Photoelectrochemical characterization of annealed cadmium selenide photoelectrode using sulphide–polysulphide electrolyte

ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 655–658

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs

Photoelectrochemical characterization of annealed cadmium selenide photoelectrode using sulphide–polysulphide electrolyte P.P. Hankare a, P.A. Chate b,, D.J. Sathe a a b

Solid State Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India Department of Chemistry, J.S.M. College, Alibag 402201, India

a r t i c l e in fo

abstract

Article history: Received 24 May 2008 Received in revised form 5 January 2009 Accepted 2 February 2009

Cadmium selenide films have been synthesized by dip method. Cadmium selenide acts as photoanode in photoelectrochemical (PEC) cells. The photoanode was annealed upto 473 K. The cell configuration is n-CdSejNaOH (1 M) +S (1 M) +Na2S (1 M) jC(graphite). Various performance parameters were examined with respect to annealed temperature. It is found that the 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 PEC cells. & 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Chalcogenides A. Inorganic compound A. Interface B. Chemical synthesis C. X-ray diffraction D. Electrical properties

1. Introduction In the present era of the beginning of 21st century, a tremendous surge of interest has developed in various fields such as photoelectrochemical (PEC) systems, surface engineering, magnetic science, telecommunications, nanotechnology, interfacial science, catalysis, etc. [1–3]. A variety of binary semiconductors, especially from the II and VI groups of the periodic table, have been extensively studied due to their potential use in photoconductive solar cells. Cadmium selenide is one of the members of the II and VI families that has a suitable band gap to match the maximum of solar spectrum and also has high photosensitivity [4–7]. Photoelectrochemical cells with active semiconductor electrolyte junction are considered to be efficient solar energy harvestors, and intensive research is going on to use such systems for production of energy [8–10]. 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 large grain size. These cells are simple in construction and have the advantage that they can be used for both solar to electrical and chemical energy conversions. Presently, one of the best materials where tailoring of band gap is possible is from the group of Cd-chalcogenides [11–12]. Such kind of films principle role may also play thin nano-sheets on the interface separating the films also the substrate [13].

 Corresponding author.

E-mail address: [email protected] (P.A. Chate). 0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.02.002

In PEC cells, photo-induced electron–hole pairs are used to generate electricity in the same way as in solid-state solar cells. The cell is usually made up of semiconductor electrode, an appropriate electrolyte and a metal electrode that converts solar energy directly into electrical energy. Redox process with the necessary charge transfer occurs at the electrode–electrolyte interface. The reduced species is oxidized by a hole to produce oxidized species, which migrate towards the metal electrode, where it is reduced. The incident photon energy at the semiconductor electrolyte interface is the driving force, which increases the free energy of electrons in the semiconductors. The gain in free energy is converted to electrical work as the electrons traverse through the external circuit to the counter electrode. The photocurrent generated in the cell is dependent on the semiconductor band gap and the quantum efficiency of electron–hole pair separation. 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 highperformance devices and large PEC cell efficiency [14–15]. The efficiency and stability of PEC cells are strongly dependent on the preparation conditions of the photoelectrode and electrolyte and on experimental conditions during tests [16]. In this paper, we report a comparative study of cadmium selenide photoelectrodes annealed at different temperatures. Various PEC properties such as I–V characteristics, C–V measurements, power output, photoresponse, spectral response as well as barrier height measurement are studied and efficiency of the cells is discussed.

ARTICLE IN PRESS 656

P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 70 (2009) 655–658

2. Experimental details

(100) 2.1. Synthesis of cadmium selenide photoelectrode

2.2. Fabrication and characterization of PEC cell Three electrode configurations were used in the experiment. Cadmium selenide was used as photoanode, CoS-treated graphite rod as a counter electrode, a calomel electrode was used as a reference electrode and sulphide–polysulphide as the electrolyte. The area of illuminated electrode was 3.0 cm2. The type of conductivity exhibited by the photoelectrode is determined by noting the polarity of the emf developed in PEC cell under illumination. The current–voltage (I–V) characteristics in dark have been plotted. The junction ideality factor has been determined by plotting the graph of log I versus V. The Mott–Schottky plot is used to determine the flat-band potential; 1 kHz frequency 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 (FF) and power conversion efficiency of the cell are calculated from 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 a function of incident light.

3. Results and discussion 3.1. XRD studies X-ray diffraction study of the CdSe film was carried out in the range of diffraction angle 10–801 with Cu Ka1 radiation using Philips PW-1710 diffractometer (l ¼ 1.54056 A˚). The literature survey revealed that cadmium selenide has two structural phases—hexagonal wurtzite and cubic zinc blende type [18]. The XRD pattern of cadmium selenide is shown in Fig. 1. Comparison of the observed ‘d’ with standard ‘d’ values confirms that the film shows hexagonal structure (JCPDS card Nos. 19-191 and 8-459). The XRD pattern show the highest intensity reflection peak at d ¼ 3.717 A˚ (1 0 0). The diffused background is due to amorphous phase present in the CdSe thin films. Along with (1 0 0) plane, (0 0 2), (1 0 1), (1 0 2), (11 0) and (1 0 3) peaks are also observed.

Intensity (a.u.)

(101) (102) (110) (103)

10

20

30

40 50 60 Two Thetha (Degree)

70

80

Fig. 1. XRD pattern of CdSe thin film.

1

0 -1

0.1

0.2

0.3

0.4

(b)

log I

In the experiments, 10 mL (0.2 M) cadmium sulphate octahydrate solution was taken in a 100 mL beaker; 2.5 mL (1 M) tartaric acid, 10 mL (2.8 M) ammonia and 10 mL (0.25 M) sodium selenosulphate were added to the same reaction bath. Sodium selenosulphate was prepared by following the method reported earlier [17]. The pH of the reactive mixture was 11.02. The total volume was made up 50 mL with double distilled water. The temperature of the bath was maintained at 278 K using ice bath. Individual solutions were cooled at 278 K and mixed to avoid precipitation. The solution was stirred vigorously before dipping stainless-steel substrates, which were kept vertically slightly tilted in a reactive bath. The temperature of the bath was then allowed to increase up to 298 K very slowly. After 5 h, the slides were removed and washed several times with double distilled water. The photoelectrodes were dried naturally, preserved in dark desiccators over anhydrous CaCl2 and annealed at 348, 423 and 473 K for 3 h and subjected to PEC studies.

(002)

-3 (a)-As deposited

(a)

(b)-Annealed at 473K -5 Volt (V) Fig. 2. Plot of log I with voltage of cadmium selenide cells.

3.2. PEC studies A PEC cell with configuration n-CdSejNaOH (1 M) +S (1 M) +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. This suggests that cadmium selenide is an n-type conductor, which has also been proved from TEP measurement studies [19–20]. Current–voltage (I–V) characteristics of PEC cell in dark have been studied at 303 K. As the annealing temperature increases, 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 and the variation is shown in Fig. 2. The ideality factor decreases from 3.23 to 2.32 as the annealing temperature increases up to 473. This suggests improvement in removal, passivation or repair of defects in the materials and structural perfection with increase in annealing temperature. The lower value indicates small trap density at the interface [11]. The charge space layer capacitance was measured under reverse biased condition and the flat-band potential was obtained from the Mott–Schottky plot. The variation of C 2 with voltage for representative samples is shown in Fig. 3. Intercepts of plots on voltage axis determine the flat-band potential value of the junction. The flat-band potential (Vfb) value increases from 0.700 to 0.953 V as the annealing temperature increases. This is probably due to increasing crystallinity of the photoelectrode. The plot suggests the presence of two regions, which are attributed to the surface states present in the cadmium selenide photoelectrode. It also suggests that the junctions are of graded type.

ARTICLE IN PRESS P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 70 (2009) 655–658

300

9 (a)-As deposited (b)-Annealed at 473K 1/C2 X 108 (F-2cm4)

657

(a)-As deposited

250 6

200 I (mA/cm2)

3

(b)-Annealed at 473K

150 100

(b) (a) -0.8

-1

50

-0.6 Voltage (mV)

0 -0.2

-0.4

(b) (a)

0 0

100

200

300

400

500

V (mV)

2

Fig. 3. 1/C versus d.c. bias voltage of cadmium selenide cells.

Fig. 5. Power output curves for annealed cadmium selenide photoelectrode.

-7 2.6

2.8

3

3.2

-3

3.4

100

200

300

400

-7.5

-8

(b) -8.5

(b)

(b)-Annealed at 473K

-3.5 log Isc

log (Io/T2)

(a)-As deposited

(a)

-4 -9

(a)-As deposited (b)-Annealed at 473K

-9.5

(a)

1000/T (per K)

-4.5

Voc (mV)

2

Fig. 4. Plot of log(Io/T ) with 1000/T of cadmium selenide cells. Fig. 6. Plot of Isc with light intensity of annealed cadmium selenide photoelectrode.

40 (a)-As deposited (b)-Annealed at 473K

30 Current (mA)

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 increases with an 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 a representative sample is shown in Fig. 4. The non-linearity of the plots at higher temperature can be attributed to the Pool–Frankel type of conduction mechanism. From the slope of the linear region of plots, the barrier height (Fb) was determined. The barrier height value increases from 0.186 to 0.245 eV as the annealing temperature increases up to 473 K [22–23]. Fig. 5 shows the photovoltaic power output characteristics for a cell under illumination of 30 mW/cm2. From the figure, it is observed that open-circuit voltage (Voc) and short-circuit current (Isc) increase with an increase in annealing temperature. The open-circuit voltage increases from 260 to 412 mV and the shortcircuit current increases from 169 to 286 mA. The increase in Voc can be correlated to increase in flat-band potential, and partly to improved grain structure of the material itself. The increase in Isc might be due to decreased photoelectrode resistance and an increased absorbance by the material itself. The fill factor and efficiency (Z) increase as the annealing temperature increases up to 473 K. The conversion efficiency was found to be 0.99% when the photoelectrode was 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 the recombination mechanism. The main drawback in utilizing PEC cells is the absence of space charge region at the photoelectrode–electrolyte

20 (b)

10 (a)

0 400

500

600 700 Wavelength (nm)

800

900

Fig. 7. Plot of Isc with wavelength for annealed cadmium selenide photoelectrode.

interface. In this situation, the photogenerated charge carriers can move in both directions. The photoresponse of all photoelectrodes annealed at different temperatures were measured in the illumination intensity range of 10–50 mW/cm2. The open-circuit voltage and short-circuit current were measured as functions of light intensity. The photoresponse measurements showed a logarithmic variation of open-circuit voltage with incident light intensity. However, at higher intensities, saturation in open-circuit voltage was

ARTICLE IN PRESS 658

P.P. Hankare et al. / Journal of Physics and Chemistry of Solids 70 (2009) 655–658

Table 1 Various performance parameters of annealed cadmium selenide photoelectrode. Annealing temperature (K)

Voc (mV)

Isc (mA)

Z (%)

FF (%)

Fb (eV)

Vfb (V)

Rsh (O)

Rs (O)

nL

nd

As deposited 348 423 473

260 315 369 412

169 196 235 286

0.55 0.71 0.84 0.99

36.91 37.96 38.48 39.49

0.186 0.205 0.231 0.245

0.700 0.785 0.848 0.953

550 455 385 268

823 745 644 503

4.06 3.76 3.43 3.03

3.23 3.01 2.64 2.32

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 photoelectrode–electrolyte 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 cadmium selenide photoelectrode annealed at different temperatures is shown in Fig. 6. The lighted ideality factor (nd) decreased from 4.06 to 3.03 as the annealing temperature increased up to 473 K. The spectral response for all the cells has been recorded in the 400–900 nm wavelength range. The photocurrent action spectra were examined and are shown in Fig. 7. It is seen that as the annealing temperature increases, the maximum value of current shifted to higher wavelength. The photoelectrode annealed at 473 K showed the maximum current at 780 nm whereas the preannealing photoelectrode gave the maximum current at l ¼ 730 nm and decreased with an increase in wavelength. The decrease in current on the longer wavelength side may be attributed to non-optimized thickness and transition between defect levels. The maximum current obtained at l ¼ 730 nm gives a band gap value of 1.69 eV while the photoelectrode annealed at 473 K gives a band gap value of 1.58 eV. As crystallinity increases with annealing temperature, an 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. Various cell characteristics such as Voc, Isc, Z (%), FF (%), Fb, Vfb, Rs, Rsh, nL and nd are cited in Table 1 for annealed cadmium selenide photoelectrode. 4. Conclusion Cadmium selenide thin films were deposited by the dip method. The films show hexagonal structure. PEC cells can be easily fabricated using cadmium selenide photoanode annealed at different temperatures, sulphide–polysulphide as electrolyte and

CoS-treated graphite rod as a counter electrode. A saturated calomel electrode was used a reference electrode. The photoelectrode shows n-type conductivity. The resistance and band gap decreases up to 473 K. It is found that the FF and conversion efficiency for the cell are maximum for photoelectrodes annealed at 473 K. References [1] S. Street, D. Goodman, Chem. Phys. Solid Surf. B (1997) 375. [2] R.W. Siegel, E.H. Hu, M.C. Roco, WTEC panel report on R&D status and trends in nanoparticles, in: Proceedings of the Nanostructured Materials and Nanodevices Workshop, 1997. [3] K.L. Chopra, K.L. Malhotra, Thin Film Technology and Applications, TMH Publishing Co., New Delhi, India, 1984, p. 1. [4] A.J. Nelson, L.L. Kazmerski, M. Engelhardt, H. Hochst, J. Appl. Phys. 67 (1990) 1393. [5] Y. Kato, S. Kurita, P. Suda, J. Appl. Phys. 62 (1987) 3737. [6] M. Matsuoka, K. Ono, J. Vac. Sci. Technol. 7 (1989) 2. [7] P.P. Hankare, V.M. Bhuse, K.M. Garadkar, S.D. Delekar, I.S. Mulla, Mater. Chem. Phys. 82 (2003) 711. [8] S. Dass, Y.S. Chaudhary, M. Agrawal, A. Shrivastav, R. Shrivastav, V.R. Sarsangi, Ind. J. Phys. 78 (2004) 229. [9] S.S. Kale, R.S. Mane, C.D. Lokhande, K.C. Nandi, S. Han, Mater. Sci. Eng. B 133 (2006) 222. [10] M. Bouroushain, D. Karoussos, T. Kosanovic, Solid State Ion. 177 (2006) 1855. [11] P.P. Hankare, P.A. Chate, D.J. Sathe, M.R. Asabe, B.V. Jadhav, Solid State Sci. 10 (2008) 1970. [12] S.M. Pawar, A.V. Moholkar, K.Y. Rajpure, C.H. Bhosale, Sol. Energy Mater. Sol. Cells 92 (2008) 45. [13] I.V. Kityk, M. Makowska-Janusik, J. Ebothe, A.E. Hichou, B.E. Idrissi, M. Addou, Appl. Surf. Sci. 202 (2002) 24. [14] M.J. Kim, H.S. Lee, J.Y. Lee, T.W. Kim, K.H. Yoo, M.D. Kim, J. Mater. Sci. 39 (2004) 323. [15] S.M. Sze, VLSI Technology, McGraw-Hill, New York, 1988. [16] C.D. Lokhande, Sol. Cells 22 (1987) 133. [17] V.M. Bhuse, P.P. Hankare, K.M. Garadkar, A.S. Khomane, Mater. Chem. Phys. 80 (2003) 82. [18] R.B. Kale, S.D. Sartale, B.K. Chougule, C.D. Lokhande, Semicond. Sci. Technol. 19 (2004) 980. [19] C.N.R. Rao, Modern Aspects of Solid State Chemistry, Plenum press, New York, 1970, p. 531. [20] T. Caillat, M. Carle, P. Pieral, H. Scherrer, J. Phys. Chem. Solids 53 (1992) 1121. [21] L.P. Deshmukh, S.S. Holikatti, J. Phys. D 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.