In Situ, Treatment with Guanidinium Chloride Ligand Enables Efficient Blue Quantum Dot Light‐Emitting Diodes with 23.5% External Quantum Efficiency

Abstract The poor efficiency and stability of blue Quantum Dot Light‐Emitting diodes (QLED) hinders the practical applications of QLEDs full‐color displays. Excessive electron injection, insufficient hole injection, and abundant defects on the surface of quantum dots (QD) are the main issues limiting the performance of blue devices. Herein, an in situ treatment with bipolar small molecule polydentate ligand–guanidine chloride (GACl) is proposed to simultaneously suppress excessive electron injection, patch surface defects of QDs and enhance hole injection. GACl‐treated blue QLEDs exhibited a remarkable increase in maximal external quantum Efficiency (EQE) from 16.3% to a record 23.5%, accompanied by maximal luminance (36810 cd m ⁻² ), excellent maximal current efficiency (17.5 cd A ⁻¹ ), and enhanced device stability. Combining C–V and J–V characteristics, a concise physical model of hole injection is also established: Below 3 V, hole injection is controlled by the interfacial barrier, primarily through tunneling and thermionic injection; Above 3 V, the interfacial barrier is eliminated, and hole injection efficiency is governed by transport within the QD layer. This study showed a clear physical model for understanding the hole injection mechanism in QLEDs, offering valuable design strategies for improving the performance of blue‐QLEDs.