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Preparation and characterization of Sb2Se3 thin films by electrodeposition and annealing treatment
Applied Surface Science 261 (2012) 510–514
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and characterization of Sb2 Se3 thin films by electrodeposition and annealing treatment Yanqing Lai a,b , Zhiwei Chen a , Can Han a , Liangxing Jiang a , Fangyang Liu a,∗ , Jie Li a , Yexiang Liu a a b
School of Metallurgical Science and Engineering, Central South University, Changsha, 410083, China Engineering Research Center of High Performance Battery Materials and Devices, Research Institute of Central South University in Shenzhen, Shenzhen, 518057, China
a r t i c l e
i n f o
Article history: Received 27 May 2012 Received in revised form 13 August 2012 Accepted 13 August 2012 Available online 21 August 2012 Keywords: Sb2 Se3 Thin films Electrodeposition Annealing Solar cells
a b s t r a c t Antimony selenide (Sb2 Se3 ) thin films were prepared on SnO2 coated glass substrates from acidic aqueous solution by potentiostatic electrodeposition and then post annealed at 300 ◦ C in Ar atmosphere. Cyclic voltammetry (CV), energy dispersive X-ray spectroscopy (EDS), and environmental scanning electron microscope (ESEM) studies were performed on as-deposited Sb2 Se3 thin films to obtain suitable electrodeposition conditions. The annealed film shows improved crystallinity with a basic structure of orthorhombic Sb2 Se3 , and exhibits an optical absorption coefficient of higher than 105 cm−1 in the visible region and an optical band gap of 1.04 ± 0.01 eV. Photoelectrochemical (PEC) tests confirm the p-type conductivity and good photovoltaic conversion characteristics of the annealed film. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Sb2 Se3 is an orthorhombic structural semiconductor as a promising material with potential applications in thermoelectric devices [1], solar cells [2] and photoelectrochemical cells [3], optical recording material [4], lithium ion battery materials [5] and hydrogen storage materials [6] due to its good optical and electrical properties. Several methods have been employed to obtain Sb2 Se3 thin films, such as vacuum thermal evaporation [7], chemical bath deposition [8], successive ionic layer adsorption and reaction methods [9], spray pyrolysis [10], reactive pulsed laser deposition [5] and electrodeposition [11–13]. Among all these methods, electrodeposition technique is a quick and simple method for large-scale preparation of thin films with respect to economic considerations. Torane et al. reported the electrodeposition of Sb2 Se3 thin films from aqueous [11] and non-aqueous solutions [12] for the first time. Fernández and Merino pointed out the potential of Sb2 Se3 thin films for photovoltaic application [13]. Sisman et al. and our group investigated the doping mechanism and characterization of Sb-doped Bi2 Se3 [14] and Bi-doped Sb2 Se3 [15], respectively. Furthermore, Shi et al. have investigated the nucleation and growth mechanisms in the initial stages for Sb2 Se3 thin films onto the indium-doped tin oxides coated glass (ITO) substrates using chronoamperometry (CA) technique [16]. And our group has also investigated the
∗ Corresponding author. Tel.: +86 732 8830474; fax: +86 732 8876454. E-mail address: [email protected] (F. Liu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.046
deposition mechanism of the Sb2 Se3 film by a systematic cyclic voltammetric study [17]. In this paper, we report the preparation and characterization of Sb2 Se3 thin films by electrodeposition on SnO2 coated glass substrate from acidic aqueous solution and their post annealing treatment. The preparation conditions and their effects on the composition and morphology of Sb2 Se3 thin films were investigated. Their potential viability as a photovoltaic material via demonstrating clear and stable electrochemical photocurrent upon illumination. 2. Experimental The electrochemical experiments, including cyclic voltammetry (CV) and electrodeposition, were performed by EG&G PAR 2273A Potentiostat in a stagnant three-electrode cell configuration at 25 ◦ C. The cell contained a SnO2 -coated glass substrate (20 /sq) as working electrode, a pure graphite plate as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. All potentials in this work were reported with respect to this reference. The SnO2 -coated glass substrates were ultrasonically cleaned in acetone, ammonia and alcohol, then rinsed with deionized water (18.2 M cm−1 ), and subsequently dried in nitrogen flow. The electrolyte solution contained 5.5 mM K(SbO)C4 H4 O6 ·0.5H2 O (antimony potassium tartrate), 4.5 mM H2 SeO3 , and 100 mM NH4 Cl. The cyclic voltammograms were measured at a scan rate of 10 mV/s and scanned first to the negative direction. The electrodeposition time for all films is 60 min. To improved the crystallinlity,
Y. Lai et al. / Applied Surface Science 261 (2012) 510–514
Fig. 1. Cyclic voltammograms of SnO2 -coated electrode in binary Sb–Se system.
as-deposited films were annealed in the flowing Ar atmosphere (20.00 sccm) at 300 ◦ C for 3 min, and then cooled to room temperature naturally. The film chemical composition, morphology and structure were characterized by energy dispersive X-ray spectroscopy (EDS, EDAX-GENSIS60S, operated at 25k eV with acquisition time of 100 s), Environmental scanning electron microscopy (ESEM, FEI Quanta-200, at a 25-keV accelerating voltage) and X-ray diffraction (XRD) measurements obtained with a Rigaku3014 X-ray ˚ 40 kV acceleradiffractometer (with Cu K␣ radiation, = 1.54 A, tion voltage, 25 mA current), respectively. Optical properties of the films were measured by UV–vis–NIR spectrophotometer (UV-VISNIR, Varian Cary-5000) in a wavelength range of 200–2500 nm at room temperature, and electrical properties of the prepared films were characterized by photoelectrochemical (PEC) tests (photocurrent–potential and photovoltage tests). The PEC tests were carried out in 0.5 M H2 SO4 solution, where the sample, a purity graphite plate, and a SCE were used as the working, counter and reference electrodes respectively and a Newport 300 W xenon lamp was used as the light source with the light intensity kept at 100 mW/cm2 . 3. Results and discussion Fig. 1 illustrates the typical cyclic voltammogram of SnO2 -coated electrode in solution containing 5.5 mM K(SbO)C4 H4 O6 ·0.5H2 O (antimony potassium tartrate), 4.5 mM H2 SeO3 , and 100 mM NH4 Cl. It is observed that, in wide potential range from −0.4 V to −0.7 V, there is a large reduction peak located at about −0.55 V, corresponding to the co-deposition of Sb and Se according to previous studies [17]. Table 1 and Fig. 2 show the compositions and morphologies of as-deposited Sb2 Se3 thin films under different deposition conditions by EDS and SEM analysis. From the composition of samples a, b and c electrodeposited at potential of −0.50 V, −0.55 V and −0.60 V, respectively, it is found that Sb content increases while the Table 1 The compositions of Sb2 Se3 thin films under different preparation condition. Sample
a b c d e f
Potential (V) vs. SCE
−0.50 −0.55 −0.60 −0.55 −0.55 −0.55
pH
2.0 2.0 2.0 1.8 2.5 2.0
Temperature (◦ C)
25 25 25 25 25 50
Atomic percent (%) Sb
Se
27.75 42.23 44.95 45.26 33.35 33.78
72.25 57.77 55.05 54.74 66.65 66.22
511
content of Se gradually decreases with the shifting of the deposition potential negatively. Based on the electrodeposition mechanism of the film which was already investigated in our previous work [17], there is a reason that the deposition of Se under limit diffusioncontrolled, thus negative shift of the deposition potential has little effect on Se content in deposited film but can promote the deposition of Sb. Film deposited at −0.50 V mainly consists of small Se clusters (Fig. 2(a)) in combination with the film composition analysis. When the potential reaches −0.55 V, the film shows a very compact and homogeneous surface morphology having isolated grains with uniform size and well-defined boundaries (Fig. 2(b)). At a more negative potential of −0.60 V, the film return to a poor surface morphology displaying grains with uneven sizes (Fig. 2(c)). It is also indicative that pH value plays an important role on film composition and morphology. Films electrodeposited at higher pH value exhibits lower Sb content and accordingly higher Se content by comparing with the composition of sample d, b and e. Taking into account the fact that both reduction reactions of Se (IV) to Se (0) proceeded by Eq. (1) and SbO+ to Sb in the solution via Eq. (2) consumes H+ , the increase of pH value hinders these two reactions, and obviously the reduction of SbO+ to Sb is inhibited more seriously. H2 SeO3 + 4H+ + 4e− = Se + 3H2 O
(1)
SbO+ + 2H+ + 3e− = Sb + H2 O
(2)
As can be seen from Fig. 2(b), (d) and (e), the surface of the film deposited at pH of 2.0 is compact and smooth, showing a granular structure with well-defined grain (Fig. 2(b)). Uneven grains with sizes between 0.1 and 1 m, and some holes are observed on the surface of the film obtained at pH of 1.8 (Fig. 2(d)). With the increase of pH to 2.5, the films became quite rough and porous (Fig. 2(e)). In addition, it is observed that increase in deposition temperature to 50 ◦ C inhibits the incorporation of Sb into film and deteriorates of morphology (Fig. 2(f)). In order to improve the crystallinity and the morphology of the as-deposited films, the post annealing treatment was employed. Sb2 Se3 thin films deposited at deposition potential of −0.55 V, pH of 2.0 and temperature of 25 ◦ C with compact and homogeneous surface morphology and stoichiometric composition were used for the annealing treatment. The SEM morphologies of as-deposited and annealed samples of Sb2 Se3 thin films are compared in Fig. 3. A enhanced compactness surface with less grain boundaries and increased particles size after annealing was observed. The boundaries fuse to be irregular because of the particles’ diffusion by high temperature and these particles grow to larger grains resulted from recrystallization. Fig. 4 displays the typical X-ray diffraction patterns measured for electrodeposited films before and after annealing treatment. It can be seen that before heat treatment all diffraction peaks can be indexed the substrate SnO2 (JCPDS 46-1088) and no diffraction peaks of other phases were observed due to the poor crystallinity (amorphous nature) of the as-deposited sample. After annealing, the film exhibits improved crystallinity and orthorhombic structure Sb2 Se3 phase (JCPDS 65-2433) with (2 1 2) preferred orientation. In addition, minor secondary phase of Sb is also found after annealing treatment (JCPDS 35-0732), which is consistent with Sepoor and Sb-rich composition of the sample. However, this metal Sb phase is not detected before annealing by XRD, revealing that the excessive Sb atoms in the film before heat treatment are in the three-dimensional amorphous space with long-range disorder. Combined with the feature of electrodeposition, it can be speculated that excessive Sb atoms in the as-deposited film should be evenly distributed in the disorder covalent bond structure of Sb2 Se3 particles, which then gather after heat treatment and are shaped into Sb grains with certain size via physical and chemical actions
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Fig. 2. Surface morphologies of thin film electrodeposited under different preparation condition: (a) −0.50 V, pH = 2.0, 25 ◦ C; (b) −0.55 V, pH = 2.0, 25 ◦ C; (C) −0.60 V, pH = 2.0, 25 ◦ C; (d) −0.55 V, pH = 1.8, 25 ◦ C; (e) −0.55 V, pH = 2.5, 25 ◦ C; and (f) −0.55 V, pH = 2.0, 50 ◦ C.
during hear treatment. And this gathered Sb second phase is most likely to be dispersed in the grain boundaries of Sb2 Se3 thin films. Fig. 5 presents the plot of (˛ h)2 versus the photon energy h for the annealed Sb2 Se3 thin films, which is converted from the transmission spectra record in the wavelength range of 200–2500 nm, without taking into account the reflection and scattering loss. The absorption coefficient is higher than 105 cm−1 in the visible region, which supports the direct band gap nature of the material [18–21] and reveals that the Sb2 Se3 film can be considered to be a suitable material for photovoltaic solar energy conversion. The as-deposited thin films and the annealed thin films exhibited optical absorption coefficient of 1.95 × 105 cm−1 [17] and 1.61 × 105 cm−1 , respectively. As can be seen from our results, the absorption coefficient of the as-deposited films is slightly higher than that of the annealed films which may be caused by the poorer crystalline quality and higher defect density of the as-deposited samples than that of the annealed films. These defects would form some light absorption center and arouse a higher optical absorption coefficient of the as-deposited thin films. By extrapolating the linear (˛ h)2 vs. h plots to (˛ h)2 = 0, the optical band gap is estimated to be 1.04 ± 0.01 eV, in good agree with that of single crystals Sb2 Se3 [22]. Compared with the band
gap of the as-deposited films [17], it can be seen that the band gap of the annealed films are hardly changed. It indicates that annealing treatment did not make notable effects on the band gap of the films. The band gap value is also very close to that of Si (1.12 eV [23]) and CuInSe2 (1.04 eV [24]), which are widely used in the field of solar cells. Fig. 6 illustrates the photocurrent–potential curve of the annealed film by PEC test. It is observed that the film shows a photo-enhancement effect in the negative potential direction under illumination, which is a characteristic of a semiconductor with p-type conductivity. It also can be qualitatively evaluated that the annealed has good photoactivity from the significant photo-generated cathodic current. As depicted from transient photocurrent spectroscopy of the annealed film at −0.5 V vs. SCE of the inset of Fig. 6, the stability of the photocurrent of the annealed Sb2 Se3 thin film was evaluated at a constant potential of −500 mV by chopping the light with 40 s on and 40 s off. The annealed Sb2 Se3 film shows a constant photocurrent-density of 0.05 mA/cm2 , suggesting good stability. Moreover, the result of transient photocurrent spectroscopy confirms the p-type conductivity of the Sb2 Se3 film again when there is a cathodic current produced under illumination.
Fig. 3. Surface morphologies of the electrodeposited thin film before and after annealing treatment: (a) before annealing and (b) after annealing.
Y. Lai et al. / Applied Surface Science 261 (2012) 510–514
513
Table 2 The photo voltage of p-Si, n-Si and annealed Sb2 Se3 thin film. Material
Vdark (mV)
Vlight (mV)
Vphoto (mV)
Conductivity
p-Si n-Si Annealed-Sb2 Se3
−615.0 −430.0 −391.0
−603.0 −507.0 −340.0
12.0 −77.0 51.0
p-type n-type p-type
very important to remark that there are significant responses in photoelectrochemical (PEC) tests (photocurrent–potential and photovoltage tests), revealing excellent photoelectrochemical activity of the prepared sample, which is very necessary for high efficiency solar cell application. 4. Conclusions
Fig. 4. X-ray diffraction patterns of the electrodeposited film at deposition potential of −0.55 V, pH of 2.0 and temperature of 25 ◦ C before and after annealing treatment.
Antimony selenide thin films have been deposited on tin oxide glass substrates by potentiostatical electrodeposition and post annealing treatment. Thin films under different preparation conditions were obtained and their effects on composition and morphology of Sb2 Se3 thin films have been investigated combining with CV, EDS and ESEM studies. It has been demonstrated that orthorhombic Sb2 Se3 (JCPDS card no. 65-2433) thin films with improved crystallinity and good morphology is obtained after annealing treatment. The annealed Sb2 Se3 thin films have an optical absorption coefficient of higher than 105 cm−1 in the visible region and a direct optical bad gap of 1.04 ± 0.01 eV. P-type conductivity and good photoactivity of the Sb2 Se3 thin films are confirmed from the PEC test. These characteristics indicate that the obtained antimony selenide thin film is a very promising material for photovoltaic application. Acknowledgments
Fig. 5. The (˛ h)2 versus the photon energy h for the annealed Sb2 Se3 thin films; the estimated optical band gap is 1.04 ± 0.01 eV.
Photovoltage test is used to further evaluate and confirm the conductivity type of the electrodeposited Sb2 Se3 , the results are shown in Table 2. According to the test data, the conductivity type of the electrodeposited Sb2 Se3 film is still p-type. It is also
Fig. 6. The photocurrent–potential curve of the annealed film. Scan rate = 10 mV/s. The inset shows the transient photocurrent spectroscopy of the annealed film at −0.5 V vs. SCE.
This work was supported by the National High Technology Research and Development Program of China (863 Program, Grant No. 2012AA050703), the National Natural Science Foundation of China (No. 51272292) and the Fundamental Research Funds for the Central Universities (Grant No. 201021100029). References [1] H.C. Kim, T.S. Oh, D.B. Hyun, Thermoelectric properties of the p-type Bi2 Te3 –Sb2 Te3 –Sb2 Se3 alloys fabricated by mechanical alloying and hot pressing, Journal of Physics and Chemistry of Solids 61 (2000) 743–749. [2] K.Y. Rajpure, C.D. Lokhande, C.H. Bhosale, Effect of the substrate temperature on the properties of spray deposited Sb–Se thin films from non-aqueous medium, Thin Solid Films 311 (1997) 114–118. [3] B.R. Sankapal, C.D. Lokhande, Studies on photoelectrochemical (PEC) cell formed with SILAR deposited Bi2 Se3 –Sb2 Se3 multilayer thin films, Solar Energy Materials and Solar Cells 69 (2001) 43–52. [4] S. Jayakumar, C. Balasubramanian, SaK. Narayandass, D. Mangalaraj, C.P. Girija Vallabhan, Optical recording characteristics of Sb2 Se3 thin films using a CW-Ar+ laser, Thin Solid Films 266 (1995) 62–68. [5] X.M. Zhe, Z.W. Fu, Pulsed laser deposited Sb2 Se3 anode for lithium-ion batteries, Journal of Alloys and Compounds 458 (2008) 351–356. [6] J.M. Ma, Y.P. Wang, Y.J. Wang, Q. Chen, J.B. Lian, W.J. Zheng, Controlled synthesis of one-dimensional Sb2 Se3 nanostructures and their electrochemical properties, Journal of Physical Chemistry C 113 (2009) 13588–13592. [7] E.A. El-Sayad, Compositional dependence of the optical properties of amorphous Sb2 Se3−x Sx thin films, Journal of Non-Crystalline Solids 354 (2008) 3806–3811. ˜ M.T.S. Nair, P.K. Nair, Polycrystalline thin [8] Y. Rodríguez-Lazcano, Yolanda Pena, films of antimony selenide via chemical bath deposition and post deposition treatments, Thin Solid Films 493 (2005) 77–82. [9] C.D. Lokhande, B.R. Sankapal, S.D. Sartale, H.M. Pathan, M. Giersig, V. Ganesan, A novel method for the deposition of nanocrystalline Bi2 Se3 , Sb2 Se3 and Bi2 Se3 –Sb2 Se3 thin films—SILAR, Applied Surface Science 182 (2001) 413–417. [10] K.Y. Rajpure, C.H. Bhosale, Effect of Se source on properties of spray deposited Sb2 Se3 thin films, Materials Chemistry and Physics 62 (2000) 169–174. [11] A.P. Torane, K.Y. Rajpure, C.H. Bhosale, Preparation and characterization of electrodeposited Sb2 Se3 thin films, Materials Chemistry and Physics 61 (1999) 219–222.
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[12] A.P. Torane, C.H. Bhosale, Preparation and characterization of electrodeposited Sb2 Se3 thin films from non-aqueous media, Journal of Physics and Chemistry of Solids 63 (2002) 1849–1855. [13] A.M. Fernández, M.G. Merino, Preparation and characterization of Sb2 Se3 thin films prepared by electrodeposition for photovoltaic applications, Thin Solid Films 366 (2000) 202–206. [14] I. Sisman, M. Bic¸er, Structural, morphological and optical properties of Bi2−x Sbx Se3 thin films grown by electrodeposition, Journal of Alloys and Compounds 509 (2011) 1538–1543. [15] J. Li, B. Wang, F.Y. Liu, J. Yang, J.Y. Li, J. Liu, M. Jia, Y.Q. Lai, Y.X. Liu, Preparation and characterization of Bi-doped antimony selenide thin films by electrodeposition, Electrochimica Acta 56 (2011) 8597–8602. [16] X.Z. Shi, X. Zhang, Y. Tian, C.M. Shen, C.M. Wang, H.J. Gao, Electrodeposition of Sb2 Se3 on indium-doped tin oxides substrate: nucleation and growth, Applied Surface Science 258 (2012) 2169–2173. [17] Y.Q. Lai, C. Han, X.J. Lv, J. Yang, F.Y. Liu, J. Li, Y.X. Liu, Electrodeposition of antimony selenide thin films from aqueous acid solutions, Journal of Electroanalytical Chemistry 671 (2012) 73–79.
[18] F.Y. Liu, B. Wang, Y.Q. Lai, J. Li, Z.A. Zhang, Y.X. Liu, Electrodeposition of cobalt selenide thin films, Journal of the Electrochemical Society 157 (2010) D523–D527. [19] M.S. Iovu, D.V. Harea, E.P. Colomeico, Some optical properties of thermally deposited Sb2 Se3 :Sn thin films, Journal of Optoelectronics and Advanced Materials 10 (2008) 862–866. [20] Q. Xie, Z.P. Liu, M.W. Shao, L.F. Kong, W.C. Yu, Y.T. Qian, Polymer-controlled growth of Sb2 Se3 nanoribbons via a hydrothermal process, Journal of Crystal Growth 252 (2003) 570–574. [21] W. Procarione, C. Wood, The optical properties of Sb2 Se3 –Sb2 Te3 , Phys. Stat. Sol. 42 (1970) 871–878. ˇ [22] F. Kosek, J. Tulka, L. Stouraˇ c, Optical, photoelectric and electric properties of single-crystalline Sb2 Se3 , Czechoslovak, Journal of Physics 28 (1978) 325–330. [23] J.P. Collings, Simple measurement of the band gap in silicon and germanium, American Journal of Physics 48 (1980) 197–199. [24] T. Maeda, T. Takeichi, T. Wada, Systematic studies on electronic structures of CuInSe2 and the other chalcopyrite related compounds by first principles calculations, Physica Status Solidi A 203 (2006) 2634–2638.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and characterization of Sb2 Se3 thin films by electrodeposition and annealing treatment Yanqing Lai a,b , Zhiwei Chen a , Can Han a , Liangxing Jiang a , Fangyang Liu a,∗ , Jie Li a , Yexiang Liu a a b
School of Metallurgical Science and Engineering, Central South University, Changsha, 410083, China Engineering Research Center of High Performance Battery Materials and Devices, Research Institute of Central South University in Shenzhen, Shenzhen, 518057, China
a r t i c l e
i n f o
Article history: Received 27 May 2012 Received in revised form 13 August 2012 Accepted 13 August 2012 Available online 21 August 2012 Keywords: Sb2 Se3 Thin films Electrodeposition Annealing Solar cells
a b s t r a c t Antimony selenide (Sb2 Se3 ) thin films were prepared on SnO2 coated glass substrates from acidic aqueous solution by potentiostatic electrodeposition and then post annealed at 300 ◦ C in Ar atmosphere. Cyclic voltammetry (CV), energy dispersive X-ray spectroscopy (EDS), and environmental scanning electron microscope (ESEM) studies were performed on as-deposited Sb2 Se3 thin films to obtain suitable electrodeposition conditions. The annealed film shows improved crystallinity with a basic structure of orthorhombic Sb2 Se3 , and exhibits an optical absorption coefficient of higher than 105 cm−1 in the visible region and an optical band gap of 1.04 ± 0.01 eV. Photoelectrochemical (PEC) tests confirm the p-type conductivity and good photovoltaic conversion characteristics of the annealed film. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Sb2 Se3 is an orthorhombic structural semiconductor as a promising material with potential applications in thermoelectric devices [1], solar cells [2] and photoelectrochemical cells [3], optical recording material [4], lithium ion battery materials [5] and hydrogen storage materials [6] due to its good optical and electrical properties. Several methods have been employed to obtain Sb2 Se3 thin films, such as vacuum thermal evaporation [7], chemical bath deposition [8], successive ionic layer adsorption and reaction methods [9], spray pyrolysis [10], reactive pulsed laser deposition [5] and electrodeposition [11–13]. Among all these methods, electrodeposition technique is a quick and simple method for large-scale preparation of thin films with respect to economic considerations. Torane et al. reported the electrodeposition of Sb2 Se3 thin films from aqueous [11] and non-aqueous solutions [12] for the first time. Fernández and Merino pointed out the potential of Sb2 Se3 thin films for photovoltaic application [13]. Sisman et al. and our group investigated the doping mechanism and characterization of Sb-doped Bi2 Se3 [14] and Bi-doped Sb2 Se3 [15], respectively. Furthermore, Shi et al. have investigated the nucleation and growth mechanisms in the initial stages for Sb2 Se3 thin films onto the indium-doped tin oxides coated glass (ITO) substrates using chronoamperometry (CA) technique [16]. And our group has also investigated the
∗ Corresponding author. Tel.: +86 732 8830474; fax: +86 732 8876454. E-mail address: [email protected] (F. Liu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.046
deposition mechanism of the Sb2 Se3 film by a systematic cyclic voltammetric study [17]. In this paper, we report the preparation and characterization of Sb2 Se3 thin films by electrodeposition on SnO2 coated glass substrate from acidic aqueous solution and their post annealing treatment. The preparation conditions and their effects on the composition and morphology of Sb2 Se3 thin films were investigated. Their potential viability as a photovoltaic material via demonstrating clear and stable electrochemical photocurrent upon illumination. 2. Experimental The electrochemical experiments, including cyclic voltammetry (CV) and electrodeposition, were performed by EG&G PAR 2273A Potentiostat in a stagnant three-electrode cell configuration at 25 ◦ C. The cell contained a SnO2 -coated glass substrate (20 /sq) as working electrode, a pure graphite plate as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. All potentials in this work were reported with respect to this reference. The SnO2 -coated glass substrates were ultrasonically cleaned in acetone, ammonia and alcohol, then rinsed with deionized water (18.2 M cm−1 ), and subsequently dried in nitrogen flow. The electrolyte solution contained 5.5 mM K(SbO)C4 H4 O6 ·0.5H2 O (antimony potassium tartrate), 4.5 mM H2 SeO3 , and 100 mM NH4 Cl. The cyclic voltammograms were measured at a scan rate of 10 mV/s and scanned first to the negative direction. The electrodeposition time for all films is 60 min. To improved the crystallinlity,
Y. Lai et al. / Applied Surface Science 261 (2012) 510–514
Fig. 1. Cyclic voltammograms of SnO2 -coated electrode in binary Sb–Se system.
as-deposited films were annealed in the flowing Ar atmosphere (20.00 sccm) at 300 ◦ C for 3 min, and then cooled to room temperature naturally. The film chemical composition, morphology and structure were characterized by energy dispersive X-ray spectroscopy (EDS, EDAX-GENSIS60S, operated at 25k eV with acquisition time of 100 s), Environmental scanning electron microscopy (ESEM, FEI Quanta-200, at a 25-keV accelerating voltage) and X-ray diffraction (XRD) measurements obtained with a Rigaku3014 X-ray ˚ 40 kV acceleradiffractometer (with Cu K␣ radiation, = 1.54 A, tion voltage, 25 mA current), respectively. Optical properties of the films were measured by UV–vis–NIR spectrophotometer (UV-VISNIR, Varian Cary-5000) in a wavelength range of 200–2500 nm at room temperature, and electrical properties of the prepared films were characterized by photoelectrochemical (PEC) tests (photocurrent–potential and photovoltage tests). The PEC tests were carried out in 0.5 M H2 SO4 solution, where the sample, a purity graphite plate, and a SCE were used as the working, counter and reference electrodes respectively and a Newport 300 W xenon lamp was used as the light source with the light intensity kept at 100 mW/cm2 . 3. Results and discussion Fig. 1 illustrates the typical cyclic voltammogram of SnO2 -coated electrode in solution containing 5.5 mM K(SbO)C4 H4 O6 ·0.5H2 O (antimony potassium tartrate), 4.5 mM H2 SeO3 , and 100 mM NH4 Cl. It is observed that, in wide potential range from −0.4 V to −0.7 V, there is a large reduction peak located at about −0.55 V, corresponding to the co-deposition of Sb and Se according to previous studies [17]. Table 1 and Fig. 2 show the compositions and morphologies of as-deposited Sb2 Se3 thin films under different deposition conditions by EDS and SEM analysis. From the composition of samples a, b and c electrodeposited at potential of −0.50 V, −0.55 V and −0.60 V, respectively, it is found that Sb content increases while the Table 1 The compositions of Sb2 Se3 thin films under different preparation condition. Sample
a b c d e f
Potential (V) vs. SCE
−0.50 −0.55 −0.60 −0.55 −0.55 −0.55
pH
2.0 2.0 2.0 1.8 2.5 2.0
Temperature (◦ C)
25 25 25 25 25 50
Atomic percent (%) Sb
Se
27.75 42.23 44.95 45.26 33.35 33.78
72.25 57.77 55.05 54.74 66.65 66.22
511
content of Se gradually decreases with the shifting of the deposition potential negatively. Based on the electrodeposition mechanism of the film which was already investigated in our previous work [17], there is a reason that the deposition of Se under limit diffusioncontrolled, thus negative shift of the deposition potential has little effect on Se content in deposited film but can promote the deposition of Sb. Film deposited at −0.50 V mainly consists of small Se clusters (Fig. 2(a)) in combination with the film composition analysis. When the potential reaches −0.55 V, the film shows a very compact and homogeneous surface morphology having isolated grains with uniform size and well-defined boundaries (Fig. 2(b)). At a more negative potential of −0.60 V, the film return to a poor surface morphology displaying grains with uneven sizes (Fig. 2(c)). It is also indicative that pH value plays an important role on film composition and morphology. Films electrodeposited at higher pH value exhibits lower Sb content and accordingly higher Se content by comparing with the composition of sample d, b and e. Taking into account the fact that both reduction reactions of Se (IV) to Se (0) proceeded by Eq. (1) and SbO+ to Sb in the solution via Eq. (2) consumes H+ , the increase of pH value hinders these two reactions, and obviously the reduction of SbO+ to Sb is inhibited more seriously. H2 SeO3 + 4H+ + 4e− = Se + 3H2 O
(1)
SbO+ + 2H+ + 3e− = Sb + H2 O
(2)
As can be seen from Fig. 2(b), (d) and (e), the surface of the film deposited at pH of 2.0 is compact and smooth, showing a granular structure with well-defined grain (Fig. 2(b)). Uneven grains with sizes between 0.1 and 1 m, and some holes are observed on the surface of the film obtained at pH of 1.8 (Fig. 2(d)). With the increase of pH to 2.5, the films became quite rough and porous (Fig. 2(e)). In addition, it is observed that increase in deposition temperature to 50 ◦ C inhibits the incorporation of Sb into film and deteriorates of morphology (Fig. 2(f)). In order to improve the crystallinity and the morphology of the as-deposited films, the post annealing treatment was employed. Sb2 Se3 thin films deposited at deposition potential of −0.55 V, pH of 2.0 and temperature of 25 ◦ C with compact and homogeneous surface morphology and stoichiometric composition were used for the annealing treatment. The SEM morphologies of as-deposited and annealed samples of Sb2 Se3 thin films are compared in Fig. 3. A enhanced compactness surface with less grain boundaries and increased particles size after annealing was observed. The boundaries fuse to be irregular because of the particles’ diffusion by high temperature and these particles grow to larger grains resulted from recrystallization. Fig. 4 displays the typical X-ray diffraction patterns measured for electrodeposited films before and after annealing treatment. It can be seen that before heat treatment all diffraction peaks can be indexed the substrate SnO2 (JCPDS 46-1088) and no diffraction peaks of other phases were observed due to the poor crystallinity (amorphous nature) of the as-deposited sample. After annealing, the film exhibits improved crystallinity and orthorhombic structure Sb2 Se3 phase (JCPDS 65-2433) with (2 1 2) preferred orientation. In addition, minor secondary phase of Sb is also found after annealing treatment (JCPDS 35-0732), which is consistent with Sepoor and Sb-rich composition of the sample. However, this metal Sb phase is not detected before annealing by XRD, revealing that the excessive Sb atoms in the film before heat treatment are in the three-dimensional amorphous space with long-range disorder. Combined with the feature of electrodeposition, it can be speculated that excessive Sb atoms in the as-deposited film should be evenly distributed in the disorder covalent bond structure of Sb2 Se3 particles, which then gather after heat treatment and are shaped into Sb grains with certain size via physical and chemical actions
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Fig. 2. Surface morphologies of thin film electrodeposited under different preparation condition: (a) −0.50 V, pH = 2.0, 25 ◦ C; (b) −0.55 V, pH = 2.0, 25 ◦ C; (C) −0.60 V, pH = 2.0, 25 ◦ C; (d) −0.55 V, pH = 1.8, 25 ◦ C; (e) −0.55 V, pH = 2.5, 25 ◦ C; and (f) −0.55 V, pH = 2.0, 50 ◦ C.
during hear treatment. And this gathered Sb second phase is most likely to be dispersed in the grain boundaries of Sb2 Se3 thin films. Fig. 5 presents the plot of (˛ h)2 versus the photon energy h for the annealed Sb2 Se3 thin films, which is converted from the transmission spectra record in the wavelength range of 200–2500 nm, without taking into account the reflection and scattering loss. The absorption coefficient is higher than 105 cm−1 in the visible region, which supports the direct band gap nature of the material [18–21] and reveals that the Sb2 Se3 film can be considered to be a suitable material for photovoltaic solar energy conversion. The as-deposited thin films and the annealed thin films exhibited optical absorption coefficient of 1.95 × 105 cm−1 [17] and 1.61 × 105 cm−1 , respectively. As can be seen from our results, the absorption coefficient of the as-deposited films is slightly higher than that of the annealed films which may be caused by the poorer crystalline quality and higher defect density of the as-deposited samples than that of the annealed films. These defects would form some light absorption center and arouse a higher optical absorption coefficient of the as-deposited thin films. By extrapolating the linear (˛ h)2 vs. h plots to (˛ h)2 = 0, the optical band gap is estimated to be 1.04 ± 0.01 eV, in good agree with that of single crystals Sb2 Se3 [22]. Compared with the band
gap of the as-deposited films [17], it can be seen that the band gap of the annealed films are hardly changed. It indicates that annealing treatment did not make notable effects on the band gap of the films. The band gap value is also very close to that of Si (1.12 eV [23]) and CuInSe2 (1.04 eV [24]), which are widely used in the field of solar cells. Fig. 6 illustrates the photocurrent–potential curve of the annealed film by PEC test. It is observed that the film shows a photo-enhancement effect in the negative potential direction under illumination, which is a characteristic of a semiconductor with p-type conductivity. It also can be qualitatively evaluated that the annealed has good photoactivity from the significant photo-generated cathodic current. As depicted from transient photocurrent spectroscopy of the annealed film at −0.5 V vs. SCE of the inset of Fig. 6, the stability of the photocurrent of the annealed Sb2 Se3 thin film was evaluated at a constant potential of −500 mV by chopping the light with 40 s on and 40 s off. The annealed Sb2 Se3 film shows a constant photocurrent-density of 0.05 mA/cm2 , suggesting good stability. Moreover, the result of transient photocurrent spectroscopy confirms the p-type conductivity of the Sb2 Se3 film again when there is a cathodic current produced under illumination.
Fig. 3. Surface morphologies of the electrodeposited thin film before and after annealing treatment: (a) before annealing and (b) after annealing.
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Table 2 The photo voltage of p-Si, n-Si and annealed Sb2 Se3 thin film. Material
Vdark (mV)
Vlight (mV)
Vphoto (mV)
Conductivity
p-Si n-Si Annealed-Sb2 Se3
−615.0 −430.0 −391.0
−603.0 −507.0 −340.0
12.0 −77.0 51.0
p-type n-type p-type
very important to remark that there are significant responses in photoelectrochemical (PEC) tests (photocurrent–potential and photovoltage tests), revealing excellent photoelectrochemical activity of the prepared sample, which is very necessary for high efficiency solar cell application. 4. Conclusions
Fig. 4. X-ray diffraction patterns of the electrodeposited film at deposition potential of −0.55 V, pH of 2.0 and temperature of 25 ◦ C before and after annealing treatment.
Antimony selenide thin films have been deposited on tin oxide glass substrates by potentiostatical electrodeposition and post annealing treatment. Thin films under different preparation conditions were obtained and their effects on composition and morphology of Sb2 Se3 thin films have been investigated combining with CV, EDS and ESEM studies. It has been demonstrated that orthorhombic Sb2 Se3 (JCPDS card no. 65-2433) thin films with improved crystallinity and good morphology is obtained after annealing treatment. The annealed Sb2 Se3 thin films have an optical absorption coefficient of higher than 105 cm−1 in the visible region and a direct optical bad gap of 1.04 ± 0.01 eV. P-type conductivity and good photoactivity of the Sb2 Se3 thin films are confirmed from the PEC test. These characteristics indicate that the obtained antimony selenide thin film is a very promising material for photovoltaic application. Acknowledgments
Fig. 5. The (˛ h)2 versus the photon energy h for the annealed Sb2 Se3 thin films; the estimated optical band gap is 1.04 ± 0.01 eV.
Photovoltage test is used to further evaluate and confirm the conductivity type of the electrodeposited Sb2 Se3 , the results are shown in Table 2. According to the test data, the conductivity type of the electrodeposited Sb2 Se3 film is still p-type. It is also
Fig. 6. The photocurrent–potential curve of the annealed film. Scan rate = 10 mV/s. The inset shows the transient photocurrent spectroscopy of the annealed film at −0.5 V vs. SCE.
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