Emerging lighting and display technologies use phosphorescent organic light-emitting diodes (Ph-OLEDs) because they are thinner, more flexible, and less pixelated than their inorganic LED counterparts. While Ph-OLEDs can have an internal quantum efficiency of 100%, on metal electrodes the light-extraction efficiency is 5-30% primarily due to coupling to surface plasmon polariton (SPP) modes and photonic waveguide modes, SPPs, accounting for up to 50% of the loss in light-extraction efficiency. In addition to low light-extraction efficiency, efficiency roll-off in Ph-OLEDs is a significant cause of device degradation at high luminance and is due to triplet-polaron and triplet-triplet quenching processes. One way to address the efficiency roll-off issue is to accelerate the radiative decay rate of phosphorescence to reduce triplet quenching processes. Further, efficiency roll-off in blue Ph-OLEDs is very pronounced due to high triplet energies and significant triplet-polaron and triplet-triplet quenching relative to red and green Ph-OLED counterparts.
This study aims to experimentally investigate the use of silver plasmonic nanostructured films with blue organic phosphorescent films to increase the radiative decay rate of triplet emission and, therefore, to minimize triplet quenching processes that cause unstable emission. We use the host poly(N-vinylcarbazole) (PVK) with the blue phosphorescent dopant, bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) which is commonly-used in blue Ph-OLED prototypes. This host-dopant combination has been shown to improve light out coupling and enhance triplet excitation because the host assists in charge transport and excitation energy transfer, while the dopant provides color and increases intersystem crossing which improves the internal quantum efficiency. PVK:FIrpic thin film samples are spin coated onto planar silver, grating (1.6µm and 0.7µm), nanoporous (NPO) silver, and nanoparticle (NPT) silver. In addition PVK, FIrpic, and PVK:FIrpic thin films on glass are used as a reference. The silver plasmonic nanostructures are chosen due to their ability to increase light emission through light scattering. Each silver plasmonic nanostructure is prepared with 50 nm of silver using nanoimprint lithography deposition for grating (1.6µm and 0.7µm) and dewetting deposition for NPO and NPT. The samples are characterized using photoluminescence (PL) stability, PL lifetime, and PL quantum yield measurements to investigate the relationship between silver plasmonic nanostructures and improved phosphorescence stability. Preliminary data has shown a correlation between enhanced PL stability and PL lifetime of silver plasmonic nanostructures relative to a planar silver.
Catrice Carter, Zeqing Shen, Kun Zhu, Kelsey Gwynne, and Deirdre M. O'Carroll, "Photoluminescence stability of blue organic phosphorescent materials on plasmonic silver nanostructure architectures (Conference Presentation)," Proc. SPIE 10529, Organic Photonic Materials and Devices XX, 1052907 (Presented at SPIE OPTO: January 31, 2018; Published: 15 March 2018); https://doi.org/10.1117/12.2288864.5752213019001.
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