Optical properties of Tb3$mathplus$/Yb3$mathplus$ co-doped silica-hafnia glass and glass-ceramic thin film

In this paper, optical properties of Tb3+/Yb3+ co-doped 70SiO2 - 30HfO2 glass and glass-ceramic thin films prepared by dip coating on Si O2 substrate are investigated using transmittance measurements. Such systems are usually used for down-converting the high-frequency sunlight in order to tune the solar spectrum into the bandgap of the semi-conductor based photovoltaic system. The interference modulated transmission spectra tau(lambda) of 70SiO2 - 30HfO2- x Tb3+ - y Yb3+ (with molar ratio x/y = 1.8/0, 1.8/7.2, 3/12) at normal incidence were recorded in the spectral wavelength region from 250 nm up to 2500 nm. The corresponding optical constants and thicknesses were deduced by analyzing the transmittance spectra using the straightforward method proposed by Swanepoel. The dispersion of the real part of the refractive index and the absorption were then deduced. The results show thatthe increase in the concentration of rare earth ions leads to a slight increase in the refractive index, this is also observed in thickness variation.Furthermore, the energy bandgap has been determined from the absorption coefficient values using the direct transition model proposed by Tauc. The corresponding value is around 3 eV with a slightly higher bandgap for glass films as compared to glass-ceramic films.