Hisahiro Sasabe

Bio: Hisahiro Sasabe is an academic researcher from Yamagata University. The author has contributed to research in topics: OLED & Quantum efficiency. The author has an hindex of 49, co-authored 182 publications receiving 9709 citations. Previous affiliations of Hisahiro Sasabe include Osaka Prefecture University & Japan Advanced Institute of Science and Technology.

Development of high performance OLEDs for general lighting

Abstract: Since the development of the first white organic light-emitting device (OLED) in 1993, twenty years have passed. The power efficiency and lifetime of this white OLED were reportedly only <1 lm W−1 and <1 day, respectively. However, recent rapid advances in material chemistry have enabled the use of white OLEDs for general lighting. In 2012, white OLED panel efficiency has reached 90 lm W−1 at 1000 cd m−2, and a tandem white OLED panel has realized a lifetime of over 100 000 hours. What is more important in OLEDs is to shed clear light on the new design products, such as transparent lighting panels and luminescent wallpapers. These fascinating features enable OLEDs as a whole new invention of artificial lighting. In this review, we would like to overview the recent developments of white OLED, especially three key elemental technologies related to material chemistry: (1) low operating voltage technology, (2) phosphorescent OLED technology and (3) multi-photon emission (MPE) device technology.

Pyridine-Containing Bipolar Host Materials for Highly Efficient Blue Phosphorescent OLEDs

Abstract: A record high efficiency and reduced efficiency roll-off were achieved for blue phosphorescent OLEDs with novel bipolar host materials containing both carbazole electron donor and pyridine electron acceptor because of good confinement of triplet excitons on the guest molecules and improved carrier balance injected into the emissive layer.

High-efficiency blue and white organic light-emitting devices incorporating a blue iridium carbene complex.

Abstract: High-effi ciency white organic light-emitting devices (OLEDs) have great potential for energy saving solid-state lighting and eco-friendly fl at-display panels. [ 1 ] In addition, white OLEDs are expected to open new designs in lighting technology, such as transparent lighting panels or luminescent wallpapers because of being able to form paper-like thin fi lms. Phosphorescent OLED technology is an imperative methodology to realize higheffi ciency white OLEDs because phosphors, such as fac -tris(2phenylpyridine)iridium( III ) [Ir(ppy) 3 ] and iridium( III )bis(4,6(difl uorophenyl)pyridinatoN , C 2 ′ )picolinate (FIrpic) enable an internal effi ciency as high as 100% converting both singlet and triplet excitons into photons. [ 2 ] There are two effective approaches to obtain white OLEDs by using phosphors. One is to combine a blue fl uorophore and phosphors for the other colors, a so-called hybrid white OLED. [ 3 ] A key requirement is the use of a blue fl uorophore with higher triplet energy ( E T1 ) than that of the other phosphors. The blue fl uorophore also needs to have a high photoluminescent quantum yield ( η PL ). Schwartz and coworkers reported hybrid white OLEDs with a power effi ciency at 1000 cd m − 2 ( η p,1000 ) of 22 lm W − 1 (external quantum effi ciency (EQE) of 10.4%) by using N , N ′ -di-1-naphthalenylN , N ′ -diphenyl-[1,1 ′ :4 ′ ,1 ′ ′ :4 ′ ′ ,1 ′ ′ ′ -quaterphenyl]-4,4 ′ ′ ′ -diamine (4P-NPD) as a blue fl uorophore, and Ir(ppy) 3 and iridium( III ) bis(2-methyldibenzo-[ f , h ]quinoxaline)(acetylacetonate) [Ir(MDQ) 2 ( acac)] as green and red phosphors, respectively. [ 4 ]

Multifunctional Materials in High-Performance OLEDs: Challenges for Solid-State Lighting†

Abstract: Recent advances in material chemistry have enabled white organic light-emitting device (OLED) efficacy beyond fluorescent tube efficacy up to 100 lm W−1. In this short review, we explore recent developments of small molecule-based multifunctional materials in high-performance OLEDs, especially blue phosphorescent emitters, host materials, and electron-transporting materials.

Highly efficient organic light-emitting diodes from delayed fluorescence

Abstract: The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

White organic light-emitting diodes with fluorescent tube efficiency

Abstract: The development of white organic light-emitting diodes (OLEDs) holds great promise for the production of highly efficient large-area light sources. High internal quantum efficiencies for the conversion of electrical energy to light have been realized. Nevertheless, the overall device power efficiencies are still considerably below the 60-70 lumens per watt of fluorescent tubes, which is the current benchmark for novel light sources. Although some reports about highly power-efficient white OLEDs exist, details about structure and the measurement conditions of these structures have not been fully disclosed: the highest power efficiency reported in the scientific literature is 44 lm W(-1) (ref. 7). Here we report an improved OLED structure which reaches fluorescent tube efficiency. By combining a carefully chosen emitter layer with high-refractive-index substrates, and using a periodic outcoupling structure, we achieve a device power efficiency of 90 lm W(-1) at 1,000 candelas per square metre. This efficiency has the potential to be raised to 124 lm W(-1) if the light outcoupling can be further improved. Besides approaching internal quantum efficiency values of one, we have also focused on reducing energetic and ohmic losses that occur during electron-photon conversion. We anticipate that our results will be a starting point for further research, leading to white OLEDs having efficiencies beyond 100 lm W(-1). This could make white-light OLEDs, with their soft area light and high colour-rendering qualities, the light sources of choice for the future.

Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent

Abstract: Metal halide perovskite materials are an emerging class of solution-processable semiconductors with considerable potential for use in optoelectronic devices1–3. For example, light-emitting diodes (LEDs) based on these materials could see application in flat-panel displays and solid-state lighting, owing to their potential to be made at low cost via facile solution processing, and could provide tunable colours and narrow emission line widths at high photoluminescence quantum yields4–8. However, the highest reported external quantum efficiencies of green- and red-light-emitting perovskite LEDs are around 14 per cent7,9 and 12 per cent8, respectively—still well behind the performance of organic LEDs10–12 and inorganic quantum dot LEDs13. Here we describe visible-light-emitting perovskite LEDs that surpass the quantum efficiency milestone of 20 per cent. This achievement stems from a new strategy for managing the compositional distribution in the device—an approach that simultaneously provides high luminescence and balanced charge injection. Specifically, we mixed a presynthesized CsPbBr3 perovskite with a MABr additive (where MA is CH3NH3), the differing solubilities of which yield sequential crystallization into a CsPbBr3/MABr quasi-core/shell structure. The MABr shell passivates the nonradiative defects that would otherwise be present in CsPbBr3 crystals, boosting the photoluminescence quantum efficiency, while the MABr capping layer enables balanced charge injection. The resulting 20.3 per cent external quantum efficiency represents a substantial step towards the practical application of perovskite LEDs in lighting and display. A strategy for managing the compositional distribution in metal halide perovskite light-emitting diodes enables them to surpass 20% external quantum efficiency—a step towards their practical application in lighting and displays.

Recent advances in organic thermally activated delayed fluorescence materials.

Abstract: Organic materials that exhibit thermally activated delayed fluorescence (TADF) are an attractive class of functional materials that have witnessed a booming development in recent years. Since Adachi et al. reported high-performance TADF-OLED devices in 2012, there have been many reports regarding the design and synthesis of new TADF luminogens, which have various molecular structures and are used for different applications. In this review, we summarize and discuss the latest progress concerning this rapidly developing research field, in which the majority of the reported TADF systems are discussed, along with their derived structure–property relationships, TADF mechanisms and applications. We hope that such a review provides a clear outlook of these novel functional materials for a broad range of scientists within different disciplinary areas and attracts more researchers to devote themselves to this interesting research field.

Highly efficient organic devices based on electrically doped transport layers.

Abstract: Most present-day semiconductor devices use inorganic crystalline materials, with single-crystalline silicon dominating other materials like GaAs by about a factor of 1000. Despite the advantages of single-crystalline inorganic semiconductors like high room-temperature mobility (up to 1000 cm2/(V s)) and high stability, these materials are less suitable for low-cost and large-area applications. Additionally, silicon is an indirect semiconductor and therefore is not well suited for optoelectronic applications like light-emitting diodes. Solar cells from silicon are expensive and require a large amount of energy for their fabrication, leading to a long energy payback time. As an alternative, organic semiconductors have recently gained much attention (for review articles, see refs 1 -3 (OLEDs), ref 4 (organic electronics in general), and refs 5 and 6 (organic solar cells)). Originally, much of the research concentrated on single crystals, which can have mobilities of a few cm2/(V s) at room temperature and even much higher values at low temperature, as shown in the pioneering work of Karl et al.7 However, for practical applications, thinfilm organic semiconductors with disordered morphology, such as evaporated small-molecule compounds or polymers processed from solution, are prevailing. Organic semiconductors are already broadly applied as photoconductors for copiers and laser printers. * Corresponding author. E-mail: leo@iapp.de. Web address: www.iapp.de. 1233 Chem. Rev. 2007, 107, 1233−1271