Electrochromism of nitrogen-doped tungsten oxide thin films Gamze Atak a,b,⇑ , _ Ilknur Bayrak Pehlivan b , José Montero b , Daniel Primetzhofer c , Claes G. Granqvist b , Gunnar A. Niklasson b a Hacettepe University, Department of Physics Engineering, 06800 Beytepe, Ankara, Turkey b Department of Materials Sciences and Engineering, The Ångström Laboratory, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden c Department of Physics and Astronomy, The Ångström Laboratory, Uppsala University, P.O. Box 516, SE-75120 Uppsala, Sweden article info Article history: Available online xxxx Keywords: Tungsten oxide Electrochromism Nitrogen-doping Sputter deposition Thin film abstract Tungsten-oxide-based thin films were prepared by reactive DC magnetron sputtering in the presence of oxygen and nitrogen. Nitrogen contents up to 12 at.% were documented by Rutherford backscattering spectrometry and time-of-flight elastic recoil detection analysis. Optical and electrochemical measure- ments showed that films with up to 4 at.% of nitrogen were as transparent as undoped tungsten oxide films and displayed enhanced electrochromic properties manifested in an increase in the coloration effi- ciency by as much as 20%. Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the E-MRS Fall Meeting, 2019. 1. Introduction Electrochromic thin films are integral parts of devices that are capable of varying their absorptance of visible light and solar energy. The prime applications of such devices are in smart win- dows for energy-efficient buildings, variable-reflectance mirrors, and non-emissive information displays [1,2]. Today’s smart win- dows typically include thin films based on tungsten oxide and nickel oxide separated by a solid inorganic thin-film ion conductor or a layer of a polymer electrolyte [3,4]. Absorptance modulation is accomplished when a voltage is applied between the tungsten oxide and nickel oxide thin films, via transparent electrical conduc- tors, so that ions and charge-balancing electrons are shuttled between the two films. Smart windows are manufactured with sizes up to several square meters and must have excellent optical properties with great uniformity and no discernible haze, and it is obvious that thin-film deposition technology is crucial [3,5]. The most common technique for making electrochromic films of tungsten oxide and nickel oxide is magnetron sputtering [3,5], which employs argon- based plasma to dislodge atoms from a target material—usually based on metallic tungsten or nickel—and deposits these atoms on an adjacent substrate of flat glass or flexible polymer foil. Sputtering is usually carried out under reactive conditions and the sputter plasma contains oxygen in order to produce oxidic thin films, for example stoichiometric or slightly sub-stoichiometric WO 3 . Additional or other gases can be used in the plasma in order to optimize optical and structural properties and to increase thin- film deposition rate; for example, Ar + O 2 + CF 4 has been used to produce fluorinated tungsten oxide films [6], and Ar + O 2 +H 2 [7,8],H 2 O [9],O 2 +H 2 O [10], and Ar + O 2 +H 2 O [11] have been used to make hydrated nickel oxide films. Sputtering in the presence of nitrogen may open new possibil- ities, which, generally speaking, have not been researched in much detail, although Nakagawa et al. [12] and Sun et al. [13] prepared tungsten-oxide-based electrochromic thin films by deposition in Ar + O 2 +N 2 and Ar + O 2 +N 2 O, respectively. Furthermore, Lin et al. [14] produced nickel-oxide-based electrochromic thin films by deposition from NiO in Ar + N 2 . Nitrogen-doping on the level of a few percent was found to enhance the optical modulation of nitrogen-doped tungsten oxide somewhat [13], whereas the bene- ficial effect of nitrogen doping was very significant in the case of nickel oxide [14]. In the present work, we investigate nitrogen- doped tungsten oxide thin films and verify that up to 4 at.% of nitrogen can improve their electrochromism. 2. Experimental techniques Tungsten oxide thin films with various amounts of nitrogen were prepared by reactive DC magnetron sputtering in a https://doi.org/10.1016/j.matpr.2020.01.332 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the E-MRS Fall Meeting, 2019. ⇑ Corresponding author at: Hacettepe University, Department of Physics Engineering, 06800 Beytepe, Ankara, Turkey. E-mail address: gbaser@hacettepe.edu.tr (G. Atak). Materials Today: Proceedings xxx (xxxx) xxx Contents lists available at ScienceDirect Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr Please cite this article as: G. Atak, _ I.Bayrak Pehlivan, J. Montero et al., Electrochromism of nitrogen-doped tungsten oxide thin films, Materials Today: Pro- ceedings, https://doi.org/10.1016/j.matpr.2020.01.332