Elsevier

Current Applied Physics

Volume 16, Issue 2, February 2016, Pages 207-210
Current Applied Physics

Cyclic voltammetry studies of copper, tin and zinc electrodeposition in a citrate complex system for CZTS solar cell application

https://doi.org/10.1016/j.cap.2015.11.017Get rights and content

Highlights

  • The electrodeposition behavior of Cu, Sn and Zn unitary electrolyte were investigated using cyclic voltammetry.

  • The reduction potentials of Cu, Sn and Zn were changed in citrate complex electrolytes.

  • In citrate complex system and pH 4.7, the reduction potentials of Cu, Sn and Zn were changed from −0.2 V, −0.5 V, −1.2 V to −0.5 V, −0.7 V, −0.7 V respectively.

  • The complexing agent could be narrow the reduction potential, so it is possible to make Cu–Sn–Zn precursors easily in single bath.

Abstract

Cu2ZnSnS4 (CZTS) has attracted considerable attention as the next generation thin film solar cell to replace CIGS because of its price and availability. The electrodeposition method is one of the fabrication methods. The reduction behaviors of each Cu, Sn and Zn from the unitary system were examined. Cyclic voltammtry (CV) was performed to analyze the behaviors. Trisodium citrate was used as the complexing agent to reduce the difference in the reduction potentials of each material. The effects of pH on the stability of the complexes were also investigated and pH 4.7 was selected to minimize the concentration of H3Cit and Cit3−. The reduction potential of Cu was lowered from −0.2 V (vs. Ag/AgCl) to −0.5 V. The reduction potential of Sn was lowered from −0.5 V (vs. Ag/AgCl) to −0.7 V. The reduction potential of Zn was changed from −1.2 V (vs. Ag/AgCl) to −0.7 V. The change in reduction potential in a complex system can allow the fabrication of CZTS thin films from a Cu, Sn and Zn mixed single bath using an electrodeposition method.

Introduction

Cu–In–Ga–S (CIGS) is the most promising absorber materials for high efficiency thin film solar cells to replace expensive silicon solar cells [1], [2]. However, the steady drop of silicon price and the shortage of CIGS consisting materials such as In and Ga, high price and unstable supply due to limited reserve, make CIGS less competitive in photovoltaic industry. In recent years, cost-effective and environmentally-friendly materials have attracted significant attention to replace CIGS thin film solar cells, of which, Cu2ZnSnS4 (CZTS) is one of those materials. Inexpensive and abundant Sn and Zn can replace the expensive and rare In and Ga. The structure of CZTS is similar to that of CIGS; the band gap energy of CZTS is 1.4–1.5 eV and the optical absorption coefficient is higher than 104 cm−1. CZTS is quite suitable for thin film solar cells [3]. Many methods are used to prepare thin film solar cells, which are divided mainly into vacuum and non-vacuum processes [4]. Sputtering, evaporation and pulse laser deposition are categorized as vacuum processes [5], [6], [7]. The advantages of vacuum processes are easy control of the film composition, surface morphology and suitability for multi-layer deposition. On the other hand, the expensive equipment and source materials are limitations. Therefore, those processes are unsuitable for the mass production of thin film solar cells. Spray pyrolysis, sol–gel and electrodeposition are categorized as non-vacuum processes [8], [9], [10], [11], [12], [13], [14], [15]. They are inexpensive and suitable to the mass production; however, in contrast to vacuum process, the uniformity of composition and surface morphology are difficult to control. The electrodeposition method is an inexpensive process for preparing thin film solar cells. The electrodeposition of CZTS is carried out in a Cu, Sn and Zn mixed solution but it is extremely difficult to control the composition of thin films. The final goal of this study is the fabrication of CZTS using electrodeposition methods and proper selection of the electrolyte is the key. The difference in the reduction potentials of Cu, Sn and Zn are more than 1 V, and it is very difficult to deposit those materials from a single bath. On the other hand, the difference in the reduction potentials of Cu, Sn and Zn can be reduced using proper complexing agents. Complexing agents can change the deposition potentials as well as prevent the precipitation of bare ions in less acidic electrolytes by forming ionic complex species in the electrolytes. However, the studies of the reduction potentials of Cu, Sn and Zn in complexing systems are unavailable. In this study, the reduction behavior of each material with and without a complexing agent from a unitary bath was investigated. Trisodium citrate was used as the complexing agent for Cu, Sn and Zn [16], [17]. CV was used to investigate the reduction potential of each materials. The effects of pH on the stability of the complex were also investigated.

Section snippets

Experimental procedures

A standard three electrode cell system was used for the CV experiments. A Pt rod with a 2 mm diameter was used as the working electrode, a Pt mesh electrode was used as the counter electrode and an Ag/AgCl electrode was used as the reference electrode. Cupric sulfate, stannous sulfate and zinc sulfate were used as the source of Cu, Sn and Zn, respectively. The concentration of each element was varied from 2.5 mM to 7.5 mM 100 mM LiCl was used as the common electrolyte for all experiments and

Results and discussions

Trisodium citrate was used as the complexing agent for three components. The stable species of citrates were varied with pH. The stability of citrates were investigated with Chem-EQL program and shown in Fig. 1. At low pH most dominant citrate species is H3Cit and it cannot make complex with three components since it forms stable H3Cit in the solution. As pH was increased, H2Cit became dominant followed by HCit2−. At high pH most dominant citrate species is Cit3−. For the stable complex of

Conclusions

CV was used to analyze the reduction behaviors of Cu, Sn and Zn from a unitary system. Trisodium citrate was used as the complexing agent to reduce the difference in the reduction potentials of each material. The pH of the solution affected the stability of the complexes and pH 4.7 was chosen for the citrate complex system to minimize the concentration of H3Cit and Cit3−. The difference in the reduction potentials of each component was narrowed at pH 4.7. The reduction potential of Cu was

Acknowledgments

This work was supported by the National Research Foundation of Korea (NFR) grant funded by the Korea government (MSIP). (No. NFR-2014R1A2A2A01007428).

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