8 October 2014 Surface plasmon polariton modification in top-emitting organic light-emitting diodes for enhanced light outcoupling
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We report on the enhanced light outcoupling efficiency of monochrome top-emitting organic light-emitting diodes (OLEDs). These OLEDs incorporate a hole transport layer (HTL) material with a substantially lower refractive index (∼ 1.5) than the emitter material or the standard HTL material (∼ 1.8) of a reference device. This low-index HTL is situated between the opaque bottom metal contact (anode) and the active emission layer. Compared to an HTL with common refractive index, the dispersion relation of the surface plasmon polariton (SPP) mode from the opaque metal contact is shifted to smaller in-plane wavenumbers. This shift enhances the outcoupling efficiency as it reduces the total dissipated power of the emitter. Furthermore, the excitation of the coupled SPPs at the thin transparent metal top contact (cathode) is avoided by using an ultrathin top electrode. Hence, the coupling of the electroluminescence from the emitter molecules to all non-radiative evanescent modes, with respect to the emitter material, is reduced by at least a factor of two, additionally increasing the outcoupling efficiency. Furthermore, for sufficiently high refractive index contrast the shift of the SPP at the anode/organic interface can lead to in-plane wavenumbers smaller than the wavenumber within the organic emitter layer and outcoupling of all excited modes by high index light extraction structures, e.g. microlens, seems feasible. In accordance to optical simulations, the external quantum efficiency is enhanced by about 20 % for monochrome green emitting OLEDs with low refractive index HTL compared to a reference sample.
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Cornelius Fuchs, Cornelius Fuchs, Tobias Schwab, Tobias Schwab, Martin Wieczorek, Martin Wieczorek, Malte C. Gather, Malte C. Gather, Simone Hofmann, Simone Hofmann, Karl Leo, Karl Leo, Reinhard Scholz, Reinhard Scholz, "Surface plasmon polariton modification in top-emitting organic light-emitting diodes for enhanced light outcoupling", Proc. SPIE 9183, Organic Light Emitting Materials and Devices XVIII, 91831Z (8 October 2014); doi: 10.1117/12.2061585; https://doi.org/10.1117/12.2061585

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