Spontaneous orientation polarization (SOP) can lead to interfacial charge accumulation and exciton-polaron quenching in OLEDs. Well-considered is the case of electron-transport layer (ETL) SOP which results in accumulation at the emissive layer (EML)-ETL interface. This work considers two device architecture driven approaches to engineer accumulation and reduce quenching. First, the impact of EML SOP is considered using a polar host material. Second, addition of a blocking layer at the hole-transport layer-EML interface is demonstrated as a means to tune charge accumulation. These results underscore the importance of SOP in OLEDs while offering architecture-based methods for altering associated charge dynamics.
The degree of strong exciton-photon coupling in organic microcavities is continuously tuned by engineering molecular transition dipole moment (TDM) orientation through the choice of thin film processing conditions. Microcavities based on amorphous organic thin films of 4, 4’-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) achieve ultrastrong coupling in a metal reflector microcavity with a Rabi splitting greater than 1.0 eV. By fabricating BSB-Cz optical microcavities as a function of substrate temperature during deposition, a ~20% variation in Rabi splitting is realized. This study adds a new axis for control over the strength of the exciton-photon interaction and polariton formation.
Polar organic semiconducting molecules can exhibit a preferential alignment of their permanent dipole moments, inducing a giant surface potential via spontaneous orientation polarization (SOP). Previous work has shown how blends of polar and non-polar molecules offer one approach to either engineer SOP and its deleterious effect on OLED efficiency. Here, the emphasis is instead on composite films where both components exhibit SOP. In these systems, the observed SOP follows a linear combination of neat film behaviors. This study expands the avenues for control over SOP via molecular blending that can be further exploited for applications.
An optimization scheme to minimize angular color shifts in OLEDs is developed using a combination of optical simulations and experimental measurements of device performance. This minimization does not compromise other critical device operation parameters, such as efficiency and the angular intensity profile. By considering both bottom- and top-emitting OLEDs, this study utilizes strong feedback between simulation and experiment to identify stack architectures that have a minimum color change with viewing angle while still maintaining high power efficiency.
We demonstrate efficient solar cells based on an active layer of methylammonium lead iodide (MAPbI3) co-deposited via vapor transport deposition (VTD). In VTD organic and metal-halide precursor vapors are transported by carrier gas to a cooled substrate where film formation occurs. Our system design enables control over a range of process parameters allowing co-deposition and composition tuning of the final perovskite film. The impact of VTD processing conditions on film composition and solar cell performance will be discussed, with emphasis on tuning the lead iodide content in MAPbI3 films to realize power conversion efficiencies >10%.
Spontaneous orientation polarization (SOP) reflects preferential molecular alignment leading to vertical orientation of permanent electric dipole moments (PDMs) and an overall polarization field. In OLEDs, SOP can induce carrier accumulation in the emissive layer, leading to exciton-polaron quenching (even at low-bias) that limits performance. We examine methods to engineer PDM alignment and mitigate the impact of SOP. We consider the behavior of blends of polar materials and the role of thin film processing conditions. The impact of deposition rate and substrate temperature are considered within a rate-temperature-superposition framework, providing a quantitative tool to predict SOP and its impact on OLEDs.
Preferential alignment of molecular permanent dipole moments, known as spontaneous orientation polarization (SOP), is present in many materials employed in the active layers of organic light-emitting devices (OLEDs). This phenomenon leads to the formation of bound polarization charge, which is compensated by polaron accumulation at voltages below turn-on. While most prior work has focused on polarization in the device electron transport layer (ETL), here we examine the impact of emissive layer SOP by systematically probing polaron accumulation and exciton-polaron quenching in phosphorescent OLEDs. To gain a deeper understanding of polaron accumulation, device capacitance is systematically probed as a function of voltage across samples with polar and nonpolar emissive layers. We find that capacitance measurements can be used to track not only the number of accumulated charges, but also its location within the device active layers. This study provides an analysis framework that allows further insights on the charge accumulation process in OLEDs, thus improving our understanding of SOP in OLEDs
Amorphous films with preferential molecular alignment can exhibit spontaneous orientation polarization (SOP). In OLEDs, SOP is frequently observed in the electron-transport layer (ETL), leading to exciton-polaron quneching and peak efficiency loss. We find that the efficiency reduction scales directly with the degree of ETL SOP. We also show that quenching can be tuned by mixing the polar ETL with a non-polar host. Finally, we demonstrate how film processing conditions can serve as an additional axis of control, with elevated substrate temperature and reduced deposition rate during film growth leading to more isotropic molecular orientation, and a reduction in SOP-induced quenching.
Much effort has been directed at understanding organic light-emitting device (OLED) efficiency and the role of bimolecular quenching in efficiency roll-off. Quenching is less widely discussed at low-bias, where populations are reduced. Here, we describe lock-in-based photoluminescence measurements on working phosphorescent OLEDs to demonstrate that this assumption is not generally valid, and that significant exciton-polaron quenching is present even prior to turn-on. Exciton-polaron quenching arises with holes accumulated due to spontaneous orientation polarization in the electron transport layer. This low-bias quenching is found to strongly determine maximum achievable efficiency, suggesting a need to refine materials selection and device design rules.
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.
KEYWORDS: Nitrogen, Adsorption, Microsoft Foundation Class Library, Optical lithography, Gases, Oxygen, Sulfur, Contamination, Deep ultraviolet, Natural surfaces
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.
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