Norimasa Yokoyama

Low driving voltage characteristics of triphenylene derivatives as electron transport materials in organic light-emitting diodes

Triphenylene-based electron transport materials (ETMs), designated Bpy-TP1-4, with a coplanar molecular structure and a large electron affinity were designed and synthesized for use in organic light-emitting diodes (OLEDs). Spectroscopic ellipsometry measurements clarified that the deposited thin films of these ETMs have optical anisotropy, indicating that the molecules in the deposited thin films tend to be mostly oriented parallel to the substrate. Green OLEDs containing these ETMs allowed a lower driving voltage than that for OLEDs containing tris(8-hydroxyquinolinato)aluminum (Alq3) with a random orientation. In particular, the OLED containing Bpy-TP2 showed a significantly lower driving voltage, achieving lower power consumption when compared with conventional ETMs such as Alq3 and 1,3,5-tris(2-phenyl-1H-benzo[d]imidazol-1-yl)benzene (TPBi). Also, the operational lifetime of a blue OLED containing Bpy-TP2 is equivalent to that of an OLED with TPBi. The increased driving voltage of the device containing Bpy-TP2 is significantly suppressed compared to that of the OLED containing TPBi.

Benzene substituted with bipyridine and terpyridine as electron-transporting materials for organic light-emitting devices

New electron-transporting materials for organic light-emitting devices (OLEDs) based on trisubstituted benzene with both bipyridine and terpyridine, 1,3-bisbipyridyl-5-terpyridylbenzene (BBTB) and 1-bipyridyl-3,5-bisterpyridylbenzene (BTBB), were developed. Glass transition temperatures of BBTB and BTBB were 93 °C and 108 °C, respectively, and BTBB was completely amorphous with no melting point. Electron mobilities of BTBB exceeded the order of 10−4 cm2 V−1 s−1, while those of BBTB were very high and reached 10−3 cm2 V−1 s−1 at an electric field of approximately 500 kV cm−2. These high mobilities contributed to a low voltage operation. For example, in the case of the conventional aluminum trisquinolinol (Alq)-based fluorescent OLED with BTBB, current densities of 3.5 mA cm−2 and 100 mA cm−2 were reached at voltages of 3.0 V and 4.5 V, respectively. In addition, ionization potentials of BBTB (6.33 eV) and BTBB (6.50 eV) were sufficiently large to confine holes in common emissive layers.

Bipyridyl-substituted benzo[1,2,3]triazoles as a thermally stable electron transporting material for organic light-emitting devices

We developed new electron-transporting materials (ETMs) for organic light-emitting devices (OLEDs) based on benzo[1,2,3]triazole and two bipyridines. Four derivatives based on the same skeleton were synthesized with four different substituents: phenyl (BpyBTAZ-Ph), biphenyl (-BP), m-terphenyl (-mTP), and o-terphenyl (-oTP). These BpyBTAZ compounds have good thermal stabilities, and their decomposition temperatures were greater than 410 °C, which is significantly higher than that of tris(8-quinolinolato)aluminium (Alq), the conventional OLED material. BpyBTAZ compounds show preferable amorphous nature, and moreover, the glass transition temperatures (Tgs) of both BpyBTAZ-TP compounds exceed 100 °C. Furthermore, BpyBTAZ-BP exhibits no melting point and is fully amorphous. The electron affinities of the materials are as large as 3.3 eV and their electron mobility is sufficiently high. These characteristics accounted for a reduction in the operational voltage of OLEDs with BpyBTAZ compounds compared with the reference device with Alq as an ETM. Specifically, the electron mobility of all the BpyBTAZ compounds exceeds 1 × 10−4 cm2 V−1s−1 at an electric field of 1 MV cm−1. In addition, it was revealed that both BpyBTAZ-TP-based devices showed longer luminous lifetimes and smaller voltage increases during continuous operation at 50 mA cm−2, compared with the Alq reference device.

P‐226: New Tris Bipyridyl Derivative as Hole‐Blocking and Electron‐Transporting Materials for Organic Light‐Emitting Devices

We demonstrate that tris bipyridyl benzene (TbpyB) works well as an electron transporting and hole-blocking material for organic light-emitting diodes. Using TbpyB for phosphorescent organic light-emitting diodes (OLEDs), we can obtain comparable high external quantum efficiency and a lower operation voltage without any additional hole-blocking layer that must be needed usually. In other words, the TbpyB layer worked as a double functioned (hole-blocking and electron-transporting) layer, and TbpyB provides more simple fabrications of phosphorescent OLEDs that work at a lower power consumption.

P-165: Wet-Processable Triphenylamine Dendrimers as Hole-Transporting and Hole-Injection Materials for Organic Light-Emitting Devices

We demonstrated two wet-processable triphenylamine dendric nonamers: the nitrogen-atom-centered nonamer (TPA9-1) and the phenyl-centered nonamer (TPA9-2). The materials were found to have high glass transition temperature (Tg) up to almost 200°C. The fractional difference of the central units of the molecular structures caused different adaptability due to the different ionization potentials (Ip). TPA9-1 (N-atom-centered), whose Ip was smaller than that of TPA9-2 (phenyl-centered), was suitable as hole injection layer material, and TPA9-2 was suitable as hole-transporting layer material. Computational chemistry provides a ready explanation for the Ip difference between the two materials.