Real time spectroellipsometry and a two-layer optical analysis have been applied to obtain alloy composition(x)and optical gap(Eg)depth profiles with ∼3 Å resolution and sensitivities better than ±0.01 inxand ±0.02 eV inEgfor graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition. Graded amorphous silicon–carbon alloy(a-Si1−xCx:H)layers incorporated at thei–pinterfaces ofa-Si:Hn-i-psolar cells have been studied using these methods, and the layer characteristics have been related to improvements in solar cell performance. ©1998 American Institute of Physics.