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Effect of temperature on the performance of photoelectrochemical cells formed with TiNi oxide electrodes
~
Solid State Communications, Vol.48,No.l, pp.;7-23,]983 Printed in Great Britain.
EFFECT OF TEMPERATURE
ON THE PERFORMANCE OF PHOTOELECTROCHEMICAL WITH TiNi OXIDE ELECTRODES V.B.Chougule
Energy Conversion
0038-]098/83 $3.00 + .00 Pergamon Press Ltd.
CELLS FORMEB
and S.H. Pawar
Laboratory, Bepartment Kolhapur 416004,
of Physics, INDIA
Shivaji University,
(Received ]2 July ]983 by S. Amelinckx) TiNi oxide electrodes are formed by oxidizing TiNi alloys of various compositions in air at different temperatures. These electrodes have been used as photoanodes in photoelectrochemical cells. The effect of temperature on these cells is studied by making measurements on the speed of response, spectral response and photovoltaic output characteristic of the cells. It is found that an optimum temperature exists for these cells. The operation of these cells at elevated temperature therefore means an increase in the conversion efficiency as the infrared region of the solar spectrum may be utilized for heating these cells.
INTRODUCTION
of an optimum temperature for the best performance of a PEG cell is brought out.
The conversion efficiency of a PEC cell depends on its spectral response and quantum efficiency. Hide band-gap materialso such as TiO2, SrTiO S etc. are stable against photo-corrosion. Their efficiency is however limited as their spectral response is restricted to the uv region. The semiconductor electrolyte interface is an important element in a PEC cell. The charge transfer taking place at the interface has a direct bearing on the performance of the cell. The rate of charge transfer may be improved by operating the cell at elevated temperatures. The electrical and optical properties of a PEG cell are modified by the change in temperature. The increase in temperature of the PEG cell may be brought about by utilizing the near infrared region of the spectrum which otherwise is wasted. This will add to the conversion efficiency considerably [i]. The effect of temperature on the performance of a PEC cell has many aspects [2]. The temperature dependence of spectral response is one of them. Changes in the flat band potential with temperature are mainly due to the change in potential across the Helmholtz layer. The electron affinity and the redox potential also change with temperature. All these factors will modify the current voltage characteristics, the power output and efficiency of these cells with change in temperature.
5XPERIMBNTAL TiNi alloys have been prepared in an argon arc furnace by melting appropriate amounts of Ti and Ni powders. The ingots are then cut suitably into plates of 2 mm thickness. These plates are oxidized in air at different temperatures. The composition and treatmentwise nomenclature is given in table i. Photo-electrochemical cells are set up by using these electrodes as photoanodes, i M NaOH is the electrolyte and a platinum wire serves as the counter electrode. The temperature of the cell is controlled within + 1 °. Table i. Composition and treatment nomenclature of TiNi oxide electrodes Composition 700°C o f TiNi alloy water atomic ~ Ni quenched.
Temperature dependence of the perform ance of PHC cells is studied by measuring the photovoltaic output characteristics. The speed of response is also affected by the changes in temperature. The results are discussed in terms of carrier generation-recombination process which is temperature dependent. The existance
700°C furnace cooled
525°C furnace cooled
45
A1
A2
A3
48
B1
B2
B3
49.2
C1
C2
C3
49.7
D1
D2
D3
50
E1
E2
E3
50.2
F1
F2
F3
50.7
G1
G2
G3
51.5
H1
H2
H3
52.5
I1
12
13
The speed of response characteristics of different cells are measured at diff]7
18
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
erent temperatures. The current voltage measurements are made at different temperatures with a constant intensity of illumination of i13 mw/cm 2, from room temperature to 80°O with an external load resistance in the circuit. The reverse saturation current I o is measured by applying a bias voltage in the same range of temperature. P~SULTS AND DISCUSSION Effect of temperature on the photovoltaic output characteristics : The current voltage curves at different temperatures are shown in fig.l for a typical cell FI/I M NaOH/Pt. It is observed that the curves shift outward as the temperature rises from 25°C to 60o0 due to increase in both short circuit current and the open circuit voltage. However after the temperature range of 50o-60oc both the current and voltage decrease. ~imilar results have been obtained by Vernon et al pS] and Rajeshwar et al [4] in case o{ ~chottky barrier cells 5ased on silcon and n-GaAs PEG cells respectively. The observed variations in the Voc and Isc are attributed to the effect of temperature on the electrical properties of the semiconductor and to the possible changes at the semiconductor-electrolyte interface. Assuming that the observations for the solid state Schottky barrier cells hold true in the case of PEG cells also, the observed changes can be explained. Agrawala et al [53 have observed that l)the wavelength response shifts towards red with increasing temperature because of the bandgap narrowing, 2) The hole diffusion length increases with increasing temperature and S) the absorption coefficient increases because of the shift in the spectral response. The initial increase in Isc upto 50oc is attributed to the increase in absorption coefficient of the semiconductor [6]. The increase in Isc however is limited because of the changes in series and shunt resistance of the cell with temperature. From figure 1 it is observed that the value of Rsh is maximum in the temperature range 50°C-60°C. At higher temperatures Rsh decreases reducing the current in the cell. The bulk resistance of the electrode decreases with temperature. The observed peak is therefore, a result of these two competing factors. At still higher temperatures Rsh becomes smaller and the current decreases. The decreases in Voc at higher temperatures as observed from figure i can be explained by applying the gchottk Z barrier model through the equation [7j I = Iph - lo(ex p qv/nkT)
(i)
where I o is the reverse saturation current density, V is the bias voltage, Iph is the photocurrent density and n is the diode quality factor. Rearranging this equation one obtains
Vol. 48, No. |
VOC : nkT in Iph (2) q In From figure 2 it is seen that In inoreas~ with temperature. As In increases Voc decreases. The observed decrease in Voc with increasing temperature is 2.66 m V / ° O Similar decrease in Voc is observed by Wysocki et al. [84 . From the relation 2, the variation o~ voc can be understood further. Voc depends on T as well as the value of I o. The initial rise in Voc upto 50°0 can be attributed to the increase in temperature T. After 50°0 the rise in In with temperature becomes sharp and Iph/Io becomes important hence Voc decreases.
0
0-1
V ( votts ) 0.2
0.3
0-4
- 50°C
50°C
< ~12
16
20
Fig I. Current-voltage curves with external load resistance for the cell FI/ M NaOH/P t at different temperatures. Figures 3(a,b,c) show the variation of short circuit current density, open circuit voltage and maximum power with temperature for the cells FI/I M NaOH/Pt, Hp/I M NaOH/Pt, and 0 2 / 1 M NaOH/P t respectively. The output power derived from all these cells is maximum in the. temperature range 50o0 - 60°C. Maximum power output is directly related to the conversion efficiency of the cell. This means that the conversion efficiency of these cells attains a maximum value in the temperature range of 500-6000. Similar results have been obtained by Robbins et al in case of CuIn~ 2 PEC cells [g]. This observation of a peak in the power is a
Vol. 48, No. 20
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
]
40
0
T e m p e r a t u r e (°C) 60
I .
:
I --
80
100
I
I
19
(h)
C
24 -12
0-6
18 -24
-0-4
-36
0.2 6 -48
I
0 20
Fig 2. Plot of reverse saturation current v e r s u s temperatures for the cells E2) E2/I M NaGH/Pt, H2) H211 M Na,O~/Pt, G2) G2/1 M NaOH/Pt, F2) F2ll M NaOH/P t and I2) I2/I m NaGH/P t .
(a)
40
I
60 Temperature (°C)
I
80
(c)
24 0-6
-0-8
18
o< o <
3-4 o m
24
12
:>6
0.2 18 0.4
< o
~
<
A < O
12
m
5
0 20
i 40
i 60
i 80
0
Temperature ( ° C ) 0.2
Fig 3. Output characteristics, Isc,Voc and maximum power at different temperatures for the cells 0
20
I I 60 40 Temperature (°C)
I
80
a) FI/I M NaOH/P t b) H2/I M NaC~IlP t c) C 2 / 1 ~4 NaOH/P t
2O
Vol. 48, No. I
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
resultant effect of changes in the electrical and optical properties of the semiconductor-electrolyte junction. The position of the maximu~ efficiency at 500-6000 is beneficial in the sense that the red and near-infrared radiations may now be used to increase the temperature of the cell which otherwise would be wasted. This is equivalent to increasing the spectral response of the cells to the near infrared region through the visible region.
(b)
-3,0
The galue of In is found to increase with temperature in agreement with the following relation [i0]
j
F2
O H -~ 4 .0 _I
In = ACT 2 exp (-~b/kT)
V
(3)
J
where A e is Richardson's constant and ~b is the barrier height at equilibrium. The validity of the relation 3 isascertained by the linear nature of log Io/T 2 versus I/T plots as shown in figure 4(a) and 4(b). Linearity of the plots indicates the validity of the relation for the oxide electrodes. The slope of this plot gives the value of ~b and is found to be of the order of 0.25 volt. These are in fair agreement with those observed from the photovoltaic characteristics of the PHC cells. In a PEC cell @b is the difference between the ECB and Eredox. The calculated values of ~b are listed in Table 2.
J
J
J
%
-5.0
-6.0
I 3.0
2"8
I 3.2
I 3.4
loo___oo(o K ~-1 T
Fig 4. Plots of Lop(lo/T2)versus for different cells. a) 12/'1 ;4 NaOH/Pt, F2/I ~ NaOH/P t, H2/I M NaOH/Pt,
(a)
(l/T)
E2/i M Na~IPt b) FI)FI/I M NaOH/Pt, F2)F2/I M Na0H/Pz, Fs)F3/I M NaOH/P t .
-3-
Table 2.
Sr. No.
Barrier height ~b for different cells Cell
~b calculated/ (volt)
-4.
G
2
H2
~
-5.(
-5.0
2.8
I 3.0
I 3.2
I__~_~(o K )-I T
I 3-4
1 2 3 4 5 6 7
E2/1M F2/I M G2/I M H2/I M I2/i M FI/I M F3/I M
NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt
O.213 0.248 O. 127 O. 213 O. 144 0.096 0.184
Bffect of Temperature on the Speed of Response Characteristics : Figure 5 shows the speed of response characteristics of a typical cell FI/I M NaOH/Pt at different temperatures. From these curves it is observed that the
Vol. 48, No. ]
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
steady state value of Isc under illumination increase upto 50°C and again decrease to a smaller value at 80°C. The time required to achieve steady state is observed to be longest at 50°C. It is also observed that the decay of Isc is also slow if the rise of Isc is slow. This change in the rise time and decay time may be attributed to a thermally activated recombination process. With increasing temperature the short circuit 24
21
(a)
2.3
/,
2-~ 18
u
8¢c J
°so°c
o
2.1
82°C
O
O O
0
0
60
120
180 240 Tirne(sec.)
300
360
420
Fig 5. Short circuit current rise and decay curves at different temperatures for a typical cell FI/I M NaOH/P t increases hence the time required to attain steady value is also longer. A similar nature for the rise and decay of open circuit voltage is observed for these cells. A typical graph of log Voc versus log time is presented in fig.6(a) and 6(b) for the cell B]/I M NaOH/Pt at different temperatures auring rise (6a) and during decay (6b). The graphs for temperatures between 50°C to 60o0 have larger slopes i.e. the values of 'a'are greater than that at other temperatures. At still higher temperatures 'a' is very small indicating an abrupt rise and decay of voltage with time. The decrease in the values of Voc with increasing temperature observed from the graph is due to the increase in the reverse saturation current with temperature. The decay of Voc in a PHC cell has been attributed to a time dependent process in the electrolyte by Lokhande and Pawar [i13. Under light excitation the electrons and holes generated at the semiconductorliquid junction are separated due to the local battery of the junction. For PEC cell with n type semiconductors as photoanode, the electrons are forced into the bulk of the semicohductors and ~he holes into the electrolyte, giving rise to a voltage with negative polarity at the semiconductor electrode. When the light is cut off, the PEC cell being open externally the excess electrons in a semi-
2.0
I 0.8
I 1-6
I 2"4
Log Time
2.3{ (Io)
162°C 2.2
25°C
~
3g.5Oc
73°c
u2-1
8o°c 2-0
1.cJ 0
--
~
I 0.8
82°C
I 1.6 Log Time
I 2-4
Fig 6. Plot of Log Voc versus log time at different temperatures for typical cell BI/I M NaOH/P t a) Rise b) Decay
22
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
conductor will cross the barrier and annihilate positive charges in the electrolyte. This being a time dependant process there persists a voltage for a long time even after the light is cut off. The voltage decay curves are sensitive to temperatures. This is attributed to the lower value of the potential barrier at higher temperature hence a relatively faster decay. P ffect of Composition and Heat Treatment on the Barrier Height The barrier height at the semiconductor electrolyte junction is influenced strongly by the presence of surface states. This implies that the barrier height is influenced also by the pre-history of the sample. Table 1 gives the values of the barrier heights
Vol. 48, No. l
sured at each wavelength. Figure 7 shows a plot of photocurrent against wavelength for the sample F2. It is observed that the curve has a photocurrent maximum peak at about 355 nm. The threshold wavelength for the onset of photocurrent for Fp is 470 nm,where as f o r TiO9 it is 400-nm. This shift in the spehtral response may be attributed tb the presence of Ni in the lattice of TiO 9 formed on the outer surface of the-electrode. Similar curves are obtained for other samples except in that their peak positions and threshold wavelengths are slightly different depending on the composition and treatment of the electrode. The effect of temperature on the spectral response is however small and cannot be brought out clearly owing to the small changes in the photocurrent.
1-2
~s
<(
0'8
/\
L
o a.
O4
o/° 0"0
320
Spectral Response Characteristics
~
,
4O0
"a
44o
c
F;. 4m
A (nm)
The spectral response of the PBC cells formed with TiNi oxide electrodes is studied by measuring the short circuit current at various wavelengths between 520 nm and 320 nm. The steady values of the photocurrents were mea-
Fig 7. Plot of photocurrent versus wavelength for typical cell £2/1 M NaOH/P t
R£F~R£NCHS I. ;4.~.Nrighton,D.L.~vlorse,A.£~.511is, O.~..~inley and H.B.Abrahmson,J.A~. Chem.Soc.9_~8, 44 (1976). 2. M.A. 6utler and u.s. Ginley, Nature 27___33,524 (1978). 3. S.M.Vernon and ~.A. Anderson, Appl. Phys. Lett. 26, 707 (1975). 4. K.~ajeshwar, P.Singh and R. Thapar, 3. Electrochem.Soc. 128,1760 (1981).
5. A. Agrawala,V.K. Tiwary,S.K.Agarwal and S.C.Jain, Solid State Electron,
23,121 (1980). 6. M.A. B u t l e r , J. Appl. Phys.48, 19&4 (1977). 7. E.H.Rhoderick, "Metal ~emiconductor Contacts" Chapter 2,Clarendron Press Oxford (1978).
Vol. 48, NO. 1
PHOTOELECTROCHEMICAL CELLS FOR~IEDWITH TiNi OXIDE ELECTRODES
8. J, Wysocki and P. Rapport, J. Appl. Phys. 31, 371 (1960). 9. M. Robbins, K. Bachman, V. Lambrecht, F.A.Thlel et al. J.Electrochem.&oc. I~5, 83 (1978). I0. P.Singh,K. Rajeshwar, J.Dubow,R. Job, J.Am.Chem.&oc. 102,4676 (1980).
23
11. C.D.Lokhande and S.H.Pawar, " S o l a r Energy Materials" in press (1982). 12. V.B.Chougule and S.H. Pawar, Mat. Res. Bull. (submitted). 13. B. Heichman and C. Byvic, J. ElectroChem. S oc. !2_8, 2601 (1981).
Solid State Communications, Vol.48,No.l, pp.;7-23,]983 Printed in Great Britain.
EFFECT OF TEMPERATURE
ON THE PERFORMANCE OF PHOTOELECTROCHEMICAL WITH TiNi OXIDE ELECTRODES V.B.Chougule
Energy Conversion
0038-]098/83 $3.00 + .00 Pergamon Press Ltd.
CELLS FORMEB
and S.H. Pawar
Laboratory, Bepartment Kolhapur 416004,
of Physics, INDIA
Shivaji University,
(Received ]2 July ]983 by S. Amelinckx) TiNi oxide electrodes are formed by oxidizing TiNi alloys of various compositions in air at different temperatures. These electrodes have been used as photoanodes in photoelectrochemical cells. The effect of temperature on these cells is studied by making measurements on the speed of response, spectral response and photovoltaic output characteristic of the cells. It is found that an optimum temperature exists for these cells. The operation of these cells at elevated temperature therefore means an increase in the conversion efficiency as the infrared region of the solar spectrum may be utilized for heating these cells.
INTRODUCTION
of an optimum temperature for the best performance of a PEG cell is brought out.
The conversion efficiency of a PEC cell depends on its spectral response and quantum efficiency. Hide band-gap materialso such as TiO2, SrTiO S etc. are stable against photo-corrosion. Their efficiency is however limited as their spectral response is restricted to the uv region. The semiconductor electrolyte interface is an important element in a PEC cell. The charge transfer taking place at the interface has a direct bearing on the performance of the cell. The rate of charge transfer may be improved by operating the cell at elevated temperatures. The electrical and optical properties of a PEG cell are modified by the change in temperature. The increase in temperature of the PEG cell may be brought about by utilizing the near infrared region of the spectrum which otherwise is wasted. This will add to the conversion efficiency considerably [i]. The effect of temperature on the performance of a PEC cell has many aspects [2]. The temperature dependence of spectral response is one of them. Changes in the flat band potential with temperature are mainly due to the change in potential across the Helmholtz layer. The electron affinity and the redox potential also change with temperature. All these factors will modify the current voltage characteristics, the power output and efficiency of these cells with change in temperature.
5XPERIMBNTAL TiNi alloys have been prepared in an argon arc furnace by melting appropriate amounts of Ti and Ni powders. The ingots are then cut suitably into plates of 2 mm thickness. These plates are oxidized in air at different temperatures. The composition and treatmentwise nomenclature is given in table i. Photo-electrochemical cells are set up by using these electrodes as photoanodes, i M NaOH is the electrolyte and a platinum wire serves as the counter electrode. The temperature of the cell is controlled within + 1 °. Table i. Composition and treatment nomenclature of TiNi oxide electrodes Composition 700°C o f TiNi alloy water atomic ~ Ni quenched.
Temperature dependence of the perform ance of PHC cells is studied by measuring the photovoltaic output characteristics. The speed of response is also affected by the changes in temperature. The results are discussed in terms of carrier generation-recombination process which is temperature dependent. The existance
700°C furnace cooled
525°C furnace cooled
45
A1
A2
A3
48
B1
B2
B3
49.2
C1
C2
C3
49.7
D1
D2
D3
50
E1
E2
E3
50.2
F1
F2
F3
50.7
G1
G2
G3
51.5
H1
H2
H3
52.5
I1
12
13
The speed of response characteristics of different cells are measured at diff]7
18
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
erent temperatures. The current voltage measurements are made at different temperatures with a constant intensity of illumination of i13 mw/cm 2, from room temperature to 80°O with an external load resistance in the circuit. The reverse saturation current I o is measured by applying a bias voltage in the same range of temperature. P~SULTS AND DISCUSSION Effect of temperature on the photovoltaic output characteristics : The current voltage curves at different temperatures are shown in fig.l for a typical cell FI/I M NaOH/Pt. It is observed that the curves shift outward as the temperature rises from 25°C to 60o0 due to increase in both short circuit current and the open circuit voltage. However after the temperature range of 50o-60oc both the current and voltage decrease. ~imilar results have been obtained by Vernon et al pS] and Rajeshwar et al [4] in case o{ ~chottky barrier cells 5ased on silcon and n-GaAs PEG cells respectively. The observed variations in the Voc and Isc are attributed to the effect of temperature on the electrical properties of the semiconductor and to the possible changes at the semiconductor-electrolyte interface. Assuming that the observations for the solid state Schottky barrier cells hold true in the case of PEG cells also, the observed changes can be explained. Agrawala et al [53 have observed that l)the wavelength response shifts towards red with increasing temperature because of the bandgap narrowing, 2) The hole diffusion length increases with increasing temperature and S) the absorption coefficient increases because of the shift in the spectral response. The initial increase in Isc upto 50oc is attributed to the increase in absorption coefficient of the semiconductor [6]. The increase in Isc however is limited because of the changes in series and shunt resistance of the cell with temperature. From figure 1 it is observed that the value of Rsh is maximum in the temperature range 50°C-60°C. At higher temperatures Rsh decreases reducing the current in the cell. The bulk resistance of the electrode decreases with temperature. The observed peak is therefore, a result of these two competing factors. At still higher temperatures Rsh becomes smaller and the current decreases. The decreases in Voc at higher temperatures as observed from figure i can be explained by applying the gchottk Z barrier model through the equation [7j I = Iph - lo(ex p qv/nkT)
(i)
where I o is the reverse saturation current density, V is the bias voltage, Iph is the photocurrent density and n is the diode quality factor. Rearranging this equation one obtains
Vol. 48, No. |
VOC : nkT in Iph (2) q In From figure 2 it is seen that In inoreas~ with temperature. As In increases Voc decreases. The observed decrease in Voc with increasing temperature is 2.66 m V / ° O Similar decrease in Voc is observed by Wysocki et al. [84 . From the relation 2, the variation o~ voc can be understood further. Voc depends on T as well as the value of I o. The initial rise in Voc upto 50°0 can be attributed to the increase in temperature T. After 50°0 the rise in In with temperature becomes sharp and Iph/Io becomes important hence Voc decreases.
0
0-1
V ( votts ) 0.2
0.3
0-4
- 50°C
50°C
< ~12
16
20
Fig I. Current-voltage curves with external load resistance for the cell FI/ M NaOH/P t at different temperatures. Figures 3(a,b,c) show the variation of short circuit current density, open circuit voltage and maximum power with temperature for the cells FI/I M NaOH/Pt, Hp/I M NaOH/Pt, and 0 2 / 1 M NaOH/P t respectively. The output power derived from all these cells is maximum in the. temperature range 50o0 - 60°C. Maximum power output is directly related to the conversion efficiency of the cell. This means that the conversion efficiency of these cells attains a maximum value in the temperature range of 500-6000. Similar results have been obtained by Robbins et al in case of CuIn~ 2 PEC cells [g]. This observation of a peak in the power is a
Vol. 48, No. 20
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
]
40
0
T e m p e r a t u r e (°C) 60
I .
:
I --
80
100
I
I
19
(h)
C
24 -12
0-6
18 -24
-0-4
-36
0.2 6 -48
I
0 20
Fig 2. Plot of reverse saturation current v e r s u s temperatures for the cells E2) E2/I M NaGH/Pt, H2) H211 M Na,O~/Pt, G2) G2/1 M NaOH/Pt, F2) F2ll M NaOH/P t and I2) I2/I m NaGH/P t .
(a)
40
I
60 Temperature (°C)
I
80
(c)
24 0-6
-0-8
18
o< o <
3-4 o m
24
12
:>6
0.2 18 0.4
< o
~
<
A < O
12
m
5
0 20
i 40
i 60
i 80
0
Temperature ( ° C ) 0.2
Fig 3. Output characteristics, Isc,Voc and maximum power at different temperatures for the cells 0
20
I I 60 40 Temperature (°C)
I
80
a) FI/I M NaOH/P t b) H2/I M NaC~IlP t c) C 2 / 1 ~4 NaOH/P t
2O
Vol. 48, No. I
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
resultant effect of changes in the electrical and optical properties of the semiconductor-electrolyte junction. The position of the maximu~ efficiency at 500-6000 is beneficial in the sense that the red and near-infrared radiations may now be used to increase the temperature of the cell which otherwise would be wasted. This is equivalent to increasing the spectral response of the cells to the near infrared region through the visible region.
(b)
-3,0
The galue of In is found to increase with temperature in agreement with the following relation [i0]
j
F2
O H -~ 4 .0 _I
In = ACT 2 exp (-~b/kT)
V
(3)
J
where A e is Richardson's constant and ~b is the barrier height at equilibrium. The validity of the relation 3 isascertained by the linear nature of log Io/T 2 versus I/T plots as shown in figure 4(a) and 4(b). Linearity of the plots indicates the validity of the relation for the oxide electrodes. The slope of this plot gives the value of ~b and is found to be of the order of 0.25 volt. These are in fair agreement with those observed from the photovoltaic characteristics of the PHC cells. In a PEC cell @b is the difference between the ECB and Eredox. The calculated values of ~b are listed in Table 2.
J
J
J
%
-5.0
-6.0
I 3.0
2"8
I 3.2
I 3.4
loo___oo(o K ~-1 T
Fig 4. Plots of Lop(lo/T2)versus for different cells. a) 12/'1 ;4 NaOH/Pt, F2/I ~ NaOH/P t, H2/I M NaOH/Pt,
(a)
(l/T)
E2/i M Na~IPt b) FI)FI/I M NaOH/Pt, F2)F2/I M Na0H/Pz, Fs)F3/I M NaOH/P t .
-3-
Table 2.
Sr. No.
Barrier height ~b for different cells Cell
~b calculated/ (volt)
-4.
G
2
H2
~
-5.(
-5.0
2.8
I 3.0
I 3.2
I__~_~(o K )-I T
I 3-4
1 2 3 4 5 6 7
E2/1M F2/I M G2/I M H2/I M I2/i M FI/I M F3/I M
NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt NaOH/Pt
O.213 0.248 O. 127 O. 213 O. 144 0.096 0.184
Bffect of Temperature on the Speed of Response Characteristics : Figure 5 shows the speed of response characteristics of a typical cell FI/I M NaOH/Pt at different temperatures. From these curves it is observed that the
Vol. 48, No. ]
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
steady state value of Isc under illumination increase upto 50°C and again decrease to a smaller value at 80°C. The time required to achieve steady state is observed to be longest at 50°C. It is also observed that the decay of Isc is also slow if the rise of Isc is slow. This change in the rise time and decay time may be attributed to a thermally activated recombination process. With increasing temperature the short circuit 24
21
(a)
2.3
/,
2-~ 18
u
8¢c J
°so°c
o
2.1
82°C
O
O O
0
0
60
120
180 240 Tirne(sec.)
300
360
420
Fig 5. Short circuit current rise and decay curves at different temperatures for a typical cell FI/I M NaOH/P t increases hence the time required to attain steady value is also longer. A similar nature for the rise and decay of open circuit voltage is observed for these cells. A typical graph of log Voc versus log time is presented in fig.6(a) and 6(b) for the cell B]/I M NaOH/Pt at different temperatures auring rise (6a) and during decay (6b). The graphs for temperatures between 50°C to 60o0 have larger slopes i.e. the values of 'a'are greater than that at other temperatures. At still higher temperatures 'a' is very small indicating an abrupt rise and decay of voltage with time. The decrease in the values of Voc with increasing temperature observed from the graph is due to the increase in the reverse saturation current with temperature. The decay of Voc in a PHC cell has been attributed to a time dependent process in the electrolyte by Lokhande and Pawar [i13. Under light excitation the electrons and holes generated at the semiconductorliquid junction are separated due to the local battery of the junction. For PEC cell with n type semiconductors as photoanode, the electrons are forced into the bulk of the semicohductors and ~he holes into the electrolyte, giving rise to a voltage with negative polarity at the semiconductor electrode. When the light is cut off, the PEC cell being open externally the excess electrons in a semi-
2.0
I 0.8
I 1-6
I 2"4
Log Time
2.3{ (Io)
162°C 2.2
25°C
~
3g.5Oc
73°c
u2-1
8o°c 2-0
1.cJ 0
--
~
I 0.8
82°C
I 1.6 Log Time
I 2-4
Fig 6. Plot of Log Voc versus log time at different temperatures for typical cell BI/I M NaOH/P t a) Rise b) Decay
22
PHOTOELECTROCHEMICAL CELLS FORMED WITH TiNi OXIDE ELECTRODES
conductor will cross the barrier and annihilate positive charges in the electrolyte. This being a time dependant process there persists a voltage for a long time even after the light is cut off. The voltage decay curves are sensitive to temperatures. This is attributed to the lower value of the potential barrier at higher temperature hence a relatively faster decay. P ffect of Composition and Heat Treatment on the Barrier Height The barrier height at the semiconductor electrolyte junction is influenced strongly by the presence of surface states. This implies that the barrier height is influenced also by the pre-history of the sample. Table 1 gives the values of the barrier heights
Vol. 48, No. l
sured at each wavelength. Figure 7 shows a plot of photocurrent against wavelength for the sample F2. It is observed that the curve has a photocurrent maximum peak at about 355 nm. The threshold wavelength for the onset of photocurrent for Fp is 470 nm,where as f o r TiO9 it is 400-nm. This shift in the spehtral response may be attributed tb the presence of Ni in the lattice of TiO 9 formed on the outer surface of the-electrode. Similar curves are obtained for other samples except in that their peak positions and threshold wavelengths are slightly different depending on the composition and treatment of the electrode. The effect of temperature on the spectral response is however small and cannot be brought out clearly owing to the small changes in the photocurrent.
1-2
~s
<(
0'8
/\
L
o a.
O4
o/° 0"0
320
Spectral Response Characteristics
~
,
4O0
"a
44o
c
F;. 4m
A (nm)
The spectral response of the PBC cells formed with TiNi oxide electrodes is studied by measuring the short circuit current at various wavelengths between 520 nm and 320 nm. The steady values of the photocurrents were mea-
Fig 7. Plot of photocurrent versus wavelength for typical cell £2/1 M NaOH/P t
R£F~R£NCHS I. ;4.~.Nrighton,D.L.~vlorse,A.£~.511is, O.~..~inley and H.B.Abrahmson,J.A~. Chem.Soc.9_~8, 44 (1976). 2. M.A. 6utler and u.s. Ginley, Nature 27___33,524 (1978). 3. S.M.Vernon and ~.A. Anderson, Appl. Phys. Lett. 26, 707 (1975). 4. K.~ajeshwar, P.Singh and R. Thapar, 3. Electrochem.Soc. 128,1760 (1981).
5. A. Agrawala,V.K. Tiwary,S.K.Agarwal and S.C.Jain, Solid State Electron,
23,121 (1980). 6. M.A. B u t l e r , J. Appl. Phys.48, 19&4 (1977). 7. E.H.Rhoderick, "Metal ~emiconductor Contacts" Chapter 2,Clarendron Press Oxford (1978).
Vol. 48, NO. 1
PHOTOELECTROCHEMICAL CELLS FOR~IEDWITH TiNi OXIDE ELECTRODES
8. J, Wysocki and P. Rapport, J. Appl. Phys. 31, 371 (1960). 9. M. Robbins, K. Bachman, V. Lambrecht, F.A.Thlel et al. J.Electrochem.&oc. I~5, 83 (1978). I0. P.Singh,K. Rajeshwar, J.Dubow,R. Job, J.Am.Chem.&oc. 102,4676 (1980).
23
11. C.D.Lokhande and S.H.Pawar, " S o l a r Energy Materials" in press (1982). 12. V.B.Chougule and S.H. Pawar, Mat. Res. Bull. (submitted). 13. B. Heichman and C. Byvic, J. ElectroChem. S oc. !2_8, 2601 (1981).