No abstract

We demonstrated efficient red organic light-emitting diodes based on a host emitting system of 9,10-di(2-naphthyl)anthracene (ADN) co-doped with 4-(dicyano-methylene)-2-t-butyle-6-(1,1,7,7-tetramethyl-julolidyl-9-enyl)-4H-pyran (DCJTB) as a red dopant and 2,3,6,7tetrahydro-1,1,7,7-tetramethyl-1H,5H,1 1H-10(2-benzothiazolyl)-quinolizine-[9,9a,1gh] coumarin (C545T) as an assistant dopant. The typical device structure was glass substrate/ITO/4,4 ,4-tris(N-3-methylphenyl-N-phenylamino) triphenylamine(m-MTDATA)/N,N -bis-(naphthalene-1-yl)-N,N -diphenylbenzidine(NPB)/[ADN: DCJTB: C545T/Alq 3 /LiF/Al]. It was found that C545T dopant did not emit by itself but did assist the energy transfer from the host (ADN) to the red emitting dopant. The red OLEDs realized by this approach not only enhanced the emission color, but also significantly improved the EL efficiency. The EL efficiency reached 3.5 cd A −1 at a current density of 20 mA cm −2 , which is enhanced by three times compared with devices where the emissive layer is composed of the DCJTB doped ADN. The saturated red emission was obtained with CIE coordinates (x = 0.618, y = 0.373) at 621 nm, and the device driving voltage is decreased as much as 38%. We attribute these improvements to the assistant dopant (C545T), which leads to the more efficient energy transfer from ADN to DCJTB. These results indicate that the co-doped system is a promising method for obtaining high-efficiency red OLEDs.

Pentacene-based organic thin film transistors (OTFTs) with WO 3 /Au as source/drain (S/D) electrodes were fabricated. In comparison with the pentacene-based OTFT having Au-only S/D electrodes, the performance of a device having WO 3 /Au S/D electrodes has been considerably improved. The saturation mobility increased from 0.39 to 2.8 cm 2 (V s) −1 , the threshold voltage reduced from −21.3 to −11.2 V, the on/off ratio shifted from 6.7 × 10 5 to 1.5 × 10 7 and the threshold swing varied from 3.75 to 1.82 V dec −1 . The improvement was attributed to (i) the significant reduction of contact resistance by using the WO 3 interlayer and (ii) the strong heat radiation during the thermal evaporation of WO 3 . Our results indicated that using WO 3 /Au as S/D electrodes is an effective approach to improving pentacene-based OTFT's performance.

A dual electron-transport layer (d-ETL) with hole-blocking function was successfully applied to a 2-(t-butyl)-9,10-di(2-naphthyl)anthracene (TBADN)-based blue organic light-emitting device (OLED) for improving chromaticity and electron injection. The d-ETL was constructed by sandwiching a 1,3-bis[2-(2,2 -bipyridin-6-yl)-1,3,4-oxadiazol-5-yl]benzene (Bpy-OXD) layer between the emission layer (EML) and the most common ETLs of tris(8-hydroquinoline) aluminum (Alq 3 ) and 4,7-diphyenyl-1,10-phenanthroline (BPhen). Due to the hole-blocking function of Bpy-OXD and thereby the better confinement of carriers within the EML, d-ETL devices showed much better chromaticity and less current-induced color shift as compared to the corresponding single ETL ones. Moreover, the d-ETL devices revealed higher power efficiency (increased by ∼30%) and lower driving voltage, indicating an enhanced electron injection. This could be explained by the fact that an interlayer of Bpy-OXD provided a stepped energy level which greatly facilitated electron injection and hence enhanced injection current.

Zn 1−x Cr x O (x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.09) nanoparticles were synthesized, by an auto-combustion method. Structural, optical, and magnetic characteristics of Cr-doped ZnO samples calcined at 600 °C have been analyzed by using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), UV-Vis spectroscopy and vibrating sample magnetometer (VSM). The XRD data confirmed the hexagonal wurtzite structure of pure and Cr-doped ZnO nanoparticles. The calculated values of grain size using Scherrer's formula are in the range of 30.7-9.2 nm. The morphology of nanopowders has been observed by FESEM, and EDS results confirmed a systematic increase of Cr content in the samples and clearly indicate with no impurity element. The band gaps, computed by UV-Vis spectroscopy, are in the range of 2.83-2.35 eV for different doping concentrations. By analyzing VSM data, significantly enhanced room temperature ferromagnetism is identified in Cr-doped ZnO samples. The value of magnetization is a 12 times increased of the value reported by Daun et al. (2010). Room temperature ferromagnetism of the nanoparticles is of vital prominence for spintronics applications.