Michael Slootsky

Stacked white organic light emitting devices consisting of separate red, green, and blue elements

We demonstrate a white organic light-emitting device where individual red, green, and blue (R, G, and B) phosphorescent organic light-emitting devices are vertically stacked and electrically interconnected by a compound MoO3/Li-doped charge generation layer. For the order of B, G, and R cells positioned relative to the indium tin oxide anode, the device yields a peak total external quantum efficiency (EQE) and power efficiency (PE) of ηext=(36±2)% at a current density of J=82 μA/cm2 and ηp=21±1 lm/W at J=17 μA/cm2, respectively. The EQE and PE of the device roll off to (32±2)% and 13±1 lm/W at 1000 cd/m2, corresponding to J=2 mA/cm2. At this luminance, the device shows Commission Internationale de L’Eclairage chromaticity coordinates of (0.45, 0.36) and a color rendering index of 63.

Enhanced efficiency in high-brightness fluorescent organic light emitting diodes through triplet management

We demonstrate suppressed singlet-triplet (S-T) quenching, and hence increased quantum efficiency, in high-brightness fluorescent organic light emitting diodes (OLEDs) by reducing the guest triplet population through the introduction of a triplet manager molecule into the emission layer (EML). As an example, an OLED whose EML consists of the red fluorophore, 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran doped into the host, tris(8-hydroxyquinoline) Al (Alq3) is blended with the triplet manager, 9,10-di(naphtha-2-yl)anthracene. The manager triplet energy is less than that of the host or dopant, leading to efficient triplet removal from the dopant without affecting the radiative singlet population. Measurements suggest the complete suppression of S-T quenching using the triplet management strategy, leading to >100% increase in the steady-state OLED external quantum efficiency.

Enhancing waveguided light extraction in organic LEDs using an ultra-low-index grid

Improved outcoupling of light into substrate modes of an organic LED (OLED) by an ultra-low index of refraction (n=1.15), porous SiO(2) grid (UltraLIG) fabricated using glancing-angle deposition is demonstrated. Outcoupling into the substrate for electrophosphorescent tris(2-phenylpyridine) iridium [Ir(ppy)(3)]-based OLEDs grown on substrates with the UltraLIG is increased by 48% over a conventional device at a luminance of 100 cd/m(2). With efficient light outcoupling at the substrate-air interface, the UltraLIG devices attain eta(EQE)=22.5% and eta(P)=64 lm/W at their peak efficiencies, a nearly threefold increase over an analogous conventional OLED.

Full-wave simulation of enhanced outcoupling of organic light-emitting devices with an embedded low-index grid

Enhancement of light outcoupling into substrate modes by a grid of low-refractive-index material embedded into the organic layer of an organic light-emitting device (OLED) is analyzed using full-wave electromagnetic simulations. The low-index grid (LIG) redirects modes normally trapped within the high-index organic and indium tin oxide layers (waveguide modes) into the substrate where they can be further extracted into free space using methods such as microlens arrays or roughened surfaces. This increases the external quantum and power efficiencies without affecting the electroluminescent spectrum. The dependence on grid geometry, dimensions, and refractive index is explored to optimize the structure. Simulations show that up to 50% more light can be extracted from the high-index region using an ultralow-index (n=1.03) grid than a conventional device, and provided efficient substrate-to-air outcoupling, the external quantum efficiencies of LIG OLEDs can reach >50%.

Photochemical origins of burn-in degradation in small molecular weight organic photovoltaic cells

Using a combination of Fourier transform infrared (FTIR) spectroscopy and physics-based degradation models, we find that the early aging of small molecular weight organic photovoltaic (OPV) cells is due to photochemical degradation of the C60 acceptor layer. Planar and mixed boron subphthalocyanine chloride (SubPc)-donor/C60-acceptor heterojunctions show significant changes in their IR absorption spectra after aging under illumination, which we find is due to exciton-mediated photo-oligomerization of C60. The consequent decrease in exciton diffusion length in the C60 layer results from a decreased exciton lifetime for C60 oligomers (e.g. C120 and C180) compared to that of the monomer. The model describes the short-circuit current reduction in planar SubPc/C60 OPV cells during early aging, and explains the lack of degradation in analogous mixed heterojunctions.

ELECTROPHOSPHORESCENT ORGANIC LIGHT EMITTING CONCENTRATOR

We demonstrate threefold directional light concentration from an organic light-emitting diode luminaire for use in spot lighting and other applications where high intensity illumination is required. The concentrating luminaire comprises four triangular, large-area green electrophosphorescent organic light emitting diodes (PHOLEDs) deposited on plastic substrates and assembled into a pyramidal structure with an open base that serves as the light exit aperture. The PHOLED surfaces are highly reflective to direct the emission from the devices to the aperture independent of the original emission position within the pyramid. The far-field intensity profile of the concentrator has a ‘batwing’ distribution that meets the requirements for general lighting for uniform illumination of planar surfaces. Light: Science & Applications (2014) 3, e181; doi:10.1038/lsa.2014.62; published online 20 June 2014

Enhanced light extraction from organic light-emitting devices using a sub-anode grid (Presentation Recording)

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.

Enhancing optical nonlinearity through engineered exciton coupling in organic-inorganic nanocomposites

We show enhancement in optical nonlinear absorption by engineering the coupling between excitons of 3,4,7,8-naphthalenetetracarboxylic dianhydride (NTCDA) and ZnO nanowires. Energy transfer between the excitonic systems and exciton scattering are found to be competing processes.

Formation of hybrid polaritons in an organic-inorganic microcavity at room temperature

We demonstrate hybridization of organic and inorganic excitons via strong coupling to a common microcavity mode. The system consists of 3,4,7,8-napthalenetetracarboxylic dianhydride (NTCDA) and ZnO nanocrystals embedded in a dielectric microcavity held at room temperature.

Triplet management in organic light emitting diodes and lasers

By doping a molecule with low triplet energy in the active regions of organic light emitting diodes and lasers, we demonstrate suppressed triplet-induced losses, thus overcoming a significant obstacle to electrically pumped organic lasers.