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The electrochemical deposition and formation of cadmium sulphide thin film electrodes in aqueous electrolytes
J. Electroanal. Chem., 119 (1981) 409--412
409
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Preliminary note THE ELECTROCHEMICAL DEPOSITION AND FORMATION OF CADMIUM SULPHIDE THIN FILM ELECTRODES IN AQUEOUS ELECTROLYTES
J.F. McCANN and M. SKYLLAS KAZACOS Physics Department, University of New South Wales, P.O. Box1, Kensington, 2033, New South Wales (Australia)
(Received 20th August 1980; in revised form 15th December 1980)
INTRODUCTION CdS films have the potential to play an important role in practical solar energy conversion devices such as Cu2 S/CdS and CdS/CdTe heterojunction cells [1,2] and as electrodes in photoelectrochemical cells [3]. Cadmium sulphide cannot be used as an efficient photoelectrode by itself as its direct band gap of 2.4 eV [4] is too large for efficient solar energy conversion. However, in photoelectrochemical cells CdS can be used either in a layered electrode arrangement, a heterodiode arrangement such as n-CdS/n-GaAs [5] or in a mixed electrode such as CdSxSey [6]. A m e t h o d for the cathodic deposition of CdS from a non aqueous system has been reported by Baranski and Fawcett [ 7 ]. In addition the anodic formation of CdS films on Cd sheet or cast rods has been reported by Miller and Heller [8] and Peter [9]. In this paper a novel m e t h o d for the cathodic deposition of CdS films from an aqueous electrolyte as well as the anodic formation of a CdS film on a thin electrochemically deposited Cd film are described. EXPERIMENTAL The thin films of CdS described in this study were prepared on strips of Ti metal. The electrolytes were prepared with AR grade reagents and distilled water. A standard three electrode electrochemical system was used to control the potential of the electrodes under examination via a Pine Instrument RDE-3 potentiostat unless otherwise specified. A carbon rod and a SCE were used as the counter electrode and reference electrode respectively in the potentiostated system. The CdS counter electrode used was prepared by the m e t h o d of Hodes et al. [10]. Cell current--voltage (i--V) curves were obtained by varying a load resistor between the CdS and CoS electrodes. In this instance the cell voltage was determined by measuring the voltage across the total external resistance across the cell and the cell current was determined by measuring the voltage developed across a high precision 1 ~2 resistor. A Fluke model 8020A digital voltmeter was used t o m o n i t o r the respective voltages. 0022-0728/81/0000--0000/$ 02.50, © 1981, Elsevier Sequoia S.A.
410
A 300 W General Electric ELH lamp was used to illuminate the CdS electrodes (intensity ca. 150 mW cm -2 ). RESULTS AND DISCUSSION The cathodic deposition of CdS films was achieved by the electroreduction of sodium thiosulphate in the presence of Cd 2÷. Figure 1 shows the voltammogram obtained for a titanium electrode in a 5 mM solution of Na2 $2 03 in 0.2 M NH3/0.2 M NH4 Cl buffer, in the potential range 0 to -- 1.6 V vs. SCE. A single reduction wave can be observed at approximately -- 1.1 V, before the current increases rapidly due to the evolution of H2. The reduction wave is presumably due to the reaction $2032- + 3H20 + 8e- -~ 2S2- + 6OH-
(I)
In Fig. 2, a voltammogramis presented for the titanium electrode in a 1 M NH3/1 M NH4 CI solution containing 5 mM Na2 $203,0.2 M CdCI2 and 0.2 M EDTA. The reduction wave corresponding to the reduction of $2032can be observed as the potential is scanned negative from 0 V vs. SCE, but at potentials more negative than -- 1.3 V, the current increases rapidly due to the reduction of Cd2+. Reversingthe scan from this point gives rise to an anodic peak corresponding to the reoxidation of Cd. Cadmium deposition does not occur to any great extent for potentials greater than -- 1.3 V in this electrolyte since cadmium ions are present in the form of an EDTA-NH3 complex which inhibits the deposition of Cd. It is apparently the inhibition of cadmium deposition t h a t makes the cathodic deposition of CdS films possible by the m e t h o d described below in detail. The starting solution consists of 60 ml of 0.1 M CdCI2 • H2 O/0.1 M EDTA/ 10 mM Na2S203/0.2 M NH3/0.2 M NH4C1. Two Ti strips which are either etched in 2% HF or abraded beforehand are fixed at different positions in the plating solution with one strip being connected to the potentiostat and the other strip being left disconnected at this point. The potentiostat is then
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~
"-
o
,
L -1 -I0!
-2
-1'8 -1'6 -14
-1'2 -10 -0'8 -0'6 -()4 -0'2 E vs SCEIVOLTS
-3
-1'6 -14
-12 -io -o8 -o'6 -o'4 -0'2
o
E vs SCE/VOLTS
Fig. 1. C o n t r o l l e d p o t e n t i a l s w e e p scan o f a Ti e l e c t r o d e in a 5 m M s o l u t i o n o f Na 2 S 2 O , in 0.2 M N H 3 / 0 . 2 M NH4C1 b u f f e r , at 1 V r a i n - ' scan rate. Fig. 2. C o n t r o l l e d p o t e n t i a l s w e e p cycle o f a Ti e l e c t r o d e in 5 m M N a 2 S 2 0 3 / 0 . 2 M CdSO 4 / 0.2 M E D T A in 1 M NH 3/1 M NH4C1 at a scan rate o f 1 V r a i n - ' .
411
adjusted so that when it is switched on it will fix the potential of the Ti strip at - - 1 . 2 V vs. SCE. After turning on the potentiostat a greyish film of Cd starts depositing on the Ti electrode. A m m o n i a (28%, w/w) is now added to the solution (1--2 ml) until the Cd 2+ reduction wave shifts to more negative potentials and CdS is the only material depositing on the Ti. The other Ti strip is now shorted to the Ti strip on which deposition is taking place and the potential is adjusted to -- 1.15 V. Provided the correct a m o u n t of NH3 has been added a CdS film will start depositing on the newly connected Ti as well as continue to deposit on the original Ti strip. Films grown by us were typically deposited over 30 min. Under visible examination the films appeared to be uniform and did n o t show a t e n d e n c y to peel off the substrate when subjected to a steady stream of distilled water. In the above process the a m o u n t of NH3 added to the electrolyte is fairly critical. If too little NH3 is added the CdS deposited contains codeposited Cd. This can be demonstrated by shorting the CdS electrode to a CoS electrode in 1 M Na2 S/1 M S/1 M NaOH. When Cd is present in large excess there is a significant dark voltage between the electrodes (about 0.25 V) and a significant dark current (say 1--6 mA) which drops rapidly with time as the excess Cd forms CdS by an anodic process. On the other hand if excess NH3 is added the deposition of CdS is retarded. Further development will u n d o u b t e d l y establish the o p t i m u m NH3 concentration. It also appears to be important to deposit the CdS on a clean substrate and n o t one that has been used for potential scans. For the anodic formation of CdS thin film electrodes on a Ti substrate the bath for the deposition of the Cd film consists of 0.05 M CdC12/0.2 M EDTA/ 1 M NaOH. The reduction of Cd 2+ in this electrolyte occurs at potentials more negative than -- 1.3 V vs. SCE. Cadmium is deposited on Ti at -- 1.595 V for about 3 min. The Cd coated Ti is then placed in a 1 M S/1 M Na2 S/1 M NaOH electrolyte and shorted to an appropriate counter electrode such as CoS. The electrodes are left in this configuration for at least 1 h and up to 12 h before use. Alternatively, the anodic CdS films can be formed under potentiostatic control as has been described by other workers [8,9]. The important factor in this process is apparently the constitution of the Cd plating solution. Of the various Cd plating baths that have been tried by us the Cd2÷/EDTA/NaOH bath has been the basis for anodic CdS films with the higher photoactivities. The uncorrected normalised photocurrent--wavelength responses of Cd8 films prepared by cathodic deposition and by dissolution are shown in Fig. 3(a) and Fig. 3(b), respectively. The responses are similar to each other and are typical of those expected for CdS, i.e. a rapid decline in the photoresponse commencing at a b o u t 480 nm until about 600 nm where it is negligible. The decline in the photoresponses between 475 nm and 400 nm in the curves is due to the drop-off in intensity of the quartz iodine lamp in this wavelength range. The photocurrent .--voltage curves of two CdS[1 M Na2 S/1 M S/1 M NaOH tCoS cells are shown in Fig. 4. In Fig. 4(a) the CdS photoelectrode is an unannealed unetched cathodically deposited CdS film on Ti. This electrode exhibited a reasonable open circuit voltage (ca. 0.42 V) and fill factor (0.53). In Fig. 4(b)
412 10
06
10
(a)
OZ
~o5 400
450
500
550
600
Wavetength / nm
C'b)
(b) c 08
4
O6
3
~2
OZ 02 400
450
500
Wavelength / nm
550
600
G
01
02
03
Oh
0'5
ECELL/VOLTS
Fig. 3. (a) Normalized photocurrent spectral response curve for CdS film prepared by cathodic deposition in a solution of 0.05 M S/1 M Na2S/1 M NaOH. Potential -- 0.74 V vs. SCE. (b) As (a) except CdS film prepared by dissolution of Cd in 1 M S/1 M Na2S. Potential -- 0.74 V vs. SCE. Fig. 4. Current--voltage characteristics of CdS--CoS cell in 1 M S/1 M Na~ S/1 M NaOH. Electrode area ca. 2 cm 2 ; illumination intensity ca. 150 mW cm -2 . (a) CdS prepared by cathodic deposition; (b) CdS prepared by dissolution of Cd in S/S 2 - . t h e C d S p h o t o e l e c t r o d e is a n u n a n n e a l e d u n e t c h e d a n o d i c a l l y f o r m e d C d S film on a Ti substrate. This electrode exhibited a reasonable open circuit voltage (0.45 V ) b u t a p o o r fill f a c t o r ( 0 . 3 2 ) s u g g e s t i n g t h a t the resistance o f
the anodically formed film is much higher than the cathodically deposited CdS. However, it should be possible to improve considerably the performance characteristics o f b o t h t y p e s o f f i l m s b y annealing and surface treatments. ACKNOWLEDGEMENTS
This work was supported by the National Energy Research Development and Demonstration Council. M. Skyllas Kazacos has been supported by a Queen Elizabeth II Fellowship. The authors thank Prof. D. Haneman for useful discussions. REFERENCES 1 2 3 4 5 6 7 8 9 10
D.M. Perkins, Adv. Energy Cony., 7 (1968) 265. Y.Y. Ma, A.L. Fahrenbruch and R.H. Bube, Appl. Phys. Lett., 30 (1977) 423. C.C. Tsou and J.R. Cleveland, J. Appl. Phys., 51 (1980) 455. M. Balkanski and R.D. Waldxon, Phys. Rev., 112 (1958) 123. S. Wagner and J.L. Shay in A. Heller (Ed.), S e m i c o n d u c t o r Liquid-Junction Solar Cells, The Electrochemical Society, Princeton, N.J., 1977, p. 231. R.N. Noufi, P.A. Kohl and A.J. Bard, J. Electroehem. Soc., 125 (1978) 375. A.S. Baranski and W.R. Fawcett, J. Electrochem. Soe., 127 (1980) 766. B. Miller and A. Heller, Nature, 262 (1976) 680. L.M. Peter, Electroehim. Aeta, 23 (1978) 165. G. Hodes, J. Manassen and n . Cahen, J. Appl. Eleetrochem., 7 (1977) 181.
409
Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands Preliminary note THE ELECTROCHEMICAL DEPOSITION AND FORMATION OF CADMIUM SULPHIDE THIN FILM ELECTRODES IN AQUEOUS ELECTROLYTES
J.F. McCANN and M. SKYLLAS KAZACOS Physics Department, University of New South Wales, P.O. Box1, Kensington, 2033, New South Wales (Australia)
(Received 20th August 1980; in revised form 15th December 1980)
INTRODUCTION CdS films have the potential to play an important role in practical solar energy conversion devices such as Cu2 S/CdS and CdS/CdTe heterojunction cells [1,2] and as electrodes in photoelectrochemical cells [3]. Cadmium sulphide cannot be used as an efficient photoelectrode by itself as its direct band gap of 2.4 eV [4] is too large for efficient solar energy conversion. However, in photoelectrochemical cells CdS can be used either in a layered electrode arrangement, a heterodiode arrangement such as n-CdS/n-GaAs [5] or in a mixed electrode such as CdSxSey [6]. A m e t h o d for the cathodic deposition of CdS from a non aqueous system has been reported by Baranski and Fawcett [ 7 ]. In addition the anodic formation of CdS films on Cd sheet or cast rods has been reported by Miller and Heller [8] and Peter [9]. In this paper a novel m e t h o d for the cathodic deposition of CdS films from an aqueous electrolyte as well as the anodic formation of a CdS film on a thin electrochemically deposited Cd film are described. EXPERIMENTAL The thin films of CdS described in this study were prepared on strips of Ti metal. The electrolytes were prepared with AR grade reagents and distilled water. A standard three electrode electrochemical system was used to control the potential of the electrodes under examination via a Pine Instrument RDE-3 potentiostat unless otherwise specified. A carbon rod and a SCE were used as the counter electrode and reference electrode respectively in the potentiostated system. The CdS counter electrode used was prepared by the m e t h o d of Hodes et al. [10]. Cell current--voltage (i--V) curves were obtained by varying a load resistor between the CdS and CoS electrodes. In this instance the cell voltage was determined by measuring the voltage across the total external resistance across the cell and the cell current was determined by measuring the voltage developed across a high precision 1 ~2 resistor. A Fluke model 8020A digital voltmeter was used t o m o n i t o r the respective voltages. 0022-0728/81/0000--0000/$ 02.50, © 1981, Elsevier Sequoia S.A.
410
A 300 W General Electric ELH lamp was used to illuminate the CdS electrodes (intensity ca. 150 mW cm -2 ). RESULTS AND DISCUSSION The cathodic deposition of CdS films was achieved by the electroreduction of sodium thiosulphate in the presence of Cd 2÷. Figure 1 shows the voltammogram obtained for a titanium electrode in a 5 mM solution of Na2 $2 03 in 0.2 M NH3/0.2 M NH4 Cl buffer, in the potential range 0 to -- 1.6 V vs. SCE. A single reduction wave can be observed at approximately -- 1.1 V, before the current increases rapidly due to the evolution of H2. The reduction wave is presumably due to the reaction $2032- + 3H20 + 8e- -~ 2S2- + 6OH-
(I)
In Fig. 2, a voltammogramis presented for the titanium electrode in a 1 M NH3/1 M NH4 CI solution containing 5 mM Na2 $203,0.2 M CdCI2 and 0.2 M EDTA. The reduction wave corresponding to the reduction of $2032can be observed as the potential is scanned negative from 0 V vs. SCE, but at potentials more negative than -- 1.3 V, the current increases rapidly due to the reduction of Cd2+. Reversingthe scan from this point gives rise to an anodic peak corresponding to the reoxidation of Cd. Cadmium deposition does not occur to any great extent for potentials greater than -- 1.3 V in this electrolyte since cadmium ions are present in the form of an EDTA-NH3 complex which inhibits the deposition of Cd. It is apparently the inhibition of cadmium deposition t h a t makes the cathodic deposition of CdS films possible by the m e t h o d described below in detail. The starting solution consists of 60 ml of 0.1 M CdCI2 • H2 O/0.1 M EDTA/ 10 mM Na2S203/0.2 M NH3/0.2 M NH4C1. Two Ti strips which are either etched in 2% HF or abraded beforehand are fixed at different positions in the plating solution with one strip being connected to the potentiostat and the other strip being left disconnected at this point. The potentiostat is then
-O.c
~
"-
o
,
L -1 -I0!
-2
-1'8 -1'6 -14
-1'2 -10 -0'8 -0'6 -()4 -0'2 E vs SCEIVOLTS
-3
-1'6 -14
-12 -io -o8 -o'6 -o'4 -0'2
o
E vs SCE/VOLTS
Fig. 1. C o n t r o l l e d p o t e n t i a l s w e e p scan o f a Ti e l e c t r o d e in a 5 m M s o l u t i o n o f Na 2 S 2 O , in 0.2 M N H 3 / 0 . 2 M NH4C1 b u f f e r , at 1 V r a i n - ' scan rate. Fig. 2. C o n t r o l l e d p o t e n t i a l s w e e p cycle o f a Ti e l e c t r o d e in 5 m M N a 2 S 2 0 3 / 0 . 2 M CdSO 4 / 0.2 M E D T A in 1 M NH 3/1 M NH4C1 at a scan rate o f 1 V r a i n - ' .
411
adjusted so that when it is switched on it will fix the potential of the Ti strip at - - 1 . 2 V vs. SCE. After turning on the potentiostat a greyish film of Cd starts depositing on the Ti electrode. A m m o n i a (28%, w/w) is now added to the solution (1--2 ml) until the Cd 2+ reduction wave shifts to more negative potentials and CdS is the only material depositing on the Ti. The other Ti strip is now shorted to the Ti strip on which deposition is taking place and the potential is adjusted to -- 1.15 V. Provided the correct a m o u n t of NH3 has been added a CdS film will start depositing on the newly connected Ti as well as continue to deposit on the original Ti strip. Films grown by us were typically deposited over 30 min. Under visible examination the films appeared to be uniform and did n o t show a t e n d e n c y to peel off the substrate when subjected to a steady stream of distilled water. In the above process the a m o u n t of NH3 added to the electrolyte is fairly critical. If too little NH3 is added the CdS deposited contains codeposited Cd. This can be demonstrated by shorting the CdS electrode to a CoS electrode in 1 M Na2 S/1 M S/1 M NaOH. When Cd is present in large excess there is a significant dark voltage between the electrodes (about 0.25 V) and a significant dark current (say 1--6 mA) which drops rapidly with time as the excess Cd forms CdS by an anodic process. On the other hand if excess NH3 is added the deposition of CdS is retarded. Further development will u n d o u b t e d l y establish the o p t i m u m NH3 concentration. It also appears to be important to deposit the CdS on a clean substrate and n o t one that has been used for potential scans. For the anodic formation of CdS thin film electrodes on a Ti substrate the bath for the deposition of the Cd film consists of 0.05 M CdC12/0.2 M EDTA/ 1 M NaOH. The reduction of Cd 2+ in this electrolyte occurs at potentials more negative than -- 1.3 V vs. SCE. Cadmium is deposited on Ti at -- 1.595 V for about 3 min. The Cd coated Ti is then placed in a 1 M S/1 M Na2 S/1 M NaOH electrolyte and shorted to an appropriate counter electrode such as CoS. The electrodes are left in this configuration for at least 1 h and up to 12 h before use. Alternatively, the anodic CdS films can be formed under potentiostatic control as has been described by other workers [8,9]. The important factor in this process is apparently the constitution of the Cd plating solution. Of the various Cd plating baths that have been tried by us the Cd2÷/EDTA/NaOH bath has been the basis for anodic CdS films with the higher photoactivities. The uncorrected normalised photocurrent--wavelength responses of Cd8 films prepared by cathodic deposition and by dissolution are shown in Fig. 3(a) and Fig. 3(b), respectively. The responses are similar to each other and are typical of those expected for CdS, i.e. a rapid decline in the photoresponse commencing at a b o u t 480 nm until about 600 nm where it is negligible. The decline in the photoresponses between 475 nm and 400 nm in the curves is due to the drop-off in intensity of the quartz iodine lamp in this wavelength range. The photocurrent .--voltage curves of two CdS[1 M Na2 S/1 M S/1 M NaOH tCoS cells are shown in Fig. 4. In Fig. 4(a) the CdS photoelectrode is an unannealed unetched cathodically deposited CdS film on Ti. This electrode exhibited a reasonable open circuit voltage (ca. 0.42 V) and fill factor (0.53). In Fig. 4(b)
412 10
06
10
(a)
OZ
~o5 400
450
500
550
600
Wavetength / nm
C'b)
(b) c 08
4
O6
3
~2
OZ 02 400
450
500
Wavelength / nm
550
600
G
01
02
03
Oh
0'5
ECELL/VOLTS
Fig. 3. (a) Normalized photocurrent spectral response curve for CdS film prepared by cathodic deposition in a solution of 0.05 M S/1 M Na2S/1 M NaOH. Potential -- 0.74 V vs. SCE. (b) As (a) except CdS film prepared by dissolution of Cd in 1 M S/1 M Na2S. Potential -- 0.74 V vs. SCE. Fig. 4. Current--voltage characteristics of CdS--CoS cell in 1 M S/1 M Na~ S/1 M NaOH. Electrode area ca. 2 cm 2 ; illumination intensity ca. 150 mW cm -2 . (a) CdS prepared by cathodic deposition; (b) CdS prepared by dissolution of Cd in S/S 2 - . t h e C d S p h o t o e l e c t r o d e is a n u n a n n e a l e d u n e t c h e d a n o d i c a l l y f o r m e d C d S film on a Ti substrate. This electrode exhibited a reasonable open circuit voltage (0.45 V ) b u t a p o o r fill f a c t o r ( 0 . 3 2 ) s u g g e s t i n g t h a t the resistance o f
the anodically formed film is much higher than the cathodically deposited CdS. However, it should be possible to improve considerably the performance characteristics o f b o t h t y p e s o f f i l m s b y annealing and surface treatments. ACKNOWLEDGEMENTS
This work was supported by the National Energy Research Development and Demonstration Council. M. Skyllas Kazacos has been supported by a Queen Elizabeth II Fellowship. The authors thank Prof. D. Haneman for useful discussions. REFERENCES 1 2 3 4 5 6 7 8 9 10
D.M. Perkins, Adv. Energy Cony., 7 (1968) 265. Y.Y. Ma, A.L. Fahrenbruch and R.H. Bube, Appl. Phys. Lett., 30 (1977) 423. C.C. Tsou and J.R. Cleveland, J. Appl. Phys., 51 (1980) 455. M. Balkanski and R.D. Waldxon, Phys. Rev., 112 (1958) 123. S. Wagner and J.L. Shay in A. Heller (Ed.), S e m i c o n d u c t o r Liquid-Junction Solar Cells, The Electrochemical Society, Princeton, N.J., 1977, p. 231. R.N. Noufi, P.A. Kohl and A.J. Bard, J. Electroehem. Soc., 125 (1978) 375. A.S. Baranski and W.R. Fawcett, J. Electrochem. Soe., 127 (1980) 766. B. Miller and A. Heller, Nature, 262 (1976) 680. L.M. Peter, Electroehim. Aeta, 23 (1978) 165. G. Hodes, J. Manassen and n . Cahen, J. Appl. Eleetrochem., 7 (1977) 181.