Thickness controlling bandgap energy, refractive index and electrical conduction mechanism of 2D tungsten diselenide (WSe${}_2$) thin films for photovoltaic applications

This paper reports thickness-dependent structural, morphological, optical, and electrical conduction mechanism of room temperature electron beam evaporated 2D WSe${}_2$ thin films on glass substrate. The thickness of the WSe${}_2$ films was varied from 100 to 400 nm. XRD results showed the fact that the WSe${}_2$ films are crystallized in a hexagonal structure. The nanostructure nature is verified from morphological studies. It was found that the microstrain decreases, while the crystallite size rises as the thickness of WSe${}_2$ film increases. The thickness-dependent optical constants and energy bandgap was studied using spectroscopic ellipsometry (SE). The refractive index and extinction coefficient dispersion curves of WSe${}_2$ film with various thicknesses display two strong absorption peaks A and B below 800 nm at 580 nm and 770 nm, which are belong to excitonic absorption features. Further, the results illustrate that the optical constants and optical bandgap of thin WSe${}_2$ films are strongly correlated with the film thickness. In addition, the electrical conduction mechanism in different temperature regimes is explained in terms of Arrhenius activated thermal conduction, Mott's variable-range hopping (VRH), and Seto's grain boundary effect models. The most interesting finding is that the film exhibits wide absorption coefficient (10${}^6$ cm$^{-1}) and energy gap value closely matches the solar spectrum, making it an excellent candidate for photovoltaic materials as an absorber layer.