Non-toxic complexing agent Tri-sodium citrate’s effect on chemical bath deposited ZnS thin films and its growth mechanism

Journal of Alloys and Compounds 535 (2012) 53–61

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Non-toxic complexing agent Tri-sodium citrate’s effect on chemical bath deposited ZnS thin films and its growth mechanism G.L. Agawane a, Seung Wook Shin b, A.V. Moholkar c, K.V. Gurav a, Jae Ho Yun d,⇑⇑, Jeong Yong Lee b, Jin Hyeok Kim a,⇑ a

Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea Department of Materials Science and Engineering, KAIST, Daejeon 305-701, South Korea Electrochemical Mat. Lab., Department of Physics, Shivaji University, Kolhapur 416 004, India d Photovoltaic Research Group, KIER, Jang-Dong, Yuseong-Gu, Daejeon 305-343, South Korea b c

a r t i c l e

i n f o

Article history: Received 9 March 2012 Received in revised form 14 April 2012 Accepted 22 April 2012 Available online 28 April 2012 Keywords: Chemical bath deposition Non-toxic complexing agent ZnS Buffer layer Thin film solar cells Growth mechanism

a b s t r a c t This study demonstrates the growth and characterizations of chemical bath deposited zinc sulfide (ZnS) thin films prepared at pH 10. Aqueous zinc acetate and thiourea were used as precursors along with the non-toxic complexing agent, Na3-citrate. The effects of different concentrations of Na3-citrate from 0 to 0.2 M on the structural, morphological, compositional, chemical, and optical properties of ZnS thin films were studied. It was revealed through field emission scanning electron microscopy studies that an increase in the concentration of Na3-citrate leads to an improvement of the uniformity of the ZnS thin films and decrease in the grain size. Atomic force microscopy showed that the RMS value decreases with an increase in Na3-citrate concentration. X-ray diffraction study revealed that crystallinity of ZnS thin films improves upon increasing concentration of Na3-citrate and that the films exhibit a hexagonal polycrystalline ZnS phase while deposited with 0.2 and 0.1 M Na3-citrate. X-ray photoelectron spectroscopy revealed that the signal intensity decreases for Zn 2p3/2 and S 2p1/2 as the concentration of Na3-citrate decreases from 0.2 to 0 M. It was shown by ultraviolet–visible spectroscopy that approximately 80% transmission in the visible region and absorption edge shifts towards blue when the concentration of Na3-citrate increases from 0 to 0.2 M. The band gap energy of the ZnS film deposited without Na3-citrate was found to be 3.53 eV, while it increases from 3.73 to 3.80 eV with a decrease in Na3-citrate concentration from 0.2 to 0.025 M. The growth mechanism of CBD–ZnS thin film was found to be dependent on Na3-citrate concentration. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, polycrystalline Cu-chalcopyrite’s such as CuInSe2, Cu(In,Ga)Se2 (CIGS), and Cu(In,Ga)(S,Se) have attracted considerable interest with regard to photovoltaic devices because of their excellent conversion efficiency in laboratory-sized solar cells [1,2]. Until now, the highest conversion efficiency (over 20.3%) for CIGS based thin film solar cells (TFSCs) has been reported when chemical bath deposition (CBD) CdS thin films are employed as a buffer layer [3]. In general CIGS TFSCs, the buffer layer is employed between the absorber layer and a transparent conducting oxide (TCO) layer, because it adjusts the appropriate interface charge and protects the modification and lattice mismatch between two layers, which results from chemical species in the sensitive surface of the absorber layer and junction ⇑ Corresponding author. Tel.: +82 62 530 1709; fax: +82 62 530 1699. ⇑⇑ Corresponding author. Tel.: +82 42 860 3199; fax: +82 42 860 3539. E-mail addresses: [email protected] (J.H. Yun), [email protected] (J.H. Kim). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.04.073

regions. In addition, the buffer layer protects the TCO layer from the damage caused during physical deposition [4,5]. However, the band gap energy of CdS is 2.42 eV [6], and light with a wavelength lower than 520 nm cannot be transmitted to the absorber layer; this is responsible for the drop in quantum efficiency [7]. This characteristic implies that there is great scope for further improvement in the short circuit current, which can be achieved by replacing CdS with other wide band gap buffer materials. Additionally, CBD-CdS causes serious environmental problems due to the large amount of Cd-containing waste occurred resulting from the deposition process [8,9]. Moreover, mass Cd poisoning incidents occurred in the early- to mid-20th century all over the world. For example, Cd-laden waste water from mining operations in the Toyama Prefecture in Japan resulted in an outbreak of a painful and sometimes fatal affliction commonly referred to as ‘‘itai–itai’’ (translated literally as ‘‘ouch–ouch’’) disease [10,11]. Therefore, the preparation of a Cd-free buffer layer has became one of the major objectives in the research field of CIGS based TFSCs, as well as in the larger community of science and technology experts.

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Cd-free buffer materials, such as ZnO, Zn(OH)2, ZnS, ZnSe, In2S3, and InSe, have been investigated as an alternative buffer layer to CdS [12–16]. Among these materials, ZnS is considered to be the most promising buffer layer material because of its non-toxicity, low cost, and the possibility of more effective photocurrent generation resulting from its wider band gap energy of 3.65 eV [5] compared to that of CdS, which is 2.42 eV [6]. Additionally, ZnS is an important II–VI compound semiconductor, due to its higher transmission and high refractive index over a wide spectrum. It also has wide spread applications in optoelectronic fields, especially in deep-blue light emitting devices [18] and as a base material for phosphors [19,20]. ZnS films can be deposited by several physical methods such as pulsed-laser deposition [21], RF reactive magnetron sputtering [22], chemical vapor deposition [23], atomic layer deposition [24], as well as some chemical methods including spray pyrolysis [25], sol–gel [26], and electrodeposition [27]. Although the physical techniques result in better quality thin films, these methods require a vacuum, high quality targets, and high energy; all of which make for a significant hurdle, economically. Commercial level solar cells fabrication is relatively convenient, but ZnS buffer layers deposited by physical methods exhibit low photovoltaic conversion efficiency [7,35]. The synthesis of thin films deposited by CBD method is simple and fast, in that deposition of the films taking place at room temperature and normal atmospheric pressure is advantageous from both ecological and economic viewpoints. The deposition of ZnS thin films requires complex Zn2+ ions and a supply of sulphide ions, which can be obtained from thiourea or thioacetamide. Since the Ksp of ZnS has very low value 1024.7 [28], it is very difficult to grow ZnS thin films by CBD. Many researchers therefore introduced complexing agents, which control the Zn2+ ion concentration during the deposition process in order to prepare the uniform ZnS thin films [8,9]. In addition, the morphological properties of a CBD-ZnS buffer layer are found to be strongly dependent on the appropriate complexing agent [29]. Hydrazine hydrate (HH) plays a vital role as a side complexing agent [28], which improves growth rate and interfacial adherence of the ZnS thin films [28,30]. However, it does suffer drawbacks in the form of its highly flammable, toxic, and carcinogenic nature. Therefore, considerable efforts have been made to replace HH with non-toxic complexing agents such as Na2EDTA and Tartaric acid during deposition process [5,12]. Although ZnS thin films are deposited successfully using the above mentioned non-toxic complexing agents, the deposited thin films indicated poor crystallinity, rough morphology, and discontinuous microstructures. These characteristics indicate low efficiency in the TFSCs [33]. Citrate anions ððC6 H5 O7 Þ3 Þ in solution can be obtained from one to three carboxylate groups, depending on the pH of the reaction solution [34]. Since citrate anions give three carboxylate groups, they can easily form strong complexes with Zn2+ ions and even Fe3+, Ca2+, Ag+, and Mg2+ ions in a basic pH solution [34]. Recently, the use of Na3-citrate as a complexing agent has demonstrated the feasibility of improving coverage of the absorber layer and creating a denser microstructure of ZnS thin film [5,29]. Although the Na3-citrate has several advantages during the CBD-ZnS deposition process, there are no reports of a direct comparison of the morphological, structural, compositional, and optical properties of ZnS thin films with different concentrations of Na3-citrate. Herein, this paper reports on the preparation and characterization of ZnS thin films via the CBD method using the non-toxic complexing agent Na3-citrate at a temperature of 80 °C. The effects of Na3-citrate with various concentrations from 0 to 0.2 M on the morphological, compositional, structural, and optical properties of CBD-ZnS thin films are investigated. Further, various ZnS growth mechanisms are proposed for both the absence and presence of various concentrations of Na3-citrate.

2. Experimental details The chemicals used for the deposition were an analytical grade and purchased from Sigma–Aldrich. It includes zinc acetate, thiourea, and liquor ammonia (25%) and the non-toxic complexing agent Tri-sodium citrate (Na3-citrate) was used to control the unwanted release of Zn2+ ions. The solutions were prepared in deionized water and films were deposited on 26  76  2 mm glass substrates. The reaction bath solution was prepared using 40 mL of a 0.2 M zinc acetate dihydrate (Zn(CH3COO)22H2O) and 40 mL of a 0.4 M thiourea (SC(NH2)2). Different concentrations of the non-toxic complexing agent Na3-citrate were used, ranging from 0 to 0.2 M. The pH was adjusted to 10 by adding 25% ammonia (NH4OH) solution. Finally, a sufficient amount of deionized water was added to make the total volume of the deposition bath 100 mL. Prior to deposition, the substrates were ultrasonically cleaned with acetone followed by rinsing in methanol, isopropyl alcohol, and deionized water for 10 min, respectively. The glass substrates were mounted vertically on a specially designed substrate holder. The temperature of the reaction bath was then maintained at 80 °C. After 4 h, the glass substrates were removed, washed several times with deionized water, dried naturally, and kept in an air-tight container. The resultant films were homogenous, pin-hole free, and well adhered to the glass substrate. The cross-sectional and morphological study of the deposited thin films were observed via field emission scanning electron microscopy (FE-SEM, Model: JSM-6701F, Japan) operated at room temperature. The surface relief of the deposited films was observed by atomic force microscopy (AFM, Digital Instrument, Nanoscope III, USA). The structural properties of the films were observed by high-resolution Xray diffraction spectroscopy (XRD, X’pert PRO, Philips, Eindhoven, Netherlands) operated using a grazing incidence diffraction mode. The chemical binding energy investigation of the ZnS thin films was carried out by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo Scientific, UK). The chemical composition was examined using an energy dispersive spectrometer (EDS) system attached to a FESEM (JEOL, JSM-7500F, Japan). The optical transmittance of the thin films was observed using UV–visible spectroscopy (Cary 100, Varian, Mulgrave, Australia).

3. Results and discussion 3.1. Characterization of chemically grown ZnS thin films Fig. 1 (a–j) shows FE-SEM images obtained for ZnS thin films deposited on the glass substrate in order to study the surface morphology and tilted cross-sectional view of the thin films. The FE-SEM images show that the ZnS thin film deposited at 0 M Na3-citrate is thicker and exhibit non-uniformity with aggregation of different shape and size grains into a bigger flat grain while the ZnS thin films deposited from 0.025 to 0.2 M Na3-citrate are uniform in nature with a compact microstructure. The ZnS thin film deposited at 0.2 M Na3-citrate consists of densely packed small ZnS grains, is free from pinholes, and displays more homogeneity, as well as very thin thickness 70 nm. The thin films densely packed nature and uniformity is increased with an increase in concentration of the complexing agent from 0 to 0.2 M Na3-citrate; this resulted from a sufficient amount of citrate ion, which holds the Zn2+ ions. The thin films deposited with 0 M Na3-citrate shows a grain size of more than 100 nm, while the ZnS film deposited at 0.2 M Na3-citrate reveals well-adhered and spherical shaped ZnS grains having a diameter of about 20 nm. The ZnS thin films deposited at 0.025, 0.05, and 0.1 M Na3-citrate exhibit microstructures consisting of small grains of about 30 nm and a small amount of ZnS cluster formations. The thickness of the ZnS thin films (Fig. 1 (k)) is seen to decrease from 140 to 70 nm as the concentration of Na3-citrate is increased from 0 to 0.2 M. In the reaction bath containing 0 M Na3-citrate, the homogeneous nucleation process dominates the ZnS thin film growth which gives rise to less number of nucleation centers and finally the thin film forms with various spherical shaped grains on the substrate (Fig. 1 (c–h)). In contrast, in the reaction bath containing a sufficient amount of complexing agent, the ZnS thin film growth is dominated by the heterogeneous ZnS nucleation process, which leads to uniformly thin film formation (Fig. 1 (i,j)). AFM studies are carried out in order to obtain microscopic information on the surface structure and extent of surface relief. Fig. 2 shows 3D images (a–e) and RMS values (f) of the ZnS thin

G.L. Agawane et al. / Journal of Alloys and Compounds 535 (2012) 53–61

(a)

55

(b)

ZnS Glass

(c)

(d)

ZnS Glass (e)

(f)

ZnS Glass Fig. 1. Surface and tilted view FE-SEM images of the ZnS thin films deposited with various concentrations of Na3-citrate viz. 0 M (a,b), 0.025 M (c,d), 0.05 M (e,f), 0.1 M (g,h), and 0.2 M (i,j). Fig. 1(k) shows films thickness variation with various concentrations of the Na3-citrate complexing agent.

films deposited from 0 to 0.2 M Na3-citrate. The AFM study indicates that the ZnS film has rough surface with high RMS value of 43 nm when deposited at 0 M Na3-citrate this is due to, at 0 M Na3-citrate the presence of Zn2+–citrate ions is none, therefore, homogeneous growth of ZnS thin film competes with the heterogeneous ZnS thin film growth and dominates it, resulting in fewer, but larger ZnS grains. Additionally, the growth of these grains is larger and preferentially three-dimensional which forms films with greater surface roughness. While the films deposited from 0.025 to 0.2 M Na3-citrate exhibit smooth surfaces with decrease in RMS values from 30 to 3 nm. The thin films deposited with 0.2 M Na3-citrate show more smoothness and less surface roughness than the films deposited at 0.025, 0.1, and 0.2 M Na3-citrate. The RMS values decrease as the concentration of Na3-citrate’s increases. An increase in the concentration of Na3-citrate improves the uniformity of ZnS thin films and decreases the grain size of the ZnS. This in turn indicates that the thin film deposited at 0.2 M Na3-citrate shows more homogeneity, less roughness with low RMS value. From the above mentioned morphological studies, it may be suggested that the concentration of Na3-citrate plays an

important role in controlling the morphology of CBD-ZnS thin films. Reports exist on the structure of cubic (zinc blende), hexagonal (wurtzite), and both cubic and hexagonal ZnS phases deposited by CBD [5,9,29]. The cubic phase ZnS is stable at room temperature while hexagonal phase ZnS is stable at high temperature [32]. Fig. 3 shows the XRD patterns of ZnS thin films deposited from 0 to 0.2 M Na3-citrate. No strong peaks corresponding to cubic or hexagonal ZnS structures are observed for the films deposited at 0, 0.025, and 0.1 M Na3-citrate; confirming the amorphous nature of deposited thin films. However, XRD patterns of the ZnS thin films deposited at 0.2 and 0.1 M Na3-citrate exhibit a strong peak at 28.55° corresponding to the hexagonal (0 0 1 6) plane [JCPDS data file No.: 89–2423 (ZnS/Hex.)] [31]. The peak cannot be clearly distinguished as belonging to either cubic or hexagonal phases, because the angle position is similar for both. Therefore, d-spacing value measurements are carried out to clarify the phase formation of ZnS thin films using HR-TEM study (results are not shown here). Our previous HR-TEM study confirms that films deposited at 0.1 and 0.2 M Na3-citrate have a d-spacing of 0.3130 nm, which is

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(g)

(h) ZnS Glass

(i)

(j) ZnS Glass

150

(k)

140 130

Thickeness (nm)

120 110 100 90 80 70 60 0.00

0.05

0.10

0.15

0.20

Concentration (M) Fig. 1 (continued)

similar to the (0 0 1 6) plane of a hexagonal ZnS phase [37]. These structural studies suggest that the ZnS thin films deposited at 0.1 and 0.2 M of Na3-citrate are grown as a pure hexagonal structure. It is well known that the thickness of the film plays an important role to determine the crystalline structure of the film [29,32,37]. From our FE-SEM results (Fig. 1), it can be seen that the thickness of the film deposited 0.1 and 0.2 M is 74 and 70 nm, respectively, which is in turn very low, therefore, except the peak obtained from the (0 0 1 6) plane there are no other peaks for hexagonal or cubic ZnS phase. Additionally, in order to clearly confirm the deposited ZnS structure, XRD analysis was carried out using the grazing angle. The peak intensity of the (0 0 1 6) plane is lower for the ZnS thin film deposited at 0.1 M Na3-citrate, while it is enhanced for the film deposited at 0.2 M Na3-citrate. The reason for enhancement of the peak can be drawn; as the Na3-citrate’s concentration increases, it gives rise to formation of more Zn2+–citrate ions which

increase the possibility of formation of pure crystalline ZnS due to controlled release of Zn2+ towards S in an aqueous solution [32], which results into enhancement of the peak. No characteristic peaks resulting from a ZnO secondary phases are detected. XPS survey is performed to reveal the state of the constituent elements as well as Zn–S, Zn–OH, and Zn–O binding energies in the ZnS thin films. Fig. 4 shows the XPS core level spectra of Zn 2p3/2 (a) and S 2p1/2 (b) of the ZnS films deposited using 0 to 0.2 M Na3-citrate. A weak signal corresponding to Zn–OH or Zn–S bonding is observed for the film deposited without Na3-citrate, this is due to highly amorphous nature of the ZnS thin film (XRD results) and the peak intensity is very low. The thin film deposited at 0.025 M Na3-citrate displays two binding energies positioned at 1022.5 and 1022.8 eV, attributed to Zn–OH and Zn–S bonds, respectively [27]. An investigation of the Zn–OH binding energy in the ZnS thin films in basic bath is unavoidable due to the

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(a)

(c)

(b)

(d)

50

(f) (e)

RMS (nm)

40 30 20 10 0 0.00

0.05

0.10

0.15

0.20

Concentration (M) Fig. 2. 3D AFM images of ZnS thin films with various concentrations of Na3-citrate viz. 0 M (a), 0.025 M (b), 0.05 M (c), 0.1 M (d), and 0.2 M (e) as well as a graph of RMS values against the concentration of Na3-citrate (f).

incorporation of OH ions surrounding to the Zn2+ ion in the reaction bath and higher Ksp of Zn–OH as compared to that of Zn–S [27]. The signals for only Zn–S bonding (Fig. 4(a)) corresponding to 1022.3 and 1022.5 eV for the Zn 2p3/2 state are observed for the ZnS films deposited at 0.05, 0.1, and 0.2 M Na3-citrate. In addition, the investigations of Zn–OH bodings at lower concentration of Na3-citrate which in turn citrate ions compare to the higher concentrations of citrate ions is a result of uncontrolled growth of Zn2+ ions. The binding energies are observed at 161.9 and 162.5 eV for Zn–S bonding resulting from the S 2p1/2 (Fig. 4(b)) in films deposited with various concentrations of Na3-citrate. The peak intensity for the Zn–S bonding is increased as the concentration of citrate ions increased. This shows that at the higher concentration of citrate ions the film gives rise to only Zn–S bonding with its higher degree of Zn–S saturation. In addition, the shift of Zn–S bonding from the 162.5 to 161.9 eV with an increasing Na3-cit-

rate’s concentration is takes place, which shows that the S 2p1/2 core level may be turned to S 2p3/2 core level [37]. For the both spectrum no shoulder on the low or high energy side is observed. Our XPS results enlisted for the CBD-ZnS thin films are very similar to the results reported elsewhere [32,37]. Table 1 shows the compositional analysis of CBD-ZnS thin films deposited using 0 to 0.2 M Na3-citrate, studied via the EDS technique. It can be seen here that the atomic percentage of Zn in the ZnS films deposited at 0 M Na3-citrate is very high, and can be attributed to the presence of excess amounts of oxygen in the film (results are not shown here), which is detected from Zn(OH)2 (Fig. 4(a)). As the concentration of Na3-citrate increases from 0.025 to 0.2 M, the Zn content in the ZnS thin films decreases from 71.54 to 55.04 at.%. This decrease in Zn content may be attributed to the controlled growth of Zn2+ ions, due to sufficient amount of citrate ions which forms the bonds with Zn2+ ions. The Zn:S atomic

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G.L. Agawane et al. / Journal of Alloys and Compounds 535 (2012) 53–61 Table 1 Elemental analysis of the ZnS thin films deposited without Na3-citrate and with various concentrations of Na3-citrate.

0.0 M

Elements

0M

0.025 M

0.05 M

0.1 M

0.2 M

Zn (at.%) S (at.%)

71.54 28.46

57.65 42.35

57.04 42.96

55.47 44.53

55.04 44.96

Intensity (arb.unit)

0.025 M

0.05 M ZnS Hex. (0016)

0.1 M ZnS Hex. (0016)

0.2 M 20

30

40

50

60

2 θ (o) Fig. 3. XRD patterns of the ZnS thin films at different concentrations of Na3-citrate.

ratio of the ZnS film deposited at 0.2 M Na3-citrate is close to 50 at.%. Meanwhile, the S content in the ZnS films is found to increase from 28.46 to 44.96 at.% as the concentration of Na3-citrate increases from 0 to 0.2 M. The film deposited at 0.2 M Na3-citrate show good stoichiometry compare to the other films, implying that in this film the citrate ions allows S2 ions to grow by controlling Zn2+ ions.

In the buffer layer thin films, the uniformity, thickness and roughness decides the percentage of transmission. The information about physical properties, like band gap energy, and band structure is determined from optical characterization of the thin films [38]. Fig. 5 shows the UV–visible transmittance spectra (a) in the wavelength region from 300 to 800 nm and plots the (aht)2 vs photon energy (ht) (b) for ZnS films deposited at various concentrations of Na3-citrate and without Na3-citrate. The ZnS films deposited without Na3-citrate (Fig. 5a) do not exhibit a sharp absorption edge, while films deposited with Na3-citrate show a sharp absorption edge at 300 nm due to an improved uniformity. The average transmittance of the ZnS thin films deposited at 0 M Na3-citrate and with Na3-citrate is approximately 75 and 85%, respectively. Similar kind of nature of increase in transmittance for the films deposited with Na3-citrate is reported elsewhere [37]. In addition, it is well known that transmittance of the thin film is strongly related to the thickness and surface condition (RMS) observed in the film [24]. From the FE-SEM and AFM results, the uniformity of ZnS thin films is found to be improved while grain size and thickness decreases with increasing amount of Na3-citrate concentration. These characteristics of ZnS thin films are responsible for the reduced scattering centers due to which the transmittance of ZnS thin films is improved. While the ZnS film deposited without Na3-citrate are thicker, having a grain size larger than 100 nm, this results in more scattering and low transmittance without a sharp absorption edge. Fig. 5(b) shows the band gap energy of the ZnS thin films from which a linear variation can be seen in the values of (aht)2 vs ht. The film deposited at 0 M Na3-citrate shows relatively narrow band gap energy of 3.53 eV while for the ZnS films

(b)

(a) Zn 2p3/2

S 2p1/2

0.0 M

0.0 M 0.025 M

Intensity (arb.unit)

Intensity (arb.unit)

0.025 M

0.05 M

0.1 M Zn-S bonding (1022.3-1022.5 eV)

1020

1022

0.1 M

Zn-(OH)2bonding (1022.5-1022.8 eV)

S-Zn bonding (161.9,

0.2 M

1018

0.05 M

1024

Binding energy (eV)

1026

-162.5 eV) 1028

156

158

0.2 M 160

162

164

166

168

170

Binding energy (eV)

Fig. 4. High-resolution XPS spectra of Zn 2p3/2 (a) and S 2p1/2 (b) of the ZnS thin films deposited with various concentrations of Na3-citrate.

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100

(a)

Transmittance (%)

80

60

40

0.0 M 0.025 M 0.05 M 0.1 M 0.2 M

20

0

300

400

500

600

700

800

Wavelength (nm) Fig. 5. UV–visible transmittance spectra (a) from 300 to 800 nm of the ZnS thin films deposited with various concentrations of Na3-citrate and the plot of (aht)2 vs ht (b) of the ZnS thin films showing variation in band gap energy with various concentrations of Na3-citrate.

deposited using 0.025 to 0.2 M Na3-citrate, the optical band gap energy decreases from 3.80 to 3.73 eV, respectively, and films tend to turn pure ZnS. The band gap energy shown by films deposited at 0.1 and 0.2 M Na3-citrate is 3.75 and 3.73 eV, respectively, which shows that pure ZnS formation. Although, these two films shows slight large band gap than the bulk ZnS (3.65 eV) [5], this is in good agreement with our XRD results, which confirm the hexagonal ZnS phase for the films deposited with 0.1 and 0.2 M Na3-citrate as having band gap energy of 3.75 and 3.73 eV, respectively [29].

these Zn2+ and S2 ions, released from an aqueous solution react with each other and deposition of ZnS thin films takes place as below:

3.2. Growth mechanisms of CBD-ZnS thin films with different concentration of Na3-citrate

in the above reaction, as the concentration of Na3-citrate increases, the position of equilibrium moves towards the right-hand side of the reaction (Eq. (6)) to generate more protonated citrate anions. This sufficient amount of citrate anions complexes all the Zn2+ ions as follows:

ZnS precipitation can take place even at low concentrations of Zn2+ and S2 ion, due to its very low Ksp value 1024.7 [28]. Therefore, considerably controlled growth of unwanted Zn2+ ions release in the hot reaction bath by the use of suitable complexing agents is necessary to obtain uniform ZnS thin films [29]. Generally, ZnS thin films are deposited on a glass substrate using an aqueous alkaline bath composed of Zn–salt (Zincate ions), SC(NH2)2 with ammonia as a complexing agent, which can be understood from following equations [34]:

½Zn  salt ! Zn2þ þ salt  ðOHÞ

ð1Þ

Zn2þ þ NH3 ! ½ZnðNH3 Þn2þ

ð2Þ

when Zinc salt dissociates in an aqueous solution it gives rise to Zn2+ ions. Further these charged ions react with the ammonia, producing ammoniated Zinc ions [34]. This complexed mixture of Zinc and ammonia is heated at a certain temperature, giving rise to positively charged Zn2+ ions as:

½ZnðNH3 Þn2þ ! Zn2þ þ nNH3

ð3Þ

2

negatively charged S formation takes place by the reaction of hydroxide ion with thiourea in an aqueous solution as follows,

SCðNH2 Þ2 þ 2OH ! S2 þ CH2 N2 þ 2H2 O

ð4Þ

Zn2þ S2 ! ZnS ðon substrate or in reaction bathÞ

ð5Þ

when the reaction bath contains Na3-citrate as a complexing agent this in turn gives rise to citrate ions according to the pH of solution, the possible reaction that occurs in an aqueous bath is:

½C6 H5 O7 3 þ H2 O $ ½C6 H5 O7 H2 þ OH

ð6Þ

2

½ZnðcitrateÞn2þ ! Zn2þ þ ncitrate

Existing work has shown that the growth of ZnS thin films mainly takes place via three different mechanisms, which depend on the amount of complexing agent added in the reaction bath [34–36]: (i) Cluster by cluster (Fig. 6a): When the reaction bath contains excess Zn2+ ions, these ions form clusters and further homogeneous precipitation of ZnS in the reaction bath takes place. Therefore, ZnS film deposited without Na3-citrate exhibit larger grains, voids, and rough surface. (ii) Mixed (Fig. 6b): The building units in the reaction bath are Zn2+ and S2 ions. When the solution does not contain a sufficient amount of complexing agent, the preferred homogeneous mechanism is followed by a heterogeneous mechanism. In this case, the ZnS film exhibits uniformity but small amount of large ZnS grains form on the film due to aggregation of the elementary units. This accounts for changes in the surface morphology, uniformity, and composition of the deposited films. The films formed by this mechanism shows thick thickness while the ZnS thin film growth follows mixed mechanism route.

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Fig. 6. Schematic illustrations to describe the formation of ZnS thin film (a) Cluster by cluster growth (b) Mixed growth (cluster by cluster and ion by ion) (c) Ion by ion growth.

(iii) Ion by ion (Fig. 6c): When the bath contains a sufficient amount of the Na3-citrate complexing agent, then all the Zn2+ ions are bonded with citrate anions and this gives rise to the preferred heterogeneous precipitation of ZnS thin film, which is superior to homogeneous nucleation. The citrate anions give rise to only Zn2+ ions and, all the Zn2+ ions react with S2- ions, which promotes only heterogeneous ZnS thin film growth on the glass substrate via ion by ion deposition, according to (Eq. (5)). The ZnS films grown by this mechanism exhibit smaller grains as well as a smooth and uniform surface without large ZnS grain formation. Our experimental results of FE-SEM and AFM studies of ZnS thin films deposited with different concentrations of Na3-citrate resembles a noticeable relationship between the theoretical growth mechanism and obtained results. The ZnS thin film deposited without Na3-citrate exhibits non-uniform morphology, high RMS values, and many voids. However, the ZnS thin films deposited at 0.025, 0.05 and 0.1 M Na3-citrate possess denser microstructures and smoothness consisting of some ZnS clusters. ZnS thin film deposited at 0.2 M Na3-citrate shows the smoothest morphology without formation of large grain ZnS. The reason for this dramatic change in the films can be is that when the ZnS thin film deposition takes place without any complexing agent in a reaction bath, only NH3 plays the role of the complexing agent. As the temperature of the reaction bath increases, NH3 reacts with Zn2+ ions, which produces ½ZnðNH3 Þn2þ ions. However, the NH3 does not fully react with Zn2+ ions and there are more free Zn2+ ions, this forms more number of Zincate ions and further it gets easily precipitated as Zn(OH)2 in the reaction bath [28]. This also gives rise to preferentially homogeneous nucleation, which grows larger ZnS clusters on the glass substrate due to which the ZnS thin film exhibits rough morphology consisting of many voids (Fig. 1a, b). Films with this growth behavior are well suited to the cluster by cluster mechanism (Fig. 6a). In the case of a reaction bath with a low concentration of the complexing agent, there is a higher presence of Zn–[complexing agent]2+ ions, in terms of ½ZnðNH3 Þn2þ and ½ZnðcitrateÞn 2þ , and a small amount of Zn2+ ions compared to a reaction bath having 0 M complexing agent. This increase in the Zn–[complexing

agent]2+ ions promotes a slow rate of reaction between Zn2+ and S2, this leads to the less Zn–(OH)2 bodings and high concentrations of Zn–S bonds. The high concentrations of formatted Zn–S bonding resulting from increased concentration of the Na3-citrate which can easily adheres to the substrate surface, resulting in a heterogeneous reaction on the substrate. However, a complete reaction of Zn–[complexing agent]2+ for Zn2+ ions does not occur in the hot reaction bath due to an insufficient complexing agent concentration. Therefore, upon increasing the Na3-citrate, ZnS thin films are grown with a denser microstructure and smoothness consisting of some ZnS clusters, as compared to film deposited without Na3-citrate (Fig. 1(c), (d), (e), (f), (g), and (h)). The ZnS thin films these behavioral properties are significantly similar to those achieved via the mixed growth mechanism (Fig. 6b). In the reaction bath having a high concentration of the complexing agent, all the positively charged Zn2+ ions react with the complexing agent and are formatted to the Zn–[complexing agent]2+, which dominantly leads to a heterogeneous reaction on the surface of the substrate. Therefore, ZnS thin film with a high concentration of the complexing agent exhibits a dense microstructure, smooth morphology, improved crystallinity, and decreased voids without formation of ZnS clusters (Fig. 1(i,j)). This type of film growth behavior is well suited to the ion by ion growth mechanism (Fig. 6 c). From the above discussion, it is suggested that ZnS thin films deposited with high concentration of Na3-citrate (0.2 M) show very uniform compact nature of surface with very fine grains and reduced grain size, excellent distribution, no voids with very rarely some over grains, an improved crystallinity, low thickness, good stoichiometry, and higher transmittance in the visible region with optimal band gap energy. 4. Conclusions This work presents a simple way to achieve deposition of ZnS thin films in an aqueous zinc acetate and thiourea using non-toxic complexing agent Na3-citrtate. This paper also studies the precursor effects of various Na3-citrtate concentrations on the growth of ZnS thin films. From FE-SEM, the film thickness and surface morphology are found to be strongly dependent on the concentration of Na3-citrate, and films deposited with complexing agent shows thin thickness which is principally needed as a buffer layer for TFSCs. Structural characterization shows that films are of an amorphous nature, except for those deposited at 0.1 and 0.2 M Na3-citrate, which shows a polycrystalline hexagonal structure. The XPS technique reveals Zn–S bonds between 1022.3 and 1022.5 eV. Films are nearly stoichiometric, except those deposited without a complexing agent. The films are highly transparent, the percentage of transmission is 85% for the films deposited with Na3-citrate, and films exhibit good adherence to the glass substrates. The direct band gap of the ZnS thin films varies from 3.73 to 3.80 eV, depending upon the concentration of Na3-citrate, and it is confirmed by XRD results. On the basis of these results, growth mechanisms are proposed that underlie the formation of the various forms of ZnS thin films for various concentrations of Na3-citrate. The above results prove that, for the fabrication of TFSCs, the uniform ZnS buffer layer with a thickness of 70 nm can be prepared at 0.2 M Na3-citrate in an alkaline bath following above experimental conditions. Acknowledgment This work was supported by the Center for Inorganic Photovoltaic Materials (2012-0001170) grant funded by Korea government, and by the grant from the Fundamental R&D program for Core

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Technology of Materials funded by the Ministry of Knowledge Economy (10037233).

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