Efficient and spectrally stable pure blue light‐emitting diodes enabled by phosphonate passivated CsPbBr₃ nanoplatelets with conjugated polyelectrolyte‐based energy transfer layer

Lead halide perovskites exhibit a very wide color gamut due to their extremely narrow emission spectra, typically characterized by a full‐width at half‐maximum (FWHM) of less than 20 nm. Significant advancements have been made in developing highly efficient and stable green, red, and near‐infrared perovskite light‐emitting diodes (PeLEDs). However, achieving efficient and stable pure blue‐emitting PeLEDs remains a significant challenge. In this work, we successfully synthesized monoanionic octyl‐phosphonate capped CsPbBr 3 nanoplatelets (OPA‐NPLs) using a combination of octyl‐phosphonic acid and oleylamine at room temperature, diverging from common approaches that necessitate complex high‐temperature methods, such as hot injection, to accommodate short‐chain ligands. The OPA‐NPLs exhibit pure blue photoluminescence at 462 nm with a FWHM of 14 nm. Compared with CsPbBr 3 nanoplatelets synthesized using oleic acid, OPA‐NPLs demonstrate significantly improved thermal stability and higher photoluminescence quantum yield (PLQY) of 90%. Additionally, we introduced Poly[(9,9‐bis(3′‐(( N,N ‐dimethyl)‐ N ‐ethylammonium)‐propyl)‐2,7‐fluorene)‐alt‐2,7‐(9,9‐dioctylfluorene)]dibromide (PFN‐Br), a conjugated polyelectrolyte material, as a hole transport layer. This facilitated energy transfer between PFN‐Br and the CsPbBr 3 nanoplatelets. The resulting device demonstrated an electroluminescence peak at 462 nm, an extremely narrow FWHM of 14 nm, and a maximum external quantum efficiency (EQE) of 4%. Notably, the device maintained pure blue emission without spectral peak shift even during degradation caused by excess joule heating.