Light outcoupling efficiency of top-emitting organic light-emitting diodes

Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 84 (2004) and may be found at http://link.aip.org/link/?APPLAB/84/2986/1We report results obtained from modeling the light outcoupling efficiency of top–emitting organic light-emitting diode (OLED) structures and compare them with results from conventional substrate-emitting structures. We investigate two types of emissive material, small molecule and conjugated polymers, and study three different cathode materials; aluminum, silver, and calcium. We show that top-emitting OLEDs may have outcoupling efficiencies comparable to their substrate-emitting counterparts, and that the choice of cathode material is critical to the optical performance of the device

Coupled surface plasmon-polariton mediated photoluminescence from a top-emitting organic light-emitting structure

Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 85 (2004) and may be found at http://link.aip.org/link/?APPLAB/85/182/1We report strong photoluminescence from a top-emitting organic light-emitting structure where emission takes place through a thin (55 nm) silver film. We show that this emission is mediated via coupled surface plasmon-polariton modes. Our results show that the addition of a dielectric grating to otherwise planar structures, such as surface-emitting organic light-emitting diodes, may offer a way to increase the external efficiency of top-emitting organic light-emitting diodes

Efficiency of radiative emission from thin films of a light-emitting conjugated polymer

J. A. E. Wasey, A. Safonov, I. D. W. Samuel, and William L. Barnes, Physical Review B, Vol. 64, article 205201 (2001). "Copyright © 2001 by the American Physical Society."We examine the efficiency of radiative emission from thin layers of light-emitting conjugated polymers. We compare our experimental results for photoluminescence of the conjugated polymer poly(2-methoxy, 5-(2′-ethyl-hexyloxy) 1,4 phenylenevinylene) (MEH-PPV) with those of a theoretical model, finding good agreement between the two. The specially developed model takes into account several factors including absorption in the emissive layer, a spread of emitter sites within the layer, and the broad emission spectrum of the polymer. We find that the photoluminescence quantum efficiency for radiative emission of a bare MEH-PPV film on a glass substrate is ∼25%. We then apply our model to study electroluminescent devices. We show that for these structures the efficiency of radiative emission is ∼10%. There is thus potential for considerable improvement in efficiency for both systems through recovery of some of the wasted waveguided light. Finally we use our model to reexamine some controversial results that indicate the probability of singlet exciton formation to be 0.4±0.05, and thus greater than the 0.25 expected from spin statistics. Our reanalysis supports a probability >0.25. We conclude by discussing the limitations of present models, including our own, in predicting the performance of realistic light-emitting diodes

Newtype single-layer magnetic semiconductor in transition-metal dichalcogenides VX 2 (X = S, Se and Te)

We present a newtype 2-dimensional (2D) magnetic semiconductor based on transition-metal dichalcogenides VX2 (X = S, Se and Te) via first-principles calculations. The obtained indirect band gaps of monolayer VS2, VSe2, and VTe2 given from the generalized gradient approximation (GGA) are respectively 0.05, 0.22, and 0.20 eV, all with integer magnetic moments of 1.0 μB. The GGA plus on-site Coulomb interaction U (GGA + U) enhances the exchange splittings and raises the energy gap up to 0.38~0.65 eV. By adopting the GW approximation, we obtain converged G0W0 gaps of 1.3, 1.2, and 0.7 eV for VS2, VSe2, and VTe2 monolayers, respectively. They agree very well with our calculated HSE gaps of 1.1, 1.2, and 0.6 eV, respectively. The gap sizes as well as the metal-insulator transitions are tunable by applying the in-plane strain and/or changing the number of stacking layers. The Monte Carlo simulations illustrate very high Curie-temperatures of 292, 472, and 553 K for VS2, VSe2, and VTe2 monolayers, respectively. They are nearly or well beyond the room temperature. Combining the semiconducting energy gap, the 100% spin polarized valence and conduction bands, the room temperature TC, and the in-plane magnetic anisotropy together in a single layer VX2, this newtype 2D magnetic semiconductor shows great potential in future spintronics