~
Chapter 7
Photoelectrochemical Studies of GeSxSe1-x (x= 0, 0.25, 0.5, 0.75, 1)
Single Crystals
Chapter -7: Photoel ectrochemical Studies 262
7.1 INTRODUCTION
So far in all the previous chapters, the growth and characterizations of
GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) has been discussed in detail. From all the
above investigations it is quite apparent that GeSxSe1-x possesses the semiconducting
behaviour and a band gap around 1.2 – 2.0 eV. Generally, it is observed that the solar
radiation reaching earth shows the maxima around 1.5 eV. Therefore if any device to
be used has solar radiations as the input quantity, then the highest efficiency can be
achieved provided the band gap of semiconductors lies near this maxima. In this
regards, it can be said from the optical characterization of GeSxSe1-x (I2) (x=0, 0.25,
0.5, 0.75, 1) single crystals that it has an appropriate value of a direct band gap which
lies close to the maxima of incident solar radiations. So the construction of a solar
cells and its performance evaluation has been carried out for all these crystals.
The twenty-first century has been predicted to be the "age of light," and, in
anticipation of this, we have been interested in light-related chemical phenomena, that
is, using light to induce chemical and electrochemical reactions. We have focused our
main attention on reactions that might be useful for maintaining our environment,
including hydrogen production, carbon dioxide reduction, and the destruction of
pollutants. This article will focus principally on hydrogen production, due to the
increasing interest in hydrogen as a clean energy storage medium.
The total amount of solar energy impinging on the earth’s surface in one year
is about 3 1024 J, or approximately 104 times the worldwide yearly consumption of
energy. The search for the efficient conversion of solar energy into other useful forms
is, in view of the increasing anxiety over the exhaustion of fossil energy resources and
attendant global warming, one of the most important challenges for future research
and technology development.
In systems designed for the purpose of converting solar energy into electricity
and/or chemicals, two principal criteria must be met. The first is absorption, by some
chemical substance, of solar illumination, leading to the creation of electrons and
holes. The second is the effective separation of these electron–hole pairs with little
energetic loss, before they lose their input energy through recombination.
Chapter -7: Photoel ectrochemical Studies 263
Another well-known example is the solar photovoltaic (PV) cell, in which the
photogenerated electron–hole pairs are driven efficiently in opposite directions by an
electric field existing at the boundary between n- and p-type semiconductors or at that
between a semiconductor and a metal (Schottky junction). A potential gradient can
also be created at the interface between a semiconducting material and a liquid
electrolyte. Hence, if a semiconductor is used as an electrode that is connected to
another (counter) electrode, photoexcitation of the semiconductor can generate
electrical work through an external load and simultaneously drive chemical (redox)
reactions on the surfaces of each electrode. Similarly, when semiconductor particles
are suspended in a liquid solution, excitation of the semiconductor can lead to redox
processes in the interfacial region around each particle, but no electrical work is done,
because the oxidation and reduction reactions are short-circuited. These types of
systems have drawn the attention of a large number of investigators over the past
twenty years, primarily in connection with the conversion of solar energy to electrical
energy and chemically stored energy [1].
A survey of chalcogenides of the cheaply available materials suggests that
among the low band gap semiconductors [2], little attention has been focused on
chalcogenides of the fourth group metals germanium and tin [GeS, GeSe, SnS, SnSe]
for their use in the conversion of solar energy to electrical energy or chemical energy
via photoelectrochemical (PEC) cells. Photo-activity of polycrystalline samples of p-
SnSe [3] and of p-SnS polycrystalline thin films [4] have been evaluated using
photoelectrochemical techniques. In all cases the electron hole separation and the
electron transfer kinetics were found to be low, most probably due to the small size of
the crystallites. To overcome these difficulties created by the presence of the grain
boundaries which act as recombination centres, one should investigate the photo
electrochemistry of single crystals when they are available. In this context, Nagard et
al [5] carried out a detailed photo electrochemical characterisation of p- GeSe single
crystals grown by the chemical vapour transport (CVT) technique using I2 as the
transporting agent.
7.2 PRIMARY COMPONENTS OF PEC SOLAR CELL
Photoelectrochemical cells are solar cells and extract electrical energy from
light, including visible light. Each cell consists of a semiconducting photo anode and
Chapter -7: Photoel ectrochemical Studies 264
a metal cathode immersed in an electrolyte. A typical PEC has three primary
components.
Semiconductor electrode
Counter electrode
Electrolyte
7.2.1 Preparation of semiconductor electrode
A glass rod of 0.5 cm in diameter and 10 to 12 cm in length with a narrow
bore of diameter 0.05 cm was used to prepare the electrode. One end of this narrow
bore glass rod was flattened by hot gas blow. The flat portion was used as a platform
for resting the crystal. The narrow bore was used as a passage for traversing a good
conducting copper wire. The copper wire was flattened at one end for getting a
contact with the crystal.
In the present work, a semiconductor electrode was fabricated in such a way
that the contacting material (adhesive silver paste) provided good ohmic contact
between the copper wire and the backside of the crystal. The whole assembly was
then kept in an oven for few hours at 100 ºC for baking. After proper setting of the
crystal on the copper wire terminal, the semiconductor was covered with an epoxy
resin (araldite) leaving a light exposed an area of 2-5 mm2 for exposure to light
source. The so prepared complete device semiconductor electrode is shown in Figure
7.1.
7.2.2 Counter electrode
A counter electrode in PEC solar cells is required to complete the
electrochemical reactions in a cell for better performance of PEC solar cell. Generally
Platinum or graphite is widely used material for the same. Many materials have been
investigated electrochemically as counter electrode materials, by Allen and Hickling
[6]. Platinum is the standard counter electrode for PEC systems but its widespread use
is impractical due to high cost and limited supplies. We can also use copper grid,
tungsten Carbide etc. In present investigations, copper grid has been used in place of
platinum as the counter electrode.
Chapter -7: Photoel ectrochemical Studies 265
Figure 7.1: The Semiconductor electrode.
7.2.3 Selection of appropriate electrolyte
The selection of electrolyte in a PEC solar cell is extremely important because
it actually is a source for the electrochemical reactions leading to the photo-effects.
The electrolyte consists of the oxidized and reduced species. These species should be
ionic in nature, which help in transfer of photo-generated carrier from the photo-
electrode to the counter electrode. To obtain a workable photoconversion from PEC
solar cell, the selection of suitable electrolyte is very important. The electrolyte
decides the band bending in the semiconductor near the interface and hence the
efficiency of photoconversion.
Among all electrolytes listed in Table 7.1, it was observed that electrolyte with
the composition 0.025 MI2 + 0.5 MNaI+ 0.5 M Na2SO4 gave the minimum dark
voltage ‘VD’ and dark current ‘ID’ and as well provided the maximum value of
photocurrent (Iph) and photovoltage (Vph) for the electrodes which are used to fabricate
PEC solar cell in present investigations.
In this case, a mixture of iodine (I2), sodium iodide (NaI) and sodium sulphate
(Na2SO4) was employed as an electrolyte. All the chemical products were of reagent
grade and the electrolyte solutions were prepared using triple distilled water. The
solutions were not stirred during the measurement. Here photoelectrodes have been
Araldite
Sample crystal
Glass Rod
Copper Wire
Chapter -7: Photoel ectrochemical Studies 266
prepared using GeSxSe1-x (I2) (0, 0.25, 0.5, 0.75, 1) single crystals having visibly
smooth surfaces.
Table 7.1: lists of prepared electrolytes for present work.
1 0.025MI2 + 2MKI + 0.5MNa2SO4 +0.5MH2SO4
2 0.025MI2 + 2MKI + 0.5MNa2SO4
3 0.01MI2 + 2MKI + 0.5MNa2SO4
4 0.025MI2 + 2MKI
5 0.025MI2 + 1MKI + 0.5MNa2SO4 +0.5MH2SO4
6 0.025MI2 + 2MNaI + 0.5MNa2SO4
7 0.025MI2 + 0.5MNaI + 0.5MNa2SO4
8 0.025MI2 + 2MNaI + 2MNa2SO4 +0.5MH2SO4
9 0.025MI2 + 1MKI + 2MNa2SO4 +0.5MH2SO4
10 0.025MI2 + 1MNaI + 2MNa2SO4 +0.5MH2SO4
11 0.025MI2 + 2MNaI
12 0.1MK4[Fe(CN)6] + 0.1MK3[Fe(CN)6]
13 1MK4[Fe(CN)6] + 0.1MK3[Fe(CN)6]
14 0.1MK4[Fe(CN)6] + 1MK3[Fe(CN)6]
15 0.1MK4[Fe(CN)6] + 1MK3[Fe(CN)6]
16 0.1MFeCl3 + 0.1MFeCl2
17 0.05MFeCl3 + 0.1MFeCl2
18 0.01MFeCl3 + 0.1MFeCl2
19 1MFeCl3 + 0.1MFeCl2
7.3 EXPERIMENTAL SET OF PHOTOELECTROCHEMICAL SOLAR
CELL FOR V-I CHARACTERISTIC
The semiconductor electrode prepared in the manner outlined above was
immersed in an appropriate electrolyte contained in a corning glass beaker. A copper
grid (3 cm 3 cm) was used as the counter electrode. A schematic diagram of the
photoelectrochemical solar cell is shown in Figure 7.2.
The cell was illuminated with light from a Xenon lamp from different
intensities. The intensity of illumination was altered by changing the distance between
the light source and the electrode.
Chapter -7: Photoel ectrochemical Studies 267
Figure 7.2: The schematic diagram of PEC solar cell used to measure V-I characteristic.
The incident intensity of illumination was measured using ‘Suryampi’ or Solar
meter (TES electrical electronic corporation TES 1332A). Photocurrent and
photovoltage were recorded using digital multimeters (Protek, 506 & RISH
multimeter, 18S) with accuracy of 0.1 mV/µA. To vary the power point on the V-I
characteristics, a series of variable resistance of different values has been used.
In ideal cases and practical cases, the V-I characteristics of PEC solar cell is
shown in Figure 7.3. The V- I characteristics of practical cases largely deviate from
ideal characteristics.
7.4 CHARACTERISTIC PARAMETERS OF PEC SOLAR CELLS
There are various parameters available from which we can judge or evaluate
the performance of PEC solar cell [7]. The most general parameters, which are used in
even day to day life for deciding the quality of the PEC solar cells are efficiency,
current and voltage specifications. Besides these, there are some other parameters
which one must study in detail to improve the performance of such cells.
SemiconductorElectrode
Container
Electrolyte
Counter Electrode
Voltmeter
Ammeter
Potentiometer
Chapter -7: Photoel ectrochemical Studies 268
Figure 7.3: Ideal and Practical I-V characteristic of solar cell.
Some important parameters which have been used in present investigation for
characterization of GeSxSe1-x (0, 0.25, 0.5, 0.75, 1) single crystal based PEC solar cell
are given below.
Short circuit current (Isc)
Open circuit voltage (Voc)
Photoconversion efficiency (η %)
Fill factor (F.F.)
Quantum efficiency (q)
7.4.1 Short circuit current (Isc)
When a PEC solar cell is illuminated by the polychromatic radiations, the
electron – hole pairs are generated in the semiconductor which take part in the
electrochemical reactions ( oxidation or reduction) consequently leading to the flow
of current in the external circuit. The current measured directly across the electrode
under zero load condition is called short circuit current. Mathematically the short
circuit current can be expressed as;
0 exp 1ocsc
eVI I
kT
(7.1)
Photo Voltage
Ph
oto
Cu
rren
t
Imp
Vmp
Pmp (maximum Power)
Practical Characteristics
Ideal Characteristics
Chapter -7: Photoel ectrochemical Studies 269
where, I0 = Reverse saturation current
k = Boltzmann constant
T = Operating temperature (Room temperature)
Voc = Open circuit voltage
This parameter depends on the band gap of the semiconductor; smaller the band gap
greater is the expected short circuit current.
7.4.2 Open circuit voltage (Voc)
It is very important to know the maximum voltage, which is obtained from
these PEC cells. The voltage measured across the working electrodes and the counter
electrodes of a PEC solar cell under open circuit conditions means infinite load is
known as the open circuit voltage. The mathematical representation of open circuit
voltage is given below;
lnoc L
nkTV I
e
(7.2)
where, n is the ideality factor
kT
e= 0.0259 volt (at 300K)
IL= Intensity of illumination
7.4.3 Photoconversion efficiency (η %)
This parameter is the most important characterizing parameter of any solar
cell. This parameter is defined as the ratio of electrical power generated and the
optical indent on the cell. In case of PEC solar cells this parameter can be
experimentally calculated using equation;
mp mp
L
V J
I
(7.3)
where, mp
mp
IJ
Area
IL = intensity of incident illumination
Jmp= current density at maximum power point
Chapter -7: Photoel ectrochemical Studies 270
Vmp= voltage at maximum power point
7.4.4 Fill factor (F.F.)
The normal photovoltage – photocurrent characteristic of a solar cell is shown
in Figure 7.3. From this it can be seen that on the photovoltage and photocurrent
becomes maximum. From this figure it has been seen that this maximum power point
is always less for practical solar cells as compared to the ideal conditions. Fill factor is
a parameter giving an idea about the deviation of the practical photovoltage-
photocurrent characteristics of a solar cell. It can be expressed as;
. .mp mp
sc oc
J VF F
J V
(7.4)
where, Jsc = the short circuit current density
Jmp = the current density at maximum power point and
Vmp = the voltage at maximum power point.
Practically, the value of this parameter is always less than 1. Thus, by evaluating this
parameter, we can predict how ideal the behavior of a solar cell is.
7.4.5 Quantum efficiency
The quantum efficiency ( q ) is defined as;
q Number of photo-generated electrons/ unit area (7.5)
Number of incident photons/ unit area
This parameter can be evaluated while investigating the behaviour of PEC
solar cells under the illumination of monochromatic radiations. It is quite clear that
the value of quantum efficiency in ideal condition is 100% but practically this value is
found to be comparatively low.
7.5 PHOTOCONVERSION CHARACTERISTIC OF GeSxSe1-x (I2) (x= 0,
0.25, 0.5, 0.75, 1) PEC SOLAR CELLS
The photo-electrode fabricated using GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1)
single crystals have been used as working semiconductor electrodes for the absorption
of incident radiations. The electrolyte having concentration [0.025MI2 + 0.5M NaI +
Chapter -7: Photoel ectrochemical Studies 271
0.5 M Na2SO4] have been used as the ionic conduction medium to support the charge
transfer mechanism for PEC solar cells. Copper wire used as a counter electrode. The
Xenon lamp has been used as a source of polychromatic light for the investigation of
the photoconversion characteristic of GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) based
PEC solar cells.
Figure 7.4 (a): Photovoltage (Vph) vs. Current Density (J) for GeSe (I2) crystal under different levels of illumination.
0
50
100
150
200
250
300
0 20 40 60 80 100 120 140 160
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
110 mW/cm290 mW/cm270 mW/cm250 mW/cm230 mW/cm210 mW/cm2
GeSe(I2)
0
50
100
150
200
250
300
0 20 40 60 80 100 120 140 160 180 200
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
120 mW/cm2100 mW/cm280 mW/cm260 mW/cm240 mW/cm220 mW/cm2
GeS e (I2)
Chapter -7: Photoel ectrochemical Studies 272
Figure 7.4 (b): Photovoltage (Vph) vs. Current Density (J) for GeS0.25Se0.75 (I2) crystal under different levels of illumination.
Figure 7.4 (a) – Figure 7.4 (e) depicts the photovoltage (Vph) – photocurrent density
(J) characteristics of the GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) electrodes obtained at
various intensities in the range 10mW/cm2 – 120mW/cm2. It is quite apparent from
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100 120 140
Cu
rren
t D
en
sit
y (
A/c
m2
)
Photovoltage (mV)
110 mW/cm290 mW/cm270 mW/cm250 mW/cm230 mW/cm210 mW/cm2
GeS0.25Se0.75 (I2)
0
20
40
60
80
100
120
140
160
0 20 40 60 80 100 120 140
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
120 mW/cm2
100 mW/cm2
80 mW/cm2
60 mW/cm2
40 mW/cm2
20 mW/cm2
GeS0.25Se0.75 (I2)
Chapter -7: Photoel ectrochemical Studies 273
Figure 7.4(a) – Figure 7.4(e) that the photovoltage characteristic deviates from the
expected ideal behaviour. Also it can be said that the characteristics show the
diverging behaviour with increase in intensity. This is quite obvious because the
increase in intensity of incident illumination directly means that the number of quanta
of photons incident on the semiconducting materials surface increases.
Figure 7.4(c): Photovoltage (Vph) vs. Current Density (J) for GeS0.5Se0.5 (I2) crystal under different levels of illumination.
0
20
40
60
80
100
120
140
160
180
200
220
240
0 20 40 60 80 100 120 140 160 180
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
110 mW/cm290 mW/cm270 mW/cm250 mW/cm230 mW/cm210 mW/cm2
GeS0.5Se0.5 (I2)
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100 120 140 160
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
120 mW/cm2100 mW/cm280 mW/cm260 mW/cm240 mW/cm220 mW/cm2
GeS0.5Se0.5 (I2)
Chapter -7: Photoel ectrochemical Studies 274
This leads to the absorption of the quanta in the semiconductor, which
subsequently enhance the generation of electron-hole pairs. The similar behaviour is
observed for all the electrodes.
Figure 7.4(d): Photovoltage (Vph) vs. Current Density (J) for GeS0.75Se0.25 (I2) crystal under different levels of illumination.
0
20
40
60
80
100
120
140
160
180
0 10 20 30 40 50 60 70 80 90 100
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
110 mW/cm290 mW/cm270 mW/cm250 mW/cm230 mW/cm210 mW/cm2
GeS0.75Se0.25 (I2)
0
20
40
60
80
100
120
140
160
180
0 10 20 30 40 50 60 70 80 90 100
Cu
rren
t D
en
sit
y (
A/c
m2)
Photovoltage (mV)
120 mW/cm2100 mW/cm280 mW/cm260 mW/cm240 mW/cm220 mW/cm2
GeS0.75Se0.25 (I2)
Chapter -7: Photoel ectrochemical Studies 275
Figure 7.4(e): Photovoltage (Vph) vs. Current Density (J) for GeS (I2) crystal under different levels of illumination.
Various characterizing parameters like short circuit current (Isc), open circuit
voltage (Voc), efficiency () and fill factor (F.F) for all the samples of GeSxSe1-x (I2)
(x = 0, 0.25, 0.5, 0.75, 1) have been evaluated and given in Table 7.1 (a), (b), (c), (d)
& (e) respectively. The further investigations have been carried out to study the effect
of incident illumination on various parameters.
0
20
40
60
80
100
120
140
160
180
200
0 10 20 30 40 50 60 70 80 90 100
Cu
rren
t D
en
sit
y (
A/c
m2)
Photo Voltage (mV)
110 mW/cm2
90 mW/cm2
70 mW/cm2
50 mW/cm2
30 mW/cm2
10 mW/cm2
GeS(I2)
0
20
40
60
80
100
120
140
160
180
0 10 20 30 40 50 60 70 80 90
Cu
rren
t D
en
sit
y (
A/c
m2)
PhotoVoltage (mV)
120 mW/cm2100 mW/cm280 mW/cm260 mW/cm240 mW/cm220 mW/cm2
GeS(I2)
Chapter -7: Photoel ectrochemical Studies 276
Table 7.1 (a): Characteristic parameters of GeSe (I2) based PEC solar cell with intensity illumination.
Intensity
(mW/cm2)
Short circuit
current
Isc (A)
Open circuit
voltage
Voc (mV)
Power max.
Pmax
(A mV)
Fill
Factor
(F.F)
Efficiency
(%)
10 2.8 72 75 0.3720 0.185
20 3.1 78 92.4 0.3821 0.131
30 3.4 95 117.6 0.3641 0.108
40 3.6 99 123.2 0.3457 0.095
50 5.0 113 420.0 0.7434 0.088
60 5.5 119 560.0 0.8556 0.084
70 5.8 134 599.5 0.7714 0.077
80 6.5 135 688.2 0.7843 0.072
90 7.0 136 710.4 0.7462 0.071
100 7.5 138 721.5 0.6971 0.065
110 7.8 140 737.0 0.6749 0.059
120 8.0 181 757.5 0.5231 0.060
Table 7.1 (b): Characteristic parameters of GeS0.25Se0.75 (I2) based PEC solar cell with intensity illumination.
Intensity
(mW/cm2)
Short circuit
current
Isc (A)
Open circuit
voltage
Voc (mV)
Power max.
Pmax
(A mV)
Fill
Factor
(F.F)
Efficiency
(%)
10 5.2 92 247.5 0.5173 0.505
20 5.3 93 256.5 0.5204 0.261
30 5.4 94 264.0 0.5201 0.179
40 5.5 96 275.0 0.5208 0.140
50 5.7 98 300.0 0.5371 0.122
60 5.8 100 318.6 0.5493 0.108
70 6.1 101 346.0 0.5616 0.100
80 6.3 102 365.4 0.4903 0.090
90 6.5 104 384.0 0.5976 0.087
100 6.9 108 442.0 0.6538 0.090
110 7.4 115 503.2 0.5913 0.093
120 7.5 122 510.0 0.5574 0.086
Chapter -7: Photoel ectrochemical Studies 277
Table 7.1 (c): Characteristic parameters of GeS0.5Se0.5 (I2) based PEC solar cell with intensity illumination.
Intensity
(mW/cm2)
Short circuit
current
Isc (A)
Open circuit
voltage
Voc (mV)
Power max.
Pmax
(A mV)
Fill
Factor
(F.F)
Efficiency
(%)
10 4.7 87 193.2 0.4724 0.394
20 4.8 91 206.8 0.4734 0.211
30 5.0 93 220.5 0.4741 0.150
40 5.1 95 225.0 0.4643 0.114
50 5.3 97 274.4 0.5337 0.112
60 5.4 98 280.0 0.5297 0.095
70 5.5 99 300.9 0.5526 0.087
80 5.9 100 313.6 0.5315 0.080
90 6.1 102 330.0 0.5303 0.074
100 6.3 105 359.6 0.5436 0.073
110 6.9 135 502.2 0.5391 0.093
120 8.5 368 585.0 0.1870 0.099
Table 7.1 (d): Characteristic parameters of GeS0.75Se0.25 (I2) based PEC solar cell with intensity illumination.
Intensity
(mW/cm2)
Short circuit
current
Isc (A)
Open circuit
voltage
Voc (mV)
Power max.
Pmax
(A mV)
Fill
Factor
(F.F)
Efficiency
(%)
10 6.5 70 202.5 0.4451 0.413
20 6.7 72 226.0 0.4685 0.23
30 7.0 75 227.9 0.4341 0.155
40 7.1 76 234.6 0.4348 0.199
50 7.2 77 235.4 0.4246 0.096
60 7.3 78 249.2 0.4317 0.084
70 7.4 80 252.6 0.4101 0.073
80 7.5 84 261.0 0.3933 0.066
90 7.6 84 264.3 0.3837 0.059
100 7.7 86 279.3 0.3913 0.057
110 7.8 87 281.4 0.3805 0.052
120 7.9 89 300.4 0.3880 0.051
Chapter -7: Photoel ectrochemical Studies 278
Table 7.1 (e): Characteristic parameters of GeS (I2) based PEC solar cell with intensity illumination.
Intensity
(mW/cm2)
Short circuit
current
Isc (A)
Open circuit
voltage
Voc (mV)
Power max.
Pmax
(A mV)
Fill
Factor
(F.F)
Efficiency
(%)
10 2.6 64 66.0 0.3966 0.134
20 2.7 65 83.6 0.4764 0.085
30 2.8 66 80.5 0.4356 0.054
40 3.0 67 86.4 0.4299 0.044
50 3.2 68 99.9 0.4591 0.04
60 3.5 70 117.0 0.4776 0.047
70 3.7 73 125.4 0.4643 0.039
80 3.9 74 133.3 0.4619 0.034
90 4.2 75 159.1 0.5051 0.036
100 4.5 76 187.2 0.5474 0.038
110 4.8 78 209.1 0.5585 0.038
120 5.5 86 275.0 0.5814 0.046
Figure 7.5 (a) & Figure 7.5(b) shows the variation of short circuit current with
intensity of incident polychromatic illumination for all the grown samples. From these
figures it is quite clear that the short circuit current increase with the intensity of
incident illumination. But the important fact is observed from Figure 7.5 (a) & Figure
7.5(b) that the increase is found to be nearly linear upto 120 mW/cm2 intensity. This
can be explained as follows.
The absorption of incident radiations leads to the generation of electron-hole
pairs within the semiconducting materials. It is always essential that the
photogenerated carriers within the semiconductor should take part in the charge
transfer mechanism through the electrolyte and the counter electrode. This process
can be divided in two steps.
❇ The efficient generation of carriers with in the semiconductor due to the
absorption of incident radiations.
❇ The oxidation-reduction which can also be called charge transfer reaction at
semiconductor – electrolyte interface and the electrolyte – counter electrode
interface.
If both the process occurs at the same rate, then the photocurrent always
increases linearly with the increase in the intensity of incident radiations. But is the
Chapter -7: Photoel ectrochemical Studies 279
charge transfer mechanism across the two electrodes becomes slower that the
photogeneration mechanism, then there will not be a transfer of all photogenerated
carriers from semiconductor electrode to the counter electrode. This results into the
nonlinear behaviour of the characteristics which means that the short circuit current
will start saturating after some intensity of light.
Based on the electrochemical kinetics and from Figure 7.5 (a) & Figure 7.5(b),
it can be seen that the mechanism of charge carriers within the semiconductor
dominates the overall charge transfer mechanism for GeSe (I2), GeS0.25Se0.75 (I2),
GeS0.5S0.5 (I2), GeS0.75Se0.25 (I2) and GeS(I2) based electrodes upto 120 mW/cm2
intensity if illumination. The nonlinear behaviour of Isc demonstrate that the
recombination of photogenerated carriers at the semiconductor electrolyte interface is
limiting the rate of overall charge reactions over the higher values of light intensities
employed. According to Kline et al. [8] and Biceli et al. [9] the observed deviation
from linearity of the short circuit current with respect to the incident light intensity
could mainly be attributed to the existence of numerous recombination centers. The
recombination centers associated with samples having surface steps results in a lower
quantum yield [10-12] at low intensity and limit the photocurrent at higher intensity.
Bulk and space charge mechanism which account for the deviation from the linearity
[10]. All these facts supported from the observation that the surface are generally
stepped. It can be concluded that the GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) based
PEC solar cells should be operated in the range 10 mW/cm2 to 100 mW/cm2 for better
photoconversion characteristic.
Similarly the open circuit voltage with intensity of incident radiations for all
the electrodes has been show in Figure 7.6 (a) & Figure 7.6 (b). It is quite clear from
Figure 7.5 (a), Figure 7.5(b) & Figure 7.6 (a), Figure 7.6(b) that variation of short
circuit current and open circuit voltage with intensity of incident polychromatic
illuminations is more or less of similar nature. This is well expected. The variation in
photoconversion efficiency with the intensity of incident polychromatic illumination
is show in Figure 7.7 (a) & Figure 7.7(b) for all the electrodes. As expected the
efficiency decreases with the intensity of illumination as per the above discussion. But
the maximum efficiency in all the electrodes is found to be around10 mW/cm2, which
is not relevance with the discussion of charge transfer mechanism given above.
Chapter -7: Photoel ectrochemical Studies 280
Figure 7.5 (a): The variation of short circuit current (Isc) with the intensity of incident illumination for GeSe (I2), GeS0.25Se0.75 (I2) and GeS0.5Se0.5 (I2) single
crystals.
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80 100 120 140
Isc
(A
)
Intensity (mW/cm2)
GeSe(I2)
0
1
2
3
4
5
6
7
8
0 20 40 60 80 100 120 140
Isc
(A
)
Intensity (mW/cm2)
GeS0.25Se0.75 (I2)
0
1
2
3
4
5
6
7
8
9
0 20 40 60 80 100 120 140
Isc
(A
)
Intenisty (mW/cm2)
GeS0.5Se0.5 (I2)
Chapter -7: Photoel ectrochemical Studies 281
Figure 7.5 (b): The variation of short circuit current (Isc) with the intensity of incident illumination for GeS0.75Se0.25 (I2) and GeS (I2) single crystals.
0
1
2
3
4
5
6
7
8
9
10
0 20 40 60 80 100 120 140
Isc
(
A)
Intensity (mW/cm2)
GeS0.75Se0.25 (I2)
0
1
2
3
4
5
6
0 20 40 60 80 100 120 140
Isc
(
A)
Intensity (mW/cm2)
GeS (I2)
Chapter -7: Photoel ectrochemical Studies 282
Figure 7.6 (a): The variation of open circuit voltage (Voc) with intensity of incident illumination for GeSe (I2), GeS0.25Se0.75 (I2) and GeS0.5Se0.5 (I2) single crystals.
0
20
40
60
80
100
120
140
160
180
200
0 20 40 60 80 100 120 140
Vo
c (
mV
)
Intensity (mW/cm2)
GeSe(I2)
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
Vo
c (
mV
)
Intensity (mW/cm2)
GeS0.25Se0.75 (I2)
0
50
100
150
200
250
300
350
400
0 20 40 60 80 100 120 140
Vo
c (
mV
)
Intensity (mW/cm2)
GeS0.5Se0.5 (I2)
Chapter -7: Photoel ectrochemical Studies 283
Figure 7.6 (b): The variation of open circuit voltage (Voc) with intensity of incident illumination for GeS0.75Se0.25 (I2) and GeS (I2) single crystals.
It indicates that there are some other parameters which also influence the
charge transfer reactions within the PEC solar cells. From the Tables 7.1 (a), (b), (c),
(d) & (e) it can be seen that the fill factor does not show a large variation with the
intensity of incident illumination.
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140
Vo
c (
mV
)
Intensity (mW/cm2)
GeS0.75Se0.25 (I2)
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100 120 140
Vo
c (
mV
)
Intensity (mW/cm2)
GeS (I2)
Chapter -7: Photoel ectrochemical Studies 284
Figure 7.7 (a): The variation of efficiency (%) with intensity of incident
illumination for GeSe (I2), GeS0.25Se0.75 (I2) and GeS0.5Se0.5 (I2) single crystals.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0 20 40 60 80 100 120 140
Eff
icie
ncy
(
%)
Intensity (mW/cm2)
GeSe(I2)
0
0.1
0.2
0.3
0.4
0.5
0.6
0 20 40 60 80 100 120 140
Eff
icie
ncy
(
%)
Intensity (mW/cm2)
GeS0.25Se0.75 (I2)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 20 40 60 80 100 120 140
Eff
icie
ncy
(
%)
Intensity (mW/cm2)
GeS0.5Se0.5 (I2)
Chapter -7: Photoel ectrochemical Studies 285
Figure 7.7 (b): The variation of efficiency (%) with intensity of incident illumination for GeS0.75Se0.25 (I2) and GeS (I2) single crystals.
7.6 MOTT-SCHOTTKY EVALUATIONS
7.6.1 Capacitance Measurements
The capacitance of solid / liquid interface in the PEC solar cells vary from a
few F to pF. It becomes highly difficult to measure the values accurately using
normal laboratory capacitance meters.
To measure the space charge capacitance in the abovementioned range, the
Hewlett Packard LCR meter was used.
The schematic diagram for impedance measurements is demonstrated in Figure 7.8. A
saturated calomel electrode (SCE) was used as a reference electrode and platinum grid
as a counter electrode.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 20 40 60 80 100 120 140
Eff
icie
ncy
(
%)
Intensity (mW/cm2)
GeS0.75Se0.25 (I2)
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0 20 40 60 80 100 120 140
Eff
icie
ncy
(
%)
Intensity (mW/cm2)
GeS (I2)
Chapter -7: Photoel ectrochemical Studies 286
Figure 7.8: Schematic diagram of impedance measurement.
7.6.2 Mott – Schottky Plots
Capacitance measurements were undertaken with GeSxSe1-x (I2) (x= 0, 0.25,
0.5, 0.75, 1) electrodes at various potentials. Capacitance data from these electrodes
were carried out to construct the Mott Schottky plots (1/C2SC versus V). Figure 7.9
present such plots for GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) single crystal electrodes
respectively using the electrolyte (0.025MI2 + 0.5M NaI + 0.5 M Na2SO4).
In the graphs of 1/C2SCE versus VSCE the voltage axis intercepts give the flat
band potentials Vfb which in the present case obtained value of flat band potential is
shown in Table 7.2. The acceptor concentration (nA) for GeSxSe1-x (I2) (x= 0, 0.25,
0.5, 0.75, 1) can be determined from the slopes of the straight line portions of the
Mott- Schottky plots in Figure 7.9 using the formula;
𝑛𝐴 = 2 𝑒𝜀𝜀0 × 𝑆𝑙𝑜𝑝𝑒 −1 (7.6)
where ‘nA’ is the accepter concentration, ‘e’ is the charge of electron taken
as1.62 × 10−19 Coulomb, ε is the dielectric constant of the material, ε0 is the
permittivity with a value of 8.854 × 10-12 Fm-1.
Battery
Capacitance Bridge
SCE
V
A C
SamplePt
Chapter -7: Photoel ectrochemical Studies 287
Figure 7.9: Mott - Schottky Plot for GeSxSe1-x (I2) (x = 0, 0.25, 0.5, 0.75, 1) single crystals.
The dielectric constant ε for GeSxSe1-x (I2) (x = 0, 0.25, 0.5, 0.75, 1) single crystals
have been evaluated by using the relation;
𝜀 =𝐶𝑑
𝐴𝜀0 (7.7)
0.00E+00
2.00E+00
4.00E+00
6.00E+00
8.00E+00
1.00E+01
1.20E+01
1.40E+01
1.60E+01
1.80E+01
2.00E+01
2.20E+01
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
1/C
2[F
-2
cm
4]
Applied Potential VSCE [V]
GeSe(I2)
GeS0.25Se0.75 (I2)
GeS0.5Se0.5 (I2)
GeS0.75Se0.25 (I2)
GeS(I2)
Chapter -7: Photoel ectrochemical Studies 288
where, ‘C’ is the capacitance, ‘d’ is the thickness of crystal, and ‘A’ is the area
of contact. Upon inserting the values of all the parameters in equation (7.6), the
acceptor concentration nA for P- type GeSxSe1-x (I2) (x = 0, 0.25, 0.5, 0.75, 1)
compounds are evaluated.
7.6.3 Energy band location
From the values of the band gaps for GeSxSe1-x (I2) (x = 0, 0.25, 0.5, 0.75, 1)
single crystal the position of the valance band and conduction band edges for all the
electrodes in the electrolyte reported can be estimated. The procedure for this
estimation is outlined below:
For entire samples, the difference between Ev and Ef can be obtained from equation;
𝑛𝐴 =𝑁𝐴 exp (𝐸𝑣−𝐸𝑓 )
𝑘𝑇 (7.8)
where ‘nA’ is the acceptor concentration (all acceptor impurities assumed to be
completely ionized), ‘Ev’ is he energy at the top of the valance band, ‘Ef’ is the Fermi
level energy and ‘NA’ is the density of effective states in valance band which is given
by ;
𝑁𝐴 = 2
ℎ3 (2𝜋𝑚ℎ∗ 𝑘𝑇)3 2 (7.9)
where 𝑚ℎ∗ is the effective mass of holes.
Taking the values of effective mass for GeSxSe1-x (I2) (x = 0, 0.25, 0.5, 0.75, 1) from
TEP measurements described in Chapter 4, the values of NA have been estimated for
these which are presented in Table 7.2.
From equation 7.8 we can write;
𝐸𝑓 −𝐸𝑣 = 𝑘𝑇𝑙𝑛 𝑁𝐴
𝑛𝐴 (7.10)
Using this relation, Ef – Ev has been evaluated and from the values of Vfb, the
values of Ev for all the compounds have been estimated and are reported in Table 7.1.
Now from the values of band gap for all the samples reported in Chapter 5, the
position of conduction band minima for all materials have been obtained and are
represented in Table 7.2.
Chapter -7: Photoel ectrochemical Studies 289
The band bending, (Vb) is an important since it gives the maximum open
circuit voltage (Voc) obtainable from photoelectrochemical cell. Vb and Vfb are related
as ;
Vb =Vf, redox – Vfb (7.11)
The values of Vb for all electrodes have been determined using this relation and are
listed in Table 7.2.
Further, substituting the values of nA and Vb from Table 7.2 in to the equation ;
𝑊 = 2𝜀𝜀0𝑉𝑏
𝑒𝑁𝐴
1/2
(7.12)
The width of the space charge region ‘W’ has been evaluated for all the electrodes in
given electrolyte. These values are also shown in Table 7.2.
The values of Vf, redox in this table have been measured using pH meter with SCE
electrode.
Chapter -7: Photoel ectrochemical Studies 290
Table 7.2: Summary of results obtained from Mott-Schottky plots for GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) single crystals.
Properties GeSe (I2) GeS0.25Se0.75 (I2) GeS0.5Se0.5 (I2) GeS0.75Se0.25 (I2) GeS (I2)
Type P P P P P
Electrolyte used 0.025MI2 + 0.5 M NaI + 0.5 M Na2SO4
Flat Band Potential (eV) 0.702 0.825 1.05 1.238 1.45
Band Bending (Vb) (eV) -0.416 -0.539 -0.764 -0.952 -1.164
Carrier Concentration (nA) (m-3)
1.55 x 10 24
1.525 x 10 24
1.461 x 10 24
1.3682 x 10 24
1.01 x 10 24
Density of states in valance band
(m-3
) 4.74 x 10
24 4.74 x 10
24 4.74 x 10
24 4.74 x 10
24 4.74 x 10
24
Depletion width (W) (m) 2.13 x 10
-6 2.36 x 10
-6 2.78 x 10
-6 2.89 x 10
-6 3.7 x 10
-6
Conduction band edge (Ec) (eV) -0.8468 -0.7544 -0.5812 -0.571 -0.271
Valance band edge (Ev) (eV) 0.6732 0.7956 1.0196 1.205 1.4099
Redox Fermi level of the
electrolyte Ef , redox 0.286 0.286 0.286 0.286 0.286
Chapter -7: Photoel ectrochemical Studies 291
7.7 CONCLUSION
† From the Photovoltage – Photocurrent characteristic of all the electrodes at
various intensities it can be seen that photoconversion characteristic shows the
diverging behaviour with intensity and it deviate from the expected ideal
behaviour.
† The short circuit current increases with intensity of illumination that is
because of the fact that the charge transfer in those materials is due to the
absorption of incident radiations and the oxidation-reduction processes at
semiconductor – electrolyte interface and electrolyte – counter electrode
interface. Open circuit voltage also shows the same.
† The photoconversion efficiency (n) of the pure GeSe is higher than that of the
GeSxSe1-x (I2) (x = 0.25, 0.5, 0.75, 1) based PEC solar cell, and as the content
of sulpher increases the efficiency of the PEC solar cell decreases [13].
† The type of Mott-Schottky plots firmly confirm the p-type behaviour of the
single crystals of GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1).
† From Mott-Schottky plots various parameters have been calculated. The Fermi
energy level is close to the top of the valence band which again confirm that
GeSxSe1-x (I2) (x= 0, 0.25, 0.5, 0.75, 1) single crystals having p-type
semiconducting nature.
Chapter -7: Photoel ectrochemical Studies 292
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