In the ever-evolving landscape of display technologies, OLEDs have emerged as frontrunners, offering unparalleled advantages such as high efficiency, wide viewing angles, improved contrast, faster refresh rates, and the capability for flexible and foldable displays. Central to these advancements are the emitters – the molecules that emit light when electrically excited. A recent study by researchers at the University of St Andrews in Scotland, and the University of Mons=Hainaut in Belgium, delves into the potential of acenes as acceptors in thermally activated delayed fluorescence (TADF) emitters, particularly spotlighting a phenoxazine-naphthalene emitter for OLED applications.
Thermally activated delayed fluorescence (TADF) is a mechanism that allows for the harvesting of both singlet and triplet excitons – essential for high-efficiency OLEDs. Excitons are excited states formed when electrons and holes recombine. In traditional fluorescent emitters, only singlet excitons contribute to light emission, capping the efficiency. TADF overcomes this limitation by enabling the conversion of triplet excitons into singlet excitons, which can then emit light, thereby harnessing 100% of the excitons generated. This principle sets the foundation for designing emitters that are not only efficient but also free from expensive and scarce heavy metals like platinum and iridium, commonly used in phosphorescent emitters.
The research presents a naphthalene-based TADF emitter, 14-PXZ-Nap-PXZ, which exhibits promising photophysical properties, including a photoluminescence quantum yield of 48% and a delayed lifetime of 22.7 ms in a doped film. These characteristics are critical for OLED performance, with the quantum yield indicating the efficiency of light emission and the delayed lifetime signifying the duration of light emission after excitation.
The researchers employed density functional theory (DFT) computations to analyze 36 naphthalene-based compounds, identifying four with promising TADF traits. These compounds exhibit small singlet-triplet splitting energy (ΔEST), crucial for efficient TADF. The study also explored the synthesis, crystal structures, electrochemical properties, absorption, and photoluminescence of these emitters, providing comprehensive insights into their potential in OLED applications.
The significance of these findings is multifold. Firstly, the research underscores the potential of naphthalene, a linear acene, as an efficient acceptor in TADF emitters. By demonstrating that naphthalene can effectively function in this capacity, the study opens new avenues for exploring other acenes in OLED emitter design. Secondly, moving away from heavy metal-based emitters to organic materials like naphthalene could significantly reduce the cost and environmental impact of OLED production. Thirdly, the phenoxazine-naphthalene emitter showcased in this study, with its high photoluminescence quantum yield and extended delayed lifetime, holds the promise for the development of highly efficient OLEDs. Lastly, the insights gained from this study can be leveraged in a wide range of applications beyond display technologies, including lighting and photonic devices.
While the study marks a significant advancement, several challenges remain. The efficiency roll-off at higher luminance values and the need for further optimization underline the complexities involved in translating these findings into commercial applications. Future research should focus on enhancing the stability and scalability of these emitters and exploring other acenes that might offer similar or superior properties.
Lee, O., Sharma, N., Slawin, A., Olivier, Y., Samuel, I., Gather, M., & Zysman-Colman, E. (2023). Evaluation of acenes as potential acceptors in thermally activated delayed fluorescence emitters and the promise of a phenoxazine-naphthalene emitter for OLEDs. ChemRxiv. https://doi.org/10.26434/chemrxiv-2023-gpxww