Key challenges in the decades-long research on organic electroluminescence include the development of efficient blue emitters and the extension of the operational lifetimes for organic light-emitting devices (OLEDs). In particular, despite the dedicated efforts of numerous researchers, the development of stable blue emitters remains a challenging hurdle.

Our group, in collaboration with Samsung Electronics, has been investigating degradation pathways for years, focusing on the identification of key degradation intermediates.^[1-4] Extracting degradation products directly from devices is notoriously difficult, and existing mechanisms reported by other groups vary widely depending on the materials used. A recurring critique during revisions has been the generalizability of our findings and experimental methods, highlighting the need for robust, universal insights. Various degradation mechanisms have been reported by different groups, and while some are widely accepted, the complexity of OLEDs makes it challenging to generalize these theories (Fig. 1a).^[5,6]

Building on these challenges, the complexity of device architectures, material diversity, and factors influencing chemical stability make it difficult to unify and define a single degradation mechanism. However, efforts to uncover previously unexplored mechanisms not only deepen our understanding of degradation but also provide valuable insights for designing more stable materials in the future, driving the advancement of OLEDs technology.

Our study investigates the degradation mechanisms of multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, promising candidates for blue emissive dopants, to identify pathways linked to device lifetimes and guide future molecular designs.^[7] By simulating excitonic and polaronic degradation in devices using photolysis and electrolysis techniques, we isolated intramolecular cyclized byproducts and characterized them by mass spectrometry and ¹H NMR spectroscopy. A clear correlation between the operational lifetime and the faradaic yield highlights radical cationic dopants as key species contributing to lifetime reduction. In contrast, excitonic degradation exhibited no such relationship, distinguishing this mechanism from previously known pathways.

Through quantum chemical calculations informed by our experimental observations of degradation products, we identified the reaction pathways underlying their formation. Quantum chemical calculations performed at KAIST revealed that intramolecular C-C bond formation between π-rings in MR-TADF materials serves as the rate-determining step, playing a pivotal role in the degradation process. This mechanism is further supported by the observation of a secondary kinetic isotope effect when deuteration—a widely recognized strategy for enhancing OLED material stability—was applied.^[8-10] In contrast to the significant performance enhancements observed in some OLEDs emitters with deuteration, our MR-TADF materials exhibited only modest stability improvements. We attribute this to the predominant role of C-C bond formation in driving material instability (Fig. 1b).

How can more stable molecules be incorporated into the design of MR-TADF structures? While this study does not primarily focus on molecular design, reviewer feedback prompted us to explore potential strategies. We propose that introducing substitutions at specific positions on the peripheral rings or incorporating steric hindrance could effectively prevent intramolecular C-C bond formation, offering a promising approach to improving stability. While further degradation studies and supporting evidence are needed, our work remains committed to uncovering new possibilities and mechanisms. We believe these findings, together with future contributions from other researchers, will ultimately provide a strong foundation for designing fully optimized and stable molecules.

REFERENCES:

[1] Kim, S., Bae, H. J., Park, S., Kim, W., Kim, J., Kim, J. S., Jung, Y., Sul, S., Ihn, S.-G., Noh, C., Kim, S. & You, Y. Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers. Nat. Commun. 9, 1211 (2019).
[2] Hwang, S., Moon, Y. K., Jang, H. J., Kim, S., Jeong, H., Lee, J. Y. & You, Y. Conformation-dependent degradation of thermally activated delayed fluorescence materials bearing cycloamino donors. Commun. Chem. 3, 53 (2020).
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[8] Kondakov, D. Y., Brown, C. T., Pawlik, T. D. & Jarikov, V. V. Chemical reactivity of aromatic hydrocarbons and operational degradation of organic light-​emitting diodes. J. Appl. Phys. 107, 024507 (2010).
[9] Abe, T., Miyazawa, A., Konno, H. & Kawanishi, Y. Deuteration isotope effect on nonradiative transition of fac-tris (2-phenylpyridinato) iridium (III) complexes. Chem. Phys. Lett. 491, 199-202 (2010).
[10] Jung, S., Cheung, W. -L., Li, S. -j., Wang, M., Li, W., Wang, C., Song, X., Wei, G., Song, Q., Chen, S. S., Cai, W., Ng, M., Tang, W. K. & Tang, M. -C. Enhancing operational stability of OLEDs based on subatomic modified thermally activated delayed fluorescence compounds. Nat. Commun. 14, 6481 (2023).