In recent years the efficiency of CdTe based solar cells has been significantly improved, mainly by the increase of the short circuit current value. This has been achieved by removing the CdS window layer and by adding a CdSexTe1-x layer, grading the band gap of the CdTe. On the other hand, ultra-thin absorbers allow to reduce the materials impact in terms of availability, cost and environmental impact.In this paper we present and study the fabrication of an ultra-thin CdTe based device with band gap grading. Moreover, we present a novel method to generate CdSexTe1-x compounds: by annealing a thin part of the absorber layer in a selenium atmosphere (selenization). This technique allows to narrow the absorber band gap to 1.41 eV, to obtain the desired selenium profile and to considerably increase the external quantum efficiency at long wavelengths.We observe a large increase in the spectral response and an explicit improvement in the short circuit current for CdTe cells with an absorber thickness of 2 ?m and without CdS. Current density values of 26 mA/cm2 have been achieved, which are the highest reported for thin CdTe absorbers.
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