Internal interfaces and phase boundaries play an important role in electronic devices. This holds especially for organic electronics due to the large number of organic and inorganic layers in such devices. Very often complicated layer structures with a wide variety of different materials are used to optimize device performances. The electronic and morphologic properties of these materials have to be matched up precisely demanding a detailed understanding of the underlying mechanisms at interfaces.
Furthermore, numerous types of mixed layers are applied in different functions in organic electronic devices, e.g. doped transport- and emission-layers in organic light emitting diodes and bulk-hetero-junctions in organic photovoltaic. In these mixed systems, a fundamental understanding of the interactions that affect the morphology and electronic properties is of great importance.
Interfaces of organic semiconductors
We investigate interfaces of organic semiconductors using in-situ infrared spectroscopy in ultra-high vacuum (UHV). With that technique, we are able to measure IR spectra of interfaces during controlled layer deposition in UHV. By evaluation of the spectral changes for interface layers compared to the spectra of the pure layers, we identify the charge transfer between the different materials. Moreover, it is possible to quantify the amount of transferred charges per dopant molecule even with thickness resolution.
By comparing experimental spectra to calculations, also a possible preferential orientation of the molecules can be determined for different interfaces. The relative molecular orientation at interfaces is crucial both for energy- and charge-transport across the interface. Furthermore, by performing temperature dependent measurements, we can influence the morphology of the system under investigation and can learn about the involved mechanisms, e.g. diffusion.
The controlled specific modification of the electronic and morphological properties of interfaces using self-assembled monolayers and polyelectrolytes represents the overall goal of the interdisciplinary research network.
Doping of organic semiconductors
The diffusion of molecules is particularly important for the issue of doping of organic semiconductors. Common problems are the unwanted agglomeration and diffusion of doping molecules, both of which generally lead to a decrease in device efficiency.
In analogy to the studies at interfaces, the charge transfer efficiency in doped layers can be carried out by the careful quantitative analysis of vibrational modes. For this purpose, shifts in the excitation energies as well as changes in intensity of the vibrational bands are evaluated to draw conclusions about the ratio between charged and neutral molecules.
Our research on the morphology and electronic properties of organic semiconductors at interfaces and in mixed phases is funded by the Federal Ministry of Education and Research (BMBF) within the InterPhase project (FKZ 13N13657).
Charge-Transfer – Solvent Interaction Predefines Doping Efficiency in p-Doped P3HT-Films, Chemistry of Materials, 28(12):4432, 2016.(Link)
Lars Müller, Diana Nanova, Tobias Glaser, Sebastian Beck, Annemarie Pucci, Anne K. Kast, Rasmus R. Schroeder, Eric Mankel, Patrick Pingel, Dieter Neher, Wolfgang Kowalsky, and Robert Lovrincic
The Swiss-Army-Knife Self-Assembled Monolayer: Improving Electron Injection, Stability, and Wettability of Metal Electrodes with a One-Minute Process, Advanced Functional Materials, 26(18):3172, 2016.(Link)
Milan Alt, Malte Jesper, Janusz Schinke, Sabina Hillebrandt, Patrick Reiser, Tobias Rödlmeier, Iva Angelova, Kaja Deing, Tobias Glaser, Eric Mankel, Wolfram Jaegermann, Annemarie Pucci, Uli Lemmer, U. H. F. Bunz, W. Kowalsky, G. Hernandez-Sosa, R. Lovrincic, and M. Hamburger
Why Inverted Small Molecule Solar Cells Outperform Their Noninverted Counterparts, Advanced Functional Materials, 25(41):6511-6518, 2015.(Link)
Diana Nanova, Michael Scherer, Felix Schell, Johannes Zimmermann, Tobias Glaser, Anne Katrin Kast, Christian Krekeler, Annemarie Pucci, Wolfgang Kowalsky, Rasmus R. Schröder, and Robert Lovrincic
Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light, Journal of Physical Chemistry C, 119, 12545, 2015.(Link)
Sebastian Beck, David Gerbert, Tobias Glaser, and Annemarie Pucci
- Prof. Wolfgang Kowalsky and Dr. Robert Lovrincic (TUBS, IHF)
- Prof. Wolfram Jaegermann and Dr. Eric Mankel (TUD, MaWi, Surface Science)
- Prof. Rasmus R. Schröder (Uni HD, Bioquant)
- Prof. Uwe H. F. Bunz (Uni HD, OCI)