Novel Ir(III) Complexes with NHC-Based Ancillary Ligands for Efficient Nondoped OLEDs

Nondoped organic light-emitting diodes (OLEDs) are of paramount importance for display and lighting applications owing to their advantages of facile fabrication and outstanding stability. However, nondoped OLEDs achieving extraordinary electroluminescence (EL) performance and low turn-on voltage (V_on) remain sparse. Here, three Ir(III) complexes featuring N-heterocyclic carbene (NHC) auxiliary ligands functionalized with electron-deficient aromatic sulfonyl or phosphine oxide groups are reported as promising emitters for nondoped OLEDs. All Ir(III) complexes exhibit green emission with relatively high neat film efficiency. Although the photoluminescence spectra of three complexes reveal similarities, there are distinct differences in the nondoped EL performance. The nondoped device N3 based on tBu-Ir-ISO displays the most eminent EL performances and presents a low V_on of 2.1 V, a power efficiency of 30.7 lm W^-1, and a maximum current efficiency of 27.0 cd A^-1, which can be attributed to steric hindrance and balanced carrier-transporting ability induced by electron-deficient substituents. Moreover, doped devices D1-D3 also realize excellent EL performance. It is believed that the strategy reported herein is a simple and efficient way of constructing excellent Ir(III) complexes for nondoped phosphorescent OLEDs.