Recent advances in organic optoelectronics, particularly in efficient organic light-emitting devices (OLEDs), have called for new electro-active 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 by artificial intelligence methods, 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.
About 25 percent world’s electric energy is used for lightning technologies. Until a half of the electric energy would be saved after development of efficient organic light emitting diodes-based lightning. The project is contributed with energy saving technologies –lightning by organic light emitting diodes and respectively with factor of social-economic benefit. The results collected during the studies will be also heavily disseminated among the scientific society.
Project funding:
KTU Research and Innovation Fund
Project results:
Pyridinyl-carbazole fragments containing low-molar-mass compounds as host derivatives H1 and H were synthesized, investigated and used for preparation of electro-phosphorescent organic light emitting devices (PhOLEDs). The materials demonstrated high stability against thermal decomposition with the decomposition temperatures of 361-386 °C and were suitable for preparation of thin amorphous and homogeneous layers having very high values of glass transition temperatures of 127-139°C. It has been determined that triplet energy values of the derivatives are, correspondingly, 2.82 eV for the derivative H1 and 2.81 eV for the host H2.
The new derivatives were tested as hosts of emitting layers in blue as well as in green phosphorescent OLEDs. The blue device with 15 wt. % of the iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2?]picolinate (FIrpic) emitter doping ratio in host material H2 exhibited the best overall characteristics having power efficiency of 24.9 lm/W, current efficiency of 23.9 cd/A, and high value of 10.3 % of external quantum efficiency at 100 cd/m2. The most efficient green PhOLED with 10 wt% of Ir(ppy)3 {tris(2-phenylpyridine)iridium(III)} in H2 host showed power efficiency of 34.1 lm/W, current efficiency of 33.9 cd/A, and high value of 9.4 % for external quantum efficiency at high brightness of 1000 cd/m2, which is required for lightning applications. These results confirmed that some of the new host materials are very promising components for developments of highly efficient phosphorescent devices.
Solution processable hole transporting materials (HTMs) are indispensable component for the cost-effective and large-area roll-to-roll fabrication of organic light emitting diodes (OLEDs). Hole transport layers should display high triplet energy, good hole injection properties along with potential electron blocking capability. In this paper, we report a series of novel fluorene-based HTMs, 9,9-diethyl-2,7-bis(2-(trifluoromethyl)phenyl)-9H-fluorene (3), 9,9-diethyl-2,7-bis(3-(trifluoromethyl)phenyl)-9H-fluorene (4) and 9,9- diethyl-2,7-bis(4-(trifluoromethyl)phenyl)-9H-fluorene (5) containing symmetrical and asymmetrical substitution of trifluoromethyl pendants at different positions. The synthesized HTMs have an exceptional solubility in common organic solvents and possess good thermal stability to form morphologically stable films. The relevant molecular energy level alignment (HOMO-LUMO), adequate ionization potential and high triplet energies promote materials performance as a good hole transporting layer (HTL). Solution-processed yellow phosphorescent OLEDs fabricated by utilizing these HTMs with a conventional yellow emitter Iridium(III)bis(4-phenylthieno[3,2-c]pyridinato-N,C2′)acetylacetonate (PO-01) doped in 4,4?-bis(N-carbazolyl)-1,1?-biphenyl (CBP) host. The best device with molecule 4 showed an improvement of 54% in current efficiency (CE) from 23.3 to 35.8 cd/A and 14% in external quantum efficiency (EQE) from 11.3 to 12.9%, compared to reference device containing N, N?-Bis(naphthalen-1-yl)-N,N?-bis(phenyl)benzidine (NPB) as HTL. Compounds 3 and 5 based devices also showed a competitive performance with the reference device. The effect of solvent was also studied. These outcomes recommend that this type of solution processable HTMs will be promising contender for high-efficiency OLED devices.
Period of project implementation: 2021-04-01 - 2021-12-31
Project partners: Vytautas Magnus University
