Highly Efficient, Solution-Processed, Single-Layer, Electrophosphorescent Diodes and the Effect of Molecular Dipole Moment

A new family of highly soluble electrophosphorescent dopants based on a series of tris-cyclometalated iridium(III) complexes (1-4) of 2-(carbazol-3-yl)-4/5-R-pyridine ligands with varying molecular dipole strengths have been synthesized. Highly efficient, solution-processed, single-layer, electrophosphorescent diodes utilizing these complexes have been prepared and characterized. The high triplet energy poly(9-vinylcarbazole) PVK is used as a host polymer doped with 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) for electron transport. Devices with a current efficiency of 40 cd A(-1) corresponding to an EQE of 12% can thus be achieved. The effect of the type and position of the substituent (electron-withdrawing group (CF3) and electron-donating group (OMe)) on the molecular dipole moment of the complexes has been investigated. A correlation between the absorption strength of the singlet metal-to-ligand charge-transfer ((MLCT)-M-1) transition and the luminance spectral red shift as a function of solvent polarity is observed. The strength of the transition dipole moments for complexes 1-4 has also been obtained from TD-DFT computations, and is found to be consistent with the observed molecular dipole moments of these complexes. The relatively long lifetime of the excitons of the phosphorescence (microseconds) compared to the charge-carrier scattering time (less than nanoseconds), allows the transition dipole moment to be considered as a "quasi permanent dipole". Therefore, the carrier mobility is sufficiently affected by the long-lived transition dipole moments of the phosphorescent molecules, which are randomly oriented in the medium. The dopant dipoles cause positional and energetic disorder because of the locally modified polarization energy. Furthermore, the electron-withdrawing group CF3 induces strong carrier dispersion that enhances the electron mobility. Therefore, the strong transition dipole moment in complexes 3 and 4 perturbs both electron and hole mobilities, yielding a reduction in exciton formation and an increase in the device dark current, thereby decreasing the device efficiency.