4 research outputs found
Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films with Solar Cell Performance
The structure of titanyl phthalocyanine (TiOPc) thin
films is correlated
with photovoltaic properties of planar heterojunction solar cells
by pairing different TiOPc polymorph donor layers with C<sub>60</sub> as an acceptor. Solvent annealing and the insertion of two different
templating layers, namely 1H,1H,2H,2H-perfluorodecyltrichlorosilane
(FDTS) and CuI, prove to be effective methods to control the TiOPc
thin film structure. The crystal phase of TiOPc thin films was identified
by combining X-ray reflectivity (XRR) measurements with spectroscopic
techniques, including absorption and micro-Raman measurements. Implementation
of a donor layer with an absorption spectrum extending into the near-infrared
(NIR) led to solar cells with external quantum efficiencies (EQEs)
above 27% from Ī» = 600 ā 890 nm, with the best device
yielding a power conversion efficiency (PCE) of 2.6%. Our results
highlight the need to understand the relationship between processing
parameters and thin film structure, as these have important consequences
on device performance
Mesoporous Oxide-Diluted Magnetic Semiconductors Prepared by Co Implantation in Nanocast 3D-Ordered In<sub>2</sub>O<sub>3ā<i>y</i></sub> Materials
Mesoporous In<sub>2</sub>O<sub>3ā<i>y</i></sub> materials have been implanted using Co ions to induce
a moderate
ferromagnetic response at room temperature, forming a āmesoporous
oxide-diluted magnetic semiconductorā (MODMS). X-ray photoemission
spectroscopy (XPS) reveals that implantation results in up to 1 at.
% Co (for 6 Ć 10<sup>15</sup> ions/cm<sup>2</sup> at 40 keV)
and 15 at. % Co (for 1 Ć 10<sup>17</sup> ions/cm<sup>2</sup> at
60 keV). This is in both cases accompanied by a pronounced increase
in the amount of oxygen vacancies with respect to the pristine, nonimplanted,
In<sub>2</sub>O<sub>3ā<i>y</i></sub>. Further increase
in the ion fluence (up to 2 Ć 10<sup>17</sup> ions/cm<sup>2</sup> at 60 keV) results in the collapse of the mesoporous structure,
i.e., loss of the 3D-ordered porous configuration. XPS also reveals
that virtually no metallic Co is formed at 40 keV, while a mixture
of Co<sup>2+</sup> and Co<sup>0</sup> states is detected after implantation
at 60 keV. Most of the Co<sup>2+</sup> is incorporated in the bixbyite
structure of the In<sub>2</sub>O<sub>3ā<i>y</i></sub> matrix. These results are consistent with previous models suggesting
that the origin of the obtained ferromagnetic response in oxide-diluted
magnetic semiconductors can be ascribed to ferromagnetic exchange
interactions mediated by oxygen vacancies. This work constitutes the
first report on MODMS prepared by nanocasting followed by implantation
of transition metal ions
Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO<sub>3</sub>āBaTiO<sub>3</sub> Superlattices
The
detailed understanding of magnetoelectric (ME) coupling in multiferroic
oxide heterostructures is still a challenge. In particular, very little
is known to date concerning the impact of the chemical interface structure
and unwanted impurities that may be buried within short-period multiferroic
BiFeO<sub>3</sub>āBaTiO<sub>3</sub> superlattices during growth.
Here, we demonstrate how trace impurities and elemental concentration
gradients contribute to high ME voltage coefficients in thin-film
superlattices, which are built from 15 double layers of BiFeO<sub>3</sub>āBaTiO<sub>3</sub>. Surprisingly, the highest ME voltage
coefficient of 55 V cm<sup>ā1</sup> Oe<sup>ā1</sup> at
300 K was measured for a superlattice with a few atomic percent of
Ba and Ti that diffused into the nominally 5 nm thin BiFeO<sub>3</sub> layers, according to analytical transmission electron microscopy.
In addition, highly sensitive enhancements of the cation signals were
observed in depth profiles by secondary ion mass spectrometry at the
interfaces of BaTiO<sub>3</sub> and BiFeO<sub>3</sub>. As these interface
features correlate with the ME performance of the samples, they point
to the importance of charge effects at the interfaces, that is, to
a possible charge mediation of ME coupling in oxide superlattices.
The challenge is to provide cleaner materials and processes, as well
as a well-defined control of the chemical interface structure, to
push forward the application of oxide superlattices in multiferroic
ME devices
Improving the Magnetic Properties of CoāCoO Systems by Designed Oxygen Implantation Profiles
Oxygen
implantation in ferromagnetic Co thin films is shown to be an advantageous
route to improving the magnetic properties of CoāCoO systems
by forming multiple nanoscaled ferromagnetic/antiferromagnetic interfaces
homogeneously distributed throughout the layer. By properly designing
the implantation conditions (energy and fluence) and the structure
of the films (capping, buffer, and Co layer thickness), relatively
uniform O profiles across the Co layer can be achieved using a single-energy
ion implantation approach. This optimized configuration results in
enhanced exchange bias loop shifts, improved loop homogeneity, increased
blocking temperature, reduced relative training effects and increased
retained remanence in the trained state with respect to both Co/CoO
bilayers and O-implanted Co films with a Gaussian-like O depth profile.
This underlines the great potential of ion implantation to tailor
the magnetic properties by controllably modifying the local microstructure
through tailored implantation profiles