Rational Molecular Design of Soluble Carbazole Phosphonic Acid for Hybrid Hole Transport Layers Enabling High‐Efficiency Sky‐Blue Perovskite LEDs

Abstract The application of carbazole phosphonic acid (CPA) molecules for interfacial modification in optoelectronic devices is often constrained by poor solubility in non‐polar solvents, limiting their processability. To address this issue, a novel CPA molecule, (4‐(3,6‐bis(7H‐dibenzo[c,g]carbazol‐7‐yl)‐9H‐carbazol‐9‐yl)butyl)phosphonic acid (BFC‐BPA), is synthesized, featuring an optimized molecular structure. This design incorporates carbazole and 7H‐dibenzo[c,g]carbazole as terminal groups, linked by a four‐carbon chain, with phosphonic acid functioning as the anchoring group. BFC‐BPA exhibits excellent solubility in chlorobenzene (CB), enabling efficient blending with poly(9‐vinylcarbazole) (PVK) to form a hybrid hole transport layer (HTL). This integration enhances the NiO x /HTL interface and improves the wettability of the organic HTL, facilitating the formation of high‐quality quasi‐2D perovskite films with superior crystallinity. The hybrid HTL not only regulates hole injection and transport but also ensures balanced charge transport in perovskite light‐emitting diodes (PeLEDs). This synergistic design enables high‐efficiency sky‐blue PeLEDs, achieving a maximum external quantum efficiency (EQE) of 18.57% and an average EQE increase from 6.94% to 17.01%, compared to bare PVK‐based devices. This study highlights the importance of rational molecular design in overcoming solubility challenges, enhancing CPA functional versatility, and expanding their potential applications in advanced optoelectronic devices.