Study of Positive Ageing Effect in Halide Perovskite Quantum Dots Prepared via Novel Alendronic Acid‐Based Surface‐Passivating Ligand

Caesium lead bromide (CsPbBr 3 ) perovskite quantum dots (QDs) are one of the promising materials for the fabrication of green light‐emitting diodes (LEDs) for their near‐unity photoluminescence quantum yield (PLQY) with narrow emission and high brightness. However, its ionic nature makes it susceptible to degradation from moisture, heat and light. Various surface‐passivating ligands were attempted, but with limited success in rendering long‐term stability. Here, we have developed a novel bisphosphonate ligand, alendronate (ALEN), by alkylating with hexyl chains of acidic protons of alendronic acid, which is otherwise sparingly soluble in common organic solvents. The ligand was characterised by spectroscopic techniques such as NMR, FT‐IR, and mass spectroscopies. The CsPbBr 3 QDs prepared with ALEN‐oleylamine ligand combination showed narrow particle‐size distribution as determined by high‐resolution transmission electron microscopy and dynamic light scattering measurements. The photostability of the QDs in solution and thin film was analysed by photoluminescence spectroscopy, revealing an interesting phenomenon called positive ageing effect, in which the PLQY of QD dispersion gradually increased by 140% from its initial value of 7 to 16.94% after two and a half months of storage. The defect passivation through multidentate binding sites of ALEN is the most plausible reason for such behaviour in ALEN‐stabilised CsPbBr 3 QDs.