The effect of conventional process parameters on the deposition of μc-Si:H solar cells is reviewed. Then, an approach to solar cell optimization is presented in which hidden, internal parameters are adjusted rather than conventional, external process parameters. The investigation focuses on deposition at low H2 dilution ratio and low total gas flow. A hidden parameter is identified through time resolved optical emission spectroscopy on SiH emission: Transient depletion of the SiH4 source gas leads to uncontrolled deposition conditions during the first 90 s after plasma ignition. There hardly is any effect on plasma properties and deposited film properties for the remainder of deposition after the transient depletion phase. As demonstrator a 9.5 % efficient single junction μc-Si:H solar cell was deposited from a pure SiH4 flow. A reinterpretation of the role of H2 dilution is discussed.