6 research outputs found
Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se<sub>2</sub> Grain Boundaries: Correlation with Microstructure
Solar
cells containing a polycrystalline Cu(In,Ga)Se<sub>2</sub> absorber
outperform the ones containing a monocrystalline absorber, showing
a record efficiency of 22.9%. However, the grain boundaries (GBs)
are very often considered to be partly responsible for the enhanced
recombination activity in the cell and thus cannot explain the registered
record efficiency. Therefore, in the present work, we resolve this
conundrum by performing correlative electron beam-induced current–electron
backscatter diffraction investigations on more than 700 grain boundaries
and demonstrating that 58% of the grain boundaries exhibit an enhanced
carrier collection compared to the grain interior. Enhanced carrier
collection thus indicates that GBs are beneficial for the device performance.
Moreover, 27% of the grain boundaries are neutral and 15% are recombination-active.
Correlation with microstructure shows that most of the ∑3 GBs
are neutral, whereas the random high-angle grain boundaries are either
beneficial or detrimental. Enhanced carrier collection observed for
a big fraction of high-angle grain boundaries supports the “type-inversion”
model and hence the downward band bending at GBs. The decrease in
current collection observed at one of the high-angle grain boundaries
is explained by Cu being enriched at this GB and hence by the upward
shift of the valence band maximum
Infrared Resonance Tuning of Nanoslit Antennas with Phase-Change Materials
Phase-change materials
(PCMs) have been established as prime candidates
for nonvolatile resonance tuning of nanophotonic components based
on a large optical contrast between their amorphous and crystalline
states. Recently, the plasmonic PCM In3SbTe2 was introduced, which can be switched from an amorphous dielectric
state to a crystalline metallic one over the entire infrared spectral
range. While locally switching the PCM around metallic nanorod antennas
has already been demonstrated, similar tuning of inverse antenna structures
(nanoslits) has not yet been investigated. Here, we demonstrate optical
resonance tuning of nanoslit antennas with dielectric and plasmonic
PCMs. We compare two geometries with fundamentally different resonance
tuning mechanisms: tuning the resonance of aluminum slit antennas
by change of the refractive index (dielectric PCM Ge3Sb2Te6), and creating slit-like volumes of amorphous
In3SbTe2 and modifying the slit geometry directly
(plasmonic PCM In3SbTe2). While the tuning range
with the plasmonic PCM is about 3.4 μm and only limited by fabrication,
the resonances with the dielectric PCM feature a three times larger
quality factor compared to resonances obtained with the plasmonic
PCM
Specific Heat of (GeTe)<sub><i>x</i></sub>(Sb<sub>2</sub>Te<sub>3</sub>)<sub>1–<i>x</i></sub> Phase-Change Materials: The Impact of Disorder and Anharmonicity
Phase-change
materials (PCM) are bad glass formers, and their rapid
crystallization is accompanied by a drastic change in optical and
electrical properties, which opens opportunities for novel nonvolatile
data storage devices. Many of these materials are located on the pseudobinary
line between GeTe and Sb<sub>2</sub>Te<sub>3</sub> and form a metastable
rock-salt-like atomic arrangement in which Te atoms occupy one of
the two sublattices and the other is randomly filled with Ge and Sb
atoms as well as vacancies. The resulting disorder has profound impact
on, for example, transport properties, causing disorder-induced localization
of charge carriers. Here we discuss the impact of disorder on thermal
properties. We have investigated several PCMs from the pseudobinary
line between GeTe and Sb<sub>2</sub>Te<sub>3</sub>. A significant
enhancement of the specific heat is found for the disordered rock-salt-like
phase compared with the ordered trigonal phase, in which Ge and Sb
atoms occupy separate layers. The magnitude of this enhancement is
correlated with the fraction of stoichiometric vacancies in the Ge/Sb
sublattice. The additional contribution to the specific heat is shown
to consist of a reversible fraction and an irreversible fraction,
which are attributed to anharmonic lattice dynamics and irreversible
vacancy ordering, respectively. These findings underline the prominent
role of vacancy ordering in electrical and thermal transport
Impact of Pressure on the Resonant Bonding in Chalcogenides
Resonant
bonding has been appreciated as an important feature in some chalcogenides.
The establishment of resonant bonding can significantly delocalize
the electrons and shrink the band gap, leading to low electrical resistivity
and soft optical phonons. Many materials that exhibit this bonding
mechanism have applications in phase-change memory and thermoelectric
devices. Resonant bonding can be tuned by various means, including
thermal excitations and changes in composition. In this work, we manipulate
it by applying large hydrostatic-like pressure. Synchrotron X-ray
diffraction and density functional theory reveal that the orthorhombic
lattice of GeSe appears to become more symmetric and the Born effective
charge has significantly increased at high pressure, indicating that
resonant bonding has been established in this material. In contrast,
the resonant bonding is partially weakened in PbSe at high pressure
due to the discontinuity of chemical bonds along a certain lattice
direction. By controlling resonant bonding in chalcogenides, we are
able to modify the material properties and tailor them for various
applications in extreme conditions
Dithiocarbamate Self-Assembled Monolayers as Efficient Surface Modifiers for Low Work Function Noble Metals
Tuning
the work function of the electrode is one of the crucial
steps to improve charge extraction in organic electronic devices.
Here, we show that <i>N</i>,<i>N</i>-dialkyl dithiocarbamates
(DTC) can be effectively employed to produce low work function noble
metal electrodes. Work functions between 3.1 and 3.5 eV are observed
for all metals investigated (Cu, Ag, and Au). Ultraviolet photoemission
spectroscopy (UPS) reveals a maximum decrease in work function by
2.1 eV as compared to the bare metal surface. Electronic structure
calculations elucidate how the complex interplay between intrinsic
dipoles and dipoles induced by bond formation generates such large
work function shifts. Subsequently, we quantify the improvement in
contact resistance of organic thin film transistor devices with DTC
coated source and drain electrodes. These findings demonstrate that
DTC molecules can be employed as universal surface modifiers to produce
stable electrodes for electron injection in high performance hybrid
organic optoelectronics
Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials
Dislocations
have been considered to be an efficient source for scattering midfrequency
phonons, contributing to the enhancement of thermoelectric performance.
The structure of dislocations can be resolved by electron microscopy
whereas their chemical composition and decoration state are scarcely
known. Here, we correlate transmission Kikuchi diffraction and (scanning)
transmission electron microscopy in conjunction with atom probe tomography
to investigate the local structure and chemical composition of dislocations
in a thermoelectric Ag-doped PbTe compound. Our investigations indicate
that Ag atoms segregate to dislocations with a 10-fold excess of Ag
compared with its average concentration in the matrix. Yet the Ag
concentration along the dislocation line is not constant but fluctuates
from ∼0.8 to ∼10 atom % with a period of about 5 nm.
Thermal conductivity is evaluated applying laser flash analysis, and
is correlated with theoretical calculations based on the Debye–Callaway
model, demonstrating that these Ag-decorated dislocations yield stronger
phonon scatterings. These findings reduce the knowledge gap regarding
the composition of dislocations needed for theoretical calculations
of phonon scattering and pave the way for extending the concept of
defect engineering to thermoelectric materials