Reduction of secondary phases in Cu 2 SnSe 3 absorbers for solar cell application Zeguo Tang a,⇑ , Yuki Nukui b , Kiichi Kosaka b , Naoki Ashida b , Hikaru Uegaki b , Takashi Minemoto b a Ritsumeikan Global Innovation Research Organization, Ritsumeikan University, Shiga, Japan b College of Science and Engineering, Ritsumeikan University, Shiga, Japan article info Article history: Received 07 March 2014 Received in revised form 11 April 2014 Accepted 15 April 2014 Available online 24 April 2014 Keywords: Cu 2 SnSe 3 thin films Secondary phases Carrier concentration abstract The creation of secondary phases, such as Cu 2x Se and SnSe, and their influence on electrical properties of Cu 2 SnSe 3 (CTSe) thin films fabricated by selenization of Cu–Sn metal precursors are investigated. The Cu 2x Se content in CTSe films is estimated via deconvolution of grazing incidence X-ray diffraction (GIXRD) patterns, and the results suggest that the Cu 2x Se content increases with the increasing Cu/Sn ratio in metal precursors. We also found that using Se and SnSe mixture powders as Se source is an effective approach to suppress the creation of Cu 2x Se secondary phase. Meanwhile, selective etching of Cu 2x Se is realized by potassium cyanide (KCN) solution. Hall measurement results reveal that the secondary phase of Cu 2x Se rather than SnSe makes major contribution to the high carrier concentration (larger than 10 18 cm 3 ) of CTSe films. The approach to further decrease the carrier concentration in CTSe films is discussed. Ó 2014 Published by Elsevier B.V. 1. Introduction Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin-film solar cells have potential to readily extend the production capacity to more than 100 GW per year due to their earth abundant raw materials compared to Cu(In, Ga)Se 2 (CIGS) thin-film solar cell whose production capacity may be limited to several tens of GW per year by reserve of rare metal indium and gallium [1,2]. However, up to now, the top efficiency of CZTSSe thin-film solar cell is 12.6% [3], much lower than the record of CIGS thin-film solar cell [4]. Preparation of pure single-phase CZTSSe thin films is challenging owing to the narrow phase stability region and a variety of secondary phases, i.e. Cu 2 SnS 3 (CTS), Cu 2 SnSe 3 (CTSe), ZnS, ZnSe, etc., are formed during the fabrication of CZTSSe thin films [5–7]. Considering the expectation that synthesis of ternary compound is much easier than that of quaternary, it is worthwhile exploring the feasibility of CTS and CTSe as absorber layer for solar cell. Additionally, to further improve the efficiency for advancing the full potential of thin-film solar cell, tandem configurations are required and the band gap of bottom layer is desired to be smaller than 1 eV. CTSe thin film is considered as a potential candidate for bottom absorber of tandem solar cells due to its suitable band gap (E g  0.8 eV) and large optical absorption coefficient of 10 4 –10 5 cm 1 [8,9]. However, the working CTSe thin-film solar cell has not realized yet. There are some literatures to report the fabrication of CTSe thin films via various methods. Kim et al. [10] fabricated CTSe films by co-evaporation at different temperature and XRD results suggested that the secondary phases, such as SnSe, SnSe 2 and Cu 2x Se, existed in CTSe films. Meanwhile, the Hall measurement results revealed that all the films present p-type conductivity with carrier concen- tration in the order of 10 17 –10 21 cm 3 while mobility in the range of 6.3–14 cm 2 V 1 s 1 . Kuo et al [11,12] prepared CTSe thin films by D.C. sputtering using CTSe targets with different composition ratios. Similarly, peaks originated from secondary phases were observed in XRD patterns and the Hall measurement results revealed p-type conductivity with carrier concentration of 10 19 cm 3 while mobility changes from 2.86 to 10.87 cm 2 V 1 s 1 . According to aforementioned studies, the main problem of CTSe thin film is attributed to the higher carrier concentration (more than 10 18 cm 3 ), probably caused by the secondary phase of Cu 2x Se, where its intrinsic carrier concentration is in the order of 10 21 cm 3 [13]. The reported value of carrier concentration for SnSe thin films is in the order of 10 16 cm 3 and the other Sn-related secondary phase of SnSe 2 presents n-type conductivity [14–16]. Thus, the reason of high carrier concentration in CTSe films is most feasibly attributed to the existence of Cu 2x Se and its intrinsic feature. There is little work to report the relationship between secondary phases and electrical properties in CTSe films. In this work, CTSe thin films are fabricated by two-step processes http://dx.doi.org/10.1016/j.jallcom.2014.04.098 0925-8388/Ó 2014 Published by Elsevier B.V. ⇑ Corresponding author. Tel./fax: +81 77 561 4836. E-mail address: tangzg@fc.ritsumei.ac.jp (Z. Tang). Journal of Alloys and Compounds 608 (2014) 213–219 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom