Tae-Yong Noh

High-efficiency stacked white organic light-emitting diodes

We report efficient tandem white organic light-emitting diodes (WOLEDs) by using bathocuproine:Cs2CO3∕MoO3 as an effective interconnecting layer. We utilized two primary colors of sky blue and orange fluorescent emitters to obtain efficient white electroluminescence. Although single WOLEDs using two adjacent emitting layers showed a maximum current efficiency of 7.96cd∕A with Commission Internationale d’Eclairage (CIE) coordinates of (0.28, 0.34), the tandem WOLED device made by stacking two single color OLEDs in series demonstrated doubled maximum current efficiency of 17.14cd∕A with CIE coordinates of (0.28, 0.41). The stacking of different single color OLEDs in series instead of double stacking of WOLEDs can be useful to achieve highly efficient WOLEDs because it can reduce the number of layers of the devices.

Hole-transporting interlayers for improving the device lifetime in the polymer light-emitting diodes

The authors report the effect of thermal treatment of hole-transporting interlayers between a polymeric hole injection layer and an emitting layer (EML) on the luminous efficiency and the lifetime performance in blue polymer light-emitting diodes. As the thermal annealing temperature of the interlayer increased, the hole mobility of the interlayer tended to decrease, which results in reducing the hole current injected into the EML in the devices. Hence, the device luminous efficiency decreased due to lower electron-hole balance. Nevertheless, the device lifetime increased, which can be attributed to the formation of the thicker interlayer and the better defined interlayer/EML interface.

Polyaniline‐Based Conducting Polymer Compositions with a High Work Function for Hole‐Injection Layers in Organic Light‐Emitting Diodes: Formation of Ohmic Contacts

It is a great challenge to develop solution-processed, polymeric hole-injection layers (HILs) that perform better than small molecular layers for realizing high-performance small-molecule organic light-emitting diodes (SM-OLEDs). We have greatly improved the injection efficiency and the current efficiency of SM-OLEDs by introducing conducting polymer compositions composed of polyaniline doped with polystyrene sulfonate and perfluorinated ionomer (PFI) as the HIL. During single spin-coating of conducting polymer compositions, the PFI layer was self-organized at the surface and greatly increased the film work function. It enhanced hole-injection efficiency and current efficiency by introducing a nearly ohmic contact and improving electron blocking. Our results demonstrate that solution-processed polyaniline HILs with tunable work functions are good candidates for reducing process costs and improving OLED performance.

Photocurrent mechanism in a hybrid system of 1-thioglycerol-capped HgTe nanoparticles

Photoluminescence, absorption, and photocurrent measurements were made for a hybrid system of 1-thioglycerol-capped HgTe nanoparticles synthesized by colloidal method to investigate the photocurrent mechanism in this hybrid system. Absorption and photoluminescence spectra taken for the capped HgTe nanoparticles reveal strong exciton peaks in the near-infrared wavelength range. The wavelength dependence of the photocurrent for these capped nanoparticles is very close to that of the absorption spectrum. For the photocurrent mechanism of the hybrid system, on the basis of our experimental results and energy diagram for the 1-thioglycerol-capped HgTe nanoparticles, it is suggested in this letter that holes among electron-hole pairs created by incident photons in the HgTe nanoparticles are transferred to capping 1-thioglycerol while electrons are strongly confined in these nanoparticles and that the holes contribute to the photocurrent flowing in the medium of 1-thioglycerol.

A stable blue host material for organic light-emitting diodes

We have developed a high performance fluorescent host material, 6-anthracene-9-yl-2,3-di-p-tolyl benzo[b]thiophene (ATB), for blue organic light-emitting diodes. ATB formed a stable amorphous solid state film with a high glass transition temperature (Tg=116°C). The multilayer devices fabricated using ATB as a blue host material showed higher power efficiency (6.4lm∕W at 1000cd∕m2) than the conventional device using 2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN) (4.3lm∕W at 1000cd∕m2). The half lifetime of the ATB device (6480h at 1000cd∕m2) was also enhanced compared to the TBADN device (5341h at 1000cd∕m2).

Photoluminescence characteristics of Mn- and Pr-doped ZnS nanoparticles optically annealed with UV illumination

Undoped, Mn-doped, Pr-doped, and Mn- and Pr-codoped ZnS nanoparticles were synthesized by a wet chemical method, and these nanoparticles were optically annealed in air or vacuum. In the photoluminescence (PL) spectra of the optically annealed nanoparticles, a sulfur–vacancy-related PL band at around 420 nm, a PL band associated with the 4T1→6A1 transition of Mn2+, and PL peaks associated with Pr-related complexes were investigated during optical annealing. The integrated PL intensities for the nanoparticles annealed in air increased more markedly with the optical annealing time than those for the nanoparticles annealed in vacuum. This increase may be explained by the photooxidation of the surface, the enhancement of the crystal quality, and the gradual diffusion of luminescent ions into the crystal lattice.

Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition

Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition is investigated in this study. For an undoped Si nanowire biased at 3 V, photocurrent excited by the 633-nm wavelength light is stronger in intensity than that excited by the 325-nm wavelength light, and photoresponses are rapid when the light is switched on and off. In contrast, for the n- and p-type Si nanowires, photocurrent excited by the 633-nm wavelength light is not measurable, although one excited by 325-nm wavelength light is still detectable. And photoresponses obtained for the doped Si nanowires are slower, compared with the undoped Si nanowire. Photocurrent phenomena observed in the undoped, n- and p-type Si nanowires are discussed in this paper.

Improved Performance of Polymeric Light-Emitting Diodes with an Electron Blocking Layer

In this paper, we describe a new approach for fabrication of high efficient polymeric light-emitting diodes (PLEDs). In the device configuration of ITO/HTL/EBL/EML/BaF2:Ca:Al (ITO: indium tin oxide, HTL: hole transport layer, EBL: electron blocking layer, EML: emitting layer), EBL contains cross-linkable moieties in order to make the layer which is insoluble to layering of an additional emitting polymer. The devices with EBL exhibit strong blue emissions and higher efficiency values than those in devices without EBL. The synthesis, characterization, device fabrication, and electroluminescence (EL) properties of devices will be presented.