Contents lists available at ScienceDirect Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb Germanium substitution effect on the property and performance of Cu 2 ZnSnSe 4 thin films and its solar cell having absorber layer made by sputtering with single metallic target plus selenization Albert Daniel Saragih a,b , Dong-Hau Kuo b, ⁎ a Department of Mechanical Engineering, Bandung State Polytechnic, Bandung 40012, Indonesia b Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ARTICLE INFO Keywords: Germanium CZTSe Semiconductor Doping Films ABSTRACT We report on the germanium substitution effect on the Cu 2 ZnSnSe 4 solar cell performance with absorber layer prepared by sputtering with a single metallic target plus further selenization. We synthesized the Cu 2 Zn (Sn 1-x Ge x )Se films with the [Ge]/([Ge] + [Sn]) percentages of 0%, 5%, 10%, 15%, and 20% when different x ratios were 0, 0.05, 0.1, 0.15, and 0.2. Defect chemistry was studied by measuring the structural, electrical, and optical properties of CZTGSe as a function of dopant concentration. The enhanced device performance was shown with the increased Ge content to CZTSe. The solar cell was fabricated with a stack structure of Ag/ITO/ ZnO/CdS/CZTGSe/Mo/SLG. The efficiencies of CZTGSe films at the Ge percentages of 0%, 5%, 10%, 15%, and 20% were 1.90, 2.35, 3.32, 4.64, and 4.24%, respectively. The further improvement in conversion efficiency of 7.23% was achieved by incorporating a NaF layer between the Mo bottom electrode and CZTGSe absorber to form a Mo-NaF bilayer. 1. Introduction Cu(In 1-x Ga x )Se 2 (CIGSe) semiconductor compound has shown the energy conversion efficiencies about 21.7% on laboratory scale [1], while the highest efficiency reported for the sulfur-selenium alloy Cu 2 ZnSn(S,Se) 4 (CZTSSe) has reached 12.6% [2]. Based on non-toxicity and abundance of the constituent elements, kesterite CZTSe or CZTSSe has attracted attention as an alternative absorber material to replaced CIGSe. CZTSSe has been proved to be promising materials for solar cell applications due to high absorption coefficient (10 4 cm -1 ) in the visible range [3–5] tunable band gap from 1.1 eV (CZTSe) to 1.5 (CZTS) [6–8]. Compared to CIGSe, the CZTSSe thin film solar cell efficiency is quite low which can be achieved due to the limiting factors such as open circuit voltage, secondary phases, back contact barrier, and detrimental defect level [9,10]. Understanding the limitations of this material system is essential for further improvements in device performance. One route to improving device performance of CZTSe absorber is through material alloying, where various elements can be incorporated into the tetragonal crystal lattice to modify the optoelectronic proper- ties of the absorber [9]. The natural p-type behavior of CZTSe is as- cribed to its intrinsic defects and therefore the understanding of such defects can accelerate progress in the solar cell performance. Current knowledge of the electronic properties and especially defect properties is mostly based on theoretical studies carried out using density func- tional theory [11]. Ag-alloyed CZTSe has demonstrated improved performance for CZTSe absorbers through improved optoelectronic properties, im- proved minority carrier life time, improved grain growth. In addition, Ag-alloyed CZTSe also is beneficial for band gap tuning/grading of the absorber for improved the performance [9,12]. Previous studies have examined the crystallographic and optical properties of (Cu,Ag) 2 ZnSnS 4 and (Cu,Ag) 2 ZnSnSe 4 [13] and found that the introduction the 5% or 10% Ag into CZTSe layer gave 7.1% [9] or 10.2% [14] efficiencies, respectively. An alternative approach to improve the performance is through Sn/Ge alloying in CZTSSe. For CZTSSe, the absorber band gap (E g ) is determined by Cu d orbital and S/Se p orbital anti-bonding (valence band maximum-VBM) and Sn s orbital and S/Se p hybrid or- bital anti-bonding (conducting band-maximum-CBM) [6,9]. Hence, E g tuning can be achieved through modification of the desired cation/ anion interactions in the lattice. The most common technique for band gap tuning in CZTSSe is through S/Se substitution; for CZTS relative to CZTSe, the p level is lower for S than Se, reducing the VBM, while the shorter bond length of Sn–S relative to Sn–Se increases the CBM through increased level repulsion of the anti-bonding state [6,9]. https://doi.org/10.1016/j.mseb.2019.114437 Received 7 November 2017; Received in revised form 13 August 2019; Accepted 17 October 2019 ⁎ Corresponding author. E-mail address: dhkuo@mail.ntust.edu.tw (D.-H. Kuo). Materials Science & Engineering B 250 (2019) 114437 Available online 26 October 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved. T