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 (Cs₂CO₃), <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>_70%) of <i>o</i>-MeO-DMBI-doped device is 45 h, more than 20 times longer than that of the Cs₂CO₃-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^–2)

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 SnO₂ active layer derived from a soluble tin­(II) 2-ethylhexanoate precursor. Densely packed polycrystalline SnO₂ 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 Al₂O₃ 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 cm² V^–1 s^–1, a current on/off ratio of 2.2 × 10⁶, a threshold voltage of 1.72 V, and a subthreshold swing of 0.26 V dec^–1 have been achieved, and the driving capability is demonstrated by implementing a single SnO₂ 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 iridium­(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 iridium­(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 × 10³ cd/m², 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 iridium­(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^–3 Pa) the device output, saturation source-drain currents (<i>I</i>_DS), 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 (Alq₃), 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>-bis­(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)