Edward Wrzesniewski

White phosphorescent organic light-emitting devices with dual triple-doped emissive layers

We demonstrate high efficiency white organic light-emitting devices with two adjacent emissive layers each doped with three phosphorescent emitters (blue, green, and red). Efficient charge and exciton confinement is realized by employing charge transport layers with high triplet energy, leading to a maximum external quantum efficiency of (19±1)%. Using the p-i-n device structure, we have achieved a peak power efficiency of (40±2) lm/W and (36±2) lm/W at 100 cd/m2, a color rendering index of 79, and Commission Internationale de L’Eclairage coordinates of (0.37, 0.40) for the white light emission.

Enhancing Light Extraction in Top‐Emitting Organic Light‐Emitting Devices Using Molded Transparent Polymer Microlens Arrays

The light extraction efficiency in organic light-emitting devices (OLEDs) is enhanced by up to 2.6 times when a close-packed, hemispherical transparent polymer microlens array (MLA) is molded on the light-emitting surface of a top-emitting device. The microlens array helps to extract the waveguided optical emission in the organic layers and the transparent top electrode, and can be manufactured in large area with low cost.

Enhancing light harvesting in organic solar cells with pyramidal rear reflectors

We report enhanced light absorption in semi-transparent organic solar cells by using pyramidal rear reflectors to induce light trapping in the photoactive layer. Pyramidal rear reflectors with a base angle of 30° were molded from a transparent polymer on planar substrates. Compared with a planar rear reflector, the pyramidal structure leads to a more than 2.5 times longer path length in the active layer for the incident light. Experimental demonstration showed an 11%–75% enhancement in the photocurrent and overall efficiency of the solar cells, depending on the device size and active layer thickness.

Transparent oxide/metal/oxide trilayer electrode for use in top-emitting organic light-emitting diodes

The most commonly used transparent electrode, indium-tin oxide (ITO), is costly and requires methods of deposition that are highly destructive to organic materials when it is deposited on top of the organic layers in top-emitting organic light-emitting devices (OLEDs). Here we have employed a trilayer electrode structure consisting of a thin layer of metal sandwiched between two MoO3 layers, which can be deposited through vacuum thermal evaporation without much damage to the organic active layers. Such MoO3/Au/MoO3 trilayer electrodes have a maximum transmittance of nearly 90% at 600 nm and a sheet resistance of <10 ohms per square (Ω/sq) with a 10-nm thick Au intermediate layer. Using these trilayers as the top transparent anode, we have fabricated top-emitting OLEDs based on either a fluorescent or phosphorescent emitter, and observed nearly identical emission spectra and similar external quantum efficiencies as compared to the more conventional bottom-emitting OLEDs based on the commercial ITO anode. The power efficiency of the top-emitting devices is 20% to 30% lower than the bottom-emitting devices due to the somewhat inferior charge injection in the top-emitting devices. The performance and emission characteristics of these devices indicate that this trilayer structure is a promising candidate as a transparent anode in top-emitting OLEDs.

Printed Microlens Arrays for Enhancing Light Extraction From Organic Light-Emitting Devices

The light out-coupling efficiency of organic light-emitting devices was enhanced using microlens arrays fabricated by a direct printing technique. The high surface-free energy of a glass substrate was modified through the use of a hydrophobic silane coupling agent thus achieving a high contact angle for liquid droplets. A transparent monomer mixture of multifunctional thiol and ene was employed as a lens material. The light out-coupling efficiency was improved by 30% using printed microlens arrays without altering the electroluminescent spectrum.

Enhancing light extraction in organic light-emitting devices via hemispherical microlens arrays fabricated by soft lithography

We demonstrate enhanced light extraction in organic light-emitting devices (OLEDs) by using microlens arrays fabricated by a soft lithography technique. A large-area and close-packed polystyrene (PS) monolayer was formed on the SiO2 substrate using a convective-capillary assembly method, and a polydimethylsiloxane polymer was used to obtain a concave template, from which microlens arrays were fabricated from a photopolymerizable transparent optical adhesive. The microlens contact angle and array fill factor both depend on the size of PS microspheres, and nearly close-packed, hemispherical microlens arrays with microlens contact angle of (85 ± 5 deg) and array fill factor of (85 ± 3)% were obtained with 100-μm PS microspheres. The enhancement in the light-extraction efficiency in OLEDs when such fabricated microlens arrays were attached to the light-emitting surface depends on the contact angle of microlens, device size, and detailed multilayer structure of the OLED. For a large-area (12 × 12 mm) fluorescent OLED with a near close-packed hemispherical microlens array, a maximum enhancement of (70 ± 7)% in the light-extraction efficiency was achieved.

High Efficiency White Organic Light-Emitting Devices

We report two efficient white OLED structures with efficacy up to 100 lm/W. One structure involves using three different emitters in the same emitting layers, while the other integrates a blue-emitting OLED with down-conversion phosphors.