Development of indium (In) doped SnSe thin films for photovoltaic application Mohan Reddy Pallavolu a , Sujaya Kumar Vishwanath b , Sang Woo Joo a,⇑ a School of Mechanical Engineering and Center for Research Facilities, Yeungnam University, Gyeongsan 38541, Republic of Korea b School of Materials Science & Engineering, Nanyang Technological University, Singapore 639798, Singapore article info Article history: Received 13 August 2020 Received in revised form 18 September 2020 Accepted 19 September 2020 Available online 24 September 2020 Keywords: In-SnSe films Single-phase orthorhombic-SnSe Selenization Photovoltaic application abstract The undoped and indium doped SnSe (In-SnSe) thin films were successfully prepared by the two-stage process (sputtering and rapid thermal annealing process). The physical properties of the prepared thin films were investigated. Crystallographic patterns of the undoped and doped SnSe thin films showed the single-phase orthorhombic-SnSe. The diffraction peaks slightly shifted towards the higher diffraction angles as well as Raman peaks also shifted towards higher Raman shift by doping of indium. Morphology of the In-SnSe films changed significantly by changing the indium percentage. The optical bandgap of SnSe and In-SnSe films was varied in between 1.06 eV and 1.50 eV, and the electrical properties of In- SnSe films were significantly changed with dopant concentration. The obtained physical properties of In-SnSe thin films are suitable for solar cell device fabrication. Ó 2020 Elsevier B.V. All rights reserved. 1. Introduction In recent years, tin mono selenide (SnSe) has attracted attention in photovoltaic applications because of its non-toxic and earth- abundant nature. The high absorption coefficient (>10 4 cm À1 ) and the bandgap (0.9 eV–1.6 eV) are the main features which make SnSe as an excellent absorber in thin-film solar cells [1,2]. The bin- ary SnSe can replace the widely used thin-film solar cells such as quaternary Cu(In, Ga)Se 2 (CIGS), CZTSe, and ternary CISe semicon- ductors due to the less number of elements and easy processing. However, the efficiency of SnSe solar cells is still far from the the- oretical efficiency (~32%) of the Shockley-Queisser limit [3]. The main factors, which affect the total device efficiency, are the pres- ence of secondary phases and bandgap alignment. The bandgap of SnSe can be varied by changing the composition ratio and the exis- tence of secondary phases [4]. Various techniques such as sputtering [4], evaporation [5,6], two-stage process [7], etc., have been employed to develop the SnSe thin films. The development of single-phase SnSe or doped SnSe thin-film absorbers is very important in thin-film solar cells [8]. Particularly, the two-stage process is employed for the devel- opment of single-phase absorber materials. In this process, the growth parameters can be easily optimized. In the past decade, reports [8] on single-phase formation and device fabrication stud- ies are less. Therefore, we focused on the single-phase formation and doping with the optimization of physical properties to produce efficient absorbers. The previously reported efficiency of SnSe solar cells is 1.4% and the low efficiency of SnSe was explained by certain limitations such as secondary phases, antisite defects, short carrier lifetime, etc. [9]. To improve the efficiency, limitations of imperfections and doping are majorly employed in the case of CZTSe and CISe. By doping of In to the SnSe, the electro-optical properties were improved and this could enhance the device efficiency of solar cells. The main reason for doping with In is that the ionic radius of In 3+ is consistent with the radius of Sn 2+ cation of the host SnSe. Due to the introduction of acceptor and donor energy levels in SnSe, the band structure of doped SnSe can slightly change. There- fore, we aimed to develop In doped SnSe thin films by the two- stage process and investigated the physical properties of In-SnSe films. 2. Experimental details The undoped and In-SnSe thin films were prepared by the two- stage process (sputtering + selenization). The tin metallic layers were deposited on cleaned glass substrates by DC (direct current) sputtering with 500 nm thickness under the Ar atmosphere at a working pressure of 5 mTorr and 150 W of DC power. Subse- quently, the selenization process was processed at different amounts (0.5 mg, 1 mg, and 1.5 mg) of indium boats (100 mesh, https://doi.org/10.1016/j.matlet.2020.128714 0167-577X/Ó 2020 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: swjoo@yu.ac.kr (S.W. Joo). Materials Letters 281 (2020) 128714 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue