11 research outputs found
Multifunctional Moth-Eye TiO<sub>2</sub>/PDMS Pads with High Transmittance and UV Filtering
This work reports
a facile fabrication method for constructing multifunctional moth-eye
TiO<sub>2</sub>/polydimethylsiloxane (PDMS) pads using soft nano-imprinting
lithography and a gas-phase-deposited thin sacrificial layer. Mesoporous
TiO<sub>2</sub> nanoparticles act as an effective UV filter, completely
blocking high-energy UVB light and partially blocking UVA light and
forming a robust TiO<sub>2</sub>/PDMS composite pad by allowing the
PDMS solution to easily fill the porous TiO<sub>2</sub> network. The
paraboloid-shaped moth-eye nanostructures provided high transparency
in the visible spectrum and also have self-cleaning effects because
of nanoroughness on the surface. Furthermore, we successfully achieved
a desired multiscale-patterned surface by partially curing select
regions using TiO<sub>2</sub>/PDMS pads with partial UVA ray blockers.
The ability to fabricate multifunctional polymeric pads is advantageous
for satisfying increasing demands for flexible and wearable electronics,
displays, and solar cells
Multifunctional Moth-Eye TiO<sub>2</sub>/PDMS Pads with High Transmittance and UV Filtering
This work reports
a facile fabrication method for constructing multifunctional moth-eye
TiO<sub>2</sub>/polydimethylsiloxane (PDMS) pads using soft nano-imprinting
lithography and a gas-phase-deposited thin sacrificial layer. Mesoporous
TiO<sub>2</sub> nanoparticles act as an effective UV filter, completely
blocking high-energy UVB light and partially blocking UVA light and
forming a robust TiO<sub>2</sub>/PDMS composite pad by allowing the
PDMS solution to easily fill the porous TiO<sub>2</sub> network. The
paraboloid-shaped moth-eye nanostructures provided high transparency
in the visible spectrum and also have self-cleaning effects because
of nanoroughness on the surface. Furthermore, we successfully achieved
a desired multiscale-patterned surface by partially curing select
regions using TiO<sub>2</sub>/PDMS pads with partial UVA ray blockers.
The ability to fabricate multifunctional polymeric pads is advantageous
for satisfying increasing demands for flexible and wearable electronics,
displays, and solar cells
Robust Microzip Fastener: Repeatable Interlocking Using Polymeric Rectangular Parallelepiped Arrays
We
report a highly repeatable and robust microzip fastener based
on the van der Waals force-assisted interlocking between rectangular
parallelepiped arrays. To investigate zipperlike interlocking behaviors,
various line arrays were fabricated with three different spacing ratios
(1, 3, and 5 of 800 nm in width) and width of parallelepipeds (400
nm, 800 nm, and 5 μm with the spacing ratio of 1). In addition,
the different rigidity of line arrays was inspected for a repeatable
microzip fastener. The normal and shear locking forces were measured
with variation of the material rigidity as well as geometry of the
array, in good agreement with a proposed theory based on the contact
area and force balance. The maximum adhesion forces as high as ∼8.5
N cm<sup>–2</sup> in the normal direction and ∼29.6
N cm<sup>–2</sup> in the shear direction were obtained with
high stability up to 1000 cycles. High stability of our fastening
system was confirmed for preventing critical failures such as buckling
and fracture in practical applications
Observation of Enhanced Hole Extraction in Br Concentration Gradient Perovskite Materials
Enhancing hole extraction inside
the perovskite layer is the key factor for boosting photovoltaic performance.
Realization of halide concentration gradient perovskite materials
has been expected to exhibit rapid hole extraction due to the precise
bandgap tuning. Moreover, a formation of Br-rich region on the tri-iodide
perovskite layer is expected to enhance moisture stability without
a loss of current density. However, conventional synthetic techniques
of perovskite materials such as the solution process have not achieved
the realization of halide concentration gradient perovskite materials.
In this report, we demonstrate the fabrication of Br concentration
gradient mixed halide perovskite materials using a novel and facile
halide conversion method based on vaporized hydrobromic acid. Accelerated
hole extraction and enhanced lifetime due to Br gradient was verified
by observing photoluminescence properties. Through the combination
of secondary ion mass spectroscopy and transmission electron microscopy
with energy-dispersive X-ray spectroscopy analysis, the diffusion
behavior of Br ions in perovskite materials was investigated. The
Br-gradient was found to be eventually converted into a homogeneous
mixed halide layer after undergoing an intermixing process. Br-substituted
perovskite solar cells exhibited a power conversion efficiency of
18.94% due to an increase in open circuit voltage from 1.08 to 1.11
V and an advance in fill-factor from 0.71 to 0.74. Long-term stability
was also dramatically enhanced after the conversion process, i.e.,
the power conversion efficiency of the post-treated device has remained
over 97% of the initial value under high humid conditions (40–90%)
without any encapsulation for 4 weeks
Facile Multiscale Patterning by Creep-Assisted Sequential Imprinting and Fuel Cell Application
The capability of
fabricating multiscale structures with desired
morphology and incorporating them into engineering applications is
key to realizing technological breakthroughs by employing the benefits
from both microscale and nanoscale morphology simultaneously. Here,
we developed a facile patterning method to fabricate multiscale hierarchical
structures by a novel approach called creep-assisted sequential imprinting.
In this work, nanopatterning was first carried out by thermal imprint
lithography above the glass transition temperature (<i>T</i><sub>g</sub>) of a polymer film, and then followed by creep-assisted
imprinting with micropatterns based on the mechanical deformation
of the polymer film under the relatively long-term exposure to mechanical
stress at temperatures below the <i>T</i><sub>g</sub> of
the polymer. The fabricated multiscale arrays exhibited excellent
pattern uniformity over large areas. To demonstrate the usage of multiscale
architectures, we incorporated the multiscale Nafion films into polymer
electrolyte membrane fuel cell, and this device showed more than 10%
higher performance than the conventional one. The enhancement was
attributed to the decrease in mass transport resistance because of
unique cone-shape morphology by creep-recovery effects and the increase
in interfacial surface area between Nafion film and electrocatalyst
layer
Precise Morphology Control and Continuous Fabrication of Perovskite Solar Cells Using Droplet-Controllable Electrospray Coating System
Herein, we developed a novel electrospray
coating system for continuous
fabrication of perovskite solar cells with high performance. Our system
can systemically control the size of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> precursor droplets by modulating the applied electrical potential,
shown to be a crucial factor for the formation of perovskite films.
As a result, we have obtained pinhole-free and large grain-sized perovskite
solar cells, yielding the best PCE of 13.27% with little photocurrent
hysteresis. Furthermore, the average PCE through the continuous coating
process was 11.56 ± 0.52%. Our system demonstrates not only the
high reproducibility but also a new way to commercialize high-quality
perovskite solar cells
Precise Morphology Control and Continuous Fabrication of Perovskite Solar Cells Using Droplet-Controllable Electrospray Coating System
Herein, we developed a novel electrospray
coating system for continuous
fabrication of perovskite solar cells with high performance. Our system
can systemically control the size of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> precursor droplets by modulating the applied electrical potential,
shown to be a crucial factor for the formation of perovskite films.
As a result, we have obtained pinhole-free and large grain-sized perovskite
solar cells, yielding the best PCE of 13.27% with little photocurrent
hysteresis. Furthermore, the average PCE through the continuous coating
process was 11.56 ± 0.52%. Our system demonstrates not only the
high reproducibility but also a new way to commercialize high-quality
perovskite solar cells
Hybrid Surface-Phonon-Plasmon Polariton Modes in Graphene/Monolayer h‑BN Heterostructures
Infrared transmission measurements
reveal the hybridization of
graphene plasmons and the phonons in a monolayer hexagonal boron nitride
(h-BN) sheet. Frequency-wavevector dispersion relations of the electromagnetically
coupled graphene plasmon/h-BN phonon modes are derived from measurement
of nanoresonators with widths varying from 30 to 300 nm. It is shown
that the graphene plasmon mode is split into two distinct optical
modes that display an anticrossing behavior near the energy of the
h-BN optical phonon at 1370 cm<sup>–1</sup>. We explain this
behavior as a classical electromagnetic strong-coupling with the highly
confined near fields of the graphene plasmons allowing for hybridization
with the phonons of the atomically thin h-BN layer to create two clearly
separated new surface-phonon-plasmon-polariton (SPPP) modes
Carbon Nanotubes versus Graphene as Flexible Transparent Electrodes in Inverted Perovskite Solar Cells
Transparent
carbon electrodes, carbon nanotubes, and graphene were
used as the bottom electrode in flexible inverted perovskite solar
cells. Their photovoltaic performance and mechanical resilience were
compared and analyzed using various techniques. Whereas a conventional
inverted perovskite solar cells using indium tin oxide showed a power
conversion efficiency of 17.8%, the carbon nanotube- and graphene-based
cells showed efficiencies of 12.8% and 14.2%, respectively. An established
MoO<sub>3</sub> doping was used for  carbon electrode-based devices.
The difference in the photovoltaic performance between the carbon
nanotube- and graphene-based cells was due to the difference in morphology
and transmittance. Raman spectroscopy, and cyclic flexural testing
revealed that the graphene-based cells were more susceptible to strain
than the carbon nanotube-based cells, though the difference was marginal.
Overall, despite higher performance, the transfer step for graphene
has lower reproducibility. Thus, the development of better graphene
transfer methods would help maximize the current capacity of graphene-based
cells
Plasmonic Organic Solar Cells Employing Nanobump Assembly <i>via</i> Aerosol-Derived Nanoparticles
We report the effect of a nanobump assembly (NBA) constructed with molybdenum oxide (MoO<sub>3</sub>) covering Ag nanoparticles (NPs) under the active layer on the efficiency of plasmonic polymer solar cells. Here, the NPs with precisely controlled concentration and size have been generated by an atmospheric evaporation/condensation method and a differential mobility classification and then deposited on an indium tin oxide electrode <i>via</i> room temperature aerosol method. NBA structure is made by enclosing NPs with MoO<sub>3</sub> layer <i>via</i> vacuum thermal evaporation to isolate the undulated active layer formed onto the underlying protruded NBA. Simulated scattering cross sections of the NBA structure reveal higher intensities with a strong forward scattering effect than those from the flat buffer cases. Experimental results of the device containing the NBA show 24% enhancement in short-circuit current density and 18% in power conversion efficiency compared to the device with the flat MoO<sub>3</sub> without the NPs. The observed improvements are attributed to the enhanced light scattering and multireflection effects arising from the NBA structure combined with the undulated active layer in the visible and near-infrared regions. Moreover, we demonstrate that the NBA adopted devices show better performance with longer exciton lifetime and higher light absorption in comparison with the devices with Ag NPs incorporated flat poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Thus, the suggested approach provides a reliable and efficient light harvesting in a broad range of wavelength, which consequently enhances the performance of various organic solar cells