Sang-Hyun Eom

Effects of triplet energies and transporting properties of carrier transporting materials on blue phosphorescent organic light emitting devices

We have studied the effects of the hole transporting layers and electron transporting layers on the device efficiencies of iridium(III) bis[(4,6-di-fluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic) doped 3,5′−N,N′-dicarbazole-benzene (mCP) host blue phosphorescent organic light emitting diodes. We found that the device efficiency is very sensitive to the hole transporting materials used and both the triplet energy and carrier transport properties affect the device efficiency. On the other hand, there is no apparent correlation between the device efficiency and the triplet energy of the electron transporting material used. Instead, the device efficiency is affected by the electron mobility of the electron transporting layer only.

High efficiency blue phosphorescent organic light-emitting device

We have demonstrated a substantial enhancement in the efficiency of iridium (III) bis[(4,6-di-fluorophenyl)-pyridinate-N,C2′]picolinate based blue phosphorescent organic light-emitting devices (PHOLEDs). The efficiencies of PHOLEDs with conventional electron transport materials are low due to their low electron mobilities as well low triplet energies. High triplet energy electron transporting material with high electron mobility was used as a hole blocker to achieve efficient exciton confinement and good charge balance in the device thereby achieving a high current efficiency of 49cd∕A and an external quantum efficiency of 23%.

Efficient deep-blue phosphorescent organic light-emitting device with improved electron and exciton confinement

We report a significant improvement in the efficiency of deep-blue phosphorescent organic light-emitting devices based on the electrophosphorescent dye bis(4′,6′-difluorophenylpyridinato)tetrakis (1-pyrazolyl) borate (FIr6). Using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as the hole transport layer (HTL), we achieved a maximum external quantum efficiency of ηEQE=(18±1)%, which is approximately 50% higher than ηEQE=12% in a previously reported device with bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl as the HTL. The maximum luminous power efficiency was also improved from (14±1)lm∕W to (18±1)lm∕W. We attribute this efficiency improvement to the enhanced electron and exciton confinement provided by TAPC.

White phosphorescent organic light-emitting devices with dual triple-doped emissive layers

We demonstrate high efficiency white organic light-emitting devices with two adjacent emissive layers each doped with three phosphorescent emitters (blue, green, and red). Efficient charge and exciton confinement is realized by employing charge transport layers with high triplet energy, leading to a maximum external quantum efficiency of (19±1)%. Using the p-i-n device structure, we have achieved a peak power efficiency of (40±2) lm/W and (36±2) lm/W at 100 cd/m2, a color rendering index of 79, and Commission Internationale de L’Eclairage coordinates of (0.37, 0.40) for the white light emission.

Effect of the charge balance on high-efficiency blue-phosphorescent organic light-emitting diodes.

The charge balance in blue-phosphorescent devices was studied using single-carrier devices, and the results show that the transport is highly hole dominant. The effect of the charge balance on the device performance was further demonstrated using different electron-transport materials with different electron mobilities. By optimization of the charge balance, a maximum current efficiency of 60 Cd A(-1) at a luminance of 500 cd m(-2) was achieved.

Low voltage and very high efficiency deep-blue phosphorescent organic light-emitting devices

We report on very high efficiency deep-blue phosphorescent organic light-emitting devices (PHOLEDs) based on iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6). Dual emissive layers consisting of an N,N′-dicarbazolyl-3,5-benzene layer doped with 4wt% FIr6 and a p-bis(triphenylsilyly)benzene layer doped with 25wt% FIr6 were employed to maximize exciton generation on FIr6 molecules. Combined with the p-i-n device structure, we achieved a low turn-on voltage of 3.2V and very high power efficiencies of 25±2lm∕W at 100cd∕m2 and 20±2lm∕W at 1000cd∕m2 for such deep-blue PHOLEDs with peak emission at a wavelength of 458nm.

Enhancing Light Extraction in Top‐Emitting Organic Light‐Emitting Devices Using Molded Transparent Polymer Microlens Arrays

The light extraction efficiency in organic light-emitting devices (OLEDs) is enhanced by up to 2.6 times when a close-packed, hemispherical transparent polymer microlens array (MLA) is molded on the light-emitting surface of a top-emitting device. The microlens array helps to extract the waveguided optical emission in the organic layers and the transparent top electrode, and can be manufactured in large area with low cost.

Near infrared organic light-emitting devices based on donor-acceptor-donor oligomers

We report strong and efficient near infrared emission from organic light-emitting devices (OLEDs) based on two donor-acceptor-donor oligomers. These oligomers have fluorescent quantum yields of up to 20% and their energy gap can be tuned by changing the strengths of the donor and acceptor components. Electroluminescence with peak emission wavelengths of 692 and 815 nm were observed from the two oligomers studied here. External quantum efficiencies up to 1.6% and electrical-to-optical power efficiencies up to 7.0 mW/W were achieved in OLEDs based on these near-infrared emitters.

Efficient near-infrared organic light-emitting devices based on low-gap fluorescent oligomers

We report efficient near-infrared (NIR) organic light-emitting devices (OLEDs) based on fluorescent donor-acceptor-donor conjugated oligomers. The energies of the highest occupied and lowest unoccupied molecular orbitals of these oligomers are controlled by the donor and acceptor components, respectively; hence the energy gap and therefore the emission wavelength can be tuned by changing the strengths of the donor and acceptor components. External quantum efficiencies (EQEs) up to 1.6% and power efficiencies up to 7.0 mW/W are achieved in NIR OLEDs based on 4,9-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-6,7-dimethyl-[1,2,5]thiadiazolo[3,4-g]-quinoxaline (BEDOT-TQMe2), in which the electroluminescence peaks at a wavelength of 692 nm but extends to well above 800 nm. With a stronger acceptor in the oligomer, 4,8-bis(2,3-dihydrothieno-[3,4-b][1,4]dioxin-5-yl)benzo[1,2-c;4,5-c′]bis [1,2,5]thiadiazole (BEDOT-BBT) based devices show longer wavelength emission peaked at 815 nm, although the maximum EQE is red...

Ultraviolet-violet electroluminescence from highly fluorescent purines

We report efficient ultraviolet (UV)-violet organic light-emitting devices (OLEDs) based on highly fluorescent donor–acceptor purine molecules, which can generate tunable emission from 350 nm to 450 nm in solution by using different electron donor and acceptor arrangements on the heterocycles as reported previously. Here, external quantum efficiencies (EQEs) up to ηEQE = 1.6% are achieved for the multilayer OLEDs based on purine 2, with UV emission peaked at 393 nm, as compared to previously reported purine 1 based OLEDs with ηEQE = 3.1% and peak emission at 433 nm. The efficiencies of the OLEDs based on the two purine molecules are among the highest reported to date with emission peak wavelengths below 450 nm. By using a range of charge transport and host materials, we show that appropriate energy level alignment in multilayer OLED devices is imperative to achieve UV emission and prevent undesired emission from other layers or interfaces.