Thermally activated delayed fluorescence (TADF) emitters have recently attracted considerable attention as dopants for organic light-emitting diodes (OLEDs), and are considered promising alternatives to fluorescent and phosphorescent dopants. TADF emitters require a small singlet-triplet energy gap (ΔEST). This study presents a molecular design strategy to achieve a small ΔEST. An electroluminescent dopant containing two electron-donating carbazolyl groups and an electron-accepting triphenyltriazine, m-bisCzTRZ, was prepared. The electron-donating moieties were introduced at the meta positions of the two phenyl rings in triphenyltriazine. Transient photoluminescence decay measurements confirmed that m-bisCzTRZ emitted blue TADF in a solid-state host layer and thus it showed promise as a blue dopant. The experimental ΔEST value for m-bisCzTRZ was small (0.09 eV), as determined from fluorescence and phosphorescence spectra of a m-bisCzTRZ-doped host. These observations suggested that the use of meta-linked electron-donating and electron-accepting units could be a useful strategy to obtain TADF emitters.
A series of carbazole-phthalonitrile hybridized light-emitting materials having expanded π-systems at the carbazole moieties were synthesized, and their physical properties such as cyclic voltammogram, UV-vis absorption spectra and photoluminescence spectra in solution and the solid states. These measurements showed that 5-carbazolyl-1, 3-phtalonitiriles have more largely expanded π-systems than 2-carbazolyl-1, 3-phtalonitiriles to exhibit bathochromic shifts in UV-vis absorption and emission spectra. Amino-substituted derivatives served as acid-sensors in CH2Cl2, and the intensities of their photoluminescence increased in accordance to the amount of 10-camphorsulfonic acid added.
An 8-inch oxide-TFT-driven flexible display using inverted organic light-emitting diodes (iOLEDs) with an inverted device structure was demonstrated. We employed iOLEDs with an air-stable electron injection layer and longer lifetime. An oxide-TFT backplane having good electrical performances (mobility ~7 cm2/Vs, on/off ratio >107) was also fabricated on a plastic substrate at low temperature (i.e., below 160 °C). The fabricated flexible iOLED display showed clear and stable color moving images and produced uniform RGB emissions from each pixel, even when it was bent.
The dependence on the roll-off property of carrier accumulation at the heterojunction interface between the emission layer (EML) and carrier transport layers was investigated in organic light-emitting diodes (OLEDs). The roll-off property was mitigated in an OLED having an indium tin oxide (ITO, 145 nm, anode)/40 wt%-molybdenum(VI) trioxide (MoO3)-doped 9,10-diphenyl-anthracene (DPA, 30 nm)/DPA (10 nm)/5 wt%-5,6,11,12-tetraphenyl-naphthacene (rubrene)-doped DPA (40 nm)/DPA (20 nm)/14 wt%-cesium (Cs)-doped DPA (20 nm)/aluminum (Al, 100 nm, cathode) structure, although a conventional OLED having an ITO (145 nm, anode)/N,N´-Di-[(1-naphthyl)-N,N´-diphenyl]-1,1´-biphenyl)-4,4´- diamine (α-NPD, 20 nm)/5 wt%-rubrene-doped DPA (42 nm)/tris(8-hydroxyquinoline) aluminum (Alq3, 20 nm)/lithium fluoride (LiF, 0.5 nm)/Al (100 nm, cathode) structure exhibited a typical roll-off property. This difference can be attributed to a decrease in the electron accumulation at the interface between the EML and electron transport layer (ETL), following a reduction in the distinct heterojunction structures at the interface in the OLED.
We report p-type doping of 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) by solution processing. We confirm the presence of F4-TCNQ in the doped films and observe the Fermi level shift towards the highest occupied molecular orbital (HOMO) as the dopant concentration increases. Furthermore we note that the doped film conductivities, as well as the threshold voltages and ON/OFF ratios of transistors incorporating those films, show trends as a function of F4-TCNQ concentration consistent with p-type doping. The number of charge carriers in the film increases with doping, but no dependence on the doping level is observed for the extracted field-effect mobility. The latter is likely due to a lack of control over the morphology formation during the film crystallization and not to fundamental factors governing charge transport. The present findings may serve as a basis for the development of higher performance organic solar cells using SQ as a donor material.
We proposed a novel method for micro-patterning organic semiconductor layers by using a laser irradiation system. The method was demonstrated with a precursor that changes solubility after thermal conversion. For micro-scale patterning, brief irradiation was crucial to prevent heat diffusion in the substrate. We found that preheating on a hot-plate is important for shortening the laser irradiation time necessary for the thermal conversion. The patterning resolution also depended on the kind of substrates as well as the laser exposure time. High-resolution line patterns with a width of 12 μm were successfully achieved on ITO substrates. Organic light-emitting diodes utilizing a patterned emitting layer, with line widths of 20 μm and spacing of 85 μm, were demonstrated.
We investigated the relationship between eye-position distribution and head-orientation while viewing dynamic images in an 85 inch 8K ultrahigh-definition television (UHDTV). Participants watched video clips of several types of scenes made for UHDTV without any restriction of head movements. The result showed a correlation between eye and head orientation, which is similar to the coordinative movements of the eye and head for static-image viewing in previous studies. The distribution of eye positions was biased in the direction of head orientation both in horizontal and vertical directions. When the head orientated to the left/right (or up/down), the eyes also tended to direct in the left/right (or up/down) relative to the head. These findings suggest that eye-head coordination is similar between static and dynamic images and that head orientation could be used to evaluate the viewing condition and image contents of wide field of view display such as that of an UHDTV.