8 research outputs found
Stretchable Polymer Dielectrics for Low-Voltage-Driven Field-Effect Transistors
A stretchable and mechanical robust
field-effect transistor is
essential for soft wearable electronics. To realize stretchable transistors,
elastic dielectrics with small current hysteresis, high elasticity,
and high dielectric constants are the critical factor for low-voltage-driven
devices. Here, we demonstrate the polar elastomer consisting of poly(vinylidene
fluoride-hexafluoropropylene) (PVDF-HFP):poly(4-vinylphenol)
(PVP). Owing to the high dielectric constant of PVDF-HFP, the device
can be operated under less than 5 V and shows a linear-regime hole
mobility as high as 0.199 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> without significant current hysteresis. Specifically,
the PVDF-HFP:PVP blends induce the vertical phase separation
and significantly reduce current leakage and reduce the crystallization
of PVDF segments, which can contribute current hysteresis in the OFET
characteristics. All-stretchable OFETs based on these PVDF-HFP:PVP
dielectrics were fabricated. The device can still keep the hole mobility
of approximately 0.1 cm<sup>2</sup>/(V s) under a low operation
voltage of 3 V even as stretched with 80% strain. Finally, we successfully
fabricate a low-voltage-driven stretchable transistor. The low voltage
operating under strains is the desirable characteristics for soft
and comfortable wearable electronics
Thiophene and Selenophene Donor–Acceptor Polyimides as Polymer Electrets for Nonvolatile Transistor Memory Devices
We report the nonvolatile memory characteristics of n-type <i><i>N,N</i></i>′-bis(2-phenylethyl)perylene-3,4:9,10-tetracarboxylic
diimide (BPE-PTCDI) based organic field-effect transistors (OFET)
using the polyimide electrets of poly[2,5-bis(4-aminophenylenesulfanyl)selenophene–hexafluoroisopropylidenediphthalimide]
(PI(APSP-6FDA)), poly[2,5-bis(4-aminophenylenesulfanyl)thiophene–hexafluoroisopropylidenediphthalimide]
(PI(APST-6FDA)), and poly(4,4′-oxidianiline-4,4′-hexafluoroisopropylidenediphthalic
anhydride) (PI(ODA-6FDA)). Among those polymer electrets, the OFET
memory device based on PI(APSP-6FDA) with a strong electron-rich selenophene
moiety exhibited the highest field-effect mobility and I<sub>on</sub>/I<sub>off</sub> current ratio of 10<sup>5</sup> due to the formation
of the large grain size of the BPE-PTCDI film. Furthermore, the device
with PI(APSP-6FDA) exhibited the largest memory window of 63 V because
the highest HOMO energy level and largest electric filed facilitated
the charges transferring from BPE-PTCDI and trapping in the PI electret.
Moreover, the charge transfer from BPE-PTCDI to the PI(APSP-6FDA)
or PI(APST-6FDA) electrets was more efficient than that of PI(ODA-6FDA)
due to the electron-donating heterocyclic ring. The nanowire device
with PI(APSP-6FDA) showed a relatively larger memory window of 82
V, compared to the thin film device. The present study suggested that
the donor–acceptor polyimide electrets could enhance the capabilities
for transferring and store the charges and have potential applications
for advanced OFET memory devices
Synthesis of Oligosaccharide-Based Block Copolymers with Pendent π‑Conjugated Oligofluorene Moieties and Their Electrical Device Applications
We report the synthesis and electric
device applications of oligosaccharide-based
diblock copolymers consisting of a maltoheptaose (MH) block and a
poly(4-oligofluorenylstyrene) block (PStFl<sub><i>n</i></sub>, <i>n</i> = 1 or 2), referred to as MH-<i>b</i>-PStFl<sub><i>n</i></sub>. MH-<i>b</i>-PStFl<sub><i>n</i></sub> was prepared by the Cu(I)-catalyzed click
reaction of azido-terminated PStFl<sub><i>n</i></sub> (PStFl<sub><i>n</i></sub>-N<sub>3</sub>), which was obtained from
the azidation reaction of the bromo-terminated PStFl<sub><i>n</i></sub> (PStFl<sub><i>n</i></sub>-Br), with excess ethynyl-terminated
MH in the THF/DMF mixture solvent. The resulting diblock copolymers
self-assembled to spherical microdomains with sub-10 nm sizes in both
bulk and thin film state after annealing process. Thereafter, the
MH-<i>b</i>-PStFl<sub><i>n</i></sub> thin film
(∼50 nm) with the self-assembled nanoscale spherical aggregates
was used as the charge storage layer for the pentacene-based field-effect
transistor type memory devices. The MH-<i>b</i>-PStFl<sub><i>n</i></sub>-based devices had the excellent hole mobility
(0.25–0.52 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>) and the high ON/OFF current (<i>I</i><sub>ON</sub>/<i>I</i><sub>OFF</sub>) ratio of 10<sup>7</sup>–10<sup>8</sup>, of which the MH-<i>b</i>-PStFl<sub>1</sub>-based one
had the higher mobility than that of the MH-<i>b</i>-PStFl<sub>2</sub>-based one because the pentacene crystal in the former device
possessed the larger grain size and fewer boundaries. On the other
hand, the MH-<i>b</i>-PStFl<sub>2</sub>-based device showed
a larger memory window than the MH-<i>b</i>-PStFl<sub>1</sub>-based one because the stronger electron-donating effect of the difluorenyl
group in MH-<i>b</i>-PStFl<sub>2</sub> increased the charge
storage capability of its related device. All the memory devices showed
a long-term retention time over 10<sup>4</sup> s with the high <i>I</i><sub>ON</sub>/<i>I</i><sub>OFF</sub> ratio of
10<sup>6</sup>–10<sup>8</sup>. Among these devices, the MH-<i>b</i>-PStFl<sub>1</sub>-based device showed a good WRER endurance
over 180 cycles. This work not only demonstrates the tunable electrical
memory characteristics by adjusting the π-conjugation length
of the oligofluorenyl side chain in the polymer electret but also
provides a promising approach for developing the next-generation “green
electronics” using natural materials
A Rapid and Facile Soft Contact Lamination Method: Evaluation of Polymer Semiconductors for Stretchable Transistors
Organic stretchable electronics have
attracted extensive scientific
and industrial interest because they can be stretched, twisted, or
compressed, enabling the next-generation of organic electronics for
human/machine interfaces. These electronic devices have already been
described for applications such as field-effect transistors, photovoltaics,
light-emitting diodes, and sensors. High-performance stretchable electronics,
however, currently still involve complicated processing steps to integrate
the substrates, semiconductors, and electrodes for effective performance.
Herein, we describe a facile method to efficiently identify suitable
semiconducting polymers for organic stretchable transistors using
soft contact lamination. In our method, the various polymers investigated
are first transferred on an elastomeric poly(dimethylsiloxane) (PDMS)
slab and subsequently stretched (up to 100%) along with the PDMS.
The polymer/PDMS matrix is then laminated on source/drain electrode-deposited
Si substrates equipped with a PDMS dielectric layer. Using this device
configuration, the polymer semiconductors can be repeatedly interrogated
with laminate/delaminate cycles under different amounts of tensile
strain. From our obtained electrical characteristics, e.g., mobility,
drain current, and on/off ratio, the strain limitation of semiconductors
can be derived. With a facile soft contact lamination testing approach,
we can thus rapidly identify potential candidates of semiconducting
polymers for stretchable electronics
Side-Chain Engineering of Isoindigo-Containing Conjugated Polymers Using Polystyrene for High-Performance Bulk Heterojunction Solar Cells
Developing
organic photovoltaic systems that possess high efficiency,
high reproducibility, and low cost remains a topic of keen investigation.
From a molecular design perspective, developing a “multicomponent”
copolymerization synthetic approach could potentially afford macromolecular
materials encompassing all of the aforementioned desired parameters.
Herein, we describe the synthesis of a series of poly(isoindigo-dithiophene)-based
conjugated polymers with varying amounts of low molecular weight polystyrene
(PS) side chains (<i>M</i><sub>n</sub> = 1300 g/mol) via
random copolymerization. We observed better solubility with polymers
containing the PS side chains (when compared to their non-PS-side-chain
counterparts), hence leading to better batch-to-batch reproducibility
in terms of molecular weights. Furthermore, the PS-side-chain-decorated
copolymers also demonstrated better thin film processability, without
affecting the electronic and optical properties, when the molar percentage
of the PS-containing repeating units were ≤10%. Bulk heterojunction
solar cell devices fabricated with these PS-containing copolymers
demonstrated significantly improved performances [maximum power conversion
efficiencies (PCE) > 7% and open circuit voltages (<i>V</i><sub>OC</sub>) ≥ 0.95 V], compared to the highest reported
performance (PCE = 6.3% and <i>V</i><sub>OC</sub> = 0.70)
based on similar isoindigo-containing polymers. Taken together, the
synthesis, processing, and device performances of PS-containing copolymers
represent a new approach in molecular engineering to achieve a balance
between the optical/electronic properties and solubility/processability
of reproducible polymeric systems
Atmospheric Pressure Plasma Jet-Assisted Synthesis of Zeolite-Based Low‑<i>k</i> Thin Films
Zeolites
are ideal low-dielectric constant (low-<i>k</i>) materials.
This paper reports on a novel plasma-assisted approach
to the synthesis of low-<i>k</i> thin films comprising pure-silica
zeolite MFI. The proposed method involves treating the aged solution
using an atmospheric pressure plasma jet (APPJ). The high reactivity
of the resulting nitrogen plasma helps to produce zeolite crystals
with high crystallinity and uniform crystal size distribution. The
APPJ treatment also remarkably reduces the time for hydrothermal reaction.
The zeolite MFI suspensions synthesized with the APPJ treatment are
used for the wet deposition to form thin films. The deposited zeolite
thin films possessed dense morphology and high crystallinity, which
overcome the trade-off between crystallinity and film quality. Zeolite
thin films synthesized using the proposed APPJ treatment achieve low
leakage current (on the order of 10<sup>–8</sup> A/cm<sup>2</sup>) and high Young’s modulus (12 GPa), outperforming the control
sample synthesized without plasma treatment. The dielectric constant
of our zeolite thin films was as low as 1.41. The overall performance
of the low-<i>k</i> thin films synthesized with the APPJ
treatment far exceed existing low-<i>k</i> films comprising
pure-silica MFI
Conjugated Polymer-Mediated Polymorphism of a High Performance, Small-Molecule Organic Semiconductor with Tuned Intermolecular Interactions, Enhanced Long-Range Order, and Charge Transport
We use 6,13-bis(triisopropylsilylethynyl)pentacene
as a model small
molecule organic semiconductor and two conjugated polymer additives
to demonstrate conjugated polymer-mediated polymorphism of a small
molecule organic semiconductor for the first time. The conjugated
polymer additives, used with a slow solution crystallization approach,
yield crystal structures that are not accessible by nonconjugated
polymer additives and impart excellent long-range order. In both of
the small molecule semiconductor/conjugated polymer blends studied
here, previously unreported polymorphs of a small molecule semiconductor
have been identified which also leads to improved charge transport
in the absence of external alignment. These results open up a new
exciting avenue to manipulate unit cell structure, long-range order,
and charge transport of high performance, solution-processed, small
molecule organic semiconductors
Understanding Polymorphism in Organic Semiconductor Thin Films through Nanoconfinement
Understanding
crystal polymorphism is a long-standing challenge
relevant to many fields, such as pharmaceuticals, organic semiconductors,
pigments, food, and explosives. Controlling polymorphism of organic
semiconductors (OSCs) in thin films is particularly important given
that such films form the active layer in most organic electronics
devices and that dramatic changes in the electronic properties can
be induced even by small changes in the molecular packing. However,
there are very few polymorphic OSCs for which the structure–property
relationships have been elucidated so far. The major challenges lie
in the transient nature of metastable forms and the preparation of
phase-pure, highly crystalline thin films for resolving the crystal
structures and evaluating the charge transport properties. Here we
demonstrate that the nanoconfinement effect combined with the flow-enhanced
crystal engineering technique is a powerful and likely material-agnostic
method to identify existing polymorphs in OSC materials and to prepare
the individual pure forms in thin films at ambient conditions. With
this method we prepared high quality crystal polymorphs and resolved
crystal structures of 6,13-bis(triisopropylsilylethynyl)pentacene
(TIPS-pentacene), including a new polymorph discovered via in situ
grazing incidence X-ray diffraction and confirmed by molecular mechanic
simulations. We further correlated molecular packing with charge transport
properties using quantum chemical calculations and charge carrier
mobility measurements. In addition, we applied our methodology to
a [1]benzothieno[3,2-<i>b</i>][1]1benzothiophene (BTBT)
derivative and successfully stabilized its metastable form