The effect of process parameters on the deposition of μc-Si:H solar cells is reviewed. Then, our approach to solar cell optimization is presented, in which in situ diagnostics are used to study the process stability. We compared a highly H2-diluted condition with a pure SiH4 flow condition, using optical emission spectroscopy (OES) to characterize the plasma and Raman spectroscopy to characterize the film. In low dilution conditions we measured a sharp drop of SiH emission in the first 90 s after plasma ignition, indicating uncontrolled deposition for the first ∼50 nm of film. This instability could be prevented by filling the chamber with pure H2 and switching on the SiH4 flow only shortly before plasma ignition. In the steady-state deposition phase following the transient depletion phase the H2 flow hardly effected plasma emission and deposited film crystallinity. This is explained by a depletion of the SiH 4. The pure SiH4 flow deposition was applied into a μc-Si:H solar cell and yielded a solar energy conversion efficiency of 9.5%. In the discussion an updated view on the role of the process parameters total flow and dilution ratio is presented. © 2006 Materials Research Society.