10 research outputs found
Improved Performance in Diketopyrrolopyrrole-Based Transistors with Bilayer Gate Dielectrics
There has been significant progress
in the past 2 decades in the field of organic and polymer thin-film
transistors. In this paper, we report a combination of stable materials,
device architecture, and process conditions that resulted in a patterned
gate, small channel length (<5 μm) device that possesses
a scaled field-induced conductivity in air that is higher than any
organic/polymer transistor reported thus far. The operating voltage
is below 10 V; the on-off ratio is high; and the active materials
are solution-processable. The semiconducting polymer is a new donor–acceptor
polymer with furan-substituted diketopyrrolopyrrole and thienyl-vinylene-thienyl
building blocks in the conjugated backbone. One of the major striking
features of our work is that the patterned-gate device architecture
is suitable for practical applications. We also propose a figure of
merit to meaningfully compare polymer/organic transistor performance
that takes into account mobility and operating voltage. With this
figure of merit, we compare leading organic and polymer transistors
that have been hitherto reported. The material and device architecture
have shown very high mobility and low operating voltage for such short
channel length (below 5 μm) organic/polymer transistors
Chemical Understanding of the Mechanisms Involved in Mitigation of Charged Impurity Effects by Polar Molecules on Graphene
It
is well-known that the transport properties of monolayer graphene
are degraded by charged impurities present between graphene and either
a given substrate or air. Such impurities cause charge scattering
of holes and electrons in graphene. In previous work, our group has
used both fluoropolymer thin films and polar vapor molecules to dramatically
improve graphene field-effect transistor (FET) device characteristics,
including Dirac voltage and mobility. We attributed the graphene device
improvements to mitigation of charged impurities and defects due to
electrostatic interaction with the dipoles of the applied fluoropolymers
and polar molecules. In this work, we present theoretical support
to this hypothesis, in the form of computational chemical simulations
involving the interaction of polar molecules and impurities on a graphene
sheet. We examine two types of impurities which may occur at graphene
interfaces: ionic impurities and molecular dipole impurities. Upon
introduction of polar vapor molecules to an impurity/graphene system,
we observed a dramatic reduction in the electrostatic potential in
the plane of the graphene from the impurity. The magnitude of potential
reduction scales with the average dipole moment of each polar molecule.
We were able to determine two separate mechanisms which contribute
to the total potential reduction, impurity displacement, and electrostatic
screening of the impurity. The respective impacts of the mechanisms
vary with distance from the impurity. Additionally, in the case of
the molecular dipole impurity, the orientation of the impurity atop
graphene is a key factor that determines the potential impact
High-Speed, Inkjet-Printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary Ring Oscillators
The materials combination of inkjet-printed
single-walled carbon
nanotubes (SWCNTs) and zinc tin oxide (ZTO) is very promising for
large-area thin-film electronics. We compare the characteristics of
conventional complementary inverters and ring oscillators measured
in air (with SWCNT p-channel field effect transistors (FETs) and ZTO
n-channel FETs) with those of ambipolar inverters and ring oscillators
comprised of bilayer SWCNT/ZTO FETs. This is the first such comparison
between the performance characteristics of ambipolar and conventional
inverters and ring oscillators. The measured signal delay per stage
of 140 ns for complementary ring oscillators is the fastest for any
ring oscillator circuit with printed semiconductors to date
Transformation of the Electrical Characteristics of Graphene Field-Effect Transistors with Fluoropolymer
We report on the improvement of the electronic characteristics
of monolayer graphene field-effect transistors (FETs) by an interacting
capping layer of a suitable fluoropolymer. Capping of monolayer graphene
FETs with CYTOP improved the on–off current ratio from 5 to
10 as well as increased the field-effect mobility by as much as a
factor of 2 compared to plain graphene FETs. Favorable shifts in the
Dirac voltage toward zero with shift magnitudes in excess of 60 V
are observed. The residual carrier concentration is reduced to ∼2.8
× 10<sup>11</sup> cm<sup>–2</sup>. Removal of the fluoropolymer
from graphene FETs results in a return to the initial electronic properties
before depositing CYTOP. This suggests that weak, reversible electronic
perturbation of graphene by the fluoropolymer favorably tune the electrical
characteristics of graphene, and we hypothesize that the origin of
this improvement is in the strongly polar nature of the C–F
chemical bonds that self-organize upon heat treatment. We demonstrate
a general method to favorably restore or transform the electrical
characteristics of graphene FETs, which will open up new applications
Efficient Polymer Solar Cells Enabled by Low Temperature Processed Ternary Metal Oxide as Electron Transport Interlayer with Large Stoichiometry Window
Highly efficient organic photovoltaic
cells are demonstrated by
incorporating low temperature solution processed indium zinc oxide
(IZO) as cathode interlayers. The IZOs are synthesized using a combustion
synthesis method, which enables low temperature processes (150–250
°C). We investigated the IZO films with different electron mobilities
(1.4 × 10<sup>–3</sup> to 0.23 cm<sup>2</sup>/(V·s)),
hydroxide–oxide content (38% to 47%), and surface roughness
(0.19–5.16 nm) by modulating the ternary metal oxide stoichiometry.
The photovoltaic performance was found to be relatively insensitive
to the composition ratio of In:Zn over the range of 0.8:0.2 to 0.5:0.5
despite the differences in their electrical and surface properties,
achieving high power conversion efficiencies of 6.61%–7.04%.
Changes in composition ratio of IZO do not lead to obvious differences
in energy levels, diode parameters and morphology of the photoactive
layer, as revealed by ultraviolet photoelectron spectroscopy (UPS),
dark current analysis and time-of-flight secondary ion mass spectrometry
(TOF-SIMS) measurements, correlating well with the large IZO stoichiometry
window that enables efficient photovoltaic devices. Our results demonstrate
the robustness of this ETL system and provide a convenient approach
to realize a wide range of multicomponent oxides and compatible with
processing on flexible plastic substrates
Enhanced Photoresponse in Metasurface-Integrated Organic Photodetectors
In
this work, we experimentally demonstrate metasurface-enhanced
photoresponse in organic photodetectors. We have designed and integrated
a metasurface with broadband functionality into an organic photodetector,
with the goal of significantly increasing the absorption of light
and generated photocurrent from 560 up to 690 nm. We discuss how the
metasurface can be integrated with the fabrication of an organic photodiode.
Our results show large gains in responsivity from 1.5× to 2×
between 560 and 690 nm
25 GHz Embedded-Gate Graphene Transistors with High‑K Dielectrics on Extremely Flexible Plastic Sheets
Despite the widespread interest in graphene electronics over the past decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this article, we report detailed studies on the electrical and mechanical properties of vapor synthesized high-quality monolayer graphene integrated onto flexible polyimide substrates. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobilities of 3900 cm<sup>2</sup>/V·s, and importantly, 25 GHz cutoff frequency, which is more than a factor of 2.5 times higher than prior results. Mechanical studies reveal robust transistor performance under repeated bending, down to 0.7 mm bending radius, whose tensile strain is a factor of 2–5 times higher than in prior studies. In addition, integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene
Long-Term Stability and Reliability of Black Phosphorus Field-Effect Transistors
Black
phosphorus has been recently suggested as a very promising
material for use in 2D field-effect transistors. However, due to its
poor stability under ambient conditions, this material has not yet
received as much attention as for instance MoS<sub>2</sub>. We show
that the recently demonstrated Al<sub>2</sub>O<sub>3</sub> encapsulation
leads to highly stable devices. In particular, we report our long-term
study on highly stable black phosphorus field-effect transistors,
which show stable device characteristics for at least eight months.
This high stability allows us to perform a detailed analysis of their
reliability with respect to hysteresis as well as the arguably most
important reliability issue in silicon technologies, the bias-temperature
instability. We find that the hysteresis in these transistors depends
strongly on the sweep rate and temperature. Moreover, the hysteresis
dynamics in our devices are reproducible over a long time, which underlines
their high reliability. Also, by using detailed physical models for
oxide traps developed for Si technologies, we are able to capture
the channel electrostatics of the black phosphorus FETs and determine
the position of the defect energy band. Finally, we demonstrate that
both hysteresis and bias-temperature instabilities are due to thermally
activated charge trapping/detrapping by oxide traps and can be reduced
if the device is covered by Teflon-AF
25 GHz Embedded-Gate Graphene Transistors with High‑K Dielectrics on Extremely Flexible Plastic Sheets
Despite the widespread interest in graphene electronics over the past decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this article, we report detailed studies on the electrical and mechanical properties of vapor synthesized high-quality monolayer graphene integrated onto flexible polyimide substrates. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobilities of 3900 cm<sup>2</sup>/V·s, and importantly, 25 GHz cutoff frequency, which is more than a factor of 2.5 times higher than prior results. Mechanical studies reveal robust transistor performance under repeated bending, down to 0.7 mm bending radius, whose tensile strain is a factor of 2–5 times higher than in prior studies. In addition, integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene
Logic-Gate Devices Based on Printed Polymer Semiconducting Nanostripes
The applications of organic semiconductors
in complex circuitry
such as printed CMOS-like logic circuits demand miniaturization of
the active structures to the submicrometric and nanoscale level while
enhancing or at least preserving the charge transport properties upon
processing. Here, we addressed this issue by using a wet lithographic
technique, which exploits and enhances the molecular order in polymers
by spatial confinement, to fabricate ambipolar organic field effect
transistors and inverter circuits based on nanostructured single component
ambipolar polymeric semiconductor. In our devices, the current flows
through a precisely defined array of nanostripes made of a highly
ordered diketopyrrolopyrrole-benzothiadiazole copolymer with high
charge carrier mobility (1.45 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for electrons and 0.70 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for holes). Finally, we demonstrated
the functionality of the ambipolar nanostripe transistors by assembling
them into an inverter circuit that exhibits a gain (105) comparable
to inverters based on single crystal semiconductors