We report on development of flexible PCPDTBT:PCBM solar cells with integrated diffraction gratings on the bottom electrodes. The presented results address PCPDTBT:PCBM solar cells in an inverted geometry, which contains implemented grating structures whose pitch is tuned to match the absorption spectra of the active layer. This optimized solar cell structure leads to an enhanced absorption in the active layer and thus improved short-circuit currents and power conversion efficiencies in the fabricated devices. Fabrication of the solar cells on thin polyimide substrates which are compatible with the lithographically processed grating structures are done in order to obtain the efficiency enhancement in thin, flexible devices.
In-situ grown organic nanofibers have been prepared on metal electrodes patterned by electron beam lithography. A
systematic investigation shows that the light emission from these nanofibers driven by an AC gate voltage depends nonlinearly
on the amplitude of the AC gate voltage and linearly on the frequency of the gate voltage, which indicates that a
model involving thermally assisted charge-carrier tunneling can be applied. The photoluminescence spectra of parahexaphenylene
(p6P) and α-sexithiophene (6T) nanofibers illustrate that the emission color of the in-situ grown
nanofibers can be tuned by depositing two types of discontinuous organic layers on the same platform.
Electroluminescence from two nanofiber thin films suggests that the relative light emission contribution from the two
organic molecules can be varied by changing, e.g., the nominal thickness of the two materials.
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.
Organic crystalline nanofibers made from phenylene-based molecules exhibit a wide range of extraordinary optical
properties such as intense, anisotropic and polarized luminescence that can be stimulated either optically or
electrically, waveguiding and random lasing. For lighting and display purposes, the high quantum yield and the easy
tunability of the color by changing the molecular building blocks are especially important.
The application of such nanostructures as electrically driven light-emitters requires integration with suitable metal
electrodes for efficient carrier injection. Here, we demonstrate the implementation of a method for achieving such
nanostructure integration. The method relies on growing the nanostructures directly between metal electrodes on a
substrate that has been specially designed to guide the nanostructures growth. We present results in terms of
morphological characterization and demonstrate how appropriate biasing with an AC gate voltage enables
electroluminescence from these in-situ grown organic nanostructures.
Specially designed surface micro- and nanostructures allow one to steer the bottom up self-organized growth of crystalline nanoaggregates from wide bandgap organic molecules, which possess extraordinary optoelectronic properties. Polarized light-emitting para-hexaphenylene nanofiber arrays exemplify such "self-growing" nanophotonic devices. The methodology behind this growth is an alternative to transfer of nanofiber arrays from specific growth substrates onto device platforms. We compared the optical properties of transferred and in situ grown nanofibers in terms of polarization function and emission homogeneity and also studied the temperature dependence of the emission spectra of transferred nanofiber arrays. Both types of nanofibers show the same spatial emission characteristics along their long axes and also the same polarization ratio. However, in nanofiber arrays, the polarization ratio decreases in the case of structured surface-grown nanofibers since the mutual orientation of the nanofibers is less perfect than for transferred fibers.
Para-hexaphenylene (p6P) molecules have the ability to self-assemble into organic nanofibers, which exhibit a
range of interesting optical and optoelectronic properties such as intense, polarized luminescence, waveguiding and
lasing. The nanofibers are typically grown on specific single-crystalline templates, such as muscovite mica, on which
mutually parallel nanofibers are self-assembled upon vapor deposition of the organic material under high vacuum
conditions. Besides such single-crystalline templates, the nanofibers can also be grown on non-crystalline gold
surfaces, on which the orientation of the nanofibers can be manipulated by structuring the gold surface prior to parahexaphenylene
(p6P) deposition. In this work it is demonstrated, how such organic nanofiber growth can be controlled
by modifying the design of the underlying gold structures prior to growth. Here, the investigated designs include
pinning lines and gratings. We demonstrate how gold gratings fabricated on an insulating substrate can enable
electrical contact to in-situ grown p6P nanofibers. Furthermore, the electrical characteristics of in-situ grown fibers are compared to that of transferred p6P nanofibers. The transferred nanofibers are initially grown on muscovite mica, and
subsequently transferred onto a target substrate by drop casting, and electrodes are applied on top by a special shadow
mask technique.
Nanofibers made from para-hexaphenylene (p6P) molecules hold unique optoelectronic properties, which make them
interesting candidates as elements in electronic and optoelectronic devices. Typically these nanofibers are grown on
specific single-crystalline substrates, on which long, mutually parallel nanofibers are formed. However, the lack of
ability to further process these substrates restrains their use in devices. In this work, a novel method for in-situ growth of
p6P nanofibers on nano- and micro-structured gold surfaces is presented. The substrates are prepared by conventional
microfabrication techniques such as lithography, etching and metal deposition, which increase their potential as device
platforms. The results presented here demonstrate, that both the growth direction and the nanofiber length can be
controlled by placement of nano- and micro-structured lines on the substrate. It is shown that the preferred growth
direction of the nanofibers is perpendicular to these structures whereas their length scales are limited by the size and
placement of the structures. This work therefore demonstrates a new technique, which can be useful within future
organic nanofiber based applications.
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