Browsing by Author "Liu, S. W."

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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
Graded-host phosphorescent light-emitting diodes with high efficiency and reduced roll-off
(A I P Publishing LLC, 2012) Liu, S. W.; Sun, X. W.; Demir, Hilmi Volkan
We demonstrated graded-host phosphorescent organic light-emitting diodes with high efficiency and reduced efficiency roll-off. The emissive layer of the graded host device consists of both electron and hole transport type hosts, 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and 4,4-,4- tris(Ncarbazolyl) triphenylamine, respectively, with graded composition, and the phosphorescent red emitter bis(2-phenylquinoline) (acetylacetonate) iridium(III), which wasuniformly doped into the graded hostmatrix. The graded host device shows improved quantum efficiency and power efficiency with significantly reduced efficiency rolloff as compared to the unipolar-host and double layer heterojunction host devices.
Open Access
Improved performance of organic light-emitting diodes with MoO3 interlayer by oblique angle deposition
(Optical Society of America, 2011) Liu, S. W.; Divayana, Y.; Sun, X. W.; Wang, Y.; Leck, K. S.; Demir, Hilmi Volkan
We fabricated and demonstrated improved organic light emitting diodes (OLEDs) in a thin film architecture of indium tin oxide (ITO)/molybdenum trioxide (MoO3) (20 nm)/ N,N'-Di(naphth-2-yl)-N,N'-diphenyl-benzidine (NPB) (50 nm)/tris-(8-hydroxyquinoline) (Alq(3)) (70 nm)/Mg:Ag (200 nm) using an oblique angle deposition technique by which MoO3 was deposited at oblique angles (theta) with respect to the surface normal. It was found that, without sacrificing the power efficiency of the device, the device current efficiency and external quantum efficiency were significantly enhanced at an oblique deposition angle of theta = 60 degrees for MoO3. (C) 2011 Optical Society of America
Open Access
On the triplet distribution and its effect on an improved phosphorescent organic light-emitting diode
(AIP Publishing, 2012-08-28) Liu, S. W.; Divayana, Y.; Abiyasa, A. P.; Tan S.T.; Demir, Hilmi Volkan; Sun, X. W.
We reported phosphorescent organic light-emitting diodes with internal quantum efficiency near 100% with significantly reduced efficiency roll-off. It was found that the use of different hole transporting layer (HTL) affects the exciton distribution in the emission region significantly. Our best device reaches external quantum efficiency (EQE), current, and power efficiency of 22.8% +/- 0.1%, 78.6 +/- 0.2 cd/A, 85 +/- 2 lm/W, respectively, with half current of 158.2 mA/cm(2). This considerably outperforms the control device with N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) (HTL) and 4,4'-N,N'-dicarbazole-biphenyl (host) with maximum EQE, current and power efficiency of 19.1% +/- 0.1%, 65.6 +/- 0.3 cd/A, 67 +/- 2 lm/W, respectively, with half current of only 8.1 mA/cm(2).