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Copper-phthalocyanine thin film photoelectrochemical cells
Applied Surface Science 4 8 / 4 9 (1991) 517-520 North-Holland
517
Copper-phthalocyanine thin film photoelectrochemical cells M a n a b u Takeuchi, Masayoshi Masui and Yoshihiro Momose Faculty of Engineering, lbaraki Unit,ersiO', 4-12-1 Nakanarusawa, Hitachi 316, Japan Received 13 August 1990: accepted for publication 28 August 1990
Copper-phthalocyanlne (CuPc) thin layers were prepared by vacuum evaporation and RF sputtering, and their photoelectrochemical behavior in NaCI solution was studied. The photoelectrochemical cells with evaporated CuPc layers showed photosensitivity in the visible region, while the cell with RF-sputtered CuPc layers showed little photoresponse. The cell with evaporated CuPc layers over-coated with an RF-sputtered thin CuPc layer was fairly stable in NaCI solution. However, the energy conversion efficiency was low due to the high resistivity of the CuPc layers.
1. Introduction
Photoelectrolysis of water with single crystal TiO 2 was proposed by Fujishima and Honda in 1972, in which water was split into hydrogen and oxygen by solar energy using the photochemical effect in semiconductors [1]. Studies on the direct conversion of solar energy into electricity, instead of electrolysis of water, have also been made [2]. Generally, photochemical cells should satisfy the following conditions in order to convert sola; energy into electricity efficiently: (1) Most of the solar radiation must be absorbed by the photoelectrochemical cell. (2) Electrons and holes must be separated without recombination. (3) Photoelectrochemical cells must be stable in the solution used. There are few materials which satisfy all these conditions. For example, TiO2 is stable in many solutions, but it absorbs only UV light [3]. While Cu20 absorbs visible light, it is not stable in most solutions [4]. Copper-phthalocyanine is a blue pigment and is also a p-type semiconductor. Recently many attempts have been made to utilize phthaIocyanines as solid-state solar cells because of their stability and spectral fitting to sun light [5]. However, relatively little information is available
in the literature on the electrochemical behavior of this material [6]. We have tried to use copper-phthalocyanine (hereafter, CuPc) layers as semiconductor electrodes for photoelectrochemical cells. CuPc layers were prepared by vacuum evaporation and RF sputtering and their photoelectrochemical behavior in NaCI solution was investigated. First, the experimental procedures for the preparation of the CuPc layers and for the electrochemical measurements are described. Then experimental results are given. Finally, we discuss photoelectrochemical cell performance.
2. Experimental
2.1. Specimens High-purity (a-form) CuPc powder was used as the source material in this study. Thin CuPc layers were prepared by two different methods. The first was vacuum evaporation. Thin layers of CuPc were prepared by vacuum evaporation onto ITO covered glass substrates using a conventional apparatus. The substrates were kept at room temperature during the deposition. The second was RF sputtering. Thin layers were deposited from a CuPc powder target onto ITO covered glass substrates
0169-4332/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
518
M. Takeuchi et al. / CuPc thin film photoelectrochemicalcells
u s i n g a conventional diode-type RF-sputtering system. Sputtering was carried out in at, atmosphere of Ar at a pressure of 1 Tort. The thickness of both the evaporated and RF-sputtered thin layers was about 0.5 p.m. Photoelectr~des were fabricated from these CuPc layers as follows: A lead wire was fitted to the edge of the ITO layer with Ag paint. The electrode surface was made insulating by silicone resin except for the front face (10 m m × 10 ram), which was exposed to the solution. The electrode was mounted along the edge of the glass tube with silicone resin. In this manner two types of cells with evaporated and RF-sputtered CuPc layers were prepared. 2.2. Photoelectrochemical measurements
The photoelectrochemical measurements were made using a simple cell with three electrodes. The CuPc working electrode, a Pt counter electrode and an A g - A g C I reference electrode (RE), to which all potentials were referenced, were immersed in 0.1N NaCI aqueous solution. N 2 gas was bubbled through the electrolyte for over one hour before and continuously during each measurement. The electrolyte was stirred to promote bubbling. An incandescent lamp (150 W) was used as the light source, and projected through a quartz window onto the working electrode.
1.2
.=E ,ta
0.6 ". (b]
0 &Of
500
, ........ r ...... 600 700 Wavelength (nm)
800
Fig. I. Absorption spectra for evaporated and RF-spunered CuPc layers: (a) evaporated CuPc layer, (b) RF-sputtered CuPc layer. The resistivity of evaporated and RF-sputtered CuPc layers was of the order of 106 and 107 f~ cm, respectively. 3.2. Photoelectrochemical behavior o f CuPc layers
The I - V characteristics in the dark and under white light illumination were measured on the two
lOOt
~ 50i
'
3. R e s u l t s a n d d i s c u s s i o n
3.1. Fundamental properties of CuPc layers
'Vvs ILO dark
The vacuum evaporated CuPc layers were the same blue color as the source material and showed X-ray diffraction peaks corresponding to t~-CuPc. The RF-sputtered layers were light yellow rather than blue and showed no X-ray diffraction peak. Absorption spectra of both the layers are shown in fig. 1. It may be concluded from these results that the evaporated layers are a-CuPc, while R F sputtering alters the CuPc. The RF-sputtered film was hard like a carbon film.
!
5!
Fig. 2. ! - V characteristics in the dark and under white light illumination for the cell with the evaporated CuPc layer.
519
M. Takeuchi et al. / CuPc thin film photoelectrochemical celb
15 E
.~
<= lO
lig,,
|
on
off
on
off
t 10in ghf
off
on
off
~o =
/,00
L
I
i
500
I
*
600
Wavetength
on
I
700 (nm)
Fig. 3. Spectral response of the short-circuit photocurrent (cathodic) for the cellwith the evaporatedCuPc layer.
~OI ,~0
,
1
,
,
2 Time
,
/+
3
(h)
Fig. 4. Short-circuit current (cathodic) and open-circuit voltage
types of cells, evaporated and RF-sputtered CuPc layers, The result for the cell with the evaporated CuPc layer is given in fig. 2. This cell showed moderate photoresponse under cathodic potential, which means negative-charge (electron) transport from the CuPc electrode into the liquid electrolyte. This indicates that the CuPc layer electrode works as a p-type semiconductor in the NaCI solution. The cell with the RF-sputtered CuPc layer showed little photoresponse, which is not surprising in view of the alteration of CuPc during the RF-sputtering process. The spectral dependence of the short-circuit photocurrent is given in fig. 3 for the cell with the evaporated CuPc layer. The cell responds to a broad wavelength range, which is desirable from the standpoint of efficient solar energy utilization. The rise and decay of the short-circuit photocurrent and 'open-circuit voltage associated with irradiation.and cessation of illumination are shown in fig. 4 for the evaporated CuPc layer cell. The cell showed some voltage and a small current even in the dark. The-latter may be explained as follows. In the case of a p-type semiconductor immersed in a liquid electrolyte, thermally excited electrons are transferred from the semiconductor into the electrolyte along the bent band. As a result a small cathodic current is usually observed in the dark. The former may be due to trapped carriers in the CuPc layer. However, there is also a possibility of a chemical reaction of the CuPc layers with the NaC! solution, which leads to a deterioration of the
responseassociatedwithwhitelight illumination.
cells. The decay of the short-circuit photocurrent in the cell with the evaporated CuPc layer was measured in order to examine the cell stability. The output current deteriorated somewhat in the NaC1 solution as shown in fig. 5. The decay rate was decreased by over-coating the evaporated CuPc layer with a very thin (less than 10 nm) RF-sputtered CuPc layer. No change was observed in the X-ray diffraction pattern and in the absorption spectrum of the CuPc layers after 20 h operation in the NaCI solution. Finally, the load characteristics of the cells with the evaporated CuPc layer and with the RFsputtered over-coating layer under white light illumination are shown in fig. 6. The fill factor is 0.5 I=
~=0.4
0.3 (b)
#.
(al
0.1 I
I
I
5
10
15
Time
I hI
Fig. 5. Decay of short-circuitphotocurrent(cathodic) for the ceils with the evaporated CuPc laver (curve a) and with the RF-sputteredover-coatinglayer(curveb).
520
M. Takeuchi et al. / CuPc thin film photoclectrochemical cells
quently, the energy c o n v e r s i o n efficiency o f these types o f cells is very limited ( 1 0 - 4 - 1 0 - 3 ) , T h e resistivity o f the C u P c layers m u s t b e d e c r e a s e d to i m p r o v e the cell p e r f o r m a n c e .
3OO k
4. Conclusion
20C
P h o t o e l e c t r o c h e m i c a l cells with e v a p o r a t e d C u P c layers s h o w p h o t o r e s p o n s e to visible light a n d are fairly stable in an N a C l solution. H o w ever, the energy c o n v e r s i o n efficiency is low because o f the high resistivity o f the C u P c layers.
References 0
01
0.2
0.3 V
0.4
0.5
IV)
Fig. 6. Load characteristics for the cells with the evaporated CuPc layer (curve a) and with the RF-sputtered over-coating layer (curve b). less t h a n 0.25 for b o t h the cells. This m a y b e a t t r i b u t e d to a high resistance o f b o t h the e v a p o r a t e d a n d R F - s p u t t e r e d C u P c layers. C o n s e -
[1] A. Fujishima and K. Honda, Nature (London) 238 (1972) 37. [21 A.B. Ellis, S.W. Kaiser and M.S. Wrighton, J. Am. Chem. Soc. 98 (1976) 6418. [3] A.J. Nozik, Nature (London) 257 (1975) 383. [4] M. Takeuchi, F.L. Weichman, K. Morosawa, M. Kawakami and H. Nagasaka, Appl. Surf. Sci. 33/34 (1988) 972. [5] A.K. Ghosh, D.L. Morel, T. Feng, R.F. Shaw and C.A. Rowe, Jr., J. Appl. Phys. 45 (1974) 230. [6] M. Kaneko, Makromol. Chem. 189 (1988) 2419.
517
Copper-phthalocyanine thin film photoelectrochemical cells M a n a b u Takeuchi, Masayoshi Masui and Yoshihiro Momose Faculty of Engineering, lbaraki Unit,ersiO', 4-12-1 Nakanarusawa, Hitachi 316, Japan Received 13 August 1990: accepted for publication 28 August 1990
Copper-phthalocyanlne (CuPc) thin layers were prepared by vacuum evaporation and RF sputtering, and their photoelectrochemical behavior in NaCI solution was studied. The photoelectrochemical cells with evaporated CuPc layers showed photosensitivity in the visible region, while the cell with RF-sputtered CuPc layers showed little photoresponse. The cell with evaporated CuPc layers over-coated with an RF-sputtered thin CuPc layer was fairly stable in NaCI solution. However, the energy conversion efficiency was low due to the high resistivity of the CuPc layers.
1. Introduction
Photoelectrolysis of water with single crystal TiO 2 was proposed by Fujishima and Honda in 1972, in which water was split into hydrogen and oxygen by solar energy using the photochemical effect in semiconductors [1]. Studies on the direct conversion of solar energy into electricity, instead of electrolysis of water, have also been made [2]. Generally, photochemical cells should satisfy the following conditions in order to convert sola; energy into electricity efficiently: (1) Most of the solar radiation must be absorbed by the photoelectrochemical cell. (2) Electrons and holes must be separated without recombination. (3) Photoelectrochemical cells must be stable in the solution used. There are few materials which satisfy all these conditions. For example, TiO2 is stable in many solutions, but it absorbs only UV light [3]. While Cu20 absorbs visible light, it is not stable in most solutions [4]. Copper-phthalocyanine is a blue pigment and is also a p-type semiconductor. Recently many attempts have been made to utilize phthaIocyanines as solid-state solar cells because of their stability and spectral fitting to sun light [5]. However, relatively little information is available
in the literature on the electrochemical behavior of this material [6]. We have tried to use copper-phthalocyanine (hereafter, CuPc) layers as semiconductor electrodes for photoelectrochemical cells. CuPc layers were prepared by vacuum evaporation and RF sputtering and their photoelectrochemical behavior in NaCI solution was investigated. First, the experimental procedures for the preparation of the CuPc layers and for the electrochemical measurements are described. Then experimental results are given. Finally, we discuss photoelectrochemical cell performance.
2. Experimental
2.1. Specimens High-purity (a-form) CuPc powder was used as the source material in this study. Thin CuPc layers were prepared by two different methods. The first was vacuum evaporation. Thin layers of CuPc were prepared by vacuum evaporation onto ITO covered glass substrates using a conventional apparatus. The substrates were kept at room temperature during the deposition. The second was RF sputtering. Thin layers were deposited from a CuPc powder target onto ITO covered glass substrates
0169-4332/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
518
M. Takeuchi et al. / CuPc thin film photoelectrochemicalcells
u s i n g a conventional diode-type RF-sputtering system. Sputtering was carried out in at, atmosphere of Ar at a pressure of 1 Tort. The thickness of both the evaporated and RF-sputtered thin layers was about 0.5 p.m. Photoelectr~des were fabricated from these CuPc layers as follows: A lead wire was fitted to the edge of the ITO layer with Ag paint. The electrode surface was made insulating by silicone resin except for the front face (10 m m × 10 ram), which was exposed to the solution. The electrode was mounted along the edge of the glass tube with silicone resin. In this manner two types of cells with evaporated and RF-sputtered CuPc layers were prepared. 2.2. Photoelectrochemical measurements
The photoelectrochemical measurements were made using a simple cell with three electrodes. The CuPc working electrode, a Pt counter electrode and an A g - A g C I reference electrode (RE), to which all potentials were referenced, were immersed in 0.1N NaCI aqueous solution. N 2 gas was bubbled through the electrolyte for over one hour before and continuously during each measurement. The electrolyte was stirred to promote bubbling. An incandescent lamp (150 W) was used as the light source, and projected through a quartz window onto the working electrode.
1.2
.=E ,ta
0.6 ". (b]
0 &Of
500
, ........ r ...... 600 700 Wavelength (nm)
800
Fig. I. Absorption spectra for evaporated and RF-spunered CuPc layers: (a) evaporated CuPc layer, (b) RF-sputtered CuPc layer. The resistivity of evaporated and RF-sputtered CuPc layers was of the order of 106 and 107 f~ cm, respectively. 3.2. Photoelectrochemical behavior o f CuPc layers
The I - V characteristics in the dark and under white light illumination were measured on the two
lOOt
~ 50i
'
3. R e s u l t s a n d d i s c u s s i o n
3.1. Fundamental properties of CuPc layers
'Vvs ILO dark
The vacuum evaporated CuPc layers were the same blue color as the source material and showed X-ray diffraction peaks corresponding to t~-CuPc. The RF-sputtered layers were light yellow rather than blue and showed no X-ray diffraction peak. Absorption spectra of both the layers are shown in fig. 1. It may be concluded from these results that the evaporated layers are a-CuPc, while R F sputtering alters the CuPc. The RF-sputtered film was hard like a carbon film.
!
5!
Fig. 2. ! - V characteristics in the dark and under white light illumination for the cell with the evaporated CuPc layer.
519
M. Takeuchi et al. / CuPc thin film photoelectrochemical celb
15 E
.~
<= lO
lig,,
|
on
off
on
off
t 10in ghf
off
on
off
~o =
/,00
L
I
i
500
I
*
600
Wavetength
on
I
700 (nm)
Fig. 3. Spectral response of the short-circuit photocurrent (cathodic) for the cellwith the evaporatedCuPc layer.
~OI ,~0
,
1
,
,
2 Time
,
/+
3
(h)
Fig. 4. Short-circuit current (cathodic) and open-circuit voltage
types of cells, evaporated and RF-sputtered CuPc layers, The result for the cell with the evaporated CuPc layer is given in fig. 2. This cell showed moderate photoresponse under cathodic potential, which means negative-charge (electron) transport from the CuPc electrode into the liquid electrolyte. This indicates that the CuPc layer electrode works as a p-type semiconductor in the NaCI solution. The cell with the RF-sputtered CuPc layer showed little photoresponse, which is not surprising in view of the alteration of CuPc during the RF-sputtering process. The spectral dependence of the short-circuit photocurrent is given in fig. 3 for the cell with the evaporated CuPc layer. The cell responds to a broad wavelength range, which is desirable from the standpoint of efficient solar energy utilization. The rise and decay of the short-circuit photocurrent and 'open-circuit voltage associated with irradiation.and cessation of illumination are shown in fig. 4 for the evaporated CuPc layer cell. The cell showed some voltage and a small current even in the dark. The-latter may be explained as follows. In the case of a p-type semiconductor immersed in a liquid electrolyte, thermally excited electrons are transferred from the semiconductor into the electrolyte along the bent band. As a result a small cathodic current is usually observed in the dark. The former may be due to trapped carriers in the CuPc layer. However, there is also a possibility of a chemical reaction of the CuPc layers with the NaC! solution, which leads to a deterioration of the
responseassociatedwithwhitelight illumination.
cells. The decay of the short-circuit photocurrent in the cell with the evaporated CuPc layer was measured in order to examine the cell stability. The output current deteriorated somewhat in the NaC1 solution as shown in fig. 5. The decay rate was decreased by over-coating the evaporated CuPc layer with a very thin (less than 10 nm) RF-sputtered CuPc layer. No change was observed in the X-ray diffraction pattern and in the absorption spectrum of the CuPc layers after 20 h operation in the NaCI solution. Finally, the load characteristics of the cells with the evaporated CuPc layer and with the RFsputtered over-coating layer under white light illumination are shown in fig. 6. The fill factor is 0.5 I=
~=0.4
0.3 (b)
#.
(al
0.1 I
I
I
5
10
15
Time
I hI
Fig. 5. Decay of short-circuitphotocurrent(cathodic) for the ceils with the evaporated CuPc laver (curve a) and with the RF-sputteredover-coatinglayer(curveb).
520
M. Takeuchi et al. / CuPc thin film photoclectrochemical cells
quently, the energy c o n v e r s i o n efficiency o f these types o f cells is very limited ( 1 0 - 4 - 1 0 - 3 ) , T h e resistivity o f the C u P c layers m u s t b e d e c r e a s e d to i m p r o v e the cell p e r f o r m a n c e .
3OO k
4. Conclusion
20C
P h o t o e l e c t r o c h e m i c a l cells with e v a p o r a t e d C u P c layers s h o w p h o t o r e s p o n s e to visible light a n d are fairly stable in an N a C l solution. H o w ever, the energy c o n v e r s i o n efficiency is low because o f the high resistivity o f the C u P c layers.
References 0
01
0.2
0.3 V
0.4
0.5
IV)
Fig. 6. Load characteristics for the cells with the evaporated CuPc layer (curve a) and with the RF-sputtered over-coating layer (curve b). less t h a n 0.25 for b o t h the cells. This m a y b e a t t r i b u t e d to a high resistance o f b o t h the e v a p o r a t e d a n d R F - s p u t t e r e d C u P c layers. C o n s e -
[1] A. Fujishima and K. Honda, Nature (London) 238 (1972) 37. [21 A.B. Ellis, S.W. Kaiser and M.S. Wrighton, J. Am. Chem. Soc. 98 (1976) 6418. [3] A.J. Nozik, Nature (London) 257 (1975) 383. [4] M. Takeuchi, F.L. Weichman, K. Morosawa, M. Kawakami and H. Nagasaka, Appl. Surf. Sci. 33/34 (1988) 972. [5] A.K. Ghosh, D.L. Morel, T. Feng, R.F. Shaw and C.A. Rowe, Jr., J. Appl. Phys. 45 (1974) 230. [6] M. Kaneko, Makromol. Chem. 189 (1988) 2419.