Synergistic effect of In doping on electrical and thermal properties of Cu2SnSe3 thermoelectric system

Abstract

Cu 2 SnSe 3 has been considered as a potential thermoelectric material owing to its tunable transport properties and its phonon-glass-electron-crystal (PGEC) characteristics. Here, p -type pure and In-doped Cu 2 SnSe 3 samples are synthesized by the solid-state sintering technique. Cubic structure with $$F\overline{4}3m$$ F 4 ¯ 3 m space group is maintained for all the samples, and a linear increase in lattice parameter with increasing In concentration has been observed. The nature of electrical resistivity changes from semiconducting to metallic behavior for samples with x > 0.10. The decrease in both electrical resistivity and Seebeck coefficient with an increase in x is attributed to the increased hole concentration. Such a scenario is confirmed from the room-temperature Hall effect measurements. Indium doping also reduces the thermal conductivity of the Cu 2 SnSe 3 system as a result of increased phonon scattering due to the mass fluctuation. Concurrently, enhancement of thermoelectric power factor ( PF ) and figure of merit ( ZT ) is achieved with In doping at Sn site of Cu 2 SnSe 3 . The maximum ZT of 0.04 has been exhibited by the sample with x = 0.25 at 400 K, which is six times higher than that of the undoped Cu 2 SnSe 3 .

Full text

Synergistic effect of In doping on electrical and thermal
properties of Cu
2
SnSe
3
thermoelectric system
Riya Thomas
1
, Ashok Rao
1,
* , Zhao-Ze Jiang
2
, and Yung-Kang Kuo
2,
*
1
Department of Physics, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal 576104, India
2
Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan
Received: 14 October 2020
Accepted: 27 January 2021
ÓThe Author(s) 2021
ABSTRACT
Cu
2
SnSe
3
has been considered as a potential thermoelectric material owing to its
tunable transport properties and its phonon-glass-electron-crystal (PGEC)
characteristics. Here, p-type pure and In-doped Cu
2
SnSe
3
samples are synthe-
sized by the solid-state sintering technique. Cubic structure with F43mspace
group is maintained for all the samples, and a linear increase in lattice param-
eter with increasing In concentration has been observed. The nature of electrical
resistivity changes from semiconducting to metallic behavior for samples with
x[0.10. The decrease in both electrical resistivity and Seebeck coefficient with
an increase in xis attributed to the increased hole concentration. Such a scenario
is confirmed from the room-temperature Hall effect measurements. Indium
doping also reduces the thermal conductivity of the Cu
2
SnSe
3
system as a result
of increased phonon scattering due to the mass fluctuation. Concurrently,
enhancement of thermoelectric power factor (PF) and figure of merit (ZT)is
achieved with In doping at Sn site of Cu
2
SnSe
3
. The maximum ZT of 0.04 has
been exhibited by the sample with x= 0.25 at 400 K, which is six times higher
than that of the undoped Cu
2
SnSe
3
.
1 Introduction
The search for environment friendly and renewable
energy sources is on the rise and could hold the key
to combating issues like climate change, dwindling
fossil fuel reserves, and growing global energy
demands. Thermoelectric (TE) technology, which
directly interconverts electrical and thermal energy
based on the well-known thermoelectric effects
(Seebeck and Peltier effects), can be utilized as a
pollution-free means of power generation and
refrigeration [1–4]. However, the adoption of TE
devices is not widespread due to its insufficient
conversion efficiency. The performance of a TE
device is determined by the materials transport
properties, namely the electrical conductivity (r), the
Seebeck coefficient (S), and thermal conductivity (j),
where thermal conductivity is a sum of lattice and
electronic contributions (j
L
?j
e
). The efficiency of
TE materials at absolute temperature Tis gauged by
the dimensionless TE figure of merit, ZT ¼S2r
jT[1,2].
https://doi.org/10.1007/s10854-021-05402-x
J Mater Sci: Mater Electron
Address correspondence to E-mail: ashokanu_rao@rediffmail.com; ykkuo@gms.ndhu.edu.tw
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

The simultaneous enhancement of electrical conduc-
tivity and the Seebeck coefficient, along with the
reduction in thermal conductivity, is a difficult chal-
lenge as these physical quantities are correlated and
usually conflict with each other [2,3]. This hinders
the use of TE materials for commercial applications.
Thus, in the last few years, efforts have been made to
attain high ZT value by utilizing different classes of
materials like skutterudites, clathrates, complex
chalcogenides, Bi
2
Te
3
, half-Heuslers, etc., and by
employing various strategies like elemental doping,
composite engineering, alloying, and nanostructuring
[5–10].
The Cu-based multinary chalcogenides have
recently attained a lot of consideration as promising
TE materials because of their tunable transport
properties. Besides, this class of materials has the
advantages of low toxicity and high elemental
abundance and therefore are considered to be eco-
friendly and cost-effective candidates for TE appli-
cations [10–12]. Among the Cu-based thermoelectric
materials, the diamond-like ternary selenide, Cu
2-
SnSe
3
, belonging to the I
2
IVVI
3
family, is a p-type TE
material having a narrow direct bandgap of *0.84
eV [13]. Cu
2
SnSe
3
obeys the phonon-glass-electron-
crystal (PGEC) concept, which is usually exhibited by
the widely investigated caged compounds like
clathrates and skutterudites [1,12–17]. This concept
was proposed by Slack, which states that to achieve
high TE performance in a material, the glass-like
thermal conductivity and crystal-like electron trans-
port must co-exist.
Cu
2
SnSe
3
has been investigated from theoretical as
well as experimental aspects for its peculiar transport
properties [15,16]. In Cu
2
SnSe
3
, the highly distorted
crystal structure allows achieving a very low lattice
thermal conductivity. The Cu–Se bond in Cu
2
SnSe
3
is
known to form an electrically conductive framework;
thus, it dominates the p-type electrical transport of
Cu
2
SnSe
3
with little contribution from Sn. Shi et al.
suggested that the optimization of electrical proper-
ties can be achieved by doping different cations at the
Sn site with a lower valence electron number than Sn
[16]. Besides, substitution could also cause mass
fluctuations that could lead to a stronger phonon
scattering, and in turn, lower thermal conductivity. A
record ZT of 1.14 reported by Shi et al. [16]inCu
2-
Sn
0.9
In
0.1
Se
3
at 850 K is close to the values obtained
for the state-of-the-art TE materials like skutterudites
and commercial Bi
2
Te
3
-based compounds [17,18]. A
drop in thermal conductivity was observed by dop-
ing Ga at Sn site as a result of mass fluctuation.
Doping with Ga also led to the enhancement in car-
rier concentration and electrical conductivity; thus,
an improved ZT value of 0.43 at 700 K was attained
[19]. Zhang et al. investigated the effect of Fe doping
and reported a high ZT of 1.1 at *820 K for x= 0.05
and 0.1 as a result of enhanced power factor in the
Cu
2
In
1-x
Fe
x
Se
3
system [20]. Doping with Mn, Pb, and
Sb has also resulted in the enhancement of ZT
[21–23]. Li et al.synthesized (Ag, In)-co-doped Cu
2-
SnSe
3
and a maximum ZT of 1.42 in Cu
1.85
Ag
0.15-
Sn
0.9
In
0.1
Se
3
at 823 K was achieved [24]. The
stoichiometric tuning of Cu in Cu
2
SnSe
3
has also been
reported as an effective approach to attain a higher
ZT value [25].
In this study, the effect of doping on the TE
transport properties in the temperature range
10–400 K by employing In as a dopant at the Sn site
of Cu
2
SnSe
3
has been studied. Generally, grain
boundaries and point defects are the major scattering
factors for electrons and phonons at low tempera-
tures, while electron–phonon and phonon–phonon
scatterings become dominant at higher temperatures.
Therefore, the electrical and thermal transport mea-
surements can provide valuable information regard-
ing the various scattering mechanisms. Besides, the
detailed investigation of the TE properties of Cu
2
In
1-
x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) revealed a
significant increase in the power factor (PF =S
2
r)
together with a simultaneous decrease in thermal
conductivity. This led to an enhancement in ZT by six
times as compared to the pristine Cu
2
SnSe
3
.
2 Experimental
Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25)
samples were synthesized by the conventional solid-
state reaction route [26–29] followed by furnace sin-
tering. Commercial high-purity powders of Cu
(99.7%, Loba Chemie), Sn, In, and Se (99.999%, Alfa
Aesar) taken in the desired stoichiometric ratio were
ground well and sealed in evacuated (10
-3
Torr)
quartz tubes. These were then sintered in a muffle
furnace at 500 °C for 72 h and allowed to cool natu-
rally to room temperature.
The crystallinity and phase structure of the sam-
ples were analyzed by the X-ray diffraction (XRD)
using Rigaku Miniflex equipped with
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

monochromatic Cu-Karadiation. The binding ener-
gies of the constituent elements of Cu
2
In
1-x
Sn
x
Se
3
were investigated using X-ray photoelectron spec-
troscopy (XPS) of Thermo Scientific equipped with
monochromated X-ray source of Al Ka. The
microstructure of finely polished surfaces of the as-
synthesized samples was examined by field emission
scanning electron microscopy (FESEM) using a JEOL
JSM-7000F equipped with energy-dispersive X-ray
spectroscopy (EDS; INCA, Oxford Instruments)
which was utilized for the elemental analysis.
The measurement of electrical resistivity with
respect to temperature (10–400 K) for the as-prepared
samples was performed by employing the standard
four-probe technique in closed-cycle refrigeration
(CCR). The temperature-dependent Seebeck coeffi-
cient and thermal conductivity were determined
simultaneously by adopting a direct heat-pulse
technique. Further details of the measurement tech-
niques are given elsewhere [30]. The carrier concen-
tration and mobility of the samples were determined
by employing the van der Pauw method using Hall
Effect Measurement System (HMS-5500) with a
magnetic field of ±0.5 T at room temperature.
3 Results and discussion
3.1 Structural and compositional
characterization
The room-temperature XRD patterns of pristine and
In-doped Cu
2
SnSe
3
samples are shown in Fig. 1a. It is
observed that all the samples have maintained the
cubic structure with space group F43m;consistent
with earlier reports [20–23]. It is noticed that traces of
SnSe as impurity phase can be seen in samples with
x= 0, 0.05, 0.10, and 0.15 in Cu
2
In
1-x
Sn
x
Se
3
. The
peaks for the In-doped samples show a slight shift
towards the lower 2hside. This can be clearly
observed in the enlarged (111) peaks, as shown in the
inset of Fig. 1a, indicative of enlargement of the unit
cell with In doping. Further, the quantitative analysis
of the experimental XRD data was carried out by
employing the Rietveld refinement technique using
the FULLPROF program [31,32], and the fitting
parameters are given in Table 1. The observed (Y
obs
)
and calculated (Y
cal
) patterns, along with their dif-
ference, are depicted in Fig. 1b, and the simulated
patterns fit well with the obtained XRD data. The
deduced lattice parameter a(= b=c) shows a linear
increase with an increase in x(Fig. 1c), which is in
agreement with the Vegard’s law. This is due to the
larger ionic radius of In than Sn, which leads to the
expansion of the crystal lattice. Similar results were
also reported for In doping on the Sn site of Cu
2
SnS
3
[33].
Figure 2a presents the micrographs of FESEM
performed on the polished surfaces of Cu
2
In
1-x
Sn
x-
Se
3
for x= 0 and 0.25 samples. Minimal pores or
cracks are visible in the samples’ surface morphol-
ogy, which indicates the compactness and homo-
geneity of the samples. The morphology of all the
synthesized samples is similar with the size of the
clusters ranging from several hundred nanometers to
several micrometers, and these clusters do not exhibit
any obvious variation with increasing in In content.
The results of the EDS mapping carried out on
x= 0.25 sample given in Fig. 2b suggests that all the
elements (Cu, Sn, In, and Se) present are evenly dis-
tributed with no detectable phase segregation. The
elemental analysis suggests that the influence of the
secondary phase or uneven elemental distribution on
the thermoelectric properties of our studied samples
is negligible.
The valence states of Cu, Sn, In, and Se in Cu
2
In
1-
x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) are con-
firmed by employing X-ray photoelectron spec-
troscopy (XPS), and the resulting survey and core-
level spectra of the respective elements are presented
in Fig. 3a. The absence of any other peak except Cu,
Sn, Se, In, O, and C in Fig. 3a indicates the high
purity of the prepared samples. Figure 3b shows the
doublet peaks located at binding energies *932.1
and *951.9 eV with a standard separation of
19.8 eV. It is revealed from the literature that these
peaks correspond to Cu 2p
3/2
and 2p
1/2
, respectively,
confirming the presence of Cu
?
in the samples
[34,35]. No peak is seen around 942 eV, and this
excludes the presence of Cu
2?
in the samples. For Sn
3dspectrum in Fig. 3c, the two peaks at 486.3 (Sn 3d
5/
2
) and 494.7 eV (Sn 3d
3/2
) refer to the existence of
Sn
4?
. Figure 3d exhibits the core-level spectrum of In
in the doped samples, where the doublet states at
binding energies, 444.6 eV and 452.2 eV correspond
to In 3d
5/2
and 3d
3/2
of In
3?
, respectively [30,31]. The
asymmetric peak of Se 3dat *54.7 eV signifies Se
(Fig. 3e) with the valence state of -2. These results
obtained agree well with those available in the liter-
ature [34–37].
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

3.2 Thermoelectric transport properties
The variation of electrical resistivity with tempera-
ture q(T) in the range of 10–400 K for pure and In-
doped Cu
2
SnSe
3
samples is illustrated in Fig. 4a.
Several features in q(T) are observed with an increase
in In concentration. Firstly, the samples with x=0,
0.05, and 0.10 show a semiconducting behavior
where the electrical resistivity decreases with an
increase in temperature (dq/dT \0). With further
increase in In content, i.e., for x= 0.15, 0.20, and 0.25,
the nature of q(T) changes to heavily doped semi-
conductor or metallic behavior (dq/dT [0). Sec-
ondly, the overall value of q(T) decreases with an
increase in x, where the q(T)ofx= 0.25 is nearly two
orders of magnitude smaller than that of the pristine
sample. The room-temperature q(T) reduces from
0.0765 Xcm for x= 0 to 0.0343, 0.0196, 0.0024, 0.0018,
Fig. 1 aRoom-temperature XRD pattern, bRietveld refinement of Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) samples, and
clinearly varying lattice parameter as a function of x
Table 1 Crystal structure
parameters of Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20,
0.25) samples obtained from
refinement of XRD
xin Cu
2
In
1-x
Sn
x
Se
3
Crystal structure Lattice parameter a(A
˚)v
2
R
wp
%
0Cubic F43m5.6995 1.17 11.0
0.05 Cubic F43m5.7004 1.59 11.2
0.10 Cubic F43m5.7050 1.50 11.6
0.15 Cubic F43m5.7075 1.39 11.8
0.20 Cubic F43m5.7081 1.40 11.2
0.25 Cubic F43m5.7093 1.49 11.5
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

and 0.0010 Xcm for samples with x= 0.05, 0.10, 0.15,
0.20, and 0.25, respectively. More holes are expected
to be created with an increase in In concentration in
the samples that act as an electron acceptor due to the
substitution of ?3 valent In on the ?4 valent Sn site.
This results in an enhancement in hole concentration,
which in turn leads to the drop in electrical resistiv-
ity. Such a decreasing trend in electrical resistivity
with an increase in dopant concentration was also
reported in the literature [19–21,38]. The q(T)of
heavily doped samples (x= 0.15, 0.20, and 0.25)
shows a T*1.5 dependence (inset of Fig. 4a), indi-
cating the dominance of acoustic phonon scattering
in the highly degenerate state.
The observations of electrical resistivity are further
supported by room-temperature Hall measurements,
and the results are presented in Table 2. It can be
observed that the carrier concentration pis effectively
enhanced with In doping, reflecting the acceptor role
of the In atoms in the Cu
2
In
1-x
Sn
x
Se
3
system. The
overall decrease in carrier mobility in In-doped
samples as compared to the undoped can be attrib-
uted to the extra ionized impurity scattering and
enhanced carrier scattering. A similar trend in Hall
measurements is also observed for Ga and Zn-doped
Cu
2
SnSe
3
[19,38].
The plot of Seebeck coefficient S(T) vs. temperature
for the Cu
2
In
1-x
Sn
x
Se
3
series is illustrated in Fig. 4b.
It can be observed that all studied samples exhibit
Fig. 2 aFESEM micrographs
of x= 0 and 0.25 of
Cu
2
In
1-x
Sn
x
Se
3
samples,
bEDS mapping of
Cu
2
In
0.25
Sn
0.75
Se
3
for the
respective elements
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

positive values of S(T) for the whole temperature
range suggesting the p-type thermoelectric conduc-
tion for Cu
2
In
1-x
Sn
x
Se
3
, in agreement with the Hall
measurements. Besides, it is observed that S(T)
increases with increasing temperature. The almost
linear increase of S(T) for doped samples with tem-
perature indicates the heavily doped semiconducting
behavior and follows the trend as reported for doped
Cu
2
SnSe
3
[19–21]. However, S(T) decreases signifi-
cantly with the increase in x, and this is attributed to
the inverse proportionality of S(T) with respect to p.
For example, the room-temperature value of S(T)
decreases from *134 lVK
-1
for the pristine sample
to *36 lVK
-1
for the x= 0.25 sample. For heavily
doped semiconductors, this inverse relationship can
be described by Mott’s equation with an assumption
of a single parabolic band (SPB) model and is given
as [23,38]:
Fig. 3 aXPS survey spectrum of Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) samples. High-resolution spectra of bCu 2p,cSn
3d,dIn 3dand eSe 3din Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) samples
Fig. 4 aTemperature-dependent resistivity with insets showing T
1.5
dependency, bSeebeck coefficient of Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05,
0.10, 0.15, 0.20, 0.25) samples, and cS*p
-1/3
dependence of linear band approximation model
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

S¼8k2
Bp2
3eh2mp
3n

2=3Tð1Þ
where eis the elementary charge, k
B
is the Boltzmann
constant, his the Planck’s constant, and m
*
is the
effective mass of the electron. The features of ther-
moelectric conduction are elucidated by determining
the effective mass m
*
and Fermi energy of the sam-
ples at room temperature from the experimental S
and pvalues by using Eqs. (1) and (2), respectively
[23,39].
EF¼p2k2
BT
3eS ð2Þ
From the results presented in Table 2, it is noticed
that m
*
shows a clear increase with an increase in
dopant concentration, which is in accordance with
the decrease in hole mobility at elevated carrier
concentrations. The negative values of E
F
suggest that
the Fermi energy is below the valence band, and it
shifts into deeper levels with the increase in x.In
addition, the electrical properties of Cu
2
In
1-x
Sn
x
Se
3
are also examined by exploring the dependence of
Svs. pat 300 K (Fig. 4c). It can be observed that the
linear band dispersion model proposed by Ioffe [40]
gives a good fit for the experimental data, suggesting
that all the samples follow the relation of S*p
-1/3
.
This indicates that In doping at the Sn site has weak
influences on the band structure; instead, the elec-
trical transport in these samples is predominantly
governed by carrier concentration. This model was
also applied for Cu
2
Ga
1-x
Sn
x
Se
3
[19], filled skut-
terudites, and clathrates [41,42].
The temperature dependence of total thermal
conductivity j(T) for the pristine and doped samples
is depicted in Fig. 5a. The thermal conductivity of all
studied samples exhibits a similar trend with
temperature, which is the characteristic behavior of
crystalline solids [43]. It is seen that j(T) increases
sharply with increasing temperature, followed by a
distinct maximum (Umklapp peak) at *35 K.
Above this temperature, j(T) gradually decreases.
This behavior is attributed to the grain boundary
scattering at low temperatures and the Umklapp
process at high temperatures. Further inspection of
the thermal transport behavior is carried out by
evaluating j
e
from the Wiedemann–Franz law: j
e
(-
T)=L
0
T/q; where L
0
is the Lorenz number and qis
the electrical resistivity at temperature T. Kim et al.
have proposed a simplified assessment of L
0
for
Table 2 Carrier concentration p, hall mobility l, density of states
(DOS) effective mass m
*
/m
e
, and Fermi energy E
F
of
Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25)
xp(10
20
cm
-3
)l(cm
2
V
-1
s
-1
)m*/m
e
E
f
(eV)
0 0.160 5.106 0.423 -0.055
0.05 0.405 4.550 0.724 -0.059
0.10 0.753 4.235 1.034 -0.063
0.15 6.821 3.809 1.656 -0.171
0.20 9.432 3.626 1.862 -0.189
0.25 17.112 3.577 2.567 -0.203
Fig. 5 aTemperature-dependent total thermal conductivity and
blattice thermal conductivity with inset showing the electrical
contribution for Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20,
0.25) samples
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

semiconductors from S(T) data which is expressed in
Eq. (3)[9],
L0¼1:5þexp S
jj
116
 ð3Þ
Here L
0
is in the units of 10
–8
WXK
-2
, and Sis in
lVK
-1
. The electronic contribution of thermal con-
ductivity j
e
(T) is obtained using Eq. (3) and is shown
in the inset of Fig. 5b. The value of j
e
(T) for the
pristine sample is very low due to its high electrical
resistivity; however, j
e
(T) increases with increasing
In concentration. Since the measured j(T) is the sum
of electronic and lattice contributions, the lattice
thermal conductivity j
L
(T) can be obtained from
j
L
(T)=j(T)-j
e
(T). Figure 5b demonstrates the
temperature dependence of j
L
, and it is much larger
than j
e
, indicating that the lattice contribution dom-
inates heat conduction in the Cu
2
In
1-x
Sn
x
Se
3
system.
In addition, j
L
(T) decreases with increasing temper-
ature following roughly a T
-1
law at high tempera-
tures. This suggests that the phonon–phonon
Umklapp process is predominant, and no obvious
bipolar diffusion effect is involved in the total ther-
mal conductivity of Cu
2
In
1-x
Sn
x
Se
3
. Besides, it is
noticed that the lattice thermal conductivity decrea-
ses systematically with increasing In content. The
mass fluctuations induced by the In doping could
cause lattice distortions and create point defects,
which would lead to enhanced phonon scattering
and, in turn, lower the thermal conductivity.
3.3 Power factor and figure of merit
An important quantity for optimization of thermo-
electric materials, namely the power factor (PF), is
expressed as PF =S
2
/q. The temperature depen-
dence of PF is obtained by using the experimental
results and is shown in Fig. 6a. With increasing In
content, an enhancement in the value of PF for the
Cu
2
In
1-x
Sn
x
Se
3
series is achieved, mainly due to the
significant reduction of the electrical resistivity. The
highest PF of *185 lWm
-1
K
-2
for the x= 0.25
sample is attained, which is more than four times
larger than that of the pristine sample
(*41 lWm
-1
K
-2
) at 400 K.
Based on the measured electrical and thermal
transport parameters, the dimensionless figure of
merit was calculated as ZT ¼S2
qj T:The ZT as a
function of temperature for Cu
2
In
1-x
Sn
x
Se
3
(x=0,
0.05, 0.10, 0.15, 0.20, 0.25) is depicted in Fig. 6b. The
enhancement in PF, along with the reduction in j(T),
leads to an increase in ZT with the increase in x. More
than a factor of six enhancement in ZT is achieved in
comparison with the pristine sample, with the max-
imum ZT *0.04 for the x= 0.25 sample at 400 K.
Moreover, further improvement in ZT is expected at
higher temperatures ([400 K), which warrants fur-
ther investigation of high-temperature TE measure-
ments for these samples.
Fig. 6 aTemperature dependence of power factor and bZT of
Cu
2
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) samples
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

4 Conclusion
In this communication, In is selected as a dopant at
the Sn site of Cu
2
SnSe
3
and its effect on the structural
and thermoelectric properties is meticulously inves-
tigated in the temperature regime of 10–400 K. Cu
2-
In
1-x
Sn
x
Se
3
(x= 0, 0.05, 0.10, 0.15, 0.20, 0.25) samples
prepared by the solid-state reaction sintering tech-
nique adopt a cubic structure which is analyzed by
X-ray diffraction studies. The quantitative investiga-
tion of the XRD patterns by the Rietveld refinement
technique reveals the increase in lattice parameter
awith increasing In concentration, indicating the
successful substitution of Sn by In. The SEM–EDS
measurements were performed to study the mor-
phology of the samples. The SEM–EDS images reveal
the homogeneous distribution of the constituent ele-
ments in Cu
2
In
1-x
Sn
x
Se
3
with little porosity. The
valence states of the elements of all the prepared
samples are confirmed by XPS, and the results agree
well with the literature. From the analysis of the
measured Seebeck coefficient of the Cu
2
In
1-x
Sn
x
Se
3
series, we conclude that In acts as an acceptor dopant
and has a strong influence on the thermoelectric
properties of Cu
2
SnSe
3
. As the In content increases,
both the electrical resistivity and Seebeck coefficient
decrease. This is due to the increase in hole concen-
tration, as In has one less valence electron than that of
Sn. The enhancement in carrier concentration is
confirmed by Hall measurements at room tempera-
ture. The positive values of S(T) and the Hall coeffi-
cient indicate the p-type conduction in all the studied
samples. The increase in electrical conductivity with
In doping leads to a significant increase in PF. The
mass fluctuations introduced by In doping increases
the phonon scattering, which in turn reduces the
thermal conductivity. The maximum ZT obtained for
the x= 0.25 sample is about six times higher than that
of the pristine sample. Moreover, the ZT of the Cu
2-
In
1-x
Sn
x
Se
3
system might be further improved by a
more effective reduction in j(T) by using a nanos-
tructuring approach.
Acknowledgements
This research was financially supported by the
Council of Scientific and Industrial Research Grant
(sanction no.: 03(1409)/17/E MR-II), Department of
Science and Technology, India, DST-FIST Grant (SR/
FIST/PS-1/2017/8), and the Ministry of Science and
Technology of Taiwan under Grant Nos. MOST-106-
2112-M-312 259-002-MY3 and MOST 107-2112-M-259-
004 (YKK).
Funding
Open access funding provided by Manipal Academy
of Higher Education, Manipal.
Open Access This article is licensed under a Crea-
tive Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution
and reproduction in any medium or format, as long
as you give appropriate credit to the original
author(s) and the source, provide a link to the Crea-
tive Commons licence, and indicate if changes were
made. The images or other third party material in this
article are included in the article’s Creative Commons
licence, unless indicated otherwise in a credit line to
the material. If material is not included in the article’s
Creative Commons licence and your intended use is
not permitted by statutory regulation or exceeds the
permitted use, you will need to obtain permission
directly from the copyright holder. To view a copy of
this licence, visit http://creativecommons.org/licen
ses/by/4.0/.
References
1. D.M. Rowe (ed.), CRC Handbook of Thermoelectrics (CRC
Press, Boca Raton, 1995)
2. G.J. Snyder, E.S. Toberer, Nat. Mater. 7, 105 (2008)
3. X.F. Zheng, C.X. Liu, Y.Y. Yan, Q. Wang, Renew. Sustain.
Energy Rev. 32, 486 (2014)
4. G.D. Mahan, Appl. Mater. 4, 104806 (2016)
5. C. Gayner, K.K. Kar, Prog. Mater. Sci. 83, 330 (2016)
6. G. Chen, M.S. Dresselhaus, G. Dresselhaus, J.-P. Fleurial, T.
Caillat, Int. Mater. Rev. 48, 45 (2003)
7. W. Liu, J. Hu, S. Zhang, M. Deng, C. Han, Y. Liu, Mater.
Today Phys. 1, 50 (2017)
8. Z.-G. Chen, G. Han, L. Yang, L. Cheng, J. Zou, Prog. Nat.
Sci. Mater. Int. 22, 535 (2012)
9. W. Kim, J. Mater. Chem. C 3, 10336 (2015)
10. P. Qiu, X. Shi, L. Chen, Energy Storage Mater. 3, 85 (2016)
11. E.J. Skoug, J.D. Cain, D.T. Morelli, J. Electron. Mater. 41,
1232 (2012)
12. D.T. Morelli, E.J. Skoug, MRS Proc. 1166, 157 (2009)
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

13. G. Marcano, C. Rinco´ n, L.M. De Chalbaud, D.B. Bracho,
G.S. Pe´ rez, J. Appl. Phys. 90, 1847 (2001)
14. T.-R. Wei, Y. Qin, T. Deng, Q. Song, B. Jiang, R. Liu, P. Qiu,
X. Shi, L. Chen, Sci. China Mater. 62, 8 (2019)
15. L. Xi, Y.B. Zhang, X.Y. Shi, J. Yang, X. Shi, L.D. Chen, W.
Zhang, J. Yang, D.J. Singh, Phys. Rev. B 86, 155201 (2012)
16. X. Shi, L. Xi, J. Fan, W. Zhang, L. Chen, Chem. Mater. 22,
6029 (2010)
17. G.S. Nolas, D.T. Morelli, T.M. Tritt, Annu. Rev. Mater. Sci.
29, 89 (1999)
18. O. Yamashita, S. Tomiyoshi, K. Makita, J. Appl. Phys. 93,
368 (2003)
19. J. Fan, H. Liu, X. Shi, S. Bai, X. Shi, L. Chen, Acta Mater.
61, 4297 (2013)
20. J. Zhang, L.L. Huang, X.G. Zhu, Z.M. Wang, C.J. Song, H.X.
Xin, D. Li, X.Y. Qin, Scr. Mater. 159, 46 (2019)
21. X. Lu, D.T. Morelli, J. Electron. Mater. 41, 1554 (2012)
22. S. Prasad, A. Rao, B. Gahtori, S. Bathula, A. Dhar, C.-C.
Chang, Y.-K. Kuo, Phys. B 520, 7 (2017)
23. K.S. Prasad, A. Rao, N.S. Chauhan, R. Bhardwaj, A. Vish-
wakarma, K. Tyagi, Appl. Phys. A 124, 98 (2018)
24. Y. Li, G. Liu, T. Cao, L. Liu, J. Li, K. Chen, L. Li, Y. Han, M.
Zhou, Adv. Funct. Mater. 26, 6025 (2016)
25. S. Prasad, A. Rao, B. Christopher, R. Bhardwaj, N.S. Chau-
han, S.A. Malik, N. Van Nong, B.S. Nagaraja, R. Thomas, J.
Alloys Compd. 748, 273 (2018)
26. K. Shan, Z.-Z. Yi, X.-T. Yin, D. Dastan, F. Altaf, H.
Garmestani, F.M. Alamgir, Surf. Interfaces 21, 100762 (2020)
27. W. Hu, T. Li, X. Liu, D. Dastan, K. Ji, P. Zhao, J. Alloys
Compd. 818, 152933 (2020)
28. K. Shan, Z.-Z. Yi, X.-T. Yin, D. Dastan, H. Garmestani, Dalt.
Trans. 49, 8549 (2020)
29. K. Shan, Z.Z. Yi, X.T. Yin, D. Dastan, H. Garmestani, Dalt.
Trans. 49, 6682 (2020)
30. Y.-K. Kuo, B. Ramachandran, C.-S. Lue, Front. Chem. 2,1
(2014)
31. K. Shan, Z.Z. Yi, X.T. Yin, L. Cui, D. Dastan, H. Garmestani,
F.M. Alamgir, J. Alloys Compd. 855, 157465 (2021)
32. K. Shan, Z.Z. Yi, X.T. Yin, D. Dastan, S. Dadkhah, B.T.
Coates, H. Garmestani, Adv. Powder Technol. 31, 4657
(2020).
33. Q. Tan, W. Sun, Z. Li, J.-F. Li, J. Alloys Compd. 672, 558
(2016)
34. J.M. Song, Y. Liu, H.L. Niu, C.J. Mao, L.J. Cheng, S.Y.
Zhang, Y.H. Shen, J. Alloys Compd. 581, 646 (2013)
35. S. Li, D. Pan, J. Cryst. Growth. 358, 38 (2012)
36. V.B. Ghanwat, S.S. Mali, C.S. Bagade, R.M. Mane, C.K.
Hong, P.N. Bhosale, Energy Technol. 4, 835 (2016)
37. J. Lei, W. Guan, D. Zhang, Z. Ma, X. Yang, C. Wang, Y.
Wang, Appl. Surf. Sci. 473, 985 (2019)
38. C. Raju, M. Falmbigl, P. Rogl, P. Heinrich, E. Royanian, E.
Bauer, R.C. Mallik, Mater. Chem. Phys. 147, 1022 (2014)
39. Y. Shen, C. Li, R. Huang, R. Tian, Y. Ye, L. Pan, K. Kou-
moto, R. Zhang, C. Wan, Y. Wang, Sci. Rep. 6, 32501 (2016)
40. A.F. Ioffe, Semiconductor Thermoelements and Thermoelec-
tric Cooling (Infosearch Limited, London, 1957).
41. T. Caillat, A. Borshchevsky, J.P. Fleurial, J. Appl. Phys. 80,
4442 (1996)
42. D.J. Singh, W.E. Pickett, Skutterudite antimonides: quasilin-
ear bands and unusual transport. Phys. Rev. B 50, 11235
(1994)
43. T.M. Tritt, Thermal Conductivity: Theory, Properties, and
Applications (Kluwer Academic, New York, 2004).
Publisher’s Note Springer Nature remains neutral with
regard to jurisdictional claims in published maps and
institutional affiliations.
J Mater Sci: Mater Electron
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

1.
2.
3.
4.
5.
6.
Terms and Conditions
Springer Nature journal content, brought to you courtesy of Springer Nature Customer Service Center GmbH (“Springer Nature”).
Springer Nature supports a reasonable amount of sharing of research papers by authors, subscribers and authorised users (“Users”), for small-
scale personal, non-commercial use provided that all copyright, trade and service marks and other proprietary notices are maintained. By
accessing, sharing, receiving or otherwise using the Springer Nature journal content you agree to these terms of use (“Terms”). For these
purposes, Springer Nature considers academic use (by researchers and students) to be non-commercial.
These Terms are supplementary and will apply in addition to any applicable website terms and conditions, a relevant site licence or a personal
subscription. These Terms will prevail over any conflict or ambiguity with regards to the relevant terms, a site licence or a personal subscription
(to the extent of the conflict or ambiguity only). For Creative Commons-licensed articles, the terms of the Creative Commons license used will
apply.
We collect and use personal data to provide access to the Springer Nature journal content. We may also use these personal data internally within
ResearchGate and Springer Nature and as agreed share it, in an anonymised way, for purposes of tracking, analysis and reporting. We will not
otherwise disclose your personal data outside the ResearchGate or the Springer Nature group of companies unless we have your permission as
detailed in the Privacy Policy.
While Users may use the Springer Nature journal content for small scale, personal non-commercial use, it is important to note that Users may
not:
use such content for the purpose of providing other users with access on a regular or large scale basis or as a means to circumvent access
control;
use such content where to do so would be considered a criminal or statutory offence in any jurisdiction, or gives rise to civil liability, or is
otherwise unlawful;
falsely or misleadingly imply or suggest endorsement, approval , sponsorship, or association unless explicitly agreed to by Springer Nature in
writing;
use bots or other automated methods to access the content or redirect messages
override any security feature or exclusionary protocol; or
share the content in order to create substitute for Springer Nature products or services or a systematic database of Springer Nature journal
content.
In line with the restriction against commercial use, Springer Nature does not permit the creation of a product or service that creates revenue,
royalties, rent or income from our content or its inclusion as part of a paid for service or for other commercial gain. Springer Nature journal
content cannot be used for inter-library loans and librarians may not upload Springer Nature journal content on a large scale into their, or any
other, institutional repository.
These terms of use are reviewed regularly and may be amended at any time. Springer Nature is not obligated to publish any information or
content on this website and may remove it or features or functionality at our sole discretion, at any time with or without notice. Springer Nature
may revoke this licence to you at any time and remove access to any copies of the Springer Nature journal content which have been saved.
To the fullest extent permitted by law, Springer Nature makes no warranties, representations or guarantees to Users, either express or implied
with respect to the Springer nature journal content and all parties disclaim and waive any implied warranties or warranties imposed by law,
including merchantability or fitness for any particular purpose.
Please note that these rights do not automatically extend to content, data or other material published by Springer Nature that may be licensed
from third parties.
If you would like to use or distribute our Springer Nature journal content to a wider audience or on a regular basis or in any other manner not
expressly permitted by these Terms, please contact Springer Nature at
onlineservice@springernature.com