High performance n-type FETs have been accomplished by using novel heterocyclic systems with trifluoromethylphenyl groups. To enhance intermolecular interactions, selenophene rings were introduced. Some FET devices showed higher electron mobilities than 0.1 cm2V-1s-1. The mobilities of the selenophene-containing materials were higher than those of the corresponding thiophene analogues. The relationship between the structures and FET characteristics have been investigated. The threshold voltages were reduced by introducing heterocyclic units with higher electron affinity.
Two types of flexible organic light-emitting diode (OLED) displays are described. One is a passive matrix (PM) display with 128 (RGB) x 72 pixels, and the other is an active-matrix (AM) panel with 4 x 4 green pixels using organic thin-film transistors (OTFTs). The emitting layer of these displays is based on high-efficiency phosphorescent materials. The PMOLED display showed a clear color video image even when it was bent. The AMOLED panel could actively drive the OLED pixels. These results suggest that the OTFT-driven OLED display would be a promising candidate for rollable mobile displays.
We have fabricated highly efficient organic light-emitting diodes (OLEDs) using novel hole-transporting emissive materials with triphenylamine moiety. The novel emissive materials have a high glass transition temperature ranging from 141 - 152 degrees Celsius, which is attributed to nonplanar molecular structure. The OLEDs consist of an emitting layer of the novel emissive material and an electron-transport layer of tris(8-qunolinolato) aluminum (Alq3). Emission colors of the OLEDs were bluish-green and greenish-yellow. High external quantum efficiency of 1.2 - 2% was obtained at a luminance of 300 cd/m2, and good durability in a continuous operation at room temperature and high temperatures was achieved.
We have introduced acene such as anthracene, naphthacene and pentacene into the main chain of polyfluorene to improve stability and to realize color tuning to pure blue, green and red. High molecular weight acene-containing polyfluorenes are successfully synthesized by the Ni mediated Yamamoto-coupling reaction. Average molecular weights of the copolymers are Mw equals 42000 - 96000 (determined by GPC). By the introduction of 10% of acene moiety, the device stability could be significantly improved, indicating the torsion between the acene and fluorene units suppresses interchain aggregation; the copolymers exhibits stable blue, green and red emission in the anthracene-, naphthacene- and pentacene-containing polyfluorenes, respectively. Onset voltage for light emission is 5 - 30 V and the maximum luminance is 240 - 5200 cd/m2. As a side effect an unique color change on UV irradiation could be observed as well as the reason for its appearance determined.
The electronic structures of 8-hydroxyquinoline aluminum (Alq3)/electron injection layer/Al interfaces, used in organic electroluminescent devices, were measured by ultraviolet photoelectron spectroscopy (UPS). LiF and alkaline earth fluorides (CaF2, SrF2 and BaF2) were used as an electron injection layer. Shifts of the highest occupied molecular orbital (HOMO) level and the vacuum level of Alq3 layer due to the insertion of the fluorides were observed. These shifts indicate that the alkaline earth fluoride layers as well as the LiF layer at the Alq3/Al interface reduce the barrier height for electron injection from the Al to Alq3. The reduction of the barrier height is consistent with the driving voltage in the organic EL device in which these fluorides are used as the electron injection layers. We believe that lowering in the driving voltage in organic EL devices with the thin insulator layers, such as LiF and alkaline earth fluorides, is attributable to the reduction of the barrier height.
Both directionality and intensity of emitted light from the organic electroluminescent (EL) device were strongly modified using a Fabry-Perot microcavity. The microcavity EL device exhibited a single mode emission that was significantly enhanced and the emission was sharply directed along the direction normal to the device surface. The luminous efficiency in the normal direction was much higher than that of the noncavity EL device. The sharply directed pure green emission exceeded 40000 cd/m2 at a driving current of 800 mA/cm2.