This work reports one low-priced ways for production of CuFeS₂ films on conductive glass surface. CuFeS₂ films were prepared by electrodeposition and post-sulfurization treatment at 220 ^oC for 30 h in N₂ atmosphere for the first time. CuFeS₂ films were prepared by electrodeposition with sulfate, ascorbic acid, complexing agent sodium citrate as raw materials. When the deposition potential is -1.0 V and the deposition time is 20 min, CuFeS₂ thin films with good phrase formation can be obtained at pH 4 and 0.02M Na₂S₂O₃·H₂O. The phase of samples were characterized using X-ray diffraction (XRD). The morphology of samples were characterized via scanning electron microscopy (SEM). The composition of samples were characterized by energy dispersive spectrometer (EDS). The crystallinity of CuFeS₂ thin films prepared under these conditions are relatively good. The microcosmic morphology of the samples is flaky crystal. It is shows that post-sulfurization method is helpful for the phase formation of CuFeS₂ films electrodeposited.

Keywords: CuFeS₂, Thin film, Photoelectric, Electrodeposition, Solar energy

Recently, Chalcopyrite CuFeS₂ was classified as indispensable and multi-functional materials [1] due to their abundance [2], their higher conversion efficiency [3], higher absorption coefficients [3] and lower toxicity [2, 3] compared with other semiconducting materials. It was found to be a semiconductor with a zero or narrow band gap [4]. It has been considered as a prospective material with wild applications including energy storage, solar energy generation [5-8]. The preparation methods of optoelectronic materials including chemical co-reducton [9], vacuum evaporation [10], solution-chemical method [11-13], hydro-thermal method [3, 5, 8, 14], electrodeposition [15], flash evaporation technique [4], high energy ball milling [16, 17], thermal-injection synthesis [2] and so on.
Se Hoon Kim studied the influence of surface roughness on the efficiency of a flexible organic solar cell [18]. At the same time, Cuan-Lin Chiu researched the influence of doping iron ions into Cu(In,Ga)Se films [19]. Thin-film batteries also has been studied for a long time. Kegao Liu has prepared many kind of absorption layer films, such as CuInSe₂ [20], CuInS₂ [21] and ZnS [22] successfully, for example. As a kind of ternary optoelectronic material, CuFeS₂ has strict technological requirements in the preparation process, and the process parameters of electrodeposition have great influence on the properties of CuFeS₂ preparation [15], so it is necessary to explore an optimum techno-   logical condition.
Eelectrodeposition method has the advantages of non-vacuum and low cost, the deposition process can be carried out at room temperature, and a uniform and dense film can be prepared [15]. The thin film can be crystallized by the sulfurization treatment, and the film is crystal structure of chalcopyrite. The electrodeposited CuFeS₂ is often prepared by sulfurization treatment to improve the composition and crystallization, thus improving the photoelectric performance of the film.
This work used CuSO₄·5H₂O, (NH₄)₂Fe(SO₄)·6H₂O, and Na₂S₂O₃·H₂O as the basic raw material, CuFeS₂ thin films were prepared by using the constant potential method. The effects of the pH value and the con-   centration of Na₂S₂O₃·H₂O on the phase formation and morphology of the films were investigated and characterized.

Solution preparation: 0.01 M CuSO₄·5H₂O, 0.01 M (NH₄)₂Fe(SO₄)·6H₂O, 0.01 M ascorbic acid, 0.01 M complexing agent sodium citrate and a certain con-   centration of Na₂S₂O₃·H₂O. The CuFeS₂ thin films were electrodeposited at different pH and concentra-   tions of Na₂S₂O₃·H₂O on the FTO-coated glass surface. The deposition time was 20 min and deposition potential was -1.0 V. The films were prepared via using electrodeposition and post-sulfurization at 220 ^oC for 30 h in an N₂ atmosphere. The electrodeposition of thin films was obtained via the PARSTAT 2273 electroche-mical workstation. All the experiments can be guaranteed to be variable unique.
The crystal structure of samples were analyzed by the Bruker D8 Advance XRD system with Nifiltered Cu-Kα (λ=1.5059 Å). The samples’ surface morphology were analyzed by using scanning electron microscope (SEM) with a model of JSM-7610F made by Japan Electronics Co., Ltd. The composition of the film was detected by using energy dispersive spectrometer (EDS) with a model of JSM-7610F made by Japan Electronics Co., Ltd. The resistivity was measured by Four Probe Resistivity Meter.

The effect of pH of system on phase formation and morphology of CuFeS₂ thin films
The crystal structures of the synthesized products were characterized by XRD. Fig. 1 shows the XRD patterns of CuFeS₂ thin films deposited on FTO substrate. FTO substrate is one kind of conductive glass. Through the phase retrieval, it can be seen that there are three peaks at 2θ angles with 29°, 48° and 57°. These three peaks are matched with the three strong peaks of copper iron sulfide. The peaks correspond to (112), (204) and (312) crystal planes of the chalcopyrite structured CuFeS₂ (Card No.74-1737) at pH 4, which is in consistent with other reports [5]. Additional peaks resulting from FTO (mostly SnO₂, Card No.46-1088) have been identified as F. Each phase of the sample contains a glass substrate. It may account for the thickness of the film. It can also be observed from the Fig. 1 that the impurity phase CuS (Card No.75-2233) appears in the other samples except the sample with pH 4. Only at pH 4 can the pure phase of the target product be obtained. Therefore, the optimal pH value of the system is 4.
Fig. 2 shows the morphology of the synthetic CuFeS₂ prepared by electrodeposition after sulfurization. It was found that the surface of the film was relatively uniform, the degree of crystallization was relatively good, and the particles were relatively small at pH 4. It can be seen the microcosmic morphology of the samples is flaky crystal. The thickness of the thin film varied from 1 to 5 μm.
For the above study, it is concluded that the structure and morphology of films depend on pH mostly.

The effect of concentration of Na₂S₂O₃·H₂O on phase formation and morphology of CuFeS₂ thin films
The crystal structures of the synthesized products were characterized by XRD. Fig. 3 shows XRD patterns of CuFeS₂ thin films with different concentration of Na₂S₂O₃·H₂O after sulfurization. It can be seen that there are three diffraction peaks at 2θ angles with 29°, 48° and 57°, except that the concentration of Na₂S₂O₃·H₂O is 0.00 M. These three peaks are matched with the three strong peaks of copper iron sulfide. The diffraction peaks correspond to (112), (204) and (312) crystal planes of the chalcopyrite structured CuFeS₂ (Card No.74-1737), additional peaks resulting from FTO (mostly SnO₂, Card No.46-1088) have been identified as F. It can also be observed from the Fig. 3 that the impurity phase CuS (Card No.74-1234) appears in sample of 0.01 M and 0.03 M. It is concluded that the purity phase could be prepared when sample’s concentration of Na₂S₂O₃·H₂O is 0.02 M. Therefore, the optimal concentration of Na₂S₂O₃·H₂O is 0.02 M.
Fig. 4 shows the morphology of the synthetic CuFeS₂ prepared by electrodeposition after sulfurization. It was found that all the samples showed flake products except that the concentration of Na₂S₂O₃·H₂O is 0.00 M. In order to determine the composition of the flake product further, the energy spectrum of the flake target product was detected.
From Fig. 5, it was found that the ratio of copper - iron - sulfur atom was roughly in accordance with the atomic ratio of the target product 1:1:2. The thin film’s resistivity is 2.016 Ω·cm.

In summary, CuFeS₂ films has been prepared successfully on conductive glass surface by two-step approach: electrodeposition and post-sulfurization. CuFeS₂ films were prepared by electrodeposition and post-sulfurization treatment at 220 ^oC for 30 h in N₂ atmosphere. There are four highlights in this work. The first one, when deposition potential is -1.0 V and the deposition time is 20 min, CuFeS₂ thin films can be obtained at pH 4 and 0.02 M Na₂S₂O₃·H₂O. Secondly, the crystallinity of CuFeS₂ thin films prepared under these conditions are relatively good. The microcosmic morphology of the samples is flaky crystal by SEM. Furthermore, the sample’s phase has been confirmed as chalcopyrite structure. At the same time, the ratio of copper - iron - sulfur atom by EDS was roughly in accordance with the atomic ratio of the target product. These results are meaningful for proving that post-sulfurization is a useful measure for preparation of CuFeS₂ films. 

This work was financially supported by the National Natural Science Foundation of China (No.51272140) and the Innovation Team of the Co-Innovation Center for Green Building of Shandong Province in Shandong Jianzhu University.