We report results obtained from modeling the light outcoupling efficiency of top-emitting organic light-emitting diode ͑OLED͒ 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. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1712036͔Organic light emitting diodes ͑OLEDs͒ are being commercially exploited, owing to the many appealing features they possess, notably the ease with which they may be fabricated. Conventional OLEDs are substrate emitting, with light emitted typically through a glass substrate. For display applications active matrix driven pixels are required and a good way to accomplish this is to make top-emitting OLEDs, i.e., for the emission to take place through the cathode. This would allow OLEDs to be incorporated onto a silicon substrate, thus enabling easier integration of light emitting and control components. Top-emitting OLEDs have recently been shown to offer an efficient way to generate light. 1-3Most aspects of OLED performance that impinge on device efficiency have now been optimized, but there is still scope for improving light outcoupling. A schematic of the structure we consider is shown in Fig. 1͑a͒ with the organic light-emitting layer sandwiched between a reflective anode and a composite metal/indium-tin-oxide ͑ITO͒ cathode. We model the conventional substrate-emitting OLED structure ͓Fig. 1͑b͔͒ for comparison.In order to optimize devices it is important to study the details concerning the amount of power lost to various decay channels. The decay of excitons within the emissive organic layer may result in outcoupled light, in power lost to guided modes, 4 including surface plasmon-polariton ͑SPP͒ modes, 5 or lost as heat to one of the electrodes. SPPs result from the coupling between the free charges at the surface of a metal and electromagnetic radiation. 6 This interaction leads to longitudinal surface charge density fluctuations that propagate along the interface combined with an oscillating EM field that decays exponentially away from the metallic surface.Here we investigate how the position of the emitters within the organic layer changes the strength of the coupling to different modes, and thus the optical efficiency of the device. To do this we make use of a specially adapted classical technique to calculate the power lost by an emissive dipole in a multilayered structure, 7 with the dipole field being represented by a sum of plane waves. Each plane wave is characterized by a different in-plane wave vector, k x , where k x is the component of the wave vector parallel to the interfaces. By calculating the power dissipated by the dipole as a function of k x we are a...
Results obtained from modeling the light out‐coupling efficiency of an organic light‐emitting diode (OLED) structure containing the recently developed first‐generation fac‐tris(2‐phenylpyridine) iridium‐cored dendrimer (Ir‐G1) as the emissive organic layer are reported. Comparison of the results obtained for this material with those of corresponding structures based upon small‐molecule and polymer emissive materials is made. The calculations of out‐coupling efficiency performed here take account of many factors, including the photoluminescence quantum yield (PLQY) of the emissive materials. Further, how each material system might perform with regard to out‐coupling efficiency when a range of possible PLQYs are considered is shown. The calculations show that the very high efficiency of dendrimer‐based OLEDs can be attributed primarily to their high PLQY.
We show that the transmission of light through metallic hole arrays supported by a glass substrate can be tuned by depositing a controlled number of LangmuirBlodgett layers on top of the hole array. Enhanced transmission is achieved when the number of overlayers is such that the surface plasmon-polariton modes on the two sides of the metal hole array have matched wavevectors. Dye molecules introduced into some of these overlayers allow us to explore the relationship between molecular fluorescence and the transmission properties of the structure, through measurement of the fluorescence lifetime of the molecules. We find there to be little change in the fluorescence lifetime between enhanced and non-enhanced transmission regimes and offer an explanation of our findings in terms of changes in the photonic mode density.
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