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Research


Since the beginning of the 21st century there has been an explosion of interest in a new class of materials, commonly known as Organic Semiconductors. The enormous progress in this field has been driven by the expectation to realize new applications, such as large area, flexible light sources and displays, low-cost printed integrated circuits or plastic solar cells from these materials. Owing to the large efforts of both academic and industrial research laboratories, organic semiconductor devices have progressed rapidly and meanwhile lead to first commercial products incorporating organic semiconductors. Our group focuses on the fundamental physics behind this rapidly developing field of organic electronics.

Photophysics and light extraction in organic light-emitting diodes (OLEDs)

LOGO-OLED

Like in their inorganic counterparts, the working principle of organic light-emitting diodes relies on the radiative recombination of injected electron–hole pairs in a solid, a process termed electroluminescence. In detail, however, the nature of electronic excitations and the structural peculiarities of organic molecular materials lead to marked differences concerning their opto-electronic properties. This holds true particularly for emitter materials in OLEDs, where spin-selection rules and molecular orientation are important parameters in determining their efficiency of converting electrical current into visible light. Our group is developing methods to analyse and improve OLED efficiency by combining experimental studies with numerical simulation. Recent research highlights have been the discovery of non-isotropic emitter orientation in molecular guest-host systems and its implementation in phosphorescent OLEDs as well as devices based on thermally activated delayed fluorescence.

Molecular donor/acceptor solar cells

Owing to the excitonic nature of photoexcitations in organic semiconductors, the working mechanism of organic solar cells relies on the donor-acceptor (D/A) concept enabling photoinduced charge transfer at the interface between two organic materials with suitable energy-level alignment. However, the introduction of such a heterojunction is accompanied by additional energy losses compared to an inorganic homojunction cell due to the presence of a charge-transfer (CT) state at the D/A interface. We are investigating the interrelation of film structure and morphology, electronic structure at interfaces as well as charge and exciton transport properties of a number of prototypical molecular D/A pairs and their performance in organic photovoltaic cells (OPVs). In particular, we develop quantitative methods to extract information on the mechanisms of charge generation and recombination in OPVs as well as their energy losses determining the overall power conversion efficiency.

Charge transport in organic field-effect transistors (OFETs)

OFET mit Rubren-Kristall

Due to the intrinsically very low charge carrier density in organic semiconductors electrical conduction can be enabled by field-effect doping at the interface to a dielectric material. Such field-effect transistors are on the one hand the building blocks for logic circuits but on the other hand also serve as an important tool to study transport properties in a variety of materials. Traditionally, organic semiconductors have been classified as either p- or n-type, depending on the nature of mobile excess carriers detectable in thin film devices. In our work, we focus on the growth behaviour of organic semiconductors on surfaces terminated with highly ordered monolayers of insulating molecules and their electrical characteristics in OFETs. We find that many organic semiconductor materials show ambipolar transport with comparable charge carrier mobilities for electrons and holes, provided that surface traps are sufficiently well passivated and that proper contact materials with low injection barrier are chosen.

 Further Reading

  • Organic Semiconductors, W. Brütting, in: R.G. Lerner, G.L. Triggs (Hrsg.): Encyclopedia of Physics, Wiley-VCH (2005) 1866-1876 [PDF]
  • Grundlagen der organischen Halbleiter, W. Brütting, W. Rieß, Physik Journal 7(5) (2008) 33-38. [PDF]
  • More light from OLEDs, Europhysics News 42 (4) (2011) 20-24. [PDF]
  • Physics of Organic Semiconductors 2nd Edition, W. Brütting, C. Adachi, Wiley-VCH (2012). [Wiley-VCH]
  • Mehr Licht durch orientierte Farbstoffmoleküle, T.D. Schmidt, T. Lampe, W. Brütting, Nachrichten aus der Chemie 64 (5) (2016) 514-518 [Link]

 Collaborative Research Projects

  • German Ministry for Education and Research (BMBF) - "OLYMP - Organische Lichtemittierende Systeme auf Basis von energie- und kosteneffizienten Materialien und Prozessen") [Web-Link]
  • Bavarian State Ministry of Science, Research and the Arts - “Solar Technologies go Hybrid (SolTech) [Web-Link]
  • German Research Foundation (DFG) - Priority Program SPP 1355 (“Elementary Processes of Organic Photovoltaics”) [Web-Link]
  • German Ministry for Education and Research (BMBF) - "Designprinzipien in der organischen Elektronik" [PDF]