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978-3-8439-1177-1, Reihe Physik

Torben Menke
Molecular Doping of Organic Semiconductors - A Conductivity and Seebeck Study

177 Seiten, Dissertation Technische Universität Dresden (2013), Hardcover, A5

Zusammenfassung / Abstract

This work, carried out at Technische Universität Dresden, Germany, aims at improving the understanding of the fundamental physics behind molecular doping of organic semiconductors, being a requirement for efficient devices like organic light-emitting diodes (OLED) and organic photovoltaic cells (OPV). The underlying physics is studied by electrical conductivity and thermoelectrical Seebeck measurements and the influences of doping concentration and temperature are investigated. Doped layers are prepared in vacuum by thermal co-evaporation of host and dopant molecules and measured in-situ.

The fullerene C60, known for its high electron mobility, is chosen as host for five different n-dopants. Two strongly ionizing air-sensitive molecules (Cr2(hpp)4 and W2(hpp)4) and three air-stable precursor compounds (AOB, DMBI-POH and o-MeO-DMBI-I) which form the active dopants upon deposition are studied to compare their doping mechanism. High conductivities are achieved, with a maximum of 10.9 S/cm. Investigating the sample degradation by air-exposure, a method for regeneration is proposed, which allows for device processing steps under ambient conditions, greatly enhancing device fabrication possibilities.

Various material combinations for p-doping are compared to study the influence of the molecular energy levels of host (MeO-TPD and BF-DPB) and dopant (F6-TCNNQ and C60F36). Corrections for the only estimated literature values for the dopant levels are proposed. Furthermore, the model system of similar-sized host pentacene and dopant F4 -TCNQ is studied and compared to theoretical predictions.

Finally, a model is developed that allows for estimating charge carrier mobility, density of free charge carriers, doping efficiency, as well as the transport level position from combining conductivity and Seebeck data.