Sei-Yong Kim

Resistive switching mechanism of TiO2 thin films grown by atomic-layer deposition

The resistive switching mechanism of 20- to 57-nm-thick TiO2 thin films grown by atomic-layer deposition was studied by current-voltage measurements and conductive atomic force microscopy. Electric pulse-induced resistance switching was repetitively (> a few hundred times) observed with a resistance ratio ⪢102. Both the low- and high-resistance states showed linear log current versus log voltage graphs with a slope of 1 in the low-voltage region where switching did not occur. The thermal stability of both conduction states was also studied. Atomic force microscopy studies under atmosphere and high-vacuum conditions showed that resistance switching is closely related to the formation and elimination of conducting spots. The conducting spots of the low-resistance state have a few tens times higher conductivity than those of the high-resistance state and their density is also a few tens times higher which results in a ∼103 times larger overall conductivity. An interesting finding was that the area where the ...

Highly Efficient Organic Light‐Emitting Diodes with Phosphorescent Emitters Having High Quantum Yield and Horizontal Orientation of Transition Dipole Moments

also used as the co-host of the emitting layer (EML) with the molar ratio of 1:1 to exploit the exciplex forming character for low driving voltage, and good electron-hole balance. [ 7,9,10 ] The detailed structure of the OLEDs is; indium tin oxide (ITO) (70 nm)/TAPC (75 nm)/TCTA (10 nm)/TCTA:B3PYMPM: 8.4 mol% dye (30 nm)/B3PYMPM (45 nm)/LiF (0.7 nm)/Al (100 nm). The photoluminescence (PL) spectra of the TCTA:B3PYMPM co-host fi lms doped with the three dyes with the doping concentration of 8.4 mol% are shown in Figure 2 a. The peak wavelength of the PL of Ir(ppy) 3 , Ir(ppy) 2 acac, and Ir(ppy) 2 tmd were 513 nm, 520 nm, 524 nm, respectively. The orientations of the transition dipole moments of the dyes in the host were determined through the analysis of the angledependent PL spectra of the fi lms. [ 7,11 ] Figure 2 b shows the measured angle-dependent PL intensities of the p-polarized light emitted from the 30 nm-thick fi lms composed of TCTA:B3PYMPM:green dyes (0.46:0.46:0.08 molar ratio) at 520 nm close to the PL maxima of the green dyes. The angledependent PL spectra were analyzed using the classical dipole model where the emission from excitons is considered as the dissipated power from oscillating dipoles. [ 12,13 ] Birefringence

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has not been thoroughly considered for phosphorescent OLEDs because of the lack of an apparent driving force for a molecular arrangement in all but a few cases, even though horizontally oriented transition dipoles can result in efficiencies of over 30%. Here we use quantum chemical calculations to show that the preferred orientation of the transition dipole moments of heteroleptic iridium complexes (HICs) in OLEDs originates from the preferred direction of the HIC triplet transition dipole moments and the strong supramolecular arrangement within the co-host environment. We also demonstrate an unprecedentedly high EQE of 35.6% when using HICs with phosphorescent transition dipole moments oriented in the horizontal direction.

Highly Enhanced Light Extraction from Surface Plasmonic Loss Minimized Organic Light‐Emitting Diodes

Extremely high light out-coupling efficiency from a transparent organic light-emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micropatterns with optimal light extraction condition are fabricated based on the well-matched analysis of optical simulations.

Enhancement of the short circuit current in organic photovoltaic devices with microcavity structures

We demonstrate that the use of a microcavity structure can overcome the trade-off between exciton diffusion and optical absorption in planar heterojunction organic photovoltaic cells. Optical simulation based on the copper phthalocyanine (CuPc)-fullerene donor-acceptor system showed that the microcavity device with the spacer layer confines a large electric field inside the cavity so that high external quantum efficiency can be achieved even with a 10-nm-thick CuPc layer, which is comparable to the exciton diffusion length of the layer. The optimized microcavity device leads to an enhancement of the short circuit current of up to 51.6% compared with the conventional device.

Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells

Enhancing the short circuit current (JSC) by extending the absorption to the near-infrared (NIR) spectrum, which has 50% of the total solar photon flux, is a remaining task in small molecular solar cells. Here, high efficiency NIR absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is explained by the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an X-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.

High efficiency p-i-n top-emitting organic light-emitting diodes with a nearly Lambertian emission pattern

We report high efficiency phosphorescent green p-i-n top-emitting organic light-emitting diodes consisting of metal anodes (Ag and Al), a rhenium oxide p-dopant, a rubidium carbonate n-dopant, and a semitransparent Ag cathode. Significantly high peak current efficiencies of 88 and 73 cd/A are demonstrated for the devices using Ag and Al anodes, respectively, through the optimization of organic layer thickness. The electroluminescence intensities of the Ag-based device with viewing angles show a nearly Lambertian distribution, whereas those of the Al-based device exhibit a relatively strong angular dependence, which is mainly attributed to the change in the resonance wavelength of the microcavity of the devices. Efficiencies, emission spectra, and angular dependence of the emission of the devices are further successfully analyzed using an optical model.

Luminescence from oriented emitting dipoles in a birefringent medium.

We present an optical model to describe the luminescence from oriented emitting dipoles in a birefringent medium and validate the theoretical model through its applications to a dye doped organic thin film and organic light emitting diodes (OLEDs). We demonstrate that the optical birefringence affects not only far-field radiation characteristics such as the angle-dependent emission spectrum and intensity from the thin film and OLEDs, but also the outcoupling efficiency of OLEDs. The orientation of emitting dipoles in a birefringent medium is successfully analyzed from the far-field radiation pattern of a thin film using the model. In addition, the birefringent model presented here provides a precise analysis of the angle-dependent EL spectra and efficiencies of OLEDs with the determined emitting dipole orientation.

An efficient interconnection unit composed of electron-transporting layer/metal/p-doped hole-transporting layer for tandem organic photovoltaics

We report an efficient interconnection unit (ICU) consisting of an electron transporting layer/metal/p-doped hole transporting layer (p-HTL) structure for tandem organic photovoltaic (TOPV) cells. The ICU satisfies all the requirements of optical transparency and low voltage loss and for functioning as an optical spacer. The variation of the short circuit current and open circuit voltage (VOC) of the TOPV cells with increasing thickness of the p-HTL in the ICU followed the theoretical predictions, proving that the ICU does not disturb the electrical characteristics of the TOPV cells up to a p-HTL thickness of 100 nm with minimal VOC loss (∼3%).

Estimation of the mean emission zone in phosphorescent organic light-emitting diodes with a thin emitting layer.

We presented an approach to estimate the emission zone (EZ) positions in high efficiency phosphorescent OLEDs with a thin emitting layer. Two devices with different distances between the emitting layer and the cathode (i.e. they are optically different), but exhibiting same current density-voltage characteristics (i.e. they are electrically the same) were used for this purpose. Mean EZ positions in the OLEDs were extracted from the comparison of the experimental luminous intensity ratio vs. the current density with the calculated intensity ratio vs. the EZ position. The validity of the approach was confirmed by the agreement between calculated and experimental spectral changes.