9:15 AM - EL02.05.03
Dipolar Doping of Organic Semiconductors to Enhance Carrier Injection
Alexander Hofmann1,Simon Züfle2,3,Kohei Shimizu4,Markus Schmid1,Vivien Wessels1,Lars Jäger1,Stéphane Altazin3,Keitaro Ikegami4,Motiur Khan5,Dieter Neher5,Hisao Ishii4,Beat Ruhstaller2,3,Wolfgang Bruetting1
Universität Augsburg1,ZHAW Zürcher Hochschule für Angewandte Wissenschaften2,Fluxim AG3,Chiba University4,Universität Potsdam5
The strong dipole moment of polar organic semiconductor materials like tris-(8-hydroxyquinolate)aluminum (Alq3), if not oriented perfectly isotropic, will lead to the buildup of a giant surface potential (GSP), and thus to a macroscopic dielectric polarization of the organic film. Despite this being a known fact , the implications of such high potentials on charge transport and injection within and into an organic layer stack have only been studied recently. For example, we could explain and reproduce in silico the dependence of electron current in organic light emitting diodes (OLEDs) depending on the sign of the GSP of the electron transport layer (ETL).
In this contribution, we will discuss the influence of the GSP of the hole transport layer (HTL) on hole injection into organic layers.
Unfortunately, most hole conducting materials are either non-polar or show an isotropic orientation, hence we resort to the new concept of dipolar doping to tune the GSP of a hole conducting layer.
Therefor, we chose the prototypical organic materials N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) as host and Alq3 as dopant and varied the doping ratios to tune the GSP in the HTL of a prototypical OLED.
The mixtures are investigated in single-layer, monopolar devices with only the HTL as well as bilayer OLEDs with Alq3-doped NPB as HTL and neat Alq3 as ETL, respectively. The latter are treated as metal insulator semiconductor (MIS) structures following and applying our recently published method of charge extraction by linearly increasing voltage (CELIV) on polar OLEDs[3, 4].
Characterization is then done electrically using current-voltage characteristics, impedance spectroscopy, CELIV and time of flight, as well as optically with ultraviolet photoelectron spectroscopy.
For all device types, we find an optimum in device performance and carrier injection for moderate doping concentrations of about 5%. By comparing all different methods with a focus on charge injection barriers, we reveal a complex relationship of carrier transport, substrate workfunction, modified injection and the effect of polarization, effectively manipulating charge carrier injection across the metal-organic interface and transport in the device.
 Noguchi, Y., Brütting, W., & Ishii, H. (2019). Spontaneous orientation polarization in organic light-emitting diodes. Japanese Journal of Applied Physics, 58(SF), SF0801. https://doi.org/10.7567/1347-4065/ab0de8
 Altazin, S., Züfle, S., Knapp, E., Kirsch, C., Schmidt, T. D., Jäger, L., … Ruhstaller, B. (2016). Simulation of OLEDs with a polar electron transport layer. Organic Electronics, 39, 244–249. https://doi.org/10.1016/j.orgel.2016.10.014
 Züfle, S., Altazin, S., Hofmann, A., Jäger, L., Neukom, M. T., Schmidt, T. D., … Ruhstaller, B. (2017). The use of charge extraction by linearly increasing voltage in polar organic light-emitting diodes. Journal of Applied Physics, 121(17). https://doi.org/10.1063/1.498290
 Züfle, S., Altazin, S., Hofmann, A., Jäger, L., Neukom, M. T., Brütting, W., & Ruhstaller, B. (2017). Determination of charge transport activation energy and injection barrier in organic semiconductor devices. Journal of Applied Physics, 122(11). https://doi.org/10.1063/1.4992041