Preparation routes based on magnetron sputtering for tungsten disulfide (WS2) films for thin-film solar cells

The semiconductor tungsten disulfide (WS2) exhibits van der Waals bonding, crystallizes in a layer-type structure and is of interest as an absorber layer for thin-film solar cells. In this review article different preparation routes for WS2 thin films, based on magnetron sputtering, are reviewed. Films prepared by direct magnetron sputtering, though exhibiting quite a good structural quality, are not or only poorly photoactive. This can be attributed to the generation of recombination centers, especially sulfur vacancies, during the ion bombardment of the films, due to the low defect-formation energy of tungsten disulfide, an intrinsic property of transition metal dichalcogenides. A promising preparation route, which leads to photoactive WS2 films, is a two-step process, where, in a first step, a sulfur-rich, X-ray amorphous tungsten sulfide is deposited at low substrate temperatures onto a thin metal film (Ni, Co). This film sandwich is after wards annealed in an ampoule in a sulfur atmosphere or in flowing gas with a sufficient H2S partial pressure. From in-situ transmission electron microscopy and energy-dispersive X-ray diffraction, it was found that the WS2 film crystallization with a pronounced (001) texture is closely related to the formation of the liquid (eutectic) metal-sulfur phase. Based on these in-situ investigations the growth of the 2-dimensional WS2 nanosheets from an amorphous WS3+x precursor can be described as an amorphous solid-liquid-crystalline solid process (SLS), somewhat similar to the well-known vapor-liquid-solid (VLS) process for the growth of whiskers or nanorods and nanotubes. Research opportunities, to overcome current limitations for a broad use of WS2 (and MoS2) as thin-film solar cell absorbers are given.