Hirohiko Fukagawa

Highly Efficient and Stable Red Phosphorescent Organic Light‐Emitting Diodes Using Platinum Complexes

There have been many reports on phosphorescent organic light-emitting diodes (PHOLEDs) because of their relatively high emission effi ciencies compared with those of conventional fl uorescent OLEDs. [ 1–3 ] In addition to their effi ciency, the driving voltage and operational stability are important factors in the application of PHOLEDs to displays and lighting. PHOLEDs often have a charge-trapping problem at the sites of dopant molecules owing to the large band gap ( E g ) difference between the host and dopant molecules. [ 4 , 5 ] An increase in the driving voltage caused by doping a phosphorescent dye is often observed, particularly in red PHOLEDs using a conventional carbazole-based host material such as 4,4 ′ -bis(9-carbazolyl)2,2 ′ -biphenyl (CBP). When a dopant with a narrow E g is doped into a host with a wide E g , the difference in the highest occupied molecular orbital (HOMO) levels and/or lowest unoccupied molecular orbital (LUMO) levels between the dopant and the host signifi cantly increases. Subsequently, the dopant becomes a deep trap for hole and/or electron transport in the emitting layer. The maximum reported power effi ciency (PE) of red PHOLEDs was about 10 lm W − 1 in the early phase of their development, where the host/dopant combination was CBP/ Ir(piq) 3 (tris[1-phenylisoquinolinato-C2,N]iridium(III)). [ 6 ]

Highly efficient, deep-blue phosphorescent organic light emitting diodes with a double-emitting layer structure

We have demonstrated a highly efficient, deep-blue organic light-emitting diode (OLED) using a host material with a high triplet energy. The OLED device that we have prepared utilizes a phosphorescent guest material, iridium(III)bis(4′,6′,-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate, exhibits a peak quantum efficiency of about 15.7%. We employed a double-emitting layer (DEL) structure that distributes the carrier recombination region within the device. In this DEL structure, the emission mechanism is such that the energy transfers from the host material in one emitting layer, and the other emitting layer provides for direct charge trapping in the guest material. This DEL structure proved to be quite useful in achieving the reported device characteristics.

Does the molecular orientation induce an electric dipole in Cu-phthalocyanine thin films?

The effect of the molecular orientation on the molecular electronic structure is studied on the Cu-phthalocyanine∕graphite system via film thickness dependences of metastable-atom electron spectra and ultraviolet photoelectron spectra. We observed a decrease in the vacuum-level position and a corresponding band-bending-like shift in the highest occupied state only for thick films where the molecular tilt angle increases gradually with the film thickness. These shifts are explained by electric dipoles produced in the film by a gradient of the intermolecular electronic interaction along the surface normal due to the continuous increase in the molecular tilt angle. The result indicates that the change in the molecular orientation is an important origin of the band-bending-like shift in the molecular electronic states even if the molecule has no permanent electric dipole.

Pyridoindole Derivative as Electron Transporting Host Material for Efficient Deep-blue Phosphorescent Organic Light-emitting Diodes

Organic light-emitting diodes (OLEDs) are very attractive for full-color fl at-panel displays and lighting applications. OLED characteristics, such as device stability and the power effi ciency, must be improved for them to become practical. Low-powerconsumption OLEDs can be fabricated by i) improving the current effi ciency and ii) reducing the driving voltage. There have been many reports on phosphorescent OLEDs (POLEDs) because their emission effi ciencies are higher than that of conventional fl uorescent OLEDs. [ 1–3 ] For fl uorescent OLEDs, a 25% internal quantum effi ciency can be obtained from 25% of singlet spin states since the singlet and triplet excitons are generated at a ratio of about 1:3 under electrical excitation. On the other hand, POLEDs show an internal quantum effi ciency of about 100% since the remaining 75% of triplet spin states can also emit light. [ 2 ] In fact, a near theoretical limitation of greenlight emission was obtained using factris(2-phenylpyridinato) iridium( III ) [Ir(ppy) 3 ] as the phosphorescent emitter. [ 4 ] By using phosphorescent dyes, red and green POLEDs with higher power effi ciencies have already been reported. [ 4–6 ] Compared to the higher power effi ciency of red and green POLEDs previously reported, the power effi ciency of blue POLEDs is poor, especially in deep-blue POLEDs. Blue light is not only one of the three primary colors from which white light can be obtained, [ 7 ]

Highly efficient and air-stable inverted organic light-emitting diode composed of inert materials

The feasibility of a highly efficient and air-stable organic light-emitting diode (OLED) was examined. A phosphorescent OLED not containing an air-sensitive material was fabricated by employing an inverted structure with an air-stable electron injection layer. Efficient electron injection from the bottom cathode to the emitting layer was demonstrated from the fact that the device characteristics of the inverted OLED were almost the same as those of a conventional OLED. No dark spot formation was observed after 250 days in the inverted OLED encapsulated by a barrier film with a water vapor transmission rate of 10−4 g m−2 day−1.

Highly efficient and stable organic light-emitting diodes with a greatly reduced amount of phosphorescent emitter

Organic light-emitting diodes (OLEDs) have been intensively studied as a key technology for next-generation displays and lighting. The efficiency of OLEDs has improved markedly in the last 15 years by employing phosphorescent emitters. However, there are two main issues in the practical application of phosphorescent OLEDs (PHOLEDs): the relatively short operational lifetime and the relatively high cost owing to the costly emitter with a concentration of about 10% in the emitting layer. Here, we report on our success in resolving these issues by the utilization of thermally activated delayed fluorescent materials, which have been developed in the past few years, as the host material for the phosphorescent emitter. Our newly developed PHOLED employing only 1 wt% phosphorescent emitter exhibits an external quantum efficiency of over 20% and a long operational lifetime of about 20 times that of an OLED consisting of a conventional host material and 1 wt% phosphorescent emitter.

Vacuum sublimed α,ω-dihexylsexithiophene thin films: Correlating electronic structure and molecular orientation

In order to correlate the molecular orientation of organic thin films with charge injection barriers at organic/metal interfaces, the electronic structure and molecular orientation of vacuum sublimed thin films of α,ω-dihexylsexithiophene (DH6T) on the substrates Ag(111), highly oriented pyrolytic graphite (HOPG), and tetratetracontane (TTC) precovered Ag(111) were investigated. Results from metastable atom electron spectroscopy, ultraviolet photoelectron spectroscopy, and x-ray diffraction were used to derive growth models (including molecular orientation and conformation) of DH6T on the different substrates. On Ag(111), DH6T exhibits a transition from lying molecules in the monolayer/bilayer range to almost standing upright molecules in multilayers. This is accompanied by a shift of the molecular energy levels to a lower binding energy by 0.65 eV with respect to the vacuum level. The unit cell of standing DH6T on lying DH6T on Ag(111) is estimated to be similar to the DH6T bulk phase. On HOPG, DH6T grow...

16.4: Low‐Temperature Fabrication of Flexible AMOLED Displays Using Oxide TFTs with Polymer Gate Insulators

We have developed InGaZnO4 TFTs with polymer gate insulators that can be formed by spin-coating on plastic substrates at temperatures below 130 °C. A 5-inch QVGA flexible OLED display has been fabricated by means of ink-jet printing on a TFT backplane, and it has successfully displayed clear color video images while in a bent state.

Operational lifetimes of organic light-emitting diodes dominated by Förster resonance energy transfer

Organic light-emitting diodes are a key technology for next-generation information displays because of their low power consumption and potentially long operational lifetimes. Although devices with internal quantum efficiencies of approximately 100% have been achieved using phosphorescent or thermally activated delayed fluorescent emitters, a systematic understanding of materials suitable for operationally stable devices is lacking. Here we demonstrate that the operational stability of phosphorescent devices is nearly proportional to the Förster resonance energy transfer rate from the host to the emitter when thermally activated delayed fluorescence molecules are used as the hosts. We find that a small molecular size is a requirement for thermally activated delayed fluorescence molecules employed as phosphorescent hosts; in contrast, an extremely small energy gap between the singlet and triplet excited states, which is essential for an efficient thermally activated delayed fluorescent emitter, is unnecessary in the phosphorescent host.

Molecular design of hole-transporting material for efficient and stable green phosphorescent organic light-emitting diodes

We examined the hole-transporting-material (HTM)-dependent device characteristics of green phosphorescent organic light-emitting diodes (PHOLEDs) using an electron-transporting host. The emission efficiency of the PHOLEDs was proportional to the optical band gap and the triplet energy of the HTMs. On the other hand, the operational stability of the PHOLEDs was not proportional to the emission efficiency. By analyzing the device characteristics in relation to the molecular structure of HTMs, amine derivative with dibenzothiophene was found to be effective HTMs suitable for highly efficient and stable green PHOLEDs.