Single-step electrodeposition of CIS thin films with the complexing agent triethanolamine
Introduction
Thin film solar cells have been gaining popularity due to the feasibility of manufacturing and CuInSe2 (CIS) is one of the particularly interested objectives [1], [2]. CIS thin film solar cell has been one of the potential candidates to be developed especially in the electroplating solutions in the pursuit of low cost and large scale productions. The absorption range in CuInSe2 for photons with band gap greater than about 1 eV is in the order of 1 μm. This value is comparable to the depletion width of CuInSe2 cells fabricated on substrates with an acceptor concentration of about 1016 cm−3 [3]. Various techniques, such as evaporation [4], [5], [6], sputtering [7], molecular beam epitaxy [8], electrodeposition [9], [10], [11], chemical bath deposition [12], and chemical vapor deposition [13] have been employed for depositing CIS thin films. Some efforts have been dedicated to the development of non-vacuum deposition methods for the preparation of the CIS absorber layer [14], [15], [16]. Among these processes, electrodeposition is a low-cost, low-temperature alternative approach with the capability for large area processing [10], [13], [17]. For all the techniques mentioned above, the control of the stoichiometry of ternary (CIS) materials is a challenging task.
The standard reduction reactions for Cu, In, and Se ions are as the followings (vs. normal hydrogen electrode; NHE) [18]:Cu2+ (aq) + 2e− → Cu (s)In3+ (aq) + 3e− → In (s)H2SeO3 (aq) → HSeO2+ (aq) + OH− (aq)HSeO2+ (aq) + 3H+ (aq) + 4e−→ Se (s) + 2H2O (l)where E is the potential, R the ideal gas constant, F the Faraday's constant, and , , , , , , and refer to the activities of species Cu, Cu2+, In, In3+, Se, HSeO2+, and H+, respectively (the activity is assumed to be 1 in pure solid state).
From the equations above, it is evident that a single-step electrodeposition of CuInSe2 from aqueous solution containing CuCl2, InCl3, and SeO2 is technically demanding since the reduction potential range of these metal ions is considerably large. The following equations can be proposed for the formation mechanism of CIS films. Two stages of reduction occur in Eqs. (8), (9), while the chemical reaction in Eq. (10) depicts the formation of CIS, along with the byproduct chemical reactions in Eqs. (11), (12) [19]:
Cu2+ + Se4+ + 6e− → CuSe2CuSe + 2e− → Cu2Se + Se2−Cu2Se + 2In3+ + 3Se2− → 2CuInSe22In3+ + 3Se2− → In2Se3Se4+ + 2Se2− → 3Se
The use of complexing agents in CIS electrodeposition has been reported [20]. The compositional manipulation of electrodeposited CuInSe2 in 1:1:2 ratio from an aqueous solution containing CuCl2, InCl3, and SeO2 with TEA as the complexing agent has been reported by our group and other authors [21], [22]. In addition to compositional manipulation, in this study the electrodeposition factors such as concentrations of metal ion species and the complexing agent, deposition potential, deposition time, annealing temperature, and annealing time were investigated and are found to be significant parameters in the deposition of CuInSe2 thin films. As the growth mechanism has been investigated [19], [23], it is hopefully expected that the understanding of electrodeposition of CIS in this work can be brought into broader practical applications.
Section snippets
Experimental details
The electrodeposition of CuInSe2 thin films was conducted using an EG&G potentiostat (model Versastat II) in a three-electrode cell; Ag/AgCl as the reference electrode, a platinum plate as the counter electrode, and indium-doped tin oxide (ITO) glass as the working electrode. ITO glass was cleansed in an ultrasonic bath with ethanol solution, rinsed with deionized water, and purged with nitrogen gas [24]. The aqueous solutions for the electrodeposition experiments were prepared with 5–10 mM CuCl2
Deposition potential
CuInSe2 thin films were obtained using a single-step electrodeposition process with various deposition potentials for 600 s from solutions containing 5 mM CuCl2, 22 mM InCl3 and 22 mM SeO2. The XRD patterns and SEM images are shown in Fig. 1, Fig. 2, respectively.
For the deposition potential of −0.5 V, the XRD pattern in Fig. 1(a) shows that there is a chalcopyrite peak among weak signals of CuInSe2, representing a poor crystalline structure. From the EDS investigation, the atomic ratio of the
Conclusions
Thin polycrystalline films of CuInSe2 were grown using single-step electrodeposition onto ITO substrate from an aqueous bath containing CuCl2, InCl3, SeO2, and complexing agent TEA at room temperature. From the experimental results, the solution of 5 mM CuCl2, 22 mM InCl3, and 22 mM SeO2 at pH 1.5 is presumed the optimum composition for electrodeposition of CIS. The deposition is also found conducted best at −0.8 V for 10 min and the annealing process is performed best at 275 °C for 30 min. The X-ray
Acknowledgements
We gratefully acknowledge the financial support of this work by the National Science Council of Taiwan (NSC 101-2113-M-006-003), and the assistance from National Cheng Kung University (D102-32004).
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Both contributed equally as the first authors of this paper.