High Triplet Energy Hosts for Blue Thermally Activated Delayed Fluorescence OLEDs

Karolis Kazlauskas^a, Dovydas Banevičius^a, Gediminas Kreiza^a, Saulius Juršėnas^a, Francesco Rodella^b, Peter Strohriegl^b

^aInstitute of Photonics and Nanotechnology, Vilnius University, Lithuania, Saulėtekio av. 3, LT-10257 Vilnius,, Lithuania

^bMacromolecular Chemistry I, University of Bayreuth, Germany, Bayreuth, Germany

Proceedings of International Conference on Emerging Light Emitting Materials (EMLEM22)

Materials for next generation LEDs and lasers:

Limasol, Cyprus, 2022 October 3rd - 5th

Organizers: Maksym Kovalenko, Maryna Bodnarchuk and Grigorios Itskos

Oral, Karolis Kazlauskas, presentation 037
DOI: https://doi.org/10.29363/nanoge.emlem.2022.037
Publication date: 15th July 2022

The last decade has witnessed a huge progress in thermally activated delayed fluorescence (TADF) emitters employed for fabrication of the latest generation OLEDs.[1,2] TADF allows harvesting of up to 100% triplet excitons through reverse intersystem crossing (RISC) process in organic materials without employing precious metals, thus rivalling the efficiencies of state-of-the-art phosphorescent OLEDs.[3,4] Even though TADF-OLEDs have demonstrated an upper limit of internal quantum efficiency (100%) across the whole visible range, their short operational lifetime in the blue range restricts the development and commercialization of these devices.[5] Half-life of blue-emitting devices typically does not exceed a few hours at a practical luminance of 1000 cd/m².[6] Among the key factors affecting the device stability are the degradation of TADF emitter and host material.

The design of efficient blue emitter–host combinations is one of the greatest challenges in organic light-emitting diode (OLED) research. The choice of the host material crucially affects device efficiency, roll-off and lifetime. High triplet energy hosts are in high demand, especially for thermally activated delayed fluorescence (TADF) emitters, due to their high exciton energies. The hosts are far less investigated than the emitters and require further progress.[7]

In this work, we introduce two novel hosts based on acridine donor and triazine acceptor (ATRZ) exhibiting good thermal and morphological properties and high triplet energies 3.07-3.27 eV. The donor and acceptor combination enables transport of both electrons and holes. Application of these bipolar hosts in blue TADF-OLEDs resulted in a blue electroluminescence peaking at 480-488 nm, a maximum EQE of 10%, and a low efficiency roll-off. The device based on ATRZ host was found to express the highest stability, i.e. the longest operational lifetime, which even surpassed that of OLED based on the benchmark mCBP host.

References:

[1] Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P. Recent advances in organic thermally activated delayed fluorescence materials. Chem. Soc. Rev. 2017, 46, 915–1016.

[2] Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, 1605444.

[3] Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtekar, K. T.; Bhalla, V.; Santos, J.; Bryce, M. R.; Monkman, A. P. Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters. Adv. Mater. 2013, 25, 3707–3714.

[4] Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234–238.

[5] Kim, M.; Jeon, S. K.; Hwang, S.-H.; Lee, J. Y. Stable Blue Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes with Three Times Longer Lifetime than Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2015, 27, 2515–2520.

[6] Su, L.; Cao, F.; Cheng, C.; Tsuboi, T.; Zhu, Y.; Deng, C.; Zheng, X.; Wang, D.; Liu, Z.; Zhang, Q. High Fluorescence Rate of Thermally Activated Delayed Fluorescence Emitters for Efficient and Stable Blue OLEDs. ACS Appl. Mater. Interfaces 2020, 12, 31706–31715.

[7] Rodella, F.; Saxena, R.; Bagnich, S.; Banevičius, D.; Kreiza, G.; Athanasopoulos, S.; Juršėnas, S.; Kazlauskas, K.; Köhler, A.; Strohriegl, P. Low efficiency roll-off blue TADF OLEDs employing a novel acridine–pyrimidine based high triplet energy host. J. Mater. Chem. C 2021, 9, 17471–17482.

Acknowledgements:

The research at Vilnius University was funded by a grant (no. S-MIP-21-12) from the Research Council of Lithuania (LMTLT).

Funding through the EU Marie Skłodowska-Curie ITN TADFlife grant (GA. 812872) is acknowledged.

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