EP02.04.03 : Correlating Optical and Electrical Dipole Characterization to Pinpoint Phosphorescent Dye Orientation

5:00 PM–7:00 PM Apr 3, 2018 (America - Denver)

PCC North, 300 Level, Exhibit Hall C-E

Thomas Morgenstern1 Markus Schmid1 Alexander Hofmann1 Markus Bierling1 Lars Jäger1 Wolfgang Bruetting1

1, University of Augsburg, Augsburg, , Germany

Deducting the exact molecular alignment of heavy metal phosphors, such as Ir-complexes, performing as emissive species in organic light emitting diodes has been in the focus of research within the last few years. Although substantial progress has been made using angular dependent spectroscopy[1], exciton lifetime measurements[2] and several computational approaches[3,4], the exact morphology is still not unveiled.

In order to pinpoint the dye alignment, we introduce a novel measurement technique, identifying the preferential orientation of the permanent dipole moment, which is present in many common emissive molecules. Therein the experimental procedure is based on impedance spectroscopy, measuring interfacial polarization induced by alignment of electrical dipoles. This technique has previously been used to identify the preferential alignment of Tris(8-hydroxyquinolinato)aluminium, which has no indication for optical anisotropy. [5]

The application of this new measurement technique reveals a huge interfacial polarization of the emissive guest host system for heteroleptic dyes, while its homoleptic counterparts show no evidence for this effect. For heteroleptic Ir-complexes the resultant polarization depends on the guest-host composition, whereupon the dependence is non-linear indicating concentration dependent processes like aggregation.

Combining both, optical and electrical experiments, allows for a more detailed insight into the morphological behavior of heteroleptic dye molecules. It is important to note that due to the amorphous nature of the film not only specific molecular alignments, but a widespread distribution has to be taken into account. The results reveal a very distinct alignment of the molecular C2 symmetry axis at low guest concentrations, wherupon the values are in agreement with common predictions for molecular orientation.

While the presented film characterization technique is of huge importance for the in depth understanding of Ir-dye molecules, it can also be applied to further polar organic systems. Thus, enabling a new characterization technique for thin films in organic electronics.

[1] M. Flämmich, J. Frischeisen, D. S. Setz, D. Michaelis, B. C. Krummacher, T. D. Schmidt, W. Brütting, N. Danz, Organic Electronics 2011, 12, 1663.
[2] R. Mac Ciarnain, D. Michaelis, T. Wehlus, A. F. Rausch, S. Wehrmeister, T. D. Schmidt, W. Brütting, N. Danz, A. Bräuer, A. Tünnermann, Scientific reports 2017, 7, 1826.
[3] C. Tonnelé, M. Stroet, B. Caron, A. J. Clulow, R. C. R. Nagiri, A. K. Malde, P. L. Burn, I. R. Gentle, A. E. Mark, B. J. Powell, Angewandte Chemie (International ed. in English) 2017, 56, 8402.
[4] C.-K. Moon, K.-H. Kim, J.-J. Kim, Nature communications 2017, 8, 791.
[5] L. Jäger, T. D. Schmidt, W. Brütting, AIP Advances 2016, 6, 95220.