10 research outputs found
Ultrathin, Flexible Organic–Inorganic Hybrid Solar Cells Based on Silicon Nanowires and PEDOT:PSS
Recently,
free-standing, ultrathin, single-crystal silicon (c-Si)
membranes have attracted considerable attention as a suitable material
for low-cost, mechanically flexible electronics. In this paper, we
report a promising ultrathin, flexible, hybrid solar cell based on
silicon nanowire (SiNW) arrays and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS). The free-standing, ultrathin c-Si membranes of different
thicknesses were produced by KOH etching of double-side-polished silicon
wafers for various etching times. The processed free-standing silicon
membranes were observed to be mechanically flexible, and in spite
of their relatively small thickness, the samples tolerated the different
steps of solar cell fabrication, including surface nanotexturization,
spin-casting, dielectric film deposition, and metallization. However,
in terms of the optical performance, ultrathin c-Si membranes suffer
from noticeable transmission losses, especially in the long-wavelength
region. We describe the experimental performance of a promising light-trapping
scheme in the aforementioned ultrathin c-Si membranes of thicknesses
as small as 5.7 μm employing front-surface random SiNW texturization
in combination with a back-surface distribution of silver (Ag) nanoparticles
(NPs). We report the enhancement of both the short-circuit current
density (<i>J</i><sub>SC</sub>) and the open-circuit voltage
(<i>V</i><sub>OC</sub>) that has been achieved in the described
devices. Such enhancement is attributable to the plasmonic backscattering
effect of the back-surface Ag NPs, which led to an overall 10% increase
in the power conversion efficiency (PCE) of the devices compared to
similar structures without Ag NPs. A PCE in excess of 6.62% has been
achieved in the described devices having a c-Si membrane of thickness
8.6 μm. The described device technology could prove crucial
in achieving an efficient, low-cost, mechanically flexible photovoltaic
device in the near future
Plasmonic Effects of Au/Ag Bimetallic Multispiked Nanoparticles for Photovoltaic Applications
In
recent years, there has been considerable interest in the use
of plasmons, that is, free electron oscillations in conductors, to
boost the performance of both organic and inorganic thin film solar
cells. This has been driven by the possibility of employing thin active
layers in solar cells in order to reduce materials costs, and is enabled
by significant advances in fabrication technology. The ability of
surface plasmons in metallic nanostructures to guide and confine light
in the nanometer scale has opened up new design possibilities for
solar cell devices. Here, we report the synthesis and characterization
of highly monodisperse, reasonably stable, multipode Au/Ag bimetallic
nanostructures using an inorganic additive as a ligand for photovoltaic
applications. A promising surface enhanced Raman scattering (SERS)
effect has been observed for the synthesized bimetallic Au/Ag multispiked
nanoparticles, which compare favorably well with their Au and Ag spherical
nanoparticle counterparts. The synthesized plasmonic nanostructures
were incorporated on the rear surface of an ultrathin planar c-silicon/organic
polymer hybrid solar cell, and the overall effect on photovoltaic
performance was investigated. A promising enhancement in solar cell
performance parameters, including both the open circuit voltage (<i>V</i><sub>OC</sub>) and short circuit current density (<i>J</i><sub>SC</sub>), has been observed by employing the aforementioned
bimetallic multispiked nanoparticles on the rear surface of solar
cell devices. A power conversion efficiency (PCE) value as high as
7.70% has been measured in a hybrid device with Au/Ag multispiked
nanoparticles on the rear surface of an ultrathin, crystalline silicon
(c-Si) membrane (∼12 μm). This value compares well to
the measured PCE value of 6.72% for a similar device without nanoparticles.
The experimental observations support the hope for a sizable PCE increase,
due to plasmon effects, in thin-film, c-Si solar cells in the near
future
Gold–Copper Nano-Alloy, “<i>Tumbaga</i>”, in the Era of Nano: Phase Diagram and Segregation
Gold–copper (Au–Cu)
phases were employed already
by pre-Columbian civilizations, essentially in decorative arts, whereas
nowadays, they emerge in nanotechnology as an important catalyst.
The knowledge of the phase diagram is critical to understanding the
performance of a material. However, experimental determination of
nanophase diagrams is rare because calorimetry remains quite challenging
at the nanoscale; theoretical investigations, therefore, are welcomed.
Using nanothermodynamics, this paper presents the phase diagrams of
various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron,
dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron,
and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes
investigated, that the congruent melting point of these nanoparticles
is shifted with respect to both size and composition (copper enrichment).
Segregation reveals a gold enrichment at the surface, leading to a
kind of core–shell structure, reminiscent of the historical
artifacts. Finally, the most stable structures were determined to
be the dodecahedron, truncated octahedron, and icosahedron with a
Cu-rich core/Au-rich surface. The results of the thermodynamic approach
are compared and supported by molecular-dynamics simulations and by
electron-microscopy (EDX) observations
High Efficiency Hybrid Silicon Nanopillar–Polymer Solar Cells
Recently,
inorganic/organic hybrid solar cells have been considered as a viable
alternative for low-cost photovoltaic devices because the Schottky
junction between inorganic and organic materials can be formed employing
low temperature processing methods. We present an efficient hybrid
solar cell based on highly ordered silicon nanopillars (SiNPs) and
poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS).
The proposed device is formed by spin coating the organic polymer
PEDOT:PSS on a SiNP array fabricated using metal assisted electroless
chemical etching process. The characteristics of the hybrid solar
cells are investigated as a function of SiNP height. A maximum power
conversion efficiency (PCE) of 9.65% has been achieved for an optimized
SiNP array hybrid solar cell with nanopillar height of 400 nm, despite
the absence of a back surface field enhancement. The effect of an
ultrathin atomic layer deposition (ALD), grown aluminum oxide (Al<sub>2</sub>O<sub>3</sub>), as a passivation layer (recombination barrier)
has also been studied for the enhanced electrical performance of the
device. With the inclusion of the ultrathin ALD deposited Al<sub>2</sub>O<sub>3</sub> between the SiNP array textured surface and the PEDOT:PSS
layer, the PCE of the fabricated device was observed to increase to
10.56%, which is ∼10% greater than the corresponding device
without the Al<sub>2</sub>O<sub>3</sub> layer. The device described
herein is considered to be promising toward the realization of a low-cost,
high-efficiency inorganic/organic hybrid solar cell
Gold–Copper Nano-Alloy, “<i>Tumbaga</i>”, in the Era of Nano: Phase Diagram and Segregation
Gold–copper (Au–Cu)
phases were employed already
by pre-Columbian civilizations, essentially in decorative arts, whereas
nowadays, they emerge in nanotechnology as an important catalyst.
The knowledge of the phase diagram is critical to understanding the
performance of a material. However, experimental determination of
nanophase diagrams is rare because calorimetry remains quite challenging
at the nanoscale; theoretical investigations, therefore, are welcomed.
Using nanothermodynamics, this paper presents the phase diagrams of
various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron,
dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron,
and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes
investigated, that the congruent melting point of these nanoparticles
is shifted with respect to both size and composition (copper enrichment).
Segregation reveals a gold enrichment at the surface, leading to a
kind of core–shell structure, reminiscent of the historical
artifacts. Finally, the most stable structures were determined to
be the dodecahedron, truncated octahedron, and icosahedron with a
Cu-rich core/Au-rich surface. The results of the thermodynamic approach
are compared and supported by molecular-dynamics simulations and by
electron-microscopy (EDX) observations
Atomically Resolved Anisotropic Electrochemical Shaping of Nano-electrocatalyst
Catalytic
properties of advanced functional materials are determined
by their surface and near-surface atomic structure, composition, morphology,
defects, compressive and tensile stresses, etc; also known as a structure–activity
relationship. The catalysts structural properties are dynamically
changing as they perform via complex phenomenon dependent on the reaction
conditions. In turn, not just the structural features but even more
importantly, catalytic characteristics of nanoparticles get altered.
Definitive conclusions about these phenomena are not possible with
imaging of random nanoparticles with unknown atomic structure history.
Using a contemporary PtCu-alloy electrocatalyst as a model system,
a unique approach allowing unprecedented insight into the morphological
dynamics on the atomic-scale caused by the process of dealloying is
presented. Observing the detailed structure and morphology of the
same nanoparticle at different stages of electrochemical treatment
reveals new insights into atomic-scale processes such as size, faceting,
strain and porosity development. Furthermore, based on precise atomically
resolved microscopy data, Kinetic Monte Carlo (KMC) simulations provide
further feedback into the physical parameters governing electrochemically
induced structural dynamics. This work introduces a unique approach
toward observation and understanding of nanoparticles dynamic changes
on the atomic level and paves the way for an understanding of the
structure–stability relationship
Corrosion Protection of Platinum-Based Electrocatalyst by Ruthenium Surface Decoration
A comprehensive
insight into the electrochemical performance of PtCu<sub>3</sub> electrocatalyst
nanoparticles with and without Ru decoration is provided. The online
dissolution investigation using the highly sensitive online analytical
methodology of electrochemical flow cell coupled to inductively coupled
plasma mass spectrometry reveals that the addition of Ru nanoparticles
inhibits Pt dissolution presumably because of three effects: (i) suppression
of Pt oxide formation, (ii) sacrificial corrosion of Ru, and (iii)
lowering of local surface pH. The Ru nanoparticles, however, also
lead to a decrease of the amount of crystal structure ordering, which
in turn is one of the reasons for the increase of the corrosion of
Cu. By measuring the potential of total zero charge it is shown that
Ru decoration does not alter the electrochemical properties of the
native Pt surface. Finally, Ru decoration of the Pt-based electrocatalyst
is shown to present a viable approach to enhance the platinum corrosion
resistance, which is confirmed by thin-film rotating disc electrode
accelerated degradation tests
Adjusting the Operational Potential Window as a Tool for Prolonging the Durability of Carbon-Supported Pt-Alloy Nanoparticles as Oxygen Reduction Reaction Electrocatalysts
A current trend in
the investigation of state-of-the-art Pt-alloys
as proton exchange membrane fuel cell (PEMFC) electrocatalysts is
to study their long-term stability as a bottleneck for their full
commercialization. Although many parameters have been appropriately
addressed, there are still certain issues that must be considered.
Here, the stability of an experimental Pt-Co/C electrocatalyst is
investigated by high-temperature accelerated degradation tests (HT-ADTs)
in a high-temperature disk electrode (HT-DE) setup, allowing the imitation
of close-to-real operational conditions in terms of temperature (60
°C). Although the US Department of Energy (DoE) protocol has
been chosen as the basis of the study (30,000 trapezoidal wave cycling
steps between 0.6 and 0.95 VRHE with a 3 s hold time at
both the lower potential limit (LPL) and the upper potential limit
(UPL)), this works demonstrates that limiting both the LPL and UPL
(from 0.6–0.95 to 0.7–0.85 VRHE) can dramatically
reduce the degradation rate of state-of-the-art Pt-alloy electrocatalysts.
This has been additionally confirmed with the use of an electrochemical
flow cell coupled to inductively coupled plasma mass spectrometry
(EFC-ICP-MS), which enables real-time monitoring of the dissolution
mechanisms of Pt and Co. In line with the HT-DE methodology observations,
a dramatic decrease in the total dissolution of Pt and Co has once
again been observed upon narrowing the potential window to 0.7–0.85
VRHE rather than 0.6–0.95 VRHE. Additionally,
the effect of the potential hold time at both LPL and UPL on metal
dissolution has also been investigated. The findings demonstrate that
the dissolution rate of both metals is proportional to the hold time
at UPL regardless of the applied potential window, whereas the hold
time at the LPL does not appear to be as detrimental to the stability
of metals
Electrochemical Dissolution of Iridium and Iridium Oxide Particles in Acidic Media: Transmission Electron Microscopy, Electrochemical Flow Cell Coupled to Inductively Coupled Plasma Mass Spectrometry, and X‑ray Absorption Spectroscopy Study
Iridium-based
particles, regarded as the most promising proton
exchange membrane electrolyzer electrocatalysts, were investigated
by transmission electron microscopy and by coupling of an electrochemical
flow cell (EFC) with online inductively coupled plasma mass spectrometry.
Additionally, studies using a thin-film rotating disc electrode, identical
location transmission and scanning electron microscopy, as well as
X-ray absorption spectroscopy have been performed. Extremely sensitive
online time-and potential-resolved electrochemical dissolution profiles
revealed that Ir particles dissolve well below oxygen evolution reaction
(OER) potentials, presumably induced by Ir surface oxidation and reduction
processes, also referred to as transient dissolution. Overall, thermally
prepared rutile-type IrO<sub>2</sub> particles are substantially more
stable and less active in comparison to as-prepared metallic and electrochemically
pretreated (E-Ir) analogues. Interestingly, under OER-relevant conditions,
E-Ir particles exhibit superior stability and activity owing to the
altered corrosion mechanism, where the formation of unstable Ir(>IV)
species is hindered. Due to the enhanced and lasting OER performance,
electrochemically pre-oxidized E-Ir particles may be considered as
the electrocatalyst of choice for an improved low-temperature electrochemical
hydrogen production device, namely a proton exchange membrane electrolyzer
Metal–Support Interaction between Titanium Oxynitride and Pt Nanoparticles Enables Efficient Low-Pt-Loaded High-Performance Electrodes at Relevant Oxygen Reduction Reaction Current Densities
In the present work,
we report on a synergistic relationship between
platinum nanoparticles and a titanium oxynitride support (TiOxNy/C) in the
context of oxygen reduction reaction (ORR) catalysis. As demonstrated
herein, this composite configuration results in significantly improved
electrocatalytic activity toward the ORR relative to platinum dispersed
on carbon support (Pt/C) at high overpotentials. Specifically, the
ORR performance was assessed under an elevated mass transport regime
using the modified floating electrode configuration, which enabled
us to pursue the reaction closer to PEMFC-relevant current densities.
A comprehensive investigation attributes the ORR performance increase
to a strong interaction between platinum and the TiOxNy/C support. In particular, according
to the generated strain maps obtained via scanning transmission electron
microscopy (STEM), the Pt-TiOxNy/C analogue exhibits a more localized strain in Pt
nanoparticles in comparison to that in the Pt/C sample. The altered
Pt structure could explain the measured ORR activity trend via the
d-band theory, which lowers the platinum surface coverage with ORR
intermediates. In terms of the Pt particle size effect, our observation
presents an anomaly as the Pt-TiOxNy/C analogue, despite having almost two times
smaller nanoparticles (2.9 nm) compared to the Pt/C benchmark (4.8
nm), manifests higher specific activity. This provides a promising
strategy to further lower the Pt loading and increase the ECSA without
sacrificing the catalytic activity under fuel cell-relevant potentials.
Apart from the ORR, the platinum-TiOxNy/C interaction is of a sufficient magnitude
not to follow the typical particle size effect also in the context
of other reactions such as CO stripping, hydrogen oxidation reaction,
and water discharge. The trend for the latter is ascribed to the lower
oxophilicity of Pt-based on electrochemical surface coverage analysis.
Namely, a lower surface coverage with oxygenated species is found
for the Pt-TiOxNy/C analogue. Further insights were provided by performing a
detailed STEM characterization via the identical location mode (IL-STEM)
in particular, via 4DSTEM acquisition. This disclosed that Pt particles
are partially encapsulated within a thin layer of TiOxNy origin