Tuning the Electronic Structure of Organic/Metal Contacts via Additive Interfacial Segregation


All organic-based devices, i.e., "organic electronics", require good electrical contacts between the
organic semiconductor and an electrode, most commonly a metal. It has been established early-on
that the nature of the metal/organic electrical contact often determines the overall device
performance and stability. Tuning the interfacial energy level alignment can generally be achieved by
depositing interlayers between the organic semiconductor and the metal contact. However, understanding and eventually adjusting the energy level alignment at buried interfaces formed by
depositing a metal electrode on top of a pre-formed organic semiconductor poses formidable
challenges. Very recently, we have discovered that the evaporation of a metal contact onto a polymer
blend could induce phase separation and segregation within the polymer film. Aluminum evaporation
induced segregation of additives to form (partial) interlayers at the polymer/metal interface and
significantly increased the performance of a photovoltaic cell. In the proposed project we aim to
establish a new paradigm for optimizing the energy level alignment of buried metal/organic interface
in organic electronic devices by inducing additive segregation to the organic/metal interface. The
additives will be judiciously selected to segregate to the organic/metal interface and to form the
desired interfacial interactions. The presence of the interlayers will be confirmed using mainly X-ray
photoelectron spectroscopy (XPS) and electron microscopy. The resulting electronic structure will be
measured by ultraviolet photoelectron spectroscopy (UPS), and also high kinetic energy photoemission, which allows probing composition and energy level alignment up to 30 nm below the sample surface. Finally, the positive effect of energy level tuning on device performance will be established through comprehensive characterization of photovoltaic cells and thin film transistors. Revealing the fundamental mechanism(s) allowing this type of interfacial energy level tuning will enable establishing reliable knowledge-based further development of the method for the use in a broad range of organic electronics applications.

Principal Investigators
Koch, Norbert Prof. Dr. techn. (Details) (Structure, Dynamics and electronic Properties of Molecular Systems)

Participating external organizations

Duration of Project
Start date: 01/2015
End date: 12/2017

Research Areas
Experimental Condensed Matter Physics

Research Areas
Experimentelle Physik, kondensierte Materie

Publications
Salazar Alarcón, L. et al. Growth of 1, 4-benzenedimethanethiol films on Au, Ag, and Cu: Effect of surface temperature on the adsorption kinetics and on the single versus multilayer formation. The Journal of Physical Chemistry C 117, 17521-17530 (2013).

Shamieh, B., Obuchovsky, S. & Frey, G. L. Spontaneous generation of interlayers in OPVs with silver cathodes: enhancing Voc and lifetime. Journal of Materials Chemistry C 4, 1821-1828, doi:10.1039/C5TC04141D (2016).

Aqua, T. et al. Role of Backbone Charge Rearrangement in the Bond-Dipole and Work Function of Molecular Monolayers. The Journal of Physical Chemistry C 115, 24888-24892, doi:10.1021/jp208411f (2011).

Hong, J.-P. et al. Tuning of Ag work functions by self-assembled monolayers of aromatic thiols for an efficient hole injection for solution processed triisopropylsilylethynyl pentacene organic thin film transistors. Applied Physics Letters 92, 131 (2008).

Oehzelt, M., Koch, N. & Heimel, G. Organic semiconductor density of states controls the energy level alignment at electrode interfaces. Nature Communications 5, 4174, doi:10.1038/ncomms5174
https://www.nature.com/articles/ncomms5174#supplementary-information (2014).

Deckman, I., Obuchovsky, S., Moshonov, M. & Frey, G. L. Chemical Composition of Additives That Spontaneously Form Cathode Interlayers in OPVs. Langmuir 31, 6721-6728, doi:10.1021/acs.langmuir.5b00884 (2015).

Vinokur, J. et al. Dynamics of Additive Migration to Form Cathodic Interlayers in Organic Solar Cells. ACS Applied Materials & Interfaces 9, 29889-29900, doi:10.1021/acsami.7b06793 (2017).

Zhou, Y. et al. A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics. Science 336, 327-332, doi:10.1126/science.1218829 (2012).

Koch, N. Electronic structure of interfaces with conjugated organic materials. physica status solidi (RRL) – Rapid Research Letters 6, 277-293, doi:10.1002/pssr.201206208 (2012).

Last updated on 2020-15-03 at 23:10