External EL quantum efficiencies at theoretical limits are currently being achieved using phosphor luminescent materials. This is because triplet exciton production efficiency has reached 100% because singlet and triplet excitons are produced at a ratio of 1:3, and intersystem transitions from singlet to triplet levels occurs at probability of 100%. While phosphor luminescent devices have excellent organic EL performances, triplet excitons become deactivated at high current densities, which sharply reduces the light emission efficiency, and they are limited to compounds that contain precious metals, such as Ir and Pt. Conventional luminescent materials have many advantages, including excellent high-current density characteristics and a wide diversity of materials, but 75% of the triplet excitons are completely heat-deactivated so that less than 25% of singlet excitons can be used for EL light emission. Therefore, by exploiting the advantages of the luminescent and phosphor luminescent processes, it is proposed to facilitate a high-efficiency reverse energy shift from triplet to singlet levels, and thus theoretically achieve an exciton production efficiency of greater than 25% in a luminescent process. We propose to develop a new EL light emitting mechanism that improves the exciton production efficiency of luminescent materials by thermally activated delayed fluorescence (TADF) (see FIG. 1). At this point, preliminary experiments have succeeded in molecular design and material synthesis with pure aromatic organic compounds that contain no metals, in which the energy difference between the singlet exciton state and triplet exciton state is less than or equal to 0.1 eV. As a typical example, FIG. 2 shows the temperature dependence of the luminescence emission spectrum and emission intensity in a thin film. Phenomena unique to TADF were discovered in which the delayed luminescence emission spectrum matched the fluorescence spectrum and the PL emission intensity increased with increasing temperature. The foothold provided by these materials will enable advances in molecular design, progress in the development of materials with smaller energy differences between the singlet and triplet exciton states, the realization of high-efficiency TADF, and expansion toward new organic EL mechanisms. The use of TADF to achieve equal or superior luminescence characteristics to those of phosphor luminescence is a ground-breaking technological innovation, which can be positioned as a third-generation luminescent material that will have great scientific and industrial significance.
|FIG1: Novel Electroluminescence Mechanism using TADF||FIG2: Temperature Dependence of PL Intensity with Transient PL in TADF Material|
|FIG3: OLED Characteristics of Liquid OLED and Prototype Devices|
Conventional organic EL elements use completely solid-state thin films. However, ultrathin (on the order of 100 nm) films are generally required to achieve effective charge injection and transport. Consequently, device short-circuiting and instability have been major problems. As long as solid-state thin films are used, it will be extremely difficult to completely avoid these problems. This is acknowledged to be a major problem for practical applications of organic EL elements. The current proposal is to replace organic solids with liquid organic semiconductors. In the past, it was believed that the liquid state was highly susceptible to thermal oscillations, which made it an unsuitable charge transport medium from the perspective of charge transport. However, charge-transporting organic compounds, such as etylhexyl carbazole (EH-Cz) and pyrene derivatives with long-chain alkyl groups, etc., have exhibited mobilities of around 10?5 cm2/V s in time-of-flight measurements, even in the liquid state (at room temperature); this demonstrates their charge transport capabilities. We are using this property to test organic EL element structures (Appl. Phys. Lett, 95, 053304 (2009)). As shown in Fig. 3, a liquid compound is sandwiched between electron/hole-injecting electrodes, electrons and holes are injected into the light-emitting layer by applying a voltage between the two electrodes, and the EL emission from the liquid compound is observed. This suggests that a paradigm shift toward liquid semiconductors is possible in organic device structures, without necessarily using the organic solid state. This technological breakthrough provides new solutions for long-held hopes to achieve greater flexibility and to suppress element degradation. The use of such liquid organic semiconductors is expected to make it possible to avoid problems with cracks, even in response to stresses when folding, etc., which will lead to flexible devices being developed. By actively exploiting fluidity to inject/efflux liquid organic semiconductors between electrodes, as in pigment lasers, it will be possible to constantly provide new organic semiconductor materials f the light-emitting section, which essentially makes it possible to realize devices that do not have any element degradation. This research will demand a thorough investigation into the possibilities of liquid organic semiconductors, allowing for expanded research in everything from the molecular design of liquid organic semiconductor materials, to the elucidation of device physics, and to flexible devices and degradation-free devices.
The ultimate condensed state in organic devices is the organic single-crystal thin film, and its use in organic EL elements is expected to vastly enhance element performance. Figure 4 (top) shows the light-emitting state of an organic single-crystal EL element using 1,4-bis(4-methylstyryl)benzene (BSB-Me). A current density of 100 mA/cm2 was obtained using an applied voltage of merely 1.5 V (Appl. Phys. Lett., 96 053301 (2010)). In conventional thin-film EL elements, carrier injection is started by applying an intense electrical field of 20 MV/m or greater, but in elements using organic single crystals as the carrier transport layer, the element can be driven at low field intensities that are an order of magnitude lower (<2 MV/m). Additionally, monocrystals make it possible to markedly reduce the charge trap concentration, even in FETs that contain monocrystalline thin films, as shown in Fig. 4 (bottom). Ambipolar devices have been successfully realized in which both electrons and holes achieve mobilities of 1 cm2/V s and intense EL emission has been observed from an FET (Appl. Phys. Lett., 95, 103307 (2009)). The injection of current densities in excess of 1 kA/cm2 has also been achieved in organic monocrystals, presenting conditions under which we can expect to use current excitation to propagate lasers from organic semiconductors in the future. Advancement to an ultimate condensed state of organic monocrystals is an important research topic, anticipating future increases in organic device performances. It is very difficult to control mass productivity, thickness, and crystal orientation, etc., with conventional monocrystal growth technologies based on sublimation and recrystallization. We are thus considering developing new film formation techniques, such as laser annealing and organic epitaxial growth. We are currently having success in achieving a nearly three-fold increase in mobility by using laser annealing an F8T2 thin film using a semiconductor laser (Appl. Phys. Lett., 95, 073303 (2009)); this technology holds a great deal of promise for practical applications. Based on the above tests for organic EL elements, we hope to continue our investigations to develop next-generation organic semiconductor devices, such as organic semiconductor lasers, organic solar cells, organic transistors, and organic memory.
|FIG4: OLED and OFET with Organic Single Crystals|
It will be necessary to control the orientation of molecules to maximize the potential of organic molecules. It has been believed until now that organic amorphous films formed by vacuum deposition were isotropic, and that the molecules in those films had three-dimensionally random orientations. However, using variable angle spectroscopic ellipsometry (VASE), we have clearly shown that molecules with elongated or planar skeletal structures will orient parallel with the substrate in amorphous films formed by vacuum deposition (Org. Elec., 10. 127 (2009)) We observed a high correlation between skeletal morphology such as length and planar, etc. of molecules and the degree of orientation of molecules, and further discovered that they oriented parallel in isotropic films even when doped. Molecular orientation in these amorphous thin films is expected to vastly improve the characteristics of organic EL devices (i.e., greatly enhance the charge transport characteristics and improve the light-extraction efficiency). This project aims to implement a fundamental review of molecular skeletons in hole-transport materials and electron-transport materials, and to improve the efficiency of high-speed charge transfer and light extraction by using amorphous molecular-oriented films.