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Physica B 673 (2024) 415506
Available online 15 November 2023
0921-4526/© 2023 Elsevier B.V. All rights reserved.
Phase transition, structural, optical and thermoelectric properties of spin
coated Cu
x
S thin lms
N. Aghad
a
, A. Narjis
b
,
**
, L. Amiri
a
, S. Elmassi
a
,
*
, Ayman S. Alo
c
, L. Nkhaili
a
, A. Alsaad
d
,
A. Tihane
b
, Rachid Karmouch
e
, Hind Albalawi
f
, A. Outzourhit
a
a
Materials, Energy and Environment Laboratory, Faculty of Sciences Semlalia, Cadi Ayyad University, PB 2390, Marrakech, Morocco
b
Materials and Renewable Energy Laboratory, Faculty of Sciences, IbnouZohr University, Agadir, 80000, Morocco
c
Physics Department, College of Science, Taibah University, Medina, 42353, Saudi Arabia
d
Department of Physical Sciences, Jordan University of Science & Technology, P.O. Box 3030, Irbid, 22110, Jordan
e
Department of Physics, Science College, Jazan University, Jazan, Saudi Arabia
f
Department of Physics, College of Sciences, Princess Nourah Bint Abdulrahman University (PNU), P.O. Box 84428, Riyadh, 11671, Saudi Arabia
ARTICLE INFO
Keywords:
Copper sulphide
Thin lms
Spin coating
Optical properties
Thermoelectric properties
ABSTRACT
Copper sulphide (Cu
x
S) thin lms were deposited by the spin coating method using copper acetate (Cu₂(CH₃-
COO)₄) and thiourea (CH₄N₂S) as precursors. This simple technique was shown to produce thin lms with high
quality in terms of tunable stoichiometry and physical properties. In fact, the room temperature structural,
optical and thermoelectric properties were studied. It is observed that the annealing during 1 h at 250 ◦C and
300 ◦C results in the transition from the covellite (CuS) to the chalcocite (Cu
2
S) and djurleite (Cu
1.95
S) phases,
respectively. Optical measurements show the formation of absorbing lms in the visible range with a trans-
mittance lower than 50 % for all phases. The covellite phase is found to be thermoelectrically interesting with a
power factor of 12.3
μ
WK
−2
m
−1
, while the conductivity of the djurleite phase (281.7 Ω
−1
cm
−1
) needs to be
improved to match its high Seebeck coefcient (21.3
μ
VK
−1
).
1. Introduction
Copper sulde is a p-type semiconductor, belonging to the I–VI
family, with the chemical formula Cu
x
S, where x varies from 1 to 2. Its
bonds are established between the sulde anions, S
2−
, and copper cat-
ions, Cu
2+
. It can be found at different compositions according to the
Cu/S ratio, namely the stoichiometric covellite (CuS) and chalcocite
(Cu
2
S) phases and the non-stoichiometric phases, like the digenite phase
where x =1.765–1.79 and the djurleite phase where x =1.93–1.96.
These compounds can be classed as intrinsic, weakly or heavy doped
semiconductors depending on the phase stoichiometry. Cu
2
S is usually
reported as an intrinsic semiconductor. It showed a semiconductor
character with a room temperature resistivity of 1 Ωcm [1].
The group VI is widely known by the chalcogen family. It consists of
non metal (O, S, Se …), metal (Uuh) and metalloid (Po, Te) elements.
These elements have six electrons on the valence level and have a strong
tendency to capture two electrons to form two covalent bonds. They
form bonds with metals and non-metal elements in binary and ternary
compounds, such as Cu
x
S and ZnCu
x
S. S, Se and Te are known to form a
wide range of non-stoichiometric phases with copper.
The structure of the copper sulde compounds is quite complicated.
The stoichiometric structures of Cu
2
S and CuS are not consistent with
their formulations as Cu(I) and Cu(II) suldes. Chalcocite Cu
2
S can be
formed in its low-temperature form with a rather complex structure or in
its high temperature form of disordered rearrangements of Cu atoms in a
close-packed array of S atoms. Likewise, the compound CuS, which oc-
curs as the mineral covellite, has one-third of its metal ions trigonally
surrounded by three neighboring S atoms and the remainder have four S
neighbors, tetrahedrally arranged [2,3]. CuS thin lms are commonly
reported to exhibit crystalline hexagonal, orthorhombic and rarely cubic
structures [4]. In some studies, it is amorphous [5]. The formed struc-
ture depends on the deposition method and the preparation parameters
like the deposition time, solvent, solution pH and temperature.
Involving copper sulphide compounds in technology includes several
uses. Indeed, they have been considered as prominent semiconductors
for the water splitting technique owing to their excellent optical
* Corresponding author.
** Corresponding author.
E-mail addresses: a.narjis@uiz.ac.ma (A. Narjis), elmassisaid@gmail.com (S. Elmassi).
Contents lists available at ScienceDirect
Physica B: Condensed Matter
journal homepage: www.elsevier.com/locate/physb
https://doi.org/10.1016/j.physb.2023.415506
Received 10 August 2023; Received in revised form 18 October 2023; Accepted 13 November 2023
Physica B: Condensed Matter 673 (2024) 415506
2
properties. Their energy band gap is 1.63–1.87 eV in Ref. [6] and 2.8 eV
in Ref. [7]. Chemical and electronic properties promote photo-generated
carriers, resulting in the reduction/oxidation of water to release H
2
and
O
2
. The proposed material, for the water splitting technique, is ZnS [8].
ZnS is a n-type semiconductor with a band gap tending towards 3.5–3.7
eV which protects the CuS and makes it possible to increase the
electron-hole lifetime by applying the PN heterojunction. On the other
hand, Cu
2
S lms are known to exhibit a narrow band gap energy at room
temperature (Eg =1.2 eV). They absorb light in the near infra-red and
visible wavelength range. This is why they are widely used as absorbent
lms in photovoltaic devices. They can be covered by large gap lms,
like CdS which has a band gap energy of 2.4 eV.
Over the course of few last years, many efforts have been made for
the development of Cu
2
S thin lms based solar cells. In this context, a
Cu
2
S/CdS based photovoltaic cell was the most promising solar energy
conversion device due to its high conversion efciency (more than 10
%), easy deposition and fabrication at low cost [9].
Moreover, Cu
2-x
S are environment-friendly, abundant and low-cost
compounds, widely investigated in terms of room and high room tem-
perature thermoelectric (TE) applications. As Phonon-Liquid-Electron-
Crystals (PLEC), their performance appears in their superionic con-
ducting character. In fact, phonon vibrations are eliminated during heat
transfer and high ionic conductivity of copper is maintained. In partic-
ular, the digenite Cu
1.8
S phase is the promising compound for TE ap-
plications due to its electrical and thermal stability [10]. Besides, it is
recognized as a stable phase with a complex crystal structure.
Improvement of TE properties can be achieved throughout increasing
the electrical conductivity (
σ
) and/or the Seebeck coefcient (S), or
reducing the thermal conductivity (λ), which can be achieved by
adjusting the carrier concentration. This optimization is generally car-
ried out by annealing [11–14], or doping with certain elements, such as
Na, Ti, Se or Ag. The TE dimensionless gure of merit (ZT =T
σ
S
2
/λ,
where T is the measuring temperature) was reported to be 1.05 for the
digenite phase at 500 ◦C [15] and 0.8 for the djurleite phase at 530 ◦C
[16].
In this work, the annealing is reported to result in transition of the
phase of spin coated Cu
x
S lms. Structural, optical and TE properties are
studied for uses in the corresponding applications.
2. Experimental
Copper acetate (Cu₂(CH₃COO)₄) and thiourea (CH₄N₂S) were taken as
precursors for preparing the Cu
x
S lms. Thiourea is the precursor source
of sulde allowing the solution to became more viscous. Copper acetate
(0.5 M) was dissolved in 4 ml of ethanol, the solution was stirred at 40 ◦C
for 15 min and then the thiourea (1 M) was added. The solution was
subjected to an aggressive stirring for 30 min until obtaining a homog-
enous solution. This solution was ltered to avoid the settled particles.
The sol-gel method consists of two steps. During the hydrolyzation
process, hydroxyde ions attach to the metallic ions. The corresponding
reactions are:
Cu2++2OH−⥄→Cu(OH)2(1)
CS(NH2)2+HO−→CH2N2+HS−+H2O(2)
HS−+HO−→S2−+H2O(3)
Then the partially hydrolyzed molecules get attached together to
form a complex macro molecule (condensation process), according to
the reaction:
Cu(OH)2+S2−→CuS+2HO−(4)
Glass substrates were cleaned by detergent liquid, acetone for 15 min
and distilled water for 20 min in aultrasonic device. Each cleaned sub-
strate was dried at 100 ◦C in an oven. The solution was deposited by the
spin coating technique. The substrate was installed on the holder, then
the solution was dropwise added until covering the surface. The rst
step was spread out the solution at 2000 RPM and next step was the
minimizing of the thickness of coating material at speed up to 4000
RPM. The as-deposited lms were found to be green-black. Then, they
were annealed in netrogen atmosphere for 1 h at 150, 200, 250 and
300 ◦C. The color was found to change to yellow-white, which was also
reported in Ref. [17].
The lm thickness was measured by a Michelson interferometer.
Light splits into two arms because of difference of refractive indices of
glass and the deposited material. The resulting interferometry pattern
leads to measure the thickness of the lm by the relation e =λd/2i,
where e is the lm thickness, i the fringe, d the fringe offset and λ the
wavelength of the used monochromatic light. In the present work, the
lms thickness was found to be approximately 1
μ
m.
Fig. 1 shows the schematic picture of preparation process.
The synthesized thin lms were characterized by X-Ray diffraction
(XRD, Smart Lab SE diffractometer, Rigaku Japan, λ (Cu
K
α
), =1.5418
Å). The Scanning Electron Microscopy was performed using a Tescan
Vega3, which is coupled with an EDAX analyzer. The optical properties
were shown in the UV–Vis-IR wavelength range (300–2000 nm) by using
the UV-3101PC Shimadzu spectrophotometer. The room temperature
electrical measurements were taken using Ecopia HMS-3000 Hall mea-
surements. Finally, the Seebeck coefcient was measured by an own lab-
fabricated device, as described in Ref. [18].
3. Results and discussion
3.1. Structural characterization
Fig. 2 shows the XRD patterns for various annealing temperatures
under nitrogen gas. For annealing at 150 ◦C, the covellite (CuS) nano-
particles (NPs) are shown with the hexagonal structure. The peak cor-
responding to (006) plane is found to be the most intense, which
indicates the preferred growth of CuS NPs along this direction. No other
peaks related to the impurity phases such as Cu
2
O, CuO
2
, Cu
2
S, and
Cu
2
S
3
are observed, which indicates that the synthesized CuS NPs are
highly pure and single-phase at 150 ◦C. At 200 ◦C, the XRD pattern
approximately shows peaks at the same angles, which corresponds to the
covellite phase and the plane (006) is still the preferred growth direc-
tion. At 250 ◦C, the XRD pattern matches the Cu
2
S phase with a cubic
structure. The plan (200) is found to be the preferred growth direction.
This compound also contains a negligible amount of hexagonal crystals,
by virtue of the small peak at 29.30◦. At 300 ◦C, the intensity of the peak
at 32.25◦is found to decrease with appearance of new peaks at 27.74◦
and 46.11◦, which matches the Cu
1.9
5S cubic structure. The preferred
growth direction for the Cu
1.95
S NPs is the plane (111).
The mean crystallite size, D, was calculated for each annealing
temperature using the Debye Scherrer formula:
D=0.9λ
FWHMcos θ(5)
where λ is the Cu(K
α
) wavelength, FWHM is the full width at half
maximum of the most intense peak and θ is the Bragg angle. Table 1
displays the found results. It is shown that the Cu
2
S phase exhibits the
highest crystallinity, which is obtained by annealing at 250 ◦C. How-
ever, further increase in the annealing temperature leads to the forma-
tion of a less crystalline phase.
For annealing at temperature 150 ◦C, the deposited lm consists of
the covellite phase CuS. The percentage of Cu
2+
ions is greater than that
of the Cu
+
ions [19]. Consequently the following reaction dominates:
Cu2++S2−→CuS (6)
Once the annealing temperature increases to 250 ◦C, the corre-
sponding phase is the chalcocite Cu
2
S, where Cu
+
ions dominate over
N. Aghad et al.
Physica B: Condensed Matter 673 (2024) 415506
3
the Cu
2+
ones because of evaporation of sulfur [19]. The dominated
reaction is:
2Cu++S2−→Cu2S(7)
A further increase in the annealing temperature leads to a non-
stoichiometric phase, which is the djurleite Cu
1.95
S phase. In fact, the
sulfur atoms in the chalcocite phase approximately maintain their po-
sitions, whereas the copper ions move through the available space, so
that the distinction between interstitial and lattice ions is lost [20]. This
gives rise to a liquid-like ionic mobility. Consequently, the following
reaction occurs:
Cu2S→Cu1.95S+Cu0.05 (8)
The phase transition temperature depends on the preparation
method and the involved chemical elements as shown in Table 2. To
explain this dependence, the transition mechanism should be consid-
ered. The phase transition in an inert medium occurs by distortion of the
lattice as in the case of Ref. [21]. However, this mechanism is accom-
panied by another one for the synthesis or annealing in a non-inert
environment. In particular, oxygen (existing in the water, in the pre-
sent study) tears off sulfur atoms, resulting in the formation of SO
2
gas,
which accelerates the phase transition and explains the difference in the
transition temperatures. A similar study, conducted by Li et al. [22],
brought hydrogen gas together with argon, which resulted in a larger
loss of S atoms. On the other hand, the transition from the
cubic-chalcocite to the cubic-djurteite phase in the present study may be
mainly due to the similarity of the geometry of lattices and to the fact
that some copper atoms are mobile between the interstitial sites. Some
of them leave the crystal lattice and participate in the raction (8).
The covellite and djurleite phases are the TE ones as it is widely
recognized in the literature data. They were, therefore, selected for the
morphological study. Fig. 3-a shows the morphology for the annealing at
200 ◦C. The surface is found to be homogenous. It consists of grains and
grain boundaries with appearance of white spots. Each spot consists of a
phase with different chemical compositions. Fig. 3-b shows a granula-
tion of small grain uniformly dispersed. The size goes to the nanometric
size. The small size was expected because of the non-stoichiometric
composition at 300 ◦C, which exercises a further stress on grains.
The EDX spectra were taken to assess the chemical composition.
Fig. 4 shows the obtained spectra, with the detailed results shown in
Table 3. The composition patterns conrm the chemical elements of the
synthesized lms. The other elements such as silicon, oxygen, calcium,
magnesium are from the glass substrate. The division of atomic percent
of copper and sulfur gives the obtained stoichiometry in Table 3, which
matches the XRD results. 200 ◦C corresponds to the CuS phase and
300 ◦C corresponds to the Cu
1.95
S and 200 ◦C phase. On the other hand,
the EDX results show more effect of the substrate. Thus, the synthesized
lms are porous which is due to the formation of clusters. The porosity of
the synthesized lms was predicted, taking into account that the
Fig. 1. Schematic picture displaying the preparation process.
Fig. 2. XRD patterns of the spin coated Cu
x
S lms for various annealing tem-
peratures under nitrogen gas.
Table 1
Phase, preferred direction and crystallite size for each annealing temperature.
Phase T (◦C) Preferred direction Crystallite
Size (nm)
CuS Hexagonal 150 (006) 17.5
CuS Hexagonal 200 (006) 17.5
Cu
2
S Cubic 250 (200) 104.9
Cu
1.95
S Cubic 300 (200) 13.1
N. Aghad et al.
Physica B: Condensed Matter 673 (2024) 415506
4
annealing transforms the Cu
x
S nanoparticles into nanoplates [23].
3.2. Hall effect measurements
It has been well established by several authors (e.g. Ref. [24]) that
the digenite Cu
1.8
S phase is metallic, CuS is semi metal and chalcocite
Cu
2
S is a p-type semiconductor, while when the composition increases
from x =1.8 to x =2 the resistivity of the material increases. The p-type
conduction has been attributed to free holes from acceptor levels of
copper vacancies. The density of these vacancies decreases by increasing
x. In the present work, the higher resistivity corresponds to the smaller x
value. The resistivity corresponds to the semiconducting range
[10
−4
-10
3
] Ωcm as shown in Table 4. Moreover, the carrier concentra-
tions are around 10
21
cm
−3
, which means that the synthesized lms are
heavily doped semiconductors, which is suitable for TE applications.
3.3. Optical properties
The transmission of Cu
x
S thin lms annealed at 150 ◦C and 250 ◦C
varies in such a way that it becomes more and more remarkably peaked
in the visible range (specially in the range 400–700 nm) as shown in
Fig. 5-a, while a slight decrease in the IR range (700–2500 nm) is
observed, which was previously observed for copper sulphide thin lms
[4]. The two recorded peaks are similar in terms of the sharp rise and
decrease. However the transmittance of the sample annealed at 200 ◦C is
quite high than that of the sample annealed at 150 ◦C because of
evaporation of sulfur. By increasing the annealing temperature to 250
and 300 ◦C, the transmission increases with changing the copper per-
centage in the compound, as shown in Ref. [25], which conrms the
phase transition from Cu
2
S to Cu
1.95
S. Continuous evaporation of sulfur
leads to an increase in the transmission. A similar sharp increase and
decrease is observed for all samples, which can be explained in terms of
the single reectivity oscillations [1]. The absorption versus wavelength
is shown in Fig. 5-b. For short wavelengths, the high absorption was
recorded for the djurleite (Cu
1.95
S) phase (annealing at 300 ◦C), which is
followed by the absorption of CuS (150–200 ◦C) and Cu
2
S (250 ◦C),
respectively. This wavelength range corresponds to the inter-band
transition. For near-infrared wavelength, the high absorption was
recorded for the covellite phase (T =150–200 ◦C), which is widely re-
ported as a good absorbent [25]. Cu
2
S and Cu
1.95
S are shown to exhibit
low absorption. This absorption is probably due to the free carriers.
The theory of optical absorption gives the relation between the ab-
sorption coefcient,
α
, and the photon energy, h
ν
, for direct and indirect
transitions. The Tauc law is expressed by the formula:
α
h
ν
=A(h
ν
−Eg)n(9)
where n =1/2 for the direct and n =2 for the indirect transition. Here h
ν
is the photon energy and Eg the optical band gap. A is the constant which
is related to the effective masses associated with the valence and con-
duction bands. The absorption coefcient was calculated from the
following equation.
α
=1
dLn(100
Tr )(10)
where d is the thickness and Tr the transmitance. The experimental
values of (
α
h
ν
)
2
against h
ν
are plotted in Fig. 6. It is seen that the found
values (listed in Table 5) are comparable to these reported in the liter-
ature. In fact, Cu
x
S thin lm shows different values of the energy band
gap depending on the composition. CuS is an indirect band gap, which
were reported to be in the range 1.55–2.02 eV [26], whereas Cu
2
S is
known to exhibit narrow indirect gap, commonly less than 1.5 eV.
Finally the djurleite phase Cu
1.95
is commonly reported to exhibit a band
gap around 2 eV.
3.4. Thermoelectric properties
The Seebeck coefcient and the power factor values are shown in
Table 6. All samples exhibit positive S values in the whole measured
temperature range, suggesting that the main carriers are holes. Cu
x
S
lms exhibit a p-type conduction.
The seebeck coefcient increases with increasing the carrier
Table 2
Annealing temperature for the phase transition of copper sulphide compounds as taken from the literaure data.
Synthesis method
Annealing atmosphere Duration (h) Annealing temperature for the formation Ref.
Digenite phase Djurleite phase Chalcocite phase
Hot-injection N
2
1 250 Not found 300 [21]
Thermal evaporation N
2
1 350 400 Not found [14]
Sol gel N
2
1 400 Not found 300 [12]
Spin coating N
2
1 250 300 Not found This work
Fig. 3. SEM images for annealing at: (a) 200
◦C and (b): 300 ◦C.
N. Aghad et al.
Physica B: Condensed Matter 673 (2024) 415506
5
concentration, which seems to contradict the theoretical approach. The
underlying cause is the phase transition. Cu
x
S undergoes to variation in
the band structure as reported in Ref. [27]. Annealing at 150, 200 and
250 ◦C results in high carrier densities, which results in increasing the
electrical conductivity. However, the low carrier mobility results in low
electrical conductivity even with high densities. The highest power
factor corresponds, in this study, is for the sample annealed at 200 ◦C.
Many investigations have worked on Cu
2
S, Cu
1.97
S and Cu
1.8
S phases,
owing to the advantages of liquid-like copper ions for high TE
Fig. 4. EDX spectra for annealing at 200 and 300
◦C.
Table 3
EDX results for annealing at 200 and 300 ◦C
Elements Annealing at 200 ◦C Annealing at 300 ◦C
Cu 52 66
S 48 34
Cu/S ratio 1.08 1.9
Table 4
Carrier density and resistivity of Cu
x
S lms.
Annealing temperature (◦C) 150 200 250 300
phase CuS CuS Cu
2
S Cu
1.95
S
ρ
( ×10
−3
Ωcm) 3.6 3.5 5.6 42
Carrier Density ( ×10
21
cm
−3
) 1.4 1.1 1.3 0.4
Fig. 5. (a) Transmittace and (b) Absorption of the synthesized Cu
x
S lms.
Fig. 6. (ah
ν
)
1/2
versus h
ν
for various annealing temperatures.
N. Aghad et al.
Physica B: Condensed Matter 673 (2024) 415506
6
performances, including very strong phonon scattering and additional
reduction of specic heat due to the suppression of transverse phonon
modes.
The chalcocite Cu
2
S lm, annealed at 300 ◦C, exhibits the worst
electrical conductivity. This may be because the dominance of the
amorphous phase. On the other hand, the lms annealed at 150 ◦C and
200 ◦C exhibit the highest conductivities although the band gap of the
lm annealed at 250 ◦C is signicantly lower, with the biggest crystal-
lite. Indeed, the chemical formula, crystal symmetry and crystal defects
also play an important role in the conductivity of the crystal lattice. In
Ref. [14], the conductivity of the Cu
1.96
S chalcocite lm (with Eg =1.4
eV) was four times higher than that of the CuS covellite lm (Eg =1.55
eV). In the present case, the defects in the Cu
2
S lm may cause the
mobility of the carriers to be very low, which results in the poor elec-
trical conductivity.
The variation in the deposition conditions opens the door for re-
searchers to seek materials with suitable optical, electrical and TE
properties [28–31]. In the present case, the deposition of the djurleite
phase needs further optimizations in order to improve its TE properties.
4. Conclusion
Cu
x
S thin lms were successfully synthesized by the spin coating
method. Structural, optical and TE properties are observed to be tunable
by annealing. In fact, high absorbing lms were obtained with a band
gap energy ranging from 1.4 to 1.8 eV. Annealing is shown to result in
the phase transition into the chalcocite (Cu
2
S), then the djurleite
(Cu
1.95
S) phase. The biggest crystallite size was observed to be formed in
the Cu
2
S phase. The energy band gap reveals the formation of high
absorbing lms. The covellite phase presents a TE power factor (12.3
μ
WK
−2
m
−1
), which is the best. The electrical conductivity of the djur-
leite phase (12.3 Ω
−1
cm
−1
) has to be improved to match the Seebeck
coefcient towards high TE performances.
Credit author statment
N. Aghad: Software, A. Narjis: Supervision, Writing-Reviewing and
Editing, L. Amiri: Software, S. Elmassi: Editing, Ayman S. Alo:
Reviewing and Editing, L. Nkhaili: Methodology, A. Alsaad: Reviewing
and Editing, A. Tihane: Reviewing and Editing, Rachid Karmouch:
Reviewing and Editing, H. Albalawi: Reviewing and Editing, A. Out-
zourhit: Supervision.
Declaration of competing interest
The authors declare that they have no known competing nancia-
linterestsor personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
No data was used for the research described in the article.
Acknowledgements
Prof. Alsaad acknowledges Jordan University of Science and Tech-
nology (JUST) in Jordan, the Deanship of Scientic research, particu-
larly for the support [grant number 325/2021].
This research was funded by the Princess Nourah bint Abdulrahman
University Researchers Supporting Project number (PNURSP2023R29),
Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.
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Table 5
Band gap for various phases.
Annealing temperature (◦C) Phase Eg (eV)
150 Covellie CuS 1.7
200 Covellite CuS 1.7
250 Chalcocite Cu
2
S 1.4
300 Djurleite Cu
1.95
S 1.8
Table 6
TE properties of the synthesized Cu
x
S lms.
Annealing temperature (◦C) 150 200 250 300
Carrier density ( ×10
21
cm
−3
) 1.4 1.1 1.3 0.4
Seebeck coefcient (
μ
VK
−1
) 21.3 20.9 21.5 20.1
Conductivity Ω
−1
cm
−1
281.7 286.62 178.01 23.53
Mobility (cm
2
V
−1
s
−1
) 1.26 1.63 0.85 0.37
Power factor (
μ
WK
−2
m
−1
) 9.9 12.3 8.2 1
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