Optical constants of Cu(In,Ga)Se2 thin films from normal incidence transmittance and reflectance

The present contribution reports on the optical constants of Cu(In,Ga)Se-2 thin films deposited by co-evaporation with five representative In/Ga-ratios varying from pure CuInSe2 to pure CuGaSe2 The optical properties of the films are characterized by spectral transmittance and reflectance at normal incidence. Fitting the optical spectra with a model of flat layers yields the optical constants for wavelengths between 350 and 2000 nm. We compare our results to recently published bulk data that are obtained by spectral ellipsometry. The comparison shows a good agreement between the refractive indices, but significantly lower extinction coefficients for our thin films. The reason for the observed differences most probably lies in the surface versus bulk sensitivity of the different determination methods.