Hydrogenated microcrystalline silicon (m c-Si:H) solar cells with different thicknesses were deposited on specular stainless steel substrates and on textured Ag/ZnO back reflectors using RF and modified very high frequency glow discharge at various deposition rates. Raman spectra and X-ray diffraction patterns exhibit a significant increase of microcrystalline volume fraction and in grain size with film thickness. Atomic force microscopy reveals an increase in the size of microstructural features and the surface roughness with increasing thickness. Based on these results, we believe that the increase of the microcrystalline phase with thickness is the main reason for the deterioration of cell performance with the thickness of the intrinsic layer. To overcome this problem, we have developed a procedure of varying the hydrogen dilution ratio during deposition. Using this method, we have been successful in controlling the microstructure evolution and achieved an initial active-area efficiency of 8.4% for a c-Si:H single-junction solar cell, and 13.6% for an a-Si:H/a-SiGe:H/m c-Si:H triple-junction solar cell.