title: Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials.
abstract: Controlling the alignment of the emitting molecules used as dopants in organic light-emitting diodes is an effective strategy to improve the outcoupling efficiency of these devices. To explore the mechanism behind the orientation of dopants in films of organic host materials, we synthesized a coumarin-based ligand that was cyclometalated onto an iridium core to form three phosphorescent heteroleptic molecules, (bppo)2Ir(acac), (bppo)2Ir(ppy) and (ppy)2Ir(bppo) (bppo represents benzopyranopyridinone, ppy represents 2-phenylpyridinate, and acac represents acetylacetonate). Each emitter was doped into a 4,4'-bis(N-carbazolyl)-1,1'-biphenyl host layer, and the resultant orientation of their transition dipole moment vectors was measured by angle-dependent p-polarized photoluminescent emission spectroscopy. In solid films, (bppo)2Ir(acac) is found to have a largely horizontal transition dipole vector orientation relative to the substrate, whereas (ppy)2Ir(bppo) and (bppo)2Ir(ppy) are isotropic. We propose that the inherent asymmetry at the surface of the growing film promotes dopant alignment in these otherwise amorphous films. Modelling the net orientation of the transition dipole moments of these materials yields general design rules for further improving horizontal orientation.
authors.affiliation: Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Jurow
authors.affiliation: Institute of Physics, University of Augsburg, 86135 Augsburg, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Mayr
authors.affiliation: Institute of Physics, University of Augsburg, 86135 Augsburg, Germany.
authors.forename: Tobias D
authors.initials: TD
authors.lastname: Schmidt
authors.affiliation: Institute of Physics, University of Augsburg, 86135 Augsburg, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Lampe
authors.affiliation: Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
authors.forename: Peter I
authors.initials: PI
authors.lastname: Djurovich
authors.affiliation: Institute of Physics, University of Augsburg, 86135 Augsburg, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Brütting
authors.affiliation: Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA.
authors.forename: Mark E
authors.initials: ME
authors.lastname: Thompson