A method for manufacturing a CdTe based thin film solar cell device with a graded refractive index profile within the CdTe-based absorber layer. The method comprises the following steps: a) providing a transparent substrate comprising a front electrode, b) forming a doped CdTe based absorber layer on the substrate, c) performing an activation treatment after step b). The doped CdTe based absorber layer in step b) is formed as a doped CdTe based absorber layer stack comprising a first and a second layer. The first layer is formed as a first doping element containing layer comprising vanadium as the first doping element by depositing a first doping element-rich layer and subsequently depositing a CdSe layer or a CdSeTe layer, or by depositing a CdSe layer or a CdSeTe layer each doped with the first doping element. The second layer is formed by depositing a CdTe layer. A CdTe based thin film solar cell device with a graded refractive index profile.