ᐊ P3-8
Open Readings 2021 • P3-9
P3-10 ᐅ

TADF BEHAVIOUR OF CARBAZOLE AND PHENOTHIAZINE SUBSTITUTED PYRIDINES TOWARDS OLED APLICATIONS

Karolis Leitonas1, Dmytro Volyniuk1, Viktorija Andruleviciene1, Pavel Arsenyan2, Brigita Vigante2, Juozas Vidas Grazulevicius1

1 Department of Polymer Chemistry and Technology, Kaunas University of Technology, K. Barsausko g. 59, LT-51423, Kaunas, Lithuania

2 Latvian Institute of Organic Synthesis, Aizkraukles 21, LV-1006, Riga, Latvia.

[email protected]

Organic compounds exhibiting thermally activated delayed fluorescence (TADF) in recent years have attracted significant interest in the field of organic optoelectronics. [1] Organic light emitting diodes (OLEDs) can reach 100% internal quantum efficiency (IQE) when TADF compounds are used as emitters due to harvesting of triplet excitons through the reverse intersystem crossing (RISC). Since the first publication on application of TADF materials in OLEDs, multiple-donor approach is widely used in the design of efficient TADF emitters. [2] High external quantum efficiency (EQEs) of TADF based OLEDs is directly related to emissive triplet harvesting via T1 →S1 RISC process in purely organic compounds with small singlet-triplet energy splitting (ΔEST) and high photoluminescence quantum yields (PLQYs) in the solid state. [3] Some multiple-donor-acceptor TADF emitters with small ΔEST and high PLQYs in the solid-state were previously developed and showed efficient and relatively stable electroluminescence. [4]

Here we report on a pair of multiple donor substituted dicyanopyridines with three donors of one-type (carbazole) or of two types (carbazole and phenothiazine) in their molecular structures. [5] The compounds were characterized by efficient conventional green and dual orange TADF which results from recombination of one or two intramolecular charge transfer states, respectively. OLEDs based on the phenothiazine and carbazole containing emitter exhibiting dual TADF (Fig. 1) showed low device life-times and low maximum external efficiency of 3.1 (for the non-doped device) and of 5 % (for the doped device). OLEDs based on the carbazolyl multiple substituted dicyanopyridine exhibiting normal TADF showed relatively high device life-times and high maximum external efficiency of 8.1 (for non-doped device) and of 25% (for doped one). Such device stability and efficiency effects are partly related to ultra-long emission decay (up to milliseconds) which can enhance probability of exciton-exciton and exciton-polaron annihilations under electrical excitation.

Figure 1
Fig. 1. Steady-state (dashed white line) and time-resolved (recorded in 0-5 ns range after excitation) emission spectra of carbazole and phenothiazine donors containing dicyanopyridine: Contour plot (left) and 3D plot (right) of 10-4 mg/ml toluene solution.

Acknowledgments: This work was supported by the project of scientific co-operation program between Latvia, Lithuania and Taiwan "Polymeric Emitters with Controllable Thermally Activated Delayed Fluorescence for Solution-processable OLEDs" (grant No. S-LLT-19-4).

[1] Y. Liu, C. Li, Z. Ren, S. Yan, M. R. Bryce, Nat. Rev. Mater. 18020 (2018) 1-20.

[2] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 2012, 492, 234-238

[3] K. Sato, K. Shizu, K. Yoshimura, A. Kawada, H. Miyazaki, C. Adachi, Phys. Rev. Lett., 2013, 110, 247401.

[4] S. K. Jeon, H. L. Lee, K. S. Yook, J. Y. Lee Adv. Mater. 2019, 31, 1803524

[5] P. Arsenyan. B. Vigante, K. Leitonas, D. Volyniuk, V. Andruleviciene, L. Skhirtladze, S. Belyakov, J. V. Grazulevicius, J. Mater. Chem. C, 2021 (Accepted)