Sequential solid-state multiligand exchange of FAPbI3 quantum dots for more efficient and stable photovoltaic devices

Despite their favorable bandgap for photovoltaic applications, ligand-passivated perovskite quantum dots (PQDs) face challenges related to reduced photogenerated carrier mobility and separation, primarily due to long insulating surface ligands. This limitation significantly hampers their efficiency and performance. In this study, we present a sequential solid-state multiligand exchange process for FAPbI3 PQDs, utilizing a solution of 3-mercaptopropionic acid (MPA) and formamidinium iodide (FAI) in methyl acetate (MeOAc) to replace the long-chain octylamine (OctAm) and oleic acid (OA) ligands. Stable FAPbI3 PQDs with an average size of ∼11 nm were synthesized via a modified ligand-assisted reprecipitation method, followed by liquid/solid purification with MeOAc, achieving ∼85% ligand removal, confirmed by 1H NMR. Subsequently, 1H NMR showed the passivation of nanocrystals with short-chain MPA and FAI ligands. We demonstrate that this sequential multiligand exchange process significantly enhances the current density of n-i-p solar cells by approximately 2 mA cm-2 and achieves a 28% improvement in power conversion efficiency. Notably, the ligand-exchanged solar cells exhibit reduced hysteresis and improved stability. Photoluminescence and electrochemical impedance spectroscopy reveal that hybrid MPA/FAI passivation improves thin-film conductivity and quality by reducing inter-dot spacing and defects, thereby mitigating vacancy-assisted ion migration. The surface-engineered FAPbI3 PQDs, enabled by this multiligand exchange approach, demonstrate significant potential for advancing next-generation photovoltaic technologies.