Polycrystalline WS2 films were grown by pulsed laser deposition (PLD) system at relatively low temperature. The main objective of this study is to optimize the growth conditions for polycrystalline WS2 films at relatively low temperature to use them for photovoltaics (PVs). Different growth conditions and substrates are used and examined systematically. It is found out that films grown on strontium titanate SrTiO3 (STO) substrate have the best structural properties when compared to other substrates examined in this work. X-ray diffraction and optical characterizations of these films reveal crystallographic growth and very promising optical properties for PVs. Furthermore, it was observed that higher growth temperature (>300°C) has an unfavorable effect on the layers by creating some tungsten metallic droplets.
The rising interest and research on alternative, abundant, and nontoxic absorber materials like tungsten disulfide (WS2) could play a major role to advance the photovoltaics field and renewable energy market [
The p-type WS2 is an indirect band gap material (1.29 eV) with direct transitions start about 1.7 eV. Along the layers, it has excellent transport properties; some reports estimated the diffusion length to be in the range of 200
In thin-film form, WS2 has been grown by various techniques like sputtering [
In this work, good-quality polycrystalline WS2 thin films have been grown at relatively low temperatures of 250°C and 300°C using PLD technique on STO substrates. XRD data shows preferred orientation for the as deposited films. The measured structural and optical properties are presented and discussed.
All the thin films of WS2 that were studied for this paper were deposited on soda-lime glass, quartz, STO, and sapphire substrates at different temperature using the PLD/MBE-2300 system from PVD Products, Inc. KrF (248 nm) excimer laser (Compex Pro 205) with pulse duration of 20 ns and a repetition with rate of 10 Hz was used for ablation of the target, which was held in a rotating carousel to enable uniform ablation of the target surface. The incident angle of the laser with respect to the target was ~45° with laser energy of 300 mJ; the partial pressure was sustained at 2 × 10−6 mTorr during the growth process. A WS2 target of 99.9% purity from American Elements Co. is used. The as deposited films were characterized using X-ray diffraction (XRD) and ellipsometer for structural and optical data.
Figure
XRD patterns of WS2 films on STO and soda-lime glass substrates at different temperatures ranged between 200°C and 400°C at 50°C increment.
Yet, there are sharp peaks for the WS2 grown on STO substrates. The large peaks at
Figure
XRD patterns of polycrystalline WS2 films on STO substrates at different temperatures ranged between 200°C and 400°C at 50°C increament.
The optical characterization of the as grown samples was carried out using an ellipsometer. Figure
Figure
The absorption coefficients of the grown WS2 films.
The thicknesses and roughnesses are also estimated from ellipsometry measurements. Figures
Thickness variation of WS2 films for the temperature 200°C−400°C.
The ratio values between the roughness and thickness of WS2 films (200°C−400°C).
Polycrystalline WS2 films were grown by PLD at relatively low temperature. This is part of the initial work to use WS2 for PVs. The main objective of this initial study is to optimize the growth conditions for polycrystalline WS2 films at relatively low-temperature. As known, the low-temperature growth of polycrystalline is desirable as PVs are largely multilayer. Different growth conditions and substrates are used and examined systematically. It is found out that films grown on STO substrate resulted in the best structural properties when compared to other substrates examined in this work. This is mainly due to the small lattice mismatch of 3%. X-ray diffraction and optical characterizations of these films reveal crystallographic growth and very promising optical properties for PVs. Furthermore, it was observed that higher growth temperature (>300°C) has an unfavorable effect on the layers by creating some tungsten metallic droplets.