Molecular Alignment of Homoleptic Iridium Phosphors in Organic Light-Emitting Diodes

The orientation of facial (fac) tris-cyclometalated iridium complexes in doped films prepared by vacuum deposition is investigated by altering the physical shape and electronic asymmetry in the molecular structure. Angle-dependent photoluminescence spectroscopy and Fourier-plane imaging microscopy show that the orientation of roughly spherical fac-tris(2-phenylpyridyl)iridium (Ir(ppy)₃) is isotropic, whereas complexes that are oblate spheroids, fac-tris(mesityl-2-phenyl-1H-imidazole)iridium (Ir(mi)₃) and fac-tris((3,5-dimethyl-[1,1′-biphenyl]-4-yl)-2-phenyl-1H-imidazole)iridium (Ir(mip)₃), have a net horizontal alignment of their transition dipole moments. Optical anisotropy factors of 0.26 and 0.15, respectively, are obtained from the latter complexes when doped into tris(4-(9H-carbazol-9-yl)phenyl)amine host thin films. The horizontal alignment is attributed to the favorable van der Waals interaction between the oblate Ir complexes and host material. Trifluoromethyl groups substituted on one polar face of the Ir(ppy)₃ and Ir(mi)₃ complexes introduce chemical asymmetries in the molecules at the expense of their oblate shapes. The anisotropy factors of films doped with these substituted derivatives are lower relative to the parent complexes, indicating that the fluorinated patches reinforce horizontal alignment during deposition. High efficiencies obtained from organic light emitting diodes prepared using the Ir dopants is attributed, in part, to improved outcoupling of electroluminescence brought about by molecular alignment.