General illumination consumes 22% of the electricity generated in the U.S. This huge proportion is partly due to the ubiquity of artificial lighting but also the inefficiency of converting electrical energy to light. Incandescent lightbulbs convert a mere 5% of the supplied power into light (most of the rest emerging as heat) whereas the more efficient fluorescent bulbs achieve about 20% efficiency. Improving the efficiency of these light sources is difficult since in all cases the emission of light is essentially a byproduct of an energetic excitation process. In contrast, solid state lighting utilizes materials which directly convert electrical energy to light with little production of heat and therefore have the potential for far higher efficiency, with over 70% demonstrated in the infrared. New materials based on direct bandgap semiconductors and organic light emitters may permit this level of efficiency for general lighting. In both cases, however, understanding the nanoscale structure of the material is critical to achieving high efficiency. This is particularly evident in the case of organic molecular compounds, where weak inter-molecular interactions can permit the photophysical properties of a solid to be tuned by changing the chemical structure of the molecular building block.

We report the synthesis and characterization of 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluoro-7,7,8,8-tetracyanoquinodimethane (F3TCNQ-Ad1), a substituted analog of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), designed for p-type conductivity doping. The dopant is designed as a model for substituted alternatives to F4TCNQ that maintain similar electronic properties with the goal of engineering dopants with superior fabrication characteristics over F4TCNQ. We describe the design strategy for F3TCNQ-Ad1 based on molecular modeling predictions that substitution of a single fluorine atom of F4TCNQ has little effect on the electronic properties of the molecule. Photophysical and electrochemical characterization reveal that the adamantyl substituent in F3TCNQ-Ad1 does not significantly alter the electronic properties of the substituted dopant relative to F4TCNQ. Unfortunately, F3TCNQ-Ad1 degrades under standard sublimation conditions, preventing sublimation deposition processing. Instead, hole-only devices were made via solution-processing of the p-doped films with the structure glass/ITO/2.3 x 10(3) angstrom PVK:(MTDATA:dopant)/2.0 x 10(2) angstrom Au/1.0 x 10(3) angstrom Al, where dopant is either F4TCNQ or F3TCNQ-Ad1. We demonstrate that F3TCNQ-Ad1 increased the conductivity of the films by at least 1000 times compared to an undoped device.

It has been demonstrated that the oxidative doping of a.oo-diphenylpolyenes generates polaronic and/or bipolaronic intermediates which can be directly observed by optical spectroscopy. Furthermore, the absorption frequencies have been shown to correlate with conjugation length and the nature of the substituents. Species are stabilized by donor substituents and destabilized by acceptor substituents. We now report conductivity and ESR studies on iodine doped α,ω-diphenylpolyenes, containing donor p,p’-disubstituents. The implications of these studies can provide insight into electrical conductivity and optical nonlinearity of doped organic materials.

Data from a series of phosphorescent blue organic light-emitting devices with emissive layers consisting of either 4,4'-bis(N-carbazolyl)-2,2'-biphenyl (CBP):6% bis[(4,6-difluorophenyl) pyridinato-N,C-2](picolinato) iridium(III) (Flrpic) or bis(9-carbazolyl) benzene (mCP):6% Flrpic show that the triplet energy of the hole and electron transport layers can have a larger influence on the external quantum efficiency of an operating device than the triplet energy of the host material. A maximum external quantum efficiency of 14% was obtained for CBP: 6% Flrpic devices which is nearly double all other published CBP: 6% Flrpic results. A new host material, 4-(diphenylphosphoryl)-N,N-di-p-tolylaniline (DHM-A2), which has a triplet energy lower than that of Flrpic is also reported. Devices fabricated using DHM-A2 show improved performance (lower drive voltage and higher external quantum efficiency) over devices using 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), a high performance ambipolar DHM-A2 analogue with a triplet energy greater than Flrpic. Nearly 18% external quantum efficiency was obtained for the DHM-A2:5% Flrpic devices. The results suggest modified design rules for the development of high performance host materials: more focus can be placed on molecular structures that provide good charge transport (ambipolarity for charge balance) and good molecular stability (for long lifetimes) rather than first focusing on the triplet energy of the host material.

We report on a joint theoretical and experimental investigation of the electronic structure of a series of bis(diphenylphosphine oxide) derivatives containing a central aromatic core with high triplet energy. Such molecules can serve as host material in the emissive layer of blue electro-phosphorescent organic devices. The aromatic cores considered in the theoretical study consist of biphenyl, fluorene, dibenzofuran, dibenzothiophene, dibenzothiophenesulfone, or carbazole, linked to the two phosphoryl groups in either para or meta positions. With respect to the isolated core molecules, it is found that addition of the diphenylphosphine oxide moieties has hardly any impact on the core geometry and only slightly reduces the energy of the lowest triplet state (by, at most, similar to 0.2 eV). However, the diphenylphosphine oxide functionalities significantly impact the ionization potential and electron affinity values, in it way that is different for para and meta Substitutions. Excellent comparison is obtained between the experimental UPS and IPES spectra of the para biphenyl and meta dibenzothiophene and dibenzothiophenesulfone compounds and the simulated spectra. In general, the phosphine oxide derivatives present triplet energies that are calculated to be at least 0.2 eV higher than those of currently widely used blue phosphorescent emitters.

Conductivity doping of charge transporting layers is becoming increasingly attractive for improving power efficiency in OLEDs. However, the number of commercially available organic molecular p-dopants is limited. The electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) is the most utilized p-dopant. F4-TCNQ can be used as a dopant for most hole transporting materials (HTM), but it is very volatile, which makes it difficult for vacuum processing, and has a low sticking coefficient. Here we present the design of novel anchored molecular dopants based on the TCNQ core. We first review how the reduction potential of TCNQ core is affected by substitution with alkyl groups of different electronic properties. Electron donating groups have negative effect on the reduction potential of the acceptor. However, attaching electron withdrawing groups such as halogens counteracts the effect of electron donating groups. Using gas phase theoretical calculations we determined that trifluorinated TCNQ can be anchored through a σ-coupled alkyl chain to an inert molecular anchor without sacrificing the electron affinity.

The efficiency and stability of blue organic light emitting devices (OLEDs) continue to be a primary roadblock to developing organic solid state white lighting. For OLEDs to meet the high power conversion efficiency goal, they will require both close to 100% internal quantum efficiency and low operating voltage in a white light emitting device.1 It is generally accepted that such high quantum efficiency, can only be achieved with the use of organometallic phosphor doped OLEDs. Blue OLEDs are particularly important for solid state lighting. The simplest (and therefore likely the lowest cost) method of generating white light is to down convert part of the emission from a blue light source with a system of external phosphors.2 A second method of generating white light requires the superposition of the light from red, green and blue OLEDs in the correct ratio. Either of these two methods (and indeed any method of generating white light with a high color rendering index) critically depends on a high efficiency blue light component.3

We report blue phosphorescent organic light-emitting devices (OLEDs) using an ambipolar host, N-(4-diphenylphosphoryl phenyl) carbazole (MPO12), doped with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). The external quantum efficiency and operating voltage is 9.1(±0.1)% and 4.8V, respectively, measured at a brightness of 800cd∕m2 with no outcoupling enhancement. By varying the layer structure of the OLEDs, we show that MPO12 is capable of transporting both electrons and holes, in contrast to previous demonstrations using diphosphine oxides, which only transported electrons. The improved hole transport results in improved device efficiency.

We show that the inductive electron-withdrawing effect of diphenylphosphoryl (Ph2P=O) groups lowers both the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of a carbazole chromophore. This improves electron injection from a cathode without affecting the high triplet exciton energy (E-T approximate to 3.0 eV) of the host material. Three new carbazole derivatives, 3,6-bis(diphenylphosphoryl)-9-ethylcarbazole (PO 10), 3,6-bis(diphenylphosphoryl)-9-phenylcarbazole (PO9), and N-(4-diphenylphosphoryl phenyl) carbazole (MPO12), were investigated as host materials in blue phosphor-doped organic light-emitting devices (OLEDs). Photophysical characterization showed all three carbazole derivatives exhibit monomer UV fluorescence (367-385 nm) in solution and contributions from molecular aggregates or excimers in solid-state films (378-395 nm). The polar MPO12 derivative exhibited solvatochromism and had the highest propensity for aggregate formation in the solid state. Testing of OLEDs using PO9, PO10, and MPO12 as host materials for the sky blue organometallic phosphor iridium(III) bis(4,6-(difluorophenyl)-pyridinato-N,C-2') picolinate (FIrpic) gave external quantum efficiencies (EQE) and operating voltages at a similar current density (J = 13 mA/cm(2)) of 6-8% at <7 V. The best device performance was exhibited using MPO12 as the host when an appropriate hole-blocking layer was implemented. The higher performance of MPO12 was attributed to the ambipolar charge-transporting character of the polar carbazole derivative. However, exciton relaxation on nonradiative aggregate states of all host materials studied may limit further improvements in device efficiencies.

Phosphine oxide substitution of small molecules with high triplet exciton energies allows development of vacuum sublimable, electron transporting host materials for blue OLEDs. Heteroaromatic building blocks (carbazole, dibenzofuran and dibenzothiophene) with ET ~ 3 eV were incorporated into phosphine oxide (PO) structures. External quantum efficiencies (EQEs) at lighting brightness (i.e., 800 cd/m2) reached as high as 9.8% at 5.2V for OLEDs using the heteroaromatic PO hosts doped with the sky blue phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2,) picolinate (FIrpic). Comparing device properties at a similar current density (i.e., J = 13 mA/cm2) showed the dibenzothiophene-bridged PO compound exhibits the highest EQEs and lowest operating voltages at all phosphor dopant levels. These results are explained with respect to the effects of the inductive phosphine oxide substituents on electrochemical, photophysical and electroluminescence properties of the substituted heteroaromatic building blocks.

The chemical composition, density, and modulus of PU foam was examined as a function of processing temperature (25 °C, 45 °C, and 85 °C). Degradation of uretoneimine cross-links and the emergence of carbodiimide functional groups are monitored and the remaining composition in the PU foam is determined using FTIR spectroscopy. The density and modulus of the PU foams after thermal processing are measured and discussed with respect to the change in chemical composition of the materials. The data indicates that the composition of chemical cross-links, the density, and the modulus all decrease as a function of increasing processing temperature. The modulus of the PU foam shows a linear dependence of the processing temperature decreasing by 20% at the highest temperature. The results suggest that processing temperature may be used to target defined physical and mechanical properties of PU foams without changing the composition of reactants used.