Post-deposition in situ passivation of AgBiS2 nanocrystal inks for high-efficiency ultra-thin solar cells†

Ternary chalcogenide AgBiS₂ nanocrystals (NCs) have emerged as a new environmentally friendly material for non-toxic solution-processed solar cells, with a record efficiency of ∼9%. To date, however, this has been achieved with NCs that undergo a ligand exchange process exclusively in the solid-state increasing the manufacturing complexity and cost. Improving surface passivation has been the main route towards high performance nanocrystal based solar cell devices, with current strategies relying on methods that only diversify the types of passivating ligands in solutions or stepwise ex situ additional ligand treatment. Herein, we report a post-deposition in situ passivation strategy for AgBiS₂ NC inks involving a multifunctional molecular agent that serves to provide effective colloidal dispersibility of the nanocrystal ink, as well as to passivate nanocrystal surfaces after film deposition via in situ dissociation of chloride ions as atomic surface passivants. We show that, upon improved colloidal dispersibility and surface passivation, AgBiS₂ NCs yield thin films free from morphological defects with low trap-state density and balanced charge carrier mobilities. As a result, this process leads to ultrathin-film solar cells with a fill-factor of 72% and a power conversion efficiency in excess of 10%, setting a new record for eco-friendly, solution-processed ultrathin solar cells.