A profound comprehension of the nanostructure of hydrogenated amorphous silicon (a-Si:H) and the defect states in this material is currently still lacking despite several decades of research on this topic. Investigating the nature of defects in a-Si:H devices is especially important in view of the poorly understood Staebler-Wronski effect (SWE). Therefore we present the latest insights into the gap states in the a-Si:H bandgap by manipulating these sub gap states in three different ways: via the application of a voltage bias, via in situ light soaking, and via in situ annealing. To monitor the changes in the sub gap absorption we employ an advanced application of Fourier Transform Photocurrent Spectroscopy (FTPS). A recently published optical model that removes interference fringes from the photocurrent spectrum is presented to obtain the absorption coefficient. For FTPS measurements on solar cells the photocurrent spectrum is scaled to the external quantum efficiency (EQE) of the solar cell. Further, a mathematical data fitting routine is employed to accurately quantify the sub gap absorption and the changes therein during light soaking and annealing. The high sensitivity of FTPS is particularly helpful in identifying the sub gap states that play a role in the SWE in a-Si:H devices. (C) 2014 Elsevier B.V. All rights reserved.