Organic photovoltaic cell performance is limited in part by a short exciton diffusion length (LD). While state-of-the-art devices address this challenge using a morphology-optimized bulk heterojunction (BHJ), longer LD would relax domainsize constraints and enable higher efficiency in simple bilayer architectures. One approach to increase LD is to exploit long-lived triplet excitons in fluorescent materials. Though these states do not absorb light, they can be populated using a host-guest triplet-sensitized architecture. Photogenerated host singlets undergo energy transfer to a guest, which rapidly forms triplets that are transferred back to the long-lived host triplet state. Previous efforts have been focused on Pt- and Irbased guests. Here, a host-guest pairing of metal-free phthalocyanine (H2Pc) and copper phthalocyanine (CuPc) is explored, advantageous as the guest also has strong and complementary optical absorption. In optimized devices (20 vol.% CuPc), the short-circuit current is enhanced by 20%. To probe the origin of the enhancement, the exciton LD is measured using a device-based methodology that relies on fitting ratios of donor-to-acceptor internal quantum efficiency as a function of layer thickness. Compared with the neat H2Pc, the LD of the 20 vol.% CuPc doped layer increases from (8.5 ± 0.4) nm to (13.4 ± 1.6 nm), confirming the increased device current comes from enhanced exciton harvesting.
Efficiency roll-off and intrinsic luminance degradation are two of the primary limitations of organic light-emitting devices (OLEDs). While both phenomena have been studied separately in detail, they are rarely considered together. Previous analyses of OLED degradation have largely neglected changes in efficiency roll-off and bimolecular quenching, and the magnitude of these changes and their impact on device lifetime remains unclear. We present experimental and modeling results to quantify the magnitude of these changes, which we find range from ~2% to above 10% in magnitude and increase in importance at high brightness or in devices with significant exciton-exciton annihilation.
Studies of strong exciton-photon coupling in organic materials have progressed at a rapid pace since the first observation
of microcavity polaritons in tetra-(2,6-t-butyl)phenol-porphyrin zinc less than ten years ago. Current research is driven
by the potential for new optoelectronic devices based on polaritonic phenomena such as ultrafast optical amplifiers and
switches, enhanced nonlinear optical materials, and coherent light emitters, known as polariton lasers. This paper
reviews experimental advances related to strong coupling in thermally evaporated organic materials, and their potential
application in future optoelectronic devices.
A method was developed to measure hydrocarbons to 1 part-per-trillion (ppt) concentration levels with a gas chromatograph and flame ionization detector (GC/FID). This method was used to measure purifier siloxane removal efficiencies from air under dry and humid conditions. Several media types were examined: activated carbon (AC), bead-shaped activated carbon (BAC) and a proprietary inorganic material (PIM). Under dry conditions, all three materials removed the siloxane challenge to below 1ppt. The AC material had a removal efficiency of 286 ppt under humid conditions. The BAC and PIM removed the siloxane challenge to below 1 ppt under humid conditions. After media saturation was reached under humid conditions, the materials were regenerated and siloxane removal efficiencies were re-examined. Only the PIM material was regenerable to below 1ppt efficiency levels.
We demonstrate efficient (ηp=11±1 lm/W at 1000 cd/m2), bright electrophosphorescent white organic light emitting devices (WOLEDs) employing three dopants in a 9-nm-thick inert host matrix. The emissive layer consists of 2 wt.% iridium (III) bis(2-phenyl quinolyl-N,C2') acetylacetonate (PQIr), 0.5 wt.% fac-tris(2-phenylpyridine) iridium (Ir(ppy)3) and 20 wt.% bis(4’,6’-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) co-doped into a wide energy gap p-bis(triphenylsilyly)benzene (UGH2) host. Devices were characterized in terms relevant to both display and general lighting applications, and have a peak total power efficiency of 42±4 lm/W at low intensities, falling to 10±1 lm/W at a drive current of 20 mA/cm2 (corresponding to 1.4 lm/cm2 for an isotropic illumination source). The Commission Internationale de l’Eclairage coordinates shift from (0.43,45) at 0.1 mA/cm2 to (0.38,0.45) at 10 mA/cm2, and a color rendering index >75 is obtained. Three factors contribute to the high efficiency: thin layers leading to low voltage operation, a high quantum efficiency blue dopant, and efficient confinement of charge and excitons within the emissive region. The highest occupied and lowest unoccupied energy levels of component layers will be discussed to elucidate charge and exciton confinement within the emissive layer. Additionally, we will explain energy transfer between dopants based on photoluminescent transient analysis of triple-doped thin films.
Adsorption and desorption rates of a 6-component hydrocarbon mixture and SO2 have been studied on the surfaces of Ultra High Purity (UHP) components under the presence of parts-per-billion (ppb) contaminant levels. The dry-down rates are monitored to sub parts-per-trillion (ppt) levels. In the hydrocarbon test, stainless steel components are confirmed to be more effective than Teflon during dry-down. Dry-down rates for hydrocarbons on stainless steel (SS) surfaces depend on the molecular weight of the contaminant; heavier molecules take longer to dry-down. The dry-down study for SO2 revealed that it will desorb from Teflon surfaces quicker than it will desorb from stainless steel. The result of UHP valves tested for outgassing indicates that Extreme Clean Dry Air (XCDA) was able to remove hydrocarbons to lower levels and cleanup faster than with a N2 purge.