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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.


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

http://en.wikipedia.org/wiki/Visible_light


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.

http://en.wikipedia.org/wiki/Cathode

http://en.wikipedia.org/wiki/Electrolyte


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


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



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.


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


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


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


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 +


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)


GeS e (I2)


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)


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)


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)


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)


GeS0.5Se0.5 (I2)


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)


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)


GeS0.75Se0.25 (I2)


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)


GeS(I2)


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


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


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


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.


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)


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)


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)


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)


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)


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)


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


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)


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.


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.


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


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.


REFERENCES

[1] Akira Fujishima, Donald A. Tryk, Photochemical and Photo-Electrochemical

Water Splitting, in Energy Carriers and Conversion Systems, [Ed. Tokio

Ohta], in Encyclopedia of Life Support Systems (EOLSS), Developed under

the Auspices of the UNESCO, Eolss Publishers, Oxford ,UK,. (2006).

[2] M. Sharon, K. Basavaswaran and N. P. Sathe., J. Sci. Ind. Res., 44 (1985)

593.

[3] M. Sharon and K. Basavaswaran., Solar Cells, 25 (1988) 97.

[4] K. Mishra, K. Rajeshwar, A. Weiss, M. Murley, R. D. Engelken,M. Slayton

and H. E. Mc Cloud., J. Electrochem. Soc., 136, (1989), p. 1915.

[5] N. L. Nagard, C. Levy Clement, A. Katty and R. M. A. Lieth., Mater. Res.

Bull., 25, (1990) 495.

[6] P. Allen and A. Hickling., Trans. Faraday Soc., 53 (1975) 1926.

[7] M. A. Bulter and D. S. Ginley., J. Mater. Sci., 15 (1980).

[8] G. Kline, K. Kam, R. Ziegler and B. A. Parkinson., “Further studies of the

photoelectrochemical properties of the group IV transition metal

dichalcogenides”. Solar Energy Mater., 6 (1983) 337.

[9] L. P. Bicelli, P. Pederferri and G. Razzni., Hydrogen energy process., V(1980)

1055.

[10] W. Kautek, H. Gerischer and H. Tribush, Ber Bunsenges., Phys.Chem.,

83(1979) 1000.

[11] H. J. lewerenz, A. Heller and F. J. Disalvo., J. Am. Chem. Soc., 102 (1980)

1877.

[12] T. E. Frautak, D. Canceled and B. A. Parkinson., J. Appl. Phys., 51 (1980)

6018.

[13] G. K. Solanki, Trupti Patel, Sandip Unadkat, Dipika B. Patel, Ruchita R. Patel,

N. N. Gosai and Yunus Gufur Mansur., Journal of Science., 1 (1) (2010) 39-

43.

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