Functionalization of small, rod like organic molecules can be used to optimize organic devices. Here we report on nanofiber formation and thin film growth of a methoxy-functionalized para-quaterphenylene (1,4'''-Dimethoxy- 4,1':4',1'':4'',1'''-quaterphenylene, MOP4) on prototypical dielectric substrates such as muscovite mica, phlogopite mica, highly ordered pyrolytic graphite (HOPG), and on the alkali halide NaCl. The nanofibers consist of lying molecules, the films of upright standing ones. The grown samples are characterized by polarized optical microscopy (fluorescence, birefringence, bireflectance), by atomic force microscopy (AFM), and by Kelvin probe force microscopy (KPFM) to gain insight into their structure and epitaxial relation with the substrates.
We report an optimized inverted bulk-heterojunction (P3HT:PCBM) organic solar cell geometry in
order to both efficiently trap incident light within in the cell (increasing light absorption) and at the
same time provide efficient transport of the generated carriers to the electrodes (reducing the active
layer thicknesses). To address these issues, we have used two approaches. The first one consists of
including diffraction gratings that increase the light path length in the cell and thus enhance absorption
in wavelength intervals matching the absorption peak of the organic active layer on the bottomelectrode,
while the second approach includes Ag nanoparticles embedded on the solar cell topelectrode,
which scatter the incident light into the solar cell active layer.
The solar cells containing either gratings or nanoparticles exhibit a significant enhancement on the
power conversion efficiency. Furthermore, the solar cells do not contain the rare metal indium, but
employ a PEDOT:PSS based transparent electrode.
Environmentally stable, non-toxic squarylium dyes with strong absorption maxima in the red and near infrared
spectral region are known for almost fifty years. Despite the fact that their optoelectronic properties distinguish
them as promising materials for organics based photovoltaic cells, they have regained attention only very
recently. For their application in heterojunction solar cells knowledge of their nanoscopic morphology as well
as nanoscopic electrical properties is paramount. In this paper thin films from two different squarylium dyes,
from squarylium (SQ) and from hydroxy-squarylium (SQOH) are investigated. The thin films are either solution
casted or vacuum sublimed onto substrates such as muscovite mica, which are known to promote self-assembly
into oriented, crystalline nanostructures such as nanofibers. Local characterization is performed via (polarized)
optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Kelvin probe
force microscopy (KPFM).
The morphological stability of organic nanowires over time and under thermal load is of major importance for
their use in any device. In this study the growth and stability of organic nanowires from a naphthyl end-capped
thiophene grown by organic molecular beam deposition is investigated via atomic force microscopy (AFM). Aging
experiments under ambient conditions already show substantial morphological changes. Nanoscopic organic
clusters, which initially coexist with the nanowires, vanish within hours. Thermal annealing of nanowire samples
leads to even more pronounced morphology changes, such as a strong decrease in nanowire number density, a
strong increase in nanowire height, and the formation of new types of crystallites. This happens even before
sublimation of organic material starts. These experiments also shine new light on the formation process of the
nanowires.
Nanofibers from light-emitting organic molecules such as para-phenylenes have already demonstrated a promising
application potential in nanophotonic devices and can act as waveguides or nanolasers. Here, the basic mechanisms
for self-assembly of three different green- and green/blue-light emitting thiophene/phenylene co-oligomers
into nanofibers are investigated. Under well defined conditions in high vacuum the molecules are deposited on
cleaved mica surfaces. The effect of substrate surface energy as well as epitaxy on the overall film morphology
is studied and significant differences between different co-oligomers are found.
Aligned ensembles of nanoscopic nanofibers from organic molecules such as para-phenylenes for photonic applications
can be fabricated by self-assembled molecular growth on a suited dielectric substrate. Epitaxy together
with alignment due to electric surface fields determines the growth directions. In this paper we demonstrate
how aligned growth along arbitrary directions can be realized by depositing the molecules on a micro-structured
and gold covered Silicon surface, consisting of channels and ridges. For the correct combination of ridge width
and deposition temperature fibers grow perpendicular to the ridge edge, emitting light polarized along the ridges
after UV excitation.
Organic nanofibers from semiconducting conjugated molecules are well suited to meet refined demands for advanced
applications in future optoelectronics and nanophotonics. In contrast to their inorganic counterparts,
the properties of organic nanowires can be tailored at the molecular level by chemical synthesis. Recently we
have demonstrated the complete route from designing hyperpolarizabilities of individual molecules by chemically
functionalizing para-quaterphenylene building blocks to the growth and optical characterization of nonlinear,
optically active nanoaggregates. For that we have investigated nanofibers as grown via organic epitaxy. In the
present work we show how chemically changing the functionalizing end groups leads to a huge increase of second
order susceptibility, making the nanofibers technologically very interesting as efficient frequency doublers. For
that the nanofibers have to be transferred either as individual entities or as ordered arrays onto specific target
substrates. Here, we study the applicability of contact printing as a possible route to non-destructive nanofiber
transfer.
Nanofibers from symmetrically and unsymmetrically functionalized p-quaterphenylenes are fabricated by a bottom-up process on muscovite mica. The symmetrically functionalized p-quaterphenylenes emit intense, polarized blue light after unpolarized UV-excitation. Upon implementing electron push-pull functional groups like chlor and methoxy groups to the molecular building block new properties of the nanoaggregates have been generated: the nanofibers exhibit increased non-linear optical properties, acting, e.g., as frequency doublers after
excitation with NIR femtosecond laser pulses. Depending on the
growth conditions the chloro-methoxy-p-quaterphenylene forms
either parallel nanofibers or nano-branches on a muscovite mica
substrate, adding another degree of freedom for the design of, e.g.,
resonator structures.
The synthesis of molecules consisting of various combinations of phenylene and thiophene groups and the subsequent
vacuum growth of needle-shaped nanoaggregates on specific surfaces allows us a systematic investigation of
the transition between single parallel and multiple aligned needle ('nanofiber') growth. The former growth mode
is observed for blue light emitting phenylene fibers, whereas the latter growth mode appears for single crystalline
fibers made from green- and orange-light emitting oligo-thiophenes and thiophene/phenylene co-oligomers. In all
cases the tailored bottom-up growth results in strongly polarized light emission along specific surface directions.
The results are compared to those found for nanoaggregates made from less rod-like organic molecules, namely
rubrene and POPOP.
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