ag thickness to 22.5 and 27 nm, all of the SAS trilayer films show lower resistivities of 0.075
235
and 0.018 Ω.cm which are close to the resistivity of a 4.5 nm thick Ag metal on the SLG (Fig.
236
7b). This is due to the complete surface coverage of the Ag film on top the SnS and also, the
237
reduced electrical scattering effect of the grain boundaries lead to an increase in both the
238
electrical conductivity and mobility [27].
239
4. Conclusion
240
We successfully deposited the SnS/Ag/SnS films onto SLG and Mo-coated SLG
241
substrates by RF and DC sputtering methods at 200 ℃. The structural, optical, and electrical
242
properties of the deposited films are significantly varied with the thickness of the Ag interlayer.
243
The EDS elemental composition of the SAS trilayer films showed the [S/Sn] ratio in between
244
1.08 – 1.13. The observed SnS (111) and Ag (111) preferred planes in the XRD spectra revealed
245
the presence of Ag interlayer in the sandwich structure. The micro-Raman spectra show an
246
improvement in the crystallinity of the SnS with the increase of Ag thickness. The presence of
247
the Ag interlayer between two SnS layers without inter-diffusion of Ag was confirmed by the
248
SIMS depth profile analysis. The optical absorption coefficient increased by a factor of 18 in the
249
SAS trilayer films than that of the SnS films without Ag. The optical band gap reduces from 1.53
250
eV to 1.28 eV with the increase of Ag thickness. The hole-mobility increases from 1.31 to 81.6
251
cm2.V-1s-1. While the the electrical resistivity is decreased from 6060 to 0.018 Ω.cm with a
252
simultaneous increase in the hole-carrier concentration by more than six orders from 1.94 × 10
253
to 4.15 × 10
254
optical and electrical properties than the single SnS films.
$
cm-3. The SAS trilayer films deposited at Ag-18 nm showed the best structural,
11
255 256
Acknowledgement: This research was supported by the Technology Development Program
257
to Solve Climate Changes of the National Research Foundation (NRF) funded by the
258
Ministry
259
2017M1A2A2087577) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP)
260
and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980).
of
Science,
ICT
&
Future
Planning
(NRF-2016M1A2A2936759,
NRF-
261 262 263
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F. Djeffal, H. Ferhati, A. Benhaya, A. Bendjerad, Effects of high temperature annealing in enhancing the optoelectronic performance of sputtered ITO/Ag/ITO transparent electrodes, Superlattices Microstruct. 130 (2019) 361–368. doi:https://doi.org/10.1016/j.spmi.2019.05.007.
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K.R. Catchpole, A. Polman, Design principles for particle plasmon enhanced solar cells, Appl. Phys. Lett. 93 (2008) 191113. doi:10.1063/1.3021072.
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S. Gedi, V.R. Minnam Reddy, T.R. Reddy Kotte, S.-H. Kim, C.-W. Jeon, Chemically synthesized Ag-doped SnS films for PV applications, Ceram. Int. 42 (2016) 19027–19035. doi:https://doi.org/10.1016/j.ceramint.2016.09.059.
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T.S. Reddy, M.C.S. Kumar, Co-evaporated SnS thin films for visible light photodetector applications, RSC Adv. 6 (2016) 95680–95692. doi:10.1039/C6RA20129F.
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K. Kawano, R. Nakata, M. Sumita, Effects of substrate temperature on absorption edge and photocurrent in evaporated amorphous SnS2films, J. Phys. D. Appl. Phys. 22 (1989) 136–141. doi:10.1088/0022-3727/22/1/019.
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A. Wangperawong, P.-C. Hsu, Y. Yee, S.M. Herron, B.M. Clemens, Y. Cui, S.F. Bent, Bifacial solar cell with SnS absorber by vapor transport deposition, Appl. Phys. Lett. 105 (2014) 173904.
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Y. Zhao, X. Liu, D.Y. Lei, Y. Chai, Effects of surface roughness of Ag thin films on surface-enhanced Raman spectroscopy of graphene: spatial nonlocality and physisorption strain, Nanoscale. 6 (2014) 1311–1317. doi:10.1039/C3NR05303B.
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K.H. Choi, J.Y. Kim, Y.S. Lee, H.J. Kim, ITO/Ag/ITO multilayer films for the application of a very low resistance transparent electrode, Thin Solid Films. 341 (1999) 152–155. doi:https://doi.org/10.1016/S0040-6090(98)01556-9.
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R.E. Banai, H. Lee, M.A. Motyka, R. Chandrasekharan, N.J. Podraza, J.R.S. Brownson, M.W. Horn, Optical properties of sputtered SnS thin films for photovoltaic absorbers, IEEE J. Photovoltaics. 3 (2013) 1084–1089. doi:10.1109/JPHOTOV.2013.2251758.
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B. Ghosh, R. Bhattacharjee, P. Banerjee, S. Das, Structural and optoelectronic properties of vacuum evaporated SnS thin films annealed in argon ambient, Appl. Surf. Sci. 257 (2011) 3670–3676. doi:https://doi.org/10.1016/j.apsusc.2010.11.103.
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T. Gotoh, Control of carrier concentration in SnS films by annealing with S and Sn, Phys. Status Solidi. 213 (2016) 1869–1872. doi:10.1002/pssa.201532986.
336 337 338
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S.S. Hegde, A.G. Kunjomana, P. Murahari, B.K. Prasad, K. Ramesh, Vacuum annealed tin sulfide (SnS) thin films for solar cell applications, Surfaces and Interfaces. 10 (2018) 78–84. doi:https://doi.org/10.1016/j.surfin.2017.12.003.
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K. Deraman, S. Sakrani, B.B. Ismatl, Y. Wahab, R.D. Gould, Electrical conductivity measurements in evaporated tin sulphide thin films, Int. J. Electron. 76 (1994) 917–922. doi:10.1080/00207219408925998.
342 343 344
[29]
E. Ringe, M.R. Langille, K. Sohn, J. Zhang, J. Huang, C.A. Mirkin, R.P. Van Duyne, L.D. Marks, Plasmon Length: A Universal Parameter to Describe Size Effects in Gold Nanoparticles, J. Phys. Chem. Lett. 3 (2012) 1479–1483. doi:10.1021/jz300426p.
345 346
Figure and table captions:
347
Fig. 1. The surface SEM images of the Ag on SnS film deposited at various Ag film thickness (a)
348
Ag-0 nm, (b) Ag-9 nm, (c) Ag-18 nm, (d) Ag- 27 nm and (e) the cross-sectional image of
349
SnS/Ag/SnS grown at Ag-27 nm. ( The arrow represents the Ag particles)
350
Fig. 2. (a) X-ray diffraction spectra and (b) high magnification X-ray spectra of the SnS/Ag/SnS
351
trilayer films as a function of Ag interlayer thickness.
352
Fig. 3. The micro-Raman spectra of the SAS trilayer films as a function of Ag interlayer
353
thickness.
354
Fig. 4. Secondary ion mass spectroscopy (SIMS) depth profiles of Sn, S, Ag, and Mo elements in
355
the SAS trilayer film deposited at 9 nm thick Ag.
14
356
Fig. 5. The spectra of (a) optical transmittance vs. wavelength, (b) reflectance vs. wavelength,
357
and (c) the absorption coefficient (α) vs. hν for SnS/Ag/SnS trilayer films as a function of Ag
358
interlayer thickness.
359
Fig. 6. Tauc plots of (αhν)2 vs. hν for the sputtered SnS/Ag/SnS trilayer films.
360
Fig. 7. The plots of (a) the electrical mobility (●) and hole concentration (■) vs. Ag film
361
thickness and (b) resistivity (■) vs. Ag film thickness of the SnS/Ag/SnS trilayer films.
362
Table 1. Experimental details of the SnS/Ag/SnS trilayer films deposited at 200 ℃ by the
363
sputtering method.
364
Table 2. The EDS data of the SnS/Ag/SnS trilayer films as a function of Ag interlayer thickness.
365 366
15
Layer
Material
Sputter power
Working pressure
Substrate temperature
Substrate to target distance
Deposition time
Growth rate
Thickness
(W)
(m Torr)
(℃)
(mm)
(s)
(nm/s)
(nm)
Bottom
SnS
RF-100
30
200
65
300
1
300
Center
Ag
DC-100
8
200
75
20 – 60
0.45
9 – 27
SnS
RF-100
30
200
65
300
1
300
Top
Table 1. Experimental details of the SnS/Ag/SnS trilayer films deposited at 200 ℃ by the sputtering method.
Layer structure
Ag Thickness
Crystallite size of SnS (111)
Ag
Sn
S
(nm)
(nm)
(at. %)
(at. %)
(at. %)
SnS
0
34.5
0
48.07
51.93
1.08
SnS/Ag/SnS
9
40.1
1.87
47.83
50.30
1.05
SnS/Ag/SnS
13.5
41.6
2.24
46.60
51.17
1.09
SnS/Ag/SnS
18
43.1
3.26
46.47
50.27
1.08
SnS/Ag/SnS
22.5
38.9
5.19
44.89
49.92
1.11
SnS/Ag/SnS
27
37.1
6.18
43.99
49.83
1.13
[S/Sn]
Table 2. The EDS data of the SnS/Ag/SnS trilayer films as a function of Ag interlayer thickness.
Fig. 1 (a)
(b)
200 nm
200 nm
(e)
(d)
(c)
200 nm
200 nm
o
Mo
@ 200 C
∗ Ag (111)
∗ Ag (200)
(b)
@ 200 C ♦ (202) ♦ (251)
∗ Ag (200) ♦ (002) ♦ (211) ♦ (112) ♦ (122)
♦ (111)
o
Mo
∗ Ag (111) ♦ (131)
Ag- 27 nm
♦ (120) ♦ (101)
(a)
♦ (110)
Fig. 2
Ag- 27 nm
Ag- 22.5 nm
Intensity (a.u)
Intensity (a.u)
Ag- 22.5 nm
Ag- 18 nm Ag- 13.5 nm Ag- 9 nm
Ag- 18 nm
Ag- 13.5 nm
Ag- 0 nm Ag- 9 nm ∗ Ag #01-1164 ♦ SnS #039-0354 • Sn2S3 #030-1379
10
20
30 40 50 2θ (degrees) A
60
70
∗ Ag #01-1164 ♦ SnS #039-0354
38 40 42 44 46 2θ (degrees) A
♦218.9
♦ 181.9
♦SnS
Ag-27 nm Ag-22.5 nm
Intensity (a.u)
♦92.0
Fig. 3
Ag-18 nm
Ag-13.5 nm
Ag-9 nm
Only SnS
200
400
600 -1
Raman shift (cm )
800
Fig. 4 7
10
6
Intensity [cps]
10
S
5
Sn
4
Ag
10 10
S Mo Ag Sn
3
10
Mo
2
10
1
10
0
10
0
200
400
600
800 1000 1200 1400
Depth profile [nm]
Fig. 5 80 Ag- 0 nm
Reflectance (%)
60
40
Ag- 0 nm Ag- 9 nm
20 Ag- 18 nm
40
20 Ag- 9 nm Ag- 18 nm Ag- 27 nm
Ag- 27 nm
800
1000
1200
1400
600
800
Wavelength (nm)
1000
1200
Wavelength (nm) 12 -1
4
0 600
(b)
60
α × 10 (cm )
Transmittance (%)
(a)
10
(c)
8 Ag-27 nm
6 Ag-18 nm
4 Ag-9 nm
2 Ag-0 nm
0.8 1.0 1.2 1.4 1.6 1.8 2.0
hν (eV)
1400
4
Ag-9 nm
9
9
8
Ag-0 nm
(b) 16 12
2
(αhν) × 10 (eV/cm)
(a) 16 12
20
2
20
2
(αhν) × 10 (eV/cm)
2
Fig. 6
8 4
1.36 eV
1.53 eV 0
0 1.0 1.2 1.4 1.6 1.8 2.0 2.2
1.0 1.2 1.4 1.6 1.8 2.0 2.2
(hν) (eV)
8 4
(d) 16
9
9
12
Ag-18 nm
20
12
2
(c) 16
(αhν) × 10 (eV/cm)
2
20
2
(αhν) × 10 (eV/cm)
2
(hν) (eV)
8 4
1.32 eV 0
Ag-27 nm
1.28 eV 0
1.0 1.2 1.4 1.6 1.8 2.0 2.2
(hν) (eV)
1.0 1.2 1.4 1.6 1.8 2.0 2.2
(hν) (eV)
4
19
18
10
17
10
16
10
15
10
14
10
0
90 80 70 60 50 40 30 20 10 0 5 10 15 20 25 30
Ag thickness (nm)
Resistivity (Ω.cm)
(a)
10
2
10
Mobility (cm /V.s)
-3
Hole concentration (cm )
Fig. 7 (b)
2
10
0 10 SnS/Ag/SnS trilayer
-2
10
Ag/SLG
-4
10
-6
10
0
5
10 15 20 25 30
Ag thickness (nm)
Highlights: •
SnS/Ag/SnS (SAS) trilayer films showed a preferred (111) peak of the SnS with improved crystallinity with an increase Ag thickness from 9 to 27 nm.
•
The SAS trilayer films showed a larger optical absorption coefficient by a factor of 18 than that of the SnS films without Ag.
•
The direct optical energy band gap decreased from 1.53 to 1.28 eV with Ag thickness.
•
All SAS trilayer films showed a p-type conductivity with enhanced electrical properties with Ag thickness.
Declaration of interest Date: 13th November 2019
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The work described in our manuscript has not been published previously or not under consideration for publication elsewhere. Its publication is approved by all authors who contributed in this work. If accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copy right holder.
Thank you so much,
Regards, Prof. Jeha Kim, Date: 11th September 2019 Department of Energy Convergence Engineering, Cheongju University, 298 Daesung-ro, Cheongwon-gu, Chungbuk Cheongju city, 28503, Republic of Korea. Tel: +82-43-229-7986 E-mail: [email protected]
S1567-1739(20)30002-X
DOI:
https://doi.org/10.1016/j.cap.2020.01.002
Reference:
CAP 5128
To appear in:
Current Applied Physics
Received Date: 12 September 2019 Revised Date:
13 November 2019
Accepted Date: 2 January 2020
Please cite this article as: V.K. Arepalli, T.D. Nguyen, J. Kim, Influence of Ag thickness on the structural, optical, and electrical properties of the SnS/Ag/SnS trilayer films for solar cell application, Current Applied Physics (2020), doi: https://doi.org/10.1016/j.cap.2020.01.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V. on behalf of Korean Physical Society.
1
Influence of Ag Thickness on the Structural, Optical, and Electrical
2
Properties of the SnS/Ag/SnS Trilayer Films for Solar Cell Application
3
Vinaya Kumar Arepalli1, Tien Dai Nguyen2, and Jeha Kim1*
4
1
Department of Energy Convergence Engineering, Cheongju University, Cheongju, Korea.
5
2
Institute of Theoretical and Applied Research, Duy Tan University, Hanoi 100000, Vietnam.
6 7 8 9 10 11 12 13
* corresponding author ([email protected]) Keywords: RF-sputtering; SnS/Ag/SnS trilayer films; Ag interlayer; sandwich structure
Abstract: We fabricated the SnS/Ag/SnS (SAS) trilayer thin films by a sputtering method at 200 ℃.
14
The structural, optical, and electrical properties of the films were studied by varying the Ag
15
interlayer thickness from 9 to 27 nm. The EDS analysis revealed that all SAS trilayer films
16
showed an increase in the atomic percentage of Ag from 1.87 to 6.18. The X-ray diffraction
17
studies confirmed that SAS films with Ag-18 nm thickness showed a preferred (111) peak of the
18
SnS with improved crystallinity. The optical absorption coefficient of the SAS films increased by
19
a factor of 18 when compared to the SnS films without Ag. Also, the optical band gap decreased
20
from 1.53 to 1.28 eV with Ag thickness. All SAS films exhibited the p-type conductivity with
21
increased hole- concentration from 1.94 ×
22
1.31 to 81.6 cm2.V-1s-1.
to 4.15 ×
cm-3 and also the mobility from
23 24
1. Introduction
25
Tin monosulphide (SnS) is IV–VI group semiconductor compound and has attracted
26
more attention recently due to its potential application as a p-type absorber material in the
27
heterojunction solar cells [1]. It has a direct optical energy band gap (1.2–1.4 eV) with the high 1
28
absorption coefficient (>104 cm-1) [2]. The constituent elements of the SnS are earth-abundant,
29
non-toxic, and inexpensive [2]. According to Alber’s group investigations, the carrier
30
concentration of the SnS was nearly 1018 cm-3 [3]. Moreover, the theoretical energy conversion
31
efficiency of the SnS is >25% [4]. The highest efficiency of the SnS based solar cell is 4.36%
32
with the hole density of 1015 cm-3 and the resistivity of 280 Ω.cm which are far from the
33
theoretical limit [3, 5]. Moreover, the SnS suffers from the loss of light absorption at near
34
bandgap region due to its indirect bandgap [6]. The important two typical issues to improve the
35
SnS thin film based solar cell performance are the higher light absorption in the active SnS layer
36
and the lower electrical resistivity. In general, the optical absorption increases with the SnS film
37
thickness. But the thickness of the SnS is limited below its carrier diffusion length (<900 nm)
38
due to the charge carrier recombination at a higher thickness (>900 nm) [7]. Moreover, there is a
39
need to reduce the film thickness (<1 µm) of any solar absorber to use at a large scale and more
40
economical way. Despite the optical properties, the electrical properties such as the mobility,
41
carrier concentration and electrical resistivities are also necessary to get improved. The lower
42
electrical resistivity (5 Ω.cm) [8] with the high carrier concentration (>1018) can be expected to
43
make an efficient solar absorber material. To meet these requirements, we adopted the
44
fundamental idea of a metal-semiconductor system in which the metal nanoparticles embedded
45
in a semiconductor. When the sun light is striking on a metal-semiconductor interface, the metal
46
nanoparticles exhibit a collective oscillation of electrons known as the localized surface plasmon
47
resonance (LSPR) through the excitation of the conduction electrons at the interface between the
48
metal and the semiconductor [9-11]. Thus, the light can be trapped into a thin semiconductor
49
layer, thereby the absorption is increased.
2
50
Noble metal nanoparticles such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al)
51
embedded in a semiconductor or dielectric material show the enhanced optical properties [12].
52
Thus, they gain much attention in the applications of plasmonic solar cells [13], surface plasmon
53
enhanced sensors [14], and photonic devices [15]. Among those, silver (Ag) is a potential metal
54
that can be used to improve the opto-electronic properties in the active solar absorber due to its
55
lowest optical absorption coefficient in the visible region and its superior conductivity when
56
compared with other metals [16]. The amount of light either absorbed or scattered depends on
57
the size and shape of the Ag nanoparticles. As per the previous literature reports, Ag can exhibit
58
plasmonic effect with different shapes such as cylindrical, hemispherical, spherical, cubic,
59
triangular nanoprism [6, 29]. The Ag nanoparticles with a diameter between 100 – 150 nm in a
60
hemispherical shape are suited well for the light trapping [17]. Till date, we have not seen the
61
thickness effect of the Ag interlayer on the structural, optical, and the electrical properties of the
62
SnS thin films. Thus, we prepared a trilayer films with a sandwich structure of Ag interlayer
63
between the top and bottom SnS layers by using a sputtering method. The trilayer film structure
64
was denoted as SnS/Ag/SnS (SAS). The structural, optical, and electrical properties of the as-
65
grown SAS films were investigated.
66
2. Experimental section
67
The SnS/Ag/SnS (SAS) trilayer films were deposited on both the soda lime glass (SLG)
68
and Mo/SLG substrates (25 × 25 mm2) by sputtering method. In this, SnS (purity; 99.99%) and
69
Ag (purity; 99.999%) targets were used. Prior to deposition, the substrates were ultrasonically
70
cleaned with acetone, methanol, and deionized water for 30 min and dried with N2 gas. Then the
71
substrates were placed inside the sputter chamber. The substrate temperature was maintained at
72
200 ℃. The distances between the substrate to both the SnS and Ag targets were 65 and 75 mm,
3
73
respectively. By using a turbomolecular pump (TMP), the base pressure of the sputter chamber
74
around 6.0 ×
75
with 30 sccm flow rate through a mass flow controller. Each top and bottom SnS layers were
76
deposited at 200 ℃ by using 100 W radio frequency (RF) power at 30 mTorr working pressure
77
for 5 min. Whereas, the center Ag layer was deposited at 200 ℃ by using 100 W direct current
78
(DC) power at 8 mTorr working pressure for various deposition times from 20 s to 60 s. The
79
above experimental details are shown in Table 2.
Torr was achieved. The argon (purity; 99.999%) was used as a sputter gas
80
The surface and cross-sectional morphologies were characterized by using scanning
81
electron microscopy (JEOL, JSM-7610F). The elemental composition of the SAS trilayer films
82
was measured using an energy dispersive spectrometer. The X-ray diffraction (SmartLab, Rigaku)
83
was used to study the structural properties with a CuKα radiation of λ = 1.54 Å. The elemental
84
concentration depth profiles were measured by a secondary ion mass spectroscopy (CAMECA,
85
IMS 6F) with O2 primary ion at 1 keV DUO (+3 kV) / Secondary (+2 kV) without oxygen
86
flooding/Cs. The optical analysis was performed by using UV-Vis-NIR spectrophotometry
87
(Perkin Elmer, Lambda 1050) in the wavelength range of 300 – 1400 nm. The structural defects
88
were characterized by using micro-Raman spectroscopy (Uninanotech, UR1207J) with a diode-
89
pumped solid-state (DPSS) laser at 532 nm excitation wavelength. The electrical properties such
90
as carrier concentration, mobility, and resistivity of the SnSAg/SnS trilayer films were evaluated
91
from the Hall measurement system (ECOPIA, HMS-3000) using Van-der Pauw method.
92 93
3. Results and Discussion
94
Based on the growth rate of Ag (0.45 nm/s), the SnS/Ag/SnS (SAS) trilayer films were
95
prpepared by varying the Ag interlayer thickness from 9 – 27 nm while maintaining the SnS film
4
96
thickness of 300 nm constant on both sides. Also, these films were compared with SnS film (600
97
nm) without Ag.
98
3.1. Microstructure
99
Fig. 1 shows the surface FESEM image of a bilayer films that consist of the Ag film on
100
top of the SnS (Ag/SnS) with various Ag layer thicknesses. Also, the morphology of the SnS
101
film without Ag is compared with the Ag layer on top of the SnS. As shown in Fig. 1a the
102
surface feature of the SnS film without Ag seems to have a densely packed plate-like grain
103
structure. The grain size is around 100 – 200 nm with slightly roughed surface. Whereas, the
104
morphology of the Ag layer on the SnS film is critically dependent on its growth nature at
105
particular temperature (200 ℃). To observe the growth behavior based on the growth rate (0.45
106
nm/s), the Ag was deposited on top of the SnS film at various deposition times: 20, 40, and 60 s.
107
The corresponding thicknesses of the Ag layer are 9, 18, and 27 nm, respectively. It is clearly
108
shown that an increase of Ag layer thickness results in transformation of its shape from an island
109
to a continuous film. The morphology of the Ag at 9 nm thickness (Fig. 1b), the nucleation and
110
the growth have been proceeded already. Thus, the Ag islands are observed on the surface of the
111
SnS as shown in Fig. 1b. Moreover, the average size of the Ag island is ≤100 nm and are
112
disconnected to each other (arrow marks). After 40 s, the thickness of the Ag becomes 18 nm
113
and the islands are randomly connected to each other by a coalescence phenomenon. As a result,
114
an incomplete surface coverage is observed (Fig. 1c) with an ellipsoid shape of slightly larger Ag
115
particles (≥100 nm). Finally, as shown in Fig. 1d, a fully covered continuous Ag film is
116
observed due to the agglomeration of the particles for 60 s growth time with 27 nm thickness.
117
Fig. 1e shows the cross-sectional image of SAS trilayer film on Mo-coated SLG in which two
118
SnS layers seperated by a small interfacial line that could be the Ag layer.
5
119
The elemental composition of the SAS trilayer films with varying Ag thickness was
120
analyzed by the EDS measurement. The atomic percentage of Sn, S, and Ag are listed in Table 2.
121
It is observed that, with increasing Ag thickness, the atomic percentage of Ag increases from
122
1.87 to 6.18. Also, the [S/Sn] ratio of the SAS films slightly increases with Ag thickness. It
123
indicates the presence of Ag layer affects slightly on the composition of the SnS. Moreover, the
124
SAS trilayer film at 18 nm of Ag shows the same [S/Sn] ratio as the SnS film (1.08) without Ag.
125
3.2. Structural phase identification
126
The structural phase of the SAS trilayer films was investigated by X-ray diffraction
127
pattern shown in Fig. 2a. The observed peaks at 2θ of 22.1°, 30.6°, 31.6°, 39.1°, 45.4°, and 66.0°
128
are assigned to (110), (120), (101), (111), (131), (002), and (251), respectively. These are well
129
matched with the standard JCPDS #39-0354 of the orthorhombic crystal phase of the SnS. All
130
samples show the polycrystalline nature with a preferred orientation along the (111) plane.
131
Importantly, from Fig. 2b, the SAS trilayer films grown at Ag of 22.5 nm and 27 nm the peaks at
132
38.1° and 45.3° are well matched with the (111) and (200) planes of the cubic Ag and are
133
consistent with the standard JCPDS #01-1164. It is clearly shown that the (111) peak intensity of
134
the SnS increases with an increase of Ag layer thickness from 0 nm to 18 nm and followed by a
135
dcrease of Ag thickness to 27 nm. It indicates an improvement in the crystallinity of the SnS
136
films due to the formation of new nucleation centers of the SnS on the Ag interlayer [18]. The
137
crystallite size that corresponds to the (111) peak of the SAS trilayer films can be evaluated from
138
Scherrer relation as follows [19] ,
139 140 141
D=
.
----------------(1)
where β is the full width at the half maxima (FWHM) of the SnS and Ag preferred (111) peaks, is the X-ray wavelength (1.54 Å) and
is Bragg’s angle. The evaluated crystallite sizes of the 6
142
both SnS and Ag are shown in Table 2. The crystallite size increases from 34.5 nm to 43.1 nm
143
for the SAS trilayer films with an increase of Ag thickness from 0 to 18 nm and followed by
144
gradually decreases to 37.1 nm at the Ag thickness of 27 nm. Other impurity phases such as SnS2
145
and Sn2S3 are not detected in the films. Based on the XRD results, the SAS trilayer structure with
146
18 nm-thick Ag interlayer exhibited the improved crystallinity than the other structures.
147
The micro-Raman analysis was used to know the secondary phases in the SAS trilayer
148
films. Fig. 3 shows the micro-Raman spectra of the SAS trilayer films. Usually, the
149
orthorhombic SnS shows 12 Raman active modes of 4Ag, 2B1g, 4B2g, 2B3g [27]. All SAS trilayer
150
films exhibit the major Raman intense peaks at 92 and 218.9 cm−1 which are assigned to Ag
151
Raman mode of the SnS [20]. Also, the minor peaks appeared at 139 cm–1 belong to the B2u
152
mode of the SnS [21]. Besides, the peak intensities are amplified with an increase of Ag
153
thickness from 9 nm to 18 nm due to the plasmon-induced electric field enhancement of Ag
154
interlayer film [22]. However, secondary impurity phases such as SnS2, Sn2S3, and Ag8SnS6
155
were not detected in the SAS films.
156
Fig. 4 shows SIMS elemental depth distribution profiles of Sn, S, Ag, and Mo in the
157
SAS trilayer film at 9 nm thick Ag. The SIMS depth profile shows a well-defined top and bottom
158
SnS layers. The green area in the elemental profiles indicates the Ag interlayer region between
159
the top and SnS layers without interfacial reactions. The constant atomic concentrations of Sn
160
and S atoms in the top and the bottom SnS films clearly denote the identical composition and
161
thickness of the top and bottom SnS layers. In addition, there is no evidence of either the Ag
162
diffusion into top and bottom SnS layers or the tin or sulfur diffusion into the Ag layer at 200 ℃.
163
3.3. Optical Properties
7
164
Fig. 5a represents the optical transmittance spectra of the SAS trilayer films deposited by
165
varying the Ag film thickness. In the transmittance spectra, all films show an apparent shift of
166
fundamental absorption edge in the wavelength range of 800 – 900 nm indicating the more
167
absorption of photons in the near IR region. When compared to the SnS film without Ag, the
168
SAS trilayer film deposited at the Ag thickness of 9 nm shows the higher reduction in the
169
transmittance. However, as the Ag thickness increases from 9 nm to 27 nm, the transmittance of
170
the SAS trilayer films futher decreases gradually. This can be attributed to the light scattering
171
effect of Ag film due to the increased surface roughness of Ag interlayer in the sandwich
172
structure [23]. Fig. 5b denotes the optical refelctance spectra of the SAS trilayer films without
173
and with Ag interlayer. The SnS film without Ag (Ag-0 nm) shows the higher reflectance due to
174
more surface smoothness compared to the SAS trilayer films with Ag thicknesses from 9 to 27
175
nm. Also, there is no proper fall of the fundamental absorption edge within the wavelength range
176
of 800 – 1300 nm for the SnS films in the reflection spectra. It can be because of the presence of
177
sputter induced ionic defects. While, as inserting the Ag layer between two SnS layers reduces
178
the optical reflection which is associated with the scattering effect of the Ag particles. Moreover,
179
with increasing Ag thickness from 9 to 27 nm, the reflectance gradually decreases, while, the
180
fundamental absorption edge shifts towards the longer wavelength side due to the transformation
181
of the shape and the increase of size of the Ag particles (Fig. 1). The larger Ag particles show the
182
higher scattering effect that increases the optical path of incident light which is beneficial for the
183
solar absorber.
184 185 186
Fig. 5c shows the optical absorption coefficient (α) of the SAS trilayer films evaluated from the above transmittance and reflectance spectra by using the following relation [24].
α = - ln (
)
8
--------------(2)
= thickness, R= reflectance and ! = transmittance of the trilayer film. As shown in Fig.
187
Where
188
5c the absorption coefficient is enhanced by a factor of 18 in the SAS trilayer films (3.24 × 10
189
cm-1) from that of 0.18 × 10 for the SnS films without Ag interlayer (Ag-0 nm). An abrupt
190
increase of absorption coefficient with the thickness of Ag from 0 nm (α = 0.18 × 10 cm-1) to
191
Ag of 9 nm (α = 1.94 × 10 cm-1) is due to the light scattering effect by the Ag islands.
192
However, the absorption coefficient increases slightly on further increase of the Ag thickness
193
from 9 to 27 nm. This can be understood by the change of Ag particle size and shape due to the
194
agglomeration of Ag islands. As a result, the surface of the bottom SnS layer is gradually
195
covered by Ag interlayer which shows a reduction in the light scattering effect of the Ag
196
particles.
197 198
The direct optical band gap of the SAS trilayer films can be estimated from the following relation [24] αhν = A (hν-Eg) k
199
-------------(3)
200
Where Eg is the optical band gap, k = ½, hν is the incident photon, and A is constant of the direct
201
allowed transitions. According to Tauc plot, as shown in Fig. 6, an intercept of tangent has
202
drawn towards X-axis in the (αhν)2 vs. hν spectra gives the optical energy band gap (Eg) of the
203
SAS trilayer films. The observed band gaps of 1.53, 1.36, 1.32, and 1.28 eV are assigned to the
204
SAS trilayer films with Ag-0 nm, Ag-9 nm, Ag-18 nm, and Ag-27 nm thicknesses, respectively.
205
These band gaps are consistant with the previously reported direct optical energy band gaps [25,
206
26]. The decrease in the band gap is attributed to the increased crystallite size of the SnS (Table
207
2) and the density of the Ag particles and their shapes in the sandwich structure (Fig. 1).
208
3.3. Electrical properties
9
209
The electrical properties such as resistivity, mobility, and hole concentration of the SAS trilayer
210
films are strongly affected by the thickness of the Ag in the sandwich structure. The change in
211
the electrical properties of the SAS trilyaer films was determined by using the Hall effect
212
measurement as a function of increasing Ag interlayer thickness and are shown in Fig. 7. As Ag
213
thickness increases, all the SAS films exhibit a p-type conductivity with an increase of hole-
214
carrier concentration (Fig. 7a) from 1.94 × 10
215
mobility (Fig. 7a) increases from 1.31 to 81.6 cm2.V-1s-1. This is attributed to the transformation
216
of Ag grain shape from island to a continuous film by the aggregation of particles with increase
217
Ag thickness and also the increase of inplane Ag grain size (Fig. 1). The hole-carriers travel
218
intitally through randomily distributed Ag islands (Ag-9 nm) which are not connected together.
219
The hole-carriers can travel much easier as Ag thickness increases to 18 and 22.5 nm. Because,
220
some of the Ag particles are connected together due to the agglomeration of these densily
221
distributed Ag islands. However, the hole-carrier mobility and concentration show a better
222
improvement at 27 nm thickess of Ag due to the formation of a continuous Ag film.
to 4.15 × 10
$
cm-3. Similarly, the hole-
223
As shown in Fig. 7b the electrical resistivity drastically decreases from 6060 to 0.018
224
Ω.cm with Ag thickness. Moreover, the resistivities of the SAS trilayer films are very high when
225
compared to the resistivity of the pure Ag metal on SLG as increasing Ag thickness. The only
226
SnS (Ag-0 nm) film shows the high electrical resistivity of 6060 Ω.cm is due to the surface
227
defects, poor crystallinity, and the higher surface roughness which can strongly reflect the
228
electrical properties. In the case of the SAS trilayer film with the Ag thicknesses of 9 and 13.5
229
nm, a fairly high resistivities 4460 and 2080 Ω.cm, respectively are observed, due to the
230
disconnection of the Ag islands (Fig. 1). However, it is noteworthy that the insertion of an 18-
231
nm-thick Ag layer between the SnS layers reduces the resistivity to 18.3 Ω.cm which is suitable
10
232
to act as an active SnS solar absorber [8]. The reason could be the improved crystallinity [28] of
233
the SAS trilayer films (Fig. 2) due to the increased crystallite size (Table 2). On further increase
234
of Ag thickness to 22.5 and 27 nm, all of the SAS trilayer films show lower resistivities of 0.075
235
and 0.018 Ω.cm which are close to the resistivity of a 4.5 nm thick Ag metal on the SLG (Fig.
236
7b). This is due to the complete surface coverage of the Ag film on top the SnS and also, the
237
reduced electrical scattering effect of the grain boundaries lead to an increase in both the
238
electrical conductivity and mobility [27].
239
4. Conclusion
240
We successfully deposited the SnS/Ag/SnS films onto SLG and Mo-coated SLG
241
substrates by RF and DC sputtering methods at 200 ℃. The structural, optical, and electrical
242
properties of the deposited films are significantly varied with the thickness of the Ag interlayer.
243
The EDS elemental composition of the SAS trilayer films showed the [S/Sn] ratio in between
244
1.08 – 1.13. The observed SnS (111) and Ag (111) preferred planes in the XRD spectra revealed
245
the presence of Ag interlayer in the sandwich structure. The micro-Raman spectra show an
246
improvement in the crystallinity of the SnS with the increase of Ag thickness. The presence of
247
the Ag interlayer between two SnS layers without inter-diffusion of Ag was confirmed by the
248
SIMS depth profile analysis. The optical absorption coefficient increased by a factor of 18 in the
249
SAS trilayer films than that of the SnS films without Ag. The optical band gap reduces from 1.53
250
eV to 1.28 eV with the increase of Ag thickness. The hole-mobility increases from 1.31 to 81.6
251
cm2.V-1s-1. While the the electrical resistivity is decreased from 6060 to 0.018 Ω.cm with a
252
simultaneous increase in the hole-carrier concentration by more than six orders from 1.94 × 10
253
to 4.15 × 10
254
optical and electrical properties than the single SnS films.
$
cm-3. The SAS trilayer films deposited at Ag-18 nm showed the best structural,
11
255 256
Acknowledgement: This research was supported by the Technology Development Program
257
to Solve Climate Changes of the National Research Foundation (NRF) funded by the
258
Ministry
259
2017M1A2A2087577) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP)
260
and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010012980).
of
Science,
ICT
&
Future
Planning
(NRF-2016M1A2A2936759,
NRF-
261 262 263
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345 346
Figure and table captions:
347
Fig. 1. The surface SEM images of the Ag on SnS film deposited at various Ag film thickness (a)
348
Ag-0 nm, (b) Ag-9 nm, (c) Ag-18 nm, (d) Ag- 27 nm and (e) the cross-sectional image of
349
SnS/Ag/SnS grown at Ag-27 nm. ( The arrow represents the Ag particles)
350
Fig. 2. (a) X-ray diffraction spectra and (b) high magnification X-ray spectra of the SnS/Ag/SnS
351
trilayer films as a function of Ag interlayer thickness.
352
Fig. 3. The micro-Raman spectra of the SAS trilayer films as a function of Ag interlayer
353
thickness.
354
Fig. 4. Secondary ion mass spectroscopy (SIMS) depth profiles of Sn, S, Ag, and Mo elements in
355
the SAS trilayer film deposited at 9 nm thick Ag.
14
356
Fig. 5. The spectra of (a) optical transmittance vs. wavelength, (b) reflectance vs. wavelength,
357
and (c) the absorption coefficient (α) vs. hν for SnS/Ag/SnS trilayer films as a function of Ag
358
interlayer thickness.
359
Fig. 6. Tauc plots of (αhν)2 vs. hν for the sputtered SnS/Ag/SnS trilayer films.
360
Fig. 7. The plots of (a) the electrical mobility (●) and hole concentration (■) vs. Ag film
361
thickness and (b) resistivity (■) vs. Ag film thickness of the SnS/Ag/SnS trilayer films.
362
Table 1. Experimental details of the SnS/Ag/SnS trilayer films deposited at 200 ℃ by the
363
sputtering method.
364
Table 2. The EDS data of the SnS/Ag/SnS trilayer films as a function of Ag interlayer thickness.
365 366
15
Layer
Material
Sputter power
Working pressure
Substrate temperature
Substrate to target distance
Deposition time
Growth rate
Thickness
(W)
(m Torr)
(℃)
(mm)
(s)
(nm/s)
(nm)
Bottom
SnS
RF-100
30
200
65
300
1
300
Center
Ag
DC-100
8
200
75
20 – 60
0.45
9 – 27
SnS
RF-100
30
200
65
300
1
300
Top
Table 1. Experimental details of the SnS/Ag/SnS trilayer films deposited at 200 ℃ by the sputtering method.
Layer structure
Ag Thickness
Crystallite size of SnS (111)
Ag
Sn
S
(nm)
(nm)
(at. %)
(at. %)
(at. %)
SnS
0
34.5
0
48.07
51.93
1.08
SnS/Ag/SnS
9
40.1
1.87
47.83
50.30
1.05
SnS/Ag/SnS
13.5
41.6
2.24
46.60
51.17
1.09
SnS/Ag/SnS
18
43.1
3.26
46.47
50.27
1.08
SnS/Ag/SnS
22.5
38.9
5.19
44.89
49.92
1.11
SnS/Ag/SnS
27
37.1
6.18
43.99
49.83
1.13
[S/Sn]
Table 2. The EDS data of the SnS/Ag/SnS trilayer films as a function of Ag interlayer thickness.
Fig. 1 (a)
(b)
200 nm
200 nm
(e)
(d)
(c)
200 nm
200 nm
o
Mo
@ 200 C
∗ Ag (111)
∗ Ag (200)
(b)
@ 200 C ♦ (202) ♦ (251)
∗ Ag (200) ♦ (002) ♦ (211) ♦ (112) ♦ (122)
♦ (111)
o
Mo
∗ Ag (111) ♦ (131)
Ag- 27 nm
♦ (120) ♦ (101)
(a)
♦ (110)
Fig. 2
Ag- 27 nm
Ag- 22.5 nm
Intensity (a.u)
Intensity (a.u)
Ag- 22.5 nm
Ag- 18 nm Ag- 13.5 nm Ag- 9 nm
Ag- 18 nm
Ag- 13.5 nm
Ag- 0 nm Ag- 9 nm ∗ Ag #01-1164 ♦ SnS #039-0354 • Sn2S3 #030-1379
10
20
30 40 50 2θ (degrees) A
60
70
∗ Ag #01-1164 ♦ SnS #039-0354
38 40 42 44 46 2θ (degrees) A
♦218.9
♦ 181.9
♦SnS
Ag-27 nm Ag-22.5 nm
Intensity (a.u)
♦92.0
Fig. 3
Ag-18 nm
Ag-13.5 nm
Ag-9 nm
Only SnS
200
400
600 -1
Raman shift (cm )
800
Fig. 4 7
10
6
Intensity [cps]
10
S
5
Sn
4
Ag
10 10
S Mo Ag Sn
3
10
Mo
2
10
1
10
0
10
0
200
400
600
800 1000 1200 1400
Depth profile [nm]
Fig. 5 80 Ag- 0 nm
Reflectance (%)
60
40
Ag- 0 nm Ag- 9 nm
20 Ag- 18 nm
40
20 Ag- 9 nm Ag- 18 nm Ag- 27 nm
Ag- 27 nm
800
1000
1200
1400
600
800
Wavelength (nm)
1000
1200
Wavelength (nm) 12 -1
4
0 600
(b)
60
α × 10 (cm )
Transmittance (%)
(a)
10
(c)
8 Ag-27 nm
6 Ag-18 nm
4 Ag-9 nm
2 Ag-0 nm
0.8 1.0 1.2 1.4 1.6 1.8 2.0
hν (eV)
1400
4
Ag-9 nm
9
9
8
Ag-0 nm
(b) 16 12
2
(αhν) × 10 (eV/cm)
(a) 16 12
20
2
20
2
(αhν) × 10 (eV/cm)
2
Fig. 6
8 4
1.36 eV
1.53 eV 0
0 1.0 1.2 1.4 1.6 1.8 2.0 2.2
1.0 1.2 1.4 1.6 1.8 2.0 2.2
(hν) (eV)
8 4
(d) 16
9
9
12
Ag-18 nm
20
12
2
(c) 16
(αhν) × 10 (eV/cm)
2
20
2
(αhν) × 10 (eV/cm)
2
(hν) (eV)
8 4
1.32 eV 0
Ag-27 nm
1.28 eV 0
1.0 1.2 1.4 1.6 1.8 2.0 2.2
(hν) (eV)
1.0 1.2 1.4 1.6 1.8 2.0 2.2
(hν) (eV)
4
19
18
10
17
10
16
10
15
10
14
10
0
90 80 70 60 50 40 30 20 10 0 5 10 15 20 25 30
Ag thickness (nm)
Resistivity (Ω.cm)
(a)
10
2
10
Mobility (cm /V.s)
-3
Hole concentration (cm )
Fig. 7 (b)
2
10
0 10 SnS/Ag/SnS trilayer
-2
10
Ag/SLG
-4
10
-6
10
0
5
10 15 20 25 30
Ag thickness (nm)
Highlights: •
SnS/Ag/SnS (SAS) trilayer films showed a preferred (111) peak of the SnS with improved crystallinity with an increase Ag thickness from 9 to 27 nm.
•
The SAS trilayer films showed a larger optical absorption coefficient by a factor of 18 than that of the SnS films without Ag.
•
The direct optical energy band gap decreased from 1.53 to 1.28 eV with Ag thickness.
•
All SAS trilayer films showed a p-type conductivity with enhanced electrical properties with Ag thickness.
Declaration of interest Date: 13th November 2019
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
The work described in our manuscript has not been published previously or not under consideration for publication elsewhere. Its publication is approved by all authors who contributed in this work. If accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copy right holder.
Thank you so much,
Regards, Prof. Jeha Kim, Date: 11th September 2019 Department of Energy Convergence Engineering, Cheongju University, 298 Daesung-ro, Cheongwon-gu, Chungbuk Cheongju city, 28503, Republic of Korea. Tel: +82-43-229-7986 E-mail: [email protected]