Nam Sung Cho

Highly Efficient Red Phosphorescent Dopants in Organic Light‐Emitting Devices

Phosphorescent organic light-emitting diodes (PHOLEDs) have been developed for more than 10 years. As a result, the highly effi cient red PHOLED materials are now utilizing in commercial active-matrix organic light-emitting diodes (AMOLEDs) and lighting. Nevertheless, the development of much more effi cient PHOLEDs is still required due to a dramatic reduction of power consumption by LED backlight in thin fi lm transistor-liquid crystal display (TFT-LCDs) and the generation of new lighting applications with comparable effi ciency of fl uorescent lamps. There have been many studies about improving the effi ciency of red PHOLEDs by developing host and dopant materials. However, the radiative and nonradiative rate constants ( k r , k nr ) have a strong dependence on the energy gap ( Δ E ) between the emissive excited state and the ground state in accordance with “the energy gap law”, which indicates that k nr increases with a decrease in Δ E . Hence it is obvious that highly effi cient red emission with smaller Δ E tends to give larger k nr and smaller k r , leading to the lower emission quantum yield. [ 1 , 2 ]

White transparent organic light-emitting diodes with high top and bottom color rendering indices

Reported in this work is the fabrication of white transparent organic light-emitting diodes (TOLEDs) with high color rendering indices (CRIs). An architecture in which the green and red emission layers are sandwiched between two blue emissions layers was used. By tuning the thicknesses of the green and red emission layers, CRI and power efficiency values of 90/20.5 and 87/6.8 lm/W were achieved in the bottom and top emissions, respectively. The study results suggest an effective engineering approach for realizing high CRI white TOLED lighting sources.

Healing Graphene Defects Using Selective Electrochemical Deposition: Toward Flexible and Stretchable Devices

Graphene produced by chemical-vapor-deposition inevitably has defects such as grain boundaries, pinholes, wrinkles, and cracks, which are the most significant obstacles for the realization of superior properties of pristine graphene. Despite efforts to reduce these defects during synthesis, significant damages are further induced during integration and operation of flexible and stretchable applications. Therefore, defect healing is required in order to recover the ideal properties of graphene. Here, the electrical and mechanical properties of graphene are healed on the basis of selective electrochemical deposition on graphene defects. By exploiting the high current density on the defects during the electrodeposition, metal ions such as silver and gold can be selectively reduced. The process is universally applicable to conductive and insulating substrates because graphene can serve as a conducting channel of electrons. The physically filled metal on the defects improves the electrical conductivity and mechanical stretchability by means of reducing contact resistance and crack density. The healing of graphene defects is enabled by the solution-based room temperature electrodeposition process, which broadens the use of graphene as an engineering material.

Random nanostructure scattering layer for suppression of microcavity effect and light extraction in OLEDs.

In this study, we investigated the effect of a random nanostructure scattering layer (RSL) on the microcavity and light extraction in organic light emitting diodes (OLEDs). In the case of the conventional OLED, the optical properties change with the thickness of the hole transporting layer (HTL) because of the presence of a microcavity. However, OLEDs equipped with the an RSL showed similar values of external quantum efficiency and luminous efficacy regardless of the HTL thickness. These phenomena can be understood by the scattering effect because of the RSL, which suppresses the microcavity effect and extracts the light confined in the device. Moreover, OLEDs with the RSL led to reduced spectrum and color changes with the viewing angle.

Flexion bonding transfer of multilayered graphene as a top electrode in transparent organic light-emitting diodes

Graphene has attracted considerable attention as a next-generation transparent conducting electrode, because of its high electrical conductivity and optical transparency. Various optoelectronic devices comprising graphene as a bottom electrode, such as organic light-emitting diodes (OLEDs), organic photovoltaics, quantum-dot LEDs, and light-emitting electrochemical cells, have recently been reported. However, performance of optoelectronic devices using graphene as top electrodes is limited, because the lamination process through which graphene is positioned as the top layer of these conventional OLEDs is a lack of control in the surface roughness, the gapless contact, and the flexion bonding between graphene and organic layer of the device. Here, a multilayered graphene (MLG) as a top electrode is successfully implanted, via dry bonding, onto the top organic layer of transparent OLED (TOLED) with flexion patterns. The performance of the TOLED with MLG electrode is comparable to that of a conventional TOLED with a semi-transparent thin-Ag top electrode, because the MLG electrode makes a contact with the TOLED with no residue. In addition, we successfully fabricate a large-size transparent segment panel using the developed MLG electrode. Therefore, we believe that the flexion bonding technology presented in this work is applicable to various optoelectronic devices.

Optical Effects of Graphene Electrodes on Organic Light-Emitting Diodes

We performed optical simulations and experiments to investigate the internal optics of graphene anode organic light-emitting diodes (OLEDs). The efficiencies, emission distribution, and spectral characteristics of four-layered graphene anode OLEDs were extracted and compared to those of ITO anode OLEDs. Unlike the case of the ITO anode OLED, the efficiencies and emission distributions of graphene anode OLEDs showed a weak dependency on the thickness of the organic layers. Furthermore, marginal changes in the emission spectra were observed. These results were ascribed to the negligible presence of a microcavity effect in the graphene anode OLED.

Colored semi-transparent organic light-emitting diodes

Semi-transparent, colored organic light-emitting diodes (OLEDs) were demonstrated by comprising a microcavity-embedded structure that uses an organic layer sandwiched between thin metal layers as the cathode. Without bias, the colored OLEDs exhibited various colors depending on the metal/organic/metal cathode configuration, by means of internal interference effects under ambient illumination. By varying the thickness of the organic layer, the transmittance and reflectance of the colored OLEDs could be controlled. The influence of the microcavity cathode on the light-emitting performances of OLEDs, such as the efficiencies and the electroluminescence spectra, was also studied.

Band gap tuning of new light emitting conjugated polymers

Abstract A series of copolymers, poly{9,9-bis(2 ′ -ethylhexyl)fluorene-2,7-diyl- co -2,5-bis(2-thienyl-1-cyanovinyl)-1-(2 ′ -ethylhexyloxy)-4-methoxybenzene-5 ″ ,5 ‴ -diyl} has been synthesized from the monomers, 2,7-dibromo-9,9-bis(2 ′ -ethylhexyl)fluorene and 2,5-bis(2-(5 ′ -bromothienyl)-1-cyanovinyl)-1-(2 ″ -ethylhexyloxy)-4-methoxybenzene (BTCVB) using the Ni(0) mediated polymerization. The synthesized copolymers showed the absorption maxima at about 380 nm and the absorption between 425 and 600 nm increased as the fraction of the thiophene-containing monomer (BTCVB) increased. In photoluminescence (PL), the emission maxima of the copolymers were red-shifted as the fraction of BTCVB increased, despite the similar absorption characteristics were shown in the UV–vis spectra. The copolymer containing 15 mol% of BTCVB showed a maximum PL emission at 620 nm.

Design and fabrication of two-stack tandem-type all-phosphorescent white organic light-emitting diode for achieving high color rendering index and luminous efficacy

White organic light-emitting diodes (WOLEDs) are regarded as the general lighting source. Although color rendering index (CRI) and luminous efficacy are usually in trade-off relation, we will discuss about the optimization of both characteristics, particularly focusing on the spectrum of a blue emitter. The emission at a shorter wavelength is substantially important for achieving very high CRI (> 90). The luminous efficacy of a phosphorescent blue emitter is low as its color falls in the deeper blue range; however, that does not show any significant influence on the WOLEDs. WOLEDs with different blue dopants are compared to confirm the calculation of the CRI and luminous efficacy, and the optimized WOLEDs exhibit luminous efficacy of 38.3 lm/W and CRI of 90.9.

Light diffusing effects of nano and micro-structures on OLED with microcavity

We examined the light diffusing effects of nano and micro-structures on microcavity designed OLEDs. The results of FDTD simulations and experiments showed that the pillar shaped nano-structure was more effective than the concave micro-structure for light diffusing of microcavity OLEDs. The sharp luminance distribution of the microcavity OLED was changed to near Lambertian luminance distribution by the nano-structure, and light diffusing effects increased with the height of the nano-structure. Furthermore, the nano-structure has advantages including light extraction of the substrate mode, reproducibility of manufacturing process, and minimizing pixel blur problems in an OLED display panel. The nano-structure is a promising candidate for a light diffuser, resolving the viewing angle problems in microcavity OLEDs.