Effects of CdCl2 heat treatment on the morphological and chemical properties of CdTe/CdS thin films solar cells

The morphological and chemical changes of CdTe surface in CdTe/CdS/ITO/glass solar cells, due to CdCl2 solution treatment followed by annealing in air from 340 to 420 °C were studied using SEM and XPS. SEM micrographs showed that the supersaturated CdCl2 samples exhibited recrystallization and a progressive increase in grain sizes upon annealing. The grain sizes of the samples increased up to about 3–4 μm when annealed above 410 °C. XPS spectra for the S 2p, Cd 3d and for the Te 3d were recorded and analyzed for the as-grown structures and for structures annealed at different temperatures with and without CdCl2 treatment. The spectra clearly show that annealing the CdCl2 supersaturated samples removes TeO3 peaks, while annealing the structures without CdCl2 treatment enhances the TeO3 peaks. The I–V measurements revealed that Voc higher than 680 mV and Jsc close to 20 mA/cm2 were obtained for CdTe/CdS solar cells annealed under optimum conditions of 410 °C for 15 min. The improvement of Voc and Jsc after CdCl2 heat treatment can be attributed to dissolving the oxidations at the free surfaces and recrystallizations of CdTe layer.