Browsing by Subject "Phosphorescence"

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    Colloidal quantum dot light-emitting diodes employing phosphorescent small organic molecules as efficient exciton harvesters
    (American Chemical Society, 2014) Mutlugun, E.; Guzelturk, B.; Abiyasa, A. P.; Gao, Y.; Sun X. W.; Demir, Hilmi Volkan
    Nonradiative energy transfer (NRET) is an alternative excitation mechanism in colloidal quantum dot (QD) based electroluminescent devices (QLEDs). Here, we develop hybrid highly spectrally pure QLEDs that facilitate energy transfer pumping via NRET from a phosphorescent small organic molecule-codoped charge transport layer to the adjacent QDs. A partially codoped exciton funnelling electron transport layer is proposed and optimized for enhanced QLED performance while exhibiting very high color purity of 99%. These energy transfer pumped hybrid QLEDs demonstrate a 6-fold enhancement factor in the external quantum efficiency over the conventional QLED structure, in which energy transfer pumping is intrinsically weak.
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    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.
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    Singlet and Triplet Exciton Harvesting in the Thin Films of Colloidal Quantum Dots Interfacing Phosphorescent Small Organic Molecules
    (American Chemical Society, 2014) Guzelturk, B.; Hernandez Martinez P.L.; Zhao, D.; Sun X.W.; Demir, Hilmi Volkan
    Efficient nonradiative energy transfer is reported in an inorganic/organic thin film that consists of a CdSe/ZnS core/shell colloidal quantum dot (QD) layer interfaced with a phosphorescent small organic molecule (FIrpic) codoped fluorescent host (TCTA) layer. The nonradiative energy transfer in these thin films is revealed to have a cascaded energy transfer nature: first from the fluorescent host TCTA to phosphorescent FIrpic and then to QDs. The nonradiative energy transfer in these films enables very efficient singlet and triplet state harvesting by the QDs with a concomitant fluorescence enhancement factor up to 2.5-fold, while overall nonradiative energy transfer efficiency is as high as 95%. The experimental results are successfully supported by the theoretical energy transfer model developed here, which considers exciton diffusion assisted Förster-type near-field dipole-dipole coupling within the hybrid films. © 2014 American Chemical Society.