Thickness controlling bandgap energy, refractive index and electrical conduction mechanism of 2D Tungsten Diselenide (WSe2) thin films for photovoltaic applications

This paper reports thickness-dependent structural, morphological, optical, and electrical conduction mechanism of room
temperature electron beam evaporated 2D WSe2thin films on glass substrate. The thickness of the WSe2
films was variedfrom 100 to 400 nm. XRD results showed the fact that the WSe2
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 WSe2
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 WSe2
film withvarious 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 WSe2
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 (106 cm−1)and energy gap value closely matches the solar spectrum, making it an excellent candidate for photovoltaic materials as an absorber layer.