Organic light emitting diodes (OLEDs) are an indispensable technology for the next generation of flat panel displays and lighting applications. In this report, we introduce a novel pathway to realize the ultimate electroluminescence efficiency by using simple aromatic compounds displaying efficient thermally activated delayed fluorescence (TADF). This technology can be described as the third generation of emitters after the first generation of fluorescence and the second generation of phosphorescence. While one supposes that a singlet excited state (S₁) level should be higher than a triplet excited state (T₁) level, the proper design of organic molecules enables the formation of a very small energy gap (δE_ST) between them. A molecule displaying efficient TADF requires a small δE_ST, resulting in an efficient T₁ → S₁ reverse intersystem crossing (ISC). Such an excited state can be realized by a spatially separated donor and acceptor system. We review a series of TADF molecules with their electroluminescence performance.

Organic single-crystal semiconductors offer systems of π-conjugated small molecules, periodically arranged on a macroscopic length scale. Therefore, such solids are the most suited, both to study the fundamental physics of charge transport in weakly interacting molecular assemblies and to fabricate the best-performing organic devices for printed and flexible electronics. The degree of fundamental intermolecular electronic charge coherence is discussed using the results of the Hall effect on various single-crystal transistors under varied temperatures. As a result the extent of the coherence depends on the molecular system, so that such an approach gives a prescription to design new molecular compounds because fully coherent materials exhibit a high carrier mobility of approximately 10 cm²/Vs.

Organic light-emitting devices (OLED) are solid-state light-emitting devices based on organic semiconductors. Recent rapid advances in material chemistry have enabled the use of a white OLED for general lighting and large-area flat panel displays. White OLED panel efficacy has reached 90 lm/W, and a tandem white OLED panel has realized a lifetime of over 100,000 hours at 1000 cd/m². LG launched a 55″ size OLED TV in 2013, and OLEDs are expected to have bigger breakthroughs. In this paper, we explored the recent progress of high-performance OLEDs for next-generation lighting.

pn-control techniques, by impurity doping, both for single and co-deposited films consisting of two kinds of organic semiconductors have been established. A series of fundamental junctions such as Schottky junctions, pn-homojunctions, p⁺, n⁺/metal ohmic junctions, n⁺p⁺-ohmic homojunctions, and tandem cells showing 2.4% efficiency have been successfully fabricated in co-deposited organic semiconductor films by impurity doping alone. 3^rd co-evaporant molecule induced phase separation/crystallization of co-deposited films, which drastically enhances the magnitude of photocurrent density, was demonstrated.

A novel optical second harmonic generation (SHG) technique that can visualize carrier dynamics in organic devices is introduced, where nonlinear polarization induced in the active layers, due to the presence of a static electric field that originates from mobile carriers, is directly probed along with the carrier motion by laser irradiation. The visualized carrier dynamics are very helpful to analyze and model carrier behaviors in a variety of organic devices such as Organic transistors, Organic electroluminescence devices, Organic solar cells and so forth. The principle of the SHG technique is described with some experimental examples, and the future scope is briefly discussed.

An organic semiconductor with having high layer crystallinity is the key to achieving high performance organic field-effect transistors. In order to realize “printed electronics” with organic semiconductors, we present two novel print production technologies, both of which are designed to promote self-organization of molecular materials to form layered crystals.

With the rapid growth of organic electronics, various techniques to evaluate material properties and device characteristics have been developed. Among these methods, this article reports on those that examine electronic structures such as photoemission spectroscopy and those that explore carrier behaviors in devices. A brief perspective will be also provided.

Recently, white LEDs fabricated from blue LED chips combined with yellow phosphors have been widely used in daily life. In this paper, I review the improvements in efficiency of blue and white LEDs, which are composed of GaN-based semiconductors.