Hydrogenated amorphous silicon (a-Si : H) films are prepared by plasma-enhanced chemical vapor deposition (PECVD) with a triode electrode configuration in which a SiH4–H2 glow-discharge plasma is confined spatially away from the substrate. Although the deposition rate (0.1–0.5 Å/s) is lower than that of the conventional diode PECVD process (2.5 Å/s), the light-induced degradation in conversion efficiency (Δη/ηini) of a single-junction solar cell is substantially reduced (e.g., Δη/ηini ~ 10% at an absorber thickness of ti = 250 nm), and efficiencies after light soaking (LS) maintain >9% for ti = 180–390 nm. By applying the improved a-Si : H layers as top cell absorbers in a-Si : H/hydrogenated microcrystalline silicon (µc-Si : H) tandem solar cells, the light-induced degradation can be reduced further (e.g., Δη/ηini ~ 5% at ti = 250 nm). As a result, we obtain confirmed stabilized efficiencies of 9.6% (LS condition: 100 mW/cm2, 50 °C, 1000 h) and 11.9% (LS condition: 125 mW/cm2, 48 °C, 310 h) for a-Si : H single-junction and a-Si : H/µc-Si : H tandem solar cells, respectively. Copyright © 2012 John Wiley & Sons, Ltd.