Blue Phosphorescent OLED Stability Study

Today organic light-emitting devices (OLEDs) are used in billions of displays generating $38 billion in annual revenue in 2021. In 1986 the demonstration of an efficient planar heterojunction device (1) started OLEDs on a path to commercialization. Further efficiency improvements were realized by utilizing a fluorescent guest and host emissive layer (EML) system (2). The internal quantum efficiency (IQE) of these early fluorescent OLEDs was limited to approximately 25% as triplet excitons cannot emit radiatively in these materials. In contrast, in a phosphorescent OLED (PHOLED) system, first demonstrated in 1998, all the singlet excited state excitons are converted into the triplet excited state through inter-system crossing (S₁ to T₁) via the presence of a heavy metal atom (3).<br/>Red and green PHOLEDS, pioneered by Universal Display Corporation (UDC), have been used in commercial OLED applications for several years now, however the device lifetime of high efficiency blue has until recently been a challenge. Here we will present a case study on a series of blue emitting imidazo-phenanthridine benzimidazole iridium (III) complexes, where an aromatic carbon is replaced with nitrogen on the ligand resulting in a dramatic blue shift in emission. The effect on device stability was probed by substituting aryl and alkyl groups on the ligand in close proximity to the nitrogen lone pair. Further modifications required to improve sublimation for the preparation of vacuum deposited devices through the use of ancillary ligands will also be described. Our results show that device stability is highly dependent on the type of substituent near the nitrogen, indicative of its reactivity in device operation. <br/> <br/><b>References</b><br/> <br/>Tang, C.W., VanSlyke, S. A. <i>Appl Phys Lett.</i>, 51 (12), 913–5 (1987).<br/>Tang, C.W., VanSlyke, S. A., Chen, C. H., <i>J. of Appl. Phys.</i>, 65 (9), 3610–3616 (1989).<br/>Baldo, M., A., O’Brien, D. F., You, Y., Shoustikov, A., Sibley, S., Thompson, M. E., Forrest, S. F., <i>Nature</i>, 395 (6698), 151–154 (1998).