We investigate the temperature dependence (from 16 K to 300 K) of an organic polariton laser, consisting of an amorphous organic semiconductor thin film in a planar microcavity. The polariton lasing threshold does not change between 45 K and room temperature, but increases when the temperature decreases below 45 K. This is accompanied by an energy relaxation bottleneck along the lower polariton branch at the lower temperature. This temperature dependence behavior is attributed to the competition between direct radiative pumping from exciton reservoir and phonon-induced relaxation at different temperature and explains the anomalously high thresholds in organic lasers.
We demonstrate a method for extracting waveguided light trapped in the organic and indium tin oxide layers of bottom emission organic light emitting devices (OLEDs) using a patterned planar grid layer (sub-anode grid) between the anode and the substrate. The scattering layer consists of two transparent materials with different refractive indices on a period sufficiently large to avoid diffraction and other unwanted wavelength-dependent effects. The position of the sub-anode grid outside of the OLED active region allows complete freedom in varying its dimensions and materials from which it is made without impacting the electrical characteristics of the device itself. Full wave electromagnetic simulation is used to study the efficiency dependence on refractive indices and geometric parameters of the grid. We show the fabrication process and characterization of OLEDs with two different grids: a buried sub-anode grid consisting of two dielectric materials, and an air sub-anode grid consisting of a dielectric material and gridline voids. Using a sub-anode grid, substrate plus air modes quantum efficiency of an OLED is enhanced from (33±2)% to (40±2)%, resulting in an increase in external quantum efficiency from (14±1)% to (18±1)%, with identical electrical characteristics to that of a conventional device. By varying the thickness of the electron transport layer (ETL) of sub-anode grid OLEDs, we find that all power launched into the waveguide modes is scattered into substrate. We also demonstrate a sub-anode grid combined with a thick ETL significantly reduces surface plasmon polaritons, and results in an increase in substrate plus air modes by a >50% compared with a conventional OLED. The wavelength, viewing angle and molecular orientational independence provided by this approach make this an attractive and general solution to the problem of extracting waveguided light and reducing plasmon losses in OLEDs.
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