The electronic state of thin films of yttrium, yttrium hydrides and yttrium oxide Trygve Mongstad a,n , Annett Thøgersen a,b , Aryasomayajula Subrahmanyam c , Smagul Karazhanov a a Institute for Energy Technology, NO-2027 Kjeller, Norway b SINTEF Materials and Chemistry, P.O. Box 124 Blindern, 0314 Oslo, Norway c Semiconductor Laboratory, Department of Physics, Indian Institute of Technology Madras , Chennai 600036, India article info Article history: Received 28 February 2014 Received in revised form 13 April 2014 Accepted 14 May 2014 Keywords: Yttrium hydride Photochromism XPS Work function abstract Thin films of yttrium hydride have for almost 20 years been under investigation for optoelectronic and solar energy applications due to the hydrogen-induced switching in electronic state from the metallic elemental yttrium and yttrium dihydride to the transparent semiconductor material yttrium trihydride. In this study, we investigate the electronic structure of yttrium, yttrium hydride and yttrium oxide by using X-ray photoelectron spectroscopy and kelvin probe measurements. The investigated samples have been prepared by reactive sputtering deposition. We show that the electronic work function of transparent yttrium hydride is of 4.76 eV and that the recently discovered photochromic reaction lowers the electronic work function of the transparent hydride by 0.2 eV. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Thin films of yttrium hydride came into the light of attention with the invention of the metal hydride-based switchable window in 1996 [1]. The switchable window effect has later been subject for intense research and development, and has enabled a new class of smart windows and optical hydrogen sensors [2]. Yttrium hydride is generally considered to belong to the class of the rare-earth metal hydrides, for which the metal–hydrogen phase diagram is divided in three phases [3]; the metallic phase with dissolved small amounts of hydrogen is called the α-phase; the metallic dihydride (YH 2 ) is known as the β-phase; and the semiconducting trihydride (YH 3 ) is known as the γ-phase. YH 3 is a semiconductor with a band gap of 2.6 eV and therefore partly transparent to visible light [1]. Thin films of yttrium capped by a thin layer of Pd are gasochromic; H can be loaded and unloaded reversibly through the Pd cap layer and the optical state of the YH x films can be controlled by regulating the H 2 pressure in the environment of the sample. Recently we reported that thin films of yttrium hydride are not only gasochromic, but can also be photochromic: Films of reactively deposited transparent yttrium hydride exposed to light will gradually increase the optical absorption and the transparency will be reduced by up to 50% under illumination. The films will return to the initial transparent state under relaxation in dark conditions [4]. This effect has so far only been visible in films prepared by reactive sputtering, where both hydrogen and oxygen are incorporated in the films during the process. The reactively deposited and oxygen-containing yttrium hydride films differ from hydrogenated yttrium films in the crystal structure; they have a cubic lattice similar to YH 2 , whereas transparent YH 3 normally is found in a hexagonal structure [5,6]. The currently available results suggest that the photochromic effect is accompanied by a small structural change [7], but at the same time, changes in resistivity and the spectral response in relation to the band gap suggest that it originates from an electronic effect [8]. The electronic structure of yttrium hydride was first investi- gated experimentally by Fujimori and Schlapbach, who reported of a hydrogen-induced state at 6 eV in the valence band spectra. The Y 3d doublet core levels was found to shift by 0.7 eV and 1.2 eV, respectively, when going from pure Y to YH 2.1 and YH 2.1 to YH 3 . More recently, shifts of 0.4 eV and 1.5 eV was reported by Hayoz et al. [9]. Dús and Nowicka [10] have investigated the dynamic changes in the work function of yttrium films upon hydrogen uptake, but we have not been able to find absolute values for the work function of yttrium hydride in the literature. 2. Experimental Four different thin film samples were investigated in this work; elemental yttrium metal, black yttrium hydride (the YH 2 electronic Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.05.037 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ47 99228200; fax: þ47 63899964. E-mail address: trygve.mongstad@ife.no (T. Mongstad). Solar Energy Materials & Solar Cells 128 (2014) 270–274