The influence of the power density, P-D on the density and structure of defects of undoped a-Si:H thin films, deposited by r.f.-PECVD, is studied by the constant photocurrent method, (CPM). and by slow positron beam spectroscopy, respectively. Deep defect density, N-DD, remains approximately constant at 10(16) cm(-3), typical of device quality material, for P-D in the range 7-20 mW(.)cm(-3), as calculated from CPM. Out of this range. N-DD increases roughly one order of magnitude for both lower and higher power densities. Positron annihilation spectroscopy reveals the predominance of two kinds of vacancy type defects in the films: large vacancy clusters or voids for P(D)similar to7 mW(.)cm(-3) and small vacancy type defects as P-D increases to similar to30 mW(.)cm(-3). The behaviour of solar cells, prepared with a-Si:H intrinsic layers with similar deep defect densities, varies with the S parameter of the a-Si:H film, S-film as S-film increases, which can be related to a lower material density, the solar cell characteristics degrades.