Jonghee Lee

Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells

The performance of both organic light-emitting diodes (OLEDs) and organic solar cells (OSC) depends on efficient coupling between optical far field modes and the emitting/absorbing region of the device. Current approaches towards OLEDs with efficient light-extraction often are limited to single-color emission or require expensive, non-standard substrates or top-down structuring, which reduces compatibility with large-area light sources. Here, we report on integrating solution-processed nano-particle based light-scattering films close to the active region of organic semiconductor devices. In OLEDs, these films efficiently extract light that would otherwise remain trapped in the device. Without additional external outcoupling structures, translucent white OLEDs containing these scattering films achieve luminous efficacies of 46 lm W−1 and external quantum efficiencies of 33% (both at 1000 cd m−2). These are by far the highest numbers ever reported for translucent white OLEDs and the best values in the open ...

Highly rigid and twisted anthracene derivatives: a strategy for deep blue OLED materials with theoretical limit efficiency

New highly twisted and rigid blue materials composed of anthracene with xylene as the core unit and either naphthalene or phenyl end units were synthesized. Non-doped devices using BDNA displayed 5.26% maximum external quantum efficiency with CIE color coordinates (x, y: 0.159, 0.072).

Enhanced efficiency and reduced roll-off in blue and white phosphorescent organic light-emitting diodes with a mixed host structure

We report a significant enhancement and a reduced efficiency roll-off in blue and white phosphorescent organic light-emitting diodes (PHOLEDs) based on a mixed host structure. The combination of a hole-transport-type host and an electron-transport-type host as a mixed host emissive layer resulted in an effective charge carrier injection and distribution of the recombination zone, and therefore highly efficient blue PHOLEDs with peak external quantum and power efficiencies of 21.6% and 44.9 lm/W were achieved. Moreover, white PHOLEDs with a mixed host structure showed a power efficiency of 37 lm/W, which is over four times higher than 8.7 lm/W in a single host structure device at a luminance of 1000 cd/m2.

Influence of doping profile on the efficiency of blue phosphorescent organic light-emitting diodes

We demonstrate that electroluminescent efficiency of blue phosphorescent organic light-emitting diodes could be improved by changing doping profile in the emissive layer (EML) of the device containing iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate (FIrpic) and N,N′-dicarbazolyl-3,5-benzene (mCP). By examining the changes of recombination zone and current density with a partially doped EML, we found that most of the recombination occurred at the EML close to the electron transporting layer (ETL). Electron injection and transport were enhanced by increasing doping ratio at the EML close to the ETL and such a stepwise doping profile resulted in almost 40 % improved power efficiency compared to that of a conventional device.

Effects of interlayers on phosphorescent blue organic light-emitting diodes

We demonstrate that the electroluminescence efficiencies of blue organic light-emitting diodes can be significantly enhanced by the incorporation of interlayers at the hole transporting layer (HTL)/emitting layer (EML) and/or EML/electron transporting layer (ETL) interfaces. Blue light-emitting iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C2′) picolinate was doped in an m-bis-(triphenylsilyl)benzene (UGH3) host and hole transporting wide band gap materials were introduced between the HTL and the EML as interlayers in order to block triplet exciton quenching and reduce electron overflow. The effects of adding a second undoped UGH3 interlayer at the EML/ETL interface were also studied. When the appropriate interlayers were added, the device performances were found to be dramatically enhanced, with peak external quantum and power efficiencies of 20.1% and 29.2lm∕W.

Improved performance of blue phosphorescent organic light-emitting diodes with a mixed host system

We have investigated a relationship between device performances and host properties in the emissive layer (EML) of iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate doped blue phosphorescent organic light-emitting diodes (PHOLEDs). We found that the recombination zone as well as charge carrier injection properties of the EML could be systematically engineered through a mixed host system. Therefore, light-emitting performances of blue PHOLEDs were greatly improved with external quantum and power efficiencies of 20.5% and 43.5 lm/W at a luminance of 56 cd/m2, respectively.

A Facile Route to Efficient, Low‐Cost Flexible Organic Light‐Emitting Diodes: Utilizing the High Refractive Index and Built‐In Scattering Properties of Industrial‐Grade PEN Substrates

An industrial-grade polyethylene naphthalate (PEN) substrate is explored as a simple, cost-effective platform for high-efficiency organic light-emitting diodes (OLEDs). Its high refractive index, combined with the built-in scattering properties inherent to the industrial-grade version, allows for a significant enhancement in outcoupling without any extra structuring or special optical elements. Flexible, color-stable OLEDs with efficiency close to 100 lm W(-1) are demonstrated.

Single nucleotide polymorphisms in a gene for translation initiation factor (eIF4G) of rice (Oryza sativa) associated with resistance to Rice tungro spherical virus.

Rice tungro disease (RTD) is a serious constraint to rice production in South and Southeast Asia. RTD is caused by Rice tungro spherical virus (RTSV) and Rice tungro bacilliform virus. Rice cv. Utri Merah is resistant to RTSV. To identify the gene or genes involved in RTSV resistance, the association of genotypic and phenotypic variations for RTSV resistance was examined in backcross populations derived from Utri Merah and rice germplasm with known RTSV resistance. Genetic analysis revealed that resistance to RTSV in Utri Merah was controlled by a single recessive gene (tsv1) mapped within an approximately 200-kb region between 22.05 and 22.25 Mb of chromosome 7. A gene for putative translation initiation factor 4G (eIF4G(tsv1)) was found in the tsv1 region. Comparison of eIF4G(tsv1) gene sequences among susceptible and resistant plants suggested the association of RTSV resistance with one of the single nucleotide polymorphism (SNP) sites found in exon 9 of the gene. Examination of the SNP site in the eIF4G(tsv1) gene among various rice plants resistant and susceptible to RTSV corroborated the association of SNP or deletions in codons for Val(1060-1061) of the predicted eIF4G(tsv1) with RTSV resistance in rice.

Growth and structural properties of m-plane ZnO on MgO (001) by molecular beam epitaxy

The growth of wurtzite ZnO on cubic MgO (001) substrates by molecular beam epitaxy is reported. ZnO epilayers exhibit the nonpolar m-plane orientation based on x-ray diffraction and transmission electron microscopy analysis. Comparative studies of ZnO grown by pulsed laser deposition on MgO (001) result in preferred growth in the polar c-plane orientation. Investigation by high resolution transmission electron microscopy confirms an abrupt ZnO∕MgO interface and monolayer spacing consistent with m-plane growth of ZnO. X-ray diffraction pole figures indicate two preferred rotational domains in-plane with a twist angle of 30°.

Polarity control of ZnO films on (0001) Al2O3 by Cr-compound intermediate layers

This letter presents a reliable and very easy method for selective growth of polarity controlled ZnO films on (0001) Al2O3 substrates by plasma-assisted molecular-beam epitaxy. Cr-compound intermediate layers are used to control the crystal polarity of ZnO films on (0001) Al2O3. ZnO films grown on rocksalt structure CrN/(0001) Al2O3 shows Zn polarity, while those grown on rhombohedral Cr2O3∕(0001) Al2O3 shows O polarity. Possible interface atomic arrangements for both heterostructures are proposed.