The use of light-emitting diodes as lighting source in different applications have rapidly increased over the past decades. In particular, organic light-emitting diodes are promising sustainable future generation displays owing to their flexibility, safety, less energy consumption, and long-life characteristics. To achieve full potential of organic LEDs, significant improvement of the light extraction efficiency of photons generated internally in the planar organic light-emitting diodes is highly desirable because nearly half of the emitted light is trapped as waveguide mode and surface plasmon polariton mode in the organic-indium tin oxide layer and at the organic-metal interface respectively. Among the methods employed to reduce the plasmonic loss and improve the extraction efficiency of waveguide modes, graded photonic super-crystals (GPSCs) have attracted remarkable research attention.
In a recent paper published in the Journal of the Optical Society of America B, researchers at the University of North Texas: Khadijah Alnasser (Ph.D. Student), Dr. Safaa Hassan, Steve Kamau (Ph.D. Student), and led by Professor Yuankun Lin in collaboration with Professor Hualiang Zhang at the University of Massachusetts Lowell explored the effect of using graded photonic super-crystals in improving light extraction from organic light-emitting diodes by minimizing plasmonic loss. Light extraction efficiency into glass substrates from OLED where the organic/Al interfaces were patterned with triangular, square, and octagonal graded photonic super-crystals were simulated and compared.
The research team observed that patterning the organic-Al interface with graded photonic super-crystals improved the light extraction efficiency into the glass substrate. The graded patterns functioned to destroy the plasmonic resonance conditions thus aiding the scattering of plasmon polaritons into the glass. Area fraction of the graded regions was identified as the main factor affecting the light extraction efficiency and an increase in the graded area resulted in a corresponding increase in extraction efficiency. For instance, area fractions of 53.9%, 78.5%, and 90.7% yield efficiencies of 68.5%, 72.9%, and 78.9% for octagonal, square, and triangular graded photonic super-crystals respectively. Furthermore, organic light-emitting diodes patterned with triangular graded photonic super-crystals exhibited an increase in the extraction efficiency above the targeted 70% due to the increase in efficiency in certain wavelength range.
In summary, Professor Lin and his colleagues successfully enhanced light extraction efficiency from organic light-emitting diodes by reducing plasmonic loss using graded photonic super-crystals. The mechanism behind the plasmonic loss was attributed to the decoupling of p-polarized surface plasmon polaritons into scattered light and destruction of the plasmonic standing waves. Light extraction efficiency increased at all wavelengths. Triangular graded photonic super-crystals with high-grade area fraction exhibited the highest light extraction efficiency into the glass substrates as compared to its square and octagonal counterparts. Having achieved an efficiency above the targeted 70%, the study marks a new dawn in the research of organic light-emitting diodes that will increase their use as efficient light sources in future generation displays.
Alnasser, K., Hassan, S., Kamau, S., Zhang, H., & Lin, Y. (2020). Enhanced light extraction from organic light-emitting diodes by reducing plasmonic loss through graded photonic super-crystals. Journal of the Optical Society of America B, 37(5), 1283.