Optical characterization of the coloration process in electrochromic amorphous and crystalline WO3 films by spectroscopic ellipsometry

•Coloration processes of amorphous and crystalline WO3 films are studied by SE.•Amorphous WO3 can store more H+ ions compared to crystalline one at same voltage.•The optical parameters of crystalline WO3 are much more sensitive to the H+ ions.•Difference of electrochromic mechanisms between amorphous and crystalline WO3 is proved by SE.