Unlocking the Full Potential of Electron-Acceptor Molecules for Efficient and Stable Hole Injection.

Understanding the hole-injection mechanism and improving the hole-injection property are of pivotal importance in the future development of organic optoelectronic devices. Electron-acceptor molecules with high electron affinity (EA) are widely used in electronic applications, such as hole injection and p-doping. Although p-doping has generally been studied in terms of matching the ionization energy (IE) of organic semiconductors with the EA of acceptor molecules, little is known about the effect of the EA of acceptor molecules on the hole-injection property.

Unravelling the electron injection/transport mechanism in organic light-emitting diodes.

Although significant progress has been made in the development of light-emitting materials for organic light-emitting diodes along with the elucidation of emission mechanisms, the electron injection/transport mechanism remains unclear, and the materials used for electron injection/transport have been basically unchanged for more than 20 years. Here, we unravelled the electron injection/transport mechanism by tuning the work function near the cathode to about 2.0 eV using a superbase.

Understanding coordination reaction for producing stable electrode with various low work functions.

The realisation of a cathode with various work functions (WFs) is required to maximise the potential of organic semiconductors that have various electron affinities. However, the barrier-free contact for electrons could only be achieved by using reactive materials, which significantly reduce the environmental stability of organic devices. We show that a stable electrode with various WFs can be produced by utilising the coordination reaction between several phenanthroline derivatives and the electrode.

Universal Strategy for Efficient Electron Injection into Organic Semiconductors Utilizing Hydrogen Bonds.

Molecular n-dopants that can lower the electron injection barrier between organic semiconductors and electrodes are essential in present-day organic electronics. However, the development of stable molecular n-dopants remains difficult owing to their low ionization potential, which generally renders them unstable. It is shown that the stable bases widely used in organic synthesis as catalysts can lower the electron injection barrier similar to that in conventional n-doping in organic optoelectronic devices.

Effects of Energy-Level Alignment on Characteristics of Inverted Organic Light-Emitting Diodes.

Inverted organic light-emitting diodes (iOLEDs) without the use of alkali metals have attracted extensive attention owing to the demand for the realization of flexible OLEDs that do not require stringent encapsulation. In this paper, we discuss the correlation between the characteristics of iOLEDs and the energy-level alignment at cathode/organic layer interfaces examined by ultraviolet photoelectron spectroscopy. Two similar electron-transporting materials having different orbital energies, 2,8-bis(diphenylphosphoryl)dibenzo[ b, d]thiophene (PPT) and 2,8-bis(diphenylphosphoryl)dibenzo[ b, d]thiophene sulfone (PPT-S), are inserted between the cathode/polyethyleneimine and the emitting layer in the iOLED.

Effect of Host Moieties on the Phosphorescent Spectrum of Green Platinum Complex.

Highly efficient, operationally stable, and pure-color organic light-emitting diodes (OLEDs) are of considerable significance for developing practical wide-color-gamut displays. Further, we have demonstrated the feasibility of an efficient pure green phosphorescent OLED (PHOLED) by employing a narrow-band platinum complex and a top-emitting structure. The utilization of the thermally activated delayed fluorescence (TADF) material as the phosphorescent host is expected to serve as a promising solution for obtaining operationally stable PHOLEDs with high color purity.

Long-Lived Flexible Displays Employing Efficient and Stable Inverted Organic Light-Emitting Diodes.

Although organic light-emitting diodes (OLEDs) are promising for use in applications such as in flexible displays, reports of long-lived flexible OLED-based devices are limited due to the poor environmental stability of OLEDs. Flexible substrates such as plastic allow ambient oxygen and moisture to permeate into devices, which degrades the alkali metals used for the electron-injection layer in conventional OLEDs (cOLEDs). Here, the fabrication of a long-lived flexible display is reported using efficient and stable inverted OLEDs (iOLEDs), in which electrons can be effectively injected without the use of alkali metals.

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.

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.

Charge reorganization energy and small polaron binding energy of rubrene thin films by ultraviolet photoelectron spectroscopy.

The hole–phonon coupling of a rubrene monolayer on graphite is measured by means of angle resolved ultraviolet photoelectron spectroscopy. Thus, the charge reorganization energy λ and the small polaron binding energy is determined, which allows insight into the nature of charge transport in condensed rubrene.