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Abstract:
This work focuses on fundamental processes which influence the efficiencies of organic
solar cells and LEDs, for instance the formation and decay dynamics of excitons,
their diffusion, the charge transfer at interfaces between organic materials and
inorganic electrodes and the correlated energy level alignment at these interfaces.
These processes are investigated on the basis of four model systems, which represent
different parts of a solar cell or an LED, by means of time-resolved photoelectron
spectroscopy which facilitates the measurement of occupied and unoccupied states
as well as the acquisition of ultrafast processes.
ZnO is a promising material for transparent electrodes and as an active LED
medium, therefore the processes in optically excited ZnO are of great interest. The
investigations show that the electronic structure of the O-terminated ZnO surface
is strongly influenced by the adsorption of hydrogen and that the exciton formation
slows down at higher electron densities at the surface as the electron phonon coupling
is screened.
The SP6/ZnO interface can serve on the one hand as a model system for charge
transfer processes, however SP6 in thick films represents a potential LED medium. In
addition to the already known relaxation processes which have been observed before
by time-resolved optical spectroscopy, photoelectron spectroscopy reveals another
ultrafast component. The interaction of long-lived triplet states results in this system
in the emission of electrons. Furthermore, photoelectron spectroscopy allows to draw
conclusions on the absolute energies of the excited states.
The energy level alignment at the interface between a metal electrode and pi-
conjugated molecules is investigated on dicyanovinyl-substituted oligothiophenes on
gold. The gold surface influences not only the electronic structure of the monolayer,
it also significantly affects the lifetimes of excited states that increase with the
distance to the metal surface.
As a model system for a polymer semiconductor P3HT was established. The
investigation of two films with different amounts of crystalline fractions shows that
the relaxation dynamics proceeds faster in the film that features higher crystallinity
and therefore superior transport properties.
These results give an insight into the complex interrelated relaxation processes
of optically excited states. The detailed comprehension of these processes promises
their targeted utilization to optimize the efficiency of organic solar cells and LEDs.
