Woo Sik Jeon

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 ]

Low roll-off efficiency green phosphorescent organic light-emitting devices with simple double emissive layer structure

Using an Ir(ppy)3 metal complex doped in 4,4′4″-tris(N-carbazolyl)-triphenylamine hole and bis[2-(2-hydroxyphenyl)-pyridine]beryllium electron transport host materials, simple three layer green phosphorescent organic light-emitting devices comprising double emissive layers have been fabricated. A low driving voltage value of 3.3 V to reach a luminance of 1000 cd/m2 and maximum current- and power-efficiency values of 58.7 cd/A and 65.1 lm/W, and maximum external quantum efficiency (EQE) value of 18.6% are reported in this device. EQE exceeding 15% over a wide range of current density from 0.03 to 25 mA/cm2 is noticed. We demonstrate a low roll-off current efficiency value of 12% at a luminance of 10 000 cd/m2 in a highly efficient simple double emissive layer device, imperative to high brightness applications.

Highly efficient bilayer green phosphorescent organic light emitting devices

We present a highly efficient green phosphorescent device comprising only of two organic layers. A host material bis[2-(2-hydroxyphenyl)-pyridine]beryllium having a good electron transporting and energy transfer characteristics, and a wide band gap hole transport material N,N′-di(4-(N,N′-diphenyl-amino)phenyl)-N,N′-diphenylbenzidine lead to the fabrication of a simplified high efficiency device. The driving voltage value of 3.3V to reach a luminance of 1000cd∕m2 is reported. The maximum current- and power-efficiency values of 38.30cd∕A and 46.60lm∕W are demonstrated in this device. Results reveal a practical way to fabricate highly efficient truly bilayer organic devices for trouble-free manufacturing processes.

Open-circuit voltage dependency on hole-extraction layers in planar heterojunction organic solar cells

The open-circuit voltage (Voc) dependency on hole-extraction layers (HELs) with different energy levels and mobility was investigated in a single stack heterojunction subphthalocyanine chloride (SubPc)/C60 organic solar cells. The HELs having about 0.2–0.3 eV higher highest occupied molecular orbital (HOMO) level than that of a donor material can significantly enhance the Voc in SubPc/C60 device due to a corresponding built-in potential increase. The high mobility of HELs can also increase Voc with increasing Jsc according to the simple diode equation. Among all HELs we utilized, N,N,N′,N′-tetra(biphenyl-4-yl)biphenyl-4,4′-diamine (TBBD) illustrates a largest increase in Voc (from 0.90 to 1.15 V) with an improvement in efficiency compared to a reference SubPc/C60 device without HEL. This increase is mainly attributed to easy and rapid extraction of holes by TBBD due to its proper HOMO level and high mobility.

Low voltage efficient simple p-i-n type electrophosphorescent green organic light-emitting devices

We present simple p-i-n structures with double-emitting and mixed-emitting layers for highly efficient phosphorescent green devices. Using a wide band-gap hole transporting material of 4,4′4″-tris(N-carbazolyl)-triphenylamine and a wide band-gap electron transporting material of bis[2-(2-hydroxyphenyl)-pyridine]beryllium, the bilayered p-i-n structure with no heterointerface barriers has been realized. A very low onset voltage value of 2.4 V corresponding to the energy of 2.4 eV of green electroluminescence, which is close to the photon energy of dopant emitting molecules (2.3–2.4 eV), is achieved in this simple p-i-n device configuration. Maximum current- and power-efficiency values of 53.3 cd/A and 61.4 lm/W and low rolloff of current efficiency (6%) are demonstrated in the simple p-i-n green phosphorescent devices, promising for the practical and economical high brightness applications.

Efficient multiple triplet quantum well structures in organic light-emitting devices

We demonstrate the multiple quantum well (MQW) structures with the charge control layers (CCLs) to produce highly efficient red phosphorescent organic light-emitting devices (OLEDs). Various triplet quantum well devices from a single to five quantum wells are realized using wide band-gap hole and electron transporting layers, narrow band-gap host and dopant materials, and CCLs. Triplet energies in such MQW devices are confined at the emitting layers. The maximum external quantum efficiency of 14.8% with a two quantum well device structure is obtained. The described MQW device concept has been proposed to be very useful to future OLED display and lighting applications.

Efficiency Control in Iridium Complex-Based Phosphorescent Light-Emitting Diodes

Key factors to control the efficiency in iridium doped red and green phosphorescent light emitting diodes (PhOLEDs) are discussed in this review: exciton confinement, charge trapping, dopant concentration and dopant molecular structure. They are not independent from each other but we attempt to present each of them in a situation where its specific effects are predominant. A good efficiency in PhOLEDs requires the triplet energy of host molecules to be sufficiently high to confine the triplet excitons within the emitting layer (EML). Furthermore, triplet excitons must be retained within the EML and should not drift into the nonradiative levels of the electron or hole transport layer (resp., ETL or HTL); this is achieved by carefully choosing the EML’s adjacent layers. We prove how reducing charge trapping results in higher efficiency in PhOLEDs. We show that there is an ideal concentration for a maximum efficiency of PhOLEDs. Finally, we present the effects of molecular structure on the efficiency of PhOLEDs using red iridium complex dopant with different modifications on the ligand to tune its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies.

Low-Voltage, Simple-Structure, High-Efficiency p–i–n-Type Electrophosphorescent Blue Organic Light-Emitting Diodes

We present simple p–i–n structures with single-emitting and mixed emitting layers for highly efficient phosphorescent blue organic devices. Using a high-triplet-energy-hole-transporting material of 1,1-bis(4-methylphenyl)-aminophenyl-cyclohexane (TAPC) and a high-triplet-energy-electron-transporting material of 1,3,5-tris(m-pyrid-3-ylphenyl)benzene (Tm3PyPB), the p–i–n structure has been realized by doping with MoO3 as a p-dopant and Cs2CO3 as an n-dopant. A very low onset voltage of 3.0 V and a driving voltage of 4.0 V to obtain a brightness of 1000 cd/m2 are achieved in this p–i–n device configuration. A maximum external quantum efficiency of 23.9% and a power efficiency of 36.7 lm/W are reported.

63.3: High Resolution OTFT-OLED on Plastic Substrate Using Self-organized Process

A high resolution (2 inch, 80 dpi) organic thin-film transistor driven light-emitting diode has been developed on plastic substrate. The pentacene OTFT array was fabricated using self-organized process. The TFT in the array exhibited an average field-effect mobility of ∼1 cm2/Vs, an on/off current ratio of 107, and a threshold voltage of ∼ −8 V after the bank formation with parylene-C / SiNx, which is good enough to drive OLEDs. An AMOLED was driven in the bent state of 100 mm radius.

Thermal Annealing Effect of Subphthalocyanine (SubPc) Donor Material in Organic Solar Cells

We report that the S-shaped kink of current density-voltage (J-V) characteristics, which reduce fill factor of organic photovoltaic cells, can be removed by substrate heating during donor layer deposition process. Subphthalocyanine (SubPc) donor film in combination with buckminsterfullerene (C60) acceptor film has been studied in a planar bilayer donor/acceptor heterojunction by J-V characterization under AM 1.5 simulated illuminations with various thicknesses of SubPc donor film. The substrate heating process enhances hole mobility of SubPc film, resulting 8.7% of efficiency enhancement with markedly improvement of S-shaped kink in J-V curves.