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Sb thin films for solar energy applications
Solar Energy Materials and Solar Cells 28 (1993) 293-303 North-Holland
Solar Energy Materials and Solar Cells
Preparation and characterization of TiO2/Sb thin films for solar energy applications W a h e e d A. B a d a w y Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt Received 30 November 1990; in revised form 4 June 1992 Pure and antimony-incorporated TiO 2 thin films were prepared using a spray-CVD method. The method allows for convenient incorporation of foreign atoms into the oxide matrix during film growth. The foreign atoms in the oxide film affects both the photovoltaic and photoelectrochemical properties of the n-Si/oxide heterojunction. The characteristics of the prepared oxide films were affected significantly by the presence of antimony on the oxide matrix. The increased conductivity of the Sb-containing oxide layers is reflected in the improved photovoltaic properties of the prepared n-Si/TiO2-Sb heterojunctions, e.g. fill factor and solar conversion efficiency. The photoelectrochemical properties of the prepared devices revealed that the charge transfer step at the oxide/electrolyte interface leads to a deterioration of the cell quality. However, this drawback has been offset by the improved properties of the heterojunction.
I. Introduction
Due to the very interesting properties of TiO2, the material attracts the attention of large number of research groups. The oxide can serve as an excellent protective layer in photoelectrolysis cells and is used in different spectroscopic applications. The properties of TiO 2 that make it a corrosion resistant material sets TiO 2 apart as a predominantly important component in solar energy applications. Pure titanium oxide was used as suitable photoanode for the photoelectrolysis of water [1-4]. Incorporation of foreign atoms into the metal oxide matrix is known to induce substantial modifications of its semiconductor characteristics. This phenomenon has been sporadically investigated in connection with semiconductor electrodes for photoelectrolysis devices [5-10]. The incorporation was essentially aimed at extending the response of some wide band gap materials like TiO 2 or Nb205 to the visible region of the spectrum. Pure TiO 2 can absorb only a small portion of the solar spectrum. Doping of TiO 2 single crystals with tungsten was found to shift the absorption of the material towards the ultraviolet region [11]. Correspondence to: W.A. Badawy, Department of Chemistry, Faculty of Science, University of Kuwait, P.O. Box 5969, 13060 Safat, Kuwait. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
294
W.A. Badawy / TiOe/ Sb thin films
Addition of chromium showed the opposite effect [2,12,13]; the photoresponse of the material shifted from ca. 400 to ca. 550 nm. Doping of polycrystalline TiO 2 with Cd was found to extend the photoactive range of the oxide into the visible region of the spectrum [14]. Although it is economical and practical in different applications to employ thin films of the material on different substrates, a systematic study of the preparation, electro-, and photoelectro-chemical properties of pure and doped TiO 2 has not yet been made. Some investigations concerning the preparation and solid state characteristics of pure and In-doped TiO 2 and SnO 2 thin films have been conducted in our laboratory [15-19]. In this work, pure and TiO 2 doped with antimony were investigated. The solid state characteristics of the prepared oxide films were studied. The photovoltaic and photoelectrochemical properties of the films as heterojunctions with n-Si were traced. The effect of Sb-incorporation on the whole characteristics of the system was discussed.
2. Experimental The preparation of pure and antimony doped oxide films were carried out using the spray-CVD (chemical vapour deposition) technique described elsewhere [15,20]. The spray solution consisted of 0.5 M TiC14 dissolved in ethanol. For Sb-doping SbC15 was added to the spraying solution. This was then 0.5 M both TiC14 and SbC15. The pyrolysis reactions of TiC14 and SbC15 on the heated substrate are given by TiCl4(g ) + 2 HzO(g ) 45°°c TiO2(s ) + 4 HCI(g),
(1)
2 SbC15(g) + 5 H20(g )
(2)
450°C
Sb2Os(s ) + 10 HCI(g).
The hydrolysis water was supplied from the solution itself since the concentration of ethanol was 96%. Glass-like, transparently clear films were deposited on glass, quartz, Si and GC (glassy carbon) substrates. The concentration of Sb in the prepared oxide films was determined by ESCA and found to be ca. 1%. For optical measurements the glass specimens (1.5 × 2.0 cm) were employed. The measurements were carried out using a double beam spectrophotometer (Perkin-Elmer Lambda UV/VIS) over the 200-900 nm wave length range. For photovoltaic applications, TiO 2 films were deposited on polished silicon wafers (Wacker-Burghausen, Germany) typically 0.2 cm 2, of orientation (100), resistivity 0.6-1.4 II cm. They were pretreated with a standard solution of formic a c i d / H 2 0 2 and hydrofluoric acid [21]. The forward contacts of the photovoltaic cells were made by evaporating two rectangular gold contacts (0.15 × 0.30 cm) onto the TiO 2 surface. The back contacts were made by using an In-Ga alloy followed by silver paste. For the electrochemical measurements, the films were deposited onto glassy carbon (GC) discs (Sigri, Meitingen, Germany, diameter 0.8 cm, thickness 0.3 cm;
W.A. Badawy / TiO2/Sb thin films
295
polished with diamond paste down to 1 ~m; washed in alcohol and acetone in an ultrasonic bath). The discs were mounted in stainless steel holders used in rotating disc electrodes and insulated with teflon tape so that the oxide coated circular front was exposed to the electrolyte. The same experimental parameters were always chosen for the preparation of TiO2 or T i O 2 / S b on glass, Si or GC in order to minimize structural fluctuations. The current density-potential behaviour of the different electrodes was traced using a conventional potentiostatic set-up. Illumination was carried out using a 150 W xenon lamp coupled with an Oriel solar simulator and filter, to simulate the solar spectrum. The photovoltaic current density-potential characteristics were corrected for R s (the series resistance) as described previously [20]. Other experimental parameters were as reported elsewhere [15,20].
3. Results
3.1. Conductivity measurements The conductivity of the prepared films was found to increase as the film thickness increases. Table 1 presents the resistance of pure and Sb-incorporated TiO 2 films of different thicknesses. Specific conductance calculations showed that the film homogeneity depends on the thickness. For TiO2 films thicker than 100 nm, a value of 1 x 10 -3 ~ - t cm-1 for the specific conductance of films prepared from ethyl acetate solutions was obtained [15]. The films prepared from alcoholic solutions are less conducting. The presence of Sb in the TiO2 matrix increases the conductivity of the film by about three orders of magnitude (cf. table 1) even for very thin oxide layers.
3.2. Optical behaviour of the films 3.2.1. Transmittance measurements Fig. 1 shows the transmittance data of pure and 1% Sb-incorporated TiO 2 films. The film thickness was 100 nm and the % transmittance ( % T ) was presented as a function of the wave length (A) in the range 200-900 nm. The results show that
Table 1 Resistance of pure and Sb-incorporated TiO 2 films as a function of the film thickness Thickness (nm) 50 100 150 200 250
Resistance of the film (lq)
,
Pure TiO2
TiO 2 / S b
1.2×107 1.0× 107 0.8× 107 0.7X 107 0.7 X 107
1.5 X 104 0.9X 104 0.7× 104 0.7x 104 0.5 x 104
W.A. Badawy / TiO2/Sb thin films
296
100 . . . . . . . . . . . 80
60 o\O /,0
20
--
- - - -
i
300
I
i
400
500
i
600
i
700
i
800
~,nrn
Fig. 1. Transmittance (% T) of pure TiO 2 ( - - - - - - ) and 1% Sb incorporated TiO 2 ( ) layers (ca. 100 nm thickness) as a function of the wave length (A) of the incident radiation.
both the pure and Sb-incorporated TiO 2 films are perfectly transparent in the visible and near IR region. A % T of more than 90 was measured. At shorter wave lengths (3, < 380 nm) the absorbance of the film increases and below 300 nm the film absorbs all the incident radiation and the % T becomes zero.
3.2.2. Energy-gap calculations For energy-gap measurements the oxide films were deposited on quartz substrates (1 × 2 cm2). The transmission ratio for two layers of thickness d t and d 2 is given by TI_ 2 =
e -~ae,
(3)
where TI_ 2 is the transmission ratio, Ad the difference between d 2 and dl, and a the absorption coefficient, aAd can be determined accurately without determining Ad [22]. By plotting ( a A d ) 2 versus the photon energy hv, the energy gap of the absorbing layer can be extrapolated. The variation of the absorption coefficient with photon energy for direct allowed transition is given by a = o~0(hv - Eg) 1/2,
(4)
where hv and E , are the photon and gap energies, respectively, a 0 is a constant independent of the photon energy [23]. Photons of energy hv >1Eg can be absorbed by the oxide film. Photons of energy hv <~Eg cannot be absorbed, i.e. in this case, O/=0.
Fig. 2 presents the relation between ( a A d ) 2 and photon energy for pure and Sb-incorporated films. The value of the band-gap energy was obtained by extrapolating the most straight portion of the line to (aAd) 2= O. The energy gap was found to be independent of the film thickness [19]. The presence of Sb in the TiO 2 matrix increases the band gap energy of the oxide by about 5%. The increase of
W.A. Badawy / TiO2/Sb thin films i
297 i
10.0
8.C
6.0
l..O
2.0 / ¢
'~
~t
I
I
/..0
/..5 E , eV
Fig. 2. Square of the absorption coefficient (a) and the thickness (Ad) of pure TiO2 (X TiO 2-Sb ( ) as a function of the incident photon energy.
x ) and
the extrapolated value of the band-gap energy may be attributed to some structural changes in the polycrystalline TiO 2. The relatively large band-gap energy of pure TiO 2 (ca. 3.6 eV) is due to the polycrystalline nature of the prepared material [24].
3.3 Photovoltaic characteristics of the n-Si / TiO2 and n-Si / TiO2-Sb heterojunctions Typical power characteristics of both n-Si/TiO 2 and n-Si/TiO2-Sb solar cells under 100 mW cm -2 simulated solar spectrum (AM1) are presented in fig. 3. The figure shows that the incorporation of Sb in the TiO 2 material even with the low concentration used in these investigations (1% Sb) had a significant improvement on the power characteristics of the solar cells. The fill factor (FF) was increased from ca. 0.6 (pure TiO2) to ca. 0.8 (1% Sb). This improvement manifests itself in the conversion efficiency of the prepared cells, increasing from ca. 10% (pure TiO 2) to ca. 13.8% (TiO2-Sb). The presence of Sb in the TiO 2 barrier layer showed its main effect in the open-circuit potential of the photovoltaic cell as an increase of ca. 100 to 130 mV was measured. On the other hand, the photocurrent or saturation current was not significantly affected by the presence of Sb. Its value lies between 27 and 30 mA cm-2.
3.4. Electro- and photoelectro-chemical behaviour of the prepared oxide films 3.4.1. The electrochemical behaviour For these investigations, the oxide films were deposited onto GC substrates and mounted as electrodes in the holders of the rotating disc system. The current density-potential behaviour of the rotating disc oxide electrodes at 298 K with a
298
W.A. Badawy / Ti02 / Sb thin films
'30 • . -- ~ _ _ _ ~ ~ ~
//,
-/
/
20
l~oto_current
/
,,( E
lO
0.0
/,11 -~-~~// 600
r ,
.
071(current
I
i
~oo
2o0
i ,
200
O0
V, mV Fig. 3.;Typica! power chasacteristics of n-Si/TiO 2 ( - - ----r) and n-Si/TiO2.-Sb ( ) photovoltaic cells. Illumination intensity, 100 mW cm-2 simulated solar spectrum (AMI).
'
i
3.0?
~
2(3-
1,0-
if '
~
'
' i
i/ /III
I
/
~a ~mV
-1.0 .
-2.0
i
z.,O0
200
. 2--<"
//
/ -//////#
-200
'
/ //
,/# /
?E
-3.0
1 ,/ / / I l i I Fig. 41 Current 'density'potential curves of pure ( - - - - - ) and 1% Sb-incorporated ( .... ) TiO 2 rotating disc electrodes (ca. i00 nm thickness) deposited' on GC. The electrolyte was 0.05 M K3Fe(CN') 6 70.05 M K4Fe(CN) 6/0.05 M KNO3, with a scan rate of 50 mV s- 1 and 2500 rpm at 298 K.
W.4. Badawy / TiO2/Sb thin films
299
(a] cathodic
\
anodic
150
\
<
\
100
\
/
/
"o
/
/
///
c
/
I -
I
I
20
-
I
1.2 .
.
I
0.0
lc
(b)
I
10 potential
.
.
10
, mV
I
I
20
"la
.
/'
O.B
~,
0.a
~
0.0
-,
/ /
o~-o.'-
_
Oe
eo o
I
- "-00
i
-200 '~'c'mV
/
.
/ "/ 0 / " o
o
-0.8
/b
o
I
I
I
0.0
200
I
I
"00
~ a ' mV
Fig. 5. (a) Current density-potential curves of pure ( - - - - - ) and 1% rotating disc near the equilibrium potential in the Fe(CN) 3 - / 4 - redox M V s -1 and rotation speed 2500 rpm. (b) Tafel plots obtained with incorporated ( e - - - - - - e) TiO 2 electrodes under the same
Sb-incorporated ( ) TiO 2 solution at 298 K. Scan rate 50 pure ( o o ) and 1% Sb conditions of fig. 4.
300
W.A. Badawy / TiO2/Sb thinfilms
// n-Type TiO2
~
(pure
// ]
//] //
EF - _
-
T
-T-Sb - incorporated
Tia2
rio 2
m
Eleclrolyte
Fig. 6. Effectof Sb-incorporationon the space charge layerof the TiO2 semiconductingfilm. speed of rotation of 2500 rpm and a scan rate of 50 mV s- 1 is plotted in fig. 4. The test solution consisted of 0.05 M K3Fe(CN)6/0.05 M K4Fe(CN)6/0.5 M KNO 3. The results showed that the over-potential near the equilibrium potential obeys the linear approximation of the Butler-Volmer equation [25] and the charge transfer resistance RCT can be calculated according to R c r = "qcr/J = ( R r / F ) (1/njo ) ,
(5)
where */ca- is the charge transfer overpotential, j is the current density, J0 is the exchange current density, n is the number of electrons involved in the electrode process, while F, R, and T have their usual meanings. Fig. 5a shows that the presence of Sb in the TiO z matrix decreases the charge transfer resistance by ca. 35%. At higher overpotentials ]~71>_-120 mV the electrode reactions can be described by the Tafel equation. Analysis of the results according to J =J0 e x p [ ( a n F / R T ) ~ c T ]
(6)
is presented in fig. 5b. In eq. (6), a is the charge transfer coefficient. The plots of fig. 5b concerning pure TiO 2 and TiO2-Sb have slopes between 400 mV/decade (pure TiO 2) and 180 mV/decade (TiO2-Sb). These high values of the Tafel slopes are due to charge transfer limitations across the electrolyte/n-type semiconductor interface, shown schematically in fig. 6 [26]. 3.4.2. The photoelectrochemical behaviour of the n-Si / T i O 2 and n-Si/TiO2-Sb photoanodes The changes in the electrochemical properties of the TiO 2 layers as a result of incorporation of antimony are reflected in the photoelectrochemical behaviour of
W.A. Badawy / TiO 2 / S b thin films
25m
/
m
301
Pholo-currenl
. _ _
20
15
11 /
5.0
0.0
Dark current
1/,?
60O
f
200
zOO
I
0,0
200
V, mV
Fig. 7. Current density-potential curves of n-Si/TiO 2 ( - - - - - - ) and n-Si/TiO 2-Sb ( ) photoanodes in 0.05 M K4Fe(CN) 6/0.05 M KNO 3 at 198 K. Scan rate 50 mV s-1, rotation speed 2500 rpm and illumination intensity 100 mW cm -2 (AM1).
the prepared photoanodes. Fig. 7 represents the photocurrent-potential behaviour or n-Si/TiO2/electrolyte and n-Si/TiO2-Sb/electrolyte systems. The first feature is that the Sb-incorporation had no effect on the saturation current, since the value of the saturation current density under AM1 illumination lies between 26 and 29 mA cm -2, which is about the same as that obtained from corresponding photovoltaic cells. The improved charge transfer characteristics manifest themselves mainly in the fill factor of photoelectrochemical cell. An increase of ca. 25% was calculated which in turn is reflected in the improved efficiency of the cell. The photoelectrochemical conversion efficiency was increased from 6.7% (pure TiO2) to 8.1% (TiO2-Sb). The open-circuit potential did not show a significant increase as that observed in the corresponding photovoltaic cells. Values of ca. 550 mV for pure TiO2, and ca. 600 mV for TiO2-Sb cells were measured.
4. Discussion
Incorporation of Sb into the TiO 2 is accompanied by an increase in the conductivity of the oxide film. The rate of reactionless one-electron transfer depends on a quantum mechanical frequency factor related to the tunnelprobabil-
W.A. Badawy / TiO2/ Sb thin films
302
ity of the electron and the density of electronic states in the TiO 2 energetically suitable for the redox system. Increasing the concentration of charge carriers in the semiconducting TiO 2 film is likely to increase the tunnel probability of decreasing the space-charge layer thickness (cf. fig. 6). This corresponds to an increase of the electrochemical rate constant of the redox reaction occurring at the electrode/ electrolyte interface. The electrochemical charge transfer process then leads to a deterioration of the cell quality. The slopes of the Tafel plots, which are related to the charge transfer coefficient a and normally used to characterize charge transfer processes involving adsorption/chemisorption of intermediates [27], are given by b = O.059/an,
at 298 K,
(7)
where b is the Tafel slope. The presence of Sb in the TiO 2 matrix increases the rate of charge transfer and hence better values of b and a are obtained. The improvement of the electrochemical performance of the cell achieved by the incorporation of Sb is reflected in the general photoelectrochemical behaviour, i.e. better fill factor and solar conversion efficiency could be obtained. The results presented in this work demonstrate the usefulness of the concept underlying the analysis of the photoelectrochemical systems [25]. According to this concept, heterojunction photoelectrochemical devices like n-Si/TiO 2 are well described as a combination of the photovoltaic and the involved electrochemical characteristics. The presence of Sb-incorporation in the TiO 2 film was shown to improve the photovoltaic characteristics of the n-Si/TiO 2 heterojunction and at the same time to increase the rate of the charge transfer process at the TiO2/ electrolyte interface. An increase in the solar conversion efficiency of more than 25% suggested that TiO 2 incorporated with foreign atoms like antimony may lead to heterojunctions with acceptable conversion efficiencies for practical use.
5. Conclusion
Sb-incorporation in the TiO2 matrix leads to a slight decrease in the transmittance and a significant increase in the conductivity of the TiO 2 films. It increases the rate of the charge transfer step and improves the catalytic properties of the oxide, which may lead to improved photovoltaic and photoelectrochemical systems of practical relevance.
References [1] [2] [3] [4] [5]
A. Fujishima and K. Honda, Nature 238 (1972) 37. A.K. Ghosh and H.P. Maruska, J. Electrochem. Soc. 124 (1977) 1516. M.P. Maruska and A.K. Ghosh, Sol; Energy Mater. 1 (1979) 237. Y. Matsumoto, J. Kurimoto, Y. Amagasaki and E. Sats, J. Electrochem. Soc. 127 (1980) 2360. L. Harris and R. Wilson, Am. Rev. Mater. Sci. 8 (1978) 99.
W.A. Badawy / Ti02 / Sb thin films [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
303
M.P. Maruska and A.K. Ghosh, Sol. Energy 20 (1979) 433. J.G. Mavroides, Mater. Res. Bull. 13 (1978) 1379. K. Rajeshwar, P. Singh and J. Du Bow, Electrochim. Acta 23 (1978) 1117. A.J. Nozik, Ann. Rev. Phys. Chem. 29 (1978) 189. M. Tomkiewicz and H. Fay, Appl. Phys. 18 (1979) 1. M.S. Wrighton, D.S. Ginley, P.T. Wolczanski, A.B. Ellis, D.L. Morse and A. Linz, Proc. Natl. Acad. Sci. USA 72 (1975) 1518. A.K. Ghosh and M.P. Muruska, in: Solar Energy, Eds. J.B. Berkowitz and I.A. Lesk, The Electrochem. Soc. Softbound Proceedings Series (Princeton, NJ, 1976) p. 92. A. Monnier and J. Augustynski, J. Electrochem. Soc. 127 (1980) 1576. C. Stalder, A. Monnier and J. Augustynski, Extended Abstracts of the 2nd International Conference on the Photochem. Conversion and Storage of Solar Energy, Cambridge, England, August 10-12 (1978) p. 81. W.A. Badawy and E.A. E1-Taher, Thin Solid Films 158 (1988) 277. W.A. Badawy, J. Electroanal. Chem. 281 (1990) 85. W.A. Badawy, H.H. Afify and E.M. Elgiar, J. Electrochem. Soc. 135 (1990) 1392. W.A. Badawy, Extended Abstracts of the 40th International Society of Electrochemistry Meeting, Vol. II, CI, Kyoto, Japan, September 17-22 (1989) p. 682. W.A. Badawy, R.S. Momtaz and E.M. Elgair, Phys. Stat. Sol. (a) 118 (1990) 197. W.A. Badawy, F. Decker and K. Doblhofer, Sol. Energy Mater. 8 (1983) 363. D. Pulfrey, IEEE Trans. Electron Dev. ED-25 (1978) 1308. A.K. Abbas and M.T. Mohammed, J. Appl. Phys. 59 (1986) 1641. J. Bardeen, F.J. Slatt and L.J. Hall; Photoconductivity Conference 1954 (Wiley, New York, 1956) p. 146. W.A. Badawy, Ind. J. Technol. 29 (1991) 235. A.J. Bard and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications (Wiley, New York, 1980) ch. 3. W.A. Badawy, Ind. J. Technol. 24 (1986) 118. K. Uosaki and H. Kita, J. Electrochem. Soc. 130 (1983) 985; H. Gerischer, Discussion Section, J. Electrochem. Soc. (Dec. 1983).
Solar Energy Materials and Solar Cells
Preparation and characterization of TiO2/Sb thin films for solar energy applications W a h e e d A. B a d a w y Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt Received 30 November 1990; in revised form 4 June 1992 Pure and antimony-incorporated TiO 2 thin films were prepared using a spray-CVD method. The method allows for convenient incorporation of foreign atoms into the oxide matrix during film growth. The foreign atoms in the oxide film affects both the photovoltaic and photoelectrochemical properties of the n-Si/oxide heterojunction. The characteristics of the prepared oxide films were affected significantly by the presence of antimony on the oxide matrix. The increased conductivity of the Sb-containing oxide layers is reflected in the improved photovoltaic properties of the prepared n-Si/TiO2-Sb heterojunctions, e.g. fill factor and solar conversion efficiency. The photoelectrochemical properties of the prepared devices revealed that the charge transfer step at the oxide/electrolyte interface leads to a deterioration of the cell quality. However, this drawback has been offset by the improved properties of the heterojunction.
I. Introduction
Due to the very interesting properties of TiO2, the material attracts the attention of large number of research groups. The oxide can serve as an excellent protective layer in photoelectrolysis cells and is used in different spectroscopic applications. The properties of TiO 2 that make it a corrosion resistant material sets TiO 2 apart as a predominantly important component in solar energy applications. Pure titanium oxide was used as suitable photoanode for the photoelectrolysis of water [1-4]. Incorporation of foreign atoms into the metal oxide matrix is known to induce substantial modifications of its semiconductor characteristics. This phenomenon has been sporadically investigated in connection with semiconductor electrodes for photoelectrolysis devices [5-10]. The incorporation was essentially aimed at extending the response of some wide band gap materials like TiO 2 or Nb205 to the visible region of the spectrum. Pure TiO 2 can absorb only a small portion of the solar spectrum. Doping of TiO 2 single crystals with tungsten was found to shift the absorption of the material towards the ultraviolet region [11]. Correspondence to: W.A. Badawy, Department of Chemistry, Faculty of Science, University of Kuwait, P.O. Box 5969, 13060 Safat, Kuwait. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
294
W.A. Badawy / TiOe/ Sb thin films
Addition of chromium showed the opposite effect [2,12,13]; the photoresponse of the material shifted from ca. 400 to ca. 550 nm. Doping of polycrystalline TiO 2 with Cd was found to extend the photoactive range of the oxide into the visible region of the spectrum [14]. Although it is economical and practical in different applications to employ thin films of the material on different substrates, a systematic study of the preparation, electro-, and photoelectro-chemical properties of pure and doped TiO 2 has not yet been made. Some investigations concerning the preparation and solid state characteristics of pure and In-doped TiO 2 and SnO 2 thin films have been conducted in our laboratory [15-19]. In this work, pure and TiO 2 doped with antimony were investigated. The solid state characteristics of the prepared oxide films were studied. The photovoltaic and photoelectrochemical properties of the films as heterojunctions with n-Si were traced. The effect of Sb-incorporation on the whole characteristics of the system was discussed.
2. Experimental The preparation of pure and antimony doped oxide films were carried out using the spray-CVD (chemical vapour deposition) technique described elsewhere [15,20]. The spray solution consisted of 0.5 M TiC14 dissolved in ethanol. For Sb-doping SbC15 was added to the spraying solution. This was then 0.5 M both TiC14 and SbC15. The pyrolysis reactions of TiC14 and SbC15 on the heated substrate are given by TiCl4(g ) + 2 HzO(g ) 45°°c TiO2(s ) + 4 HCI(g),
(1)
2 SbC15(g) + 5 H20(g )
(2)
450°C
Sb2Os(s ) + 10 HCI(g).
The hydrolysis water was supplied from the solution itself since the concentration of ethanol was 96%. Glass-like, transparently clear films were deposited on glass, quartz, Si and GC (glassy carbon) substrates. The concentration of Sb in the prepared oxide films was determined by ESCA and found to be ca. 1%. For optical measurements the glass specimens (1.5 × 2.0 cm) were employed. The measurements were carried out using a double beam spectrophotometer (Perkin-Elmer Lambda UV/VIS) over the 200-900 nm wave length range. For photovoltaic applications, TiO 2 films were deposited on polished silicon wafers (Wacker-Burghausen, Germany) typically 0.2 cm 2, of orientation (100), resistivity 0.6-1.4 II cm. They were pretreated with a standard solution of formic a c i d / H 2 0 2 and hydrofluoric acid [21]. The forward contacts of the photovoltaic cells were made by evaporating two rectangular gold contacts (0.15 × 0.30 cm) onto the TiO 2 surface. The back contacts were made by using an In-Ga alloy followed by silver paste. For the electrochemical measurements, the films were deposited onto glassy carbon (GC) discs (Sigri, Meitingen, Germany, diameter 0.8 cm, thickness 0.3 cm;
W.A. Badawy / TiO2/Sb thin films
295
polished with diamond paste down to 1 ~m; washed in alcohol and acetone in an ultrasonic bath). The discs were mounted in stainless steel holders used in rotating disc electrodes and insulated with teflon tape so that the oxide coated circular front was exposed to the electrolyte. The same experimental parameters were always chosen for the preparation of TiO2 or T i O 2 / S b on glass, Si or GC in order to minimize structural fluctuations. The current density-potential behaviour of the different electrodes was traced using a conventional potentiostatic set-up. Illumination was carried out using a 150 W xenon lamp coupled with an Oriel solar simulator and filter, to simulate the solar spectrum. The photovoltaic current density-potential characteristics were corrected for R s (the series resistance) as described previously [20]. Other experimental parameters were as reported elsewhere [15,20].
3. Results
3.1. Conductivity measurements The conductivity of the prepared films was found to increase as the film thickness increases. Table 1 presents the resistance of pure and Sb-incorporated TiO 2 films of different thicknesses. Specific conductance calculations showed that the film homogeneity depends on the thickness. For TiO2 films thicker than 100 nm, a value of 1 x 10 -3 ~ - t cm-1 for the specific conductance of films prepared from ethyl acetate solutions was obtained [15]. The films prepared from alcoholic solutions are less conducting. The presence of Sb in the TiO2 matrix increases the conductivity of the film by about three orders of magnitude (cf. table 1) even for very thin oxide layers.
3.2. Optical behaviour of the films 3.2.1. Transmittance measurements Fig. 1 shows the transmittance data of pure and 1% Sb-incorporated TiO 2 films. The film thickness was 100 nm and the % transmittance ( % T ) was presented as a function of the wave length (A) in the range 200-900 nm. The results show that
Table 1 Resistance of pure and Sb-incorporated TiO 2 films as a function of the film thickness Thickness (nm) 50 100 150 200 250
Resistance of the film (lq)
,
Pure TiO2
TiO 2 / S b
1.2×107 1.0× 107 0.8× 107 0.7X 107 0.7 X 107
1.5 X 104 0.9X 104 0.7× 104 0.7x 104 0.5 x 104
W.A. Badawy / TiO2/Sb thin films
296
100 . . . . . . . . . . . 80
60 o\O /,0
20
--
- - - -
i
300
I
i
400
500
i
600
i
700
i
800
~,nrn
Fig. 1. Transmittance (% T) of pure TiO 2 ( - - - - - - ) and 1% Sb incorporated TiO 2 ( ) layers (ca. 100 nm thickness) as a function of the wave length (A) of the incident radiation.
both the pure and Sb-incorporated TiO 2 films are perfectly transparent in the visible and near IR region. A % T of more than 90 was measured. At shorter wave lengths (3, < 380 nm) the absorbance of the film increases and below 300 nm the film absorbs all the incident radiation and the % T becomes zero.
3.2.2. Energy-gap calculations For energy-gap measurements the oxide films were deposited on quartz substrates (1 × 2 cm2). The transmission ratio for two layers of thickness d t and d 2 is given by TI_ 2 =
e -~ae,
(3)
where TI_ 2 is the transmission ratio, Ad the difference between d 2 and dl, and a the absorption coefficient, aAd can be determined accurately without determining Ad [22]. By plotting ( a A d ) 2 versus the photon energy hv, the energy gap of the absorbing layer can be extrapolated. The variation of the absorption coefficient with photon energy for direct allowed transition is given by a = o~0(hv - Eg) 1/2,
(4)
where hv and E , are the photon and gap energies, respectively, a 0 is a constant independent of the photon energy [23]. Photons of energy hv >1Eg can be absorbed by the oxide film. Photons of energy hv <~Eg cannot be absorbed, i.e. in this case, O/=0.
Fig. 2 presents the relation between ( a A d ) 2 and photon energy for pure and Sb-incorporated films. The value of the band-gap energy was obtained by extrapolating the most straight portion of the line to (aAd) 2= O. The energy gap was found to be independent of the film thickness [19]. The presence of Sb in the TiO 2 matrix increases the band gap energy of the oxide by about 5%. The increase of
W.A. Badawy / TiO2/Sb thin films i
297 i
10.0
8.C
6.0
l..O
2.0 / ¢
'~
~t
I
I
/..0
/..5 E , eV
Fig. 2. Square of the absorption coefficient (a) and the thickness (Ad) of pure TiO2 (X TiO 2-Sb ( ) as a function of the incident photon energy.
x ) and
the extrapolated value of the band-gap energy may be attributed to some structural changes in the polycrystalline TiO 2. The relatively large band-gap energy of pure TiO 2 (ca. 3.6 eV) is due to the polycrystalline nature of the prepared material [24].
3.3 Photovoltaic characteristics of the n-Si / TiO2 and n-Si / TiO2-Sb heterojunctions Typical power characteristics of both n-Si/TiO 2 and n-Si/TiO2-Sb solar cells under 100 mW cm -2 simulated solar spectrum (AM1) are presented in fig. 3. The figure shows that the incorporation of Sb in the TiO 2 material even with the low concentration used in these investigations (1% Sb) had a significant improvement on the power characteristics of the solar cells. The fill factor (FF) was increased from ca. 0.6 (pure TiO2) to ca. 0.8 (1% Sb). This improvement manifests itself in the conversion efficiency of the prepared cells, increasing from ca. 10% (pure TiO 2) to ca. 13.8% (TiO2-Sb). The presence of Sb in the TiO 2 barrier layer showed its main effect in the open-circuit potential of the photovoltaic cell as an increase of ca. 100 to 130 mV was measured. On the other hand, the photocurrent or saturation current was not significantly affected by the presence of Sb. Its value lies between 27 and 30 mA cm-2.
3.4. Electro- and photoelectro-chemical behaviour of the prepared oxide films 3.4.1. The electrochemical behaviour For these investigations, the oxide films were deposited onto GC substrates and mounted as electrodes in the holders of the rotating disc system. The current density-potential behaviour of the rotating disc oxide electrodes at 298 K with a
298
W.A. Badawy / Ti02 / Sb thin films
'30 • . -- ~ _ _ _ ~ ~ ~
//,
-/
/
20
l~oto_current
/
,,( E
lO
0.0
/,11 -~-~~// 600
r ,
.
071(current
I
i
~oo
2o0
i ,
200
O0
V, mV Fig. 3.;Typica! power chasacteristics of n-Si/TiO 2 ( - - ----r) and n-Si/TiO2.-Sb ( ) photovoltaic cells. Illumination intensity, 100 mW cm-2 simulated solar spectrum (AMI).
'
i
3.0?
~
2(3-
1,0-
if '
~
'
' i
i/ /III
I
/
~a ~mV
-1.0 .
-2.0
i
z.,O0
200
. 2--<"
//
/ -//////#
-200
'
/ //
,/# /
?E
-3.0
1 ,/ / / I l i I Fig. 41 Current 'density'potential curves of pure ( - - - - - ) and 1% Sb-incorporated ( .... ) TiO 2 rotating disc electrodes (ca. i00 nm thickness) deposited' on GC. The electrolyte was 0.05 M K3Fe(CN') 6 70.05 M K4Fe(CN) 6/0.05 M KNO3, with a scan rate of 50 mV s- 1 and 2500 rpm at 298 K.
W.4. Badawy / TiO2/Sb thin films
299
(a] cathodic
\
anodic
150
\
<
\
100
\
/
/
"o
/
/
///
c
/
I -
I
I
20
-
I
1.2 .
.
I
0.0
lc
(b)
I
10 potential
.
.
10
, mV
I
I
20
"la
.
/'
O.B
~,
0.a
~
0.0
-,
/ /
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_
Oe
eo o
I
- "-00
i
-200 '~'c'mV
/
.
/ "/ 0 / " o
o
-0.8
/b
o
I
I
I
0.0
200
I
I
"00
~ a ' mV
Fig. 5. (a) Current density-potential curves of pure ( - - - - - ) and 1% rotating disc near the equilibrium potential in the Fe(CN) 3 - / 4 - redox M V s -1 and rotation speed 2500 rpm. (b) Tafel plots obtained with incorporated ( e - - - - - - e) TiO 2 electrodes under the same
Sb-incorporated ( ) TiO 2 solution at 298 K. Scan rate 50 pure ( o o ) and 1% Sb conditions of fig. 4.
300
W.A. Badawy / TiO2/Sb thinfilms
// n-Type TiO2
~
(pure
// ]
//] //
EF - _
-
T
-T-Sb - incorporated
Tia2
rio 2
m
Eleclrolyte
Fig. 6. Effectof Sb-incorporationon the space charge layerof the TiO2 semiconductingfilm. speed of rotation of 2500 rpm and a scan rate of 50 mV s- 1 is plotted in fig. 4. The test solution consisted of 0.05 M K3Fe(CN)6/0.05 M K4Fe(CN)6/0.5 M KNO 3. The results showed that the over-potential near the equilibrium potential obeys the linear approximation of the Butler-Volmer equation [25] and the charge transfer resistance RCT can be calculated according to R c r = "qcr/J = ( R r / F ) (1/njo ) ,
(5)
where */ca- is the charge transfer overpotential, j is the current density, J0 is the exchange current density, n is the number of electrons involved in the electrode process, while F, R, and T have their usual meanings. Fig. 5a shows that the presence of Sb in the TiO z matrix decreases the charge transfer resistance by ca. 35%. At higher overpotentials ]~71>_-120 mV the electrode reactions can be described by the Tafel equation. Analysis of the results according to J =J0 e x p [ ( a n F / R T ) ~ c T ]
(6)
is presented in fig. 5b. In eq. (6), a is the charge transfer coefficient. The plots of fig. 5b concerning pure TiO 2 and TiO2-Sb have slopes between 400 mV/decade (pure TiO 2) and 180 mV/decade (TiO2-Sb). These high values of the Tafel slopes are due to charge transfer limitations across the electrolyte/n-type semiconductor interface, shown schematically in fig. 6 [26]. 3.4.2. The photoelectrochemical behaviour of the n-Si / T i O 2 and n-Si/TiO2-Sb photoanodes The changes in the electrochemical properties of the TiO 2 layers as a result of incorporation of antimony are reflected in the photoelectrochemical behaviour of
W.A. Badawy / TiO 2 / S b thin films
25m
/
m
301
Pholo-currenl
. _ _
20
15
11 /
5.0
0.0
Dark current
1/,?
60O
f
200
zOO
I
0,0
200
V, mV
Fig. 7. Current density-potential curves of n-Si/TiO 2 ( - - - - - - ) and n-Si/TiO 2-Sb ( ) photoanodes in 0.05 M K4Fe(CN) 6/0.05 M KNO 3 at 198 K. Scan rate 50 mV s-1, rotation speed 2500 rpm and illumination intensity 100 mW cm -2 (AM1).
the prepared photoanodes. Fig. 7 represents the photocurrent-potential behaviour or n-Si/TiO2/electrolyte and n-Si/TiO2-Sb/electrolyte systems. The first feature is that the Sb-incorporation had no effect on the saturation current, since the value of the saturation current density under AM1 illumination lies between 26 and 29 mA cm -2, which is about the same as that obtained from corresponding photovoltaic cells. The improved charge transfer characteristics manifest themselves mainly in the fill factor of photoelectrochemical cell. An increase of ca. 25% was calculated which in turn is reflected in the improved efficiency of the cell. The photoelectrochemical conversion efficiency was increased from 6.7% (pure TiO2) to 8.1% (TiO2-Sb). The open-circuit potential did not show a significant increase as that observed in the corresponding photovoltaic cells. Values of ca. 550 mV for pure TiO2, and ca. 600 mV for TiO2-Sb cells were measured.
4. Discussion
Incorporation of Sb into the TiO 2 is accompanied by an increase in the conductivity of the oxide film. The rate of reactionless one-electron transfer depends on a quantum mechanical frequency factor related to the tunnelprobabil-
W.A. Badawy / TiO2/ Sb thin films
302
ity of the electron and the density of electronic states in the TiO 2 energetically suitable for the redox system. Increasing the concentration of charge carriers in the semiconducting TiO 2 film is likely to increase the tunnel probability of decreasing the space-charge layer thickness (cf. fig. 6). This corresponds to an increase of the electrochemical rate constant of the redox reaction occurring at the electrode/ electrolyte interface. The electrochemical charge transfer process then leads to a deterioration of the cell quality. The slopes of the Tafel plots, which are related to the charge transfer coefficient a and normally used to characterize charge transfer processes involving adsorption/chemisorption of intermediates [27], are given by b = O.059/an,
at 298 K,
(7)
where b is the Tafel slope. The presence of Sb in the TiO 2 matrix increases the rate of charge transfer and hence better values of b and a are obtained. The improvement of the electrochemical performance of the cell achieved by the incorporation of Sb is reflected in the general photoelectrochemical behaviour, i.e. better fill factor and solar conversion efficiency could be obtained. The results presented in this work demonstrate the usefulness of the concept underlying the analysis of the photoelectrochemical systems [25]. According to this concept, heterojunction photoelectrochemical devices like n-Si/TiO 2 are well described as a combination of the photovoltaic and the involved electrochemical characteristics. The presence of Sb-incorporation in the TiO 2 film was shown to improve the photovoltaic characteristics of the n-Si/TiO 2 heterojunction and at the same time to increase the rate of the charge transfer process at the TiO2/ electrolyte interface. An increase in the solar conversion efficiency of more than 25% suggested that TiO 2 incorporated with foreign atoms like antimony may lead to heterojunctions with acceptable conversion efficiencies for practical use.
5. Conclusion
Sb-incorporation in the TiO2 matrix leads to a slight decrease in the transmittance and a significant increase in the conductivity of the TiO 2 films. It increases the rate of the charge transfer step and improves the catalytic properties of the oxide, which may lead to improved photovoltaic and photoelectrochemical systems of practical relevance.
References [1] [2] [3] [4] [5]
A. Fujishima and K. Honda, Nature 238 (1972) 37. A.K. Ghosh and H.P. Maruska, J. Electrochem. Soc. 124 (1977) 1516. M.P. Maruska and A.K. Ghosh, Sol; Energy Mater. 1 (1979) 237. Y. Matsumoto, J. Kurimoto, Y. Amagasaki and E. Sats, J. Electrochem. Soc. 127 (1980) 2360. L. Harris and R. Wilson, Am. Rev. Mater. Sci. 8 (1978) 99.
W.A. Badawy / Ti02 / Sb thin films [6] [7] [8] [9] [10] [11] [12] [13] [14]
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
303
M.P. Maruska and A.K. Ghosh, Sol. Energy 20 (1979) 433. J.G. Mavroides, Mater. Res. Bull. 13 (1978) 1379. K. Rajeshwar, P. Singh and J. Du Bow, Electrochim. Acta 23 (1978) 1117. A.J. Nozik, Ann. Rev. Phys. Chem. 29 (1978) 189. M. Tomkiewicz and H. Fay, Appl. Phys. 18 (1979) 1. M.S. Wrighton, D.S. Ginley, P.T. Wolczanski, A.B. Ellis, D.L. Morse and A. Linz, Proc. Natl. Acad. Sci. USA 72 (1975) 1518. A.K. Ghosh and M.P. Muruska, in: Solar Energy, Eds. J.B. Berkowitz and I.A. Lesk, The Electrochem. Soc. Softbound Proceedings Series (Princeton, NJ, 1976) p. 92. A. Monnier and J. Augustynski, J. Electrochem. Soc. 127 (1980) 1576. C. Stalder, A. Monnier and J. Augustynski, Extended Abstracts of the 2nd International Conference on the Photochem. Conversion and Storage of Solar Energy, Cambridge, England, August 10-12 (1978) p. 81. W.A. Badawy and E.A. E1-Taher, Thin Solid Films 158 (1988) 277. W.A. Badawy, J. Electroanal. Chem. 281 (1990) 85. W.A. Badawy, H.H. Afify and E.M. Elgiar, J. Electrochem. Soc. 135 (1990) 1392. W.A. Badawy, Extended Abstracts of the 40th International Society of Electrochemistry Meeting, Vol. II, CI, Kyoto, Japan, September 17-22 (1989) p. 682. W.A. Badawy, R.S. Momtaz and E.M. Elgair, Phys. Stat. Sol. (a) 118 (1990) 197. W.A. Badawy, F. Decker and K. Doblhofer, Sol. Energy Mater. 8 (1983) 363. D. Pulfrey, IEEE Trans. Electron Dev. ED-25 (1978) 1308. A.K. Abbas and M.T. Mohammed, J. Appl. Phys. 59 (1986) 1641. J. Bardeen, F.J. Slatt and L.J. Hall; Photoconductivity Conference 1954 (Wiley, New York, 1956) p. 146. W.A. Badawy, Ind. J. Technol. 29 (1991) 235. A.J. Bard and L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications (Wiley, New York, 1980) ch. 3. W.A. Badawy, Ind. J. Technol. 24 (1986) 118. K. Uosaki and H. Kita, J. Electrochem. Soc. 130 (1983) 985; H. Gerischer, Discussion Section, J. Electrochem. Soc. (Dec. 1983).