Triethanolamine-facilitated one-step electrodeposition of CuAlSe2 thin films and the mechanistic studies utilizing cyclic voltammetry

Journal of Electroanalytical Chemistry 762 (2016) 73–79

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Triethanolamine-facilitated one-step electrodeposition of CuAlSe2 thin films and the mechanistic studies utilizing cyclic voltammetry Mu-Tao Hsieh, Chien-Ting Chen, Thou-Jen Whang ⁎ Department of Chemistry, National Cheng Kung University, No. 1, University Road, Tainan 70101, Taiwan

a r t i c l e

i n f o

Article history: Received 4 November 2015 Received in revised form 23 December 2015 Accepted 26 December 2015 Available online 28 December 2015 Keywords: Triethanolamine Electrodeposition CuAlSe2 (CASe) Cyclic voltammetry (CV) Indium doped tin oxide (ITO)

a b s t r a c t The electrochemical behavior of the compositional ingredients CuCl2, AlCl3 and SeO2 individually or collectively in acidic aqueous solution was investigated on a glassy carbon electrode using the technique of cyclic voltammetry. Co-electrodeposition mechanism of CuAlSe2 was attempted to be unraveled through cyclic voltammetric study of the electroactive species. The formation of CuAlSe2 was found to be achieved by the generation of the compounds Al2Se3 and Cu2Se via reductions and/or chemical reactions of the precursors. Triethanolamine as the complexing agent was introduced in the electrodeposition of CuAlSe2 on indium doped tin oxide glass substrates in order to assist the deposition and the stoichiometric composition of CuAlSe2 thin films was fabricated. The deposited thin films were characterized by X-ray diffraction, scanning electron microscopy, UV–Vis spectrometry, Auger electron spectroscopy and energy dispersive X-ray spectrometry analysis. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In the pursuit of a stable and sustainable energy for the generations to come, solar cells have been favored over other energy sources since it is not restricted by geographical environment, inexhaustible in supply, and no hazardous waste generated. Therefore, it is one of the prospective and promising alternatives to the presently used sources. Siliconbased material is the main stream of commercial solar cells due to its advantages of better efficiency and high yield rate [1]. However, the pursuit of cost-effectiveness and higher absorption coefficient for alternative materials has become more and more imperative. Thin film solar cells have been developed since previous decades and the trend is attributed to its ease of fabrication, flexibility, and cost reduction, one of the materials for thin films is the I–III–VI family [2–4]. CuAlSe2 (CASe) has been widely studied owing to the wide band gap (Eg = 2.67 eV) and relative stability for the application as a buffer layer in thin-film solar cells and detectors. As a semiconductor of chalcopyrite structure CASe exhibits tetragonal crystal system with space group I42d and similar cell parameters to CuInSe2 [5,6], it is feasible to fabricate either n-type or p-type CuAlSe2 on suitable substrates. Therefore CASe is growing in popularity recently in search of an excellent window layer or buffer layer for photovoltaic cells [7]. Techniques of elemental co-evaporation [8], chemical vapor transport (CVT) [9], metal organic chemical vapor deposition (MOCVD) [10], selenization of metallic precursors [11], and molecular beam epitaxy (MBE) [12] have been proposed for preparing CuAlSe2 thin films. Apart from these methods, the approach of electrodeposition ⁎ Corresponding author. E-mail address: [email protected] (T.-J. Whang).

http://dx.doi.org/10.1016/j.jelechem.2015.12.041 1572-6657/© 2015 Elsevier B.V. All rights reserved.

for CuAlSe2 is rarely proposed to date, however, due to the regular features of cost-effectiveness, time-saving, and displaying high-quality in fabrication with appropriate parameters [13], the electrodeposition approach was therefore adopted by the authors to fabricate CuAlSe2 thin films. Bhattacharya first introduced triethanolamine (TEA) in the electrodeposition bath as the complexing agent for the deposition of CuInSe2 thin films [14]. Similarly, the single step of co-electrodeposition of CuAlSe2 from aqueous solution containing CuCl2, AlCl3 and SeO2 is challenging due to the considerably large reduction potential range of these precursors. In order to bring deposition potentials of electroactive species closer for a better co-deposition environment, complexing agents were used to shift the potential of copper ions in more cathodic direction. Additionally, the copper ions in the complex form of Cu2 +–TEA may impede the reaction between itself and other species in solution such as the direct chemical reactions. Generally the advantages of bringing in a complexing agent in electrodeposition bath are to bring the potentials of reducible species closer, to improve adhesion of deposits on substrates, and to enhance the morphological features of the films. The frequently used complexing agents for electrodeposition of alloy or semiconductor thin films consist of TEA [15], ethylenediamine [16], sodium citrate [17], tartaric acid [18,19] and potassium thiocyanate [20]. In this article, the main purpose is to unravel the possible mechanism of the single step electrodeposition of CuAlSe2 thin films by the cyclic voltammetric study from aqueous solution containing CuCl2, AlCl3 and SeO2 with TEA as the complexing agent, and to further optimize the experimental condition for the deposition process. To the best of our knowledge, the electrodeposition mechanism of CuAlSe2 has not yet been discussed. The results suggested that by the generation of the

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compounds Al2Se3 and Cu2Se via reductions and/or chemical reactions of the precursors under proper manipulation of experimental conditions, CASe thin films with fairly accurate stoichiometric ratio can be fabricated by single-step electrodeposition with the facilitation of TEA. XRD (X-ray diffractometer) and UV–Vis spectra, SEM (scanning electron microscope) images, AES (Auger electron spectroscope) and EDX (energy dispersive X-ray spectrometer) analysis have been employed for the characterization of the deposited films. 2. Experimental details Indium doped tin oxide (ITO; In2O3:SnO2) coated glass was purchased from RITEK Co. (Taiwan), triethanolamine, copper (II) chloride dihydrate (CuCl2·2H2O, 99.9%), aluminum chloride hexahydrate (AlCl3·6H2O, 99.8%) and selenium dioxide (SeO2, 99.9%) were obtained from Hayashi pure chemical Co. (Japan), all chemicals were used as received without further purification. All solutions were prepared by using deionized water produced from Deionized water system. ITO coated glass was cleansed in an ultrasonic bath with ethanol solution and rinsed with deionized water, finally purged with nitrogen gas. CHI electrochemical analyzer model 627C was used to record cyclic voltammograms (CV) at a scan rate of 50 mV/s. The three-electrode cell consisted of a platinum wire (BASi MW-4130; 6 cm) as the counter electrode, an Ag/AgCl electrode (BASi MF-2052) as the reference electrode and a glassy carbon working electrode (CH Instruments, Inc. CHI104) as the working electrode. The supporting electrolyte K2SO4 was used in all solutions to maintain constant ionic strength and to increase the conductivity of the solutions. Stream of nitrogen gas was introduced into each solution for 5 min before each scan for deaeration. All voltammograms were first scanned to negative (cathodic) direction in unstirred solutions at room temperature unless otherwise stated. The aqueous solutions for electrodeposition experiments were prepared with 5 mM CuCl2·2H2O, 30 mM AlCl3·6H2O, 22 mM SeO2 and the complexing-agent concentrations of TEA ranged from 0.05 M to 1.0 M. The pH of all electrolyte solutions were adjusted with 0.1 M HCl to pH 1.47–2.47 in order to prevent the hydrolysis of the electroactive species and to improve the solubility. Nitrogen gas was bubbled into the solution for 5 min before each electrochemical study or electrodeposition process. Potential was applied to the solutions from − 0.5 V to −0.8 V, and the deposition time was set to 15 min. The electrodeposition of CuAlSe2 thin films were performed using an EG&G potentiostat model Versastat II with a three-electrode cell, which consists of a platinum plate (1 × 2.4 cm2) as the counter electrode, an Ag/AgCl (BASi MF2052) as the reference electrode and an ITO glass as the working electrode. An IBM compatible desktop PC was connected to the potentiostat to monitor and record the data of potential and current. To improve the crystallinity, a NEY 2-525 furnace was used the annealing of the deposited films and the temperature was selected from 275 °C to 300 °C. The morphology of the films deposited on the working electrode was recorded with a scanning electron microscope (SEM, HitachiS4100), and the crystallinity of the films was scanned with an X-ray diffractometer (XRD, Shimadzu-7000S) using Cu Kα radiation. Compositions of the films were analyzed with an energy dispersive X-ray spectrometer (EDX) equipped with the SEM. The surface chemical components were investigated by an Auger electron spectroscope system (AES, Thermo Scientific Microlab 350).

to the following equation: E ¼ E0MZþ −


a z−pq  MLq RT RT lnβ þ ln zF zF ðaLp− Þq

ð2Þ

where β is the stability of the complex and aMLz−pq and aLp− are the activq and Lp−, respectively [21]. From the equation it can be easities of MLz−pq q ily understood that, with higher stability of the complex, lower activity of , and/or higher activity of Lp− the reduction equilibrium potential MLz−pq q would be shifted to a lower value. TEA is a weak base which is represented in the chemical formula of N(CH2CH2OH)3. The Kb constant of TEA is 5.75 × 10−7 [22] in aqueous solution at 25 °C, which indicates the nitrogen atoms on TEA are substantially protonated by hydrogen ions in the pH region of experiment condition. Hence, the chelating ability of TEA is attributed to the interaction of oxygen atoms on hydroxyl groups with copper ions. The decimal logarithm of stability constant of Cu2+ and TEA complex is 4.07–4.4 [23, 24], it is apparent that TEA is a good candidate for chelating agent toward Cu2+ in a ratio of 1:1 [24,25] since the complexing products can significantly shift the reduction potential of Cu ions toward cathodic direction for the co-electrodeposition process. Moreover, since the reaction between Cu2+–TEA complex and other species in solution is slow and hence the direct chemical reactions can be alleviated [14]. 3.2. Mechanism investigation for the co-deposition of CASe by cyclic voltammetry Fig. 1 shows the merged cyclic voltammograms of three individual solutions each containing 5 mM CuCl2, AlCl3, or SeO2 with 0.08 M TEA. The curve a was recorded for the electroactive species H2SeO3 composed of three major peaks, the onset potential starting from 0 V toward cathodic direction is the 4-electron reduction of H2SeO3 to elemental Se, as shown in Eq. (3) [26]. The next peak occurred at − 0.7 V is the 6electron reduction of H2SeO3 to H2Se, as revealed in Eq. (4). The following wave at −0.81 V is the reduction of Se to H2Se, as revealed in Eq. (5). This curve differs from other ordinary CV curves since the oxidation peak is apparently absent from the curve, it is attributed to the inertness of Se element which makes it unlikely to be oxidized once being deposited and the oxidation potential is beyond the electrochemical window of this scan [21]. The reduction peak of Cu2+/Cu+ can be observed near − 0.1 V vs. Ag/AgCl and the reduction peak of Cu+/Cu appeared at −0.35 V vs. Ag/AgCl in curve b for the solution of CuCl2. In the anodic part of the curve for copper ions, the slightly converged peaks at 0.07

3. Results and discussion 3.1. TEA introduced as the complexing agent in electrodeposition process The formation of a complex from a metal ion Mz+ with a ligand Lp− Mzþ þ qLp− → MLq z−pq ←

ð1Þ

causes the reduction equilibrium potential shifting to an extent according

Fig. 1. Merged cyclic voltammograms of deaerated pH 2.35 solutions each containing 5 mM CuCl2, AlCl3, and SeO2 with 0.08 M TEA, first cathodic scan to −1.0 V and anodic scan to 0.8 V vs. Ag/AgCl at a rate of 50 mV/s. A glassy carbon electrode was used as the working electrode, Ag/AgCl electrode as the reference electrode and platinum wire as the counter electrode.

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and 0.14 V correspond to the consecutive oxidations of elemental Cu and Cu+ species. With the inclusion of TEA in each solution, the following phenomena were observed: (1) the gap of oxidation potentials of two redox couples for copper Cu2 +/Cu+ and Cu+/Cu decreases, (2) the area under each peak decreases and the redox currents became smaller. These observations were resulted from the interaction of TEA and copper ions. In Fig. 1c, no reduction or oxidation can be observed in the curve of Al3+ species which agrees with the expected outcome being carried out in aqueous medium. H2 SeO3 þ 4e þ 4Hþ → SeðgrayÞ þ 3H2 O

ð3Þ

H2 SeO3 þ 6e þ 6Hþ → H2 Se þ 3H2 O

ð4Þ

Se þ 2e þ 2Hþ → H2 Se

ð5Þ

←

←

←

Shown in Fig. 2 is the cyclic voltammogram of the mixture of the three precursors, the curve clearly reflects the peaks of each species, with the negative potential part of the curve tilted upward due to the baseline of H2SeO3 and Cu(II) species. Each individual peak was assigned and marked in the diagram for clarity. The peak located at around 0.25 V in anodic scan is apparently absent from the examination of Fig. 1 and the comparison of these two figures. It provoked the inquisitiveness for further investigation of the following discussion. Fig. 3 reveals the first (a; blue curve) and four successive (b; red curve) CV scans of the aqueous solution containing Cu2+ and H2SeO3 species. The broad peak started from the onset potential of 0.2 V in the first cathodic scan is the reduction of Cu(II) species to Cu(I), which is overlapped with the formation reaction of binary compound copper(II) selenide CuSe according to Eq. (6). It is a combined consecutive reduction of H2SeO3 and Se in the presence of Cu2+. The peak at − 0.3 V is the subsequent reduction of Cu(I) to elemental Cu(0), in second loops onward the peak is observed to shift toward anodic direction at around −0.2 V due to the deposition of elemental Cu on the electrode surface. In the cathodic curve of the second and subsequent scans, the peak near 0.15 V is absent from the first scan, which is attributed to the reduction of copper(II) selenide to copper(I) selenide Cu2Se in the presence of Cu2+ ions, which can be represented by Eq. (7) [27]. The fact that the current and hence charge under the peak increased with the proceeding of scans indicates the accumulation of the solid compound. The same phenomenon can be observed in the anodic scan of the loops, the peak at 0.25 V is the oxidation of copper(I) selenide, the

Fig. 2. Cyclic voltammogram of a deaerated pH 2.35 solution containing 5 mM CuCl2, 5 mM AlCl3, and 5 mM SeO2 with 0.08 M TEA, first cathodic scan to −1.0 V and anodic scan to 0.8 V vs. Ag/AgCl at a rate of 50 mV/s. A glassy carbon electrode was used as the working electrode, Ag/AgCl electrode as the reference electrode and platinum wire as the counter electrode.

Fig. 3. First (a; blue curve) and four successive (b; red curve) CV scans of a deaerated pH 2.35 solution containing 5 mM CuCl2 and 5 mM SeO2 with 0.08 M TEA, first cathodic scan to −1.0 V and anodic scan to 0.8 V vs. Ag/AgCl at a rate of 50 mV/s. A glassy carbon electrode was used as the working electrode, Ag/AgCl electrode as the reference electrode and platinum wire as the counter electrode. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

reverse reaction of Eq. (7) [28]. The significant peak located at 0.1 V with a tiny shoulder in anodic scan is the subsequent oxidations of Cu and Cu(I). Copper(I) selenide is one of the binary compounds for CASe, which will be discussed in later section. Cu2þ þ H2 SeO3 þ 4Hþ þ 6e → CuSe þ 3H2 O

ð6Þ

CuSe þ Cu2þ þ 2e → Cu2 Se

ð7Þ

←

←

The merged cyclic voltammograms displayed in Fig. 4 is for a comparison between the single species H2SeO3 (a; blue curve) and the mixture of Al3+ and H2SeO3 species (b; red curve). The individual peaks of H2SeO3 scan were each specified in the initial part of this section. In the case of the mixture, the shape of curve is virtually identical to that of the H2SeO3 species with a substantial current increase. However, the peak at − 0.82 V in the cathodic scan for H2SeO3 is absent in the curve for the mixture of Al3 + and H2SeO3. It suggests that the reduction of

Fig. 4. Merged cyclic voltammograms of a deaerated pH 2.35 solution containing 5 mM SeO2 (a; blue curve) and a solution containing 5 mM AlCl3 and 5 mM SeO2 (b; red curve), each solution with 0.08 M TEA. First cathodic scan to −1.0 V and anodic scan to 0.8 V vs. Ag/AgCl at a rate of 50 mV/s. A glassy carbon electrode was used as the working electrode, Ag/AgCl electrode as the reference electrode and platinum wire as the counter electrode. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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elemental Se to hydrogen selenide H2Se in aqueous solution was suppressed to a certain extent in the case of the Al3+/H2SeO3 mixture. The underlying reason for this occurrence can be explained by the following reactions: when H2Se was generated by reduction of H2SeO3 at −0.7 V as expressed by Eq. (4). In the absence of Al3+, hydrogen selenide would react with H2SeO3 to form the less stable allotrope of red Se [29] according to Eq. (8), which was distinctively observed on the electrode when the number of scan cycles increases. On the other hand, the competition reaction arises in the presence of Al3+ producing aluminum selenide Al2Se3 on the electrode, as shown in Eq. (9) which suppresses the reaction in Eq. (8). In summary, the production of aluminum selenide Al2Se3 at the expense of H2Se by reacting with Al3+ causes the decline of the peak at −0.82 V. Furthermore, CV of the solid Se deposited on the GCE by a 20-cycle scan of SeO2 solution was performed to confirm the aforementioned phenomenon, as shown in Fig. 5a. Reduction of elemental Se to H2Se is clearly depicted at −0.81 V with a shoulder, which is the reduction of adsorbed H2SeO3 to H2Se according to Eq. (4). The peak located at 0.13 V of the anodic scan is the oxidation of H2Se to HeSeO3, the reverse reaction of Eq. (4). When AlCl3 was included in the scan, in Fig. 5b, the fact that the formerly peaked current-potential curve changes to a steep-slope curve down to the switching potential can be inferred that the depletion of H2Se is caused by the chemical reaction of Al3+ and H2Se. A downsized oxidation peak area of H2Se also suggests the consumption of H2Se through the

Fig. 5. (a) Cyclic voltammogram of a deaerated pH 2.35 solution containing 0.5 M K2SO4 on a GCE deposited with a 20-cycle of 5 mM SeO2 (inset) (b) Cyclic voltammogram of a deaerated pH 2.35 solution containing 5 mM AlCl3 and 0.5 M K2SO4 on a GCE deposited with a 20-cycle of 5 mM SeO2. First cathodic scan to −1.0 V and anodic scan to 1.0 V vs. Ag/AgCl at a rate of 100 mV/s for both voltammograms.

chemical reaction. 2H2 Se þ H2 SeO3 → 3SeðredÞ þ 3H2 O ←

3H2 Se þ 2Al

3þ

→ Al2 Se3 þ 6Hþ ←

ð8Þ ð9Þ

The overall reactions from the electrodeposition process can be illustrated by the following equation, the theoretical molar ratio of the three constituents is Cu:Al:Se = 2:2:4, the ratio provides a quantitative suggestion regarding the preparation of the solutions. Nevertheless, due to non-stoichiometric reactions such as the production of compounds Cu2 − xSe [27], the efficiency of individual reduction, and the incompleteness of chemical reaction, consequently the practical situation of optimal component ration vary from the theoretical one. Cu2 Se þ Al2 Se3 → 2CuAlSe2 ←

ð10Þ

3.3. Electrodeposition of CASe thin films and the characterization From the electrochemical study in the previous section, the reduction of copper species started from 0 V down to − 0.5 V vs. Ag/AgCl while for H2SeO3 the reduction extended further to around −0.8 V vs. Ag/AgCl. In order to investigate the effect of deposition potential on the formation of CASe thin films, various potentials were employed in the deposition process from − 0.5 V to − 0.8 V at an interval of −0.1 V. The XRD patterns of CASe films deposited with these potentials with 0.08 M TEA were exhibited in Fig. 6. These XRD scans were conducted after annealing process since the as-deposited films obtained were amorphous in structure. Annealing promotes the uniformity of crystalline structures of the layers by heat treatment of the films after the deposition process to improve the crystallization of CuAlSe2. However, Se evaporation arises at higher temperature, therefore it may cause the break-down of the composition of the CuAlSe2 layers and the crystalline feature. From the XRD patterns in this figure, it was found that the crystalline signals of CuAlSe2 were less strong in the preferred orientation (112) plane of the chalcopyrite structure as the potential proceeded in the negative direction. The potential being applied majorly determines the species being reduced and hence the chemical reactions, the composition of the deposits, and the morphology of the films [30]. The morphology and surface grains from the deposited films under different potentials can

Fig. 6. XRD patterns of thin films deposited at potential (a) −0.5 V (b) −0.6 V (c) −0.7 V (d) −0.8 V vs. Ag/AgCl with solutions containing 5 mM CuCl2, 30 mM AlCl3, and 22 mM SeO2 after 300 °C/1 h annealing. The peak marked with an asterisk represents the diffraction originated from ITO substrates.

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be inspected from the SEM images in Fig. 7. Left and right panels of the images provide a visual comparison of the annealing effect produced on the film morphology, apparently more compact structures can be observed after heat treatment of the films. In the case of − 0.5 V vs. Ag/ AgCl in Fig. 7a, small bead-like grains of crystalline structure of CuAlSe2 were evenly spread throughout the surface from the nucleation process. Whereas in the case of −0.6 V vs. Ag/AgCl shown in Fig. 7b, the bigger multinuclear, cauliflower-like grains were found distinctively on the surface of the film among the small bead-like grains. For the case of −0.7 V vs. Ag/AgCl in Fig. 7c, the film morphology is similar to that of

77

Fig. 7b but little more multinuclear, cauliflower-like grains were observed. For the film deposited at the most negative potential of − 0.8 V vs. Ag/AgCl, the bigger multinuclear, cauliflower-like grains were more distantly scattered among the grains which was caused by the evolution of hydrogen selenide, as shown in Fig. 7d. Regarding the deposition time and film thickness, it was found that the optimal time duration in the deposition process is 15 min for the fabrication of CASe on ITO substrates. Since the films may not be thick enough to be applicable in practical endeavors with shorter deposition time. On the other hand, the CuAlSe2 layers may not be firmly adhesive to, even

Fig. 7. SEM images of thin films deposited at potential (a) −0.5 V (b) −0.6 V (c) −0.7 V (d) −0.8 V vs. Ag/AgCl with solutions containing 5 mM CuCl2, 30 mM AlCl3, and 22 mM SeO2. Left panels were taken before annealing, and right panels with 300 °C/1.0 h annealing.

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begin to peel off from ITO substrates with longer deposition time. The surface chemical components of the as-prepared deposits were investigated by AES spectra as shown in Fig. 8. The peaks appeared at around 1340 and 1388 eV are attributed to KLL emissions from Al3+ in Al2Se3, which are shifted from around 1390 eV for elemental Al. The peaks emerged at around 1192, 1308, and 1350 eV can be assigned to LMM emissions from Se and Se2− in Cu2Se and Al2Se3. The inset in Fig. 8 illustrates the LMM emissions of Cu+ located at around 918 eV, which is approximately one eV shifted from elemental Cu. The optical energy gap of CASe thin films can be evaluated from the Tauc–Sunds equation through data treatment of UV–Vis absorbance by plotting (αhν)2 vs. hν with the following relationship [15,31]:

α¼

h  n=2 i A hv−Eg hv

ð11Þ

where α is the absorbance, A is a constant, h is the Planck's constant, ν is the frequency of light, n = 1 for direct and n = 4 for indirect electron transfer, and Eg is the band gap of the object concerned. From the results shown in Fig. 9, the optical energy gap was evaluated at 2.19, 2.18, 2.20, and 2.34 eV for the film which was deposited at −0.5, −0.6, −0.7, and − 0.8 V, respectively. It suggests that wider optical band gap of CASe thin films can be acquired through electrodeposition at more negative potential and the band gap energies of these nanoparticles are inversely related to the grain size of these particles. The characterization of CASe thin films can be summarized in Table 1, the average particle size was calculated utilizing the Scherrer equation from the peak information of XRD spectra [32]. It is distinct to observe the significant potential influence exerted on the average particle size of the films, that is, smaller particle size resulted from more negative potential. The atomic ratios of the deposited films at various potentials reveal that the fairly accurate stoichiometric result of the CuAlSe2 films was attained, with little insufficient Se in the composition due to the evaporation of Se at high temperature in the ambient atmosphere without the presence of inert gas or Se vapor. In order to get the stoichiometric result of CuAlSe2, high contents of H2SeO3 must be provided in the solution [33]. Besides, since the species Al3+ in aqueous medium is too active to be reduced in electrochemical window, so the requirements of relative high concentrations of Al3+ and H2SeO3 must be met in the preparation of solutions. And it is worth to note that the grain size is inversely related to the optical band gap, which can be ascribed to the quantum size effect of the compact crystalline structure [34]. By applying the Bragg's law: nλ = 2d(hkl)sinθ, the inter-planar

Fig. 9. Plot of (αhν)2 vs. hν for CASe thin films deposited from. The dotted lines represent the extrapolation of the abscissa of hν axis from the linear fit of the curves.

spacing d(hkl) can be evaluated. For example, the calculated interplanar spacings of the (1 1 2) plane of CuAlSe2 are 3.30 Å, 3.30 Å, 3.28 Å, and 3.30 Å from the corresponding XRD peaks deposited at −0.5, −0.6, −0.7, and −0.8 V, respectively. 4. Conclusions Cyclic voltammograms of solution containing CuCl2, AlCl3 and/or SeO2 by individual or combined scans reveal the development mechanism of ternary compound CASe can be demonstrated by the formation of compounds Al2Se3 and Cu2Se via reductions and/or chemical reactions of the precursors. The introduction of complexing agent TEA creates an appropriate environment for co-deposition reaction by effectively chelating with Cu(II) ions among all electroactive species. The chalcopyrite structures of CuAlSe2 were obtained with these deposition processes. The crystallinity of the as-deposited thin films was improved after annealing and it was confirmed by the examination of the

Table 1 XRD and EDX analysis of CASe thin film deposited on In2O3: SnO2 glass substrates. XRD

Fig. 8. AES spectra of as-prepared CASe thin films showing KLL emissions from aluminum and LMM emissions from selenium. Inset shows the LMM emissions from copper.

EDX (at.%)

UV–Vis

Deposition Intensity of FWHM D potential peak (112) (in radians) (nm) (V vs. Ag/AgCl)

Cu

Al

Se

Band gap (eV)

−0.5 −0.6 −0.7 −0.8

31.1 30.7 27.7 26.4

28.9 29.0 28.1 28.6

40.0 40.3 44.2 45.0

2.19 2.18 2.20 2.34

180 250 263 166

0.0048 0.0061 0.0079 0.0157

29.9 23.6 18.1 9.1

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