Optical Properties of Seloo-Xbix Thin Films Deposited On Glass Substrates by Flash Evaporation Method

Abstract

The global need for sustainable energy production is pushing scientific research towards the development of inexpensive thin film solar cells which can compete with established commercial silicon-based technologies. These solar cells must be nontoxic and readily available away from cadmium telluride and gallium doped copper indium diselenide. For example, selenium and bismuth elements are nontoxic and readily available on the market. Further, selenium-bismuth alloy system is photosensitive to electromagnetic radiation. The optical properties of this system especially the absorption coefficient and the optical band gap energy are very important in the absorption of light energy. The study investigated optical properties of flash evaporated amorphous selenium-bismuth Se10o-xBix ex = 0,1,2,3, & 4 at. %) thin films deposited on cleaned glass substrates at temperatures of 51°C, 55°C, 59°C and evacuated to 3.0 x 10-smbar. The study objectives were; to investigate the effect of film thickness and the effect of substrate temperature on the optical properties of selenium-bismuth alloy thin films. Thin films of thicknesses 350 ± 10nm, 400 ± 10nm, 450 ± 10nm and 500 ± 10nm, measured on a surface profiler were selected. Their transmittance and reflectance were measured on SolidSpec.3700 Deep Ultra Violet Spectrophotometer (200nm - 3000nm). It was observed that addition of bismuth led to increase in the retlectance (R), absorption coefficient (a), refractive index (n), extinction coefficient (k), real (E1) and imaginary (E2) parts of the dielectric constant for the deposited thin films. However, the transmittance (T) and band gap energy (Eg) decreased at specific wavelengths or photon energies. At a wavelength of 750nm or 1.655eV and film thickness of 350 ± 10nm, the optical constants calculated were; 0.683 :::::T: ::::0:.:815, 1.263eV:::::: Eg ::::::1.352eV, 7.350:::::: E1 ::::::9.014, 0.167:::::: E2 ::::::0.651, 2.722:::::: n < 2.962, 0.028:::::: k ::::0::.093, 0.180:::::: R :::::0:.331, and 1.641 x 104cm-1 :::::a: ::::2:.0:11 x 104cm-1 . The optical constants were also observed to vary with film thickness and substrate temperature. The deposited amorphous selenium-bismuth alloy thin films have improved sensitivity to sun light as can be observed from the high absorption coefficient in the order ofl04cm-1 . The band gap energy and the absorption coefficient values exhibited by the films are good for the formation of selenium-bismuth thin film solar cells. VI