16 research outputs found
Engineering Colloidal Perovskite Nanocrystals and Devices for Efficient and Large-Area Light-Emitting Diodes
ConspectusColloidal metal halide perovskite nanocrystals
(PNCs) have high
color purity, solution processability, high luminescence efficiency,
and facile color tunability in visible wavelengths and therefore show
promise as light emitters in next-generation displays. The external
quantum efficiency (EQE) of PNC light-emitting diodes (LEDs) has been
rapidly increased to reach 24.96% by using colloidal PNCs and 28.9%
using on-substrate in situ synthesized PNCs. However,
high operating stability and a further increase of EQE in PNC-LEDs
have been impeded for three reasons: (1) Colloidal PNCs consist of
ionic crystal structures in which ligands bind dynamically and therefore
easily agglomerate in colloidal solution and films; (2) Long-alkyl-chain
organic ligands that adhere to the PNC surface improve the photoluminescence
quantum efficiency and colloidal stability of PNCs in solution but
impede charge transport in PNC films and limit their electroluminescence
efficiency in LEDs; (3) Unoptimized device structure and nonuniform
PNC films limit the charge balance and reduce the device efficiency
in PNC-LEDs.In this Account, we summarize strategies to solve
the limitations
in PNCs and PNC-LEDs as consequences of photoluminescence quantum
efficiency in PNCs and the charge-balance factor and out-coupling
factor in LEDs, which together determine the EQE of PNC-LEDs. We introduce
the fundamental photophysical properties of colloidal PNCs related
to effective mass of charge carriers and surface stoichiometry, requirements
for PNC surface stabilization, and subsequent research strategies
to demonstrate highly efficient colloidal PNCs and PNC-LEDs with high
operating stability.First, we present various ligand-engineering
strategies that have
been used to achieve both efficient carrier injection and radiative
recombination in PNC films. In situ ligand engineering
reduces ligand length and concentration during synthesis of colloidal
PNCs, and it can achieve size-independent high color purity and high
luminescent efficiency in PNCs. Postsynthesis ligand engineering such
as optimized purification, replacement of organic ligands with inorganic
ligands or strongly bound ligands can increase charge transport and
coupling between PNC dots in films. The luminescence efficiency of
PNCs and PNC-LEDs can be further increased by various postsynthesis
ligand-engineering methods or by sequential treatment with different
ligands. Second, we present methods to modify the crystal structure
in PNCs to have alloy- or core/shell-like structure. Such crystal
engineering is performed by the correlation between entropy and enthalpy
in PNCs and result in increased carrier confinement (increased radiative
recombination) and reduced defects (decreased nonradiative recombination).
Third, we present strategies to boost the charge-balance factor and
out-coupling factor in PNC-LEDs such as modification of thickness
of each layer and insertion of additional interlayers, and out-coupling
hemispherical lens are discussed. Finally, we present the advantages,
potential, and remaining challenges to be solved to enable use of
colloidal PNCs in commercialized industrial displays and solid-state
lighting. We hope this Account will help its readers to grasp the
progresses and perspectives of colloidal PNCs and PNC-LEDs, and that
our insights will guide future research to achieve efficient PNC-LEDs
that have high stability and low toxicity
Synergetic Influences of Mixed-Host Emitting Layer Structures and Hole Injection Layers on Efficiency and Lifetime of Simplified Phosphorescent Organic Light-Emitting Diodes
We
used various nondestructive analyses to investigate various
host material systems in the emitting layer (EML) of simple-structured,
green phosphorescent organic light-emitting diodes (OLEDs) to clarify
how the host systems affect its luminous efficiency (LE) and operational
stability. An OLED that has a unipolar single-host EML with conventional
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)
showed high operating voltage, low LE (∼26.6 cd/A, 13.7 lm/W),
and short lifetime (∼4.4 h @ 1000 cd/m<sup>2</sup>). However,
the combined use of a gradient mixed-host EML and a molecularly controlled
HIL that has increased surface work function (WF) remarkably decreased
operating voltage and improved LE (∼68.7 cd/A, 77.0 lm/W) and
lifetime (∼70.7 h @ 1000 cd/m<sup>2</sup>). Accumulated charges
at the injecting interfaces and formation of a narrow recombination
zone close to the interfaces are the major factors that accelerate
degradation of charge injection/transport and electroluminescent properties
of OLEDs, so achievement of simple-structured OLEDs with high efficiency
and long lifetime requires facilitating charge injection and balanced
transport into the EML and distributing charge carriers and excitons
in EML
Room-Temperature-Processable Wire-Templated Nanoelectrodes for Flexible and Transparent All-Wire Electronics
Sophisticated
preparation of arbitrarily long conducting nanowire
electrodes on a large area is a significant requirement for development
of transparent nanoelectronics. We report a position-customizable
and room-temperature-processable metallic nanowire (NW) electrode
array using aligned NW templates and a demonstration of transparent
all-NW-based electronic applications by simple direct-printing. Well-controlled
electroless-plating chemistry on a polymer NW template provided a
highly conducting Au NW array with a very low resistivity of 7.5 μΩ
cm (only 3.4 times higher than that of bulk Au), high optical transmittance
(>90%), and mechanical bending stability. This method enables fabrication
of all-NW-based electronic devices on various nonplanar surfaces and
flexible plastic substrates. Our approach facilitates realization
of advanced future electronics
Electroluminescence from Graphene Quantum Dots Prepared by Amidative Cutting of Tattered Graphite
Size-controlled graphene quantum dots (GQDs) are prepared via amidative
cutting of tattered graphite. The power of this method is that the
size of the GQDs could be varied from 2 to over 10 nm by simply regulating
the amine concentration. The energy gaps in such GQDs are narrowed
down with increasing their size, showing colorful photoluminescence
from blue to brown. We also reveal the roles of defect sites in photoluminescence,
developing long-wavelength emission and reducing exciton lifetime.
To assess the viability of the present method, organic light-emitting
diodes employing our GQDs as a dopant are first demonstrated with
the thorough studies in their energy levels. This is to our best knowledge
the first meaningful report on the electroluminescence of GQDs, successfully
rendering white light with the external quantum efficiency of ca.
0.1%
Color Purifying Optical Nanothin Film for Three Primary Colors in Optoelectronics
Numerous optical films have been
developed to implement optoelectronics
with advanced performance. In this study, we propose a color purifying
optical nanothin film that improves the performance of optoelectronics
by filtering the white light to have a spectrum composed of pure three
primary colors of red, green, and blue. It was experimentally confirmed
that a wider color gamut that covers 176.33% of the sRGB could be
expressed when the suggested optical nanothin film was applied to
a display system consisting of a white back light unit and color filters.
Furthermore, a full width half-maximum value of 20 nm on average was
observed in the three primary colors. When this film was applied to
light recording optoelectronics, such as cameras, it acts as a multiband-pass
filter that increases the sensitivity of the three primary colors.
The principle of this optical nanothin film is based on multiple light
resonance inside the film. A theoretical analysis and simulations
were conducted to design the structure of the nanothin film and optical
characteristics were verified by both experiment and simulation. Because
it is fabricated by in situ thermal evaporation it provides advantages
in terms of fabrication time and cost, and it also has potential to
be fabricated with well-established deposition equipment such as a
sputter apparatus. The results of this paper show that nanoscaled
thin films sufficiently control the optical phenomenon with a simple
structure, implementing advanced optoelectronics
Ultra-High-Resolution Organic Light-Emitting Diodes with Color Conversion Electrode
The
implementation of ultra-high-resolution displays is one of the important
technologies for advanced displays. In this paper, an ultra-high-resolution
organic light-emitting diode display is implemented without the fine
metal mask method, but via a color conversion electrode. A red and
green color ultra-high-resolution organic light-emitting diode display
with a pixel size of 5 μm was experimentally realized without
changing any aspects of the structure of the OLED display except for
the precisely fine-patterned color conversion electrode. Furthermore,
nanometer-scale pixel size can be expected through this method. The
color conversion electrode is a multilayer structured nanometer thick
conductive optical film, and its applicability was confirmed based
on a theoretical analysis and optical simulation. The ultra-high-resolution
display with color conversion electrode could be a basic technology
for the development of advanced high-resolution displays
Highly Conductive Transparent and Flexible Electrodes Including Double-Stacked Thin Metal Films for Transparent Flexible Electronics
To keep pace with
the era of transparent and deformable electronics, electrode functions
should be improved. In this paper, an innovative structure is suggested
to overcome the trade-off between optical and electrical properties
that commonly arises with transparent electrodes. The structure of
double-stacked metal films showed high conductivity (<3 Ω/sq)
and high transparency (∼90%) simultaneously. A proper space
between two metal films led to high transmittance by an optical phenomenon.
The principle of parallel connection allowed the electrode to have
high conductivity. In situ fabrication was possible because the only
materials composing the electrode were silver and WO<sub>3</sub>,
which can be deposited by thermal evaporation. The electrode was flexible
enough to withstand 10 000 bending cycles with a 1 mm bending
radius. Furthermore, a few μm scale patterning of the electrode
was easily implemented by using photolithography, which is widely
employed industrially for patterning. Flexible organic light-emitting
diodes and a transparent flexible thin-film transistor were successfully
fabricated with the proposed electrode. Various practical applications
of this electrode to new transparent flexible electronics are expected
Laminated Graphene Films for Flexible Transparent Thin Film Encapsulation
We introduce a simple, inexpensive,
and large-area flexible transparent
lamination encapsulation method that uses graphene films with polydimethylsiloxane
(PDMS) buffer on polyethylene terephthalate (PET) substrate. The number
of stacked graphene layers (<i>n</i><sub>G</sub>) was increased
from 2 to 6, and 6-layered graphene-encapsulation showed high impermeability
to moisture and air. The graphene-encapsulated polymer light emitting
diodes (PLEDs) had stable
operating characteristics, and the operational lifetime of encapsulated
PLEDs increased as <i>n</i><sub>G</sub> increased. Calcium
oxidation test data confirmed the improved impermeability of graphene-encapsulation
with increased <i>n</i><sub>G</sub>. As a practical application,
we demonstrated large-area flexible organic light emitting diodes
(FOLEDs) and transparent FOLEDs that were encapsulated by our polymer/graphene
encapsulant
Graphenes Converted from Polymers
Because the direct formation of large, patterned graphene layers on active electronic devices without any physical transfer process is an ultimate important research goal for practical applications, we first developed a cost-effective, scalable, and sustainable process to form graphene films from solution-processed common polymers directly on a SiO<sub>2</sub>/Si substrate. We obtained few-layer graphene by heating the thin polymer films covered with a metal capping layer in a high-temperature furnace under low vacuum in an Ar/H<sub>2</sub> atmosphere. We find that the metal capping layer appears to have two functions: prevention of vaporization of dissociated molecules and catalysis of graphene formation. We suggest that polymer-derived graphene growth directly on inert substrates in active electronic devices will have great advantages because of its simple, inexpensive, and safer process
Flexible and Transparent Metallic Grid Electrodes Prepared by Evaporative Assembly
We propose a novel approach to fabricating
flexible transparent
metallic grid electrodes via evaporative deposition involving flow-coating.
A transparent flexible metal grid electrode was fabricated through
four essential steps including: (i) polymer line pattern formation
on the thermally evaporated metal layer onto a plastic substrate;
(ii) rotation of the stage by 90° and the formation of the second
polymer line pattern; (iii) etching of the unprotected metal region;
and (iv) removal of the residual polymer from the metal grid pattern.
Both the metal grid width and the spacing were systematically controlled
by varying the concentration of the polymer solution and the moving
distance between intermittent stop times of the polymer blade. The
optimized Au grid electrodes exhibited an optical transmittance of
92% at 550 nm and a sheet resistance of 97 Ω/sq. The resulting
metallic grid electrodes were successfully applied to various organic
electronic devices, such as organic field-effect transistors (OFETs),
organic light-emitting diodes (OLEDs), and organic solar cells (OSCs)