Electrical Interfaces for Organic Nanodevices

Optoelectronic applications of organic semiconductor materials is a research
field, which recently came to the large scale consumer market in display technologies.
Organic semiconductors are mainly applied in amorphous form offering
fabrication control on a large scale. Crystalline organic semiconductors,
where the molecular packing is more crucial, have not yet had a major impact
in commercial products. This thesis describes development of new ways
to electrically contact organic semiconductors. In particular, crystalline organic
para-hexaphenylene (p6P) nanofibers have been used as a representative
component for the organic nanofiber class.
Organic light emitting devices based on nanofibers cannot readily be fabricated
by conventional methods developed for thin film devices. A novel design
of layered top contacts, separated by an insulating layer, was fabricated using
three different approaches. Creating the separator by partly oxidizing an Al
cathode anodically is considered the most promising implementation, however
further development would be necessary.
During the project a group of collaborators managed to obtain electrically
stimulated light emission in organic p6P nanofibers, by using an AC-gated
organic field-effect transistor (OFET) implementation.
The electrical properties of arrays of p6P nanofibers were investigated asgrown
and modeled theoretically. The developed model, assuming hopping-like
transport of charge carriers, was used to estimate the distance between hopping
sites. A distance of 23±5 nm was extracted and found to be in good agreement
with transmission electron microscopy (TEM) studies.
Graphene, a one atom thin 2D crystal of carbon, has several properties
relevant for electrodes: it is atomically flat, optically transparent, does not
oxidize, and has high electrical and thermal conductivity. In this project the
use of graphene as an electrode material for organic electronics was investigated.
For this purpose a fabrication process compatible with contamination sensitive
cleanroom equipment was developed. First the process was applied to fabricate
arrays of OFET templates and p6P applied as the organic semiconductor. The
tested devices exhibited large injection barriers and significant hysteresis of the
electrical characteristics. Therefore the device design was found unsuitable to
elucidate the possible advantages of graphene electrodes in OFETs.
Secondly the electrode fabrication method was applied to realize electrodes
for dielectrophoresis experiments. Robust electrodes with multi-layer graphene
contact pads and few-layer graphene electrode edges were made. Carbon nanotubes
were assembled with dielectrophoresis between electrodes. Optimization
of the dispersion prevented the graphitic electrodes from being washed off, and
the same samples could be reused for several experiments. During the experiments it was discovered that thin films of p6P on graphitic
substrates can form crystalline domains. Molecular orientations on the samples
were probed by fluorescence and white light polarization experiments. It was
found that blue reflected light has the same polarization as fluorescence from
the samples. This can be used to probe molecular orientations in these samples
and completely avoid the bleaching effect of UV-excitation. An investigation
of the morphological and molecular orientations within the domains, in relation
to the graphitic lattice, showed growth of two different crystalline phases.
One of the phases was found comparable to the β-phase typically observed on
mica substrates. The morphology of the other phase had formed nanofiber-like
aggregates on the substrates with typical dimensions up to 500×20 nm2. A
possible application was demonstrated by growing nano-aggregates of p6P on
a suspended graphene membrane, which could be used for TEM studies of the
as-grown crystalline properties of p6P.