Violet-blue LEDs based on p-GaN/n-ZnO nanorods and their stability.

In this paper, we report a fabrication, characterization and stability study of p-GaN/n-ZnO nanorod heterojunction light-emitting devices (LEDs). The LEDs were assembled from arrays of n-ZnO vertical nanorods epitaxially grown on p-GaN. LEDs showed bright electroluminescence in blue (440 nm), although weaker violet (372 nm) and green-yellow (550 nm) spectral components were also observed. The device characteristics are generally stable and reproducible. The LEDs have a low turn-on voltage (∼5 V). The electroluminescence (EL) is intense enough to be noticed by the naked eye, at an injection current as low as ∼ 40 µA (2.1 × 10(-2) A cm(-2) at 7 V bias). Analysis of the materials, electrical and EL investigations point to the role of a high quality of p-n nano-heterojunction which facilitates a large rectification ratio (320) and a stable reverse current of 2.8 µA (1.4 × 10(-3) A cm(-2) at 5 V). Stability of EL characteristics was investigated in detail. EL intensity showed systematic degradation over a short duration when the LED was bias-stressed at 30 V. At smaller bias (<20 V) LEDs tend to show a stable and repeatable EL characteristic. Thus a simple low temperature solution growth method was successfully exploited to realize nanorod/film heterojunction LED devices with predictable characteristics.

[Spectral characteristics of novel small molecular fluorene derivative for organic light-emitting device].

Double-layer organic light-emitting devices (OLEDs) based on a blend system of novel small molecule fluorene material 6,6'-(9H-fluoren-9,9-diyl) bis(2,3-bis(9,9-dihexyl-9H-fluoren-2-yl) quinoxaline) (BFLBBFLYQ) and hole transporting material N, N'-biphenyl-N, N'-bis-(3-methylphenyl)-1, 1'-biphenyl-4, 4'-diamine (TPD) were fabricated. The structure of the double-layer device was ITO/BFLBBFLYQ : TPD/tris(8-hydroxyquinolinato) aluminum(Alq)/Mg : Ag. The photoluminescence (PL) spectra of BFLBBFLYQ and TPD were located at 447 and 414 nm, respectively. The spectral characteristics of the blend system and the double-layer device were investigated, which indicated that a new long wavelength emission peaking at 530 nm was appeared both in PL spectra and electroluminescence (EL) spectra. The exciplex between BFLBBFLYQ and TPD may play the role in long wavelength emission in the blend device and the spin-coated film. Based on the absorption spectra of a red fluorescent dye 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as probe and the PL spectra of the blend system showing good overlap, energy transfer from the blend system to DCJTB could be expected. There fore, DCJTB could be selected as a molecular dopant to investigate the influence on EL spectra and the recombination of the devices. It was found that the excitons recombine at the interior Alq layer near to the BFLBBFLYQ : TPD layer.