Effect of [Al] and [In] molar ratio in solutions on the growth and microstructure of electrodeposition Cu(In,Al)Se2 films

Applied Surface Science 273 (2013) 723–729

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Effect of [Al] and [In] molar ratio in solutions on the growth and microstructure of electrodeposition Cu(In,Al)Se2 films Kuo-Chan Huang, Chien-Lin Liu, Pin-Kun Hung, Mau-Phon Houng ∗ Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan 701, Taiwan

a r t i c l e

i n f o

Article history: Received 15 January 2013 Received in revised form 25 February 2013 Accepted 26 February 2013 Available online 6 March 2013 Keywords: Cu(In,Al)Se2 Cyclic voltammerty Surface morphology Optical energy band gap

a b s t r a c t In this paper, the cyclic voltammetric studies were used to realize the element’s reduction potential and chemical reaction mechanism for presuming the formation routes of quaternary Cu(In,Al)Se2 crystals. Thereafter, the prior adjustment of deposited potential from −0.6 V to −1.0 V can be identified a suitable potential as co-electrodeposition. The material characteristics of Cu(In,Al)Se2 films are dominated by the percentage of aluminum content. Thus, the influence of aluminum and indium concentrations in solutions on the percentage composition, surface morphology, structural and crystal properties, and optical energy band gap of Cu(In,Al)Se2 films were investigated. Energy dispersive X-ray spectroscopy (EDS) indicated that the ratio of Al to (Al + In) in Cu(In,Al)Se2 films varied from 0.21 to 0.42 when adjusting aluminum and indium concentrations in solutions. Scanning electron microscopy (SEM) shows that the surface morphology changed from round-like structures into cauliflower-like structures and became rough when the aluminum concentration increased and indium concentration decreased in solutions. X-ray diffraction (XRD) patterns revealed three preferred growth orientations along the (1 1 2), (2 0 4/2 2 0), and (1 1 6/3 1 2) planes for all species. The (˛h)2 versus h plots (UV–Visible) shows that the optical energy band gap of the Cu(In,Al)Se2 films can be successfully controlled from 1.17 eV to 1.48 eV by adjusting the aluminum and indium concentrations. Furthermore, the shift of the (1 1 2) peak in the XRD patterns and variation of optical band gap are evidence that the incorporation of aluminum atoms into the crystallitic CuInSe2 forms Cu(In,Al)Se2 crystals. © 2013 Elsevier B.V. All rights reserved.

1. Introduction CuInSe2 (CIS) and related chalcopyrite-based solar cells are leading candidates for low-cost and high-efficiency thin film solar cells. However, a band gap of approximately 1.04 eV for CuInSe2 material is less than the optimum band gap value (approximately 1.4 eV) for the solar spectrum. The energy band gap (Eg ) of the CIS material can be increased to match the solar spectrum by alloying with III or VI elements. The energy band gap of CuIn1−x Gax Se2 (CIGS) material can be controlled in the range of 1.04 eV (CuInSe2 ) to 1.7 eV (CuGaSe2 ) by changing the Ga/(Ga + In) ratio from 0 to 1. A 19.9% efficiency was shown using a small-area CIGS absorber layer with an energy band gap of approximately 1.2 eV [1]. The highest efficiency of CIGS solar cells (20.3%) was recently reported by the German Center for Solar Energy and Hydrogen Research [2]. Nevertheless, Ga is an expensive rare material and has a device efficiency performance degradation with a great band gap (Eg > 1.3 eV) for CIGS solar cell. This phenomenon was attributed to a higher Ga

∗ Corresponding author. Tel.: +886 6 275 7575x62342; fax: +886 6 234 5482. E-mail address: [email protected] (M.-P. Houng). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.121

addition, which increases the defect density, leading to losses in open circuit voltage and current recombination [3,4]. CuIn1−x Alx Se2 (CIAS) was considered an alternative candidate to wider band gap CuIn1−x Gax Se2 solar cells because it requires less aluminum (Al) than gallium (Ga) to achieve a similar band gap. This indicates that the change in lattice constant with Al alloying is smaller than Ga alloying for a similar band gap [3,5]. Fewer changes in the structural properties of CuInSe2 can prevent excess defects and the compressive stress effect. The band gap of CIAS material can be controlled from 1.04 eV (CuInSe2 ) to 2.67 eV (CuAlSe2 ) by changing the Al/(Al + In) ratio from 0 to 1 [6]. CIAS films have been prepared using several techniques including co-evaporation [3,4,7], sputtering [8], and selenization [9,10]. The highest efficiency of 16.9% for CIAS solar cells was demonstrated using the co-evaporation method [11], but such vacuum techniques are difficult to scale because of high manufacturing costs and low material utilization efficiencies. The electrodeposition technique is an appealing method that offers non-vacuum production, low costs, low temperature processes, and low material waste for large-area thin film fabrications. The one-step electrodeposition technique is the most promising electrochemical method for fabricating CIS-based films such as CIS, CIGS, and CIAS for solar cell application. However, the composition

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The XRD patterns of the CIAS films were obtained using a Rikagu Ultima IV diffractometer with Bragg-Bretano focusing ˚ radiation. The 0.02◦ step size geometry and CuK␣ ( = 1.54056 A) 2 scans were used to identify the phases present in the CIAS films. Raman measurements were performed at room temperature in a backscattering configuration using a Jobin Yvon T6400 micro-Raman spectrometer with a 633 nm laser as the light excitation source. The optical characterizations of the CIAS films were measured using a UV–VIS spectrometer (Hitachi U4100) with an integrating sphere. 3. Results and discussion Fig. 1. Cyclic voltammograms of water and unitary Cu, In, Al and Se systems.

of CIS-based films is difficult to control in solutions for singlepotential deposition. The variation of electrolyte concentrations is limited because of the correspondent plating potential of each element. The reduction potentials of each element in solutions are determined by cyclic voltammetric research. Previous studies [12–14] have shown that the reduction potentials of Cu2+ and H2 SeO3 are more positive than III3+ (III = In, Ga, Al), which indicates that the deposition of Cu and Se is easier than that of In, Ga, and Al for CIS-based films. Therefore, the electrolyte concentrations are often adjusted to contain a lower Cu2+ concentration or higher III3+ concentration. In addition, the complexing agents are added into electrolyte to change the reduction potential of each element to obtain a better co-electrodeposition environment for achieving the desired Cu/III ratio (approximately 1:1) to form CIS-based films. There are few studies have fabricated CIAS films with the electrodeposition technique. Electrochemical behaviors during the one-step electrodeposition of the quaternary CIAS system were studied in this study by using cyclic voltammetric research. Moreover, this study investigated the effect of Al and In concentrations in precursor solutions and variation of the Al/(Al + In) composition ratio in CIAS films on the characteristics of CIAS films. 2. Experiment Cyclic voltammetry and electrodeposition of CIAS films were performed potentiostatically in a three-electrode cell configuration. The working electrode was a Mo-coated soda lime glass substrate, the reference electrode was a saturated calomel electrode (SCE), and the counter electrode was a platinum plate. Substrates were ultrasonically cleaned in acetone, methyl alcohol, and deionized water, and then purged with nitrogen gas before the CIAS electrochemical experiments. The precursor chlorine-based solutions of 2–2.5 mM CuCl2 , 2–5 mM InCl3 , 5–12.5 mM AlCl3 , 5 mM H2 SeO3 , and 5 mM sodium dodecyl sulfate (SDS) were prepared. A Princeton Applied Research potentiostat/galvanostat VersaSTAT 4 was used for the cyclic voltammetric research and CIAS film electrodepositions. The cyclic voltammograms were measured at a scan rate of 10 mV/s in the negative direction of the potential. The potential of the CIAS film electrodepositions was adjusted from −0.6 to −1.0 V versus the SCE electrode to identify an optimal potential for the one-step electrodeposition environment. The Al and In concentrations in the solutions were varied for the electrodeposition of CIAS films with a constant potential for 30 min. The pH values of the solutions were adjusted to 1.5 with diluted HCl. The temperatures of the solutions were maintained at 25 ◦ C, and the solutions were magnetically stirred during electrodeposition. The surface morphology of the CIAS films was examined with a HITACHI SU8000 scanning electron microscope (SEM), and the chemical composition of the CIAS films was determined using energy-dispersive X-ray spectroscopy (EDS).

3.1. Electrochemical behavior of electrodeposition CuIn1−x Alx Se2 films 3.1.1. Cyclic voltammograms of unitary systems Typical cyclic voltammograms of the unitary system blank solution, CuCl2 solution, InCl3 solution, AlCl3 solution, and H2 SeO3 solution are shown in Fig. 1 to define the reduction potential of each element. The position and intensity of the peaks represent the reduction potential and reduction amount of each element, respectively. In the cyclic voltammetric curve of blank solution, there is only one reduction peak at −0.64 V which is attributed to the reduction of H+ to H2 . In the Cu system, two reduction peaks are observed in the cyclic voltammetric curve, and the individual chemical reactions are expressed as Eqs. (1) and (2). The prioritized reduction peak for Cu2+ /Cu+ is observed at −0.01 V, and the second peak for Cu+ /Cu is observed at −0.34 V. In the In system, there is only one indium reduction peak at −0.85 V, which is attributed to the reduction of In3+ to In, as expressed in Eq. (3). Moreover, this similar phenomenon is observed in the cyclic voltammograms of the Al system. The only reduction peak for Al3+ /Al is observed at −1.1 V, and is expressed in Eq. (4). Throughout the potential scanning region of the cyclic voltammograms for each metal element, the findings revealed that the reduction potential of Al is highly negative compared to In and Cu. This result indicates that the electrochemical insertion of Al atoms into CIS films is difficult to accomplish and may explain why the formation of CIAS films by one-step electrodeposition has rarely been reported for several years. Cu2+ + e− ↔ Cu+ +

−

Cu + e ↔ Cu

(1) (2)

3+

−

+ 3e ↔ In

(3)

3+

−

(4)

In

Al

+ 3e ↔ Al

In the Se system, three reduction peaks are observed in the cyclic voltammetric curve. Peak (A) at −0.27 V is attributed to the four electron reductions of H2 SeO3 to Se, as expressed in Eq. (5) [14]. Peak (B) at −0.67 V can be attributed to the six electron reductions of H2 SeO3 to H2 Se, as shown in Eq. (6) [15]. Peak (C) at −0.89 V can be attributed to the reaction between Se and H+ in solution, forming H2 Se, as shown in Eq. (7) [16]. H2 SeO3 + 4H+ + 4e− ↔ Se + 3H2 O

(5)

H2 SeO3 + 6H+ + 6e− ↔ H2 Se + 3H2 O

(6)

Se + 2H+ + 2e− ↔ H2 Se

(7)

3.1.2. Cyclic voltammograms of binary systems Fig. 2 shows the cyclic voltammograms of the binary system CuCl2 + H2 SeO3 , InCl3 + H2 SeO3 , and AlCl3 + H2 SeO3 solutions. The three reduction peaks (A), (B), and (C) are observed in the Cu–Se system. The corresponding chemical reactions of these three reduction peaks are inferences according to the XRD analysis shown in Fig. 3.

K.-C. Huang et al. / Applied Surface Science 273 (2013) 723–729

Fig. 4. Cyclic voltammograms of ternary Cu–Al–Se and Cu–In–Se systems and quaternary Cu–In–Al–Se system.

Fig. 2. Cyclic voltammograms of binary Cu–Se, In-Se and Al–Se systems.

Cu–Se films were deposited at potentials of −0.2, −0.45, and −0.7 V, which corresponded to the positions of the reduction peaks. Peak (A) is attributed to the Cu2+ reaction with H2 SeO3 to form copper selenides (Cu3 Se2 ), as expressed by Eq. (8). Peak (B) is presumed to result from a portion of the copper selenides (Cu3 Se2 ) that transfer into Cu2 Se and CuSe, as expressed in Eq. (9). At this potential region, the copper selenides (Cu3 Se2 , Cu2 Se, and CuSe) coexist in the Cu–Se film. Peak (C) can be attributed to the transformation from CuSe to red Se suspensions according to XRD analysis, as shown in Eq. (10). This chemical reaction is explained by the promotion of the dissolution of the prior product H2 Se by Cu2+ . Thereafter, Cu2+ recombines with the dissolved selenium ions to form CuSe, as shown in Eq. (11) [16]. CuSe reacts with the dissolved H2 SeO3 again to form red Se suspensions in solution. 3Cu2+ + H2 SeO3 + 8H+ + 14e− ↔ Cu3 Se2 + 6H2 O +

(8)

−

Cu3 Se2 + H2 SeO3 + 4H + 4e ↔ Cu2 Se + CuSe + Se + 3H2 O +

2+

2CuSe + H2 SeO3 + 4H ↔ 2Cu H2 Se + Cu

2+

+

↔ CuSe + 2H

+ Se + 3H2 O

725

(9) (10) (11)

Fig. 2 shows the reduction peak of In2 Se3 at −0.72 V, as expressed in Eq. (12), and the reduction peak of Al2 Se3 at −0.85 V, as shown in Eq. (13). This positive shift is similar to the results of previous studies [13,17]. The contribution of In and Ga ions can promote the reduction of H2 SeO3 to H2 Se. Thereafter, H2 Se combines with In and Ga to form In2 Se3 and Ga2 Se3 products in solution because of larger formation free energies. The formation free energies of In2 Se3 and Ga2 Se3 are −386 kJ/mol and −418 kJ/mol [13,17]. The concept of electronegativity indicates that larger electronegativity variations of combined elements cause stronger bonding strengths. The electronegativity of Al, In, Ga, and Se is 1.61, 1.78, 1.81, and 2.5, respectively. Therefore, we infer that Al2 Se3 possesses larger formation free energy than In2 Se3 and Ga2 Se3 . Furthermore, they shift

the reduction potential of Al2 Se3 to a more positive direction and reveal its higher current density. 3H2 Se + 2In3+ ↔ In2 Se3 + 6H+

(12)

+

(13)

3+

3H2 Se + 2Al

↔ Al2 Se3 + 6H

3.1.3. Cyclic voltammograms of ternary and quaternary systems Fig. 4 shows the cyclic voltammograms of ternary systems and quaternary system: CuCl2 + InCl3 + H2 SeO3 and CuCl2 + AlCl3 + H2 SeO3 solutions, and CuCl2 + InCl3 + AlCl3 + H2 SeO3 solution, respectively. Two similar cyclic voltammetric curves are apparent in the two ternary systems, which display three reduction peaks. Peak (A) at approximately −0.23 V is attributed to Cu2+ combining with H2 SeO3 to form Cu3 Se2 . Peak (B) at approximately −0.61 V is attributed to the transformation of Cu3 Se2 to Cu2 Se and CuSe. Peak (C) at −0.72 and −0.78 V are attributed to the reduction of In2 Se3 and Al2 Se3 , respectively. Integration of our ternary cyclic voltammetric research, In and Al elements incorporate into films through the formation of In2 Se3 and Al2 Se3 precursor and then reacted with Cu3 Se2 , Cu2 Se and CuSe to form CuInSe2 and CuAlSe2 phase. This study assumes that the formation of CuInSe2 films may proceed as one of the following reactions: 2Cu3 Se2 + 3In2 Se3 ↔ 6CuInSe2 + Se

(14)

Cu2 Se + 2CuSe + 2In2 Se3 ↔ 4CuInSe2 + Se

(15)

Moreover, the formation of CuAlSe2 films may proceed as one of the following reactions: 2Cu3 Se2 + 3Al2 Se3 ↔ 6CuAlSe2 + Se

(16)

Cu2 Se + 2CuSe + 2Al2 Se3 ↔ 4CuAlSe2 + Se

(17)

In the quaternary system, the cyclic voltammetric curve shows similar reduction peaks to the ternary system. Peak (a) at −0.24 V and peak (b) at −0.35 V corresponded to the reduction of Cu3 Se2 , Cu2 Se, and CuSe, respectively. Peak (c) at −0.76 V is attributed to the simultaneous reduction of In2 Se3 and Al2 Se3 . The formation of CuInAlSe2 films may proceed as one of the following reactions: 2Cu3 Se2 + 3xAl2 Se3 + 3(1 − x)In2 Se3 ↔ 6CuIn(1−x) Alx Se2 + Se (18) Cu2 Se + 2CuSe + 2xAl2 Se3 + 2(1 − x)In2 Se3 ↔ 4CuIn(1−x) Alx Se2 + Se

Fig. 3. XRD patterns of Cu–Se thin films deposited with various potentials.

(19)

The Cu3 Se2 , Cu2 Se and CuSe are prior reacted with In2 Se3 and Al2 Se3 to simultaneously form CuInSe2 and CuAlSe2 phase, subsequently CuInSe2 and CuAlSe2 phase incorporated together to form CuInAlSe2 films. The x value (range: 0–1) is related

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to the better quality of CIAS films and absence of the Cu element at more negative potentials. Therefore, this study maintains the electrodeposited potential at −0.8 V to investigate the following experiments. 3.2. Characteristics of CIAS films fabricated using various Al and In concentrations in solutions

Fig. 5. XRD patterns of CIAS thin films with various potentials: (a) −0.6 V, (b) −0.7 V, (c) −0.8 V, (d) −0.9 V and (e) −1.0 V.

to co-electrodeposition parameters such as deposited potential, deposited time, and ion concentration. According to these cyclic voltammetric studies, the possible chemical reactions and formation potential of quaternary CIAS films were investigated. Thus, this study electrodeposited CIAS films by adjusting potentials from −0.6 to −1.0 V versus the SCE electrode to confirm the feasibility of fabricating CIAS films by using the co-electrodeposition method and discover the optimal electrodeposition potential. The quality and crystallization of the CIAS films are analyzed using the XRD measurement. In Fig. 5, the XRD patterns show the peaks related to the polycrystalline CIAS films with chalcopyrite structures and preferred orientations along the (1 1 2), (2 0 4/2 2 0), and (1 1 6/3 1 2) planes [5,18]. Moreover, the two peaks at 25.2◦ and 40.51◦ individually correspond to the second-phase Cu3 Se2 and Mo substrate. The intensity of the preferred orientation (1 1 2) peak increases, and the second-phase Cu3 Se2 peak gradually decreases as the deposited potential increasing to more negative direction. This phenomenon can be attributed

3.2.1. Composition and surface morphology of CIAS films The influence of ion concentration variations in solution on CIAS film properties was studied and is shown in Table 1. The chemical composition of the CIAS films was obtained using EDS measurements, and the ratio of Al to (Al + In) in the CIAS films was evaluated. According to the results from the cyclic voltammograms, the reduction potentials for Cu and Se are more positive than that of In and Al. Thus, the elementary composition of In and Al is normally less than that of Cu and Se because the deposition of Cu and Se precedes than that of In and Al. In order to increase the percentage composition of Al content in CIAS films, the Al and In concentration in solutions were adjusted to attain a high-Al-content CIAS film. The CIAS samples (a to d) were electrodeposited using increasing Al concentrations from 5 to 12.5 mM at a constant potential of −0.8 V for 30 min. EDS analysis showed that the percentage composition of Al content increased and the ratio of Al to (Al + In) increased as the Al concentration was raised from 5 to 10 mM. When the Al concentration is higher than 10 mM, excess Al ions induce a colloidal state in solution and promote the collection of particles in solution. Therefore, the quality and the percentage composition of Al content in the CIAS film degraded. The appropriate Al concentration in solution was identified from the results of this experiment. Thereafter, the lower In concentration in solutions were adjusted from 4 to 2 mM in the samples (e to g). The In concentration was modified because the Al atom is substituted for the In atom in the crystal place of the CIAS chalcopyrite structure. The composition percentages of Al and In contents in CIAS

Fig. 6. SEM micrographs of CIAS thin films with various aluminum concentrations: (a) 5 mM, (b) 7.5 mM, (c) 10 mM, and (d) 12.5 mM.

K.-C. Huang et al. / Applied Surface Science 273 (2013) 723–729

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Table 1 Different chemical solutions prepared and the corresponding compositional analysis and optical energy band gap of CIAS thin films. Sample

a b c d e f g

Concentration (mM)

Composition (at%)

CuCl2

InCl3

AlCl3

H2 SeO3

Cu

In

Al

Se

2 2 2 2 2.5 2.5 2.5

4 4 4 4 4 3 2

5 7.5 10 12.5 10 10 10

5 5 5 5 5 5 5

22.27 22.0 22.06 20.92 24.46 26.63 27.72

22.06 19.42 19.59 21.43 18.26 16.55 12.34

5.90 7.42 9.11 6.03 7.37 7.75 8.76

49.76 51.16 49.24 51.62 49.91 49.07 51.18

Al/(Al + In)

Eg (eV)

0.21 0.27 0.33 0.22 0.28 0.32 0.42

1.17 1.22 1.35 1.17 1.23 1.35 1.48

films have a tradeoff relationship. The EDS results demonstrate the tradeoff relationship between the In and Al contents. The ratio of Al to (Al + In) is improved as the In concentration decreases in solution. Moreover, the composition percentage of Cu content is affected by modifying the In concentration. Compared with the influence of Al and In concentration in solutions, the ESD results show that the influence of reducing the In concentration is more pronounced than that of increasing the Al concentration in solutions. The surface morphologies of the CIAS films were analyzed with the SEM measurements. Figs. 6 and 7 show the SEM images of the CIAS films under various Al and In concentrations. The SEM images show that the surface morphologies of the CIAS films have round-like structures. For all the species, the thickness of CIAS films electrodeposited at −0.8 V for 30 min is approximately 1 ␮m which observed by the SEM cross-section images (not shown here). Fig. 6(a)–(c) shows that the surface morphologies of the CIAS films are smooth and compact, but the surface of the CIAS films is gradually covered by large particles as the Al concentrations in solutions increase. This phenomenon may occur because higher Al concentrations cause a colloidal state in solution and promote the gathering of ions, forming large particles. Fig. 7 shows that the rough and cauliflower-like structures on the surface morphologies of the CIAS films are apparent as In concentrations in solutions decrease. The rough surface of the CIAS films is attributed to the poor structure and formation of copper selenide from the absent In content and excess Cu content [19]. By integrating EDS analysis and SEM microscopy, this study confirmed that the surface morphologies of the CIAS films are affected by the Al and In concentrations in solution. A typical surface of CIAS films is composed of round-like structures. The surface morphologies of the CIAS films gradually become rough and turn into cauliflower-like structures as the Al concentration increases and In concentration decreases in solution. The excess Al concentration or absent In concentration in solution induces the degradation of the surface morphology and quality of CIAS films.

3.2.2. The structural and crystal analysis of CIAS films Figs. 8 and 9 show the individual XRD patterns of the CIAS films with various Al and In concentrations. They show the three peaks that are associated with the polycrystalline CIAS films with chalcopyrite structures and the growth orientation along the (1 1 2), (2 2 0), and (3 1 2) planes. The peaks at 25.2◦ and 59.3◦ are associated with the second-phase Cu3 Se2 and the peak at 40.51◦ is associated with the Mo substrate in the XRD patterns. The inset in Figs. 8 and 9 shows the enlargement of the preferred orientation (1 1 2) peak, which shifts to higher scattering angles as the ratio of Al to (Al + In) increases. This feature is attributed to the substitution of Al atoms for In atoms in the CIAS structure, which induces the reduction of d-spacing and convinces the incorporation of Al atoms into the CIS lattice to form CIAS crystals [7,20,21]. The lattice constants a and c and the structure parameter  = c/a calculated from the interplanar spacing (d) of (1 1 2) and (2 2 0) reflection with various Al/(Al + In) ratio are shown in Table 2. The

Fig. 7. SEM micrographs of CIAS thin films with various indium concentrations: (a) 4 mM, (b) 3 mM, and (c) 2 mM.

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K.-C. Huang et al. / Applied Surface Science 273 (2013) 723–729

Table 2 The composition and lattice constants of CIAS thin films electrodeposited with various aluminum and indium concentrations in solutions. Sample

a b c d e f g

Composition (at%)

Al/(Al + In)

Cu

In

Al

Se

22.27 22.0 22.06 20.92 24.46 26.63 27.72

22.06 19.42 19.59 21.43 18.26 16.55 12.34

5.90 7.42 9.11 6.03 7.37 7.75 8.76

49.76 51.16 49.24 51.62 49.91 49.07 51.18

0.21 0.27 0.33 0.22 0.28 0.32 0.42

˚ Lattice constants (A) a

c

c/a

5.762 5.746 5.724 5.759 5.743 5.725 5.693

11.55 11.48 11.39 11.53 11.46 11.40 11.32

2.004 1.997 1.989 2.002 1.995 1.991 1.986

Fig. 9 indicated that the crystallization of the CIAS films was significantly influenced by varying the In concentration in solutions. 3.2.3. The optical energy band gap of CIAS films Examining the optical energy band gap is another significant method for confirming the formation of CIAS films. The optical energy band gap of the initial CIS material is 1.04 eV, and it can be modified to higher values through the incorporation of Al atoms into the crystallitic CIS to form CIAS crystals. The optical energy band gap was estimated from (˛h)2 versus h plots, and the absorption coefficient ˛ for CIAS films was calculated using the Manifacier model [22]: ˛= Fig. 8. XRD patterns of CIAS thin films with various aluminum concentrations: (a) 5 mM, (b) 7.5 mM, (c) 10 mM, and (d) 12.5 mM.



1 1−R ln t T



(20)

where t is the thickness of CIAS films, and T and R are optical transmission and reflectance data, respectively (not shown here). The variation of ˛ with h near the main absorption edge of direct bandgap CIAS films is expected to follow relation [23]:

lattice constants a and c are employed to characterize the tetragonal distortion in the x–y direction and z direction, respectively. Furthermore, the structural parameter  indicates the deformation of the unit cell to a length c. It can be seen that the increase in Al concentration and decrease in In concentration correlate with decrease in the lattice constants and structure parameter. Such results are presumably attributed to the replacement of In atom by Al atom, result from the atomic size of Al is smaller than that of In. Fig. 8 shows that three growth orientation peaks do not have apparent changes when the Al concentration is adjusted from 5 to 10 mM. While the Al concentration achieves 12.5 mM, but the intensity of the three peaks decreased because of the colloidal state in solution, which decreases film quality. Fig. 9 shows the broad peaks that correspond to the (1 1 2), (2 2 0), and (3 1 2) planes of the chalcopyrite phase, which reveals that the poor crystallization of the CIAS films is formed in low In concentration solutions. Moreover, copper selenides (Cu3 Se2 ) developed because of the excess Cu content when the In concentration decreased. A comparison of Fig. 8 and

where A depends on the transition nature, effective mass, and refractive index, and Eg is the energy band gap. This relationship indicates that the fundamental absorption edge is attributed to allowed direct transitions between parabolic bands. The optical energy band gap was evaluated by plotting (˛h)2 versus h from Eq. (21) and by extrapolating the linear portion of the spectrum (˛h)2 = h to zero. Figs. 10 and 11 show the dependence of (˛h)2 on h for CIAS films with various Al and In concentrations in solutions. The optical energy band gap of CIAS films was obtained from Figs. 10 and 11, and are listed in Table 1. The optical energy band gap of the CIAS films increased from 1.17 to 1.35 eV when increasing the Al concentrations from 5 to 10 mM in solution (samples a to c). The optical energy band gap decreased to 1.17 eV when the Al concentration

Fig. 9. XRD patterns of CIAS thin films with various indium concentrations: (a) 4 mM, (b) 3 mM, and (c) 2 mM.

Fig. 10. (˛h)2 versus h plots for CIAS thin films with various aluminum concentration.

˛=

A 1/2 (h − Eg ) h

(21)

K.-C. Huang et al. / Applied Surface Science 273 (2013) 723–729

Fig. 11. (˛h)2 versus h plots for CIAS thin films with various indium concentration.

exceeded 12.5 mM in solution (sample d), which is associated with the colloidal state in higher Al concentration solutions and declines the percentage composition of Al content in the CIAS film. Moreover, the optical energy band gap of CIAS films increased from 1.23 to 1.48 eV when the In concentration was reduced from 4 to 2 mM in solution (samples e to g). This enhancement of the optical energy band gap is attributed to the ratio of Al to (Al + In), which is improved from 0.28 to 0.42 as the In concentration is decreased in solution. However, the Al/(Al + In) ratio in the 0.28–0.42 range shows a smaller optical energy band gap value to that of previous studies [3,7,24]. This result occurs because the crystallization and surface morphology of the electrodeposited CIAS films are inadequate compared to that of the vacuum process, which may induce structural defects and the second phase, affecting the main absorption edge of the optical energy band gap. This study reports the successful control of the optical energy band gap of CIAS films from 1.17 to 1.48 eV by adjusting various Al and In concentrations in solution. 4. Conclusion In this paper, cyclic voltammetric studies of quaternary Cu(In,Al)Se2 films are investigated to identify their chemical mechanism in solutions and their suitable potential for coelectrodeposition. The formation of quaternary Cu(In,Al)Se2 films may occur because the pre-products Cu3 Se2 , Cu2 Se, and CuSe react with In2 Se3 and Al2 Se3 , forming the CuInSe2 and CuAlSe2 compound phase. Thereafter, the CuInSe2 compound phase reunites with the CuAlSe2 compound phase and forms the final CIAS crystals. Moreover, the suitable potential for co-electrodeposited CIAS films is −0.8 V, which is defined according to XRD analysis. ESD analysis indicates that the percentage composition of Al content in CIAS films is successfully increased by adjusting the Al and In concentrations in solution. The surface morphology of the CIAS films changes from typical round-like structures to cauliflower-like structures and gradually becomes rough with increasing Al concentration and decreasing In concentrations in solution. However, a smooth and compact surface morphology for CIAS films can be

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obtained with a suitable solution recipe. The XRD patterns show three preferred growth orientations for the CIAS films and the peaks shift toward higher scattering angles, showing the incorporation of Al atoms in CIS lattice to form CIAS crystals. The increase in Al concentration and decrease in In concentration correlate with decrease in the lattice constants and structure parameter. Furthermore, the evaluated optical energy band gap of CIAS films is evidence of the formation of CIAS crystals. The optical energy band gap can be successfully controlled from 1.17 to 1.48 eV by adjusting the Al and In concentrations in solutions. These characteristics of CIAS films show the practicability of the fabricated CIAS films through the co-electrodeposition method. However, the crystallization of CIAS films fabricated using the co-electrodeposition method is less adequate than that of vacuum methods. The posterior enhancement of the crystallization and fabrication of solar cell devices will be studied in the future. Acknowledgment This work was supported by the National Science Council (NSC) of Taiwan under Contract Number NSC-97-2221-E-006-239-MY2. References [1] I. Repins, M.A. Contreras, B. Egaas, J. Scharf, C.L. Perkins, B. To, R. Noufi, Progress in Photovoltaics 16 (2008) 235. [2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Progress in Photovoltaics: Research and Applications 19 (2011) 894. [3] P.D. Paulson, M.W. Haimbodi, S. Marsillac, R.W. Birkmire, W.N. Shafarmana, Journal of Applied Physics 91 (2002) 12. [4] T. Hayashi, T. Minemoto, G. Zoppi, I. Forbes, K. Tanaka, S. Yamada, T. Araki, H. Takakura, Solar Energy Materials and Solar Cells 93 (2009) 922. [5] M. Sugiyama, A. Umezawa, T. Yasuniwa, A. Miyama, H. Nakanishi, S.F. Chichibu, Thin Solid Films 517 (2009) 2175. [6] J. López-García, C. Maffiotte, C. Guillén, Solar Energy Materials and Solar Cells 94 (2010) 1263. [7] S. Yamada, K. Tanaka, T. Minemoto, H. Takakura, Journal of Crystal Growth 311 (2009) 731. [8] D.C. Pernga, J.W. Chena, C.J. Wua, Solar Energy Materials and Solar Cells 95 (2011) 257. [9] J. López-García, C. Guillén, Thin Solid Films 517 (2009) 2240. [10] J.H. Yuna, R.B.V. Chalapathy, J.C. Lee, J. Song, K.H. Yoon, Solid State Phenomena 124 (2007) 975. [11] S. Marsillac, P.D. Paulson, M.W. Haimbodi, R.W. Birkmire, W.N. Shafarmanb, Applied Physics Letters 81 (2002) 7. [12] T.J. Whang, M.T. Hsieh, Y.C. Kao, S.J. Lee, Applied Surface Science 255 (2009) 4600. [13] Y. Lai, F. Liu, Z. Zhang, J. Liu, Y. Li, S. Kuang, J. Li, Y. Liu, Electrochimica Acta 54 (2009) 3004. [14] M. Kemell, M. Ritala, H. Saloniemi, M. Leskela, T. Sajavarra, E. Rauhalh, Journal of the Electrochemical Society 147 (2000) 1080. [15] Y. Lai, F. Liu, J. Li, Z. Zhang, Y. Liu, Journal of Electroanalytical Chemistry 639 (2010) 187. [16] K.K. Mishra, K. Rajeshwar, Journal of Electroanalytical Chemistry 271 (1989) 279. [17] J. Liu, F. Liu, Y. Lai, Z. Zhang, J. Li, Y. Liu, Journal of Electroanalytical Chemistry 651 (2011) 191. [18] B. Kavitha, M. Dhanam, Materials Science and Engineering B 140 (2007) 59. [19] T.P. Hsieh, C.C. Chuang, C.S. Wu, J.C. Chang, J.W. Guo, W.C. Chen, Solid-State Electronics 56 (2011) 175. [20] Y.B.K. Reddy, V.S. Raja, B. Sreedhar, Journal of Physics D: Applied Physics 39 (2006) 5124. [21] E. Halgand, J.C. Bernède, S. Marsillac, J. Kessler, Thin Solid Films 480 (2005) 443. [22] J.C. Manifacier, M.D. Murcia, J.P. Fillard, Thin Solid Films 41 (1977) 127. [23] M.A. Green, Solar Cells: Operating Principles, Technology, and System Applications, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1982. [24] D. Prasher, P. Rajaram, Thin Solid Films 519 (2011) 6252.