Molecular design and synthesis of highly soluble hole transporting materials based on phenylnaphthalene cores for solution-processible OLEDs

Highly soluble small molecules are crucial to realizing large-area solution-processible organic light-emitting diodes with consistent performance. In this study, a series of solution-processable hole transporting materials (HTMs) based on 1,4- and 2,6-phenylnaphthalene (NP) cores were synthesized. The cores had an asymmetric and twisted structure, which enhanced the solubility and thermal stability of the materials. Density functional theory calculations and MD simulations revealed that the 2,6-NP core exhibited a coplanar core structure, leading to a reduced hole reorganization energy and enhanced hole mobility compared to those of the 1,4-NP core. Furthermore, the 2,6-NP-core materials exhibited a higher solubility than the 1,4-NP-core materials. 2,6-NPNP also exhibited the highest solubility, approximately 50 times higher than that of the commercialized HTM N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB). Remarkably, when 2,6-NPNP was used as the HTM, the turn-on (V_on = 2.54 V) and operating voltages (V_op = 5.67 V @ 1000 nit) of the solution-processed OLED were lower than that of the vacuum-deposited device using NPB as the HTM. Furthermore, the external quantum efficiency, current efficiency, and power efficiency of the former were 1.63 %, 4.49 cd A⁻¹, and 3.78 lm W⁻¹, respectively, which were higher than those of NPB.