Recent advances in organic optoelectronics, particularly in efficient organic light-emittng devices (OLEDs), have called for new electroactive organic materials as well as for new device technologies. Small OLED-based displays already generate hundreds of millions of dollars. Larger OLED displays will penetrate the television market in the not-too-distant future. Nowadays white displays play important role in lightening. Further advances of these devices substantially rely on development and studying of high-performance organic charge-transport and emitting materials, theoretical understanding of charge and energy transport in the organic systems and their well-balanced application in OLED devices. The phosphorescent OLEDs (PhOLEDs) have been demonstrated with an internal quantum efficiency of nearly 100%, which is much higher than that of fluorescent OLEDs. On the other hand the PhOLEDs suffer from high cost and rarity of the noble-metal complex. Besides, the PhOLEDs are mainly doped devices, whereas non-doped devices are more attractive because they are easier to be employed for mass production and the devices are more reliable. Thus, it is of great value to develop efficient fluorescent materials. Most thin films prepared from traditional fluorophores are usually weakly emissive due to aggregation caused quenching. Since the emitters in the OLEDs are used as solid state, it is required to overcome the aggregation caused quenching effect in order to manufacture non-doped electroluminescence devices. Tasks of the project: synthesis of new aggregation induced emission emitters and hole transporting materials for fluorescent organic light emitting diodes. Characterization of hole transporting layer materials and emitters in devices, formation, characterization and optimization of the OLED devices, in order to discover the most effective materials and create the most efficient prototypes of the devices.
Project funding:
KTU Business Support Fund
Project results:
In this project among all lowmolar-mass derivatives under investigation in OLEDs field, carbazole-based materials have been studied at length for their interesting physical properties, including good charge injection and transport, electroluminescence, improved thermal and morphological stabilities as well as film forming properties. The article is written from structural organic chemist’s point of view and is divided in several parts: V-shaped 3(2)-substituted carbazoles and derivatives with 9-carbazolyl rings, star-shaped 3,6(2,7)-substituted carbazoles and branched twin derivatives containing (di) arylcarbazolyl fragments. Observed that 2,7- and 3,6-diaryl substituted carbazoles act very effectively as hole transporting materials, enhancing the quantum efficiency and lowering the driving voltage of the organic light emitting devices. Among various carbazole containing host materials, 3(9)-aryl carbazoles and 3,6-diaryl substituted derivatives were very effective as the host materials for blue (EQE < 24%), green (EQE < 20%) and red (EQE < 19%) phosphorescent organic light emitting diodes.
In the next step, the naphthyl substituted triphenylamine and its derivatives with bromine atoms were synthesized and investigated. In the past studies, introducing halogen atoms into the electroactive derivatives has been demonstrated on leading to enhancements of carrier mobilities in their layers. The respective glass transition temperatures of the materials were estimated to be in a range 65–137 °C, which can provide morphologically-stable amorphous films for applications in organic light emitting diodes. The compounds possess adequate ionization potentials (5.5–5.75 eV), high hole drift mobilities (> 10?3 cm2/V·s) and suitable triplet energies (?2.4 eV), which make them suitable hole transporting materials for use in red phosphorescent organic light-emitting diodes. A superior peak efficiency of 17.9% (31.4 cd/A and 26.9 lm/W) was achieved in a device having hole transporting layer of tris[4-(1-naphthyl)phenyl]amine. Furthermore, the device gave efficiencies of 17.7% and 16.6% recorded at luminance levels of 102 and 103 cd/m2. The efficiency drop from the maximum to the value recorded at the luminance of 103 cd/m2 for the device was only 7%.
More ower tetramer of triphenylamine and similar compounds having bromine atoms have been synthesized, characterized and tested as hole injecting or transporting layers of organic light-emitting diodes (OLEDs). Very high glass transition temperatures of the derivatives were determined to be in a range of 99-163?, which can form stable amorphous films for applications in the devices. The materials have suitable ionization potentials (5.8-5.2 eV) and adequate triplet state energies (2.37-2.44 eV), which make them useful hole injecting/transporting layer materials for application in red phosphorescent devices. A simplified tri-layer device architectures of red OLEDs were prepared by adopting N-(1-diphenylamino[4,4′-biphenyl])-N,N-bis(4-bromophenyl)amine (1) or N,N-bis (1-diphenylamino[4,4′-biphenyl])-N-(4-bromophenyl)amine (2) as hole transporting layer (HTL) material. Tris(1-diphenylamino[4,4′-biphenyl])amine (3) was selected for the hole injecting layer (HIL) to combine with 1 or 2 to construct step-wise hole injection in the devices. Peak efficiencies of the device with a single 1-based HTL were 12.5%, 18.6 cd/A, and 13.1 lm/W, all of which were slightly lower than those of a step-wise device having an additional 3-based HIL (i.e. 14.0%, 22.8 cd/A, and 17.8 lm/W). Similarly, the device with 2-based HTL exhibited peak efficiencies up to 13.5%, 21.3 cd/A, and 17.6 lm/W; while those of the device with an additional 3-based HIL achieved higher values of 13.8%, 22.6 cd/A, and 18.4 lm/W.
Period of project implementation: 2018-09-17 - 2019-09-16
Project coordinator: Kaunas University of Technology
