Sub-Cap Modulated Photo Current Spectroscopy (SGMPCS) is an excellent tool in order to investigate the band gap defect density of the absorber layer, directly on Cu(In-x,Ga1-x)(Se-y,S1-y)(2) (CIGSS) based solar cells. This technique is essentially sensitive to defect states located in the absorber layer, which has the lowest band gap of the heterojunction solar cell. It allows the determination of the sigma-N(E) product, where sigma is the defect Optical Cross Section (OCS) and N(E) is its Density Of States (DOS). We have developed an analytical model, allowing to derive the above product from the imaginary part of the ac photocurrent of the solar cell, under reverse applied dc bias. We have then applied this model to study the defect density of the co-evaporated CIGS (i.e. y = 1) absorber layer of a heterojunction solar cell. Two different defect distributions have been exhibited by SGMPCS, the properties of which vary with thermal annealing. Correlation with Admittance Spectroscopy allows us to derive an estimation of the defect OCS.