Raman scattering analysis of the surface chemistry of kesterites: Impact of post-deposition annealing and Cu/Zn reordering on solar cell performance

Abstract

Post-deposition annealing (PDA) of chalcogenide-based solar cells has shown to enhance optoelectronic properties, though the mechanism for such improvement is still poorly understood. In this work Cu2ZnSnSe4 kesterite-based solar cells are fabricated and subjected to different PDA temperatures in order to study the structure/function relationships and explain the responsible mechanisms. Diffusion and recrystallization phenomena at the absorber/buffer interface during the PDA are investigated using multiwavelength Raman spectroscopy and photoluminescence, and correlated to the optoelectronic properties. By using varying laser excitations (442, 532, 785nm), the selective acquisition of structural information at different depths is obtained, thus providing insights of the differences in the composition and defect concentration in the device cross section. The results show that PDA treatments of completed devices induce a redistribution of atoms (Cu and Zn) within the surface and sub-surface of the absorber layer. The absorber surface becomes more Cu-depleted and Zn-enriched, creating optoelectronically beneficial defects VCu and ZnCu, which are partly responsible for the performance improvement. Recrystallization effects of the CdS layer are observed, leading to a better absorber-buffer interface and potentially a better band alignment. Additionally, this work opens the possibility of tuning the defect concentration in the absorber with the PDA temperature.