Photoactive Tungsten Disulfide WS2 Nanosheets, Prepared by Rapid Crystallization from Liquid Sulfides

We have shown, that photoactive, highly 001 textured tungsten disulfide WS2 films can be prepared both on insulating as well as metallic substrates. The WS2 films are crystallized from X ray amorphous WS3 x films by a rapid annealing process in an H2S atmosphere with the assistance of metal sulfide droplets, which are formed, when the eutectic temperature in the system metal sulfur is exceeded. As promoters Ni, Co and Pd can be used, where the eutectic temperatures with sulfur are in the range from 620 to 680 C. From ex situ and in situ analysis a modified model for the formation of the WS2 nanosheets was derived, which resembles the well known VLS vaporliquid solid model for the formation of nanorods or whiskers , except that in our systems the precursor is also an amorphous solid, and instead of 1 dimensional rods, 2 dimensional nanosheets are grown. Especially, the preparation on metallic layers W, TiN , which was not successful before, offers now the opportunity, to prepare thin film solar cells from these WS2 films. First experiments to prepare thin film solar cells show, that short circuits occur, most probably at grain boundaries, which have to be avoided or passivate

Metal sulfide assisted rapid crystallization of highly 001 textured tungsten disulphide WS2 films on metallic back contacts

Highly 001 textured, photoactive WS2 films, which could be deposited before only on insulating substrates, have been prepared on polycrystalline metal layers, which can be used as back contacts for electronic WS2 devices, for instance for thin film solar cells. The WS2 films were prepared by the amorphous solid liquid crystalline solid aSLcS crystallization process from a Ni S eutectic. However, normal polycrystalline metallic films can not be used as back contact layers, caused by the diffusion of the thin metal promoter Ni into the back contact layer, before a nickel sulfur eutectic can be formed. Preventing the diffusion of the metal promoter into the back contact allows using the well known metal promoter assisted crystallization to grow photoactive films on different back contacts, like TiN O. Additionally, WS2 films on tungsten layers could be grown by Ni induced sulfidation of W films. Based on these experiments the model of the rapid nickel sulfide induced crystallization was modified Ni assists the growth of WS2 crystallites already at temperatures between 500 and 600 amp; 8201; C leading to WS2 films with differently oriented crystallites. At temperatures above the Ni S eutectic temperature 637 amp; 8201; C liquid NiSx droplets induce, as reported earlier, a rapid recrystallization, which leads to WS2 films with strong 001 orientatio

In situ energy dispersive X ray diffraction of metal sulfide assisted crystallization of strongly 001 textured photoactive tungsten disulfide thin films

Highly 001 textured tungsten disulphide WS2 thin films were grown by rapid metal Ni, Pd sulfide assisted crystallization of amorphous reactively sputtered sulfur rich tungsten sulfide WS3 x and by sulfurization of tungsten metal films. The rapid crystallization was monitored by real time in situ energy dispersive X ray diffraction EDXRD . Provided, that a thin nickel or palladium film was deposited prior to the deposition of WS3 x or W the films crystallized very fast about 20 nm s at temperatures above the metal sulfide eutectic temperature. After crystallization, isolated MeSx crystallites are located on the surface of the WS2 layer, which was proved by scanning electron microscopy. The metal sulfide assisted crystallized WS2 layers exhibit a pronounced 001 orientation with large crystallites up to 2 amp; 956;m. The in situ EDXRD analysis revealed distinct differences of the two crystallization routes from tungsten and from amorphous, sulfur rich WS3 x precursors, respectively. The crystallized WS2 films showed photoactivity and high hole mobilities. Combined with the high absorption coefficient of 10 5 cm 1 and a direct band gap of 1.8 eV these properties make such films suitable for absorber layers in thin film solar cell