Effect of complexing agent on the

Applied Surface Science 256 (2010) 2884–2889

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Effect of complexing agent on the photoelectrochemical properties of bath deposited CdS thin films S.B. Patil, A.K. Singh * Defence Institute of Advanced Technology (DU), Girinagar, Pune 411025, M.S., India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 24 September 2009 Received in revised form 13 November 2009 Accepted 13 November 2009 Available online 20 November 2009

In the present paper photoelectrochemical (PEC) performance of bath deposited CdS thin films based on complexing agents i.e. ammonia and triethanolamine (TEA) has been discussed. Effect of annealing has also been analyzed. The as-deposited and annealed (at 523 K for 1 h in air) films were characterized by Xray diffraction (XRD), ultraviolet–visible (UV–vis) absorption spectroscopy, SEM, electrochemical impedance spectroscopy (EIS), and PEC properties. XRD studies revealed that the films were nanocrystalline in nature with mixed hexagonal and cubic phases. TEA complex resulted in better crystallinity. Further improvement in the crystallinity of the films was observed after air annealing. The marigold flower-like structure, in addition to flakes morphology, was observed with TEA complex, whereas for ammonia complex only flakes morphology was observed. The UV–vis absorption studies revealed that the optical absorption edge for the films with ammonia and TEA complex was around 475 nm and 500 nm, respectively. Annealing of the films resulted in red shift in the UV–vis absorption. The PEC cell performance of CdS films was found to be strongly affected by crystallinity and morphology of the films resulted due to complexing agent and annealing. The air annealed film deposited using TEA complex showed maximum short circuit current density (Jsc) and open circuit voltage (Voc) i.e. 99 mA/ cm2 and 376 mV respectively, under 10 mW/cm2 of illumination. The films deposited using TEA complex showed good stability under PEC cell conditions. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Complexing agent TEA Ammonia Marigold flower structure EIS PEC

1. Introduction Recently, many research groups have concentrated on the low cost and high efficiency materials for solar cells, as solar energy is the most promising future energy source. After Honda–Fujishima effect a variety of photoelectrochemical (PEC) solar cells have been fabricated and investigated. Thin film based PEC solar cells [1] have wide applications due to their low fabrication cost, highthroughput processing techniques and ease of junction formation with an electrolyte. The semiconductors with bandgap close to the maximum wavelength in the visible spectrum are promising materials for the PEC cells. CdS, belonging to II–VI group, is one of the promising semiconductors, having bulk bandgap 2.42 eV. It also acts as window layer in CdTe [2] and CIS as well as CIGS [3] based solar cells. Numbers of techniques have been employed to deposit CdS thin films viz vacuum evaporation [4,5], sputtering [6], CVD [7,8], electrodeposition [9,10], pulsed-laser deposition [11], spray pyrolysis [12,13], SILAR [14], chemical bath deposition (CBD) [15–19]. Out of these techniques CBD is one of the widely used synthesis routes for formation of inorganic metal chalcogenide/

* Corresponding author. Tel.: +91 020 24304173. E-mail address: [email protected] (A.K. Singh). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.043

oxide semiconductors thin films on substrates due to various advantages such as large deposition area, relatively low temperature processes, no restriction on use of substrate, reproducibility and most importantly low cost of equipment. Despite being hazardous in nature, CdS has attracted wide attention [20]. It degrades into soluble cadmium cations under PEC conditions which is hazardous [21]. This would limit the use of CdS as a photoelectrode material for PEC cell unless its characteristics are modified. Hence, in addition to the cost of the device and conversion efficiency the stability to degradation is an important parameter for the CdS based PEC cells. Review of literature reveals that properties of the films can be modified by variety of ways. Doping has been adopted as one of the methods for improving conversion efficiency [17,19,22]. Deposition techniques also affect the performance of cells. Annealing has also been used to modify the characteristics of films. Recently Hilal et al. [23] have shown that different cooling rates resulted in enhancement of the conversion efficiency as well as stability of the CdS photoelectrode. Tiwari and Tiwari [24] have shown that surface treatment could increase the stability of the CdS photoelectrode. In the present work, we report the effect of complexing agent on both conversion efficiency and stability to degradation of bath deposited CdS photoelectrode under PEC conditions, as well as, the effect of annealing on cell performance.

S.B. Patil, A.K. Singh / Applied Surface Science 256 (2010) 2884–2889

2. Experimental Thin films of CdS were bath deposited using two different complexing agents viz ammonia and TEA. In both the cases 20 mM aqueous solution of cadmium sulphate and 60 mM aqueous solution of thiourea were used as precursors. In one bath cadmium sulphate was complexed by aqueous ammonia. Initially white precipitate was observed which disappeared on further addition of aqueous ammonia, pH of this solution was adjusted to 11, and aqueous solution of thiourea was then added. While in other bath cadmium sulphate was complexed by TEA, and aqueous ammonia was used to adjust pH to 11. In this case, no white precipitate was observed. Then aqueous solution of thiourea was added to this solution. All solutions were prepared in de-ionized (DI) water of resistivity 18.2 MV cm. Both the baths were then kept at 333 K with constant slow stirring and substrates were kept vertically in the bath. Amorphous glass microslides and stainless steel (SS) plates were cleaned by labogent detergent and rinsed with DI water. Further, these substrates were boiled in dilute chromic acid for 15 min and rinsed thoroughly with DI water and finally ultrasonicated in DI water for 20 min. Deposition was carried out for 45 min. During the precipitation, heterogeneous reaction occurred and deposition of CdS took place on the substrates. Asdeposited films were heat treated at 523 K for 1 h in air. Asdeposited and annealed films, using ammonia as a complexing agent, are designated as S1 and S2, respectively, while as-deposited and annealed films, using TEA complex, are designated as S3 and S4, respectively, hereafter. Films deposited on amorphous glass microslides were used for characterizing structural, morphological and optical properties and films on SS plates were used for EIS and PEC properties. Crystallographic study was carried out using Bruker AXS, Germany (Model D8 Advanced) diffractometer in the scanning range of 20–608 (2u) using Cu Ka radiations of wavelength 1.5045 A´˚ . JEOL ASM 6360A scanning electron microscope (SEM) was used to study the morphology of the films and the elemental analysis. Room temperature optical absorption spectra of samples were recorded in the wavelength range of 350–750 nm using Ocean Optics HR4000 high-resolution spectrometer. EIS and PEC properties were studied using VersaSTAT3 Potentiostat/Galvanostat, Princiton Applied Research. Three-electrode system was used for EIS studies in which films on SS substrate were used as a working electrode, platinum mesh as a counter electrode and saturated calomel electrode (SCE) as a reference electrode. Films were placed in the Teflon holder with a Teflon washer having 10 mm internal diameter determining the exposed area. Sulfide/ polysulfide of composition NaOH (0.1 M), S (0.1 M) and Na2S (0.1 M) was used as electrolyte. The frequency range examined was 1 MHz to 10 mHz. The PEC cell was fabricated with three-electrode system in which CdS films on steel substrate was used as a working electrode (active photoelectrode), graphite as counter electrode and SCE as a reference electrode. Sulfide/polysulfide electrolyte of same composition was used as redox couple. A tungsten filament lamp of 230 V/50 W was used as a source of white light to illuminate the PEC cell. The measured illumination intensity was 10 mW/cm2 at the interface.

precipitation occurs. The film growth takes place via ion-by-ion condensation of materials or by adsorption of colloidal particles from the solution on the substrate. Complexing agents help to control the reaction rate in chemical bath deposition. Generally, slow reactions results in adherent and good quality films. The formation of CdS thin films using ammonia as a complexing agents proceeds via following steps [26]: CdðNH3 Þ4 2þ þ 2OH ! CdðOHÞ2ads þ 4NH3

(1)

CdðOHÞ2ads þ S¼CðNH2 Þ2 ! ½CdðS¼CðNH2 Þ2 ðOH2 Þads

(2)

½CdðS¼CðNH2 Þ2 ÞðOHÞads ! CdS þ CN2 H2 þ 2H2 O

(3)

The proposed reaction for the TEA complexing agent is as follows: CdðTEA2þ þ 2OH ! CdðOHÞ2ads þ TEA

(4)

CdðOHÞ2ads þ S¼CðNH2 Þ2 ! ½CdðS¼CðNH2 Þ2 ðOH2 Þads

(5)

½CdðS¼CðNH2 Þ2 ÞðOHÞads ! CdS þ CN2 H2 þ2H2 O

(6)

Heterogeneous reaction occurred during the precipitation and deposition of CdS took place on the substrates. As grown films, using ammonia as a complexing agent, were uniform but spongy in nature, whereas in case of TEA films were well adherent and uniform. 3.2. Structural analysis The crystal quality of the samples was studied by recording the XRD patterns in the range of 20–608. Fig. 1 shows the diffraction pattern of the samples (S1–S4). All the samples are polycrystalline in nature with mixed phases of hexagonal and cubic structures (JCPDS data sheet no. 00-043-0). All samples (S1–S4) show broad hump between 208 and 408 of 2u which is due to the amorphous glass substrate. The crystallinity is very poor for S1 as can be seen from the diffraction pattern. It can be attributed to fast precipitation of the CdS molecules resulted by ammonia complex. The diffraction pattern reveals that the crystallinity is better for sample S3. It may be attributed to slow precipitation of the CdS molecules caused by relatively stable TEA complex. TEA complex breaks slowly with temperature to release the cadmium ions,

3. Result and discussion 3.1. Growth mechanism The precipitation of metal chalcogenides in chemical bath deposition technique occurs only when the ionic product of ions (both anion and cation) exceeds the solubility product of metal chalcogenides (i.e. CdS in our case) [15,25]. Generally ions combine to form nuclei on the substrate, as well as, in the solution and

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Fig. 1. X-ray diffraction pattern of the samples (S1–S4).

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which will then react with hydroxyl ions (OH) and finally with thiourea to form CdS molecule. The average crystallite sizes were calculated using Scherrer’s formula and are presented in Table 1. The obtained films are nanocrystalline in nature. Annealing of the films led to an enhancement in the crystallinity and hence in the crystallite size as can be seen from the diffraction pattern [27]. The smaller crystallite generally merges into larger one at elevated temperature by coalescence process. Stoichiometry of the CdS films has been confirmed by EDAX spectra and is shown in Fig. 2. The presence of Si peak in the EDAX spectra can be attributed to Si from the glass substrate.

Table 1 PEC performance parameters of bath deposited CdS thin films based on complexing agent and heat treatment. Sample S1 Ammonia (as-deposited) S2 Ammonia (annealed) S3 TEA (as-deposited) S4 TEA (annealed)

Crystallite size (nm)

Jsc (mA/cm2)

Voc (mV)

h (%)

FF (%)

9

22

86

0.006

29.18

12

35

108

0.012

31.75

19

46

297

0.057

41.52

23

99

376

0.158

42.43

3.3. Optical absorption studies Fig. 3 shows the room temperature optical absorption spectra of the films recorded in the range of 350–750 nm without taking into account scattering and reflection losses. It is clear from the figure that the optical absorption edge of the sample S1 is around 475 nm, which is red shifted to around 500 nm after air annealing

i.e. the absorption edge of S2. Whereas for the sample S3, the optical absorption edge observed at around 500 nm and red shifted to around 525 nm after air annealing i.e. the absorption edge of S4. The significant difference in the absorption edge of samples S1 and S3 is due to an improved crystallinity i.e. larger crystallite size, for S3, resulted by TEA complex. Annealing led to an enhancement in the optical absorption which may be attributed to an improvement in the crystallinity i.e. an enhancement in the crystallite size and reduction in the crystal imperfections. The results are in good agreement with reported values [28,29]. 3.4. Morphological studies SEM micrographs of the films (S1–S4) are shown in Fig. 4a–d. The SEM micrograph of the S1 reveals that ammonia complex resulted in flake like morphology which is spongy in nature and is very sensitive to photoelectrode degradation. Tiwari and Tiwari [24] have reported the spongy nature of the film deposited using ammonia complex. TEA complex, i.e. for sample S3, resulted in marigold flower-like structure in addition to flakes morphology. The average flower size was found to be 5 mm. Patil et al. [30] have observed such type of morphology for which the growth mechanism is unclear. Maximum surface area offered by flowerlike morphology is advantageous for PEC based solar cells. Annealing of the films did not show any significant changes in the surface morphology. 3.5. EIS studies To investigate the properties of the interface formed between the CdS photoelectrode and sulfide/polysulfide electrolyte EIS

Fig. 2. EDAX spectra of the samples (S1–S4).

Fig. 3. Room temperature optical absorption spectra of the samples (S1–S4).

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Fig. 4. SEM micrograph of the sample (a) S1, (b) S2, (c) S3 and (d) S4.

studies were carried out in dark at open cell voltage. Fig. 5a shows the Nyquist plot of the samples S1 and S2. The experimental data were compared with different equivalent circuits with the ZSimpWin 3.21 EChem Software and the best fit is shown in the inset of the figure. The presence of the term Q (leaking capacitor) may be due to spongy nature of the film which causes diffusion of Cd2+ ions across the double layer capacitance to the electrolyte. The Nyquist plots of samples S3 and S4 are shown in Fig. 5b. Inset of the figure shows the equivalent circuits. This behavior of the Nyquist plots can be attributed to the porous structure formed by randomly oriented flakes of the photoelectrode [31,32].

3.6. PEC properties When a photoelectrode is immersed in an electrolyte an electrode–electrolyte interface is formed which is called PEC cell. As soon as photoelectrode is immersed in an electrolyte charge transfer takes place across the interface, which in turns forms depletion region across the interface. This depletion region is driving force for the current–voltage characteristics in dark and under illumination. Fig. 6 shows the J–V characteristics under 10 mW/cm2 of illumination. As the photovoltage behavior is cathodic, seen from the figure, all samples are of n-type. It is clear

Fig. 5. Nyquist plots of the sample (a) S1 and S2 and (b) S3 and S4. Inset of the figure shows the equivalent circuits. Frequency range examined is 10 mHz to 1 MHz.

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To reduce the heating effect [33] by constant illumination for long time chopped light was used to study stability of the photoelectrode to degradation and transient response. Light was chopped after every 15 s and experiment was carried out for 5 min. The plots of Jsc versus time are shown in Fig. 7. It can be seen from the figure that the transient response to the light for all samples is good. Decreasing Jsc with time was found in case sample S1 and S2, as can be seen from the figure. It can be attributed to the spongy nature resulted from ammonia complex. The EIS studies also supported the photoelectrode degradation in terms of leaking capacitor. Samples S3 and S4 showed no such decrement in the Jsc with time as can be seen from the figure. 4. Conclusion

Fig. 6. Photo J–V plots of the samples (S1–S4) under 10 mW/cm2 of illumination.

from photo J–V plots that ammonia complex, for S1, resulted in lower values of both Jsc and Voc. It can be attributed to the low crystallinity of the sample and thus presence of large number of recombination centers surrounding the small crystallites as well as low optical absorption. For TEA complex, S3, both the Jsc and Voc are increased significantly which is due to improvement in the crystallite size and enhancement in the optical absorption which in turns produces more photocarriers. This increment is also due to flower-like structure which enhances the interface area i.e. depletion region. The improvement in the PEC cell performance by increasing the surface area by etching the photoelectrode and making pits has been reported by Tiwari and Tiwari [24]. Annealing of the films resulted in to grain growth by fusing the smaller crystallites and hence reducing the grain boundaries which in turns reduces the recombination centers for minority charge carriers and trapping centers for majority charge carriers. This leads to decrease in series resistance and hence increases both the Jsc and Voc. The values of Jsc, Voc, efficiency (h) and fill factor (FF) are reported in Table 1. The annealing temperature 523 K has been chosen as it is the optimal annealing temperature which enhances PEC cell performance of CdS thin films [23,29].

Fig. 7. The plot of Jsc versus time. The illumination was alternately on and off for 15 s.

The nanocrystalline thin films of CdS were successfully deposited by chemical bath deposition method. Triethanolamine complex led to better crystallinity and marigold flower-like morphology. The improved crystallinity, as well as, surface area led to enhanced optical absorption. Due to increased surface area and optical absorption, in case of triethanolamine complex, the short circuit current density, as well as, open circuit voltage were increased which in turn increased overall conversion efficiency. Further improvement in the cell parameters was observed by annealing of the films. The films deposited using triethanolamine complex showed good stability under photoelectrochemical cell conditions. In short, triethanolamine complex showed reliability as a complex to deposit CdS thin films by chemical bath deposition for photoelectrochemical cell. Acknowledgements Authors are thankful to Vice Chancellor, Defence Institute of Advanced Technology, Girinagar, Pune- 411025 (India) for granting permission to publish this work. Authors are also thankful to University of Pune and HEMRL Pune for providing the SEM and XRD characterizations. References [1] W. Yao, S. Yu, S. Liu, J. Chen, X. Liu, F. Li, J. Phys. Chem. B 110 (24) (2006) 11704. [2] J. Heo, H. Ahn, R. Lee, Y. Han, D. Kim, Sol. Energy Mater. Sol. Cells 75 (2003) 193. [3] C.J. Hibberd, K. Ernits, M. Kaelin, U. Mu¨ller, A.N. Tiwari, Prog. Photovolt.: Res. Appl. 16 (2008) 585. [4] K. Senthil, D. Mangalaraj, Sa.K. Narayandass, Appl. Surf. Sci. 169–170 (2001) 476. [5] S. Shiv Shankar, S. Chatterjee, M. Sastry, Phys. Chem. Commun. 6 (9) (2003) 36. [6] A. Podesta´, N. Armani, G. Salviati, N. Romeo, A. Bosio, M. Prato, Thin Solid Films 511–512 (2006) 448. [7] X. Shen, A. Yuan, F. Wang, J. Hong, Z. Xu, Solid State Commun. 133 (2005) 19. [8] H. Uda, H. Yonezawa, Y. Ohtsubo, M. Kosaka, H. Sonomura, Sol. Energy Mater. Sol. Cells 75 (2003) 219. [9] Y. Luan, M. An, G. Lu, Appl. Surf. Sci. 253 (2006) 459. [10] S. Chen, M. Paulose, C. Ruan, G.K. Mor, O.K. Varghese, D. Kouzoudis, C.A. Grimes, J. Photochem. Photobiol. A: Chem. 177 (2006) 177. [11] B. Ullrich, J.W. Tomm, N.M. Dushkina, Y. Tomm, H. Sakai, Y. Segawa, Solid State Commun. 116 (2000) 33. [12] Y.Y. Ma, R.H. Bube, J. Electrochem. Soc. 124 (1977) 1430. [13] S. Mathew, P.S. Mukerjee, K.P. Vijayakumar, Thin Solid Films 254 (1995) 278. [14] C.D. Lokhande, B.R. Sankapal, H.M. Pathan, M. Muller, M. Giersig, H. Tributsch, Appl. Surf. Sci. 181 (2001) 277. [15] R.S. Mane, C.D. Lokhande, Mater. Chem. Phys. 65 (2000) 1. [16] J.N. Ximello-Quiebras, G. Contreras-Puente, J. Aguilar-Herna´ndez, G. SantanaRodriguez, A. Arias-Carbajal Readigos, Sol. Energy Mater. Sol. Cells 82 (2004) 263. [17] H. Khallaf, G. Chai, O. Lupan, L. Chow, S. Park, A. Schulte, Appl. Surf. Sci. 255 (2009) 4129. [18] H. Khallaf, I.O. Oladeji, G. Chai, L. Chow, Thin Solid Films 516 (2008) 7306. [19] H. Khallaf, G. Chai, O. Lupan, L. Chow, S. Park, A. Schulte, J. Phys. D: Appl. Phys. 41 (2008) 185304. [20] M. Fleischer, A.F. Sarofim, D.W. Fassett, P. Hammond, H.T. Shacklette, I.C.T. Nisbet, S. Epstein, Environ. Health Perspect. 7 (1974) 253. [21] A.H. Zyoud, N. Zaatar, I. Saadeddin, C. Ali, D. Park, G. Campet, H.S. Hilal, J. Hazard. Mater. 173 (2010) 318.

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