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. The flexible display employing iOLEDs can emit light for over 1 year with simplified encapsulation, whereas a flexible display employing cOLEDs exhibits almost no luminescence after only 21 d with the same encapsulation. These results demonstrate the great potential of iOLEDs to replace cOLEDs employing alkali metals for use in a wide variety of flexible organic optoelectronic devices.

The feasibility of a highly efficient and air-stable organic light-emitting diode (OLED) was examined. A phosphorescent OLED not containing an airsensitive 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 .

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.

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. This extremely low-work function cathode allows direct electron injection into various materials, and it was found that organic materials can transport electrons independently of their molecular structure. On the basis of these findings, we have realised a simply structured blue organic light-emitting diode with an operational lifetime of more than 1,000,000 hours. Unravelling the electron injection/transport mechanism, as reported in this paper, not only greatly increases the choice of materials to be used for devices, but also allows simple device structures.

high reliability, their internal electroluminescence quantum effi ciency ( η int ), defi ned as the number of photons generated per injected carrier, is limited to 25% because of the exciton-branching ratio of singlet excited states under electrical excitation. [ 2 ] To increase the value of η int , several methods were proposed, such as employing phosphorescent materials with heavy atoms, utilizing triplet-triplet exciton annihilation for extra singlet generation, and employing thermally activated delayed fl uorescence (TADF) materials. [3][4][5][6] In particular, an η int value of 100% is expected by employing phosphorescent and TADF materials. In 1999, effi cient electrophosphorescence was fi rst demonstrated using iridium complexes, and an η int value of almost 100% was achieved afterwards. [ 7,8 ] Many highly effi cient phosphorescent OLEDs (PHOLEDs) with values of η int of about 100% were demonstrated by using a device architecture that can confi ne charges and excitons inside the phosphorescence-emitting layer. [ 9,10 ] Although PHOLEDs have a longer history than OLEDs fabricated with TADF materials, the basic concept for improving both the effi ciency and operational stability is not clear; this is the greatest challenge for the practical application of PHOLEDs.In recent years, several PHOLEDs with high effi ciency and high operational stability were reported. [ 11,12 ] The confi guration of the emitting layer, i.e., the combination of the phosphorescent dopant and the surrounding host material, was proposed as a key parameter determining the effi ciency and stability of PHOLEDs. Particular metal complexes such as bis(benzo[h]quinolin-10-olato-kN,kO)beryllium( II ) (Bebq 2 ) and bis[2-(2-hydroxyphenyl) benzothiazolato]zinc( II ) [Zn(BTZ) 2 ] are effective hosts for red PHOLEDs; however, an effective host for green/blue PHOLEDs with high operational stability was not yet reported. If the role of effective hosts such as metal complexes in a highly effi cient and stable PHOLED is revealed, the development of useful host materials for green/blue PHOLEDs is expected to be accelerated.Herein, to investigate a possible strategy for realizing highly effi cient and stable PHOLEDs, we examine the energy-transfer mechanism in such PHOLEDs. A highly effi cient green PHOLED that exhibits a half lifetime of over 10000 h with an initial luminance of 1000 cd m -2 is realized by using a Phosphorescent organic-light emitting diodes (PHOLEDs) exhibit an internal quantum effi ciency of 100% and their early practical realization is expected. One of the main challenges in PHOLEDs is to improve the operational stability of green and blue devices. Triplet exciton dynamics in stable PHOLEDs, which are different from those in unstable PHOLEDs, are shown. An effi cient and stable green PHOLED is demonstrated by employing a suitable host that surrounds the phosphorescent dopant. The transient photoluminescence characteristics show that the triplet excitons of the host, which are generally unstable, are rapidly transferred to the dop...