Systematic studies have been carried out on the transition from the amorphous to the microcrystalline phase in intrinsic Si:H as a function of the accumulated film thickness and the effect of this transition on p–i–n solar cell performance [J. Koh, Y. Lee, H. Fujiwara, C. R. Wronski, and R. W. Collins, Appl. Phys. Lett. 73, 1526 (1998)]. Guided by a deposition phase diagram obtained from real-time spectroscopic ellipsometry, cell structures having i layers deposited with different H2-dilution levels and thicknesses were investigated. For these structures, the fill factors are controlled by the bulk i layers. From the systematic changes in the fill factors, specifically their initial and degraded steady-state values and their degradation kinetics, the effects of the transition from the amorphous to the microcrystalline phase within the Si:H layers are identified, and insights are obtained into the properties of these structurally graded materials. © 1999 American Institute of Physics.