An atmospheric-pressure plasma deposition system was developed and used to deposit intrinsic hydrogenated microcrystalline silicon (mu c-Si:H) thin films for mu c-Si:H thin film solar cells. The helium (He) gas concentration in plasma increased Raman crystallinity (X-c) of mu c-Si:H, which was a critical parameter to determine photovoltaic performance. In addition, dependence of X-c on the He concentration is shown to be closely related to the atomic hydrogen flux during film growth, which is evidenced by plasma diagnostics derived from in situ optical emission spectroscopy measurements. Those analysis contributed to achieve impressive photovoltaic performance - namely, a power conversion efficiency of 4.60%, an open-circuit voltage of 0.52 V, a short-circuit current density of 13.62 mA/cm(2), and a fill factor of 0.65, which is the first experimental demonstration of mu c-Si: H thin film solar cells fabricated with the atmospheric-pressure plasma deposition under very high working-pressure regime at 13.3 kPa.