Elsevier

Journal of Alloys and Compounds

Volume 670, 15 June 2016, Pages 289-296
Journal of Alloys and Compounds

A mechanochemical route to single phase Cu2ZnSnS4 powder

https://doi.org/10.1016/j.jallcom.2016.02.058Get rights and content

Highlights

  • Mechanochemical synthesis of phase pure kesterite powder.

  • Determination of the cation distribution using neutron diffraction.

  • Partial disorder for copper and zinc observed.

Abstract

With respect to absorber materials in solar cells, Cu2ZnSnS4 (CZTS) has been a focus of interest in recent years. In this work, a new route leading to single phase CZTS powders is presented. For structural characterization X-ray and neutron powder diffraction measurements were performed. Further structural and compositional analysis of the CZTS powder was carried out by means of X-ray absorption near edge spectroscopy (XANES) and wavelength-dispersive X-ray spectroscopy (WDS). The obtained CZTS powder with an actual composition of Cu2.00(4)Zn1.02(2)Sn0.99(2)S4.00(8) adopts the kesterite-type structure. A detailed cation distribution analysis using the average neutron scattering length method revealed a partial disorder of copper and zinc on the (2c) and (2d) sites.

Introduction

The quaternary chalcogenide Cu2ZnSnS4 recently gained attraction as a prospective absorber material for thin film photovoltaic applications. As it consists merely of earth-abundant, non-toxic and low-cost elements, it would be a suitable alternative to other chalcogenide-based absorber materials such as CdTe or CIGS (CuInxGa(1-x)Se2) that are currently used in thin films. It is a direct band gap p-type semiconductor with an optical band gap energy value of 1.5 eV and has a large absorption coefficient in the order of 10−4 cm−1 [1], [2], [3]. Up to now record efficiencies of CZTS-based thin films reached values up to 8.4 % [4]. Yet, compared to the currently used chalcopyrite materials, efficiencies are significantly lower.

In order to enhance the quality and the efficiency of CZTS thin film photovoltaics it is necessary to gain a deeper insight into the absorber material. Systematic analysis of the semiconductor compound and its structural, chemical, and physical properties has been in the focus of interest in the last few years [5], [6], [7], [8], [9], [10]. Due to the formation of secondary phases it is challenging to prepare phase-pure CZTS powder, which is important for detailed experiments concerning the correlations between structural and electronic properties. Consequently, the main motivation of the here-presented research is the development of a new chemical route for the synthesis of phase-pure stoichiometric kesterite powder and its structural characterization.

CZTS is a quaternary semiconductor belonging to the I2-II-IV-VI4 compound family. It adopts a tetragonal structure that can be derived from the zinc blende type by doubling the c-axis of the cubic sphalerite unit cell and by substituting the cations. A substitution scheme from binary ZnS to ternary Cu2ZnSnS4 is shown in Fig. 1.

The correct crystal structure of CZTS has been a controversial issue. Two main structure types are described for quaternary AI2BIICIVXVI4 compounds, stannite (see Fig. 2a) and kesterite (see Fig. 2b) [11]. In these types the sulfur atoms form a ccp array where half of the tetrahedral voids are occupied by the cation species. The structures are closely connected but differ in the distribution of Cu+, Zn2+, and Sn4+.

First reports on the crystal structure of natural specimen suggested the kesterite-type structure, space group I4¯ (No. 82) [11], with a complete ordering of Cu+ and Zn2+. Cu fully occupies the Wyckoff position 2a (0, 0, 0) whereas remaining Cu and Zn occupy the positions 2c (0, 1/2, 1/4) and 2d (0, 1/2, 3/4), respectively. Thus, the structure can be described by stacking cation layers Cu/Sn Cu/Zn Sn/Cu Cu/Zn Cu/Sn along the c-axis. Recently several groups could confirm that Cu2ZnSnS4 adopts the kesterite-type structure [5], [12], [13].

Using conventional X-ray diffraction methods, Cu+ and Zn2+ are not distinguishable due to their isoelectronic characteristic. As there is a significant difference in the neutron scattering length (bCu = 7.718(4) fm, bZn = 5.680(5) fm), neutron diffraction is the method of choice. Neutron diffraction studies of Cu2ZnSnS4 powder samples confirm the kesterite-type structure, yet report a partial [14] or complete disorder [5] of Cu and Zn on the 2c and 2d positions. Kesterite-type phases exhibiting a statistical distribution of Cu/Zn can be described in space group I4¯2m (No. 121) with Zn and Cu occupying the 4d Wyckoff position (1/2, 0, 1/4) which is called disordered kesterite [15] (see Fig. 2c).

It was also suggested by ab initio calculations [16], [17] that point defects CuZn and ZnCu have very low formation energies, which underlines the possibility for Cu/Zn disorder. Recent studies on CZTS thin films determined the critical temperature for the transition from ordered to disordered kesterite to be at 260 ± 10 °C [18], which is also visible by a kink in the temperature dependent lattice parameter variation [6].

As four elements are present in the material, the formation of secondary phases such as CuS, Cu2S, ZnS, SnS, SnS2 and Cu2SnS3 seems probable. According to theoretical and experimental work [1], [19] the homogeneity region of the CZTS phase in the ternary phase diagram is rather small. Due to its small enthalpy of formation ZnS is likely to form and has been found to have a detrimental influence on solar cell performance [7], [8], [20]. Identification and quantification of the secondary phases ZnS and Cu2SnS3 by X-ray and neutron diffraction is difficult because of diffraction pattern overlap. As reported in Ref. [7] it is possible to identify the most important secondary phases with X-ray absorption spectroscopy (XAS).

In this study we investigate chemical composition and structural properties of a stoichiometric CZTS powder synthesized by a newly developed mechanochemical process. Phase purity and composition was determined by WDS and XANES. X-ray and neutron diffraction measurements were used to identify the crystal structure with a closer look to the cation distribution of the powder sample. Two refinement strategies will be discussed.

Section snippets

Synthesis

The quaternary sulfide with the general formula Cu2ZnSnS4 was prepared by mechanical milling in a Fritsch Planetary Mono Mill PULVERISETTE 6 starting from the corresponding binary sulfides CuS, ZnS, and SnS followed by an annealing step.

Starting chemicals were prepared either by precipitation (CuS), sulfidation of the oxide (ZnS) or solid state reaction of the elements (SnS). Copper monosulfide (CuS) was precipitated from a 0.1 M Cu(NO3)2-solution (Merck, 99.5 %) with H2S (Air Liquide, 99.5 %)

Mechanochemical synthesis and annealing

Attempts to prepare single-phase CZTS have been met with varying levels of success. Up to this time, bulk material of the quaternary sulfide has been usually synthesized by solid state reactions of the pure elements in evacuated and sealed silica ampoules [5] according to the standard technique described in Ref. [28]. Due to the high sulfur vapor pressure it is necessary to apply a defined temperature program and homogenization is obtained by a second annealing step at 750 °C. All these factors

Conclusions

A mechanochemical synthesis route to CZTS was successfully developed. The synthesis process includes a milling step, which is essential to produce phase pure powder, and an annealing step at 500 °C. This reaction temperature is favorable as it is comparable to the temperatures actually used during thin films growth and is close to the real technical process conditions.

WDS measurements confirmed a stoichiometric chemical composition of Cu2.00(4)Zn1.02(2)Sn0.99(2)S4.00(8) and no secondary phases

Acknowledgement

The authors thank C. Behr, PD Dr. R. Milke (FU Berlin), and K. Neldner (HZB) for support with the microprobe analyses. Financial support from the MatSEC graduate school of the Helmholtz Zentrum Berlin (HZB) in cooperation with the Dahlem Research School is gratefully acknowledged.

References (34)

  • S. Schorr et al.

    In-situ investigation of the structural phase transition in kesterite

    Phys. Status Solidi A

    (2009)
  • J. Just et al.

    Determination of secondary phases in kesterite Cu2ZnSnS4 thin films by x-ray absorption near edge structure analysis

    Appl. Phys. Lett.

    (2011)
  • S. Chen et al.

    Defect physics of the kesterite thin-film solar cell absorber Cu2ZnSnS4

    Appl. Phys. Lett.

    (2010)
  • S. Chen et al.

    Abundance of CuZn+SnZn and 2CuZn+SnZn defect clusters in kesterite solar cells

    Appl. Phys. Lett.

    (2012)
  • A.J. Jackson et al.

    Ab initio thermodynamic model of Cu2ZnSnS4

    J. Mater. Chem. A

    (2014)
  • S.R. Hall et al.

    Kesterite, Cu2(Zn,Fe)SnS4, and stannite, Cu2(Fe,Zn)SnS4, structurally similar but distinct minerals

    Can. Mineral.

    (1978)
  • L. Choubrac et al.

    Structure flexibility of the Cu2ZnSnS4 absorber in low-cost photovoltaic cells: from the stoichiometric to the copper-poor compounds

    Inorg. Chem.

    (2012)
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