zz
Chapter-VI
PHOTOELECTROCHEMICAL
STUDIES OF Cd1-XNixSe THIN FILMS
Chapter-VI
6.1 Introduction
The scientific scenario of solar cells has been dominated by inorganic
solid-state material device, specially doped forms of crystalline and
amorphous silicon. However, this dominance is now being challenged by the
emergence of a new generation of devices based on polycrystalline or
nanocrystalline materials. These offer the prospect of cheaper fabrication
together with other attractive features, such as flexibility.1 In the present
power-crisis, all scientists are being motivated to contribute for the
development of alternative solar cells using novel materials, including those
related to economical as well as to health and environmental concerns. In
view
of
this,
solar
energy
conversion
can
be
achieved
by
photoelectrochemical process which is the most intensive example of this
approach.
Semiconductor- electrolyte interface may be used for photoelectrolysis,
photocatalysis and photoelectrochemical power generation.2-4
The direct
conversion of solar energy into electrical current using semiconductorelectrolyte interface was first demonstrated by Gerischer and Eills.5-6 Since
then a large number of metal as well as mixed chalcogenide and oxides have
been used as photoelectrode in PEC cells. The stability and efficiency of PEC
cells are mainly dependent on preparation conditions for photoelectrode,
electrolyte and experimental conditions set during the experiment.7 The basic
requirements of good thin film photoelectrode for PEC cells are low
resisitivity and larger grain size. Large grain size leads to reduction of grain
boundary area of the thin film leading to an efficient energy conversion. The
low restitivity of the photoelectrode is required to minimize the series
resistance of the PEC cell which leads to lower the short circuit current.
8-9
Polycrystalline semiconductor film can be used without any drastic decrease
in efficiency. This is because of the intimate and perfect contact of liquid
electrolyte with crystalline grains. Thus PEC cell provides an economical
chemical route for trapping solar energy. It consists of a photosensitive n or ptype semiconductor electrode and a counter electrode dipped in a suitable
134
Chapter-VI
electrolyte. Charge transfer takes place at equilibrium and corresponding
potential difference is developed in the both phases.
A Schottky barrier with a space charge ionized donor or acceptor ion
is formed within the semiconductor and minority carriers which are present in
too low concentration. Upon illumination of this barrier with light of suitable
wavelength, electron-hole pairs are generated and separated by a barrier at the
interface; holes are drawn into electrolyte whereas the electrons travel through
the barrier into the semiconductor. In short-circuit condition, current is
proportional to the intensity of the incident light whereas at the open circuit
conditions these electrons and holes do not recombine.
The alloyed/mixed semiconductor materials are known to function
effectively in conversion of solar energy into electrical energy.
10-11
The
properties of the mixed material can be tailored to desired level by smooth
variation of the compositional parameter ‘x’.
A photoelectrochemical approach of trapping solar energy has
developed mostly by polycrystalline isoelectronic ternary materials such as
CdZnSe, CdSSe, etc. It was therefore planned to test PEC properties of
deposited Cd1-xNixSe thin films as photoelectrode with suitable electrolyte.
The present chapter mainly describes the photoelectrochemical
investigations on Cd1-xNixSe thin films. The results and interpretation of
fabricated PEC cells can be examined in terms of current-voltage (I-V),
capacitance-voltage (C-V) characteristics in dark, built-in-potential, power
output curves, photo-response and spectral response.
6.2 Experimental Details
6.2.1 Fabrication of PEC Cell
In the present investigation, photoelectrochemical cell was fabricated
using Cd1-xNixSe thin film as photoanode, sulphide-polysulphide as an
electrolyte and CoS treated graphite rod as a counter electrode. A saturated
calomel electrode was used as reference electrode. Corning glass cuvette
having ‘H’ shape was used to construct the cell. An electrolyte solution offers
135
Chapter-VI
an advantage of stabilization against photoelectrode dissolution.12-13 The
details of cell fabrications have already been discussed in section 3.5.2
6.2.2 Electrical Characterization of PEC Cell
The electrical properties of PEC cell were examined in order to know
about the charge transfer mechanism occurring across electrode-electrolyte
interface. I-V, C-V characteristics in dark, measurement of built-in-potential,
power output characteristics under illumination were studied. A wire wound
potentiometer was used to vary the voltage across the junction and current
flowing through the junction was measured with a current meter. The same
circuit was used to determine the capacitance of the junction. The barrier
height was examined from temperature dependence of reverse saturation
current at different temperatures; the lighted ideality factor was calculated.
The junction ideality for all the cells were determined by plotting the graph of
log I versus V. Photoelectrochemical activities were studied under 30
mW/cm2 light illumination. The illumination intensity was measured by Meco
Lux meter.
6.2.3 Optical Characterization of PEC Cell
Photoresponse for all the samples were measured to determine the light
ideality factor. The short circuit current and open circuit voltage were
measured as a function of incident light intensity. Spectral response was
determined by measuring the short-circuit current as well as open circuit
voltage as a function of incident wavelength (400 -1000 nm).
6.3 Results and Discussion
For efficient conversion of incident light into electrical energy, an
ohmic contact between the photoelectrode material and the substrate is very
important. A contact is said to be ohmic, if it is non-injecting and has a linear
current-voltage relation in both directions.14 Therefore, the nature of contact
between the photoelectrode and the substrate was examined for all samples.
Fig.6.1 shows the variation plot of current with voltage. I-V relations were
found to be linear in both directions, suggesting the ohmic contact between
photoelectrode and substrates.
136
Chapter-VI
0.4
X=0.0
0.3
X=0.6
0.2
2
Current (mA/cm )
X=0.3
0.1
0
-600
-400
-200
0
200
400
600
-0.1
-0.2
-0.3
-0.4
Voltage (mV)
Fig.6.1 Plot of current versus voltage to evaluate nature of contact
between photoelectrode and substrate.
137
Chapter-VI
6.3.1 Electrical Properties
a) I-V characteristics in Dark
Current-Voltage (I-V) characteristics of the PEC have been studied at
303 K. The dark voltage and dark current were found to develop. The polarity
of this dark voltage was positive towards the semiconductor electrode. The
dark voltage is developed due to difference between the two half cell potential
of a cell; 15
E = E Cd1-xNixSe-EGraphite ----------------------------------6.1
Where ECd1-xNixSe, EGraphite are the half cell potentials of photoelectrode and
counter electrode respectively. Half cell potential is developed when the
electrode is directly in contact with the electrolyte. But,
E Cd1-xNixSe > E Graphite ----------------------------------------6.2
After illumination of the junction, the magnitude of voltage increases
with increase in positive polarity towards the thin film. The sign of this
photovoltage indicates that Cd1-xNixSe is a p-type conductor which has also
been proved from TEP measurement studies. Existence of some dark current
shows that there is some deterioration of the photoelectrode materials in the
electrolyte.16-17 Considering semiconductor/electrolyte interface as the analog
of a Schottky barrier cell, the current transport through the interface is defined
by Bulter-Volmer relation as; 18
I =Io {exp [(1-β)VF/RT]-exp (-βVF/RT)---------------------6.3
Where, Io is equilibrium exchange current density, V is the over voltage, β is a
symmetry factor, F is Faraday constant and R is universal gas constant. A
value of β equal to 0.5 indicates presence of a symmetrical barrier which
results in a symmetrical I versus V curve. This interface is called nonrectifying. If β ≠ 0.5, the curves would not be symmetrical and the interface
has rectifying properties called as Faradic rectification. The characteristic are
non-symmetrical indicating the formation of rectifying type junction.18,19 In
the present investigation, β factor was found to be greater than 0.5 for all
compositions suggesting the rectifying nature of the interface. 20 The dynamic
current-voltage characteristics are shown in Fig.6.2. The junction ideality
138
Chapter-VI
factor (nd) can be determined from the plot of log I with voltage (mV) and the
variation is shown in Fig.6.3. Linear nature of plot was used for the estimation
of junction ideality factor. The ideality factor was found to be minimum for x
= 0.3 composition. (Table 6.1). The higher values of nd suggest the dominance
of series resistance as well as the structural imperfection induced by
dissimilarity in the Cd and Ni atomic size and their resulting arrangement in
the solid during lattice construction. Defect levels, introduced in this manner
inside the valence band and energy gap acts as carrier traps or recombination
centers. The junction ideality factor has a minimum value for x = 0.3
suggesting lowest trap density at the photoelectrode-electrolyte interface.
21
1
X=0.3
0.8
X=0.6
X=0.0
0.6
0.4
2
Current (mA/cm )
X=0.9
0.2
0
-500
-300
-100
-0.2
100
300
500
-0.4
-0.6
-0.8
Voltage (mV)
Fig. 6.2 Current-voltage characteristics of Cd1-xNixSe photoelectrode (in dark)
139
Chapter-VI
0
0
100
200
300
400
500
-0.5
log I
-1
-1.5
X=0.3
X=0.6
X=0.0
X=0.9
-2
-2.5
Voltage (mV)
Fig.6.3 Plot of log I versus Voltage of Cd1-xNixSe cells.
b) C-V Characteristics in Dark
The C-V Characteristics of junctions provides useful information such
as type of conductivity, values of flat band potential (Vfb), donor density, band
bending depletion layer width position of bond edges etc. The flat band
potential (Vfb) of a semiconductor gives information of the relative position of
the Fermi levels in photoelectrode as well as the influence of electrolyte and
charge transfer process across the junction. The intrinsic bond bending of the
interface is used to determine ability of photoelectrode to operate under the
short circuit condition. This is also useful to measure the maximum open
circuit voltage (Voc) that can be obtained from a cell. Measured capacitance is
the sum of the capacitance due to depletion layers and Helmholtz layer in
electrolyte which is neglected by assuming high ionic concentration. 22 Under
such circumstances, Vfb can be obtained using Mott-Schottky equation;
C-2 = [2/qεε0Nd] (V- Vfb-kT/q) ----------------------------6.4
where symbols have their usual meaning. The charge space layer capacitance
was measured under reverse biased condition and the flat band potential was
140
Chapter-VI
obtained from the Mott-Schottky plot. The variation of C-2 with Voltage for
representative samples is shown in Fig 6.4. The linear regions of these plots
were extrapolated to the voltage axis, which gives the flat band potentials
(Vfb). The sign of flat band potential indicates the nature of the material. In
this investigation, it can be concluded that Cd1-xNixSe material with varying
composition (x) is p-type. A variation of flat band potential versus
composition (x) is displayed in Fig.6.5. It is observed that flat band potential
is enhanced to more
positive value as nickel content in the electrode
increased upto x=0.3, thereafter Vfb diminishes linearly upto x=1. This may be
due to decreased electron affinity as a result of introduction of Ni+2 ions in the
lattice of CdSe, an increased amount of surface adsorption and creation of
new donor level which shifts the Fermi level thus increasing the amount of
band bending.
7
X=0.0
X=0.1
X=0.3
X=0.6
X=0.9
5
4
8
-2
4
1/C X10 (F cm )
6
2
3
2
1
0
100
300
500
Voltage (mV Vs. SCE)
700
Fig. 6.4 1/C2 versus d.c.bias voltage of Cd1-xNixSe cells.
141
Chapter-VI
750
Vfb (mV)
650
550
450
350
250
0
0.2
0.4
0.6
0.8
1
Composition parameter (X)
Fig. 6.5 Plot of Vfb against composition parameter (x)
c) Built-in-Potential Measurement
The Built-in-Potential (also called barrier-height, Φβ) was determined
by measuring the reverse saturation current (Io) flowing through the junction
at different temperature from 363 to 303 K. The reverse saturation current
flowing through junction is related to temperature as; 23
Io = AT2 exp (Φβ / kT) -----------------------------------6.5
Where, A is Richardson constant, k is Boltzmann constant, Φβ is the barrier
height in eV. The reverse saturation current exhibits an exponential variation
with temperature. Fig. 6.6 shows plots of log (Io/T2) versus 1000/T for
representative samples. The values of built in potentials of the photoelectrode
were determined from the slope of the linear region of the plots. The barrier
height value decreases up to x = 0.3 (0.180 eV) and then increases. (Table
6.1)
142
Chapter-VI
-7.5
2.6
2.8
3
3.2
3.4
-8
2
log (Io/T )
-8.5
-9
x=0.6
x=0.9
x=0.0
X=1.0
-9.5
x=0.3
-10
-1
1000/T (K )
Fig.6.6 Plot of log (Io/T2) with 1000/T Cd1-xNixSe cells
180
X=0.0
160
X=0.3
140
X=0.6
2
Current ( µ A/cm )
X=1.0
120
X=0.9
100
80
60
40
20
0
0
100
200
300
Voltage (mV)
Fig.6.7 Power output curves for Cd1-xNixSe photoelectrode
143
Chapter-VI
d) Power Output Characteristics
Fig.6.7 shows the photovoltaic power output characteristics of various
cells recorded under 30mW/cm2 illumination intensity. The various cell
parameters like open circuit voltage (Voc), short-circuit current (Isc), fill factor
(ff), series resistance (Rs), conversion efficiency (η) and shunt resistance (Rsh)
were determined.
The open circuit voltage, short-circuit current, fill factor and efficiency
increase up to x = 0.3, but decrease thereafter. The series resistance and shunt
resistance decrease up to x = 0.3, but increase thereafter. The open circuit
voltage and short-circuit current is found to be 260 mV and 161 µA/cm2
respectively. The calculations show that the fill factor is 48.12 % and
conversion efficiency is 0.65 % at x = 0.3 (Table 6.1). Under constant
illumination, the maximum efficiency is given by; 24
ηmax = (Vredox-Vfb) (e/Eg) -------------------------------------6.6
where symbols have usual meaning. The variation of Isc and Voc with
composition parameter is shown in Fig.6.8 and 6.9. At x = 0.3, the flat band
potential value is more positive as well as comparative band gap, results in
enhancement in power efficiency. The high short circuit current was due to
decreased photoelectrode resistance and increased the absorbance by the
material. The improvement in open circuit voltage was due to increase in flat
band potential. The low efficiency in the present investigation might be due to
the high series resistance of the PEC cell, low thickness of the film and
interface states which are responsible for the recombination mechanism.25 The
series resistance and shunt resistance were calculated from the slope of the
power output characteristics using the relation;
(dI/dV)I = 0 = (1/Rs) --------------------------------------6.7
(dI/dV)V = 0 = (1/Rsh) -------------------------------------6.8
The values of Rs and Rsh were found to be 826 and 543 Ω respectively.
The main drawback in utilizing PEC cell is the absence of space charge
region at the photoelectrode-electrolyte interface. In this situation, the
photogenerated charge carriers can move in both the direction. Lu and Kamat
144
Chapter-VI
26
reported that the photogenerated electrons in n-type material either
recombine readily with holes or leak out into the electrolyte, instead of
flowing through external circuit. The variation of fill factor and efficiency
with compositional parameter (x) is shown in Fig.6.10 and Fig.6.11
respectively.
6.3.2 Optical Proprieties
a) Photoresponse
To study, the response of the PEC cell towards light, the cell was
illuminated with light of different intensity. The open circuit voltage and short
circuit current were measured as a function of light intensity. Fig.6.12 shows
variation of Isc as a function of light intensity, whereas, Fig.6.13 shows the
variation of Voc as a function of light intensity. The photoresponse
measurements showed a logarithmic variation of open circuit voltage with the
incident light intensity. However, at higher intensities, saturation in open
circuit voltage was observed, which can be attributed to the saturation of the
electrolyte interface, charge transfer and non-equilibrium distribution of
electrons and holes in the space charge region of the photoelectrode. But short
circuit current follows almost a straight line path. The photoelectrodeelectrolyte interface can be modelled as a Schottky barrier solar cell
27
and it
is therefore possible to represent the current-voltage relationship as;
I = Iph - Id = Iph-[Io exp (qV/ndkT)-1] ---------------------6.9
Where, I is the net current density, Iph is the photocurrent densities, Id is the
dark current density, Io is the reverse saturation current density, V is the
applied bias voltage and nd is the junction ideality factor. In bias voltage
condition V>3kT/q and at equilibrium open circuit conditions;
Iph = Id and V = Voc Thus,
Voc= (nLkT/q) ln (Isc/Io) --------------------------------6.10
Where, Voc is open circuit voltage, Isc is short circuit current. As Isc >> Io, a
plot of log Isc against Voc should give a straight line and from the slope of the
145
Chapter-VI
Voc (mV)
300
200
100
0
0.2
0.4
0.6
0.8
1
Composition parameter (X)
Fig. 6.8 Plot of Voc with composition parameter
2
Isc (µ A/cm )
200
150
100
50
0
0.2
0.4
0.6
0.8
1
Composition parameter (X)
Fig.6.9 Plot of Isc with composition parameter
146
Chapter-VI
50
48
%ff
46
44
42
40
0
0.2
0.4
0.6
0.8
1
Coposition parameter (X)
Fig. 6.10 Plot of %fill factor with compositional parameter
0.7
% effiencey
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
Composition parameter (X)
Fig. 6.11 Plot of % efficiency with compositional parameter
147
Chapter-VI
line the lighted ideality factor can be determined. The plot of log Isc with Voc
for representative Cd1-xNixSe photoelectrode is shown in Fig.6.14. The
junction ideality factor was calculated for all the photoelectrodes and found to
be minimum for Cd0.7Ni0.3Se composition. The observed value being 3.45 for
x = 0.3 photoelectrode. (Table 6.1)
b) Spectral Response
The spectral response of photoelectrochemical cell is one of the most
powerful techniques to measure the performance of the cell qualitatively.
Therefore, the spectral response of a cell has been recorded in the 400 to 1000
nm wavelength range. The photocurrent action spectra were examined and are
shown in Fig. 6.15 It is seen that spectra attains maximum value of current at
around wavelengths 721 nm and 725 nm for CdSe
and Cd0.7Ni0.3Se
respectively and decreases with increase in wavelength. The decrease in
current on shorter wavelength side may be due to absorption of light in the
electrolyte and high surface recombination of photogenerated minority
carriers. The decrease in current on longer wavelength side may be attributed
to non-optimized thickness and transition between defect levels. The
maximum current is obtained corresponding to λ = 721 nm and λ = 775 nm
giving band gap value 1.72 eV for CdSe and 1.60
eV for Cd.0.7Ni0.3Se
agreeing with the results of optical absorption studies.
The various PEC cell characteristics such as Voc, Isc, η%, ff%, Φβ, Vfb,
Rs, Rsh, nL, nd are cited in Table 6.1 for Cd1-xNixSe photoelectrode.
148
Chapter-VI
250
x=0.0
x=0.9
2
Isc (µ A/cm )
200
x=0.6
x=0.3
150
100
50
0
0
20
40
60
Light intensity (mW/cm2 )
Fig. 6.12 Plot of Isc with light intensity of Cd1-xNixSe photoelectrode
250
x=0.0
x=0.9
x=0.6
x=0.3
Voc (mV)
200
150
100
50
0
10
20
30
40
50
60
2
Light intensity (mW/cm )
Fig. 6.13 Plot of Voc with light intensity of Cd1-xNixSe photoelectrode
149
Chapter-VI
-3
log Isc
50
150
250
350
-4
X=0.0
X=0.9
X=0.6
X=0.3
-5
Voc (mV)
Fig.6.14 Plot of log Isc with Voc for Cd1-xNixSe photoelectrode
50
x=0.3
x=0.6
2
Isc (µA/cm )
40
x=0.9
x=0.0
30
20
10
0
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. 6.15 Plot of Isc with wavelength for Cd1-xNixSe photoelectrode
150
Chapter-VI
Table 6.1 PEC cell performance parameters of Cd1-xNixSe photoelectrode
Composition.
Voc
Isc
(mV) (µA/cm2)
η % ff %
Фβ
(eV)
Rsh
Rs
(mV) (Ω)
(Ω)
Vfb
nL
nd
CdSe
210
132
0.37 40.86 0.234
549
640
974 5.56 4.84
Cd0.9Ni0.1Se
228
135
0.43 44.65 0.216
590
630
959 4.48 4.78
Cd0.8Ni0.2Se
244
142
0.55 46.25 0.119
659
607
923 3.79 4.73
Cd0.7Ni0.3Se
260
161
0.65 48.12 0.180
710
543
826 2.89 4.64
Cd0.6Ni0.4Se
240
148
0.56 47.15 0.182
652
586
892 2.98 4.72
Cd0.5Ni0.5Se
230
145
0.48 46.26 0.184
575
596
908 3.18 4.81
Cd0.4N0.6Se
220
142
0.40 45.12 0.185
526
607
923 3.48 4.87
Cd0.3Ni0.7Se
205
125
0.36 44.54 0.187
469
664
1010 3.94 5.02
Cd0.2Ni0.8Se
194
109
0.30 43.63 0.189
421
718
1092 4.43 5.21
Cd0.1Ni0.9Se
179
101
0.23 42.75 0.190
362
745
1133 4.88 5.42
NiSe
160
71
0.15 41.89 0.195
304
846
1287 5.37 5.61
151
Chapter-VI
6.4 Conclusions
The PEC cell can be easily fabricated using Cd1-xNixSe photoelectrode,
sulphide-polysulphide as electrolyte and CoS treated graphite rod as a counter
electrode. A saturated calomel electrode was used as reference electrode. The
various performance parameters were determined with respect to the
composition parameter (x). It is found that the fill factor and efficiency is
maximum for Cd0.7Ni0.3Se composition. This is due to low resistance, high
flat band potential, maximum open circuit voltage as well as maximum short
circuit current. The barrier height was examined from the temperature
dependence of the reverse saturation current. The lighted ideality factor was
found to be minimum for Cd0.7Ni0.3Se photoelectrode. A cell utilizing
photoelectrode of this composition showed a wider spectral response and high
short circuit current.
152
Chapter-VI
References
1)
S.Pass, Y.S.Chaudhary, M. Agrawa, A. Shrivastav, R. Shrivastav,
V.R.Satsangi, Ind. J. Phys., 78A (2004) 229.
2)
M.Ramrakhiani, Physics News, Sept-Dec (1998) 115.
3)
B.Miller, A.Heller, M.Robbins, S.Mebzones, K.C.Change, J.Thomson,
J. Electrochem. Soc., 124 (1977) 1019.
4)
M.T.Gurierrez, J.Ortega, Sol. Ener. Mater., 20 (1990) 387.
5)
P.J.Holes, “The Electrochemistry of Semiconductors”, Academic
Press, (1992).
6)
H.Gerischer, Electrochem., 58 (1975) 263.
7)
A.D.Eills, S.W.Kaiser, M.S.Wrighton, J. Phys. Chem., 80 (1976) 1325.
8)
C.D.Lokhande, Solar Cells, 22 (1987) 133.
9)
V.V.Killedar, C.D.Lokhande, C.H.Bhosale, Ind. J. Pure Appl. Phys.,
36 (1998) 643.
10)
D.Das, Proc. Conf. Phys. & Tech. Semiconductor Dev. Interg.
Circuits, 1523 (1992) 323.
11)
P.Mitechell, D.G.Denure, Thin Solid Films, 16 (1973) 285.
12)
L.L.Kazmerski, M.S.Ayyagari, G.A.Sanborn, J. Appl. Phys., 46
(1975) 4865.
13)
L.P.Deshmukh, B.M.More, C.B.Rotti, G.S.Shahane, Mater. Chem.
Phys., 45 (1996) 145.
14)
L.P.Deshmukh, G.S.Shahane, Inter. J. Electrno., 83 (1997) 341.
15)
J.O.M.Bockris, A.K.Reddy, in “Modern Electrochemistry”, Vol-2
(Eds) J.O.M.Bockris, A.K.Reddy, Plenum Press, New York, (1973).
16)
L.P.Deshmukh, S.S.Holikatti, J. Phys.D:, Appl. Phys., 27 (1994) 1786.
17)
L.P.Deshmukh, Ind. J. Pure Appl. Phys., 36 (1998) 302.
18)
A.M.A.Dhafiri, A.A.I.Al-Bassam, Sol. Ener. Mater. Sol. Cells, 33
(1994) 177.
19)
A.J.Nozik, Ann. Rev. Phys Chem., 29 (1978) 189.
20)
M.A.Butler, J. Appl. Phys., 48 (1977) 1914.
153
Chapter-VI
21)
A.Aruchami, G. Aravamudan, G.V.Subba Rao, Bull. Mater. Sci.,
4 (1982) 483.
22)
P.K.Mahapartra, A.R.Dubey,Sol. Ener. Mater. Sol. Cells, 32(1994) 29.
23)
K.C.Chang, B.Miller, J. Electrochem. Soc., 124 (1977) 696.
24)
K.Y.Rajpure, S.M.Bamane, C.D.Lokhande, C.H.Bhosale, Ind. J. Pure
Appl. Phys., 37 (199) 413.
25)
D.Lue, P.Kamat, J. Phys.Chem., 97 (1993) 1073.
26)
K.Rajeshwar, L.Thomson, P.Singh, R.C.Kainthala, K.L.Chopra, J.
Electrochem. Soc., 128 (1981) 1744.
27)
H.Gerischer, Electro. Anal. Chem., 150 (1983) 553.
154