  We present a first-principles study of the structural, electronic, and optical properties of hydrogenated amorphous silicon (a-Si:H). To this end, atomic configurations of a-Si:H with 72 and 576 atoms respectively are generated using molecular dynamics. Density functional theory calculations are then applied to these configurations to obtain the electronic wave functions. These are analyzed and characterized with respect to their localization and their contribution to the density of states, and are used for calculating ab-initio absorption spectra of a-Si:H. The results show that both the size and the defect structure of the configurations modify the electronic and optical properties and in particular the value of the band gap. This value could be improved by calculating quasi-particle (QP) corrections to the single-particle spectra using the G$_0$W$_0$ method. We find that the QP corrections can be described by a set of scissors shift parameters, which can also be used in calculations of larger structures. 
Ab-initio analysis of structural, electronic, and optical properties of   a-Si:H