Controlling charge balance and exciton recombination by bipolar host in single-layer organic light-emitting diodes

Highly efficient single-layer organic light-emitting diodes with reduced efficiency roll-off are demonstrated by using a bipolar host material of 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD) doped with iridium complexes as the emissive layer. For example, the green single-layer device, employing fac-tris(2-phenylpyridine)iridium Ir(ppy)₃ as dopant, shows a peak current efficiency of 45.57 cd A^-1, corresponding to external quantum efficiency (EQE) of 12.42%, and still exhibits efficiencies of 45.26 cd A^-1 and 40.42 cd A^-1 at luminance of 1000 and 10 000 cd m^-2, respectively. In addition, the yellow and red single-layer devices, with bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1- phenyl-1H-benzoimidazol-N, C³)iridium(acetylacetonate)(fbi) ₂Ir(acac) and bis(1-phenylisoquinolinolato-C²,N) iridium(acetylacetonate) (piq) ₂Ir (acac) as emitter, also show high EQE of 7.04% and 7.28%, respectively. The transport properties of o-CzOXD film are well investigated by current-voltage measurement, from which both hole and electron mobility are determined. It is found that the o-CzOXD shows appealing bipolar transport character, which is favor for the balanced charge distribution in the whole doped zone. More importantly, the multifunctional role of hole trapping and electron transporting of the iridium complex in o-CzOXD further balances the charge carriers and broadens the recombination zone. As a result, the recombination of electrons and holes is significantly improved and the triplet-triplet annihilation and triplet-polaron quenching processes are effectively suppressed, eventually leading to the high efficiency as well as the reduced efficiency roll-off.