Plasma driven nano-morphological changes and photovoltaic performance in dye sensitized 2D-layered dual oxy-sulfide phase WS2 films
The present work examined dye sensitized 2D layered tungsten disulfide (WS2) as a photo-anode in solar cells with no use of nanocrystalline metal oxides as electron acceptors such as titanium dioxide. It is observed that coating WS2 directly onto a fluorine doped tin oxide (FTO) transparent conductor, annealed at 530 °C, resulted in a mixed oxy-sulfide dual phase as confirmed by transmission electron microscopy and X-ray diffraction studies. Further studies on the surface morphology of the dual phase WS2–WO3 film showed a random distribution of platelets which further shaped into precisely regulated hexagonal platelets upon plasma treatment. High resolution transmission electron microscopic studies elucidated two different phases, WS2 and WO3, with d-spacing values of 0.26 nm and 0.37 nm, respectively. A well-defined grain boundary was also observed which separated the oxy-sulfide phase in the sample. The dual WS2–WO3 phase films showed optical absorption in the wavelength range of 350 nm–800 nm with a systematic increase in plasma exposure duration. Photovoltaic devices fabricated using the WS2–WO3 mixed phase photo-anodes resulted in 0.61% efficiency (η) which further was observed to be sensitive to the plasma exposure as it was observed that the 20 minute plasma treated sample increased the η value to 0.67%. Plasma treatment on the dual-phase samples orients and modifies the shapes of the platelets with a significant change in the surface which eventually influences the charge transport in resulting photovoltaic devices and thus the variation with respect to exposure duration.