Studies on photoelectroactive (CdHg)Se films in different redox systems

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1 Indian Journal of Chemical Technology Vol. 20, November 2013, pp Studies on photoelectroactive (CdHg)Se films in different redox systems Md Rashid Tanveer* & Shoeb A Ansari Department of Chemistry, St. Andrew's P.G. College, Gorakhpur , India Received 7 April 2011 ; accepted 19 August 2013 Electrosynthesis of (CdHg)Se films has been carried out on titanium substrate by electrochemical codeposition method. Photoelectrochemical characteristics of these films studied in different redox solutions reveal that I 3 /I 2 redox system is the most suitable solution for their characterization. Impedance measurements are used for the estimation of corrosion behaviour of the deposited films. The rate of corrosion is found to be comparatively lower when the electrodeposited films are tested in I 3 /I 2 redox solution. Photoaction spectral studies are used to study light intensity dependence on photoactivity and also in the determination of band gap. The band gap values of the electrodeposited films are found to be independent to the nature of redox solution. Keywords: Band gap, Cadmium mercury selenide, Corrosion, Impedance, Photoactivity, Redox solution Solar power, which has the brightest future, is the least popular for the main conventional sources like bio energy and wind because of the high capital costs and relatively low current releases. The solar cells are currently being used by satellites to get energy and photovoltaic modules are being used for street lightning and many other places. Their cost is much higher than that of power generated by conventional means. The conventional sources are cheaper but they are limited on earth and highly polluting. Thus, unless there is tremendous technical advances, solar power will remain peripheral to the larger energy scene. In order to have a hope of replacing fossil fuels, there is need to develop materials that can be easily mass produced and convert enough sunlight to electricity in comparably low cost. In this connection cadmium mercury selenide and cadmium mercury tellurides are attracting attention 14. They have various practical applications 58 ranging from infrared detection to solar energy conversion. These materials are tunable in composition over a wide range. A few methods have been reported for preparing (CdHg)Se. Single crystal of (CdHg)Se is grown by the Bridgman method 9. Recently, thin film (CdHg)Se has been grown epitaxially on CdSe substrates 10. The electrochemical codeposition method may also be employed with advantage for the preparation of these films 11. This method is currently *Corresponding author. rashidtanveer1@gmail.com being used by many workers 12,13 for other related systems also. This technique is relatively easy, less expensive and has negligible waste of materials. The present paper reports an electrochemical codeposition technique for the formation of (CdHg)Se films and study of their photoelectrochemical characteristics with an objective to identify the suitable redox solution for their characterization. Impedance behaviour of the (CdHg)Se films has been examined in different redox solutions. These data have been used for the determination of corrosion rates of the electrodeposited films. The band gap values of the deposited thin films are measured from photoaction spectra. The nature of semiconductivity is assessed by the current voltage behaviour in dark as well as under illumination. Experimental Procedure Electrochemical codeposition of (CdHg)Se was carried out using flag shaped titanium cathode with crosssection area of 1 cm 1 cm. The potential of the cathode was manually kept at a desired value using a power supply. The current flowing between the cathode and the platinum counter electrode was measured by means of multimeter. Solutions of CdSO 4, HgCl 2 (CDH, India) and SeO 2 (Fluka Chemika, Switzerland) were prepared in double distilled water. Illumination was done with the help of a 1000 W tungsten lamp. Band gap values were calculated from photoaction spectrum for which Model f /3.4 monochromator (Applied Photophysics,

2 412 INDIAN J. CHEM. TECHNOL., NOVEMBER 2013 London) was used. Electrochemical impedance measurements were performed with EG & G Princeton Applied Research, USA (Model 378) consisting of a two phase lockinamplifier (Model 5208) and a potentiostat/galvanostat (Model 273) using compatible personal computer. Results and Discussion The currentvoltage behaviour of the electrochemical system containing 0.05 M CdSO 4, M SeO 2 and variable composition of HgCl 2 has been investigated with the objective of identification of the deposition potential range within which formation of (CdHg)Se films may be expected. The results presented in Fig. 1 show that the relevant electrochemical activity is confined to the voltage range from 0.40 to 0.75 V versus SCE. The films of (CdHg)Se are electrodeposited at different deposition potentials within this range and tested for their photoelectroactivity in I 3 / I 2 redox system. These studies lead us to infer that a potential of 0.65 V vs SCE is needed for obtaining (CdHg)Se films which exhibit maximum photoresponse. Electrosynthesis of (CdHg)Se may be represented as: H 2 SeO 3 + Cd 2+ + Hg H + + 8e (CdHg)Se + 3H 2 O This reaction is analogous to CdSe formation by electrochemical codeposition 12. The above reaction actually occurs in following two steps: H 2 SeO 3 + 4H + + 4e Cd 2+ + Hg 2+ +Se + 4e Se + 3H 2 O (CdHg)Se A semiconductor upon optical excitation generates two oppositely charged carriers, namely the electrons and the holes. Effective separation of these charge carriers results in the production of photoeffect 14. From the point of view of photoelectric conversion, it is desirable that the majority carriers move towards the bulk of semiconductor while the minority carriers particulate in oxidative or reductive reactions with redox species available in the solution 15. Thus, nature of redox system have profound infuence on separation of charge carriers and production of photoeffect of the electrodeposited films. In order to investigate this aspect, (CdHg)Se films obtained by using electroplating solution containing M SeO 2, 0.05 M CdSO 4 and M HgSO 4 are tested in I 3 /I 2, Fe 2+ /Fe 3+, [Fe(CN) 6 ] 4 /[Fe(CN) 6 ] 3, Ce 3+ /Ce 4+ and polysulphide redox solutions. Results (Table 1) clearly show that (CdHg)Se films exhibit maximum photoresponse in Ce 3+ /Ce 4+ redox solutions. In this system enhanced band bending of semiconductor perhaps occur, leading to increased photoactivity. However, in this redox solution, relatively slow build up rate of photopotential is observed on illumination. On the other hand, the electrodeposited films exhibit reasonable photoactivity with fairly high build up rates in I 3 /I 2 redox solution. In order to ascertain the ability of these materials to withstand impairment in optical activity, these films Table 1 Effect of redox systems on photoactivity of (CdHg)Se films Redox systems Conc. M E D E L E p (CH 3 COO) 2 Cd KI 5 mm I 2 K 4 [Fe(CN) 6 ] K 3 [Fe(CN) 6 ] Ce 2 (SO 4 ) Ce(SO 4 ) 2 NaOH Na S 2 S Fig. 1 Current voltage behaviour in 0.05M CdSO 4 and M SeO 2 solutions containing different concentrations of HgCl 2 FeSO FeCl 3 E D Dark potential. E L Potential obtained on illumination of (CdHg)Se Films. E P Photopotential.

3 TANVEER & ANSARI : PHOTOELECTROACTIVE (CdHg)Se FILMS 413 are subjected to uninterrupted illumination. It is found that electrodeposited films exhibit relatively higher stability in I 3 /I 2 redox solution. The band gap depends on composition of the electroplating solution and should be independent to the nature of redox solution. In order to investigate this aspect, photoaction spectral studies of (CdHg)Se films are carried out in different redox solutions. The photoaction spectra were taken in the form of variation in the photopotential E P with the wave length λ of light irradiated on the film. These data are then used to construct E P 2 verses λ plots. The results are presented in Fig. 2. These plots are used to calculate the threshold frequency (λ T ). Now band gap (E g ) values are calculated using the following relationship: E g (ev) = hc / λ T e (1) where h is the Plank s constant; c, the velocity of light; and e, the electronic charge. The band gap values obtained from these studies are found to be comparable. The slight deviation in the band gap values may be due to different extent of adsorption of redox species on the electrodeposited films. Light intensity dependence of photopotential has been examined at λ = 436 nm, the wavelength which generates maximum photoeffect. A linear relation exists between photopotential and lni L. This behaviour indicates semiconducting nature of the electrodeposits 16. The data are also consistent with the following equation: E P = (AkT/e) ln I L + B (2) where E P is the photopotential in ; A, the ideality factor; I L, the light intensity;k, the Boltzman constant; T, the Absolute temperature; e, the electronic charge; and B, the another constant. The value of A is unity in ideal case. In reality the ideality factors are often larger due to recombination centers and traps responsible for deviation from electroneutrality. For the electrodeposited films examined in I 3 /I 2 redox solution and presented in Fig. 3, the ideality factor is found to be The value of A is also calculated for other redox solutions. The ideality factor is found to be comparable in different redox solutions. In all cases, A is found to be greater than unity which shows that the deposited films are indeed not ideal. Fig. 3 Light intensity dependence of photopotential for (CdHg)Se films at λ = 436 nm Fig. 2 E P 2 versus λ plot for (CdHg) Se films in different redox solutions Fig. 4 Photocurrent photopotential characteristics of (CdHg)Se films

4 414 INDIAN J. CHEM. TECHNOL., NOVEMBER 2013 The photocurrentphotovoltage characteristics of a typical (CdHg)Se film, observed in redox I 3 / I 2 solution, are shown in Fig. 4. Power characteristics of same film are presented in Fig. 5. Fill factor (F.F.) is estimated using the following relationship 17 : (E I ) F.F. P = P max (3) (E I ) P max P max where (E P ) max and (I P ) max are the maximum photopotential and maximum photocurrent respectively. (E P I P ) max is the maximum output. Efficiency (η) is commonly expressed as 18 : ( IP ) max (EP ) max η= 100 P (4) Fill factor and efficiency obtained for the electrodeposited film under consideration are found to be and 0.023% respectively. The efficiency is obviously not attractive from the point of view of photoelectric conversion and needs to be improved by orders of magnitude before these deposited films could be considered of some interest for practical photoelectrochemical conversion. The temperature of redox solution is also found to have effect on the activity of semiconductor. Some electrodeposited films are tested for their photoactivity at different temperatures of I 3 /I 2 redox solution. Enhanced photoresponse is observed at elevated temperature. This perhaps is because of decrease in resistance of semiconductor at higher temperature. But at higher temperature, evaporation of redox solution is an unavoidable problem. The corrosion of material is an essential reaction in the electrochemistry of semiconductor and many workers has already dealt with its negative aspect to PEC solar energy conversion. We have estimated corrosion current i corr and corrosion rate R corr of these materials in different redox solutions from impedance studies. If R P is the polarization resistance of (CdHg)Se semiconducting film, then corrosion current i corr may be expressed as: i corr = RT/ FR P (5) where R is the gas constant; T, the absolute temperature; and F, the faraday constant. The corrosion rate (R corr ) is generally expressed in milli inch per year (mpy) as: R corr (mpy) = 3 i corr (EW) / da (6) where EW is the equivalent weight of the electroactive material; d, the density; and A, the area of crosssection. The corrosion rate may also be expressed as: R corr (gs 1 ) = i corr. (EW) / F (7) Results (Table 2) show that relatively low corrosion rates are obtained when (CdHg)Se electrodeposited films are tested in I 3 /I 2 redox solution. Rs value included in this table is the resistance of solution. Table 2 Impedance parameters of (CdHg)Se films in different redox solutions Redox solution (CH 3 COO) 2 KI I 2 Conc. M 5 mm R s R p i corr R corr Ω kω µa gs K 4 [Fe(CN) 6 ] K 3 [Fe(CN) 6 FeSO 4 FeCl 3 Ce(SO 4 ) 3 Ce(SO 4 ) Fig. 5 Performance of phtoelectrochemical cell based on (CdHg)Se films NaOH Na 2 S S R S Resistance of solution. R P Polarization resistance of semiconductor films. I corr Corrosion current.

5 TANVEER & ANSARI : PHOTOELECTROACTIVE (CdHg)Se FILMS 415 Conclusion The above investigations clearly show that although (CdHg)Se semiconducting films exhibit better photoresponse in the Ce 3+ /Ce 4+ redox solution, the overall results show that I 3 /I 2 system the most suitable redox couple for characterization of (CdHg)Se films. In this redox solution, the (CdHg)Se film exhibits reasonable stability. Acknowledgement One of the authors (MRT) thankfully acknowledges University Grants Commission, New Delhi for providing financial assistance in the form of minor project [F. No. 8 3 (38)/2011 (MRP/NRCB) dated 23 rd December 2011]. References 1 Bhuse V M, Mater Chem Phys, 106 (2007) Jacobs R N, Benson J D, Stoltz A J Almeida L A, Farrel S, Brill G, Salmon M & Newell A, J Crystal Growth, 366 (2013) Duan H, Dong Y Z, Luo J, Huang Y, Chen X S & Lu W, J Phys Chem Solids, 74 (2013) Duan H, Dong Y Z, Lin Z P, Huang Y & Chen X S, Solid State Commun, 152 (2012) Xue L, Zhou P, Zhang C X, Sun L Z & Zhong J X, J Phys Chem Solids, 74 (2013) Jozwikow K, Jozwikowska A, Kopytko M, Rogalski A & Jaroszewies L R, Infrared Phys Technol, 55 (2012) Rhiger D R, Semiconductors Semimetals, 84 (2011) Yang Z S & Chang H T, Solar Energy Materials Solar Cells, 94 (2010) Broerman J G, Proceedings, Eleventh International Conference on the Physics of Semiconductors (Polish Scientific, Warsaw), Vol. 2, 1972, Kong H Z, Shi W D, Wang D C & Wang Chin B K, Phys (Engl Ed), 8 (1988) Aruchamy A & Wrighton M S, J Phys Chem, 84 (1980) Cheng K W & Liang C J, Solar Energy Materials Solar Cells, 94 (2010) Delekar S D, Patil M K, Jadhav B V, Sanadi K R & Hankare P P, Solar Energy, 84 (3) (2010) Chandra S, Photoelectrochemical Cells (Gordon and Breach Inc 1980), chap Russak, J Electrochem Soc, 129 (1982) Tributch H, Structure and Bonding 49, edited by C K Jorgensen (SpringerVerlog Berlin Heidberg), 1982, Archer M D, J Appl Electrochem, 5 (1975) Gerischer H, Pure Appl Chem, 82 (1980) 2049.