Pyridine-containing phenanthroimidazole electron-transport materials with electron mobility/energy-level trade-off optimization for highly efficient and low roll-off sky blue fluorescent OLEDs

Abstract

By incorporating the pyridine ring to the different positions of the rigid phenanthroimidazole skeleton, three new pyridine-containing phenanthroimidazole derivatives, 1-phenyl-2-(4-(pyridin-3-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (CPI-p3Py), 2-phenyl-1-(4-(pyridin-3-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (NPI-p3Py) and 1,2-bis(4-(pyridin-3-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (CNPI-p3Py), were designed and synthesized as electron-transport materials (ETMs). Their photophysical properties, energy levels and electron mobilities can be readily regulated through changing the connecting position. Through optimizing electron mobility and energy levels trade-offs, CNPI-p3Py exhibits not only lower-lying HOMO/LUMO energy levels, but also a relatively high electron mobility of around 10⁻³ cm² V⁻¹ s⁻¹, which is 2-fold greater than that of the widely used material BPhen. More interestingly, the blue fluorescent OLEDs with CPI-p3Py or CNPI-p3Py as an electron-transporting layer (ETL) exhibited superior performance compared to that using Bphen, remarkably lowering the driving voltage and improving efficiencies. In particular, the device with CNPI-p3Py as an ETL showed a maximum current efficiency of 15.17 cd A⁻¹, a maximum power efficiency of 10.65 lm W⁻¹, a maximum external quantum efficiency (EQE) of 7.75% and low efficiency roll-off even at an illumination-relevant luminance of 10 000 cd m⁻² by the efficient utilization of the up-conversion of a triplet excited state, which are among the most efficient ETMs and proposes a new strategy for the molecular design of high-performance ETMs.