First-Principles DFT Insights into the Stabilization of Zinc Diphosphide (ZnP2) Nanocrystals via Surface Functionalization by 4-Aminothiophenol for Photovoltaic Applications

The resurgence of interest in zinc phosphide compounds as efficient solar absorbers has initiated increasing efforts to improve their stability under humid and oxygen-rich conditions. Although organic functionalization has been suggested as a promising strategy to passivate zinc phosphide nanoparticles, fundamental atomic-level insights into the adsorption processes and structures at zinc diphosphide (ZnP2) surfaces are still lacking. In this study, the interactions between 4-aminothiophenol and the low-Miller index surfaces of monoclinic ZnP2 have been investigated by means of density functional theory calculations. A bidentate adsorption mode, in which 4-aminothiophenol binds through both its functional groups via Zn-N and Zn-S bonds, was predicted to be the strongest form of interaction, and monolayer-functionalized ZnP2 surfaces were found to be highly stable under adsorbate-rich conditions. Changes in the equilibrium morphology of ZnP2 nanocrystallites upon functionalization and effects of humidity are also discussed. The results are expected to contribute toward the rational design of ZnP2-based materials for photovoltaic (PV) devices.