The optical absorption coefficient of amorphous and microcrystalline silicon was determined in a spectral range 400-3100 nm and a temperature range 77-350 K. Transmittance measurement and Fourier transform photocurrent spectroscopy were used. The measured data served as an input for our optical model of amorphous/microcrystalline tandem solar cell. Differences in the current generated in the amorphous and microcrystalline parts were computed, for an operating temperature between -20°C and +80°C. Optical spectra of microcrystalline silicon were compared to the spectrum of silicon on sapphire (without hydrogen and hydrogenated) and the observed difference was interpreted in terms of a different density of defects and higher disorder of microcrystalline Si. © 2004 Elsevier B.V. All rights reserved.