36 research outputs found
Air Stable Organic Salt As an n‑Type Dopant for Efficient and Stable Organic Light-Emitting Diodes
Air-stable and low-temperature-evaporable
n-type dopants are highly desired for efficient and stable organic
light-emitting diodes (OLEDs). In this work, 2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium
iodide (<i>o</i>-MeO-DMBI-I), a thermally decomposable precursor
of organic radical <i>o</i>-MeO-DMBI, has been employed
as a novel n-type dopant in OLEDs, because of its air stability, low
decomposition temperature, and lack of atom diffusion. The n-type
electrical doping is evidenced by the rapid increase in current density
of electron-only devices and the large improvement in conductivity,
originated from increased electron concentration in electron-transport
layer (ETL) and reduced electron injection barrier. A highly efficient
and stable OLED is created using <i>o</i>-MeO-DMBI as an
n-type dopant in Bphen. Compared with the control device with its
high-temperature-evaporable n-type dopant cesium carbonate (Cs<sub>2</sub>CO<sub>3</sub>), <i>o</i>-MeO-DMBI-doped device
showed an incredible boom in current efficiency from 28.6 to 42.2
cd/A. Moreover, the lifetime (<i>T</i><sub>70%</sub>) of <i>o</i>-MeO-DMBI-doped device is 45 h, more than 20 times longer
than that of the Cs<sub>2</sub>CO<sub>3</sub>-doped device (2 h).
The enhanced efficiency and stability are attributed to the improved
balance of holes and electrons in the emissive layer, and the eliminated
atom diffusion of cesium
Electric Field inside a Hole-Only Device and Insights into Space-Charge-Limited Current Measurement for Organic Semiconductors
It is known that the electric field
is nonuniform inside organic electronic devices. However, the physics
a few nanometers near interfaces and factors that influence the electric
field distribution are still not fully understood. Moreover, the mobility
might be nonuniform inside the device, since it is electric-field
dependent. However, this has been overlooked in the apprehension of
the space-charge-limited (SCL) current in the commonly used Mott–Gurney
equation. Here, we carry out 3D multiparticle Monte Carlo simulations
to study the electric field and energy diagram in a hole-only device
under bias. Coulomb potential is obtained from the solution of the
1D Poisson equation of our system. The influences of the injection
barrier, the energetic disorder and the applied bias are studied in
detail. The SCL current is compared with that from the Mott−Gurney
equation. It is found that the apparent charge mobilities are close
to those calculated using the transit time corresponding to the cross
point of asymptotes to the plateau and trailing edge of the current
in the double logarithmic plot in the time-of-flight (TOF) simulations,
which means that the space-charge-limited current (SCLC) measurement
can be reliable for organic semiconductors
Thermally Activated Delayed Fluorescence Sensitized Phosphorescence: A Strategy To Break the Trade-Off between Efficiency and Efficiency Roll-Off
Materials with thermally activated
delayed fluorescence (TADF)
realized 100% internal quantum efficiency (IQE) but suffered significant
efficiency roll-off. Here, an exciton dynamics study reveals that
materials with TADF may play opposite roles in affecting the efficiency
roll-off: decreasing the triplet density due to the fast reverse intersystem
crossing, on the one hand, and increasing the triplet density due
to the weakened singlet radiation. We show theoretically and experimentally
that TADF-sensitized phosphorescence can break this trade-off by exploiting
the efficient Förster energy transfer and simultaneously achieve
100% IQE and low efficiency roll-off (with a critical current density
of 460 mA cm<sup>–2</sup>)
High-Mobility Solution-Processed Tin Oxide Thin-Film Transistors with High‑κ Alumina Dielectric Working in Enhancement Mode
Solution-processed metal oxide thin-film
transistors (TFTs) operating
in enhancement mode are promising for the next-generation flat panel
displays. In this work, we report high-mobility TFTs based on SnO<sub>2</sub> active layer derived from a soluble tinÂ(II) 2-ethylhexanoate
precursor. Densely packed polycrystalline SnO<sub>2</sub> thin films
with moderate oxygen vacancies and only a few hydroxides are obtained
via systemically optimizing precursor concentrations and processing
conditions. The utilization of a solution-processed high-κ Al<sub>2</sub>O<sub>3</sub> insulating layer could generate a coherent dielectric/semiconductor
interface, hence further improving the device performance. TFT devices
with an average field-effect mobility of 96.4 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>, a current on/off ratio of
2.2 × 10<sup>6</sup>, a threshold voltage of 1.72 V, and a subthreshold
swing of 0.26 V dec<sup>–1</sup> have been achieved, and the
driving capability is demonstrated by implementing a single SnO<sub>2</sub> TFT device to tune the brightness of an organic light-emitting
diode. It is worth noting that these TFTs work in enhancement mode
at low voltages less than 4 V, which sheds light on their potential
application to the next-generation low-cost active matrix flat panel
displays
Molecular Understanding of the Chemical Stability of Organic Materials for OLEDs: A Comparative Study on Sulfonyl, Phosphine-Oxide, and Carbonyl-Containing Host Materials
Chemical
stability of organic materials on service toward excitons and charge
carriers is intrinsically associated with the operational stability
and economics of state-of-the-art organic light-emitting devices.
Here we conducted comprehensive experiments and theoretical calculations
to comparatively investigate the intrinsic chemical stability of organic
materials, which contain typical electron-accepting moieties of sulfonyl,
phosphine-oxide, and carbonyl group. The materials with a diphenylsulfonyl
moiety suffered a fatal chemical instability originating from the
cleavage of C–S single bond whether under UV irradiation or
in electrical-stressed devices. The material with a dibenzothiophene-<i>S</i>,<i>S</i>-dioxide moiety exhibited significantly
improved chemical stability because of effective shielding of the
weak C–S single bond in a ring. In contrast, the commercially
used carbonyl-containing compound demonstrated the highest chemical
stability with negligible degradation under the same condition. Quantum
chemical calculations fully supported the experimental results and
suggested that the bond strength of the weak chemical bonds of the
molecules would determine the intrinsic chemical stability of the
organic materials in their excited and charged states, which might
be a plausible origin of the limited stability of high-energy blue-emitting
materials and devices. Several implications have been drawn for the
design of new blue-emitting materials
High-Efficiency Organic Light-Emitting Diodes Based on Sublimable Cationic Iridium(III) Complexes with Sterically Hindered Spacers
Cationic iridiumÂ(III)
complexes show great promise as phosphorescent
materials, while their utilization in organic light-emitting diodes
is severely hindered by the inferior sublimability and low device
performance. Here we devise a judicious strategy to develop high-efficiency
sublimable cationic iridiumÂ(III) complexes by simultaneously introducing
sterically hindered spacers into the negative counterions, major and
ancillary ligands. We have exploited a novel series of yellow and
red emitters, investigated their photophysical properties, electrochemical
behaviors, and thermal stabilities, and finally fabricated organic
light-emitting diodes by vacuum evaporation deposition. Record-high
device performance has been achieved with a superior external quantum
efficiency of 16%, excellent power efficiency of 49 lm/W, maximum
brightness over 27.3 × 10<sup>3</sup> cd/m<sup>2</sup>, very
low turn-on voltage below 2.5 V, and quite small efficiency roll-off.
Our study represents a significant advance in the development of sublimable
cationic iridiumÂ(III) complexes and evidences their promising applications
in state-of-the-art optoelectronic devices
Co-Actions of Ambient Pressure and Gas Molecular Adsorption on the Carriers’ Transport in Polycrystalline Pentacene Thin-Film Transistors
Organic transistors have proved to have potential applications
in pressure sensors. However, few reports consider the coactions of
pressure and ambient gas adsorption on the characteristics of the
sensitive transistors. In this article, pentacene polycrystalline
thin films were fabricated as the active layer of organic transistors,
and the effects of ambient pressure and the gas adsorption on the
carriers’ transport characteristics have been investigated.
It was found that during the process from one atmosphere to vacuum
(∼5 × 10<sup>–3</sup> Pa) the device output, saturation
source-drain currents (<i>I</i><sub>DS</sub>), changed with
pressure not monotonously but with an unexpected reversible minimum
peak. Considering the variation of gas adsorption quantity and the
distance between pentacene grains with pressure, we established models
to understand the nature of the pressure sensitivity. We found that
in low pressures the adsorption of gas molecules in grain boundaries
was the main factor that affects device performance, whereas in high
pressures, the shortening of the distance between pentacene grains
was the main factor. Our research will benefit the understanding of
charge-transport nature and, more importantly, give some instructions
on using and designing highly sensitive pressure sensors
Charge Transport in Mixed Organic Disorder Semiconductors: Trapping, Scattering, and Effective Energetic Disorder
The effects of trapping and scattering on the transporting
properties
of organic disorder semiconductors have been studied by time-of-flight
(TOF) method. Tris-(8-hydroxyquinoline)-aluminum (Alq<sub>3</sub>),
2,2′,2″-(1,3,5-benzenetriyl)-tris-(1-phenyl-1<i>H</i>-benzimid-azole) (TPBi), and <i>N</i>,<i>N</i>-diphenyl-<i>N</i>,<i>N</i>-bisÂ(1-naphthyl)-(1,1′-biphenyl)-4,4′diamine
(NPB) are doped into 4,4′-<i>N</i>,<i>N</i>′-dicarbazolebiphenyl (CBP) to form traps and scatters with
various energy level differences. It is found that the low scatters
significantly reduce the mobility and make the TOF transients, while
the deep traps and high scatters would not significantly reduce the
mobility and change the nondispersive behavior of the TOF transients.
The main difference between deep traps and high scatters is that the
deep traps induce a great reduction of the photocurrent, while the
high scatters do not obviously decrease the photocurrent. The experimental
results are well explained by the Miller–Abrahams hopping model
and the effective energetic disorder. Furthermore, a theoretical method
is established to determine the demarcation between the shallow trap
(low scatter) and the deep trap (high scatter) in terms of energy
level differences. These results may shed light on the understanding
of charge transport in mixed organic semiconductors
Microphotographs of PARP-positive chondrocytes in the resting zone and in the proliferative & hypertrophic zone of growth plate of apex vertebrae in AIS patient(Magnification: 400×)
<p><b>Copyright information:</b></p><p>Taken from "Histomorphological study of the spinal growth plates from the convex side and the concave side in adolescent idiopathic scoliosis"</p><p>http://www.josr-online.com/content/2/1/19</p><p>Journal of Orthopaedic Surgery and Research 2007;2():19-19.</p><p>Published online 11 Nov 2007</p><p>PMCID:PMC2186319.</p><p></p> PARP-positive chondrocytes (arrows) in the resting zone of convex side(A). PARP-positive chondrocytes (arrows) in the resting zone of concave side(B). PARP-positive chondrocytes (arrows) in the proliferative & hypertrophic zone of convex side(C). PARP-positive chondrocytes (arrows) in the proliferative & hypertrophic zone of concave side(D)
Microphotographs of TUNEL-positive chondrocytes in the resting zone and in the proliferative & hypertrophic zone of growth plate of apex vertebrae in AIS patient(Magnification: 400×)
<p><b>Copyright information:</b></p><p>Taken from "Histomorphological study of the spinal growth plates from the convex side and the concave side in adolescent idiopathic scoliosis"</p><p>http://www.josr-online.com/content/2/1/19</p><p>Journal of Orthopaedic Surgery and Research 2007;2():19-19.</p><p>Published online 11 Nov 2007</p><p>PMCID:PMC2186319.</p><p></p> TUNEL-positive chondrocytes (arrows) in the resting zone of convex side(A). TUNEL-positive chondrocytes (arrows) in the resting zone of concave side(B). TUNEL-positive chondrocytes (arrows) in the proliferative & hypertrophic zone of convex side(C). TUNEL-positive chondrocytes (arrows) in the proliferative & hypertrophic zone of concave side(D)