Studies on process induced optoelectronic and structural modifications of CdS/CZTS heterojunction interface affecting solar cell efficiency

Modification in the structure and electronic properties of the heterojunction interface between p-Cu₂ZnSnS₄ (CZTS) films and n-CdS films induced by inevitable process related annealing used in the fabrication of solar cells has been studied. XRD and XPS studies show Cu and Cd cross diffusion across the CdS/CZTS interface in the as-formed state and after annealing at 200°C for 10-60 min. These modifications shift the absorption edge of n-type heterojunction layer towards the lower energy side diminishing the spectrally important light radiation reaching the p-CZTS absorber which reduces carrier generation and short circuit current. Diffusing Cd into Cu vacancy site result in forming a low field p^- n^- n⁺ buried homojunction within p-CZTS as observed by dark current-voltage characteristics. For the CdS/CZTS junction it is shown as an undesirable effect for efficient generation and collection of photo-excited carriers.