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Comparative study of zinc selenide photoelectrode annealed at different temperatures
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Solid State Sciences 10 (2008) 1970e1975 www.elsevier.com/locate/ssscie
Comparative study of zinc selenide photoelectrode annealed at different temperatures P.P. Hankare a, P.A. Chate b,*, D.J. Sathe a, M.R. Asabe a, B.V. Jadhav a a
Solid state Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India b Department of Chemistry, J.S.M. College, J.S.M. College Road, Alibag 402201, Maharashtra, India Received 16 September 2007; received in revised form 9 February 2008; accepted 1 March 2008 Available online 7 March 2008
Abstract Zinc selenide films have been synthesized by chemical bath deposition method. Zinc selenide acts as photoanode in PEC cells. The photoanode is annealed up to 473 K. The cell configuration is n-ZnSejNaOH (1 M) þ S (1 M) þ Na2S (1 M)jC(graphite). The various performance parameters were examined with respect to annealed temperature. It is found that fill factor and efficiency are maximum for photoelectrode annealed at 473 K. This is due to low resistance, high flat band potential, maximum open circuit voltage as well as maximum short-circuit current. The barrier height was examined from the temperature dependence of the reverse saturation current. The lighted ideality factor was found to be minimum for photoelectrode annealed at 473 K. A cell utilizing annealed photoelectrode showed a wider spectral response. The utility of this work is in improving the efficiency of the PEC cell. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Chemical bath deposition; Annealed photoelectrode; Flat band potential; Photoelectrochemical cell; Barrier height
1. Introduction Photoelectrochemical (PEC) cells with active semiconductor electrolyte junction are considered to be efficient solar energy harvestors, and intensive research is going on to use such a system for production of energy. PEC system can be characterized not only by the semiconductors but often also by electrolytic limitations and substantial improvements of the PEC energy conversions which are attained by understanding and optimizing solution phase phenomenon. The properties of such systems are critically dependent on the interface formed between the semiconductor and the electrolyte; hence from the material science point of view, the microstructure of semiconductor surface is of main importance. Since any practical application of solar energy conversion has to rely on polycrystalline semiconductor films, the electrode behavior of such layers developed by soft growth * Corresponding author. E-mail address: [email protected] (P.A. Chate). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.03.001
technique like chemical bath deposition has to be determined in detail [1e3]. The metal chalcogenide thin films can be used as photoanode in PEC cells. The basic requirement of a good thin film photoelectrode for PEC cells is low resistivity and larger grain size [4,5]. Thermal treatment is necessary for fabrication process of several kinds of opto-electronic devices and PEC cells. The role of thermal annealing process is very important in achieving high performance devices and PEC cell efficiency [6,7]. By considering these criteria, zinc selenide thin films are prepared. Zinc selenide seems to be a promising material since it has a direct band gap and is transparent over a wide range of visible spectrum [8]. Many methods have been employed for the preparation of zinc selenide thin films [9e16]. In this paper, we report the comparative study of the zinc selenide photoelectrode annealed at different temperatures. Various PEC properties such as IeV characteristics, CeV measurements, power output, photoresponse, spectral response as well as barrier height measurement are studied.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
2. Experimental details
1971
(111)
2.1. Preparation of zinc selenide photoelectrode
2.2. Fabrication of PEC cell Three electrode configurations are used in the experiment. Zinc selenide as photoanode, CoS-treated graphite rod as a counter electrode. A calomel electrode was used as the reference electrode and sulphideepolysulphide as the electrolyte. 2.3. Characterization of PEC cell The area of illuminated electrode was 3.0 cm2. The type of conductivity exhibited by the film is determined by noting the polarity of the emf developed in PEC cell under illumination. The currentevoltage (IeV) characteristics in dark have been plotted. The junction ideality factor has been determined by plotting the graph of log I versus V. The MotteSchottky plot is used to determine the flat band potential. A frequency of 1 KHz is used to determine the flat band potential. The power output characteristic has been obtained for a PEC cell at a constant illumination of 30 mW/cm2. The fill factor and power conversion efficiency of the cell are calculated from the photovoltaic power output characteristics. The barrier height was examined from temperature dependence of reverse saturation current at different temperatures. Light ideality factor was measured from photoresponse. Spectral response was determined by measuring short-circuit current as well as open circuit voltage as function of incident light.
Intensity (a.u.)
(220)
10
20
30
40
50
60
70
80
Two Thetha (Degree) Fig. 1. XRD pattern of ZnSe thin film.
thus control Zn2þ concentrations. The dissociation of sodium selenosulphate as well as Znetartarate complex in alkaline medium takes place. At room temperature, it forms clear solution and no film or precipitate is observed. As the temperature increases slowly, the kinetic energy increases, as a result decomposition of sodium selenosulphate and metal complex takes place in alkaline medium which favors the formation of ZnSe thin film. The deposition process is based on the slow release of Zn2þ ions and Se2 ions in the solution by ion-by-ion basis on the glass substrate. There are several soluble and insoluble species of Zn2þ possible in a reaction bath containing OH. The pH of reactive mixture is less than 7.5 or greater than 13.7 when soluble species such as Zn2þ or ZnO2 is present, respectively. If the pH of the bath is in between 7.5 and 13.7, then insoluble Zn(OH)2 may be present [18]. The presence of Zn(OH)2 in the reactive mixture is unavoidable due to aqueous alkaline nature of the bath. The amount of Zn(OH)2 increases with increasing temperature of the bath. This results in the inclusion of Zn(OH)2 in the ZnSe film to give Znx(Se,OH)y thin film rather than ZnSe film [19]. An increase in deposition temperature favors the homogenous precipitation rather than the film formation, which causes saturation to occur. Both hydrazine hydrate and ammonia are necessary for the formation of ZnSe thin
0.2 (a)-As deposited (b)-Annealed at 473K
-0.4
3. Results and discussion
(311) (222)
(b) 0.15 0.1
I (mA/cm2)
All the chemicals used for the deposition were of analytical grade. It includes zinc sulphate heptahydrate, tartaric acid, liquor ammonia, hydrazine hydrate, sodium sulfite and selenium powder. All the solutions were prepared in double distilled water. Sodium selenosulphate was prepared by following the method reported earlier [17]. In actual experimentation, 10 mL (0.2 M) zinc sulphate heptahydrate solution was taken in a 100 mL beaker. Tartaric acid 2.5 mL (1 M), 25 mL (2.8 M) ammonia, 25 mL (2%) hydrazine hydrate and 10 mL (0.25 M) sodium selenosulphate were added in the reaction bath at room temperature. The pH of the reactive mixture is 11.45. The beaker was kept in oil bath. The stainless steel substrate was mounted vertically on a specially designed substrate holder and rotated in the reactive mixture with a speed of 55 2 rpm. The temperature of the bath was then allowed to increase slowly up to 333 K. After 120 min, the slides were removed, washed several times with double distilled water, and dried naturally preserved in a dark desiccator over anhydrous CaCl2. The films were annealed at 348, 423 and 473 K for 3 h and subjected to photoelectrochemical studies.
(200)
(a) 0.05 0 -0.2
0
0.2
0.4
-0.05
3.1. Growth mechanism
-0.1
V (Volt)
2þ
In the reaction bath, Zn ions are complexed with tartaric acid in the form of water-soluble Znetartarate complex and
Fig. 2. Currentevoltage characteristics of annealed zinc selenide photoelectrode (in dark).
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
1972
-7.6
-3 0
0.2
2.6
0.4
2.8
3
3.2
3.4
(b) -8
log (Io/T2)
-4
log I
(a)
(b)
-8.4
-5 -8.8 (a)-As deposited
(a)-As deposited -9.2
-6
(a)
(b)-Annealed at 473K
(b)-Annealed at 473K
1000/T (per K)
Volt (V) 2
Fig. 5. Plot of log(Io/T ) with 1000/T zinc selenide cells.
Fig. 3. Plot of log I with voltage of zinc selenide cells.
films and hydrazine hydrate might be playing a complexing and/or catalytic role in the film process [20], which improves compactness and adherence of the films. 3.2. XRD studies The X-ray diffraction study of ZnSe film was carried out in the range of the diffraction angle 10 e80 with Cu Ka1 radi˚ ). ation using Philips PW-1710 diffractometer (l ¼ 1.54056 A The literature survey revealed that zinc selenide has two structural phases such as hexagonal wurtzite and cubic zinc blende type. The XRD pattern of ZnSe film is shown in Fig. 1. Comparison of observed ‘d’ with standard ‘d’ values confirms that the film shows cubic structure (JCPDS-1463). The XRD pattern shows the highest intensity reflection peak at ˚ (111). The diffused background is due to amord ¼ 3.271 A phous glass substrate and also to some amorphous phase present in the ZnSe thin films. Along with (111) plane, (200), (220), (311), and (222) peaks are also observed.
3.3. Photoelectrochemical studies 3.3.1. Conductivity type A PEC cell with configuration n-ZnSejNaOH (1 M) þ S (1 M) þ Na2S (1 M)jC (graphite) was formed. Even in the dark, PEC cell shows dark voltage and dark current. The polarity of this dark voltage was negative towards the semiconductor electrode. The sign of the photovoltage gives the conductivity type of zinc selenide. This suggests that zinc selenide is an n-type conductor which has also been proved from TEP measurement studies [14]. 3.3.2. IeV characteristics in dark Currentevoltage (IeV) characteristics of PEC cell in dark have been studied at 303 K and shown in Fig. 2. As the annealing temperature increase, the current increases. The characteristics are non-symmetrical indicating the formation of rectifying type junction [21]. The junction ideality factor (nd) can be determined from the plot of log I with voltage 35
4
(a)-As deposited (b)-Annealed at 473K
(a)-As deposited 28
(b)
(b)-Annealed at 473K
(a)
2
1
I ( A/cm2)
1/C2 x 108 (F-2cm4)
3 21
14
(b) 7 (a)
-0.7
-0.6
-0.5
-0.4
-0.3
Voltage (mV) Fig. 4. 1/C2 versus d.c. bias voltage of zinc selenide cells.
0 -0.2
0
0
50
100
150
200
V (mv) Fig. 6. Power output curves for annealed zinc selenide photoelectrode.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
1973
-3.5
120
0 (b)
-3.8
90
50
100
150
200
250
(a)-As deposited (b)-Annealed at 473K
log Isc
Isc ( A)
(a)
60
-4.1
(b)
-4.4 (a) 30
-4.7 (a)-As deposited (b)-Annealed at 473K
-5
0 0
20
40
Voc (mV)
60
Fig. 9. Plot of log Isc with Voc for annealed zinc selenide photoelectrode.
Light Intensity (mw/cm2) Fig. 7. Plot of Isc with light intensity of annealed zinc selenide photoelectrode.
and the variation is shown in Fig. 3. The ideality factor decreases from 3.85 to 2.64 as the annealing temperature increases up to 473. This suggests that there is improvement in removal, passivation or repair of defects with the materials and structural perfection with increase in annealing temperature. The lower value indicates the less trap density at the interface. 3.3.3. CeV characteristics in dark The flat band potential is a very useful quantity in photochemistry as it facilitates location of the energetic position of the valence and conduction band edge of a given semiconductor material. This depends upon the applied bias voltage according to MotteSchottky equation: ð1:1Þ C2 ¼ ½2=q330 nd V Vfb kT=q where symbols have their usual meaning. The charge space layer capacitance was measured under reverse biased
condition and the flat band potential is obtained from the MotteSchottky plot. The variation of C2 with voltage for representative samples is shown in Fig. 4. Intercepts of plots on voltage axis determine the flat band potential value of the junction. The flat band potential (Vfb) value increases from 0.515 to 0.628 V as the annealing temperature increases. This is probably due to increase in crystallinity of the photoelectrode. The plot suggests the presence of two regions which are attributed to the surface states present in the zinc selenide thin film. It also suggests that the junctions are graded type. 3.3.4. Barrier height measurements The barrier height was determined by measuring the reverse saturation current (Io) through the junction at different temperatures from 363 to 303 K. The reverse saturation current flowing through junction is related to temperature as follows [22,23]: Io ¼ AT 2 exp ðFb =kTÞ
250
ð1:2Þ
16 (a)-As deposited
(b) 200
(b)-Annealed at 473K 12
150
Current ( A)
Voc (mV)
(a)
100
(b)
8
4
50
(a)-As deposited (b)-Annealed at 473K
(a)
0
0
0
20
40
60
Light Intensity (mw/cm2) Fig. 8. Plot of Voc with light intensity of annealed zinc selenide photoelectrode.
400
500
600
700
800
900
Wavelength (nm) Fig. 10. Plot of Isc with wavelength for annealed zinc selenide photoelectrode.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
1974
Table 1 Various performance parameters of annealed zinc selenide photoelectrode Annealing temperature (K)
Voc (mV)
Isc (mA)
h%
ff%
Vb (eV)
Vfb (V)
Rsh (U)
Rs (U)
nL
nd
As deposited 348 423 473
153 164 176 191
20 24 28 33
0.13 0.15 0.17 0.22
29.41 31.42 34.54 35.89
0.171 0.183 0.198 0.209
0.515 0.559 0.592 0.628
1452 1339 1215 1108
1941 1832 1756 1621
6.78 6.41 6.09 5.80
3.85 3.39 2.98 2.64
where A is the Richardson constant, k is the Boltzmann constant, Vb is the barrier height in eV. The reverse saturation current increases with increase in annealing temperature. To determine the barrier height of the photoelectrode, a graph of log(Io/T2) with 1000/T was plotted. The plot of log(Io/T2) with 1000/T for representative sample is shown in Fig. 5. The non-linearity of the plots in higher temperature can be attributed to Pool-Frankel type of conduction mechanism. From the slope of the linear region of plots, the barrier height (Vb) was determined. The barrier height value increases from 0.171 to 0.209 eV as the annealing temperature increases up to 473 K. 3.3.5. Power output characteristics Fig. 6 shows the photovoltaic power putout characteristics for a cell under illumination of 30 mW/cm2. The maximum power output of the cell is given by the largest rectangle that can be drawn inside the curve. From the figure, it is observed that open circuit voltage (Voc) and short-circuit current (Isc) increases with increase in annealing temperature. The open circuit voltage increases from 153 to 191 mV and shortcircuit current also increases from 20 to 33 mA, respectively. The increase in Voc could be correlated to increase in flat and potential, and partly due to improved grain structure of the material itself. The increase in the Isc might be due to decreased photoelectrode resistance and an increased absorbance by the material itself. The calculation shows that the fill factor (ff) and efficiency (h) increases as the annealing temperature increases up to 473 K. The conversion efficiency was found to be 0.22% when the photoelectrode is annealed at 473 K. The power conversion efficiency is still low. This is because of high series resistance and interface states which are responsible for recombination mechanism. The main drawback in utilizing PEC cell is the absence of space change region at the photoelectrodeeelectrolyte interface. In this situation, the photogenerated charge carriers can move in both the direction. 3.3.6. Photoresponse The photoresponse of all zinc selenide photoelectrodes annealed at different temperatures was measured in the illumination intensity range of 10e50 mW/cm2. The open circuit voltage and short-circuit current were measured as function of light intensity. Fig. 7 shows variation of short-circuit current as a function of light intensity, whereas, Fig. 8 shows the variation of open circuit voltage as a function of light intensity. The photoresponse measurements showed a logarithmic variation of open circuit voltage with the incident light intensity.
However, at higher intensities, saturation in open circuit voltage was observed, which can be attributed to the saturation of the electrolyte interface, charge transfer and non-equilibrium distribution of electrons and holes in the space charge region of the photoelectrode. But short-circuit current follows almost a straight line path. The photoelectrodeeelectrolyte interface can be modeled as a Schottky barrier solar cell [24]. The plot of log Isc against Voc should give a straight line and from the slope of the line the lighted ideality factor can be determined. The plot of log Isc with Voc for zinc selenide photoelectrode annealed at different temperatures is shown in Fig. 9. The lighted ideality factor (nd) decreases from 6.78 to 5.80 as the annealing temperature increases up to 473 K. 3.3.7. Spectral response The spectral response for all the cells has been recorded in the 400e900 nm wavelength range. The photocurrent action spectra were examined and are shown in Fig. 10. It is seen that as the annealing temperature increases, maximum value of current shifted to the higher wavelength. The photoelectrode annealed at 473 K shows maximum current at 460 nm whereas the preannealing photoelectrode gave maximum current at l ¼ 440 nm and decreases with increase in wavelength. The decrease in current on longer wavelength side may be attributed to non-optimized thickness and transition between defect levels. The maximum current is obtained corresponding to l ¼ 440 nm which gives band gap value of 2.81 eV while photoelectrode annealed at 473 K gives band gap value of 2.69 eV. As the annealing temperature increases crystallinity, improvement in crystal structure of the photoelectrodes was observed. The increase in short-circuit current is due to decreased photoelectrode resistance and increased absorbance of the material. The various cell characteristics such as Voc, Isc, h%, ff%, Vb, Vfb, Rs, Rsh, nL, nd are cited in Table 1 for annealed zinc selenide photoelectrode. 4. Conclusions The zinc selenide thin films were deposited by using chemical bath deposition method. The films show cubic structure. The PEC cell can be easily fabricated using zinc selenide photoanode annealed at different temperatures, sulphideepolysulphide as a electrolyte, CoS-treated graphite rod as a counter electrode. A saturated calomel electrode was used as a reference electrode. The photoelectrode shows n-type conductivity. The resistance and band gap decreases up to 473 K. It is found that the fill factor and conversion efficiency for the cell are maximum for a photoelectrode annealed at 473 K.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
References [1] S. Dass, Y.S. Chaudhary, M. Agrawal, A. Shrivastav, R. Shrivastav, V.R. Sarsangi, Ind. J. Phys. 78 (2004) 229. [2] S.S. Kale, R.S. Mane, C.D. Lokhande, K.C. Nandi, Sun-Hwan-Han, Mater. Sci. Eng., B 133 (2006) 222. [3] M. Bouroushain, D. Karoussos, T. Kosanovic, Solid State Ionics 177 (2006) 1855. [4] V.V. Killedar, C.D. Lokhande, C.H. Bhosale, Ind. J. Pure Appl. Phys. 36 (1998) 643. [5] D. Das, Proceedings of Conference of Physics and Technology in Semiconductor Devices and Integrated Circuits, vol. 1523 (1992) 323. pp. [6] M.J. Kim, H.S. Lee, J.Y. Lee, T.W. Kim, K.H. Yoo, M.D. Kim, J. Mater. Sci. 39 (2004) 323. 7] S.M. Sze, VLSI Technology, McGraw-Hill, New York, 1988. [8] P.K. Kalita, B.K. Sarma, H.L. Das, Bull. Mater. Sci. 23 (2000) 313. [9] N. Katsumura, K. Maemura, T. Mori, J. Saraie, J. Cryst. Growth 159 (1996) 85. [10] R. Islam, D.R. Rao, J. Mater. Sci. Lett. 13 (1994) 1637. [11] A.M. Chaparro, M.T. Gutierez, J. Herero, Electrochim. Acta 47 (2001) 977. [12] A. Chandramohan, T. Mahalingam, J.P. Chu, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 81 (2004) 371.
1975
[13] R.B. Kale, C.D. Lokhande, Mater. Res. Bull. 39 (2004) 1829. [14] P.P. Hankare, P.A. Chate, S.D. Delekar, M.R. Asabe, I.S. Mulla, J. Phys. Chem. Solids 67 (2006) 2310. [15] P. Poulopoulos, S. Baskoutas, V. Karoutsos, M. Angelakeris, N.K. Flevaris, J. Phys.: Conf. Ser. 10 (2005) 259. [16] P. Poulopoulos, S. Baskoutas, V. Karoutsos, M. Angelakeris, N.K. Flevaris, Chem. Phys. Lett. 417 (2006) 461. [17] V.M. Bhuse, P.P. Hankare, K.M. Garadkar, A.S. Khomane, Mater. Chem. Phys. 80 (2003) 82. [18] C.A. Estrada, P.K. Nair, M.T.S. Nair, R.A. Zingaro, E.S. Meyers, J. Electrochem. Soc. 141 (1994) 802. [19] B. Mokili, Y. Charreire, R. Cortes, D. Lincot, Thin Solid Films 288 (1996) 21. [20] C.D. Lokhande, P.S. Patil, H. Tributsch, A. Ennaoui, Sol. Energy Mater. Sol. Cells 55 (1998) 379. [21] L.P. Deshmukh, S.S. Holikatti, J. Phys. D: Appl. Phys. 27 (1994) 1786. [22] M.A. Butler, J. Appl. Phys. 48 (1977) 1914. [23] A. Aruchami, G. Aravamudan, G.V. Subba Rao, Bull. Mater. Sci. 4 (1982) 483. [24] K. Rajeshwar, L. Thomson, P. Singh, R.C. Kainthala, K.L. Chopra, J. Electrochem. Soc. 128 (1981) 1744.
Solid State Sciences 10 (2008) 1970e1975 www.elsevier.com/locate/ssscie
Comparative study of zinc selenide photoelectrode annealed at different temperatures P.P. Hankare a, P.A. Chate b,*, D.J. Sathe a, M.R. Asabe a, B.V. Jadhav a a
Solid state Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India b Department of Chemistry, J.S.M. College, J.S.M. College Road, Alibag 402201, Maharashtra, India Received 16 September 2007; received in revised form 9 February 2008; accepted 1 March 2008 Available online 7 March 2008
Abstract Zinc selenide films have been synthesized by chemical bath deposition method. Zinc selenide acts as photoanode in PEC cells. The photoanode is annealed up to 473 K. The cell configuration is n-ZnSejNaOH (1 M) þ S (1 M) þ Na2S (1 M)jC(graphite). The various performance parameters were examined with respect to annealed temperature. It is found that fill factor and efficiency are maximum for photoelectrode annealed at 473 K. This is due to low resistance, high flat band potential, maximum open circuit voltage as well as maximum short-circuit current. The barrier height was examined from the temperature dependence of the reverse saturation current. The lighted ideality factor was found to be minimum for photoelectrode annealed at 473 K. A cell utilizing annealed photoelectrode showed a wider spectral response. The utility of this work is in improving the efficiency of the PEC cell. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Chemical bath deposition; Annealed photoelectrode; Flat band potential; Photoelectrochemical cell; Barrier height
1. Introduction Photoelectrochemical (PEC) cells with active semiconductor electrolyte junction are considered to be efficient solar energy harvestors, and intensive research is going on to use such a system for production of energy. PEC system can be characterized not only by the semiconductors but often also by electrolytic limitations and substantial improvements of the PEC energy conversions which are attained by understanding and optimizing solution phase phenomenon. The properties of such systems are critically dependent on the interface formed between the semiconductor and the electrolyte; hence from the material science point of view, the microstructure of semiconductor surface is of main importance. Since any practical application of solar energy conversion has to rely on polycrystalline semiconductor films, the electrode behavior of such layers developed by soft growth * Corresponding author. E-mail address: [email protected] (P.A. Chate). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.03.001
technique like chemical bath deposition has to be determined in detail [1e3]. The metal chalcogenide thin films can be used as photoanode in PEC cells. The basic requirement of a good thin film photoelectrode for PEC cells is low resistivity and larger grain size [4,5]. Thermal treatment is necessary for fabrication process of several kinds of opto-electronic devices and PEC cells. The role of thermal annealing process is very important in achieving high performance devices and PEC cell efficiency [6,7]. By considering these criteria, zinc selenide thin films are prepared. Zinc selenide seems to be a promising material since it has a direct band gap and is transparent over a wide range of visible spectrum [8]. Many methods have been employed for the preparation of zinc selenide thin films [9e16]. In this paper, we report the comparative study of the zinc selenide photoelectrode annealed at different temperatures. Various PEC properties such as IeV characteristics, CeV measurements, power output, photoresponse, spectral response as well as barrier height measurement are studied.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
2. Experimental details
1971
(111)
2.1. Preparation of zinc selenide photoelectrode
2.2. Fabrication of PEC cell Three electrode configurations are used in the experiment. Zinc selenide as photoanode, CoS-treated graphite rod as a counter electrode. A calomel electrode was used as the reference electrode and sulphideepolysulphide as the electrolyte. 2.3. Characterization of PEC cell The area of illuminated electrode was 3.0 cm2. The type of conductivity exhibited by the film is determined by noting the polarity of the emf developed in PEC cell under illumination. The currentevoltage (IeV) characteristics in dark have been plotted. The junction ideality factor has been determined by plotting the graph of log I versus V. The MotteSchottky plot is used to determine the flat band potential. A frequency of 1 KHz is used to determine the flat band potential. The power output characteristic has been obtained for a PEC cell at a constant illumination of 30 mW/cm2. The fill factor and power conversion efficiency of the cell are calculated from the photovoltaic power output characteristics. The barrier height was examined from temperature dependence of reverse saturation current at different temperatures. Light ideality factor was measured from photoresponse. Spectral response was determined by measuring short-circuit current as well as open circuit voltage as function of incident light.
Intensity (a.u.)
(220)
10
20
30
40
50
60
70
80
Two Thetha (Degree) Fig. 1. XRD pattern of ZnSe thin film.
thus control Zn2þ concentrations. The dissociation of sodium selenosulphate as well as Znetartarate complex in alkaline medium takes place. At room temperature, it forms clear solution and no film or precipitate is observed. As the temperature increases slowly, the kinetic energy increases, as a result decomposition of sodium selenosulphate and metal complex takes place in alkaline medium which favors the formation of ZnSe thin film. The deposition process is based on the slow release of Zn2þ ions and Se2 ions in the solution by ion-by-ion basis on the glass substrate. There are several soluble and insoluble species of Zn2þ possible in a reaction bath containing OH. The pH of reactive mixture is less than 7.5 or greater than 13.7 when soluble species such as Zn2þ or ZnO2 is present, respectively. If the pH of the bath is in between 7.5 and 13.7, then insoluble Zn(OH)2 may be present [18]. The presence of Zn(OH)2 in the reactive mixture is unavoidable due to aqueous alkaline nature of the bath. The amount of Zn(OH)2 increases with increasing temperature of the bath. This results in the inclusion of Zn(OH)2 in the ZnSe film to give Znx(Se,OH)y thin film rather than ZnSe film [19]. An increase in deposition temperature favors the homogenous precipitation rather than the film formation, which causes saturation to occur. Both hydrazine hydrate and ammonia are necessary for the formation of ZnSe thin
0.2 (a)-As deposited (b)-Annealed at 473K
-0.4
3. Results and discussion
(311) (222)
(b) 0.15 0.1
I (mA/cm2)
All the chemicals used for the deposition were of analytical grade. It includes zinc sulphate heptahydrate, tartaric acid, liquor ammonia, hydrazine hydrate, sodium sulfite and selenium powder. All the solutions were prepared in double distilled water. Sodium selenosulphate was prepared by following the method reported earlier [17]. In actual experimentation, 10 mL (0.2 M) zinc sulphate heptahydrate solution was taken in a 100 mL beaker. Tartaric acid 2.5 mL (1 M), 25 mL (2.8 M) ammonia, 25 mL (2%) hydrazine hydrate and 10 mL (0.25 M) sodium selenosulphate were added in the reaction bath at room temperature. The pH of the reactive mixture is 11.45. The beaker was kept in oil bath. The stainless steel substrate was mounted vertically on a specially designed substrate holder and rotated in the reactive mixture with a speed of 55 2 rpm. The temperature of the bath was then allowed to increase slowly up to 333 K. After 120 min, the slides were removed, washed several times with double distilled water, and dried naturally preserved in a dark desiccator over anhydrous CaCl2. The films were annealed at 348, 423 and 473 K for 3 h and subjected to photoelectrochemical studies.
(200)
(a) 0.05 0 -0.2
0
0.2
0.4
-0.05
3.1. Growth mechanism
-0.1
V (Volt)
2þ
In the reaction bath, Zn ions are complexed with tartaric acid in the form of water-soluble Znetartarate complex and
Fig. 2. Currentevoltage characteristics of annealed zinc selenide photoelectrode (in dark).
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
1972
-7.6
-3 0
0.2
2.6
0.4
2.8
3
3.2
3.4
(b) -8
log (Io/T2)
-4
log I
(a)
(b)
-8.4
-5 -8.8 (a)-As deposited
(a)-As deposited -9.2
-6
(a)
(b)-Annealed at 473K
(b)-Annealed at 473K
1000/T (per K)
Volt (V) 2
Fig. 5. Plot of log(Io/T ) with 1000/T zinc selenide cells.
Fig. 3. Plot of log I with voltage of zinc selenide cells.
films and hydrazine hydrate might be playing a complexing and/or catalytic role in the film process [20], which improves compactness and adherence of the films. 3.2. XRD studies The X-ray diffraction study of ZnSe film was carried out in the range of the diffraction angle 10 e80 with Cu Ka1 radi˚ ). ation using Philips PW-1710 diffractometer (l ¼ 1.54056 A The literature survey revealed that zinc selenide has two structural phases such as hexagonal wurtzite and cubic zinc blende type. The XRD pattern of ZnSe film is shown in Fig. 1. Comparison of observed ‘d’ with standard ‘d’ values confirms that the film shows cubic structure (JCPDS-1463). The XRD pattern shows the highest intensity reflection peak at ˚ (111). The diffused background is due to amord ¼ 3.271 A phous glass substrate and also to some amorphous phase present in the ZnSe thin films. Along with (111) plane, (200), (220), (311), and (222) peaks are also observed.
3.3. Photoelectrochemical studies 3.3.1. Conductivity type A PEC cell with configuration n-ZnSejNaOH (1 M) þ S (1 M) þ Na2S (1 M)jC (graphite) was formed. Even in the dark, PEC cell shows dark voltage and dark current. The polarity of this dark voltage was negative towards the semiconductor electrode. The sign of the photovoltage gives the conductivity type of zinc selenide. This suggests that zinc selenide is an n-type conductor which has also been proved from TEP measurement studies [14]. 3.3.2. IeV characteristics in dark Currentevoltage (IeV) characteristics of PEC cell in dark have been studied at 303 K and shown in Fig. 2. As the annealing temperature increase, the current increases. The characteristics are non-symmetrical indicating the formation of rectifying type junction [21]. The junction ideality factor (nd) can be determined from the plot of log I with voltage 35
4
(a)-As deposited (b)-Annealed at 473K
(a)-As deposited 28
(b)
(b)-Annealed at 473K
(a)
2
1
I ( A/cm2)
1/C2 x 108 (F-2cm4)
3 21
14
(b) 7 (a)
-0.7
-0.6
-0.5
-0.4
-0.3
Voltage (mV) Fig. 4. 1/C2 versus d.c. bias voltage of zinc selenide cells.
0 -0.2
0
0
50
100
150
200
V (mv) Fig. 6. Power output curves for annealed zinc selenide photoelectrode.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
1973
-3.5
120
0 (b)
-3.8
90
50
100
150
200
250
(a)-As deposited (b)-Annealed at 473K
log Isc
Isc ( A)
(a)
60
-4.1
(b)
-4.4 (a) 30
-4.7 (a)-As deposited (b)-Annealed at 473K
-5
0 0
20
40
Voc (mV)
60
Fig. 9. Plot of log Isc with Voc for annealed zinc selenide photoelectrode.
Light Intensity (mw/cm2) Fig. 7. Plot of Isc with light intensity of annealed zinc selenide photoelectrode.
and the variation is shown in Fig. 3. The ideality factor decreases from 3.85 to 2.64 as the annealing temperature increases up to 473. This suggests that there is improvement in removal, passivation or repair of defects with the materials and structural perfection with increase in annealing temperature. The lower value indicates the less trap density at the interface. 3.3.3. CeV characteristics in dark The flat band potential is a very useful quantity in photochemistry as it facilitates location of the energetic position of the valence and conduction band edge of a given semiconductor material. This depends upon the applied bias voltage according to MotteSchottky equation: ð1:1Þ C2 ¼ ½2=q330 nd V Vfb kT=q where symbols have their usual meaning. The charge space layer capacitance was measured under reverse biased
condition and the flat band potential is obtained from the MotteSchottky plot. The variation of C2 with voltage for representative samples is shown in Fig. 4. Intercepts of plots on voltage axis determine the flat band potential value of the junction. The flat band potential (Vfb) value increases from 0.515 to 0.628 V as the annealing temperature increases. This is probably due to increase in crystallinity of the photoelectrode. The plot suggests the presence of two regions which are attributed to the surface states present in the zinc selenide thin film. It also suggests that the junctions are graded type. 3.3.4. Barrier height measurements The barrier height was determined by measuring the reverse saturation current (Io) through the junction at different temperatures from 363 to 303 K. The reverse saturation current flowing through junction is related to temperature as follows [22,23]: Io ¼ AT 2 exp ðFb =kTÞ
250
ð1:2Þ
16 (a)-As deposited
(b) 200
(b)-Annealed at 473K 12
150
Current ( A)
Voc (mV)
(a)
100
(b)
8
4
50
(a)-As deposited (b)-Annealed at 473K
(a)
0
0
0
20
40
60
Light Intensity (mw/cm2) Fig. 8. Plot of Voc with light intensity of annealed zinc selenide photoelectrode.
400
500
600
700
800
900
Wavelength (nm) Fig. 10. Plot of Isc with wavelength for annealed zinc selenide photoelectrode.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
1974
Table 1 Various performance parameters of annealed zinc selenide photoelectrode Annealing temperature (K)
Voc (mV)
Isc (mA)
h%
ff%
Vb (eV)
Vfb (V)
Rsh (U)
Rs (U)
nL
nd
As deposited 348 423 473
153 164 176 191
20 24 28 33
0.13 0.15 0.17 0.22
29.41 31.42 34.54 35.89
0.171 0.183 0.198 0.209
0.515 0.559 0.592 0.628
1452 1339 1215 1108
1941 1832 1756 1621
6.78 6.41 6.09 5.80
3.85 3.39 2.98 2.64
where A is the Richardson constant, k is the Boltzmann constant, Vb is the barrier height in eV. The reverse saturation current increases with increase in annealing temperature. To determine the barrier height of the photoelectrode, a graph of log(Io/T2) with 1000/T was plotted. The plot of log(Io/T2) with 1000/T for representative sample is shown in Fig. 5. The non-linearity of the plots in higher temperature can be attributed to Pool-Frankel type of conduction mechanism. From the slope of the linear region of plots, the barrier height (Vb) was determined. The barrier height value increases from 0.171 to 0.209 eV as the annealing temperature increases up to 473 K. 3.3.5. Power output characteristics Fig. 6 shows the photovoltaic power putout characteristics for a cell under illumination of 30 mW/cm2. The maximum power output of the cell is given by the largest rectangle that can be drawn inside the curve. From the figure, it is observed that open circuit voltage (Voc) and short-circuit current (Isc) increases with increase in annealing temperature. The open circuit voltage increases from 153 to 191 mV and shortcircuit current also increases from 20 to 33 mA, respectively. The increase in Voc could be correlated to increase in flat and potential, and partly due to improved grain structure of the material itself. The increase in the Isc might be due to decreased photoelectrode resistance and an increased absorbance by the material itself. The calculation shows that the fill factor (ff) and efficiency (h) increases as the annealing temperature increases up to 473 K. The conversion efficiency was found to be 0.22% when the photoelectrode is annealed at 473 K. The power conversion efficiency is still low. This is because of high series resistance and interface states which are responsible for recombination mechanism. The main drawback in utilizing PEC cell is the absence of space change region at the photoelectrodeeelectrolyte interface. In this situation, the photogenerated charge carriers can move in both the direction. 3.3.6. Photoresponse The photoresponse of all zinc selenide photoelectrodes annealed at different temperatures was measured in the illumination intensity range of 10e50 mW/cm2. The open circuit voltage and short-circuit current were measured as function of light intensity. Fig. 7 shows variation of short-circuit current as a function of light intensity, whereas, Fig. 8 shows the variation of open circuit voltage as a function of light intensity. The photoresponse measurements showed a logarithmic variation of open circuit voltage with the incident light intensity.
However, at higher intensities, saturation in open circuit voltage was observed, which can be attributed to the saturation of the electrolyte interface, charge transfer and non-equilibrium distribution of electrons and holes in the space charge region of the photoelectrode. But short-circuit current follows almost a straight line path. The photoelectrodeeelectrolyte interface can be modeled as a Schottky barrier solar cell [24]. The plot of log Isc against Voc should give a straight line and from the slope of the line the lighted ideality factor can be determined. The plot of log Isc with Voc for zinc selenide photoelectrode annealed at different temperatures is shown in Fig. 9. The lighted ideality factor (nd) decreases from 6.78 to 5.80 as the annealing temperature increases up to 473 K. 3.3.7. Spectral response The spectral response for all the cells has been recorded in the 400e900 nm wavelength range. The photocurrent action spectra were examined and are shown in Fig. 10. It is seen that as the annealing temperature increases, maximum value of current shifted to the higher wavelength. The photoelectrode annealed at 473 K shows maximum current at 460 nm whereas the preannealing photoelectrode gave maximum current at l ¼ 440 nm and decreases with increase in wavelength. The decrease in current on longer wavelength side may be attributed to non-optimized thickness and transition between defect levels. The maximum current is obtained corresponding to l ¼ 440 nm which gives band gap value of 2.81 eV while photoelectrode annealed at 473 K gives band gap value of 2.69 eV. As the annealing temperature increases crystallinity, improvement in crystal structure of the photoelectrodes was observed. The increase in short-circuit current is due to decreased photoelectrode resistance and increased absorbance of the material. The various cell characteristics such as Voc, Isc, h%, ff%, Vb, Vfb, Rs, Rsh, nL, nd are cited in Table 1 for annealed zinc selenide photoelectrode. 4. Conclusions The zinc selenide thin films were deposited by using chemical bath deposition method. The films show cubic structure. The PEC cell can be easily fabricated using zinc selenide photoanode annealed at different temperatures, sulphideepolysulphide as a electrolyte, CoS-treated graphite rod as a counter electrode. A saturated calomel electrode was used as a reference electrode. The photoelectrode shows n-type conductivity. The resistance and band gap decreases up to 473 K. It is found that the fill factor and conversion efficiency for the cell are maximum for a photoelectrode annealed at 473 K.
P.P. Hankare et al. / Solid State Sciences 10 (2008) 1970e1975
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