Organic and Molecular Electronics

Over recent years, a wide variety of challenges aiming for electrical pumping of organic laser diodes have been addressed. However, organic laser diodes have difficulty gaining widespread application due to their high lasing thresholds under electrical pumping. A variety of organic semiconductors have been developed to reduce the lasing threshold, which include not only small molecular materials but also conjugated polymers. In this article, we demonstrate that bis-styrylbenzene derivatives show promising characteristics for very low lasing thresholds and discuss the design considerations for organic lasing molecules. Introduction The dissolution of luminescent organic dye molecules in an organic solvent followed by their excitation with high-energy light (greater than their lasing threshold) produces a laser beam. Due to their broad tunabilities, dye lasers have made remarkable advancements.1-5 In recent years, organic solid-state lasers involving the dispersion of an organic dye within a solid/crystal matrix have also been actively studied. In 1989, we proposed an idea to create an organic semiconductor laser diode using triplet excitons of rare earth complexes such as a europium (Eu) complex.6 This system would produce an electrically excited laser with a low threshold by forming a population inversion using the long excitation lifetime of Eu. There have been many studies reporting amplified spontaneous emission (ASE) oscillation from an organic solid thin film and laser oscillation from an organic thin film with an oscillator structure. The thin films in these studies were made by doping a polymer or small-molecule medium with an appropriate laser dye.7-11 These studies provide evidence that organic dyes have suitably high stimulated emission coefficients and that the formation of a solid thin film waveguide can provide the necessary architecture for a solidstate device with a low lasing threshold. More recently, laser oscillations from organic semiconductor dyes, which take electrical excitation into consideration, have also been actively studied.12,13 This interest was generated because organic light-emitting diodes (OLEDs) have achieved an internal luminescence efficiency of nearly 100% through the use of a triplet excited state at the luminescence center. This excited state leads to the realization of an organic semiconductor laser that is an extension of OLEDs a practical study subject of organic electronics.14 In addition, light-emitting transistors have also become capable of producing highly efficient electroluminescence (EL), prompting new electrical excitation developments of organic semiconductors.14-21 In this article, we will discuss materials issues for the production of an organic semiconductor laser. This process requires the development of a new laser material with a focus on a low lasing threshold. Derivatives of bis-styrylbenzene (BSB) have demonstrated extremely good ASE oscillation characteristics in organic solid thin films.15,16 As a result, we will introduce new organic semiconductor materials with similar ASE characteristics in organic solid thin films by using BSB analogs as active materials. The Mechanism and Goal of Organic LEDs The external quantum efficiency (ηext) of OLEDs is based on four factors: (i) the ratio of electron and hole injection, transport, and recombination (ν), (ii) exciton formation efficiency (ηr), (iii) internal electroluminescence quantum yield (φp), and (iv) light out-coupling efficiency (ηp) (Figure 1).