Fabrication of highly crystallized Cu 2 SnS 3 thin films through sulfurization of Sn-rich metallic precursors Haitao Zhang, Meng Xie, Shu Zhang ⇑ , Yong Xiang ⇑ State Key Laboratory of Electronic Thin Films & Integrated Devices, School of Energy Science and Engineering, University of Electronic Science and Technology of China, 2006 Xiyuan Ave., West High-Tech Zone, Chengdu, Sichuan 611731, China article info Article history: Received 6 January 2014 Received in revised form 3 March 2014 Accepted 4 March 2014 Available online 13 March 2014 Keywords: Cu 2 SnS 3 Thin films Solar energy materials Columnar grains Sn loss abstract Cu 2 SnS 3 is an emerging material with great potential for applications in thin film solar cells. However, the conversion efficiency of Cu 2 SnS 3 -based solar cells is mainly limited by the poor crystallization and micro- structure of the absorber layer. Here we report a fabrication route that can form compact, single-phase Cu 2 SnS 3 films through sulfurization of Sn-rich metallic precursors. In the high quality films, deposited both on bare soda lime glass substrates and on Mo-coated glass substrates, columnar grains with width and length of several micrometers extend through the thickness of the films. After sulfurization, the com- position of sulfide films obtained from Sn-rich precursors self-adjusts to the stoichiometry of Cu 2 SnS 3 . Furthermore, the impurity phases and pores in films can be inhibited efficiently by using precursors with Cu/Sn ratio close to 2. The band gap energy value of the fabricated Cu 2 SnS 3 thin film is determined to be 1.35 eV, suggesting the application potential of this thin film in solar cells. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction The compound semiconductor Cu 2 ZnSnS 4 (CZTS) has attracted widespread attention as a promising candidate for the next gener- ation of thin film solar cells, and solar cells based on Se-substituted CZTS have achieved the record conversion efficiency of 12.6% [1]. However, binary or ternary impurity phases are formed easily dur- ing the fabrication of CZTS, because the quaternary compound is only stable in a small region of chemical potential [2,3]. In addition, SnS x tends to re-evaporate from CZTS during high temperature annealing due to its high vapor pressure, which also induces the formation of impurity phases [4,5]. Therefore, fabrication of high- quality CZTS thin films is of great challenge [6]. On the other hand, the ternary compound Cu 2 SnS 3 has a structure similar to CZTS, and is also considered a promising candidate for thin film solar cell ab- sorber materials due to its suitable band gap energy (in a range of 0.96–1.35 eV dependent on different crystal structures [7–10]) for single-junction solar cells [11], adequately high absorption coeffi- cient (>10 4 cm À1 ) [7–9], and environmentally friendly compo- nents. The stable range of Cu 2 SnS 3 is the widest in the chemical potential phase space of the Cu–Sn–S system [12]. Additionally, Cu 2 SnS 3 has an excellent conductivity (3.43 S/cm at room temper- ature) and supercell crystal structure, which also leads to their application in lithium batteries [13–15]. Cu 2 SnS 3 has been prepared through various methods, such as evaporation [16,17], sputtering [8], electrodeposition [18], succes- sive ionic layer adsorption and reaction (SILAR) [19], solvothermal synthesis [15,20,21], spray pyrolysis [22], and solid reactions [9,23,24]. Although pure Cu 2 SnS 3 can be synthesized through most of these methods, the highest conversion efficiency of Cu 2 SnS 3 - based thin film solar cells has only reached 2.92% thus far [17]. One possible reason for this low efficiency is the poor crystalliza- tion and unfavorable morphology of the Cu 2 SnS 3 films. For an ideal absorber layer of solar cells, columnar grains through the thickness of the film are desirable, and they can avoid grain boundaries along the horizontal direction that usually act as recombination centers for minority carriers [25]. Therefore, a high degree of crystalliza- tion is very important for Cu 2 SnS 3 thin films. In this study, we report a sulfurization route that can efficiently prepare highly crystallized and compact Cu 2 SnS 3 thin films with- out impurity phases. Based on the understanding of the chemical and physical changes during the film formation, we optimized the fabrication procedure and achieved control over the composi- tion, phase purity, crystallization, and morphology of sulfurized Cu 2 SnS 3 thin films by simply adjusting the Cu/Sn ratios of the precursors. http://dx.doi.org/10.1016/j.jallcom.2014.03.014 0925-8388/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding authors. Tel.: +86 28 61831556; fax: +86 28 61831080. E-mail addresses: shuzhang@uestc.edu.cn (S. Zhang), xiang@uestc.edu.cn (Y. Xiang). Journal of Alloys and Compounds 602 (2014) 199–203 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom