We present two different investigations showing the influence of hydrogen in hot-wire chemical vapor deposition (HWCVD). First, dissimilarities in the growth kinetics of hydrogenated amorphous silicon (a-Si:H) films by the plasma enhanced chemical vapor deposition (PECVD) and HWCVD are discussed. A series of a-Si:H films with varying substrate temperature (T,) was deposited by HWCVD and PECVD. In comparing the initial growth, which was measured by in-situ kinetic ellipsometry for both deposition methods, we conclude that, in the PECVD process, a faster coalescence takes place as a result of a larger surface mobility of the adsorbed precursor radicals at the growing surface. This dissimilarity can be explained by different silane dissociation processes which yield to a higher ratio of atomic hydrogen to silicon radicals and a lower hydrogen coverage of the film growing surface in the case of HWCVD. Therefore, dense high quality a-Si:H films deposited by HWCVD are formed at higher substrate temperatures and lower hydrogen dilutions compared to PECVD. The second investigation deals with the effect of atomic hydrogen on polished and textured silicon wafers. The hydrogen treatment can decrease the surface defect density without deteriorating the properties of the bulk material. Therefore, the fill factor and the open circuit voltage of (n)a-Si:H/(p)c-Si heterojunction solar cells increase and the intrinsic conversion efficiency reaches up to 15.2%.