Yi-Lu Chang

Highly Efficient Blue Phosphorescence from Triarylboron-Functionalized Platinum(II) Complexes of N-Heterocyclic Carbenes

The first examples of BMes(2)-functionalized NHC chelate ligands have been achieved. Their Pt(II) acetylacetonate complexes have been synthesized and fully characterized. These NHC-chelate Pt(II) compounds display highly efficient blue or blue-green phosphorescence in solution (Φ = 0.41-0.87) and the solid state (Φ = 0.86-0.90). Highly efficient electroluminescent devices based on these new Pt(II) compounds have also been fabricated.

White Organic Light-Emitting Diodes for Solid-State Lighting

Lighting consumes a significant amount of generated electrical power in developing countries, and it uses over 20% of the energy supplied in developed countries. Therefore, semiconductor-based light sources with high energy efficiencies are critical technologies for the reduction of global carbon footprint. As an emerging lighting technology, organic light-emitting diode (OLED) has received huge worldwide attention in recent years, partially driven by its success in the flat-panel display market and partially driven by its technology virtues such as an unique thin, flat, foldable form factor. In this review, we will provide an overview on the current status of OLEDs for lighting applications. Specifically, a detailed overview of the state-of-the-art white OLED design concepts including their working principles will be presented. A brief overview on the current status of out-coupling techniques suitable for white OLEDs will also be discussed.

Highly efficient blue phosphorescent and electroluminescent Ir(III) compounds

Three new Ir(III) compounds with deep-blue phosphorescence have been synthesized. These molecules have the general formula of Ir(C∧N)2(L∧X), where C∧N = 2′,6′-difluoro-2,3′-bipyridine (dfpypy) and L∧X = ancillary ligand such as 2-picolinate, pic (1), acetylacetonate, acac (2), or dipivaloylmethanoate, dpm (3). The ancillary ligands have been found to significantly destabilize both HOMO and LUMO levels of the Ir(III) complexes, compared to Ir(dfpypy)3, without significantly changing the phosphorescence energy. Compounds 1–3 emit bright blue phosphorescence with λmax = 440–460 nm and quantum efficiencies of 0.60–0.95 in solution and the solid state. Double-layer electroluminescent devices using compounds 1–3 as the dopant, CDBP (4,4′-bis(9-carbazolyl)-2,2′-dimethylbiphenyl) as the host/hole transporting layer, and TPBi (1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene) as the electron transport layer have been fabricated. These EL devices show pure blue colour with high efficiency. The EL device of compound 3 at the doping level of 20 wt% shows the best performance with EQE of 10–15% at the brightness of 10–1000 cd m−2 and the maximum current efficiency of 22 cd A−1.

In Situ Solid-State Generation of (BN)2-Pyrenes and Electroluminescent Devices

New BN-heterocyclic compounds have been found to undergo double arene photoelimination, forming rare yellow fluorescent BN-pyrenes that contain two BN units. Most significant is the discovery that the double arene elimination can also be driven by excitons generated electrically within electroluminescent (EL) devices, enabling the in situ solid-state conversion of BN-heterocycles to BN-pyrenes and the use of BN-pyrenes as emitters for EL devices. The in situ exciton-driven elimination (EDE) phenomenon has also been observed for other BN-heterocycles.

Silicon Nanocrystal OLEDs: Effect of Organic Capping Group on Performance

The synthesis of highly luminescent, colloidally-stable and organically-capped silicon nanocrystals (ncSi) and their incorporation into a visible wavelength organic light-emitting diode (OLED) is reported. By substituting decyl chains with aromatic allylbenzene capping ligands and size-selecting visible emitting ncSi, superior packing density, enhanced charge transport, and an improved photoluminescence absolute quantum yield of the ncSi is obtained in the active layer of an OLED.

Exciton-stimulated molecular transformation in organic light-emitting diodes.

An exciton-stimulated molecular transformation in an organic light-emitting diode (OLED) on a time scale of a few seconds under electrical bias is shown to reach nearly 100% under standard operating conditions, leading to color switching. It is reversible in both a thin film and an OLED when sufficient thermal energy is supplied. Such an exciton-stimulated molecular transformation suggests a new process which may be exploited for applications such as electrochromic and memory devices.

Organic Light-Emitting Diodes

The organic light-emitting diode (OLED) is considered the ultimate technology for displays because of its unique form factor and its ability to produce vibrant colors. It is being actively studied for use as the most desired broadband light source for next-generation solid-state lighting. Already, small active matrix OLED (AMOLED) displays have become popular in smartphones worldwide and even large-sized (55 in.) AMOLED televisions are now commercially available. The key features of OLED include high energy efficiency, high color quality, environmental friendliness, and most distinctively, an ultrathin and flexible form factor, which provides a unique opportunity for a plethora of innovative designs such as wearable screens, semitransparent displays, and lighting panels. In this article, the fundamental concepts and current status of OLEDs will be presented. Keywords: organic light-emitting diodes; electroluminescence; phosphorescence; fluorescence; excitons

Highly efficient greenish-blue platinum-based phosphorescent organic light-emitting diodes on a high triplet energy platform

We have demonstrated high-efficiency greenish-blue phosphorescent organic light-emitting diodes (PHOLEDs) based on a dimesitylboryl-functionalized C^N chelate Pt(II) phosphor, Pt(m-Bptrz)(t-Bu-pytrz-Me). Using a high triplet energy platform and optimized double emissive zone device architecture results in greenish-blue PHOLEDs that exhibit an external quantum efficiency of 24.0% and a power efficiency of 55.8 lm/W. This record high performance is comparable with that of the state-of-the-art Ir-based sky-blue organic light-emitting diodes.

C60:LiF nanocomposite for high power efficiency fluorescent organic light-emitting diodes

To reduce the driving voltage, and hence enhance the power efficiency of OLEDs, the mobility of the various carrier transport layers needs to be increased. Buckminsterfullerene (C(60)) has been proposed to be one possible alternative conductive electron transport layer (ETL) to enhance the power efficiency in OLEDs, due to its high conductivity and the formation of an ohmic contact with the LiF/Al cathode. The optical properties of a nanocomposite of C(60) with LiF (C(60):LiF) and its potential as an efficient ETL in OLEDs was studied. With proper optimization of the device structure, a more than 50% improvement in the power efficiency, without sacrificing the high EQE, in optimized fluorescent OLEDs with the use of C(60):LiF nanocomposite ETL was achieved.

Considerations in device design and materials selection in organic light emitting diodes

The development of high performance organic light emitting diodes (OLEDs) for display and lighting applications has attracted considerable research interest from both academia and industry. In this work, the designs of simplified phosphorescent OLEDs with exceptionally high efficiency are discussed. It is found that discrete blocking layers and a double emission zone are unnecessary to achieve high efficiency in optimized phosphorescent OLEDs. Due to the elimination of these redundant layers, the device structure can be highly simplified. It is also shown that single-layer, two organic component devices are feasible with state-of-the-art efficiency.