SKM 2023 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 7: Poster I

HL 7.25: Poster

Monday, March 27, 2023, 13:00–15:00, P2/EG

Spontaneous orientation polarization in phosphine oxides and how to control it — •Albin Cakaj, Markus Schmid, Alexander Hofmann, and Wolfgang Brütting — Institut für Physik, Universität Augsburg, Germany

The orientation of small organic molecules in amorphous thin films prepared by vapor-deposition is an important aspect in further improvement of organic semiconductor devices. This feature can influence the optical and electrical properties of organic light emitting diodes (OLEDs) drastically. Due to their asymmetric molecular structure, microscopic properties like the permanent dipole moment (PDM) can show preferential alignment and lead to a macroscopic film polarization in the organic layer. This so-called spontaneous orientation polarization (SOP) affects the charge injection and accumulation behaviour in a device. In our studies, we investigated the electrical and optical orientation of a group of organic molecules, characterized by P=O double bonds, and how to control it by their design and the film-growth conditions. Many of these phosphine oxides have a large PDM, due to a strong electronegative oxygen. However, the magnitude of their SOP is often affected by the presence of multiple molecular conformers. Thus, using less polar species with only one P=O bond yields superior performance with record-high SOP of up to 160mV/nm, even though optical measurements hint to almost no preferred orientation on average.