Dual-Function Self-Assembled Molecules as Hole-Transport Layers for Thermally Evaporated High-Efficiency Blue Perovskite Light-Emitting Diodes

Thermally evaporated perovskite light-emitting diodes (PeLEDs) hold immense potential for future applications in the display industry. However, the performance of blue PeLEDs is far behind, one of the most important reasons is the lack of suitable hole-transporting materials. Herein, the study designs and synthesizes a new class of self-assembled monolayer (SAM) materials, namely, (2-(3,6-bis(4-formylphenyl)-9H-carbazol-9-yl)ethyl)phosphonic acid (C-2PACz) and (2-(3,6-bis(4-(methylsulfonyl)phenyl)-9H-carbazol-9-yl)ethyl)phosphonic acid (S-2PACz). First, the phosphonic acid is induced to form bidentate bonds with ITO. Second, the strong electron-withdrawing groups are integrated to increase the electron cloud density of the termini contacting with perovskite, which enhances the electrostatic interaction with the Pb²⁺, reduces the interfacial defects. These advantages improve their carrier transport ability and reduce the non-radiative recombination at the interface. Meanwhile, it is found that compound C-2PACz possessing the smaller steric hindrance makes the SAMs have a more homogeneous film and a better interfacial passivation effect. By employing C-2PACz as hole-transporting layer in blue PeLEDs with metal halides as the emitting layer, the device exhibits a high brightness (1843 cd m⁻²) and a maximum external quantum efficiency (10.41% @65.59 cd m⁻²), which among the best of reported thermally evaporated sky-blue PeLEDs. The work provides new insights into design strategies for dual-function SAMs to achieve higher performance in PeLEDs.