20 nm thick anatase niobium-doped titanium dioxide (TNO) nanofilm with resistivity of 2.9 × 10⁻³ Ω·cm, visible-light transmittance of over 92% and work function of 4.43 eV was obtained by radio-frequency magnetron sputtering method and a diluted H₂ atmosphere annealing. A rapid annealing in a forming gas atmosphere of 3%H₂ and 97%N₂ at 500 °C reduced the resistivity by 3 orders of magnitude. Furthermore, the utilization of a “glass cover” (GC) in the annealing process decreased the resistivity by approximately 3 times and increased the refractive index by approximately 3.6% for the anatase TNO nanofilm. The secondary ion mass spectrometry measurement confirmed that the GC restrained the hydrogen diffusion from the surface into TNO occurred during the annealing. Employed as the antireflection layer in hydrogenated amorphous Si solar cells, the TNO nanofilm annealed with GC enabled an open-circuit voltage of 0.91 V, a gain of 6.5% in short-circuit current density and a reduction of 30.3% in series resistance, resulting in a gain of approximately 12.8% in photoelectric conversion efficiency. These results suggest that moderate H incorporation is beneficial for improving the electrical and optical characteristics of TNO thin films.