Investigation of Mg dopant in Cu 2 SnSe 3 thin films for photovoltaic applications Albert Daniel Saragih, Dong-Hau Kuo * Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan article info Article history: Received 29 January 2016 Received in revised form 2 May 2016 Accepted 8 May 2016 Available online 15 May 2016 Keywords: Co-sputtering Thin films Cu 2 SnSe Electrical property abstract Mg-doped Cu 2 SnSe 3 films were prepared by DC magnetron co-sputtering at room temperature for 1 h with two different targets of Cu and Sn or Cu-Mg and Sn based upon the formula of (Cu 2x Mg x )SnSe 3 at x ¼ 0, 0.1, 0.2, and 0.3, abbreviated as Mg-x-Cu 2 SnSe 3 , or the [Mg]/([Cu]þ[Mg]) composition ratios in the Cu-Mg target at 0, 0.05, 0.1, and 0.15, followed by a selenization procedure at 550  C under the Se at- mosphere. Mg-doped Cu 2 SnSe 3 films were a cubic structure. The direct optical band gaps of Mg-x-CTSe films at x ¼ 0, 0.1, 0.2, and 0.3 were estimated to be 1.19 eV,1.18 eV,1.19 eV, and 1.21 eV, respectively. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped Cu 2 SnSe 3 as a function of dopant concentration. Mg-x-CTSe films showed p-type at x ¼ 0 and 0.1 and n-type at x ¼ 0.2 and 0.3. The explanation based upon the Mg-to-Cu antisite donor defect for the change in electrical property was declared. Mg doping in controlling the electrical properties of CTSe films is fulfilled, as we did for its bulks. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The fastest growing sector of the photovoltaic (PV) market is based on ‘thin film’ PV technologies. In the most part, the thin film materials are p-type semiconductor compounds with the major systems being Cu(In,Ga)(S,Se) 2 (CIGSSe), CuInS 2 (CIS) and CdTe. It has been demonstrated as an effective approach to prepare high efficiency CIGS thin-film solar cells with efficiency of over 20% [1e3]. Meanwhile, other promising absorbers like Cu 2 (Zn,Sn)(S,Se) 4 (CZTSSe) for thin-film solar cells are also prepared by selenization [4e7]. Cu 2 SnSe 3 (CTSe) thin film is considered as a potential candidate for bottom absorber of tandem solar cells due to its suitable band gap of 0.7e1.7 eV and high absorption coefficient of 10 4 e10 5 cm 1 [8e10]. According to G. Marcano et al. [8] in the undoped CTSe sample with Se-rich and Cu-poor, based on defect chemistry considerations, selenium interstitials Se i , copper va- cancies V Cu , and Sn atoms in Cu sites Sn Cu are expected to be the dominant defect species. V Cu is at a single and Se i at a double acceptor state in Cu 2 SnSe 3 . On the other hand, Sn Cu , which is ex- pected to be a triple-charged donor, is probably the compensating donor whose concentration was estimated from the electrical measurements. G. S. Babu et al. [11] explained that for the undoped CTSe thin film, with the Cu-poor and Se-rich, the possible defect levels are selenium interstitials (Se i ), copper vacancies (V Cu ), and tin on copper sites (Sn Cu ). The acceptor state with the low activation energy is attributed to the defect level Se i . The acceptor state with the high activation energy is attributed to V Cu  . Various methods are reported for the preparation of CTSe in the forms of thin film. CTSe thin films have been deposited by using different techniques such as DC magnetron [12] from the targets composed of Cu 2 Se and SnSe 2 [13] and co-evaporation [9,10]. Our study in bulk Mg-x-CTSe had been done to give an expla- nation for structural, compositional characteristics, and electrical properties. Mg-x-CTSe pellets at x ¼ 0, 0.05, and 0.1 exhibited p- type conductivity and they were n-type at x ¼ 0.15 and 0.2 [14]. In this work, we intend to follow our bulk work to demonstrate the feasibility of Mg doping in changing the electrical properties of CTSe thin films from higher conductivity to lower conductivity for better photovoltaic application. The fabrication of Mg-doped CTSe thin films with different Mg ratios is quite different from that of Mg-doped CTSe bulks, which can have better composition control through the powder mixing. However, the Mg-doped CTSe films involving four components are difficult to be prepared with the designed composition by vacuum deposition techniques. We executed with a two-target sputtering method to form the Mg-Cu- Sn metallic thin films, following with a selenization procedure to * Corresponding author. E-mail address: dhkuo@mail.ntust.edu.tw (D.-H. Kuo). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2016.05.085 0925-8388/© 2016 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds 683 (2016) 542e546