The increasing complexity of hydrogenated amorphous silicon (a-Si:H) based solar cells requires continuous extending and testing of the computer models which are used for their simulation. The calculation of the defect states density in the bandgap of a-Si:H based on the defect pool model (DPM) has been rally implemented in the ASA (Amorphous Semiconductor Analysis) computer program developed at Delft University of Technology. We used the technique of inverse modelling to verify the DPM and to calibrate it by fitting the dark and illuminated J-V characteristics of a single-junction a-Si:H solar cell fabricated at Utrecht University. The DPM was also used in simulating the absorption coefficient of a-Si:H layers. Using the DPM for the defect states distribution, a good agreement between simulated and measured data was obtained. The values of the material parameters needed for obtaining the best fits are more realistic than using a conventional model for the defect states distribution.