2, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
With its low mineral extraction costs, a reported direct bandgap of 1.35-1.4 eV and high minority carrier mobility, the WSe2 material system is seeing renewed interest as an absorber in next-generation solar cell material development. When preparing WSe2 thin films via the selenization of tungsten metal, the lattice expansion required to accommodate WSe2 formation presents a large activation barrier that requires very high temperatures (> 900 oC) and long reaction times (24-72 hrs) to overcome. It is known that tungsten oxide may be reacted with various selenium sources to form WSe2 but literature reports still include processing steps that involve high temperatures, reducing atmospheres, and/or oxidative pre-treatments of tungsten oxide. In this work, we report a three-step non-vacuum process for the fabrication of compositionally high quality WSe2 thin films via the selenization of tungsten oxide that does not require a reducing atmosphere, temperatures above 550 oC, or any pre-treatment of the tungsten oxide prior to selenization. Hydrated tungsten oxide was prepared via a chemical bath reaction between Na2WO4 and diethyl sulfate and deposited onto Corning glass substrates using a centrifuge method. The product phase, monoclinic-WO3 ● 2 H2O or orthorhombic-WO3 ● H2O, could be controlled by varying the Na2WO4 concentration in the bath, and using the latter phase as the tungsten source resulted in more effective conversion to WSe2. Films of orthorhombic-WO3 ● H2O were heated in a graphite susceptor containing elemental selenium using a two-stage heating profile (250 oC for 15 minutes and 550 oC for 30 minutes) under a static argon atmosphere. Finally, this heating was repeated in a graphite susceptor with no elemental selenium. X-ray diffraction analysis revealed that the product film was exceptionally well ordered with a (002) plane orientation. The Raman spectrum was consistent with previously reported WSe2 peak frequency assignments and the closely-spaced E12g and A1g major peaks were very well resolved. X-ray photoelectron spectroscopy and photoluminescence studies are in progress and will be discussed.