On the growth process of Cu 2 ZnSn(S,Se) 4 absorber layer formed by selenizing Cu–ZnS–SnS precursors and its photovoltaic performance Jianjun Li a , Yi Zhang a,n , Hongxia Wang b , Li Wu c , Jiguo Wang a , Wei Liu a , Zhiqiang Zhou a , Qing He a , Yun Sun a,n a Institute of Photoelectronic Thin Film Devices and Technology and Tianjin Key Laboratory of Thin Film Devices and Technology, Nankai University, Tianjin 300071, PR China b School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia c The MOE Key Laboratory of Weak-Light Nonlinear Photonics, School of Physics, Nankai University, Tianjin 300071, PR China article info Article history: Received 5 February 2014 Received in revised form 31 July 2014 Accepted 5 September 2014 Keywords: CZTSSe Stacking order Growth properties Selenization Solar cell abstract Sputtering and subsequent sulfurization (or selenization) is one of the methods that have been extensively employed to fabricate Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin films. However, there are limited reports on the effect of precursor stacking order of the sputtered source materials on the properties of the synthesized CZTSSe films. In this work, the morphology and crystallization process of the CZTSSe films which were prepared by selenizing Cu–ZnS–SnS precursor layers with different stacking sequences and the adhesion property between the as-synthesized CZTSSe layer and Mo substrate have been thoroughly investigated. It has been found that the growth of CZTSSe material and the morphology of the film strongly depend on the location of Cu layer in the precursor film. The formation of CZTSSe starts from the diffusion of Cu–Se to Sn(S,Se) layer to form Cu–Sn–(S,Se) compound, followed by the reaction with Zn(S,Se). The investigation of the morphology of the CZTSSe films has shown that large grains are formed in the film with the precursor stacking order of Mo/SnS/ZnS/Cu, which is attributed to a bottom-to-top growth mechanism. In contrast, the film made from a precursor with a stacking sequence of Mo/ZnS/ SnS/Cu is mainly consisted of small grains due to a top-to-bottom growth mechanism. The best CZTSSe solar cell with energy conversion efficiency of 3.35% has been achieved with the selenized Mo/ZnS/ SnS/Cu film, which is attributed to a good contact between the absorber layer and the Mo substrate. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The rare, thus high cost of In and Ga elements which are used in Cu(In,Ga)Se 2 (CIGS) based light absorbing material [1,2] has recently raised significant concern on the production scale of CIGS based thin film solar cells. To solve this issue, a substitute material based on Cu 2 ZnSn(S,Se) 4 (CZTSSe) which uses earth-abundant, thus much cheaper elements Zn and Sn to replace In and Ga in CIGS has received considerable attention since 2009 [3,4]. CZTSSe is derived from CIGS compound and normally adopts kesterite or stannite structure [5,6]. Fundamentally speaking, CZTSSe has a desirable optoelectronic prop- erty for PVs including an optimal band gap of 1.0–1.5 eV [6–8] depending on the ratio of Se/S [9], and a high absorption coefficient larger than 10 4 cm À1 [5,7,9] in visible spectrum range. In practice, CZTSSe film can be made using the approaches that have been applied in CIGS film fabrication, such as co-evaporation, sputtering combined with post-sulfurization or selenization, electro-deposition, spin-coating combined with subsequent annealing, and so on [3,4]. Among them, sputtering is one of the promising methods which are viable for large-scale production of CZTSSe thin films. In sputtering deposition, variable combination of target sources based on either metal or binary metal sulfides have been used to make the precursor for CZTSSe film [10–14]. Katagiri et al. have reported CZTSSe solar cells with power conversion efficiency of 6.77% which were made by co- sputtering Cu, ZnS and SnS precursor [15]. Chawl and Clemens have further improved the power conversion efficiency of CZTSSe solar cells to 9.3% by co-sputtering Cu x (S,Se) y , Zn x (S,Se) y and Sn x (S,Se) y composites [16]. Since different stacking order of the sputtering source materials can be adopted for deposition of the precursor layers, it brings up the question of whether the different stacking order has any impact on the property of the synthesized light absorber material. Fernandes et al. have reported the synthesis of CZTS thin films which were made by sulfurizing the sputtered metal precursors with the stacking order of Mo/Zn/Sn/Cu and Mo/Zn/Cu/Sn. They have found that the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.09.023 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding authors. Tel.: þ86 22 23508572 8018; fax: þ86 22 23508912. E-mail addresses: yizhang@nankai.edu.cn (Y. Zhang), suny@nankai.edu.cn (Y. Sun). Solar Energy Materials & Solar Cells 132 (2015) 363–371