Abstract We have carried out a comparative study of the sub-gap optical absorption, α(hν), determined by photothermal deflection spectroscopy (PDS) and photocurrent spectroscopy (CMP) applied to a series of n-type and p-type a-Si:H films. For undoped films the defect-related shoulder of α(hν) is apprecibly lower from CPM than from PDS indicating that in these films the number of defects in bulk is much lower than that at the surface and substrate interface. We derived the density of states distribution N(E) from α(hν). in n-type films N(E) shows broad maxima about 1 eV below the conduction band which we associate with doubly occupied dangling-band (D −) levels. In contrast, the spectra of p-type samples produce maxima near 0.4 eV. We discuss possible origins of these defect states in p-type a-Si:H films.