Porous silicon was formed on mono- and multicrystalline Si substrates by stain etching in aqueous HF/HNO3 solutions. Optical properties of the resulting films were investigated by reflectance spectroscopy and related to the main etch parameters. Porous Si films have been shown able to reduce surface reflectance to under 3% in a wavelength range larger than 400-800 nm. Efficient anti-reflection coatings were developed and used in multicrystalline Si solar cells with area up to 12.8 X 12.8 cm(2). Photovoltaic conversion efficiency up to 12% was obtained under standard AM1.5G simulated sunlight.