The efficiency of a bulk hetero junction (BHJ) solar cell critically depends upon quality of its inter-faces. The imperfect interfaces can lead to S-kink in the current–voltage characteristics that reduce the efficiency of BHJ solar cells. In this letter, using PCDTBT:PCBM based BHJ solar cells, we demonstrate that non-destructive X-ray reflectivity is powerful technique to estimate the electron density profile across the BHJ solar cells. A direct correlation is observed between the enhanced electron density at PEDOT:PSS/PCDTBT:PCBM interface and appearance of S-kink in J–V characteristics, which is also supported by X-ray photoelectron spectroscopy and Kelvin probe measurements.