Preparation and characterization of Bi-doped antimony selenide thin films by electrodeposition

Electrochimica Acta 56 (2011) 8597–8602

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Preparation and characterization of Bi-doped antimony selenide thin films by electrodeposition Jie Li, Bo Wang, Fangyang Liu ∗,1 , Jia Yang, Jiyu Li, Jun Liu, Ming Jia, Yanqing Lai ∗ , Yexiang Liu School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 26 April 2011 Received in revised form 11 July 2011 Accepted 13 July 2011 Available online 23 July 2011 Keywords: Bi doping Sb2 Se3 Thin film Electrodeposition Solar cell

a b s t r a c t Bi-doped antimony selenide (Sb2−x Bix Se3 ) thin films have been prepared by potentiostatical electrodeposition and post annealing treatment. Cyclic voltammetry (CV) was used to investigate the electrochemical behaviors of electrodeposition. The suitable deposition potential for film preparation was determined to be about −0.40 V vs. SCE combining with CV, energy dispersive X-ray spectroscopy (EDS), environmental scanning electron microscope (ESEM) studies. After annealing, film shows improved crystallinity and a basic orthorhombic Sb2 Se3 structure but having a larger d-spacing due to the substitution of Bi for Sb in Sb2 Se3 lattice. The annealed film exhibits an absorption coefficient of larger than 105 cm−1 in the visible region, an direct optical band gap of 1.12 ± 0.01 eV, the n-type conductivity, an carrier concentration of 1.1 × 1019 cm−3 and an flat band potential of −0.40 ± 0.03 V vs. SCE. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Antimony selenide (Sb2 Se3 ) is an attractive semiconductor as an absorber material for thin film solar cells due to its suitable optical band gap (∼2.00 eV) [1–3]. However, the ideal single-junction solar cell maintains a band gap range between 1.10 and 1.70 eV [4]. In order to match with the solar energy spectrum, the band gap of Sb2 Se3 thin film should be lowered. One possible approach is to obtain ternary Sb2−x Bix Se3 thin films by bismuth doping in Sb2 Se3 , due to Bi2 Se3 thin film has a low band gap value of 0.35–0.79 eV [5–8]. This is very similar to the case of Cu(In,Ga)Se2 , one of the materials holding the highest record efficiency (20.3%) in the CuInSe2 -based thin film solar cells [9], whose band gap can be tuned by partly substituting In atoms with Ga atoms in CuInSe2 lattice. Up to now, several methods have been employed to obtain Sb2 Se3 thin films, such as chemical bath deposition [7], arrested precipitation technique [10], reactive pulsed laser deposition [11], spray deposition [12], vacuum thermal evaporation [13–15], and electrodeposition [2,16–18], as well as successive ionic layer adsorption and reaction method [19]. Among these techniques, electrodeposition is one of the most cost-efficient and scalable methods [20]. Torane et al. reported for the first time the electrodeposition of Sb2 Se3 thin films from aqueous [16] and non-aqueous

∗ Corresponding authors. Tel.: +86 732 8830474; fax: +86 732 8876454. E-mail address: [email protected] (F. Liu). 1 ISE member. 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.07.042

solutions [18]. Fernández et al. [2] pointed out the potential application of Sb2 Se3 thin films in photovoltaic areas. Recently, Sisman et al. [21] reported the electrodeposition of Sb-doped bismuth selenide thin films, which have low band gaps ranging from 0.24 to 0.38 eV and are suitable for thermoelectric application. To the best of our knowledge, the preparation of Bi-doped Sb2 Se3 thin films by electrodeposition has not been reported yet. In this paper, we report the preparation of Bi-doped Sb2 Se3 thin films by electrodeposition and annealing. A systematic cyclic voltammetric study was performed to understand the electrochemical behaviors of electrodeposition. The structural, morphological, compositional, optical and electrical properties of the films have also been investigated. 2. Experimental The electrochemical experiments, including the cyclic voltammetry (CV) and the electrodeposition, were controlled by an EG&G PAR 2273A Potentiostat in a stagnant three-electrode bath at 25 ◦ C. The bath contained a SnO2 -coated glass substrate as the working electrode, a pure graphite plate as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. All potentials were reported with respect to this reference. The SnO2 coated substrates were ultrasonically cleaned in acetone, ammonia and alcohol, then rinsed with deinonized water and finally dried by air blow. The electrolytes contained 0.5–1.5 mM Bi(NO3 )3 ·5H2 O, 2.5–3.5 mM K(SbO)C4 H4 O6 ·2H2 O, 6.0 mM SeO2 and 3.5 M NH4 Cl in deionized water, and the pH value was adjusted to 1.5 by adding drops of concentrated HCl solution. The cyclic voltammograms

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deposit–substrate interaction as the same reaction via Eq. (2) [25]. After Peak A1, there is a slow increase in current with further shifting potential negatively to −0.40 V, at which the current shows a much steeper increase than before, and then develop into a peak at −0.53 V (Peak A2). Peak A2 can be assigned to the six-electron reduction of Se(IV) to Se(−II) (Eq. (3)) [24–27]. Se(IV) + 4e−  Se(0) −

Se(IV) + 6e  Se(−II)

Fig. 1. Cyclic voltammograms of SnO2 -coated electrode in Blank solution (3.5 M NH4 Cl), Se solution (6.0 mM SeO2 + 3.5 M NH4 Cl), Sb solution (2.5 mM K (SbO)C4 H4 O6 + 3.5 M NH4 Cl) and Sb–Se solution (2.5 mM K(SbO)C4 H4 O6 + 6.0 mM SeO2 + 3.5 M NH4 Cl).

were measured at a scan rate of 10 mV/s and first scanned in the negative direction. The electrodeposition time was 30 min. To improve the crystallinity, as-deposited films were rapidly heated in the flowing Ar atmosphere (2.00 sccm) at the rate of 10 ◦ C/s from room temperature to 300 ◦ C, then kept at 300 ◦ C for 3 min, and finally cooled to room temperature naturally. The film composition, morphology and structure were characterized by energy dispersive X-ray spectroscopy (EDS, EDAXGENSIS60S), environmental scanning electron microscope (ESEM, FEI Quanta-200) and X-ray diffraction (XRD, Rigaku 3014), respectively. Optical properties of prepared thin films were measured by UV–VIS–NIR spectrophotometer (UV-VIS-NIR, Varian Cary 5000). Electrical properties of annealed thin films were characterized by photoelectrochemical (PEC) and Mott–Schottky tests. The PEC test (photocurrent–potential characteristic) was carried out in 0.5 M H2 SO4 solution, which has already been efficiently utilized for CuInSe2 [22] and CdTe [23] thin films. A 300 W xenon lamp was used as the light source with the light intensity kept at 100 mW/cm2 . The Mott–Schottky plot of the annealed film was also measured with an EG&G PAR 273A Potentiostat and an EG&G PAR 5210 lock-in amplifier, superimposing on an AC signal with the amplitude of 10 mV at frequency at 10 kHz to the linear voltage sweep. All measurements were conducted in a Pyrex electrolytic cell with SCE as refer electrode and purity graphite plate as counter electrode. 3. Results and discussion Fig. 1 shows the typical cyclic voltammograms of SnO2 -coated electrode in the blank, Se, Sb and Sb–Se solutions. For the blank solution that only contains 3.5 M NH4 Cl, it is observed from the inset that there is a very small increase in current beginning from −0.15 V, probably corresponding to the hydrogen evolution (Eq. (1)). 2H(I) + 2e−  H2 ↑

(1)

For the Se solution containing 6.0 mM SeO2 and 3.5 M NH4 Cl, a cathodic peak observed at −0.27 V (Peak A1) is attributed to the bulk Se deposition via four-electron reduction of Se(IV) to Se(0) (Eq. (2)), as reported in the literature [24,25]. An initial reductive feature, initialing from 0.05 V before Peak A1, corresponds to the pre-deposition of selenium on SnO2 substrate caused by the

(2) (3)

For the Sb solution of 2.5 mM K(SbO)C4 H4 O6 and 3.5 M NH4 Cl, a cathodic peak (B) and two anodic peaks (C1 and C2) are observed at −0.45 V, −0.02 V and 0.50 V, respectively. Peak B is associated with the reduction of Sb(III) to Sb(0), while Peaks C1 and C2 may be attributed to the dissolution of Sb(0) from electrode to the solution in form of different ionic species or with the dissolution of different antimony phases, previously formed during the cathodic scan. In comparison with the blank solution, there is a negative potential shift from −0.15 V to −0.35 V for the initial reduction feature, suggesting that the hydrogen evolution is somewhat inhibited. This is probably due to the competitive adsorption of Sb(III) and H(I) ions, which results in the decrease in H(I) concentration near the cathode surface with the adding of Sb(III). For the Sb–Se solution containing 2.5 mM K(SbO)C4 H4 O6 , 6.0 mM SeO2 and 3.5 M NH4 Cl, the initial reductive feature for the pre-deposition of selenium is observed again. But it holds a smaller current density between 0.05 V and −0.39 V in comparison with the Se solution. The decrease in current indicates that Sb(III) ions inhibit both the pre- and bulk deposition of selenium, which can be explained by the competitive adsorption of Sb(III) and Se(IV) ions near the cathode surface. In this experiment, we did not observe the induced underpotential deposition feature for Sb–Se compounds [5], but we could infer that the following reductive peak at −0.40 V (Peak D) can be assigned to the formation of Sb–Se compounds via observing the color change of working electrode surface. Sb2 Se3 is used to present the stoichiometry of animony selenide for simplicity, no matter what the stoichiometry is. With further negative shift of potential, the generated Se(−II) by Eq. (3) may interact with Sb(III) to form Sb2 Se3 according to Eq. (4). We can hardly discern individual peaks for Eqs. (5) and (4) from Fig. 1, probably because they overlap with each other. 2Sb(III) + 3Se(−II)  Sb2 Se3

(4)

2Sb(III) + 3Se(0) + 6e−  Sb2 Se3

(5)

Fig. 2 presents cyclic voltammograms of SnO2 -coated electrode in the Bi and Bi-Se solutions. Additionally, cyclic voltammograms of the blank and Se solutions are added for comparison. For the Bi solution that contains 1.5 mM Bi(NO3 )3 and 3.5 M NH4 Cl, a cathodic Peak E and an anodic Peak F are observed at −0.30 V and −0.04 V, respectively, corresponding to the reductive and oxidative reaction by Eq. (6) [6,28–30]. Bi(III) + 3e−  Bi(0)

(6)

For the Bi–Se solution that contains 1.5 mM Bi(NO3 )3 , 6.0 mM SeO2 and 3.5 M NH4 Cl, the similar reductive feature for the predeposition of selenium is found again in the range of 0.05 V to −0.17 V with a larger current density in comparison with the Se solution (inset in Fig. 2). The increase in current density can be explained by the fact that Bi(III) ions could promote the prereduction and four-electron bulk reduction of selenium, which probably caused by the large free energy released in the formation of Bi–Se compounds, via the co-deposition of Bi(III) and Se(0) as Eq. (7). Regardless of the stoichiometry of bismuth selenide compounds, Bi2 Se3 is presented here for simplicity. When negatively scanning below −0.23 V, the current density decreases sharply, followed by a weak Peak G1 at −0.31 V relating to the formation of

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Fig. 2. Cyclic voltammograms of SnO2 -coated electrode in Blank solution (3.5 M NH4 Cl), Se solution (6.0 mM SeO2 + 3.5 M NH4 Cl), Bi solution (1.5 mM Bi(NO3 )3 + 3.5 M NH4 Cl) and Bi–Se solution (1.5 mM Bi(NO3 )3 + 6.0 mM SeO2 + 3.5 M NH4 Cl).

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Fig. 4. Film atomic composition of as-deposited films at various potentials between −0.20 V and −0.60 V from the solution containing 1.5 mM Bi(NO3 )3 , 2.5 mM K(SbO)C4 H4 O6 , 6.0 mM SeO2 and 3.5 M NH4 Cl.

2Bi(III) + 3Se(0) + 6e−  Bi2 Se3

(7)

K(SbO)C4 H4 O6 , 1.5 mM Bi(NO3 )3 , 6.0 mM SeO2 and 3.5 M NH4 Cl. The cyclic voltammograms of the Sb–Se solution and the Bi–Se solution are used for comparison. For the Sb–Bi–Se solution, only one reductive Peak M is observed at −0.41 V, probably indicating the co-deposition of Sb, Bi and Se via Eq. (10) or (11) (For simplicity, Sb2−x Bix Se3 is used to present Sb–Bi–Se compounds). The Sb–Bi–Se compounds with grey surface color were determined by EDS. In the reverse scan, an oxidative Peak N at 0.11 V is observed, related to the stripping or dissolution of Sb–Bi–Se compounds. By comparing the Sb–Se solution with the Bi–Se solution, it can be concluded that the electrochemical formation of Bi2 Se3 is much easier than Sb2 Se3 , and the addition of Bi(III) ions in Sb–Se system can promote the reduction of Sb2 Se3 .

2Bi(III) + 3Se(−II)  Bi2 Se3

(8)

(2 − x)Sb(III) + xBi(III) + 3Se(0) + 6e−  Sb2−x Bix Se3

(10)

Bi2 Se3 + 6e−  2Bi(0) + 3Se(−II)

(9)

(2 − x)Sb(III) + xBi(III) + 3Se(−II)  Sb2−x Bix Se3

(11)

Bi–Se compounds proceeded by the reaction of Eq. (7). The following reductive peak at −0.38 V (Peak G2) is also associated with the formation of Bi–Se compounds, but it may undergo a different route as Eq. (8), via the reaction of Bi(III) with Se(−II), where the Se(−II) comes from the Se six-electron reduction of Eq. (3). The third peak located at −0.46 V (Peak G3) is probably attributed to the reduction of “bound” Se in Bi–Se compounds to soluble selenide as Eq. (9), which is similar to the results reported by Ham et al. [28]. On the reverse scanning, an obvious oxidation peak, labeled as Peak H, probably indicates the stripping and oxidation of complex Bi–Se compounds.

Fig. 3 displays the typical cyclic voltammograms of SnO2 -coated electrode in the ternary Sb–Bi–Se solution that contains 2.5 mM

Fig. 3. Cyclic voltammograms of SnO2 -coated electrode in Sb–Se solution (2.5 mM K(SbO)C4 H4 O6 + 6.0 mM SeO2 + 3.5 M NH4 Cl), Bi–Se solution (1.5 mM Bi(NO3 )3 + 6.0 mM SeO2 + 3.5 M NH4 Cl) and Sb–Bi–Se solution (2.5 mM K(SbO)C4 H4 O6 + 1.5 mM Bi(NO3 )3 + 6.0 mM SeO2 + 3.5 M NH4 Cl).

The results of the cyclic voltammetry above represent that the suitable potential range for the preparation of Sb–Bi–Se compound thin films may be between −0.28 V and −0.60 V vs. SCE. Fig. 4 shows the atomic composition of thin films electrodeposited at various potentials from −0.20 V to −0.60 V. As is seen from Fig. 4, film electrodeposited at −0.20 V mainly consists of selenium. For deposition potential negative than −0.30 V, film composition almost keep constant. This phenomena implies that potentials play little role in film composition and the electrodeposition process may be controlled by the ion diffusion independent on the potential in this range of −0.30 V to −0.60 V. Therefore, we can conclude that ternary Sb–Bi–Se compound can be prepared in a wide potential range, and the film composition could be adjusted by changing reactant concentrations in our electrodepositon system. Fig. 5 shows the dramatic evolution of surface morphologies of electrodeposited films at varies deposition potentials from −0.20 V and −0.60 V. Film deposited at −0.20 V mainly consists of very small Se clusters (Fig. 5(a)) in combination with EDS composition analysis. Uneven grains with sizes between 1 and 5 ␮m, and some cracks and holes are observed on the surface of the film obtained at −0.30 V (Fig. 5(b)). When the potential reaches −0.40 V, the film shows compact and homogeneous surface morphology with isolated grains of uniform size (Fig. 5(c)). At more negative potential of −0.50 V, the film returns to a poor surface morphology displaying grains with uneven sizes (Fig. 5(d)). Further shifting the potential negative to −0.60 V, the film shows a loose flocculent structure (Fig. 5(e)). This

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Fig. 5. Surface morphology of thin film electrodeposited at different deposition potential: (a) −0.20 V, (b) −0.30 V, (c) −0.40 V, (d) −0.50 V and (e) −0.60 V.

morphology is probably caused by the high electrode reaction rate and excessive concentration polarization [24,25]. Hence, the optimal potential for the preparation of Sb–Bi–Se compound in this experimental system can be confirmed to be −0.40 V vs. SCE in combination with CV, EDS and ESEM studies. Fig. 6 displays the typical X-ray diffraction patterns measured for the electrodeposited film before and after annealing treatment. The annealing treatment at 300 ◦ C does not obviously change film composition. And according to Fig. 6, as-deposited sample is

poor crystallinity without any diffraction peaks, except for peaks for SnO2 substrate (JCPDS card No. 77-0452). After annealing, new sharp diffraction peaks are visible, which can match with the orthorhombic structure Sb2 Se3 phase (JCPDS card no. 150861). However, carefully comparing with the standard peaks of orthorhombic Sb2 Se3 (marked as black dotted lines in the insets), the experimental peaks for annealed film show a shift to smaller 2 value. For example, three strong peaks of (2 1 1), (2 2 1), and (3 0 1) planes shift their 2 value from 28.199◦ , 31.159◦ , 32.220◦ to

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Fig. 6. X-ray diffraction patterns of SnO2 -coated substrate, as-deposited and annealed film. The black dotted lines in the inset present the standard peaks of Sb2 Se3 (JCPDS card no. 15-0861).

27.980◦ , 30.957◦ , 32.059◦ , respectively. These shifts demonstrate the increase of d-spacing in crystal lattices, which can be understood as the incorporation of Bi along with Sb into Sb2 Se3 lattice due to Bi atom has larger radius than Sb atom. Therefore, we can conclude the formation of Bi-doped Sb2 Se3 alloy with the basic crystal structure of the orthorhombic Sb2 Se3 . Fig. 7 shows the optical absorption coefficient (˛) of the annealed film as a function of photon energy (h), which is converted from the transmission spectra recorded in the wavelength range of 200–3000 nm, without taking into account the reflection loss. The sharp line near 4.00 eV (∼310 nm) is attributed to the light source change and the lower wave length absorption by the substrate [31], while the curve fracture at 1.56 eV (∼800 nm) is caused by switching of light detector. As seen in Fig. 8, the absorption coefficient is larger than 105 cm−1 in the visible region, supporting the direct band gap nature of material [25,32]. Based on the direct interband transition, the optical band gap is estimated to

Fig. 7. Optical absorption coefficient (˛) of the annealed film as a function of photon energy (h). The inset shows the plot of (˛h)2 vs. h; the estimated optical band gap is 1.12 eV ± 0.01 eV.

be 1.12 ± 0.01 eV by extrapolating the linear (˛h)2 vs. h plots to (˛h)2 = 0, as depicted from the inset of Fig. 7. This estimated value demonstrates that bismuth doping is a suitable approach to lower the band gap value of Sb2 Se3 thin films [1–3]. The optical properties exhibit that Bi-doped Sb2 Se3 films can be considered to be a promising absorber material for photovoltaic solar energy conversion. Fig. 8 illustrates the photocurrent–potential curve of the annealed film. This figure exhibits that the anode photocurrent is increased in the direction of the anode potential, which is a characteristic of a semiconductor with n-type conductivity [33,34]. Fig. 9 shows the Mott–Schottky plot for the annealed film in 0.5 M H2 SO4 solution. According to Mott–Schottky equation [35,36], the carrier concentration of 1.1 × 1019 cm−3 is obtained from the slope of the Mott–Schottky plot and the assumption that the dielectric constant of the film is equal to 10 for Sb2 Se3 [37] approximately, and the flat band potential can be determined to be about −0.40 ± 0.03 V by extrapolating the linear C−2 vs. potential plots to C−2 = 0. The annealed film also shows n-type conductivity

Fig. 8. The photocurrent–potential curve of the annealed film. Scan rate = 10 mV/s.

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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Fig. 9. The Mott–Schottky plot for the annealed film.

[17] [18]

from the positive slope of the Mott–Schottky plot, in agreement with that obtained from photocurrent–potential characterization.

[19]

4. Conclusion

[21] [22] [23]

Bi-doped antimony selenide thin films have been obtained on tin oxide glass substrates by potentiostatical electrodeposition and post annealing treatment. The electrochemical behaviors of electrodeposition were revealed by cyclic voltammetry (CV) investigation. The suitable deposition potential for film preparation was determined to be about −0.40 V vs. SCE combining with CV, composition and morphology study. The annealed film exhibited improved crystallinity with a basic structure of orthorhombic Sb2 Se3 (JCPDS card no. 15-0861), but having larger d-spacing due to the incorporation of Bi along with Sb into Sb2 Se3 lattice. The absorption coefficient and the optical band gap were estimated as larger than 105 cm−1 in the visible region and 1.12 ± 0.01 eV. Conductivity type, carrier concentration and flat band potential were determined by PEC characterization to be n-type, 1.1 × 1019 cm−3 and about −0.40 ± 0.03 V, respectively.

[20]

[24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

Acknowledgements This work was supported by the Natural Science Foundation of Hunan Province in China (Grant no. 09JJ3110) and the Research Fund of Young Scholars for the Doctoral Program of Higher Education in China (Grant no. 200805331121).

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