Silicon Based Thermoelectric Cells with Built in Solar Cells and CNT Composite Based Thermoelectric Cells for Measurement of Gradient of Temperature

Abstract

Thermoelectric cells and Solar cells are source of harvesting energy from heat and light respectively. In this study both are fabricated on a single cell using n-Si and p-Si single crystal strips.Later, temperature gradient cells are also fabricated . The fabricated Phtoto-thermoelectric cells and Temperature gradient celss are investigated, showing improvement in seebeck effect.

Full text

Silicon Based Thermoelectric Cells with Built in Solar Cells and CNT
Composite Based Thermoelectric Cells for Measurement of Gradient of
Temperature
Kh.S.Karimov
1,2,a
, Muhammad Abid
1,b
, Cheong Kuan Yew
3,c
, Nisar Ahmed
1,d
,
Muhammad Mehran Bashir
1,e*
, Zameer Abbas
1,f
1
GIK Institute of Engineering Sciences and Technology, Topi District Swabi, KPK 23640, Pakistan
2
Physical Technical Institute of Academy of Sciences, Rudaki Ave. 33, Dushanbe, 734025,
Tajikistan
3
School of Materials & Mineral Resources Engineering Engineering Campus, Universiti Sains
Malaysia14300 Nibong Tebal, Penang, Malaysia
a
Khasansangink@gmail.com,
b
abid@giki.edu.pk,
c
cheong@gmail.com,
d
nisarahmed@giki.edu.pk,
e
mehran@giki.edu.pk,
f
zameer@giki.edu.pk
Keywords: Photo-thermoelectric, Cell, Solar Cell, Thermoelectric cell, Built-in, Module, Non-
Tracking, Compound Parabolic Concentrator, Schottky Junction, Sensor, Gradient of Temperature,
Carbon Nano-Tubes, Seebeck Coefficient, Adhesive, Composites.
Abstract:
Thermoelectric cells and Solar cells are source of harvesting energy from heat and light
respectively. In this study both are fabricated on a single cell using n-Si and p-Si single crystal
strips.Later, temperature gradient cells are also fabricated . The fabricated Phtoto-thermoelectric
cells and Temperature gradient celss are investigated, showing improvement in seebeck effect.
1. Introduction
At present approximately 90 per cent of the world’s power is generated by heat engines. These
engines usually convert energy of fossil fuel into heat . The efficiency of these generators are in
the range of 30–40 percent . It is known that semiconductor thermoelectric cells working on the
base of Seebeck effect , are used for conversion of heat energy into electric and for cooling at
thermoelectric refrigerators [1]. Тhermoelectric efficiency (Z) is determined by the following
expression [2] :
Z = α
2
ϭ / k
tot
. (1)
where α is Seebeck coefficient, ϭ is conductivity and k
tot
= k
el
+ k
ph
is total thermal conductivity
that is equal to sum of the electron (k
el
) and phonon (k
ph
) thermal conductivities. The increase of the
efficiency of the thermoelectric generators depends , first of all, on decrease of phonon thermal
conductivity (k
ph
). In this way the layered chalcogenides with complex crystal structure are
investigated intensively. Last years on the thin layered of thickness of 11 µm thermoelectric cells
on the base of n-Si/SiGe- p-B4C/B9C deposited on the silicon substrate of thickness of 5 µm it
was observed high efficiency of 15% [3] .
At present a number of an integrated photo-thermoelectric generators were designed and
fabricated ,for example , for low power microelectronics on MEMS with micro lenses to achieve
high efficiency for the thermoelectric conversion [4].
It was observed a photothermoelectric effect at graphene interface field-effect transistors [5].
Potentially it can allow to design the graphene-based optoelectronics, such as photothermocouples
and photovoltaics. In [4,5] actually light energy was converted into heat energy that in turn created
difference in temperature between illuminated and non-illuminated parts of the sample that was the
Advanced Materials Research Vol. 1024 (2014) pp 385-392
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reason of generation of Seebeck voltage. In [6] it was fabricated grapheme based samples having
the structure of the field effect transistor with source, drain and gate.. Under effect of the laser light
it was observed photothermoelectric effect, that was sum of thermoelectric or Seebeck effect and
photovoltaic effect due to charges separation by the electric field of the applied to gate voltage. It
may allow to develop graphene-based optoelectronic devices [6].
The photo-thermoelectric generator integrating dye-sensitized solar cells (DSSC) with
thermoelectric modules was fabricated and investigated. Actually the sunlight illuminating the
solar cell produce electric power and at the same time heat the cell that reduce the photoelectric
conversion efficiency. This heat energy can be converted into electric power by thermoelectric
cells. For fabrication of the DSSC the nano-TiO2 powder was used. Commercial nano-Cu powder
was used as the medium with high thermal conductivity,that can effectively transfer heat produced
by sunlight in DSSC to the thermoelectric generator (TEG). Intensity of light illumination was
equal to 100 mW/cm
2
. It was observed that photoelectric conversion efficiency of DSSC was
equal to 4.83% and power output was equal to 4.83 mW/cm
2
.The thermoelectric conversion
efficiency of the thermoelectric modules was equal to 1.53%. The total output power was 4.97
mW/cm
2
, i.e. it increased on 2.87% output with respect of the use of DSSCs alone [7]. In this paper
we present the results of the investigations of the experimental model of the photo-thermoelectric
cells integrating solar cells with built-in thermoelectric cells based on n-Si and p-Si .
It is reasonable to investigate possibility to use CNT in the thermoelectric cells for measurement
of the temperature gradient due to relatively cheapness and availability of the CNTs. In this paper
the results of investigation of temperature gradient cells fabricated on the base of CNT composites
with polymer adhesive are presented.The composition of composite may be changed and carbon
nano-tubes (CNT),p-Si and silicone adhesive (Hero Gum) is used for the sake of comparison.
2. Experimental
Single crystal strips of planes of (100) of p-type and n-type samples with dopants concentrations
of 10
18
cm
-3
and 10
17
cm
-3
respectively . The sizes of strips were 3.5 cm and 2 cm in length and
width .On strip’s area of 2 cm x 2 cm it was fabricated Schottky junction solar cells (photo-voltaic
cell) by deposition of the thin films , 100 nm, of Cu (Fig.1(a)). Copper film comprised of the fingers
of 1 mm in width, the distances between fingers were equal to 0.5 mm (Fig.1(b)). The contacts to
the strip shaped photo-thermoelectric (PTE) cells were made by silver paste.
(a)
386 Advancement of Materials and Nanotechnology III

(b)
(c)
Figure 1: Side view (a) and top view (b) of single photo-thermoelectric cell (c) Schematic diagram
of the temperature gradient sensor based on CNT composite with polymer adhesive: glass substrate
(1), layer of composite (2), heater (3), thermocouples (4 &5)
Photo-thermoelectric module contained thermoelectric cells of n-Si and p-Si connected in series
(Fig.3). Non-tracking combined parabolic concentrator (CPC) with concentration ratio of 3.8,
designed in [8] was used to increase intensity of illumination of light source ( Fig.4). The CPC
played double role, first of all for concentration of light to the solar cells, secondly ,for shading of
thermoelectric cells from light source . As a light source the filament bulb of power of 100 W was
used. For measurement of the temperature and voltage it was used FLUKE 87. Currents were
measured by HIOKI 3256. Intensity of illumination was measured by KYOCERA JIM100.
Commercially produced (Sun Nanotech Co Ltd., China) CNTs (multi-walled carbon nanotubes
,MWNTs ) powder was used for fabrication of the composites . The diameter of MWNTs in the
powder particles varied between 10-30 nm. The temperature gradient cells were fabricated by the
following way. On the glass substrate it was deposited thin layer of the composite of CNT (50
wt.%) and polymer (GMSA) adhesive. The liquid adhesives are commercially available. The
length, width and thickness of the composites layers were equal to 45 mm ; 10 mm and 100 µm
respectively. For measurement of the temperature it was used thermocouples that were played the
role of electrodes as well as for measurement of voltage and current. The thermocouples were fixed
at the cell’s surface by the silver paste. Fig.1(c) shows schematic diagram of the temperature
gradient (∆T) sensor based on CNT composites with adhesive polymer.
For fabrication of the sensors commercially produced (Sun Nanotech Co Ltd., China) CNTs
powder was used . The diameter of multiwalled nanotubes (MWNTs) varied between 10-30 nm.
The silicon powder was obtained by milling of the p-Si crystal wafer with impurity (boron)
concentration of 10
16
cm
–3
. Average size of the particles in silicone powder was equal to 1.99
µm. that was found by size analyzer SA-CP 3 . As adhesive materials the Hero Gum (based on
silicone) and GMSA (based on organic polymer) were used. The composites were obtained by
Advanced Materials Research Vol. 1024 387

mixing of the components. As the substrates medical glasses were used that were cleaned by
methanole and dried before deposition of the blends.
Figure 2: Photo-thermoelectric module containing photo-thermoelectric cells of n-Si and p-Si
connected in series
Figure 3: Non-tracking combined parabolic concentrator (CPC) with concentration ratio of 3.8
integrated with photo-thermoelectric module
388 Advancement of Materials and Nanotechnology III

3. Results and discussion
Table 1 shows the results obtained at investigation of the properties of the photo-thermoelectric
cells on the base of n-Si and p-Si strips without of use of combined parabolic concentrator. Estimate
showed that in the case of use of the CPC in average the short circuit currents (Isc) and voltage of
the thermoelectric cells (Vtec) increase about of 3 times and voltage of the solar cells (Vsc)
increased on 1.3 times. Total open circuit voltage and short circuit current of the four photo-electric
cells in the module with CPC was equal to 3.7 mV and 0.6 µA respectively. As it is known
thermal conductivity of the bulk Si is high (150 W m
-1
K
-1
at room temperature) [9], that gives
figure of merit (ZT) <0.01 at 300 K [10]. Due to low ZT actually efficiency of the thermoelectric
cells fabricated on the base of bulky Si is much lower than 1%. In [11] it was investigated the
nanowires with an average diameter of approximately 100 nm. It was found that by using of
roughened Si nanowires, the thermal conductivity related to the phonons can be reduced to 1.6
Wm
-1
K
-1
, due to decrease of the mean free path lengths of phonons [11]. It can allows to increase
ZT up to 1 if diameter of nanowires will be decreased. This will make Si nanowires attractive for
applications in the thermoelectric generators.
Table 1: The properties of the photo-thermoelectric cells on the base of n-Si and p-Si.
Cell
Material
Intensity
(mW/cm
2
)
V
tec
(mV)
V
sc
(mV)
V
tot
(mV)
I
sc
(µA)
∆T
(
o
C)
Seebeck coefficient
(µV/
o
C)
n-Si 12 -0.2 -0.1 -0.3 0.2 1.3 -152
p-Si 12 0.2 0.4 0.6 0.2 1.2 167
Fig. 4 shows Open-circuit voltage and short-circuit current-temperature gradient relationships for
the CNT- adhesive composite while Fig.5 shows Seebeck coefficient-temperature gradient
relationship for the CNT- adhesive composite. It is seen that open-circuit voltage and short-circuit
current increased in 5 and 5.3 times, Seebeck coefficient decreased in 1.3 time as temperature
gradient increased in 7.5 times.
0 5 10 15 20 25 30 35 40
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Magnitude
∆T (oC)
I,
µ
A
V , mV
Figure 4: Open-circuit voltage (V) and short-circuit current (I)-temperature gradient (∆T)
relationships for the CNT-adhesive composite.
Advanced Materials Research Vol. 1024 389

0 5 10 15 20 25 30 35 40
30
35
40
45
Seebeck Coefficient
α
α
α
α
, (µ V / o C)
∆
T (o C)
Figure 5: Seebeck coefficient (α)-temperature gradient (∆T) relationship for the CNT- adhesive
composite.
20 40 60 80 100
230
235
240
245
250
255
260
265
Resistance
R , (Ω)
T (o C)
Figure 6: Resistance (R)-temperature (T) relationship for the CNT- adhesive composite.
Fig. 6 and Fig. 7 show resistance-temperature and Seebeck coefficient-temperature relationships
for the CNT-adhesive composite. It is seen that resistance and Seebeck coefficient decrease with
increase of temperature in 1.13 and 1.43 times respectively. At room temperature conditions
(T=25
o
C) Seebeck coefficient of the different samples were in the range from 40µV/
o
C to 45µV/
o
C.
It was stated that the limitation in the sizes of the particles , as it was observed in the case of nano-
particles, have led to the increase of efficiency of the thermoelectric cells fabricated on the base of
these materials in comparison with the ordinary materials of the same composition.
390 Advancement of Materials and Nanotechnology III

20 30 40 50 60 70
25
30
35
40
45
Seebeck Coefficient
α
(
µ
V/ oC)
T ( oC)
Figure 7: Seebeck coefficient (α)-temperature (T) relationship for the CNT- adhesive composi
4. Conclusion
The experimental model of the photo-thermoelectric integrating with built-in thermoelectric cells
based on n-Si and p-Si single crystal strips showed that both solar cells and thermoelectric cells
actually connected in series having good thermal and electric linkage, the total voltages of the
photo-thermoelectric cells was equal to sum of the solar cell’s and thermoelectric cell’s voltages . It
allows to increase total efficiency of the photo-thermoelectric cells.
The investigation of thin film sensor of temperature gradient based on composites of carbon nano-
tubes (CNT) and polymer adhesive showed that with increase of temperature gradient from 5
o
C
up to 38
o
C, the voltage and current of the sensor increases accordingly to 5 and 5.3 times while the
Seebeck coefficient decreases in 1.3 time. Resistance-temperature and Seebeck coefficient-
temperature behavior showed that the composite’s behavior concerned to the semiconductor
properties. Thermoelectric properties of the composite mostly depend on the properties of the
CNTs, while adhesive make the composite firm and the properties stable. The sensor can be used in
instrumentation for measurement of the temperature gradient.
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