Improving the Stability of Colloidal CsPbBr3 Nanocrystals with an Alkylphosphonium Bromide as Surface Ligand Pair

TL;DR

This paper introduces a phosphonium-based ligand, TTP-Br, that significantly enhances the stability and photoluminescence efficiency of CsPbBr3 nanocrystals, making them more suitable for optoelectronic devices.

Contribution

The study demonstrates the use of TTP-Br as a surface ligand to improve stability and quantum yield of CsPbBr3 nanocrystals, with detailed theoretical and experimental validation.

Findings

PLQY increased from 60% to over 90% with TTP-Br treatment

TTP-Br-capped NCs retained 90% of PLQY after six weeks in air

LEDs with TTP-Br NCs achieved 17.2% external quantum efficiency

Abstract

In this study, we synthesised a phosphonium-based ligand, trimethyl(tetradecyl)phosphonium bromide (TTP-Br), and employed it in the post-synthesis surface treatment of Cs-oleate-capped CsPbBr3 NCs. The photoluminescence quantum yield (PLQY) of the NCs increased from 60% to more than 90%, as a consequence of replacing Cs-oleate with TTP-Br ligand pairs. Density functional theory calculations revealed that TTP+ ions bind to the NC surface by occupying Cs+ surface sites and orienting one of their P-CH3 bonds perpendicular to the surface, akin to quaternary ammonium passivation. Importantly, TTP-Br-capped NCs exhibited higher stability in air compared to didodecyldimethylammonium bromide-capped CsPbBr3 NCs (which is considered a benchmark system), retaining 90% of their PLQY after six weeks of air exposure. Light-emitting diodes fabricated with TTP-Br-capped NCs achieved a maximum external quantum efficiency of 17.2 %, demonstrating the potential of phosphonium-based molecules as surface ligands for CsPbBr3 NCs in optoelectronic applications.