Efficient Warm-White OLEDs Based on the Phosphorescent bis-Cyclometalated iridium(III) Complex

The starburst carbazole derivative and phosphorescent bis-cyclometalated iridium(III) complex (IC2) were used for the preparation of multilayered warm-white organic light-emitting diodes (OLEDs), the emission spectra of which are modulated by the thickness of the phosphorescent layer. It was shown that the electroluminescence spectra of the fabricated devices are more extended into the visible region compared with the photoluminescence spectra of both component materials. The observed extension of the electroluminescence spectra can be assigned to the phosphorescent emission of the low-energy exciplex formed at the interface of the emissive layers. The quantum-chemical calculations performed by the DFT and (TD) DFT methods support the formation of the low-energy triplet exciplex at the interface of the IC2 layer and the neighboring layer of the starshaped carbazole-based compound, (4,4',4-tris[3-methylphenyl(phenyl)amino] triphenylamine, tri(9-hexylcarbazol-3-yl)amine (THCA). Indeed, the triplet excited state of such bimolecular complex corresponds to intermolecular charge transfer between IC2 and THCA. The experimentally observed electrophosphorescence of these exciplexes is induced by strong spin-orbit coupling in the THCA:IC2 complexes due to the Ir(III) heavy atom effect. With dependence on the iridium(III)-complex film thickness (5-9 nm), the CIE coordinates changed from (0.41, 0.41) to (0.52, 0.47), corresponding to the warm white and orange color. The brightness of the fabricated OLEDs at the 15 V bias was in the range from 500 to 6000 cd/m(2).