Non-vacuum Preparation of wse 2 Thin Films via the Selenization of Hydrated Tungsten Oxide Prepared using Chemical Solut
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.451
Non-vacuum Preparation of wse2 Thin Films via the Selenization of Hydrated Tungsten Oxide Prepared using Chemical Solution Methods Christopher L. Exstrom1, Scott A. Darveau1, Megan E. Falconer1, Jessica R. Blum1, Whitney M. Colling1, and Natale J. Ianno2 1
Department of Chemistry, University of Nebraska at Kearney, Kearney, NE 68849-1150, U.S.A.
2 Department of Electrical & Computer Engineering, University of Nebraska-Lincoln, Lincoln, NE 685880511, U.S.A.
ABSTRACT
It is known that tungsten oxide may be reacted with selenium sources to form WSe 2 but literature reports include processing steps that involve high temperatures, reducing atmospheres, and/or oxidative pre-treatments of tungsten oxide. In this work, we report a non-vacuum process for the fabrication of compositionally high quality WSe 2 thin films via the selenization of tungsten oxide under milder conditions. Tungsten source materials were various hydrated WO3 and WO2.9 compounds that were prepared using chemical solution techniques. Resulting films were selenized using a two-stage heating profile (250 oC for 15 minutes and 550 oC for 30 minutes) under a static argon atmosphere. Effects of the starting tungsten oxide phase on WSe2 formation after single and double selenization cycles were investigated using Raman spectroscopy and X-ray diffraction (XRD). After two selenization cycles, hydrated WO3 was converted to (002)-oriented WSe2 that exhibits well-resolved peaks for E12g and A1g phonon modes. Only a single selenization cycle was required to convert amorphous WO2.9 to WSe2. All selenizations in this work were achieved in non-reducing atmospheres and at lower temperatures and shorter times than any non-laser-assisted processes reported for WO3-to-WSe2 conversions.
INTRODUCTION With its low mineral extraction costs [1], a reported direct bandgap of 1.35-1.5 eV [2] and high minority carrier mobility [3], the WSe 2 material system is seeing renewed interest in next-generation solar cell absorber material development. Heterojunction photovoltaic (PV) devices based on WSe 2 single-crystal absorbers have had reported efficiencies exceeding 8% [4]. Fabrication of WSe 2 films via the selenization of tungsten has been shown to be effective at temperatures > 900 oC and reaction times longer than 24 hrs [5] due to a large activation energy barrier that accompanies the lattice expansion. So-called “soft selenization” of tungsten [3,6] at temperatures of 450-560 oC over 24 hrs has yielded WSe2 films of non-specific orientation and there is no evidence for electronic properties that would be suitable for
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PV applications. More recently, a laser-assisted selenization method has been successfully executed at 250 oC, but this is useful only in conju
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