Strategy to improve the performance of solution-processed phosphorescent organic light-emitting diodes using heteroleptic green Ir(III) complexes bearing multi-functional units

For solution-processed phosphorescence organic light-emitting diodes (PHOLEDs), two multi-functional units functionalized green triplet emitters Ir(ET)-Pic and Ir(HT)-Pic were designed and synthesized. They were synthesized by employing a 2-ethylhexyl substituted amide-bridged phenylpyridine moiety applied as main ligand and picolinate (Pic) as an ancillary ligand. In addition, the 1,3-bis(N-carbazolyl)benzene (mCP) and 1,3,4-oxadiazole (OXD) groups were introduced into 3-position of main ligand via non-conjugated benzyl ether linkage. Ir (ET)-Pic and Ir(HT)-Pic exhibited an intensive green photoluminescence (PL) emission at 502 nm, narrowed full-width at half maximum (FWHM), high PL quantum yields (PLQYs) and good solubility in organic solvents. Hence, the hole injection layer optimized device using Ir(ET)-Pic and Ir(HT)-Pic as dopants showed excellent performances with a maximum external quantum efficiency (EQEmax) of 21.5 and 19.6%, and the maximum current efficiency (CEmax) of 79.4 and 69.2 cd A-1 with Commission International L'Eclairage (CIE x,y) color coordinates of (0.36, 0.60) and (0.31,0.62), respectively. In addition, we found that the presence of bulky mCP unit in the Ir(III) complex affect the hole transport characteristics in the emissive layer. The result findings, the construction of multi-functional Ir(III) complexes are an attractive design strategy for fabrication of highly efficient green solution-processed PHOLEDs.