Green and red phosphorescent organic light-emitting diodes with ambipolar hosts based on phenothiazine and carbazole moieties: photoelectrical properties, morphology and efficiency

The two low-molar mass compounds 10,10'-(9-ethyl-9H-carbazole-3,6-diyl)bis(10H-phenothiazine) and 10-(9-ethyl-9H-carbazol-3-yl)-10H-phenothiazine were synthesized as ambipolar hosts for phosphorescent organic light emitting diodes. The structure and properties of these compounds were studied by X-ray crystallographic analysis, extensive UV-vis and fluorescence spectrometry, cyclic voltammetry and theoretical calculations. These compounds exhibited triplet energies of 2.6 eV; the shallow ionization potentials in the interval of 5.10-5.25 eV. The ambipolar semiconductor properties of the amorphous layers for both compounds were proved by the experimental and Marcus hooping theory methods. Importantly, the compounds appeared to be ideally suited for the green and red phosphorescent organic light-emitting diodes as the hosts for Ir(III) dyes. The maximum power and external quantum efficiencies up to 47.5/40.6 lm W-1 and 20.0/10.5% were observed for the green and red devices, respectively. The quenching processes such as a triplet-triplet annihilation and triplet-polaron quenching contributing to the roll-off of the quantum efficiency were investigated. The considerable widths of the exciton formation zone of 13 and 17 nm were estimated for the best performance devices. The morphology of the physical vapor deposited layers of the different composition mixtures of the hosts and Ir(III) emitters was examined using atomic force microscopy. Based on the obtained morphological information the preliminary correlation between the changes in a surface morphology and the widths of the exciton formation zone were established.