Browsing by Author "Dev, K."

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Open Access
An efficient non-Lambertian organic light-emitting diode using imprinted submicron-size zinc oxide pillar arrays
(AIP, 2013) Liu, S. W.; Wang, J. X.; Divayana, Y.; Dev, K.; Tan S.T.; Demir, Hilmi Volkan; Sun, X. W.
We report phosphorescent organic light-emitting diodes with a substantially improved light outcoupling efficiency and a wider angular distribution through applying a layer of zinc oxide periodic nanopillar arrays by pattern replication in non-wetting templates technique. The devices exhibited the peak emission intensity at an emission angle of 40° compared to 0° for reference device using bare ITO-glass. The best device showed a peak luminance efficiency of 95.5 ± 1.5 cd/A at 0° emission (external quantum efficiency - EQE of 38.5 ± 0.1%, power efficiency of 127 ± 1 lm/W), compared to that of the reference device, which has a peak luminance efficiency of 68.0 ± 1.4 cd/A (EQE of 22.0 ± 0.1%, power efficiency of 72 ± 1 lm/W). © 2013 American Institute of Physics.
Open Access
Highly flexible, electrically driven, top-emitting, quantum dot light-emitting stickers
(American Chemical Society, 2014) Yang X.; Mutlugun, E.; Dang, C.; Dev, K.; Gao, Y.; Tan, S.T.; Sun X.W.; Demir, Hilmi Volkan
Flexible information displays are key elements in future optoelectronic devices. Quantum dot light-emitting diodes (QLEDs) with advantages in color quality, stability, and cost-effectiveness are emerging as a candidate for single-material, full color light sources. Despite the recent advances in QLED technology, making high-performance flexible QLEDs still remains a big challenge due to limited choices of proper materials and device architectures as well as poor mechanical stability. Here, we show highly efficient, large-area QLED tapes emitting in red, green, and blue (RGB) colors with top-emitting design and polyimide tapes as flexible substrates. The brightness and quantum efficiency are 20 000 cd/m2 and 4.03%, respectively, the highest values reported for flexible QLEDs. Besides the excellent electroluminescence performance, these QLED films are highly flexible and mechanically robust to use as electrically driven light-emitting stickers by placing on or removing from any curved surface, facilitating versatile LED applications. Our QLED tapes present a step toward practical quantum dot based platforms for high-performance flexible displays and solid-state lighting. © 2014 American Chemical Society.
Open Access
Light extraction efficiency enhancement of colloidal quantum dot light-emitting diodes using large-scale nanopillar arrays
(Wiley-VCH Verlag, 2014) Yang, X.; Dev, K.; Wang, J.; Mutlugun, E.; Dang, C.; Zhao Y.; Liu, S.; Tang, Y.; Tan S.T.; Sun, X. W.; Demir, Hilmi Volkan
A colloidal quantum dot light-emitting diode (QLED) is reported with substantially enhanced light extraction efficiency by applying a layer of large-scale, low-cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, efficient method of non-wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current efficiency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum efficiency (EQE) of 9.34% reaches the highest EQE value for green-emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light-extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the first account for QLEDs offering detailed insight into the light extraction efficiency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high-performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.
Open Access
Low-cost, large-scale, ordered ZnO nanopillar arrays for light extraction efficiency enhancement in quantum dot light-emitting diodes
(IEEE, 2014) Yang, X.; Dev, K.; Wang, J.; Mutlugün, E.; Dang, C.; Zhao, Y.; Tan, S. T.; Sun, X. W.; Demir, Hilmi Volkan
We report a QLED with enhanced light outcoupling efficiency by applying a layer of periodic ZnO nanopillar arrays. The resulting QLED reaches the record external quantum efficiency (EQE) of 9.34% in green-emitting QLEDs with a similar device structure.
Open Access
Room-temperature larger-scale highly ordered nanorod imprints of ZnO film
(Optical Society of American (OSA), 2013) Kyaw, Z.; Wang J.; Dev, K.; Tiam Tan, S.; Ju, Z.; Zhang, Z.-H.; Ji, Y.; Hasanov, N.; Liu W.; Sun X.W.; Demir, Hilmi Volkan
Room-temperature large-scale highly ordered nanorod-patterned ZnO films directly integrated on III-nitride light-emitting diodes (LEDs) are proposed and demonstrated via low-cost modified nanoimprinting, avoiding a high-temperature process. with a 600 nm pitch on top of a critical 200 nm thick Imprinting ZnO nanorods of 200 nm in diameter and 200 nm in height continuous ZnO wetting layer, the light output power of the resulting integrated ZnO-nanorod-film/semi- transparent metal/GaN/InGaN LED shows a two-fold enhancement (100% light extraction efficiency improvement) at the injection current of 150 mA, in comparison with the conventional LED without the imprint film. The increased optical output is well explained by the enhanced light scattering and outcoupling of the ZnOrod structures along with the wetting film, as verified by the numerical simulations. The wetting layer is found to be essential for better impedance matching. The current-voltage characteristics and electroluminescence measurements confirm that there is no noticeable change in the electrical or spectral properties of the final LEDs after ZnO-nanorod film integration. These results suggest that the low-cost high-quality large-scale ZnOnanorod imprints hold great promise for superior LED light extraction. ©2013 Optical Society of America.